  This paper presents a modified multi model predictive control algorithm for
the control of riser outlet temperature and regenerator temperature for the
fluid catalytic cracking unit (FCCU). The models of the fluid catalytic
cracking unit are estimated using subspace identification (N4SID) algorithm.
The PRBS signal is applied as an input signal to estimate the FCCU models.
Since the estimated model does not give 100% fit; especially for nonlinear
systems having more than one operating conditions, multi-model approach is
proposed. In multi model, more than one model of FCCU used in MPC design. The
main advantages of proposed method are that it can handle hard input and output
constraints and it can be used for multi input multi output processes (MIMO)
without increasing the complexity in control design. MATLAB/Simulink is used to
estimate the models of FCCU and simulate the results for the controller. The
simulation results show that the proposed algorithm provides better result for
both reference tracking and disturbance rejection.
