118
UNDERGRADUATE STUDENT’S GUIDE PROJECT PREPARATION BRIGHT STAR UNIVERSITY - BRAGA
UNDERGRADUATE STUDENT’S GUIDE
PROJECT
PREPARATION
BRIGHT STAR UNIVERSITY - BRAGA
TH
E S
TU
DE
NT
NA
ME
BA
CH
EL
OR
20
18
Libya
Ministry of Higher Education
Bright Star University – Braga
Bright Star University – Braga
Faculty of Technical Engineering
Department of Chemical Engineering
THE PROJECT TITLE
Chlor- Alkali production by electrochemical
process
By
Saada Hamad Abobaker Zubi 21152892
Huda Almabrok Abd Alkareem 21152921
Aisha Juma Omar Elwerfalli 211521003
Supervised by
Yusuf Ahmed Al Mestiri
DEGREE OF BACHELOR IN CHEMICAL ENGINEERING
FACULTY OF TECHNICAL ENGINEERING
i
THE PROJECT TITLE
Chlor- Alkali production by electrochemical
process
By
Saada Hamad Abu Baker Zubi 21152892
Huda Almabrok Abd Alkareem 21152921
Aisha Juma Omar Elwerfalli 211521003
Supervised By
Yusuf Ahmed Al Mestiri
Project Report Submitted as Partial Fulfillment of the Requirements for the
Degree of Bachelor in chemical Engineering
Month, 2018
ii
ABSTRACT
The chlor-alkali industry is the industry produces chlorine gas (Cl2), hydrogen gas (H2), and
alkali; sodium hydroxide (NaOH) and to less extent potassium hydroxide (KOH) by
electrolysis of a salt solution using direct current (DC). The salt solution is sodium chloride
or potassium chloride. The main technologies applied for chlor-alkali production are
mercury cell, diaphragm cell and membrane cell techniques. The amount of 59electricity
required for the electrolysis process depends on electrolytic cell parameters such as current
density, voltage, and anode and cathode material.
iii
DEDICATION
To the utmost knowledge lighthouse, to our greatest and most honored prophet Mohamed
May peace and grace from Allah be upon him. To the Spring that never stops giving, to my
mother who weaves my happiness with strings from her merciful heart. To my mother. To
whom he strives to bless comfort and welfare and never stints what he owns to push me in
the success way who taught me to promote life stairs wisely and patiently, to my dearest
father. To whose love flows in my veins, and my heart always remembers them, to my
brothers and sisters. To those who taught us letters of gold and words of jewel of the utmost
and sweetest sentences in the whole knowledge. Who reworded to us their knowledge
simply and from their thoughts made a lighthouse guides us through the knowledge and
success path, To our honored teachers and professors.
iv
ACKNOWLEDGEMENTS
I would like to express my appreciation to my supervisor, (Yusuf Ahmed Al Mestiri) who
has cheerfully answered my queries, provided me with materials, checked my examples,
assisted me in a myriad way with the writing and helpfully commented on earlier drafts of
this project. Also, I am also very grateful to my friends, family for their good humor and
support throughout the production of this project
v
APPROVAL
This project report is submitted to the Faculty of Technical Engineering, Bright Star
University – Braga, and has been accepted as partial fulfillment of the requirement for the
degree of bachelor in Chemical Engineering. The members of the Examination Committee
are as follows:
________________________________________
Supervisor
The supervisor name Yusuf Ahmed Al Mestiri
Department of Chemical Engineering
Faculty of Technical Engineering
Bright Star University – Braga
____________________________________________
Examiner 1
The examiner name
Department of Chemical Engineering
Faculty of Technical Engineering
Bright Star University – Braga
____________________________________________
Examiner 2
The examiner name
Department of Chemical Engineering
Faculty of Technical Engineering
Bright Star University – Braga
vi
DECLARATION
I hereby declare that the project report is my original work except for quotations and
citations, which have been duly acknowledged. I also declare that it has not been
previously, and is not concurrently, submitted for any other degree at Bright Star University
– Braga or at any other institution.
___________________________________
Saada Hamad Abu Baker Zubi
SN 21152892
Date:
Huda Almabrok Abd Alkareem
SN 21152921
Date:
Aisha Juma Omar Elwerfalli
SN 211521003
Date:
vii
TABLE OF CONTENTS
ABSTRACT…………………………………………………………………………… ii
DEDICATION………………………………………………………………………… iii
ACKNOWLEDGEMENTS…………………………………………………………… Iv
APPROVAL…………………………………………………………………………… V
DECLARATION……………………………………………………………………… vi
LIST OF TABLES……………………………………………………………………. Xi
LIST OF FIGURES…………………………………………………………………… xii
Introduction……………………………………………………………………………. 1
CHAPTER 1: CHLOR-ALKALI RAW MATERIAL ……………………………. 2
1.1 Brine Preparation………………………………………………………………. 2
1.2 Chlor-Alkali raw material chemical treatment……………………………......... 2
1.2.1 Primary purification………………………………………………………… 3
1.2.2 Secondary purification: membrane cell technique ………………………… 5
1.3 Control of nitrogen compounds in the brine…………………………………… 6
1.4 Brine dechlorination and resaturation…………………………………………. 7
1.5 Chlorate destruction: membrane cell technique…………………....................... 9
CHAPTER 2: CHLOR-ALKALI ELECTROLYSIS PROCESS TECHNIQUES. 10
2.1 Mercury Cell Process Technique………………………………………………. 11
2.1.1 General description…………………………………………………………. 11
2.1.2 The mercury cathode electrolyzer and decomposer………………………... 12
2.1.3 Mercury cell chemical reactions…………………………………………… 13
2.1.4 The problem………………………………………………………………… 14
2.2 The Diaphragm Cell Technique ………………………………………………... 14
viii
2.2.1 General description………………………………………………………… 14
2.2.2 The cell…………………………………………………………………….. 16
2.2.3 Diaphragm cell chemical reactions………………………………………… 16
2.3 The Membrane Cell Process Technique………………………………………. 17
2.3.1 General description…………………………………………………………. 17
2.3.2 The Cell……………………………………………………………………. 20
2.3.3 Monopolar and bipolar electrolyzer………………………………………… 21
2.3.4 Membrane cell chemical reactions…………………………………………. 24
CHAPTER 3: CHLORINE, HYDROGEN AND CAUSTIC SODA PROCESSING……. 27
3.1 Chlorine processing, storage and handling……………………………………... 27
3.1.1 General description…………………………………………………………. 27
3.1.2 Material……………………………………………………………………. 28
3.1.3 Cooling……………………………………………………………………… 29
3.1.3.1 Chlorine Cooling…………………………………………........................... 29
3.1.4 Filtration and boosting……………………………………………………… 31
3.1.4.1 filtration………………………………………………………..................... 31
3.1.4.2 boosting………………………………………………………..................... 32
3.1.5 Drying……………………………………………………………………… 32
3.1.6 Cleaning of dry chlorine…………………………………………................ 33
3.1.7 Compression………………………………………………………………. 33
3.1.8 Liquefaction………………………………………………………………… 34
3.1.9 handling and storage………………………………………………………... 35
3.1.10 Vaporization………………………………………………………………… 36
3.2 Hydrogen processing, storage and handling……………………………………. 36
3.3 Caustic soda processing…………………………………………....................... 37
ix
CHAPTER 4: CHLOR-ALKALI MATERIAL AND ENERGY BALANCE…… 39
4.1 Chlor-alkali material balances…………………………………………………. 39
4.1.1 Mercury cell technique material balance…………………………………… 40
4.1.2 Diaphragm cell technique material balance………………………………… 41
4.1.3 Membrane cell technique material balance………………………………… 42
4.1.4 Chlorine drying material balance…………………………………………… 43
4.2 Chlor-alkali energy balance……………………………………………………. 44
4.2.1 Energy balance over mercury cell…………………………………………. 45
4.2.2 Energy balance over diaphragm cell………………………………………... 46
4.2.3 Energy balance over membrane cell………………………………………. 47
CHAPTER 5: PROCESS DESIGN…………………………………………………. 49
5.1 Heat exchanger process design………………………………………………… 49
5.2 The tube-side pressure drop………………………………...…………………. 54
5.3 The pressure drop in shell side………………………………………………… 55
CHAPTER 6: THE COST …………………………………………………………. 56
6.1 Cost Economy of Salt in Chlor-alkali Manufacture …………………………… 56
6.2 Cost of Salt……………………………………………………………………… 58
6.3 Cost of Sodium Chloride and Brine Treatment………………………………… 58
6.4 Typical Brine Purification Cost………………………………………………… 59
6.5 Cost of Salt and Brine Treatment………………………………………………. 60
6.6 Elements of High Quality Solar Salt Production………………………………. 61
6.7 Chlor-Alkali Costs of Production and Products Costs…………………………. 62
6.7.1 Costs of Production………………………………………………………… 62
6.7.2 Chlor-Alkali Products Costs………………………………………………. 64
6.7.3 Variable Costs to Produce an ECU of Caustic and Chlorine…..................... 65
x
CHAPTER 7: INSTRUMENTATION AND CONTROL…………………………. 69
7.1 Process Control…………………………………………………………………. 69
7.2 The Closed Loop ………………………………………………………………. 72
7.2.1 Instrument Uses……………………………………………………………. 72
7.2.2 The control function………………………………………………………… 73
7.2.3 Factors of process control…………………………………………………. 73
7.2.4 Definition…………………………………………………………………… 75
7.2.5 Types of Signals………………………………………………..................... 75
7.2.6 Control Modes……………………………………………………………… 76
7.2.7 How a feedback control system works……………………………………. 78
CHAPTER 8: USES OF PRODUCTS ……………………………………………… 81
8.1 Important uses of Chloride………………………………………....................... 81
8.2 Important uses of Hydrogen …………………………………………………… 81
8.3 Important uses of sodium hydroxide …………………………………………... 82
CHAPTER 9: ENVIRONMENT AND SAFETY MEASURES…………………… 83
9.1 Environmental Relevance of the Chlor-Alkali industry………………………... 83
9.2 Safety measures………………………………………………………………… 84
9.2.1 General measures…………………………………………………………. 85
9.2.2 In the chlor-alkali plant……………………………………………………... 86
9.2.3 In the loading area…………………………………………………………... 86
Reference………………………………………………………………………………. 88
Appendix A: Material Safety Data for Chlorine………………………………………. 89
Appendix B: Material Safety Data for Hydrogen……………………………………… 94
Appendix C: Material Safety Data for Sodium Hydroxide……………………………. 97
xi
LIST OF TABLES
Table 1 Typical compositions of sodium chloride used in chlor-alkali electrolysis. 3
Table 2 Typical impurities with sources and effects on the membrane cell
technique and brine specifications………………………………………...
6
Table 3 The differences between typical configurations of monopolar and bipolar 23
Table 4 Main typical characteristics of the different electrolysis techniques……... 25
Table 5 Typical data for chlor-alkali cells………………………………………… 45
Table 6 Costs of Production for the Chlor-Alkali Industry………………………. 63
Table 7 Capital Costs for 500 Ton per Day Chlorine Production Plant…………... 63
xii
LIST OF FIGURES
Figure 1 Simple brine treatment block diagram…………………………………… 3
Figure 2 Flow diagram of a possible layout for the brine system used in the
membrane cell technique………………………………………………… 5
Figure 3 Typical flow diagrams of the three cells techniques……………………… 10
Figure 4 Mercury cell………………………………………………………………. 11
Figure 5 Diaphragm cell……………………………………………………………. 16
Figure 6 Process flow of the membrane cell process………………………………. 18
Figure 7 Flow diagram of the integration of the membrane and mercury cell
techniques………………………………………………………………… 19
Figure 8 Schematic view of a membrane…………………………………………. 21
Figure 9 Simplified scheme of monopolar and bipolar electrolyzer………………. 22
Figure 10 Electrolyzer architecture…………………………………………………. 24
Figure 11 Chlorine processing………………………………………………………. 29
Figure 12 Modern Concept of Chlorine Cooling……………………………………. 31
Figure 13 Chlorine drying process…………………………………………………... 34
Figure 14 Mass and Energy Balance………………………………………………… 40
Figure 15 Mercury cell material balance……………………………………………. 42
Figure 16 Diaphragm membrane cell material balance……………………………… 43
Figure 17 Membrane cell material balance…………………………………………. 44
Figure 18 Chlorine drying…………………………………………………………… 44
Figure 19 Mercury cell energy balance……………………………………………… 46
Figure 20 Diaphragm membrane cell energy balance………………………………. 47
Figure 21 Membrane cell energy balance…………………………………………… 48
Figure 22 Heat exchanger…………………………………………………………… 49
xiii
Figure 23 Temperature correction factor: one shell pass; two or more even tube
Passes……………………………………………………………………. 51
Figure 24 Shell-bundle clearance……………………………………………………. 52
Figure 25 Tube-side heat-transfer factor……………………………………………. 55
Figure 26 The higher is the cost of salt……………………………………………… 58
Figure 27 Variable Costs to Produce an ECU of Caustic and Chlorine……………... 65
Figure 28 The Impact the Cost of Salt………………………………………………. 66
Figure 29 Total Costs to Produce an ECU of Caustic and Chlorine………………… 66
Figure 30 The process………………………………………………………………. 71
1
INTRODUCTION
Electrolysis reactions are the basic foundations of today's modern industry. There are
various elements, chemical compounds that are only produced by electrolysis and are an
electrochemical process in which a direct current is passed between two electrodes through
an ionized solution (electrolyte). It deposits positive ions (cations) on the negative electrode
(cathode) and negative ions (anions) on the positive electrode (anode). The processes are
used in many industries. Here are three examples: chlor-alkali industries, sodium and
Aluminum industries [1]. The chlor-alkali electrolysis process industry is used in the
manufacture of chlorine, sodium hydroxide (caustic soda) solution (an alkali) and hydrogen
hence the term chlor-alkali. The term chlor-alkali industry also includes the production of
chlorine with potassium or lithium hydroxide. The primary product is chlorine; chlorine is
one of the more abundant chemicals produced by industry and has a wide variety of
industrial uses [2]. The technologies applied for chloro-alkali production are mercury cell,
diaphragm cell and membrane cell. In all 3 methods, the chlorine (Cl2) is produced at the
positive electrode (anode) and the caustic soda (NaOH) and hydrogen (H2) are produced,
directly or indirectly, at the negative electrode (cathode).The 3 processes differ in the
method by which the anode products are kept separate from the cathode products. Chlor-
alkali raw material is sodium chloride salt solution or brine as feed and to a lesser extent
potassium or lithium chlorides salt. In Libya Abu Kammash chlor-alkali plant produced
51.6 caustic soda tons per year, 45.7 chlorine ton per year by mercury cell technique,
unfortunately the plant is permanently closed. Chlor-Alkali products are precursors for
many leading chemical industries all over the world [7].
2
CHAPTER 1
CHLOR-ALKALI RAW MATERIAL
The basic raw material is the natural occurring salt lakes of sodium chloride (brine), sabkha
and rock salts obtained by mechanical mining as sodium chloride and are all raw materials
for chlor-alkali industry. Solar evaporation of seawater or brine is another source of raw
material salts. Sodium chloride (NaCl) is the major raw material in chlor-alkali industry
with the exception when potassium or lithium hydroxide is the desired products. Mercury
cell and membrane cell plants mostly used vacuum salt and rock salt solution; diaphragm
cell plants used solution-mined brine.
1.1- Brine Preparation
When solid salt is the raw material, a dissolving operation becomes necessary, and this may
be carried out in open or closed vessels. The water and/or depleted brine can be sprayed
onto the salt or introduced at the base of the saturator for progressive saturation when
running through it. In the latter case, the saturated brine overflows the equipment at the top.
Modern saturators are closed vessels to reduce emissions of salt spray or mist, as well as of
mercury in the case of the mercury cell technique. Sodium chloride (NaCl) concentrations
in the saturated brine reach values of 310–315 g/l.
1.2- Chlor-Alkali Raw Material Chemical Treatment
The natural brine solution and the prepared solution by mixing rock salt of sodium chloride
with water contain traces of divalent ions such as magnesium, calcium, sulfate, and barium
ions that must be removed before the electrochemical process in the electrolytic cell to
prevent fouling. Figure 1illustrates a simple brine treatment block diagram, table 1 shows
typical sodium chloride chemical composition used in chlor-alkaly electrolysis.
3
Figure 1 Simple brine treatment block diagram
Table 1. Typical compositions of sodium chloride used in chlor-alkali electrolysis [7]
1.2.1-Primary purification
The initial stage of purification uses sodium carbonate and sodium hydroxide to precipitate
calcium and magnesium ions as calcium carbonate (CaCO3) and magnesium hydroxide
(Mg(OH)2). Metals (iron, titanium, molybdenum, nickel, chromium, vanadium, and
tungsten) may also precipitate as hydroxide during this operation. The usual way to reduce
the concentrations of metals is to specify maximum concentration values in the purchase
specifications for the salt. Sulphate ion is precipitate by adding calcium chloride (CaCl2) or
barium salts as barium carbonate (BaCO3) or barium chloride(BaCl2). The precipitation of
4
barium sulphate can take place simultaneously with the precipitation of calcium carbonate
and magnesium hydroxide, whereas the precipitation of calcium sulphate requires a
separate vessel. When vacuum salt is used as raw material, only a part of the brine stream
might be treated in the primary purification unit, while the total stream is usually treated
when using other salt types. Some plants using vacuum salt omit primary brine purification
completely. The sulphate content can also be reduced without the use of expensive barium
salts by purging a part of the brine, by cooling the brine stream and crystallizing
Na2SO4·10H2O, by precipitating the double salt Na2SO4·CaSO4, by ion exchange, or by
nanofiltration combined with purging of the brine. In the diaphragm cell technique, the
removal of sulphate is not always necessary because sulphate can be removed from the cell
liquor as pure Na2SO4 during the concentration process. In the case of the membrane cell
technique, the use of barium salts is generally avoided to protect the membrane against
potential precipitations . Chemical chlor-alkali reaction treatment is shown in the chemical
equations below.
Calcium is precipitated as calcium carbonate by reaction with sodium carbonate.
Ca+2 + Na2CO3 ------------> CaCO3(s) + 2Na+
Magnesium is precipitated as magnesium hydroxide by reaction with caustic soda.
Mg+2 + 2NaOH ------------> Mg(OH)2(s) + 2Na+
Sulphates are precipitated as barium sulphate by reaction with barium carbonate.
SO4-2 + BaCO3 ------------> BaSO4(s) + CO3-2
Strontium, is precipitated mainly as strontium carbonate.
Sr+2 + CO3-2 ------------> SrCO3(s)
The solids are then removed using a combination of clarifying and filtration steps. This step
will also remove heavy metals if they are present, pH is usually maintained in the 9–11
range at this stage to optimize the removal of the contaminants, and control is achieved by
regulating the hydroxide feed rate. The purified brine should ideally contain [ 5].
•Calcium (Ca2+): < 2 mg/l
•Magnesium (Mg2+): < 1 mg/l
•Sulphate (SO42-): < 5 g/l
5
Following the filtration steps, the brine is purified by ion exchange chelation, which
reduces the calcium and magnesium to ug/l or parts per billion (ppb) levels. Current
technology calls for brine with 20 ppb levels of calcium and magnesium to enable the
membranes to have a useful performance.
Before the brine enters electrolysis cells, it is usually acidified with hydrochloric acid to
maintain a pH less than 6 (pH < 6), which increases the lifetime of the anode coating and
reduces the formation of oxygen, hypochlorite ion and chlorate ion.
1.2.2- Secondary purification: membrane cell technique
To maintain the high performance of the ion-exchange membrane in the membrane cell
technique, the feed brine must be purified to a greater degree than in the mercury or
diaphragm cell techniques. The precipitation step alone is not enough to reduce the levels
of calcium and magnesium, and additional softening is thus required. Figure 2 shows the
flow diagram of a possible layout for the brine system used in the membrane cell technique.
The secondary brine purification generally consists of polishing filtration and brine
softening in an ion-exchange unit. The polishing filtration generally consists of candle-type,
plate frame or pressure leaf filters (either with or without a cellulose-based pre-coat) in
order to sufficiently reduce suspended particles and protect the ion-exchange resin from
damage. The ion-exchange chelating resin treatment is designed to decrease the sum of
magnesium and calcium concentrations to less than 20 μg/l; part per billion levels (ppb).
Figure 2 Flow diagram of a possible layout for the brine system used in the membrane cell
technique
6
Table 2 indicates typical specifications required for metals, sulphate and other impurities.
These specifications can vary if the users want to operate at a low current density (< 4
kA/m2) or at a high current density. The specifications are more stringent for high current
densities. Specifications also depend on the interaction of impurities. While the presence of
one impurity may not be harmful, its synergistic combination with others may be (e.g. the
combination of aluminum, calcium and silica). The resin is periodically regenerated with
high-purity hydrochloric acid and sodium hydroxide solutions. Generally, one resin
exchange column is in operation while another resin exchange column is regenerated.
1.3 Control of nitrogen compounds in the brine
The presence of some nitrogen compounds in the brine gives rise to the formation of
nitrogen trichloride (NCl3), which is an explosive substance. Techniques are applied to
reduce the concentration of nitrogen compounds in the brine.
Table 2. Typical impurities with sources and effects on the membrane cell technique and brine
specifications
Impurity Source Typical
upper limit of
brine
specifications
Effects Mechanism
Ca2+ +
Mg2+
Salt 20 pbb Ca CE
Mg V
Ca: Precipitation with various
anions near the cathode side of the
membrane, precipitation with silica
and iodine in the membrane
Mg: Fine precipitation with OH-
near the anode side of the
membrane, precipitation with silica
in the membrane
St2+ Salt 0.1–4 ppm CE Precipitation with hydroxide on the
cathode side of the membrane,
precipitation with silica and iodine
in the membrane
Ba2+ Salt 0.05–0.5
ppm
CE Very fine precipitation with iodine
in the mem- brane, precipitation
with silica in the membrane
Al3+ Salt 0.1 ppm CE Precipitation with silica in the
membrane, precipitation of
calcium/strontium aluminosilicates
7
near the cathode side of the
membrane
Fe3+ Salt, pipe
work, tank
material,
anti-caking
agent
0.05–0.1
ppm
V Deposition on the cathode,
precipitation with hydroxide on the
anode side of the membrane or in
the membrane (depending on pH of
the brine)
Hg2+ Parallel
operation of
mercury cell
plant
0.2 ppm
heavy metals
V Deposition on the cathode
Ni2+ Salt, pipe
work, tank
material,
cathode
0.2 ppm
heavy metals
V Deposition on the cathode,
absorption in the membrane
ClO3- Process side
reactions
10 g/l (as
NaClO3)
0 Chlorination of the ion-exchange
resin
I2
(e.g.H2IO6)
Salt 0.1–0.2 ppm CE, V Very fine precipitation with
calcium, strontium or barium in the
membrane,
precipitation with sodium on the
cathode side of the membrane
F- Salt 0.5 ppm V Destruction of the anode coating
SO42- Salt
dichlorination
with NaHSO3
< 4–8 g/l (as
Na2SO4)
CE Precipitation with sodium near the
cathode side of the membrane,
anode coating with barium
SiO2(e.g.
SiO32- )
Salt 10 ppm CE Silica itself is harmless, but in the
presence of magnesium, calcium,
strontium,
barium or aluminum, silicates can
be formed (see above)
Suspended
solids
Salt 0.5–1 ppm V Precipitation on the anode side of
the membrane
Total
organic
carbon
Salt 1–10 ppm V Increased foaming, over plating
NB: CE = current efficiency decreases; O = other effects; V = cell voltage increases. Source: [ 7] •Brine
dichlorination and resaturation
1.4 Brine dechlorination and resaturation
Mercury and membrane cell plants usually operate with brine recirculation and resaturation.
Some plants operating on a once-through basis, one of them use mercury cell technique, the
membrane cell technique or use both the mercury and the membrane cell technique.
8
Diaphragm cell plants always use a once through brine circuit, but some employ brine
saturation using the salt recovered from the caustic evaporators.
In recirculation circuits, the depleted brine leaving the electrolyzer is first dechlorinated:
• Only partially for the mercury cell technique (leaving active chlorine in the brine
keeps the mercury in its oxidized form as HgCl3- and HgCl4
2- and avoids the
presence of metallic mercury in the brine purification sludge.
• Totally for the membrane cell technique (necessary here because the active chlorine
can damage the ion-exchange resins of the secondary brine purification unit).
For this purpose, the brine containing 0.4–1 g/l of dissolved chlorine is generally acidified
to pH 2–2.5 and sent to an air-blown packed column or sprayed into a vacuum system of
50–60 kPa to extract the majority of the dissolved chlorine to a residual concentration of
10–30 mg/l. The chlorine-containing vapors are subsequently fed back to the raw chlorine
collecting unit or directed to the chlorine absorption unit. The water that evaporates from
the dechlorinated brine is condensed in a cooler. The condensate can be sent back to the
brine system or chemically dechlorinated.
For the membrane cell technique, complete dichlorination is achieved by passing the brine
through an activated carbon bed, by catalytic reduction, or by using chemical reducing
agents such as sulphate. Residual levels were reported to be < 0.5 mg/l or below the
detection limit. No such dichlorination treatment is required for the diaphragm system,
since any chlorine passing through the diaphragm reacts with caustic soda in the catholyte
compartment to form hypochlorite or chlorate.
If the saturation is carried out with impure salt (followed by a primary purification step of
the total brine flow), the pH of the dechlorinated brine is then brought to an alkaline value
with caustic soda to reduce the solubilization of impurities from the salt. If the saturation is
carried out with pure salt (with subsequent primary purification on a small part of the flow),
there is no alkalization step prior to the resaturation (only in the purification phase). In the
case of mercury or membrane cell plants operating with solution-mined brine, brine
resaturation is achieved by evaporation. During this step, sodium sulphate precipitates and
can be recovered, purified and used for other purposes. In the case of diaphragm cells, the
catholyte liquor (10–12 wt-% NaOH, 15–17 wt-% NaCl) is directly used or transferred to
the caustic evaporators, where solid salt and 50 wt-% caustic are recovered together. Fresh
9
brine may be saturated with recycled solid salt from the caustic evaporators before entering
the diaphragm electrolyzer.
1.5Chlorate destruction: membrane cell technique
In order to reduce the build-up of chlorate in the brine circuit, which could have negative
effects on the ion-exchange resins, the caustic quality, and on emissions to the
environment, some membrane cell plants operate a chlorate destruction unit prior to the
dichlorination. Techniques include the reduction of chlorate to chlorine with hydrochloric
acid at temperatures higher than 85°C and the catalytic reduction of chlorate to chloride
with hydrogen.
10
CHAPTER 2
CHLOR-ALKALI ELECTROLYSIS PROCESS
TECHNIQUES
There are three basic techniques for the electrolytic production of chlorine, caustic soda and
hydrogen. These three techniques are the mercury cell, the diaphragm cell and the
membrane cell. The three techniques differ from each other in terms of electrode reactions,
the amount of electricity and in the way the produced chlorine and caustic soda/hydrogen
are kept separate. The products of the electrolysis are formed in a fixed stoichiometric ratio,
which is 1070–1128 kg of caustic soda; NaOH (100 wt- %) and approximately 28 kg of
hydrogen gas (H2) per one ton of chlorine gas (Cl2) produced. This product combination is
often referred to as the electrochemical unit (ECU). Figure 3 is a simple block diagram of
the three chlor-alkali process techniques.
Figure 3 Typical flow diagrams of the three cells techniques
11
2.1- Mercury Cell Process Technique
2.1.1 General description
The mercury cell technique includes an electrolysis cell and a horizontal or vertical
decomposer as shown in figure 4. A 25 wt-% sodium chloride brine having pH 2-5 enters
the electrolysis cell at 60–70 °C. The temperature can be achieved by preheating the
saturated brine, and is increased in the cell by the heat of resistance to approximately 75–85
°C. At this temperature, the conductivity of the brine solution and the fluidity of the
mercury are higher compared to operation at ambient temperature. The cell operation
voltage is 3.15-4.80 V and current density 2.20-14.50 kA/m2, total electrical power 3
MW/h per one ton produced chlorine gas (Cl2) [7].
In the electrolysis cell, the brine flows through an elongated trough that is slightly inclined.
In the bottom of this trough the cathode which is a shallow film of mercury metal (Hg)
flows along the brine cell together with the brine. Closely spaced above the cathode, a
titanium anode assembly is suspended as shown in figure 4 [1].
Figure 4. Mercury cell
Electric direct current flowing through the cell decomposes sodium chloride in the brine
passing through the narrow space between the electrodes, liberating chlorine gas (Cl2) at the
anode and metallic sodium (Na) at the cathode. The chlorine gas is accumulated above the
12
anode assembly and is discharged to the purification process. As it is liberated at the
surface of the mercury cathode, the sodium (Na) immediately forms an amalgam with
mercury (Na/Hg) (a mixture of metal sodium and mercury metal). The concentration of the
amalgam is maintained at 0.2–0.4 wt-percent sodium so that the amalgam flows freely.
Sodium concentrations higher than 0.5 wt-percent can cause increased hydrogen evolution
in the cells [1]. The liquid amalgam flows from the electrolytic cell to a separate reactor,
called the decomposer or denuder where it reacts with demineralized water in the presence
of a graphite catalyst to form sodium hydroxide and hydrogen gas. The decomposer
operates at a temperature of approximately 90-130 ºC, which is caused by the chemical
reactions in the decomposer and the input of warm amalgam from the electrolyzer. The
sodium-free mercury is fed back into the cell and is reused. The depleted brine anolyte
leaving the cell is saturated with chlorine and must be partially dechlorinated before being
returned to the dissolvers.
The sodium hydroxide is produced from the decomposer at a concentration of
approximately50 wt-percent [1]. For its operation, the mercury cell depends on the higher
overpotential of hydrogen on mercury to achieve the preferential release of sodium rather
than hydrogen. However, impurities such as vanadium (V), molybdenum (Mo), and
chromium (Cr) at the 0.01–0.1 ppm level and other elements (Al, Ba, Ca, Co, Fe, Mg, Ni,
W) at the ppm level that can contaminate the mercury surface may lack this overpotential
protection and can cause localized release of hydrogen into the chlorine. There is a risk that
the hydrogen concentration in the chlorine can increase to the point at which the cell and
downstream chlorine handling equipment contain explosive mixtures [1].
Mercury cells are usually operated to maintain a 21–22 wt-% concentration of salt in the
spent brine discharged from the cell. This corresponds to the decomposition of 15–16 % of
the salt during a single pass. Further salt decomposition to a lower concentration in the
brine would decrease brine conductivity, with the attendant loss of electrical efficiency.
However, in plants with once-through brine systems, approximately 40 % of the salt is
electrolyzed in the cells [1].
2.1.2 The mercury cathode electrolyzer and decomposer
The cell is made of an elongated, slightly inclined trough and a gas-tight cover. The trough
is made of steel, and its sides are lined with a protective, non-conductive coating to prevent
13
contact with the anolyte, to confine brine-cathode contact to the mercury surface, and to
avoid the corrosive action of the electrolyte. Modern electrolyzer is10-25 m long and 1 -2.5
m wide. As a result, the cell area today can be greater than 30 m2. The size of the cells can
be varied over a broad range to give the desired chlorine production rate. At the design
stage, computer programs can be used to optimize the cell size, number of cells, and
optimum current density as a function of the electricity cost and capital cost [1]. The steel
base is made as smooth as possible to ensure mercury flow in an unbroken film. In the
event of a break in the mercury surface, caustic soda will be formed on the bare (steel)
cathode, with simultaneous release of hydrogen, which will mix with the chlorine. Because
hydrogen and chlorine can forma highly explosive mixture, great care is necessary to
prevent hydrogen formation in the cell.
Characteristics of the cathode: The cathode is made by a shallow layer of mercury which
flows from one extremity of the cell to the other because of the slight inclination from the
horizontal of the cell. Characteristics of the anode: Electrolytic cell anodes made of
titanium coated with ruthenium oxide (RuO2) and titanium oxide (TiO2) were used of
reduces energy consumption by about 10% and their life expectancy is higher. In recent
years there have been competitive developments in detailed anode geometry, all with the
aim of improving gas release in order to reduce ohmic losses and increase the homogeneity
of the brine to improve anode coating life. Compartments for collecting the chlorine gas
and weirs for separating the mercury and brine streams, washing the mercury and
permitting the removal of thick mercury “butter” that is formed by impurities. The whole
electrolyzer is insulated from the floor to prevent stray ground currents. Usually, several
electrolyzers are placed in series by means of electrically connecting the cathode of
electrolyzer to the anodes of the next electrolyzer. Individual cells can be by-passed for
maintenance and replacement. The decomposer may be regarded as a short-circuited
electrical cell in which the graphite catalyst is the cathode and sodium amalgam the anode.
2.1.3 Mercury cell chemical reactions:
• Anode (positive electrode): Titanium
• Anode reaction (oxidation): 2Cl-(aq) → Cl2(g) + 2e-
• Cathode (negative electrode): Mercury flowing along bottom of cell
• Cathode reaction (reduction): 2Na+(aq) + 2e- → 2Na(s)
14
• The overall reaction is
2Na+(aq) +2Cl-(aq) →2Na(s) +Cl2(g)
Na(s)+ Hg(l) → Na/Hg(l)
2Na/Hg + 2H2O(l) → 2Na+ + 2OH- + H2(g) + 2Hg(l)
2.1.4The problem:
Industrial installations consist of some 200 mercury cells in series, each one measuring 15
m x 2 m x 0.3 m, the volume of each cell 9 m3 and the total volume is 1800 m3. The
process, which takes place at a very high voltage, uses an enormous quantity of electricity:
3 MW/h per ton Cl2.This method only produces a fraction of the chlorine and sodium
hydroxide used by industry as it has certain disadvantages: mercury is expensive and toxic,
and whilst it is recirculated, some always escapes with the produced sodium hydroxide and
the spent brine with which it reacts to form mercury (II) chloride (HgCl2), if the effluent is
discharge into lakes, rivers and seas lead to the accumulation of high levels of mercury in
fish, which absorbed the mercury compound but could not re-excrete it, Now a days the
spent brine is treated before discharge, the mercury being precipitated as mercury(II)
sulphide (HgS). In recent years a large share of chlorine and sodium hydroxide production
has been produced in two other types of cells, which do not use mercury: the diaphragm
cell and the membrane cell.
2.2 The Diaphragm Cell Technique
2.2.1 General description
In the diaphragm cell, a 25 wt-% sodium chloride brine having pH2.5-3.5 enters anode
compartment as shown in figure 5. The brine is decomposed to approximately 50 wt-% of
its original concentration in a passage through the cell. Cell voltage 2.90-3.60 V and current
density 0.80-2.70 kA/m2. Heating caused by passage of current through the diaphragm cell
raises the operating temperature of the electrolyte to 80-99 ºC. On the negatively charged
15
cathode surface caustic soda (NaOH) and hydrogen gas (H2) are formed directly and
chlorine gas (Cl2) is formed at the anode. To separate the chlorine from the sodium
hydroxide, the two half-cells were traditionally separated by a porous asbestos diaphragm,
which needed to be replaced every two months. This was environmentally detrimental
owing to the need of disposing of large quantities of asbestos. Such frequent replacement is
fortunately no longer necessary, the asbestos having now been replaced in part by metal
oxide with polymers resulting in diaphragms with a much longer life. Diaphragm cells
generally produce cell liquor that contains 10–12 wt-% NaOH and 15–17 wt-% NaCl.
Generally, this solution is evaporated to 50 wt-% sodium hydroxide. The caustic liquor
produced may contain lower concentrations of about 7 wt-% sodium chloride if it is used
directly without further concentration [7]. During evaporation, most of the sodium chloride
precipitates, except a residual of approximately 1.0 wt-%. The salt generated is very pure
and is typically used to make more brine. This high quality sodium chloride is sometimes
used as a raw material for mercury or membrane cells. Low concentrations of oxygen (0.5–
2.0 vol-%) in chlorine are formed by the electrolytic decomposition of water. Furthermore,
chlorate is formed in the cell liquor by anodic oxidation and the disproportionate of
hypochlorous acid. (0.04–0.05 wt-% before concentration and ~ 0.1 wt-% after
concentration). However, a high amount of steam may be necessary for the caustic soda
concentration, and the quality of the caustic soda and chlorine produced are low.
Figure 5 Diaphragm cell
16
2.2.2 The cell
Various designs of diaphragm cells have been developed and used in commercial
operations, an example is the monopolar diaphragm cell the typical anode areas per cell
range from 20 to100 m2and the cell voltage 2.90–3.60 V; current density 0.8–2.70
kA/m2[5].Cathodes used in diaphragm cells consist of carbon steel with an active coating
which lowers the hydrogen overpotential, thus providing significant energy savings. The
coatings consist of two or more components. At least one of the components is leached out
in caustic to leave a highly porous nickel surface [5]. The coatings have to be robust, as a
powerful water jet is used to remove the diaphragm at the end of its lifetime from the
cathode mesh, which can adversely affect the coatings. Anodes used in diaphragm cells
consist of titanium coated with a mixture of ruthenium dioxide, titanium dioxide and tin
dioxide. The lifetime of the coatings is at least 12 years [7]. The most commercially
accepted design is that of an expandable anode, which allows compression of the anode
structure during cell assembly and expansion when the cathode is in position. The spacers
initially placed over the cathode create a controlled gap of a few millimeters between the
anode and cathode. Minimization of the gap leads to a reduced power consumption. The
lifetime of the diaphragm can be several years. Their service life has also increased due to a
change in composition. The earliest diaphragms were made of sheets of asbestos paper.
Due to the potential exposure of employees to asbestos and emissions to the environment,
efforts have been made to replace the asbestos with other diaphragm materials. The basis of
the material used is the same in all asbestos-free diaphragms, i.e. a fluorocarbon polymer,
mainly polytetrafluoroethylene (PTFE). The differences lie in the fillers used and the way
the hydrophobic PTFE fibers are treated and deposited in order to form a permeable and
hydrophilic diaphragm [5]. The diaphragm cell is now technologically the most advanced
of all three cells and has a high electrochemical performance.
2.2.3 Reactions in the diaphragm cell are as follows:
• Anode (positive electrode): carbon (graphite) or titanium coated with Ru-Ti oxide.
• Anode reaction (oxidation): 2Cl-(aq) → Cl2(g) + 2e-
17
• Cathode (negative electrode): steel mesh
• Cathode reaction (reduction): 2H2O(l) + 2e- → H2(g) + 2OH-(aq)
• Na+ migrates across diaphragm to cathode compartment combining with OH- to
form NaOH.
• 2OH-(aq) + 2Na+ → 2NaOH(aq)
• Overall cell reaction (showing Na+ spectator ions):
2.3The Membrane Cell Process Technique
2.3.1- General description
In this technique, the anode and cathode are separated by an ion-conducting membrane
figure 6. Cell voltage is 2.35-4 V and current density 1.0-6.50A kA/m2. A 28 wt-% brine
having pH 2-4 flows through the anode compartment, where chloride ions are oxidized to
chlorine gas. The sodium ions, together with approximately 3.5 to 4.5 moles of water per
mole of sodium, migrate through the membrane to the cathode compartment, which
contains a caustic soda solution [5]. The water is electrolyzed at the cathode, releasing
hydrogen gas and hydroxide ions. The sodium and hydroxide ions combine to produce
caustic soda, which is typically kept at 32 ± 1 wt-% in the cell by diluting a part of the
product stream with demineralized water to a concentration of approximately 30 wt-percent
and subsequent recycling to the catholyte inlet. Caustic soda is continuously removed from
the circuit. Depleted brine is discharged from the anode compartment and resaturated with
salt. The membrane largely prevents the migration of chloride ions from the anode
compartment to the cathode compartment; therefore, the caustic soda solution produced
contains little sodium chloride (i.e. approximately 50 mg/l). Back-migration of hydroxide is
also largely prevented by the membrane but nevertheless takes place to a certain extent and
increases the formation of oxygen, hypochlorite and chlorate in the anode compartment,
18
thereby resulting in a loss of current efficiency of 3–7 % with respect to caustic soda
production [5].
Figure 6. Process flow of the membrane cell process
Some electrolyzer produces more diluted 23–24 wt-% caustic soda. In this case, the caustic
entering the cell has a concentration of approximately 20–21 wt-% and the heat of the
electrolysis can be used to concentrate the 23–24 wt-% caustic solution to 32–34 wt-%. The
overall energy efficiency is comparable to the aforementioned process with the 32 wt-%
caustic solution but more equipment is required for the caustic evaporation. On the other
hand, simpler and cheaper construction materials can be used in the caustic circuit around
the membrane cells. Generally, the caustic produced in a concentration of 30–33 wt-percent
is concentrated to the usual commercial standard concentration of 50 wt-percent by
evaporation using steam. Another possibility is to use the caustic produced in the
membrane cells as feed to the decomposers of mercury cells. A flow diagram of a possible
integrated plant is shown in figure 7.
19
Figure 7. Flow diagram of the integration of the membrane and mercury cell techniques
The concentration of sodium chlorate in the produced caustic soda typically ranges from
less than 10 to 50 mg/kg. The level depends on the membrane characteristics, the
operational current density and the chlorate levels in the brine. The chlorine produced in
membrane cells contains low concentrations of oxygen (0.5–2.0 vol-%). The formation of
oxygen and chlorate ion can be depressed by selecting an anode coating with suitable
characteristics and/or by decreasing the pH in the anode compartment [5]. Brine depletion
in membrane cells is two or three times greater than in mercury cells, which allows the
brine system to be smaller, resulting in significantly lower recycling rates and less
equipment needed compared to mercury cell plants of the same capacity [5]. The membrane
cell technique has the advantage of producing a very pure caustic soda solution and of
using less energy than the other techniques. In addition, the membrane cell technique uses
neither mercury, which is very toxic, nor asbestos, which is classified as toxic
(carcinogenic) [7]. The membrane cell process brine specifications are more stringent than
that of the mercury and diaphragm processes and calls for impurities to be at the parts-per-
billion (ppb) levels. If this level of purity is not reached the membrane will be damaged.
Disadvantages of the membrane cell technique are that the caustic soda produced may need
to be evaporated to increase its concentration and, for some applications, the chlorine gas
produced needs to be processed to remove oxygen, usually by liquefaction and evaporation.
20
Furthermore, the brine entering a membrane cell must be of a very high purity, which
requires additional purification steps prior to electrolysis.
2.3.2The Cell
Various designs of membrane cells have been developed and used in commercial
operations figure 6 is a membrane cell diagram. The cathode material used in membrane
cells is nickel. Like the cathodes of diaphragm cells, they are often coated with a catalyst
that is more stable than the substrate and which increases surface area and reduces the
overpotential. Coating materials include Ni-S, Ni-Al, and Ni-NiO mixtures, as well as
mixtures of nickel and platinum group metals. The cathode coatings for membrane cells
have to be more chemically resistant than those of diaphragm cells, because of the higher
caustic concentration. The anodes used consist of titanium coated with a mixture of
ruthenium dioxide, titanium dioxide and iridium dioxide.
The membranes used in the chlor-alkali industry are commonly made of per fluorinated
polymers. The membranes may have one to three layers, but generally consist of two layers
see the figure 8. One of these layers consists of a per fluorinated polymer with substituted
carboxylic groups and is adjacent to the cathodic side. The other layer consists of a per
fluorinated polymer with substituted sulphonic groups and is adjacent to the anodic side.
The carboxylate layer exhibits a high selectivity for the transport of sodium and potassium
ions and largely prevents the transport of hydroxide, chloride, hypochlorite, and chlorate
ions, while the sulphonate layer ensures good mechanical strength and a high electrical
conductivity. To give the membrane additional mechanical strength, it is generally
reinforced with polytetrafluoroethylene (PTFE) fibers. The membranes must remain stable
while being exposed to chlorine on one side and a strong caustic solution on the other.
Commercially available membranes are optimized for use in a specific strength of caustic.
Depending on the particular design, membrane sizes range from 0.2 to 5 m2.The general
economic lifetime of chlor-alkali membranes is approximately three to five years [5]. In the
design of a membrane cell, minimization of the voltage drop across the electrolyte is
accomplished by bringing the electrodes close together. However, when the gap is very
small, the voltage increases because of the entrapment of gas bubbles between the
electrodes and the hydrophobic membrane. This effect is avoided by coating both sides of
the membrane with a thin layer of a porous inorganic material to enhance the membrane's
21
ability to release the gaseous products from its surface. These improved membranes have
allowed for the development of modern cells with zero-gap or finite-gap cathode structures.
Figure 8. Schematic view of a membrane
2.3.3-Monopolar and bipolar electrolyzer
Electrolysis containing a multitude of membrane or diaphragm cells are classified as either
monopolar or bipolar. The designation does not refer to the electrochemical reactions that
take place, which of course require two poles or electrodes for all cells, but to the
electrolyzer construction or assembly. In a bipolar arrangement, the elements are connected
in series with a resultant low current and high voltage (Kirchhoff's circuit laws). The
cathode of a cell is connected directly to the anode of the adjacent cell as shown in figure 9.
In the monopolar arrangement, all anodes and cathodes are connected in parallel, forming
an electrolyzer with a high current and low voltage. The current has to be connected to
every single anodic and cathodic element, while in a bipolar electrolyzer the power supply
is connected only to the end part of the electrolyzer. Due to the long current path, ohmic
losses in monopolar electrolyzer are much higher than in equivalent bipolar electrolyzer,
leading to increased energy consumption.
22
Figure 9. Simplified scheme of monopolar and bipolar electrolyzer
Multiple electrolyzers are employed in a single direct current circuit as illustrated in figure
9. Usually bipolar electrolyzers are connected in parallel with a low current and high
voltage. Monopolar electrolyzers are often connected in series, resulting in a high current
circuit and low voltage. Table 3 shows the differences between typical configurations of
monopolar and bipolar membrane cell plants with the same production capacity.
Monopolar membrane cell plants are characterized by a larger number of electrolyzer while
the number of cells per electrolyzer is lower than in a bipolar membrane cell plant.
Table 3. The differences between typical configurations of monopolar and bipolar
23
The maximum current density of an electrolyzer is determined by the resistance (ion
conductivity) of the membrane and the hydraulic conditions of the cells (elimination of the
gas formed). The first monopolar electrolyzer worked at maximum current densities of 4
kA/m2 but bipolar electrolyzer can now be operated at current densities of 6–7 kA/m2 and
pilot cells are currently tested at up to 10 kA/m2. The trend to develop higher current
density electrolyzer aims to reduce investment costs while developments in membranes,
electrolysis technology such as anode-cathode gap reduction and catalyst developments
strive to reduce energy consumption.
Monopolar membrane electrolyzer were only commercialized to maintain existing plants
and for new plants with small capacities. That change is due to the following advantages of
bipolar electrolyzer which lead to reduced investment and operating costs, as well as to
improved safety.
• Easier manufacturing;
•Smaller copper busbars due to the lower current; the only copper current distributors
needed are the main busbars connected to the end parts of the electrolyzer;
•Possibility to operate at higher current density without dramatic effects on energy
consumption due to very efficient internal recirculation;
•Membrane area more effectively used (from 85–87 % to 90–92 %);
•Better energy performance due to smaller voltage drop;
•No need for spare bipolar electrolyzer (only some spare individual cells are necessary,
compared to spare electrolyzer required for the monopolar technique);
• Shorter duration of shutdown and start-up phases to replace membranes due to the easy
and simple filter press design;
• Higher flexibility of operation (each electrolyzer could be operated independently of the
others due to a parallel connection); no need for expensive mobile short-circuit switches for
the isolation of troubled electrolyzer (Figure 10).
• New possibility to operate bipolar electrolyzer under slight pressure on the chlorine side
(no need for a blower).
24
• Easier detection of faulty cells by monitoring of individual cell voltages.
Figure 10. Electrolyzer architecture
2.3.4 Reactions in the membrane cell are as follows:
• Anode (positive electrode) : titanium
• Anode reaction (oxidation): 2Cl-(aq) → Cl2(g) + 2e-
Cathode (negative electrode): nickel
• Cathode reaction (reduction): 2H2O(l) + 2e- → H2(g) + 2OH-(aq)
• Na+ migrates across the membrane to cathode compartment combining with OH- to
form NaOH.
• Overall cell reaction (showing Na+ spectator ions):
2H2O(l) + 2Cl-(aq) + 2Na
+(aq) → 2Na
+(aq) + 2OH
-(aq) + H2(g) + Cl2(g)
• Product is concentrated sodium hydroxide
The overall reaction for the diaphragm and membrane cells is:
25
The products of the electrolysis are formed in a fixed ratio, which is 1070–1128 kg of
NaOH (100 wt- %) and approximately 28 kg of H2 per ton of Cl2 produced. This product
combination is often referred to as the electrochemical unit (ECU).
Some side reactions occur during electrolysis, leading to a loss of efficiency [7]
At the anode, oxidation of water to oxygen and of hypochlorous acid to chlorate takes
place:
2H2O → O2 + 4H+ + 4 e- or 4OH- → O2 + 2H2O + 4e
-
12HClO + 6 H2O → 4ClO3- + 8 Cl- + 24 H+ + 3 O2 + 12 e
-
Hypochlorous acid is formed by disproportionate of chlorine in water:
Cl2 + H2O ⇌ HClO + H+ + Cl-
Chlorate is also produced by chemical reactions in the anolyte:
2HClO + ClO- → ClO3- + 2Cl- + 2H+
These four major side reactions are repressed by lowering the pH value.
The main typical characteristics of the different electrolysis technique are shown in table 4.
Table 4. Main typical characteristics of the different electrolysis techniques [7]
Criterion Mercury Diaphragm Membrane
Anode RuO2 + TiO2 coating
on Ti substrate
RuO2 + TiO2 + SnO2
coating on Ti substrate
RuO2 + IrO2 + TiO2
coating on Ti substrate
Cathode Mercury Steel (or steel coated
with activated nickel)
Nickel coated with high area
nickel-based or noble metal-
based coatings
Separator None Asbestos, polymer-
modified asbestos, or
non-asbestos
diaphragm
Ion-exchange membrane
Cell voltage 3.15–4.80 V 2.35–4.00 V 2.35–4.00 V
Current
density
2.2–14.5 kA/m2 0.8–2.7 kA/m2 1.0–6.5 kA/m2
Temperature Inlet: 50–75 °C Outlet:
80–90 °C
NI NI
pH 2–5 2.5–3.5 2–4
Cathode
product
Sodium amalgam (Na
HgX)
10–12 wt-% NaOH
and H2
30–33 wt-% NaOH and
Decomposer
product
50 wt-% NaOH and
H2
No decomposer needed No decomposer needed
Evaporator
product
No evaporation needed 50 wt-% NaOH 50 wt-% NaOH
Quality of NaCl: ~ 50 mg/kg NaCl: ~ 10 000 mg/kg NaCl: ~ 50 mg/kg
26
caustic soda
(50 wt-%
NaOH)
NaClO3: ~ 5 mg/kg
Hg: ~ 0.1 mg/kg
(15 000–17 000 mg/kg
before concentration)
NaClO3: ~ 1 000
mg/kg
(400 –500 mg/kg
before
concentration)
NaClO3: ≤ 10–50 mg/kg
Chlorine
quality
O2: 0.1–0.3 vol-%
H2: 0.1–0.5 vol-%
N2: 0.2–0.5 vol-%
O2: 0.5–2.0 vol-%
H2: 0.1–0.5 vol-%
N2: 1.0–3.0 vol-%
O2: 0.5–2.0 vol-%
H2: 0.03–0.3 vol-%
Advantages 50 wt-% high-purity
caustic directly from
cell, high-purity
chlorine and
hydrogen, simple
brine purification
Low quality
requirements of brine,
low electrical energy
consumption
Low total energy consumption,
low
investment and operating
costs, no use of mercury or
asbestos, high-purity caustic,
further
improvements expected
Disadvantages Use of mercury,
expensive cell
operation, costly
environmental
protection, large floor
space
High steam
consumption for
caustic concentration
in
expensive multi-effect
evaporators, low-purity
caustic, low chlorine
quality, some cells are
operated with
asbestos diaphragms
High-purity brine
required, low chlorine
quality, high cost of
membranes
NB: NI = No information provided
27
CHAPTER 3
CHLORINE, HYDROGEN AND CAUSTIC SODA
PROCESSING
3.1 Chlorine Processing, Storage and Handling
3.1.1 General description
Generally, for the production of dry gaseous or liquid chlorine most downstream process
units are required to get anhydrous chlorine with low hydrogen and oxygen content. The
typical chlorine gas is Cl2 97 - 99.5 vol%, O2 0.5 - 2.0 vol% and H2 0.03 - 0.3 vol%. To
get the typical chlorine specifications, it goes through a series of processes:
1. Cooling and filtration of hot wet chlorine gas.
2. Drying of “pre-dried” chlorine gas.
3. Scrubbing and cooling of dried chlorine gas, decomposition of NCl3.
4. Compression of dried chlorine gas.
5. Liquefying of compressed chlorine gas.
6. Vaporization of liquid chlorine.
7. Storage, transfer and filling.
In some applications, chlorine gas can be used as dry gas without the need for liquefaction.
Very occasionally it can be used directly from the electrolyzers. A general flow of chlorine
from the electrolyzers to storage is presented in figure 11. The chlorine process usually
takes hot, wet cell gas and converts it to a cold, dry gas. Chlorine gas leaving the
electrolyzers has a temperature of approximately 80–90 ºC and is saturated with water
vapor. It also contains brine mist, impurities such as N2, NCl3, H2, O2, CO2 and traces of
chlorinated hydrocarbons. Electrolyzers are operated at essentially atmospheric pressure
with only a few mbar differences between the pressure of the anolyte and the catholyte.
28
Figure 11. Chlorine processing
3.1.2 Material
The strong oxidizing nature of chlorine requires a careful choice of construction materials
at all stages of processing, depending on the operating conditions (temperature, pressure,
state of matter, moisture content). Most metals are resistant to dry chlorine at temperatures
below 100 °C. Above a specific temperature for each metal, depending also on the particle
size of the metal, spontaneous ignition takes place (150–250 °C for iron). Carbon steel is
the material most commonly used for dry chlorine gas (water content below 20 ppmw).
Wet chlorine gas rapidly attacks most common metallic materials with the exception of
tantalum and titanium, the latter being the preferred choice in chlor-alkali plants. However,
if the system does not remain sufficiently wet, titanium ignites spontaneously (ignition
temperature ~ 20 °C). Other construction materials such as alloys, graphite, glass, porcelain
and polymers may be used,
depending on the conditions. Oils or greases generally react with chlorine upon contact
unless they are fully halogenated [7].
29
3.1.3 Cooling
Chlorine cooling is generally designed for continuous cooling, filtration and, if required,
boosting of wet chlorine gas generated in the electrolysis cells. The purpose is to lower the
water content to the lowest possible value by means of cooling and chilling, to separate and
remove brine mist (aerosol) which is carried over from the electrolysis cells and optionally
to increase the gas pressure as may require for the downstream plant sections.
·Modern Concept
The modern concept of chlorine cooling see figure 12, is designed for:
·Indirect cooling in shell and tube heat exchangers.
·High efficiency filtration of the gas by use of glass fiber elements to separate the entrained
brine mist. With these measures the dew point of the gas could be brought to the absolute
minimum and the brine mist separation to the maximum.
The modern system is described below:
3.1.3.1 Chlorine Cooling
Cooling of the wet chlorine gas is normally done in two stages.
· First stage cooling from 95 to 40 °C by means of cooling water.
· Second stage cooling from 40 to 15 °C by means of chilled water.
·Indirect cooling through a titanium surface (usually in a single-pass vertical shell-and-tube
heat exchanger). The resultant condensate is either fed back into the brine system of the
mercury or membrane cell technique or is dechlorinated by evaporation in the case of the
diaphragm cell technique. Indirect cooling lowers the water content to approx. 4500 ppm.
Gas temperature lower than 13 °C is not recommended due to the formation of chlorine-
hydrate (white crystals; Cl2·nH2O; n = 7–8), which may plug of pipelines or filter elements.
The final temperature of the cooled gas is also subject to the temperature increase in the
blower and the remaining water content at this temperature to avoid the ignition of a Ti-Cl2
fire. Below 13°C the water vapour pressure in the gas becomes too low to be safe. Under
conditions of chlorine-hydrate formation ignition is still possible. All parts that are coming
into contact with the chlorine gas are made of titanium; these are the tube sheet as well as
the tubes, which are seamless or welded. The shell is made of normal carbon steel suitable
for cooling water. The heat exchangers are installed either vertically or horizontally with a
slope of approx. 3° in direction of the flow. The temperature split at 40 °C results in 90%
30
heat removal in the first stage and 10% in the second stage and similar heat transfer area for
both coolers. Condensate from both coolers is chlorine saturated and is recycled to depleted
brine. There is no need to provide standby heat exchangers. Also cleaning of the tubes is
normally not necessary. The advantages of this concept are:
·No possibility of NCl3 formation
·Low space requirement
·Very low maintenance requirement
·Lowest possible gas temperature and water vapour content in the gas, which reduces acid
consumption for chlorine drying.
·Low operating cost.
·Less chlorine to be condensed or absorbed
·Less chlorine-saturated water for disposal.
·Indirect cooling can be carried out in once-through, open-recirculating, or closed-loop
systems.
·Another method is direct cooling with water (or brine or other fluids). The chlorine gas is
cooled by passing it directly into the bottom of a tower. Water is sprayed from the top and
flows counter currently to the chlorine. The cooling water is generally free of traces of
ammonium salts, to avoid the formation of nitrogen trichloride. This method has the
advantage of better mass transfer characteristics and higher thermal efficiencies. Direct
cooling is usually carried out in closed-loop systems.
Figure 12. Modern Concept of Chlorine Cooling
31
3.1.4 Filtration and boosting
3.1.4.1 Filtration
Cooled and water vapour saturated chlorine gas contains remaining brine mist which must
be separated from the gas to avoid formation of sodium-sulfate during chlorine drying with
sulfuric acid. The gas is fed horizontally through a chlorine resistant glass fiber bed.
Particles are contacted and collected on individual fibers of the bed and then coalesce to
form liquid films and droplets, which are pushed through the bed by the gas flow. The
collected liquid then is drained off the downstream face of the bed by gravity. The particles
are collected in three different ways: by internal agglomeration (mainly for larger particles
> 3 micron) by direct collision for particle sizes between 1and 3 microns and by Brownian
diffusion for particles smaller than 1 micron. The filter-elements are designed for an overall
separation efficiency of 98% and a pressure drop of max. 150 mm WC (water column). The
brine mist quantity in the feed is normally less than 350 ppm NaCl. To achieve the high
separation efficiency the gas flow rate should be kept at design conditions.
The filter elements are installed vertically and fixed in a filter plate. Gas is entering the
filter vessel below the lowest point of the elements, flowing up and then horizontally
through the elements. Inside the element the filtered gas leaves the vessel at the top. The
constant saturation of the gas with water vapour is important. Therefore, the filter is
installed upstream of the blower (if any) and close to the second stage chlorine gas cooler.
To achieve saturation of overheated gas, water addition via a spray nozzle located at gas
inlet nozzle of the filter is provided. Water addition is normally not necessary. If required it
is added discontinuously. The water collected from the bottom of the filter vessel is drawn-
off and added to the condensate from the chlorine coolers. The filter elements are normally
self-cleaning and do not require maintenance. A replacement is required after 5-6 years of
operation.
The advantages of this concept are:
· Very high separation efficiency for brine mist.
· Very low maintenance and operating cost.
· High reliability.
32
3.1.4.2 Boosting
If required, the cooled and filtered chlorine gas is boosted by means of centrifugal blowers
to a pressure of up to 2000 mm WC. Boosting the entire or part of the gas stream is only
required if wet chlorine gas is branched off for the production of hydrochloric acid or iron-
chloride. Alternatively the gas stream could be branched-off after the chlorine gas
compression. The blower is made of carbon steel with hard rubber lining and titanium
impeller. Normally is the shaft sealed with a double acting mechanical seal, flushed and
cooled with water. The mechanical seal is normally very sensitive to water failures,
vibration etc. and special protections like water flow- and pressure control is required. The
improvement consists in using a labyrinth sealing with gas flushing. To avoid over heating
of the boosted gas and therefore ignition of a Ti-Cl2 fire at the blower impeller,
recirculating part of the gas stream back to the inlet of the first chlorine gas cooler must
control the capacity [4].
3.1.5 Drying
Chlorine from the cooling system is more or less saturated with water vapour. The water
content is typically 1–3 vol-%. This must be reduced in order to avoid downstream
corrosion and to minimize the formation of hydrates.
The drying of chlorine is carried out almost exclusively with concentrated sulphuric acid
(96–98 wt-%) in countercurrent contact towers in two to six stages, which reduce the
moisture content to less than 20 mg/m3 as shown in figure 13. The remaining moisture
content depends on the temperature and concentration of the sulphuric acid in the last
drying stage. For low temperature liquefaction, a lower moisture content is required, which
can be achieved by adding more equilibrium stages to the drying towers or by using
molecular sieves to levels of 3–9 mg/m3 [7]. The number of stages is usually increased to
lower the final strength of the spent sulphuric acid. For example, three stages are needed to
reach a spent acid concentration of 50–65 wt-percent while six stages are needed for a final
concentration of 30–40 wt-percent. The columns contain plastic packing resistant to
chlorine and sulphuric acid to improve fluids distribution, increase efficiency and lower
pressure drops, and thus reduce energy consumption. The heat liberated during dilution of
the circulating acid is removed by titanium heat exchangers, and the spent acid is
33
dechlorinated chemically or by stripping. The concentration of the spent acid depends on
the number of drying stages and the further potential use or method of disposal. In some
cases, the acid is reconcentrated to 96 wt-% by heating it under vacuum and then it is
subsequently recirculated. Sometimes the acid is sold or used for other purposes. Rarely, it
becomes waste.
Figure 13. Chlorine drying process
3.1.6 Cleaning of dry chlorine
When leaving the top of the drying tower, dry chlorine passes through high efficiency
demisters or a packed bed to prevent the entrainment of sulphuric acid droplets.
Further potential cleaning steps after chlorine drying include:
•adsorption on carbon beds to remove organic impurities;
• scrubbing with concentrated hydrochloric acid to remove nitrogen trichloride;
•scrubbing with liquid chlorine to remove nitrogen trichloride, organic impurities, carbon
dioxide and bromine;
•irradiation with UV destroys nitrogen trichloride and hydrogen.
34
3.1.7 Compression
After drying and potential further cleaning, chlorine gas may be compressed by a variety of
compressors, depending on the throughput and the desired pressure [7], Rotary
compressors, such as:
•Sulphuric acid liquid ring compressors for throughputs of 150 t/d per compressor and for
pressures of 4 bar or, in two-stage compressors, 12 bar;
•Screw compressors for low throughputs and for pressures of up to 16 bar.
Reciprocating compressors, such as:
•Dry ring compressors for throughputs of 200 t/d per compressor and for pressures of up to
16 bar.
Centrifugal compressors, such as:
•Turbo compressors in mono- or multi-stage operation for throughputs of up to ~ 1800 t/d
per unit and for pressures of up to 16 bar;
•Sundyne blowers for throughputs of 80–250 t/d per compressor and for pressures of up to
3 bar.
Because of heat build-up from compression, multi-stage units with coolers between stages
are usually necessary. Compressor seals are generally fitted with a pressurized purge to
inhibit the leakage of chlorine to the atmosphere. Dry chlorine at high temperatures can
react spontaneously and uncontrollably with iron. Chlorine temperatures are therefore
usually kept below 120 °C.
3.1.8 Liquefaction
Liquefaction can be accomplished at different pressure and temperature levels: at ambient
temperature and high pressure (for example 18 ºC and 7–12 bar), at low temperature and
low pressure (for example -35 ºC and 1 bar) or any other intermediate combination of
temperature and pressure. Important factors for selecting appropriate liquefaction
conditions include the composition of the chlorine gas, the desired purity of the liquid
chlorine and the desired yield. Increasing the liquefaction pressure increases the energy
consumption of compression, although the necessary energy for cooling decreases,
resulting in an overall reduction in energy consumption. The liquefaction yield is typically
limited to 90–95 % in a single-stage installation, as hydrogen is concentrated in the residual
gas and its concentration needs to be kept below the lower explosive limit. Higher yields of
35
up to 99.8 % can be achieved by multi-stage liquefaction. Typically, small volume
liquefiers which are protected against explosions are used after primary liquefaction, and
inert gas is added to keep the mixture below the lower explosive limit [7]. Another
possibility is to remove hydrogen from the system by reaction with chlorine gas in a
column, yielding hydrogen chloride which can be recovered in a hydrochloric acid unit.
The remaining chlorine gas can then be safely further condensed. This solution can be
chosen if hydrochloric acid is a saleable product or if it can be used as a feedstock for
downstream production, such as for ferric chloride.
The choice of the refrigerant in a certain stage of the liquefaction depends on the pressure
of the chlorine. When the pressure is sufficiently high, water can be used as an indirect
refrigerant. When the pressure is relatively low, other refrigerants such as
hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs), typically
chlorodifluoromethane (HCFC-22) and 1,1,1,2-tetrafluoroethane (HFC-134a) (indirect
cooling), ammonia (indirect cooling) or liquid chlorine (direct cooling) are used.
The use of HCFCs such as HCFC-22 is generally prohibited but reclaimed or recycled
HCFCs may be used for the maintenance or servicing of existing refrigeration equipment
until 31 December 2014.
The temperature of the chlorine gas in a certain stage depends mainly on the pressure after
compression. A pressure greater than 8 bar generally enables water cooling but implies an
increased hazard.
The residual chlorine in the tail gas can be used to produce hypochlorite, iron (III) chloride
or hydrochloric acid. The residual chlorine which cannot be valorized is then led to the
chlorine absorption unit. In some cases, it is recovered by absorption-desorption process
with carbon tetrachloride which is strictly prohibited. The latter has the disadvantage of
using a toxic substance with a high ozone depletion and global warming potential.
3.1.9 Handling and storage
Liquefied chlorine is stored at ambient or low temperature. The pressure corresponds to the
vapour pressure of the liquefied chlorine at the temperature in the storage tank. Pressure
storage at ambient temperatures (~ 7 bar at 20 °C) has advantages of simplicity of
operation, ease of visual external inspections, as well as lower energy and investment costs.
Low-pressure storage operating around the boiling point of liquid chlorine (-34 °C) requires
36
more complex infrastructure, particular safety measures and higher energy costs. Within a
plant and over distances of several kilometers, chlorine can be transported by pipelines,
either as a gas or a liquid. The liquid chlorine from the bulk tank can be used as a feedstock
for on-site processes or loaded into containers, or road or rail tankers.
3.1.10 Vaporization
Liquid chlorine is usually vaporized prior to use. The easiest option is to use ambient heat
by which approximately 5 kg of chlorine per hour and square meter of container surface
can be vaporized. For higher flowrates, it is necessary to use a chlorine vaporizer.
3.2 Hydrogen Processing, Storage and Handling
Hydrogen leaving the cells is highly concentrated (> 99.9 vol-%) and normally cooled to
remove water vapour, sodium hydroxide and salt. The solution of condensed saltwater and
sodium hydroxide is recycled to produce caustic, as brine make-up or is treated with other
waste water streams.
In the case of the membrane or diaphragm cell technique, the cooling is usually carried out
by one or more large heat exchangers. In the mercury cell technique, primary cooling is
carried out at the electrolyzer, allowing mercury vapour to condense into the main mercury
circuit. Further cooling and mercury removal takes place at a later stage using a variety of
techniques. Some uses of hydrogen require additional removal of traces of oxygen, which
may be achieved by react the oxygen with some of the hydrogen over a platinum catalyst.
Hydrogen may be distributed to users using booster fans or fed to the main compression
plant, which usually comprises a number of compressors and a gas holder (surge chamber).
The hydrogen gas holder is incorporated into the system to minimize fluctuations in gas
pressure from the primary stage. The hydrogen product gas stream is always kept
pressurized to avoid the ingress of air. All electrical equipment taken into the hydrogen
compression plant area must be
'intrinsically safe' (i.e. the equipment will not produce a spark) or explosion proof (i.e. a
local small explosion is contained within the equipment). A relief valve is normally
provided within the system to relieve high pressure to atmosphere. Hydrogen is normally
monitored for oxygen content, and the compression will shut down automatically in critical
situations. The hydrogen sold to distributors is usually compressed at pressures higher than
100 bar and is injected into a pipeline network. Otherwise, the hydrogen is transported in
37
dedicated tank lorries or in steel bottles at pressures of up to 300 bar. For these high
pressures, the gas is further dried and traces of oxygen are usually removed.
The main utilizations of the co-produced hydrogen are combustion to produce steam (and
some electricity) and chemical reactions such as the production of ammonia, hydrogen
peroxide, hydrochloric acid and methanol.
3.3 Caustic Soda Processing
The caustic soda solution from the three techniques is treated in slightly different ways due
to the difference in composition and concentration. In the case of the mercury cell
technique, 50 wt-percent caustic soda is obtained directly from the decomposers. The
caustic soda is normally pumped through a cooler, then through a mer
