Process selection
Introduction to process selection
General chlor-alkali process
For determining the most appropriate process to produce the specified amount of
chlorine (200,000 tonnes/year), there were only three chlor-alkali processes that were
considered with intent these being; diaphragm, mercury and membrane cell
electrolysis. All three processes followed similar concepts and involved the electrolysis
of sodium chloride solution.
The electrolytic processes all produce chlorine gas, hydrogen gas and caustic soda of
which chlorine is the primary product. However, since hydrogen gas and caustic soda
are both valuable and profitable chemicals, they along with chlorine will undergo
concentration and purification steps to increase the purities of the products and thus
their value.
Although each of the processes considered follow similar concepts with respect to their
process, their method of keeping the chlorine produced at the anode separate from the
caustic soda and hydrogen produced at the cathode, differ.
In each of the three electrolytic processes, a saturated brine solution enters the
electrolyser and by the action of direct electric (DC) current, chlorine ions are converted
to chlorine gas. The overall process reaction is as follows;
2 NaCl+2 H 2 0 →Cl 2+ H 2+ 2 NaOH
In all three methods chlorine gas is produced at the anode by the splitting of NaCl;
−¿¿
NaCl → Na +¿+Cl ¿
2 Cl−¿→Cl ¿ 2
Hydrogen gas and caustic soda (NaOH) are both produced at the cathode;
−¿¿
H 2 O → H + ¿+OH ¿
2 H +¿→ H 2 ¿
−¿→NaOH ¿
Na+¿+OH ¿
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Process selection
The first operation as always in the chlor-alkali industry is to produce purified brine. As
can be seen on each of the process flow diagrams below, the first step is the saturation
of brine from raw salt and water. Precipitants, usually NaOH and CaCO 3, are added to the
brine solution to remove any excess ions before undergoing filtration to remove sludge.
Brine is then pumped into the electrolyser.
Weak brine is produced in the electrolyser and so to improve overall efficiency,
depleted brine is pumped out of the anode and into a de-chlorination process before
entering a brine re-saturator where it combines with the fresh brine stream.
Chlorine gas is produced in the anode and exits from the top before being cooled, dried,
compressed, and liquefied and finally evaporated.
While at the cathode hydrogen gas is produced and is cooled and compressed after
leaving the electrolyser. At the same time, caustic soda is produced and is usually
concentrated to produce a purer product that makes it more profitable.
Diaphragm cell process
Diaphragm cell electrolysis is one of the three common electrolytic processes adopted
by the chlor-alkali industry. In this particular process a permeable asbestos based
diaphragm is used to separate the anode from the cathode products.
Despite it being used as a separator material, asbestos is a toxic material and a proven
carcinogen that has the potential to cause lung cancer, mesothelioma and asbestosis.
Consequently, most types of asbestos that has been previously used in Europe for a
variety of applications have been banned by the EU. There have been efforts to exclude
the use of asbestos in the production of chlorine however due to a lack of safe
alternatives and its operation being undertaken in a closed system on site, asbestos
diaphragm electrolysis continues to make a large contribution to the world’s chlorine
production.
The asbestos diaphragms that are required for the process are usually dispersed in a
bath as slurry. With the use of a vacuum the asbestos is then deposited onto the
cathodes. This process is used to form the diaphragm membrane.
The main issue with respect to the quality of the products produced in this particular
process is the composition of the caustic soda produced. Due to the specifications of the
asbestos diaphragm, along with sodium ions and water, the brine that enters the anode
migrates through the diaphragm and into the cathode compartment. As a result, this
process generally produces a caustic brine solution comprising of 11-12% NaOH, 14%
NaCl and a small percentage of sodium chlorate and thus it is essential that it is
concentrated. An evaporative process is used to increase the concentration of the
caustic soda to 50% NaOH with up to 1% NaCl. The salt separated from the caustic brine
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Process selection
is then pumped back into the brine re-saturator unit, before re-entering a heat
exchanger.
The brine temperature is increased due to the hot hydrogen gas that enters from the
cathode compartment of the electrolyser. Oxygen is then removed from the hydrogen
gas coming from the heat exchanger and so the resulting brine is ready to enter the
electrolyser while the hydrogen gas is cooled, compressed and ready for storage.
The main advantage to the process is the fact that the quality of brine required is not as
high as required in membrane cells
Issues with the process
Diaphragm electrolysers typically consume 2550 kWh per tonne of NaOH, which is 15%
lower than in mercury cells (3150kWh). However, the overall energy consumption for
the process is significantly increased when steam requirements for the caustic soda
concentration is taken into account (since only 12% NaOH caustic soda comes out of the
cell while 50% is required). Hence, this value is increased to 3260kWh which
automatically makes this process the most energy inefficient.
Additionally, the chlorine produced from the anode in the cell is typically 98% pure
(due to the oxygen content in the chlorine) which is significantly less than the required
target of 99.9%, therefore the treatment required to improve the concentration will
result in increased energy costs.
Another disadvantage to this process is the short life the diaphragm membrane
processes. Deterioration of this cell has the adverse effects, the main being the inability
of the membrane to resist back migration of OH- ions from the cathode into the anode
compartment.
Furthermore, due to the safety and environmental hazards asbestos pose, there are high
costs associated with the disposal of the asbestos membranes.
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Process selection
DIAPHRAGHM PROCESS
FLOW DIAGRAM
Membrane cell process
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Process selection
A membrane cell is a popular chlor-alkai process in which the anode and cathode
compartments are separated by an ion exchange membrane. This membrane allows
only sodium ions and small amounts of water to pass through from the anode to the
cathode. And thus migration of chloride ions and hydroxyl ions from the cathode to the
anode is prevented.
Description of process
The raw materials in this process as in the other two are salt, water and electric current
(DC). Salt is dissolved with fresh water stream in a saturator along with depleted brine.
Impurities from the salt are chemically precipitated and filtered out in the primary
brine purification steps. The filtered brine then enters an ion exchange membrane
where the remaining calcium and magnesium ions are removed.
The salt that enters the electrolyser is decomposed to produce chlorine gas. This
chlorine is then cooled, dried, compressed, liquefied and evaporated. The remaining
sodium ions in the solution migrate across the membrane to combine with hydroxyl
ions formed in the cathode. This results in caustic soda being produced at
approximately 32% NaOH. The caustic soda is cooled and evaporated to increase the
concentration to 50% NaOH, while the hydrogen ions in the cathode combine to form
hydrogen gas that leaves the electrolyser. This hydrogen gas (usually 99.9% conc) is
cooled and compressed.
The depleted brine from the anode is treated (de-chlorinated) before being sent back to
the brine saturation unit.
Advantages and Disadvantages of Membrane Cells
One of most significant advantages to this process is that fact the overall operation of the
plant requires less energy than mercury and diaphragm cells. 2520 kWh per tonne NaOH is
typically consumed whereas both mercury and diaphragm cells use over 3000 kWh.
However, capital costs are higher since the membrane cells are relatively expensive.
Additionally, due to the nature of the cell, very pure brine is required.
As a result, membrane cells produced high quality caustic soda directly from the cell (32%)
in relation to the 12% produced from diaphragm cells. Therefore less steam is required for
the concentration of the caustic soda.
The chlorine gas produced directly from the electrolyser is at around 99.3% with the rest
comprising of oxygen. This is removed by liquefaction and evaporation.
The other main advantage of this process is that mercury and asbestos are not used and so
there are no adverse effects to people and the environment unlike those found in the
mercury cell and diaphragm cell industries.
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Process selection
MEMBRANE PROCESS FLOW
DIAGRAM
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Process selection
Reasons for deciding to adopt the membrane process
There were three chlor-alkali process that were considered; membrane, diaphragm and
mercury cell. All three processes were evaluated economically and in terms of product
quality:
Energy consumption
Undoubtedly, energy consumption is an essential factor to consider when a process
plant is to be design. Below is a table that compares the energy consumptions of the
three chlor-alkali processes:
Chlor-alkali process Energy consumption (of the electrolytic
cell)
(kWh per tonne NaOH)
Mercury cell 3150
Diaphragm cell 2550
Membrane cell 2400
Table: showing typical energy consumption data for Chlor-alkali cells
As can be seen on the table above, membrane cell electrolysis consumes the least energy
at 2400 kWh per tonne NaOH giving the process an instant advantage over the other
two.
Purity of products
The other important factor to consider is the purity of which the products leave the cell
at. This can also have economical implications especially when deciding to install extra
process units to purify the products. The table below shows the purity of which each
product leaves the cell for each of the processes:
Chlor-alkali process Purity of H2 (%) Purity of Cl2 (%)
Mercury cell 99.9 99.2
Diaphragm cell 99.9 98
Membrane cell 99.9 99.3
Table: showing typical data for products leaving the electrolytic cell for each process
As can be seen above, the membrane cell produces the purest Cl 2 at 99.3% which is our
primary product of concern.
Safety and environmental
As discussed above, mercury cell process includes steps to deal with mercury pollution.
This results in more installations of unit operations as well as it being a danger on site if
not controlled. This applies to asbestos where
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Process selection
Process description
Brine purification process:
Brine treatment is an essential step for chlor-alkali processes, especially for membrane
cell which have the most exacting brine specification due to the cells sensitivity to
chemical damage from impurities.
The sodium chloride for our particular process is available via rock salt. This rock
contains impurities in the form of magnesium chloride, calcium chloride and various
insoluble. In order to optimize high current efficiencies at the electrolytes, these
impurities must be removed by a brine purification process.
The first step in this process is brine saturation. Distilled water would be produced by
passing tap water through distillation and ion exchange operations. The rock would be
transported directly into the plant via convey belts. The brine saturation step would
take place in a saturation unit, where rock salt is mixed in with distilled water. In our
particular process, the depleted brine coming from the electrolyser is also fed into this
saturation unit. Since the depleted brine also contained undesirable impurities, a
decision was made to combine the fresh feed with the depleted brine feed;
consequently, installations of additional units were avoided.
The first step after brine saturation is purification. The stream coming from the brine
saturator unit contains the following impurities;
Impurities
HCL
HOCl
NaClO3
Cl2
NaCl
CaCl2
MgCl2
H2O
Insolubles
It is essential that impurities are significantly reduced before entrance to the
electrolytic cell. This is done by chemical precipitation which is considered the
primary treatment of brine.
Chemical precipitation of MgCl 2 and CaCl2 is achieved by the addition of NaOH and
Na2CO3 as follows;
Removal of MgCl2 with the addition of NaOH:
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Process selection
−¿→ Mg(OH )2 ¿
Mg 2+¿+2 OH ¿
Removal of CaCl2 with the addition of Na2CO3:
2−¿→ CaCO3 ¿
Ca2+¿+CO 3 ¿
The NaOH is pumped into the chemical precipitation tank in the form of caustic soda
directly from the membrane cell after being cooled. It was decided, for economical
reasons, that the NaOH produced from the caustic soda would be used for the treatment
of brine for MgCl2 removal. This caustic soda can be vented off before the caustic
concentration operation as 30.3% since higher concentrations are unnecessary and
expensive.
Hydrogen processing
One of the great advantages of modern chlor alkali technologies including the
membrane cell is the purity of Hydrogen gas that leaves the cathode (>99.9%).
However, it carries the disadvantage of it being produced at relatively small amounts
with high water content and low pressure.
The H2 that leaves the electrolysis cell is at 90°C, thus the first step for the processing of
hydrogen is cooling. Usually, the cooling of hydrogen will take place in a shell ad tube
exchanger or in a direct contact column. However, since the aim is to reduce the water
content of the hydrogen gas, a conventional shell and tube heat exchanger will be used
to cool the hydrogen gas.
As a result, cool water will be purchased and constantly supplied to the heat exchanger.
This temperature of the hydrogen gas will decrease via condensation. Consequently, the
hydrogen gas leaves the heat exchanger at 73°C since only a 17°C decrease can be
achieved in these conventional coolers.
The next step for the processing of hydrogen is to pass it through a drying process to
eliminate the excess water that has been picked up with the hydrogen from the heat
exchanger. This drying process will take place in a packed drying column that uses
sulphuric acid to absorb water
The step following the cooling process is compression of the hydrogen gas; this will
usually take place in a water ring compressor. This uses the input of air and water to
compress the incoming hydrogen gas from 1 bar to 3 bar pressure.
