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 t
he global process in a part representing the process without recycle and in
apart representing the recycle is presented. This is the key issue for eva
luating the relevance of the recycle and to reduce the control system to tw
o blocks: a regulator, depending on the process without recycle, plus a rec
ycle compensator. The design of this control scheme is much easier with res
pect to a specific regulator computed for the global process and allows to
achieve relevant improvement of performance with respect to a standard PI r
egulator. This is clearly illustrated for a SISO reference case, where proc
ess parameters are changed to create situations of particular evidence, and
have been confirmed in the application to the two MIMO benchmarks, propose
d in literature: the two reactors in series (W.H. Ray, Advanced Process Con
trol, 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 ef
fect 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. (C) 1999 Elsevier Sc
ience Ltd. All rights reserved.