Investigations of afterburn control with secondary air injection during coke combustion in FCC catalyst regenerator

The coke deposited on the spent catalyst used in a fluidized bed catalytic cracking (FCC) unit in a petroleum refinery is burned off in a controlled manner in a fluidized bed regenerator. However, afterburn has been experienced in industry, linked to increase in the temperature above the dense bed in the freeboard region. The afterburn can occur due to a combination of limiting factors such as inadequate mixing, mass transfer, and availability of sufficient oxygen in the combustion chamber. Higher coke content on the spent catalyst due to operating factors and its nonuniform distribution and mixing in the bed can add to the problem. Combustion of gases rich in CO can continue in the freeboard region and the resulting high temperature can lead to loss of efficiency with deposition of hot catalyst particles on the cyclone inlet etc. This study evaluates the injection of secondary air above the dense bed as a cost-effective means of controlling the problem. For the simulations, a modified Eulerian-Lagrangian approach of Computational Particle Fluid Dynamics (CPFD) has been used to track the complex fluid dynamics and interaction of phases with particle size distribution effects. The 3D reactor model accounts for kinetics, heat and mass transfer, and mixing effects during the coke combustion process in a regenerator and maps the gas phase composition.