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EKB 1013 CHEMICAL PROCESS INDUSTRIES
Chlor-alkali IndustryProduction of Caustic Soda & Chlorine
By:Dharisinee Jegathisan
(1000043)
Production Process of Caustic Soda and Chlorine by Electrolysis
What is electrolysis??
Process of breaking down a compound
into its
constituent elements by passing
electricity
through it.
Basically chlorine and caustic soda is
produced
by electrolysis of salt water (brine).
In this process there are three types of
electrolytic cell:
* Mercury Cell * Diaphragm
Cell
* Membrane Cell
• Salt is first dissolved in the dissolution
tank.
• The obtained saturated brine(salt) is
then sent to a purification tank to
remove impurities.
• Industrial water is also purified before entering the cell.
• This shows the need of electricity in electrolytic cell.
Mercury Cell Process• Electric current flowing through the cell decomposes
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 anode
assembly and discharged to the purification process.
• As it is liberated at the surface of the mercury cathode
and the sodium immediately forms an amalgam (a
'mixture' of two metals)
• The liquid amalgam flows from the electrolytic
cell to a separate reactor, called the decomposer,
where it reacts with water in the presence of a
graphite catalyst to form caustic soda (sodium
hydroxide) and hydrogen gas.
• The sodium-free mercury is fed back into the
electrolyser and reused.
The reaction in the electrolyser is:
2 Na+ +2Cl-+ 2 Hg → 2 Na-Hg + Cl2(g)
The reaction in the decomposer is:
2 Na-Hg + 2 H2O → 2 Na++ 2 OH- + H2 (g) + 2 Hg
View animation!
View Animation!
Diaphragm Cell Process• In the diaphragm cell process, there are two compartments
separated by a permeable diaphragm.
• Brine is introduced into the anode compartment and flows
into the cathode compartment.
• Similarly to the Membrane Cell, chloride ions are oxidized at
the anode to produce chlorine, and at the cathode, water is
split into caustic soda and hydrogen.
• The diaphragm prevents the reaction of the caustic soda
with the chlorine.
• A diluted caustic brine leaves the cell.
• The caustic soda must usually be concentrated to 50% and
the salt removed. This is done using an evaporative process.
Membrane Cell
Membrane Cell Process
• This technology uses water-impermeable ion-conducting
membrane.
• The membrane is made of a special resin which permits
cations (positive ions) to pass through.
• The anode chamber of a membrane electrolytic cell is filled
with brine, and the cathode chamber with water.
• The brine in the anode chamber contains sodium (Na+)
and chloride (Cl-) ions.
• These ions migrate when a current is applied: the
positively charged sodium ions pass through the
membrane to the cathode chamber, while the negatively
charged chloride ions are discharged on the anode surface
to form chlorine gas (Cl2).
• Water in the cathode chamber partly dissociates into
hydrogen (H+) and hydroxide (OH-) ions.
• The hydrogen ions capture electrons on the cathode
surface to form hydrogen gas (H2).
• The hydroxide ions are attracted to the anode, but
blocked by the membrane, and react with the sodium ions
from the anode chamber to form caustic soda (sodium
hydroxide, NaOH). View Animation!
• The chlorine is washed and cooled to remove salt,
and further dehydrated before being delivered or
liquefied.
• Chlorine is liquefied and stored low temperature. The
liquid chlorine from the bulk tank can be used as a
feedstock.
Caustic soda is further concentrated in a vaporizer
to a concentration of about 50% for delivery.
• In the mercury cell process, 50% caustic soda is
obtained directly from the decomposers.
• The caustic soda is normally pumped through a
cooler, then through a mercury removal system and
then to the intermediate and final storage sections.
In some cases the caustic is heated before filtration.
The most common method for removal of mercury
from caustic soda is a plate (or leaf) filter with carbon
coat.
• In the case of diaphragm and membrane
technologies the caustic soda is concentrated by
evaporation before final storage.
It never gets contaminated with any sodium chloride
solution due to presence of water-impermeable ion-
conducting membrane.
The caustic soda is more quality.
Operates at low voltage.
Energy efficient.
Less environmental impact
Membrane Cell is Widely Used
Safety, Health, Environment Considerations
Safety
Wear goggles all the time to avoid contact
with gases.
Handle the gases with care in loading area
and also storage area as it is explosive.
Always follow the safety hazards.
Health
Eye irritation
Breathing difficulties
Asthma
Irritates respiratory system
Can be fatal
Environmet
Air Pollution
Water Pollution
Prevention
Treat gases before emission
Have a proper disposal of industrial waste
Practice to recycle and reuse the gases
End Products