M.S. Sidhu, K.E. Kwok, and M.W. Foley
Caustic control, nonlinear PID, adaptive control, averaging level control, minimum variance control
Caustic solution is commonly used in pulp-and-paper mills for various processes, such as bleaching and pH control. It is usually purchased as caustic concentrate, which is diluted on site to the desired concentration using common mill water and is stored in a reservoir for mill utilization. The diluted caustic has to meet the desired specifications as variations in the caustic concentration have adverse effects on the process, such as poor bleaching and washing. Because direct measurement of caustic concentration is not available, the concentration is indirectly controlled by maintaining a certain mixing temperature that causes significant nonlinearity in the process. Another challenge in the process was the level limitations imposed by the storage tank system. A multistructured control strategy combining an adaptive PID controller and optimal averaging level control was successfully implemented in this industrial caustic dilution system. The overall process operation was significantly improved.
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