Comparison of model predictive control strategies for a fluidized catalytic cracker

Abstract

A FCC model is used to compare five different Model Predictive Control (MPC) strategies. The FCC process is a complex petrochemical unit with catalyst recycling that makes its behaviour highly nonlinear. The FCC comprises a riser, a separator and a regenerator with important heat coupling due to the endothermic cracking reactions of gas oil in the riser and the exothermic combustion reactions in the regenerator. The riser and the regenerator exhibit fast and slow dynamics respectively. The temperatures at riser top and in the regenerator should be controlled by manipulation of catalyst and air flow rates. All these nonlinear and coupled characteristics render the multivariable control problem difficult and thus the FCC process constitutes a valuable benchmark for comparing control strategies. Here, the performances of Dynamic Matrix Control, Quadratic Dynamic Matrix Control, MPC control with penalty on the outputs, NonLinear MPC control, Observer Based MPC control are compared.

A Review of Modelling of the FCC Unit–Part I: The Riser

“…In the catalyst equation, F cRS is the catalyst flow rate terminating from the riser, F cSV is the flow rate of spent catalyst through the slide valve in the transport line to the regenerator and the last term represents the flow rate of unseparated catalyst entrained with gaseous components to the main fractionating vessel. It is important to note at this point that most researchers [2,5,6,207,217] are in agreement that, contrary to the riser where a steady state balance was sufficient, the holdup and residence times of the stripper section are important to its operation and the FCC unit as whole, hence dynamic balances are required in this section. The coke balance over the disengage/stripper unit allows the determination of coke-on-catalyst concentration entering the regenerator.…”

“…Q loss is the heat lost from the system through the walls of the unit to the surroundings, the simple heat exchanger equation can be used to estimate this heat loss (i.e., Q loss = U.A e f f (T ST − T air ) where symbols have their usual definitions). The adiabatic assumption, where Q loss is assumed to be zero, is also a common simplification made for this system by various workers [2,217] with great utility. However, to the best of our knowledge, no paper has provided evidence that the adiabatic assumption affects the predictions in any significant way, and heat loss to the environment is still an important aspect of the energy considerations of the process, and therefore we recommend its inclusion in the models for completeness.…”