Chlor-Alkali Market, By Products (Caustic Soda/Sodium Hydroxide, Chlorine, So...
Project paper on Chlor-Alkali process
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Approval
This is to declare that the one student of the department of ‘Chemical Engineering
& Polymer Science’ of “Shahjalal University of Science & Technology” has
completed his industrial project report on “Global Heavy Chemicals Ltd.” heading
“Industrial Project on Production of Sodium Hydroxide from sodium chloride
by Membrane Cell Technology Process”.
The report goes to the partial fulfillment of the requirements for the degree of B.Sc. in
Chemical Engineering & Polymer Science.
His involvement was much welcomed and I wish for their stunning future.
Name of the student: Registration No:
J. S. M. Mahedi 2010332037
SUPERVISOR
Md. Tamez Uddin
Associate professor,
Department of Chemical Engineering and Polymer Science
Shahjalal University of Science & Technology
Sylhet-3114, Bangladesh.
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Acknowledgement
I am grateful for the contributions from many individuals leading towards the
successful completion of our program, especially those who gave the time to share
their thoughtful criticisms & suggestions to improve it.
First, I would like to thanks Almighty to give me the opportunity to do the project
work in Global Heavy Chemicals Ltd., the pioneer Chlor-Alkali industry of
Bangladesh. I convey my respectful gratitude to our Teacher and Project Supervisor
Md. Tamez Uddin, Associate professor, Department of Chemical Engineering and
Polymer Science, Shahjalal University of Science and Technology, for his valued co-
operation in making this project paper, Communication & guidance support.
I must grateful to Md. Masudur Rahman, Process In charge, Global Heavy
Chemicals Ltd. He helped me every moment to complete my project successfully.
Without his kind co-operation it became very difficult to complete my project paper.
Whenever a problem arises he helped me as much as he can in every time.
I am thankful to Utpal Kumar Prasadi, Deputy Plant Manager Taslim and Mr. Joy for
their assist which was supportive for me. I also want to say with great thanks to
Jogesh Das for the massive support of giving approach to the Industry an also for
whole contribution. I also like to thanks to Mr. Bidduth for the best help to solve the
different calculations during the project.
Special thanks to Mr. Shishir, Mr. Rana, Mr. Noman, Mr. Gobindo, Mr. Hasan, Mr.
Sobuj and Mr. Sojib for their kind cooperation which help me to complete the project
successfully.
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Abstract
I the student of Chemical Engineering & Polymer Science of Shahjalal University of
Science & Technology was taken part an Industrial Project. The Project was carried
out in Global Heavy Chemicals Ltd. from 3 March 2016 to 30 March 2016. Global
Heavy Chemicals Ltd. is situated in Hasnabad union under Keranigonj Thana. This
industry is one of the renowned industry of Bangladesh. The main product is caustic
soda & the byproducts are chlorinated paraffin wax, sodium hypo chloride as
Clotech B, bleaching, & hydrochloric acid.
Electrolysis is one of the acknowledged means of generating chemical products from
their native state. For example, metallic copper is produced by electrolyzing an
aqueous solution of copper sulfate, prepared by leaching the bearing ores with
sulfuric acid. Sodium hydroxide has now become the top ten chemicals produced
worldwide by electrolysis process. This work represents how NaOH is produced by
Membrane cell technology process.
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Objectives
Sodium hydroxide and Chlorine are the two top ten chemicals nowadays produced
around the world from readily available Sodium Chloride. There are three methods
for the production of NaOH from NaCl. Present investigation shows that membrane
cell process are used by most of the top Sodium hydroxide producing industry. This
process is safe, environmental friendly and least energy consuming and produced
the highest purity product.
The specific aims of this work are:
1. To learn the types and characteristics of the membrane.
2. To learn the whole process of producing Sodium Hydroxide.
3. To learn how the important unit operations are carried out.
4. To do material and energy balance for the most important unit operations.
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Contents
Chapter 1: Introduction 6
1.1 Market demand of chlor-alkali product 7
1.2 Site selection 8
1.3 Plant layout 9
1.4 Chlor-alkali manufacturing process 15
1.4.1 Mercury cell process 16
1.4.2 Diaphragm cell process 17
1.4.3 Membrane cell process 18
Chapter 2: Process description 20
2.1 Primary brine purification section 22
2.2 Secondary brine purification section 26
2.3 Electrolyser section 29
2.4 Chlorine section 32
2.5 Process block diagram 35
Chapter 3: Material balance 36
Chapter 4: Energy balance 43
Chapter 5: HAZOP analysis 47
Conclusion 51
References 52
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Chlorine and Sodium hydroxide are among the top ten chemicals produced in the
world and are involved in the manufacturing of a wide variety of products used in day
to day life. These include: pharmaceuticals, detergents, deodorants, disinfectants,
herbicides, pesticides, and plastics.
That’s why the Chlor-alkali industry has now become a major branch of the chemical
industry. Its primary products are Sodium hydroxide, Chlorine and Hydrogen which
are produced from rock salt, a readily accessible raw material. This interactive unit is
concerned with a short description of the different manufacturing processes used to
produce these materials as well as a vivid description of the process used by Global
Heavy Chemicals Ltd. For producing NaOH.
1.1 Market demand of Chlor-Alkali product:
Figure shows the Chlorine and Sodium Hydroxide production since 1960 and shows
recent output to be roughly constant at ca. 10 million tons. Collectively this data
show these materials to be in high demand and that the volumes involved are large.
In fact the worldwide manufacturing capacity for each of these chemicals is
approximately 40 million tons per year i.e. Chlor-alkali industry is a big business.
Figure: Chlorine and Sodium hydroxide production since 1960
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1.2 Site Selection:
Major Site Location Factors
While many factors can be important in the selection of a plant site, three are usually
considered the most important. These are
1. The location of the markets and
2. Location of raw materials and
3. The type of transportation to be used.
Any one or all of these factors together may greatly limit the number of sites that are
feasible.
Location of Raw Materials:
One possible location is a site near the source of the raw materials. This location
should always be one of the sites considered. Global Heavy Chemicals Ltd is on
the bank of the Burigonga River. The raw material which is imported from the
nearest country India is bought to here by the river.
Location of Markets:
Plants are usually constructed close to the prospective markets. Transportation cost
will be less if the local market is near to the industry.
Transportation:
The importance of the cost of transportation has been indicated in the previous
paragraphs. The least expensive method of shipping is usually by water, the most
expensive is by truck.
Other Site Location Factors
Besides the three most important variables, others must be considered. For a given
plant any one of these may be a reason why a specific location is preferable. These
are given in below:
1. Transportation
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2. Sources and costs of raw materials
3. Prospective markets for products
4. Corporation long range planning
5. Water source-quality and quantity
6. Special incentives
7. Climatic conditions
8. Pollution requirements (Waste disposal)
9. Utilities-cost, quantity and reliability; fuel-costs, reliability and availability
10. Amount of site preparation necessary (site conditions)
11. Construction costs
12. Operating labor
13. Taxes
14. Living conditions
15. Corrosion
16. Expansion possibilities
17. Other factors
Most corporations have some long-term goals. Often these goals affect the choosing
of a plant site. This means that each plant site is not considered only for itself and
that its chosen location might not be the one that would be selected if only the
economics of the one plant had been considered. The object of long-range planning
is to optimize a whole network of operations instead of each one individually.
1.3 Plant Layout:
The laying out of a plant is still an art rather than a science. Plant Layout is the
physical arrangement of equipment and facilities within a Plant. It can be indicated
on a floor plan showing the distances between different features of the plant.
Optimizing the Layout of a Plant can improve productivity, safety and quality of
Products. Unnecessary efforts of materials handling can be avoided when the Plant
Layout is optimized. It involves the placing of equipment so that the following are
minimized:
(1) Damage to persons and property in case of a tire or explosion
(2) Maintenance costs
(3) The number of people required to operate the plant
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(4) Other operating costs
(5) Construction costs
(6) The cost of the planned future revision or expansion.
All of these goals cannot be met. For example, to reduce potential losses in case of
fire, the plant should be spread out, but this would also result in higher pumping
costs, and might increase manpower needs. The engineer must decide within the
guidelines set by his company which of the aforementioned items are most
important.
The first thing that should be done is to determine the direction of the prevailing
wind. This can be done by consulting Weather Bureau records. In Bangladesh the
prevailing winds are often from the north to south in the summer. Wind direction will
determine the general location of many things. All equipment that may spill
flammable materials should be located on the downwind side. Then if a spill occurs
the prevailing winds are not apt to carry any vapors over the plant, where they could
be ignited by an open flame or a hot surface.
For a similar reason the powerhouse, boilers, water pumping, and air supply facilities
should be located 250 ft (75 m) from the rest of the plant, and on the upwind side.
This is to minimize the possibility that these facilities will be damaged in case of a
major spill. This is especially important for the first two items, where there are
usually open flames.
Every precaution should be taken to prevent the disruption of utilities, since this
could mean the failure of pumps, agitators, and instrumentation. For this reason, it
may also be wise to separate the boilers and furnaces from the other utilities. Then,
should the fired equipment explode, the other utilities will not be damaged.
Other facilities that are generally placed upwind of operating units are plant offices,
mechanical shops, and central laboratories. All of these involve a number of people
who need to be protected. Also shops and laboratories frequently produce sparks
and flames that would ignite flammable gases. Laboratories that are used primarily
for quality control are sometimes located in the production area.
A list of items that should be placed downwind of the processing facilities is given
below
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Items That Should Be Located Upwind of the Plant
Plant offices
Central laboratories
Mechanical and other shops
Office building
Cafeteria
Storehouse
Medical building
Change house
Fire station
Boiler house
Electrical powerhouse
Electrical Substation
Water treatment plant
Cooling tower
Air compressors
Parking lot
Main water pumps
Warehouses that contain nonhazardous,
Non explosive, and
Nonflammable materials
Fired heaters
All ignition sources
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Items That Should Be Located Downwind of the Plant
Equipment that may spill inflammable materials
Blow down tanks
Burning flares
Settling ponds
Storage Facilities
Tank farms and warehouses that contain nonhazardous, nonflammable, and
nonexplosive materials should be located upwind of the plant. Those that do not fit
this category should not be located downwind of the plant, where they could be
damaged and possibly destroyed by a major spill in the processing area. Nor should
they be located upwind of the plant where, if they spilled some of their contents, the
processing area might be damaged. They should be located at least 250 ft (75m) to
the side of any processing area. Some authorities suggest this should be 500 ft.
The same reasoning applies to hazardous shipping and receiving areas.
Sometimes storage tanks are located on a hill, in order to allow the gravity feeding of
tank cars.
Care must be taken under these circumstances to see that any slop over cannot flow
into the processing, utilities, or service areas in case of a tank fire.
Spacing of Items
The OSHA has standards for hazardous materials that give the minimum distances
between containers and the distance between these items and the property line,
public roads, and buildings. These depend on the characteristics of the material, the
type and size of the container, whether the tank is above ground or buried, and what
type of protection is provided. Specific details are provided for compressed gas
equipment containing acetylene-air, hydrogen-oxygen, and nitrous oxide, as well as
liquefied petroleum gases. They also prohibit the storage and location of vessels
containing flammable and combustible materials inside buildings, unless special
precautions are taken.
Processing Area
There are two ways of laying out a processing area. The grouped layout places all
similar pieces of equipment adjacent. This provides for ease of operation and
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switching from one unit to another. For instance, if there are 10 batch reactors, these
would all be placed in the same general area, and could be watched by a minimum
of operators; if they were spread out over a wide area, more operators might be
needed. This type of scheme is best for large plants. The flow line layout uses the
train or line system, which locates all the equipment in the order in which it occurs on
the flow sheet. This minimizes the length of transfer lines and, therefore, reduces the
energy needed to transport materials. This system is used extensively in the
pharmaceutical industry, where each batch of a drug that is produced must be kept
separate from all other batches. In other industries it is used mainly for small-volume
products. Often, instead of using the grouped or flow line layout exclusively, a
combination that best suits the specific situation is used.
Elevation
If there is no special reason for elevating equipment, it should be placed on the
ground level. The superstructure to support an elevated piece of equipment is
expensive. It can also be a hazard should there be an earthquake, fire, or explosion.
Then it might collapse and destroy the equipment it is supporting as well as that
nearby. Some pieces of equipment will be elevated to simplify the plant operations.
An example of this is the gravity feed of reactors from elevated tanks. This
eliminates the need for some materials-handling equipment. Other pieces may have
to be elevated to enable the system to operate. A steam jet ejector with an inter
condenser that is used to produce a vacuum must be located above a 34 ft (10 m)
barometric leg. Condensate receivers and holding tanks frequently must be located
high enough to provide an adequate net positive suction head (NPSH) for the pump
below.
For many pumps an NPSH of at least 14 ft(4.2 m)
H2O is desirable. Others can operate when the NPSH is only 6 ft (2 m) H2O.
The third reason for elevating equipment is safety. In making explosive materials,
such as TNT, the reactor is located above a large tank of water. Then if the mixture
in the reactor gets too hot and is in danger of exploding, a quick-opening valve
below the reactor is opened and the whole batch is dumped into the water. An
emergency water tank may need to be elevated so that, in case of a power failure,
cooling water to the plant will continue to flow, and there will be water available
should a tire occur. Sometimes this tank is located on a nearby hill. An elevation
plan should be drawn to scale showing the vertical relationships of all elevated
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equipment. These drawings, as well as the plot plan, are usually sketched by the
engineer and then redrawn to scale by a draftsman.
Maintenance
Maintenance costs are very large in the chemical industry. In some cases the cost of
maintenance exceeds the company’s profit.
Construction and Building
Proper placing of equipment can result in large savings during the construction of
the plant. For instance, large columns that are field-erected should be located at one
end of the site so that they can be built, welded, and tested without interfering with
the construction of the rest of the plant.
Buildings
Included with the layout of the plant is the decision as to what types of buildings are
to be constructed, and the size of each. When laying out buildings, a standard size
bay (area in which there is no structural supports) is 20 ft x 20 ft (6m x 6m). Under
normal conditions a 20 ft (6 m) span does not need any center supports. The
extension of the bay in one direction can be done inexpensively. This only increases
the amount of steel in the long girders, and requires stronger supports. Lavatories,
change rooms, cafeterias, and medical facilities are all located inside buildings. The
minimum size of these facilities is dictated by OSHA. It depends on the number of
men employed. Research laboratories and office buildings are usually not included
in the preliminary cost estimate. However, if they are contemplated their location
should be indicated on the plot plan.
Processing Buildings
Quality control laboratories are a necessary part of any plant, and must be included
in all cost estimates. Adequate space must be provided in them for performing all
tests, and for cleaning and storing laboratory sampling and testing containers. The
processing units of most large chemical plants today are not located inside buildings.
This is true as far north as Michigan. The only equipment enclosed in buildings is
that which must be protected from the weather, or batch equipment that requires
constant attention from operators. Much of the batch equipment used today does not
fit this category. It is highly automated and does not need to be enclosed. When
buildings are used, the ceilings generally vary from 14 to 20 ft (4 to 6 m). Space
must be allowed above process vessels for piping and for access to valves. One rule
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of thumb is to make the floor to- floor heights 8- 10 ft (approximately 3m) higher than
the sides of a dished-head vertical tank.6
Packaging equipment generally must be in an enclosed building, and is often located
at one end of the warehouse. If the material being packaged is hazardous, either this
operation will be performed in a separate building, or a firewall will separate it from
any processing or storage areas.
Warehouse:
The engineer must decide whether warehouses should be at ground level or at dock
level. The latter facilitates loading trains and trucks, but costs 1520% more than one
placed on the ground.
It is usually difficult to justify the added expense of a dock-high warehouse. To size
the amount of space needed for a warehouse, it must be determined how much is to
be stored in what size containers. The container sizes that will be used are obtained
from the scope. Liquids are generally stored in bulk containers. No more than a
week’s supply of liquid stored in drums should be planned. Solids, on the other
hand, are frequently stored in smaller containers or in a pile on the ground.
Control Rooms
The control center(s) and the electrical switching room are always located in an
enclosed building. It is important that both of these services be maintained so that
the plant can be shut down in an orderly manner in the case of an emergency.
Therefore these buildings must be built so that should an external explosion occur
the room will not collapse and destroy the control center and switching center. To
avoid this, either the structure must have 3-4 ft (l-l.2 m) thick walls, or the roof must
be supported independently of the walls. The Humble Oil and Refining
Co. has specified that the building withstand a 400 psf (2,000 kg / m2) external
explosive force.
To keep any flammable or explosive vapors from entering the building, it is
frequently slightly pressurized. This prevents the possibility of an internal explosion.
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1.4 Chlor-Alkali manufacturing process:
To produce NaOH it is necessary to prevent reaction of the NaOH with the chlorine.
There are 3 types of electrolytic processes used in the production of chlorine:
1. Mercury cell technology
2. Diaphragm cell technology
3. Membrane cell process
In each process, a salt solution is electrolyzed by the action of direct electric current
that converts chloride ions to elemental chlorine. The overall process reaction is:
2NaCl + 2H2O → Cl2 + H2 + 2NaOH
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.
1.4.1 Mercury Cell Process:
The anode reaction involves chloride ion being converted to chorine gas. Mercury
flows over the steel base of the cell and, in this way, the mercury acts as the
cathode. Sodium is released in preference to hydrogen on the mercury surface, the
sodium dissolving in the mercury. This is then carried into the secondary cell where
it reacts with water to release sodium hydroxide.
In the secondary reactor, the sodium amalgam reacts with water to produce sodium
hydroxide (NaOH) and hydrogen.
The relevant equations are:
2Cl- → Cl2 + 2e-
Na+ + e- → Na
Na + Hg → Na/Hg (sodium amalgam, a dense liquid)
2Na/Hg + 2H2O → 2NaOH + H2 + 2Hg (slow reaction)
To increase the rate of this reaction, the secondary reactor contains carbon balls,
which catalyze the reaction. The sodium hydroxide is produced at up to 50%
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concentration. This is the sales specification and therefore no further purification is
required.
Fig.: Mercury cell
1.4.2 Diaphragm Cell Process:
Diaphragm cell process uses a steel cathode, and the reaction of NaOH with Cl2 is
prevented using a porous diaphragm, often made of asbestos fibers. In the
diaphragm cell process the anode area is separated from the cathode area by a
permeable diaphragm. The brine is introduced into the anode compartment and
flows through the diaphragm into the cathode compartment. Diluted caustic brine
leaves the cell. The sodium hydroxide must usually be concentrated to 50% and the
salt removed. This is done using an evaporative process with about three tones of
steam per ton of sodium hydroxide. The salt separated from the caustic brine can be
used to saturate diluted brine. The chlorine contains oxygen and is purified by
liquefaction and evaporation.
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Fig.: Diaphragm cell
1.4.3 Membrane Cell Process:
Membrane cell process is similar to the diaphragm cell process, with a Nation
membrane to separate the cathode and anode reactions. Only sodium ions and a
little water pass through the membrane. It produces a higher quality of NaOH. Of the
three processes, the membrane cell process requires the lowest consumption of
electric energy and the amount of steam needed for concentration of the caustic is
relatively small (less than one tone per ton of sodium hydroxide). In the production of
chlorine and caustic soda via the electrolytic splitting of salt (sodium chloride) both
the diaphragm and the amalgam processes have become obsolete due to a high
energy consumption and low environmental compatibility. They have been replaced
by the latest development in chlorine / caustic soda technology: the membrane
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process. This process in not just energy-efficient, it is environment friendly,
extremely safe and also produces caustic soda of a consistently high quality.
Fig.: Membrane cell
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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) level. If this level of purity is not reached the membrane will be
damaged. For this reason in GHCL the brine is purified in two section. Brine is then
sent to cell house to produce 32% Caustic Soda.
Main units of the industry are:
10 MW power plant including diesel generator & boiler house.
1800 MT water storage tank including two-pump house & cooling tower.
Bi-polar membrane cell house including rectifier, rectifier transformer, DCS
control room.
Anolyte & Catholyte tank, de-chlorination building as well as quality control
department.
Utility building including DM plant, Nitrogen plant, absorption chillers &
compressors. This block has got HCl synthesis building including storage tank
and delivery platform.
Primary & Secondary brine purification area including Salt Saturator, Reactor,
Chemical-Dosing Tanks, Main Clarifier, Anthracite Filter, Candle Filter,
Polished Brine Tank, Ion-Exchange Resin Column and Purified Brine Storage
Tank.
Automatic Salt Washer unit including separate storage area for raw & washed
salt, conveyers, small clarifier etc.
Chlorine Drying & Compression Building including Bottling area as well as
four large storage tank and delivery platform.
Caustic Evaporation and Flaking Building including bagging and storage
facility.
Hydrogen gas Compression and bottling building.
Automatic Effluent treatment plant for industrial water treatment.
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2.1 Primary brine purification section:
The main raw material for the GHCL is the solid salt which is further processed to
produce caustic soda. In GHCL plant salts are imported from the neighboring
country India. Because the composition of the salt comes from India is better from
the local salt and also have less impurities than the local salt. The composition of
raw salt is shown in Table 1.
Table 1. Composition of raw salt used in GHCL
Composition Percentage
Ca2+ 0.227%
Mg2+ 0.049%
SO4
2- 0.645%
Total Iron 13.2 ppm
NaCl 95.43%
Moisture 3.649%
Salt Washer Unit:
To purify this imported salt, there is one fully automatic modern salt washer unit. The
raw salt is initially put into the hopper by a pay loader or robot and pulled by bucket
elevator into the screw conveyer. Salt is then dissolved in the salt saturator. The
obtained saturated brine is then sent to purification process to remove impurities.
Salt Saturator:
For melting the solid salt in the salt saturator there is a continuous process of
pumping of return brine solution at about 85°c from the return brine tank which is
executed from the cell and is not converted to the caustic soda. After melting raw
salt in the salt saturator the solution is passed through a plate filter to remove the
floating substance and impurities that come from the salt feeding. Then the solution
is fed to the dosing unit.
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Regeneration:
Impurities or the mud which come from the salt decompose at the bottom of the salt
saturator and decrease the efficiency of the salt saturator. For this reason after 3-4
months the salt saturator is washed to make it clean.
Dosing:
From analysis of the raw salt the dissolved impurities are the Ca+, Mg+, SO4
2- and
mud. To remove this impurities chemical dosing is required. BaCl2 is used to remove
the SO4
2-. NaCO3 issued to remove Ca+ and also NaOH for the Mg+. After the dosing
of these chemical the salt solution is send to the reactor for the proper mixing.
In this plant five different dosing are performed and these are as follows:
1. Soda Ash(Na2CO3) dosing
2. Barium Chloride(BaCl2) dosing
3. Sodium Sulphide(Na2SO3) dosing
4. Caustic Soda(NaOH) dosing
5. Flocculants dosing
For production of 8333.33 kg/hr Sodium hydroxide, 6385.774 kg/hr salt is needed.
Chemical dosing:
Ca2+ in raw materials =
6385.774 𝑘𝑔 𝑠𝑎𝑙𝑡
ℎ𝑟
0.227 𝑘𝑔 𝐶𝑎2+
100 𝑘𝑔 𝑠𝑎𝑙𝑡
= 14.49 kg/hr
For removing Ca2+, Na2CO3 needed:
Ca2+ + Na2CO3 = CaCO3 + 2Na+
Na2CO3 needed =
106∗14.49 𝑘𝑔
111 ℎ𝑟
= 13.83 kg/hr
Amount of Mg2+ =
6385.774 𝑘𝑔 𝑠𝑎𝑙𝑡
ℎ𝑟
0.049 𝑘𝑔 𝑀𝑔2+
100 𝑘𝑔 𝑠𝑎𝑙𝑡
= 3.129 kg/hr
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NaOH needed for removing Mg2+:
MgCl2 + 2NaOH = 2NaCl + Mg(OH)2
NaOH needed =
80∗3.129 𝑘𝑔
95 ℎ𝑟
= 2.634 kg/hr
Amount of SO4
2- =
6385.774 𝑘𝑔 𝑠𝑎𝑙𝑡
ℎ𝑟
0.645 𝑘𝑔 𝑆𝑂4
2−
100 𝑘𝑔 𝑠𝑎𝑙𝑡
= 41.18 kg/hr
BaCl2 needed for removing SO4
2-:
Na2SO4 + BaCl2 = 2NaCl + BaSO4
BaCl2 needed =
208∗41.18 𝑘𝑔
142 ℎ𝑟
= 60.32 kg/hr
Procedure of making Dosing:
1. Barium Chloride (BaCl2):
Desired concentration: 0.15% by weight
Required composition:
1. 475 kg BaCl2
2. 2000-2500 Liter H2O
3. 500 kg HCl
Chemical Reaction:
BaCO3 + HCl + H2O = BaCl2 + CO2 + H2O
In this reaction pH of HCl is 3.5 to 4 where pH should be maintained at level 6. This
is done by adding 10 to 12 kg excess NaOH.
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2. Soda Ash (Na2CO3):
Desired concentration: 0.14%
Required composition:
1. Soda Ash (Na2CO3)
2. 1400 liter H2O
3. Flocculent:
Required composition:
1. 500 gm floccal
2. 1000 liter H2O
Chemical Reaction:
500 gm floccal + 1000 L H2O
Main function of flocculent is to hold up the moisture.
4. Sodium Sulphide(Na2SO3):
Desired concentration- 7% by weight.
Required composition:
1. 100 L Na2SO3
2. 200 L H2O
Reactor:
Reactor which is used here mainly a CSTR. In this reactor the following reaction
occurs:
Na2SO4 + BaCl2 → NaCl + BaSO4 ↓
Na2CO3 + Ca2+ → CaCO3 ↓ + 2 Na2+
2 NaOH + Mg2+ → Mg(OH)3 ↓
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After complete mixing of the brine and dosing solution a flocculent named Megna
floc is added to the solution. Then the solution is send to the clarifier for the
removing the precipitate of the solution and also increasing the turbidity of the
solution.
Reactor concentration range of brine is 310-315 gpl and is continuously monitored
by a Hydrometer. Reactor temperature is 60-65oC and is continuously monitored by
a Thermometer.
Clarifier:
In the clarifier the mud, precipitated produced from the dosing which are carried by
the saturated brine solution is precipitated in the bottom of the clarifier. From the
bottom of the clarifier the thick mud solution of the saturated brine is pumped to the
decanter and mud is separated and collected for disposal as waste product. The
brine solution driven from the clarifier is stored in clarified brine tank and then sends
to anthracite filter for further removal of floc particles.
Anthracite filter:
Filter medium of the anthracite filter is mainly the anthracite. In anthracite filter solid-
solid adsorption occurred. Three types of carbon: large, small and medium lies in the
anthracite filter. When the brine solution is passed through the fine anthracite filter
medium the floc particles cannot pass through the medium and get trapped in the
anthracite medium. Then the solution is stored in the anthracite filter tank to make
the process continuous.
2.2 Secondary brine purification section:
In membrane cell process brine specifications are more stringent than that of the
mercury and diaphragm processes and the impurities should be at ppb level. This is
accomplished by filtering the brine in a pre-coat type secondary filter. To meet this
need the filtered brine of primary purification section is sent to brine polishing filter
also called candle filter.
Candle filter:
Candle filter is a special type of filter in which the filter medium is activated carbon
and the filter aid is the alpha cellulose. This alpha cellulose blocks the micro level
particles from the brine solution. To maintain the layer of the alpha cellulose which is
27. 27 | P a g e
externally exerted in the upper surface of the activated carbon filter 1-2 atm pressure
is maintain continuously. If the pressure drops, there will no more alpha cellulose
layer upon the activated carbon filter. To maintain the efficiency the of the filter aid,
alpha cellulose is continuously added in the candle filter. Brine solution is feed at the
bottom of the filter and mud free solution is out at the top of the filter. After filtering in
the candle filter the turbidity becomes -3 or -4 and brine solution is 3 to 4 times
transparent than water.
Fig.: Cross-section of candle filter
Regeneration of candle filter:
The candle of the alpha cellulose is washed away by using the back flow of the air.
The new alpha cellulose is added from the pre-coat tank.
Ion Exchanger:
Multivalent ions are exchange with the 1aminodiacetic acid of ion exchange resin in
the ion-exchanger. But sodium is monovalent ion so it is not exchanged with this
resin. Na ion is replaced by Ca2+ and Mg2+. The resin used in ion-exchanger passed
only Na+ and as it is a cation exchanger so Na+ and Cl- entered into cell house.
28. 28 | P a g e
Fig.: Ion exchanger
Regeneration of ion-exchange resin:
Resin can work very well till its efficiency is high or moderate. But when
concentration of Ca2+ is less than 10 ppm and concentration of Mg2+ is 2-3 ppb the
bed is needed to regenerate. The regeneration process is as follows:
Wash-1:
At first the resin bed is washed away by demineralized water at constant flow 1600
L/h and it continue 1 hour as all ash and dust will washed.
29. 29 | P a g e
Back wash:
Back wash is done by DM water at a flow rate of 1.6m3/h over 30 minute. DM water
supplied at the bottom of the tower and resin was circulate with the tower. Water
flow is maintained at a constant rate so that resin does not overflow. After ensuring
that all brine washed away back wash was completed.
HCl regeneration:
18% concentrated HCl is then supplied in the at 600L/h flow rate over 30 to 50
minutes. By adding DM water at rate 1000L/h, 5% concentrated HCl made up. When
the pH of HCl becomes 1 HCl supply will stop. During HCl regeneration Na of 1-
aminodiacetic acid was replaced by Cl2 and the media become acidic.
Wash-2:
To remove the acidic media again DM water supplied at a flow rate 1600L/h over 1
hour. Consequently all Cl2 will replace by H+ ion of water.
NaOH regeneration:
Now 32% NaOH passed through the bed at a rate 200L/h with DM water of rate
1400L/h over 40 to 50 minutes. As a result COOH of 1aminodiacitic acid will
converted to COONa and resin regeneration will completed.
Wash-3:
Again the bed will washed away by DM water at a flow rate of 1600L/h over 1 hour
to maintain the pH 10. If pH 10 is obtained water supply should stopped.
2.3 Electrolyser section:
Membrane technology is the unique Single Element, which comprises an anode half
shell, a cathode half shell and an individual sealing system with external flanges.
The Single Elements are suspended in a frame and are pressed against each other
by a clamping device to form a "Bipolar stack”. Each Single element can be replaced
quickly and easily. The elements are assembled in the Electrolyser workshop, where
tightness tests are also carried out.
30. 30 | P a g e
Sodium hydroxide is produced (along with chlorine and hydrogen) via the Chlor-
alkali process. This involves the electrolysis of an aqueous solution of sodium
chloride. The sodium hydroxide builds up at the cathode, where water is reduced to
hydrogen gas and hydroxide ion:
2 Na+ + 2 H2O + 2 e– → H2 + 2 NaOH
More accurately:
2 Na+ + Cl– + 2 H2O + 2 e– → H2 + 2 Cl– + 2 NaOH
The Cl– ions are oxidized to chlorine gas at the anode.
Fig: Cell house
31. 31 | P a g e
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 + 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. To produce NaOH it is necessary to prevent reaction of
the NaOH with the chlorine.
Important Feature of this Membrane:
Perfluro Sulphonate Polymer act as an anode coating.
Perfluro Carboxylate Polymer act as a cathode coating.
High caustic flow is maintained as coating could not attach with the
membrane body.
Hence clorine is a heavy gas so it pulled from separator by a compressor.
This membrane is only permeable to Na+ ion.
This membrane is imported from Asai Kasai Company Japan.
Basic cell reaction:
Anode: 2Cl- - 2e- Cl2
Cathode: 2H+ + 2e- H2
32. 32 | P a g e
2.4 Chlorine section:
Generally, Chlorine production and storage are comprises of four basic section.
These are follows:
1. Drying section
2. Compression section
3. Liquefaction section
4. Storage
Very occasionally it can be used directly from the electrolysers. A general flow of
chlorine from the electrolysers to storage is presented in figure.
Figure: A general flow of chlorine from the electrolysers to storage
33. 33 | P a g e
The excess chlorine gas are then send to chlorine unit. Chlorine gas leaving the
electrolyzer is at approximately 80-90 ºC and saturated with water vapor. It also
contains brine mist, impurities such as N2, H2, O2, CO2 and traces of chlorinated
hydrocarbons. Electrolyzers are operated at essentially atmospheric pressure with
only a few milli-atmospheres differential pressure between the anolyte and the
Catholyte.
Drying section:
Drying of chlorine is carried out almost exclusively with 78% concentrated sulphuric
acid. Drying is accomplished in counter-current sulphuric acid contact towers. H2SO4
act as an adsorber and it adsorb almost all moisture present in chlorine. 98% H2SO4
charges at the top side of tower which always keeps the downward pressure
constant. Dry chlorine leaving the top of the drying tower passes through high
efficiency demisters to prevent the entrainment of sulphuric acid droplets. 78%
H2SO4 get out at the bottom of the tower and continuously collected at the jar.
Figure: Drying of Chlorine
34. 34 | P a g e
Compression section:
After drying, chlorine gas might be scrubbed with liquid chlorine or treated with ultra
violet irradiation to reduce levels of nitrogen trichloride. The dry chlorine is
compressed in a centrifugal compressor to maintain the outlet pressure at 8 bar.
Liquefaction section:
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 in a reciprocating
compressor. Freon-22 is used as refrigerator. After liquefaction gaseous chlorine is
converted to liquid chlorine and liquid Freon is converted to gaseous Freon.
Storage section:
Liquid chlorine gas is then sent to storage tank at 8 bar in four tank. Four tank is
assembled so that one is in production, one is in storage, one is in cleaning and
other is in delivery.
35. 35 | P a g e
2.5 Process diagrams:
Fig.: Block diagram of the brine purification process and electrolysis
37. 37 | P a g e
Overall material balance:
Basis: 200 MT/day Caustic Soda (NaOH) produduction
ṁ2 kg NaCl solution/hr ṁ4 kg NaOH solution/hr
.
0.2 kg NaCl/kg solution 0.32 kg NaOH/kg solution
0.8 kg H2O/kg solution 0.68 kg H2O/kg solution
aa
ṁ1 kg NaCl solution/hr ṁ3 kg NaOH solution/hr
0.3 kg NaCl/kg solution 0.28 kg NaOH/kg solution
0.7 kg H2O/kg solution 0.72 kg H2O/kg solution
Figure: Electrolyser
Area of a cell = 2.9 m2
= 2.9 * (100 cm)2
= 29000 cm2
Current density = 0.4 A/cm2
Total current needed for one cell = 0.4
𝐴
𝑐𝑚2
* 29000 cm2
= 11600 A
= 11.6 kA
Anode(Ti) Cathode(Ni)
Na+
H+
Cl-
OH-
38. 38 | P a g e
From faraday’s law, 1 F ≡ 1 mole NaOH
40 gm NaOH ≡ 96500 C
1 gm NaOH ≡
96500
40
C
200 MT NaOH ≡
96500∗200∗1000∗1000
40
C
= 4.825 * 1011 C
From Faraday’s first law of electrolysis,
Q = It
I =
𝑄
𝑡
=
4.825∗1011
24∗3600
A
= 5584490.741 A
= 5584.5 kA
So total cell needed for 200 MT NaOH plant
=
5584.5
11.6
= 482
Capacity of the plant = 200 MT NaOH/day
=
200∗1000 𝑘𝑔
24 ℎ𝑟
= 8333.33 kg/hr
= 208.33 Kmol/hr
The principle chemical reaction is
2 NaCl + H2O = 2NaOH + H2 + Cl2
Flow rate at anode side:
From the reaction
39. 39 | P a g e
1 kmol/hr NaOH ≡ 1 kmol/hr NaCl
208.33 kmol/hr NaOH ≡ 208.33 kmol/hr NaCl
At 600C temperature specific gravity of 30% NaCl = 1.2
And density ρ = 1.2 * 103 𝑘𝑔
𝑚3
NaClin =
1.2∗103 𝑘𝑔 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
𝑚3
0.3 𝑘𝑔 𝑁𝑎𝐶𝑙
1 𝑘𝑔 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
= 360 kg NaCl/m3 solution
= 6.154 kmol/m3 solution
At 850C temperature specific gravity of 20% NaCl = 1.111375
And density ρ = 1.111375 * 103 𝑘𝑔
𝑚3
NaClout =
1.111375∗103 𝑘𝑔 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
𝑚3
0.2 𝑘𝑔 𝑁𝑎𝐶𝑙
1 𝑘𝑔 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
= 222.275 kg NaCl/m3 solution
= 3.8 kmol/m3 solution
NaClin - NaClout = (6.154- 3.8) kmol/m3
= 2.354 kmol/m3
Flow rate in anode side =
208.33 𝑘𝑚𝑜𝑙
ℎ𝑟
𝑚3
2.354 𝑘𝑚𝑜𝑙
= 88.5 m3/hr
Material balance at anode side:
NaClin =
1.2∗103 𝑘𝑔 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
𝑚3
0.3 𝑘𝑔 𝑁𝑎𝐶𝑙
1 𝑘𝑔 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
*
88.5 𝑚3
ℎ𝑟
= 31861.44 kg/hr
= 544.64 kmol/hr
NaClout = NaClin – NaClconsumption
= (544.64 – 208.33)
𝑘𝑚𝑜𝑙
ℎ𝑟
41. 41 | P a g e
Flow rate of cathode side:
=
208.33 𝑘𝑚𝑜𝑙
ℎ𝑟
𝑚3
1.4896 𝑘𝑚𝑜𝑙
= 139.86 m3/hr
Material balance at cathode side:
NaOHin =
1.284∗103 𝑘𝑔
𝑚3
0.28 𝑘𝑔 𝑁𝑎𝑂𝐻
1 𝑘𝑔 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
1 𝑘𝑚𝑜𝑙
40 𝑘𝑔 𝑁𝑎𝑂𝐻
* 139.86 m3/hr
= 1257.68 kmol/hr
= 50307.2 kg/hr
NaOHout = NaOHin + NaOHproduced
= (1257.68 + 208.33)kmol/hr
= 1466.1 kmol/hr
= 58644 kg/hr
H2O in =
50307.2kg NaOH
ℎ𝑟
0.72 𝑘𝑔 H2O
0.28 𝑘𝑔 𝑁𝑎𝑂𝐻
= 129361.36 kg H2O/hr
= 7186.74 kmol/hr
H2O out = H2O in
= 7186.74 kmol/hr
H2 produced =
1
2
* NaOH kmol/hr
=
1
2
* 1466.1 kmol/hr
= 733.05 kmol/hr
= 1466.1 kg/hr
42. 42 | P a g e
NaCl needed = (31861.44 – 19674.135) kh/hr
= 12187.81 kg/hr
So raw salt needed
=
12187.81 kg
ℎ𝑟
100 𝑘𝑔 𝑟𝑎𝑤 𝑠𝑎𝑙𝑡
95.43 𝑘𝑔 𝑁𝑎𝐶𝑙
= 12771.466 kg raw salt/hr
Material balance at a glance:
m3/hr Kmol/hr Kg/hr
Anode in NaCl 88.5 544.64 31861.44
H2O 4130.19 74343.36
Anode out NaCl 336.31 19674.135
H2O 4130.19 74343.36
Cl2 104.165 7395.715
Cathode in NaOH 139.86 1257.68 50307.2
H2O 7186.74 129361.36
Cathode out NaOH 1466.1 58644
H2O 7186.74 129361.36
H2 733.05 1466.1
48. 48 | P a g e
The hazard and operability study, commonly referred to as the HAZOP study is a
systematic approach for identifying all plant or equipment hazards and operability
problems. In this technique all segment are carefully examined and all possible
deviation from normal operating conditions are identified.
Hazard assessment is vital tool in loss prevention throughout the life of a facility. A
through hazard and risk assessment of a new facility is essential during the final
design stage.
A hazard assessment during the prestart-up period should be a final check rather
than an initial assessment.
The major hazard usually include toxicity, fire, and explosions, however thermal
radiation, nose, asphyxiation and various environmental concerns also need to
be considered.
Hazard in chlor-alkali industry:
1. Chlorine Hazard:
Hazards associated with breathing of Chlorine:
Chlorine is a severe nose, throat and upper respiratory tract irritant. People exposed
to chlorine, even for short periods of time, can develop a tolerance to its odour and
irritating properties. Concentrations of 1 to 2 ppm produce significant irritation and
coughing, minor difficulty breathing and headache. Concentrations of 1 to 4 ppm are
considered unbearable. Severe respiratory tract damage including bronchitis and
pulmonary edema (a potentially fatal accumulation of fluid in the lungs) has been
observed after even relatively low, brief exposures (estimates range from 15 to 60
ppm). However, long-term respiratory system and lung disorders have been
observed following severe short-term exposures to chlorine.
Hazard associated when Chlorine comes into contact with skin:
Direct contact with the liquefied gas escaping from its pressurized cylinder can
cause frostbite. Symptoms of mild frostbite include numbness, prickling and itching
in the affected area. The skin may become waxy white or yellow. Blistering, tissue
death and gangrene may also develop in severe cases. In addition, the airborne gas
may irritate and burn the skin.
49. 49 | P a g e
Hazard associated when Chlorine hurt eyes:
Chlorine gas is a severe eye irritant. Stinging, a burning sensation, rapid blinking,
redness and watering of the eyes have been observed at concentrations of 1 ppm
and higher.
Health effects to exposure of Chlorine:
INHALATION: Despite design limitations, the small number of human
population studies conducted have not shown significant respiratory system
effects in workers with long-term, low-level (typically less than 1 ppm) chlorine
exposure and 1.42 ppm (0.15 ppm average) for an average exposure.
Chlorine workers reported a higher incidence of tooth decay (based on
medical history.
First Aid Measures:
Inhalation:
Remove to fresh air. Get medical attention for any breathing difficulty.
Ingestion:
If large amounts were swallowed, give water to drink and get medical advice.
Skin Contact:
Wash exposed area with soap and water. Get medical advice if irritation develops.
Eye Contact:
Immediately flush eyes with plenty of water for at least 15 minutes, lifting upper and
lower eyelids occasionally. Get medical attention if irritation persists.
Bleaching Hazard:
Chlorine bleach contains chlorine, a toxic gas, combined with sodium and oxygen as
sodium hypochlorite. Hazards arise when the chlorine is released from this bond.
The U.S. Food and Drug Administration reports that chlorine bleach is also a
common food tampering adulterant.
Gastrointestinal Damage
Excluding deliberate beverage tampering, accidental ingestion is relatively unlikely
because this strong-smelling, caustic liquid induces the gag reflex. However, when it
is swallowed, bleach causes corrosive damage to the throat and stomach linings. At
50. 50 | P a g e
domestic concentrations, severe tissue damage or systemic poisoning are unlikely.
Both toxicity levels and causticity are more hazardous in industrial-strength bleach
products.
Skin Damage
Undiluted bleach is corrosive. Even domestic bleach damages skin tissues and
removes essential fats. During extended contact, small amounts of toxic chlorine
may enter the body through the skin. Industrial bleach carries a much greater
corrosive hazard, and protective clothing and eye protection are required.
Lung Damage
It is relatively easy accidentally to mix bleach, used in cleaning, with other cleaning
products--for example in the toilet, sink or drain. Mixing bleach with ammonia is
particularly hazardous, releasing chlorine gas, ammonia gas and chloramines.
These gases are caustic and irritating, and inhalation damages the lungs and nasal
passages. Exposure to high concentrations of ammonia gas for longer than 15 to 30
minutes can lead to irreversible damage, even death. Because chlorine gas is water-
soluble, it forms hydrochloric or hypochlorous acid upon meeting moisture in the
mucus membranes, eyes and mouth. In the lungs, acid damage results in pulmonary
edema (release of fluid into the tissues), causing breathing difficulties. Chloramines
cause similar breathing difficulties and irritation to the eyes, nose, throat and skin.
These are the compounds that cause irritation in swimming pools.
Explosion
More likely to occur in an industrial than a domestic setting, ammonia mixed with
bleach in higher proportion may form nitrogen trichloride or hydrazine, both of which
are explosive. Exposure to hydrazine causes burning pain in the eyes, nose and
throat.
51. 51 | P a g e
Conclusion
Sodium hydroxide is produced at Global Heavy Chemicals Ltd. by membrane cell
technology process. Here the membrane used is bi-polar. The cell voltage is 2.6V
DC. The caustic concentration out from the cell is 33 wt%. The other product of the
industry is Chlorine, Hydrogen, Bleaching powder and Sodium hypo chlorite
(clotech). Explosion of Chlorine is very harmful for the employee and local being as
well as industry. So it should be controlled and minimized for better production by
insulation.
52. 52 | P a g e
References
1. Holman, J.P(1986) “Heat Transfer”, Sixth Edition, McGraw-Hill Book Co-
Singapore
2. Coulson, J.M. Richardson, J.F(1978), “Chemical Engineering Volume Six”, Third
Edition, Pergamon Press. Oxford.
3. Felder, R.M. Rousseau, R.W(2005), “Elementary Principles of Chemical
processes”, Third Edition, U.S.A
4. Perry, R.H., Green D(2003) “Chemical Engineers Hand Book”, Sixth Edition,
McGraw-Hill Book Co-New York
5. Peters M.S and Timmerhaus, K.D(1986) “Plant Design and Economics for
Chemical Engineers”, Fourth Edition, Ronald E. West. New York
