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In this paper is presented an evaluation of different control structures for a fluidized catalytic cracker. A systematic methodology for evaluating such structures was developed which is suitable for complex nonlinear processes in which the number of manipulated variables is smaller than the number of variables that are part of the specifications. It is shown that the choice of the control structure and the variables entering it is far more important than that of the multivariable algorithms to be used. Criteria for the choice of the measured and manipulated variables entering the dynamic structure are presented. The role of additional so-called slow variables in steady state control is discussed and demonstrated by examples. It is shown that linear algorithms are sufficient for dynamic control but nonlinear considerations dominate the choice of the control structure. This paper is one of the first that thoroughly discusses the compromises and choices a designer faces in designing a control system for a complex nonlinear system. Areas in need of further research are outlined.

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In this paper the impact design has on the control of a fluidized catalytic cracker (FCC) is explored. The available control options depend strongly on the availability of manipulated variables as well as on downstream equipment. Also of importance is the range in which each variable can be manipulated. An important distinction is made between the total number of manipulated variables and the effective degrees of freedom for control, especially of the product specification vector Y p . Control both in complete CO combustion when there is no CO boiler available and in partial combustion when a boiler is present is analyzed. The fast manipulated variables of air flowrate, catalyst circulation rate, feed preheat, and catalyst cooler heat removal rate are all considered. It is shown that the sufficiency of a partial control structure changes when the operating conditions change and that it strongly depends on the control objectives. Also examined is the impact of catalyst activity.

In four previous papers of this series (Ind. Eng. Chem. Res. 1995, 34 (4), 1228−1243; 1995, 34 (4), 3014−3026; 1996, 35 (7), 2215−2233; 1997, 36 (3), 747−759) the concepts of partial control were presented on the basis of the assumption that model information adequate to characterize the process is available from existing units. In this paper the more difficult problem of utilizing the limited data from a laboratory or small pilot plant for the design of a new complex process is analyzed. The fluid catalytic cracker again serves as the main example. One faces large uncertainties during such a scale-up. Reducing them by building a big pilot plant is time-consuming and expensive. It is demonstrated how that uncertainty can be compensated for through concurrent design that is focused on providing proper manipulated variables for control. This achieves one of the main goals of control, namely, to compensate for model uncertainties. A design strategy is presented for the scale-up of complex processes with large model uncertainties, utilizing the concepts of partial control developed previously.