Module: 2
Lecture: 6 Ammonia
Dr. N. K. Patel
Module: 2
Lecture: 6
AMMONIA
INTRODUCTION
Ammonia (NH3) or azane is a compound of nitrogen and hydrogen. It is a
colourless gas with a characteristic pungent smell. Ammonia contributes significantly
to the nutritional needs of terrestrial organisms by serving as a precursor to food and
fertilizers. Although in wide use, ammonia is both caustic and hazardous.
It is one of the most important nitrogenous material. It is a base from which all
the nitrogen containing compounds are derived. Mostly is produced synthetically,
but during some chemical processes obtained as by product. Either directly or
indirectly, ammonia is a building-block for the synthesis of many pharmaceuticals
and is used in many commercial cleaning products.
Gaseous ammonia was first isolated by Joseph Priestley in 1774 and was
termed as "alkaline air". Claude Louis Berthollet ascertained its composition in 1785.
The Haber-Bosch process to produce ammonia from the nitrogen in the air
was developed by Fritz Haber and Carl Bosch in 1909 and patented in 1910. Prior to
the availability of cheap natural gas, hydrogen as a precursor to ammonia
production was produced via the electrolysis of water or using the chloralkali
process.
MANUFACTURE
(a) Haber and Bosch Process
Raw materials
Basis: 1000kg of NH3 (85% yield)
Hydrogen
= 210kg
Nitrogen
= 960kg
Catalyst
= 0.2kg
Power
= 850KWH
Fuel gas for compressors = 3800Kcal
Cooling water
= 12,500kg
NPTEL
36
Module: 2
Lecture: 6 Ammonia
Dr. N. K. Patel
Sources of raw material
 Nitrogen
Nitrogen is taken form air as discussed in Lecture: 3 (Module: 2)
 Hydrogen
Hydrogen can be synthesized from any feed stock listed in the table
Feed stock
Natural gas
Coke oven gas
Fuel oil or low sulfur heavy stock
Coal
Water
Process or techniques to produce H2
Partial oxidation / steam reforming
Partial oxidation/ low temperature separation
Partial oxidation
Partial oxidation
Electrolysis
 Catalyst
Most widely used catalyst for ammonia synthesis is iron with added promoters
e.g. oxides of aluminium, zirconium or silicon at about 3% concentration and
potassium oxide at about 1%. These prevent sintering and make the catalyst more
porous. Iron catalysts lose their activity rapidly, if heated above 520°C. Also, is
deactivated by contact with copper, phosphorous, arsenic, sulfur and CO.
Purification of raw gases
The Liquid nitrogen wash is mainly used to purify and prepare ammonia
synthesis gas within fertilizer plants. It is usually the last purification step upstream of
ammonia synthesis.
The liquid nitrogen wash has the function to remove residual impurities like
CO, Ar and CH4 from a crude hydrogen stream and to establish a stoichiometric
ratio H2/N2 = 3:1. Carbon monoxide must be completely removed, since it is
poisonous for the ammonia synthesis catalyst. Ar and CH4 are inert components
enriching in the ammonia synthesis loop. If not removed, a syngas purge or
expenditures for purge gas separation are required.
If partial oxidation of coal, heavy oil or residue oil were selected as feedstock
basis for ammonia production then liquid nitrogen wash is typically arranged to
downstream of the scrubbing process.
Traces of water, carbon dioxide, solvent (methanol) are removed in the
adsorber station. Center piece of the liquid nitrogen wash is the so called ―coldbox‖.
The process equipment of the cryogenic separation is installed close-packed in the
coldbox, which is covered with a metal shell. The coldbox voidage is filled with
insulation material (perlite) to prevent heat input.
NPTEL
37
Module: 2
Lecture: 6 Ammonia
Dr. N. K. Patel
Fuel gas
Absorber
Steam
Purified gas
Heat
exchanger
HP-N2
LP-N2
for stripping
LP-N2
Feed gas
(cold)
Absorber unit
Cold box
Figure: Purification of raw gases
Raw hydrogen and HP nitrogen are fed to the liquid nitrogen wash unit. Both
streams are cooled down against product gas. Feeding raw hydrogen to the
bottom of the nitrogen wash column and some beforehand condensed liquid to the
top. Trace components are removed and separated as fuel gas. To establish the
desired H2/N2 ratio, HP nitrogen is additionally admixing inside and outside the
coldbox.
Reaction
N2(g) + 3H2(g)
2NH3(g)
ΔH = - 22.0kcals
Manufacture
The method was first developed by Haber and Bosch therefore known as
Haber and Bosch Process. The manufacture of ammonia is carried out by passing
mixture of pure hydrogen and nitrogen in the proportion of 3:1 by volume under
pressure (100-1000atm depending on conversion required). Both the gases are sent
through filter to remove compression oil and additionally through the high
temperature guard converter in which CO and CO2 are converted to CH4, and also
removal of traces of H2O, H2S, P and As. The relatively cool gas is added along the
outside of converter tube walls to provide cooling. Carbon steel is used as material
of construction for pressure vessel and internal tubes.
NPTEL
38
Module: 2
Lecture: 6 Ammonia
Dr. N. K. Patel
The preheated gas flows next through the inside of the tube which contains
promoted iron catalyst at 500-5500C. The NH3 product, with 8-30% conversion
depending on a process conditions, is removed by condensation, first with water
cooling and then NH3 refrigeration. The unconverted N2-H2 mixture is recirculated to
allow an 85-90% yield.
300 atm
1 Vol N2 + 3 Vols H2
compresor
Liquid NH3
Condenser
Water
Water
Water
Condenser
Water
NH3 Gas
Condenser
Heating coil
Separators
Liquid Ammonia
Liquid Ammonia
Filter
Recycled Gas 300 atm
Figure: Manufacturing of Ammonia by Haber Process
Block diagram of manufacturing process
Diagram with process equipment
Animation
NPTEL
39
Module: 2
Lecture: 6 Ammonia
Dr. N. K. Patel
Cost is greatly influenced by the pressure, temperature, catalyst, purity of raw
materials and most importantly heat recovery and reuse. For achieving quality
material at lower cost modification in Haber and Bosch Process are initiated.
(b) Modern method/ Killogg ammonia process
Raw material
Natural gas
Air
Reaction
2CH4 + O2
2CO + O2
N2 + 3H2
2CO + 4H2
2CO2
2NH3
Manufacture
Block diagram of manufacturing process
Diagram with process equipment
Animation
H2O
Steam
boiler
Furnace
Tube- type furnace
(methane convertor)
2 nd - stage
CO
convertor
NH3
Cold heat
exchanger
Air
Natural gas
Heat
exchanger
Plant synthesis column
Heat
exchanger
Turbo
compressor
with gas turbine
Natural
gas
Steam
boiler
Shaft methane
convertor 1 st - stage CO
CO2
Air
convertor
regenerator CO
primary
absorber
Methanator separator
Hot heat
exchanger
Natural gas
heater
Steam boiler
water heater
Reactor for organic
Hydrogen sulphide adsorber
sulphur hydrogenation
H2
Steam
turbine
Air cooler
Air
Secondary Cold
NH3 seperator
Ammonia
cooler
Figure: Manufacturing of Ammonia By Kellogg Process
In the process natural gas is used for production of nitrogen and hydrogen.
The purified nitrogen and hydrogen is thus reacted to give ammonia gas. In
commercial production sulfur free natural gas is mixed with steam in the volume
NPTEL
40
Module: 2
Lecture: 6 Ammonia
Dr. N. K. Patel
based ratio of 3.7:1 and compressed to 40atm. The mixture is preheated with the
recycled flue or effluent gases and fed into the furnace. At 800-8500C in the
presence of iron catalyst promoted with other metal oxides conversion of methane
takes place with the formation of CO. The residual gas is mixed with air and fed into
shaft converter to get complete conversion. The waste heat is utilized for the steam
generation and ethanolamine which are used in CO2 and H2S removal. The exit gas
containing poison was regenerated in the methanator at 280-3500C which ultimately
used for heating the feed water.
Purified N2 and H2 mixture was compressed to 300atm at 320 to 3800C in the
presence of catalyst converted to NH3. 14-20% conversion per pass was achieved.
NH3 condensed and separated from exit gas, whereas unconverted N2 and H2 gases
were recycled along with the fresh gases.
Ammonia synthesis is being exothermic the process requires an effective
temperature control system at every stage of reaction.
(c) Modified Haber Bosch process
The manufacture of ammonia may be carried out by the partial oxidation of
hydrocarbon derived from naphtha, natural gas or coal by oxygen enriched air in
the presence of catalyst. CO is removed by passing through ammonical solution of
cuprous formate. The remaining N2 and H2 gas are utilized for the manufacture of
ammonia by Haber process.
Modified Haber Bosch process has following steps
a)
b)
c)
d)
e)
Manufacture of reactant gases
Purification
Compression
Catalytic reaction
Recovery of ammonia and recycle of reactant gases
a) Manufacture of reactant gases
Water gas as source of H2 is prepared from coke and steam at 10000C14000C. It is cooled and purified by passing through lime and iron oxide coated
wood shavings.
C + H2O
CO + H2 ∆H = -38900cal
Producer gas is prepared by passing air through heated coke or coal bed
Resulting CO2 passed through the hot bed of the fuel which
reduced it to carbon monoxide, the nitrogen of the air remains mixed with CO. The
gas is cooled and purified. In both the cases sensible heat of the gases is utilized by
raising steam in waste heat boiler
at10000C-14000C.
NPTEL
41
Module: 2
Lecture: 6 Ammonia
C + 1/2O2
Dr. N. K. Patel
CO ∆H = -28900cal
b) Purification
Both water gas and producer gas are mixed in such a ratio so that after
purification concentration of nitrogen and hydrogen by volume becomes one is to
three (1: 3). The cold mixed gas is mixed with excess of steam, then the hot gases are
passed through horizontal converters containing catalyst consisting of iron oxide
promoted with Cr2O3 and CeO2. The exothermic conversion of CO to CO2 by steam
is carried out at an optimum temperature 4500C by the heat of reaction.
CO + H2 + H2O
CO2 + 2H2O
∆H =98,000cals.
The hot mixture of CO2, H2, N2 and CO is cooled by passing through the heat
exchanger then the cooled gas is stored. CO2 is removed by any one method which
is described (Module: 2, Lecture: 2) as method of recovery of CO2
The gases after removal of CO2, are compressed to 200atm pressure, cooled,
and treated in a pressure tower with ammonical solution of cuprous formate
(HCOOCu) which absorbs CO. The resultant gas is mixture of H2 and N2 (3vol: 1vol).
The cuprous formate solution after stripping of carbon monoxide is recycled back to
the process.
c) Compression
The purified N2 and H2 mixture at 200atm pressure is further compressed to
300atm pressure mixed with recycling gas at the same pressure and passed through
oil filters. The compressed gas mixture is then cooled by cold water followed by
refrigeration by liquid ammonia. The recycling gas in the mixed gas contained some
ammonia. This ammonia is liquefied by pressure and refrigeration hence before
allowing the gas mixture to enter into the converter, the liquid ammonia is
separated.
d) Catalytic reaction
The gas mixture then passes into the converter which is made of nickel,
vanadium, chromium steel having 7ft. height and 21 inches diameter. The seamless
cap having 3 inch wall thickness is held by bolts of nickel steel. The converter is fitted
with double coil acting as heat interchanger through the inner tube of which cold
gas mixture passes, and through the outer tube of which passes the hot outgoing
gas mixture. At the base of the coil there is resistance coil for electrical heating. In
the converter there is the contact catalyst chamber consist of three concentric
tubes which contain the granular catalyst.
The compressed gas enters through the inner coil of the heat interchanger.
After passing through the interchanger the gas is heated electrically by the
NPTEL
42
Module: 2
Lecture: 6 Ammonia
Dr. N. K. Patel
resistance coil and then goes up 1st catalyst chamber, and then down through the
2nd, and lastly up through the last. It then enters the outer coil of the central heat
exchanger, gives up the heat to the incoming gas, and then goes out of the
converter from the top.
e) Recovery of ammonia and recycle of reactant gases
The mixed outgoing gas containing19% NH3 and rest N2 and H2 going out of
the converter is cooled by cold water in the condenser. Major portion of ammonia
liquefies. The liquid NH3 is separated and the unconverted gas mixture containing
some unliquefied NH3 is compressed to 300atm pressure and then mixed with fresh
compressed gas mixture and recycled. A part of the recycled gas is rejected from
time to time to prevent the accumulation of argon and methane. The temperature
in the contact chamber is 5500C.
Kinetics and thermodynamics
N2(g) + 3H2(g)
2NH3(g)
ΔH = - 22.0kcals
The highest yield of above reaction can be obtained at high pressure and
low temperature which can be expressed as follows
√
Where, the equilibrium constant is an inverse function of the absolute
temperature
∆F= -RT ln Kp = -19000 + 9.92T ln T + 1.15 X 10-3T2 - 1.63 X 10-6T3 - 18.32T
The reaction is exothermic and similar to oxidation of SO2 is favoured by low
temperature from equilibrium stand point but reaction kinetics dictate a
compromise temperature at some higher value like 500 - 5500C in single stage
convertor.
The cost of high pressure reaction system is higher so multistage operation as
used with SO2 oxidation is not economically feasible for ammonia production.
The design problem thus reduces to an optimization of space velocity based
on the following considerations.
The fraction of NH3 (x) in the exit gas decreases with increase in flow rate or
space velocity by equation
x = fV-n
NPTEL
43
Module: 2
Lecture: 6 Ammonia
Dr. N. K. Patel
Where,
n<1 if bed is at correct temperature and mass transfer rates are improved
n>1 where bed is at too low temperature because of high velocity gas
cooling
The space time yield (Y) is
(
)(
)
Y = V.V-n = V(1-n)
In addition to very high space velocity, cooling the bed will increases the cost
of NH3 recovery because x is lower and also increases the pumping cost hence
based on these considerations an optimized cost is calculated.
Catalyst development
Iron oxide promoted by alkali is widely used as catalyst or nonferrous metal
oxides such as K2O (1-2%) and Al2O3 (2-5%). The iron oxide is fused in an electric
furnace and the promoters added. The solidified mass is ground to desired particle
size. The iron oxide is reduced to porous iron in the start-up phases of operation in the
synthesis reactor. There is a maximum operating temperature of about 6200C, above
which the catalyst fuses.
A promoted iron catalyst has recently been developed in Europe (Mont Cenis
process) which allows for very low temperature (4000C) and low pressure operation
(100atm). The life of the catalyst is not firmly established.
Process design modifications
The pressure affects conversion, recirculation rates and refrigeration of the
process. The various process used with different process parameter are as follows
 Very high pressure (900-1000atm, 500-6000C, 40-80% conversion) — Claude,
Du pont, L‘air liquide
 High pressure (600atm, 50000C, 15-25% conversion) — Casale
 Moderate pressure (200-300atm, 500-5500C, 10-30% conversion) — Haber
bosch, Kellogg, Fauser, Nitrogen Engineering Corporation
 Low pressure (100atm, 400-4250C, 8-20% conversion)
 Mont Cenis: uses a new type of iron catalyst promoted iron cyanide.
The modern trend is towards lower pressure and increased recirculation loads
because of the relatively high cost of pressure vessels. The large single train plants
NPTEL
44
Module: 2
Lecture: 6 Ammonia
Dr. N. K. Patel
using centrifugal compressors and having capacities as high as 1000 tons/day from
a single reactor at low production cost are used widely.
PROPERTIES








Molecular formula
Molecular weight
Appearance
Odour
Boiling point
Melting point
Density
Solubility
: NH3
: 17.031gm/mole
: Colourless gas
: Strong pungent
: -33.340C
: -77.730C
: 681.9kg/m3 at −33.30C (liquid)
: Soluble in water
USES
 Ammonia is major raw material for fertilizer industries
 It is used during the manufacture of Nitro compounds, Fertilizers e.g. urea,
ammonium sulfate, ammonium phosphate etc.
 It is also used in manufacture of Nitric acid, Hydroxylamine, Hydrazine, Amines
and amides, and in many other organic compounds
 It is also used in heat treating, paper pulping, as explosives and refrigerants
NPTEL
45