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