Control of Systems with Recycle by means of Compensators

The paper analyses the effects of the presence of recycle streams on the performance of the control system of chemical processes and the possibility of success of compensators. Control schemes based on a regulator designed for the process without recycle and augmented by a recycle compensator are compared with specific controllers designed for the global process; potential benefits, in terms of superiority of performance and simpler design, are clearly illustrated for a SISO reference case. Application to two multivariable processes proposed in literature, the two reactors in series (Ray, 1981) and the two reactors plus three distillation columns (Luyben and Luyben, 1995) shows that also in these more realistic cases main advantages of the compensator are retained