OPTIMIZATION OF BROWN STOCK WASHING USING ADVANCED PROCESS CONTROL, 18PaperCon

In the operation of brown stock washing, the main objective is to achieve a trade‐off between minimization of impurities in the pulp and reducing the fresh water consumption. A poor performance of the brown stock washers can significantly affect the stability of operation and result in increased operating costs due to higher energy requirement in evaporators and bleaching chemical consumption. An increased focus on the environmental impact, combined with a need for higher washing efficiency while reducing total cost of operation have resulted in greater emphasis on process optimization strategies in Kraft mills. In this paper, we present an optimization strategy based on multivariable model predictive control. The proposed control strategy involves computing optimal setpoints for (a) the dilution factor in each washing stage and (b) the defoamer charge to maintain the alkali/soda loss at the specified target while respecting the constraints on the wash liquor inventory i.e. the filtrate tank levels. This results in reduced usage of fresh water, a decrease in the alkali losses and an increase in the dry solids in the evaporation liquor, thereby reducing steam consumption in the evaporation plant. In this paper, we also present the application of this strategy at a mill in Europe. The implementation resulted in a 30% reduction in alkali losses, 20% reduction in fresh water consumption and a 12% reduction in steam consumption in the evaporation plant. 

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