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.