CAUSTIC SODA

PROCESS DESCRIPTION

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 location.

I-2 Secondary Brine Purification

1) Filtration

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 equipments.

1) Electrolyzer

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 salt particles.

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 line.

Excess 78% H2SO4 produced during the drying process is treated to remove the dissolved Chlorine before sending to the main storage.
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

Production Capacity For: 35 TPD

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