May 13, 2008
R. Hays, S.B.R. Karri, R. Cocco, T.M. Knowlton in “Circulating Fluidized Bed Technology IX” (J. Wertehr, W. Nowak, K-E Wirth, E-U Hartge eds.) TuTech Innovation GmbH, Hamburg, p. 335
Three cases are presented on the entrainment of various materials from fluidized beds in which agglomerated clumps of particles affected the measured entrainment rates. In the first case, direct evidence of clump formation was visually observed in the freeboard of a fluidized bed of 25 µm particles. Large clusters were visible in the freeboard at sizes exceeding 200 µm. In the second and third cases, the effects of agglomeration were indirectly determined from the measured entrainment fluxes. For the second case, the entrainment rate of fine material was measured for different fluidized bed heights. Higher entrainment fluxes were observed with lower bed heights (i.e., higher disengaging heights). The third case involves the replication of a phenomenon observed in an industrial scale process where the entrainment from the fluidized bed was highly variable and led to flooding of the recovery system. It was found in a batch entrainment test that with an initial high concentration of fines in the fluidized bed the entrainment flux was very low. As the fines were gradually elutriated away, the entrainment flux increased dramatically. Following the dramatic increase, the entrainment flux then exhibited the classical batch exponential decay as the fines were elutriated from the fluidized bed. Evidence suggests that fine particles in many materials may be clumping together - resulting in a reduced entrainment rate. The clumps appear to be formed in the fluidized bed and can be ejected into the freeboard intact. Thus, entrainment rate may be influenced by fines level, bed height, baffles, jet velocity from the distributor, etc.