Mercury, membrane or diaphragm
Introduction
The chloro-alkali industry is a major branch of the chemical industry. Its primary products are
chlorine, sodium hydroxide and hydrogen which are produced from rock salt, a readily
accessible raw material. This interactive teaching unit is concerned with the different
manufacturing processes used to produce these materials. These involve electrochemical cells
and, as such, this represents an exercise in applied electrochemistry.
Throughout the unit you are required to operate in small groups acting as a design team and are
asked to devise the optimum operational arrangement for a particular process. This will require
an understanding of the basic electrochemical processes but, in addition, an awareness of
economic and environmental issues will also be required.
The top 10 chemicals production for the US for 1989 is shown in Table 1. Chlorine and sodium
hydroxide were the 8th and 9th largest volume chemicals respectively for that year. Figure 1
shows the chlorine and sodium hydroxide production since 1960 and shows recent output to be
roughly constant at ca. 10 million tons. Collectively these data show these materials to be in
high demand and that the volumes involved are large. In fact the world-wide manufacturing
capacity for each of these chemicals is approximately 40 million tonnes per year i.e. chloroalkali production is big business.
Table 1 US top 10 chemicals production (1989)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10
.
Sulfuric acid
Nitrogen
Oxygen
Ethylene
Ammonia
Lime
Phosphoric acid
Chlorine
Sodium
hydroxide
Propylene
106 t
39.4
24.4
17.1
15.9
15.3
15.0
10.5
10.8
11.3
9.2
14
12
10
106 t
8
6
4
2
0
1960 1964 1968 1972 1976 1980 1984 1988
Annual Chlorine
Capacity
Caustic Soda
Production
Chlorine Production
Year
TASK 1
Given this large demand for chlorine, sodium hydroxide and hydrogen, what do you
think are the market outlets for these bulk chemicals? Form into your sub-groups (e.g.
A1, A2, A3, etc.) and together answer the following questions.
1.
Describe the principal uses of chlorine, sodium hydroxide and hydrogen.
2.
Chlorine is a highly corrosive gas that is difficult to handle. What would be the best
way to supply chlorine to the market outlets you have identified in Question 1?
Figure A
Unit 2
Class Distrib
Section
Exercise
LT (Whole cla
1.
Introduction to class
2.
Divide class into sub-groups and move to smaller
rooms
↓
↓
↓
↓
↓
↓
↓
3.
Introduction to unit
A1
A2
A3
B1
B2
B3
C
1
C
2
4.
Background Information
↓
↓
↓
↓
↓
↓
↓
↓
5.
Design teams A1, A2, A3, B1, B2, etc.
6.
Design team technical presentations (speaker 1)
↓
A1
↓
A2
↓
A3
↓
B1
↓
B2
↓
B3
↓
C
1
↓
C
2
7.
Design team marketing presentations (speaker 2)
8.
Sub-group discussion.
acetate (speaker 3)
Return to LT
9.
A
A
B
B
Preparation of summary
10.
Plenary session. Sub-group presentation (speaker
3)
11.
Assessment
LT
C
↓
Unit 2 Timetable
Table A
Time
Section
14.00
14.10
1
Introduction to class. Administration
Video 1
10
5
14.15
2
5
14.20
3
Divide class into sub-groups A, B, C and D and
move to smaller rooms
Introduction to unit
Read
Task 1
Discuss
5
5
5
Background Information (green)
Read
Tutor Summary
Group Discuss
5
5
5
14.35
14.50
4
5
Exercise
Design Teams
Task 2
Initial Information
Read and Discuss
Further Information
Read and Discuss
Complete Cell Diagrams
Discuss
5 + 10
5+5
10
15.25
6
15.35
7
15.50
8
16.00
9
Return to lecture theatre
5
16.05
10
Plenary session
Sub-group presentation
Electrochemical cell performance
Video 2
Slides
Summary
5
5
5
5
10
16.35
16.40
11
Design teams technical presentations
(speaker 1)
Design teams discussion.
Design teams marketing presentations
(speaker 2)
Sub-group discussion
Preparation of summary acetate
Preferred choice of cell
Duration (mins)
Assessment
End
10
10
10
10
5
BACKGROUND INFORMATION
Chlorine consumption is a major indicator of industrial activity. It is used as a disinfectant, in
water treatment and in processing pulp and paper. Large quantities of chlorine are needed for
polyvinyl chloride (about 15 million tonnes per year in 1990), an important material in the
building industry as well as for consumer goods. Chlorine is not readily transported or stored in
quantity. In many processes, its production forms part of an integrated petrochemicals/plastics
complex where it is used immediately on production.
Sodium hydroxide has wide applications in mineral processing, the pulp/paper industry as well
as textile and glass manufacturing. Hydrogen is used as a chemical feedstock or can be
integrated with other hydrogen plants. Electrolytically produced hydrogen is very pure and can
be compressed to around 200 atmospheres for transportation in cylinders for a variety of uses.
Market Outlets for Sodium Hydroxide
Chemicals
Rayon
Soap
Alumina
Neutralisation
Pulp Paper
Other
0
5
10
15
20
25
30
35
40
Percentage of Production
Market Oultets for Chlorine
Vinylchloride
Solvents
Propylene Oxide
Chloromethanes
Inorganics
Pulp Paper
Other
0
5
10
15
20
Percentage of Production
25
30
Industrial processes in the past
In 1870, there were two main manufacturing routes to chlorine. The Weldon process (1),
involving the oxidation of hydrochloric acid with manganese dioxide, competed with the Deacon
process (2) in which hydrochloric acid was oxidised by air over a copper based catalyst.
4HCl
+
MnO2
4HCl
+
O2
→
MnCl2
+
2H2O
Cu(cat )
2H2O
+
2Cl2
+
Cl2 (1)
(2)
Early nineteenth century production of sodium hydroxide used the LeBlanc process. This
amazing process involved burning coal, sodium sulfate (made from salt and sulfuric acid) and
calcium carbonate together in the ratio 35:100:100, extracting the product and treating the black
liquor with calcium hydroxide to form dilute sodium hydroxide solution. In 1861, the Solvay
process was introduced to make sodium carbonate from sodium chloride and calcium carbonate
by means of a multi-step synthesis. The sodium carbonate was then converted to sodium
hydroxide by reaction with lime.
This process competed with the Leblanc process until
electrolytic routes became dominant in the 1930s. Modern production involves electrolytic
routes only.
Industrial processes today
Sodium chloride, often referred to as rock salt, is the raw material required for electrolysis and
occurs naturally in plentiful supplies. In the UK there are large deposits in Cheshire, where it is
mined (see video 1). During the electrolysis of brine (sodium chloride solution), chlorine,
hydrogen and sodium hydroxide are formed at the same time, the overall equation being:
2NaCl
+
2H2O
→
Cl2(g)
+
2NaOH(aq)
+
H2(g)
This means that the three products are formed in a fixed ratio. Chlorine demand is traditionally
the factor which governs the chlor-alkali industry. However, the amount of co-produced sodium
hydroxide may or may not satisfy the demand. Thus, sodium hydroxide prices can vary greatly:
a shortfall leads to higher prices while, with a production excess, prices move downwards.
During the 1980s, the lowest price was $40 per tonne and the highest $500 per tonne. This
range of prices illustrates one of the difficulties in managing the process in such a way that it is
economic.
The process of electrolysis
When electrodes are inserted in a solution of electrolyte and a potential difference applied,
chemical reactions can be observed at the electrodes. The anode is defined as the electrode at
which oxidation occurs, while reduction occurs at the cathode. The two processes can be
represented by the following equations:
Reduction at the cathode
C+
+
e-
→
C
+
e-
Electron Flow
Oxidation at the anode
A-
→
A
The problem with electrolysis
In the process, three products are produced. It
is vital that these are not allowed to mix. Thus,
a requirement of a commercial cell for the
electrolysis of brine is that it separates the three
products effectively. Electrolysis in a simple
vessel (described as a ‘one-pot’ vessel) leads to
the reaction of chlorine with sodium hydroxide to
give unwanted sodium hypochlorite (NaClO),
sodium chlorate (NaClO3) and oxygen by the
following reactions:
Cl2
+
2HOCl
+
OHHOCl
OCl4OH-
→
→
→
→
Anode (+)
Cathode (-)
ClH+
ClO3O2
C+
+
+
+
+
HOCl
OCl2Cl2H2O
A-
+
+
2H+
4e-
Thus, in a commercial cell, sodium hypochlorite, sodium chlorate and oxygen could be formed
as bi-products.
There are three main approaches in designing such a cell. Each seeks to produce the three
products in such a way that reaction between them cannot occur.
The three cell types are the mercury cell, the membrane cell and the diaphragm cell.
Your Task
Working in your design teams (e.g. A1), you will be given information on ONE
of the cell types that might be used. Your task is to work out how the cell
could work and to examine its strengths and weaknesses in the production of
pure products.
The mercury cell – Initial information
You are a team of chemists asked to study an industrial electrochemical process. A cell has
been proposed that will convert brine (sodium chloride solution) into chlorine, hydrogen and
sodium hydroxide as separate products. Given the following chemical equations and materials
suggest how such a cell could be used to produce the desired products.
The relevant equations are:
2ClNa+
Na
2Na/Hg
+
+
+
-
e
Hg
2H2O
→
→
→
→
Cl2
+
2eNa
Na/Hg (sodium amalgam, a dense liquid)
2NaOH
+
H2(g)
+
2Hg (slow reaction)
The appropriate chemicals and equipment:
Aqueous sodium chloride, mercury, water, coated titanium electrode, a supply of electricity and
an electrochemical reactor of the following general design:
The main vessel is equipped with inlet 2 and outlet 2 for liquid at the bottom. It is also equipped
with inlet 1 and outlet 1 at a higher level. There is a facility to collect a gas at the top of the
vessel.
Liquid emerging from the lower outlet 2 is transferred to a secondary vessel with provision for
interaction with water, the collection of one gas, and outlet 3 for liquid.
A cell of this type has a power consumption of 3400 kWh (kilowatt hour) per tonne of chlorine
produced.
Your task
(1)
As a group of chemists responsible for the design of new processes for a
company, discuss, how the cell might operate to produce the three desired
products.
(2)
Decide how you will use the titanium electrode and what will be used as the other
electrode.
Associate the anode and the cathode with the correct
electrochemistry. Write out the equations for the anode and cathode reactions.
(3)
Decide how materials are to be moved around the system and how the three
products will be produced separate from each other.
(4)
Comment on the likely purity of the three products and any steps that might be
necessary to improve purity levels.
(5)
Discuss any possible drawbacks in using this cell.
After you have taken your decisions, one member of your design team (speaker one) will give a
brief presentation to the sub-group (e.g. group A). This presentation should be technical in
nature and describe the main workings of your cell and draw attention to the strengths and
weaknesses of the cell. An acetate for the overhead projector showing the basic cell outline is
available.
Further information will be made available in due course.
Once you have heard the technical presentations from the other design teams, you need to work
together to give a presentation to the Company Technical Director on why your cell should be
adopted by the Company. Elect speaker two, who will make this marketing presentation.
The relative merits of all three cells can then be discussed within the sub-group (e.g. group A)
as a whole.
Diaphragm cell – Initial information
You are a team of chemists asked to study an industrial electrochemical process. A cell has
been proposed which will convert brine (sodium chloride solution) into chlorine, hydrogen and
sodium hydroxide as separate products. Given the following chemical equations and materials,
suggest how such a cell could be used to produce the desired products.
The relevant equations are:
2Cl-
→
Cl2(g)
+
2e-
2H2O
+
2e-
→
H2(g)
+
2OH-
The sodium ions remain solvated throughout the process.
The appropriate chemicals and equipment are:
A sheet of gauze coated with asbestos and referred to as a diaphragm. It allows liquid to pass
slowly through from one side to the other. For example, aqueous sodium chloride (as solvated
sodium and chloride ions) will flow through this diaphragm. Its principal function is to separate
chlorine from sodium hydroxide and hydrogen.
Aqueous sodium chloride, coated titanium electrode, one steel electrode, a supply of electricity
and an electrochemical reactor of the following general design:
A cell of this type has a power consumption of 2900 kWh (kilowatt hour) per ton of chlorine produced.
Your task
(1)
As a group of chemists responsible for the design of new processes for a
company, discuss, how the cell might operate to produce the three desired
products. Decide where to place the diaphragm.
(2)
Decide how you will use the titanium electrode and the steel electrode. Associate
the anode and the cathode with the correct electrochemistry. Write out the
equations for the anode and cathode reactions.
(3)
Decide how materials will move around the system and how the three products
will be produced separate from each other.
(4)
Comment on the likely purity of the three products and any steps that might be
necessary to improve purity levels. In particular, consider any impurities that
might be present in the sodium hydroxide and how these might be removed.
(5)
Discuss any possible drawbacks in using this cell.
After you have taken your decisions, one member of your design team (speaker one) will give a
brief presentation to the group (e.g. group A). This presentation should be technical in nature
and describe the main workings of your cell, list the fundamental chemical equations and draw
attention to the strengths and weaknesses of the cell. An acetate for the overhead projector
showing the basic cell outline is available.
Further information will be made available in due course.
Once you have heard the technical presentations from the other design teams, you need to work
together to give a presentation to the Company Technical Director on why your cell should be
adopted by the Company. Elect speaker two, who will make this marketing presentation.
The relative merits of all three cells can then be discussed within the sub-group (e.g. group A)
as a whole.
Membrane cell – Initial information
You are a team of chemists asked to study an industrial electrochemical process. A cell has
been proposed which will convert brine (sodium chloride solution) into chlorine, hydrogen and
sodium hydroxide as separate products. Given the following chemical equations and materials,
suggest how such a cell could be used to produce the desired products.
The relevant equations are:
2Cl-
→
Cl2(g)
+
2e-
2H2O + 2e-
→
H2(g)
+
2OH-
The sodium ions remain solvated throughout the process.
The appropriate chemicals and equipment are:
A sheet of material called a membrane. It has been designed to allow only sodium ions to pass
through, there being no flow of liquid from one side to the other.
Aqueous sodium chloride, water, coated titanium electrode, one nickel electrode, a supply of
electricity and an electrochemical reactor of the following general design:
A cell of this type has a power consumption of 2700 kWh (kilowatt hour) per ton of chlorine
produced.
Your task
(1)
As a group of chemists responsible for the design of new processes for a
company, discuss, how the cell might operate to produce the three desired
products. Decide where to place the membrane.
(2)
Decide how you will use the titanium electrode and the nickel electrode.
Associate the anode and the cathode with the correct electrochemistry. Write
out the equations for the anode and cathode reactions.
(3)
Decide how materials might move around the system and how the three
products will be produced separate from each other.
(4)
Comment on the likely purity of each of the three products and any steps that
might be necessary to improve purity levels.
(5)
Discuss any possible drawbacks in using this cell.
After you have taken your decisions, one member of your design team (speaker one) will give a
brief presentation to the group (e.g. group A). This presentation should be technical in nature
and describe the main workings of your cell and draw attention to the strengths and weaknesses
of the cell. An acetate for the overhead projector showing the cell is available.
Further information will be made available in due course.
Once you have heard the technical presentations from the other design teams, you need to work
together to give a presentation to the Company Technical Director on why your cell should be
adopted by the Company. Elect speaker two, who will make this marketing presentation.
The relative merits of all three cells can then be discussed within the sub-group (e.g. group A)
as a whole.
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