Chemical Process Industries

1. SODIUM HYDROXIDE
Prepared by- Mr. Vishal B. Thakare
(M.Tech, Chemical SVNIT Surat)
Assistant Professor PARUL UNIVERSITY
Vadodara
vishalbt88@gmail.com

2. SODIUM HYDROXIDE
Sodium hydroxide (NaOH), also known as lye and caustic soda is a
highly caustic metallic base which is a white solid available in
pellets, flakes, granules, and as 50% saturated solution.
Caustic soda and chlorine are produced as co-products by the
electrolysis of brine.
In India 80% caustic soda and more than 95% of chlorine produces
by electrolysis of brine.
During electrolysis chlorine is liberated at the anode and caustic
soda along with hydrogen is produced at the cathode.

3. TYPE OF CELLS
Cells which are used for production of caustic soda are
1. Diaphragm cell
2. Mercury cathode cell
3. Membrane cell

9. Brine solution flows through pipelines to a storage reservoir and
then through a brine treatment system.
Caustic soda, soda ash, and/or barium carbonate removes calcium,
magnesium and iron salts which would clog up diaphragms. This
purified, saturated brine (25-28% NaCl) is heated and electrolyzed in
a diaphragm cell.
The cell, operating at 45-55 % decomposition efficiency, discharges
a 10- 12% solution of caustic soda with about an equal concentration
of NaCl.
Multiple effect evaporation concentrates the cell liquor to 50%
NaOH solution.

10. The precipitated salt is separated, centrifuged, washed, and then
slurried with treated brine.
Salt separator overflow is 50% caustic soda product containing 2%
NaCl and 0.1-0.5% NaClO on a dry basis.
This commercial caustic grade can be evaporated to produce
saturated 73% NaOH liquor or fused to flake, granular or stick
caustic.
Purified grade can be produced by a combination treatment of
CaCO3 to remove colloidal Fe and liquid NH3 counter-current
extraction to take out chloride and chlorate impurities.

11. In a membrane cell a cation-exchange membrane separates the
anolyte and catholyte.
Brine is fed into the anode compartment where chlorine gas is
created and the sodium ion and associated water of hydration migrate
through the membrane into the catholyte.
Unlike the diaphragm cell process, the cation-exchange membrane
prevents the migration of chloride ions into the catholyte.
Depleted brine is discharged from the anolyte to maintain a
minimum NaCl concentration.
Water is electrolyzed at the cathode and strong caustic (32-35 wt. %)
is produced either by controlling the water addition rate directly to
the catholyte or by recirculating caustic to which water has been
added.

12. The membrane is the most critical component of this cell technology,
and current efficiency, cell voltage and hence energy consumption,
are greatly dependent on its quality.
Requirements for membrane separators are as follows:
(a) Durability under the conditions of chlor-alkali electrolysis. It
should have good mechanical properties and long term stability for
practical use.
(b) High selectivity for the transport of sodium or potassium ions.
(c) Negligible transport of chloride, hypochlorite and chlorate ions.
(d) Zero back-migration of hydroxide ion.
(e) Low electrical resistance.

13. To produce the solid salt needed for the mercury cells, the
centrifuged salt is added to a salt saturator where depleted cell brine
contacts the salt and provides a replenished feed to the electrolyzing
compartment of the mercury cell.
The wet chlorine gas from the anodes is cooled to remove water and
further dried in a sulfuric acid scrubber.
The gas is condensed by one of the following pressure-temperature
combinations: • High pressure (7-10 atms.), water cooling • Medium
pressure (2-3 atms.), refrigeration at —20°C • Low pressure (5-10
atm H20), refrigeration at —40°C

14. Mercury process
Advantages:
 Pure 50% sodium hydroxide solution (without evaporation)
 Pure chlorine gas
Disadvantages:
Higher voltage than with the diaphragm process and hence 10 to
15% higher electrical energy consumption.
More stringent brine purification requirements.
 Stringent mercury contamination avoidance measures required.
Diaphragm process:
Advantages:
Utilization of less pure brine
 Lower voltage than in the mercury process

15. Disadvantages:
 Sodium hydroxide produced is both dilute and chloride-
contaminated, evaporation required.
 Chlorine gas contains oxygen
 Rigorous measures required to avoid asbestos emission
Membrane process:
Advantages:
 Pure sodium hydroxide
 Electrical energy consumption only about 77 % of that of the
mercury process

16. No mercury or asbestos used.
Disadvantages:
 Sodium hydroxide content only ca. 33% by weight
Chlorine gas contains oxygen
Very high purity brine required
Present high cost and short lifetime of the membranes.
Diaphragam cells tvpicallv analyzes as 10-11% NaOH and 15-17%
NaCl. Membrane cells produce 30-33% NaOH. Mecury cells
produce 70% NaOH