ANALYSIS OF CONTROL-STRUCTURES FOR REACTION SEPARATION/RECYCLE PROCESSES WITH 2ND-ORDER REACTIONS/

This work analyzes the effect of the process design on control structu re for a system with a reactor, two distillation columns, and two recy cle streams. The reaction A + B --> C occurs in a reactor, and since c omponent C is assumed to be the intermediate boiler, the two distillat ion columns recycle components A and B back to the reactor. A previous paper presented two workable control structures for this process. One fixed the flow rates of the two recycle streams and brought in makeup fresh feeds of components A and B on level control. The other control structure fixed the reactor effluent flow rate, controlled the compos ition of one reactant in the reactor by manipulating one fresh feed, a nd brought in the other fresh feed on reactor level control. These two structures have the undesirable feature of not being able to set dire ctly the production rate and, in the second structure, requiring a rea ctor composition measurement, which can be difficult due to the hostil e environment and can require expensive instrument maintenance. Studie s of other more complex processes have led to similar results: measure ment of composition somewhere in the reaction section is necessary for stable operation. In this paper we present an analysis that explains the fundamental problem with control structures in which one fresh fee d is fixed and no reactor composition is measured. We show that this c ontrol structure can work if modifications are made in the design from the steady-state economic optimum. This highlights the potential trad e-off between steady-state economics and dynamic controllability and i llustrates considerations that ought to be included during the concept ual design procedure. A modified control structure is proposed that pr ovides effective control of the economically optimal process design. I t permits throughput to be directly set and does not require a composi tion measurement. The basic idea is to use the flow rates of the recyc les from the separation section to infer reactor compositions. Dynamic simulation studies on both simplified and rigorous models are used to evaluate the performance of the proposed control system over a wide r ange of reactor sizes.