Open loop response to changes of coke-precursors during fluidised-bed catalytic cracking

Management of coke is very important during industrial operation of fluidised-bed catalytic cracking units; this coke can be classified into two categories. The first one is “additive coke”, which comes as precursors in feedstock from up-stream processes such as vacuum distillation; this coke is deposited on the catalyst surface independently from kinetic rates, catalyst/oil ratio or kind of catalyst. The second one is coke produced in cracking reactions, which depends on catalyst activity and catalyst/oil ratio. Both kinds of coke, additive and product, are deposited on catalyst surface and busted during regeneration. Due to normal changes in feedstock composition, the amount of coke precursors might oscillate during industrial operation. This situation would be seen as a change in the effective yield to coke in the riser and, consequently, a variation in the heat production rate during regeneration reactions, which could modify regenerator operating temperature; therefore several control studies are focused to the rejection of this disturbance. In this work, a mathematical model of industrial FCC units is used to simulate open-loop responses to different changes of coke precursors, it is found that FCC units are able to manage these disturbances without control actions, up to certain limits around the operating region; therefore control studies should consider this self-stabilisation ability prior to propose closed-loop operating policies.