Application of Multistep Newton-Type Controllers to Fluid Catalytic Cracking

Newton-type controllers have recently been extended to multistep control algorithms and implemented within a moving time horizon framework. These controllers have a great deal of flexibility in that they apply directly to nonlinear process models, can handle both input and output constraints and can be implemented in an efficient manner. Here a nonlinear process model is linearized over a time horizon and optimal controller moves are determined through solution of a quadratic program (QP). Moreover, the algorithm has a number of interesting properties, including monotonic convergence to the setpoint for sufficiently large control intervals. This paper discusses the application of these controllers to the Fluid Catalytic Cracking (FCC) process. While many studies have been devoted to these systems, there still remain industrially important and difficult control problems. Here several studies have predicted the existence of multiple steady states, and several SISO loop pairings and control strategies have been suggested. Newton-type controllers have a number of useful advantages for FCC systems, including the automatic handling of interactions, and treatment of input multiplicities that may be encountered. Moreover, we also examine the behavior of these controllers for open loop unstable systems.