(22a) Advanced Residue Fluid Catalytic Cracking (RFCC) Regenerator Analysis
A Residue Fluid Catalytic Cracking (RFCC) regenerator typically consists of two consecutive bubbling beds combusting coke on deactivated catalyst under partial burn (below stoichiometric) conditions. The RFCC regenerator may well be the largest piece of equipment in a refinery, so improvement in coke combustion efficiency in such a vessel can make it smaller and shorter and thus reduce capital investment for a new RFCC unit.
CFD analysis was employed to understand the flow patterns inside the regenerator and develop a more efficient design. A two-fluid model for gas and solids, coupled with the kinetic theory, and employing the EMMS drag force correlation were used to simulate the flow of air and particles and their respective residence time distribution in the regenerator. The new regenerator design features a compartmentalized second stage with individual air distributors supplying air to each compartment. Because of different air flow rates through these air distributors, the EMMS drag model was tuned differently for the flow in the inner and outer compartment to match the commercial observations from similar systems.
CFD results shows the radial pressure decreases at the bed interface from the regen center to the chamber wall. Residence time distributions of air showed a typical bubbling bed behavior corresponding to four CSTRs in series per compartment, while those of catalyst followed a two-CSTR behavior. Compartmentalization led to a more efficient use of oxygen in the regenerator which now required smaller fluidized bed volumes to sufficiently regenerate catalyst. The latter directly translates into the capital costs of the vessel.