Modeling and optimization of a two-stage regenerator fluid catalytic cracking unit
Williams, Linda Denise Reavis
Badgwell, Thomas A.
Doctor of Philosophy
In Petroleum refining, the Fluid Catalytic Cracking Unit (FCCU) is the critical stage in the processing of heavy hydrocarbon oils into more valuable gasoline and lighter products. In the FCCU process, the catalyst becomes deactivated in the reactor by heavy carbon deposits called coke. Catalyst activity is restored in the regenerator by burning off the coke with air. Steam formation during high temperature single-stage regeneration can cause permanent catalyst deactivation. Two stage regeneration allows steam formation to occur in the lower temperature first stage minimizing permanent catalyst deactivation. Heavier feedstocks have led to increased usage of two-stage regeneration. A new FCC model has been developed for modeling a unit that has a two-stage regenerator. The model incorporates the ten lump kinetic model of Jacob et al. for the riser and a three phase model based on the work of Gwyn et al. for the dense bed of the regenerator. The overall model, consisting of ODEs, is solved with a sequential modular algorithm. Simulation results using the model are consistent with real operating conditions. The riser spatial results show most of the reactions to occur in the first 20% of the length of the riser. The dense bed spatial results are similar to expected fluidized bed operations in that most of the bed is uniform in temperature and concentration with a small section where gradients occur. Optimization studies have determined that for maximizing gasoline yield, defined here as pounds gasoline per pound feed, lower feed rates are favored and selectivity is important. For maximizing total gasoline production, feed throughput becomes the important factor at the expense of selectivity. An economic study is included showing optimal conditions for a range in crack spread.