CFD Modeling of Fluid Catalytic Cracking Riser Reactor

Abstract

Fluid catalytic cracking (FCC) is a process that converts gas oil to more valuable products in a gas-solid circulating fluidized bed. FCC units are used widely in refineries to produce higher value gasoline from heavy oil. Because of the importance of FCCU in refining, considerable effort has been done for the modeling of this unit. In last five decades, the mathematical modeling of FCC unit have matured in many ways but the modeling continues to evolve to improve the closeness of models predictions to the real process whose hardware is ever changing to meet the needs of petroleum refining. Complexity of the FCC process because of unknown reaction mechanism, complex hydrodynamics, and strong interaction between reactor and regenerator, has made it almost impossible to develop a general model for the integrated process. Many one and two dimensional, two/three phase models of FCC risers have been developed by the researchers. These models cannot capture the complex hydrodynamics of the riser reactor. In recent years, with the increasing computational capabilities, Computational Fluid Dynamics (CFD) has become a robust modeling tool not only for FCC riser reactor systems, but also for different kind of reacting or non-reacting systems in chemical engineering. CFD can provide us with detailed information on flow processes and heat and mass transfer processes. This is a tremendous advantage over traditional methods of obtaining flow and heat transfer data in FCC riser reactors, which are usually limited to few sampling points and are mostly intrusive. In the present work a two phase flow FCC riser model incorporating a four lump kinetic scheme is presented. The two phase flow (gas-solid) in the riser is modeled using the Eulerian- Eulerian multiphase flow model. The model simulation studies are presented using two cases: gas-solid flow without reaction, and gas-solid flow with reaction. The results for the two phase flow without reaction show that the gas phase velocity decreases along the riser height as the gas loses momentum. The catalyst velocity increases as the catalyst gains momentum. The temperature of the gas phase increases as it gains heat from the hot catalyst, and the catalyst temperature decreases. The results for gas-solid flow with reaction predict the gas phase velocity increase from 4.7 to 14.7 from riser inlet to outlet due to cracking of heavy gas oil to lighter products. The model predicted gas oil conversion 62%, gasoline yield 39%, light gases yield 20%, and coke yield 3%.