Brown Stock Washing Optimization l Industries l Spartan Controls

Effective operation of the brown stock washing process can be extremely challenging.

This difficulty is caused by the inherent challenge of filtrate level control, the coupling between the filtrate tank levels, the multiple production rates within the process, and process problems such as foaming. Another difficulty is caused by the large time delay between changes in wash flows and the resulting unbleached stock cleanliness.

Further, the process may be subject to changing process limitations such as evaporator capacity. Under-washing the pulp drives up bleaching costs and results in high soda loss, but over-washing overloads the evaporators and creates large steam costs. Ideally, the system should run to an operating point that minimizes this total cost. In practice, while this operating point is often achievable, the control system must also be robust enough to cope with process upsets and limitations, and mismatched production rates.

Our team has developed a robust, optimal, and multivariable brown stock washing control strategy.

Conventional brown stock washing control strategies have limited scope, robustness, and performance. Our brown stock washing control strategy is completely developed within the Emerson DeltaV™ system, fully leveraging the DeltaV™ Model Predictive Control (MPC) Plus function block to provide model-based control with priorities.

Our solution stabilizes both carryover and the weak black liquor strength under varying process conditions. It typically employs a model-based, multivariable control strategy, usually incorporating WBL solids measurements to ensure efficient operation at all times. This multivariable control is robust and flexible enough to handle evaporator limitations, digester shut-in's, and bypassed washing equipment while still optimizing the washing operation. It has been applied to a large variety of washing equipment including atmospheric diffuser washers, pressure diffuser washers, drum washers, and press washers.