Energy economy Multiple-effect evaporators

The maximum number of effects in a multiple-effect evaporation system and a comparison of energy economy of multiple-effect evaporation with that of vapor-compression distillation are discussed. 

Energy economy and evaporative capacity are the major measures of evaporator performance. When evaporating water with steam, the economy is nearly always less than 1.0 for single-effect evaporators, but in multiple-effect evaporators it is considerably greater. Other performance variables to be considered include product quality, product losses, and decrease in performance as scaling, salting, or fouling occurs. 

Multiple-Effect Evaporation. The discussion of brine evaporation in Section 7.1.5.2A showed how installing a number of effects and using process vapor as a source of heat reduces the amount of energy that must be added to the process. The boiling point rise (BPR) of brine solutions makes some of the available temperature potential useless as a driving force for heat transfer. This limits the number of effects and the achievable steam economy. Similar limitations exist in caustic evaporation but are much more severe. 

Energy economy obtained by multiple-effect evaporation can often be equalled in a single-effect compression evaporation system. To achieve reasonable compressor costs and energy requirements, compression evaporator systems must operate with fairly low temperature differences, usually from 5 to 10°C. As a result, large heat transfer surfaces are needed partially offsetting the potential energy economy. 

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