Hydrodynamics and mixing of solids in a recirculating fluidized bed

This paper reports the voidage and mixing of solids measured in a 10-cm-diameter, 6.2-m-tall recirculating fluidized bed column. The solid were alumina with an average particle size of 120 {mu}m, a wide particle size distribution, and a density of 3460 kg/m{sup 3}. For mixing of solids, CaCl{sub 2} impregnated on alumina as a tracer and the carbon- alumina system were examined. The experimental measurements were carried out in the ranges of gas velocities from 3.5 to 4.5 m/s, solid recycle rates from 7 to 11 kg/(m{sup 2} {center dot} s), column inventories from 19 to 24 kg, and average riser voidages from 0.879 to 0.938. The experimental data for the solid fraction were correlated well (mean deviation 26.9%) with an expression similar to Kwauk et al.'s model. Mixing of solids in the riser was well correlated assuming a dilute core region, a wall region with a high concentration of solids, and interchange of solids between these two regions. The mixing model contained two parameters; the mass-exchange coefficient {ital k}{sub m} between the core and wall regions and the effective average residence time for the solids in the return leg, {ital t}{sub pf}. The experimental data obtained in this study were well correlated by the mass-exchange coefficient.