Industrial Gases Processing
Edited by
Heinz-Wolfgang Häring
1345vch00.indd I
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Further Reading
A. Züttel, A. Borgschulte, L. Schlapbach (Eds.)
Hydrogen as a Future Energy Carrier
2008
ISBN 978-3-527-30817-0
G. A. Olah, A. Goeppert, G. K. S. Prakash
Beyond Oil and Gas: The Methanol Economy
2006
ISBN 978-3-527-31275-7
1345vch00.indd II
10.11.2007 12:47:57
Industrial Gases Processing
Edited by
Heinz-Wolfgang Häring
Translated by Christine Ahner
1345vch00.indd III
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The Editor
Dr. Heinz-Wolfgang Häring
Lommelstrasse 6
81479 München
Germany
Cover Illustration:
Hydrogen plant, Oberhausen, Germany,
with kind permission of Linde AG
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statements, data, illustrations, procedural details or
other items may inadvertently be inaccurate.
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¤ 2008 WILEY-VCH Verlag GmbH & Co. KGaA,
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All rights reserved (including those of translation
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ISBN 978-3-527-31685-4
1345vch00.indd IV
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V
Foreword
Industrial gases have become over a period of more than a century ubiquitous
ingredients of our daily activities, e.g. metal fabrication, metallurgy, petrochemicals, food processing, healthcare, and many more.
In 2006, the business generated globally with industrial gases exceeded the
50 billion US dollar volume.
At The Linde Group, generations of scientists and engineers have been working
in this field, determining the physical and chemical properties, developing
processes for the production, purification and application of industrial gases, as
well as their safe handling, storage and transportation.
This book, written and compiled by numerous experts and edited by Dr.
Wolfgang Häring, is an authoritative, accurate, and useful single-source reference
for those who work in this industry, for students or simply for the users of
industrial gases. Details are also offered concerning the historical background
of these molecules.
The term “industrial gases” is herewith intended to include also the category of
“medical gases”, which, while being produced by means of “industrial” processes,
have meanwhile become real drugs and are subject to Good Manufacturing
Practices.
It is my pleasure to commend the editor as well as the authors of this outstanding
piece of technical literature for their relentless and professional quest for precision
and completeness, which for sure will be highly appreciated by all readers.
Pullach, October 2007
Dr. Aldo Belloni
Linde AG
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
1345vch00.indd V
10.11.2007 12:48:13
VII
Contents
Foreword V
List of Contributors XIII
1
Introduction 1
References 7
2
The Air Gases Nitrogen, Oxygen and Argon 9
History, Occurrence and Properties 9
Nitrogen 9
History 9
Occurrence 9
Physical and Chemical Properties 10
Oxygen 11
History 11
Occurrence 11
Physical and Chemical Properties 11
Argon 12
History 12
Occurrence 13
Physical and Chemical Properties 13
Recovery of Nitrogen, Oxygen and Argon 13
Introduction 13
Application Range of Membrane Separation, Pressure Swing
Adsorption and Cryogenic Rectification 14
Nitrogen Recovery with Membranes 16
Physical Principle 16
Membrane Technology 16
Design 17
Nitrogen and Oxygen Recovery by Means of Pressure Swing
Adsorption 18
Physical Principle 18
2.1
2.1.1
2.1.1.1
2.1.1.2
2.1.1.3
2.1.2
2.1.2.1
2.1.2.2
2.1.2.3
2.1.3
2.1.3.1
2.1.3.2
2.1.3.3
2.2
2.2.1
2.2.2
2.2.3
2.2.3.1
2.2.3.2
2.2.3.3
2.2.4
2.2.4.1
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
1345vch00.indd VII
10.11.2007 12:48:32
VIII
Contents
2.2.4.2
2.2.4.3
2.2.4.4
2.2.5
2.2.5.1
2.2.5.2
2.2.5.3
2.2.5.4
2.2.5.5
2.2.5.6
2.2.5.7
2.3
2.3.1
2.3.3
2.3.4
2.3.5
2.4
2.5
2.5.1
2.5.1.1
2.5.1.2
2.5.2
2.5.3
3
3.1
3.2
3.3
3.3.1
3.3.2
3.3.2.1
3.3.2.2
3.4
3.4.1
3.4.2
3.5
3.6
3.6.1
3.6.2
3.6.3
1345vch00.indd VIII
Properties of Molecular Sieves 18
Nitrogen Recovery 19
Oxygen Recovery 20
Cryogenic Rectification 20
Process with Air Booster and Medium-Pressure Turbine
for the Recovery of Compressed Oxygen, Nitrogen and Argon 21
Internal Compression 32
Nitrogen Generators 36
Liquefiers 37
High-purity Plants 38
Apparatus 42
Design, Assembly and Transport of the Coldbox 57
Safety Aspects 59
Introduction 59
Air Pollution 61
Ignition in Reboilers 63
Other Hazards in Air Separation Units 64
Process Analysis Air Separation Units 64
Applications of the Air Gases 67
Applications of Nitrogen 67
Applications of Nitrogen for Inerting and Purging 67
Applications of Nitrogen for Cooling, Preserving and
Deep-Freezing 74
Applications of Oxygen 83
Applications of Argon 104
References 108
The Noble Gases Neon, Krypton and Xenon 111
History and Occurrence 111
Physical and Chemical Properties 111
Recovery of Krypton and Xenon 112
Pre-enrichment in the Air Separator 113
Recovery of Pure Kr and Xe 115
Catalytic Combustion of Hydrocarbons 115
Cryogenic Separation 116
Recovery of Neon 118
Pre-enrichment 118
Fine Purification 119
Industrial Product Purities and Analytics 120
Applications of the Noble Gases Neon, Krypton and Xenon 121
Applications of Neon 121
Applications of Krypton 121
Applications of Xenon 122
References 124
10.11.2007 12:48:32
Contents
4
4.1
4.1.1
4.1.2
4.1.3
4.2
4.3
5
5.1
5.1.1
5.1.2
5.1.3
5.1.3.1
5.1.3.2
5.1.4
5.1.4.1
5.1.4.2
5.2
5.2.1
5.2.2
5.2.2.1
5.2.2.2
5.2.2.3
5.2.3
5.2.3.1
5.2.3.2
5.2.3.3
5.2.3.4
5.2.3.5
5.2.3.6
5.2.4
5.2.4.1
5.2.4.2
5.2.4.3
5.3
5.4
5.4.1
5.4.1.1
5.4.1.2
5.4.1.3
1345vch00.indd IX
IX
The Noble Gas Helium 125
History, Occurrence and Properties 125
History 125
Occurrence 125
Physical and Chemical Properties 127
Recovery 127
Applications 131
References 134
Hydrogen and Carbon Monoxide: Synthesis Gases 135
History, Occurrence and Properties 135
Introduction 135
History of Synthesis Gas 136
Hydrogen 136
History and Occurrence 136
Physical and Chemical Properties 137
Carbon Monoxide 141
History and Occurrence 141
Physical and Chemical Properties 141
Production of Synthesis Gas 143
Production of Hydrogen by Electrolysis 143
Production of Synthesis Gas from Hydrocarbons 144
Generation of Synthesis Gas by Steam Reforming 145
Synthesis Gas Generation by Partial Oxidation (PO) 146
Generation of Synthesis Gas by Autothermal Reforming
(ATR) 148
Synthesis Gas Processing 150
Water–Gas Shift Reactor 150
Removal of Carbon Dioxide and Acid Gases 150
Methanation 151
Pressure Swing Adsorption (PSA) 151
Membrane Processes 152
Cryogenic Separation Processes 153
Processes for the Production of Synthesis Gas from
Hydrocarbons 156
Reformer Plant for the Production of Hydrogen 157
Reformer Units for the Generation of CO and H2 158
PO-plant for the Production of CO and H2 159
Process Analytics 161
Applications of Hydrogen, Carbon Monoxide and Syngas 164
Applications of Hydrogen 164
Hydrogen Use in the Chemical Industry 165
Hydrogen as an Energy Carrier 166
Fuel Cells 177
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X
Contents
5.4.2
5.4.3
Applications of Carbon Monoxide 182
Applications of Synthesis Gas (Mixtures of CO and H2)
References 182
6
Carbon Dioxide 185
History, Occurrence, Properties and Safety 185
History 185
Occurrence 185
Physical and Chemical Properties 186
Safety Issues 188
Recovery of Carbon Dioxide 189
Sources of Carbon Dioxide Recovery 190
Pre-purification, Enrichment, Extraction, Capture 191
Standard Process for the Liquefaction of Carbon Dioxide 193
Compression and Water Separation 194
Adsorber Station 194
Liquefaction and Stripping of Lighter Components 194
Refrigerating Unit 194
Process Steps to Obtain High Product Purity and Recovery Rate 195
Scrubbing 196
Adsorption and Chemisorption 197
Catalytic Combustion 198
Improvement of the Carbon Dioxide Recovery Rate 198
Carbon Dioxide Recovery from Flue Gas 198
Production of Dry Ice 200
Applications 201
References 215
6.1
6.1.1
6.1.2
6.1.3
6.1.4
6.2
6.2.1
6.2.2
6.2.3
6.2.3.1
6.2.3.2
6.2.3.3
6.2.3.4
6.2.4
6.2.4.1
6.2.4.2
6.2.4.3
6.2.4.4
6.2.5
6.2.6
6.3
7
7.1
7.2
7.3
7.4
7.5
7.6
7.6.1
7.6.2
7.6.3
7.6.4
7.6.5
7.7
1345vch00.indd X
182
Natural Gas 217
History 217
Occurrence 218
Consumption 220
Natural Gas Trade 220
Composition 223
Process of Natural Gas Treatment 224
Dew-point Adjustment 224
Separation of Liquefied Petroleum Gas 225
Ethane Separation 229
Liquefaction 231
Nitrogen Separation 236
Applications 238
10.11.2007 12:48:32
Contents
8
8.1
8.2
8.2.1
8.2.2
8.2.3
8.2.4
8.2.5
8.2.6
8.2.7
8.2.8
8.2.8.1
8.2.8.2
8.2.9
8.2.9.1
8.2.9.2
8.2.10
8.2.11
8.3
8.3.1
8.3.2
8.3.3
8.3.4
8.3.5
8.4
8.5
9
9.1
9.2
9.2.1
9.2.2
9.2.3
9.2.4
9.2.5
9.3
9.3.1
9.3.2
9.3.2.1
9.3.2.2
9.3.2.3
9.3.2.4
9.3.3
1345vch00.indd XI
XI
Fuel Gases 239
Introduction 239
Acetylene C2H2 240
Acetylene and the Beginnings of Welding Engineering 240
Physical Properties 241
Acetylene Decomposition – Deflagration 243
Ignitable Mixtures 243
Liquefaction of Acetylene – Acetylene Hydrate 243
Acetylene Hydrate 244
Acetylides 244
Extraction Processes 246
Acetylene Generated via Carbide 246
Petrochemically Generated Acetylene 246
Gas Supply 246
Storage of Dissolved Acetylene in Cylinders 246
Design of a Gas Supply System 247
Autogenous Engineering Applications 247
Regulations 248
Ethene C2H4 249
Physical Properties 249
Production Processes 249
Application and Use 249
Gas Supply and Safety 249
Regulations 251
Other Fuel Gases 251
Applications 252
References 253
Specialty Gases 255
Introduction 255
Pure Gases 256
Definitions 256
Quality Criteria 256
Sources/Production 257
Purification/Processing 257
Application Examples 258
Gas Mixtures/Calibration Gas Mixtures 261
Definitions 261
Production [9.9] 263
Technical Feasibility 263
Pretreatment of Containers 263
Preparation Methods 264
Analytical Quality Assurance 267
Application Examples 269
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XII
Contents
9.4
9.4.1
9.4.2
9.5
9.6
9.6.1
9.6.2
9.6.3
9.6.4
Electronic Gases 269
Definition/Special Demands 269
Application Examples 271
Disposal 271
Transfer of Gases 272
Selection of the Materials 272
Physical Interaction Forces 272
Tightness of the Gas Supply System 273
Purging of the Gas Supply System 273
References 275
10
Gases in Medicine 277
Introduction 277
Medicinal Oxygen 278
Home-therapy 278
Hospitals and Other Fields of Application 280
Gases for Anaesthesia 280
Medical Nitrous Oxide (Laughing Gas) 280
Xenon 281
Medical Carbonic Acid (Carbon Dioxide) 281
Medical Air 282
References 282
10.1
10.2
10.2.1
10.2.2
10.3
10.3.1
10.3.2
10.4
10.5
11
11.1
11.2
11.2.1
11.2.2
11.2.3
11.3
11.3.1
11.3.2
11.3.3
11.4
Logistics of Industrial Gas Supply 283
Introduction 283
Storage and Transport of Compressed Gases 284
Fundamentals 284
Kinds of Transport and Storage for Compressed Gases 285
Efficiency of Compressed Air Gas Transport 286
Storage and Transport of Liquefied Compressed Gases 286
Fundamentals 286
Forms of Transport and Storage of Liquefied Gases 287
Efficiency of the Transport of Liquefied Gases 288
Special Forms of Supply 289
References 289
Subject Index 291
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XIII
List of Contributors
Dr. Heinz Bauer
Linde AG
Geschäftsbereich Linde Engineering
LE-GD
Dr.-Carl-von-Linde-Strasse 6–14
82049 Pullach
Germany
Sections 7.1–7.6
Dr. Harald Klein
Linde AG
Geschäftsbereich Linde Engineering
LE-HDV
Dr.-Carl-von-Linde-Strasse 6–14
82049 Pullach
Germany
Section 5.2
Dr. Michael Berger
Linde AG
Geschäftsbereich Linde Gas
Carl-von-Linde-Strasse 25
85716 Unterschleißheim
Germany
Sections 2.5, 3.6, 4.3, 5.4, 6.3,
7.7, 8.5
Dr. Matthias Meilinger
Linde AG
Geschäftsbereich Linde Engineering
LE-EC
Dr.-Carl-von-Linde-Strasse 6–14
82049 Pullach
Germany
Sections 2.1, 2.3, 2.4, 3.1, 3.2, 3.5,
5.3, 6.1
Dr. Matthias Duisberg
Umicore AG & Co. KG
Bereich HC
Postfach 1351
63403 Hanau
Germany
Section 5.4.1.3
Dr. Heinz-Wolfgang Häring
Lommelstrasse 6
81479 München
Germany
Chapter 1
Johann Raab
Linde AG
Geschäftsbereich Linde Gas
LG-THA
Seitnerstrasse 70
82049 Pullach
Germany
Sections 8.1–8.4
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
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XIV
List of Contributors
Dr. Harald Ranke
Linde AG
Geschäftsbereich Linde Engineering
LE-HPP
Dr.-Carl-von-Linde-Strasse 6–14
82049 Pullach
Germany
Section 5.1
Dieter Tillmann
Linde AG
Geschäftsbereich Linde Gas
LG-SEP
Seitnerstrasse 70
82049 Pullach
Germany
Sections 6.1, 6.2
Jaco Reijerkerk
Linde AG
Geschäftsbereich Linde Gas
Hydrogen Solutions
Seitnerstrasse 70
82049 Pullach
Germany
Section 5.4.1.2
Bernhard Valentin
Linde AG
Geschäftsbereich Linde Engineering
LE-BE
Dr.-Carl-von-Linde-Strasse 6–14
82049 Pullach
Germany
Chapter 11
Dr. Hans Schmidt
Linde AG
Geschäftsbereich Linde Engineering
LE-GDV
Dr.-Carl-von-Linde-Strasse 6–14
82049 Pullach
Germany
Sections 4.1, 4.2
Dr. Kurt Wilde
Sommerstrasse 1
82234 Weßling
Germany
Sections 2.5, 3.6, 4.3, 5.4, 6.3, 7.7, 8.5,
Chapter 9
Dr. Dirk Schwenk
Linde AG
Geschäftsbereich Linde Engineering
LE-LDV
Dr.-Carl-von-Linde-Strasse 6–14
82049 Pullach
Germany
Sections 2.2, 3.3–3.5
Dr. Joachim Wolf
Linde AG
Geschäftsbereich Linde Gas
Hydrogen Solutions
Seitnerstrasse 70
82049 Pullach
Germany
Section 5.4.1.2
Dr. Hermann Stenger
Linde Gas
Therapeutics GmbH & Co. KG
Gas Solutions Hospital Care
Edisonstrasse 2
85716 Unterschleißheim
Germany
Chapter 10
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1
1
Introduction
The history of industrial gases is inextricably linked to the rapid pace of industrialisation that marked the nineteenth century. The large-scale generation of certain
gases opened the door for new types of technologies and production processes.
Acetylene, for example, was discovered by E. Davy in 1836. A significant
landmark followed in 1862, when F. Wöhler succeeded in producing acetylene
from the reaction between calcium carbide and water. Then, in 1892, T. L. Wilson
and H. Moissan discovered a process for generating calcium carbide in an electric
furnace. This paved the way for industrial-scale production of acetylene in 1895
(see also Section 8.2). Initially, acetylene was mainly used for lighting purposes
due to its bright flame. Later, its high combustion temperature in oxygen prompted
development of autogenous cutting and welding technology, starting in 1901.
An even more important step from today’s perspective was the liquefaction of
air by Carl von Linde, marking the birth of an entirely new industry. C. v. Linde
employed the Joule–Thomson effect, decreasing the temperature of the gas by
adiabatic expansion. In 1895, he achieved continuous generation of liquid air at
a yield of three litres per hour using a laboratory plant [1.1]. The following years
saw the construction and delivery of the first small commercial air liquefaction
plants. Figure 1.1 shows a typical early air liquefier (ca. 1899).
Fig. 1.1 Typical assembly of a Linde air liquefier (ca. 1899).
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
1345vch01.indd 1
26.10.2007 10:26:39
2
1 Introduction
Fig. 1.2 Linde air separation unit for simultaneous production of
200 m3 h–1 oxygen and 1,800 m3 h–1 nitrogen (ca. 1919).
In 1902, C. v. Linde started using a rectification process to separate liquid air for
continuous oxygen production at a purity above 99%. High-purity nitrogen was
first recovered in 1905. And five years later, in 1910, simultaneous production of
oxygen and nitrogen became possible with C. v. Linde’s invention of the doublecolumn rectifier. Figure 1.2 shows one of these plants (ca. 1919).
During this period, there was particularly strong competition between C. v. Linde
and G. Claude, one of the founders of L’Air Liquide S.A in 1902, thus spurring
further development of air separation technology and resulting in important
improvements [1.2].
A century on, innovations in air separation technology have spawned some
impressively large plants: Air Liquide installed an air separation unit in 2004 to
feed pressurised gaseous oxygen (GOX) to a Sasol partial oxidation plant in South
Africa at a rate of 3,500 td–1, for instance. In 2006, Linde received a construction
order from Shell for the biggest air separation facility ever built, with eight units
producing a total of 30,000 td–1 GOX to feed Pearl, the world’s largest gas-to-liquid
(GTL) plant in Qatar. And since 2000, the four units of the Linde air separation
facility at Cantarell, Mexico have been producing a total of 40,000 td–1 pressurised
gaseous nitrogen (GAN), which is injected into the well to enhance oil recovery
(see Figure 1.3). A fifth unit is now also in operation.
The industrial-scale availability of nitrogen and hydrogen at the turn of the
19th to the 20th century enabled a host of new applications. The BASF company,
for example, succeeded in developing an ammonia synthesis from nitrogen and
hydrogen in 1913. This paved the way for mass production of fertilisers.
A good overview of the historical development and pioneers of industrial gases
may be found in [1.3–1.5].
1345vch01.indd 2
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1 Introduction
3
Fig. 1.3 Air separation unit at Cantarell, Mexico (2000).
The following table sets out some of the chronological milestones in the history
of industrial gases:
1766
1783
Production of pure hydrogen by H. Cavendish
First flight of a hydrogen-filled balloon (J. Charles) using
hydrogen generated from the reaction between iron and
sulphuric acid
1853
J. P. Joule and W. Thomson observe a temperature decrease
caused by the adiabatic expansion of compressed gases
(Joule–Thomson effect)
1868
First operation under nitrous oxide (“laughing gas”)/oxygen
anaesthetic performed by Andrews
1892
H. Moissan and Th. L. Wilson discover a method for generating
calcium carbide in an electric furnace, enabling industrial
production of acetylene in 1895
1895
C. v. Linde builds the first technical apparatus for the
liquefaction of air
1898
Liquefaction of hydrogen by J. Dewar
1898
Discovery of the noble gases neon (Ne), krypton (Kr) and
xenon (Xe) (1868: helium, He, 1894: argon, Ar)
1900
First flight of a hydrogen-filled Zeppelin airship
1901 onwards The high combustion temperature of acetylene in oxygen
inspires development of autogenous welding technology
1902
C. v. Linde employs a rectification process for technical
production of liquid oxygen
1902
G. Claude invents the piston expansion machine for air
liquefaction
1908
Liquefaction of helium by H. Kamerlingh-Onnes
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4
1 Introduction
1910
1913
1917
1922
1925
1936 onwards
1940s onwards
1942
1950s onwards
1960s onwards
1961
1961
1962
1963
1965
1980s onwards
1980s onwards
1999
C. v. Linde invents the double-column rectifier for simultaneous
production of oxygen and nitrogen
Technical ammonia synthesis from nitrogen and hydrogen by
F. Haber and C. Bosch (BASF)
First extraction of helium from natural gas in Hamilton, Canada
Technical methanol synthesis from synthesis gas by G. Patart
(BASF)
Development of Fischer–Tropsch synthesis, i.e. catalytic
synthesis of hydrocarbons using synthesis gas (mixture of
hydrogen and carbon monoxide) by F. Fischer and H. Tropsch.
Industrial application since 1932
Commercial use of the Lurgi process to generate synthesis gas
from carbon using oxygen and steam
Use of Ar and He in tungsten inert gas (TIG) welding
Use of liquid oxygen for a V2 missile
Use of carbon dioxide in metal active gas (MAG) welding
Use of high-purity electronic gases in manufacturing
semiconductor elements (contaminations in lower ppb range)
First continuous helium/neon laser
Linz-Donawitz (LD) process for steel manufacture by injecting
oxygen into the converter
First use of liquid nitrogen for cryogenic (shock) freezing
of food
Use of liquid hydrogen and liquid oxygen as fuel for space
travel (USA)
Commercial use of argon-oxygen decarburization (AOD)
process to produce austenitic stainless steel
Use of liquid helium for superconducting magnets in nuclear
magnetic resonance tomography, for particle accelerators and
fusion reactors
CO2 laser for cutting metal
First public hydrogen fuelling station for cars and buses at
Munich Airport, Germany
Which gases are classified as industrial today? According to [1.6], the term
“industrial gases” is “a collective term for combustible and non-combustible gases
generated on an industrial scale, such as hydrogen, oxygen, nitrogen, carbon
dioxide, acetylene, ethylene, noble gases, ammonia, water gas, generator gas,
city gas, synthesis gas, etc.”. Taking global market share (percentage of sales),
[1.9] identifies the major candidates here as oxygen (29%), nitrogen (17%), argon
(10%), carbon dioxide (9%), acetylene (7%), hydrogen (5%) and helium (1%). The
total share of all other industrial gases together is 22% of the global gas market.
This includes carbon monoxide, nitrous oxide (“laughing gas”), the noble gases
krypton, xenon and neon, and a large number of specialty gases and gas mixtures
for different applications. Some of the most common specialty gases here are
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1 Introduction
5
nitrogen trifluoride, silane, arsine, phosphane (phosphine), tungsten hexafluoride
and sulphur hexafluoride (see Chapter 9).
Who are the main suppliers? Companies that supply industrial gases can be
divided into two tiers according to sales. Three different supply models are used
– on-site (including pipeline), bulk and cylinder delivery.
In the first tier, with sales exceeding USD 1 billion, there are seven major
companies whose combined gas-related revenue accounted for over 75% of the
global market at the end of 2005:
AL:
BOC:
AP:
Praxair:
Linde:
TNS:
Airgas:
Air Liquide (French gas company)
BOC Gases (UK gas company)
Air Products and Chemicals, Inc. (US gas company)
Praxair, Inc. (US gas company)
Linde Gas (German gas company)
Taiyo Nippon Sanso Co. (Japanese gas company)
Airgas, Inc. (major US distributor)
Figure 1.4 [1.7] shows the global market shares of the first-tier companies in 2005.
This reflects the market situation prior to the acquisition of the BOC Group by
Linde AG to form a leading gas and engineering company under the name of
The Linde Group in 2006.
The second tier, with sales below USD 1 billion, contains a larger number of
companies such as Iwatani (Japan), Messer (Germany), Air Water (Japan), Sapio
(Italy), Cryoinfra (Mexico) and Indura (South America).
In addition, there are numerous smaller gas companies active at national or
even regional levels. The strength of local gas companies often lies in the high
costs entailed in transporting compressed gas in steel cylinders and cryogenically
liquefied gas in tank trucks. Production in the customers’ vicinity is therefore a
more economical alternative.
The value of the global industrial gas business reached USD 49 billion (EUR
39 billion) in 2005, an increase of 9% from 2004. Indeed, for the seven first-tier
Fig. 1.4 Global market shares of industrial gas companies, 2005.
1345vch01.indd 5
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6
1 Introduction
Fig. 1.5 Global gas business by end-use sector, 2005.
companies, the increase was as high as 12.1% [1.7]. The leading markets are still
North America (33%) and Western Europe (29%), which remain far ahead of the
rest of the world. The forecast for 2005 to 2010 anticipates a compound annual
growth rate of 7.8% for the industrial gas business, with the highest rates expected
in the Middle East, Eastern Europe and Asia [1.7] (see also [1.8]).
Finally, who are the end-users? The applications of industrial gases span
medicine, food, metallurgy, glass, ceramics and other minerals, rubbers and
plastics, paints, environmental protection, water treatment, chemicals, cutting
and welding, safety, semiconductors and aerospace, to name just a few. Figure 1.5
provides an overview of the main industries and market sectors supplied by gas
companies, together with a growth forecast for 2005 to 2010 [1.7] (see also [1.9]).
The chemical, healthcare and electronics industries are set to be the main growth
drivers.
Various associations have been set up to cater for the common interests of
industrial gas companies worldwide. These span all fields of activity from gas
production through storage, transport and delivery to the actual application, not
forgetting equipment manufacture [1.10]. Two of the main associations are
x the Compressed Gas Association (CGA) and
x the European Industrial Gases Association (EIGA)
The North-American CGA [1.11] was established as far back as 1913. Its European
counterpart is the EIGA [1.12]. The EIGA was preceded by the “Commission
Permanente Internationale (CPI) de l’acétylène, de la soudure autogène et des
industries qui s’y rattachent”, founded in Paris in 1923, which merged with the
EDIA, the European Dry Ice Association in 1989, maintaining the name CPI. The
institution started operating as the European Industrial Gases Association (EIGA)
in 1990 and is currently headquartered in Brussels.
These associations were founded with a view to self-regulation and to enable
joint solutions to safety issues. Right from the beginning, the main task of
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References
7
the associations was to define and introduce safety standards and implement
regulations. This was in response to various accidents – some serious – that
occurred in the early days of industrial gas production, involving oxygen and
combustible gases such as hydrogen and acetylene. Even now, safety issues still
constitute an essential part of the work of these associations. The CGA, EIGA
and JIGA, the Japanese Industrial Gas Association, are in particularly close
collaboration. Efforts are currently underway to harmonise safety standards
worldwide, defining common standards over and above those of individual
associations. These safety recommendations are primarily based on the analysis
of accidents reported to the association by the companies involved. CGA’s goal
is to adapt existing standards every five years to reflect the latest knowledge.
Section 2.3 also contains information about the safety requirements of air
separation plants.
Apart from those mentioned above, there are a number of other industrial gas
associations [1.10], including
x
x
x
x
China Industrial Gases Industry Association (CIGIA)
International Oxygen Manufacturers Association (IOMA)
Asia Industrial Gases Association (AIGA)
Gases and Welding Distributors Association (GAWDA)
Almost all German companies producing, filling or selling industrial gases are
members of the German association Industriegaseverband e.V. (IGV). The IGV
[1.13] is a member of the EIGA through its membership of the chemical industry
trade association, Verband der Chemischen Industrie e.V. (VCI).
This book focuses on the industrial gases of greatest commercial importance.
It describes their history and properties, the processes involved in generating or
separating them and their industrial and consumer applications, as well as their
distribution logistics. It also discusses the future of hydrogen technology.
At the end of each gas type chapter, the typical gas applications are listed.
For reasons of clarity they are divided into industry segments (e.g. metallurgy,
chemistry).
The most important applications are described in more detail in concrete
application examples. These are indicated in the text by capital letters in bold
type (e.g. Example A).
References
[1.1] C. Linde: Aus meinem Leben und von meiner Arbeit (1916), reprint Oldenbourg Verlag,
Munich, 1979, p. 87.
[1.2] W. Foerg: The History of Air Separation, MUST ’96, Munich Meeting on Air Separation
Technology, 1996, pp. 1–12.
[1.3] E. Almqvist: History of Industrial Gases, Kluwer Academic/Plenum Publishers,
New York, 2003.
1345vch01.indd 7
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8
1 Introduction
[1.4] Winnacker-Küchler, Chemische Technologie, Vol. 3: Anorganische Technologie, 4th edition,
Carl Hanser Verlag, 1983, pp. 566–650.
[1.5] W. Linde et al.: The Invisible Industries/The Story of the Industrial Gas Industry,
International Oxygen Manufacturers Association (IOMA), Cleveland, OH 1997.
[1.6] Römpp, 10th edition, Thieme Verlag, Stuttgart, 1997, p. 1915.
[1.7] Spiritus Consulting Ltd.: Annual Report 2005, Cornwall, UK, 2006,
www.spiritusgroup.com.
[1.8] Datamonitor: Global Industrial Gases Research Report, 2005.
[1.9] E. Gobina: C-237 – The World Industrial Gas Business, Business Communications Co.,
Inc., Norwalk, CT, Oct. 2003.
[1.10] CryoGas Staff Report: A Look at the Various Industrial Gas Associations, Jan. 2003,
pp. 28–35.
[1.11] Compressed Gas Association (CGA): www.cganet.com.
[1.12] European Industrial Gases Association (EIGA): www.eiga.org.
[1.13] Industriegaseverband e.V. (IGV): www.industriegaseverband.de.
1345vch01.indd 8
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9
2
The Air Gases Nitrogen, Oxygen and Argon
2.1
History, Occurrence and Properties
2.1.1
Nitrogen
2.1.1.1
History
Owing to its inertness, nitrogen as an element was discovered relatively late. Still
in the 17th century, air was supposed to be a homogeneous substance. Only in the
year 1770, Scheele and Priestley discovered nitrogen (dinitrogen, N2) as a component
of air that does not feed combustion. The chemical symbol N derives from the Latin
word “nitrogenium” (nitrum generating). In 1784, Cavendish obtained nitrogen
oxides and nitric acid through electric discharge in the air. Only at the beginning
of the 20th century, atmospheric nitrogen was used for the large-scale production
of calcium nitrate (Frank and Caro, 1902), nitric acid (Birkeland and Eyde, 1905) and
ammonia (Haber and Bosch, 1913). One prerequisite was the large-scale availability
of N2 through the rectification of liquefied air, successfully performed by Carl von
Linde as of 1902. N2 was liquefied by Cailletet for the first time in the year 1877.
Today about 85% of the N2-output are used for the production of fertilizers for
farming, e.g. ammonia salts, nitrates, lime nitrogen (calcium cyanamide, CaCN2),
lime ammonium nitrate and urea [2.1].
2.1.1.2
Occurrence
The content of nitrogen in the upper 16 km thick earth’s crust is assessed at a mass
fraction of about 0.03%. Thus, it belongs to the more frequently found elements.
The atmosphere with a N2-content of 78.1% volume fraction or 75.51% mass
fraction contains the largest quantities with 3.9 · 1015 t [2.2]. Smaller quantities
of N2 are found dissolved in gases of springs and rock inclusions. Bound nitrogen
occurs, for example, in nitrates and ammonium compounds. So since 1825,
natural Chile saltpetre mainly consisting of sodium nitrate, has been exploited on
a large scale as fertilizer. Today however, this happens mostly to recover the iodine
contained within. Bound nitrogen, too, exists in the proteins in all organisms and
is returned into the N2-cycle through decomposition reactions. Nitrogen oxides are
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
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2 The Air Gases Nitrogen, Oxygen and Argon
formed by the reaction of the airborne oxygen and nitrogen in flashes of lightning,
subsequently washed out by rain and deposited on the soil as nitrates (about 0.1 g
of nitrate-nitrogen on 1 m2 of soil in one year).
2.1.1.3
Physical and Chemical Properties
At atmospheric pressure and room temperature, nitrogen is a colourless, odourless
and non-flammable gas. At 0 °C and 1.013 bar, 1 L of nitrogen weighs 1.2505 g.
N2 condenses at –195.8 °C to a colourless liquid with a density of 0.812 kg L–1
that gets solid at –209.86 °C in the form of white crystals. The solubility of N2 in
water amounts to 23.2 mL per kilogram of water at 0 °C and 1 bar, at 25 °C it is
only 13.8 mL kg–1. Consequently, N2 is less soluble in water than O2.
The inversion temperature of N2 is 850 K. Inversion temperature is the temperature below which a gas cools down by adiabatic expansion (Joule-Thomson-Effect).
Therefore, N2 can be liquefied from room temperature by means of counter-cooling
of previously expanded cold gas, in contrast to H2 und He.
Nitrogen is an element of the 5th main group of the periodic system and occurs
in compounds in the oxidation stages –3 (e.g. NH3) to +5 (e.g. HNO3). In the N2molecule, both atoms are linked with a homopolar triple bond which is the reason
for the stability and the inert character of the molecule. Therefore, high activation
energy has to be supplied for the reaction of N2 with other substances, e.g. through
electric discharge or high temperature. Even at 3000 °C, there is no noticeable
dissociation into the atoms (K = c2(N) / c(N2) | 10–6). Nitrogen as molecule only
reacts with lithium at room temperature and with calcium and magnesium at
higher temperatures to the respective metal nitrides. Other metals, such as Al, Ti,
V and Cr form nitrides only at red heat. An important product from the reaction
with boron is boric nitride which is used as grinding material. Owing to its inert
character, N2 is often used as shielding gas, e.g. in chip production.
The nitrogen atom in compounds is often threefold coordinated and has a
tetragonal structure with the free electron pair in one corner of the tetrahedron.
Nitrogen occurs in varied forms in organic molecules, e.g. in amines (R–NH2),
amides (R–C(=O)–NH2), nitriles (R–C{N), oximes (R2C=N–OH) and nitrogenheterocycles (e.g. pyridine) [2.3].
One of the research aims of the past years was the nitrogen fixation under
mild conditions, especially from the aspect of the synthesis of plant-based
protein through transformation of atmospheric nitrogen into ammonium ions
by microorganisms (atmospheric fertilization). This biological nitrogen fixation is
catalyzed by the enzyme nitrogenase [2.4]. In addition to that, there are numerous
attempts to carry out nitrogen-fixation by chemical means. However, in contrast
to the Haber-Bosch-Synthesis of ammonia which requires high pressures and
temperatures (475–600 °C, 200 bar), the N2-fixation should occur under mild
conditions. Today, coordination compounds of nitrogen with molybdenum,
chrome, rhenium, tungsten, cobalt, nickel, titanium, manganese and all platinumgroup metals are known.
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2.1 History, Occurrence and Properties
11
2.1.2
Oxygen
2.1.2.1
History
Both Scheele (1772) and Priestley (1774) discovered oxygen quite independently of
each other. Scheele collected the developing gas of the thermal decomposition of
potassium nitrate and mercury oxide as O2 (dioxygen), whereas Priestley generated
oxygen and mercury from heated mercury oxide and demonstrated that at low
temperature, this reaction is running reverse to the metal oxide again. The Latin
name “oxygenium” (= acid generator) is said to come from the French chemist
Lavoisier who assumed that oxygen is contained in each acid and determines
decisively its properties. Lavoisier was the first to recognize combustion as
combination of oxygen with a fuel gas.
2.1.2.2
Occurrence
Oxygen is the most frequent element in our living space (atmosphere, hydrosphere
and earth’s crust). The weight proportion of oxygen in the upper 16 km of the earth’s
crust is assessed at 48.9%; oxygen occurs mainly in the form of compounds. The
earth’s atmosphere contains an average of 20.95% of O2 (23.1% mass fraction),
altogether about 1015 t. Up to a height of 90 km, the oxygen content of the air is
almost constant. At larger heights, O2 and N2-molecules are split into atoms due
to the ultraviolet portion of the sunlight. The concentration of the atmospheric
oxygen is in dynamic balance: Respiration and weathering consume oxygen while
oxygen is produced through assimilation (photo synthesis).Via photo synthesis,
glucose is generated from CO2 and H2O with the help of sunlight simultaneously
releasing O2. About 2.7 · 1011 t of O2 arise annually by photo synthesis. Apart
from this, oxygen results from the decomposition of water vapour in a height of
70–80 km and from the decay of CO2 in a height of approx. 115 km, however in
considerably smaller quantities. The major oxygen consumer is the sea, with the
respiration of sea organisms and the oxidation of organic material as the biggest
consumers [2.5].
2.1.2.3
Physical and Chemical Properties
At atmospheric pressure and room temperature, oxygen is a colourless and
odourless gas. At 0 °C and 1.013 bar, 1 L of oxygen weighs 1.429 g. O2 condenses
at –182.96 °C to a light-blue liquid with a density of 1.141 kg L–1 that solidifies at
–218.78 °C in the form of light-blue crystals. The solubility of O2 in water is 49.1 mL
O2 per litre at 0 °C and 1 bar, dropping to 31.1 mL at 20 °C [2.6]. The inversion
temperature of O2 is 767 °C. The average relative atomic mass is 15.9994. O2 is
paramagnetic in gaseous, liquid and solid state.
Oxygen is an element of the 6th main group (chalcogens) of the periodic table
of elements occurring in compounds mainly in the oxidation stage –2 (e.g. H2O).
Besides the biatomic form, even a triatomic form of the elementary oxygen occurs
under natural conditions, i.e. ozone (O3). Between –160 and –196 °C, O2 dimerizes
to unstable (O2)2-aggregates. With the supply of ignition energy, molecular oxygen
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2 The Air Gases Nitrogen, Oxygen and Argon
turns into an extremely reactive gas, which, under energy release, reacts with a
number of substances (e.g. carbon, hydrogen, hydrocarbons, sulphur, phosphorus,
magnesium, iron powder) to the corresponding oxides. These oxidation reactions
often happen under fire and are described as combustion. The reactions in pure
oxygen proceed much more oxidizing and are considerably more intensive than in
air (atmospheric oxygen) where O2 is diluted with N2 in a ratio of approx. 1 : 4.
Therefore, in plants with O2-concentrations above 21 vol.%, a series of safety
guidelines have to be obeyed (e.g. IGC Doc 13/02, Oxygen Pipeline Systems,)
that lay down, for instance, the range of materials and the pressure depending
maximum gas velocities for piping in oxygen service. The oxidation effect of O2 is
utilized in steel production by blowing the oxygen into molten steel which reduces
the C-content in unalloyed steel from initially 4% to . 2% (oxygen refining).
In addition, manganese, silicon and phosphorus, also contained in pig iron,
are combusted as well and removed as slag from the top. With the temperature
ranging under the ignition temperature, oxidation often occurs very slowly, for
instance, during rusting of iron and rotting of wood. The reason is the stability
of the double bond in the O2-molecule (binding energy –490.7 kJ mol–1). In
the paramagnetic normal state of the molecule (triplet state) two electrons are
arranged in two antibonding S*-orbitals according molecular orbital theory [2.7].
With this biradical structure, dehydrogenation reactions proceeding over an
intermediate peroxide radical (R–O–Ox) are easily explained. The diamagnetic
singlet oxygen (two electrons with antiparallel spin are in an antibonding S*molecule orbital) formed photochemically is an effective oxidizing agent and in
contrast to triplet oxygen, it adds to many organic double bond systems via [2+2]
and [2+4] cycloaddition reactions. Thus, selective oxidations can be carried out
on a ton scale in the odorant industry. O2 forms complexes with a lot of metals,
the most important of them is haemoglobin (iron complex), responsible for the
oxygen transport in blood due to the reversible O2 uptake.
2.1.3
Argon
2.1.3.1
History
In 1894, argon (Ar) was discovered by W. Ramsay and Lord John William
Rayleigh, who noticed that “nitrogen” isolated from the air had a higher density
(1.2567 g L–1 under normal conditions) than nitrogen recovered from ammonia
nitrite (1.2505 g L–1). Therefore, apart from nitrogen, they derived the presence of
another inert gas in the atmospheric air which is heavier than nitrogen. However,
the first to isolate argon was Cavendish 100 years before, who obtained a not
further reducible gas bubble of 1/120 of the original volume after the removal of
O2 and N2 from an air volume. Due to its inert chemical behaviour, the gas was
named argon (argos = Greek for inert). In the year 1938, fluorescent tubes filled
with argon were more and more used in the lighting industry. However, only
its application as shielding gas, e.g. for welding purposes, triggered off a high
worldwide demand for argon after 1950.
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2.2 Recovery of Nitrogen, Oxygen and Argon
2.1.3.2
13
Occurrence
Argon is the most frequent noble gas by far. Air contains an average volume
fraction of 0.93%. Furthermore water springs in larger depth (Geysire) contain
plenty of solved argon. According to the state of the art, argon is obtained by
cryogenic air separation.
2.1.3.3
Physical and Chemical Properties
At atmospheric pressure and room temperature, argon is a colourless, odourless
and non-flammable gas. At 20 °C and 1.013 bar, 1 L of Ar weighs 1.664 g. Argon
condenses at –185.88 °C to a colourless liquid with a density of 1.40 kg L–1 and
solidifies at –189.2 °C. Thus, the boiling point of argon ranges between that of N2
and O2. At 0 °C and 1 bar, the solubility of Ar is 51.5 mL per litre of water. Argon
has an average atomic mass of 39.948 and is diamagnetic in gaseous, liquid and
solid state.
Argon is a noble gas and thus appears in the 8th main group of the periodic
system. This monoatomic gas is completely inert and therefore technically used
as shielding gas against oxidation, e.g. in welding. In discharge tubes together
with other noble gases, certain colour effects can be obtained.
Attempts to generate real argon compounds such as ArF2 respect. ArF+ have
failed [2.9]. Together with water, argon forms a hydrate with a dissociation pressure
of 106 bar at 0 °C. At 0 °C and above 106 bar, this hydrate is stable. Moreover, with
hydroquinone an inclusion compound is known (clathrates) however containing
no real chemical bonds [2.8].
2.2
Recovery of Nitrogen, Oxygen and Argon
2.2.1
Introduction
Nitrogen, oxygen and argon are almost exclusively recovered from atmospheric
air. Table 2.1 shows the concentration of these gases as well as of further components relevant for air separation. Three separation methods are predominant,
namely membrane separation, pressure swing adsorption and low-temperature
rectification. These methods will be described in the following. As the cryogenic
rectification has a share of far more than 90% on the worldwide production, it
will be presented in greater detail. A typical cryogenic process for the recovery of
compressed oxygen, nitrogen and argon will be introduced. This process will be
used to classify the essential process steps, to characterize the key components and
to show how the development of these components has also evolved the processes.
Subsequent sections on nitrogen generators, high-purity plants and liquefiers will
give an impression of the creativity, by which the cryogenic separation technology
has been adapted to meet even special demands of the market.
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14
2 The Air Gases Nitrogen, Oxygen and Argon
Table 2.1 Composition of the dry atmosphere.
Volume fraction in the air
N2
78.12%
Kr
1.138 ppm
O2
20.95%
Xe
0.086 ppm
Ar
0.932%
Ne
18 ppm
CO2
~ 400 ppm
He
5.2 ppm
CO
~ 0.1 ppm
H2
0.5 ppm
N2O
0.31 ppm
2.2.2
Application Range of Membrane Separation, Pressure Swing Adsorption and
Cryogenic Rectification
The three separation techniques have different process properties, investment and
operating costs. A dedicated segment can be defined for each method, in which
it allows for the most economic gas production. Table 2.2 characterizes these
segments by their production capacity and gas purity. Of course, the numbers
given therein are no sharp limits but indicate reasonable application ranges.
Table 2.2 Application range of membrane separation, pressure swing
adsorption and cryogenic rectification.
Gas
Capacity
(mN3 h–1)
Typical purities
Preferred separation
method
Load range
N2
1–1000
< 99.5% 1)
Membrane
30–100%
5–5000
< 99.99% 1)
Pressure swing adsorption
30–100%
200–400 000
any with residual
concentrations down
to ppb range
Cryogenic rectification
60–100%
100–5000
< 95%
Vacuum pressure swing
adsorption
30–100%
1000–150 000
any with residual
concentrations down
to ppb range
O2 content mostly > 95%
Cryogenic rectification
60–100%
O2
Ar
1)
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Cryogenic rectification
Including argon.
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2.2 Recovery of Nitrogen, Oxygen and Argon
15
Conclusions:
x Oxygen is not recovered with membranes.
x Cryogenic separation is applied whenever high purity, large quantities, liquid
products or argon is required.
x Membrane and adsorption plants have a high load range and can be started
and powered up to full production within a few minutes. Especially when the
gas consumption fluctuates strongly, the flexibility of these plants reduces
the overcapacity, which has to be provided, and allows to save energy by fast
load matching. A cryogenic plant needs about 2 hours for the start from cold
condition until the beginning of production of oxygen and nitrogen.
x Membrane and adsorption plants are suitable to cover the demands of small
and medium-sized gas consumers on site. This on-site supply competes with
the delivery of liquid N2, O2 and Ar by trucks. Here the gas recovery and
liquefaction is performed in a large central cryogenic air separation unit, cf.
also Chapter 11.
The operating costs of the separation units are mainly determined by their
energy consumption. Tables 2.3 and 2.4 show the specific energy demand for the
production of N2 and O2 by the three separation methods. The figures are only
guidelines. The actual values depend on the detailed process design. Cryogenic
separation requires the smallest work, which, however, is still significantly larger
than the minimal separation work needed for a completely reversible process
[2.10].
Table 2.3 Energy needed for the production of one standard cubic meter
of nitrogen at 8 bar (kWh).
O2 content in nitrogen
2%
0.5%
Membrane
0.43
0.65
Pressure swing adsorption
0.26
0.34
0.1%
1 ppm
0.45
Cryogenic rectification
0.15–0.25
Theoretical minimal separation work
0.08
Table 2.4 Energy needed for the production of one standard cubic meter
of unpressurized oxygen (kWh).
O2 purity
90%
93%
Pressure swing adsorption
0.36
0.39
Cryogenic rectification
Theoretical minimal separation work
1345vch02.indd 15
0.32
99.5%
0.35
0.07
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16
2 The Air Gases Nitrogen, Oxygen and Argon
The economic significance of the specific energy consumption is demonstrated
by the example of a cryogenic plant for the production of 50 000 mN3 h–1 of
unpressurized oxygen, which is a common plant size. With a specific energy
requirement of 0.35 kWh per standard cubic meter of O2 the plant needs
annually 1.5 · 105 MWh, which, assuming an electricity price of 0.05 € kW–1 h–1,
corresponds to operating cost of 7.7 Mio. € per year.
2.2.3
Nitrogen Recovery with Membranes
2.2.3.1
Physical Principle
Gases penetrate a dense non porous membrane differently well according to
the following model: The surface of the membrane absorbs the gas on the highpressure side. This “solution process” is followed by the diffusion through the
membrane to the low-pressure side with ensuing desorption. The permeability
coefficient Pi describes, how good the gas “i” is transferred through the membrane.
The coefficient is the product of the solubility coefficient of Henry’s Law and the
diffusion constant of Fick’s Law [2.11, 2.12]. The flow Ji of the component i through
the membrane, having the surface A and the thickness l, is
A
Ji = Pi
(2.1)
dpi
l
with dpi being the partial pressure gradient across the membrane.
The permeability is usually given in Barrer, with
m
1 Barrer = 2.664 ⋅ 10 −9 m3N/h ⋅ 2
m ⋅ bar
The ability of the membrane to separate two components i and j from each
other is described by the ratio of their permeabilities D = Pi / Pj, which is also
called selectivity or separation factor. Nearly all membranes are most permeable
for O2. Therefore, in membrane separation nitrogen is recovered as retentate on
the pressurized side.
2.2.3.2
Membrane Technology
The efficiency of membrane separation increases with the permeability and the
selectivity. Thin membranes are economic, since according to Equation (2.1) the
gas flow is inverse proportional to the layer thickness. However thin polymeric
films, which have favorable permeability and selectivity, are too weak to withstand
the high pressure difference between permeate and retentate side. The economic
breakthrough set in with the production of ultrathin compound polymeric
membranes. These are designed as hollow fibres with a thick porous back-up
layer for mechanical stability and a thin dense non porous membrane layer for
gas separation. The porous layer only has a slight influence on gas separation.
These hollow fibres are combined in a bundle, which is arranged in a cylindrical
container [2.13]. Several of these bundles, also called modules, can be added to
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2.2 Recovery of Nitrogen, Oxygen and Argon
17
form larger units. The bundle is being stretched and stuck into a resin layer at its
end caps. Feed and retentate usually flow through the interior of the fibres, the
permeate escapes through the exterior.
Typical parameters of an industrial module are [2.14]:
x O2-permeability = 2 Barrers, O2/N2 selectivity = 6
x Feed pressure = 11 bar, permeate unpressurized
x Outer diameter of the hollow fibre < 500 µm, inside diameter < 300 µm,
thickness of the non-porous membrane layer ~ 50 nm
x Length of module = 2 m, module diameter = 300 mm,
packing density < 5000 m2 m–3
Carbon molecular sieve membranes, which are currently developed (2003) on
different porous carriers in various geometries [2.15, 2.16], are an interesting
alternative to the hollow fibre membranes. They promise higher permeability
with higher selectivity at the same time.
Oxygen ion-conducting Perowskit membranes are also subject of industrial
research. In conjunction with a suitable catalyst layer on the permeate side,
these membranes enable the oxygen separation and its reaction with methane to
synthesis gas [2.32] in one step via 1/2 O2 + CH4 o CO + 2 H2. The oxygen-ion
transport through the membrane occurs at high temperatures in the range of
800–1000 °C and is maintained either over a partial pressure gradient or via an
electric potential.
2.2.3.3
Design
Fig. 2.1 shows the principal design of a membrane process.
The air is compressed to about 6–15 bar, dried by cooling and cleaned with filters.
Before entering the membrane, the air is heated to prevent water condensation
within the hollow membrane. The membrane modules that remove not only O2 but
also CO2 and H2O by means of permeation are the heart of the plant. N2 remains
inside the hollow fibres, accumulates there and escapes at the end of the fibres.
Fig. 2.1 Flowsheet of a simple membrane process for the N2-production:
(1) compressor, (2) filter, (3) heater, (4) membrane.
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2 The Air Gases Nitrogen, Oxygen and Argon
The fibres cannot be cleaned owing to their small inside diameter. Therefore a
thorough air pre-purification with fine and coarse filters as well as activated carbon
adsorbers is necessary to provide a membrane lifetime of more than 10 years.
Membrane plants require little space, are robust, easy to operate and low-priced.
They are available for capacities in the range of ~ 1–1000 mN3 h–1, for purities of
95–99.5% and pressures of up to 13 bar. When the N2-purity is high, the product
yield is low. With 0.5% of oxygen in the nitrogen product, the N2-yield is typically
22%. With 8% of oxygen in the nitrogen product, the N2-yield increases to 64%.
N2-yield = (Conc.N2 in the product · FlowProduct) / (Conc.N2 in the air · FlowAir),
where Conc. is given in % volume fraction, flow in mN3 h–1.
There are numerous modifications of the basic process shown in Fig. 2.1, which
allow a more efficient operation with higher product purity and capacity:
x Multi-stage separations: The permeate of a membrane stage is fed back into
the feed of the previous stage.
x Combination of membrane separation with downstream catalytic combustion
of the residual oxygen.
2.2.4
Nitrogen and Oxygen Recovery by Means of Pressure Swing Adsorption
2.2.4.1
Physical Principle
Pressure Swing Adsorption (PSA) makes use of the fact that the amount of
adsorbate, which can be deposited on the adsorbent, increases with increasing
pressure. Adsorption occurs at high pressure, desorption at low pressure. This
technique is applied for the adsorptive recovery of O2 and N2 from air.
With Temperature Swing Adsorption (TSA) the adsorbent is regenerated by
heat supply. This technique is not used for N2/O2-separation. It is preferably
applied, when the component to be separated has a low partial pressure in the
feed gas. Section 2.2.5 describes how H2O and CO2 in the feed air to a cryogenic
separator are adsorbed by means of a TSA. The cycle time of PSA lies in the range
of minutes, the one of TSA in the range of hours.
2.2.4.2
Properties of Molecular Sieves
N2-recovery by PSA is performed predominantly on carbon molecular sieves
(CMS), the O2-production on zeolitic molecular sieves [2.17, 2.18].
x CMS: The binding forces of oxygen and nitrogen on the CMS surfaces do not
differ particularly, but the material has pores and cavities into which oxygen
diffuses faster than nitrogen.
x Zeolites: The recovery of oxygen on zeolites uses the fact, that nitrogen is bound
more strongly on the surface. The binding of the molecules on the surface
is characterized by adsorption isotherms measured in the laboratory. These
isotherms indicate the equilibrium loading of the component for different
partial pressures above the adsorbent. The adsorbents are designed on the basis
of these isotherms.
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2.2 Recovery of Nitrogen, Oxygen and Argon
19
Both adsorbents have high specific surfaces of ~ 800–1500 m2 g–1.
The molesieve material is filled into horizontally or vertically orientated
cylindrical vessels and the gas passes through the material either in axial or radial
direction. Design criteria are pressure drop, the so-called lifting velocity above
which the bed fluidizes, and the homogeneous distribution of the gas flow.
2.2.4.3
Nitrogen Recovery
Figure 2.2 shows a typical process for nitrogen recovery by means of PSA. Feed
air is compressed to 5–12 bar, cooled, and after removal of the condensate fed
to the adsorbers (3, 3c). When adsorber (3) is in adsorption mode, its inlet and
product valve (a) and (c) are opened and the expansion valve (b) to the atmosphere
is closed. H2O, CO2 and O2 are preferably adsorbed on the CMS. The non-adsorbed
nitrogen passes through the adsorber (3) and is collected in a buffer tank (4), which
is operated at a somewhat lower pressure. The product is finally withdrawn from
this tank. At the same time, the adsorber (3c) is regenerated. While its inlet and
product valve (ac) and (cc) are shut, the pressurized gas in the adsorber is released
into the atmosphere via expansion valve (bc). An adsorption phase lasts about
40–60 s. Then the adsorbers change roles: After equalizing the pressure of the
two vessels within 1–2 s, adsorber (3c) is pressurized within about 5 s to take over
the adsorption. The buffer tank (4) attenuates the product-pressure fluctuations
during the adsorber switching.
x Nitrogen purities are within about 98% to 99.99%. The capacity ranges between
few standard cubic meters and 5000 mN3 per hour and depends strongly on
the residual oxygen content in the product. For instance the product capacity
increases by about 30%, when the oxygen content increases from 0.5–2%.
Fig. 2.2 N2-recovery with PSA:
(1) compressor, (2) filter, (3) (3c) adsorber, (4) N2-buffer,
(5) silencer, (a) (ac), (b) (bc), (c) (cc) switching valves.
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2 The Air Gases Nitrogen, Oxygen and Argon
x With elaborate CMS even higher purities with O2-contents down to below
10 ppm may be obtained. Alternatively, very pure nitrogen can be produced by
combining the PSA with a downstream catalytic conversion stage.
x 1 m3 of CMS material is sufficient to produce about 100 mN3 h–1 of nitrogen
with an O2-content of 0.5%.
x With the standard PSA-process the product is delivered at ~ 7 bar. The product
pressure may be increased by raising the adsorption pressure to about 12 bar.
For even higher pressures a separate compressor is applied.
x PSA allows higher purities than membrane separation.
2.2.4.4
Oxygen Recovery
O2-generators use zeolitic molecular sieves. Here N2 is adsorbed and oxygenenriched air is obtained as product at the adsorber outlet. During regeneration
the adsorbed nitrogen is blown into the atmosphere. Towards the end of the
desorption phase the regenerated adsorber may be purged with oxygen to avoid
nitrogen contamination of the product at the beginning of the following adsorption
cycle.
In high capacity plants, adsorption and regeneration are often performed at
reduced pressure (VPSA = Vacuum Pressure Swing Adsorption) [2.19]: The air
compressor is replaced by a blower with a discharge pressure of about 1.5 bar,
whereas a vacuum pump generates a vacuum of typically 35–50 kPa for the
regeneration. An adsorption cycle takes about 40–60 s. VPSA-plants require less
energy than PSA-plants, since the specific capacity factor increases with decreasing
pressure. This factor is the load change dQ between adsorption and desorption
phase, related to the load Q of the adsorption phase, i.e. dQ/Q.
Although nitrogen is well adsorbed, the achievable oxygen purities are limited
to the range between 90 and 93% (at most 95%). This is due to argon, which has
a similar adsorption behaviour as oxygen, and which is concentrated in the same
proportion as oxygen to a content of about 4.5%. The product capacity of VPSA
plants ranges between 100 mN3 h–1 and 5000 mN3 h–1. O2–PSA plants are often
operated at an adsorption pressure of 3–4 bar and a desorption pressure of 1 bar.
The product capacity ranges between 5–300 mN3 h–1.
2.2.5
Cryogenic Rectification
One of the most important milestones in the history of industrial air separation
was the introduction of the so-called double column for the distillative separation
of air into its components under cryogenic conditions. Even today this principle
is still applied in numerous variations in most of the cryogenic air separators.
A frequently applied process is going to be introduced in the following by example.
It is built this way or similarly by all commercial vendors of air separation units.
Elementary concepts of process technique will be applied to explain the “key
features” of the air separator, such as concentration profiles in the columns,
argon production or refrigeration. A separate section describes so-called internal
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2.2 Recovery of Nitrogen, Oxygen and Argon
21
compression, where oxygen is pressurized without using an oxygen compressor.
This technique has been increasingly applied since 1980. Other applications of
the cryogenic separation, namely, nitrogen generators and high purity plants
as well as liquefiers will be introduced in subsequent sections. The central
components molecular sieve, compressor, turbine, heat exchanger and column
will be characterized and the technical challenges, that have to be met in design,
manufacturing and assembly of these components, will be described.
2.2.5.1 Process with Air Booster and Medium-Pressure Turbine for the Recovery
of Compressed Oxygen, Nitrogen and Argon
The process (Fig. 2.3A) is designed for delivering products as described in Table 2.5.
A plant based on such a process with a production capacity of 45 000 mN3 h–1
of oxygen typically looks like the one shown in Fig. 2.3B. Compressed gaseous
oxygen is often supplied exclusively to a bulk consumer, for example a steel mill.
The plant’s liquid production is stored in tanks. During a plant stop this liquid is
evaporated to guarantee the continuous gas supply of the bulk consumer and it
is also distributed by trucks to smaller consumers in the local area.
The plant consists of a warm section including compression, pre-cooling,
drying and pre-purification of the air, and a cold section with heat exchange and
rectification. The cold section with temperatures down to 80 K like at the top of the
low-pressure column is housed inside a “coldbox”. This is a “container” made up
of steel panels and filled with insulating material to protect the cold equipment
from cold losses.
Table 2.5 Product spectrum of the process with air booster and medium
pressure turbine (cf. also Fig. 2.3).
Product
Purity/Residual impurity
Yield
(= mass flow of component
in the product/mass flow of
component in the feed air)
Compressed gaseous oxygen,
pressure between 6 and
about 100 bar
> 99.5%
O2-yield between approx.
90% and nearly 100%
Unpressurized gaseous
nitrogen
Typically 1 ppm O2 and
100 ppm Ar
N2-yield up to approx. 60%
Compressed gaseous
nitrogen at almost 5 bar
Typically 1 ppm O2 and
100 ppm Ar
Up to 20% of the
processed air
Argon
1 ppm O2 and 1 ppm N2
Ar-yield between 60% and
95%
Liquid products
1345vch02.indd 21
All three products can be
delivered partly as liquid
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2 The Air Gases Nitrogen, Oxygen and Argon
Fig. 2.3A (legend see p. 23)
22
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2.2 Recovery of Nitrogen, Oxygen and Argon
23
Warm Section (Fig. 2.3A)
Compressing and Cooling
Air is filtered (2) and compressed to 5.7 bar in a compressor (3), leaving the
compressor’s last stage with about 100 °C. Subsequently it is cooled first with
cooling water (o) and then with chilled water (p) to temperatures between 5 and
20 °C in the direct contact cooler (4). This cooling reduces the moisture content
of the saturated air, thus reducing the expenditure for the ensuing H2O removal
in the zeolite adsorbers (6/6c). The “chilled” water is supplied by the evaporative
Fig. 2.3B Cryogenic air separator, Linde AG, Linz, Austria.
(1) Cold Box; (2) Tank; (3) Machine house; (4) Direct contact cooler;
(5) Evaporation cooler.
Fig. 2.3A Cryogenic air separator with pure
argon production and internal compression.
(1) Coldbox and battery limit for enthalpy
balance; (2) Filter; (3) Main air compressor;
(4) Direct contact cooler; (5) Evaporation
cooler; (6/6c) Adsorber; (7) Regeneration gas
heater; (8) Air booster; (10) Heat exchanger;
(11) Turbine; (12) Rectification column
(pressure section); (13), (16), (19) combined
condenser–evaporator unit; (14) Rectification
column (low-pressure section); (15) Crude
argon column; (17) Pure argon column; (18)
Reboiler; (20) Ar – process pump; (21) Internal compression pump; (22) Throttle valve.
1345vch02.indd 23
(a, b, c, i, j, k, l, o, p, r) Internal process
streams; (d) N2 gaseous; (e) Waste gas;
Pressurized – N2 gaseous; (g) Pressurized
– O2 gaseous; (h) N2 liquid; (m) Ar liquid;
(q) O2 liquid.
For simplicity, the subcooler and the purge
flows of crude and the pure argon condenser
16, 19 are omitted.
GAN = Gaseous nitrogen;
PGAN = Pressurized gaseous nitrogen;
PGOX = Pressurized gaseous oxygen;
LAR = Liquid argon; LIN = Liquid nitrogen;
LOX = Liquid oxygen.
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2 The Air Gases Nitrogen, Oxygen and Argon
cooler (5). Here it is cooled in counter-current flow with the nitrogen-rich residual
gas from the separation. This dry residual gas saturates with moisture. The thereby
required evaporation heat is withdrawn from the water and cools it. The heat and
mass transfer in the direct contact and in the evaporative cooler takes place on
irregular or structured packings often made of plastic.
Purification
The cold air still contains H2O and about 400 ppm of CO2. In the adsorber, the
air is purified such that during the ensuing cooling in the heat exchanger (10) to
about 100 K, no ice or CO2-snow is formed by desublimation. This would gradually
block the heat exchanger.
The vapour pressure PCO2 of solid CO2 at 100 K is about PCO2 = 2.2 · 10–7 bar.
Thus the average CO2 concentration xCO2 of air at 5.5 bar has to be smaller than
xCO2 < 2.2 · 10–7 / 5.5 = 40 ppb, to avoid CO2-snow even at 100 K.
The ambient air is diluted with numerous hydrocarbon components. Some
of these molecules are fully or at least partly retained by the molesieve. This is
important for the safety of the air separation unit, since all hydrocarbons are less
volatile than nitrogen and oxygen and accumulate in the liquid oxygen formed
in the bath of the main evaporator (13). Here their concentration must remain
far below the solubility and explosion limit. In particular, the molesieve adsorbs
acetylene, which has a solubility limit of only ~ 6 ppm, completely. The adsorbers
(6/6c) are arranged pairwise and are automatically switched between adsorption
and regeneration mode.
Cold Section (Fig. 2.3A)
Heat Exchange
Three fractions of air (a, b, c) with different pressures enter the heat exchanger
(10). Here, they are cooled in counter-current flow against gaseous nitrogen (d)
from the top of the low-pressure column (14), gaseous residual gas (e), compressed
nitrogen (f) from the top of the pressure column (12) and compressed oxygen
(g). The compressed oxygen enters the exchanger in liquid form, is evaporated
therein and is discharged as pressurized gas.
The first air fraction (a) passes into the heat exchanger without further boosting
and is cooled down close to its liquefaction temperature.
The second fraction (b) is further compressed in a booster (8), partly cooled
in the heat exchanger and expanded via the turbine (11) into the bottom of the
pressure column. The turbine performs work, which is transformed into electric
energy by a generator. The turbine controls the refrigeration balance of the plant,
as described in more detail further below. The more liquid products are produced,
the higher is the requirement for refrigeration. This is satisfied by increasing
expansion flow through the turbine.
The third fraction (c) enables the so-called “internal compression” of the oxygen:
Fraction (c) is boosted in compressor (9) and liquefied in the heat exchanger. The
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2.2 Recovery of Nitrogen, Oxygen and Argon
25
thereby released condensation heat vaporizes the liquid compressed oxygen which
is delivered by the pump (21). The liquefied high-pressure air is expanded into the
pressure column at a suitable tray via a throttle valve. The expanded air is a two
phase flow, being mostly liquid with only a small vapour fraction. The amount
of the liquid phase is direct proportional to the amount of internally compressed
oxygen. Typically, for one part of internally compressed oxygen about 1.3 parts
of liquid air must be fed into the columns. In Section 2.2.5.2 the principle and
benefit of internal compression will be explained in detail.
Rectification in the Pressure Column
In the pressure column (12) the gaseous and liquid air is pre-separated at a pressure
between 5 and 6 bar. The rectification is crucially determined by the volatilities of
the components, which goes hand in hand with their boiling temperatures, see
Table 2.6. Nitrogen, which is more volatile, accumulates at the top of the pressure
column. It has a typical residual oxygen content of 1 ppm. At the bottom of the
column an oxygen-enriched liquid with an O2-content between 35 and 40% is
formed.
Table 2.6 Boiling temperatures of N2, O2 and Ar at atmospheric pressure.
Boiling temperature at atmospheric pressure
N2
77.4 K
Ar
87.3 K
O2
90.2 K
Only in the low-pressure column (14), typically operated at 1.3–1.5 bar, the final
separation into pure oxygen as sump product, pure nitrogen (d) as top product and
residual gas (e) withdrawn from an intermediate stage, takes place. The residual
gas is mainly nitrogen with a small oxygen content in the range of < 1%.
The gaseous nitrogen at the top of the pressure column is liquefied in the main
condenser (13). This condenser is cooled by evaporating liquid oxygen from the
sump of the low-pressure column. Condenser and evaporator are designed as a
combined heat transfer unit, see also Section 2.2.5.6. Part of the condensate serves
as reflux for the pressure column, the rest is expanded and fed as reflux onto the
top of the low-pressure column.
The product nitrogen is alternatively withdrawn either
x in gaseous form and almost unpressurized from the low-pressure
column (d), or
x in gaseous form and pressurized from the pressure column (f), or
x in liquid form from the pressure or low-pressure column (h)
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2 The Air Gases Nitrogen, Oxygen and Argon
The oxygen-enriched liquid from the sump of the pressure column is also expanded and then, on the one hand, fed as liquid into the low-pressure column (i)
and, on the other hand, evaporated in the two combined evaporator-condenser
units (16), (19) on top of the crude argon column (15) and the pure argon column
(17). This cools the two argon columns’ head condensers, thereby providing the
necessary reflux for these columns. The evaporated oxygen – enriched liquid is
fed into the low-pressure column.
The sensible heat of the oxygen-enriched liquid from the pressure column is
also used for the heating of the sump of the pure argon column (18).
Finally, in addition to the liquid nitrogen and liquid oxygen – enriched streams
from the pressure to the low-pressure column, there is a third liquid air fraction
(k) from the pressure to the low-pressure side. This flow is adjusted such that the
operating line and the equilibrium curve, as shown in the McCabe–Thiele diagram
of the low-pressure column, move closer together. This reduces the “irreversibility”
of the rectification and increases the O2- and Ar-yield.
Rectification in the Low-pressure, Crude and Pure Argon Column
Just as in the pressure column, also in the low-pressure column oxygen is rectified
downwards and nitrogen upwards. Argon accumulates both above and below the
feeding of the evaporated crude oxygen (j) in the form of an argon concentration
bulge.
Liquid oxygen is withdrawn from the column’s sump and compressed to product
pressure by means of a pump (21).
Close to the tray with maximal argon concentration, vapour with an argon
concentration between 5% and 15% is withdrawn (l) and fed to the crude argon
column (15). The feed gas to the crude argon column typically has an N2-content
of 100 ppm and an oxygen concentration of 85–95%. The crude argon column
separates oxygen from argon. The composition at the top of the argon column
amounts to about 1 ppm of O2 and 0.5% of N2. Approx. 1/30 of the gas, which
ascends in the crude argon column, is passed on to the pure argon column (17). It
is called crude argon (f). In the pure argon column the residual nitrogen is rectified
towards the top and ejected into the atmosphere by blowing off a small amount
of waste gas (n) with a typical N2-content of 40%. The liquid argon product with
1 ppm of N2 and 1 ppm of O2 is withdrawn from the column’s sump. The head
condenser (19) ensures the reflux for the pure argon column.
With a double-column, the vapour and liquid flows within the individual column
sections are not freely adjustable, unless additional enhancement recycles are
applied. This is a specific characteristic of the double-column principle. As a
consequence, the product yields are also more or less fixed. In particular, it is not
possible to withdraw all the gas ascending to the top of the low-pressure column
as pure nitrogen. However, a fraction of up to 50% of the processed air can be
obtained as pure nitrogen (d) with an O2-content of ~ 1 ppm, if residual gas with
an O2-concentration ranging between 0.1–3% is withdrawn some trays below the
top. Thereby the liquid/vapour-ratio in the top section, the so called “nitrogen
section”, increases such, that the oxygen can be rectified downwards in this section.
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2.2 Recovery of Nitrogen, Oxygen and Argon
27
The withdrawn residual gas is warmed in the heat exchanger and is used for the
regeneration of the molesieve adsorber and for evaporative water cooling.
The Average Temperature Difference at the Main Condenser Determines
the Process Air Pressure
The main condenser (13) of the process shown in Fig. 2.3A determines significantly the pressure of the process air after the main air compressor (3) and thus
the energy consumption:
The pressure in the low-pressure column is selected to compensate the frictional
pressure drop of the residual gas (e) on its way from the low-pressure column
to the atmosphere. For example, when the low-pressure column is of the packed
type, a pressure of about 1.4 bar in the column sump is sufficient. In order
to maintain the heat flow in the main condenser – evaporator unit, a driving
temperature difference of usually 1–2 K has to be maintained between evaporator
and condenser. Therefore, the pressure in the pressure column must be adjusted
such that the temperature of the condensing nitrogen at the top of the pressure
column exceeds the temperature of the boiling oxygen at the bottom of the lowpressure column by this temperature difference.
Cryogenic Losses are Mainly Covered by the Turbine
The rectification occurs at low temperatures between 80 K and 100 K. Therefore
air must be cooled down prior to rectification and the gaseous products must be
warmed up afterwards. This process is associated with cryogenic losses which
have to be compensated by performing additional work on the system. Losses
occur owing to:
x
x
x
x
insufficient warming up of the products in the heat exchanger
discharge of liquid products
heat flux into the coldbox
work performed on the process pumps
On the other hand, an enthalpy balance along the battery limit (1) shown in
Fig. 2.3A, reveals two sources of “cryogenic gains”, for which according to the
first law of thermodynamics 6 gains = 6 losses holds:
x Work performed by the turbine.
x Throttling: Air enters the system with overpressure and leaves it nearly
unpressurized in form of the separated gaseous products. If the air and its
separation products would behave strictly like an ideal gas, whose enthalpy
does not depend upon the pressure, the throttling of the process air would not
lead to any cold production. However, they are real gases and their enthalpy
decreases with increasing pressure. This gives rise to a gain of enthalpy. This
gain gets more significant, when the pressure difference between the incoming
air and the discharged separation products is large.
The cryogenic losses of the process shown in Fig. 2.3A are mainly covered by
the turbine.
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2 The Air Gases Nitrogen, Oxygen and Argon
The original air liquefier built by C. v. Linde did not use a turbine. The cryogenic
losses were exclusively compensated by throttling the processed air, which had
been compressed up to 200 bar. This is energetically unfavorable since throttling
is an irreversible process, while the expansion in an ideal turbine, having a 100%
efficiency (efficiencies of up to 90% are achieved with modern turbines), is a
reversible process occurring without loss of exergy.
The Concentration Profile in the Low-Pressure Column is Characterized
by Two Argon Bulges
The triangular diagram, see Fig. 2.4 [2.20], visualizes the concentration profile
of the ternary system oxygen/nitrogen/argon in the low-pressure column. The
curve within the triangle indicates the composition of the three components in
the liquid on each theoretical tray within the column.
Fig. 2.4 Ternary concentration diagram for
the down-flowing liquid in the low-pressure
rectification column.
(1) Bottom of the low-pressure column.
(2) Withdrawal of the argon-rich gaseous
feed to the crude argon column.
1345vch02.indd 28
(3) Feed of oxygen enriched liquid from the
pressure column. (4) Feed of liquid air from
the pressure column. (a) Connects states of
uniform Ar-concentration; (b) connects states
of uniform N2-concentration; (c) connects
states of uniform O2-concentration.
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2.2 Recovery of Nitrogen, Oxygen and Argon
29
The diagram reveals that although the argon concentration of the ambient air is
quite small with less than one percent, it has a strong impact on the concentration
profile of the low-pressure column. It would not be adequate to describe the
separation in the low-pressure column as a binary O2–N2 rectification, in which
the presence of argon only represents a minor disturbance: In the lowest, the socalled oxygen section with about 30–40 theoretical trays, a pure binary separation
between O2 and Ar takes place. At the upper end of this section, where the argon
content reaches its maximum and where the feed gas to the crude argon column
is withdrawn, the binary O2–Ar rectification transforms within a few trays into a
ternary rectification.
The diagram also shows the second, more weakly developed argon bulge above
the feed of the oxygen enriched liquid.
Cryogenic Production of Pure Argon
Until about 1990 the production of pure argon, having a contamination of only a
few ppm of O2 and N2, was accomplished by a combination of rectification and
catalytic combustion. The O2-impurities of the argon were catalytically burnt by
addition of hydrogen to form water. Since then, the pure argon recovery solely by
means of rectification, as it is shown in Fig. 2.3A, has been established [2.21] and
become the industrial standard. Here the O2–Ar separation is performed in the
so called crude argon column. From the top of this column almost O2-free crude
argon is withdrawn and forwarded to the pure argon column, where the remaining
N2 is separated from the argon. This section describes the special demands on the
crude argon column and the plant control, resulting from the close integration of
the argon rectification into the air separation process.
The rectification in the crude argon column is almost a pure binary separation
between O2 and Ar and can be best visualized and understood in the McCabe–
Thiele Diagram, shown in Fig. 2.5. This diagram indicates for each tray the Arconcentration y of the vapour directed towards the tray and the concentration x
Fig. 2.5 McCabe–Thiele diagram for the binary O2/Ar-mixture of the crude argon column.
1345vch02.indd 29
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2 The Air Gases Nitrogen, Oxygen and Argon
of the liquid leaving this tray. These points lie on a common straight line, the
so-called operating line. Its gradient is defined by the ratio L/V = Liquid/Vapour.
The so-called phase equilibrium curve is plotted as well. This curve shows for
a given argon concentration x in the liquid the corresponding equilibrium
concentration y* of the vapour. The number of theoretical separation stages
required is determined by the well known staircase construction, to be arranged
between the equilibrium and operating line. The closer the two lines lie together,
the more separation stages are needed.
The top of the column corresponds to the upper right point in the diagram. The
crude argon at the top of the column is rectified to an O2-purity of typically 1 ppm.
Since the boiling temperatures of O2 and Ar are close together (see Table 2.6), Ar
is only slightly more volatile than O2. Therefore the rectification requires a large
number of theoretical separation stages, typically 180 stages. This is visualized by
the small gap between the operating and the phase equilibrium curve.
The minimal number of theoretical separation stages Nmin, which is required
for the crude argon column to accomplish the O2–Ar separation, is estimated with
the Fenske formula. This formula holds for infinite reflux, L/V ~ 1. This condition
is almost fulfilled since typically only 1 part out of 30 parts of ascending vapour
is withdrawn as crude argon from the top of the crude argon column, whereas
29 parts of liquid remain as reflux.
log S
where
log D
x DO2 − Ar = K O2 / K Ar is the relative volatility of the oxygen-argon mixture, defined
by the component equilibrium factor K O2 = y O* 2 / x O2 for oxygen and the
corresponding factor for argon. Here, y* is the concentration of vapour, which is
in equilibrium with its liquid, having the concentration x. The relative volatility
varies between 0.66 in the column’s sump and 0.91 in the column’s top, i.e. the
argon-richer the mixture, the more difficult the separation will be.
x O2 , top (1 − x O2 , sump )
. Inserting the
x S is the so-called separation factor S =
(1 − x O2 , top ) x O2 , sump
oxygen purity at the top, x O2 , top = 1 ppm , and the sump oxygen purity of
typically x O2 , sump ~ 90% , yields S ~ 1.11 · 10–7.
The equation is N min + 1 =
Together with the relative volatility evaluated at the top of the column, the Fenkse
equation results in Nmin ~ 159 theoretical trays.
The separation of oxygen from argon by means of rectification became only
possible after the introduction of packed columns, in which the mass transfer
between vapour and liquid phase occurs on the surface of regularly structured
packings. Here the pressure drop of roughly 0.07 kPa per theoretical stage is
significantly lower than in a sieve tray column with about 0.35 kPa per stage.
A sieve tray column with 180 sieve trays would have a total pressure drop of about
60 kPa and would result in a low condensation pressure of the argon at the top
of the column. Due to this low pressure the condensation temperature of the
argon would fall below the boiling temperature of the cooling oxygen-rich liquid,
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2.2 Recovery of Nitrogen, Oxygen and Argon
31
coming from the pressure column. Thus, there would be no driving temperature
difference to ensure the heat transfer at the crude argon condenser. In a packed
column the pressure drop amounts only to about 15 kPa and a driving temperature
difference of 1–2 K remains.
The packed crude argon column with 180 theoretical trays gets high: The height
of the packed beds sums up to approx. 40 m. To this, the height of numerous
liquid collectors and distributors has to be added. These are arranged between the
packing beds to guarantee a uniform distribution of the liquid over the packings.
Hence the crude argon column may be split up into two towers standing side by
side, the towers being connected with a pump.
The large volume of the crude argon column requires a long start-up time for
the argon production:
X Example: The crude argon column of an air separator, having an argon production
of 2000 mN3 h–1 and a process air input of 240 000 mN3 h–1, has a diameter of about
3.3 m. Approx. 36 000 mN3 of argon are stored within the column in form of a liquid
argon film on the packing surface, liquid in the distributors and liquid in the sump
of the column. Since 2000 mN3 h–1 of pure argon are transferred into the crude
argon column, the time passing until the complete column holdup is built up and
the argon production can be started, amounts to tholdup = 36 000 mN3 / 2000 mN3 h–1
= 18 h.
Argon Yield
The argon yield rises, when the argon concentration of the feed gas to the crude
argon column [(l) in Fig. 2.3A] is increased. However, the plant operator must
be careful, when he increases, in view of a high argon production, the argon
concentration in this feed gas by suitable adjustment of the process parameters.
This renders the process more sensitive to disturbances. The ternary concentration
diagram in Fig. 2.4 explains this: The maximum of the argon concentration in the
lower part of the low-pressure column lies at the point where the binary O2–Arseparation transforms into a ternary separation, and where the N2-concentration
rises from typically 100 ppm to a percent value within few stages. Operating the
plant close to this point bears the risk, that a small disturbance may provoke
a strong increase of the N2-concentration in the feed gas to the crude argon
column. There, owing to its high volatility, nitrogen is driven to the column’s top.
Here it accumulates due to the small top product extraction of about only 1/30
of the column feed. The increasing N2-content of the vapour at the column’s top
reduces its temperature, and the driving temperature difference at the crude argon
condenser drops, until the vapour flow in the crude argon column collapses.
Argon gets lost via
x the oxygen product: For example, when the oxygen purity is 99.7% the remaining
0.3% are almost exclusively argon, corresponding to an argon loss of 6.7%
x pure and impure nitrogen withdrawn from the top of the low-pressure
column
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2 The Air Gases Nitrogen, Oxygen and Argon
Fig. 2.6 Argon and oxygen yield: (1) argon yield; (2) oxygen yield.
The argon loss via the pure and impure nitrogen depends strongly upon the
amount of reflux onto the top of the low-pressure column. The loss is low when
the reflux is high and at the same time has low argon content. These are two
contradictory requirements setting up a total limit for the argon yield: High reflux
onto the low-pressure column induces a low reflux onto the pressure column with
correspondingly high argon content at the top of this column. The optimum is
a suitable compromise.
The argon yield reacts particularly sensitive to a withdrawal of product nitrogen
from the top of the pressure column, (f) in Fig. 2.3A. This discharge is often
advantageous, because it reduces the investment for a further nitrogen compressor
or makes it even obsolete. However the nitrogen withdrawn in this way is missing
as reflux onto the top of the low-pressure column. This reduces the argon yield
and to a certain extent also the oxygen yield (cf. Fig. 2.6).
2.2.5.2
Internal Compression
Industrial applications mostly require pressurized oxygen.
X Example: Linz-Donawitz-Process for steelmaking: Here oxygen at about 15 bar is
blown through a lance into the pig iron melt.
With “external compression” (Fig. 2.7), the gaseous oxygen is withdrawn from the
low-pressure column, warmed in the heat exchanger and brought to the discharge
pressure by means of a compressor.
The safe operation of an oxygen compressor requires elaborate and expensive
safety measures to avoid heat development caused by mechanical friction. In an
oxygen-rich environment this could lead to a sudden exothermal combustion of
the material in contact with oxygen. The higher the O2-pressure, the lower the
amount of heat required for the ignition (see also Section 2.3).
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2.2 Recovery of Nitrogen, Oxygen and Argon
Fig. 2.7 Internal and external compression.
(1) Heat exchanger; (2) Turbine;
(3) Rectification column (pressure section);
(4) Rectification column (low-pressure section); (5) Combined condenser–evaporator
unit; (6) Booster air compressor; (7) Internal
compression pump; (8) Oxygen compressor.
(a, b) Internal process flows; (c) Gaseous
pressurized oxygen; (d) Low-pressure
products.
33
In contrast to the exemplary process shown
in Fig. 2.3A, the turbine (2) is operated here
with unboostered process air and expanded
into the low-pressure column (4). This
modification is applied in units with low
liquid production.
GAN = Gaseous low pressure nitrogen
PGOX = Pressurized gaseous oxygen
The alternative for “external compression” is “internal compression” shown
in Fig. 2.7. It has gained broad acceptance since about 1980 owing to its safe
operation, availability, easy maintenance and low investment costs. With internal
compression, the product oxygen is not evaporated in the main condenser of
the double column, but it is pressurized as liquid and is evaporated in the heat
exchanger against condensing high-pressure air.
Q-T-Diagram
The so-called Q-T-Diagram [2.22] for external and internal compression (Fig. 2.8)
x explains the principle of internal compression
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2 The Air Gases Nitrogen, Oxygen and Argon
x enables to calculate the relation between the pressure of the condensing highpressure air and the pressure of the evaporating liquid oxygen, and
x illustrates why internal compression needs slightly more energy than external
compression
The Q-T-Diagram characterizes the heat transfer in the heat exchanger in a
similarly distinctive way as the McCabe–Thiele diagram characterizes the mass
transfer in the columns:
The upper curve of the diagram indicates the amount of heat that has been
transferred to all cold streams on the way from their entry point at the cold end of
the exchanger to an arbitrary position within the exchanger, at which the streams
have been warmed up to a temperature T. This temperature T is shown on the
horizontal axis. Correspondingly the lower curve shows the amount of heat that
Fig. 2.8 Q-T-Diagram for internal (a) and external compression (b).
(1) cold streams; (2) warm streams.
In this example O2 pressure = 14 bar and pressure of boostered air = 31 bar.
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2.2 Recovery of Nitrogen, Oxygen and Argon
35
is removed from all warm streams on their way from a position, where they take
on the temperature T down to the cold end of the exchanger.
The horizontal distance dT of both curves at a given value of the transferred heat
Q is the driving temperature difference, maintaining the heat flow from warm
to cold streams. Therefore, the two curves may not intersect. The closer they lie
together, the smaller the dT and the more exchanging surface is required in this
area for the heat exchange.
The Q-T-Diagram allows to judge the quality of the heat transfer: A transfer of
heat dQ over a finite temperature difference dT is irreversible. That means exergy,
i.e. technical available work, is lost [2.32]. This exergy loss must be compensated
by extra work performed on one of the compressors. The exergy loss dE caused
by a heat flow dQ over the temperature gradient dT = Tc – T is
dE = Ta dQ
dT
T T′
(2.2)
Here, Ta is the ambient temperature to which the technical available work refers.
From this equation one concludes that
x the loss increases with the growing temperature difference dT
x the loss is larger at low temperatures, owing to the factor T · T c in the
denominator
Comparison of Internal and External Compression
The warm and cold branches of the Q-T-Diagram for internal compression,
Fig. 2.8, are almost vertical at the temperature around 126 K. This corresponds to
the region in the heat exchanger, where the 14 bar oxygen evaporates and thereby
condenses the 31 bar high-pressure air.
The warm and cold branch of the Q-T-Diagram may not intersect. This is
guaranteed if the boiling temperature of the high-pressure air is slightly higher,
typically 1–2 K, than the boiling temperature of the oxygen. From this requirement,
the relation between the oxygen pressure and the optimal pressure of the boostered
air is constructed.
In the example, the oxygen pressure is 14 bar and is lower than the critical
pressure Pcr = 50.8 bar of oxygen, and similarly the air pressure is 31 bar being also
lower than the critical pressure Pcr = 37.7 bar of air. However, internal compression
is also applied at overcritical pressures. The sharp bends in the Q-T-Diagram will
get “rounder” then.
The Q-T-curves for internal and external compression visualize, that the
advantage of the internal compression is paid for by higher energy consumption:
For the internal compression the temperature difference between the warm and
cold streams is slightly larger in the region around the zone of the boiling oxygen.
Therefore the heat transfer in this area induces a higher loss of exergy according
to Equation (2.2). As this increased temperature difference occurs at lower
temperatures, the exergy loss will be more pronounced, as explained above.
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2 The Air Gases Nitrogen, Oxygen and Argon
2.2.5.3
Nitrogen Generators
The double-column process shown in Fig. 2.3A produces oxygen and nitrogen.
There are industrial applications requiring exclusively nitrogen. For these,
numerous modifications of the double-column system have been developed, two
of which are introduced here:
The first process is a one-column apparatus. It can be viewed as classical doublecolumn system, the low-pressure column of which is shrunk to a vessel with an
evaporator inside and no separation stages above. A detailed process description
is given in [2.24]. This apparatus is typically applied to produce nitrogen with an
oxygen concentration between 1–100 ppm and pressures up to 10 bar. The process
is designed to optimize investment cost and is suitable for small-scale consumers
with a nitrogen demand of up to about 7000 mN3 h–1.
The second process is energy-optimized and suitable for the production of
large quantities of compressed nitrogen. Since 1999, this process was realized
by Linde AG in Cantarell/Mexico as five identical plants for the production of
a total of 1 675 000 mN3 h–1 of compressed nitrogen at 120 bar. It is recovered
from 2 375 000 mN3 h–1 of process air with a power consumption of 450 MW. The
nitrogen is pressed via an 80 km long pipeline into an offshore- oil well to increase
the oil production rate.
The specific energy demand related to one standard cubic meter of pure nitrogen
at 8 bar is 0.25 kWh mN–3 for the first and 0.15 kWh mN–3 for the second process.
The specific energy consumption of industrial cryogenic nitrogen generators
typically lies within the range spanned by these two values.
Energy-Optimized Two-Column Nitrogen Generator
The two-column process (Fig. 2.9) is based on the classical double column
supplemented by a further condenser/evaporator unit (6) at the top of the lowpressure column (5). The column pressures are chosen such, that there is a driving
temperature difference to maintain the heat flow at the two condenser/evaporator
units (6) and (4):
x The pressure on the evaporation side of the heat transfer unit (6) amounts to
about 1.3 bar, which is just high enough to compensate the frictional pressure
drop of the residual gas (a) on its way from the evaporator to the atmosphere.
x A pressure of about 4 bar in the low-pressure column (5) guarantees a driving
temperature difference at the transfer unit (6).
x A pressure of about 9 bar in the pressure column (3) guarantees a driving
temperature difference at the transfer unit (4).
The additional condenser (6) produces reflux for the low-pressure column.
Consequently the need for reflux (b) from the pressure column is reduced and
more reflux is available for the pressure column. This allows to withdraw about
48% of the total nitrogen product from the top of the pressure column. The rest
is discharged from the low-pressure column (d). Both nitrogen fractions are
finally compressed by (7) and (8) to the product pressure in the warm section.
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2.2 Recovery of Nitrogen, Oxygen and Argon
37
Fig. 2.9 Double-column nitrogen generator.
(1) Heat exchanger; (2) Turbine; (3) Rectification column (pressure section);
(4, 6) combined condenser–evaporator unit; (5) Rectification column
(low-pressure section); (7, 8) Compressor.
(a) Residual gas; (b–e) Internal process flows; (f) Pressurized – gaseous N2.
The nitrogen content of the residual gas (a) is only approx. 25%. The cryogenic
losses are covered by turbine expansion (2) of part of the process air (e) into the
low-pressure column.
2.2.5.4
Liquefiers
There is a class of air separation units, specialized to deliver the products exclusively
in liquid form. The liquid products are supplied to smaller gas consumers within
the regional market via tank trucks. Usual capacities of such plants lie in the
range of 3000–20 000 mN3 h–1 of cryogenic liquids. The liquefying unit is either
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2 The Air Gases Nitrogen, Oxygen and Argon
integrated into the air separation process or modularly designed as “stand alone
unit”. A typical process for liquefying unpressurized gaseous nitrogen, equipped
with a nitrogen cycle and two turbines, is described in [2.24].
The specific energy requirement for the liquefaction is typically 0.6–0.7 kWh mN–3,
and is significantly lower than the ~ 2.7 kWh mN–3 of the first air liquefiers built
after 1895. Nevertheless, the 0.6–0.7 kWh mN–3 are still far away from the theoretical
minimum value of about 0.2 kWh mN–3 [2.25], which a reversible process would
require.
2.2.5.5
High-purity Plants
There is a growing market for high-purity gases with total impurities ranging from
< 1 ppm down to 1 ppb. Especially the semiconductor industry has a demand for
highly purified nitrogen, which is used for inertizing. Typical capacities are in the
range of 3000–50 000 mN3 h–1. Since the semiconductor surfaces are sensitive to
undesired reactions with oxygen, hydrogen and carbon monoxide concentrations
of O2, H2 and CO lower than 1 ppb are required.
Table 2.7 shows typical purities of liquid high-purity oxygen, nitrogen and argon,
which are required by industrial users and which are achieved with cryogenic
plants. The purities are usually indicated in terms of degree. The degree states,
how often the figure “9” occurs in the specified purity.
X Example: Nitrogen with a purity of 99.9999% has the degree 6.0, corresponding
to 1 ppm of residual impurities (cf. also Section 9.2.2).
Table 2.7 Typical concentrations of high-pure products.
Element
Concentration in the air
Degree
5.0–7.0
N2
Ar
6.0–9.01)
6.0
< 0.1 ppb
< 10 ppb
O2
20.95%
N2
78.08%
< 50 ppb
Ar
0.93%
< 5 ppb
< 5 ppb
H2
0.5 ppm
< 50 ppb
< 5 ppb
< 5 ppb
CO
~ < 1 ppm
< 50 ppb
< 1 ppb
< 50 ppb
CO2
~ 400 ppm
< 50 ppb
< 1 ppb
< 50 ppb
CH4
~ < 5 ppm
< 50 ppb
< 10 ppb
< 50 ppb
< 50 ppb
< 50 ppb
< 0.05 ppb
< 50 ppb
CnHm
1)
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O2
< 200 ppb
Argon as inert noble gas excluded.
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2.2 Recovery of Nitrogen, Oxygen and Argon
39
Fig. 2.10 High-purity plant.
(1) Rectification column (pressure section);
(2, 4) Combined condenser – evaporator unit; (3) Stripping column.
(a) Internal process flows; (b) Ultra high purity pressurized gaseous N2;
(c) Inert gas exhaust.
High purity plants have specific solutions in design and construction, to be
described here by the example of a nitrogen generator (Fig. 2.10). The apparatus
is derived from the one-column nitrogen generator described in [2.24]. A central
component is the pressure column (1).
Contamination with Oxygen
Theoretically the oxygen contamination of the nitrogen at the top of the column
(a) can be reduced to any arbitrary small value, provided the column has a
correspondingly high number of trays and the amount of withdrawn nitrogen
product is sufficiently low.
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2 The Air Gases Nitrogen, Oxygen and Argon
The oxygen concentration changes from one theoretical tray to the next one
above by the separation factor S
S=K⋅
V
L
(2.3)
Here K = y*/x, the component equilibrium factor, describes the ratio between the
O2-concentration of the vapour phase y* and of the liquid phase x, with both phases
being in equilibrium. L is the molar downwards liquid and V the molar upwards
vapour flow. A depletion of oxygen towards the top of the column occurs, if S < 1,
i.e. if the amount of nitrogen, withdrawn from the column’s top is small enough
to ensure that L/V > K holds. When the column is designed to be operated with
a separation factor close to 1, the nitrogen yield is high (low energy consumption)
but a large number of trays is required (high investment costs).
X Example: The K-value of oxygen in almost pure nitrogen at 9 bar is K = 0.49
(Table 2.8). With L/V = 0.65 a separation factor S = 0.75 results. Then the
O2-concentration depletes over 70 theoretical separation stages by the factor
S70 = 2.6 · 10–9, while with 100 separation stages the depletion factor is already
S100 = 5.3 · 10–13.
Table 2.8 Component equilibrium factors in pure nitrogen at 9 bar.
Component
O2
Ar
CO
Ne
H2
He
K = y*/x
0.49
0.58
0.82
26.1
27.5
114.6
All components which are less volatile than oxygen do not accumulate significantly at the top of the pressure column and do not contaminate the product. This
holds for all hydrocarbon compounds.
Contamination with Argon and Carbon Monoxide
The volatilities of argon and carbon monoxide lie between those of oxygen and
nitrogen and their separation requires special measures: They are only depleted
towards the column top if the liquid to vapour ration L/V according to Equation (2.3) is high enough. For a pressure column of 9 bar, the liquid/vapour-ratio
has to be L/V > 0.58 for Ar to be depleted and L/V > 0.82 to enable the depletion
of CO. Theoretically, arbitrary small Ar- and CO-concentrations in nitrogen could
be acquired this way. However, the energy consumption would be high, because
a large L/V ratio close to 1 allows only for a small withdrawal of nitrogen product
with a resulting large amount of waste gas. Moreover, owing to the larger amount
of air to be processed, the dimension of the apparatus and thereby its cost would
increase. If, despite its inertness, the noble gas argon is regarded as an impurity
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2.2 Recovery of Nitrogen, Oxygen and Argon
41
and has to be removed from the nitrogen product, it is nevertheless separated in
the pressure column. For example, the nitrogen withdrawal from the column is
reduced to match the specified Ar concentration. CO, however, is removed in a
more economical way from the compressed air before it is passed to the cryogenic
separation. This is performed by means of the catalytic reaction
2 CO + O2 o 2 CO2
taking place at ambient air temperature on Cu/Mn-oxide (Hopcalite). The CO2
formed is removed in a downstream adsorber.
Contamination with Hydrogen, Helium and Neon
Hydrogen can also be removed in the warm section by the catalytic reaction
2 H2 + O2 o 2 H2O
on a Pd-surface with ensuing adsorption of the water. In this “front-end-purification”, the correct arrangement of adsorptive and catalytic layers has a great
influence on the quality of the purification and the protection of the catalyst from
being poisoned by undesired chemical reactions.
The catalytic process for H2 removal competes with a cryogenic separation:
This cryogenic rectification removes not only hydrogen, which is an undesired
impurity in wafer factories, but also the two other highly volatile components
He and Ne. This requires a further stripping column (3) equipped with a few
theoretical separation stages (see Fig. 2.10). This column is operated at a slightly
lower pressure than the pressure-column, in order to ensure the heat flow at the
heat transfer unit (2). Gaseous nitrogen product is withdrawn from the bottom
of the stripping column (b). By this way the hydrogen content of the nitrogen can
be reduced theoretically to any desired small concentration.
The vapour ascending in the pressure column (1) transports the hydrogen to the
top of the pressure column. It has more or less the H2-concentration of air, approx.
500 ppb. The component equilibrium factor of hydrogen in nitrogen is K = 27.5,
see Table 2.8. Therefore the down-flowing liquid, which is in equilibrium with
the vapour, has an H2-concentration of 500 ppb/27.5 = 18 ppb. This concentration
can not be reduced by increasing the purge flow (c) of the condenser (2). Only in
the stripping column (3) the H2-content depletes downwards below the nitrogen
feed (a) by the factor (1/S) per tray. Here the separation factor is defined in
Equation (2.3). For the reflux ratio L/V = 0.65 and the K-value = 27.5 the depletion
factor is 0.024. Thus a depletion by the factor 0.0245 = 7.4 · 10–9 occurs over five
trays.
Apart from front-end purification by means of adsorption and catalysis and
purification by means of rectification, also the “downstream”-purification via
chemisorption, adsorption and catalysis is applied. This is preferably done to
convert technical nitrogen from standard to high purity.
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2 The Air Gases Nitrogen, Oxygen and Argon
X Examples:
x Chemisorption of oxygen over activated Cu through oxidation O2 + 2 Cu o
2 CuO. Regeneration with H2, CuO + H2 o Cu + H2O.
x Reaction of CO over activated Cu through CO + 2 CuO o CO2 + Cu with ensuing adsorption of the CO2. Regeneration with O2, O2 + 2 Cu o 2 CuO.
x Chemisorption of oxygen over Ni through oxidation O2 + 2 Ni o 2 NiO.
x For smaller quantities of up to about 50 mN3 h–1 so-called getters are an economic
solution. They cover a large spectrum of impurities and are easy to operate.
Irreversible chemical reactions on the surface of the getter material form stable
components. The getter material must be exchanged periodically. The getter
process guarantees high purities, when the inlet impurity exceeds the desired
final concentration at best by a factor 100.
Concerning the constructive details of high-purity plants, numerous design
rules have to be followed, in order to achieve the theoretically calculated product
purities. For example:
x High demands on the leakproofness of all components, such as plate fin heat
exchanger passages and packing edges of packed columns.
x The partial pressure gradient determines the flow of impurities. For instance,
oxygen can diffuse from the atmosphere against the flow direction of the carrying
medium through a leak into a pressurized pipeline.
x Welded instead of flanged joints, as well as X-Ray testing of the welds.
x Special cleaning of all components after manufacturing.
x Short pipes, no dead zones with zero flow.
2.2.5.6
Apparatus
Molsieve Adsorber
An important milestone in the history of air separation was the introduction of
zeolitic molecular sieve adsorbers for the removal of H2O, CO2 and acetylene from
atmospheric air since 1968. This simplified the operation of the plants, which
by then had been equipped with so called reversible heat exchangers and liquid
oxygen adsorbers for the removal of acetylene.
Meanwhile, molsieve adsorbers have reached a high degree of functionality
and reliability. In the following, specific design and operating parameters will
be presented.
The molsieve station consists of two adsorbers which are alternately in
adsorption or regeneration mode. One cycle includes
x an adsorption phase, which typically lasts between 1.5 h and up to 6 h and which
is performed at the pressure of the process air, lying in the range of 5–20 bar
x a depressurization to ambient pressure within about 10 min
x the regeneration period with dry waste nitrogen in the countercurrent direction,
divided into a heating phase with warm regeneration gas and an ensuing cooling
phase with cold regeneration gas, as well as
x a pressure build-up of about 20 minutes
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2.2 Recovery of Nitrogen, Oxygen and Argon
43
The adsorber vessel is filled with zeolitic crystals [2.26] which are bound with
ceramic material and formed to spheres of about 3 mm diameter or rods. The
crystalline components have pores with diameters in the range of approx. 1 nm.
H2O as well as other polar or easily polarizable impurities such as CO2 and
various hydrocarbons are retained in the pores. Frequently a thin layer of activated
aluminium oxide for H2O adsorption is arranged in front of the zeolitic layer.
The so called mass transfer zone of H2O and CO2 in the adsorbent is short. This
is the zone where the components are adsorbed and which is moving forward
with ongoing time. Due to this compact adsorption front the bed height can be
reduced to about 1 m with a correspondingly small pressure drop ranging between
2 and 5 kPa.
The molsieve material is housed in horizontally or vertically orientated cylindrical
vessels with axial or radial orientated gas flows being possible.
X Example: Typical amount of molsieve material needed for a plant, producing
60 000 mN3 h–1 of oxygen from 300 000 mN3 h–1 of process air is about 75 t per
adsorber.
Regeneration
Regeneration is performed by heat, which is transferred into the molsieve via the
warm regeneration gas. The amount of regeneration heat needs to be only slightly
higher than the heat released during adsorption. Most of the heat is contributed
by the adsorbed water, whereas the CO2-adsorption heat hardly counts owing to
the low CO2 content.
X Example: 300 000 mN3 h–1 of process air being purified in the molsieve at 15 °C
and 5.7 bar within an adsorption cycle of 4 h: Typically the heat for adsorption of
H2O on a zeolit is 1.2–2 times higher than the H2O condensation heat, which is
1808 kJ mN–3. For this exemplary calculation an adsorption heat of 2680 kJ mN–3 is
assumed. The adsorption heat of CO2 is ~ 2050 kJ mN–3. The water content of the
saturated air at the molsieve inlet amounts to about 3000 ppm, the CO2-content
to ~ 400 ppm. Thus the desorption heat for the water is ~ 2.7 MWh while the
desorption heat for CO2 amounts to only 0.27 MWh.
The temperature profile of the regeneration gas leaving the molsieve adsorber
(Fig. 2.11) and allows the plant operator to judge the quality of regeneration
[2.27]:
There are two temperatures of about 30 and 40 °C, the so-called holding
temperatures, at which the exit temperature of the regeneration gas pauses for a
certain time. Only afterwards during the cooling phase, where cold regeneration
gas removes the remaining heat out of the adsorber, the temperature reaches a
maximum. The first small temperature plateau is induced by the CO2-desorption,
the second more developed one has its origin in the water desorption: When the
dry and warm regeneration gas, having the temperature Treg, meets the water-
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2 The Air Gases Nitrogen, Oxygen and Argon
Fig. 2.11 Regeneration gas temperature at adsorber inlet and outlet.
ADS = Adsorbing; D = Depressurizing; H = Heating; C = Cooling; P = Pressurizing.
loaded zone, it desorbs the water molecules until it takes on the temperature
dependent saturation water load y*(T). In doing so, the gas cools down to provide
the required desorption heat Had. However, owing to this the water adsorption
capacity of the regeneration gas decreases. Thus, a steady plateau temperature T*
is developed. Only after the water has been completely desorbed, the temperature
can rise again. By an enthalpy balance one derives an implicit equation, the socalled psychrometric equation, from which the plateau temperature T* can be
calculated
y * (T * ) H ad = (TReg − T * ) c p
Here, cp is the specific heat of the regeneration gas. The CO2-desorption is
subjected to the same mechanism. The excess heat, which is transported by the
regeneration gas into the adsorber and which is not needed for the desorption,
will only be transported to the outlet of the molsieve, after all components have
been desorbed at their respective plateau temperatures. This leads to the aforesaid
temperature maximum. A maximal temperature value of about 100 °C guarantees
a complete desorption.
Numerous detailed constructive solutions have contributed to the successful
introduction of the molecular sieves:
x The adsorber is subject to temperature changes of up to 200 °C occurring
periodically over years. The apparatus is constructed to withstand this alternating
thermal stress.
x Reliable switching valves have been developed even for large units with
diameters of up to 2 m.
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45
x A uniform gas flow through the molsieve material is accomplished. Maldistribution and by-pass flow weaken the adsorptive purification.
x The molsieve material at the outer edge zones is protected from heat loss to
the atmosphere. If the regeneration temperature in the wall areas remains
significantly below 50 °C, the adsorbens cannot be fully regenerated.
Compressors
Compressors are cost-intensive components:
x The investment for compressors including drive amounts to typically 15–20%
of the total investment of an air separation unit.
x Approx. 90% of the energy consumption is charged to the compressors. Thus,
their efficiency determines considerably the operating costs of the plant.
Compressors are designed as turbo or positive displacement machines. The two
types have different operational behaviour. With turbo machines the amount of
compressed gas decreases with increasing pressure, while the machines of the
second group deliver, owing to their displacement principle, nearly a constant mass
flow independent from the discharge pressure. Piston and screw compressors are
the most prominent displacement machines.
Multiple-stage turbo compressors are by far the most frequently used machines
in cryogenic air separation. Positive displacement machines are found in niche
applications, such as piston compressors for oxygen compression and screw
compressors for small plants processing less than 4000 mN3 h–1 of air.
Turbo compressors can be of the radial or the axial type. They differ from each
other in the direction by which the compressed gas leaves the impeller. If the
amount of process air is below about 400 000 mN3 h–1, radial turbo compressors
are preferably applied in two particular designs, i.e. the integrally geared turbo
compressor and the single-shaft compressor with integrated cooling devices. Their
development has decisively pushed the evolution of industrial air separation. The
specific thermodynamic and mechanical properties of these compressors will be
introduced in the following.
Principle of the Radial Turbo Compressor
The compressor is made up of several stages, which are arranged on one or more
shafts [2.33]. These are driven via a gear either by an electric motor or a steam
turbine.
The gas enters the impeller in axial direction. Here, the energy transfer takes
place. The impeller blades accelerate the gas and forward it in radial direction into
a diffuser. The cross section of the diffuser expands along the flow direction and
the speed of the compressed gas decreases accordingly. Thus the kinetic energy
of the gas is converted into pressure according to Bernoulli’s law. The gas, which
is heated by the compression to about 100 °C, flows from the diffuser outlet to a
shell and tube heat exchanger, where it is cooled with water before entering the
next stage. Since the gas volume decreases from stage to stage due to the increasing
pressure, all dimensions shrink accordingly.
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2 The Air Gases Nitrogen, Oxygen and Argon
The advantage of the radial turbo compressors is, apart from their reliability
and compact design, their high efficiency: The compression process comes
quite close to an isothermal state change, because the gas is cooled between
subsequent compression stages. And the ideal isothermal compression is known
P 
to have the lowest possible specific energy consumption R T ln  out  , where
 Pin 
R = 8.3143 J mol–1 K–1 is the general gas constant.
The heat removed in the water cooler is, according to the first law of thermodynamics, almost equal to the work performed in the compression stage.
X Example: A compressor for 300 000 mN3 h–1 of process air with a stage pressure
ratio of Pout/Pin ~ 1.8 has a power input of about 7 MW per stage. This amount of
heat determines the heat transfer surface to be provided in the intercoolers. Therefore, it is not surprising, that the appearance of the compressors is dominated by
the large intercoolers and that these can make up almost 30% of the compressor
costs.
Design Parameters of Radial Turbo Compressors
x Diameter D of the impellers between 200 and 1600 mm.
x The speed of rotation N is limited by the strength of the impeller material. The
peripheral speed u, u = N · S · D of the impellers may not exceed a critical value
of typically 380–450 m s–1. With a peripheral speed of 450 m s–1, the maximum
speed N for an impeller with a diameter of 1.6 m is N = 5400 r min–1 and for an
impeller with a diameter of 0.2 m the maximum speed is N = 43 000 r min–1.
x Pressure ratio per compressor stage about 1.4–2.0. With an eight-stage machine,
gas can be compressed from atmospheric pressure to about 100 bar.
x Isothermal efficiencies for large compressors lie between 73–79% and for small
ones in the range of 60–70%.
Numerous technical detail solutions have contributed to the reliability and long
lifetime of turbo compressors, i.e.:
x Non-contact seals designed as labyrinth seals or “Carbon Ring Seals”.
x A special starting system in case of an electric motor drive enables a quick
compressor start within about one minute such that the voltage drop in the
supplying net does not get larger than the admissible value of 2–15%. Thus
even for a large scale plant an electrical load of some MW can be switched on
rapidly.
Underload Capacity and Flow Control by Means of Guide Vanes
In times of lower product demand, the plant shall be operated in underload
mode with reduced power consumption. This will minimize the operating costs.
The underload flexibility is accomplished by the flow control of the compressor,
allowing a partial load of typically 70%. Below this value, the compressor falls
into a non-stationary so called surging mode, in which it cannot be operated. The
majority of compressors are driven by an electric motor with fixed rotational speed.
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47
Here the flow control is performed by means of a so-called inlet guide vane in
front of the impeller. This guide vane consists of a number of stationary blades,
which can be rotated, to modify the angle, by which the gas to be compressed
hits the impeller blades. This influences the amount of kinetic energy transferred
from the impeller to the gas.
Turbines
Expansion turbines have been developed to have high efficiencies of up to 90%.
In the beginning of cryogenic air separation, energy intensive high-pressure
recycles were applied to satisfy the refrigeration demand. With the introduction
of efficient turbines, these have become obsolete.
The enthalpy-entropy diagram (see Fig. 2.12) visualizes the expansion of
gaseous air in an ideal turbine, a real turbine and a throttle. The entropy increase
dS during the expansion is accompanied by a loss of technical available work
dE = Tamb · dS. Here Tamb is the ambient temperature introduced in the exergy
definition and the technical available work refers to this temperature. The loss
of exergy must be compensated by extra work performed on the compressors
of the process. Therefore, a process where expansion occurs in an ideal turbine
with dS = 0 requires less specific energy than a process with throttle expansion,
which has the highest increase of entropy. A real turbine lies between the ideal
turbine and the throttle.
In air separation, radial turbines are applied. The gas to be expanded enters
the turbine in a radial direction and is directed via a ring of static nozzles to
the impeller wheel inside of the ring. On flowing through the nozzles, the gas
velocity increases and thus the static pressure is already reduced by about 50%
and converted into kinetic energy. Then the gas molecules hit the impeller blades
and are ejected from the impeller’s eye in an axial direction. Since there is a static
Fig. 2.12 Turbine expansion in an enthalpy–entropy diagram.
(1) Ideal turbine (100% efficiency); (2) Turbine with 80% efficiency;
(3) Throttle expansion.
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2 The Air Gases Nitrogen, Oxygen and Argon
pressure gradient between impeller inlet and outlet, these turbines are classified
as so called back pressure or reaction turbines.
The turbine power is controlled by the opening angle of the nozzles which are
designed as swivel-mounted blades. Thus, the angle, by which the gas leaves the
nozzle ring and also the free diameter of this ring are varied. This allows controlling
the throughput of gas continuously.
The expanding gas performs work on the impeller. In small turbines with few
kilowatts of power, this work is rejected by transforming it into heat via an oil
brake or a brake blower. In larger turbines the released work is used for driving
either a generator or a single compressor stage arranged on the turbine shaft.
This compressor stage often serves as booster for the process gas flowing to the
turbine.
The technical solutions for bearings and contactless seals are similar to the
ones of radial turbo compressors. However special attention is paid to the sealing
and insulation: While the turbine’s temperature is significantly below 200 K, the
oil lubricated shaft bearing has ambient temperature. For insulation purposes,
labyrinths with three chambers are arranged on the shaft between the warm
bearing and the cold impeller wheel. So-called sealing gas is blown into the middle
chamber which escapes through each neighbouring chamber, on one side towards
the bearing, on the other towards the impeller. This prevents the oil or oil mist
from creeping along the shaft towards the cold process part and, vice versa, the
cold process gas from reaching the bearing.
Design Parameters of Expansion Turbines
x Impeller diameter between about 80–600 mm.
x The speed of rotation is limited by the maximal admissible peripheral speed
of the impeller which, for reasons of stability, ranges at about 250 m s–1. With
the diameters mentioned above, typical rotational speeds of large turbines lie
in the range of 10 000 r/min–1 and speeds of small turbines in the range of
50 000 r/min–1.
x Pressure ratio Pin/Pout between 2 and 15.
x The higher the volume flow, the higher the efficiency. The best efficiencies
achievable are just below 90%.
x Gas flow between about 300 and 100 000 mN3 h–1, released power between about
3 and 3000 kW.
Liquid Turbines
In the air separation process (see Fig. 2.3A) liquid high pressure air (c), resulting
from the internal compression of oxygen, is expanded via a throttle valve (22) into
the pressure column (12). Alternatively this expansion can be performed in a so
called “dense fluid expander”. This is a turbine for the expansion of a liquid or
very dense supercritical cold fluid. A turbine expansion produces less exergy loss
than a throttle expansion. Owing to this the use of a dense fluid expander reduces
the work for gas separation or liquefaction.
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X Example: The application of a liquid turbine instead of throttle 22 (see Fig. 2.3A)
reduces the work for the production of 50 000 mN3 oxygen at 30 bar by about
510 kWh: For 50 000 mN3 of oxygen to be internally compressed at 30 bar, about
65 000 mN3 of high-pressure air at 60 bar and 100 K must be expanded to 5 bar
via the throttle respectively the liquid turbine. With an efficiency of the liquid
turbine of 80%, the entropy increase in the turbine is dSturbine = 0.34 kWh K–1
and in the throttle dSthrottle = 1.7 kWh K–1 and the enthalpy change in the turbine
dHturbine = –132 kWh.
The change of exergy due to the expansion in the turbine is
dEturbine = dHturbine – Tu · dSturbine = –132 – 300 · 0.34 = –231 kWh
The change of exergy due to the throttle expansion is
dEthrottle = dHthrottle – Tu · dSthrottle = 0 – 300 · 1.7 = –513 kWh
Thus the application of the turbine reduces the loss of technical available
work by the amount of dEturbine – dEthrottle = –213 + 531 = 282 kWh. This work is
saved at the air booster (8, 9 in Fig. 2.3A), which now has to compress a smaller
amount of air. Since the booster compression itself is also not a reversible process,
but has an isothermal efficiency of typically 72%, the work saving at the shaft
of the compressor then amounts to 282 kW/0.72 = 390 kW. If in addition the
turbine drives a generator with an efficiency of 91%, then the gain of electrical
energy amounts to 0.91 · 132 kWh = 120 kWh. Then, the total savings amount
to 390 + 120 = 510 kWh.
Heat Exchangers and Condensers
The heat exchangers in the cryogenic section of an air separator are almost
exclusively of the aluminium plate fin type.
Their advantages compared to tubular exchangers are in particular:
x High specific surface in the range of 500 to 1800 m2 m–3.
x Several process streams can be passed through one block for mutual heat
exchange. This allows the design of complex processes.
x Lower costs.
Figure 2.13A shows the structure of a plate fin heat exchanger module: The process
streams are led through passages. Up to 200 of these passages are stapled one
on top of the other. This large number of passages makes it possible to bring
several streams into thermal contact within one unit. The outer frame is formed
by 10–25 mm side bars (5, 6), which are only interrupted for passage inlets and
outlets (7). A fluid enters the passage via this inlet. Beginning from here the flow
is distributed with special fins over the entire cross-section of the passage and is
then passed over to the section of the heat transfer fins (3). The individual passages
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2 The Air Gases Nitrogen, Oxygen and Argon
Fig. 2.13A Plate fin heat exchanger, schematically.
(1) Cove plate; (2) Partition plate; (3) Heat transfer fin; (4) Distributor fin;
(5) Side bar; (6) End bar; (7) Passage outlet.
Fig. 2.13B Plate fin heat exchanger assembled from three modules.
(1) Header; (2) Module; (3) Stub pipe.
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are closed on top and bottom against each other by partition plates (2), which have
a width of 0.8–2 mm. The arrangement of the individual passages as well as the
fin types are selected according to the process requirements.
The complex arrangement of passages is possible owing to a particular
manufacturing process [2.28]:
The partition and fin plates with widths of up to about 1.5 m and lengths of
about 8 m consist of aluminium alloy. They are covered with solder the melting
point of which is lower than that of the base metal. The partition and fin plates are
stacked alternately on top of each other to from a module with a height of up to
1.2 m. In a vacuum soldering furnace, this module is heated and soldered by means
of radiant heat applied to the module surface and by ensuing heat conduction
into the core. Fluxing agents are not required for this soldering process, thus the
finished modules are free of (corrosive) residues. Due to the heating the solder
melts and connects fins, partition plates and side bars, so that after cooling these
layers are intimately connected to each other. As shown in Fig. 2.13B, several
such modules can be welded one on top of the other to form a large core. The
collectors or so-called headers (1) are manufactured from semi cylinders and
welded onto the module. Cores with dimensions of up to 1.5 m u 2.5 m u 8.0 m
are manufactured this way.
A leakage between neighbouring passages will contaminate the products.
Therefore a high leakproofness of the passages is demanded. This is tested by
means of helium which can be detected even in small concentrations. Typically
leakage flows between the pressure and low-pressure passages in the range of
typically 10–3 mN3 h–1 are tolerated.
Exchanger Volumes
The exchanger volume V [2.29] depends upon the transferred heat Q and the
average temperature difference ¢dT ² between the warm and cold streams.
This temperature difference is evaluated from the Q-T-Diagram introduced
in Section 2.2.5.2, and lies typically in the range between 3–10 K. The mean
temperature difference and the amount of transferred heat are related by
Q = Ueff Aeff ¢dT². Here Ueff (W m–2 K–1) is the mean heat transfer coefficient and
Aeff (m2) the effective heat transfer surface.
The surface density of the plate fin heat exchanger is in the range of
Aeff /V ~ 500 m2 m–3, and the effective heat transfer coefficient is in the range of
100 W m–2 K–1, which leads to the following volume estimation formula
Q
dT
V ~
C
with the constant C ~ 50 000 W K–1 m–3.
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2 The Air Gases Nitrogen, Oxygen and Argon
X Example: Air separation unit for 300 000 mN3 h–1 of process air at 6 bar. The
air is cooled down from 300 K to 100 K, thereby warming up the separated
products and the residual gas to ambient temperature. The specific heat of air is
cp ~ 1300 J K–1 mN–3. Thus the transferred heat is Q = 300 000 mN3 h · 200 K · 1300
J K–1 mN–3 = 21.7 MW. The average temperature difference is typically ¢dT² = 4.0 K.
From this, an exchanger volume of about 109 m3 is estimated.
There are numerous types of fins (3, 4). The selection of the proper type decides
on a good design. The fins must ensure good heat flow, provide the mechanical
stability of the passages and induce only a small pressure drop.
The fin plates are shaped from 0.15 to 0.6 mm thick aluminium sheets by
periodical distortion. The fin height, i.e. the clearance between the neighbouring
partition plates, ranges between 3.8 and 12 mm. The distortions form channels
with “diameters” ranging between 1.2 and 4.5 mm. The Reynolds number
calculated on the basis of this diameter lies between about 500–10 000, i.e. the flow
is in the transition range between laminar and turbulent. In order to improve the
turbulence and thus the heat transfer, additional “obstacles” are installed. To this
end, fins are either perforated or are serrated and periodically displaced against
each other with a certain clearance.
A typical pressure drop of the low-pressure gaseous products in the exchanger
is about 10 kPa.
Safe operation and a long lifetime of the plate-fin exchanger require the
adherence to some rules:
x Maximum operating pressure within the range of 100 bar, maximum design
temperature < 65 °C.
x To confine the thermal stress, the maximum temperature difference between
the different streams is limited. Rapid temperature changes are inadmissible.
In the start up phase of the air separation unit the cooling down must be
performed sufficiently slowly.
x The exchanger passages cannot be cleaned mechanically. Therefore the entering
gases shall be free of any particles.
Combined Evaporator/Condenser – Heat Transfer Units
The combined evaporator – condenser units, such as the “main condenser” (13)
or the crude argon condenser (16) in Fig. 2.3A are special applications of plate fin
heat exchangers. Figure 2.14 on the left shows this heat transfer unit designed
as bath evaporator based on the thermosyphon principle. This is a plate fin exchanger with passages for the condensation of gaseous nitrogen from the top of
the pressure column and passages for the evaporation of liquid oxygen from the
bottom of the low-pressure column. The gaseous nitrogen is directed to the core
via a header at the top (header not shown in the figure) and exhausted as liquid
over a header at the bottom. The oxygen passages are open both at the bottom
and the top of the block. The core is immersed into a bath of liquid oxygen in
the sump of the low-pressure column. The liquid oxygen enters the block via the
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2.2 Recovery of Nitrogen, Oxygen and Argon
53
Fig. 2.14 Thermosiphon and downflow evaporator.
(1) Rectification column (pressure section); (2) Rectification
column (low-pressure section); (3) Process pump.
LOX = liquid oxygen; GOX = gaseous oxygen;
GAN = gaseous pressurized nitrogen; LIN = liquid nitrogen.
open passages at the bottom and is partially evaporated inside. The two-phase
mixture in the oxygen passages communicates with the liquid oxygen bath. The
two-phase mixture has a lower density than the liquid oxygen and is therefore
discharged according to the thermosyphon effect to the top of the block. A phase
separation takes place above the block. The evaporated oxygen ascends upwards
in the column while the remaining liquid falls back into the bath and mixes with
the fresh liquid coming from the trays above.
Typical core heights range between 1–2 m. Larger heights become more and
more ineffective since the additional heat exchange surface is paid for with a
reduced driving temperature difference between the evaporator and the condenser
side. This is because the pressure of the liquid oxygen in the bottom area of
the evaporator increases due to the hydrostatic pressure. This gives rise to a
higher boiling temperature of the liquid oxygen and thereby reduces the driving
temperature difference.
X Example: With a core of 2 m height, fully immersed into the liquid bath, the hydrostatic pressure at the bottom of the evaporator amounts to 22 kPa. This corresponds
to an increase of the boiling temperature by ~ 1.5 K and reduces the driving
temperature difference in the lower area of the block by this amount. With a core
height of 3 m, the hydrostatic pressure rises to 33 kPa, corresponding to a reduction
of the driving temperature difference by already 2.2 K in the lower area.
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2 The Air Gases Nitrogen, Oxygen and Argon
An alternative to the thermosyphon evaporator is the falling film evaporator,
shown in the right part of Fig. 2.14. Here the liquid oxygen is fed onto the head
of the core by means of a process pump (3) and flows downwards co-current to
the condensing nitrogen. A thin liquid oxygen film develops on the passage walls
and partially evaporates there. At the bottom outlet of the block the evaporated
fraction is separated. The remaining liquid portion is mixed with the fresh liquid
oxygen from the bottom tray of the low-pressure column and is fed back to the top
of the block via the pump. There is no hydrostatic pressure to reduce the driving
temperature gradient. Thus core lengths of about 6 m can be chosen and the
required heat exchange surface can be housed compactly within few large cores.
The liquid oxygen film, formed on the passage walls, may not dry out within
the block. Otherwise traces of hydrocarbon components would be enriched to
a dangerous concentrate. This is avoided by a sufficiently high flow rate of the
process pump and other measurements.
Columns
Since the beginning of industrial air separation in 1902 by Carl von Linde,
rectification columns have been the central element of the cryogenic separation
units. But even nowadays their technical development has not come to a standstill.
For example at the end of the 1980’s columns with structured packings were
introduced as alternative for tray columns. Packings cause lower pressure drop
and have a larger loading range than trays. Columns are the highest equipment
within the coldbox (Table 2.9) and determine its vertical dimension.
Table 2.9 Typical column heights.
Typical
theoretical
number of trays
Typical
height
Pressure column, sieve tray
45
14–25 m
Packed low-pressure column
80
25–40 m
Pressure and low-pressure column on top of each other
45 + 80
< 70 m
Crude argon column divided into two
2 · 100
2 · (23–32 m)
Packed Columns
Figure 2.15 shows the essential elements of a packed column: vapour flows
upwards through the regular structured packings (2) in counterflow to the liquid,
wetting the packing surface. The mass and heat transfer occurs on the surface
of the packing elements (1) [2.30, 2.31]. A packing element is about 200 mm
high and consists of thin corrugated aluminium sheets with a thickness of less
than a millimetre. Several packing elements are stacked on top of each other to
form a packing bed. The corrugation of the sheets is inclined by about 45–60°
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2.2 Recovery of Nitrogen, Oxygen and Argon
55
Fig. 2.15 Assembly of a packed column.
(1) Packing element; (2) Packing bed; (3) Liquid distributor;
(4) Liquid collector; (5) Liquid feed; (6) Liquid discharge;
(7) Gas discharge.
with respect to the horizontal and the inclination within one element alternates
between ascending and descending from sheet to sheet. The gas flows along the
channels formed by the corrugations, while the liquid runs downwards along the
direction of the steepest descent. Thus the liquid distributes along the plane of the
sheet. To guarantee a distribution also in a plane vertical to this, the orientation
of the packing elements lying on top of each other is twisted by 90°. Special seals
between the packing elements and the column wall prevent a by-pass flow of the
gas and liquid along the wall.
A uniform distribution of the liquid and vapour phase over the entire crosssection (3) is crucial for the efficiency of the separation. Even if the liquid is
initially distributed in a uniform way, its maldistribution increases with the covered
distance. Therefore, the height of a packing bed is limited to about 3–6 m. The
liquid is collected at the bottom of the packing bed, is mixed and is uniformly
redistributed by means of a distributor onto the top of the next packing bed.
The specific surface of the packing ranges between 350 and 800 m2 m–3. Despite
this high surface density, the packing material occupies only about 10% of the
volume. The HETP-value, i.e. the packing height equivalent to one theoretical
tray, is between about 170 and 500 mm.
Packing columns are manufactured with diameters of up to approximately 6 m
corresponding to an oxygen production of about 100 000 mN3 h–1. Larger diameters
are possible. However then transportation may get impossible and the column
must be assembled on site instead of being assembled in a workshop.
Sieve Tray Columns
Sieve trays are well-established in air separation. Due to their comparatively simple
assembly they are an economical alternative to the packings. A sieve tray consists
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2 The Air Gases Nitrogen, Oxygen and Argon
of a perforated plate, an inlet and an outlet downcomer. The gas flows upwards
through the perforation and gets into a crossflow contact with the liquid on the
perforated plate. This liquid comes from the inlet downcomer via a weir onto the
plate and leaves the plate by another weir and outlet downcomer. The mass transfer
takes place in the bubbling zone on the perforated plate. In the downcomer a
liquid level is formed. It compensates the pressure drop of the vapour on its way
through the tray and prevents the vapour from bypassing the perforated plate via
the downcomer. For columns with large diameters several downcomers must be
arranged on one tray, since the specific weir length, that is the length of one weir
divided by the liquid flow, decreases with increasing column diameter.
The area of the holes amounts to about 5–16% of the total area of the perforated
plate. The distance between neighboured trays lies between 80 and 300 mm. It
increases with the column’s diameter and load range. The tray efficiency factor, i.e.
the separation efficiency referred to one theoretical tray, ranges between 60 and
90%. Sieve tray columns are manufactured with diameters of up to about 6 m.
Pressure Drop of Sieve Tray and Packed Columns
The pressure drop over a sieve tray is 0.3–0.5 kPa, while the pressure drop in a
packing element corresponding to one theoretical tray is only about 0.08 kPa. The
small pressure drop of a packed low-pressure column significantly reduces the
energy needed for the separation.
X Example: Low-pressure column with 70 theoretical trays: Then the sieve tray
column is made up of about 85 sieve trays with a total pressure drop of 85 · 0.35 kPa
= 29.8 kPa, while the packed column has a pressure drop of only about 70 · 0.08 kPa
= 56 kPa. Owing to the coupling of the low-pressure column and the pressure
column via the condenser – evaporator unit (see Section 2.2.5.1), an additional
pressure drop dP in the low-pressure column induces an increase of the pressure
on the pressure side by an amount of ~ 3 · dP. Thus, for the case of a sieve tray
column, the pressure on the pressure side is by about 3 · (29.8 – 5.6) kPa = 72.6 kPa
higher than for the case of a packed column. If the discharge pressure in case
of a packed low-pressure column is 5.70 bar, it increases to 6.42 bar in case of a
sieve tray column, causing an increase in the power demand of this compressor
by about 6%.
Also due to the low pressure drop of a packed crude argon column, it became
possible to recover pure argon only by means of cryogenic distillation.
Load Range of Sieve Tray and Packed Columns
The efficiency of a plant increases, when the columns can be operated with
sufficient overload or underload in order to cover either an additional or a reduced
demand of the gas consumers.
The specific over- and under-load behavior of the two column types is summarized as follows:
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2.2 Recovery of Nitrogen, Oxygen and Argon
Sieve tray column
57
Packed column
Upper load range:
x Flooding in the downcomer: Liquid load,
at which the level in the downcomer rises
close to the tray above.
x Jet flooding: Vapour load, at which the
entrainment of liquid above the tray gets
too large.
x Abrupt transition to flooding with a sharp
increase of the pressure drop, stationary
operation of column no longer possible.
x Above a certain load, the liquid develops
a “bubble layer” at the intersection
between adjacent packing elements.
With increasing load, the bubble layer
gets larger until it fills the packing
element completely and flooding sets in.
x Smooth transition to flooding accompanied by a continuous reduction of the
separation efficiency.
Lower load range:
x ~ 50%
x Weeping: Vapour flow so small, that the
liquid weeps through the perforation of
the trays, exchange effect gets lost.
2.2.5.7
x < 40%
x Tolerant towards low vapour load.
x Minimal liquid load such that dewetting
of packing surface does not occur.
Underload capacity depends strongly
upon the quality of the liquid distribution.
Low surface tension of the cryogenic
liquid is of benefit to the wettability.
Design, Assembly and Transport of the Coldbox
All cryogenic components of an air separator are arranged in a so called coldbox,
which is a container with a height of up to about 70 m filled with insulating
material. Coldboxes for the processing of up to about 50 000 mN3 h–1 of air are
assembled completely in a workshop and are transported as “packaged units” to the
erection site. With increasing plant size only submoduls are shop-assembled. Their
size is chosen according to the restrictions of the means of transport. Figure 2.16
shows the transport of the two columns (at the back) and the two condensers (at
the front) of the nitrogen generator introduced before. It gives an impression of
how challenging transportation can be.
Equipment and pipes within the coldbox are made of aluminium alloy or 18/10CrNi-steel. These materials are suitable for the application at low temperatures
and dispose high ductility even at 100 K, i.e. no tendency to propagation of
cracks, originating from notches, sharp edges or abrupt discontinuities of the
wall thickness.
The equipment and its interconnecting piping is arranged in a compact way
within the coldbox in order to reduce space requirement and to minimize the
heat flow from outside into the box. When the plant is taken into operation,
the material cools down by about 200 °C. The associated contraction of the
material has to be taken into account by suitable pipe routing and supports. The
longitudinal contraction of aluminium due to this temperature difference amounts
to 4 mm m–1, i.e. a 30 m long pipe shortens by 12 cm during the transition from
the warm to the cold operating state. These demands on the piping design are
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2 The Air Gases Nitrogen, Oxygen and Argon
Fig. 2.16 Transport of shop-assembled components.
Fig. 2.17 3D-view of the Coldbox.
(1) Rectification column (pressure section);
(2) Second section of the crude argon column;
(3) Main condenser/evaporator;
(4) Rectification column (low-pressure section);
(5) Crude argon condenser;
(6) Coldbox shell.
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2.3 Safety Aspects
59
economically met by using a CAD-system with integrated pipe stress analysis. The
design review session, within which the coldbox, as for example shown in Fig. 2.17,
is virtually inspected via the CAD-system, is a quality-assuring measure.
A good insulation of the coldbox is required since the heat penetrating into the
cryogenic area from outside must be removed again at the expense of additional
energy: The interior of the coldbox is insulated with perlite. This is a bulk material
with grain sizes of about 0.5 mm, which is filled into the coldbox from above and
which flows into all cavities of the box like a viscose liquid. Perlite is made from pulverized volcanic rock, from which the crystalline water is expelled at 1100 °C. Thus
a “cell-like glass structure” with numerous inclusions of air – filled pores develops,
having a low heat conductivity O ~ 0.03 W m–1 K–1. Heat bridges are formed, if areas
remain non-insulated after filling of perlites into the box. These are recognized by
the icing on the outer coldbox shell and cause a loss of liquid production.
2.3
Safety Aspects
2.3.1
Introduction
Apart from the main air components N2, O2, and Ar a number of trace impurities
of the air enter the cryogenic section of an air separation unit. The entry of
combustible trace components, like methane, ethane, ethylene and propane,
represents a safety hazard even for today’s air separation plants, unless suitable
precautions are applied. The hazard results from the simultaneous presence
of pure oxygen which may act as an oxidizing agent. Thus compounds like
hydrocarbons could combust heavily with oxygen, ignition provided. In principal,
it is possible that, as a consecutive reaction of a hydrocarbon fire, even the metallic
column internals installed in the immediate vicinity of the fire source will ignite.
Typical internals of a rectification column of an air separation unit are e.g. packing
or sieve trays of aluminium. The aluminothermic reaction resulting in a violent
energy release can destroy the plant and injure persons. Actually the air separation
industry has experienced only very few major accidents of this kind, mainly caused
by maloperation of the plant.
Therefore, the following measures for the safe operation of air separation plants
should be observed:
x An accumulation of combustible substances in terms of a precipitation of
a pure hydrocarbon phase should be avoided (e.g. liquid propane or solid
acetylene in liquid oxygen (LOX)). An ignition close to a hydrocarbon phase
with a subsequent combustion can transfer a lot of energy to the adjoining
metal which might catch fire.
x The hydrocarbon concentration should be far away from its explosion range
(e.g. methane in O2). Regarding hydrocarbon hazard in LOX of air separation
units the lower explosion limit is of interest.
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2 The Air Gases Nitrogen, Oxygen and Argon
To exclude a precipitation of a solid or liquid phase, the hydrocarbon is only
allowed to enrich below its solubility limit in the LOX-sump. In addition, during
the operation of a plant a sufficiently large safety margin to the solubility limit of
a combustible component should be kept. Thus in large air separation plants, a
value of typically 500 vppm (ppm volume fraction, mL m–3) of methane (actually
methane equivalents) in the liquid oxygen would cause a shut down of the plant and
a following blow off of the sump. The lower explosion limit of volatile hydrocarbons
relevant for air separation units (ASU) is in the range of some percent. Thus
only methane, ethane and ethylene reach their lower explosion limits in the LOX
without precipitating. Heavy components in gas-producing air separation units
are being 500–2000 times enriched compared to their outside air concentration,
if the components pass the front-end coolers and molsieve adsorbers completely
(e.g. Kr, Xe, CH4). In Kr/Xe-plants the enrichment is enhanced further up to a
factor of 3000–5000. To exclude any hazard pertaining hydrocarbon accumulation
combustible compounds are burnt to water and CO2 at 500 °C. This is performed
at the inlet of Kr/Xe-plants on a noble metal catalyst, simultaneously converting
N2O to N2 and O2. In contrast to the mentioned hydrocarbon removal route
an alternative Kr/Xe-producing process exists where only the oxygen as main
component of the crude Kr/Xe-feed is completely substituted by N2.
In order to avoid exceeding a certain enrichment in an ASU, in places with the
highest enrichment, i.e. mostly in the sump of the low-pressure column (upper
column) or after a falling film evaporator, a certain quantity of LOX per time unit
is discharged. In order not to exceed arithmetically an enrichment of 1000, for
example, 0.5% of the generated liquid oxygen have to be extracted continuously.
To avoid product losses, this liquid is discontinuously being flushed into the
warm O2-product pipe.
Possible ignition sources within an air separation unit are: Combustion
of a hydrocarbon (promoted combustion), particle impact, friction, adiabatic
compression, resonance and electric arc, fraction, respectively generation of fresh
surfaces, shock waves and autoignition when heating to melting temperature.
Investigations pertaining incidents in air separation units elucidated the important
role of ignitions caused by preceding hydrocarbon enrichment. Of course, first the
ignition of the hydrocarbon phase itself has to be induced. In the past, acetylene
was considered to be the most dangerous hydrocarbon, due to its tendency to
self-decomposition and low solubility in LOX (5 vppm at –183 °C). In today’s
air separation units equipped with an air prepurification by molsieve adsorbers,
however, acetylene is completely removed by adsorption upstream the cryogenic
section. To ignite aluminium, a material preferably used in the manufacturing of
air separation units, temperatures above 2000 °C have to be reached [2.35]. The
ignition of metals in oxygen is facilitated, the less susceptible to oxidation the
metals are, the smaller the material thickness, the higher the O2-concentration,
the higher the ignition energy and in general the higher the pressure. With the
help of ignition tests it can be investigated under which conditions a sample
of a certain material exhibits a propagation of combustion. Thus numerous
publications of tests exist, dealing with the ignition of package, rods, tubes and
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2.3 Safety Aspects
61
Fig. 2.18 Impingement velocity curve for carbon steel (IGC Doc 13/02/E).
reboilers made of different alloys with varying O2-purity and pressure as the most
important parameters. This results in a so-called O2-index, which describes the
limiting conditions for a propagation of combustion of metallic material. Promoted
Combustion-Tests were often carried out with thermite ignitions (thermite pill:
ferric oxide/aluminium powder: 3 : 1) [2.36–2.38].
Furthermore that, for the safety assessment regarding the oxygen tolerance,
the gas velocity is of vital importance. Background is the particle impact as
ignition mechanism which becomes more likely at high gas velocities. Therefore,
international rules and regulations stipulate the compliance with maximal gas
velocities for C-steels and stainless steel, which additionally depend on the O2pressure. For C-steel in the pressure range of 3–15 bar, for instance, the flow
velocity must not exceed 30 m s–1; above 100 up to 200 bar, 4.5 m s–1 must not be
exceeded, as shown in Fig. 2.18. These limitations are applicable to the transfer
of gases in C-steel pipes with an O2-concentration above 35%, a temperature up
to 150 °C and a probability of an impingement. Generally, ignitions by particle
impact at lower pressure with a subsequent combustion of a metal are very unlikely
for flow velocities below 50 m s–1.
2.3.3
Air Pollution
Apart from methane, in the sump of the low-pressure column, ethane and propane
are often to be found, even if only in traces in the lower vppb-range (µL m–3),
since these components are not completely retained by the upstream molsieve
adsorbers. In industrial areas the LOX-sumps may also contain ethylene in trace
amounts. In addition, a number of inert compounds like nitrous oxide enter
the cryogenic part of an air separation plant, which could lead to the blockage of
pump filters, passages of reboilers and analyzing lines. The following Table 2.10
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2 The Air Gases Nitrogen, Oxygen and Argon
Table 2.10 Air components and impurities.
Components
Concentration
in clean, dry
air (vppm)
Concentration
in industrial
air (vppm)
Retention
in molecular
sieve
adsorbers (%)
Solubility
in LOX
at –183 °C
Lower
explosion limit
(%volume
fraction) [2.40]
CH4
1.7–2.0
3–10
1
totally soluble
4.4
C2H2
< 0.001
0.01
100
4–6 vppm
2.3
C2H4
< 0.001
0.05–2
85
1.3–3%
volume frac.
2.3
C2H6
0.005
0.05–1
10
12%
volume frac.
2.7
C3H6
< 0.001
0.05–2
> 99
0.36–0.67%
volume frac.
2.0
C3H8
0.003
0.05–2
70
1.0%
volume frac.
1.7
n-C4H10
< 0.001
0.05–5
> 99.99
45 vppm
1.4
Oil
0.01–2
mg Nm–3
partly oil
(oil as aerosol)
few vppm
0.8 (naphtha)
Acetone
0.01–0.05
mg Nm–3
100
8 vppm
2.5
Methanol
0.02–0.1
100
12 vppm
5.5
0.3–3
0
mixable
10.9
> 99.99
4.1 vppm
incombustible
70–99
140–160 vppm
CO
0.1
CO2
370
N2O
0.3
He
5.2
0
incombustible
Ne
18
0
incombustible
Kr
1.1
0
> 30%
volume frac.
incombustible
Xe
0.086
0
2.7%
volume frac.
incombustible
H2
0.4
O3
1345vch02.indd 62
0.4–1
0.8–10
0
0.06–0.25
mg Nm–3
100
(decomposition on molecular sieve)
4.0
11%
volume frac.
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2.3 Safety Aspects
63
states air pollutions with their typical concentrations, their solubility in liquid
oxygen at –183 °C and their lower explosion limit in the atmosphere. At the same
pressure and temperature, the lower explosion limit of a flammable compound
is almost independent from the concentration of the inert gas, provided the O2concentration is not falling below a lower limit where no combustion can occur.
Thus the lower explosion limit of a hydrocarbon in air or pure oxygen is nearly
equal in contrast to the upper limit. In cryogenic liquid oxygen, explosion limits
are not known, so that these obtained at higher temperatures (e.g. 20 °C) are also
applied for cryogenic temperatures.
Apart from this, components such as SO2, NO2, NH3, Cl2 and HCl occur as traces
in air. However, they are almost completely retained by the spray cooler and molecular sieve adsorbers. Partly, SF6, CF4 and C2F6 break through as non-flammable
compounds which are recovered in highly enriched sumps of Kr/Xe-plants. The
component oil given in Table 2.10, is mainly generated by oil-lubricated compressors that are only used in very small plants nowadays. This undesired oil-content
can be reduced by means of suitable downstream filters (e.g. activated carbon filters). An oil-load of the compressed air of 0.005 mg of oil per standard cubic meter
of air should not be exceeded at the inlet of the molecular sieve adsorbers.
In addition, even externally generated organic aerosols, e.g. in areas with
extensive forest fires, can enter the cryogenic part of a conventional ASU. Most of
the particles with high organic content have a size of 0.1–1 µm. Effective retention
of these particles is only possible with ultrafilters or aerosol filters. Once low
volatile oil drops or aerosols on organic basis have reached the cryogenic part,
they cannot be removed by warming up the ASU, in contrast to the highly volatile
C1–C3-hydrocarbons.
2.3.4
Ignition in Reboilers
Up to now, to the air separator industry a number of incidents caused by minor
hydrocarbon combustions is known. Commonly they occurred in reboilers leading
to only slightly damaged passages. These passages were bulged or started to leak
detectible by O2-contamination from adjoining N2-passages. Only in few cases
serious aluminium combustion happened arising from the reboiler where the fire
could also spread to the packing above resulting in a total damage of the plant and
environment. With a bath reboiler (see Fig. 2.14), a safe operation is achieved by a
sufficiently deep immersion in the LOX-bath (e.g. 100% immersion depth) and by
the high liquid circulation. In the case of a downflow reboiler with cryogenic oxygen
being fed from above and gas and liquid discharging at the bottom, dry evaporation
and thus deposition of LOX-impurities such as N2O and CO2 might locally occur,
even though the solubility limit of the impurity in the fed LOX has not been reached
by far. The more liquid is discharged at the outlet of the downflow reboiler, the
less dry evaporation accompanied with the deposition of heavy components takes
place. A typical liquid/gas mass-ratio at the outlet of the downflow reboiler is 3 : 1,
since here the reboiler passages are sufficiently flushed.
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2 The Air Gases Nitrogen, Oxygen and Argon
Another measure for the safe operation of a reboiler is the reduction of the
concentration of impurities dissolved in LOX. Here, not only hydrocarbon
concentrations need to be taken into consideration, but also the concentration of
compounds inert towards O2, such as N2O and CO2 which can be depleted in LOX
by a cryogenic adsorption. The inert substances involve an indirect danger, since,
deposited as solids, they block passages in the reboilers and create cavities in which
LOX evaporates and heavy hydrocarbons can accumulate. Since conventional
molecular sieve adsorbers are designed for the retention of CO2, usually more
N2O than CO2 is found in the analyzed deriming air when air separation plants
are being defrosted every 2 or 3 years. In case of simultaneous presence in LOX,
both compounds do no longer exhibit their original solubilities (see Table 2.10),
but precipitate at lower concentrations. The reason is the formation of a common
mixed crystal [2.41, 2.42], which is possible due to the almost identical molecule
dimensions of the two compounds. Thus, for instance, maximal solubilities of
3 vppm of CO2 were found at simultaneous presence of 60 vppm of N2O or 2 vppm
of CO2 with 100 vppm of N2O [2.43]. Due to this influence on the solubility, a solid
deposition is reached earlier in LOX when the two components are simultaneous
present.
2.3.5
Other Hazards in Air Separation Units
Apart from the increased combustion hazard in the presence of pure oxygen
cold burning due to skin contact during the discharge of liquid gases can injure
involved personnel. The symptoms on the skin are very similar to burns on hot
surfaces. Moreover, in case of nitrogen leakages in air separation units, there is
the danger of suffocation.
2.4
Process Analysis Air Separation Units
Depending on concentration, kind of component to be determined and the gas
balance, a couple of process analyzers based on different measuring principles
are used in air separation plants producing O2, Ar and N2. The analytical
control ensures a reasonable and safe pant operation and monitors the product
specification. For a not continuously required analysis commonly 2–6 analyzing
points share one instrument. If necessary the analyzing points can be switched one
by one by pressing a button or in some cases automatically for an alternate analysis.
Apart from the production itself, the adjoining tanks of the liquid products and
the filling of the certified liquid gases into tank trucks are analytically monitored.
In air separation plants, personnel experienced in analytics is usually entrusted
with the maintenance and monitoring of the process and product analysis.
The analyzers are accommodated in an air-conditioned analysis room and are
recalibrated with certified calibration gas in determined intervals. The analysis
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2.4 Process Analysis Air Separation Units
65
room itself is equipped with powerful ventilation to avoid a concentration of e.g.
N2 or O2 in case of leakage however, additional monitoring of the indoor air for
O2 is customary. In contrast to process analytics, the product control during the
tanking is carried out at another place with different instruments, i.e. directly at
the tanks. Table 2.11 sets out important measuring points in air separation units
together with the applied analytics.
In addition, the water content of the regeneration gas entering the molecular
sieve adsorber is analyzed. Due to this dew point analysis the leak-proofness of the
pressurized tubes of the regeneration gas heat exchanger conducting superheated
steam is guaranteed. In normal operation, a dew point of –70 °C is indicated.
In principle, only gaseous samples and those heated to room temperatures
can be measured in the analyzer unit. If impurities in cryogenic liquids are to
be determined, the liquids have to be evaporated first. In order to avoid faulty
measuring results due to an inadequate complete evaporation leading to a reduced
content of heavy components in the gas, cryogenic liquids should be heated
with a high temperature gradient as quickly as possible. This is guaranteed via
evaporation in thin, electrically heated capillaries.
All process analyzer were periodically calibrated with certified calibration gases,
commonly stored in 10 L steel or aluminium cylinders.
If, in addition, krypton, xenon or neon are produced, analytics will extend
considerably (see Section 3.5). Due to the presently increasing size of O2-producing
air separation units a krypton/xenon recovery is more and more of economical
interest. Actually many Linde plants are equipped with such krypton/xenonenrichment columns which are integrated in the main cold box and thus belonging
to the main air separation unit. The enriched noble gas containing liquid is
continuously directed to a tank and from there often shipped by road tank cars to
a Kr/Xe-purification unit for further refurbishment.
In this sump a 4000fold enrichment pertaining to the outside air concentrations
of Kr and Xe is realized. Subsequently all heavy air impurities which are able
to pass the air purification accumulate at this location. Common sump concentrations are: 4000 vppm Kr, 350 vppm Xe, 2000 vppm CH4, 10–150 vppm N2O,
0–2 vppm CO2, balance: O2/N2. Due to plant safety reasons the methane content
must be checked. The goal is to be far away from the lower explosion limit of
4 V%. Other hydrocarbons like ethane and propane are of minor interest because
of their low air concentrations. For the hydrocarbon sump control often a total
hydrocarbon analyzer is applied.
To avoid any blocking of the heat exchanger within the Kr/Xe enrichment
column also the content of N2O and CO2 must be monitored. Their combined
solubility limit in the sump should not be exceeded.
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2 The Air Gases Nitrogen, Oxygen and Argon
Table 2.11 Process analytics in air separation plants.
Measuring point
Component
Expectation value1)
Measuring principle,
Measuring range1)
Cause
After air
prepurification
CO2 in air
< 1 ppm
IR-spectrometry,
0–10 ppm
Performance
monitoring of air
prepurification
Middle section
pressure column
O2 in N2
a few %
Paramagnetism
0–25%
Monitoring of
pressure column
Argon transition
low-pressure column
O2 in Ar
about 90%
Paramagnetism
80–100%
Monitoring of lowpressure column
Argon transition
low-pressure column
N2 in O2/Ar
100–1000 ppm
Plasma cell
0–1000 ppm
Monitoring of lowpressure column
Condenser
sump of low-pressure
column
Hydrocarbons
10–100 ppm
Methane
equivalents
FID – total hydrocarbon analyzer
0–500 ppm
with column:
C1–C3-hydrocarbons
Safety low-pressure
column
Sump Kr/Xeenrichment column
CH4 in O2
2000 ppm
Flame ionization
detector
Safety Kr/Xeenrichment column
Sump Kr/Xeenrichment column
N2O/CO2 in O2
0–150 vppm
IR-spectrometry
Safety + monitoring
Kr/Xe-enrichment
column
N2-product
(highly pure)
Ar-product
(highly pure)
O2 in N2 resp. Ar
0–10 ppm/0–50 ppb
Electrochemical
Verification
Product specification
Ar-product
N2 in Ar
0–10 ppm
Plasma cell
0–100 ppm
Verification
Product specification
Ar-product
H2, O2, N2, CO, CO2
in Ar
ppm-range
Luminescence by
high-frequency
argon discharge
Verification
Product specification
O2-, Ar-, N2-product
Hydrocarbons in O2
resp. Ar resp. N2
0.05–100 ppm
FID – total
hydrocarbon
analyser
0–100 ppm
Verification
Product specification
O2-product
Ar in O2
1 ppm
Ionization through
high-frequency
discharge of helium
(helium detector)
0–100 ppm
Verification
Product specification
O2-product,
purity for steel mills
O2
99.5–99.8%
Paramagnetism
97–100%
Verification
Product specification
1)
1345vch02.indd 66
All concentrations indicated in mol%, mol ppm or mol ppb.
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2.5 Applications of the Air Gases
67
2.5
Applications of the Air Gases
2.5.1
Applications of Nitrogen
Gaseous nitrogen (GAN) is used as a raw material or inert gas, while liquid nitrogen
(LIN) is used for refrigeration. The versatility of nitrogen means it has a vast
number of applications. These are divided in the following into two main categories
of “inerting and purging” and “cooling, preserving and deep-freezing”.
2.5.1.1
Applications of Nitrogen for Inerting and Purging
In the processing industry, nitrogen is used to:
x fill and pressurise gas shock absorbers and hydraulic springs (oil conservation)
x perform leak tests (e.g. for containers) and protect against corrosion (e.g. for
electronic components)
x purge stainless steel pipes or containers before welding to perform root shielding
(mostly together with hydrogen)
x protect and inert electric parts during manufacturing and storage (e.g. lamps,
vacuum tubes and magnetic devices)
x create a high-purity inert environment for the manufacture of semiconductors
and as a carrier gas for epitaxy, diffusion and chemical vapour deposition
(CVD)
In metallurgy, foundry technology and the steel industry, nitrogen is used to:
x activate shut-off switches in burners and furnaces (with possible subsequent
N2 purging)
x purge and stir metal melts by bubbling them through porous bed stones or
lances (recirculation, discharge of gases and slag)
x feed powdered alloy components into steel melts (via N2 jet for alloying)
x spray metal melts and gain high-quality metal powders (nozzle atomization,
powder metallurgy)
x sinter metallurgical powders under shielding gas
x purge non-ferrous metal melts to reduce the hydrogen content (e.g. aluminium
melts)
x stabilise the austenitic structure of stainless steel (with N2 as a reactive
component)
x shield metal parts during thermal treatment (e.g. annealing, sintering, hardening)
x adjust carbon transfer in heat-treatment furnaces for gas carburization (e.g. with
methanol through the formation of CO and H2) (see Example A, below)
x reduce the carbon content of electric sheet metal in decarbonising annealing
(e.g. with humidified nitrogen and H2, CO and CH4 formation)
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2 The Air Gases Nitrogen, Oxygen and Argon
x maintain the carbon content of steel parts by carbon-neutral annealing (e.g.
with cracked hydrocarbon gases before quenching and hardening)
In the chemistry/petrochemical industries, nitrogen is used to:
x make products such as ammonia, calcium cyanamide and metal nitrides (with
N2 as a reactive component)
x produce foamed plastic (with N2 as blowing agent)
x prevent oxygen and humidity accessing production processes (i.e. when
producing graphite, phosphor, sodium, plastics, rubber and synthetic fibres)
x inert and handle storage tanks holding combustible liquids (e.g. filling, covering,
emptying) (see Example B, below)
x inert gasometers before revision or scrapping
x inert tankers during emptying and filling at ports (direct LIN feed for LNG
tankers)
x inert and clean oil pipelines (pipeline scraping or “pigging”) (see Example C,
below)
x discharge stone dust in oil and natural gas drilling (high-pressure N2 application)
x accelerate oil and natural gas recovery (enhanced oil and gas recovery (EOR))
x dry technical equipment (e.g. following repairs, during standstills or before
re-starts)
In food technology, nitrogen is used to:
x ensure pest control and fire safety in silos (e.g. for corn, powdered food)
x preserve fatty or powdered food during filling, packing and storing (N2 or
mixtures with CO2 and/or O2) (see Example D, below)
x protect liquid food from oxidation during storing and filling (e.g. beer, wine,
juices)
x ensure controlled ripening of stored fruit in combination with “ripening gases”
(e.g. N2/ethylene mixtures for bananas)
x stabilise the internal pressure of thin-walled beverage cans (administering drops
of LIN before closing)
In other industries, nitrogen is used to:
x avoid self-ignition of coal during handling (especially when transporting coal
dust)
x prevent mine fires (e.g. inerting of mine drifts in coal-mining)
x regenerate adsorbers for combustible materials (e.g. by purging with heated
GAN)
x reduce the oxygen content in cooling and storage rooms (fire protection, while
still allowing access to stores)
x shield perishable and combustible goods in receptacles and silos (general
inerting)
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2.5 Applications of the Air Gases
69
x inert pre-stressed steel in pre-cast concrete elements before grouting with mortar
(e.g. in bridge construction)
x prevent any kind of oxidation, including fire and explosion protection
x prevent problems during the assembly of electronic devices (see Example E,
below)
Example A: CARBOFLEX® Nitrogen/Methanol Atmosphere System
Carburising and carbonitriding are widely used in the automotive industry,
mainly to enhance the load performance of transmission and engine parts in heat
treatment furnaces. The major advantage of these processes is that they enable the
production of parts with very hard surfaces and a ductile and tough core.
During carburising and carbonitriding, carbon and nitrogen diffuse into the
surface of a steel component to produce a hard martensitic surface layer after
quenching. The reactive carbon originates from the decomposition of carbon
monoxide, the reactive nitrogen is generated when ammonia decomposes. The
carrier gas transports the reactive components to the surface of the steel parts.
Carbon and nitrogen potential can be measured and controlled by analysing the
furnace atmosphere.
Traditionally, this carrier gas is generated by understoichiometric combustion
of either natural gas or propane via a catalyst in an endogas generator. The
disadvantages of this method include limited gas purity, restricted flexibility and
high maintenance effort.
The CARBOFLEX® system can provide carrier gas efficiently, cost effectively
and reliably, while meeting all safety requirements. The process involves cracking
Fig. 2.19 CARBOFLEX® – continuous carbon control and optimising system
on a pusher furnace.
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2 The Air Gases Nitrogen, Oxygen and Argon
methanol in a hot furnace to produce hydrogen and carbon monoxide, which,
together with injected N2, forms an appropriate carburising atmosphere.
The benefits of the CARBOFLEX® process include:
x
x
x
x
x
Highly versatile range of application (carburising, carbonitriding)
Increased process flexibility and productivity
High purity gas supply (direct injection and formation in furnaces)
Lower costs (investment, cost of operation)
Safe operation and reliable performance (e.g. continuous monitoring,
see Fig. 2.19)
x Capacity for future growth
In combination with CARBOJET® high-speed gas injection technology, the
CARBOFLEX® technique can deliver higher levels of atmosphere homogeneity
and product quality as well as cut costs even further.
Example B: Inerting
For safety reasons, the inert gas nitrogen is used to displace atmospheric oxygen
to prevent explosions during processing, storage and transport of materials that
tend to oxidize in strong exothermal reactions (gases, liquids and powders).
This inerting process can be carried out using various methods such as dilution
purging (see Fig. 2.20) or blanketing. Based on the same initial and final oxygen
content, all of these processes use different quantities of inert gas and are applied
for different periods of time. The method used mainly depends on the size and
shape of the plant or the container.
The residual oxygen content depends on the product and the safety requirements. As a rule of thumb, the oxygen content should be below 8 vol.% for gases
and flammable liquids, below 4 vol.% for metal powders and below 10 vol.% for
all other powders.
Fig. 2.20 Dilution purging.
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Nitrogen for larger inerting projects is generally supplied in liquid form.
Stationary storage tanks and evaporation units are used for long-term inerting
and require some time to be installed. Mobile high-performance evaporators and
road tankers are available for temporary applications and can be operative within
a short space of time.
Special inert gas sluices are used for inerting containers that have to be manually
or automatically filled via manholes or valves. The sluices create an inert gas
mantle over the filling aperture and prevent gas exchange between the container
and its surroundings.
Example C: Pigging
When products are transported through pipelines (such as oil pipelines or
heat exchangers in refineries), impurities or materials can be deposited in the
pipelines as a result of physical or chemical reactions. These deposits have a
negative effect on throughput, pump efficiency and heat transfer, and are therefore
undesirable.
Pigging is used to remove relatively soft or pasty deposits that accumulate in
pipelines (see Fig. 2.21). During this process, a pipeline inspection gauge (pig)
is inserted into the pipeline through a lock system or pig trap. The pig is pushed
through the pipeline or pipe network either by the usual product flow or by an
inert gas, generally nitrogen. It scrapes along the pipe walls, pushing away any
deposits in front of it.
Pig traps are regularly spaced along pipeline systems, so that the pipeline
can be pigged in sections. Although the simplest pigs are made of foam balls
or plastic materials, different types of pigs are available, including those shaped
like dumbbells with sealing elements attached to the plates. Pigs are generally
deployed by pipeline operators and industrial services companies.
Fig. 2.21 Pipeline purging.
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Nitrogen is used in almost every case as the pressure medium because it can
be supplied in large quantities to any location, and is dry and inert. If the pipeline
or network is not filled immediately afterwards, nitrogen also offers effective
protection against corrosion.
Example D: Modified-Atmosphere Packaging (MAP) for Foodstuffs
MAP is used to maintain the quality of foodstuffs and increase the shelf-life
of packaged products. The key to this technology lies in applying the gases
(generally CO2, N2 and O2) and gas mixtures required for conserving the product
in question.
The most important prerequisites for successful MAP treatment are:
x High-quality products and raw materials
x Appropriate temperature control
x Strict hygiene standards throughout processes
x Appropriate quality and safety systems (such as hazard analysis and
critical control points or HACCP)
x Gas mixtures suited to the product
x Appropriate packaging materials
The last point, in particular, is a decisive factor in ensuring the efficiency of
MAP. Packaging should generally have low oxygen/gas permeability as well as
tight sealings. If this is not the case, too much gas can be exchanged or lost via
the packaging, invalidating the benefits of the initial MAP atmosphere. To avoid
adverse effects resulting from oxygenation, the concentration of residual oxygen
in each package should be below 0.5 vol.%. MAP provides an ideal atmosphere
through evacuation and replenishment, or by purging using oxygen-free gas
mixtures. Special MAP atmospheres with high concentrations of oxygen (such
as those used for fresh meat) are exceptions to this rule. If carbon dioxide is used
in a modified atmosphere, it should have a minimum concentration of 20 vol.%
in order to capitalise on its bacteriostatic effect (see Figs. 2.22 and 2.23).
Fig. 2.22 Bacterial growth on pork in different atmospheres at 4 °C.
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Fig. 2.23 Microbial growth based on pH levels.
Example E: Inerting During the Manufacture of Electronic Devices
The electronics industry is currently the largest market in terms of revenue,
positioned even ahead of the food industry. Its products are used in almost all
areas of daily life including mechanical engineering, consumer electronics and
the automotive and aerospace industries.
When electronic products are constructed, individual components such as chips,
condensers, or conductor boards must be connected to fully functional electronic
devices. This crucial step is usually performed using reflow or wave soldering
processes. Both processes can be performed either in air or in controlled and inert
atmospheres (see Fig. 2.24). Using a controlled atmosphere, however, provides
the following substantial benefits:
x Improved wetting results
x Increased soldering quality
Fig. 2.24 Gas installation for soldering machines.
1 Shut-off valve, main
2 Other gas using source
3 Shut-off valve, soldering machine
4 Pressure regulator
5 Pressure gauge
6 Solenoid valve
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x
x
x
x
Reduced solder consumption, especially in wave soldering
Reduced environmental impact (e.g. by using milder flux material)
Larger process window, greater flexibility without adjustment
Better evaporation of dissolvers from flux material, printed surfaces and
devices
x Less contamination of printed circuit boards and frames
x Fewer and smaller voids in soldering points
x Easier handling of consecutive soldering processes
2.5.1.2
Applications of Nitrogen for Cooling, Preserving and Deep-Freezing
In the processing industry, nitrogen is used to:
x join components by cold-shrinking and positive grouting with LIN (e.g. shafts,
gear wheels, valve seats)
x extrude aluminium (cooling the extruder head and/or the running strand with
GAN or LIN)
In metallurgy, foundry technology and the steel industry, nitrogen is used to:
x temper and harden metal parts through high-pressure (HP) gas quenching after
annealing furnaces (e.g. substitution of oil baths)
x embrittle cast-iron parts and knock off lugs (e.g. open risers on ductile cast
iron)
x remove residual austenite in hardened steel parts using cooling baths (the low
temperatures are generated by passing LIN through heat exchangers in the
bath)
In the chemistry and plastics industries, nitrogen is used to:
x cool rubber, pigments and plastics for cryogenic grinding (e.g. for homogenization and recycling) (see Example F below)
x cool rubber and plastic moulds for cryogenic deflashing (e.g. in mechanical
shakers, rotary drums and jet machines) (see Example G below)
x cool rubber and plastic hoses to produce reinforced, high-pressure qualities
x cool plastic films internally during blow moulding for manufacturing plastic
containers (e.g. to enhance production performance and product quality) (see
Example H below)
x cool coated metal parts for improved mechanical debonding and decoating (e.g.
recycling of used steel belt tyres)
x generate a high vacuum using vacuum pumps and cold traps (e.g. for applying
aluminium coating to foils)
x control the temperature of chemical reactions (e.g. LIN/GAN as a cooling
medium for chemical cryogenic syntheses) (see Example I below)
x cool extrusion tools for chemical manufacturing in general (compare with the
processing industry entries listed above)
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In food technology, biology and medicine, nitrogen is used to:
x support or substitute cryogenic plants in cold stores (e.g. emergency cooling)
or in refrigerated trucks (transport cooling)
x refrigerate feed and mills for gentle cryogenic grinding of spices (with benefits
including aroma preservation and improved feed control)
x freeze food such as meat, fish or bakery products (e.g. quick freezing in tunnel,
spiral or immersion freezers, see Example J below)
x cool food during mixing or chopping (e.g. meat emulsions in cutters during
sausage production, see Example J below)
x cool food during the tumbling process (e.g. treatment of ham, coating with
sauces)
x preserve biological material (e.g. cryopreservation of cells and tissue, “cryobanks”)
x perform cryosurgery for the specific destruction/removal of tissue (e.g. warts)
x support the cooling of magnetic resonance imaging scanners (see also applications of helium)
In other applications, nitrogen is used to:
x condense solvent vapours from exhaust gas flows (over heat exchangers or by
direct GAN/LIN injection, see Example K below)
x freeze soil for sealing and stabilising purposes in civil engineering (e.g. soil
icing, bulkheading groundwater, see Example L below)
x cool fresh concrete for security-critical buildings (preventing stress cracks
while setting)
x cool new asphalt road surfaces (making roads ready to bear traffic more quickly,
reduced down-time on building sites)
x freeze liquid-carrying pipes (blockages using freeze plugs, e.g. for assembly
work)
x simulate space conditions in cryochambers and climatic chambers (e.g. high
vacuum using cryopumps)
x operate cooled wind tunnels for high-speed testing of aircraft and missiles
(mostly models)
x cool and inert electronic components during operation and storage
x cool coated electrodes in glass production (oxidation protection)
x reduce air temperature to optimise cooling during glass manufacture (e.g. with
LIN heat exchangers in the air intake)
Example F: Cryogenic Grinding
Grinding is one of the oldest known technical procedures. Grinding heat-sensitive
materials can prove particularly difficult as many types of mills generate large
amounts of heat.
Using cryogenic industrial gases such as carbon dioxide or nitrogen guarantees
reliable cooling, which in turn reduces the amount of energy required for grinding
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Fig. 2.25 Diagram of a cryogenic grinding system.
and normally increases production rates (in comparison to a plant that does not
use cryogenic grinding). These gases also provide an inert atmosphere to prevent
explosions and fire hazards during cryogenic grinding (see Fig. 2.25).
In the food industry, it is common practice to cool mills that grind spices as
this enables delicate flavours to be retained. Similarly, many rubber and (thermo-)
plastic materials can only be reduced in size if they have been embrittled prior to
entering a mill. And the recycling industry also uses embrittlement and different
thermal expansion of components to break down composite materials.
Example G: Deflashing Rubber
Pressed or injection-moulded rubber parts often have to be deflashed following
vulcanization.
Previously, this residual material was removed by hand, using knives or scissors,
for instance. Later, CO2-based cryogenic machines were used to perform this task.
Today, we often employ automatic cryogenic tumblers or cryogenic shot blast deflashing machines. Liquid nitrogen (LIN) is normally used as the cryogenic fluid.
Cryogenic deflashing is mainly used in companies that produce rubber parts
in a wide range of shapes and sizes. This may include products such as sealings,
O-rings, bellows or stoppers (see Fig. 2.26). It is also a common procedure in
companies that provide deflashing services for original manufacturers (OMs).
A shot blast deflasher system uses small deflashing plastic granules and is a
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Fig. 2.26 Parts before and after deflashing.
particularly efficient, high-quality option for removing flash. Residual flash is
removed by granules that are propelled against the LIN-embrittled flashes by a
throw wheel. The deflashing temperature, retention time and energy input (throw
wheel speed, etc.) are controlled automatically.
Different types and sizes of deflasher systems are available on the market,
including special machines for large components or tyres.
Example H: Gas-assisted Injection Moulding (GIM)
GIM technology is a special injection moulding method for thermoplastic
polymers.
It is used to manufacture parts with cavities or thick walls. As this process also
overcomes sinking, it enables thick-walled parts to be manufactured without
sink marks.
GIM can be used to cost-effectively produce complex shapes that require highquality surfaces, offering designers a high level of freedom. Examples here include
door handles for cars or other plastic car parts such as mirror housings, bumpers
or door modules. GIM parts are not restricted to the automotive industry, however.
They can also be found in other areas, for example as handles in various household
appliances (e.g. refrigerators), TV cabinets or crates for carrying bottles.
In order to create a cavity, nitrogen is injected into the melted polymer at high
pressure (typically between 80 and 350 bar). The gas forces the polymer towards
the walls of the mould, creating the hollow section. After the plastic has solidified,
the nitrogen is released from the cavity.
Different variants of this technology are available on the market. The short-shot
process and the process with overflow cavities are the two most important.
The benefits of GIM technology include:
x Improved product quality as a result of superior surfaces (elimination of sink
marks), reduced warp and improved dimensional accuracy
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Fig. 2.27 Diagram of a GIM installation: Combined with a high-pressure
evaporator, Desy® is a practical nitrogen supply system for gas-assisted
injection moulding (GIM).
x Reduced cycle time due to faster cooling and enhanced productivity
x Energy savings resulting from the lower clamping force required from the
injection moulding machine
x Reduced resin feed stock, which in turn results in lighter products
In addition to the injection moulding machine, a nitrogen supply, appropriate
high-pressure compressor and pressure-control module are required for deploying
GIM technology (see Fig. 2.27). The pressure-control module is equipped with a
precise, highly dynamic pressure valve that allows defined pressure time profiles
to be created.
To ensure high product quality (e.g. no oxidation reactions) and reliable operation
with low levels of maintenance, high-purity nitrogen with a low oxygen content
must be used.
Example I: Cooling and Heating Systems for the Chemical Industry
Many reactions in fine chemistry and the pharmaceutical industry require precise
temperature control. This is increasingly assured using multi-step syntheses across
a wide temperature range from –100 °C to + 100 °C. Low-temperature synthesis is
gaining in importance, especially for the production of active agents and special
chemical substances.
During this process, liquid nitrogen cools a heat carrier which circulates in
a secondary cycle (see Fig. 2.28). The temperature of the heat carrier is raised
either electrically or by using a heating circuit. Linde uses a special hydrocarbon
as the heat-carrier medium. This process also allows standard heat-carrier oils
(e.g. Syltherm XLT) to be used at temperatures as low as –80 °C. The nitrogen
applied for the process can be reintegrated into the inert gas network and used
again. The various elements of the system are standardised.
Depending on the heat exchanger device, cooling and heating can be performed
at 5 kW to 50 kW, within a temperature range of –110 °C to +130 °C.
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Fig. 2.28 Cooling/heating system with secondary heating circuit.
Example J: Food Freezing Solutions
Cryogenic Bottom Injection System (LIX-Shooter®)
LIX-Shooter® is a cryogenic bottom injection system that works with either liquid
nitrogen or liquid carbon dioxide (“LIX”). This system is very useful in situations
when fast and efficient cooling is required without additional process equipment. It
can be installed in new or existing process machines, requires a minimum amount
of space and cools directly into the product. The resulting low phase separation,
low product stress and reduced aroma loss means that the end product has a very
high quality. The system works by injecting a discrete amount of coolant directly
into the product mass. The coolant evaporates when it comes into contact with the
product. It then absorbs heat from the product and continues to have a cooling
effect as it passes through to the top of the process container.
The system can be used for a wide range of products that require rapid cooling,
including meat and vegetable products (e.g. prior to forming), soups, sauces, paste
and pulp products, baby food and purées.
The Tunnel Freezer
The tunnel freezer is designed to meet the exacting standards of the modern food
industry (see Fig. 2.29). It combines the highest level of hygiene with the best
available control systems. This freezer is not only highly efficient in refrigerant
consumption, but also quick and easy to clean, thus keeping costs at a minimum.
With specially designed fans for a very high heat transfer and efficient refrigerant
spraying, the tunnel freezer has a large capacity for freezing or cooling a wide
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Fig. 2.29 CRYOLINE® MT tunnel freezer (Linde).
range of products despite its small footprint. These products include meat, fish,
seafood, and bakery foodstuffs, ready meals and other convenience food. The very
low operating temperatures of liquid nitrogen (LIN) and liquid carbon dioxide
(LIC) ensure extremely fast freezing. This helps maintain the quality and shape
of food products and keeps losses at a minimum.
The tunnel freezer can be used with either LIN or LIC as a refrigerant, depending
on local availability and product requirements. Manufacturers can select the
refrigerant that best meets their specific demands.
A fully compatible industrial PC can be used to monitor trends and events
during the production cycle and download these to an external computer, where
they can be stored as traceable records.
The Cryogenic Contact Freezer
The cryogenic contact freezer is designed to process products that are delicate,
sticky or difficult to handle in an efficient and hygienic way. The freezer uses a
disposable plastic film that travels through a conventional freezing tunnel where
it comes into contact with cold plates. These plates are cooled by vaporizing liquid
nitrogen at –196 °C, which in turn quickly and effectively freezes the contact
surface of the product. This ensures that the product is free from belt marks and
wrinkles and can be easily handled for further processing. The film ensures that
soft, wet or sticky products can be readily handled without deforming or sticking,
and even liquids can be easily frozen. As the film is disposable, the freezer can be
quickly and efficiently cleaned at the end of production, ready for use the following
day. This also means that product changes do not cause expensive delays.
The system acts as a contact freezer, where heat is removed from the product
following contact with the cold plates. High-speed fans circulate the cold gas
atmosphere inside the tunnel and help to freeze the upper surface of the
product. The gas generated by vaporizing the liquid nitrogen is exhausted into
the atmosphere. As the temperature of the exhaust is controlled, the amount of
cold extracted from the nitrogen can be optimised, depending on production
requirements.
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Fig. 2.30 CRYOLINE® CS spiral freezer (Linde).
High-capacity Spiral Freezer
The cryogenic high-capacity spiral freezer is self-stacking and has the smallest
possible footprint (see Fig. 2.30). It is based on a new technology designed to create
a more efficient and cost-effective freezer. The eight-sided shape is manufactured
with a minimum of space around the belt, ensuring that the cold gas moves at
high speeds and removes the heat as quickly as possible. The nitrogen supply and
gas balance are controlled by a state-of-the-art automatic system, resulting in less
idle consumption compared with existing spiral freezers.
The system is suitable for freezing a wide range of products, including meat
patties, whole fish or fish fillets, pies, ice cream, pastries, pizza and ready-made
dishes. The spiral can also be used as a cooling unit.
Liquid nitrogen is used to maintain a very low operating temperature, ensuring
rapid freezing that preserves the quality and shape of the product and keeps weight
loss to a minimum. Cryogenic freezing is normally most economical with low to
medium volumes. This spiral freezer, however, is also suitable for large volumes
where a high level of quality is required or the special preservation properties of
cryogenic freezing are critical.
Example K: Solvent Recovery (Cryocondensation)
Almost all painting, gluing and coating processes produce solvent vapours. Solvents are recovered for reasons of cost and safety. On the one hand, recovering
solvents saves energy and process materials; on the other, it prevents environmental and health problems as well as fire hazards.
Nitrogen is commonly used as an inert carrier gas in drying units linked to
condensation recovery units. If solvents are recovered using adsorbers (e.g.
activated carbon), the adsorbent can be regenerated using nitrogen. This is useful
if flammable or explosive materials are being recovered, or if the adsorbent adsorbs
unwanted moisture from the air and therefore needs to be dried.
Nitrogen is the preferred refrigerant for solvent recovery through condensation
when very low temperatures (i.e. below approx. –40 °C) are required for recovery
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Fig. 2.31 The CRYOCON™ solvent recovery process (Linde).
or if the volumes for cleaning are very small (up to approx. 500 m³/h) and highly
loaded with solvents or other pollutants.
Waste gas emission limits as set out by national regulations (see e.g. the German
air regulations “TA-Luft”) are easier to maintain if waste gas is continuously
emitted. Cryogenic solvent recovery plants become more cost-effective if larger
amounts of nitrogen can be used as an inert gas for other operations. The
CRYOCON™ process from Linde (see Fig. 2.31) is a typical example of such an
installation.
Example L: Ground Freezing
Ground freezing is designed to make unstable, soft and water-logged soils stable
and watertight (see Fig. 2.32). The process is applied in tunnels, pits, shafts and
other special ground construction works when problems arise and conventional
stabilising and sealing methods cannot be successfully applied. In some cases,
it may make sense to combine different methods, e.g. grouting, drainage and
ground freezing.
Electrical brine freezing plants were originally used for this process and are
still common today. For smaller projects and short-term freezing, however, liquid
nitrogen ground freezing is more suitable as it provides a more flexible freezing
capacity and reduces the time needed for freezing. Equipment costs are low in
comparison to brine freezing, but energy costs are higher.
Ground freezing usually involves creating frozen soil walls in soft ground or
temporary ice barriers in existing constructions such as bore or sheet-pile walls.
The ice acts as a sealant and fulfils a temporary, static function. In this process, ice
is created by drilling or pushing freeze pipes into the ground. These pipes can be
inserted to form a closed ice wall. A distribution, circulation and controlling system
must be installed to transport liquid nitrogen (LIN) into the freeze pipes. The LIN
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Fig. 2.32 Ground freezing: Icing a gap in a sheet pile wall.
evaporates in the freeze-pipe system as it absorbs heat from the surrounding soil,
then flows into the exhaust pipe system more or less warm.
This method can create a one-metre frozen wall in approximately three days.
It is very quick and safe and is often used both in emergencies and as part of
scheduled small and medium-sized projects.
2.5.2
Applications of Oxygen
Many industrial processes are using air for combustion and chemical oxidation.
The benefits of using oxygen by enriching or replacing air include:
x Higher reaction temperatures
x Faster reaction through higher partial pressure of reaction partners
(by eliminating N2)
x Energy savings, especially at high temperatures and pressures (no N2 ballast)
x Smaller plant dimensions or higher plant performance (no N2 ballast)
x Higher yield and selectivity of reaction
x Prevention of undesired secondary reactions with N2 (e.g. formation of
nitride)
x Few or no valuable products lost with off-gas
x Reduced off-gas volumes, emissions and off-gas treatment costs
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In the processing industry and autogenous technology, oxygen is used to:
x operate high-performance burners (e.g. for gas welding, flame cutting, flame
soldering, flame heating, flame straightening or flame gouging), in particular
with the fuel gases acetylene, natural gas and hydrogen (see Examples A and
B below; see also Chapter 8)
x coat work pieces using oxy-fuel gas flame spraying (e.g. with metals or ceramics,
see Example C below; see also Chapter 8)
x derust steel sheets or rework concrete surfaces by flame blasting
x remove flash from metal die castings by explosion deburring (in a pressure
chamber with an O2/H2 or O2/CH4 mixture)
In metallurgy, foundry technology and the steel industry, oxygen is used to:
x refine pig iron in converters (decarburization: using O2 lances in the LinzDonawitz (LD) method, using O2 bottom blowing in the Oxygen Bottom
Maxhütte (OBM) method)
x treat scrap in electric arc furnaces (melting, refining and alloying with O2 lances
and O2 burners)
x increase the performance of induction furnaces in the manufacture of cast iron
(using O2 burners)
x increase the performance of shaft or cupola furnaces in the manufacture of cast
iron (O2 enrichment with wind nozzles, HIGHJET® method)
x increase the performance of blast furnaces by injecting O2 (using lances) and
fuel (e.g. oil, coal dust)
x preheat casting ladles (prevents melting baths from cooling, reduces cycle
times)
x increase the performance of furnaces in non-ferrous metallurgy (e.g. aluminium
or copper hearth-type furnaces) with additional oxy-fuel burners (see Examples
D and E below)
x boost reheating and annealing furnaces in rolling mills, processing lines and
forges (see Example F below)
In chemistry, energy engineering and environmental protection, oxygen
is used to:
x produce chemical products through oxidation of inorganic raw materials (e.g.
hydrogen peroxide from hydrogen) and organic raw materials (e.g. ethylene
oxide from ethylene)
x generate synthesis gases (e.g. mixtures of CO/H2 with possible further reaction
to alkanes and alcohols) through partial oxidation of hydrocarbons, e.g. oxidation
of heavy oil residues with O2 (see Section 5.2)
x generate synthesis gases through coal gasification, e.g. by gasification of coal
dust with O2/H2O (see Section 5.2)
x increase the performance of Claus plants (extraction of sulphur from process gases containing H2S, production of sulphur-free fuels, see Example G
below)
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x regenerate catalysts (e.g. burn off adsorbed coke in the Fluid Catalytic Cracking
(FCC) process, see Example H below)
x operate catalytic oxidations in the gas/liquid phase (e.g. production of propionic
acid) or in the heterogeneous gas phase (e.g. production of vinyl acetate
monomer – VAM)
x operate power stations with low emissions, e.g. through coal gasification with
O2 and subsequent removal/recycling/sequestration of CO2
x treat waste water in biological sewage works, (e.g. by injecting O2 into activated
sludge tanks to improve performance and minimise odours, see Example I
below)
x prevent anaerobic digestion processes in pressurised sewers (e.g. by injecting
O2 into pump stations to suppress odours and corrosion)
x treat solid waste products in gasification plants (e.g. through partial oxidation
with O2 and further off-gas treatment as a low-emission alternative to waste
incineration)
x treat flame-resistant, harmful liquids (e.g. for environmentally friendly disposal
through high-temperature combustion with O2)
x produce chemical pulp from wood chips in paper manufacturing (e.g. for
delignification, bleaching and black liquor oxidation, see Example J below)
x enable oxidative treatment of drinking water (e.g. to remove iron, manganese,
ammonia and organic substances, see Example K below)
x produce ozone for hygiene and chemical purposes (e.g. for purifying drinking
water, bleaching pulp, sterilisation, deodorisation or chemical synthesis)
In engineering, food treatment and medicine, oxygen is used to:
x substitute chemicals in meat and sausage preparation (e.g. O2 maintains a
fresh, red colour)
x preserve packaged food (e.g. O2 mixed with CO2/N2 for packaged lettuce)
x oxygenate water in fish-farming basins or containers for transporting fish (e.g.
O2 for aquaculture or emergency supply)
x provide respiratory gas in aircrafts (breathing oxygen)
x support and enable various medical applications (see Chapter 10)
In other industry segments, oxygen is used to:
x recover energy and raw materials (e.g. recycling in the pulp and plastics
industry)
x improve burnout, slag liquefaction and emission values (e.g. in waste incineration sites)
x increase the performance of melting devices and emission control units (e.g.
application of oxy-fuel burners or oxygen lances in the glass industry, see
Example L below)
x improve finish on industrial products (e.g. by fire-polishing glassware, see
Example M below)
x manufacture semiconductor components (e.g. highest purity O2 for thermal
oxidation of silicon, in combination with a carrier gas for gaseous diffusion)
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x impel rockets and missiles in the aerospace industry (O2 as an oxidant for
spacecraft fuels)
Example A: Oxy-fuel Cutting
In industrial applications, oxy-fuel cutting is the predominant flame process for
cutting mild- and low-alloy steels (see Fig. 2.33). With the right gases, a good
torch and a steady hand, this process enables an experienced operator to cut steel
anywhere, as it does not require electricity or complicated equipment. Although developed at the beginning of the last century, the basic processes still apply today.
Oxy-fuel cutting is an intensive combustion process that involves steel being
preheated to ignition temperature. This is achieved using an oxy-fuel gas flame.
An oxygen jet then burns a narrow section of the metal at the point where the
operator wants to make the cut. The jet also removes the molten combustion
products (slag) from the cut.
The purity of oxygen is a key factor in determining cutting speed. High-purity
oxygen increases productivity. The initial quality of the O2 must therefore be
maintained along the pipes and metering device until the point of use. The fuel
gas influences the quality of the cutting process, the preheating time and the
thickness of the material that can be cut effectively.
Flame temperature is an important factor when a work piece has to undergo
rapid, concentrated heating and preheating (see Fig. 2.34). In this case, the higher
the flame temperature, the more heat is transferred to the work piece. In comparison with other fuel gases, the use of acetylene allows the fastest preheating
of steel for cutting holes, and the highest cutting speed even for rusty, scaled or
primered sheets.
Fig. 2.33 Oxy-fuel cutting.
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Fig. 2.34 Flame temperatures: Advantages of the oxy-acetylene flame.
Oxy-fuel cutting is a versatile process that can be deployed for a wide range of
applications, including straight cuts, bevelling or weld-edge preparation using
multiple torches simultaneously. Another benefit of this method is that it can
easily be automated.
Although the process itself looks very easy, handling fuel gases and oxygen
actually requires considerable expertise and familiarity with the equipment and
relevant safety requirements.
Example B: Soldering and Brazing
Soldering and brazing are used to join two metal parts by means of a filler metal
that has a lower melting point than the base metal. The joint may consist of one
or more metals. The filler metal is normally distributed in the joint by capillary
action, with additional flux dissolving unwanted oxides. The necessary heat can
be provided from a number of sources, including furnaces or fuel gas torches.
In soldering, metals are joined by a filler metal with a melting point below 450 °C.
Soldering is used for copper and its alloys, zinc, steel, aluminium and its alloys.
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Fig. 2.35 Mechanised brazing process.
The most common filler metals are tin-based, although special filler metals are
required for aluminium. Soldering produces weaker joints than brazing. However,
alloying filler metals with silver considerably increases joint strength.
Due to the relatively low temperature of the flame, soldering can be performed
using simple propane/air burners. The oxy-acetylene process, however, may be a
better option for more demanding soldering tasks.
In brazing, filler metals have melting points between 450 and 1000 °C. So
more powerful torches tend to produce better results here. The oxygen source
(O2, compressed air or aspirated air) affects the heating method for the work
piece. Acetylene flames cause the surface temperature of a piece to increase
more quickly than flames generated by other fuel gases. The reducing effect of
acetylene-compressed air flames helps dissolve oxides, supporting the flux applied
to the surface of the brazing zone.
Braze welding (see Fig. 2.35) uses filler metals such as brass or bronze, which
are not distributed in the joint by capillary action. The benefits of the flexible oxyacetylene process make this a frequent choice here. Braze welding produces very
strong joints in steel and copper. It is widely used to repair cast parts.
Example C: Flame Spraying
Spraying is an interesting surface technology that is growing steadily. During this
process, a gas flame and coating material are used to create an optimal surface
on a substrate (see Fig. 2.36). It is a simple, fast and profitable procedure that can
increase a substrate’s resistance to wear, corrosion and heat. It is also possible to
increase or decrease friction or change a surface’s electrical properties. Damaged
surfaces can be repaired and faulty parts corrected.
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Fig. 2.36 Flame spraying with metal wire.
In flame spraying, the material used for coating (a wire, rod or powder, for
instance) is heated in a gas flame until it melts. The molten particles are then
transferred to the substrate by the gas jet, and the new surface is generated. In
most cases, the surface can be post-treated without difficulty.
Any material can be used to spray a substrate in flame spraying, and any solid
substrate can be coated. The fuel gas used depends on the melting temperature
of the coating material. This gas also influences the speed at which the gas jet
propels the molten coating particles and, as a result, how they adhere to the work
piece.
Oxygen is the second combustion component in flame spraying. Only an
oxy-acetylene flame can provide the required temperature and efficiency (at a
neutral flame setting) to coat high-melting materials such as molybdenum. Other
fuel gases, including ethylene, hydrogen, propylene, or propane, are suitable
alternatives for low-melting materials. In the case of powder flame spraying,
a propellant gas (e.g. air, nitrogen, argon) can be used to further accelerate the
powder particles.
Example D: Recycling of Contaminated Scrap (Oxygen Lancing)
Melting contaminated scrap is a challenging task for an air-fuel combustion
system. The metals recycling industry (e.g. aluminium, copper, lead) is forced
to recycle even highly contaminated scrap to be competitive and protect the
environment.
Oxygen lancing improves the combustion efficiency of air-fuel systems and
reduces atmospheric emissions. This technology injects oxygen primarily to burn
off feedstock contamination (e.g. oil, paint, organics, hydrocarbons).
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Fig. 2.37 Oxygen lances convert feedstock combustibles into energy.
Fig. 2.38 Oxygen lances and oxy-fuel burners in a double-pass rotary furnace.
The benefits of oxygen lancing include:
x
x
x
x
x
Fuel savings
Reduced and controlled emissions
Enhanced feedstock variations (contaminated scrap, low-grade scrap)
Simple installation
Increased production
While oxygen lancing (see Fig. 2.37) achieves elimination rates for feedstock combustibles of up to 99%, air-fuel combustion can only achieve rates between 50 and
80%. Oxy-fuel burners combined with oxygen lancing (see Fig. 2.38) are also very
efficient at reducing volatile organic combustibles (VOCs) in the flue gas stream.
Example E: Universal Rotary Tiltable Furnace (URTF)
Aluminium and dross recycling is a challenging task with regard to productivity
and metal yield. Fixed-angle rotary furnaces require specific charging, tapping
and cleaning times. Although fluxes are needed to maximise metal recovery, they
create environmental issues and increase costs.
The URTF (see Fig. 2.39) has been developed to set new, highest standards in
recycling aluminium and dross.
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It provides the following benefits:
x
x
x
x
x
x
Improved metal yields
Reduced process-cycle times
Higher productivity
Fuel savings
Reduced feedstock grades
Reduced costs for fluxes and disposal (see Fig. 2.40)
The URTF employs the dry salt process and therefore optimises the energy
required by the melting cycle. Oxy-fuel combustion can also be combined with
oxygen lancing.
Fig. 2.39 Diagram of a URTF with oxygen lancing technology: The URTF
delivers unique process technology for recycling contaminated metals.
Fig. 2.40 URTF at Stena Aluminium, Sweden: The URTF reduces costs
for fluxes and disposal.
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Example F: Reheating and Annealing Furnaces
A wide range of oxy-fuel solutions are used in reheating and annealing furnaces.
These are integrated in rolling mills, processing lines and forges, for example.
When industrial-grade oxygen is combusted with either gaseous, liquid or solid
fuels in a furnace, it generally optimises thermal efficiency and fuel burnout.
Thanks to the higher partial pressure of O2 and the exclusion of nitrogen ballast,
reactivity in furnaces improves and flue gas volume decreases (see Fig. 2.41).
At the same time, the partial pressure of H2O and CO2 in the flame rises, which
in turn causes radiation and heat transfer to increase. Typical improvements
compared with air/fuel systems include:
x up to 50% more furnace capacity
x up to 50% less specific fuel consumption
x up to 20% less flue gas volume
Fig. 2.41 Comparison of burner systems for reheating and annealing.
Top:
Oxy-fuel flame:
Optimum combustion and heat transfer,
focussed flame.
Centre: Air-fuel flame:
Dilution and cooling by N2 ballast,
reduced efficiency.
Bottom: Flameless oxy-fuel: Dilution with furnace flue gases,
reduced NOx emissions.
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Fig. 2.42 Flameless oxy-fuel at Outokumpu Stainless, Degerfors, Sweden.
The Outokumpu plate mill implemented oxy-fuel technology in its
existing walking beam furnace. The stipulated performance guarantee
resulted in a 40–50% rise in furnace heating capacity, a 25% drop in
specific fuel consumption, more uniform temperatures for correct
rolling and a drop in NOx emissions to below 70 mg/MJ.
This also enhances process flexibility and reduces CO2, NOx and SO2 emissions,
depending on the equipment and type of fuel used.
Industrial gas companies have over 40 years of experience in the field of
industrial combustion and heating. They provide technical solutions for reheating
and annealing furnaces that cover:
x
x
x
x
in-depth analysis of customer processes
process and equipment engineering
furnace revamping (including combustion technology and equipment)
installation, startup, monitoring and optimisation
The technology deployed is usually patent protected, as is the case with REBOX®
solutions (see Fig. 2.42). This cutting-edge technology comprises “Direct Flame
Impingement” and “Flameless Oxy Fuel”.
Example G: Oxygen Enrichment in Claus Plants
Oxygen enrichment helps refineries produce clean fuels. Claus plants use oxidation
to remove hydrogen sulphide (H2S) and ammonia (NH3), for instance. Increased
levels of ammonia are a by-product of producing clean fuels. Ammonia is created
when crude oil is severely hydrogenated to convert organically bound sulphur into
H2S. During this process, some of the nitrogen reacts to form ammonia.
H2S and NH3 are transported to a refinery’s Claus plant, which then processes
H2S into elemental sulphur. A Claus plant can also remove pollutants, in particular,
by decomposing ammonia. When a Claus plant’s H2S and NH3 load increases,
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Fig. 2.43 The OXYMIX® oxygen injector (Linde) and the O2 concentration: O2 concentration
in the process air is calculated using the computational fluid dynamics (CFD) method.
Fig. 2.44 The FLOWTRAIN® control system (Linde): The system is
used to meter O2 feed into Claus plants and fulfils safety requirements.
bottlenecks may occur. Oxygen enrichment can be applied to overcome this
problem.
Oxygen can be injected into the pipe that carries combustion air to the Claus
furnace, for example. This simple method of applying oxygen is mainly used for
applications requiring oxygen enrichment levels of up to 28 vol.% (see Fig. 2.43).
For higher O2 levels, oxygen is usually injected directly into the burner.
Another crucial piece of equipment is the control and safety cabinet (see
Fig. 2.44). This is required to regulate the oxygen injected into the combustion
air or burner.
Figure 2.45 shows the capacity increases that can be achieved in Claus plants
through oxygen enrichment. Given a constant sulphur load, the total gas flow
decreases as oxygen enrichment increases, due to reduced levels of N2 in the
process air. The resulting drop in pressure can be compensated by increasing the
amount of feed gas, which in turn increases the Claus plant’s capacity.
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Fig. 2.45 Claus plant capacity as a function of oxygen enrichment.
Example H: Fluid Catalytic Cracking (FCC) with Oxygen
FCC plants are used to convert vacuum gas oil (often mixed with residues
from atmospheric distillation, vacuum distillation and visbreaking) into lighter
hydrocarbon fractions. The products are a gas fraction (primarily C3/C4), a liquid
fraction (primarily gasoline) and solid coke. The coke is deposited on the catalyst
and burnt off during catalyst regeneration (see Fig. 2.46).
Fig. 2.46 Overview of an FCC process.
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Fig. 2.47 Test results from an experimental FCC plant.
Oxygen enrichment boosts the regeneration stage and offers the following
benefits:
x Increased capacity of entire plant
x Greater flexibility, particularly with regard to selecting feedstock
x Possibility to use heavier feedstock with a greater tendency to form coke
x Improved conversion rate and gasoline yield (see Fig. 2.47)
x Reduction in by-products
x Less catalyst abrasion and erosion of cyclones during catalyst separation thanks
to smaller gas streams, resulting in lower gas velocity
Example I: Waste-Water Treatment – Aerobic Purification
Many problems in waste-water treatment – both in municipal and industrial plants
– are caused by oxygen deficiency. The consequences include inadequate purification or even anaerobic decomposition processes, causing offensive odours.
The systematic input of pure oxygen at critical points of the waste-water chain
has provided a lasting solution in many cases.
SOLVOX® processes (see Fig. 2.48) rely on various O2 transfer principles
that input oxygen fast, efficiently, precisely and flexibly. SOLVOX® supports the
performance of permanent aeration devices and serves as a preliminary treatment
process (e.g. for seasonal campaigns). A high oxygen concentration can be achieved
safely and maintained according to individual requirements.
The benefits of pure oxygen and SOLVOX® processes include:
x Low investment and maintenance costs
x Optimum use of O2 and maintenance of ideal O2 concentrations
x Systematic and flexible O2 input with low operational costs
x Prevention of extensive construction work and downtime
x Improved plant performance through implementation of rapid,
cost-effective measures
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Fig. 2.48 SOLVOX®-V process (Linde).
Fig. 2.49 SOLVOX®-V oxygenation unit (Linde).
The SOLVOX®-V process, for example, relies on a venturi system (see Fig. 2.49)
and is used for pre-treating and oxygenating waste water. It is very easy to install
and can even be lowered into full tanks with no drainage required.
Example J: Oxygen Delignification – Paper Manufacture
Oxygen delignification can be regarded partly as a continuation of the pulp cooking
process and partly as the first stage in bleaching (see Fig. 2.50). It has become a
standard step in the production of bleached chemical pulp during which lignin,
the coloured substance in kraft pulp, is removed. Using oxygen delignification not
only cuts the cost of chemical pulp production, but also reduces environmental
impact.
During oxygen delignification, pulp is treated with oxygen in a pressurised
vessel at high temperatures in an alkaline environment. Delignification may vary
from 40 to 70%, depending on the wood used as a raw material and whether one
or two reactors are used in series.
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Fig. 2.50 Oxygen delignification installation.
Unbleached kraft (sulphate) pulp has a lignin content of 3 to 5% which, after
oxygen delignification, can be decreased to approximately 1.5%, or a kappa value
of 8 to 10 (indicator of residual lignin).
The benefits of oxygen delignification include:
x Additional delignification after cooking with less destruction cellulose
x Reduced emissions from bleach plants and lower consumption of bleaching
chemicals
x Higher brightness ceiling in a given bleaching sequence
x Lower shives and extractives content
x Consistent pulp strength
x Easier system closure
Example K: Drinking Water – Oxidative Conditioning
High-quality drinking water is one of life’s essentials, which is why it is subject
to stringent statutory requirements. However, low-quality raw water increasingly
has to be used as a basis for recovering this high-quality product.
In Europe, for instance, strict EU directives determine the parameters and
limits for many possible substances in water. The following concentrations of
the heavy metal ions iron and manganese as well as unhygienic ammonium ions
must not be exceeded:
x Iron (II): 0.20 mg/l
x Manganese (II): 0.05 mg/l
x Ammonium: 0.50 mg/l
Raw water usually has to be conditioned to ensure compliance with these statutory
limits (see Fig. 2.51). Industrial gases provide a range of different treatments
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Fig. 2.51 Process diagram: Pure oxygen in drinking water treatment.
for this. Applying oxygen either allows precipitation and filtering of unwanted
substances or converts them into harmless products.
The benefits of oxygen (in comparison with air) include:
x Increased plant capacity and chemical reaction rate (grade of contaminant
elimination)
x Prevention of clogged filters by degassing nitrogen (extended uptime, lower
operating costs)
x Closed systems with one-step pressurising (hygiene and energy aspects)
Example L: Oxygen-Enhanced Combustion – Glass Manufacture
Glass manufacturers are constantly under pressure to enhance production,
extend furnace life, improve glass quality and meet increasingly strict emission
regulations. Oxygen has been used for many years to successfully overcome these
challenges.
All oxygen-enhanced combustion processes are based on the full or partial
replacement of air through oxygen. They increase flame temperature by eliminating nitrogen and increasing oxygen concentration. At the same time, they raise
concentrations of CO2 and H2O in the vicinity of the flame for upgraded thermal
radiation. Both effects boost heat transfer in the furnace and significantly improve
glass quality and furnace performance. Other effects are described below.
Oxygen enrichment: This is the most basic way of using oxygen in glass melting
applications. Enrichment is typically used in furnaces nearing the end of their
campaigns and suffering from regenerator plugging or impending collapse.
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Fig. 2.52 Flames from vertical crown burners (CGM™ system).
Oxygen lancing: This is also used for enrichment, but is typically associated with
more severe or complex conditions. Lancing involves the precise injection of
oxygen at the point where it is most needed. Enrichment is not as accurate and is
less efficient than selective lancing, which injects more total oxygen.
Oxy-fuel boosting: In contrast to enrichment and lancing, boosting involves the
addition of an oxidant and a fuel. Boosting technologies can be used to recover
glass furnaces and extend furnace life. They are also designed to increase furnace
throughput and/or improve quality. CGM™ boost technology (see Fig. 2.52) is an
example of a boosting solution.
Fig. 2.53 Energy consumption of air fuel and oxy fuel installations as
a function of the flue gas temperature and air preheating temperature.
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All oxy-fuel (AOF) fired melting: As one of the most popular oxygen applications,
AOF fired melting eliminates the need for a combustion air preheater or heat
recovery device. This process is one of the most efficient ways of reducing nitrogen
oxides (NOx) and particulate emissions from glass furnaces. It also reduces energy
consumption significantly. Figure 2.53 shows that increased air preheating boosts
fuel savings (conventional technology). However, fuel costs can be reduced further
by using advanced oxygen systems (AOF technology).
Figure 2.54 shows fuel on AOF installation with two types of burners.
Burner technology: Customers can choose from a large variety of burners (see
Fig. 2.55). These are based on the ‘tube-in-tube’ principle, whereby the fuel jet is
conically shielded by an oxygen beam. The flame can be adapted to furnace and
melting requirements by changing the velocity of each partner. Where necessary,
oxygen can be injected in stages to increase flame length (‘flame staging’).
The benefits of oxygen/AOF systems in glass manufacture include:
x Reduced emissions (e.g. NOx and particles)
x Improved heat transfer (with enhanced glass quality)
x Lower capital costs (no air preheating necessary, smaller filter unit, smaller
furnace)
x Increased productivity (faster melting, higher throughput, extended furnace
life)
x More stable process (better control over operations, smaller process variations)
x Extended useful life (e.g. replacement of combustion air at the end of a process
cycle)
x Energy savings (see graph above)
Fig. 2.54 Typical All Oxy Fuel installation including combustion equipment.
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Fig. 2.55 Typical oxy-fuel burners used for Oxy Fuel Boosting and
All Oxy Fuel installations (Linde).
Application M: Fire-Polishing Glass
Tableware and flacon manufacturing is an important part of the glass industry.
In addition to cost reduction, modern glass production must also:
x
x
x
x
Deliver optimum glass quality (particularly surface quality)
Make machined glass resemble hand-made glass
Eliminate acid-polishing
Eliminate further mechanical processing
Advanced fire-polishing and fusing can fulfil these requirements. Flames from
specially designed burners impinge the glass directly to remelt a thin surface
layer. This process can be used to eliminate or reduce the need for etching, thus
decreasing the environmental impact and paving the way for cleaner, safer working
environments.
Traditional post-mixing burners have at least two inlets, one for a fuel gas and one
for oxygen and/or air. The fuel and oxidant are fed separately through the burner
and only mixed once they have passed their respective exit nozzles. Combustion
takes place in a second step.
Modern premixing burners function differently. The fuel gas and oxygen/air
are mixed in an upstream section. This gas mixture is then delivered directly to
the burner, which has just one inlet. Combustion takes place immediately once
the mixture has passed the joint nozzle system.
Figure 2.56 shows the difference in performance between the two systems. The
pre-mixing option provides a number of benefits regardless of the gas used.
Furthermore, the heat-transfer rate of traditional air-fuel burners is approximately 50% of the rate associated with comparable oxy-fuel burners.
Figure 2.57 shows different pre-mixing burners for oxy-fuel application.
Figures 2.58 and 2.59 show typical fire-polishing installations.
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Fig. 2.56 Axial distribution of heat transfer for oxy-hydrogen and oxy-methane flames.
Fig. 2.57 Typical designs for pre-mixing HYDROPOX® burners.
The benefits of pre-mixing oxy-fuel burners for fire-polishing glassware include:
x Removal of all pressing burrs and sharp edges
x Significantly higher brilliance compared with acid polishing
x Removal of cold waves
x No deformation of glassware due to short polishing times and precise heating
points
x Ability to polish decorative surfaces with deep relieves
x Ability to polish thick-walled glass parts internally and externally
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Fig. 2.58 Fire polishing with HYDROPOX® following the forming process.
Fig. 2.59 Total removal of burrs and other consumer-relevant flaws with HYDROPOX®.
2.5.3
Applications of Argon
Although the noble gas argon is mainly used for its low reactivity (as is the case
with nitrogen), it can provide higher inertness than nitrogen as well as lower
thermal conductivity and improved solubility in water and oils.
Argon is more common and cost-effective than the noble gases helium, neon,
krypton and xenon.
Argon is usually applied in its gaseous state.
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In the processing industry, welding and engineering, argon is used to:
x perform tungsten inert gas (TIG) welding with a non-consumable tungsten
electrode and an additional filler metal (e.g. with the shielding gases argon (Ar),
argon/helium (Ar/He), argon/hydrogen (Ar/H2), see Example A below)
x perform metal inert gas (MIG) welding of non-ferrous metals with a consumable
electrode (e.g. aluminium with inert gases such as Ar, Ar/He, see Example B
below)
x perform metal active gas (MAG) welding of construction steel or galvanized
sheet metal with a consumable electrode and active gases such as argon/carbon
dioxide (Ar/CO2) or argon/oxygen (Ar/O2) (see Example B below)
x perform MAG welding of stainless steel with admixtures of CO2, O2 and He (to
improve welding speed and weld appearance, see Example B below)
x perform MAG high-performance welding with shielding gas mixtures based
on Ar/He with admixtures of CO2 or O2 (see Example B below)
x perform laser cutting with pure argon (e.g. to cut metals such as titanium that
would otherwise react with N2/O2)
x purge receptacles of stainless steel or reactive metals (e.g. titanium, tantalum,
zirconium) before welding (mostly in mixture with hydrogen, compare root
shielding and use of nitrogen)
In metallurgy, foundry technology and the steel industry, argon is used to:
x perform large-scale metallurgical processes with additional oxygen injection
(e.g. for controlled decarburisation of cast iron and stainless steel melts with
the argon-oxygen-decarburisation (AOD) process)
x purge and stir metal melts by bubbling them through porous bed stones or
lances (recirculation, discharge of gases and slag)
x feed powdered alloy components into steel and other metal melts (injection of
calcium or magnesium compounds with an argon jet for desulfurisation)
x atomise molten metals with high-pressure jets for producing high-quality metal
powders (powder metallurgy)
x sinter metallurgical powders under high pressures and temperatures (e.g. hot
isostatic pressing (HIP))
x heat or anneal stainless steel (e.g. in furnaces, to avoid nitrogen uptake)
x prevent oxidation of hot, light metal alloys (e.g. when die casting aluminium
or magnesium)
In the lighting and electronic industry, argon is used:
x to fill incandescent light bulbs (partly mixed with nitrogen to protect the tungsten
filament and prolong bulb life)
x to fill fluorescent lamps and luminous electric discharge tubes (with a small
amount of neon to create the characteristic blue light)
x to protect electric equipment during storage, operation and shutdowns (e.g.
rectifiers and overload protection switches)
x as a base gas in detectors that measure radioactivity (e.g. Geiger counters)
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In the semiconductor industry, high-purity argon is used to:
x grow monocrystalline silicon (e.g. for manufacturing wafers or photovoltaic
cells)
x manufacture semiconductor components (e.g. by supporting processes such as
diffusion, epitaxy, chemical vapour deposition (CVD) or ion implantation)
In other industries, argon is used to:
x insulate energy-efficient windows (e.g. double glazing, see low thermal conductivity)
x protect foodstuffs and beverages during treatment and storage (e.g. modified
atmospheres)
Example A: Tungsten Inert Gas (TIG) Welding
In addition to aluminium and magnesium, TIG welding is used to weld stainless
steel as well as carbon- and low-alloy steel. It is mainly used to weld thin metals
(under 6 mm in thickness).
In TIG welding, an electric arc is used to heat and melt the material. The electric
arc burns between the burner electrode and the work piece (see Fig. 2.60). The
shielding gas flows through a gas nozzle positioned concentrically around the
electrode.
The shielding gas is primarily used in TIG welding to protect the hot and
molten parts of the work piece, the filler metal and the electrode from the harmful
influence of the surrounding air. Shielding gas also affects the characteristics of
the arc and the appearance of the weld. Argon and helium (or mixtures of these
gases) are typical shielding gases used here. Hydrogen or nitrogen may also be
beneficial under certain conditions.
TIG welding is typically used for welding pipes, pressure vessels and heat
exchangers.
Since it can be used to weld thin metals and small objects, the process is also
used in the electronics industry. TIG welding ensures very high weld quality, with-
Fig. 2.60 TIG welding.
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107
out slag and with very little spatter. It is an extremely versatile method, suitable for
most weldable materials and any welding position and joint configuration.
Example B: Metal Inert Gas (MIG)/Metal Active Gas (MAG) Welding
MIG and MAG are the most common welding methods. The high levels of
productivity offered by these methods and the fact that they are simple to automate
have contributed to their popularity. The methods together are often referred to
as gas metal arc welding (GMAW).
During MIG/MAG welding, a metallic wire is fed through the welding gun and
melted in an electric arc (see Fig. 2.62). The wire acts as both the current-carrying
electrode and the weld metal filler wire. Electrical energy is supplied from a welding
power source. A shielding gas that flows through the gas nozzle protects the arc
and the pool of molten material. The shielding gas is either inert (MIG) or active
(MAG). An inert gas such as argon or helium does not react with the molten material. Active gases, on the other hand, participate in the process between the arc and
the molten material and can stabilise and broaden the arc. An example of an active
gas here is argon containing a small proportion of carbon dioxide or oxygen.
Ionisation energy is an important property of gases (see Fig. 2.61). It primarily
determines the temperature of the electric arc and, therefore, the attainable heat
transfer and welding speed. Helium has considerably higher ionisation energy
than argon, which is why helium is often added to argon to improve the process
performance. Helium also affects the weld pool and improves the penetration
profile (bead profile) of the weld.
Fig. 2.61 Dissociation (dark grey bar) and ionisation (light grey bar) energy of gases.
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2 The Air Gases Nitrogen, Oxygen and Argon
Fig. 2.62 MIG/MAG welding.
References
[2.1]
[2.2]
[2.3]
[2.4]
[2.5]
[2.6]
[2.7]
[2.8]
[2.9]
[2.10]
[2.11]
[2.12]
[2.13]
[2.14]
[2.15]
[2.16]
[2.17]
[2.18]
[2.19]
[2.20]
[2.21]
[2.22]
[2.23]
[2.24]
[2.25]
[2.26]
[2.27]
1345vch02.indd 108
Ullmann’s, 6th edition, 23, p. 175, Wiley-VCH, Weinheim, 2003.
Römpp, 10th edition, Keyword: Stickstoff, Thieme Verlag, Stuttgart, 1996.
Ullmann’s, 6th edition, 23, p. 178, Wiley-VCH, Weinheim, 2003.
Römpp, 10th edition, Keyword: Stickstoff-Fixierung, Thieme Verlag, Stuttgart, 1996.
Römpp, 10th edition, Keyword: Sauerstoff, Thieme Verlag, Stuttgart, 1996.
Ullmann’s, 6th edition, 24, p. 562, Wiley-VCH, Weinheim, 2003.
Hollemann-Wiberg: Lehrbuch der Anorganischen Chemie, 101st edition, W. de Gruyter,
Berlin, 1995, pp. 508–510.
Römpp, 10th edition, Keyword: Argon, Thieme Verlag, Stuttgart, 1996.
Ullmann’s, 6th edition, 23, p. 227, Wiley-VCH, Weinheim, 2003.
H. Hausen: Zeitschr. tech. Phys. 1932, 13 (6), 271–277.
D. R. Paul, Y. P. Yampolskii (Eds.): Monography on polymeric gas separation membranes:
Polymeric Gas Separation Membranes, CRC Press, Boca Raton, Fla., 1994.
Ullmann’s, 6th edition, 21, p. 243, Wiley-VCH, Weinheim, 2003.
E. Staude: Membranen und Membranprozess, VCH, Weinheim, 1992.
State of the art 1990 are membrane fibres with properties as listed: P. S. Puri, Gas. Sep.
Purif. 1990, 4, 29.
J. Membrane Sci. 2001, 193, 1–18.
Sep. Purif. Technol. 2002, 28, 29–41.
M. Grahl, P. Leitgeb: Oxygen Production by Pressure Swing Adsorption, MUST‚ 96,
Munich Meeting on Air Separation Technology, p. 135.
Nitrogen Production Based on Pressure Swing Adsorption, 96 Munich Meeting on Air
Separation Technology, p. 185.
G. Beysel, P. Leitgeb, G. Scholz: LINDE Reports on Science and Technology 1988, 44, 3–7.
J. G. Stichlmair, J. R. Fair: Distillation, Wiley-VCH, New York, 1998.
Kryogene Argongewinnung, EP 377117 B2, US 5019145, Inventors: H. Corduan,
W. Rohde, LINDE AG.
E. Blass: Entwicklung verfahrenstechnischer Prozesse, Springer-Verlag, Berlin, 1997.
K. Stephan, F. Mayinger: Thermodynamik, Vol. 1, 14th edition, Springer-Verlag, Berlin,
1992.
Ullmann’s, 6th edition, 23, p. 175, Wiley-VCH, Weinheim, 2003.
H. Hausen, H. Linde: Tieftemperaturtechnik, 2nd edition, Springer-Verlag, Berlin, 1985.
W. Kast: Adsorption aus der Gasphase, VCH, Weinheim, 1988.
S. Weiß (Hrsg.): Verfahrenstechnische Berechnungsmethoden, Deutscher Verlag für
Grundstoffindustrie, Stuttgart, 1996.
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[2.28] W. Diery, LINDE Reports on Science and Technology 1988, 44, 72–82.
[2.29] The standards of the brazed aluminium plate-fin heat exchanger manufacturers’
association. www.alpema.org.
[2.30] K. Sattler: Thermische Trennverfahren, 2nd edition, VCH, Weinheim, 1995.
[2.31] Ullmann’s, 6th edition, Wiley-VCH, Weinheim, 2003.
[2.32] D. Igkesia, J. Spivey, H. Fleisch (Eds.): Studies in Surface Science and Catalysis 136:
Natural Gas Conversion VI, Proceedings of the 6th Natural Gas Conversion Symposium,
Elsevier, Amsterdam, 2001, pp. 45–56.
[2.33] Borsig Taschenbuch, Deutsche Babcock- Borsig Aktiengesellschaft, Berlin, 1994.
[2.34] R. Smith: Chemical Process Design, McGraw-Hill. Inc. New York, 1995.
[2.35] Compressed Gas Association, CGA G – 4.8.2000: Safe use of aluminium-structured
packing for oxygen distillation, 2nd edition, Chap. 7.2, 2000, CGA, Arlington, VA, USA.
[2.36] B. R. Dunbobbin, J. G. Hansel, B. L. Werley: Oxygen Compatability of High Surface
Area Materials, in Flammability and Sensitivity of Materials in Oxygen-Enriched
Atmospheres: Vol. 5, ASTM STP 1111, 1991, pp. 338–351.
[2.37] R. Zawierucha, J. F. Million, S. L. Cooper, K. McIlroy, J. R. Martin: Compatibility
of Aluminium Packing with Oxygen Environments Under Simulated Operating
Conditions, in Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres:
Vol. 6, ASTM STP 1197, 1993, pp. 255–275.
[2.38] R. Zawierucha, J. F. Million: Promoted Ignition-Combustion Tests of Brazed Aluminum
Heat Exchanger Samples in Gaseous and Liquid Oxygen Environments, in Flammability
and Sensitivity of Materials in Oxygen-Enriched Atmospheres: Vol. 9, ASTM STP 1395,
2000, pp. 373–383.
[2.39] Oxygen Pipeline Systems, EIGA, IGC Doc 13/02/E.
[2.40] K. Nabert, G. Schön: Sicherheitstechnische Kennzahlen brennbarer Gase und Dämpfe,
2nd edition 1963, 6th suppl. 1990, Deutscher Eichverlag, Braunschweig.
[2.41] E. J. Miller, S. R. Auvil, N. F. Giles, G. M. Wilson: Air Products – Presentation for the
12th Intersociety Meeting, March 5–9, 2000, Atlanta, GA.
[2.42] L. Vegard: Naturwissenschaften 1931, 19, 443.
[2.43] Linde in-house report, 2000.
1345vch02.indd 109
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111
3
The Noble Gases Neon, Krypton and Xenon
3.1
History and Occurrence
In 1897/1898 W. Ramsay found neon, krypton and xenon (Ne, Kr, Xe) by liquefying
and fractionating air according to the C. v. Linde method. Neon discovered due to
its red spectral lines derives its name from the Greek word “neos” = new, krypton
from the Greek word “kryptos” = hidden and xenon from “xenos” = guest, stranger
[3.1]. The first neon tubes filled with a helium/neon mixture were manufactured
by the “Chemische Fabrik Griesheim-Elektron” in 1913. Up to the middle of the
last century the demand for pure neon was quite low, demonstrated by a quantity
of 10 L liquid neon shipped in 1960 for the first time.
Worldwide, the same concentrations are found in the air: neon is contained
with 18 vppm, krypton with 1.14 vppm and xenon with 0.086 vppm (see Table
2.1). Except helium, which occurs enriched in some natural gas sources, all
other noble gases are extracted from air. In view of the low concentrations of
neon, krypton and xenon large quantities of air have to be processed in order to
produce these noble gases on an industrial scale. Radon (Rn) as the heaviest and
radioactive noble gas develops from radioactive decay processes and occurs in
extremely small traces in the air (6 · 10–14 vppm). In medicine, radon serves as
D-source in cancer therapy.
3.2
Physical and Chemical Properties
Neon, krypton and xenon together with He, Ar and Rn belong to the 8th main
group of the periodic table of elements (noble gases) and at atmospheric pressure
they are colourless, odourless, non-combustible, monoatomic gases. The outer
electron shell is completely filled (noble gas configuration), responsible for the
inert character of noble gases. Due to their inertness noble gases are used as
filling gases for light bulbs. The larger the atom, the more easily polarizable is
the electron shell, resulting in stronger interatomic forces (van-der-Waals-forces).
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
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3 The Noble Gases Neon, Krypton and Xenon
Neon boils at –246.1 °C, krypton at –153.4 °C and the heavy xenon at –108.2 °C at
1 bar [3.2]. Solid xenon at –241 °C and 330 000 bar conducts the electric current
like a metal. 1 L of water dissolve 10.5 mL of neon at 20 °C.
For a long time it was believed that noble gases are not able to form compounds.
In 1962, the American N. Bartlett was the first to synthesize a compound of the
composition “XePtF6” by a reaction of xenon with PtF6. In the same year, Hoppe
succeeded in synthesizing the first binary noble gas compound XeF2. Noble
gas compounds can be subdivided into three types: (1) short-living noble gas
containing molecules, (2) molecules with covalent bonds, (3) inclusion compounds
and clathrates (e.g. noble gas hydrates). The thermodynamic stability of the noble
gas halogenides complies with the following rule: The larger the atomic mass
of the noble gas and the smaller the halogen, the more stable is the compound:
XeF2 > KrF2 and XeF2 > XeCl2 > XeBr2. Additionally the stability of noble gas
compounds increases with the decreasing oxidation stage of the noble gas atom.
Apart from covalent xenon fluorides and oxides, krypton fluorides are known, too.
Except for the xenon fluorides, all other noble gas compounds are endothermal,
thus decompose into their elements under release of energy. Chemical compounds
of neon as for the next heavier noble gas argon are not known [3.3].
3.3
Recovery of Krypton and Xenon
Krypton and xenon, which have a concentration of 1.138 ppm and 0.086 ppm in the
atmospheric air, are mainly recovered as secondary products from air separation.
The worldwide demand increases annually. In 2001, the global annual Xe-production amounted to about 6800 mN3, the Kr-production to about 67 000 mN3. Owing to
its low concentration in the air the xenon price is high with typical 4000–8000 €
per mN3 and is strongly fluctuating. The price for krypton, which occurs about 13
times more frequently in the air than xenon, is lower by about this factor.
Typically the noble gases Kr and Xe are recovered in two steps:
In the first step, a pre-enrichment of about 4000 ppm of Kr and 400 ppm of Xe
in liquid oxygen is achieved in the sump of an additional column (4), (Fig. 3.1).
This column is integrated into the air separation unit.
In a second step, the products are extracted from this pre-concentrate in a unit,
operating independently from the air separator. The production rate of this preenriched liquid is low.
X Example: Separation of 300 000 mN3 h–1 of process air. The Kr-yield shall be 85%
and the Kr-purity in the pre-enriched liquid oxygen shall be 4000 ppm. Then the
amount of withdrawn pre-concentrate is: 300 000 mN3 h–1 · 0.85 · (1.138/4000)
= 73 mN3 h–1.
Due to this low production rate, the pre-product is often stored in a tank, which
is periodically transported to a central fine-purification plant. In order to keep
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3.3 Recovery of Krypton and Xenon
113
transportation cost low, efforts are made to obtain high Kr and Xe concentration
in the pre-product.
3.3.1
Pre-enrichment in the Air Separator
Figure 3.1 shows a typical process for Kr/Xe-pre-enrichment:
Since the boiling temperatures of Kr and Xe are higher than that of oxygen
(Table 3.1), these two components accumulate at the point of the highest oxygen
concentration, i.e. at the bottom of the low-pressure column (2). From here liquid
oxygen, containing most of the Kr/Xe from the processed air, is fed (a) to an
enrichment column (4) and is evaporated in the sump of this column by means of
a suitable heating medium (b), for example by means of gaseous air cooled down
close to its dew temperature. Gaseous product oxygen (c) is withdrawn from the
Table 3.1 Boiling temperatures (K) of components relevant for the
Kr/Xe-extraction at a pressure of 1.013 bar.
N2
Ar
O2
77.3
87.3
90.2
CH4
Kr
CF4
Xe
C2H4
N2O
C2H6
C2F6
SF6
C3H8
111.6 119.8 145.2 165.1 169.4 184.6 184.5 194.2 209.9 231.1
Fig. 3.1 Diagram of a two-column air separator with noble gas production.
(1) Rectification column (pressure section); (2) Rectification column
(low-pressure section); (3, 5, 7) Combined condenser – evaporator unit;
(4) Kr/Xe enrichment column; (6) He/Ne enrichment column.
(a, b, e, f, g, h, j) Internal process flows; (c) Product O2 gaseous;
(d) Kr–Xe Primary Product liquid; (i) He–Ne Primary product gaseous.
GOX = gaseous oxygen; LIN = liquid nitrogen.
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114
3 The Noble Gases Neon, Krypton and Xenon
top and liquid oxygen (d), enriched with Kr/Xe, is withdrawn from the sump of
this column. A second liquid oxygen fraction (e) is fed to the top of the enrichment
column in order to reduce the Kr – content of the oxygen product (c) and thereby
the krypton loss. This second fraction is extracted a few theoretical trays above
the sump of the low-pressure column, where it has only a low concentration of
Kr/Xe and CH4.
The pre-enriched product (d) typically contains 85% of the krypton, which is
fed to the air separation unit via the process air and about 95% of the xenon.
The krypton yield is lower than the xenon yield, because Kr is more volatile than
Xe (see Table 3.2).
The Kr/Xe concentration in the sump of the enrichment column is controlled
by the amount of withdrawn sump product (d). The smaller the flow, the higher
will be the concentration. Thus the Kr/Xe-concentration could be theoretically
increased up to the solubility limit of krypton (30%) and xenon (2%). The maximal
admissible concentration is however determined by the solubility and the
explosion limit of the hydrocarbon components, which accumulate in the sump
product as well. The most prominent component is CH4, which is contained
in ambient air with up to 5 ppm. Owing to its low volatility (Table 3.2), CH4
accumulates together with Kr/Xe in the liquid bath. The methane concentration
in the Kr/Xe pre-product must remain well below its lower explosion limit of
about 4.4%.
In order to reduce the CH4 content in the Kr/Xe-pre-product, the enrichment column (4) is divided into an upper and lower section. This allows the
discharge of a part of the methane, which enters the enrichment column via
stream (a), over the top of the column together with the oxygen product. This
is achieved by suitable adjustment of the liquid and vapour flow L und V in the
two sections.
A component is driven upwards in the column, whenever the vapour flow V is
so large, that for the separation factor S the inequality
S=K⋅
V
>1
L
(3.1)
(see Section 2.2.5.5) holds. Here K is the component equilibrium factor K-value
(see Table 3.2). The reflux (e) to the head of the enrichment column, which is poor
of CH4 and Kr/Xe, is adjusted such, that methane is driven upwards and krypton
Table 3.2 Component equilibrium factor of Kr, Xe and CH4 in liquid oxygen at 1.4 bar.
CH4
Kr
Xe
0.29
0.11
0.0028
The component equilibrium factor K = y*/x of a component describes the mass transfer
equilibrium between the liquid phase with the concentration x and the vapor phase with
the concentration y* (see Table 2.8, Section 2.2.5.5).
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3.3 Recovery of Krypton and Xenon
115
downwards. According to Equation (3.1), this is guaranteed if the liquid/vapourL
ratio (L/V)top in the upper section lies in the interval K Kr <   < K CH4 , i.e.
 V  top
ranging within the limits of 0.11 to 0.29 according to Table 3.2.
3.3.2
Recovery of Pure Kr and Xe
Kr/Xe-fine-purification plants are available in numerous designs, combining the
separation techniques cryogenic rectification, catalysis, adsorption, chemisorption
or membrane separation. They may be operated continuously or, due to the small
capacities, in batch mode.
In the following, a continuous process will be described, the process steps of
which are realized this way or similarly in a number of plants.
The feed to the purification unit is liquid oxygen enriched with about 400 ppm
of Xe and 4000 ppm of Kr. It contains numerous additional impurities (cf.
Table 2.1), e.g.
x hydrocarbons < 5000 ppm (concentration in units of the so-called CH4-equivalent)
x Nitrous oxide (laughing gas), N2O < 200 ppm
x Greenhouse gases CF4 (tetrafluoromethane), C2F6 (hexafluoroethane) and SF6
(sulphur hexafluoride) < 500 ppb
High concentrations of CF4, C2F6 and SF6 in krypton and xenon are not tolerable
for applications in the lighting industry and reduce the Kr/Xe-price. Table 3.4
summarizes typical admissible contaminations. The concentration of these
greenhouse gases in the air is low and typically ranges between 0.1–100 ppt
(1 ppt = 1 part/1012 parts). However, since the boiling temperatures of the above
mentioned three components are similarly high as those of krypton and xenon
(Table 3.1), they accumulate together with krypton and xenon. The potential
accumulation of Kr and Xe with these greenhouse gases can be estimated from
the ratio between the concentration of Kr/Xe in ambient air and the concentration
of these greenhouse gases in ambient air.
X Example: 100 ppt of CF4 and 1.138 ppm of krypton in the ambient air. With
a cryogenic separation of krypton and xenon, the CF4 preferably accumulates
in krypton owing to its volatility. Thus, the CF4-contamination of the krypton
product amounts to 100 ppt/1.138 ppm = 88 · 10–6 = 88 ppm. According to the
specification in Table 3.4 this concentration is too high.
3.3.2.1
Catalytic Combustion of Hydrocarbons
Since in the Kr/Xe enrichment steps, the hydrocarbons would enrich as well and
would form an explosive mixture, they have to be removed first. This is done by
combustion to water and CO2 in an exothermal reaction with oxygen on a Pd- or
Pt-catalyst at a temperature of about 400–500 °C. To this end the cryogenic liquid
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116
3 The Noble Gases Neon, Krypton and Xenon
crude product (d), Fig. 3.1, is pressurized to a supercritical state at 55 bar by means
of a pump, heated to ambient temperature and then expanded to about 6 bar.
The compression to the supercritical state prior to heating is required for safety
reasons. If the evaporation would be performed at an undercritical pressure, an
inadmissible hydrocarbon accumulation would occur in the evaporating liquid
phase.
The residual concentration of hydrocarbons after the combustion is small and
typically < 1 ppm. However in subsequent steps, hydrocarbons will be enriched
again. Thus, depending on the purities required, hydrocarbons may have to be
further separated in the downstream rectification.
N2O is also converted on the catalyst into oxygen and nitrogen, whereas
the thermally stable greenhouse gases CF4, C2F6 and SF6 are only partially
decomposed.
The hot gas from the catalyst’s outlet is being cooled and its water and CO2
content, resulting from the combustion, is removed in an adsorber.
An alternative to the combustion of hydrocarbons in the oxygen-rich crude
product is to replace oxygen by nitrogen in this product. Hydrocarbons, embedded
into nitrogen can be concentrated higher without exceeding safety limits. The
exchange of oxygen against nitrogen takes place in a cryogenic stripping column.
This process is especially advantageous in connection with an ensuing, purely
cryogenic rectification, as the heating necessary for the catalysis and the following
cooling down to cryogenic temperatures is not required. Such a plant was built
by the Linde AG in Duisburg in 2001 [3.4].
3.3.2.2
Cryogenic Separation
In the following a process will be described that purifies krypton and xenon by
means of cryogenic rectification in five columns.
The feed gas is cooled before it is fed to the first column. It contains mostly
oxygen with typical 4000 ppm of Kr and 400 ppm of Xe and additional impurities
of hydrocarbons and greenhouse gases with concentrations below 1 ppm.
Figure 3.2 shows schematically the sequence of the five columns for the
isolation of Kr/Xe. In this figure, the multi-component mixture to be separated is
characterized in a simplified way as a six-component mixture (A, Kr, B, C, Xe, D).
The pseudo component A combines all components that are more volatile than
Kr, i.e. mainly O2 und CH4. B and C represent the components with volatilities
in between those of the key components Kr and Xe and D summarizes the
components less volatile than Xe.
In Column “1” oxygen and CH4 is separated to the top of the column, while
Kr/Xe together with all other low volatile components collect in the bottom
fraction. A strong enrichment of all low volatile components occurs in this bottom
fraction. With the flows indicated in Fig. 3.2, the enrichment factor amounts to
3000/14 = 124. Column “2” splits krypton from xenon. Column “3” separates the
less volatile components from krypton and isolates the latter as a pure product at
the column’s top. Column “4” segregates xenon from its less volatile companions
and column “5” isolates xenon as pure sump product.
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3.3 Recovery of Krypton and Xenon
117
Fig. 3.2 Diagram of a rectifying Kr/Xe fine scrubbing.
(1–5) Diagram of columns, the figures on the arrows are flows in arbitrary units.
The flow to the Xe-fine-purification column “5” is by about a factor 10–7
smaller than the flows involved with the columns of the air separation unit. The
rectification on such a small scale requires special solutions:
x Electrical heaters in the sump of the columns guarantee sufficient vapour and
liquid load to maintain the rectification.
x The pressure in the columns is controlled by condensing the vapour at the
column heads with liquid or cold gaseous nitrogen.
x Measurement and controlling of the small flows is largely avoided. The gas flow
exhausted from the top of the columns is controlled via the temperature profile
developing in the column.
x Pure xenon solidifies at about 161 K and tends to freeze in the cold parts of the
plant. This is avoided by operating the columns at elevated pressure and thereby
at elevated temperature, by wrapping the column shells with electrical heating
bands and by careful pipe routing.
The Kr and Xe product obtained in the cryogenic plant is warmed up and stored
and supplied to the consumers in cylinders at pressures of about 60 bar for Xe
and about 140 bar for Kr. The pressure of a Xe-cylinder must be lower, because Xe
is near its critical state (Tcrit = 289.8 K, Pcrit = 58.4 bar). Thus when a Xe-cylinder
is heated, its pressure will increase more than the pressure in a Kr-cylinder
(Tcrit = 209.4 K, Pcrit = 55.0 bar). A preceding cleaning of the cylinders by evacuation
and purging is necessary to obtain the product purities as specified in Table 3.4.
1345vch03.indd 117
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118
3 The Noble Gases Neon, Krypton and Xenon
Numerous modifications of the process shown in Fig. 3.2 exist:
x A second catalytic combustion of hydrocarbons downstream of the krypton
and xenon enrichment
x Chemisorption of fluorocarbons, SF6 and other components
x Adsorption
3.4
Recovery of Neon
The industry’s demand for neon is exclusively covered from air separation
plants, where neon is a by-product. The neon content of the atmospheric air is
18 ppm (see Table 2.1). In contrast the recovery of helium from atmospheric
air is of minor importance. The most important helium source is natural gas
(cf. Chapters 4 and 7).
The global annual production of neon amounts to about 250 000 mN3 and its
market price is subject to strong fluctuations, ranging typically between 30 and
200 € mN–3.
Neon production is performed in two steps. In the first step, a neon enriched
gas with about 50% of Ne, 3% of H2, 16% of He and 31% of N2 (see Fig. 3.1) is
obtained in a small column (6) integrated into the air separator. The neon enriched
gas is often stored in pressure cylinders and transported to a central fine-purification plant. There, in the second step, neon is recovered with the required purity.
3.4.1
Pre-enrichment
Since the boiling temperatures of neon, helium and hydrogen are significantly
lower than those of nitrogen (see Table 3.3), they accumulate at the head condenser
(3) of the pressure column (see Fig. 3.1) in a non-condensing gas phase, which is
extracted. The amount of this extracted inert gas (f) is typically 1/1000 of the processed air. Thus the Ne-concentration increases by a factor of 1000 from 18 ppm in
the ambient air to ~ 1.8% in the inert gas. This inert gas is fed into the Ne-concentration column (6). Neon further enriches in its head condenser (7), which is cooled
with about 80 K cold liquid nitrogen (g) and (h). The non-condensing proportion
(i) with a Ne content of about 50% is withdrawn from the head condenser and is
discharged as pre-enriched product after being warmed up in an exchanger.
Table 3.3 Boiling temperatures (K) of the components involved in the Ne-extraction
(at a pressure of 1.013 bar).
1345vch03.indd 118
He
H2
Ne
N2
4.3
20.4
27.1
77.3
26.10.2007 10:27:13
3.4 Recovery of Neon
119
In order to avoid Ne-losses, the liquid nitrogen (j), which is fed to the lowpressure column as reflux, is withdrawn a few trays below the top of the pressure
column. For such processes the Ne-yield is significantly over 90%, provided that
none of the process air bypasses the pressure column in some way.
3.4.2
Fine Purification
Even with large air separation plants the amount of neon pre-enriched product is
small, so that the final purification is often operated in a batch mode.
X Example: Separation unit processing 200 000 mN3 h–1 of air. Pre-enriched product
with 50% (= 500 000 ppm) of neon. Then the production rate of this pre-product
is = (18 ppm/500 000 ppm) · 200 000 = 7 mN3 h–1.
Various processes exist, combining adsorption, catalysis, partial condensation
and rectification for the isolation of neon. For example, the process applied by
the Linde AG either in batch or continuous mode combines these unit operations
in the following way [3.5]:
x Catalytic oxidation of the hydrogen in the pre-product. For this oxidation oxygen
is added. The arising water is removed by ensuing adsorption.
x Pre-separation of nitrogen by condensation at 66 K. The non condensed part
has a residual N2 content < 2%.
x After adsorption of the remaining nitrogen on a silica gel adsorber a mixture
of 76% of Ne and 24% of He remains.
x Compression of this mixture to 180 bar and cooling down to 50 K.
x Throttle expansion down to 25 bar into a separator. The condensate in the
separator has a neon content of about 97%.
x The condensate is fed to the top of a rectification column, operated at 1.3 bar
and about 28 K. From its sump pure neon is withdrawn, warmed up and filled
into gas cylinders at a pressure of about 150 bar.
The cryogenic part of the apparatus, designed for the production of a few standard
cubic meters per hour, is housed in two containers with a volume of about
1 m3. Due to the vacuum- (~ 1 Pa) and the super-insulation, the heat flow into
the equipment is small and the low temperatures can be obtained via the Joule
Thomson effect.
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3 The Noble Gases Neon, Krypton and Xenon
3.5
Industrial Product Purities and Analytics
Table 3.4 shows typical demands of industrial consumers on the product purity
of inert gases.
The control of the Kr/Xe-purification process and of the product require a
complex analyzing equipment. The following compounds represent impurities
and should be detectible in the range of trace amounts: Kr in Xe, Xe in Kr, CO2,
N2, O2, N2O, C1–C4-hydrocarbons, SF6, CF4, C2F6. In case of Ne-production also
components like He, N2 and H2 must be analyzed.
The gas chromatographs used for process and product control are bearing FID(Flame Ionisation Detector), TCD- (Thermal Conductivity) and PDD-detectors
(Pulse Discharge Detectors = ionization of the compounds to detect by means of
a high-frequency discharge of helium). Owing to the demand on the detection of
many impurities and thus on great variability, these analytical appliances resemble
more conventional laboratory gas chromatographs. They require more analytical
expert knowledge and maintenance than automatically running process analyzers.
Owing to the high enrichment in the production of xenon – xenon is contained in
the air with only 0.086 mol ppm – nearly all imaginable air impurities are found
Table 3.4 Typical Kr, Xe and Ne purities.
Components
Kr
Xe
Kr
99.995–99.9998%
0.1–35 ppm
Xe
0.1–35 ppm
99.995–99.9995%
Ne
99.996–99.999%
He
8–35 ppm
N2
0.1–5 ppm
1–5 ppm
1–4 ppm
O2
0.1–1 ppm
< 1 ppm
0.5–1 ppm
CO2
0.1–1 ppm
0.1–1 ppm
0.5–1 ppm
CO
0.5–1 ppm
CH4
0.1–1 ppm
0.1–1 ppm
CF4
0.1–0.5 ppm for lamp
industry
< 1 ppm for lighting industry
C2F6
1345vch03.indd 120
Ne
0.5–1 ppm
< 1 ppm for lighting industry
SF6
< 1 ppm
< 1 ppm for lighting industry
no requirements for
insulating glass
H2O
1 ppm
1 ppm
0.5–1 ppm
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3.6 Applications
121
in the process, provided they are not eliminated by upstream purging stages like
spray coolers, adsorbers and catalytic reactors.
The analytics of a Kr/Xe purification process starts with the control of the CH4and N2O-abatement after the catalytic reactor. This sampling point is crucial for
the plant safety. Downstream in the first column where the light components N2
and O2 are withdrawn overhead, the O2-content in the sump and the Kr-content
in the top of the column are periodically checked. In the following columns all
of the above mentioned impurities must be analyzed applying in some different
separation cases so called heart-cut methods on GC. Analyzing the tiny pure xenon
column causes sometimes problems: An unreduced sample flow may exceed the
continuous xenon production and thus interferes with the rectification process.
Commonly the pure krypton and xenon is filled in 50 L cylinders. An analytical
check of the cylinders requires a good mixing of the content in advance by rolling
the cylinders over several hours.
3.6
Applications of the Noble Gases Neon, Krypton and Xenon
Compared to the relatively widespread noble gases helium and argon, the
noble gases neon, krypton and xenon are less common and harder to obtain.
Nevertheless, they are used in a wide range of modern technologies, in particular
lighting, optics and electronics, on account of their special properties. They are
preferably used in a gaseous state and in mixtures.
3.6.1
Applications of Neon
Neon is used:
x with helium in helium-neon lasers (e.g. laser pointers)
x as a refrigerant for special cooling devices
x as a filling gas for gas discharge lamps, low-consumption glow lamps
(night lights) and stroboscopic lamps
x as a filling gas for plasma display panels (PDP) (see Example A below)
x as a filling gas for over-voltage protection and lightning protection devices
x as a filling gas for thyratron tubes (similar to amplifier tubes/triodes)
3.6.2
Applications of Krypton
Krypton is used:
x as a filling gas for incandescent lamps and flash bulbs
x as a filling gas for halogen lamps (with halogen components, see Example B
below)
x with fluorine, neon and helium for eximer lasers (see Example C below)
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3 The Noble Gases Neon, Krypton and Xenon
x as a filling gas for insulating glass panes (see Example D below)
x as a filling gas for detectors designed to measure radiation (e.g. Geiger
counters)
3.6.3
Applications of Xenon
Xenon is used:
x as a filling gas for plasma display panels (PDP) (see Example A below)
x as a filling gas for flash bulbs and gas discharge lamps
x as a filling gas for halogen lamps (with halogen components, see Example B
below)
x as a filling gas for thyratron tubes (similar to amplifier tubes/triodes)
x as a filling gas for xenon high-pressure lamps (e.g. lamps for floodlight units
and cinema projectors)
x with hydrogen chloride and helium for eximer lasers (see Example C below)
x as a propellant for small thrusters used to position satellites in orbit (Solar
Electric Propulsion (SEP), see Example E below)
x as an anaesthetic gas
x with oxygen in Computer Aided Tomography (CAT) scanners for mapping
blood flow
x as a contrast agent in Computer Aided Tomography to obtain pictures of the
brain o enzephalography
Example A: Plasma Display Panels
Plasma televisions comprise two glass plates, one with vertical conductive lines
and one with horizontal lines, and a neon-xenon gas mixture positioned between
these plates. When placed together, the two plates form a grid. Electric currents are
passed through the horizontal and vertical lines, causing the gas to emit ultraviolet
light, which excites fluorescent materials and create the picture (see Fig. 3.3).
Fig. 3.3 A mixture of neon and xenon is used for plasma TV screens.
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3.6 Applications
123
Example B: Halogen Lamps
Krypton and xenon bulbs are used in automotive headlights. The noble gases
provide an inert medium that allows to reduce the size of the bulbs. When mixed
with halogen components, the bulbs burn brighter and longer, use less power and
generate more light. In xenon headlights (discharge lamps), xenon gas creates a
bright, blue-tinted light.
Example C: Excimer Lasers
Excimer lasers work with an ultraviolet light beam. The gas mixture in a laser
tube contains fluorine gas (F2) and hydrogen chloride (HCl) as halogen donors
as well as argon, krypton and xenon as active noble gases, and neon as a buffer
gas. The excited monohalides ArF (193 nm), KrF (248 nm) and XeCl (308 nm) are
the active laser molecules. Excimer lasers are primarily used for vision correction
(cold beam) (see Fig. 3.4), in the lithography process for manufacturing computer
chips (short wavelength) and for microstructuring.
Fig. 3.4 Eximer lasers are used for correcting vision (cold beam).
Example D: Insulating Glass Panes
Making buildings as energy efficient as possible cuts costs. Windows with good
insulating properties make an important contribution to saving energy. Filling the
airspace between window panes with noble gas (argon or, even better, krypton)
greatly improves insulating capabilities by reducing circulation between the panes
and minimizing heat conduction. This is caused by heavy-atom krypton which does
not move around as quickly as nitrogen and oxygen in air. In addition, krypton
is monatomic and does not oscillate. This limited atomic mobility reduces heat
loss from buildings.
Example E: Solar Electric Propulsion
Ion beams are used in one of several types of spacecraft propulsion. In this
particular process, xenon flows into the ion engine where it is electrically charged
and pushed around by an electrical voltage. Two grids electrified to almost
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3 The Noble Gases Neon, Krypton and Xenon
1300 volts accelerate xenon ions to very high velocities and shoot them out of the
engine. As the ions race away from the engine, they push back on the spacecraft,
propelling it in the opposite direction. As a heavy ion, xenon provides ten times
more push than chemical propellants.
References
[3.1]
[3.2]
[3.3]
[3.4]
[3.5]
1345vch03.indd 124
Römpp, 10. Aufl., Stichworte: Neon, Krypton, Xenon, Thieme Verlag, Stuttgart, 1996.
Ullmann’s, 6. Aufl., 23, S. 218, Wiley-VCH, Weinheim, 2003.
Ullmann’s, 6. Aufl., 23, S. 227, Wiley-VCH, Weinheim, 2003.
Kryogene Kr-Xe-Gewinnung: EP 1082577 B1, US 6351970 B1.
Ullmann’s, 6. Aufl., 23, S. 215, Wiley-VCH, Weinheim, 2003.
26.10.2007 10:27:13
125
4
The Noble Gas Helium
4.1
History, Occurrence and Properties
4.1.1
History
In 1868, helium (He; from Greek “helios”, the sun) was discovered due to a pale
yellow line at 587.6 nm during the spectroanalytical examination of the solar
prominences. In 1882, it was also detected in the spectral analysis of lava from
the Vesuvius and as gas occlusion in the uranium mineral uraninite in 1889. Only
in 1895, helium could be produced in its pure form in larger quantities from the
mineral cleveite. And in 1908, H. Kamerlingh-Onnes at Leiden succeeded for the
first time in liquefying helium [4.1].
4.1.2
Occurrence
Helium is produced in radioactive decay processes and emitted as D-radiation.
Therefore it is found in all uranium minerals. The helium-method for the
age determination of minerals is also based on the formation of D-particles.
Consequently, as a decay product of radioactive processes, helium is also found
in many natural gases. The largest deposits of He-rich natural gases are located in
the USA, Siberia, Algeria and Canada (see Table 4.1). In Europe, helium occurs in
Polish natural gas with a molar fraction of about 0.4% and in gas from the North
Sea with a molar fraction of up to 0.12% [4.2]. Table 4.2 gives an overview of the
worldwide production of He and its reserves.
The helium content of ambient air amounts to 5.24 ppm. The total quantity of
helium in the atmosphere is in a stationary equilibrium between the gas escaping
into space on the one hand and the helium supplied by radioactive minerals and the
solar wind on the other hand [4.2]. In the sun, helium is represented with a molar
fraction of about 8%, whereas hydrogen amounts to about 92%. In the universe, helium is the second most abundant element with a molar fraction of more than 25%.
Helium is an end product of the hydrogen-nuclear fusion in fixed stars [4.1].
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
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4 The Noble Gas Helium
Table 4.1 Helium content of some natural gas sources and assessed reserves (1984) [4.2].
Source
Location, field name
Composition, % molar fraction
He
CmHn
N2
Reserves,
106 mN3
CO2
USA/Wyoming, Tip Top Field
0.4–0.8
1200
USA/New Mexico, Beautiful Mountain
4.05
49
45
0.9
USA/Alaska, South Barroweast
2.54
90.2
6.8
0.3
USA/Texas, Young Regular
1.17
66.2
31.1
0.1
USA/Kansas, Hugoton
0.44
81.8
17.6
Canada/Alberta, Worsley
0.53
93
6.0
0.5
Canada/Ontario, Norfolk
0.36
91.5
8.1
0.1
Netherlands, Groningen
0.05
83
14
0.9
Netherlands, De Wijk
0.05
94.3
5.6
Poland, Ostrow
0.4
56
43
0.3
North Sea, Indefatigable
0.05
96
3.3
0.5
Germany/Niedersachsen, Apeldorn
0.12
25.9
73.3
0.2
Russia, Urengoi
0.055
94.2
5
0.03
Russia, Orenburg
1270
6000
3400
Algeria, Hassi R’Mel
0.19
93.6
5.8
0.2
2400
Australia, Palm Valley
0.21
97.5
2.3
0.1
900
Table 4.2 Helium – Global production and reserves (in 106 mN3 ) [4.3].
Country
Production
Exploitable
reserves
2001
2002
USA
92
90
4300
9400
Algeria
15
15
2100
3200
Canada
2100
China
1200
Poland
1
1
42
300
Russia
4
4
1800
7000
Other countries
Total
1345vch04.indd 126
Total
reserves
3000
112
110
26 000
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4.2 Recovery
127
4.1.3
Physical and Chemical Properties
At atmospheric pressure and room temperature, helium is a colorless, odorless,
nonflammable monoatomic gas of the group of noble gases with the atomic
number 2. As natural isotopes, 3He and 4He occur, with 3He amounting to a portion of only 1.38 · 10–6. Gaseous He is characterized by high thermal conductivity
(0.143 W m–1 K–1), low density (0.1785 kg m–3, values at 0 °C and 1.013 bar), low
solubility in metals and high diffusivity. The critical point of 4He is at 5.20 K and
2.275 bar and at 3.31 K and 1.146 bar for 3He.
The liquefied helium is subdivided into two states: He I and He II with a sharp
transition point of 2.18 K at 5.04 kPa, the so-called O-point. He I behaves like a
normal liquid, whereas He II exhibits interesting properties of a superfluid or
quantum fluid. During expansion of liquid He I below this pressure, the previously
even surface forms a sharp meniscus at the wall of the container since at the
O-point the viscosity decreases by the factor 106 and the thermal conductivity
rises by the same factor. The thermal conductivity of He II is about 200 times
higher than that of copper at 20 °C. Close to the absolute zero point, the viscosity
turns zero and He II becomes an inviscid superfluid. He II flows over obstacles,
which lie higher than the surface of the liquid, to reach the lowest level. If two
containers of different temperatures are filled with He II and connected to each
other by a capillary or another He II-film, He II flows from the cold container
into the warmer one.
Helium is the only substance that remains liquid in close proximity of the
absolute zero point at atmospheric pressure.
For the production of solid helium the liquid has to be compressed to about
30 bar at 1 K, at 24 °C to about 117 000 bar. Solid helium is the softest solid known;
slight pressure fluctuations lead to changes of the interatomic distances in the
crystal lattice and thus to changes of the thermodynamic properties. It is the only
known example of a so-called quantum solid.
Helium is the lightes of the noble gases. The monoatomic gas is absolutely
inert and in contrast to the heavy noble gases, it does not form any chemical
compound.
4.2
Recovery
Owing to its very low portion in the air (5.24 ppm), helium is mainly recovered
from natural gases (cf. also Chapter 7). In general, helium recovery plants are
economic at He concentrations of about 0.2% molar fraction and more in the
natural gas. In a pre-cleaning step, H2O, CO2, H2S and other trace components
which would solidify during cool down are removed. Then the gas is partially
condensed and the remaining gaseous helium is enriched again. The helium
content of this so-called “raw helium” usually ranges between 50 and 90% molar
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4 The Noble Gas Helium
Fig. 4.1 Nitrogen removal and crude helium extraction from natural gas.
(a) Heat exchanger; (b) High-pressure column; (c) Condenser;
(d) Low-pressure column; (e) Crude He-separator; (f) Heat exchanger; (g) Pump.
fraction. A simple but typical modern process for the recovery of raw helium as
part of nitrogen removal from natural gas is shown in Fig. 4.1 [4.2]:
The pre-cleaned natural gas is cooled against cold product flows in heat
exchanger (a) and fed into the sump of the high-pressure column (b). The
condenser (c) supplies both the high-pressure column (b) and the low-pressure
column (d) with reflux. The non-condensable portion contains the helium which
is drawn from the raw helium separator (e) and discharged at the battery limit
after preheating. After further cooling down in the heat exchanger (f), the sump
product of the high-pressure column (b) is fed into the low-pressure column (d),
where the final separation into nitrogen and methane takes place. Nitrogen leaves
the column overhead and is heated against the cooled reflux in the heat exchanger
(h) and discharged at the battery limit after further heating in (f) and (a). The
reboiler (c) of the low-pressure column (d), which serves also as condenser for
the high-pressure column (b), depletes the nitrogen. The methane-rich fraction
is pumped to higher pressure (g) and is also discharged to battery limit after
heating in (f) and (a).
Until recently, in the USA raw helium with a molar fraction of about 70% was
stored as strategic reserve for future demand in exploited natural gas deposits.
For the production of high purity helium from raw helium, pressure swing
adsorbers are used, since helium is only poorly adsorbable, whereas methane
and nitrogen are relatively well adsorbed. Thus, with zeolitic molecular sieves
and fine-pored activated carbons, purities of 99.999–99.9999% molar fraction of
helium can be adjusted without additional purification steps. A typical Pressure
Swing Adsorber Unit (PSA) is shown in Fig. 4.2 [4.2]:
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4.2 Recovery
129
Fig. 4.2 Diagram of a four-bed pressure swing adsorber station.
FC = Flow control; FT = Volumetric flow measurement; PC = Pressure
control; PT = Pressure measurement; G1–G4 = Adsorbers 1 to 4.
b = Buffer tank for residual gas.
After volumetric flow measurement (FT) the crude helium gas is fed to the
station. At the outlet of each individual adsorber G1–G4 the pressure is measured
(PT) and maintained in the pure helium flow (PC). The desorption of impurities
from the adsorbent respectively the regeneration of the loaded adsorber is
controlled via purging steps with pure helium and via flow respectively pressure
control of the residual gas (FC). Pressure or flow fluctuations of the residual gas
are dampened by a buffer tank (b).
The individual adsorbers go through the following steps cyclically, with the
whole cycle taking a few minutes:
1. Adsorption
2. Pressure balance with another adsorber
3. Pressure reduction cocurrent to adsorption to provide purge gas for another
adsorber
4. Desorption through further pressure reduction to the residual gas system
countercurrent to adsorption
5. Purging with pressure relief gas of another adsorber
6. Pressure build-up by pressure balance with another adsorber
7. Final pressure build-up with pure helium to adsorption pressure
For PSA cf. also Section 2.2.4.
If hydrogen and neon are contained in the crude gas to the helium recovery
plant, the separation of these components in a pressure swing adsorber station
is not possible, since they behave similar to helium. Hydrogen can then be
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4 The Noble Gas Helium
Fig. 4.3 Diagram of a helium fine scrubbing and liquefaction plant.
(a) He-recycle compressor; (b) Cooler; (c) Plate fin heat exchanger;
(d) Expansion turbines; (e) Adsorbers; (f) Throttle valve;
(g) Liquid-He-tank.
combusted to water, for instance, via a de-oxo-plant, adding atmospheric oxygen.
The separation of neon is only possible in another cryogenic step which is also
used for the liquefaction of helium in order to allow to transport larger volumes.
Such a process is shown in Fig. 4.3 [4.2].
The large-scale liquefaction of helium is carried out with the help of a helium
cycle with a compressor (a) and cooler (b), in which cold is supplied at about
80 K by liquid nitrogen (c), and below by expansion turbines (d). Depending on
the size of the plant, more or less expansion turbines are used. At about 80 K,
traces of N2, O2, CO2 and CH4 are removed via a first adsorber (e), traces of H2
and Ne in a second adsorber (f) at about 20 K. The process heat exchangers (c)
are usually aluminium plate fin heat exchangers with very narrow temperature
differences in order to minimize exergy losses. At the inlet of the throttle valve (g)
the fluid is in a supercritical state. Only after throttling, the helium can be stored
as liquid in tank (h) at pressures slightly above atmospheric pressure. The flash
gas and the He evaporated due to heat leaks is heated in the heat exchangers (c)
and recirculated to the suction side of the compressor (a).
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4.3 Applications
131
Fig. 4.4 Diagram of a helium collecting tank.
(a) Cooling pipes; (b) Vacuum-superinsulation;
(c) Outer casing; (d) Heat-radiation shield;
(e) Container for liquid helium; (f) Support.
Cryogenic tanks (h) are built up to a volume of 100 m3. Their insulation loss
amounts to about 1% of the design liquid inventory per day. These low heat losses
at storage temperatures of about 4 K are possible due to superinsulation, thermal
radiation shields and deep vacuum in the clearances of the double-walled container.
The heat radiation shields transport the heat to the pipes in which either nitrogen
or helium itself evaporates. A typical layout is shown in Fig. 4.4 [4.2].
4.3
Applications
On the one hand, gaseous helium (GHe) is particularly popular for applications
that require an inert gas offering high thermal conductivity and low density.
Liquid helium (LHe), on the other, is used when extremely low temperatures are
essential.
It is used in the processing industry
x to increase the quality and performance of shielding gas welding processes
such as
– tungsten inert gas/TIG
– metal inert gas/MIG
– metal active gas/MAG (GHe combined with Ar/CO2)
(see Example A below)
x to weld aluminium alloys and stainless steel (GHe combined with Ar/CO2)
x to operate CO2 lasers (GHe mixed with N2 and CO2)
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4 The Noble Gas Helium
x to operate Excimer lasers (GHe mixed with F2 or HCl)
x to purge and shield semiconductors during fabrication (GHe in high purities)
x to protect and cool optical fibres during manufacture (with purification and
recycling of GHe)
x to provide a carrier gas in thermal spraying and related coating technologies
(e.g. Cold Spray)
It is used in other industries
x to temper and harden steel parts by quenching after an annealing furnace
(e.g. instead of using an oil bath)
x to perform quality control and research on pure metals and superconductive
wires at very low temperatures (cf. boiling point of LHe at 4.2 K)
x to cool superconductive magnet coils in
– magnetic resonance imaging (MRI) scanners used in medical diagnostic
equipment (see Example B below)
– nuclear magnetic resonance (NMR) spectrometers used for chemical
analysis
– particle accelerators used for nuclear research
x to cool the primary coolant circuit of pebble bed reactors (in nuclear power
stations)
x to run leak tests on pipelines, heat exchangers, receptacles and food packages
by means of thermal conductivity detectors or mass spectrometers
x to lift toys, air balloons and air ships
x to purge and pressurise rocket fuel systems (see Example C below)
x to create respiratory mixtures for deep-sea diving and medical care
x to inflate automotive airbags (as passenger safety devices)
Example A: Welding with Helium Gas Mixtures
Helium is valued not only for its inertness and high thermal conductivity, but also
for its high ionisation energy.
These properties make it a popular shielding gas for electric arc welding. It
enables higher electric arc temperatures and improves heat transfer to the work
piece. It also reduces the formation of pores and fusion faults, at the same time
improving gap bridging.
Unlike argon, helium is rarely used as a pure shielding gas here. Helium
mixtures, however, are proving extremely useful results in many electric arc
welding applications using materials such as steel, aluminium, stainless steel
and copper.
Nowadays, shielding gases with helium are widely used to increase the
productivity of many arc welding processes.
In the case of laser welding, pure helium can be used as a shielding gas for the
laser beam. Helium has an advantage over argon here. It does not absorb the laser
radiation with the result that it does not weaken the laser beam.
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4.3 Applications
133
Fig. 4.5 Gaseous helium as shielding gas for laser welding stainless steel.
But helium mixtures are also used for many kinds of cutting-edge laser
technologies. The correct composition of the gas mixtures creates the basis for
increased quality and productivity also in laser welding.
Example B: Superconductivity, Magnetic Resonance Imaging/MRI
The discovery of superconductive materials has led to the development of some
of the most remarkable diagnostic and research equipment ever.
Such materials have zero resistance for electricity when they fall below a specific
critical temperature. Cryogenic gases play an important role to reach and maintain
these temperatures of superconductivity.
Liquid helium helps to cool certain metal alloys below this critical temperature.
These alloys are used to build the superconductive coils that are needed to create
the strong magnetic field for the operation of MRI scanners.
Fig. 4.6 Liquid helium cools superconductive magnets in MRI scanners.
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4 The Noble Gas Helium
MRI scanners give doctors a detailed view inside the body of a patient, allowing
the diagnosis of many diseases without invasive surgery.
Example C: Purging and Pressurizing of Rocket Fuel Systems
In rocket science, helium is used to boost liquid hydrogen and liquid oxygen out
of the tanks into the engines during the take off. It is also applied to purge liquid
hydrogen systems in space launch vehicles.
For example, the Saturn V booster used in the Apollo program has consumed
vast quantities of helium for this pressurizing purpose. The helium was extracted
from natural gas and partly stored in an exploited gas field in Texas. (Please note:
In 1903, helium was found in natural gas, which has remained the main source
of commercial helium supply ever since.)
Fig. 4.7 Saturn V rocket launch (Apollo 16 program): Helium is used
for fuelling and pressurising liquid hydrogen and liquid oxygen tanks
in space rockets.
References
[4.1] Römpp, 10th edition, Keyword: Helium, Thieme Verlag, Stuttgart, 1996.
[4.2] Ullmann’s, 6th edition, Vol. 23, pp. 215–273, Wiley-VCH, Weinheim, 2003.
[4.3] US Geological Survey, Mineral Commodity Summaries, January 2003.
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5
Hydrogen and Carbon Monoxide: Synthesis Gases
5.1
History, Occurrence and Properties
5.1.1
Introduction
Synthesis gas is the term for gas mixtures mainly composed of carbon monoxide
(CO) and hydrogen (H2). In the chemical industry it is applied as raw material for
the production (“synthesis”) of various products, for example, for the production
of methanol or even ammonia (by synthesis of N2 and H2). Synthesis gases are
obtained in many ways (see Section 5.2). Fossil fuels such as carbon, petroleum,
petroleum residues, natural gas as well as wood, peat and biomass reacting with
water vapour, air, O2 or CO2 serve as feed materials. Apart from CO and H2, synthesis gas may contain CO2, H2O, N2, methane and higher-boiling hydrocarbons.
Terminologically as well as regarding the practical production process, the
boundaries to other industrial gases such as water gas and generator gas are fluid.
Table 5.1 Examples of H2, CO and synthesis gas products (see also Section 5.4).
H2
Various hydrogenations, as e.g. the fat hydrogenation in the
food industry and the desulphurization in refineries, energy
carrier for fuel cells and a future hydrogen infrastructure
H2 and N2
Ammonia
CO
Phosgene, polycarbonate, formic acid
H2 and CO in split flows
Acetic acid, intermediate products for the production of
polyurethane foams
Mixtures of H2 and CO
Oxo-alcohols, reduction gas for the steel industry,
fuel for gas turbines, synthetic fuels from natural gas
(Fischer-Tropsch-Synthesis: Gas to Liquids: GTL)
Mixtures of H2, CO and CO2
Methanol
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
In the past, synthesis gas was almost exclusively produced from coal and then
called water gas. Generator gas is understood as the gas mixture with high nitrogen
content developing during the coal gasification with air mixed with water vapour.
Today natural gas is the most important basic feed for the production of synthesis
gas. Meanwhile, first small plants for the “CO2-neutral” generation of synthesis
gas or pure hydrogen through the gasification of waste products and biomass are
operated worldwide.
Table. 5.1 gives a first overview of the main fields of application of synthesis
gas (cf. also Section 5.4.3).
5.1.2
History of Synthesis Gas
In 1780, Felice Fontana [5.1] discovered that combustible gas develops if water
vapour is passed over carbon at temperatures over 500 °C. This CO and H2
containing gas was called water gas and mainly used for lighting purposes in the
19th century. A more detailed historical overview of this epoch is given in [5.2]. As
of the beginning of the 20th century, H2 /CO-mixtures were used for syntheses of
hydrocarbons and then, as a consequence, also called synthesis gas. In 1921, Patart
reported for the first time on the synthesis of methanol, after he had carried out
the reaction of H2 /CO-mixtures under pressure and at 400 °C on ZnO-contacts
[5.3]. The systematic examination of the reaction conditions of the synthol-process
developed by Fischer and Tropsch in 1922 led to the direct hydrocarbon synthesis
on ferric catalysts in 1926 [5.4], the so-called Fischer-Tropsch- or FT-synthesis [5.5].
In 1943/44, this was applied for large-scale production of artificial fuels from
synthesis gas in Germany, with coal as a basic feed. Nowadays, synthesis gas is
mainly used for production of the products listed in Table 5.1 and increasingly
in energy engineering again. With the gasification of heavy hydrocarbons and
the combustion of the generated synthesis gas in gas turbines for generation of
electric energy the original application of “water gas” in the energy sector has
been reached again.
5.1.3
Hydrogen
5.1.3.1
History and Occurrence
In the second half of the 17th century, Boyle produced hydrogen for the first time
through dissolving iron in sulphuric acid. In 1766, Cavendish published precise
values for the specific weight and the density of hydrogen. Owing to its very low
density, hydrogen was already used as filling gas for balloons by C. Charles in
1783. Also in the year 1783, Lavoisier termed the combustible product of water
separation “hydrogenium” (= water generator), from which H is derived as the
symbol of the element. In 1898, Dewar applied the Linde-Process for the first
liquefaction of hydrogen. A sad peak of the degree of fame was the fire of the
hydrogen-filled airship “Hindenburg” in Lake Hurst in 1937. In 1963, the USA
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137
started a rocket driven by hydrogen and oxygen in Cape Kennedy for the first time
ever. In order to avoid greenhouse gases and other atmospheric contaminants,
at least in conurbations, hydrogen has been considered as an energy source for a
modern traffic infrastructure for about 20–30 years, cf. also Section 5.4.1.2.
Molecular hydrogen is contained in traces in the ground-near atmospheric
air with 0.5 vppm. However, with increasing height its content increases as
well, until in some 100 km the ultrathin gas layer consists almost exclusively of
hydrogen [5.6]. In bound state, hydrogen is a component of water (mass portion
of hydrogen 11.2%) and many other compounds, such as hydrocarbons. While the
hydrogen content in the upper 16 km of the earth’s crust, including hydrosphere
and atmosphere, is assessed at a mass portion of 0.88% (thus hydrogen is ninth
with regard to its frequency on earth) H represents the most frequent element of
the whole planetary system inside and outside the Milky Way. The sun consists
of a mass portion of hydrogen (protons) of 84%. The fusion of these hydrogen
nuclei to helium forms its energy source [5.7].
5.1.3.2
Physical and Chemical Properties
With only one proton and one electron, hydrogen is the lightest of all chemical
elements. At ambient temperature, molecular hydrogen, H2, is a colourless and
odourless gas. At 0 °C and 1 bar, one litre weighs 0.0899 g. Consequently, H2 is
14 times lighter than air and with this, a suitable filling gas for balloons. The
lift of 1 m3 of H2 in air amounts to 1.2 kg. Disadvantageous for this application
is its high combustibility, since hydrogen in a concentration range of 4–77% of
volume fraction in air forms an explosive mixture. At –252.78 °C (1 bar), hydrogen
condenses to a colourless liquid, it freezes at –259.15 °C.
Its high diffusion capability owing to its low molar mass respectively its solubility
in a number of metals has to be considered in the handling of hydrogen and
selection of materials, e.g. regarding the embrittlement of steels [5.50, 5.51]. On
the other hand, the good solubility of H2 in metallic platinum and palladium
respectively its extraordinarily high diffusion selectivity due to dense membranes
of these noble metals can be utilized for the separation of H2.
Two other isotopes of hydrogen exist. On earth, heavy hydrogen (D, deuterium),
which is stable, occurs in the natural isotopic distribution with an atomic fraction
of 0.015%; tritium (T) is a weak E-radiator with a half-life of about 12.3 years, on
earth represented in the natural isotopic distribution with an atomic fraction of
only about 10–12–10–13%.
The Diagrams 5.1 and 5.2 show a comparison of the energy content of different
energy sources. From this, it can be construed that the mass-related energy content
of H2 is very high, the volume-related energy content for gaseous hydrogen,
however, is relatively low. Therefore, an important goal of development is the
discovery of a competitive storage medium respectively storage method for
hydrogen.
With ortho-hydrogen (o-H2: parallel nuclear spin) and para-hydrogen (p-H2:
anti-parallel nuclear spin) hydrogen disposes of two nuclear spin isomers. At
room temperature, the equilibrium fraction of p-H2 is 25%. This mixture is
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Diagram 5.1 Mass-related energy content of different energy sources in kWh kg–1.
Diagram 5.2 Volume-related energy content of different energy sources in kWh m–3.
called normal-hydrogen (n-H2). At low temperatures, however, the p-portion
increases strongly whereas the equilibrium is reached only very slowly. Since
the conversion from ortho to para-form is exothermal, further cooling has to be
provided after liquefaction of hydrogen to reach the equilibrium. If the hydrogen
is in equilibrium, it is also called e-H2. Figure 5.1 shows the p-content of the
e-hydrogen in dependency on the temperature. Further important physical
properties of n- and e-hydrogen are listed in Table 5.2A and B [5.8, 5.9].
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139
Table 5.2A Physical properties of n- and e-hydrogen.
Unit
n-hydrogen
(o- 75%, p- 25%)
e-hydrogen
g mol–1
2.016
2.016
kg m–3
J mol–1 K–1
W m–1 K–1
0.0899
Cp = 28.6, Cv = 20.2
0.1645
0.0899
Cp = 28.6, Cv = 20.2
0.1645
x Temperature
x Density (liquid)
x Density (gas)
K
kg m–3
kg m–3
20.37
70.00
1.319
20.43
70.81
1.316
Heat of evaporation
J mol–1
898
916
Liquid at boiling point (101.3 kPa)
x Molar heat
x Enthalpy1)
x Thermal conductivity
J mol–1 K–1
J mol–1
W m–1 K–1
Cp = 22.0, Cv = 6.51
–7918
0.117
Cp = 23.5, Cv = 7.97
–7932
0.123
Gas at boiling point (101.3 kPa)
x Specific heat capacity
x Enthalpy1)
x Thermal conductivity
J mol–1 K–1
J mol–1
W m–1 K–1
Cp = 23.49, Cv = 12.8 Cp = 22.4, Cv = 12.1
–7020
–7016
0.0185
0.0180
K
kPa
kg m–3
33.00
1339
30.09
32.98
1310
40.16
K
kPa
kg m–3
kg m–3
kg m–3
13.81
6.14
86.7
76.4
0.12
13.95
7.03
86.7
77.2
0.13
Molar mass
Properties at 273.15 K, 101.3 kPa
x Density
x Molar heat
x Thermal conductivity
Boiling point (101.3 kPa)
Critical point
x Temperature
x Pressure
x Density
Triple point
x
x
x
x
x
Temperature
Pressure
Density (solid)
Density (liquid)
Density (gas)
1)
The reference point for the enthalpy (incl. transformation heat) is zero for the ideal gas at 0 K.
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Table 5.2B Physical properties of hydrogen in combustion.
Combustion (with air at 101.3 kPa)
Ignition limits
% volume fraction
4–77
Ignition temperature
K
858
Minimal ignition energy
mJ
0.02
Flame temperature
K
2591
–1
Laminar combustion velocity
ms
Lower calorific value (volumetric)
kWh m–3
Lower calorific value (gravimetric)
2.75
–1
kWh kg
3.0
33.3
Fig. 5.1 p-fraction of e-hydrogen depending on the temperature.
Chemical Properties
In air, H2 combusts to water with a hardly visible, weakly bluish flame (detonating
gas reaction). The chemical bond between hydrogen and oxygen is very strong and
can only be ruptured by adding considerable energy ('Hf (H2O): –286 kJ mol–1).
The hydrogen bridge bonds occurring between the hydroxyl groups (-OH) of two
water-molecules are important intermolecular bonds that sharply increase the
melting and boiling points of light molecules.
Hydrogen combines with almost any other element. Metal compounds with
negatively charged hydrogen are called metal hydrides (e.g. CaH2, NaH, LiH).
With water, they form metal hydroxides and gaseous hydrogen. Through thermal
cracking of ethane or naphtha (dehydration), for example, ethylene, propene and
H2 are obtained. In addition to that, the hydration of unsaturated hydrocarbons,
as for instance in fat hardening, plays an important role. Hydrogen has a reducing
effect on a lot of metal oxides when heated. Thus CuO with H2, for example, reacts
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141
to Cu and H2O. In the 1H-NMR-spectroscopy the H-atom with the nuclear spin
of 1/2 is used as a probe for the structural analysis of organic molecules as well
as in the medical nuclear spin tomography.
5.1.4
Carbon Monoxide
5.1.4.1
History and Occurrence
Carbon monoxide CO, is a gas that develops in each incomplete combustion
of carbon-containing compounds (e.g. coal, petroleum, natural gas). Carbon
monoxide was discovered by Lasonne in 1776 when glowing coal with zinc oxide,
and in 1796 by Priestley when glowing coal with hammer blow (Fe3O4) [5.10].
Originally, the gas had been interpreted as hydrocarbon, in 1801 Clement and
Desormes determined the chemical composition. In 1877, CO was liquefied for the
first time by L. Cailletet. The carbon monoxide amounts of the earth’s atmosphere
are for the most part based on the bacterial emission of CO in the soil and the sea.
In addition, CO is found in volcanic gases. Anthropogenic CO-emissions result
from the incomplete combustion of fossil fuels.
Typically, industrial air contains about 0.3 vppm of carbon monoxide. Near busy
roads the mean value of CO amounts to about 0.6 vppm with peak concentrations
of some vppm of CO occurring (immission values). In the exhaust gas of Ottoengines without catalytic converters, CO is contained with a mass portion of about
1.4% (emission value). The reduction of this CO-quantity by means of a catalytic
converter installed in the exhaust system amounts to about 90%.
5.1.4.2
Physical and Chemical Properties
Carbon monoxide with the molar mass of 28.01 g mol–1 is colourless, odourless
and tasteless and does not irritate the respiratory tracts. It is highly toxic, slightly
lighter than air, poorly soluble in water (solubility: 23 mL L–1 at 20 °C and 1 bar)
and combustible. Together with air, carbon monoxide forms explosive mixtures in
the concentration range of a CO-volume fraction of 10.9–76%. It combusts in air
with a bluish, very hot flame. The calorific value is 12.69 kJ m–3 resp. 12.93 kJ kg–1.
Carbon monoxide liquefies at 1 bar and –191.5 °C and solidifies at –199 °C. In the
laboratory, carbon monoxide (= anhydride of formic acid) is obtained by dripping
concentrated sulphuric acid into formic acid at temperatures over 100 °C [5.11]. In
engineering, it is obtained by separation from synthesis gas. In steel production,
CO occurs as reducing agent of ferric oxide, where it exhausts as blast-furnace
gas in large quantities. There are numerous complex bonds of transition metals
with CO called carbonyl complexes. Examples are Ni(CO)4 and Fe(CO)5. The
possibility of a carbonyl formation at high CO-partial pressure has to be taken into
consideration when choosing the material of equipment and piping. The lifespan
of CO in the atmosphere is assessed at 1–2 months, with the reaction of CO with
OH-radicals to CO2 and Hx being regarded as decomposition reaction [5.7].
CO is a highly toxic gas with a threshold limit value (TLV) of 30 vppm. The reason
for its toxicity is its property to displace the oxygen from the haemoglobin-complex
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
of blood, since the affinity of haemoglobin (Hb) to CO is about 300 times higher
than to O2. A concentration of 660 vppm of CO is enough to block about half of
the Hb for the oxygen transport. The bond of CO to haemoglobin is reversible, the
CO-decomposition however occurs very slowly. In CO-free air the whole adsorbed
CO can be replaced by O2 again, thus re-establishing the functional capability of
the haemoglobin. Through ventilation with pure O2, this process is accelerated
considerably. The symptoms of a CO-intoxication mainly arise through oxygen
deficiency in the tissue. The haemoglobin of a heavy smoker of cigarettes can reach
a CO-saturation of up to 15% in the course of a day. Other important properties
of carbon monoxide are listed in Table 5.3 [5.12–5.15].
Table 5.3 Physical properties of carbon monoxide.
Unit
Carbon monoxide
g mol–1
28.010
kg m–3
J mol–1 K–1
W m–1 K–1
1.250
Cp = 29.05, Cv = 20.68
0.02324
K
81.65
K
74.15
K
kPa
kg m–3
132.29
3496
301
x Temperature
x Pressure
K
kPa
68.05
15.25
Explosion range (in air at 101.3 kPa)
% volume
fraction
10.9–76
Ignition temperature (in air at 101.3 kPa)
°C
605
Threshold limit value (TLV)
ppm
30
Molar Mass
Properties at 273.15 K, 101.3 kPa
x Density
x Molar heat
x Thermal conductivity
Boiling point (101.3 kPa)
x Temperature
Melting point (101.3 kPa)
x Temperature
Critical point
x Temperature
x Pressure
x Density
Triple point
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143
5.2
Production of Synthesis Gas
The term synthesis gas stands for a multitude of different gas mixtures consisting
of H2, CO and N2, partially with traces of hydrocarbons, CO2 and Ar. Similarly
high as the number of the different and technically relevant synthesis gases is the
number of the practically applied production processes [5.16–5.18].
For some years, central importance has been attributed to hydrogen as a future
energy carrier. However, the use of this energy carrier represents a solution to the
problem of carbon dioxide emissions only if hydrogen is generated from regenerative energy sources like sun, wind or renewable organic raw materials, without
carbon dioxide as a by-product [5.19]. All production processes from regenerative
energy sources are not yet on a technical level that would enable to meet the current
demand for hydrogen, aside from the demand that would arise with the utilization as an energy carrier of the future (cf. also Section 5.4.1.2). Nevertheless, the
development regarding the use of hydrogen as energy carrier is progressing rapidly,
although at present the conventional hydrogen production from hydrocarbons is
still to prevailing [5.20]. These production processes from hydrocarbons as well as
the electrolysis of water for the production of hydrogen are explained in detail.
5.2.1
Production of Hydrogen by Electrolysis
A technically far-developed process is the electrolysis of water for the production
of hydrogen, although only about 5% of the hydrogen is produced by means
of electrolytic processes worldwide [5.21]. The efficiency is limited to smaller
hydrogen capacities up to about 5000 mN3 h–1 and the availability of cheap electric
energy, for example from water power.
Core element is the electrolyser consisting of two electrodes: the negatively
charged cathode and the positively charged anode immersed in an electrolyte
solution. Since pure water is a poor ionic conductor, alkaline media such as
potassiumhydroxide (KOH)-solutions are predominantly used in technical
electrolysers. Figure 5.2 shows the structure of an alkaline electrolyser [5.22]. The
diaphragm, which is permeable to OH–-ions but separates the generated gases, is
arranged between the electrodes. The following electrochemical reactions occur
at the electrodes after the application of voltage:
Cathode:
2 H2O + 2 e– o H2 + 2 OH–
Anode:
2 OH– o ½ O2 + H2O + 2 e–
The complete process running in the electrolyser results from the sum of these
two reactions:
Electrolyser:
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Fig. 5.2 Alkaline electrolysis cell.
The concentration of the electrolyte in the solution amounts to a mass fraction
of usually 25–30%, the temperature is about 80 °C and the pressure depending on
the required product pressure is 1–30 bar. The electrodes are corrosion-resistant,
good electric conductors and support the formation of the gases by means of
structured surface.
5.2.2
Production of Synthesis Gas from Hydrocarbons
In the production of synthesis gases from hydrocarbons, the components hydrogen
and carbon monoxide usually appear as complementary products, carbon dioxide
can be obtained as a by-product as well. Apart from hydrocarbons and steam
(steam reforming), some processes require carbon dioxide (CO2 reforming) as
well as oxygen or air (partial oxidation and autothermal reforming) as feedstock.
Usually, the process selection depends on two factors:
1. The desired product composition, usually characterized by the so-called H2/COratio in the raw synthesis gas.
2. The feedstock available (natural gas, residual gases from refineries, LPG
(Liquefied Petroleum Gas), naphtha, heavy oils, distillation residues, pitch,
coal, carbon dioxide, oxygen) and process utilities (steam, cooling water, …).
For a systematic structure, it is practical to distinguish between the actual synthesis
gas generation and the synthesis gas processing. In processes requiring more or less
pure oxygen (partial oxidation, autothermal reforming), the air separation plant
that may have to be installed has to be considered with regard to the selection of
the process and the economic evaluation [5.23].
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5.2 Production of Synthesis Gas
5.2.2.1
145
Generation of Synthesis Gas by Steam Reforming
In this process, light feedstock like natural gas, LPG, refinery residual gas (ROG)
or naphtha are converted into carbon monoxide and hydrogen under the addition
of water vapour. Based on the reaction of methane (CH4), representing the main
component of natural gas, with water vapour the most important aspects of the
reaction process will be explained:
CH4 + H2O CO + 3 H2
'HR = 206 kJ mol–1
(5.1)
This so-called reforming reaction is strongly endothermal, and due to the
chemical equilibrium the conversion is limited. According to the Le Chatelier
principle, a reaction system with heat supply at high temperatures and low pressure
is best for the realization of high conversion rates. In the so-called steam reformer,
high-temperature resistant tubes filled with nickel catalyst are heated by means
of an external furnace. Apart from the reforming reaction, the weakly exothermal
water–gas shift reaction, also determined by the chemical equilibrium, takes place
in the reforming tubes parallel to the reaction (5.1):
CO + H2O CO2 + H2
'HR = –41 kJ mol–1
(5.2)
For higher hydrocarbons, the reforming proceeds according to the general
scheme:
CnHm + n H2O n CO + (n + m/2) H2
(5.3)
Due to the sensibility of the nickel catalyst for poisoning, the feedstock has
to be purified from catalyst poisons like sulphur and chlorides before the
reforming. First with the addition of hydrogen the organic sulphur components
like mercaptanes and thiophenes are hydrogenated to hydrogen sulphide (H2S).
The hydrogen required is taken from the product flow and compressed for
recirculation. After the hydrogenation over a cobalt/molybdenum catalyst (CoMo),
the hydrogen sulphide can be easily removed with zinc oxide (ZnO) adsorbents
by means of chemisorption:
H2S + ZnO o ZnS + H2O
Especially for higher hydrocarbons, an over-stoichiometric steam to carbon ratio
S/C has to be adjusted at the inlet of the reforming tube to avoid soot formation on
the nickel catalyst. The higher the steam flow, the higher the methane conversion
due to the chemical equilibrium, however, at the expense of a higher firing duty.
Efficient values for the S/C ratio in the generation of hydrogen-rich synthesis gases
range between 2.5–3.5, for the reformer outlet temperature values of 850 to 920 °C
are common which are limited, however, due to the metallurgy of the reformer
tubes, especially at higher pressures. A low reformer pressure is advantageous
for a high methane conversion. Nevertheless, usually a high natural gas pressure
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Fig. 5.3 Basic types of radiation zones in steam reformers.
should be utilized to avoid energy-intensive product compression of synthesis gas.
Nowadays steam reformers with pressures of up to 40 bar are feasible due to the
enhancement of the mechanical stability of the materials at high temperatures.
For the generation of a CO-rich synthesis gas, the S/C-ratio has to be reduced
because of the water–gas shift equilibrium, however, due to the risk of soot
formation a minimum value, depending on the selected catalyst, must not be
fallen below. Downstream the process, carbon dioxide is scrubbed out of the raw
synthesis gas, which is then to be compressed and recycled to the reformer. With
this scheme and the import of additional carbon dioxide, the CO-content in the
synthesis gas can be further increased [5.24].
Up to a synthesis gas capacity equivalent to about 6000 mN3 h–1, so-called cantype reformers are the most economical alternative for the technical realization
of a steam reformer; for capacities exceeding this value, box reformers with the
reforming tubes arranged in rows are more economical. Figure 5.3 shows the
so-called radiation zone of reformer types used in practice which differ from each
other in the arrangement of the catalyst tubes and burners in the furnace box.
Due to the high outlet temperatures of the reformer, only about 50% of the
fired heat is absorbed by the reformer tubes. The so-called convection zone
follows the radiation zone. Here, the energy of the flue gas, which is still up to
1000 °C hot, is used for the preheating of feedstock, for the generation of steam
and optionally for the preheating of combustion air. Nowadays, steam reformers
with up to 1000 tubes and a hydrogen capacity of up to 300 000 mN3 h–1 are used
in technical plants [5.21].
5.2.2.2
Synthesis Gas Generation by Partial Oxidation (PO)
Contrary to the catalytic steam reforming, the non-catalytic partial oxidation
process is also suitable for heavier hydrocarbons such as heavy oils, pitch or coal.
Usually, oxidation takes place with pure oxygen. If air is used, the effort for the
separation of nitrogen from the synthesis gas would be higher than for the air
separation. The reaction is strongly exothermal and is realized uncooled in the
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5.2 Production of Synthesis Gas
147
so-called PO-reactor (or gasification reactor, as the process itself is often called
gasification) under oxygen deficiency. Special developed burners ensure a good
mixing and complete oxygen conversion:
CnHm + n/2 O2 o n CO + m/2 H2
(5.4)
For the atomisation of liquid feedstock, a certain quantity of steam is added to
the hydrocarbon stream.
Natural gas can be used as feed for the partial oxidation as well, especially if
cheap oxygen is available and/or H2/CO-ratios are required smaller than those
achievable with the steam reforming process:
CH4 + ½ O2 o CO + 2 H2
'HR = –36 kJ mol–1
(5.5)
In the case of oxygen excess, too much of the feed is fully oxidized to undesired
CO2 under large development of heat.
CH4 + 2 O2 o CO2 + 2 H2O
'HR = –803 kJ mol–1
(5.6)
Insufficient oxygen results in a too low outlet temperature with a high soot
formation rate:
CH4 o C + 2 H2
'HR = 75 kJ mol–1
(5.7)
2 CO o C + CO2
'HR = –173 kJ mol–1
(5.8)
The oxygen quantity determines the adiabatic reactor outlet temperature. Due
to the high temperature, the reactor has to be brick lined inside with heat-resistant
materials.
After the actual oxidation in the combustion zone, the gas mixture passes through
the so-called reaction zone, in which mainly the reforming reaction (5.1) described
in the section on steam reforming and the water–gas shift reaction (5.2) occur.
Having passed this zone, the gas has a temperature of typically 1200–1400 °C and
requires cooling. Figure 5.4 shows a PO-reactor with combined quench cooling
and synthesis gas cooler. In the quench chamber, the gas is cooled by quenching
with water. Thereby, the temperature of the water dew point of the synthesis gas
is reached. In the synthesis gas cooler, the process heat of the hot gases is used
for the generation of high-pressure steam. Downstream, the cooled or quenched
synthesis gas passes the so-called scrubber, where the possibly formed soot is
scrubbed out with water.
In process plants, two PO-technologies are applied, which differ from each other,
amongst others, in their way of soot-recirculation: the Texaco-method and the
Shell-method. Soot-recirculation is essential, especially when heavy hydrocarbons
are used [5.25]. Contrary to steam reforming, these two processes can be applied
at significantly higher pressures (Texaco: 85 bar, Shell: 65 bar). Since it is a non-
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Fig. 5.4 Texaco PO-reactor with quench and heat recovery boiler.
catalytic process, the pre-treatment of the feedstock for the removal of sulphur
from the hydrocarbons is not required, a sophisticated acid gas removal from the
synthesis gas is necessary instead.
5.2.2.3
Generation of Synthesis Gas by Autothermal Reforming (ATR)
Autothermal reforming is often regarded as a combination of partial oxidation
and steam reforming. The energy required for endothermal steam reforming is
generated through partial oxidation of the hydrocarbons in presence of steam in
the upper part of the reactor (reactions (5.4) to (5.6)).
Figure 5.5 shows a typical autothermal reformer with burner and adiabatic
catalyst fixed bed in the lower part. As in the case of steam reforming with catalyst
tubes fired from outside the endothermal methane reaction occurs on a nickel
catalyst. Due to the lack of heat supply, the gas mixture cools down and reaches
almost the value of the methane and water–gas shift equilibrium according to
reactions (5.1) and (5.2) at the outlet. Typical values for the outlet temperature vary
from 900 °C to 1100 °C [5.26] depending on the application. Since the autothermal
reformer has to be brick lined similar to the PO-reactor, there is no limitation to
pressures of max. 40 bar compared to steam reformers with catalyst tubes.
Besides hydrocarbons, autothermal reformers are also fed with pre-reformed H2and CO-rich gas mixtures from a steam reformer (the so-called primary reformer).
In this case the ATR is also called secondary reformer. For the generation of synthesis
gases for the ammonia synthesis, compressed air instead of pure oxygen is used.
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5.2 Production of Synthesis Gas
149
Fig. 5.5 Autothermal reactor with burner and catalyst bed.
Besides the oxygen required for the partial oxidation, the nitrogen required for
the ammonia synthesis is fed as well.
In more recent reactor concepts, the hot synthesis gas at the outlet of the
autothermal reformer is used for the heating of the catalyst tubes in the primary
reformer. In general, this system is called convective reformer. Figure 5.6 shows an
example of the so-called tandem reformer for the generation of methanol synthesis
gas.
Fig. 5.6 Tandem reformer.
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
5.2.3
Synthesis Gas Processing
Usually, the gas mixture from the synthesis gas generation section has not the
composition and purity required for the respective application. Therefore, the
synthesis gas generation is followed by the raw synthesis gas processing. The
process steps presented here can also be applied for the processing of the desired
synthesis gas mixture from so-called residual gases of refineries and petrochemical
plants. In this case, the synthesis gas plant only consists of the synthesis gas
processing section, while the generation part is not required; this represents the
most economical variant for the generation of synthesis gas.
5.2.3.1
Water–Gas Shift Reactor
This reactor used for the conversion of CO according to the water–gas shift
reaction (5.2) is found in each plant for the production of hydrogen and ammonia
synthesis gas. Furthermore, it is used to increase the H2/CO-ratio in H2/COplants. Depending on the temperature range, a distinction is made between hightemperature shift, medium-temperature shift and low-temperature shift (HTS,
MTS and LTS). Since the water–gas shift reaction is an exothermal equilibrium
reaction, the residual-CO content is the lowest at low temperatures. However, the
reaction rate at the catalyst as well as the dew point of the synthesis gas determine
the limits of the technical feasibility [5.27]. Usually, the reactors are designed as
adiabatic fixed beds with the temperature rising during the reaction. The mediumtemperature shift reaction can be carried out in isothermal reactors with steam
generation [5.28]. For sulphur containing synthesis gases from partial oxidations,
stable and sulphur-resistant catalysts are available for the so-called Dirty Shift.
5.2.3.2
Removal of Carbon Dioxide and Acid Gases
Usually, carbon dioxide and acid sulphuric components like H2S and COS are
removed using chemical or physical scrubbing processes. There is a variety of
solvents applied. Representative of the chemical solvents, the amine-containing
detergents MEA (monoethanolamines) and the activated aMDEA (methyldiethanolamines) from BASF shall be mentioned. Besides that, the so-called
Benfield process with hot potash (K2CO3) as solvent is of technical importance.
In the scrubbing column of a chemical wash process the carbon dioxide reacts
with the solvent; in the so-called regeneration column the bound CO2 is stripped
out again through steam generated by energy supply in the bottom. The energy
is provided by the cooling of synthesis gas in the process or by condensing lowpressure steam. The process flow diagram of an aMDEA-scrubbing process
according to the BASF process can be found in Section 6.2.2.
In the case of sulphur containing synthesis gases from a partial oxidation, amine
scrubbing processes can be applied as well in which the sulphuric components
H2S and COS are scrubbed out together with CO2. In the case of higher sulphur
contents (coal and heavy oil gasification) the sulphur components have to be
removed selectively. A physical scrubbing process such as the so-called Rectisol
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Fig. 5.7 Rectisol scrubbing with selective CO2- and H2S-removal.
process is applicable to these conditions. Here, the acid gas components are
scrubbed out in the scrubbing column with cold methanol. With an adapted
process flow arrangement in the regeneration part, a sulphur-rich fraction can be
obtained which is applicable for processing to elemental sulphur in a Claus reactor.
Contrary to chemical scrubbing, physical scrubbing is applicable for feedstocks
at high pressure and high content of acid gas components to be scrubbed. The
process flow chart of a Rectisol-scrubbing with scrubbing columns, stripper,
hot regeneration and methanol/water separation is depicted in Fig. 5.7. The
refrigeration plant required for this process is not shown.
5.2.3.3
Methanation
After a water–gas shift reaction at low temperatures (LTS) and the following CO2removal, the synthesis gas can be purified in a so-called methanation to remove
smaller traces of CO and CO2. Due to the exothermal reversed steam reforming
reaction (5.1) smaller quantities of methane are formed which can be accepted
as inert component, for example, in the ammonia synthesis, however, only at the
expense of a so-called purge gas flow from the synthesis loop [5.18]. Usually, this
last purification step is not suitable for the generation of high-purity hydrogen.
5.2.3.4
Pressure Swing Adsorption (PSA)
This adsorptive purification method is suitable for the production of high-purity
hydrogen from raw synthesis gas and residual gases from refineries. From the
crude hydrogen at high pressure with CO, CO2, H2O, CH4 (in residual gases higher
hydrocarbons as well) all components are adsorbed, however, the adsorption of
hydrogen is only very low. On this selectivity the separation effect of the pressure
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Fig. 5.8 Hydrogen yield as function of the residual and adsorption pressure.
swing adsorption is based. The unadsorbed, passing hydrogen is highly-pure and
under high pressure. After the adsorption phase, the adsorbed components are
desorbed by means of pressure decrease, finally accumulating at low pressure
in the so-called tail or purge gas. Usually, this tail gas is combustible and can be
compressed, for instance, to be fed into the fuel gas system. In case higher tail gas
pressure and desorption pressure is chosen for the PSA-design, compression is not
required, however, at the expense of the hydrogen yield, which decreases rapidly
with increasing tail gas pressure. This effect is shown in Fig. 5.8. A very common
practice in steam reformer plants is the utilisation of the tail gas of the PSA as fuel
gas for reformer firing. Based on a suitable combination of pressure compensation
and purging steps, the hydrogen loss in the tail gas can be minimized to the effect
that large plants with up to 16 adsorbers reach H2-yields of more than 90% (for
PSA see also Sections 2.2.4 and 2.2.5.6).
5.2.3.5
Membrane Processes
When processing synthesis gases with a membrane process, the light component
hydrogen passes from the synthesis gas mixture through a membrane. This
permeation is a combination of diffusion and solubility of the gas, e.g. in a polymer
membrane. Due to the required pressure difference between the feedstock side
and the permeate, up to 98% of pure hydrogen is obtained at low pressure, with
the yield decreasing with increasing purity [5.29]. This effect is depicted in Fig. 5.9
for different pressure conditions between raw synthesis gas and permeate. The
heavier components such as carbon monoxide and hydrocarbons are retained and
accumulate in the retentate. Apart from the hydrogen recovery from purge gases,
membrane methods are suitable for the hydrogen recovery from H2-containing
residual gases from refineries and for the adjustment of a desired H2/CO-ratio in
combination with other processes like a pressure swing adsorption or a cryogenic
separation (hybrid process). For example, from the raw synthesis gas from a partial
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Fig. 5.9 Hydrogen yield and purity in membrane processes.
oxidation of natural gas (H2/CO ~ 1.8) the ratio with a value of almost 1 required
for an oxo-alcohol synthesis can be adjusted with a membrane process.
5.2.3.6
Cryogenic Separation Processes
Separation and purification processes at low temperatures make use of the big
difference of the boiling points of hydrogen and the other components contained
in the raw synthesis gas, like CO, CH4 (in the case of residual gases higher
hydrocarbons as well) or N2. All processes have in common the necessity to
remove upstream water and carbon dioxide traces completely from the feedstock
to prevent these components from freezing at low temperatures, thus avoiding
the blockage of equipment and piping. Nowadays, in modern processes this
drying is done almost always with adsorption at molsieves, where the adsorber
beds are regenerated thermally with heated gas. All equipment and piping of the
cryogenic part are compactly cased in the so-called coldbox. In order to reduce the
heat penetration into the coldbox, it is filled with insulation material, e.g. perlite,
cf. also Section 2.2.5.7.
Depending on the composition of the synthesis gas, two different kinds of
processes are technically applied for the separation of raw synthesis gas into the
components H2 and CO:
x Condensation processes
x Methane scrubbing
With both processes almost all desired CO-purities are achievable; according to the
requirements, the purity of the raw hydrogen can be improved with a downstream
pressure swing adsorption.
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Fig. 5.10 Cryogenic condensation process.
(1, 2) Plate fin heat exchanger; (3) Separator; (4) Hydrogen stripper;
(5) CO/CH4-separation column; (6) Compressor.
Fig. 5.10 shows a condensation process with two separation columns. The dried
raw synthesis gas is cooled in compact aluminium plate fin heat exchangers (1, 2)
to such an extent that the gas phase, the raw hydrogen, can be separated from the
CO/CH4-rich liquid phase at feedstock pressure (3). The “refrigeration” required
for cooling is provided by raw hydrogen to be heated and the evaporating CO. In
the hydrogen stripper (4), hydrogen still dissolved in the liquid phase is separated.
The CO/CH4-mixture of the stripper bottom is separated in the second column
(5), thus reaching the CO-purity at the top through the reflux of liquefied and
sub-cooled pure CO from the multistage CO-compressor (6). The mixture from
the top of the hydrogen stripper and the bottom of the CO/CH4-column results in
an waste gas at low pressure and is suitable to be used as fuel gas. Condensation
processes are preferably used for the separation of CO-rich high pressure synthesis
gases with low methane content from partial oxidations. The raw hydrogen can
be purified further in a PSA-plant.
For raw synthesis gas from steam reformers with a higher H2/CO-ratio and
a higher residual methane content, the methane scrubbing for the extraction of
pure CO and crude hydrogen is more suitable than the condensation process.
The process flow diagram is shown in Fig. 5.11. In the first, so-called scrubbing
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Fig. 5.11 Cryogenic methane scrubbing.
(1, 2) Plate fin heat exchanger; (3) Scrubbing column; (4) Hydrogen stripper;
(5) CO/CH4-separation column; (6) Compressor; (7) Expansion turbine.
column (3), the major part of the hydrogen is separated from the raw synthesis
gas, which is cooled in the plate fin heat exchangers (1, 2), by scrubbing out the
other components. Sub-cooled and highly pure, liquid methane is used as solvent,
which is cooled with evaporating CO for the removal of the heat of solution. In the
hydrogen stripper (4), the bottom product of the first column is purified from the
still dissolved hydrogen. The CO/CH4-mixture of the stripper bottom is separated
in the third column (5). The CO-purity at the top of this column is adjusted via the
reflux of sub-cooled pure CO. A refrigeration balance of the process is achieved
by the CO-loop through compressor (6) and expansion turbine (7). Excessive
methane is released at low pressure together with the top product of the hydrogen
stripper and usually used as fuel gas. Hydrogen can be delivered for numerous
applications without further purification.
The energy requirement of cryogenic separation methods is mainly determined
by the energy required by the CO-compressor. There are a lot of variations for both
processes, for example, too much nitrogen in the raw synthesis gas reduces the
CO-purity. In such cases, an additional separation column for N2-separation is
required, although this increases the energy requirement considerably. If besides
methane higher hydrocarbons are found in the raw synthesis gas as well they can
be separated by means of a suitable process arrangement.
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Fig. 5.12 Nitrogen scrubbing.
A technically important process for the production of ammonia synthesis gas
is the nitrogen wash process. The synthesis gas, usually from a rectisol scrubbing
process and mainly consisting of hydrogen, is purified from CO, Ar and CH4.
High-pressure nitrogen, e.g. from the air separator for the PO-unit, is cooled and
expands into the synthesis gas, due to which the “refrigeration” required for the
process is generated [5.30].
The nitrogen serves as solvent for the removal of CO, simultaneously the
H2/N2 ratio of almost 3, required for the ammonia synthesis, is adjusted (see
Fig. 5.12).
5.2.4
Processes for the Production of Synthesis Gas from Hydrocarbons
This section exemplarily describes selected complete processes for the generation
of synthesis gases of different composition.
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5.2.4.1
157
Reformer Plant for the Production of Hydrogen
Figure 5.13 shows the process flow diagram of a modern hydrogen plant with
steam reforming, HT-shift and pressure swing adsorption (PSA) [5.31]. Feedstock
is natural gas that has to be pretreated because of the contained sulphur traces.
Before feeding to the reformer, the natural gas is heated to about 380 °C with a
small quantity of recompressed hydrogen against hot synthesis gas. After the
addition of steam and further heating against flue gas, the hydrocarbon/steam
mixture is fed to the reformer. In the radiation zone, the tubes filled with the
catalyst are heated by burners from outside. Hot gas at about 850 °C leaving the
reformer contains with S/C = 2.7 about 50% H2, 10% CO, 5% CO2, 30% H2O and
5% CH4. After cooling down to about 330 °C, during which high-pressure steam
is generated, the gas passes the adiabatic catalyst bed of the HT-shift reactor. The
outlet flow still contains about 3% residual CO. Under further energy recovery
(preheating of feedstock, preheating of boiler feed water) and the discharge of
waste heat to air and cooling water, the synthesis gas is cooled down to the PSAentrance temperature of about 30–40 °C. In the PSA-plant, high-purity hydrogen
is produced, the PSA-tail gas together with additional natural gas is used for the
firing of the steam reformer. In the radiation zone, about 50% of this heat is
absorbed by the catalyst tubes. The flue gas is routed into the convection zone
at about 1000 °C, where it is cooled down to a stack temperature of about 150°
(feedstock superheating, steam generation and superheating, air preheating).
The steam generated in the plant is partially required as process steam, the rest
is available for export.
Fig. 5.13 Steam-reformer plant for the generation of hydrogen from natural gas.
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Fig. 5.14 Photography of a steam-reformer plant.
Over the last 10–20 years, this type of plant has gained global acceptance,
although there are slight differences in the various processes with regard to energy
integration. Before PSA-plants with efficient H2-yield were available, process
configurations with low-temperature shift, CO2-scrubbing and methanation were
common. For modern hydrogen plants for the application in refineries, however,
hydrogen purities are required that are only achievable by means of pressure swing
adsorption [5.32]. The picture in Fig. 5.14 shows a modern steam reformer plant
for hydrogen generation.
Steam reformer plants are also suitable for the processing of light, liquid
hydrocarbons like naphtha or LPG. The liquid feedstock is evaporated against
cooling process gas and then, together with hydrogen, it is further heated for
desulphurization. For the reforming in the catalyst tubes a special catalyst with
alkaline components, usually potash, has to be chosen. Thereby, soot formation
on the catalyst can be avoided. An alternative to this is the so-called pre-reformer
that generates a pre-reformed mixture of CH4, CO and H2 at temperatures around
450–550 °C in an adiabatic fixed bed reactor from a hydrocarbon/steam mixture.
Since the methane content is still high at these temperatures due to the chemical
equilibrium, the gas is downstream fed to a conventional steam reformer.
5.2.4.2
Reformer Units for the Generation of CO and H2
The block diagram of a H2/CO unit (HyCO unit) is shown in Fig. 5.15.
For the synthesis gas generation unit, the aspects described in the previous
section apply. In contrast to the hydrogen plant, however, carbon dioxide is removed
by means of chemical scrubbing process, then it is compressed and, completely
part of it, fed to the steam reformer. In case additional CO2 is available, a further
reduction of the H2/CO ratio in the raw synthesis gas is possible by means of
CO2 import. This process does not allow the exclusive generation of CO, but H2
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159
Fig. 5.15 Steam-reformer plant for the generation of CO- and H2 from natural gas.
is always generated as a by-product. Thus, the most economical process is always
the production of both gases.
After the scrubbing, the synthesis gas has to be dried in an adsorber station
and purified from CO2-traces. The separation to pure CO and raw hydrogen takes
place in a methane scrubbing, the fuel gas produced there is used for firing of
the reformer. In a PSA-unit, the raw hydrogen is processed to the required purity,
with the tail gas being also used as fuel gas in the reformer. Contrary to the more
simple hydrogen plant shown in Fig. 5.13, the H2/CO plant requires additional
energy for the CO and CO2 compressors. Furthermore, thermal energy from the
synthesis gas to be cooled is required for the regeneration of the solvent, which
results in reduced steam export.
5.2.4.3
PO-plant for the Production of CO and H2
The generation of synthesis gas from heavy oil by means of steam reforming is not
possible. Usually, a PO-process for the gasification of heavy oil is chosen instead.
Figure 5.16 shows the required process steps of a PO plant for the production
of hydrogen and carbon monoxide. As by-product, the rectisol-wash supplies
a H2S-rich gas suitable for being processed to elementary sulphur. Due to the
inevitable soot formation in the PO-reactor, the soot has to be scrubbed out of the
synthesis gas and then be separated from the soot water. In the Texaco-process,
usually naphtha is used for this, which is mixed with a part of the heavy oil and
then evaporated. The soot does not evaporate, together with the heavy oil, it is
fed back to the PO-reactor. In a condensation process, the CO-rich synthesis gas
is separated into pure CO and raw hydrogen, the purity of the product hydrogen
is adjusted in a PSA-unit. In the process described, the tail gas of the PSA is
recompressed and recycled to the scrubbing unit, thus a CO-yield of almost 100%
is achieved. The photo in Fig. 5.17 shows a modern PO-plant for the production
of CO and H2.
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Fig. 5.16 PO-plant for the generation of CO- and H2-generation from heavy oil.
Fig. 5.17 Photography of a PO-plant.
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161
5.3
Process Analytics
A number of process appliances is used for the production monitoring in
synthesis gas plants. Depending on concentration and kind of component to be
examined, they are based on different measuring principles. Analytics support
the control of process steps such as partial oxidation, CO2-scrubbing, adsorption
on molecular sieve and product purities. Apart from the process analyzers, a
number of gas analyzers with electrochemical sensors are applied to ensure that
operating staff is not exposed to explosive methane/H2/air mixtures or increased
O2-concentrations, and that the threshold limit value (TLV) of 30 vppm for CO is
not permanently exceeded. Such gas analyzers are installed at potential leakage
points (e.g. natural gas compressor, reformer, hydrogen compressor). Special safety
measures are to be taken for the analysis room. Via methane and H2-monitoring,
the non-explosion-proof analyzers are switched dead e.g. at a concentration of 50%
of the lower explosion limit (LEL) in the room. Further measures are: Limiting
the feeding of combustible or toxic media into the analysis room by a measuring
orifices; ventilation has to ensure that in case of a line break 50% LEL are not
exceeded. An alarm system outside the analysis room indicates: Failure ventilation,
combustible gas > 20% LEL, CO-alarm, oxygen deficiency, collected faults for gas
alarm system, O2-monitoring and smoke alarm. The safety regulation for analysis
rooms according to DIN EN 61 285 has to be obeyed.
In the following, a typical example of a synthesis gas plant on the basis of
partial oxidation is described. The process analytics of which is specified in
Table 5.4. The feedstock consists of natural gas with methane (87.7% mole
fraction), ethane (6.8% mole fraction), propane (1.5% mole fraction), nitrogen
(1.3% mole fraction), carbon dioxide and hydrogen (1% mole fraction each) as
the main components. The process steps are as follows: After a catalytic sulphur
removal the non-catalytic partial oxidation takes place at 1400 °C generating CO,
CO2, H2, H2O. After a soot scrubbing, a partial flow of the product is directed
from the partial oxidation reactor to the shift reactor, in which the adjustment
of the desired CO/H2-ratio in the synthesis gas takes place. This includes the
conversion of CO with water vapour to CO2 and H2 on a Cu-catalyst. After an
ensuing CO2-scrubbing on methyldiethanolamine basis, the combined synthesis
gas flows reach a membrane separation unit, consisting of a bundle of organic
high-performance capillaries, where the hydrogen is separated as permeate. The
separated hydrogen is fine scrubbed in a pressure swing unit (molecular sieve
filling). The retentate (residual H2, CH4, N2, CO) of the membrane is passed to
the compression as synthesis gas product.
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28.4%
2.2%
51%
0.45%
17.5%
70.5%
24.4%
3%
0.4%
0.4%
CO:
CO2:
H2:
CH4:
H2O:
H 2:
CO2:
CO:
CH4:
N2:
H2:
CO:
CH4:
N2:
CO2:
H2:
N2:
CH4:
CO:
Synthesis gas after
partial oxidation and
soot separator
Synthesis gas after
shift reactor
Synthesis gas after
CO2-scrubbing
Hydrogen 18 bar
Outlet pressure swing
adsorption
99.98%
200 ppm
1 ppm
1 ppm
66.4%
32.2%
0.6%
0.5%
0.07%
Composition of gas mixture1)
Measuring point
Process gas chromatograph:
Flame ionization after
CO-methanization
redundant: infrared
Infrared spectrometry
Process gas chromatograph:
Thermal conductivity
Process gas chromatograph:
Thermal conductivity
Measuring principle
Table 5.4 Process analysis in a synthesis plant based on partial oxidation.
0–50%
0–5%
0–10%
0–70%
0–3%
CO: 0–3 ppm (GC)
CO: 0–10 (IR)
CO2: 0–0.2%
CO: 0–5%
CO:
CO:
CO2:
H2:
CH4:
Measuring range1)
Hydrogen product control
Control
CO2-scrubbing
Adjustment H2/CO-ratio
in the synthesis gas to
membrane separation:
1.5–2.5 (mol/mol)
Monitoring of partial
oxidation
Cause
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
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1)
Conductivity: < 0.2 µS cm–1
Boiler feed water
All concentrations indicated in % molar fraction or ppm molar fraction.
Electrodes
pH-electrodes
pH:
Boiler feed water
9–10
Amperometry
dissolved O2 < 0.1 mg L–1
Boiler feed water
0–0.5%
0–8%
5–10
Cond.: 0–5 µS cm–1
pH:
diss. O2: 0–0.2 mg L–1
O2:
ZrO2-probe
O2: 0%
Composition: variable
Off-gas for combustion
O2:
Paramagnetism
71.5%
16.3%
8.9%
2.5%
N2:
H2O:
CO2:
O2:
Flue gas
0–70%
0–70%
0–3000 ppm
0–0.5 ppm
0–0.5 ppm
0–3%
Measuring range1)
H 2:
CO:
CO2:
COS:
H2S:
CH4:
48.7%
49.4%
0.11%
0.87%
0.7%
0.15%
H2:
CO:
CO2:
CH4:
N2:
Ar:
Synthesis gas 65 bar,
Retentate from
membrane separation
Measuring principle
Process gas chromatograph
S-components: Flame photometry
Other components:
Thermal conductivity
Composition of gas mixture1)
Measuring point
Table 5.4 (continued)
Suppression of corrosion
Suppression of corrosion
Suppression of corrosion
Exclusion of
reverse flow of air
Verification of the
completeness of
combustion
Product control
Synthesis gas
Cause
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
5.4
Applications of Hydrogen, Carbon Monoxide and Syngas
Typical applications of hydrogen, carbon monoxide and synthesis gases are listed
in the following chapter.
In the case of hydrogen they are divided by industry segments (Section 5.4.1).
The use of hydrogen in the chemical industry/refineries is described in more
detail (Section 5.4.1.1).
Encouraged by the rapidly increasing importance of hydrogen as a sustaining
and emission-free energy carrier, Section 5.4.1.2 additionally deals with some aspects of the speedily developing hydrogen technology. The ensuing Section 5.4.1.3
discusses the application of hydrogen in fuel cells.
5.4.1
Applications of Hydrogen
The applications of gaseous hydrogen (GH2) take advantage of its low density,
its high thermal conductivity and/or its reducing/hydrogenating effect. The
application of liquid hydrogen (LH2) is especially determined by its energy density
and the environmentally friendly overall balance regarding recovery, handling
and combustion.
In the processing industry/metallurgy hydrogen is used
x as a welding gas component for TIG-welding (Ar/H2-mixtures)
x as a root shielding gas component for electric arc welding (together with
nitrogen or argon)
x as a shielding gas component for the generation of float glass (in mixture with
N2)
x to provide a shielding atmosphere in the stainless steel manufacturing
x to boost bell-type annealing furnaces for sheet metal coils in cold rolling mills
(reduced heating and cooling phase with H2)
x for high-pressure gas quenching after annealing furnaces for tempering and
hardening (e.g. substitute of oil baths)
In chemistry/refining hydrogen is used
x to gain a synthesis gas for the generation of ammonia (with N2) or methanol
(with CO)
x to hydrogenate higher-boiling oil distillates for the recovery of fuels (see
Section 5.4.1.1)
x to hydrogenate inedible oils to produce soaps, ointments and other special
chemicals
x to hydrogenate and remove sulphur compounds (e.g. desulphurization of fuels)
x to hydrogenate unsaturated compounds and functional groups such as aldehydes, ketones or nitriles
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165
In environmental protection/energy engineering hydrogen is used
x to substitute existing combustion engines by H2-operated drive systems to
reduce exhaust gas pollution
x to develop new LH2/GH2-storage systems for motor cars and busses to supply
the necessary infrastructure (e.g. tanks and filling stations)
x to develop carbon-free power stations and energy supply chains (avoidance of
CO2-emissions and “greenhouse gases”) (see Section 5.4.1.2)
x to operate fuel cells for mobile (“hydrogen-vehicles”) and stationary applications
(e.g. energy emergency supply) (see Section 5.4.1.3)
x to serve as a fuel for missiles/spacecrafts (LH2/LOX-engines)
x to refrigerate high-speed turbines in power stations
In others industries hydrogen is used to
x to hydrogenate unsaturated fatty acids (e.g. production of margarine from
vegetable oils by fat hardening in autoclaves)
x to remove nitrogen compounds from drinking water (e.g. denitration)
x to remove oxygen from cooling water in power stations (to avoid stress cracking
corrosion in piping, boilers and heat exchangers)
x to serve as a buoyancy medium for balloons
x to serve in highest purities as shielding or carrier gas for the manufacturing of
semiconductor components (e.g. ion implantation, epitaxy)
5.4.1.1
Hydrogen Use in the Chemical Industry
Oil refineries are both producers and consumers of hydrogen, the volumes
consumed usually exceeding those internally produced. Hydrogen is needed
as a reactant in hydrogenating processes, as hydro cracking and hydro treating.
Since refineries are shifting their output to larger quantities of low-sulphur, lowaromatics and brighter products, demand for hydrogen will continue to grow.
The main processes for production of hydrogen are:
x Platforming as the main hydrogen producing process in the refining process
x Recovery from refinery gases, e.g. by pressure swing adsorption after platforming step
x Steam reforming of methane as an external hydrogen source
x Gasification of oil refining residues and recovery from synthesis gas
The main processes for the consumption of hydrogen are:
x Hydro-treating
0– 90 Nm3 H2/m3 feed
x Hydro-desulphurization 15–275 Nm3 H2/m3 feed
20–100 Nm3 H2/m3 feed
x Hydro-refining
0–100 Nm3 H2/m3 feed
x Hydro-dealkylation
300–450 Nm3 H2/m3 feed
x Hydro-cracking
Basic Chemicals and Catalyst Production: Hydrogen is a reactant in the manufacture of basic chemicals and intermediates as well as specialty chemicals and
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Fig. 5.18 Hydrogenation of phenol to cyclohexanone.
pharmaceuticals. Synthesis gas, a mixture of hydrogen and carbon monoxide, is
employed above all for synthesis of alkanes, methanol and in the hydroformylation
of olefins to aldehydes and alcohols. Ammonia is manufactured from a different
synthesis gas mixture of hydrogen and nitrogen.
Hydrogenation is generally of high importance and involves the homogeneous
or heterogeneous catalytic addition of hydrogen to organic compounds. Examples
are:
x Hydrogenation of adipic acid dinitrile to hexamethylene diamine
x Hydrogenation of benzene to cyclohexane
x Hydrogenation of phenol to cyclohexanone (see Fig. 5.18)
Industrial hydrogen is also used in the manufacture of catalysts for reducing metal
oxides to the active metallic form and to regulate chain length in the polymerization
of propylene to polypropylene and in the manufacture of polyethylene.
5.4.1.2
Hydrogen as an Energy Carrier
Introduction: The crude oil era cannot last forever, although exactly when it will
end is something no-one can reliably predict at present. However, experts [5.33]
estimate that global oil production is almost certain to peak within the decade
(between 2000 and 2010) and push the fossil fuel supply and demand curve out
of balance as a result (Fig. 5.19). For every barrel of crude oil discovered in new
reserves, four barrels are now consumed. Peak oil production will mark a decisive
turning point in market perceptions surrounding the availability of crude oil [5.34].
For the first time, depletion of fossil fuel reserves will start to have a direct impact
on national economies, with increasing global energy demands inevitably also
inflating oil prices, as is already the case. Particularly given that petrol and diesel
engines alone consume half of today’s entire crude oil production, researchers
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Fig. 5.19 Global oil production will probably exceed its vertex in this decade.
This is one of the significant reasons, to look for alternative energy sources.
(Source: Jolin J. Campbell and Jean H. Laherrère, “The End of Cheap Oil”,
Scientific American, March 1998).
and developers worldwide are searching for suitable energy carriers and sources
to ensure a sustainable supply of energy for automotive transport. And since
global markets will continue to require enhanced mobility for people, goods and
services, tomorrow’s energy sources must also be expandable.
Increased mobility has also exacerbated the CO2 problem (see Section 6.1.1).
Experts meanwhile agree that our use of fossil fuels is a significant factor in climate
change, if not the main cause [5.35–5.37]. In view of these facts, the spotlight is
increasingly turning to research and technology players to find new, sustainable
energy carriers and sources (wind, water, sun, biomass and biogas). The most
promising energy carrier of the future is currently hydrogen.
Hydrogen and Mobility: Hydrogen is not a source of energy, but an energy carrier
that has to be produced – similar to electricity. However, it has a significant
advantage over the latter: hydrogen stores well. This means it is an ideal way to
store electricity. However, its real potential lies in mobile applications. It is the
only sustainable, zero-emission fuel source capable of powering an unlimited
number of vehicles and other applications. Hydrogen is also able to absorb a
local energy surplus and release it in a different place on demand. This enables a
reliable, constant flow of energy – even when drawing from energy sources that
are not continuously available. As an energy carrier, H2 could therefore play an
important role both medium and long term in securing our mobility. Because it is
a zero-emission fuel, it can power cars, aeroplanes, buses and ships, for instance,
with water as the only waste product. Since transport accounts for a major part of
our energy consumption and draws almost exclusively on fossil fuels, this is the
most significant potential application of this new energy carrier.
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Fig. 5.20 The carbon share of energy carriers has decreased continuously
over the course of history. The ultimate goal is to completely do without
carbon and abandon CO2 emissions as a consequence.
Feasibility: Any new, cutting-edge energy carrier can only be successful if it fulfils
certain criteria. For instance, it must reduce CO2 emissions throughout the
entire energy chain. With hydrogen, this can be achieved by generating it either
from sustainable energy sources or from fossil fuels (coal, oil, natural gas) with
CO2 capture and storage [5.38]. Figure 5.20 shows the carbon-hydrogen ratio of
various typical hydrocarbons. Energy carriers of the future must also guarantee
unrestricted availability.
Only traces of free hydrogen are present in the lower part of earth’s atmosphere, whereas chemically bound hydrogen as a component of water and other
compounds is widely distributed. In fact, on the average, every sixth atom in the
earth’s crust (including the hydrosphere and atmosphere) is a hydrogen atom [5.39]
Releasing it from these compounds requires energy. For decades hydrogen has
been a key element in chemical industry. The technologies required to produce
hydrogen are already available and widely used in industry (see Section 5.2). When
implementing hydrogen applications for use by the general public, the challenge
particularly lays in the future energy source, rather than in the development of
the energy transport means (read hydrogen or electricity).
Diverse Supply Models: Although there are many possible hydrogen supply
routes from today’s perspective there will not be one ideal route for all purposes.
Rather than following a one-size-fits-all production path for all applications, a far
more varied strategy is called for here. The main issue is diversification of supply.
Initially, fossil fuels will continue to be the main source of energy production
for electricity or hydrogen. However, in the medium and long term, the use of
sustainable energy sources such as wind and hydropower, biomass and solar
energy will account for a growing share of global energy and subsequent hydrogen
production until they finally supplant fossil sources (Fig. 5.21).
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Fig. 5.21 Overview of hydrogen infrastructure for automotive application:
Several production and supply paths are conceivable; particularly a broad diversification of production is the basis of a meaningful hydrogen infrastructure [5.55].
The best hydrogen production process must be selected on an individual basis
and respectively take the application, location and region into account. Typical
selection criteria include production capacity, availability of raw materials, demand
patterns, product purity and pressure, energy and operating costs, investment costs
and the value of by-products. Factors such as plant reliability or expandability as
demand rises may also play a decisive role. Analysis of emissions over the entire
energy chain (well-to-wheel analysis) [5.40, 5.41] shows that certain methods can
already reduce emissions by using natural gas in hydrogen production. Ultimately,
however, it is only methods that use sustainable energy sources or fossil fuels
with CO2 capture and storage that are able to meet long-term CO2 reduction
targets. Some supply paths may show higher or equivalent greenhouse gas (GHG)
emissions than the current petrol or diesel-driven options. These paths would
therefore only be acceptable for a relatively short transitional period. The idea is
to leverage “low hanging fruit” approaches to push technological advances and
drive down costs. The main conventional methods of hydrogen production are
steam reforming, partial oxidation (POX) and electrolysis (see Section 5.2). The
most economical method of hydrogen production is currently steam reforming
of natural gas [5.42, 5.43]. CO2 generation is unavoidable in both first mentioned
processes. The raw material used (fossil or regenerative) determines whether the
CO2 produced results in a net increase in the atmosphere. Electrolysis has been
in commercial use for over eighty years in its conventional form. It provides an
obvious way to capitalize on regenerative energy sources (i.e. electricity generated
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
with regenerative sources). Electrolysis will therefore gain in importance in the
long term.
Sustainability: Hydrogen production from sustainable energy sources (sun,
wind, water, and biomass) is an almost perfect cycle, both sustainable and
environmentally friendly. The current obstacle is that installed capacities of energy
collection from sustainable sources are not sufficient to meet total global energy
demand cost effectively. However, hydrogen production from sustainable energy
is already cost-effective at locations that meet certain preconditions. A well-known
example of this is the plan to build a hydrogen economy in Iceland by 2030. On one
hand, geothermal springs and hydropower mean sustainable energy is available
in abundance, and, on the other, importing crude oil is particularly expensive
for such a remote island. Construction, maintenance and operation of the grid
networks currently accounts for a large proportion of electricity costs, for instance
[5.44]. These costs could be reduced with distributed, sustainable facilities with
hydrogen as the storage medium.
Supply Chain and Transport: To successfully build a hydrogen infrastructure,
existing resources such as roads, natural gas pipelines and electricity grids must
be integrated in the new infrastructure concept [5.45]. There are two basic options
for distributing hydrogen to customers via a fuelling station network: the hydrogen
can either be directly produced at fuelling stations or concentrated in large-scale
plants and then transported to the fuelling stations. Taking these two general
distribution paths as the departure point, there are currently five hydrogen supply
paths under consideration:
x Hydrogen is produced from natural gas in concentrated large scale, reforming
plants (the same principle would also work biogas-, or biomass-based) and then
transported as a liquid or as a compressed gas via the road to the H2 fuelling
stations.
x Hydrogen is produced in concentrated large scale, reforming plants attached
to a gas pipeline network. It is then transported by pipeline to the H2 fuelling
stations.
x Hydrogen is produced directly at the fuelling station using small scale gas
reforming facilities. It is then compressed before distribution.
x Hydrogen is produced directly at the fuelling station using small scale electrolysers and compressed before distribution.
x Hydrogen for early, short-term applications at fuelling stations can be supplied
from an existing nearby H2 pipeline system serving refineries and chemical
parks (after having been purified).
All these supply paths must be weighed up in terms of overall energy consumption,
emissions and cost. Producing hydrogen via electrolysis has the advantage that the
electricity can be generated from almost all other energy sources, so it allows for
a high degree of diversification across energy sources. Large scale concentrated
hydrogen production is well suited to refinery locations, where steam reformers
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usually already produce industrial quantities of hydrogen for desulphurising
fuels and hydrogenating crude oil fractions. Reformers could also be installed at
locations with high demand such as urban centres, keeping transport distances
short. Liquid hydrogen is transported to the fuelling stations in trailers or superinsulated tankers such as have been in daily use for many years. This is the
most cost-effective option for distributing high quantities of hydrogen, although
it requires significant investment. The technology for hydrogen liquefaction is
already established (see Section 5.2.3.6). Looking beyond liquefaction, industrialscale compressed gaseous hydrogen can also be distributed via a pipeline network
similar to that currently used for natural gas. The disadvantage of this path is a
loss of flexibility. Liquid hydrogen can be easily turned into gaseous hydrogen at
the station, but the reverse operation is currently only economically feasible on
an industrial scale.
Storage plays a key role in the hydrogen supply chain. Hydrogen is an extremely
light gas and its low density poses technical challenges to its adoption as a fuel.
At normal pressure, three thousand litres of gaseous hydrogen contain the same
amount of energy as one litre of petrol, for example. To store and transport the
gas effectively, it therefore has to be highly compressed. So it is either compressed
(CGH2, compressed gaseous hydrogen) or cooled down to –253 °C so it liquefies
(LH2, liquid hydrogen). In terms of mass (gravimetric energy density), LH2 has
an outstanding storage value of 120 MJ/kg (petrol: 44 MJ/kg), but in terms of
volume (volumetric energy density) it performs less well at 9 MJ/L (petrol: 33 MJ/L)
Fig. 5.22 Important elements of hydrogen infrastructure:
Transport vehicles for compressed gaseous and liquid hydrogen.
To transport an equal amount of hydrogen gaseous transport vehicles
have to travel up to seven times more frequent [5.55].
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
(see Section 5.1.3.2). This means that a car would either have to be fitted with a
relatively large fuel tank to cover the same distance or it would have to refuel at
much shorter intervals. Whether future vehicles will consume more or less energy
than today’s vehicles depends very strongly on the type of drive train. With fuel
cell electric drive trains the best fuel economy is possible [5.41].
The mobile storage challenge is best exemplified by a transport comparison:
around six times more hydrogen – and therefore energy – can be transported by
lorry trailer when the hydrogen is in liquid form. Figure 5.22 shows the different
hydrogen transport capacities of two 40-tonne trailers with a net payload of
approximately 500 kg (gas) and 3500 kg (liquid). In practice the differences are
even larger, since the pressure differential with the customers’ stationary hydrogen
tanks mean gas tankers cannot be completely emptied, but have to return for
refilling with a certain residual pressure. The higher investments, personnel
expenses and traffic volume involved in storing gaseous hydrogen must also be
factored into economic viability assessments.
Fuelling: The cost-effectiveness of different delivery systems must be taken into
account when assessing fuelling, since for example the costs of distributing
compressed hydrogen are higher than those of liquid hydrogen. Compressed
hydrogen tanks make sense wherever volume and weight are not key factors,
for example storage facilities on industrial premises. When considering storage
systems for liquid hydrogen evaporation rates have to be taken into account. The
latter is inevitable – despite super-insulation – a small portion of liquid hydrogen
always gasifies inside the storage vessel due to the impact of external temperature.
When, after some time, the maximum pressure is reached, hydrogen must be
released to reduce pressure inside the storage vessel (boil-off gas). It is therefore
important to develop vessels whose boil-off rates are as low as possible. The boiloff effect is heavily dependent on the volume to surface ratio of the vessel [5.46].
For example a 10 000 litres storage vessel used at a refuelling station can have
a value of 0.1 vol.% per day after two weeks; an 80 litres passenger car tank can
have a value of 3 vol.% per day after 4 days [5.47].
Significant progress has been made in the development of liquid hydrogen
storage systems for vehicles. Linde AG has developed an innovative tank system,
for example, which significantly extends the window before the maximum
pressure is reached and hydrogen has to be blown off because of the tank design
[5.47]. However, pressure has only to be released if the vehicle is not driven for
an extended period. As soon as the vehicle is started, hydrogen is consumed and
the tank pressure drops, so the boil-off clock is reset. Hence for fleet vehicles
in constant use, for example, boil-off is rarely an issue. Alternatively, fuel cells
could be used to easily convert evaporated hydrogen into electricity, thus avoiding
pressure build-up if a vehicle is not driven for a long time.
Alternative hydrogen storage methods such as chemical bonding in alanates
and metal hydrides or graphite storage based on nanotechnology are still in
development and currently nowhere near the efficiency of liquid and gaseous
hydrogen storage.
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Safety: Particularly when it comes to personal mobility, the safety of a new energy
carrier has to match that of established fuels, mainly petrol. The light hydrogen
molecules volatilize very rapidly in the atmosphere which is a major advantage.
Petrol evaporates more slowly and is heavier than air, so stays on the ground
longer, where it is most likely to ignite. They also burn differently. As a liquid,
petrol can leak and spread on the ground, burning with a wide flame that radiates
very strong heat. Hydrogen, on the other hand, originating from the safety valve,
burns with a narrow, almost upright flame that emits little heat [5.48]. However,
in contrast to bright petrol flames, hydrogen flames are barely visible in daylight.
The absence of carbon ensures soot-free combustion when burning hydrogen
only producing water vapour. Hydrogen thus fares well when its combustion
properties are compared with those of the energy carriers commonly used today
(see Section 5.1.3).
We can already draw on around thirty years’ experience surrounding the safety
of gases in cars and the supporting infrastructure. This shows that liquid gas
vehicles do not present any increased risk as long as the regulations in force are
adhered to. Recent testing, for instance by TÜV SÜD [5.49], confirm that this
also applies to hydrogen.
In all hydrogen systems, safety concerns focus on parts that gas flows around
or through, such as containers, pipelines, fuelling nozzles and valves. Hydrogen
storage is therefore already subject to an almost seamless series of quality assurance measures, covering construction, manufacture and expert inspections.
The properties of metallic equipment are influenced by hydrogen that entered
the metallic matrix. Some metallic materials are sensitive to degradation mechanisms caused by the latter. In order to prevent damages on steel pressure equipment induced by gaseous hydrogen, material selection, design and fabrication of
such equipment should be thoroughly assessed [5.50, 5.51].
Strict safety regulations apply to both the on-board components and to the supply
infrastructure. Extensive experience is also available in this area, since natural
gas, liquefied petroleum gas (LPG) and hydrogen have already seen decades of
industrial use for power generation and as fuel. Safety measures and guidelines
for novel mobile applications of hydrogen technology are being developed and
to some extent already in place [5.52], and experience shows hydrogen is at least
as safe to use as a vehicle fuel as conventional options such as petrol and diesel
[5.48, 5.49].
Costs: Alongside availability and greenhouse gas (GHG) related issues, cost is
a decisive factor governing the viability of a new energy carrier. Every well-towheel-analysis must include the given requirements concerning local, regional or
long-distance conversion steps and transport options. Given that some hydrogen
production paths include the CO2 emissions at some location along the path it is
evident that the evaluation should extend beyond the fuelling station to include
the wider picture. As a rule, low-emission and, especially GHG-free-energy
carriers will entail higher costs. However, it should also be noted that fossil fuels
in general are also becoming more expensive in the medium term and the cost
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
gap between regenerative and fossil fuels is closing. All evaluations must also
take account of the usage efficiency profile. In the case of vehicles, for example,
this includes efficient use of hydrogen thanks to hydrogen-optimised internal
combustion engines or fuel-cell systems (see Section 5.4.1.3). General cost analyses
[5.53–5.56] show that:
x Viewed across the entire supply chain, hydrogen liquefaction expenses are
partially compensated by lower transport costs compared with the compressed
hydrogen option.
x Hydrogen management at the fuelling station determines the costs of fuelling
with compressed or liquid hydrogen.
x Local supply is influenced by yet unclear unit costs, and current electricity and
natural gas prices.
x In the long-term, pipeline delivery will only enable low hydrogen supply costs
if there is a high substitution rate.
A detailed examination of hydrogen supply costs at fuelling stations [5.55] shows
that a hydrogen infrastructure can only be established successfully if it is integrated
in the current filling station infrastructure. The report concludes that central
(large scale and concentrated) hydrogen production and liquefaction followed by
liquid hydrogen (LH2) delivery to the fuelling stations in cryogenic containers is
the most cost-efficient option, both for liquid and compressed hydrogen (CGH2)
fuelling (Fig. 5.23).
Fig. 5.23 Logistics of hydrogen can be arranged according to the supply
concept of conventional fuel; additional to conventional fuels, refuelling
stations are supplied with liquid hydrogen and are able to supply both
liquid-, and compressed hydrogen vehicles.
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Fig. 5.24 In order to allow for long distance travel, up to the year 2020
approximately 2800 hydrogen dispensing refuelling stations would be
needed in various European conurbations. Until 2030 the network would
need to be continuously extended [5.55].
The costs of a comprehensive European infrastructure of hydrogen fuelling
stations, including production and distribution, were determined for the first
time in a study by E4tech [5.57]. This clearly demonstrates that the costs in real
terms depend on the strategy and technical parameters adopted in developing
the network. The study concludes that expansion of the fuelling network should
concentrate on a few major European urban areas, ideally in countries such as
France, Germany and the United Kingdom. Figure 5.24 shows how the supply
network in these countries would have to be expanded to support long-distance
travel in the future.
Applications: Engineers in all the major automobile companies are working on
drive-train concepts for hydrogen-powered vehicles. They are currently pursuing
two main strategies to advance hydrogen technology – fuel cells and internal
combustion engines. Developers now have around six hundred vehicles on the
road worldwide. Some vehicle manufacturers are primarily focusing on the
conventional spark ignited internal combustion engine, which is relatively simple
to modify so that it can also operate on hydrogen. This strategy benefits from
the technically evolved status of internal combustion engines, which hydrogenfuelled engines can of course draw on, as well as the possibility of transitional
options until the hydrogen fuelling station network is dense enough to ensure
long-distance travel. A car with a hydrogen combustion engine can switch to
petrol when the hydrogen tank is empty. However, viewed long-term, everything
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
points to intensive use of fuel cell technologies (see Section 5.4.1.3). Fuel cell
development is therefore also regarded as a key success factor in the widespread
use of hydrogen in transport. In stationary usage, fuel cells could also operate
heating systems and mini power plants, for instance.
Looking beyond cars, there are numerous other potential uses of hydrogen as
an alternative vehicle fuel, with hydrogen-powered buses, commercial vehicles
and forklifts already operational. The first H2-propelled submarines are also in
active use. Hydrogen in submarines is stored in metal hydrides, since the relatively
high weight of hydride storage units is not a disadvantage. Hydrogen-powered
fuel cells are also silent, and low temperature operated, helping submarines to
remain undetected.
The aviation industry is also refining concepts to implement hydrogen as
an energy carrier [5.58]. Since weight reduction is essential in aviation, liquid
hydrogen is the only viable option for hydrogen-fuelled aviation. A European
project designed an airport infrastructure concept in which large quantities of
liquid hydrogen are produced directly on-site. A decade ago a wide-ranging industry
consortium, including Linde, was already involved in the German development of
the Sänger horizontal take-off space transportation system, fuelled by a mixture of
liquid and solid hydrogen (slush hydrogen), since this has an even higher density
than liquid hydrogen [5.59]. There are also designs for the use of hydrogen to
fuel the on-board electrical system of conventional aircrafts, operated by an APU
(auxiliary power unit).
Essentially, hydrogen is potentially suitable for all applications that currently run
on batteries and require minimal recharging times whilst offering a maximum of
power output and operating hours. Hydrogen is expected to grow in popularity for
portable applications: as a replacement for batteries and accumulators in handheld electrical appliances and as portable power generators. Micro fuel cells are
viewed as alternative energy suppliers for electronic devices such as notebooks,
torches, cameras or pocket computers, for instance. These devices currently require
between 5 and 15-watt electrical power. Two economic aspects make fuels highly
attractive in this area – the high volume sales and high proceeds that energy
carriers can generate here. Annual European demand is estimated at over half a
million fuel cells, and due to high battery costs, strong manufacturer loyalty and
the significant consumer benefit, this market segment is also prepared to pay a
price premium per kilowatt hour.
Portable power sets appear to open up another promising niche application
for hydrogen technologies and fuel cells. They are used as portable generators,
emergency power backup’s, uninterruptible power supplies, drive units for
special vehicles and fixed generators for aeroplanes, boats and mobile homes.
These applications are much closer technically to the requirements posed by road
transport, for example, but standards here with regard to performance, lifetime
and costs are far less strict. These niche markets could therefore help to drive
the move towards mass production of fuel cells and with that the widespread
implementation of hydrogen technologies.
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5.4.1.3
177
Fuel Cells
The Function of Fuel Cells: In future energy supply scenarios fuel cells will play
an important role. Fuel cell systems are more environmentally friendly than
conventional systems and their size can be adapted to the market requirements,
ranging from fuel cells used to drive cars to those for electricity generation in
power plants.
Fuel cells directly convert chemical into electrical energy – without noise emission or open combustion [5.60]. Fuel cells use hydrogen or hydrogen-containing
gases as fuel and pure oxygen or air as oxidant. In contrast to the detonating
oxy-hydrogen reaction, which releases thermal energy by way of explosion, the
electrochemical reactions in a fuel cell release energy in the form of electrical
current and only in minor amounts as heat. In an ideal case only pure water
is formed as the reaction product. In order to control the generation of energy,
the gases are separately converted on catalyst layers, which function as anodes
and cathodes. Hydrogen (H2) is chemically oxidized at the anode. The electrons
released in this process flow through the external circuit to perform electrical
work before they reduce oxygen molecules to oxonium anions (O2–) at the cathode.
Hydronium ions (protons, H+), which are produced by the oxidation of hydrogen,
move through the gas-impermeable electrolyte and recombine with oxonium
anions to form water. Figure 5.25 shows the structure of a polymer electrolyte
membrane fuel cell (PEMFC), the fuel cell with the highest perspective for use
in car engines.
Oxidation reaction at the anode:
2 H2 o 4 H+ + 4 e–
Reduction reaction at the cathode:
O2 + 4 H+ + 4 e– o 2 H2O
Fig. 5.25 Schematic of the structure of a PEMFC.
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Fig. 5.26 Schematic of Grove’s original gas battery apparatus [5.61].
Table 5.5 Development steps of fuel-cell technology.
Year
Development
1800
Johannes v. Ritter produces hydrogen and oxygen through the electrolysis of a sulfuric
acid solution
1839
Sir W. Grove recognizes the reversibility of Ritter’s experiments and constructs the first
prototype of a fuel cell (see Fig. 5.26 and [5.61])
1889
Ludwig Mond and Charles Langer construct a fuel cell using air and coal gas, on the
basis of Grove’s experiments
1894
William Ostwald points out the relevance of Grove’s discovery in a discourse
1939
Francis T. Bacon constructs the first fuel cell with an alkaline electrolyte
(200 °C hot caustic potash solution and pressurized hydrogen).
1950s Siemens and Varta investigate AFC technology
1960s The PEMFC is tested as an energy source in NASAs Gemini space travel program
1960s Use of alkaline fuel cells in the Apollo space travel program
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1982
Submarines with fuel cells
1994
The first cogeneration power plant on phosphoric acid fuel cell (PAFC)-basis comes
into service in Hamburg
1997
Daimler-Benz introduces a public-service bus (new electric bus, NEBUS) with fuel
cells
1997
Start of the NECAR (new electric car) model ranges as an experimental platform for
automobile applications
2000
Introduction of a go-cart operated by direct methanol fuel cell (DMFC)
2002
Crossing the USA with a NECAR 5 from Daimler-Chrysler
2003
Daimler-Chrysler, Ford Honda, Nissan and Toyota announce first demonstration fleets
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179
History: The working principal of the fuel cell is known since the 19th century:
in 1839 the physicist Sir William Robert Grove discovered the reversibility of the
electrolysis of water at a platinum wire surface (Fig. 5.26).
In the following years the development of fuel cells was mainly performed by
scientific and research institutions. Due to the high costs the applications were
limited to niche markets as military applications in submarines. For the Apollo
space program the fuel cell was also used. An overview of the most important steps
of the development of fuel-cell technology is presented in Table 5.5 [5.63].
Growing environmental awareness in the late 20th century and the large increase
in environmental pollution due to emission of waste gases of all kinds resulted
in strongly growing interest in the low-emission fuel-cell technology. Efforts to
improve air quality led to the introduction of the Clean Air Act in California in 1988.
This law demanded a set fraction of emission-free automobiles (zero emission
vehicles) to be in operation from 2004 onward. This put the automobile industry
under strong pressure to develop clean driving techniques. Different prototypes
of fuel-cell vehicles have been developed in the past 10 years [5.62].
Types of Fuel Cells: All fuel cells consist of a gas-impermeable ion conductor as
the electrolyte, which is catalytically coated on both sides or which has catalytic
characteristics itself. This ion conductor separates the anodic and cathodic spaces
of the electrochemical cell from each other. Fuel-cell systems and their components
must meet different requirements, depending on the desired electrical output.
Fuel cells can cover a range from 0.1 W to 30 kW power plants.
Depending on the working temperature, which can range from room temperature to 1000 °C and depending on the fuel cell type, different fuels can be used like
hydrogen, alcohols or hydrocarbons (e.g. natural gas, gasoline, diesel fuel) used
as fuel and air or oxygen as oxidant. Table 5.6 summarizes different operating
conditions, the catalysts used, and application areas [5.64].
Applications for Hydrogen: The role of hydrogen as an energy carrier has been
described in Section 5.4.1.2. In the following the main applications for fuel cells
and their demand in the hydrogen quality are shortly described. The main focus in
recent fuel cell development is the use for propulsion. All major car manufacturers
are investing in this technique. For fuel cell driven cars high power densities
are required as 100 kW have to be installed within a passenger car. One other
requirement is the fast start up time and the short reaction time for load changes.
This is only fulfilled by a PEM fuel cell. As the PEMFC operates at temperatures
around 100 °C the start up time is rather short. On the other hand the low operating
temperature causes very high demand for the hydrogen quality. As the catalysts
for hydrogen oxidation in PEMFC are Platinum based all substances adsorbing
strongly on Platinum are a potential poison for the fuel cell. Especially Carbon
Monoxide (CO) has to be removed from the feed gas. Therefore the hydrogen has
to be of high purity with a maximum CO content of 1 ppmv. Furthermore NH3 is
a strong poison for the electrocatalysts. Therefore a car application demands the
highest grade of hydrogen which is stored on board.
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
Table 5.6 Different operating conditions of fuel cell types.
AFC
PEMFC
PAFC
MCFC
SOFC
Operation
temperature
[°C]
60–90
80–110
160–200
600–800
800–1000
new development:
500–600
Fuel
H2
(high purity)
H2
(high purity)
H2
natural gas
H2
natural gas
H2
natural gas,
LPG
Oxidation
O2
(high purity)
O2, Air
Air
Air
Air
Anode catalyst
(example)
Raney-Ni;
Pt/C
Pt/C;
Pt–Ru/C
Pt/C
Ni/Al;
Ni/Cr
Ni
Electrolyte
KOH
sulfonated
PTFE
phosphoric
acid
Li/Na
carbonate
Y–ZrO2
Cathode
catalyst
Raney Ni
Pt/C
Pt/C;
Pt–Co–Cr/C
NiO
La–Sr–MnO3
Application
Space craft
mobile,
portable;
automotive
stationary
stationary
stationary
AFC:
PEMFC:
PAFC:
MCFC:
SOFC:
Alkaline Fuel Cell
Polymer Electrolyte Fuel Cell
Phosphoric Acid Fuel Cell
Molten Carbonate Fuel Cell
Solid Oxide Fuel Cell
Another application of great interest is the use of fuel cells in electronic devices.
For laptops or mobile phones the energy density of hydrogen is too small. Therefore
the development concentrates on a special form of the PEMFC the Direct Methanol
fuel cell (DMFC) in which the fuel is converted within the electrocatalyst.
Further applications for fuel cells are auxiliary power units. These devices
generate additionally electric power for instance in trucks or campers. For these
applications the fuels already used should be applied. This means the feed gas
has to be reformed on site. As mentioned above the quality demands for PEMFC
are very high and therefore the fuel preparation is complex. For this application
the SOFC is more appropriate. Due to the high temperature of the SOFC not only
hydrogen can be converted but also light hydrocarbons and carbon monoxide.
Therefore only one catalytic step prior to the fuel cell is needed which converts
higher hydrocarbons to hydrogen, CO and short chain hydrocarbons. As the anode
catalyst is Ni based and the metal is in the reduced state an oxygen contact at the
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5.4 Applications
181
operating temperatures leads to formation of NiO in an exothermic reaction. This
destroys the anode catalyst. Therefore even traces of oxygen have to avoided in
the feed. Other poisons for the SOFC are higher hydrocarbons leading to coking
as well as sulphur compounds.
Another application for fuel cells is combined heat and power (CHP) generation
devices. The choice of the fuel cell type depends on the desired power generation.
For small decentralized units in the range up to 3 kWel PEMFC and SOFC are in the
focus of development. The higher temperature of the SOFC makes a SOFC more
efficient. On the other hand the applied temperatures cause material problems.
The PEMFC requires the highest gas quality what is reflected in a more complex
reforming system. The common feed for the FC system is natural gas. One residue
in the reforming process is Methane which is inert to the PEMFC electrode and
can be used as heating gas in the off gas burner. In case of the SOFC the not
converted methane can be converted itself in the SOFC.
For larger power generation units (up to 100 kWel) PAFC and MOFC are an
attractive alternative. The gas quality requirements for the PAFC are not as strict
as for the PEMFC. The electrocatalyst of the PAFC is Pt based. Due to the higher
operating temperature the PAFC can tolerate higher CO concentration compared
to the PEMFC. CO concentrations up to a few 100 ppmv CO do not affect the
performance of the FC. One major disadvantage of the PAFC is the use of a highly
corrosive electrolyte (Phosphoric Acid) which can damage the materials. Also the
liquid electrolyte leaks out of the matrix in which it is embedded with operating
time. Even higher temperatures are used for the MCFC. This type of fuel cell can
convert also CO and light hydrocarbons directly in the FC. Often a pre-reforming
is used to produce a mixture of hydrogen, CO and methane out of natural gas
or even liquid fuels. As the catalyst is Ni based like in the case of SOFC small
amounts of Oxygen can lead to major damage of the catalysts. Also sulphur in
form of H2S has to be removed prior to the FC as the catalyst is poisoned strongly
by it. A major problem of the MCFC technique is the aggressive conditions of the
operation. The electrolyte is a molten carbonate and it has to be prevented that
this corrosive material leaks out the FC unit.
Resume: The required quality of hydrogen for fuel cells strongly depends on the
type of fuel cell used. In terms of CO content the general rule is: the lower the
working temperature the lower has to be the content of CO in the hydrogen. For
fuel cells for propulsion with the highest potential to become a mass market the
hydrogen should contain less than 1 ppmv CO.
A second catalyst poison for all fuel cell types is sulphur. Sulphur in form of H2S
strongly bonds to all anode catalysts and blocks active sites for oxidation reactions.
Sulphur requires even lower maximum concentrations (< 0.5 ppmv) than CO.
For all FC types there are other contaminants which differ from type to type.
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5 Hydrogen and Carbon Monoxide: Synthesis Gases
5.4.2
Applications of Carbon Monoxide
It is used as:
x Carbon donor in the heat treatment of metals (e.g. carbonisation of iron and
steel)
x Basic compound for the organic chemistry (e.g. production of higher alcohols,
aldehydes, carbon acids)
x Reactant in the inorganic chemistry (e.g. production of phosgene, metal
carbonyls)
5.4.3
Applications of Synthesis Gas (Mixtures of CO and H2)
It is used as:
x Raw material for the methanol synthesis
x Raw material for the hydrocarbon/fuel-production (Fischer–Tropsch synthesis)
x Raw material for the formation of aldehydes and alcohols from olefins (oxosynthesis)
x Reduction gas for the production of metals from oxides or ores (in special
furnaces)
x Heat treatment gas for neutral annealing or carbonisation of iron and steel
(e.g. on site production in gas generators starting from hydrocarbons or by
cracking of CH3OH)
x Fuel for the generation of electricity in power stations
References
[5.1] M. Geitel: Das Wassergas und seine Verwendung in der Technik, 3rd edition, Siemens,
Berlin, 1900, p. 4.
[5.2] F. Ullmann: Enzyklopädie der technischen Chemie, Vol. 11, Urban & Schwarzenberg,
Berlin, 1922, p. 608 ff.
[5.3] Georges Patart, French Patent 540, p. 343 (19/08/1921, published 12/07/1922).
[5.4] F. Fischer, H. Tropsch: Ber. Dtsch. Chem. Ges. 1926, 59, 830–832.
[5.5] C. D. Frohning, H. Kölbel, M. Ralek, W. Rottig, F. Schnur, H. Schulz in J. Falbe (Ed.):
Chemierohstoffe aus Kohle, Georg Thieme Verlag, Stuttgart, 1977.
[5.6] Hollemann-Wiberg, Lehrbuch der Anorganischen Chemie, 101st edition, W. de Gruyter,
Berlin, 1995, p. 249 ff.
[5.7] Römpp, 10th edition, Keyword: Wasserstoff, Georg Thieme Verlag, Stuttgart, 1996.
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of Standards, NASA Technical Reports, NASA SP. 3089, January 1975.
[5.9] Ullmann’s, 5th edition, Vol. A 13, p. 299 ff., VCH, Weinheim, 1989.
[5.10] F. Ullmann: Enzyklopädie der technischen Chemie, Vol. 7, Urban & Schwarzenberg,
Berlin, 1919, p. 33 ff.
[5.11] Hollemann-Wiberg, Lehrbuch der Anorganischen Chemie, 101st edition, W. de Gruyter,
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[5.13] G. Sorbe: Sicherheitstechnische Kenndaten chem. Stoffe, 41. Erg. Lfg., 9/94; Handbook of
Compressed Gases, 3th ed, Compr. Gas. Assoc., Van Nostrand Reinhold, New York, 1989.
[5.14] Kirk-Othmer, 4th edition, 5, Volume 5, p. 97–101.
[5.15] L’Air Liquide: Encyclopedie des Gaz, Elsevier, Amsterdam, 1976.
[5.16] Ullmann’s, 5th edition, A13, p. 297 ff., VCH, Weinheim, 1989.
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Technologies, Nitrogen, British Sulphur Publishing, 1997, ISBN 1-873387-26-1.
[5.19] T. Dreier, U. Wagner: Perspektiven einer Wasserstoff-Energiewirtschaft. Brennstoff
Wärme Kraft 2000, 52(12), 41–46.
[5.20] C. E. G. Padro: Back to the Future. American Chemical Society 1999, 44(2), 235–239.
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2002, 6(4), 150–159.
[5.22] K. Andreassen: Hydrogen Production by Electrolysis. Hydrogen Power 1998, 91–102.
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DGMK Conference Proceedings 2000, 2, 31–24.
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1999, 78(4), 87–93.
[5.25] G. Bourbonneux: Hydrogen Production, in Petroleum Refining, 2001, Vol. 3.
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Production. Hydrocarbon Eng. 1998, 3(1), 56–65.
[5.27] M. V. Twigg: ICI Catalyst Handbook, 2nd edition, Manson Publishing, London, 1996.
[5.28] M. Lembeck: LAC – The Linde Ammonia Concept. Linde-Report on Science and
Technology 1995, 55.
[5.29] W. Baade, K. Cambell, U. Parekh: 50 Years of Continuous Innovation in Hydrogen.
Hydrocarbon Eng. 2002, 7(8), 37–39.
[5.30] R. Fabian, W. Förg: Modern Liquid Nitrogen Wash Process for the Purification of NH3
Synthesis Gas at High Pressure. Linde-Report on Science and Technology 1975, 22.
[5.31] B. Kandziora: Hydrogen Technology – Advanced Technologies. Hydrocarbon Eng. 2002,
7(8), 47–50.
[5.32] D. Steen, A. Zagoria: Hydrogen Technology – A Uniform Approach. Hydrocarbon Eng.
2002, 7(8), 40–45.
[5.33] C. J. Campbell: Peak Oil – A Turning Point for Mankind, Presentation at the Technical
University of Clausthal, Germany, December 2000.
[5.34] J. Schindler, W. Zittel (Ludwig Bölkow Systemtechnik): Written statement on the
public hearing of experts by the Enquete Commission of the German Bundestag
“Nachhaltige Energieversorgung unter den Bedingungen der Globalisierung und der
Liberalisierung” on the topic “Weltweite Entwicklung der Energienachfrage und der
Ressourcenverfügbarkeit”, October 2000.
[5.35] Norbert Strohschen (Gerling Global Reinsurance Company): Climate Change and
Environmental Protection – a Global Interest for Insurers and Reinsurers, Paper
presented at HYFORUM 2000.
[5.36] Thomas Loster: Getting to the Root of Natural Catastrophes, Allianz Global Risk
Report 3/99.
[5.37] Prof. Dr. H. Graßl: Strategies for Slowing Climate Change, revised version of
presentation made at Allianz Environmental Protection Foundation’s annual
symposium in 1997, Allianz Global Risk Report 3/99.
[5.38] G. Ondrey: Carbon Dioxide Gets Grounded, Chemical Engineering, March 2000.
[5.39] Hollemann-Wiberg, Lehrbuch der Anorganischen Chemie, 102nd edition, Walter de
Gruyter, Berlin 2007, pp. 259–260.
[5.40] Well-to-Wheels Report, Eucar Concawe JRC, Well-to-wheels analysis of future
automotive fuels and powertrains in the European context, May 2006, European
commission, http://ies.jrc.cec.eu.int/wtw.html.
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[5.41] Well-to-Wheel Analysis of Energy Use and Greenhouse Gas Emissions of Advanced
Fuel/Vehicle Systems – A European Study. L-B-Systemtechnik GmbH, Ottobrunn,
September 2002; www.lbst.de/gm-wtw.
[5.42] A. König, F. Seyfried, I. Drescher: Fuel Reforming as Part of a Hydrogen Economy,
HYFORUM 2000, The International Hydrogen Energy Forum 2000 Policy – Business –
Technology, 11–15 September 2000, Munich, Germany.
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to the Atmosphere, International Journal of Hydrogen Energy, Vol. 23, No. 12,
pp. 1087–1093.
[5.44] Iceland, Shell, DaimlerChrysler, Norsk Hydro form Company to develop hydrogen
economy, The Hydrogen & Fuel Cell Letter, March 1999.
[5.45] Hermann Scheer (Alternativer Nobelpreis 1999): Solare Weltwirtschaft, Verlag
Kunstmann, 1999.
[5.46] Andreas Züttel: Hydrogen Storage, University of Fribourg Switzerland, presentation at
The International German Hydrogen Energy Congress, Essen, Feb. 2004.
[5.47] C. J. J. Reijerkerk: Potential of cryogenic hydrogen storage in vehicles, paper published
at NHA Conference, Apr. 2004, Los Angeles.
[5.48] Dr. Michael R. Swain University of Miami, Fuel Leak Simulation, proceedings of the
2001 DOE Hydrogen Program Review.
[5.49] A. Stepken: TÜV-Süd, Presentation Wasserstoff – so sicher wie Benzin, Medienforum
Deutscher Wasserstofftag, München, Oct. 2003.
[5.50] R. A. Oriani et al.: Hydrogen Degradation of Ferrous Alloys, Noyes Publications USA
(1985).
[5.51] P. F. Timmins: Solutions to Hydrogen Attack in Steels, ASM International 1997,
ISBN 0-87170-597-4.
[5.52] Preliminary draft proposal for a regulation of the European parliament and of the
Council relating to the type-approval of hydrogen powered motor vehicles, Version 2,
July 13th 2006, http://ec.europa.eu/enterprise/automotive/pagesbackground/hydrogen/
consultation/hydrogen_draft_proposal.pdf.
[5.53] B. Hoehlein, T. Grube, C. J. J. Reijerkerk: Beitrag zur FVS-Jahrestagung 2004:
Wasserstoff und Brennstoffzellen – Energieforschung im Verbund, Wasserstofflogistik
– Produktion, Konditionierung, Verteilung, Speicherung und Betankung.
[5.54] C. J. J. Reijerkerk: Hydrogen Filling Stations Commercialisation, Final Project for
University of Hertfordshire in conjunction with University of Applied Sciences
Hamburg, Linde AG, Munich Sep. 2001.
[5.55] B. Valentin: Wirtschaftlichkeitsbetrachtung einer Wasserstoffinfrastruktur für
Kraftfahrzeuge, University of Applied Sciences München, Linde AG, Munich, Nov.
2001.
[5.56] Jaco Reijerkerk: “Wasserstofflogistik”, Page 4–5, BWK 1/2, 2006.
[5.57] David Hart: “The Economics of a European Hydrogen Automotive Infrastructure”,
presented at International Hydrogen Day, Berlin 2005.
[5.58] Andreas Westenberger: “Ausblick auf zukünftige H2-Anwendungen in der zivilen
Luftfahrt”, hydrogen.tech 2006, Munich.
[5.59] M. Bracha: Sänger – An Advanced Space Transport System, Study by Linde AG on
behalf of MBB, 1989.
[5.60] Ullmann fuel cells.
[5.61] W. R. Grove: On a New Voltaic Combination of Gases by Platinum, The London and
Edinburgh Philosophical Magazine and Journal of Science, Philos. Mag. 14 (1839) 127.
[5.62] A. E. Hammerschmidt, M. F. Waidhas: Spektrum der Wissenschaft (1999) No. 2, A44.
[5.63] a) Internet page from Smithonian Institutes http://americanhistory.si.edu/csr/fuelcells/
index.htm; b) http://dc2.uni-bielefeld.de/dc2/fc/folien/f-geschi.htm;
c) http://www.fuelcelltoday.com.
[5.64] R. Dittmeyer, W. Keim, G. Kreysa, A. Oberholz, Winnacker-Küchler: Chemische Technik,
Bd. 4, 2005.
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6
Carbon Dioxide
6.1
History, Occurrence, Properties and Safety
6.1.1
History
Carbon dioxide (CO2, carbonic acid anhydride), previously often called carbonic
acid, is a natural component of man’s environment, i.e. of its respiratory gas. In
the first half of the 16th century Paracelsus was the first to distinguish between
CO2 and air, while Black identified CO2 as an element of the respiratory air
around 1760 [6.1]. More or less at the same time, Lavoisier established proof of the
composition of CO2 through synthesis [6.2]. In the 19th century, man learned to
deal with CO2 and extended its utilization increasingly. In 1835, Thilorier carried
out first experiments for liquefying CO2 and producing dry ice [6.3]. The first
plant for the industrial generation of CO2 in Germany was erected in 1875 by the
“Maschinenfabrik Sürth” [6.4]. Since the drawing up of a property table for CO2 by
Mollier (1895) [6.5] engineering has made great progress regarding both the CO2
liquefaction plants and the application of the industrial gas CO2 [6.6].
6.1.2
Occurrence
The occurrence of CO2 on the earth should be regarded as an essential part of the
carbon cycle. Carbon is to a large extent organically bound and stored in fossil
fuels such as petroleum, natural gas and coal (cf. Chapter 7).
Large quantities of CO2 are found in the form of different carbonates in minerals
of the earth’s crust, such as limestone (about 5.5 · 1016 t) as well as in the oceans
(about 1.4 · 1014 t). And the atmosphere contains CO2 to a volume fraction of
meanwhile 0.038%, which in fact adds up to 2.7 · 1012 t.
Large quantities of CO2 are also involved in the biological carbon cycle in the
narrower sense, i.e. in the photosynthesis of CO2 to sugar (glucose) and the
respective reverse combustion reaction:
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
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6 Carbon Dioxide
light + chlorophyll
Photosynthesis: 6 CO2 + 6 H2O → C6H12O6 + 6 O2
Combustion:
C6H12O6 + 6 O2 o 6 CO2 + 6 H2O + energy
It is estimated that about 110 to 120 · 109 t of carbon are currently bound through
photosynthesis each year. By determining the CO2 content of the atmosphere and
deep ice, it has been established that in the last 100 years more CO2 has been
generated by the combustion of fossil fuel than bound through photosynthesis
[6.7, 6.8].
In recent years, carbon dioxide has fallen into disrepute as the major greenhouse
gas and thus possibly as one of the causes of global warming. The reason for
this is that energy generation through the combustion of fossil fuels producing
carbon dioxide has increased dramatically in the 20th century. In fact, since about
1900, an increase in the CO2 content of the atmosphere from an initial volume
fraction of 0.03% to the above mentioned volume fraction of 0.037% has been
observed [6.9].
The CO2 which is being traded industrially at the present time is only produced
separately through combustion in exceptional cases. In general it is recovered
from industrial off-gases that would otherwise escape directly into the atmosphere.
Only a small percentage of the traded CO2 is recovered from earth sources, e.g.
mineral waters. On the other hand, the CO2 obtained and liquefied this way has
not disappeared, but turns up sooner or later in a dispersed form. With worldwide
about 20 · 106 t a–1 in the year 2002, the industrial gas CO2 to be discussed here
makes up only a tiny share of the global CO2 cycle. Therefore the CO2 traded as
industrial gas is not subject to the Emissions Trading Directive 2003/87/EG, the
requirements established in view of complying with the Kyoto Protocol regarding
the framework agreement of the United Nations on climatic changes of December
11, 1997 for an EU-wide emission trading system.
6.1.3
Physical and Chemical Properties
Under normal conditions, carbon dioxide is an odourless and colourless gas with
the following properties (gaseous carbon dioxide):
Molar mass
Standard density
Rel. density to air
Spec. gas constant
Molar heat capacity (25 °C)
Thermal conductivity (25 °C, 1 bar)
Viscosity (gaseous, 20 °C)
Dielectric constant (0 °C, 1 bar)
1345vch06.indd 186
44.011
1.977
1.5291
0.1927
37.13
1.64 · 10–4
20.3 · 106
1.000989
kg kmol–1
kg mN–3
kJ kg–1 K–1
J mol–1 K–1
W cm–1 K–1
Pa s
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6.1 History, Occurrence, Properties and Safety
187
Fig. 6.1 Pressure/temperature diagram for CO2.
Carbon dioxide is easily liquefiable and has a triple point (see Fig. 6.1). Some of
the essential properties of the liquid carbon dioxide are listed below [6.10, 6.11]:
Critical temperature
Critical pressure
Temperature at triple point
Pressure at triple point
Boiling pressure
at 220 K
250 K
280 K
300 K
Heat of vaporization (triple point)
Melting heat (triple point)
Density (–50 °C)
Thermal conductivity (300 K, 1 bar)
Viscosity (gaseous, 300 K)
Dielectric constant (liquid, 0 °C)
304.13
73.75
216.58
5.18
5.99
17.85
41.61
67.13
347.86
9.02
1152.6
16.8 · 10–3
15.0 · 106
1.58
K
bar
K
bar
bar
bar
bar
bar
kJ kg–1
kJ mol–1
kg m–3
W m–1 K–1
Pa s
Solid carbon dioxide occurs as pressed CO2 snow, so-called dry ice, with a density
of 1300 to 1500 kg m–3.
CO2 is a very stable component that degrades only at very high temperatures
(under atmospheric pressure at 1205 °C to 0.032%, at 2367 °C to 21%, at 2843 °C
to 76.1%) into CO and O2. At even higher temperatures, carbon and O2 are
formed.
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188
6 Carbon Dioxide
CO2 o CO + 0.5 O2
'H = + 238.17 kJ · mol–1
CO2 o C + O2
'H = + 393.77 kJ · mol–1
For this reason, CO2 is a very weak oxidizing agent that reacts to carbon
monoxide and carbon only at high temperatures with strong reducing agents
such as H2, C, P, Mg, Na, K. Thus, magnesium oxide and carbon (soot) are
produced under a strong flash of light when carbon-dioxide snow is ignited with
magnesium powder. The equilibrium arising from the reaction with hydrogen
and coal plays an important role, for instance, in the production of synthesis gas
and steel (see Chapter 5).
Water–gas equilibrium:
CO2 + H2 o CO + H2O(g)
'H = + 41.19 kJ · mol–1
Boudouard-equilibrium:
CO2 + C o 2 CO
'H = + 172.58 kJ · mol–1
An aqueous solution of CO2 shows only weak acidity (solubility at 20 °C, 1 bar:
0.9 L CO2 per litre of water). The reason for this is that only 0.2% of the CO2 reacts
with water to carbonic acid H2CO3.
CO2 + H2O o H2CO3 o H+ + HCO3–
The remainder occurs as hydrated CO2. With ammonia, CO2 forms ammonium
carbamates as the result of a CO2 insertion reaction:
CO2 + 2 NH3 o H2N–C(=O)–O– + NH4+
The dehydration of the carbamate provides urea (H2N–C(=O)–NH2) which is
an important compound for the fertilizer and plastics industry [6.12].
6.1.4
Safety Issues
Gaseous CO2 is part of the human respiratory system. The CO2 concentration in
our exhaled air is about 4 to 4.5 vol.%. But inhalation of air with CO2 concentrations
higher than atmospheric will have the following physiological effects [6.13]:
x
x
x
x
x
x
1345vch06.indd 188
up to 1 to 1.5%, slight effect after several hours (TLV-TWA: 0.5%)
up to ca. 3% slightly narcotic, higher breathing and pulse rate
4 to 5% headache, higher blood pressure, signs of intoxication
5 to 10% breathing more laborious, loss of judgement
over 10% unconsciousness after 1 min
further exposure to levels between 10 and 100% will result in death
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6.2 Recovery of Carbon Dioxide
189
It should be observed that CO2 is heavier than air, so it spreads along the ground
and is collected in ditches, cellars, cavities etc.
Liquid CO2 is stored under pressure and at low temperatures.
The relevant rules and recommendations must be observed:
x national and local regulations, industrial code of practice
x European Union’s Pressure Equipment Directive 97/23/EC, and
x relevant rules and recommendations of the EIGA und CGA [6.14, 6.15]
Dry ice exists at a temperature of –78.5 °C. When handling liquid CO2 and dry
ice the wearing of personal protection equipment (protective goggles and gloves)
is mandatory.
6.2
Recovery of Carbon Dioxide
Today, most of the carbon dioxide coming on to the market as industrial gas is
recovered from CO2 sources which already exist. A CO2 source is understood to
be gases and off-gases with a significant CO2 content. About 70% of the CO2 on
the European market is actually recovered from synthesis gas plants (see also
Section 5.2.3). Only a small part of the CO2 is generated by the combustion of
fossil fuels, preferably natural gas. This usually occurs in smaller units (capacity
< 2 t h–1), which are so far away from a suitable CO2 source that the transport to
the consumer is economically not feasible.
Apart from brewery plants, today the predominant part of CO2 is produced
in plants with a capacity of 1 to 25 t h–1. This usually requires the process steps
shown in Fig. 6.2.
Fig. 6.2 Process steps for the CO2-production.
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6 Carbon Dioxide
6.2.1
Sources of Carbon Dioxide Recovery
The most important criterion for the economic efficiency of a CO2 source is the
CO2 partial pressure. Gas mixtures which are suitable for the economical recovery
of liquefied CO2 with high purity are understood to be carbon dioxide sources
preferably with a CO2 partial pressure > 1 bar. Table 6.1 shows examples of CO2
sources listed by decreasing CO2 partial pressure.
A criterion for not selecting a CO2 source is the presence of additional gas
components the removal of which may be costly. Some examples of such sourcespecific secondary components are given in Table 6.2.
Table 6.1 CO2 Sources.
Source
CO2
partial pressure
CO2-fraction from acid-gas scrubbings in ammonia or other synthesis
gas plants or H2-generation plants (cf. Section 5.2)
1.0 to 1.2 bar
CO2-containing off-gas from fermentation plants, e.g. in breweries
0.9 to 0.95 bar
CO2 from underground deposits (also in mixtures with hydrocarbons)
1 to 30 bar
Natural gas purification plants, so-called sweetening plants
1.0 to 1.2 bar
Ethylene oxide plants
0.8 to 0.95 bar
Acid neutralisation plants
around 1 bar
Lime and cement furnaces
0.2 to 0.5 bar
Flue gas
0.09 to 0.11 bar
Table 6.2 Specific impurities in different CO2 sources [6.16].
Often
occurring
impurities
H2
N2
CO
O2
Ar
CH4
Water
1345vch06.indd 190
Selection of additional components found in certain CO2 sources
CO2 fraction
of synthesis
gas plant
Natural
CO2
source
CO2 fraction
from a fermentation plant
Residual gas
of an ethylene
oxide plant
Flue gas
Alcohols
Org. acids
Aldehydes
Ketones
Amines
H2S
COS
Mercaptanes
HCN
C2+
Oil
Benzene
Toluene
H2S
R–SH
COS
Radon
Mercury
Salts
Alcohols
Org. acids
Aldehydes
Ketones
Yeasts
Germs
S-compounds
C2H4
C2+
Ethyl chloride
Vinyl chloride
Aldehydes
NOx
SO2
SO3
HCl
HCN
Amines
Metal oxides
Mercury
Dust
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6.2 Recovery of Carbon Dioxide
191
6.2.2
Pre-purification, Enrichment, Extraction, Capture
If CO2 sources with low partial pressure (< 1 bar) are utilised an additional process
step for the extraction or enrichment of the CO2 has to be applied upstream of
the liquefaction unit. Most commonly an absorption process (CO2 scrubbing)
is used but a pressure swing adsorption (PSA) process (cf. Section 5.2.3.4) or a
combination of partial condensation and wash unit can also be applied.
The scrubbing unit consists of an absorption column, where the CO2 is absorbed
in a wash liquid and a regeneration column, where the CO2 is stripped from the
wash liquid.
The feed gas enters the absorption column at the bottom and is absorbed in
the lean wash liquid, which flows countercurrently from the top to the bottom
of the column. On top of the column traces of wash liquid are removed from the
residual gas by means of fresh water.
The CO2-rich wash liquid is pumped to the upper section of the stripper. On
its way it is heated against hot-regenerated wash liquid from the stripper bottom.
The CO2 is absorbed by means of the stripping steam generated in the reboiler,
which is heated by LP steam. The lean wash liquid is withdrawn from the bottom
of the stripper, cooled against loaded wash liquid and finally against cooling water
and fed to the top of the absorber column. The stripping steam is condensed
against cooling water in the top of the regeneration column. The surplus water
is withdrawn as waste water.
This process is typical for a number of wash liquids but mainly amine solvents.
Depending on the selected solvent some additional equipment is required, i.e.
x
x
x
x
x
a filter system in a side stream to the lean wash solution
a device for the addition of anti-foam agent
a solvent storage of the complete liquid inventory
a reclaimer for removal of heat-stable salts
and a pump for the feeding of the amine solution from storage into the process
A large variety of wash liquids are available which can be divided into physical
absorbents and chemical absorbents [6.17].
Among the first group are solvents like water or methanol, where the CO2
solubility is basically a function of the CO2 partial pressure.
The chemical absorption is characterised by a temporary chemical bond at
ambient temperature which is released at higher temperatures. An important
group of these solvents are the alkanolamines.
Among these is Monoethanolamine (MEA) commonly used for the extraction
of CO2 from the flue gas, lime-kiln off-gas or blast furnace off-gas. An aqueous
solution of 15 to 30 wt-% MEA reacts with CO2 as follows:
C2H5O–NH2 + H2O + CO2 C2H5O–NH3+ + HCO3–
about 120 °C m
o about 50 °C
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6 Carbon Dioxide
Fig. 6.3 Sketch of CO2 scrubbing unit.
Advantage: high reaction velocity and high pick-up at low CO2 partial pressures.
Disadvantage: high reaction enthalpy i.e. high energy demand for regeneration,
sensitive against oxygen, i.e. higher MEA losses if O2 is present in the feed gas.
Methyldiethanolamine (MDEA) is often used for the removal of CO2 from
synthesis gases produced in steam reformers (cf. Section 5.2.2.1). The aqueous
solution of 35 to 50 wt-% MDEA reacts with CO2:
+
(C2H5O)2N–CH3 + H2O + CO2 (C2H5O)2NH–CH3 + HCO3–
about 120 °C m
o about 50 °C.
Advantage: less sensitive to O2, lower reaction enthalpy, high pick-up at high
CO2 partial pressure.
Disadvantage: low reaction velocity, this can be almost compensated by
adding an activator, e.g. piperazine, to the MDEA solution (e.g. BASF’s activated
aMDEA®).
The steam demand for the regeneration of some commercially available
absorption solutions based on amines is shown in Fig. 6.4.
To reduce the cost of the post-combustion capture of CO2 from power stations
in the future large research efforts are being made to develop new absorption
solvents or to improve the efficiency of the existing ones [6.18–6.21].
For raw gases with a CO2 partial pressure higher than 3 to 5 bar a two-stage
MDEA wash can be used, which is characterised by two wash cycles, i.e. one
with a completely regenerated wash solution and a second one with a partially
regenerated wash solution. The two-stage wash process is characterised by a
significantly lower steam demand but an increase in the required pump energy
and investment cost.
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Fig. 6.4 Specific steam consumption of different CO2 scrubbing processes.
6.2.3
Standard Process for the Liquefaction of Carbon Dioxide
A frequently used process for the purification and liquefaction of CO2 is described
(Fig. 6.5).
Fig. 6.5 Standard process for the liquefaction of CO2.
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6 Carbon Dioxide
6.2.3.1
Compression and Water Separation
The raw CO2 has approximately 1.1 to 1.4 bar at ambient temperature. It is
compressed to about 18 bar in an oil-flooded two-stage screw compressor followed
by a highly efficient oil separation, consisting of coalescers and a carbon bed.
The lubrication oil has to be suitable for food applications. Non-lubricated twostage piston compressors are occasionally used for small-capacity units. After
compression, the CO2 is cooled against cooling water to just above ambient
temperature and further cooled against evaporating refrigerant to about 10 °C.
Possibly condensed water is separated in separators which are installed before
the compressor and after the coolers. The condensate (water and probably traces
of wash solution and/or oil) is fed to the waste water system.
6.2.3.2
Adsorber Station
The residual traces of water and other components, such as odorants, are
removed in regenerative adsorbers filled with molecular sieve or silica gel and
activated carbon. Typically the unit operates according to the following automatic
sequence:
Adsorption, expansion, regeneration, cooling, pressure build-up and adsorption
again in possibly parallel operation, (see also Sections 2.2.4 and 2.2.5.6).
For the regeneration of the adsorbent either residual gas of the rectification or
dried CO2 is used. But air can also be used for regeneration of adsorbers. In this
case the cooling is normally done with dry CO2 or residual gas to sweep the air from
the adsorbent. The regeneration gas is heated to about 200 °C either electrically or
by means of steam. By these means, the components adsorbed on the molecular
sieve as well as on the activated carbon are desorbed. The regeneration gas is then
emitted to the atmosphere. If only water is desorbed by means of dry CO2, it can
be recycled to the compressor to improve the recovery rate.
6.2.3.3
Liquefaction and Stripping of Lighter Components
The purified CO2 is first cooled against cold residual gas and in the reboiler of
the stripping column. Here it serves as heating medium for the generation of
the required stripping steam. The major part of the CO2 is liquefied against an
evaporating refrigerant. The liquefied portion is fed to the top of the stripping
column. Part of the stripping steam is also reliquefied in the liquefier to increase
the CO2 recovery rate. This depends both on the portion of the lighter components,
such as N2, CH4 etc., and on pressure and temperature in the liquefier (see
Fig. 6.6). The remaining residual gas serves to pre-cool the CO2 and, if required,
to regenerate the adsorbers.
6.2.3.4
Refrigerating Unit
The liquefaction of the CO2 requires a considerable refrigeration unit; the
theoretical minimal value amounts to 85.8 kWh per ton of CO2 at –33 °C. Generally,
the total amount of refrigeration required (liquefaction, insulation losses, losses
due to temperature differences through heat exchangers) is covered by a closed
compression refrigeration unit. All modern refrigerants can be used, but often
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Fig. 6.6 CO2 recovery rate depending on the CO2 content and final liquefaction temperature.
water-free ammonia is selected for its favourable properties, such as high specific
evaporation enthalpy and volumetric refrigeration effect as well as low price and
environment-friendly behaviour.
The gaseous refrigerant is compressed either in a two-stage piston compressor
or more frequently in two screw compressors or a so-called compound compressor
from slightly over ambient pressure to about 16 to 17 bar. It is liquefied against
cooling water (often in a so-called evaporative condenser, where the main part
of the cooling duty is directly provided by evaporating water) and stored in an
accumulator. From here, it can be expanded either directly into the CO2 liquefier
or, as is common for thermodynamically efficient plants, first into a flash drum at
the compressor interstage pressure. Together with the gaseous refrigerant from
the CO2 precooler, the expansion gas is fed to the second compression stage.
The remaining liquid can first be supercooled against pure CO2 from the sump
of the stripping column and then expanded into the CO2 liquefier and evaporated
there. The flash is significantly less compared to direct expansion from liquefaction
condition to evaporation pressure.
Apart from the investment costs for the plant, a decisive cost factor of the CO2
recovery is the total energy consumption, i.e. mainly the energy demand for the
CO2 compressor and the refrigerating unit. For medium to large liquefaction
capacities (3 to 20 t h–1 liquid CO2), the specific energy demand can be estimated
with the help of the curve shown in Fig. 6.7.
6.2.4
Process Steps to Obtain High Product Purity and Recovery Rate
Today the CO2 market generally requires high product purities (total impurities
< 100 ppmv, for individual components and applications < 1 ppmv and occasionally in the lower ppbv-range). With the process shown in Fig. 6.5 not all of the
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6 Carbon Dioxide
Fig. 6.7 Specific power demand of the CO2 liquefaction.
trace components found in the different CO2 sources can be removed to achieve
the required purity.
Quality standards for CO2 used in food and beverages for example are to be
found in papers published by the following organizations:
x International Society of Beverage Technologists (isbt) [6.22]:
“Quality Guidelines and Analytical Method Bibliography for Bottlers”
x European Industrial Gases Association (EIGA) [6.14]:
Publication: Doc. 76/01/E “CO2 Specification Guide for Analytical Steps
and Frequencies”
Publication: Doc. 70/99/E “Carbon Dioxide Source Certification, Quality
Standards and Verification”
x Compressed Gas Association, Inc. [6.15]:
Publication: CGA G-6-2003 “Carbon Dioxide, 6th Edition”
Publication: CGA G-6.2
“Commodity Specification for Carbon Dioxide”
The trace components given in Table 6.2 can be removed with the following
process steps (see Fig. 6.8).
6.2.4.1
Scrubbing
Water scrubbing: All easily water-soluble components (e.g. alcohols, organic
chlorides, amines, etc.) can be separated by means of water scrubbing.
Potassium permanganate: An aqueous solution of KMnO4 is used to oxidise
organic components such as germs, yeasts and other easily oxidable matters.
Soda: An aqueous solution of Na2CO3 can be used for the removal of SO2/SO3
as well as the corresponding acids. Caustic soda can also be added to the soda
wash cycle.
All these wash processes comprise of similar equipment.
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Fig. 6.8 Process steps for the removal of trace components.
A wash unit consists of a scrubbing column in which the raw CO2 is fed in
countercurrent to a cooled water/wash solution circulation. The liquid enriched
with impurities is drawn from the sump of the column and partly replaced by fresh
water/solution at the top of the column. Water scrubbing is only applied when
the raw CO2 is not already the CO2 fraction of a gas scrubbing. It is sometimes
installed upstream of the compressor to be used as a direct gas cooler.
6.2.4.2
Adsorption and Chemisorption
Apart from the usual adsorptive dryer station, additional adsorption steps can
serve to remove trace components (e.g. sulphur compounds, alcohols, aldehydes,
ketones, ester, other odorants, aromatics, etc.). All molsieves and all types of
activated carbon are used as adsorbents, but also zinc and ferric oxides which, in
contrast to the dryers, are usually not regenerated in situ but have to be replaced
by fresh adsorbents after saturation. According to the quantity of the component(s)
to be removed, adsorption occurs in single adsorbers or in series-connected
twin-adsorbers. This series-connection (also called “Lead/Lag” configuration) is
designed in such a way that the container with the unloaded adsorbent follows the
“active” one during normal operation of the plant. When the adsorption capacity
of the “active” adsorber is exhausted, it is taken out of operation and the vessel
with the fresh adsorbent is turned into the “active” one. The loaded adsorbent can
be replaced by a fresh one while the plant is kept in operation.
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6 Carbon Dioxide
6.2.4.3
Catalytic Combustion
Today the removal of hydrocarbons, with boiling points higher than CO2, is carried
out almost exclusively by catalytic combustion. The potassium permanganate
wash once used for oxidising easily oxidable hydrocarbons is nowadays only used
for the purification of CO2 obtained in connection with fermentation processes
(e.g. in breweries).
The catalytic oxidation takes place with precious metal catalysts (platinum
and/or palladium on aluminium oxide carriers) at temperatures between 200
and 600 °C. When the reactor exit temperature reaches only about 450 °C the
remaining methane has to be removed in the CO2 stripper. If the required oxygen
is added to the CO2 in the form of air, then the nitrogen mixed with the CO2 has
also to be separated again in the CO2 stripper. The CO2 partial pressure is lower
and therefore the recovery rate is lower (see Fig. 6.6).
The CO2 is first heated against the combustion product and then, in an
electrical heater, up to the required starting temperature. If only small amounts
of combustible components are contained in the CO2, the temperature rise in the
combustion reactor is small and either the electrical heater has to provide more
energy or the countercurrent gas/gas heat exchanger requires significantly more
exchanger surface. In other words, the capital to be invested for the countercurrent
heat exchanger has to be optimized in view of the operating costs of the electric
heater.
6.2.4.4
Improvement of the Carbon Dioxide Recovery Rate
With low concentration of CO2 in the raw gas, the CO2 recovery rate at common
liquefaction temperatures is small, as is apparent from Fig. 6.6. An improvement
in the recovery rate can, for example, be achieved by means of lower liquefaction
temperatures. In order to avoid the required installation of a cascade refrigeration
unit with different refrigerants and additional compressors and equipment,
liquefied CO2 can be used as refrigerant for the cold generation in a so-called
“open” refrigeration cycle at a sufficiently low temperature level.
A large part of the CO2 in the residual gases of the stripping column and the
liquefier can be liquefied against evaporating CO2, at –50 °C, for example. The
liquefied CO2 is separated and evaporated at about 5.5 bara. The evaporation
enthalpy is exactly adequate to compensate the required liquefaction enthalpy.
The evaporated CO2 is mixed with the inlet flow of the second stage of the CO2
compressor. Thus the open CO2 cycle with only small amounts of additional
equipment (see process flows depicted in bold print in Fig. 6.9) represents an
economical alternative to a cascade refrigeration unit.
6.2.5
Carbon Dioxide Recovery from Flue Gas
For the recovery of CO2 from flue gas or calcinations kiln exhaust, further process
steps are required as shown in Fig. 6.10, in addition to the CO2 purification and
liquefaction.
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Fig. 6.9 Enhancement of CO2 recovery.
Fig. 6.10 CO2 from flue gas.
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6 Carbon Dioxide
Initially, solid particles (dust) have to be removed from the flue gas in order
not to endanger the subsequent devices, e.g. blowers or compressors. The flue
gas is then cooled down in the heat recovery (countercurrent heat exchanger
or recuperator). The blower compensates the pressure drop of the additional
downstream process steps. Before CO2 is removed from the flue gas, SO2 and NOx
have to be removed to a remaining level of about 10 ppmv SO2 and about 20 ppmv
NOx [6.23]. The NOx must be removed with a Selective Catalytic Reduction (SCR)
using aqueous ammonia. This process step takes place at about 200 to 300 °C and
must be carried-out upstream the heat recovery section. The bulk removal of SO2
is normally done by a wet flue gas desulphurisation system using limestone and
producing gypsum [6.24] or other wash processes producing SO2, e.g. SOLINOX®
[6.25]. The remaining sulphur oxides can be removed with a soda scrubbing unit
[6.26]. The resulting sodium sulfite (Na2SO3) is discharged with the waste water.
For the following CO2 scrubbing, an oxygen resistant wash solution has to be
applied, e.g. Econamine FG“ (cf. Section 6.2.2).
Compared to the recovery of CO2 from other sources, the cost of this process is
so high that flue gas or similar CO2 sources cannot be regarded as economically
attractive for CO2 recovery.
However, with the increasing political pressure regarding CO2 emissions, e.g.
from power stations burning fossil fuel and chemical plants, great efforts are
being made to develop new processes for the capture of CO2 [6.27, 6.28]. This
captured CO2 may be stored in the ground (e.g. in exploited oil or gas fields
(EOR) or aquifiers) or in the deep sea. For this sequestration the CO2 must be
compressed to pressures up to 100 bar and more. The flow rates of such CO2
streams are huge compared to the CO2 flow rates in the normal gas market, e.g.
a 600 MWthermal coal-fired power station produces about 240 t/h CO2. After the
necessary purification steps those CO2 streams will be excellent sources for the
CO2 liquefaction and will change the entire situation on the CO2 market.
6.2.6
Production of Dry Ice
If liquid CO2 is expanded from storage conditions (e.g. boiling fluid at 15 bara)
to a pressure slightly above ambient pressure (e.g. 1.1 bara, –77.6 °C), about 52%
of the CO2 occur as dry ice snow and the rest in gaseous form [6.29, 6.30]. This
dry ice snow is pressed into blocks or into pellets. The remaining expansion gas
is compressed to a little over the storage pressure by means of compressors and
liquefied in a liquefaction unit so that it can be fed back into the storage tank.
The required energy for the production of dry ice is about 170 to 210 kWh t–1,
including compression and re-liquefaction of the arising residual gas.
Dry ice block weights can vary from 5 to 20 kg per block, with a density of about
1.4–1.6 kg dm–3.
Pellets vary from 5 to 15 g per pellet, smaller pellets, 0.3 to 0.7 g are used for
dry ice blasting, bulk density is about 0.8–1.0 kg dm–3
Dry ice is shipped in insulated boxes.
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6.3
Applications
Carbon dioxide has certain special properties (see Section 6.1.3) that must be
taken into account in its application:
x Liquid CO2 can only be used at pressures over 5.2 bar (e.g. in pressurised heat
exchangers or spray nozzles)
x When pressurised liquid CO2 expands, gaseous and solid CO2 form as a result.
Solid CO2 is known as “dry ice” and available on the market as snow, slices,
pellets, nuggets and blocks
x Solid CO2 or dry ice sublimates at atmospheric pressure, allowing a number of
special applications (e.g. cooling and blasting)
x Over approximately 32 °C and 74 bar, CO2 enters a supercritical condition,
facilitating another range of special applications (e.g. dissolving and cleaning)
x Inerting and purging processes with gaseous CO2 follow the same basic pattern
as those using nitrogen (see Section 2.5.1)
In welding and the processing industry, carbon dioxide is used to:
x Inert and purge pipes and vessels for safety and maintenance purposes
x Replace harmful propellants in plastic foam (e.g. polystyrene) (see Example B)
x Pressurise spray cans as a substitute propellant for harmful chlorofluorocarbons
x Shrink and join construction components, e.g. shrink fitting and positive
grouting of shafts, gears, valve seats and other machinery components. Dry ice
is often used for this purpose
x Operate CO2 lasers, e.g. in combination with nitrogen and helium (see
Example A)
x Weld construction steel and fine-grained steel using the MAG process (preferably
in combination with Ar or Ar/O2) (see Section 2.5.3, Example B)
In the chemistry, petrochemistry, pharmaceutical and medical industries,
carbon dioxide is used to:
x Provide an inert solvent or co-reactant for chemical synthesis (e.g. of carbonates)
x Extract ingredients and separate them by chromatography (e.g. in analytical
chemistry)
x Produce inorganic carbonates (e.g. sodium carbonate/bicarbonate) and catalysts
x Provide a protective atmosphere in chemical production
x Regenerate ion exchangers (e.g. for the partial desalination of drinking water)
x Accelerate enhanced oil recovery (EOR) and natural gas recovery
x Ensure inert atmospheres during manufacturing and packaging of pharmaceuticals (e.g. pills, powders, pastes, capsules, ampoules)
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6 Carbon Dioxide
x Produce organic and pharmaceutical compounds (e.g. acetyl salicylate, or
aspirin)
x Enable various medical applications (see Chapter 10)
In food technology, biology and environmental protection, carbon dioxide is
used to:
x Shock-freeze food, e.g. meat or fish, in tunnel, spiral or rotary freezers
x Cool food during mixing, chopping and transport (e.g. by mixing dry-ice pellets
or CO2 snow into the product)
x Protect food during packaging and storage (using pure CO2 or mixtures with
N2 and/or O2)
x Produce refreshments such as fizzy or sparkling drinks (carbonating with
dissolved CO2)
x Harden foodstuffs by cooling before cutting (e.g. raw meat or ham)
x Stun animals before slaughter (e.g. pigs and fowl)
x Extract substances using supercritical CO2 (e.g. caffeine from coffee beans,
hop extracts)
x Accelerate the growth of plants in greenhouses (see assimilation, chlorophyll)
x Neutralise alkaline wastewaters (see Example C)
x adjust the calcite saturation in drinking-water recovery and supply (see avoidance
of pipe corrosion and Example D)
In other industries, carbon dioxide is used to:
x Clean surfaces by blasting with CO2 pellets (e.g. heat exchangers, building
facades)
x Remove raw oil from rocks and beaches after oil-tanker disasters (see Example E)
x Remove paint from automotive parts or other coated equipment (e.g. in
underground stations)
x Clean garments or metal parts in washing machines (substitution of chlorinated
carbons)
x Protect magnesium melts during handling
x Prevent oxidation, fire and explosions (e.g. fire-prevention in warehouses and
fire-extinguishing in waste-incineration bunkers) (see Example F)
x Clean semiconductor components, e.g. wafers, by dry-ice blasting using highest
purity CO2
x Improve pulp and paper production processes (see Example G)
Example A: Laser Processes
All laser processes basically involve the conversion of electrical energy into a light
beam of a single wavelength. This laser beam is generated in the laser resonator.
It is mostly parallel, allowing easy transfer over long distances to wherever it is
needed. At the processing area, the laser beam is focused by lenses, providing
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203
Fig. 6.11 Absorption rate [%] for laser radiation of cold metal.
the energy density needed for immediate heating, melting and even evaporation
of metals.
Many different laser types have been developed for the treatment of different
materials. The higher the absorption rate of the laser radiation, the faster the
material can be heated. While some highly reflective materials (e.g. aluminium
and copper) absorb short wavelengths best (e.g. Nd:YAG/Neodynium:Yttrium
Aluminium Garnet or diode lasers), less reflective materials (e.g. iron and steel)
can best be treated with lasers operating at longer wavelengths (e.g. CO2 lasers)
(see Fig. 6.11).
The most common application area for high-power lasers is laser cutting of
metals, since high cutting speed can be combined with high cutting precision
(see Fig. 6.12).
The advantages of laser welding include very narrow seam widths with considerably fewer weld distortions compared with traditional welding methods (see
Fig. 6.13).
Welding with CO2 and Nd:YAG lasers is becoming increasingly widespread
in industrial production. High-power CO2 lasers (2–12 kW) are used to weld
car bodies, automotive transmission components, heat exchangers and tailored
blanks.
Other laser applications include marking (e.g. product codes), drilling (e.g.
where extremely small holes are required) and surface treatment (e.g. annealing,
hardening, spraying, coating and cleaning).
Technical gases are widely used in laser applications. On one hand, laser gases
are used to generate radiation (e.g. LASERMIX® gases), and on the other process
gases (e.g. O2, N2 and He) support applications such as cutting, welding and
surface treatment.
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6 Carbon Dioxide
Fig. 6.12 Laser cutting.
Fig. 6.13 Laser welding.
As CO2 laser implies, carbon dioxide is the active component of this laser type.
The laser gas also contains helium and nitrogen. Besides these main components,
some special CO2 lasers require admixing of oxygen, hydrogen, carbon monoxide,
and/or xenon, which additionally supports the physical and chemical laser-beam
formation.
The cutting gas used (see process gases) is crucial to the result. Oxygen generally
yields good cutting performance in carbon steels and low-alloyed steels. However,
it can react with the base metal and cover the cut edge with an oxide layer. This
is why nitrogen is the gas of preference for cutting high-alloyed steels, especially
where high laser-power is available.
The welding gas (see process gases) has various functions. It protects the
focusing optics (lenses) against fumes and spatters and inhibits the formation of
a plasma cloud along the laser beam. Helium is most often used for this purpose
with CO2 lasers. The welding gas often also plays an active role in the welding
process. It increases welding speed and improves the mechanical properties of
the joint. Mixtures of helium, argon, carbon dioxide, and/or oxygen are frequently
used here.
In summary, lasers provide a precise and easily adjustable tool, removing the
need for mechanical contact with the work piece. The evolution of this equipment
is fascinating, with new applications emerging almost daily.
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205
Example B: Plastic Foaming
Foamed polymers are characterised by a cellular structure and reduced density
compared to solid material. They may be divided into open and closed-cell foams
as well as foams with homogeneous cell structures and those with a porous core
and compact (unfoamed) outer skin. Foam densities range from as low as approx.
10 kg/m3 in some polymers to close to that of compact materials.
The main advantages of foamed plastics are low consumption of raw materials,
reduced weight, excellent heat and sound-insulation and mechanical damping.
Important applications of polymer foams include packaging materials, insulation,
sound absorption and upholstery.
Economically important production processes especially include extrusion
foaming, polyurethane foaming, production of expanded polystyrene (EPS) and
polyolefines (EPP, EPE) and injection-moulded foaming.
The cellular structure of synthetic foams is created by so-called blowing agents.
Additives are also often required, particularly nucleation agents and stabilisers.
Depending on the process and desired foam density, either chemical or physical
blowing agents are applied.
Chemical blowing agents are mixed into the plastic in powder or pellet-form.
Above a certain temperature, the blowing agent disaggregates and releases
gaseous reaction products, usually nitrogen or CO2, inflating the plastic to form
a high-density foam. One of the most common chemical blowing agents is
azodicarbamide (ADC).
Physical blowing agents are metered into the molten plastic during foam extrusion or injection-moulded foaming. They may also be applied to one of the initial
components in polyurethane (PUR) foaming. Physical blowing agents are used to
create low-density foams with a more homogenous foam structure. Hydrocarbons
(particularly butane and pentane) and inert gases (e.g. CO2 and N2) are widely used
here. Inert gases have many advantages, such as being environmentally friendly,
non-flammable, non-toxic, chemically inert and inexpensive.
The blowing agent is homogeneously distributed and solved under high
pressure (usually 100 to 400 bar) in the melt or in a single reaction component
(PUR foaming). At the die exit, the pressure drops abruptly and the blowing agent
becomes highly super-saturated in the polymer. The foaming process then starts,
i.e. the existing nuclei grow and form bubbles.
The physical blowing agent selected has a strong influence on foam quality and
costs of the foamed product. Environmental safety also plays an increasingly important role. The blowing agents currently used in industrial countries have no ozone
depletion potential (ODP) and aim for very low global warming potential (GWP).
The successful application of CO2, whose use is growing in the industry, strongly
depends on a powerful pressurising and precise metering system. Starting with
a cryogenic tank, the liquid CO2 passes through an initial pressure-boosting step
such as a compressor station, followed by a mass flow-controlled high-pressure
dosing pump (usually of a diaphragm design) (see Fig. 6.14). Other specially
customised systems have also been implemented successfully.
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6 Carbon Dioxide
Fig. 6.14 High-pressure CO2 supply and metering concept for extrusion foaming.
Example C: Wastewater Neutralisation
Many industrial processes generate alkaline wastewater that has to be neutralised
prior to further internal usage or external discharge. The release of untreated waste
water into public sewers can lead to drastic consequences and serious measures
being imposed by the local authorities. The mandatory pH range varies between
pH 9 and 6.5.
One method of neutralisation is treatment with mineral acids such as hydrochloric or sulphuric acid. However, the potential drawbacks of this include complex storage and metering units, insufficient or excessive acidification, corrosion
problems and accumulation of salts such as chlorides or sulphates.
When carbon dioxide is mixed into water, it quickly forms carbonic acid and
neutralises alkaline compounds. This process is very efficient and requires only
very simple dissolving and metering devices. One common application is the
SOLVOCARB®-B process (see Fig. 6.15).
The carbon dioxide process may be used in wastewater treatment basins or
buffer tanks, for example. The gas diffuser hoses release CO2 uniformly into the
water, ensuring optimum utilisation. Fixed at the bottom of the neutralisation
tank, these perforated hoses are made of resistant elastomer. When the carbon
dioxide is switched on, the pores open and small bubbles of gas are emitted. No
additional energy source is required for CO2 introduction, which is controlled by
a pH-measurement device.
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207
Fig. 6.15 Introduction of CO2 into wastewater via the SOLVOCARB®-B process (Linde).
The benefits of carbon dioxide for wastewater neutralisation include:
x
x
x
x
x
Flat neutralisation curve, quick and safe adjustment to mandatory pH range
Easy and secure storage and dosage of CO2
Low risk of excessive acidification, corrosion or salt precipitation
Economic, safe and eco-friendly operation
Also suitable for water treatment in swimming pools and on construction
sites
Example D: Conditioning of Drinking Water
High-quality drinking water is one of life’s everyday essentials. However, as
demand rises and suitable resources shrink, low-quality raw water is increasingly
being used in drinking-water production. Drinking water is subject to stringent
statutory requirements. In the developed world, numerous directives define the
required minimum quality and associated parameters. To ensure high-quality
drinking water supplies, the raw water usually has to be treated to comply with
these regulations. Industrial gases provide a variety of conditioning methods to
achieve this.
Examples of CO2 applications in water treatment are:
x Water hardening: In case of insufficient hardness, CO2 and suitable calcium
compounds can be added to adjust the desired mineral content (cf. remineralisation)
x Utilisation following rapid decarbonisation: Excessive CO2 and/or calciumhydrogen-carbonate can be eliminated by adding calcium hydroxide. After calcite
precipitation, the pH can be adjusted as appropriate by adding small amounts
of CO2 (cf. calcite equilibrium)
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6 Carbon Dioxide
Fig. 6.16 Enrichment of drinking water with CO2 via the SOLVOCARB®-R process (Linde).
x pH regulation prior to process stages: Many processes can only be performed
within a defined pH range. CO2 is used to adjust the pH prior to flocculation
stages, for example
There are a variety of ways to add CO2 in water works, such as via reactors. One
common application is the SOLVOCARB®-R process (see Fig. 6.16). It is a closed
system in bypass operation, guaranteeing high standards of hygiene.
The benefits of carbon dioxide for the conditioning of drinking water include:
x Accurate adjustment of the desired pH and water hardness
x Improved taste
x Prevention of precipitation and limestone formation
x Prevention of corrosion in pipeline mains
Example E: Blast-Cleaning with Dry Ice and Liquid CO2
Dry-ice blasting is a non-abrasive method using dry-ice pellets. Liquid blasting
starts with CO2 liquid, which is then transformed into CO2 snow, producing an
even softer cleaning agent. In both cases, the CO2 particles are accelerated by
compressed air through a nozzle focused on the object for cleaning (see Fig. 6.17).
The process is similar to traditional gun-shot blasting using solid blasting media
such as steel or glass particles.
Today, nearly all industries use the CO2 blast-cleaning method. Example
applications include cleaning of tire moulds, electronic elements, ship inner walls,
bakery machinery, printing equipment, gear drives, foundry moulds, chemical
plants, as well as paint removal from all types of surface.
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209
Fig. 6.17 Ship restoration – removing the antifouling layer via dry-ice blasting.
CO2 pellets are available in various sizes, but the optimum diameter for this
purpose is 3 mm. They are produced in pelletisers, which press CO2 snow through
a matrix to form the pellets. These are then transported in insulated boxes, or
on-site production is also possible. In the liquid blasting method, CO2 liquid is
drawn from an insulated tank and supplied via pipeline to a so-called gun. This
consists of a dedicated chamber producing the snow and a special nozzle directing
it at the target.
The main advantage of these CO2 cleaning systems is that they operate without
any blast-media residue. The CO2 sublimates completely. For tougher jobs it
is possible to add more abrasive matter such as lava sand, leaving only minor
deposits.
There is a wide variety of cleaning systems on the market catering to different
abrasion requirements. As the expanding gas creates considerable noise at the tip
of the nozzle, ear protection and sound insulation are mandatory. Customised systems are also available for various purposes, e.g. automatic tire-mould cleaning.
Example F: Extinguishing Smouldering Fires in Waste-Incineration Bunkers
Smouldering fires can occur in many sites, such as coal-dust storage, corn silos or
wood-chip silos in chipboard factories. Conventional methods of fire-extinguishing
often fail in these types of situation, mainly because neither water (with or without
additives), foam nor powder can reach the fire if it is deep within a pile of stored
material. Gas-extinguishing is then the method of choice.
Fire-extinguishing methods for a smouldering fire may be differentiated as
follows:
Water:
x Only soaks upper layers, cannot reach fires deep in waste piles
x Makes waste sticky, so clearing by crane becomes difficult
x May overload the bunker walls due to relatively high weight
x Impedes subsequent waste-incineration in furnaces
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6 Carbon Dioxide
Fig. 6.18 CO2 injection in a waste-incineration bunker.
Nitrogen:
x Weighs less than air (molecular weight 28 kg/kmol; air 28,8)
x Rises and rapidly leaves seat of fire, diminishing its extinguishing effect
x Draws air in as it leaves the fire
Carbon dioxide:
x Weighs much more than air (molecular weight 44 kg/kmol) and accumulates
in bunker
x Remains near seat of fire for longer and does not draw in air
x Cools due to its high heat capacity
x Allows loss-free storage
This comparison shows that CO2 provides clear advantages for the fighting of
smouldering fires in container like waste-incineration bunkers (see Fig. 6.18).
Example G: Process Improvement in Pulp and Paper Production
Soap Acidulation Using CO2
Crude tall oil (CTO) is produced from soap by means of a reaction using
sulphuric acid. The consumption of sulphuric acid is normally about 200 kg per
tonne of CTO, depending on the soap quality. This represents a major portion
of total sulphur intake for a modern kraft pulp mill. Using carbon dioxide as a
pre-treatment for soap acidulation in CTO production can cut sulphuric acid
consumption by 30 to 50%.
Dissolving carbon dioxide in water forms carbonic acid, which reacts with the
CTO soap, reducing the solution’s pH from around 12 to below 8. At this pH level,
two phases separate, producing both a creamy soap oil and a bicarbonate brine in
which black liquor components are dissolved. The creamy soap oil phase is acidulated into CTO. This pre-treatment process may be used in batch or continuous
mode and implemented using the regular control system (see Fig. 6.19).
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6.3 Applications
211
Fig. 6.19 Example of soap acidulation with CO2 (pretreatment).
Benefits of carbon dioxide in soap acidulation include:
x
x
x
x
x
Better control of sulphur/sodium balance in the pulp mill
Simple process design
Use of existing soap tanks and equipment
Low investment cost (no pressurised equipment)
Improved run ability in existing CTO plant
Pulp Washing
Pulp washing is a key process stage in all types of pulp mill. Poor washing
conditions may lead to higher water and chemical consumption, reducing the
effectiveness of effluent treatment and chemical recovery operations. All these
factors tend to increase production costs and effluent problems.
CO2 pulp washing technology (see Fig. 6.21) can significantly improve pulp
washing results, producing substantial economic benefits. Key effects of CO2
pulp washing include improved ion exchange and reduced fibre swelling, which
has an impact on dewatering capability. The carbonates formed react with waterinsoluble sticky calcium soaps, which can then act as detergents.
Depending on the specific mill situation, benefits of CO2 pulp washing may
include:
x
x
x
x
x
x
x
x
1345vch06.indd 211
Increased production rate
Reduced COD from wash plant
Decreased chemical consumption
Reduced wash-water volumes
Decreased steam consumption
Lower demand for defoamer agents (see Fig. 6.20)
Reduced effluent loads
Lower maintenance costs
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6 Carbon Dioxide
Fig. 6.20 Defoamer consumption based on the pulp washing process.
Fig. 6.21 Example of a CO2 pulp washing installation.
The pH Control Concept (ACTICO®)
Adjusting pH levels with carbon dioxide instead of a strong acid has several
advantages, allowing stable and reliable control and eliminating the risk of pH
shocks in the system (see Fig. 6.22). For systems containing calcium carbonate
as filler, pH is an important parameter. Below pH 8, calcium carbonate starts to
dissolve and the calcium concentrations in the process waters increase. Using
carbon dioxide it is possible to adjust safely the right pH that is low enough for
further process requirements but high enough to secure reduced dissolution of
calcium carbonate.
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213
Fig. 6.22 pH variations in the head box without ACTICO® (Linde).
Fig. 6.23 Stable pH in the head box with ACTICO® (Linde).
The ACTICO® concept is a sophisticated combination of an automation and
CO2 injection system which can be tailored to individual paper machines. The
pH control system provides total pH control in the wet end and minimises pH
variations (see Fig. 6.23).
Benefits of ACTICO® pH control concept include:
x Automated adjustment of optimum pH with general improvements in the
whole papermaking process
x Carbon dioxide may be added in different positions from the stock preparation
to the headbox in the paper machine (see Fig. 6.24).
Process Stabiliser (ADALKA®)
Calcium carbonate is often used as a filler when high brightness levels are desired
in the final paper, as its own high brightness increases that of the final product.
In systems containing calcium carbonate, pH is an important parameter because
calcium carbonate starts to dissolve below pH 8 (see ACTICO®).
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6 Carbon Dioxide
Fig. 6.24 Example of an ACTICO® installation for controlling the pH level in the head box.
Fig. 6.25 Example of ADALKA® installation (Linde) at the incoming pulp
lines and the coated broke tower in the papermaking process.
During the ADALKA® process, a sodium bicarbonate buffering solution is added
to stock preparation to regulate and stabilise pH, alkalinity and calcium levels in
the papermaking process (see Fig. 6.25). This buffering solution is formed on-site
by combining carbon dioxide and sodium hydroxide, preferably in an alkalinity
control unit (ACU™). The solution enhances the system’s buffering capacity, allowing it to handle more acids and bases without substantial changes in pH.
The ADALKA® process stabiliser reduces the dissolution of calcium carbonate
due to the common ion effect, resulting in lower concentrations of calcium ions
in the process (see Fig. 6.26).
Benefits of ADALKA® process stabilizer include:
x Addition of bicarbonate buffering solution to reduce calcium hardness which is
a process obstacle that negatively impacts several additives and the process
x The solution may also be used in many different positions in the papermaking
process for pH stabilisation and alkalinity control
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References
215
Fig. 6.26 Reduction of calcium hardness with ADALKA® process stabiliser.
References
[6.1] J. Black: Experiments upon Magnesia Alba, Quicklime and Other Alcaline Substances,
Alembic Club, Edinburgh, 1777.
[6.2] A. L. Lavoisier: Opuscules Physiques et Chimiques, Paris, 1774.
[6.3] M. Thilorier: Ann. Chim. Phys. 1835, 60 (2), 427.
[6.4] H. Dünkelmann: Extraction and Use of Carbon Dioxide (CO2) and Dry Ice, LINDE
Reports on Science and Technology, 1968, 12.
[6.5] www.coolpage4u.de.
[6.6] E. Almquist: History of Industrial Gases, p. 93 ff., New York, Boston, Dortrecht, London,
Moscow, 2003.
[6.7] W. Stoll, F. Berit: Carbondioxide – where does it come from, where does it go?, special print,
edition 9040/II, Messer Griesheim GmbH, Düsseldorf, 1990.
[6.8] T. Banuri et al.: IPCC Third Assessment Report, Climate Change 2001, Mitigation,
Technical Summary, p. 27. www.ipcc.ch.
[6.9] D. L. Albritton et al.: IPCC Third Assessment Report, Climate Change 2001, The Scientific
Basis, Technical Summary, p. 36. www.ipcc.ch.
[6.10] Ullmann’s, 6th edition, 6, p. 394 ff., Wiley-VCH, Weinheim, 2003.
[6.11] Handbook of Chemistry and Physics, 86th edition, CRC Press, London, 2005.
[6.12] Ullmann’s, 6th edition, 6, p. 397, Wiley-VCH, Weinheim, 2003.
[6.13] EIGA Publication: IGA Doc 66/99/E, Appendix B.
[6.14] European Industrial Gas Association (EIGA). www.eiga.be/catalogue.asp.
[6.15] Compressed Gas Association (CGA). www.cganet.com/Publications.asp.
[6.16] EIGA Publication: IGC Doc 70/99/E, Appendix B.
[6.17] A. L. Kohl, R. B. Nielsen, R. B.: Gas Purification, Gulf Publishing Company, Houston,
1997.
[6.18] Publication of the German Patent Application No.: DE 10 2004 011 428 A1.
[6.19] Publication of the German Patent Application No.: DE 10 2004 011 429 A1.
[6.20] T. Mimura, K. Matsumoto, M. Iijima, S. Mitsuoka: Development and Application of Flue
Gas Carbon Dioxide Recovery technology. www.CO2cr.com.au/PUBFILES/CAP0304/
PUBFT-0209.pdf.
[6.21] International Patent Application, No.: 2004/110595 A2.
[6.22] International Society of Beverage Technologists (isbt).
www.bevtech.org/order_publications.html.
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6 Carbon Dioxide
[6.23] M. Simmonds, P. Hurst, M. B. Wilkinson, C. Watt, C. A. Roberts: A Study of Very
Large Scale Post Combustion CO2 Capture at Refining & Petrochemical Complex,
6th International Conference on Greenhouse Gas Control Technologies, pp. 39–44,
Kyoto.
[6.24] K. Muramatsu, T. Shimizu, N. Shinoda, A. Taani: Development of Mitsubishi Wet Flue
Gas Desulfurization System, Cemical Economy & Engineering Review, No. 11, Vol. 16,
Nov. 1984.
[6.25] J. Sporer, The SOLINOX® Process, Technology and Operating Experience of Regenerative
Desulfurization Method, LINDE Reports on Science and Technology, 1992, 50.
[6.26] J. D. Brady, Flue Gas Scrubbing Process for Sulfur Dioxide and Particulate Emissions
Preceding CO2 Absorption, Enviromental Progress, Vol. 6, No. 1, Feb. 1987.
[6.27] B. Mertz, O. Davidson, H. de Coninck, M. Loos, L. Meyer: Carbon Dioxide Capture
and Storage, IPCC Special Report (Summery for Policymakers & Technical Summery)
WMO, UNEP 2003.
[6.28] G. Marsh: Carbon Dioxide Capture and Storage – A Win-Win Option? Special report by
AEA Technology plc, May 2003. www.dti.gov.uk/files/file18798.pdf.
[6.29] Ullmann’s, 6th edition, 6, p. 405, Wiley-VCH, Weinheim, 2003.
[6.30] CGA Publication: G-6.9 – 2004, Dry Ice.
1345vch06.indd 216
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217
7
Natural Gas
7.1
History
Natural gas just like petroleum developed during the mesozoic (250 to 65 million
years ago) and the tertiary (65 to 1.6 million years ago) during the conversion
from organic substances of predominantly maritime origin that deposited at the
bottom of the sea. With more and more material being taken up, the pressure
on the lower layers increased sharply and the temperature rose to 100 to 200 °C.
Thus crude oil and natural gas developed from the residues of the dead organisms
(Figs. 7.1 and 7.2).
In China, natural gas was used as a fuel a number of centuries earlier than in
Western cultures, maybe even in the 5th century B.C. Methane-rich natural gas
Fig. 7.1 Natural gas development.
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
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7 Natural Gas
Fig. 7.2 Natural gas bubble.
found in brine deposits was gathered together with the brine. Here, the combustible gas was often used to evaporate the brine for the extraction of salt. Later on in
China gas pipelines of bamboo canes were built with which the distance of a day’s
journey could be bridged. Old texts already report on heating value adjustment of
the natural gas by the controlled admixture of air before being fed into the pipelines. The transport of natural gas in big leather sacks was also very common.
The ancient “perpetual fires” in the area of today’s Iraq, as they were already
mentioned in the paper of Plutarch of the period from about 100 to 125 B.C.,
presumably came from natural gas which escaped out of crevices and which was
ignited by flashes of lightning.
In the 19th century, the commercial utilization of natural gas began in Europe
and North America. An essential contribution to this was the invention of the
Bunsen burner by Robert Bunsen in the year 1885. With this device, natural gas
could be mixed with air in the proper ratio to enable safe combustion.
7.2
Occurrence
Natural gas is found in many different layers in the underground. This comprises
formations of slate, sandstone, coal and underground salt water accumulations.
On the ground of deep oceans, solid methane hydrate is found that develops
from water and methane under pressure and at low temperatures. Rich methane
hydrate deposits are situated in the North Siberian Sea. The natural gas deposits
detected today are shown in Fig. 7.3.
The development of the detected deposits over the last twenty years is to be found
in Fig. 7.4. It was above all the discoveries in Russia and around the Persian Golf
that considerably increased the known reserves.
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7.2 Occurrence
219
Fig. 7.3 Proved natural gas deposits. (Source: BP statistical review of world energy 2006).
Fig. 7.4 Development of proved natural gas reserves. (Source: BP statistical review of world energy 2006).
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7 Natural Gas
Natural gas can be subdivided into two categories: dry and wet natural gas.
Here, dry and wet do not refer to the water content, but to the content of higher
hydrocarbons. Dry natural gas is produced from relatively low depths in pure gas
deposits and disposes of a calorific value of about 35 000 kJ m–3. In contrast to
wet natural gas, the dry one is immediately available for use and needs no special
cleaning. Wet natural gas, also called associated gas, usually occurs when crude
oil is pumped from greater depths. Owing to the higher reservoir pressure, the
hydrocarbons gaseous at atmospheric pressure are dissolved in the crude oil. With
the crude oil being pumped, the light and medium-weight hydrocarbons evaporate
owing to decompression and are absorbed again by the natural gas. The carbon
dioxide possibly contained in the natural gas is not only capable of contaminating
the product. In different research project, experts try to develop techniques that
help to avoid the emission of large amounts of this greenhouse gas. Promising
seem to be recent projects in which the carbon dioxide occurring during the
drillings is pumped through the drilling tunnel back under the seabed. In these
experiments, the carbon dioxide accumulates in a porous layer of sandstone
enclosed by slate rocks.
Natural gas is one of the most important energy sources, the availability of
which is of great importance for the global economy. After its pumping together
with crude oil, natural gas is often stored in former already depleted gas fields.
In some cases, these natural reservoirs dispose of a capacity of up to one billion
cubic meters.
7.3
Consumption
The consumption of natural gas differs widely from country to country (Fig. 7.5).
Countries with large own reserves tend to handle the raw material natural gas
more generously, while countries with scarce or lacking resources are of course
more economical. Despite the considerable findings, the predicted availability of
the natural gas reserves has hardly changed. If consumption and resources develop
similarly, as it was the case in the last years, a serious natural gas shortage is to be
expected at the beginning of the second half of the 21st century (Fig. 7.6).
7.4
Natural Gas Trade
As a rule, over shorter and medium distances up to about 3000 km, natural gas is
transported as gas in pipelines. In case the laying of pipelines is not possible for
geographical or political reasons, or the distances between source and consumer
exceed 3000 km by far, then today natural gas is usually liquefied near the source,
transported by ship as LNG (liquefied natural gas) and again converted into its
gaseous state in the vicinity of the consumer (Fig. 7.7).
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7.4 Natural Gas Trade
221
Fig. 7.5 Natural gas consumption per capita. (Source: BP statistical review of world energy 2006).
Fig. 7.6 Ratio natural gas reserves/extraction. (Source: BP statistical review of world energy 2006).
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7 Natural Gas
Fig. 7.7 Cross border trade in natural gas. (Source: BP statistical review of world energy 2006).
At the beginning of the 21st century, the total amount of natural gas consumed
amounted to about 2500 · 109 m3 a–1 or 2500 BCM (see Table 7.1).
With 150 · 109 m3 or 150 BCM (corresponds to about 120 · 106 t or 120 Mtpa)
per year, the share of LNG in the consumption seems to be quite small. However,
in relation to the cross-border natural gas market, this results in a trade share of
more than 20–25% with a tendency to rise (Fig. 7.8).
Table 7.1 Regional classification of natural gas consumption in 109 m3.
Marketed
production
Imports
Consumption
120.0
121.8
739.3
North America
737.5
Latin America
133.4
12.0
12.0
133.4
Europe
283.0
117.3
263.4
428.9
22.9
0.0
45.5
68.4
FSU
719.7
131.1
–
588.7
Africa
127.2
66.3
1.2
62.1
Middle East
229.4
36.6
9.6
202.5
Asia/Oceania
272.3
76.6
106.5
302.1
2525.4
559.7
559.7
2525.4
Central Europe
Total World
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Exports
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7.5 Composition
223
Fig. 7.8 Development of the global LNG-quantities in trade.
7.5
Composition
Depending on the deposit, there are large differences in the composition of natural
gas. It is often the case that heavier natural gas, i.e. rich in higher hydrocarbons,
can be found in deep reservoirs and vice versa. Increased helium contents are
often accompanied by high nitrogen content.
Table 7.2 Composition of natural gas.
Components
Typical
Extreme
Methane CH4
80–95
50–95
% Mol. frac.
Ethane C2H6
2–5
2–20
% Mol. frac.
Propane C3H8
1–3
1–12
% Mol. frac.
Butane C4H10
0–1
0–4
% Mol. frac.
C5 Alkanes and higher hydrocarbons
0–1
0–1
% Mol. frac.
Carbon dioxide CO2
1–5
0–99
% Mol. frac.
Nitrogen N2
1–5
0–70
% Mol. frac.
Hydrogen sulfide H2S
0–2
0–6
% Mol. frac.
Oxygen O2
0
0–0.2
% Mol. frac.
Helium
0–0.1
0–1
% Mol. frac.
Other inert gases
traces
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7 Natural Gas
7.6
Process of Natural Gas Treatment
Usually, natural gas treatment on the basis of thermal process engineering
takes place in three steps (see Fig. 7.9). The first step that may consist of partial
steps just like all other subsequent steps, serves the preparation of the crude
gas for its processing. Here, for example, acid-forming gas components, such
CO2, H2S and other sulphuric compounds are removed. Usually, chemical
scrubbing with amines (MEA, DEA, MDEA) is applied in which the adsorbent
is being regenerated. Then the natural gas is dried. In case of moderate water
dew point requirements, glycol is used as wash liquor. The lowest water contents
(< 1 ppm) are achieved with the application of zeolitic molecular sieves. Finally,
mercury is removed in case aluminium will be used as material of construction
for equipment. Mercury in contact with aluminium may lead to catastrophic
corrosion.
In the central process step, the pre-treated natural gas is separated into a light
and a heavy fraction. As a rule, this separation takes place by means of partial
condensation below ambient temperature.
The light fraction always contains methane and nitrogen, sometimes even lighter
hydrocarbons. For further use it is either compressed to pipeline pressure or
liquefied and used as LNG. Beginning with ethane, the heavy fraction can contain
all higher hydrocarbons that may be isolated, if required, by means of fractioning
and then be marketed in technically pure quality.
Fig. 7.9 Diagram of natural gas treatment.
7.6.1
Dew-point Adjustment
Dew-point adjustment (see Fig. 7.10) serves the reduction of the concentration
of water and heavy hydrocarbons in natural gas to such an extent that no
condensation occurs during the ensuing transport in the pipeline. This would
lead to a multiphase flow that places higher demands on the design and laying
of the pipelines.
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7.6 Process of Natural Gas Treatment
225
Fig. 7.10 Dew point adjustment.
(1) Condenser; (2) Separator; (3) Column; (4) Reboiler; (5) Compressor.
Water can be removed down to a dew point of –30 °C by means of glycol
scrubbing. Then the water-containing glycol is regenerated through rectification
and is reused. Owing to diverse impurities, the arising waste water has to be
reconditioned before it is discharged into the environment.
Usually, the ensuing separation of higher hydrocarbons occurs through cooling
and partial condensation in a heat exchanger (1). The formed liquid phase is
separated (2) and, if required, stabilized in a column (3) through the stripping of
light components by means of a reboiler (4). Now, after being heated up again,
the remaining gas has the required margin from the dew point of the water and
the organic compounds. Usually, the top product of column (3) is fed back to the
sales gas again by means of compression (5).
7.6.2
Separation of Liquefied Petroleum Gas
By liquefied petroleum gas, also called LPG, a mixture of propane and butane is
understood that is often sold in small containers as cylinder gas. Above all, it is
important for a sufficient fuel gas supply of infrastructurally weak areas without
connection to a gas pipeline. In some countries, LPG is very popular as fuel for
motor vehicles. Moreover, LPG is a valuable raw material in the petrochemical
and chemical industry.
The commercial recovery of liquefied petroleum gas from natural gas is carried
out in plants with a capacity range of 10 000 mN3 h–1 up to over 1 000 000 mN3 h–1
of crude gas. The larger the plant, the higher the propane yield may be for an
economic optimum, since the increased investment costs are amortized faster.
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7 Natural Gas
Simple process versions such as in the case of dew point adjustment are not
suitable here, since at a typical crude gas pressure of 50 to 100 bar and a singlestage, partial condensation with ensuing separation of the developed fluid, too
much propane remains in the gas phase, thus limiting the propane separation
from the sales gas.
All modern plants dispose of a cryogenic separation in the core of the process.
Earlier concepts, based on the scrubbing of light hydrocarbons from the crude
gas by means of oil, are no longer market-relevant.
A process version with high propane yield (US-patent 4,157,904 of The
Ortloff Corporation) is shown in Fig. 7.11. The crude gas is cooled and partially
condensed under high pressure in a heat exchanger (1) that may also consist of a
heat exchanger network. If required, even an external refrigerating plant can be
used for cooling, which, as a rule, uses propane as refrigerant. The formed liquid
is separated in a separator (2), expanded, heated up again in the heat exchanger
(1) and fed to a column (4). The column (4) is operated by a reboiler in a way
that the sump product consists of propane and higher hydrocarbons (C3plus in
short). Only small amounts of ethane and lighter hydrocarbons are admitted in
the bottom product. The gas phase from the separator (2) is now divided into
two split flows. The usually larger part is expanded in a turbine (3) and works on
the pressure of the column (4), at the same time it is partly condensed and then
fed to the column. Due to this procedure, a cold flow is fed to the column that
reduces the overhead propane losses. Now the decisive step for the propane yield
is to completely condense the remaining gas flow from the separator (2) against
Fig. 7.11 C3plus separation GSP.
(1) Heat exchanger; (2) Separator; (3) Turbine; (4) Column;
(5) Booster; (6) Compressor; (7) Heat exchanger; (8) Reboiler.
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the cold overhead product of column (4) in a further heat exchanger (7) under
pressure, to subcool it and feed it to column (4) as reflux. This step is the basis for
the customary expression GSP (Gas Subcooled Process). The overhead product,
heated again to ambient temperature in heat exchanger (1) is then recompressed to
a pressure under which it can be discharged to a pipeline. Sales-gas compression
is driven by the mechanical power of the expansion turbine (3) in a booster (5)
and the external energy in a compressor (6). Now the sales gas consists of all feed
gas components with a vapor pressure higher than that of propane, thus basically
of nitrogen, methane and ethane. With this process version, a propane yield of
about 90% can be achieved efficiently.
The propane yield of the GSP-process is limited by the fact that the reflux on the
top of the column (4) is still relatively rich in propane, thus owing to the gas/liquidequilibrium propane gets lost overhead. A possibility for the enhancement of the
propane yield is shown in Fig. 7.12 (US patent 4,854,955 of the Elcor Corporation).
In contrast to the GSP-process, the crude gas flow condensed in the heat exchanger
(7) is not directly fed to column (4) after the expansion to column pressure, but
used as refrigerant in an overhead condenser (11). Propane still getting lost
in the GSP-process can now be recovered in a reflux separator (9) after partial
condensation and again be fed to column (4) by means of the reflux pump (10).
The name SFR (Split Flow Reflux) derives from the additional reflux. Due to this
economical development of the GSP-process, the propane yield can be boosted
to the range between 97 to 99% at constant power consumption.
Fig. 7.12 C3plus separation SFR.
(1) Heat exchanger; (2) Separator; (3) Turbine; (4) Column; (5) Booster;
(6) Compressor; (7) Heat exchanger; (8) Reboiler; (9) Reflux separator;
(10) Reflux pump; (11) Overhead condenser.
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7 Natural Gas
The two processes presented so far share the fact that the crude gas is completely
led through column (4). On the one hand, this makes a larger column diameter
necessary in the upper part of column (4). Moreover, the large amount of methane
in this area dilutes the hydrocarbons to be recovered, thus rendering their
separation more difficult.
In a new approach (see Fig. 7.13) differing from the GSP and SFR-processes,
the upper section of column (4) is separated and designed as a recontactor (9) (US
Patent 4,617,039 of the Pro-Quip Corporation, today incorporated in the Linde
AG). The partially condensed crude gas flow of the expansion turbine (3) is fed to
the sump of the recontactor. The overhead product of the reduced column (4) is
cooled against the top product of the recontactor (9) and at the same time partially
condensed. Here, a larger amount of ethane is condensed in a methane-lean gas.
In the recontactor (9), this liquid ethane now meets a methane-rich gas phase
with significantly lower ethane content. As a result, part of the ethane-rich reflux
of the recontactor (9) evaporates and, owing to the heat of evaporation required
for this, it leads to a significant cooling of gas and liquid in the recontactor (9).
Ultimately, in the OHR-process the lowest process temperature is no longer
caused by the outlet condition at the expansion turbine (9), but by the cooling
effect of this integrated open absorption-heat pump, through which ethane in the
column (4) is stripped under high pressure, liquefied in the condenser (7) and
re-evaporated in the recontactor (9) under low partial pressure. The permanent
ethane losses overhead of the recontactor (9) are continuously replaced by ethane
from the crude gas.
Fig. 7.13 C3plus separation OHR.
(1) Heat exchanger; (2) Separator; (3) Turbine; (4) Column; (5) Booster;
(6) Compressor; (7) Heat exchanger; (8) Reboiler; (9) Recontactor,
(10) Reflux pump.
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Apart from the usual process steps of partial condensation and rectification,
this aspect of the OHR-process brings up the variant of the self-cooling physical
scrubbing, thus opening up new variation possibilities. One of these variants is
increasing the pressure of the recontactor (9) over the pressure of the column (4).
Now, in the partially condensed top product of column (4) a separator is required
from which the reflux can be transported to the recontactor (9) by means of a
pump. As a counter move, pump (10) is no longer required and can be replaced
by a valve. Owing to this modification, the pressure in recontactor (9) can now be
chosen independently from the critical pressure of the C3plus fraction in the sump
of column (4). Instead of the previous limitation to about 30 bar, the pressure of
the methane fraction in recontactor (9) can now be maintained at 40 to 50 bar,
owing to which the expenditure for recompression in the sales gas compressor
(6) can be reduced. Although the OHR-process does allow the highest propane
yields, it is still very popular due to its attractive investment costs and the high
overall efficiency resulting from it.
7.6.3
Ethane Separation
Ethane recovered from natural gas is mainly used in ethylene plants in which
ethylene and other light alkenes are produced from ethane and even higher
hydrocarbons. Since today ethylene plants are only efficient with annual capacities
starting at 500 000 t of ethylene, even the demand of natural gas in the preceding
ethane recovery is high. With a typical ethane content of a mole fraction of 5% in
the natural gas, about 1 200 000 mN3 h–1 of natural gas are required for the ethylene
production mentioned before. Large petrochemical complexes process about two
or three times this amount.
Ethane separation from natural gas (see Fig. 7.14) is carried out in similar
processes as the propane (or liquefied petroleum gas) separation previously
described in detail. Since the patent protection for the GSP-process has expired
for some time (original patent grant in the year 1979), this process belongs to
common knowledge in the separation of natural gas. Since in the case of ethane
separation, column (4) shows a significantly colder temperature profile compared
to propane separation owing to the lighter components, the heat integration
between crude gas cooling and column heating has to be carried out differently.
The liquid from separator (2) is not heated again in heat exchanger (1), but fed
directly to column (4). For the crude gas cooling, cold liquid flows are rather
routed from column (4) and heated against crude gas. Thus, an advantageous
coupling of column heating and crude gas cooling is to be achieved, rendering
superfluous the separate sump heating of column (4) as well as a crude gas
cooling by external sources.
Similar to the propane separation, with this simple process the economic ethane
yield can only be brought to slightly over 90%. For higher yields, the further
development of the GSP will be useful, which is known under the term RSV
(Recycle Split Vapour) (US patent 5,568,737 of the Elcor Corporation, see Fig. 7.15).
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7 Natural Gas
Fig. 7.14 C2plus separation GSP.
(1) Heat exchanger; (2) Separator; (3) Turbine; (4) Column;
(5) Booster; (6) Compressor; (7) Heat exchanger.
Fig. 7.15 C2plus separation RSV.
(1) Heat exchanger; (2) Separator; (3) Turbine; (4) Column;
(5) Booster; (6) Compressor; (7) Heat exchanger.
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Here, with the otherwise same basic process, a split flow of the compressed sales
gas is recycled, completely condensed in the heat exchanger (7), subcooled and
finally fed to column (4) as reflux. This reflux is significantly leaner in ethane
compared to the reflux used in the GSP-process. Thus, better retention of the
ethane at the top of column (4) is possible owing to which ethane yields between
95 and 99% are economically achievable.
7.6.4
Liquefaction
With the liquefaction of natural gas, an energy density is obtained that corresponds
to about three times the pressure storage at 200 bar. This fact is favourably used
for storage and transport purposes.
Natural-gas liquefaction was originally used to balance the seasonally different
demand for natural gas. In these so-called peak-shaving plants, natural gas is
typically liquefied in the in-between seasons and in summer, and is stored as LNG
in large tanks. In times of peak demand, often occurring in January, natural gas
is re-evaporated and fed into the pipeline system. The first plant of this kind was
built in 1939 in West Virginia, USA. Even today, a lot of these plants are operated
mainly by local energy suppliers. Today, however, there are hardly any new projects
for peak-shaving LNG-plants, since caverns are used for the storage of highpressure natural gas. This renders the expensive liquefaction and cold storage of
the natural gas superfluous. For a large part of the existing peak-shaving plants,
refrigeration is based on expander cycles as they are common, for example, for air
separation and liquefaction. This uncomplicated technique is particularly suited
for the intermittent operation of a typical peak-shaving plant.
Apart from serving as permanent energy store, LNG is also an alternative to the
natural gas transport via pipeline. In case the laying of pipelines is not possible
for geographical or political reasons, or if distances between source and consumer
significantly exceed 3000 km, today natural gas is usually liquefied near the source,
transported by ship as LNG and converted into its gaseous state again near the
consumer. Liquefaction plants of this kind serve the permanent basic supply of
gas customers and are therefore called Base-Load-Plants.
The first Base-Load LNG-Plant was put into operation in Algeria in 1964. From
today’s point of view, the early plants disposed of a low liquefaction capacity of
below one million tons of LNG per year (< 1.0 mtpa LNG). The cold required for
the liquefaction was usually generated in a series arrangement of pure component
refrigeration cycles (pure component cascade) or in one single mixed refrigerant
cycle.
Since in the case of pure refrigerants, the temperature does not change during
evaporation at constant pressure, an energetically advantageous small temperature
difference between warm and cold flows can be realized through the application
of a number of evaporation stages (see Fig. 7.16).
Despite the high equipment cost of a pure-component cascade resulting from
the multitude and complexity of the recycle compressors, the specific energy
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7 Natural Gas
Fig. 7.16 Cooling of a typical natural gas against a pure component refrigerant cascade.
consumption is still quite high. Various companies developed improved concepts
for the reduction of energy consumption and investment costs of the refrigeration
cycles – criteria that are increasingly important for plants with a capacity of more
than 1.0 mtpa of LNG.
Over the last decades, Air Products & Chemicals, Inc. held an outstanding
market position mainly based on the C3MR-process (LNG Air Products C3MR,
see Fig. 7.17). In this process, for the first time ever applied in Brunei in 1973, the
pretreated natural gas is first cooled (1) to about –30 °C by a multi-stage propane
refrigerating unit installed around a large recycle compressor (2) and a condenser
Fig. 7.17 LNG Air Products C3MR.
(1) Heat exchanger; (2) Recycle compressor; (3) Condenser;
(4) Heat exchanger; (5) Separator; (6) Compressor; (7) Condenser.
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Fig. 7.18 Cooling of a typical natural gas against pure component
precooling with ensuing mixed refrigerant cycle.
(3). Liquefaction and subcooling of the natural gas to –160 °C occur against a mixed
refrigerant in a heat exchanger (4) of special design described in more detail at
the end of this chapter. This mixture, normally consisting of light hydrocarbons
from methane to pentane and nitrogen, is chosen in a way that the evaporation
process over a wide temperature range approaches the cooling curve of the warm
process flows (see Fig. 7.18).
In detail, the mixed refrigerant cycle is structured as follows: the gaseous
refrigerant is fed to the recycle compressor at about –30 °C and at low pressure of
about 2 to 3 bar and compressed to a high discharge pressure of more than 50 bar.
As a rule, two separate compressor casings are required for this compression, as
there is not enough space in a common casing for the multitude of compressor
impellers necessary owing to the high pressure ratio. In the aftercooler (7), the
compression heat is then discharged to the ambient. Now the C3-precooling
cycle is used to partly condense the mixed refrigerant. In the cold separator (5) a
high-boiling liquid and a low-boiling gas develop which are further cooled down
separately in the heat exchanger (4). At the same time, the gas flow of (5) is being
completely condensed and fed as refrigerant to the top of the heat exchanger (4)
after expansion. At the same pressure, the liquid phase separated in separator (5)
shows a boiling range shifted towards higher temperatures owing to the heavier
components, and is therefore suited to the cooling of the warm flows entering
the heat exchanger (4). Thus the mixed refrigerant is first separated into two
fractions in the separator (5) and then mixed again in heat exchanger (4). The
C3MR-process, extremely successful on the market is predominantly applied in
the performance range of 1–5 mtpa of LNG.
In the case of higher plant capacities per train, limiting factors are above all
the main dimensions of the mixed refrigerant cycle compressor (6) and the heat
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7 Natural Gas
Fig. 7.19 LNG Linde/Statoil MFC.
exchanger (4), and here mainly in the lower, warm range. With the aim of shifting
the capacity barrier of base-load LNG-plants upwards and reducing specific
investment and operating costs, the Linde AG company, in cooperation with the
Norwegian Statoil ASA, has developed the Mixed Fluid Cascade (MFC process)
(see Fig. 7.19). In contrast to the C3MR-process, in the MFC-process all refrigerant
flows of different composition are kept separate at any time. The cold separator (5)
is redundant. Instead of this, mixed refrigerants are used in separate closed cycles
the composition of which corresponds more or less to the gas respectively liquid
phase of the separator (5). The additional cold mixed refrigerant cycle has now a
separate refrigerant evaporator (8), serving the subcooling of the LNG, as well as
a recycle compressor (9) with aftercooler (10). Since the refrigerant completely
evaporated in the heat exchanger (8) is now led directly to the corresponding
compressor (9) and is no longer first superheated in the warm lower part of the
heat exchanger (4) and than led to compressor (6) together with the heavy fraction
of separator (5), as it is the case in the C3MR-process, both compressor (6) and
the warm lower part of heat exchanger (4) are significantly relieved. Owing to
this measure, the upper capacity barrier of a single-line LNG-plant is raised to
about 12 mtpa of LNG. Moreover, the specific energy consumption is reduced
due to precooling (1) based on a further third mixed refrigerant cycle. A first
LNG-plant according to the MFC-principle is currently being built and will be
put into commercial operation in the year 2007, with a liquefaction capacity of
4.3 mtpa of LNG.
Almost all base-load LNG-plants built so far are using a special construction
for the heat exchangers (4) respectively (8), which is termed as coil-wound heat
exchanger (see Figs. 7.20 and 7.21). Here, a number of heat-exchanger tubes,
sometimes several thousand, are wound on a central core tube (mandrel). This
process is comparable to the winding up of yarn on a bobbin. Thus, huge heating
surfaces of several ten thousand square meters can be accommodated in one
apparatus. In the tubes, the flows to be cooled down are arranged upwards. On
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Fig. 7.20 Coil-wound heat exchanger on the winding bench.
the shell side, the cold flow, in this case the evaporating refrigerant, is falling,
owing to which all tubes are equally cooled down. Owing to their mechanical
robustness, coil-wound heat exchangers are highly esteemed. For design reasons,
one shell flow only can be led against several tube flows. Brazed aluminium heat
exchangers, a frequently used alternative in cryogenic processes, are much more
flexible with regard to the flow arrangement. However, owing to the limited core
dimensions many parallel cores are required. At the time of printing, however, it
was still not possible to apply brazed aluminium heat exchangers successfully in
large base-load LNG-plants with mixed refrigerant cooling.
Besides Air Products & Chemicals, Inc., today the Linde AG is the only
manufacturer worldwide of coil-wound heat exchangers suitable for base-load
LNG-plants. Figure 7.21 shows two of these items manufactured by the Linde AG
for an LNG-plant in Australia that works according to the C3MR-process.
Fig. 7.21 Coil-wound heat exchanger ready for shipment.
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7 Natural Gas
7.6.5
Nitrogen Separation
Natural gas is a widespread energy carrier used by a multitude of consumers. Since
pipeline systems often connect a lot of providers with a lot of users, the adherence
to certain specifications for the required compatibility of the different kinds of
natural gas is necessary. Very important quality features in this connection are the
calorific value and the Wobbe number. In case natural gases with high nitrogen
content cannot be adapted to pipeline standards by mixing them with other natural
gases of suitable composition, nitrogen separation is often required. As a rule, the
separated nitrogen is emitted to the atmosphere, consequently it has to comply
with the stringent conditions regarding the residual content of hydrocarbons. The
admissible residual content of nitrogen in natural gas usually amounts to some
percent. For separation units with these demands, i.e. high purity and at the same
time high yield in relation to one gas component, even nowadays cryogenic plants
(see Fig. 7.22, N2-separation without pre-separation) are economically superior to
newer processes on the basis of adsorption or permeation.
After the pretreatment usual for cryogenic plants, the crude gas is cooled down
in a first heat exchanger (1) and partially condensed. The developing liquid, which
contains only little dissolved nitrogen, is evaporated again under a pressure as
high as possible and fed to the second stage of the two-stage sales gas compressor
(8/9). The remaining gas phase of (2) is cooled down further in the heat exchanger
(3) and fed to the lower part of a double column. The sump heating of the lower
column part (5) is also integrated into the heat exchanger (3). The task of the lower
Fig. 7.22 N2-separation without pre-separation.
(1) Heat exchanger; (2) Separator; (3) Heat exchanger; (4) Heat exchanger;
(5) Column; (6) Heat exchanger; (7) Methane pump; (8, 9) Compressor.
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column part is the separation of the crude gas into a methane-lean top product and
a nitrogen-lean sump product. Then, both primary products are fed to the upper
part of the column, with the sump product being subcooled in heat exchanger
(3) and the top product being condensed, also subcooled, in heat exchanger (4),
and finally used as reflux. Quantity and composition of this reflux are responsible
for the quality of the top product – the nitrogen to be separated – of the upper
column. The highly concentrated nitrogen is heated up in the heat exchanger chain
4–3–1 and discharged to the ambient. Now, the sump of the upper column is a
nitrogen-lean methane fraction that evaporates in heat exchanger (3) after internal
compression by means of pump (7) and is heated to ambient temperature in heat
exchanger (1). Afterwards, this flow is brought to discharge pressure to the pipeline
with the help of the sales gas compressor (8/9). The heat integration of these two
sections of column (5) occurs similarly to an air separation unit to the effect that
the operating pressure of the lower (pressure) column is chosen high enough to
enable the top product of the pressure column to be condensed in a heat exchanger
(6) against the evaporating sump product of the upper (low-pressure) column.
During the operation of a plant, quality and yield of the separated nitrogen are
only to be influenced by the discharge pressure of the methane pump (7). High
pressure of the pump results in poor separation and vice versa.
In case the nitrogen content of the natural gas falls below 25% by volume, the
process described above is no longer suitable for the separation of nitrogen with the
required volume and purity. In such cases, a process with pre-separation column
(10) (see Fig. 7.23) is applied. Here, in the first heat exchanger the feed gas is
Fig. 7.23 N2-separation with pre-separation.
(1) Heat exchanger; (2) Separator; (3) Heat exchanger; (4) Heat exchanger;
(5) Column; (6) Heat exchanger; (7) Methane pump; (8, 9) Compressor;
(10) Column.
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7 Natural Gas
deeper precondensed, and from the liquid separated in the separator a nitrogenrich overhead fraction is stripped. The nitrogen-lean methane quantity removed
via the sump of the column (10) is now large enough for the gases discharging
from separator (2) and column (10) to be further separated in the double-column
operation described above, owing to their now increased nitrogen content. The
initially surprising fact that the reduction of a low nitrogen concentration in the
crude gas involves higher expenditures than a high nitrogen concentration, is
to be explained by the fact that in the first case the concentration factor of the
nitrogen to be separated is higher and therefore represents the more demanding
processing task.
For this reason, natural gas fields with a nitrogen content increasing over the
time represent a special procedural challenge. This situation is more and more
caused by tertiary oil production in which gases, among them also nitrogen,
are re-injected in order to maintain the natural reservoir pressure. After some
time, nitrogen finds its way into the neighbouring natural gas fields and makes
a nitrogen separation from the natural gas necessary to keep the natural gas still
salable.
7.7
Applications
Natural gas is used as:
x
x
x
x
x
a fuel for industrial heating and desiccation processes
a fuel for the operation of public and industrial power stations
a household fuel for cooking, heating and providing hot water
a fuel for environmentally friendly liquid natural gas vehicles
a raw material for chemical synthesis (see also Methane and other Fuel Gases,
Section 8.5)
x a raw material for large-scale fuel production using gas-to-liquid (GTL) processes (e.g. to produce sulphur- and aromatics-free diesel with low-emission
combustion)
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8
Fuel Gases
8.1
Introduction
Combustible gases have an ignition range and an ignition temperature when
mixed with air or oxidizing substances. Combustible gases are self-igniting if the
ignition temperature is below 100 °C. Even at room temperature these gases can
react so strongly that the exothermal energy leads to flame occurrence. Instable
gases are prone to exothermal spontaneous decomposition without requiring air
or an oxidizing substance.
In general combustible gases with a high calorific value are referred to as fuel
gases (Table 8.1).
Table 8.1 Overview of important fuel gases [8.1, 8.2].
Name
of the gas
Chemical formula
Ignition
range in air
(% vol. fraction)
Ignition
temperature
(°C)
Specific
calorific value
(kJ kg–1)
Acetylene
C2H2
2.4–83.0
325
49 912
Butane
C4H10
1.5–8.5
365
49 500
1-Butene
C4H8
1.6–10.0
440
48 426
Natural gas
Mixtures esp. of
CH4, C3H8, CO2,N2
depending on
composition
depending on
composition
Ethane
C2H6
3.0–15.5
515
51 877
Ethene
C2H4
2.7–34.0
425
50 283
Methane
CH4
5.0–15.0
595
55 498
Propane
C3H8
2.1–9.5
470
50 345
Propene
C3H6
2.0–11.1
455
48 918
Hydrogen
H2
4.0–75.6
560
141 800
depending on
composition
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
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8 Fuel Gases
They are used for specific purposes on an industrial scale, such as acetylene
for welding and cutting steel.
The fuel gases acetylene and ethene are discussed below.
8.2
Acetylene C2H2
8.2.1
Acetylene and the Beginnings of Welding Engineering
The historical development of acetylene runs parallel to the history of carbide
from which it was initially formed after the addition of water.
The first reference to carbide and acetylene came from the Irish chemist Edmund
Davy in England in 1836. When trying to produce metal potassium by heating a
mixture of calcined potassium tartrate with charcoal, he obtained a black mass
as by-product – i.e. calcium carbide – which reacted with water and formed a
combustible gas. In a scientific report he described the characteristic properties
of acetylene in detail. At that time, he called the gas “bicarburet of hydrogen”,
which means more or less “double carbon bond of hydrogen”.
In 1860, the Frenchman Berthelot again came across the gas discovered by
Davy and investigated its properties more thoroughly. He named it “acetylene“
and determined its chemical formula as C2H2. He also recognized that this gas
formed the first element in a range of hydrogens whose formula he determined
as CnH2n–2.
Berthelot also succeeded in producing acetylene from organic raw materials, i.e.
from hydrocarbons and was thus to all intents and purposes one of the founders
of organic chemistry.
In 1862 F. Wöhler found the (at that time) more natural way to produce acetylene
when, searching for metal aluminum, he succeeded for the first time in producing
calcium carbide that releases acetylene when treated with water. However, Wöhler’s
discovery remained on an academic scale since the large amounts of electric
energy, which are today applied as a matter of course, were not available at that
time. The basis of the carbide industry – inexpensive energy in large amounts
– was not formed until the invention of the dynamo machine by Werner von
Siemens in the year 1866. The first electric furnaces to be powered with electrical
energy gained from water power were put into operation after 1892, which enabled
the production of calcium carbide on an industrial scale and also with suitable
purity. The German-Canadian chemist T. L. Wilson and the Frenchman and later
noble laureate H. Moissan are closely associated with these events. In 1892 Wilson
succeeded in producing calcium carbide for the first time in an electric furnace
he himself had built and is therefore regarded as the founder of the acetylene
industry. In the USA Wilson was granted a patent for his process in 1893.
At that time, acetylene won from carbide was very much in demand as it burned
with a bright shining flame. Its luminance easily surpassed that of candles,
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241
petroleum lamps and coke gas. Euphorically people referred “to the sun having
been brought to earth”. Acetylene lighting spread quickly, but the success story
hoped for by the fast expanding carbide industry was only fulfilled to a small extent.
The American Edison invented the filament lamp in 1879 with which electric
current was used directly for lighting purposes. Even the brightest acetylene lamp
did not have a chance in the long run against this competition.
At the turn of the century, however, another property of acetylene gained growing
importance in industry – its extremely high combustion temperature with oxygen,
i.e., 3160 °C. Carl von Linde’s invention of liquefying air and isolating oxygen from
it greatly accelerated the development of autogenous technology. Autogenous
technology is the process in which ferrous metals can be separated or bonded by
means of the acetylene-oxygen flame.
The biggest step forward for the autogenous technology was the ability to safely
store and transport gases under pressure in special steel cylinders. This is especially
valid for acetylene for which special measures had to be taken as a result of its
thermal instability (proneness to decomposition). Acetylene can only be stored
and transported safely in steel cylinders if it is dissolved in a suitable liquid, such
as acetone, at a maximum pressure of 25 bar and if the solvent with the acetylene
dissolved in it is absorbed inside the steel cylinder by a porous mass which does
not conduct heat well. The most urgent task of the national and international
associations, e.g. “Association for Autogenous Metal Processing e.V.”, “DVS
– German Welding Society”, “IIW – International Institute of Welding” and “EWF
– European Federation for Welding, Joining and Cutting” was the safety aspect.
The necessity of establishing rules and regulations which have to be observed by
gas producers and users was and still is obvious. The accidents, some of which
were serious, occurring initially during the production and handling of acetylene
(and oxygen) had to be prevented at all costs.
In the period before the development of petrochemistry, acetylene was the basis
of organic chemistry, as is still the case in some industries today. Calcium carbide,
the ideal energy store, can be easily won from coal, lime and electric energy and
can be stored in large quantities without difficulty. Similarly acetylene can easily
be produced from carbide. A variety of products can then be produced from the
acetylene after bringing it together with other elements and compounds. In
Germany today about 60% of the total acetylene production is supplied by the
petrochemical industry and 40% is still produced in carbide-acetylene plants.
8.2.2
Physical Properties
The main physical properties of acetylene (ethyne) are shown in Table 8.2
[8.1, 8.2].
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8 Fuel Gases
Table 8.2 Physical properties of acetylene.
Property
Symbol
Unit
Value
Molar weight
M
g mol–1
26.038
°C
bar
–80.55
1.2819
Triple point temperature
Pressure
Critical point temperature
Pressure
Density
TC
PC
UC
°C
bar
g m–3
35.18
61.9
231
Standard density at 15 °C/1bar
UN
kg m–3
1.095
Density relative to air at 15 °C/1bar
Specific heat
(at 0 °C/1.013bar)
J g–1
0.1018
Ignition temperature (1.013 bar)
x with air
x with oxygen
°C
°C
355
300
Max. flame temperature with oxygen
°C
3160
Ignition limits at room temperature
x with air
x with oxygen
% volume fraction
% volume fraction
2.3 … 82
2.5 … 93
Combustion velocity
m s–1
1.35
Formation enthalpy
kJ mol–1
226.9
kJ kg–1
kJ kg–1
50 400
48 700
Thermal conductivity
(at 0 °C, 1.013 bar)
W m–1
0.0184
Molar volume
(at 0 °C, 1.013 bar)
m3 kmol–1
22.223
Conversion figures
Data: 1 kg = 0.909 m3
kg
m3
1
0.909
Upper calorific value
Lower calorific value
1345vch08.indd 242
0.905
cP
HU
HL
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8.2 Acetylene C2H2
243
8.2.3
Acetylene Decomposition – Deflagration
Acetylene (C2H2, ethyne) has a high reactivity as a result of the triple bond. The
triple bond can be broken down through external influences, such as heat supply.
This process is called decay or decomposition, in special cases even deflagration.
When this happens the acetylene molecule decomposes into its components
carbon and hydrogen and the large amount of energy stored in the triple bond,
which is required to keep it together, is released. The energy released in the form
of heat can stimulate the neighboring molecules to decompose too, which may lead
to a chain reaction. This property is the source of the risk potential of acetylene.
The safety risk can be controlled but it requires an exact knowledge of possible
reactions to certain external influences.
Applied to modern acetylene technology, this results in the following safety
principle:
Acetylene under pressure is only permitted to be enclosed in cavities of limited
volume that can be sealed off from each other or in pipes of limited nominal
diameters and limited maximum allowable pressure. The mechanical stability of
the casing of these cavities and pipes has to be great enough to safely withstand
the possible consequences of decomposition under operating conditions.
8.2.4
Ignitable Mixtures
Table 8.2 shows that acetylene is ignitable in air within the wide range of 2.3–82.0.
It has a combustion velocity of 1.35 m s–1 and an enthalpy of 226 kJ mol–1.
Acetylene can to all intents and purposes be regarded as a gaseous explosive,
comparable to other better known explosive agents. As reference explosive one
quantity unit of TNT (trinitrotoluene) is internationally valid and here the quantity
of 1 kg of TNT is used for the comparison:
The following is applicable:
1 kg
1 kg
1 kg
1 kg
Nitroglycerine
C2H2 in decomposition
C2H2 with O2
Oxyhydrogen
to
to
to
to
1.25 kg TNT
1.72 kg TNT
2.41 kg TNT
3.14 kg TNT
8.2.5
Liquefaction of Acetylene – Acetylene Hydrate
In the aggregate state “liquid” acetylene has particularly high energy content and
therefore it has explosive character. In the vapor-pressure curve (Fig. 8.1) basic
data such as the critical point and the dew point are shown. The formation of
liquid acetylene should be absolutely avoided when handling this gas, as this state
is not manageable without elaborate measures.
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8 Fuel Gases
Fig. 8.1 Critical point and dew point of acetylene.
Liquefaction of acetylene in acetone-treated cylinders is impossible as the
pressure in the cylinder cannot reach the liquefaction pressure at low temperatures
[8.3].
8.2.6
Acetylene Hydrate
Acetylene hydrate originates through the addition of water to compressed acetylene
in a temperature range of –5 to +10 °C. Its formula is C2H2 · 5.75 H2O and it has
a waxy appearance. Its risk potential is not as high as that of liquid acetylene. It
can however ignite when hit by shock waves. Similar to liquid acetylene, it can
originate during compression at winter temperatures. This has to be avoided by
taking appropriate safety measures.
When acetylene hydrate occurs in the equipment of an acetylene plant, especially
in decomposition barriers and other safety accesories, it is less the danger of
explosion than the blockage of the gas paths through the waxy mass that has to
be taken into consideration.
8.2.7
Acetylides
Under certain circumstances acetylene combines with the metals copper, silver
and mercury to form acetylides. As a dry substance these acetylene compounds
are explosive and ignite through impact or friction. Compared to the other
acetylides, silver acetylide releases the largest amount of energy in an explosion.
Acetylides can originate and precipitate from watery saline solutions of the metals
mentioned above under certain conditions regarding temperature, pH value and
concentration. These precipitation reactions were previously used in the wet
chemical acetylene analytics.
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245
In acetylene engineering – and especially in the generation of acetylene from
carbide as well as in supply engineering – the possibility of generating acetylide is
an important factor when selecting the material for the equipment. The formation
of acetylides is not only possible in saline solutions in laboratory experiments
but it also occurs when moist crude acetylene comes into contact with metallic
silver or copper surfaces. Corrosion products encourage acetylide formation on
copper surfaces.
Tests have shown that there is considerable acetylide formation when pure
copper or copper alloys (brass) have a Cu-content > 70%. The layer thicknesses can
reach a strength that allows the separation of pure acetylide as particles, similar
to scale. With Cu contents of 70% and below, very thin acetylide layers are still
possible but there is no danger of ignition.
Fig. 8.2 Plant sketch of an acetylene generator (Type Sirius-Linde 400,
designed for a gasometer overpressure of 750 mm water column { 7.35 kPa).
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8 Fuel Gases
Regulations concerning the use of materials containing silver or copper can be
found in the appropriate guidelines, such as the TRAC 204 “Technical regulations
for acetylene plants and calcium carbide storage” or EIGA “Code of practice
acetylene”. If these rules are observed, there is no risk in using these metals in
acetylene plants [8.3].
8.2.8
Extraction Processes
8.2.8.1
Acetylene Generated via Carbide
Acetylene is traditionally generated by the reaction of calcium carbide with water.
The low-pressure development system Sirius-Linde is described in systems
engineering terms as a highly efficient system with trouble-free operation.
Acetylene is purified and dried after generation [8.3].
8.2.8.2
Petrochemically Generated Acetylene
Acetylene which is won petrochemically is usually offered in very large quantities at
the respective plant as acetylene is no longer of prime importance in the chemical
industry as a raw material for chemical products. One advantage over carbide
acetylene is that there are no by-products and therefore no facilities required for
their disposal.
The petrochemical manufacture of acetylene is based on reprocessing cracked
gases with the ensuing extraction of C2 hydrocarbons [8.4].
8.2.9
Gas Supply
8.2.9.1
Storage of Dissolved Acetylene in Cylinders
Acetylene can only be stored in a gaseous state. The supply as liquefied gas, as
with air gases (argon, nitrogen, oxygen and the like), is ruled out due to the very
high risk of explosion.
In order to prevent acetylene decomposition, initiated by a flashback or external
heating, from spreading and to bring it to a standstill, the steel cylinders have to
be specially prepared with a porous or high-porous mass. In the past, Mikropor A,
consisting of pumice, kieselguhr, charcoal and magnesium carbonate, was used
as a filling mass. A high-porous monolithic mass of 90% porosity consisting of
calcium oxide, silica flour and water has, since the 1960s, taken the place of the
previous filling mass. It is filled into the steel cylinder in viscous form and is then
steam cured in the furnace. For the improvement of the mechanical stability a
special kind of glass fiber is now used instead of asbestos fibers.
The solvent plays the main role in the safety system “acetylene cylinder“, in which
acetylene can be dissolved under pressure. Two solvents, which are comparable
regarding their protective properties in acetylene cylinders, are used world wide
– acetone and Dimethyl Formamide (DMF).
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247
Due to the differences in vapor pressure and dissolving power for acetylene,
acetone can be favorably applied in climate zones with summer and winter. DMF
should be used in warmer climate zones, where temperatures remain considerably
above freezing all the year round. The reason for this lies in the higher dissolving
power of DMF for acetylene at lower temperatures.
DMF is detrimental to health and as a liquid can be absorbed through the skin.
When handling these two solvents, the respective rules of conduct contained in
the safety specifications, laws and rules and regulations must be observed.
8.2.9.2
Design of a Gas Supply System
The design of a gas supply system depends on the one hand on the maximum
discharge quantity of an acetylene cylinder, type 40, 48, 50 (500 l per hour in normal
single-shift operation), on the other hand on the weekly average consumption
and also on cost-effective logistics.
Gas supply systems can comprise of single-cylinder units, gas cylinder manifold
units, cylinder bundle units (6 or 16 cylinders) and mobile acetylene supply plants,
e.g. containers, acetylene trailers with 8 or 16 bundles (16 cylinders per bundle)
that can provide up to 2304 kg of acetylene with one vehicle.
An acetylene supply plant consists of a high-pressure, medium-pressure and
possibly a low-pressure section. In addition to the pressure regulators, safety
devices such as manual safety cylinder or bundle connectors, non-return valves,
automatic quick acting shut-off devices, flame arrestors and multifunctional safety
devices have to be installed.
As basic rules and regulations for the planning of an acetylene supply unit EN
or ISO standards, e.g. ISO 14 114, ISO 5175, ISO 7291, ISO 2503, ISO 14 113,
ISO 15 615 have to be complied with. The initial approval of acetylene plants has
to be carried out by qualified persons and/or a notified body. Recurrent inspections
are also required.
Before planning or modifying such plants it is advisable to contact a gas supplier
who employs specialized application engineers and qualified staff.
8.2.10
Autogenous Engineering Applications
The molecule acetylene has an enormous amount of combustible energy with high
flame efficiency and ignition velocity in the acetylene flame. As soon as molecular
decomposing begins energy is released – in contrast to other fuel gases. This
energy is called heat of formation or enthalpy. Acetylene releases 8714 kJ kg–1
for use at this stage.
The energy of the first combustion phase with oxygen, the primary flame, has
to be added. Only this energy is of importance in autogenous engineering, a
considerable advantage compared to other fuel gases.
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8 Fuel Gases
Main application fields of autogenous engineering (cf. Section 8.5):
x
x
x
x
x
x
x
x
x
x
x
Flame cutting
Flame cleaning
Flame grooving
Flame straightening
Flame heating
Flame hardening
Gas welding
Hard-face welding
Flame spraying
Flame brazing
Carbon boating
8.2.11
Regulations
When operating, planning, constructing, at the initial and recurrent inspections,
during maintenance and service of acetylene gas cylinders, bundles and their
supply units national, European or international rules, orders, regulations have
to be complied with, e.g.:
1. Directive 97/23/EC of the European Parliament and of the Council of 29 May
1997
2. EC Directive 1999/92/EC of the European Parliament and of the Council of
14 Dec 1999 explosion protection
3. ISO 5175/EN 730-1 and -2
Equipment used in gas welding, cutting and allied processes – safety devices for
fuel gases and oxygen or compresses air – general specifications, requirements
and tests
4. ISO 7291/EN 961
Welding, cutting and allied processes – manifold regulators
5. ISO 2503/EN ISO 2503
Gas welding equipment – pressure regulators for gas cylinders used in welding,
cutting and allied processes up to 300 bar
6. ISO 14 113/EN ISO 14 113
Gas welding equipment – rubber and plastic hoses assembled for compressed
or liquefied gases up to a maximum design pressure of 450 bar
7. ISO 14 114/EN ISO 14 114
Gas welding equipment – acetylene manifold systems for welding, cutting and
allied processes, general requirements
8. EIGA – European Industrial Gases Association
Code of practice acetylene, IGC Doc 123/04/E
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8.3 Ethene C2H4
249
8.3
Ethene C2H4
8.3.1
Physical Properties
In Table 8.3 the fundamental physical properties of ethylene (ethene), C2H4 are
shown [8.1, 8.2].
8.3.2
Production Processes
The petrochemical production of ethene is based on reprocessing cracked gases
and their isolation and the ensuing extraction of C2 hydrocarbons [8.5].
8.3.3
Application and Use
Ethene is also used for industrial production in autogenous engineering due to
its more favorable chemical-physical properties with regard to flame temperature,
ignition velocity, flame efficiency and heat of formation. These advantages
distinguish ethane from the other more slowly burning fuel gases. The values
are however below those of acetylene.
Main application fields in autogenous engineering (cf. Section 8.5):
x
x
x
x
x
x
x
Flame cutting (cutting efficiency lies between propane and acetylene)
Flame grooving
Flame straightening
Flame heating
Gas welding
Flame spraying
Flame brazing
8.3.4
Gas Supply and Safety
The design of the gas supply depends on the one hand on the required maximum
gas amount in m3/h together with the supply pressure, on the other hand on the
weekly average consumption and in addition on cost-effective logistics. Gas supply
systems can comprise of single-cylinder units, gas cylinder manifold units as well
as cylinder bundle units (e.g. 12 cylinders per bundle).
At constant volumes of more than approx. 500 m3 per month, there is the
possibility of a supply via heat-insulated stationary tanks with downstream
evaporators. The low-temperature ethene is delivered to the customer in a liquid
state by means of special tank trucks and then transferred loss-free into the
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8 Fuel Gases
Table 8.3 Physical properties of ethylene.
Property
Symbol
Unit
Value
Molar weight
M
g mol–1
28.054
Triple point resp. melting point at 1.013 bar
Vapor pressure
Melting heat
°C
bar
kJ kg–1
–169.43
0.0012
119.45
Boiling point at 1.013 bar
Heat of evaporation
°C
kJ kg–1
–103.72
482.86
°C
bar
g/L
9.5
50.76
218
In liquid state:
x Density at the boiling point at 1.013 bar
x Specific heat at the boiling point
g L–1
kJ kg–1 K–1
567.92
2.42
In gaseous state:
x Density at 0 °C and 1.013 bar
x Specific heat at 25 °C and 1.013 bar
x Thermal conductivity at 15 °C and 1 bar
kg m–3
kJ kg–1 K–1
µW cm–1 K–1
1.261
1.54
188
Critical point temperature
Pressure
Density
TC
PC
UC
Density relative to air
Ignition temperature (1.013 bar)
x with air
°C
425
Max. flame temperature with oxygen
°C
2924
Ignition limits at room temperature
x with air
x with oxygen
% volume fraction
% volume fraction
2.7 … 34.0
2.9 … 80.0
Formation enthalpy
kJ kg–1
1865
kJ kg–1
47 600
m3 kmol–1
22.245
L
kg
m3
1
0.568
0.482
Lower calorific value
Molar volume
(at 0 °C, 1.013 bar)
Conversion figures for different states
Date: 1 L liquid = 0.568 kg = 0.482 m3
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HL
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8.4 Other Fuel Gases
251
customer tank via a rotary pump. If necessary, the liquefied ethene is evaporated
in the evaporator and conducted via pipes to the points of consumption, each of
which is equipped with a multifunctional safety device. The capacity of the ethene
storage depots ranges from 1500 kg up to 30 000 kg and higher. The admissible
operating pressure of the tank is 18 bar. The required supply pressure is adjustable
over the whole pressure range.
The fundamental rules and regulations for the design of ethene supply plants
laid down in international and national rules, orders or directives have to be
followed. The initial approval of ethene plants has to be carried out by qualified
persons and/or a notified body. Recurrent inspections are also required.
Before planning or modifying such plants it is advisable to contact a gas supplier
who employs specialized application engineers and qualified staff.
8.3.5
Regulations
When operating, planning, constructing, at the initial and recurrent inspections,
during maintenance and service of acetylene gas cylinders, bundles and their
supply units the following has to be observed (among others):
1. Directive 97/23/EC of the European Parliament and of the Council of 29 May
1997
2. EC Directive 1999/92/EC of the European Parliament and of the Council of
14 Dec 1999 explosion protection
3. ISO 5175/EN 730-1 and -2
Equipment used in gas welding, cutting and allied processes – safety devices for
fuel gases and oxygen or compresses air – general specifications, requirements
and tests
4. ISO 7291/EN 961
Welding, cutting and allied processes – manifold regulators
5. ISO 2503/EN ISO 2503
Gas welding equipment – pressure regulators for gas cylinders used in welding,
cutting and allied processes up to 300 bar
6. ISO 14 113/EN ISO 14 113
Gas welding equipment – rubber and plastic hoses assembled for compressed
or liquefied gases up to a maximum design pressure of 450 bar
8.4
Other Fuel Gases
The preceding chapters dealt exclusively with the fuel gases acetylene and ethene.
Other fuel gases, such as e.g. ammonia, chlorine ethane, chlorine methane,
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8 Fuel Gases
dimethyl ether, isobutane, isobutene, phosphine etc. are described in [8.1, 8.2,
8.6, 8.7].
8.5
Applications
Acetylene (ethyne) is used as:
x high-performance fuel gas for autogenous processes (see Section 2.5.2)
– in combination with O2, e.g. for welding, cutting, straightening, brazing,
soldering, hotforming
– in combination with compressed air, e.g. for preheating before – and past –
heating after welding
x separating agent that deposits pure carbon (e.g. for the glass and aluminium
industries)
x combustible carrier gas for flame-photometry in analytical chemistry
x basic substance for the production of organic compounds (e.g. acetaldehyde,
monomer vinyl chloride, acetanhydride)
Ethylene (ethene) is used as:
x pure fuel gas and main component in fuel gas mixtures for autogenous processes
(allowing transport and storage in the liquid phase)
x ripening gas for the controlled ripening of stored fruit (e.g. with N2/ethylene
mixtures for bananas)
x basic substance for the production of organic compounds (e.g. polyethylene,
polyvinyl chloride, polyether, polyvinyl ether, anthracene, ethylene oxide,
isoprene)
Methane is used as (see also Chapter 7):
x universal and environmentally friendly fuel gas for heating purposes
x fuel gas component for explosion deburring
x reactant to create specific atmospheres in metallurgical furnaces
x operating gas for radiation-counter tubes
x standard for calorimetric measuring
x basic substance for the production of organic compounds (e.g. via formation
of synthesis gas)
Propane is used as:
x common liquid gas for many types of general heating
x technical fuel gas for applications such as heating, warming, flame cutting
and annealing
x propellant for aerosol cans (substitute for chlorofluorocarbons or CFCs)
x refrigerant, e.g. for household or industry refrigerators
x fuel gas for hot air balloons (buoyancy through hot combustion gas products)
x basic substance for the production of propene and polypropylene
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References
253
References
[8.1] Linde AG, 82049 Höllriegelskreuth: Spezialgase-Katalog (8550/2).
[8.2] Linde AG, Division Linde Gas: catalogues, data sheets and special prints on specialty
gases and gases for metalworking (fuel gases), 82049 Höllriegelskreuth.
[8.3] H.-J. Sontag: Linde AG, 82049 Höllriegelskreuth: Handbuch des Carbidacetylens.
[8.4] Ullmann’s, 6th edition, 1, p. 215 ff., Wiley-VCH, Weinheim, 2003.
[8.5] Ullmann’s, 6th edition, 1, p. 531 ff., Wiley-VCH, Weinheim, 2003.
[8.6] F. Schuster: Handbuch der Brenngase und ihre Eigenschaften, Vieweg, Braunschweig.
[8.7] The John Zink Combustion Handbook, Printed in USA, ISBN 0-8493-2337-1.
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9
Specialty Gases
9.1
Introduction
Under the generic term “specialty gases”, far more than 100 different gaseous and
liquid substances and many thousands of different gas mixtures are subsumed.
The boundaries of the product group of specialty gases are fluid and therefore
only difficult to define. Nevertheless, in the following an attempt is made to
compile criteria defining a gas’ or a gas mixture’s characteristics classifying them
as specialty gases.
x Higher technical specification of the gas,
e.g. regarding its purity or its specific components
x Higher expenditure on the availability,
e.g. for the pre-treatment of the tanks, purification, filling
x Rare occurrence of a gas (even with regard to the application volume),
e.g. stable isotopes such as helium-3, deuterium
x Origin beyond the classical field of industrial gases
(in general, from the chemical and petrochemical industry),
e.g. chlorine, hydrogen chloride, silane and related to these
x Certain chemical (corrosivity, self ignition) and toxicological properties
x Supply in special packing such as compressed gas packing
(non-returnable containers)
In the following, specialty gases are subdivided into pure gases, gas mixtures and
the product group of electronic gases derived from the application (may be pure
gases and gas mixtures).
The pure gases/gas mixtures dealt with in this chapter are compressed gases
often filled and transported in compressed gas containers. Accordingly, the
applicable regulations (Technical Rules on Compressed Gases = TRG and Traffic
Laws, ADR) are being referred to.
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
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9 Specialty Gases
9.2
Pure Gases
9.2.1
Definitions
In general, the definitions of “gases” are almost congruent in different literature
sources.
According to ADR 2003: “Gases are substances with a vapour pressure of more than
3 bar at +50° or which are completely gaseous at +20° and a standard pressure of
1.013 bar.” From this, the definition of pure gases can be derived [9.1].
According to pr EN 13 096: “A pure gas is a substance as described above, which
occurs technically pure in the compressed gas container [9.2].
A pure gas can contain other components stemming from the production process
or added to maintain the stability of the product, provided that the concentrations
of these components do not change the classification or the transport regulations
such as the reason for filling, filling pressure or test pressure”.
According to TRG 100: “Pure gases are compressed gases consisting of only one
molecule type and occurring technically pure in the compressed gas container”
[9.3].
These definitions take already into account that 100% pure gases do not exist.
The different stages of purity are characterized by more or less large portions of
impurities stemming from production and filling processes.
9.2.2
Quality Criteria
The purity of a gas corresponds to the content of the main molecule and is given
in percent, usually with reference to the mole fraction. Another indication of
purity, the dot notation [9.4] has also gained acceptance, as already described
in Section 2.2.5.5. The dot notation serves the clear indication of the minimum
content of a gas by means of two digits separated by a dot.
x The digit in front of the dot indicates the number of “nines” in the percentage
for the content of the pure gas
x The digit behind the dot indicates the first decimal place deviating from the
“nine”, e.g.
argon 5.6 means:
minimum content of argon 99.9996% mole fraction
nitrogen 6.0 means: minimum content of nitrogen 99.9999% mole fraction
Apart from the dot notation, there are also terms that point to particularly low
impurities or to an application.
x Nitrogen “CO-free” (Zero gas for CO-gauges)
x Helium ECD (Helium for the operation of Electron Capture Detectors)
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257
Essential additional criteria are defined by listing components (specifications) with
limiting concentrations that must not be exceeded. Owing to the low proportions,
the following smaller units have gained acceptance.
ppm (part per million, 1 · 10–6)
ppb (part per billion, 1 · 10–9)
ppt (part per trillion, 1 · 10–12)
9.2.3
Sources/Production
Today, a number of large-scale units for the generation of gases can achieve the
purities (6.0 and higher) required for specialty gases. These plants are described,
inter alia, in the Sections 2.2.5.5, 3.3.2, 3.4.2, 4.2, 5.2.4 and 6.2.4. The generation
of all other gases occurs in complex chemical processes in which split streams
are branched off for further utilization.
9.2.4
Purification/Processing
The product quality achieved in large-scale plants not always is sufficient for
the different applications of specialty gases. This means the products have to
be subsequently purified. The multitude of pure gases and the descriptions of
the related individual purification-processes would go beyond the scope of this
chapter. Therefore, Table 9.1 shows only examples of processes, pure gases and
impurities to be removed [9.5].
Irrespective of the purification carried out, it is essential for maintaining the
quality of the raw material to ensure suitable pretreatment of the containers and
adequate gas transfer in the filling station. These topics are being referred to in
Sections 9.3.2.2 and 9.6. The supply of special purities is also possible on the basis
of analytical control (“selection procedure”).
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9 Specialty Gases
Table 9.1 Processes for the purification of specialty gases.
Process
Pure gas
Impurities to be removed
Notes
Distillation
cylinder to
cylinder
Gases
liquefied
under
pressure
lower-boiling components like
N2, O2
Heating of the output
cylinder, cooling of the
target cylinder over
condenser
Distillation in
the tank
i. a.
CO2
lower-boiling components like
N2, O2
Distill off top gas, isobar
if possible
Adsorption on
molecular sieve
inert
gases
CO2, moisture
Adsorption on
activated carbon
inert
gases
SO2, NO2, HCl
Heterogeneous
catalysis
Air
H2, N2,
Ar, He
CO2
CO, H2, CH4
O2
Catalyst Pd b. 300–450 °C
Catalyst Pd b. 20–120 °C
H2, HC1)
Catalyst Pd/Pt 230–350 °C
inert gas
O2
H2
N2
O2, H2O, CO, CO2, CH4, H2, N2
H2O, CO, CO2, CH4, H2
O2, H2O, CO, CO2
O2, H2O, CO, CO2, CH4, H2
Getter acc. SAES-patent
Catalyst + Absorber
Getter/Catalyst + Absorber
Getter/Catalyst + Absorber
500–600 °C
Xe
SF6
Getter/gas
scrubber
1)
HC = hydrocarbons
9.2.5
Application Examples
Here again the multitude of products renders the representation difficult. This
means that only a small part of the possible applications, as given in Table 9.2,
can be described.
In view of its importance, one field of application shall be described more closely,
namely instrumentation gases for analytical measuring methods.
Instrumentation gases are used in sample processing as extraction medium,
stripping medium or refrigerant to extract samples, to strip out highly volatile
substances or to enable the enrichment in a cold trap.
As zero gas, they are only permitted to contain the component to be measured
in a concentration not detectable for the applied measuring devices. Zero gases
serve to the adjustment of the zero point in gas-analysis devices.
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Table 9.2 Application examples for pure gases.
Gas kind
Application
Use as/for
Ar
x Insulation glass panels
x Atomic absorption spectrometer
(AAS) for metal traces
Filling gas
Shielding gas and purge gas
CO2
x Biological growth control
Anaerobic organisms
He
x Glass fiber production
Inert atmosphere and thermal
discharge
Kr
x Light and gas discharge lamps
x Insulation glass panels
Filling gas
Filling gas
Ne
x Light and gas discharge lamps
x Flatpanels in plasma technology
(Plasma-Display-Panel, PDP)
Filling gas
Filling gas
N2
x Biological growth control
x Semiconductor manufacturing
anaerobic organisms
Inert gas
N2O
x Food technology
x Flame atom absorption spectrometer
Propellant for spray cream
Operating gas
SF6
x Medium and high voltage switch
x Nonferrous metal melts (aluminium,
magnesium)
x Glass fiber production
x Insulation glass panels
Insulation and cooling gas
Shielding gas
Adjustment of the refraction gradient
Filling gas
Xe
x Light and gas discharge lamps
x Flat panels in plasma technology
x Particle accelerator for nuclear-physical
examinations
x Ion engines for e.g. satellites
Filling gas
Filling gas
Filling gas
Propellant
Usually, for this purpose are used:
x
x
x
x
Nitrogen in purity levels of up to 6.0, CO-free and ECD
Hydrogen in purity levels of up to 6.0
Helium in purity levels of up to 6.0 and ECD
Argon in purity levels of up to 6.0
Taking gas chromatography as an example, Table 9.3 shows the various applications in analytics as carrier and instrumentation gas.
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9 Specialty Gases
Table 9.3 Application of gases in the gas chromatography.
Detector
Carrier gas
Instrumentation
gas
Gas purity for measuring
range
Remarks
ppt – 100 ppb – > 10 ppm
100 ppb 10 ppm
Thermal
conductivity
detector
(TCD)
Hydrogen
5.3
5.0
Helium
Argon
Nitrogen
5.3
5.3
5.3
5.0
5.0
5.0
Flame
ionization
detector
(FID)
Hydrogen
Helium
Nitrogen
6.0
5.6
5.3
5.0
6.0
6.0
5.6
5.6
5.3
5.3
5.0
5.0
HC-free1)
Synthetic air
Electron
capture
detector
(ECD)
Flame
photometric
detector
(FPD)
Helium
Nitrogen
ECD
Nitrogen
Helium
Hydrogen
ECD
P10/P5 – gas
(%-methane
in Argon)
Hydrogen
Helium
Nitrogen
6.0
5.6
5.3
5.0
6.0
6.0
5.6
5.6
5.3
5.3
5.0
5.0
HC-free
Helium
6.0
5.6
5.3
5.0
6.0
5.6
5.3
5.0
Helium
7.0–6.0
Hydrogen
6.0
5.6
5.3
5.0
6.0
6.0
6.0
5.6
5.6
5.6
5.3
5.3
5.3
5.0
5.0
5.0
Helium
Argon
Nitrogen
Mass selective
detector
(GC-)MS
1)
1345vch09.indd 260
Helium
Nitrogen
Hydrogen
Oxygen
Methane
Helium
6.0
C
D
A
HC-free1)
Synthetic air
Atomic
emission
detector
(AED)
A
1)
Nitrogen
Helium ionization detector
(HID)
Thermoionic
detector
(TID)
B
ECD
ECD
Synthetic air
Photo ionization detector
(PID)
A
6.0
6.0
6.0
5.0
5.0
4.5
5.3
5.0
5.0
4.5
7.0–6.0
6.0
E
Hydrocarbons
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9.3
Gas Mixtures/Calibration Gas Mixtures
9.3.1
Definitions
Gas Mixtures (Generic Term)
Just as in the case of pure gases, there are again definitions from different
sources.
According to pr EN 13 099: “A gas mixture is a mixture of two or more components,
liquid or gaseous, filled in a cylinder to be withdrawn as mixture, and fulfilling
the criteria of a gas” (see Section 9.2.1) [9.6].
According to TRG 100 : “Gas mixtures are compressed gases consisting of several
kinds of molecules (components). Apart from gases fluids may also be components
of a gas mixture” [9.3].
Calibration Gas Mixtures
Generally, gas mixtures intended for the application as calibration gas mixtures
have to fulfil higher demands on manufacturing and analysis as well as on the
purity of the raw materials.
There are two official German definitions for the term “calibration gas mixtures”:
According to TRG 102 : “A calibration gas mixture is a gas mixture intended to be
used in analytical technology.” [9.7].
Notes for Table 9.3
A Hydrocarbon impurities (HC) in the instrumentation gases cause strong reference line noise and
consequently to a deterioration in the detection limit. Therefore, the HC-concentration in the instrumentation gases should be as low as possible. For the FID/FPD, a gas mixture of 40% of hydrogen,
balance helium is used as fuel gas.
B The ECD reacts very sensitively to impurities in the gases, pipes, fittings and seals from substances
with a high electron affinity like oxygen and chlorofluorocarbons (CFC). Oxygen, moisture and CFCs
deteriorate the detection limit.
C Easily ionizable HC impurities in instrumentation gases increase the reference line noise.
Therefore, the HC-share in the instrumentation gases should be as low as possible.
D Due to the interference liability of the HID, the detector should be operated under protective
atmosphere.
E Besides high purity helium as carrier and plasma gas, the spectrometer needs high purity nitrogen
as purge gas and various reagent gases, depending on which elements are to be measured.
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9 Specialty Gases
According to VDI-Guidelines 3490 P. 1: “A mostly compressed gas mixture, normally
consisting of a balance gas and one or more components” [9.8].
This VDI-definition contains further essential terms.
Balance gas: “Pure gas or gas mixture, which as the major component supplements
the components used for calibration”.
Component: “Gaseous or vaporized component of a calibration gas mixture, known
in quantity and quality, and directly used for examination of the calibration”.
In Table 9.4, some customary synonyms for these “official” terms are given.
Table 9.4 Terms and synonyms.
Calibration gas mixture
=
Balance gas
+
Component
Calibration gas
Main component
Measuring component
Standard
Carrier gas
Component
Reference standard
Residual gas
Portion
Sample gas
Component
Element
Mole Fractions and Concentration Units
For a comprehensive description of a gas mixture/calibration gas mixture, apart
from the indication of the kind of balance gas and components, even the indication
of the mole fraction resp. of the concentration is required.
x The mole fraction of a component is the ratio of the number of moles of
the component to the sum of the number of moles of all components of the
calibration gas mixture.
x The concentration of a component represents the ratio of the quantity of this
component to the volume of the mixing phase.
For a clear characterization, inter alia, the following details are possible:
x
x
x
x
Mole fraction, e.g. mol mol–1, mmol mol–1, µmol mol–1
Mass concentration, e.g. kg m–3, g m–3, mg m–3
Volume concentration, e.g. m3 m–3, L m–3, mL m–3
Mole concentration, e.g. mol m–3, mol L–1, mmol L–1
The volume indications always refer to the standard state (1.013 bar, 273.15 K);
parts by volume are based on ideal gas volumes (mole fractions).
For smaller units, see Section 9.2.2.
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9.3.2
Production [9.9]
9.3.2.1
Technical Feasibility
Physical Constraints
If gas mixtures contain condensable components (acc. to TRG 102, each gas with
a critical temperature t –10 °C, as well as each liquid) the filling pressure has
to be limited in a way that these components do not condense under the filling
pressure. As practice-oriented condensation temperatures +10 °C are determined
for summer (manufacturer-specific exception, according to TRG +5 °C) resp.
–10 °C in winter that should not be exceeded or fallen below.
Determination of the maximum filling pressure according to TRG 102 [9.7]:
Pfill + 1 ≤
100 ⋅ Pi
Ki
Pfill = Filling pressure (gauge) at +15 °C in bar
Pi = absolute vapour pressure of the condensable component i in bar
Ki = Concentration of the condensable component in the mixture in % vol
X Example:
Butane Pi = 1.3 bar at +10 °C
Ki = 10% volume concentration
Result: Pfill = 12 bar
i.e. a mixture with a butane volume fraction of 10% may only be filled up to 12 bar
(based on the temperature +10 °C).
Chemical Constraints
Gases that can react amongst themselves must not be mixed (e.g. CO2 + NH3, SO2
+ NH3). See also TRG 102, Appendix 2 “Gas mixture diagram” [9.7].
Safety-related Constraints
Mixtures of fuel gases and oxygen resp. synthetic air are only allowed to be filled,
if after all, their concentration lies in a concentration range, which has to be
sufficiently below or above the respective explosion limit. For the filling of such
mixtures (in Germany) a documented approval of the Federal Institute for Materials
Research and Testing (BAM) has to be obtained.
9.3.2.2
Pretreatment of Containers
Only consequent pretreatment of the containers [9.9] enables the production of
stable calibration gas mixtures. Since calibration gas cylinders can be used for
the most different compositions, both residues of the previous mixture and a
possibly existing moisture film on the inner surface have to be removed during the
pretreatment, if possible quantitatively. For this purpose, the containers are being
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9 Specialty Gases
heated, evacuated at regular intervals and purged with the gas used as balance
gas later on. In case of special requirements (e.g. ppm-mixtures with corrosive
components), the additional moisture measurement of the purge gas is advisable.
Only when the discharging purge gas shows the same moisture content as the
inflowing dry purge gas, the quality of the pretreatment is guaranteed.
9.3.2.3
Preparation Methods
Basically, calibration gas mixtures are produced by combining defined quantities
of different components. The preparation methods are characterized by special
features:
Method of mixing procedure:
x Static, i.e. certain gas amounts are filled one after the other into a container
(unique mixing procedure)
x Dynamic, i.e. gas flows are continuously being mixed
Method of determining quantity:
x Volumetric, i.e. by determination of volumes
x Manometric, i.e. by measurement of pressures
x Gravimetric, i.e. by determination of masses (weighing)
The combination of these procedures results in a multitude of possible methods
Here, however, only such methods are being dealt with that are suitable for a
production in compressed gas cylinders.
Dynamic-Volumetric Method
The basic principle is the blending of different volume or mass flows. The simplest
mixing arrangement consists of one pressure regulator and one flow meter per
type of gas. The low uncertainties of the content of components desired for the
calibration gas mixtures cannot be achieved with this simple arrangement. If
manufacturing tolerances of 2 to 3% rel. shall be obtained, the volume ratio
control has to be taken over by the measured variable of the respective analytical
device. Moreover, the filling into compressed gas cylinders requires a suitable
compressor.
Advantage of the method: Constant composition when filling larger numbers of
cylinders.
Disadvantage of the method: The number of components is limited in this method
(mainly binary mixtures) and the investment costs are considerable.
Manometric Method (Manometric-Static Partial Pressure Method)
This principle is applied for the production of technical gas mixtures (e.g. shielding
gases for welding). It is based on the measuring of the pressure changes after the
addition of individual components resp. the balance gas. For example, in order to
obtain a composition of 10% of A, 10% of B, balance C at 150 bar, the component A
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265
is filled into the compressed gas cylinder up to 15 bar, than component B at further
15 bar up to 30 bar and the balance gas C up to a final pressure of 150 bar.
However, problems may arise in practice, since this method requires isothermal
filling and neglects the different compressibility of the gases. Consequently, with
this method, production tolerances of only r5 … 10% rel. are achievable. Due to
the application of equations of state, deviations from the “ideal behaviour” as
well as temperature influences can be taken into account. With the help of these
corrections, the production tolerance can be increased to r2 … 5% rel.
Advantage of this method: Economical large-scale production of calibration gas
mixtures with constant composition is possible.
Disadvantage of this method: The mixture tolerance is relatively low. The use of
different equations of state leads to varying results.
Gravimetric Method
At this method, the components are filled into a compressed gas cylinder one after
the other and, after each dosing the mass increase is determined by weighing.
Thus, the direct relation of the weighed gases to the basic unit “kg” or “mol” is
given, and corrections with the help of equations of state are obsolete. For the
reasonable application of this method, some preconditions have to be fulfilled.
Requirement on the weighing equipment: On the one hand, the balances have to
dispose of a high capacity owing to the high weight of the cylinders, and on the
other hand as high a resolution as possible owing to the small “gas weights”.
Precision balance (Beam balance): With a resolution of 3 mg, for instance, these
balances dispose of a capacity of 30 kg. They are also applied for the production of
calibration gas mixtures according to the method of re-weighing with an inaccuracy
of up to 0.01% rel. (Fig. 9.1).
In the production of these extremely precise calibration gas mixtures [9.11], apart
from the inaccuracies of the balances even other influencing factors, such as gas
impurities, filling errors or lift forces have to be considered. The balance itself
only appears at the third level in the uncertainty hierarchy [9.12].
Electronic balances: In routine production, electronic balances are predominantly
used (see Fig. 9.2).
At a capacity of 150 kg, for example, electronic balances show a weighing
inaccuracy of up to r0.1 g. Meanwhile, there are also solutions for the re-weighing
method with fully-automatic connection and disconnection of the filling pipe
under the application of electronic balances [9.13].
Mass of the component to be dosed: The mass of the smallest component has to be
significantly higher than the absolute uncertainty of the balance. With electronic
balances of a capacity of 150 kg and a resolution of 0.1 g (s.a.), a production
tolerance of r 1% rel. can only be achieved with the weighed gas sample being at
least 10 g. In case this mass is not achieved owing to the desired content of the
calibration gas mixture, premixtures that are also gravimetrically produced have to
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9 Specialty Gases
Fig. 9.1 Precision balance for the production
of calibration gas mixtures.
Fig. 9.2 Filling station with electronic balances.
be used. These are gas mixtures, in which the required component occurs with a
higher percentage of mole fraction. The balance gas of the premixture corresponds
to that of the calibration gas to be produced.
Advantage of this method: Very low preparation tolerance. Independent from
pressure, temperature and compressibility. This method is supported by standards
(e.g. ISO 6142) [9.14].
Disadvantage of this method: Increased manufacturing expenditure (time-, staffand plant-consuming). Meanwhile, these disadvantages have been reduced by
automation to the extent that today, the gravimetric method is the preferred
method for calibration gas mixture production.
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267
Homogenization
After filling, the gas fractions have to be mixed completely. According to the
Brownian molecular movement, the gas molecules would mix automatically after
a certain time. To accelerate the mixing process, the gas cylinders, for instance, are
being rotated around their longitudinal axis in a mechanical device (Fig. 9.3).
A provable mixture separation of a homogeneous mixture occurs only when
the condensation temperature of a component is fallen below.
Fig. 9.3 Homogenization of gas mixtures.
9.3.2.4
Analytical Quality Assurance
The importance of the analytical control depends on the preparation tolerance
of the mixture method applied. In case of preparation methods with higher
inaccuracies it is common practice to determine the mole fraction by means
of analysis. Even if the mixture is gravimetrically produced, analytical control
cannot be renounced. On the one hand, it can never be excluded, for instance,
that systematic weighing errors or individual errors of the operator occur. On
the other hand, above all in case of corrosive components, adsorption effects and
reactions with the inner surface of the gas cylinder cannot be excluded. Such
effects can only be detected analytically.
The common analytical methods shall only be listed concisely:
x
x
x
x
x
x
x
x
x
x
1345vch09.indd 267
Wet-chemical absolute methods
Electrochemical methods
Optical methods (FTIR, IR, UV-VIS)
Special oxygen and moisture measuring systems
Gas chromatography (GC) with a multitude of detector systems
Chemiluminescence method (CLD)
Mass spectrometry (MS)
Atomic absorption spectrometry (AAS)
Ion chromatography (IC)
Inductively coupled plasma spectrophotometry (ICP)
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9 Specialty Gases
Table 9.5 Examples for the application of gas mixtures.
Application
Mixture Components
Balance gas
Notes
Emission control
at furnaces
O2
CO
SO2
NO
N2
N2
N2
N2
according to the
legal regulations of
BImSchG and
TA-Air
Exhaust-emission
check (AU)
CO, CO2, C3H8
N2
i.a. calibration gas
mixtures with official
test certificate
Car industry
x Exhaust control
x Optimization of
engines
x Development of
catalytic converters
O2
CO
NO
C3H8
N2
N2
N2
Synth. air
Indoor air monitoring
x Explosive
H2
atmospheres
CH4
C3H8
other flammable gases
x TLV
CO
(personal security,
PH3
workplace
SO2
monitoring)
NH3
other toxic gases
1345vch09.indd 268
Synth. air
Synth. air
Synth. air
Synth. air
Synth. air
N2
N2
N2
N2
Immission
measuring
Formaldehyde
NO
NO2
SO2
Benzene, toluene, xylene (BTX)
N2
N2
Synth. air
Synth. air
Synth. air/N2
Laser
CO2, N2
He
H2, CO, CO2, N2
He
F2
HCl
He
He
Filling gas
for light bulbs
N2
Ar
Leak detection
He
N2
Process monitoring
and control
Hydrocarbons C1–C6
N2, CH4 and
other hydrocarbons
Calibration gas
mixtures for
ex-alarm devices
Calibration gas
mixtures for
TLV alarm devices
Operating gases
for CO2-lasers
Operating gases
for marker laser
Operating gases
for excimer-laser
Calibration gas
mixtures for process
chromatographs
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269
Only wet-chemical analysis methods are absolute methods which refer to basic values, and thus do not require calibration. All other measuring methods are working
comparatively (ISO 6143) [9.15]. Here, apart from internal standards gravimetrically produced on precision balances (see Section 9.2.2.3, precision balances), even
worldwide recognized standards are used (Federal Institute for Materials Research
and Testing, Bundesanstalt für Materialforschung und -prüfung, BAM; National
Institute of Standards and Technology, NIST; Nederlands Meetinstituut, NMi).
9.3.3
Application Examples
Gas mixtures/calibration gas mixtures are used in a number of applications in
environmental protection, process optimization, protection of people and plants
and in research. Table 9.5 shows an excerpt of the most important applications.
9.4
Electronic Gases
9.4.1
Definition/Special Demands
In accordance with their name, electronic gases are used for the manufacturing
of semiconductor components, but also in related fields of high-technology (e.g.
manufacturing of glass fibres, solar cells, microscopical components). Due to the
high complexity of the manufacturing processes with several hundred process
steps resulting in structures in the range of less than 0.1 µm, high demands
regarding the content of gaseous impurities (particularly O2 and H2O) and particles
are placed on the gases applied.
Moreover, some of the gases have chemical properties (corrosive, highly toxic,
self-igniting) that require the use of special equipment on the compressed gas
containers (i.e. remotely controlled, pneumatically operated cylinder valves,
flow restrictors, metal-to-metal seal between cylinder valve and process lines).
Containers for electronic gases are subject to special cleaning procedures to
remove particles, organic impurities, deposits and corrosion products from their
inner surface. Depending on the chemical properties of the respective product
and the specific demands in the respective field of application, apart from the
usual steel containers also inside polished containers of steel, stainless steel or
aluminium are used.
Filling stations for electronic gases are equipped with particle filters, products
liquefied under pressure are filled by application of a distillation step. In addition, adsorptive cleaning methods are often applied (see also Section 9.2.4). Regarding the gas analysis methods used in quality control, apart from the already
described methods for specialty gases, the field of metal-trace analysis has to be
emphasized.
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9 Specialty Gases
Table 9.6 Examples for applications of electronic gases.
Application
Gas/Gas mixture
Notes
Manufacturing of semiconductors
Deposition of silicon
Deposition of monocrystalline layers (Epitaxy)
x Si-containing gases
x Doping gases
x Etching gases
Dry etching
x of Si, SiO2
x of Al, Al2O3
x of compound
semiconductors
SiH4
SiH2Cl2
SiH4, SiH2Cl2, ‚Si2H6
AsH3, PH3, B2H6
HCl
CF4, SF6, CHF3, Cl2, C2F6, C3F8,
C4F8, NF3, HBr, CH2F2, CH3F
CClF3, BCl3, Cl2
HCl, Cl2
Thermal oxidation
HCl, Cl2
Deposition from the gas
phase (CVD, Chemical
Vapour Deposition)
SiH4, NH3, N2O, SiF4, NO
Doping
x Diffusion
For the removal of
impurities
x Ion implantation
PH3
Donor o n-conductive
AsH3
Donor o n-conductive
B2H6
Acceptor o p-conductive
BCl3, BF3, AsH3 in H2, PH3 in H2
Metallization (CVD)
WF6
Cleaning of reaction chambers C2F6, C3F8, C4F8, C4F8O, NF3, SF6
Manufacturing of solar cells
Deposition of silicon
SiH4
Doping
PH3 in H2, (CH3)3B
Chamber cleaning
C2F6, NF3, SF6
Protective layer
SiH4, NH3
Si3N4-layer
Glass fiber manufacturing
Etching of surface and
removal of moisture
Cl2
Other applications
1345vch09.indd 270
Coating of flat glass
SiH4
Reflective layer
Tempering of surfaces
SiH4, NH3
Si3N4-layer
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271
9.4.2
Application Examples
In Table 9.6, selected applications are listed.
9.5
Disposal
As defined, specialty gases can be combustible, self-igniting, toxic or corrosive.
The handling of such gases requires special safety measures. This includes the
disposal of residues that has to be carried out without polluting the environment.
The selection of the disposal method depends on the gas properties, the quantities
and the local and plant-specific conditions. In Table 9.7, examples of the most
important methods for the disposal of certain gases are given. Further details,
mainly according to safety-related aspects, can be found in [9.16].
Table 9.7 Examples for gas disposal methods.
Method
Suitable for
Notes
Recovery
All gases
Preferred method for the
benefit of environmental
protection. Appropriate for
high-quality gases or gases
the disposal of which is
costly due to properties or
quantities.
Emission into the
atmosphere
Inert gases that do not pollute
the environment, like N2, O2,
noble gases
Neutralization with acids
Alkaline gases like NH3
Neutralization with lyes
Acid gases like chlorosilanes,
HCl, HBr, SO2, NO2
Fixed-bed adsorption
AsH3, COS, amines
Combustion
Hydrocarbons, CO
Combustion combined
with adsorption
Halogenated hydrocarbons, PH3
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Combustion product must
not be ecologically harmful
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9.6
Transfer of Gases
Unsuitable fittings and pipes can reduce the quality of the pure gases and
calibration gas mixtures considerably. The selection of the adequate gas supply
system depends on the chemical, physical and physiological properties of the
gases applied and the application-specific demands.
9.6.1
Selection of the Materials
Special attention has to be paid to the selection of the materials when reactive
gases are applied. For example, considerable corrosion problems are to be expected
when halogens, sulphuric compounds and nitrogen oxides are used in presence
of moisture. Some gases, such as H2S, SO2, C2H2 and C2H4 are prone to thermal
decomposition on hot metal surfaces. CO can form carbonyl compounds with
nickel, iron and chrome. Hydrogen reduces metal oxides under the formation
of water vapour.
9.6.2
Physical Interaction Forces
Adsorption and Desorption on Surfaces
Apart from the chemical influence of the fluid-contacting inner surfaces of fittings
and piping, the microscopic surface (roughness) has to be paid attention to. It
decisively influences the physical interaction forces between gas and surface, i.e.
the adsorption and desorption of gas molecules. In case of the metal materials
copper and stainless steel, mainly applied in ultra-pure gas supply systems, the
surface reactive in the adsorption/desorption of gases and vapours can be reduced
significantly by means of polishing (e.g. electro chemical polishing). Relatively
rough surfaces are found in plastics used for seals, membranes, valves and piping.
Here, such gas/surface reactions occur frequently.
Each surface, thus also the fluid-contacting inner surface of a fitting or pipe
relevant here, is covered with a layer of adsorbed gases and vapours in equilibrium
with their environment. The equilibrium state and therefore the transport of
the adsorbed gases from the wall into the gas and vice versa is a function of the
temperature and the concentration or partial pressure drop between wall and gas
flow. Moreover, the surface structure has considerable influence. The microscopic
surface active for adsorption can have many times the surface of the geometrical
surface. If, for example, a fitting was exposed to ambient air during its storage,
oxygen and moisture have accumulated on the surface which diffuse into the
ultra-pure gas during later operation.
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273
Adsorption and Desorption Out of Materials
Adsorption and desorption on and off materials are negligible in the case of metals,
in the case of plastic materials, however, as they are used for seals, membranes,
valve seats, they develop values that have to be noted. Plastics are able to adsorb
considerable amounts of gases and vapours and to desorb them again. Even the
plastic material itself can desorb volatile components such as softening agents.
Therefore, care has to be taken that independent from the application the amount
of plastic components in the ultra-pure gas systems is as low as possible.
Diffusion Through Materials
The diffusion through materials is also relevant for plastic material. Air components, for example, diffuse through plastic tubes. Therefore, they should generally
not be used in ultra-pure gas system. At first sight, the assertion that air is able to
diffuse into fittings and pipes even against higher pressure seems to be paradox.
However, an exchange of gas molecules in both directions actually takes place.
Decisive is the partial pressure difference of the involved gases inside and outside
a pipe, for example. This phenomenon called permeation occurs practically only
in plastic materials. In the case of metals, permeation (within the scope of gas
engineering) is absolutely negligible.
9.6.3
Tightness of the Gas Supply System
Utmost attention has to be paid to the tightness of the supply system. Potential
leakages in the pipe system are soldered or welded joints and screwings, e.g.
flanges, screwings and valve seats. While soldered and welded joints, provided
they are manufactured according to the rules, can be regarded as tight, in the case
of detachable connections this is only possible to a certain extent. Metal seals in
general show lower leakage rates than soft seals, and should therefore be used
exclusively. The number of screwings should be minimized and pipe connections
should generally be welded.
9.6.4
Purging of the Gas Supply System
In general, the contamination of pure gases or calibration gas mixtures owing
to desorption processes can be avoided by means of intensive purging of the gas
supply system before operation. Evacuating is only advisable if the fittings used
are vacuum-tight and the ports on the valves are not too small. When purging
fittings and piping some factors regarding the maintenance of the gas quality
have to be taken into account. For example, it is wrong to completely open the
cylinder valve at once, if a pressure regulator is connected. In this case, pressure
compensation with the cylinder pressure occurs in the high-pressure section of
the pressure regulator. Owing to back diffusion, the air in the pressure regulator
would mix with the process gas of the cylinder and contaminate it. Even continuous
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9 Specialty Gases
Table 9.8 Design of the components in specialty gas supply systems.
1345vch09.indd 274
Gas purity
Class 5.0
Class 6.0
Class 6.0 and higher,
and semiconductor
process gases
Pressure
regulator
Material brass or stainless steel, specially
cleaned, He-leak
rate d 10–8 kPa L s–1,
elastomere or a metal
diaphragm
Material predominantly
stainless steel, specially
cleaned, He-leak rate
d 10–10 kPa L s–1, stainless steel diaphragm
Material stainless steel,
electropolished, high
surface quality, He-leak
rate d 10–10 kPa L s–1,
stainless steel diaphragm, minimized
dead volume, free of
non-ferrous metals,
cleanroom assembled,
minimum particulate
emission
Valves
Diaphragm-type seal,
rarely packing seal or
O-ring seal
Diaphragm-type seal,
Bellows-type seal
Diaphragm-type seal,
bellows-type seal
electropolished,
minimized dead
volume
Piping
Material copper
or stainless steel,
specially cleaned
Material stainless steel
specially cleaned or
electropolished
Material stainless
steel, electropolished
Pipe
connections
Flux-free brazed,
orbital welded
Orbital welded
Orbital welded
Detachable
connections
Metal-to-metal sealed
tube fittings
Metal-to-metal sealed
tube fittings,
metal-to-metal sealed
VCR unions
Metal-to-metal sealed
VCR unions
Application
examples
Gas supply for the
general laboratory needs,
for gas analyzers, for
production plants using
high-quality working
gases e.g. CO2-Lasers,
for the manufacturing
of lamps, production
of special ceramics and
metals
Gas supply for laboratory
needs involving high
purity gases, for gas
analyzers using calibration gas mixtures in
the ppm-range and/or
corrosive components,
for production plants
using highest purity
gases and gas mixtures,
e.g. excimer-lasers, for
the manufacturing of
optical fibers, discrete
components and less
highly integrated circuits
Gas supply for R&D
applications involving
ultra-high purities,
e.g. in microelectronics,
for production plants
using ultra-high purity
gases and gas mixtures
as well as corrosive and
toxic process gases,
e.g. for VLSI circuits,
sensors and solar cells
26.10.2007 10:28:21
References
275
Fig. 9.4 Pressure swing purging.
purging is not very effective because of the numerous dead volumes. Therefore,
the pressure swing method is recommended (Fig. 9.4).
In Table 9.8, the most important components for specialty gas supply systems
depending on gas purity and application are shown.
References
[9.1] European Agreement on the International Transport of hazardous goods on roads
(ADR 2003) Deutscher Bundesverlag, Bonn, 2003.
[9.2] per EN 13 096, Conditions for filling gases into cylinders, draft, 1999.
[9.3] TRG 100, Technical Rules for Compressed Gases, Register of Compressed Gases,
Carl Heymanns Verlag, Cologne, 1998.
[9.4] Dot notation, in the internet at www.industriegaseverband.de.
[9.5] H. Schön: Handbuch der reinsten Gase, Springer-Verlag, Berlin, Heidelberg, New York,
2005.
[9.6] per EN 13 099, Conditions for filling gas mixtures in cylinders, draft 1999.
[9.7] TRG 102, Technical Rules for Compressed Gases, Gas Mixtures, Carl Heymanns Verlag,
Cologne, 1985.
[9.8] VDI-Guidelines 3490 Sheet 1, Measuring of gases – calibration gas mixtures – terms
and definition VDI-Verlag, Düsseldorf, 1980.
[9.9] K. Wilde, K. Studtrucker: Linde Reports 69, 1993.
[9.10] H. Schön: Before the Gas Cylinder is Filled. Linde Reports 1998, 60, 28.
[9.11] D. Heller: Production of precision gas mixtures on a high-resolution weigh beambalance. Linde Reports 77, 1998, 21.
[9.12] H. Gaier, H. Heller: Computer simulation of the error calculation for precision gas
mixtures. Linde Reports 77, 1998, 21.
[9.13] H. Schön: Methods and devices for the gravimetric preparation of calibration gas
mixtures by means of reweighing, EP Pat. W 097/42447 (1996).
[9.14] ISO 6142, Gas analysis – Preparation of calibration gas mixtures – gravimetric method,
2001.
[9.15] ISO 6143, Gas analysis, Comparison methods for determining and checking the
composition of calibration gas mixtures, 2001.
[9.16] EIGA 30/03, Disposal of Gases, European Industrial Gases Association, Brussels, 2003.
1345vch09.indd 275
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10
Gases in Medicine
10.1
Introduction
For decades medical gases1) have been regarded by the healthcare community as
low value commodities, delivered to the hospital and reordered when necessary.
However, things are evolving rapidly. The most significant change is the classification from former medical gases to pharmaceutical quality products and medical
devices. This process is largely driven in Europe by the regulatory authorities.
This means that for medical gases the same requirements are applicable as for the
traditional medical pharmaceutical industry – all for the safety of the patient.
The regulatory authorities are conducting an evaluation and assessment based
on submitted comprehensive quality, non-clinical and clinical documentation
aiming to demonstrate the benefit and safety for the patient. The authority
evaluation includes the entire finished product, i.e. active substance, excipients
and container closure system (cylinder and valves).
In US most medical gases are sold as US Pharmacopoeia products. These
products have no indications connected with them. Manufacturing of these gases
needs to be approved by the FDA (Food and Drug Administration/US regulatory
agency for medicinal products).
Medical gases and the services around them are needed in various clinical
situations and can be found in ambulances, in intensive care units and may also
be delivered right to the patient’s home.
Medical Gases can be produced in bulk and delivered in special customized
packages (compressed gas cylinders of different sizes, compressed gas bundles,
as refrigerated liquid) in mobile cryo-containers or in stationary cryo-tanks
including evaporator. In addition, a large number of gas mixtures are individually
produced by medical institutes according to specific orders and, due to lacking
authorizations, delivered as specific pharmaceutical.
If a medical gas is classified as a medical device in accordance with the European
classification criteria for medical devices, fulfilment of so-called “Essential
1) The term “medical gas” is generally used for gases in medicine, although they are designated
“medicinal gas” if being approved as pharmaceutical.
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
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10 Gases in Medicine
requirements” has to be proved with the help of a conformity assessment procedure
according to the Medical Device Directive/MDD (MPG in Germany), and the
product has to be provided with the mandatory CE-marking.
The demands on the quality of medical gases are laid down bindingly in
pharmacopoeia (e.g. European Pharmacopoeia/Ph. Eur. – US Pharmacopoeia/
USP) in the form of monographs. A pharmacopoeia monograph contains, inter
alia, requirements for the quality of a medical gas as well as for the measuring
methods to be applied in testing, e.g. its identity, content, purity. (A monograph
can be compared with a technical standard).
Production sites for the manufacturing of pharmaceutical preparations are
subject to the control of regulatory authorities. Therefore, all locations where
medical gases are produced or transferred need a manufacturing license for
the grant of which the fulfilment of the GMP-guidelines (Good Manufacturing
Practise) is a prerequisite. A complete documentation of production, quality control
and distribution channels (possibility to traceability) is a basic requirement for the
handling of medical products. Medical products have to be released by a special
qualified and authorized person (“QP”).
10.2
Medicinal Oxygen
Recovery/Processing: During the process of air separation, atmospheric air is
purified, liquefied and separated into its components, cf. Section 2.2. The recovered
oxygen (O2) is either filled into large storage tanks in liquid form (LOX med) or after
its evaporation in gaseous form (GOX med) into compressed gas cylinders.
Medical oxygen fulfils the requirements defined in the “Oxygen” monograph
of the European Pharmacopoeia. This is subject to continuous control by the
mandatory analytical processes.
Medical oxygen is one of the pharmaceutical preparations used most world
wide. Traditionally, oxygen is administered in case of prevailing or imminent
oxygen deficiency.
10.2.1
Home-therapy
Currently oxygen is the only medical gas finding wide application at the patient’s
home.
Patients benefiting from short-term oxygen therapy include cluster headache
patients, where inhalation of 100% oxygen has proven to bring relief of symptoms
in up to 82% of patients [10.1].
Long-term oxygen-therapy is indicated for all patients with a stable chronic lung
disease (such as chronic obstructive pulmonary disease, cystic fibrosis etc.), who
have an arterial PO2 (oxygen partial pressure) consistently less than or equal to
55 mm Hg when breathing air, at rest and awake [10.2]. These patients need a
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279
continuous oxygen supply for their treatment for more than 15 hours a day. The
dose is indicated in L/min (e.g. 2 L/min) in the medical prescription. There are
three modes to supply oxygen to the patient at home, offering different levels of
mobility.
Gaseous oxygen: Compressed gas cylinders with oxygen are filled at a typical
pressure of 200 bar (= 20 MPa). Therefore, 1 L (litre) of this compressed oxygen
corresponds to 200 L oxygen at ambient pressure. Typical cylinder sizes for home
supply contain 2 or 10 L. The administration of oxygen is done via pressure
regulators that are connected to or permanently integrated in the cylinder valves
(IVR, combivalves).
Oxygen concentrators are electric devices and thus require a power connection,
limiting the patient’s mobility. The ambient air sucked-in by a compressor is
filtered and pressed through sieve beds that adsorb nitrogen from the air. The
oxygen-enriched air is collected in a product tank and delivered to the patient via a
pressure regulator at the outlet of the device. Depending on the adjusted flow rate,
oxygen concentrations of up to 98% are achieved. Recently developed concentrating
devices operating with fuel cell technology deliver 100% oxygen.
Liquid oxygen: Oxygen is a refrigerated liquid (–183 °C). One litre of liquid oxygen
corresponds to 853 L of gaseous oxygen at ambient conditions of 15 °C and 1 bar.
The liquid oxygen is directly supplied to the home patient and stored there in a
special cryogenic vessel. The administration of the oxygen to the patient prior to
application (e.g. by means of nasal cannula) occurs either directly from the storage
container in gaseous state after evaporation via a downstream heat exchanger or
from a smaller portable cryogenic tank (with integrated evaporator), into which
the patient himself fills a defined quantity of liquid oxygen.
Feeding of the evaporated oxygen occurs by means of a plastic tube (Fig. 10.1).
Fig. 10.1 Mobile O2-therapy with portable vessels.
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10 Gases in Medicine
10.2.2
Hospitals and Other Fields of Application
Hospitals, rehab-clinics, emergency services and registered doctors with own
practices receive medical oxygen either in large quantities in liquid state by tank
trucks or in compressed gas cylinders (single cylinders or cylinder bundles), if
smaller quantities are required.
Oxygen is either centrally provided via pipe systems and withdrawn in the
hospital wards via quick couplings (wall outlets) or delivered directly to the patient
by means of mobile compressed gas cylinders. The cylinder pressure (e.g. 200 bar)
has to be reduced to a pressure suitable for the patients’ needs by means of pressure
regulators or an innovative cylinder valve with permanently integrated pressure
regulator and flow meter (medical device with CE-marking).
Current applications are the HBO-Chambers (Hyperbaric Oxygenation), in
which oxygen is administered under increased pressure.
10.3
Gases for Anaesthesia
10.3.1
Medical Nitrous Oxide (Laughing Gas)
Nitrous oxide (N2O) existing in the atmosphere emanates from different sources.
The main pollution of the atmosphere with nitrous oxide arises from fertilizers in
agriculture. In relation to the total emission of nitrous oxide in the atmosphere,
the share of medical nitrous oxide is less than 0.05%. Here it has to be considered
that nitrous oxide enhances the greenhouse effect, however, without any influence
on the ozone layer.
Recovery/Processing
Medical nitrous oxide is obtained by heating of ammonium nitrate:
NH4NO3 o N2O + 2 H2O (240 °C)
It is mainly supplied liquefied under pressure in compressed gas cylinders. At
high demands, nitrous oxide is also supplied by special tank trucks and filled into
storage tanks installed on-site from where the centralized supply in the hospital
is possible.
Application
For more than 150 years, nitrous oxide has been clinically used for anaesthesia
and pain treatment all over the world. Owing to the multitude of patients to be
treated, nitrous oxide can be regarded as one of the pharmaceutical products
broadest examined with regard to its spectrum of side effects and resulting
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10.4 Medicinal Carbonic Acid (Carbon Dioxide)
281
Fig. 10.2 Application of medical gases in surgery.
contra-indications. Even today, nitrous oxide is an essential part of general
anaesthesia.
The good controllability and the possibility of continuous control of the concentration render nitrous oxide a safe and easy-to-handle anaesthetic (Fig. 10.2).
10.3.2
Xenon
Xenon (Xe) represents an innovation in the field of anaesthesia. Its narcotic effect
was discovered only in 1951. Clinical studies proved the principle applicability
even as mono-anaesthetic. The marketing of xenon as finished pharmaceutical
requires authorization according to the current drug law regulations. The high
price for xenon can be reduced by its recycling from exhaled gas. In order to enable
the substitution of all anaesthetics by xenon, considerable increases in capacity
of air separation plants would be required.
10.4
Medical Carbonic Acid (Carbon Dioxide)
For recovery/processing see Section 6.2.
Application
Recently, medical carbonic acid (CO2) has gained more and more importance in
a number of medical fields of application. Especially in the promising field of
minimal invasive surgery, the stabilization of visceral cavities (aeroperitoneum,
pneumothorax) in laparoscopic and thoracoscopical interventions can not be
imagined without the application of this gas – which in case of being correctly
used reabsorbs easily.
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10 Gases in Medicine
However, even the external application of medical carbonic acid is of growing
importance, as for example in sanatoria, where carbonic acid baths have become
firmly established for the successful treatment of certain dysfunctions of the
cardiac and circulatory system as well as of the vascular system.
10.5
Medical Air
Recovery/Processing
This product can either be obtained according to the European Pharmacopoeia
monograph “Air medicinal (Aer medicinalis)” by the filling of purified and compressed ambient air, or even synthetically by mixing of oxygen and nitrogen.
Application
Medical air is indispensible for a number of therapeutical measures, in particular
in neonatology, anaesthesia and intensive medicine. It supports respiration in
case of:
x
x
x
x
anaesthesia
mechanical ventilation
artificial respiration in case of transports
supply of respiratory air in the incubator
Medical air must not be confused with the product “Compressed air for breathing
apparatuses” which is intended for non-medical purposes, and the demands on
production and quality of which are not defined in the pharmaceutical laws and
regulations (GMP, Ph. Eur.), but in a standard norm (EN 12 021). Therefore,
they differ significantly from the demands on the quality of a pharmaceutical
preparation. For these reasons, compressed air for breathing apparatuses must
not be used for medical purposes.
Since August 1, 2001, the quality requirements on air for artificial respiration
purposes according Ph. Eur. have also become valid for air used for surgical tools,
see ISO 739-1 (EN 737-3).
References
[10.1] L. Kudrow: Response of cluster headache attacks to oxygen inhalation. Headache 1981,
21: 1–4.
[10.2] C. McDonald, A. Crockett, I. Young: Adult domiciliary oxygen therapy. Position
statement of the Thoracic Society of Australia and New Zealand Med J Aust 2005, 182
(12): 621–626 [Position Statement].
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11
Logistics of Industrial Gas Supply
11.1
Introduction
In principle, logistics of customers’ supply with industrial gases is determined by
the required quantity and the transport distance. In order to adapt to the specific
requirements of the consumers, nowadays suppliers avail of several possibilities
that are schematically depicted in Fig. 11.1.
The customer receives the industrial gases either in gaseous state in compressed
gas cylinders or in liquid state, mostly cryogenic, in insulated special containers.
Bulk consumers, such as refineries or steel mills with constantly high demand
are supplied via pipelines.
Table 11.1 gives an overview of the currently relevant possibilities of industrial
gas supply.
Table 11.1 Possibilities of supply with some industrial gases as examples.
Form of supply
N2
O2
Ar
H2
CO CO2 He
Ne
Kr
Xe
Gaseous
u
u
u
u
u
u1)
u
u
u
u
u
1)
u
u
u
u
1)
u
Compressed gas cans
Compressed gas cylinders
Cylinder bundles
u
u
u
u
u
u
Compressed gas trailers
Liquid
1)
2)
2)
2)
u
u
u
u
u
2)
Pipeline system
u
u
Cryogenic jugs
u
u
u
u
Tank truck
u
u
u
u
Railroad tank car
u
u
u
u
u
u
u
2)
u
u
u
u
u
Liquefied under pressure.
Locally limited networks.
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
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11 Logistics of Industrial Gas Supply
Fig. 11.1 Logistics of industrial gases supply.
11.2
Storage and Transport of Compressed Gases
11.2.1
Fundamentals
Owing to its low specific density, gases and gas mixtures are usually stored in
compressed gas cylinders. As a rule, compressed gas cylinders consist of steel or
aluminium materials, even fibrous composite materials are used. The European
Standard EN 1089 part 3 provides for a corresponding colour label on the collar
of the cylinder, according to the content: Compressed gas cylinders for argon are
labelled dark green, cylinders for helium brown, those for oxygen white and red
for hydrogen.
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285
The TRG (Technical Rules for Compressed Gases) subdivide gases and gas
mixtures according to their chemical and physical behaviour and determine
the compressed gas cylinders to be used, including their equipment, inspection
periods, filling factors and filling pressures. Cylinders with inflammable content
are generally equipped with a left-hand thread at the outlet valve.
In general, the transport of compressed gases is carried out by lorries, and
both national and international regulations have to be followed. To mention on
national level, e.g. the
x German directive for road transport of dangerous materials (“Gefahrgutverordnung Straße”/GGVS)
x US “Hazardous Materials Regulations” (Title 49 CFR, Parts 100–199)
x European agrement concerning the international carriage of dangerous goods
by road (“Accord européen relativ au transport international des marchandises
dangereuses par route”/ADR)
The United Nation’s “Recommodations on the Transport of Dangerous Goods
– Model Regulations” provides a uniform regulatory framework which can be
applied in all countries for national or international transport by any mode of
transport.
11.2.2
Kinds of Transport and Storage for Compressed Gases
Storage and transport of compressed gases occurs in different kinds of containers,
depending on the quantities:
Small quantities are delivered to the user in handy compressed gas cans of aluminium or in small steel cylinders with volumes of 1 resp. 0.38 litres and a maximum
filling pressure of 12 resp. 200 bar. These non-returnable compressed gas containers mainly used for laboratory purposes are the smallest possible transport
unit [11.1].
As a rule, larger quantities are transported in compressed gas cylinders of steel or
aluminium, usually with a volume of 10, 20, 40 and 50 L and a maximum filling
pressure of 200 bar. Partly, even compressed gas cylinders with a maximum
filling pressure of 300 bar are used for the transport of helium, nitrogen, oxygen
and welding protective gases [11.1]. Pallets, in which up to 12 cylinders can be
transported, depending on the type, are used for the safe and efficient transport
of compressed gas cylinders. In case the demand exceeds the efficient and
reasonable supply by single cylinders, compressed gas cylinder bundles are used.
Those are 12 single compressed gas cylinders with a content of 40 or 50 L each,
connected with each other via pipe and forming one unit. The cylinders are
arranged vertically or horizontally; each bundle frame disposes of a filling and
discharge valve. Compressed gas cylinder bundles can be connected to larger
units at the consumer. The four mentioned types of compressed gas cylinders
are customary for the storage and transport of nearly all gas kinds. In the case
of helium and hydrogen, a special form of vehicles with mounted compressed
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11 Logistics of Industrial Gas Supply
gas cylinders, so-called compressed gas trailers, are applied for the supply to bulk
consumers.
As a rule, the gas is transferred into a large stationary pressure vessel, in rare
cases it is even withdrawn directly from the trailer.
However, the transport and storage means described above, are inadequate for
industrial bulk consumers of the chemical and steel industry. They are supplied
by On-site-plants (gas production plants on site, also called “over-the-fence-supply”)
respectively by a pipeline system. Such pipeline system for nitrogen, oxygen and
hydrogen, for instance, is operated by Linde in the German chemistry triangle
Buna/Leuna/Bitterfeld. This form of transport of natural gas has been popular in
Europe for a long time and enables the transport of large quantities of compressed
air over large distances (cf. Chapter 7).
11.2.3
Efficiency of Compressed Air Gas Transport
The quantity of gas to be transported is determined by container pressure and
container size. However, in the case of withdrawal of overflowing, the pressure
of the transport container in dependence on the consumer pressure can only be
insufficiently used. The efficiency of the transport of compressed gases is still
characterized by the fact that the respective pressure containers are several times
heavier than the transported quantity of gas itself. For this reason, constructions
of fiber reinforced composite materials in combination with steel or aluminium
are increasingly applied. By using such combinations of materials, the transport
capacity of a compressed gas trailer for hydrogen, for instance, can almost be
doubled at a constant gross vehicle weight rating of 40 t.
A highly efficient form of transport is the pipeline; however, the high investment
costs should be commensurate with the transported gas quantity. Therefore,
pipeline systems for industrial gases are only regionally available and to a
limited extent to bulk consumers such as to refineries (H2) or steel mills (O2),
for instance.
11.3
Storage and Transport of Liquefied Compressed Gases
11.3.1
Fundamentals
Liquefied cryogenic gases are transported and stored in thermally insulated
compressed gas vessels. The insulation of the vessels is achieved by their design
with cylinder jacket insulation. Here, the inner vessel is concentrically arranged
in the outer vessel and the insulation is located in the clearance. This helps to
minimize heat penetration into the cryogenic gas of inner vessel which results
from thermal conductivity, thermal radiation and convection:
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287
x For gases with boiling temperatures above 70 K (e.g. N2, O2, Ar) usually a perlitepowder-vacuum insulation is used
x For gases with boiling points below 70 K (e.g. H2, He, Ne) superinsulation has
become established. This consists of a number of double layers of aluminium
foil and insulation foil in the high vacuum
The quality of the insulation depends among other factors significantly on the
quality of the vacuum. Despite good insulation, stored liquefied compressed gases
are subject to heat penetration which makes the cryogenic content evaporate
slowly. The insulation quality and thus the thermal flow are described by the
evaporation rate. It is indicated in percent per day, based on the maximal filling
capacity of the vessel.
In closed storage vessels, i.e. without extraction, this evaporation rate causes a
slow pressure rise. Thus, in case of very long periods of standstill, the admissible
operating pressure can be achieved and trigger off a safety valve.
For safety reasons, only special materials, such as austenitic, low temperature
steels or aluminium alloys are used for cryogenic liquefied gases, that still dispose
of sufficient fracture toughness at low design temperatures of down to 4 K (liquid
He).
11.3.2
Forms of Transport and Storage of Liquefied Gases
The liquefaction of gases takes place in a process step following the production,
partly involving high expenditure of energy. Nevertheless, especially the air gases
nitrogen, oxygen and argon are mostly stored and transported in liquid state as
a consequence of their considerably reduced volume compared to the gaseous
state.
Cylindrical vertical tanks for liquefied compressed gases are available in a
volume range between 1500 and 80 000 L. These tanks are suitable for both the
direct extraction of liquid gas and for the extraction of compressed gas with a
subsequently installed evaporator. For the storage of huge quantities sometimes
even spherical tanks are used, for example as buffer tanks in production plants.
For the transport of liquefied cryogenic gases such as N2, O2, H2, Ar and He,
cryogenic jugs with volumes of 38 to 1000 L [11.2] are used. For lager quantities
special double-wall tank trucks are used. Optimized tank trucks at a gross vehicle
weight rating of 40 t (depending on national laws and regulations) and a volume
of 13 000 to 28 000 L are able to transport a payload of up to 22 000 kg. Road tank
trucks are mostly designed perlite-insulated; in case higher demands regarding the
evaporation rate are to be fulfilled, even superinsulated tank trucks are deployed,
see Fig. 11.2.
For the transport over very large distances, e.g. intercontinental shipping, in
addition to the superinsulation of the vessel, a so-called nitrogen shield that
cools the insulation by means of liquid nitrogen reduces the heat penetration
into the cryogenic gas of tank trucks for He and H2. As a rule, trucks for road
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11 Logistics of Industrial Gas Supply
Fig. 11.2 Special tank truck for the transport of liquid hydrogen filling vertical tanks.
transport are designed for a maximum operational overpressure of 1.7 bar
[11.1].
Transport vehicles for rail transport are available for liquid O2, N2, Ar and CO2.
For railroad tank cars, the gross vehicle weight rating amounts to 80 t and the
payload depending on the gas to about 60 000 kg (CO2).
11.3.3
Efficiency of the Transport of Liquefied Gases
The decisive advantage of liquefied gases is their significantly higher density
compared to the compressed gas. Owing to this fact, the transport capacity in
road transport is considerably higher. At identical vehicle weight, a special tank
truck for liquid hydrogen, for example, is able to transport more than six times
the quantity of hydrogen a compressed gas trailer is able to transport.
The justification for the liquid transport can be found in the favourable ratio of
payload to gross weight of the tank truck compared to the transport in compressed
gas cylinders. After liquefying, for instance, the gas volume of oxygen is reduced by
the factor of 850 at 15 °C and 1 bar [11.2]. Thus, the higher costs for the liquefaction
of gases are faced with lower transport costs. Depending on the transport distance,
the energetic extra expenditure is compensated for.
1345vch11.indd 288
26.10.2007 10:28:34
References
289
11.4
Special Forms of Supply
Carbon dioxide is stored in pressure-liquefied state in small containers like
compressed gas cylinders and cylinder bundles. Large quantities of CO2 are
transported in special tank trucks respectively railroad tank cars in cryogenic
liquefied state, analogous to the gases described in Table 11.1. For the transport
of solid CO2 (dry ice), coolers e.g. of polystyrene are used (cf. Chapter 6).
Natural gas is usually transported via pipelines. In order to be able to exploit
natural gas deposits even in more remote areas without pipelines being available
for transportation, natural gas, liquefied to cryogenic LNG is transported by tank
ships (cf. Chapter 7). This enables the efficient transport of large quantities of
LNG over far distances even in intercontinental transport.
References
[11.1] http://www.linde-spezialgase.de.
[11.2] Ullmann’s, 6th edition, 23, p. 189 ff., Wiley-VCH, Weinheim, 2003.
1345vch11.indd 289
26.10.2007 10:28:34
291
Subject Index
a
accompanying gas 220
acetylene 240
– application in autogeneous engineering
247
– critical point 244
– decomposition 243
– dew point 244
– hydrate 244
– ignitable mixtures with air 243
– liquefaction 243
– petrochemical 246
– recovery from calcium carbide 246
– storage 246
– supply system 247
acetylene generator 245
acetylene hydrate 244
acetylide 244
air, medical 282
air booster 21
air separation by cryogenic rectification
20
air separation unit
– process analysis 64
– safety 59
– safety, ignition source 60
air separator
– cold section 24
– cryogenic 23
– safety, ignition in reboilers 63
– two-column 113
– warm section 23
alkaline electrolyser, electrolysis of water
143
aluminium plate-fin type, air separation
49
ambient air, helium content 125
aMDEA 150
ammonia synthesis gas, processing, nitrogen
wash process 156
ammonium nitrate for processing of nitrous
oxide 280
Andrews 3
AOD 4
argon
– applications 104
– history 12
– occurrence 13
– properties 13
argon bulge, air separation 29
Asia Industrial Gases Association (AIGA) 7
ATR 148, see also autothermal reforming
autogeneous engineering 247
autogenous technology 241
autothermal reactor 149
autothermal reformer 148
autothermal reforming 148
– production of synthesis gas 144
b
Barrer 16
Bartlett, N. 112
Base-Load-Plants, natural gas 231
Benfield-process 150
Berthelot 240
Birkeland 9
Black 185
Bosch 4, 9
Boudouard-equilibrium 188
box reformers 146
Bunsen, Robert 218
c
C2H2 243
C2H4 249
C2plus 230
C3MR 232
C3MR-process
C3plus 226
232
Industrial Gases Processing. Edited by Heinz-Wolfgang Häring
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31685-4
1345vch12.indd 291
26.10.2007 10:28:46
292
Subject Index
Cailletet 9, 141
calibration gas mixtures 261
– fittings and pipes 272
– production 264
– production, dynamic-volumetric
method 264
– production, gravimetric method 265
– production, manometric-static
method 264
– supply systems 273
can-type reformers 146
carbon dioxide
– as greenhouse gas 186
– high purity 195
– liquefaction 193
– liquid, properties 187
– occurrence 185
– pre-purification 191
– properties 186
– recovery 189
– recovery from flue gas 198
– sources 190
carbonic acid 185, 188
– medical 281
carbonic acid anhydride 185
carbon molecular sieves, O2 production
18
carbon monoxide
– applications 182
– occurrence 141
– properties 141
carbonyl complexes 141
Caro 9
Cavendish 3, 9, 12, 136
CGA 6
Charles, C. 136
Charles, J. 3
chemical scrubbings 150
China Industrial Gases Industry
Association (CIGIA) 7
CIGIA 7
Claude, G. 2, 3
Clement 141
CMS 18, see also carbon molecular sieves
CO 141
CO2 185
CO2 reforming, synthesis gas production
144
CO2 scrubbing 191, 192
CO2 source 189
cold box 21, 23
columns, air separation 54
Commission Permanente Internationale
(CPI) 6
1345vch12.indd 292
compressed air for breathing apparatuses
282
compressed gas 287
– liquefied, storage 286
– liquefied, storage in cryogenic jugs 287
– liquefied, storage in vertical tanks 287
– pipeline 286
– pipeline systems 286
Compressed Gas Association 6
compressed gas cans 285
compressed gas cylinder 283, 284
– colour labelling 284
compressed gas cylinder bundles 285
compressed gas trailers 286
compressor, air separation 45
concentration units 262
convective reformer 149
crude argon column
– air separation 26
– McCabe–Thiele diagram 29
d
D 137
Davy, E. 1, 240
Desormes 141
deuterium 137
Dewar 3, 136
Dirty Shift 150
dot notation, pure gases
dry ice 187
– production 200
DVS 241
256
e
EIGA 6
electronic gases 269
ethane, recovery from natural gas 229
ethene 249, see also ethylene
– application in autogeneous engineering
249
– recovery 249
ethylene, properties 249
ethyne 243, see also acetylene
European Dry Ice Association (EDIA) 6
European Industrial Gases Association 6
exergy
– definition 35
– loss 35
expansion turbines, air separation 48
external compression, oxygen recovery
33
Eyde 9
26.10.2007 10:28:46
Subject Index
f
Fenske, formula by 30
Fick’s Law 16
Fischer 4, 136
Fischer–Tropsch synthesis 4, 136
flooding in the downcomer 57
Fontana, Felice 136
Frank 9
FT-synthesis 136
fuel gases 239
–
–
–
–
–
–
–
–
–
bridge bonds 140
frequency 137
heavy 137
normal 138
occurrence 136
ortho 137
para 137
production by electrolysis of water
properties 137
293
143
i
g
GAN 2
gas-to-liquid 238
gas companies, shares in the world
market 5
Gases and Welding Distributors
Association (GAWDA) 7
gases in medicine 277
gasification reactor 147
gas mixtures 261
– production 263
Gas Subcooled Process 227
GAWDA 7
generator gas 135
glycol scrubbing, natural gas 225
GOX 2
GOX med 278
GSP 226, 227
GTL 2, 238
h
Haber 4, 9
heat exchanger
– air separation 49
– coil-wound 234, 235
He I 127
He II 127
helium
– high purity, recovery 128
– liquefaction 130
– occurrence 125
– properties 127
– recovery 127
helium-method, age determination of
minerals 125
helium content, ambient air 125
Henry’s Law 16
high-temperature shift 150
Hoppe 112
HTS 150
HyCO-unit 158
hydrogen
– applications 164
1345vch12.indd 293
industrial gas companies 5
industrial gases, supply, logistics 283
Industriegasverband e.V. (IGV) 7
instrumentation gases for analytical
measuring methods 258
internal compression, oxygen recovery 33
International Oxygen Manufacturers
Association (IOMA) 7
j
Japanese Industrial Gas Association (JIGA)
7
jet flooding 57
Joule, J. P. 3
Joule–Thomson effect 1, 3
k
Kamerlingh-Onnes, H. 3, 125
Kr/Xe
– fine purification 115
– fine purification, combustion of
hydrocarbons at the catalyst 115
– fine scrubbing 117
– pre-enrichtment in the air separator
krypton 111
– occurrence 111
– recovery 112
113
l
Lasonne 141
laughing gas, see nitrous oxide
laughing gas for anaesthesia 280
Lavoisier 11, 185
Linde, C. v. 1, 3, 9
Linde air liquefier 1
Linde air separation 2
Linz-Donawitz (LD) process 4
liquefied natural gas 220
liquefied petroleum gas 225
– recovery from natural gas 225
liquefiers 37
liquid turbines, air separation 48
LNG 220
26.10.2007 10:28:46
294
Subject Index
– applications 238
low-pressure column, air separation
low-temperature shift 150
LOX 59
LOX med 278
LPG 225
LTS 150
28
m
MAG 4
McCabe–Thiele diagram
– binary O2/Ar-mixture 29
– crude argon column 29
MDEA 192
MEA 150, 191
Medical Device Directive, medical gases
278
medical gases 277
medium-temperature shift 150
membrane, nitrogen recovery 16
membrane module, parameters 17
metal hydrides 140
methanation 151
methane hydrate 218
methane scrubbing, cryogenic 155
methyldiethanolamine 192
– as chemical solvent for synthesis
gas 150
MFC process 234
Mikropor A 246
Mixed Fluid Cascade process 234
Moissan, H. 1, 3, 240
molecular sieves
– for pressure swing adsorption 18
– zeolitic, O2 recovery 18, 20
mole fraction 262
monoethanolamine 191
– as chemical solvent for synthesis
gas 150
mtpa 231
MTS 150
n
N2 10
nasal cannula 279
natural gas 217
– basic feed for synthesis gas 136
– calorific value 220
– composition 223
– dew-point adjustment 224
– dry 220
– ethane separation 229
– glycol scrubbing 224
– liquefaction 231
1345vch12.indd 294
–
–
–
–
nitrogen separation 236
occurrence 218
occurrence, detected 218
separation from liquefied petroleum
gas 225
– treatment 224
– wet 220
natural gas bubble 218
natural gas deposits 219
natural gas development 217
natural gas reserves 219
neon 111
– occurrence 111
– properties 112
– recovery 118
– recovery, fine purification 119
– recovery, pre-enrichtment 118
nitrogen
– applications 67
– chemical properties 10
– fixation 10
– history 9
– inversion temperature 10
– occurrence 9
– properties 10
nitrogenase 10
nitrogen generators 36
nitrogen recovery
– by means of PSA 19
– membranes 16
nitrogen scrubbing 156
nitrous oxide, medical, processing 280
noble gas hydrates 112
normal-hydrogen 138
o
OHR 228
OHR-process 228
OMA 7
ortho-hydrogen 137
oxygen
– applications 83
– concentrators 279
– for medical application 278
– history 11
– inversion temperature 11
– occurrence 11
– properties 11
– refining 12
p
para-hydrogen 137
partial oxidation, production of synthesis
gas 144, 146
26.10.2007 10:28:46
Subject Index
Patart 4, 136
peak-shaving plants, natural gas 231
permeabilities of membranes 16
photosynthesis 186
PO 146
PO-plant 160
– gasification of heavy oil 159
PO-reactor 147
– Texaco 148
polymeric membranes, gas separation 16
potash as a chemical solvent for synthesis
gas 150
pre-reformer 158
pressure column, air separation 25
Pressure Swing Adsorber Unit
– helium recovery 128
pressure swing adsorption 18
– nitrogen recovery 18
– oxygen recovery 18
– production of high-purity hydrogen
from synthesis gas 151
Priestley 9, 11, 141
primary reformer 148
process analytics, synthesis gas plants 161
production of hydrogen, reformer plant
157
production of pure argon 29
promoted combustion-test 61
PSA 18, 151
Puls Discharge Detector 120
pure argon column, air separation 26
pure gases 256
– dot notation 256
– fittings and pipes 272
– supply systems 273
q
Q-T-Diagram 33
quantum fluid 127
quantum solid 127
r
radon, occurrence 111
Ramsay 111
Rayleigh, J. W. 12
rectisol process 151
rectisol scrubbing 151
Recycle Split Vapor 229
RSV 229, 230
s
Scheele 9
secondary reformer 148
separation factor 40, 114
1345vch12.indd 295
295
SFR 227
Shell-method, partial oxidation 147
sieve tray columns, air separation 55
Sirius-Linde, development system for
acetylene 246
specialty gases, disposal 271
Split Flow Reflux 227
steam reformer 145, 157, 159
steam reforming, see steam reformer
superfluid 127
synthesis gas
– applications 182
– definition 135
– generation by autothermal reforming
148
– generation by partial oxidation 146
– generation by steam reforming 145
– processing 150
– processing, condensation process 154
– processing, cryogenic processes 153
– processing, membrane process 152
– processing, methanation 151
– processing, methane scrubbing 154
– processing, removal of carbon dioxide
150
– production 143
– production from hydrocarbons 144
– removal of acid gases 150
synthol-process 136
t
T 137
tandem reformer 149
Technical Rules for Compressed Gases 285
Texaco-method, partial oxidation 147
Texaco-process, partial oxidation 159
thermosiphon and downflow evaporator 53
Thilorier 185
Thomson, W. 3
TIG 4
TRG 285
tritium 137
Tropsch 4, 136
turbines, air separation 47
turbo compressor, radial, air separation 45
two-column nitrogen generator 36
v
vacuum pressure swing adsorption 20
Verband der Chemischen Industrie e.V.
(VCI) 7
VPSA 20
26.10.2007 10:28:46
296
1345vch12.indd 296
Subject Index
w
x
water–gas shift equilibrium 146, 148
water–gas shift reaction 145, 147
water–gas shift reactor 150
water gas equilibrium 188
weeping 57
welding engineering, history 240
Wilson, T. L. 1, 3, 240
Wöhler, F. 1, 240
xenon 111
– in anaesthesia 281
– occurrence 111
– properties 112
– recovery 112
XePtF6 112
26.10.2007 10:28:46