Industrial implementation of an advanced bleach plant control system

Control strategies for the kraft bleach plant are designed to reduce production of off-grade pulp and minimize chemical consumption. This paper describes a new model-based control strategy that was implemented on the first two stages of a bleach plant that processes 800 metric tons/day of softwood kraft pulp. The new strategy-based on first-principale models-controls delignification during the chlorination stage (100% chlorine dioxide) and brightness after the first extraction stage. The model-based control strategy has increased the stability of the bleaching operation while remaining robust to process upsets. Chemical consumption is significantly lower than it was under the previous control scheme (compensated-brightness control), and standard deviation for DE kappa number dropped from 1.4 to 0.6, a reduction of 57%. Application: The proposed control strategy provides significantly better bleaching performance than the widely used compensated-brightness control strategy.