Reliable, single evaporator train cuts steam usage by more than half

Crown Zellerbach's Bogalusa, LA, Mill was using four small LTV (Long Tube Vertical) evaporator trains to concentrate black liquor from 14 to 50%. The evaporation capacity of each train was about 175,000 lb/hr. Some had been in service for up to 50 years and all had relatively poor evaporation efficiency (3-4 lb evaporation/lb steam) compared to today's standards. Engineers wanted to replace the evaporators to improve system reliability and reduce maintenance and knew this could also result in lower overall energy costs for the mill. A single, 8-effect, tubular, falling film evaporation system with an evaporation capacity of over 1,000,000 lb/hr was installed at the mill. The system employs a single centrifugal pump on each effect that serves the dual purpose of recirculating liquor from a sump through the heat exchanger and effecting inter-state liquor transfer. The evaporator design includes a preheater in each of the falling film heat exchangers. Physically the preheater is a group of tubes located in the center of the evaporator heat exchanger tubesheet. Liquor is pumped upward through the preheater section and is discharged into the top liquor box. The 8-effect falling film evaporator system with a steam economy of 8.4 lb evaporation/lb steam savesmore » enough steam to eliminate the need for additional boiler capacity. It frees steam for other uses or allows less total steam generation. Operation of the tubular falling film evaporator system has been a delight - start-up began on a Friday with the old evaporators still in operation. The unit came up and ran in, reaching a level of confidence within 72 hours sufficient to shut the old system down permanently. In terms of the labor required to run it and the steam it takes to do the evaporation the system is a real money saver.« less