|Caustic Soda (at outlet of
|43 ± 1 %
|Caustic soda flakes
|Chlorine Gas (at outlet of
|99.5 Vol.% (dry base) (Excluding CO2,
0.5% vol (dry base)
|Hydrogen Gas (at outlet of
|99.9 Vol.% (dry base)
0.03% vol (dry base)
|30 – 33 % (By Weight)
I. Brine Purification Process
Brine for Ion Exchange Membrane Chlor-alkali process (IM process)
is prepared by dissolving crystal salt into the return brine
from the electrolyzers, and is purified with chemicals in
order to precipitate the impurities of raw salt. IM process,
however, requires a little more finely purified brine, and
this is performed by pre coated filter and ion exchange resins.
I-1 Primary Brine Purification
The return brine is fed from top to the bottom of Salt Saturator
and is saturated with Salt while the brine flows upwards through
the salt bed in the saturator. Crystal salt is continuously
supplied to the top of the saturator by salt conveyor. Temperature
of the return brine is controlled by Return Brine heater so
as to maintain the saturated brine temperature within 60-65
Deg.C. The saturated raw brine overflows from the saturator
into the Reactor by gravity flow. Chemicals, such as caustic
soda, barium carbonate and sodium carbonate, are added to
the saturated raw brine flow. Dissolved impurities of raw
brine (Ca, Mg and SO4) react with these chemicals and precipitate
as CaCO3, Mg(OH)2 and BaSO4 from the solution in the reactor.
This suspending solution is sent to the center well of Brine
Clarifier and then is separated to sediment and solution.
A part of sludge (Sediments collected at the bottom of Clarifier)
is recycled back to the Reactor in order to form big size
particles of sediment. Flocculent is also added to the stream
of suspending solution to make bulky sediment. Clarified brine
overflows from Brine Clarifier to Clarified Brine Tank. The
sludge from the bottom of the clarifier is filtered in the
sludge filter. The solid sludge is disposed of in suitable
I-2 Secondary Brine Purification
Two Candle precoat type ceramic filters are installed;
While one is on-lined, the other is off-lined for the back
washing. Alpha-cellulose is used as precoat material and
also body feed materials for the brine filtration. Back
washing of the filter is carried out to renew the coating
for the filter elements.
Filters output is collected into Filtrated Brine Tank.
Suspended Solid in this solution is normally kept less than
0.5 PPM (Max. 1.0 PPM).
2) Fine purification by ion exchange resin
Three Ion Exchange Resin Columns are installed; Three columns
are used something like a merry-go-round. While two are
on-lined in series, the rest one is off-lined for regeneration.
The primary column is designed to remove all multivalent
caution and the secondary column is considered as a safety
back up. Regeneration of resin is done by Hydrochloric Acid,
Caustic Soda and D.M.Water. This switching of the columns
and regeneration are carried out automatically according
to the pre-determined time table. Thus purified brine is
transferred to Purified Brine Tank.
II. Electrolysis Process
Electrolysis section is composed of electrolyser and associated
Electrolyzer is composed of a number of bi-polar type cell
frames with metal anode and the activated cathode, the ion-exchange
membrane, press unit for mounting cell frames, sub-headers
for feeding purified brine and catholyte, sub-headers for
discharging electrolysis products, hoses for connecting
cell frames with sub-headers.
2) Anolyte Circulation
Purified brine is fed to feed brine inlet sub-header. Anolyte
(dilute brine) and chlorine gas are collected to anolyte
outlet sub-header, and then flows into Anolyte Separator.
The diluted brine collected in depleted Brine Tank is sent
to De-Chlorination Tower for removal of chlorine gas. Chlorine
gas generated in Electrolyzer is separated in Anolyte Separator
and sent to Hydrochloric acid furnace.
3) Catholyte Circulation
Catholyte is circulated through cathode compartment of
cell frames to Caustic Soda Tank by pump, and catholyte
is taken out from catholyte separator to caustic soda tank
and sent out to caustic storage section. To keep the concentration
of caustic soda at designed level, demineralized water is
fed to catholyte inlet sub-header. Hydrogen gas generated
in Electrolyzer is separated from the catholyte in catholyte
separator and sent to Hydrogen gas cooling and compression
section. Heat generated at electrolyzer is removed by cooling
water at Catholyte cooler.
III. HCl Synthesis
The HCl Synthesis unit consists of combustion furnace fitted
with absorbers. The chlorine gas reacts with Hydrogen to form
HCl gas which is cooled and absorbed in DM Water. The flow
rate of DM Water is adjusted to obtain 30 - 33% HCl Solution.
The flow rates of Chlorine and Hydrogen is controlled by controllers.
Safety interlocking provisions are made which get actuated
by flame protection device. The Hydrochloric Acid Solution
is collected in a product tank from where it is pumped to
HCl Storage Tank.
IV. Cl2 Gas Cooling & Drying
Chlorine gas cooling section is composed of Primary Chlorine
Cooler, Secondary Chlorine Cooler & Chlorine Gas Mist
Eliminator. Chlorine gas coming from Electrolyser at 90 Deg.
is saturated with water vapors & contains traces of Sodium
Chloride. Chlorine gas is first cooled to 40-45 Deg.C in Primary
Chlorine Cooler (Shell & Tube heat exchanger) with cooling
water. During cooling large quantities of water vapor gets
condensed which are collected in seal pot. Further, Chlorine
gas is cooled to about 15 Deg C in Secondary Chlorine Cooler.
Entrained water vapors & salt traces after Secondary Cooler
are removed in Mist Eliminator. Small quantity of process
water is added in chlorine line in order to dissolve entrained
After this, Chlorine gas is introduced to Drying Tower consisting
of trays section and packed bed section. 98% Sulphuric Acid
is fed on the top of trays by means of metering pumps. The
acid gets diluted to 78% and 78 % H2SO4 is continuously circulated
over the packed section. The heat of dilution in dissipated
using chilled water in a cooler installed in the recirculation
Excess 78% H2SO4 produced during the drying process is treated
to remove the dissolved Chlorine before sending to the main
Chlorine gas leaving the drying tower top is then passed through
a dry chlorine filter in order to remove the remaining acid
mist for the protection of the downstream compressor.
V. Compression and Liquefaction
The dried Chlorine gas is compressed to 3.5 -4.5 kg/cm2 using
a centrifugal compressor. The compressor suction pressure
is controlled by recycling a part of the compressed gas.
A part of the compressed Chlorine gas is liquefied using
a Freon refrigeration unit. The liquid Chlorine from the liquefier
is sent to storage.
The unliquefied sniff gas from the liquefier in sent to the
HCl synthesis unit under pressure and flow control. Excess
sniff gas, if any, diverted to the waste air system.
VI. Waste Air Dechlorination/Hypo Section
Sniff Chlorine gas from chlorine facilities & leaked
chlorine gas are sucked to WAD section by WAD blowers. Absorbent
caustic soda solution is prepared in Caustic Soda tank by
dilution of 32% Caustic lye by proper adjustment of caustic
soda concentration with D.M.Water.
WAD system is a double stage system comprising of WAD Tower
Stage I & WAD Tower Stage II (packed towers) and a Polishing
Tower. Chlorine from Waste air is absorbed in these towers
with circulation of caustic soda by pumps.
Sodium Hypochlorite solution is circulated between Waste
air dechlorination tower tank and WAD tower by circulation
pump, through a chilled water PHE to remove heat which is
generated by chlorine absorbing reaction, till the conc. of
caustic soda falls from 150 gpl to 10 gpl.
VII. Hydrogen Handling Treatment and Bottling Plant
Hydrogen gas from electrolyzer is first cooled and passed
through Mist eleminator for removal of any caustic soda and
sent to HCl plant. Balance hydrogen is sent to Hydrogen bottling
plant for further treatment, compression & bottling.
VIII. Caustic Concentration Unit
32 % cell liquor produced from the Electrolyser is evaporated
by heating with steam in a triplicate effect evaporator to
produce 48% caustic lye for sale.
IX. Caustic Flaking Unit
The 48% caustic lye produced from the concentration unit
is heated to 4000 C and the molten Caustic is cooled in the
form of flakes.
LIST OF EQUIPMENT
||Primary Brine Purification
||Secondery Brine Purification
||Chlorine Gas Handling
||Chlorine Drying, Compression, Liquification
||Waste Air Dechlorination
Production Capacity For: 35 TPD
|a) Raw Material
|| 1. Salt (as 100% NaCl)
2. Electric Power Electrolysis
4. Sodium Carbonate
5. Sodium Hydroxide (100%)
6. Sodium Sulphite (100%)
Ion Exchange Resin
8. Alpha Cellulose
9. Hydrochloric Acid (as 100% HCl)
|| 1. DM Water
2. Cooling Water (Circulation rate)
4. Process Water