Constrained model predictive control of blow tank consistency

Abstract

Consistency control at the outlet of blow tanks and hi-density towers, the so-called double-dilution problem, is an area where further control development is needed. The task is to coordinate the manipulation of two input variables, coarse and fine dilution, to control the consistency, while simultaneously mid-ranging the fine dilution to keep it from saturating. This is made difficult by process nonlinearities, complex dynamics and tight manipulated variable saturation constraints. To address these problems, a model-based predictive controller (MPC) was applied to blow tank consistency control in a pulp mill. One of the unique features of the proposed MPC algorithm is that it is well suited to run on standard distributed control system (DCS) hardware of the type that already exists in most process plants. Several control strategies were tested, including MPC and the previously implemented scheme, with and without a ratio control scheme aimed at removing the dominant process nonlinearity. The results of a mill trial indicate that both controllers, when implemented as ratio controllers, give excellent disturbance rejection for production rate changes, keeping the consistency within 0.1% of target. However, MPC outperformed the conventional strategy in its response to setpoint changes and unmeasured disturbances, and was able to cope with actuator failure.