Evaporator Upgrading Experience at Irving Pulp and Paper, Limited (Part I), 1998 International Chemical Recovery Conference Proceedings

In 1994. Irving Pulp and Paper Ltd. decided to upgrade the evaporation system as a part of millwide environmental improvement project. The existing LTV S-effect evaporation and single PFR concentrator was upgraded by adding a FF #6 evaporator and a surface condenser both equipped with condensate segregation baflles. external preheaters. a high solids crystallizer and a complete stripping unit including a column with 25 bubble cap trays. feed/product heat exchangers and a steam generator as reflux condenser. The new evaporation system was designed for 445000 kg/h of 15% solids feed weak black liquor. product strong liquor solids of 72%. total steam economy of 4.5. segregation of 75% of BOD in 15% of total evaporator’s condensate and over 90% BOD removal efficiency in stripping unit. After startup of new evaporation and stripping systems it was found that the unit performance met the designed capacity. steam economy, and stripping efficiency. However. instead of 75% BOD segregation. only 50% of BOD was segregated into the foul condensate. Several process and equipment improvements were implemented to achieve the designed performance such as upgrading the steam ejectors to generate a higher vacuum. installing a recirculation pump on the cooling water line to the surface condenser for better control of condensate segregation, adding mesh pads on top of the chevron mist eliminators of the new #6 FF effect to reduce the liquor carryover. upgrading the recircualtion pumps of the PFR concentrator to extends the concentrator runtime between the washes, reducing the # of plates of the overdesigned feed/product stripper plate heat exchanger to reduce the plates fouling. and using stripped condensate at causticizing unit. Despite unit performance slightly improved with implementation of above changes, the BOD segregation was still a problem. An extensive testing program on the condensate from each effect showed that most of the BOD in the clean condensate is due to the evaporation of the black liquor organics which have boiling points higher than that of liquor volatiles such as methanol and TRS. In conclusion. it was decided to utilize new processes to reduce the BOD of the clean condensate which will be published in part II of this paper.