(448k) CFD Simulation of Gas and Solids Mixing in FCC Packing Strippers

The gas and solid mixing in fluid catalytic cracking (FCC) stripper with and without packing were investigated using computational fluid dynamics (CFD). The Eulerian-Eulerian two-fluid model coupled with the modified Gidaspow drag model was used to simulate the gas-solid flow behavior. The grid independency study and the comparison between 2D and 3D simulations were carried out. Many parameters may affect the stripping efficiency, including backing-mixing degree, the gas and solids mixing degree and available vessel area. The residence time distribution model (RTD) and axial dispersion model were utilized to obtain the parameters indicating the back-mixing degree, such as mean residence time t_m, dimensionless variance and Peclet number of gas and solids. The time-averaged bed density at various superficial gas velocities, the bubble size and gas/solid distribution indicated the gas and solids mixing degree. The dead zone, plug and well-mixed volume indicated the available vessel area. It was found that stripping efficiency depends strongly on the operating conditions as well as the internal configurations of the strippers. Higher superficial gas velocity increased stripping efficiency. Internal packing could improve break-up and redistribution of bubbles, reduce particle volume fraction and enhancing the gas-solids contacting remarkably. Compared with the empty stripper, the packing stripper exhibited a smaller , a shorter t_m and a bigger Pe, standing for a reduced back-mixing degree of gas and solids. In addition, internal packing could reduce the dead zone and well-mixed volume, increase plug volume, thus improving the utilization of the vessel area.