Seasonal model based control of processes with recycle dynamics

Plants with recycle systems have complex dynamics and can present challengi ng control problems. In this paper, the dynamics of recycle systems were ex amined 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 recycl e fraction must be carefully included. Also, the overall recycle dynamics c an be represented by seasonal time-series models. The overall transfer func tion for the recycle system might be nonrational, thus complicating control ler design. Three methods of applying model-based predictive control to rec ycle processes were compared. The first method was a Taylor series expansio n used to linearize the process model, the second method used a seasonal ti me-series model to represent the process, and the third method was a recycl e compensator that removed the effect of recycle dynamics. Although all thr ee methods gave good control, practical considerations determine the method that should be used.