Performance of control systems based on recycle compensators in integrated plants

The paper illustrates the effects of recycle streams on the controllability of integrated plants and the improvement of performance made possible by a direct compensation of the recycle. A procedure for the decomposition of the global process in a part representing the process without recycle and in a part representing the recycle is presented. This is the key issue for evaluating the relevance of the recycle and to reduce the control system to two blocks: a regulator, depending on the process without recycle, plus a recycle compensator. The design of this control scheme is much easier with respect to a specific regulator computed for the global process and allows to achieve relevant improvement of performance with respect to a standard PI regulator. This is clearly illustrated for a SISO reference case, where process parameters are changed to create situations of particular evidence, and have been confirmed in the application to the two MIMO benchmarks, proposed in literature: the two reactors in series (W.H. Ray, Advanced Process Control, McGraw–Hill, New York, 1981, pp. 219–224), and the two reactors plus three distillation columns (M.L. Luyben, W.L. Luyben, Design and control of a complex process involving two reaction steps, three distillation columns and two recycle streams, Ind. Eng. Chem. Res. 34 (1995) 3885–3898). For a full evaluation of the effectiveness of the proposed control system, the effect of the uncertainty, of the approximated structure of the compensator, as well as possible improvements by including partial adaptive features in the control system, are also taken into consideration.