3D CFD simulation of gas-solids hydrodynamics and bubbles behaviors in empty and packed bubbling fluidized beds

Abstract

Bubbling fluidized bed is extensively used in petrochemical and other process industries. The performance of the bubbling fluidized bed can be improved through the placement of various internals, among which structured packing is proposed to be a promising type of internal. To address the advantage of such packing over the bubbling fluidized bed without any internals, three-dimensional Computational Fluid Dynamics (3D CFD) simulations were performed to compare the gas-solids flow hydrodynamics and bubble behaviors in the bubbling fluidized beds with and without structured packing. A method of capturing and characterizing the bubble was proposed based on the simulation results. Experimental data from the pilot-scale bubbling fluidized bed was used to examine the accuracy of the CFD model. Simulation results show that the distributions of solids hold-up and velocity were more uniform in the bubbling fluidized bed with structured packing than that without internals. The bubble diameter and the bubble number in the packed bubbling fluidized bed were less than half and about 10 times, respectively, of those in the empty bubbling fluidized bed, indicating that much more amounts of smaller bubbles were present in the packed bed compared to the empty bed. It was found that the improved gas content in the fluidized bed at higher gas velocities was mainly due to the increased bubble diameter in empty bubbling bed while it was mainly due to the increased bubble number in packed bubbling bed. The degree of gas back-mixing was reduced slightly while the degree of solids back-mixing was reduced by around 50% in the packed bubbling fluidized bed compared to those in the empty bubbling fluidized bed. Simulation results indicate that the bubbling fluidized bed with structured packing can provide larger interfacial area between gas and solids and can significantly reduce solids back-mixing, which is beneficial for improving the performance of the bubbling fluidized bed.