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

The question of multiple steady states and their stability arises when ever one considers the control of large nonlinear systems. One such sy stem, the fluidized catalytic cracker (FCC), is the subject of this pa per. Using a new FCC model representative of present-day units and cat alysts, it is shown that the three open loop steady states typical of autothermic reactors exist for this system. Of the two higher steady s tates, the upper one is always linearly stable, at least over the pres ent range of operating conditions. The middle one is always unstable. The potential for five steady states is examined. The feasible operati ng range is established, and the impact of feed and catalyst propertie s as well as combustion promoters is shown. As there is considerable c onfusion in the literature about this subject, an effort is made to cl arify 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 o f a control system for a complex nonlinear system, and how to deal wit h model uncertainties.