Seasonal Model Based Control of Processes with Recycle Dynamics

Abstract

Plants with recycle systems have complex dynamics and can present challenging control problems. In this paper, the dynamics of recycle systems were examined by analyzing a two-stage heating process with recycle. It was found that, when a positive feedback arrangement is used to represent the linear dynamics of systems with recycle, the recycle-path time constant and recycle fraction must be carefully included. Also, the overall recycle dynamics can be represented by seasonal time-series models. The overall transfer function for the recycle system might be nonrational, thus complicating controller design. Three methods of applying model-based predictive control to recycle processes were compared. The first method was a Taylor series expansion used to linearize the process model, the second method used a seasonal time-series model to represent the process, and the third method was a recycle compensator that removed the effect of recycle dynamics. Although all three methods gave good control, practical considerations determine the method that should be used.