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.