GAS BACKMIXING IN A CIRCULATING FLUIDIZED-BED

The gas backmixing characteristics in a circulating fluidized bed (0.1 m i.d., 5.3 m high) of fluid catalytic cracking (FCC) particles have been determined. The gas backmixing coefficient has been determined ba sed on the axial dispersion model by using two tracer gases (He and CO 2) in a circulating fluidized bed (CFB). The obtained gas backmixing c oefficients by using the adsorbed CO2 gas overestimates the gas backmi xing coefficient 60-62% in the dense region, 49-62% in the transition region and 2.4-19% in the dilute region compared to the values obtaine d by the non-adsorbed He tracer gas. The gas backmixing coefficient in creases with increasing the ratio of particle to gas velocities (U-p/U -g). In the upper dilute region, the core annulus model is proposed to determine the gas backmixing coefficients and the mass transfer coeff icients between the core and the annulus regions. The gas backmixing c oefficient and mass transfer coefficient increase with increasing U-p/ U-g. (C) 1998 Elsevier Science S.A. All rights reserved.