Effective operation of the brownstock washing process can be extremely difficult. This difficulty is caused by the inherent difficulty 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.
Spartan Controls Advanced Solution’s Brownstock Washing Control Strategy stabilizes both carryover and the weak black liquor strength under varying process conditions. Our solution typically employs a model-based multivariable control strategy, typically 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-ins, 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.
Conventional brownstock washing control strategies have been offered to industry in various forms, with limited scope, robustness, and performance. Spartan’s Advanced Solutions team has developed a robust, optimal, multivariable brownstock washing control strategy that is completely developed within the DeltaV™ system. The strategy fully leverages the DeltaV MPC Plus block with its abilities to provide model based control with priorities.
Spartan’s Brownstock washing control solution was implemented at a mill in British Columbia. Over $700,00/year of chemical and steam savings were generated as a result of the project.