HYDRODYNAMICS AND MIXING OF SOLIDS IN A RECIRCULATING FLUIDIZED-BED

Voidage and mixing of solids were measured in a 10-cm-diameter, 6.2-m-tall recirculating fluidized bed column. The solids were alumina with an average particle size of 120 µm, a wide particle size distribution, and a density of 3460 kg/m3. For mixing of solids, CaCl2impregnated 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/(m2s), 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 kmbetween the core and wall regions and the effective average residence time for the solids in the return leg, tpf. The experimental data obtained in this study were well correlated by the mass-exchange coefficient, km, whose value varied between 0.02 and 0.04 s-1. In general, kmdecreases with the average residence time of solids in the riser and tpfincreases with the solid recirculation rate. In the range of experimental measurements examined in this study, a high degree of backmixing of solids was observed. © 1990, American Chemical Society. All rights reserved.