Fluid dynamic studies of a three-phase fluidized bed

Abstract

Three phase flow and mixing in the liquid phase were studied for a draft tube fluidized bed (DTFB) operating in a circulating regime. The DTFB was considered as a system consisting of five interconnected compartiments: the draft tube, annulus and three top sections. The local liquid velocities and phase hold ups were assessed from (a) tracer response studies using a 3-D column, and (b) flow visualization studies using a geometrically similar 2-D column. The drift-flux model for two-phase flow was extended to take into account the momentum transfer between gas bubbles, liquid and solid particles and used to describe the circulating flow in the DTFB. Additional momentum transfer from gas bubbles to solid particles was identified and attributed to the changes in relative velocities and drag forces induced by bubble motion in three phase flow. Mixing in the liquid phase was assessed in the individual DTFB compartments and the system as a whole using the response technique and fast conductivity measurements. Axial mixing in the liquid phase was attributed to the circulating flow between the draft tube and annulus, and to the backmixing in the top sections of the reactor.