GAS-PHASE HYDRODYNAMICS IN THE RISER OF A CIRCULATING FLUIDIZED-BED

The hydrodynamics of a circulating fluidized bed (CFB) were studied using radioactive argon as a tracer. The impulse experiments show that gas passes through the riser at velocities significantly greater than superficial gas velocities. A core annular flow model was used to describe the hydrodynamics. Axial dispersion in the core zone is negligible and mass transfer to the denser, stagnant annulus is characterized by a cross-flow coefficient, k. The Gilliland Sherwood equation for wetted wall towers, modified to account for the increase in mass transfer rate introduced by the solids, correlates the data reasonably well: Sh(c) = 0.25Sc1/2Re(c)3/4(U(p)/U(g))1/4. The ratio of the cross-sectional area of the lean core to riser cross-sectional area, PHI(g), decreases with solids mass flux but increases with gas velocity. Based on a number of experimental investigations, the effects of riser diameter and particle characteristics on PHI(g) were qualified and the data were fit with a simple two parameter model: PHI(g) = 1/1 + 1.1Fr(U(p)/U(g))0.083Fr This relationship adequately predicts gas bypassing in risers from 0.05 to 0.94 m in diameter.