Design and Control of Recycle Processes in Ternary Systems with Consecutive Reactions

Abstract

Abstract This paper explores some of the challenging problems associated with the dynamics and control of recycle systems. The process considered consists of a reactor in which consecutive reactions A → B →C occur to produce a desired product B and less valuable product C. The reactor effluent is fed to a separations section consisting of two distillation columns with recycle of the unreacted component A from one of the columns back to the reactor. Because the production of undesirable component C must be kept low, the optimum design is a small reactor with a high concentration of component A and a large recycle. A large recycle means high energy and capital costs. But the improved yield of component B justifies a large recycle stream. Changes in fresh feed flow rate are shown to have the most dramatic effect on the system when a conventional control system is used. Increasing fresh feed by 5% can lead to 100% increases in the flow rate of the recycle stream. A variable-volume control strategy is proposed to eliminate this “snowball” effect (the large amplification of disturbances in the recycle flow rate). A generic rule for liquid recycle systems is proposed: one flow rate somewhere in the recycle loop should be flow controlled.