Mercury, Membrane Or Diaphragm Introduction

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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. chloro- alkali production is big business.

Table 1 US top 10 chemicals production (1989)

106 t 1. Sulfuric acid 39.4 2. Nitrogen 24.4 3. Oxygen 17.1 4. Ethylene 15.9 5. Ammonia 15.3 6. Lime 15.0 7. Phosphoric acid 10.5 8. Chlorine 10.8 9. Sodium 11.3 hydroxide 10 Propylene 9.2 .

8 t 6

10 6 Annual Chlorine Capacity 4 Caustic Soda 2 Production Chlorine Production 0 1960 1964 1968 1972 1976 1980 1984 1988 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 A B C rooms ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓

3. Introduction to unit A1 A2 A3 B1 B2 B3 C C 1 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 C 1 2 7. Design team marketing presentations (speaker 2) A B

8. Sub-group discussion. Preparation of summary acetate (speaker 3) 9. Return to LT

10. Plenary session. Sub-group presentation (speaker LT 3)

11. Assessment

Unit 2 Timetable Table A

Time Section Exercise Duration (mins)

14.00 1 Introduction to class. Administration 10 14.10 Video 1 5

14.15 2 Divide class into sub-groups A, B, C and D and 5 move to smaller rooms 14.20 3 Introduction to unit Read 5 Task 1 5 Discuss 5

14.35 4 Background Information (green) Read 5 Tutor Summary 5 Group Discuss 5

14.50 5 Design Teams Task 2 Initial Information Read and Discuss 5 + 10 Further Information Read and Discuss 5 + 5 Complete Cell Diagrams Discuss 10

15.25 6 Design teams technical presentations 10 (speaker 1) 15.35 7 Design teams discussion. 10 Design teams marketing presentations 10 (speaker 2) 15.50 8 Sub-group discussion 10 Preparation of summary acetate Preferred choice of cell

16.00 9 Return to lecture theatre 5

16.05 10 Plenary session Sub-group presentation 5 Electrochemical cell performance 5 Video 2 5 Slides 5 Summary 10

16.35 11 Assessment 5 16.40 End

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 25 30 Percentage of Production

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 → MnCl2 + 2H2O + Cl2 (1)

4HCl + O2 Cu(cat) 2H2O + 2Cl2 (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 Electron Flow C+ + e- → C

Oxidation at the anode

A- → A + e-

The problem with electrolysis Cathode (-) Anode (+)

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 C+ A- give unwanted sodium hypochlorite (NaClO), sodium chlorate (NaClO3) and oxygen by the following reactions:

- - Cl2 + OH → Cl + HOCl HOCl → H+ + OCl- - - - + 2HOCl + OCl → ClO3 + 2Cl + 2H - - 4OH → O2 + 2H2O + 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:

- - 2Cl → Cl2 + 2e Na+ + e- → Na Na + Hg → Na/Hg (sodium amalgam, a dense liquid) 2Na/Hg + 2H2O → 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.

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

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 Formatted: Font: (Default) Arial

(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.

The relative merits of all three cells can then be discussed within the sub-group (e.g. group A) as a whole.