PRE-EVAPORATION CHOICES For INCREMENTAL EVAPORATIVE CAPACITY In EXISTING MILLS, 2017PEERS

Evaporation of weak black liquor from the pulp mill to a firing liquor concentration at the recovery boiler is one of the essential operations in an integrated chemical pulp and paper mill. For many mills, pressure to increase capacity over the years beyond the original evaporator design rates has resulted from spent brine addition from the tall oil plant, increased saltcake by-product from chlorine dioxide plants, recovered Na/S streams from chloride removal systems, pond reclamation, changes in sweetening liquor practices, and higher flows/weaker concentrations of filtrate from brownstock washing as higher dilution factors have been used to compensate for overloaded washers.

One method for providing additional capacity margin is evaporation prior to the traditional evaporators and concentrators, or pre-evaporation. Water removal from the liquor in this manner is not widely practiced in the industry, but it has been successfully accomplished in many pulp and paper mills. Pre-evaporation offers an economic alternative to a new evaporation train or a costly retrofit of an existing set, and it can be accomplished in numerous ways to suit the unique conditions of the mill. Direct use of low pressure steam or waste vapor and thermal vapor recompression of steam are choices to drive one or more pre-evaporator bodies. Another option is vapor compression with mechanical means. All three pre-evaporation choices are investigated in this paper which will compare the consumption of steam, electricity, and cooling water and the capital investment necessary for each technology option.  

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