A comparison of bleach plant control strategies

More and more kraft pulp mills are taking advantage of on-line kappa analyzers for controlling the front-end of the bleach plant. Different control strategies, based on kappa number measurement, have been proposed for the first bleaching stage. This paper compares three control strategies: constant kappa factor, constant outlet kappa number and constant percent kappa number reduction. The comparison between these three control strategies is done on the basis of kappa number at the outlet of the first tower, bleaching chemical charge and residuals. The effect of changes in tower residence time and temperature on the control strategy performance was also studied and the pros and cons of each strategy are discussed. The study showed that the constant outlet kappa number control strategy is the most aggressive one. It guarantees uniform pulp quality in terms of lignin content but chemical cost is higher than for the other two strategies. Constant kappa factor control is the least expensive in terms of chemical usage but pulp quality varies significantly when the incoming kappa number, tower residence time, or temperature is changing.