Dynamics and control of recycle systems. 3. Alternative process designs in a ternary system

Abstract

This paper is the third in a series of papers that explore the challenging problems associated with the dynamics and control of recycle systems. In this paper the authors extend this work to a more challenging and realistic system in which consecutive reactions occur in a reactor and separation is performed in two distillation columns with recycle back to the reactor from one of the columns. The consecutive reactions A[yields]B[yields]C produce a desired product B and less valuable product C. The optimum reactor size in this system is much smaller than the optimum in the single reaction system because the production of undesirable component C must be kept low. This favors a small reactor with a high concentration of component A, which means that there is a large recycle flow of unreacted component A back to the reactor from the distillation section. A large recycle means high energy and capital costs, but the improved yield of component B justifies a large recycle stream. The optimum steady-state design depends strongly on the rate constants of the desired and undesired reactions. It also depends on the relative volatilities of the three components. In this paper, the reactant component A is assumed tomore » be the most volatile, so the recycle stream comes from the first distillation column. The different process configurations display different dynamics and require different control systems for effective disturbance rejection. Changes in fresh feed flow rate are shown to have the most dramatic effect on the system. 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 recycle systems is proposed: one flow rate somewhere in the recycle loop should be flow controlled.« less