Performance of control systems based on recycle compensators in integrated plants

The paper illustrates the e€ects 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 repre- senting the process without recycle and in a part representing the recycle is presented. This is the key issue for evaluating the rele- vance 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 speci®c regulator computed for the global process and allows to achieve relevant improvement of performance with respect to a standard PI regulator. This is clearly illu- strated for a SISO reference case, where process parameters are changed to create situations of particular evidence, and have been con®rmed 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 e€ectiveness of the proposed control system, the e€ect 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.