A SMALL-SCALE REGULARLY PACKED CIRCULATING FLUIDIZED-BED .2. MASS-TRANSFER

The underlying objective of the present study is to increase gas-solid s contact in a circulating fluidized bed by the introduction of obstac les in the riser portion. The presence of such obstacles leads to supp ression of radial inhomogeneities in the solids mass flux and concentr ation, and break-up of solids clusters. At ambient conditions, gas-sol ids mass transfer was investigated for cocurrent upward flow of air an d microsize solid particles (FCC, 70 Am diameter) over a regularly str uctured inert packing introduced into the riser part of a circulating fluidized bed unit. The packed section has a height of 0.48 m, a cross -sectional area of 0.06 X 0.06 m2, and contains regularly stacked 0.01 m diameter Perspex bars as the obstacles meant to enhance the gas-sol ids contact. Gas mass fluxes used were 1.4 and 2.7 kg m-2 s-1. Solids mass fluxes were varied in the range 0 less-than-or-equal G(s) less-th an-or-equal-to 12 kg m-2 s-1. Experimental mass transfer data were obt ained by applying the method of adsorption of naphthalene vapor on FCC particles. A conservative estimate of the apparent gas-solids mass tr ansfer coefficient k(g) could be derived from the naphthalene vapor c oncentration profile along the packed section on the basis of a plug-f low-model interpretation, while assuming single-particle behaviour and neglecting intraparticle diffusion effects. Such k(g) values appear to increase with increasing gas mass flux, but decrease with increasin g solids mass flux (and consequently increasing solids volume fraction ) probably due to the corresponding increase in particle shielding. Co mparison of the present results with available literature data for sim ilar solid materials suggests that the effect of the packing inserted into the CFB is significant: the Sherwood numbers derived from the pre sent study are relatively high.