Modeling and Simulation of Fluid Catalytic Cracking Riser Reactor

Abstract

A Fluid Catalytic Cracking Unit mainly consists of a riser reactor, a catalyst stripper, and a regenerator. Riser reactor is the most important part of this unit as the cracking reactions take place in the riser. At the bottom of the riser, the gas oil feed comes in contact with the hot regenerated catalyst coming from the regenerator and instantaneous vaporization occurs and the cracking reactions start. The reactions’ by product (coke) gets deposited on the catalyst surface and decreases its activity as the catalyst moves toward the exit of the riser. At the riser exit, the deactivated (spent) catalyst is separated from the hydrocarbon products’ vapor through specially designed riser termination device and sent to the regenerator for burning off the coke from its surface. The product vapors are sent to the main fractionator for recovery. Modeling of riser helps in understanding the complex physical phenomena of this process. Complex hydrodynamics, unknown hydrocarbons in the FCC feed and involvement of different type of simultaneous reactions make riser modeling difficult. There has been lot of progress in the modeling of riser reactor. Most of the researchers have taken four to five lump model to avoid complexities in determining reaction rates. These models are easy to integrate with the material and energy balance equations. In the literature two phase or three phase flow hydrodynamics is used by various authors, heat and mass transfer resistances are ignored in most studies, and exponential catalyst deactivation model is used in most works. The present work incorporates a four lump kinetic model having two-phase flow with cluster based approach (considering pressure drop inside the riser). The effect of cracking on phase velocities, and catalyst deactivation model based on concentration of coke on catalyst is also considered. For the solution of the model, the riser is divided into a number of small volume elements along the height. Material and energy balance equations are solved in each volume element. Product yields, riser temperature, catalyst activity, phase velocities, and riser pressure profiles are plotted and discussed. The model predictions matches well with the data reported in the literature. Simulations are also done for various cluster sizes and the effect of cluster size on cluster velocity and product yields is presented.