Optimization of a brown stock washing line

Kraft pulp washing removes soluble impurities from a pulp suspension, recovers cooking chemicals and incinerates organics for energy (steam and electricity). The parameters used to describe the performance of washing or its effectiveness fall into two categories: wash loss and the dilution factor (DF). The performance of brown stock washing has effects on many sub-processes like evaporation, oxygen delignification, bleaching, and wastewater treatment. The washing results can have the positive or negative effects of these sub-processes. It is important to find the correct optimal operating point for washing because without effective washing the chemical pulp production is not economically viable. The objective of this study was to improve the efficiency of a certain Scandinavian chemical pulp mill's brown stock washing line. The information was collected by using online devices (a refractometer and a conductivity meter) and the whole line was controlled by an upper level controlling system. The results indicate that it is possible to reduce the amount of brown stock washing loss by controlling the DF of the entire brown stock washing line, by torque control of the washers, and by real time wash loss measurements. Simultaneously, the amount of used wash water is decreased, significantly improving the economic efficiency of the fiber line, especially of the evaporation and bleaching plant. Real time dissolved dry solids (DDS) measurements proved useful for controlling and monitoring the wash loss and water consumption.