Sweetening and Intermediate Liquor Impacts on Evaporator Performance, TAPPICon25

Black liquor evaporator performance is key for mill production and energy efficiency. Evaporator operational stability is likewise critically important. Feed liquor sweetening (the practice of mixing concentrated black liquor with weak black liquor feed) is a common practice to achieve operational stability when an evaporator system is prone to carryover of entrained liquor into the evaporator condensate. Intermediate liquor (45 to 50% black liquor solids) addition to the concentrator section of an evaporator train, to produce heavy product firing liquor, is also a common practice when older rising film evaporator systems, limited to 50% product solids, are still in operation at a facility. This paper investigates the impacts of sweetening and intermediate liquor addition on the performance of black liquor evaporators. A six-effect WinGEMS™ process simulation model is used to evaluate the impacts of these practices on solids profile, boiling point rise, steam pressure, economy, and evaporation capacity. Higher levels of feed liquor sweetening cause higher front-end steam pressure, lower economy, higher fouling risk, and capacity loss if steam pressure is limited. Likewise, higher levels of intermediate liquor addition cause similar impacts due to the increase in solids profile of the front-end effects. The findings underscore the importance of careful management of the liquor input streams to optimize evaporator performance.

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