CAUSTIC EVAPORATION PLANT
The purpose of the plant is to concentrate the 30 % caustic coming out of the electrolysers to 50% caustic lye. This concentration is achieved by three multi effect evaporators. These evaporators concentrate the 30% caustic to 36 %, 42 % and eventually 50 % caustic lye. Multi effect evaporators are usually fed by 3 possible feed systems:
1. Forward feed system: where feed and the heating medium both enter the first evaporator, in other words, co-currently.
2. Backward feed system: In this system the feed enters counter-currently with respect to the heating medium.
3. Mixed feed system: In this feed system the feed enters from the first evaporator, and the fresh heating medium may be introduced anywhere between the evaporators.
The caustic evaporation plant under consideration employs the backward feed system. The evaporators are shell and tube type. The caustic flows in the tube side and steam circulates on the shell side. Glass wool insulated steel lines are used for circulation of steam. The caustic is carried in stainless steel lines up to a concentration of 36 %, after which glass wool insulated Titanium lines are used for it. Steam is used as the heating medium. 30 % caustic from the electrolysers is preheated to 90 °C by the 50 % caustic coming out of the CEP plant. Steam coming from 2nd evaporator is fed into the 1st one, and caustic is concentrated to 36 %. It is sent to a vapour separator, which separates the vapours from the caustic which are cooled and sent for recovery.
The vapours separated from the 3rd evaporator are used to pre-heat the 36% caustic before it enters the 2nd evaporator; it is heated up to 110 °C. Out coming steam from the 3rd evaporator is circulated inside the 2nd one and the caustic is concentrated to 42 %, and the temperature to 120 °C. The vapour is separated and the used up steam is re-used in the 1st evaporator.
The steam and caustic from the 3rd evaporator pre-heat the 42 % caustic to 160 °C for more efficient concentration. Fresh steam enters the evaporator and concentrates the caustic to 50 % caustic lye. The out coming caustic is cooled down by using its heat to pre-heat the caustic feeds to the other evaporators. It is finally cooled by cooling water lines to 45 °C caustic lye. This is either sold as a product or sent for further concentration in the Fusion Plant.
