Recycle Effect Index: A Measure to Aid in Control System Design for Recycle Processes

Abstract

Systems with material and/or energy recycles are quite common in the chemical industry. Modeling and DCS-level controller design are quite challenging for such processes because separation of the overall process response into a direct (forward-path) response and an indirect (recycle-path) response is often difficult. Even when this is possible, one needs to know whether a feedback-only control structure is sufficient or an advanced control structure that employs a recycle compensator is necessary. Using control loop performance assessment concepts, this paper proposes a quantitative measure-the recycle effect index (REI)-to help determine whether a simple feedback control system can provide the desired quality of control.