DYNAMICS AND CONTROL OF FLUIDIZED CATALYTIC CRACKERS .2. MULTIPLE STEADY-STATES AND INSTABILITIES

The question of multiple steady states and their stability arises whenever one considers the control of large nonlinear systems. One such system, the fluidized catalytic cracker (FCC), is the subject of this paper. Using a new FCC model representative of present-day units and catalysts, it is shown that the three open loop steady states typical of autothermic reactors exist for this system. Of the two higher steady states, the upper one is always linearly stable, at least over the present range of operating conditions. The middle one is always unstable. The potential for five steady states is examined. The feasible operating range is established, and the impact of feed and catalyst properties as well as combustion promoters is shown. As there is considerable confusion in the literature about this subject, an effort is made to clarify some of the key issues. An attempt is made to show with the help of this example how to approach this problem when facing the design of a control system for a complex nonlinear system, and how to deal with model uncertainties.