(53d) Dynamic Simulation of a Fluid Catalytic Cracking Unit. Predicting the Operation Using a Hydrotreated Feed

Fluid Catalytic Cracking has been used for more than 60 years in order to convert straight-run atmospheric gas oils, vacuum gas oils, atmospheric residues, and heavy stocks recovered from other refinery operations into high-octane gasoline, light fuel oils and olefin-rich light gases. Due to the complexity of the process itself, and the need for efficient ways of handling various feedstocks, a rigorous dynamic model that predicts the reactor yields and the unit dynamics is becoming a crucial tool in determining and achieving the optimum operating conditions.

In the current work we used detailed, first-principle based modeling for the whole FCC system (reactor, riser, cyclones, stripper, regeneration, standpipe and slide valves) in order to make sure that the system dynamics are accurately represented. The reactor and the regenerator were first modeled in Aspen FCC?. Using the aforementioned ?off-line? rigorous model, we carried out a series of step changes within the operating limits of the specific unit in order to identify the reactor products as a function of the various system operating parameters (temperature, pressure, residence time, specific gravity of feed, catalyst type etc.). From these runs, yield shift variations tables for each component were calculated and used to define the total yields of the reactor and, thus, the final products in real time.

The result was a dynamic model of the FCC unit with the ability to simulate accurately the start-up, shutdown and operation of the process and predict the reactor yields for different feedstocks, like Arabian light/medium oil, hydrotreated feed etc. The process bottlenecks were identified and resolved before the operation of the unit, resulting in a smoother start-up and significant financial savings.