Pre-evaporation systems utilize low-level waste heat to evaporate weak liquor prior to the main evaporator train. Three heat sources are most often utilized in a pre-evaporator system: heat from a blow heat system, heat from a stand-alone concentrator, or heat from a continuous digester extraction liquor flash tank. The amount of evaporation achievable is dependent upon both the quantity of heat available and the number of effects in the system. Whenever a pre-evaporator is installed, its impact on the existing multiple effect evaporator system must be considered, in particular the effect of the higher feed solids.
Aside from using waste heat, one of the main advantages of a pre-evaporation system is condensate segregation. The pre-evaporator strips the bulk of the TRS and VOC's from the liquor resulting in an evaporator system condensate that is substantially cleaner.