Caustic evaporation process design

Besides the choice of evaporator type, evaporation process design includes selection of the number of effects to be installed. This choice is primarily a matter of a classical economic balance between the cost of supplying more effects and the benefits they offer in energy consumption. As the preceding section made clear, the number of effects in a caustic evaporator is never very great. For technical and economic reasons, diaphragmcell evaporators usually have three or four effects and membrane-cell evaporators have two or three. 

Evaporation process design and the characteristics of caustic evaporators are covered in Section 9.3.3. The unit cost of steam is a major factor in deciding the number of effects to be used in evaporation. Costing of steam is often somewhat arbitrary, as when low-pressure exhaust steam is available or when a plant has its own cogeneration system. Reducing the consumption of cogenerated steam (Section 12.3.3) when more electrical energy must be purchased (or less sold) may save very little money. 

When manufacturing solid sodium hydroxide on industrial scale a caustic solution containing 50 per cent NaOH is evaporated. This is either done directly into the final product or by an intermediate concentration to a 75 per cent NaOH solution. For intermediate concentration a standard single-effect vacuum evaporator is used. As no salt is separated in the course of this evaporation process, the design of the evaporator is very simple. All pumps as well as the piping must be steam jaeketed to prevent the solidification of the highly concentrated liquor. 

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