DYNAMICS AND CONTROL OF RECYCLE SYSTEMS .4. TERNARY-SYSTEMS WITH ONE OR 2 RECYCLE STREAMS

This paper is the fourth in a series of papers that explore the challenging problems associated with the dynamics and control of recycle systems. The reactions considered in previous papers were fairly simple. Only first-order reactions were considered, so there was only one fresh feed stream. In this paper, second-order kinetics are considered with two fresh-feed makeup streams. Two cases are considered: (1) instantaneous and complete one-pass conversion of one of the two components in the reactor so there is an excess of only one component that must be recycled and (2) incomplete conversion per pass so there are two recycle streams. It is shown that the generic liquid-recycle rule proposed by Luyben applies in both of these cases: ''snowballing'' is prevented by fixing the flow rate somewhere in the recycle system. An additional generic rule is proposed: fresh feed makeup of any component cannot be fixed unless the component undergoes complete single-pass conversion. In the complete one-pass conversion case, throughput can be set by fixing the flow rate of the limiting reactant. The makeup of the other reactant should be set by level control in the reflux drum of the distillation column. In the incomplete conversion case, two workable schemes were found: (1) Both recycle flow rates are fixed and both fresh-feed makeups are brought in on level control. Throughput is controlled by changing either the reactor temperature or the recycle flow rates. (2) One fresh-feed makeup controls reactor level and the other controls the composition in the reactor. Throughput is controlled by setting reactor temperature or reactor effluent flow rate.