Controlling Brown Stock Washing Production Rate Changes, 1998 Engineering Conference Proceedings

This paper compares methods of controlling brown stock washing flows during production rate changes. Wash water flow to a brown stock washing system affects both pulp washing loss and black liquor evaporation demand. The complexity of brown stock washing systems has increased with the application of new technology, the increasing demands for improved washing and the inclusion of oxygen delignification systems. Storage and mixing tanks create process lags and dead-times which need to be considered when controlling wash water flows. An optimum operating point exists, which corresponds to the minimum of the combined costs associated with washing loss and evaporation. To ensure operation is sustained as close to this optimum point as possible, a system requires stable control of stock flows, tank levels, feed-forward dilution factor control, as well as feedback control based on washing loss measurements. Sufficient filtrate surge capacity is recommended to balance accumulations in pulp storage tanks, especially during production rate changes. The impacts of production rate disturbances and the consequences of specific control strategies are demonstrated with dynamic simulations of a brown stock washing system. Various methods of controlling inter-stage filtrate flows are compared. The methods studied range from simple feed-forward PID control, to more complex filtrate management strategies including multi-variable fuzzy logic control.