The aim of this project was to enable the client to increase capacity of the caustic soda produced at Sasol Polymers. This was achieved using a two stage Falling Film Backward Feed Evaporator Plant. The caustic solution concentration was increased from 32% to a 50% solution with energy supplied by medium pressure steam. The plant capacity is in excess of 20 t/hr of 32% caustic feed solution.
The 32% feed solution is fed to the second effect evaporator where process vapour from the 1st effect provides the energy for evaporation. The evaporator is operated under a high vacuum which is provided by a two stage vacuum ejector and condenser installation, to keep the caustic liquor operating temperature at a design minimum. The caustic solution leaves the 2nd effect between 37 to 40% and is fed to the 1st effect evaporator, where medium pressure steam provides the energy input required for evaporation. The 1st effect operates under pressure to account for the boiling point elevation of the concentrated caustic solution. Plate heat exchangers recover energy from the product streams to maximise energy efficiency during processing. Process and steam condensates are returned to the client.