Recycle Compensator Facilitates Rapid Parameterization of Proportional Integral/Derivative (PI/D) Controllers for Open-Loop Unstable Recycle Processes

Processes with recycle are quite common in the process industries. It has been pointed out that such processes often exhibit unusual dynamics, such as open-loop instability, which makes the task of designing a model-based controller for them challenging. In this work, a design procedure for rapid parametrization of simple internal model control (IMC)-based proportional integral/derivative (PI/D) controller for open-loop unstable recycle processes is proposed. In the proposed strategy, first, the recycle process model is decomposed into the forward and recycle path. Thereafter, a perfect/approximate compensator is designed for the global process. The final controllers for the global process are then parametrized using the compensated system model and implemented on the system. It was observed that the compensator was able to restore stability to the open-loop unstable recycle process model and simplifies its model order considerably. Simulation results obtained revealed that the recycle compensated system displays better closed-loop performance, in terms of set-point tracking and disturbance rejection. Furthermore, the proposed method results in a closed-loop system that is less sensitive to disturbance and has a smoother control signal with higher gain and phase margins.