Elutriation from fluidized beds—II. Disengagement of particles from gas in the freeboard

Abstract

The freeboard above a fluidized bed is the dilute phase region in which the gas and particles disengage. The freeboard container is normally cylindrical and usually of the same diameter as the bed but sometimes larger. Theory is given to describe horizontal turbulent diffusion of fine particles towards the freeboard walls. On reaching the walls, the fine particles descend as a falling film which may drag down gas and thus generate a gas circulation current in the freeboard. This theory gives predictions of (1) upward flux of fine particles and (2) particle concentration, both as functions of distance above the bed surface. These predictions are in reasonable agreement with (1) measurements, using isokinetic sampling, of upward particle flux above an 0.6 m diameter bed of polymer particles of mean diameter 760 μm, and (2) measurements of particle concentration at five levels above a 0.3 m square bed containing a mixture of 73 and 370 μm particles, using entrapment of particles between horizontal shutters in the freeboard. The theory gives a working formula to predict the transport disengaging height (TDH) in reasonable agreement with published data. The theory predicts that the TDH increases with freeboard diameter. The theory predicts, and experiments confirm, that the TDH can be reduced by inserting vertical baffles into the freeboard. The circulation of freeboard gas, generated by the fine particle motion, may explain the published observations (Geldart, D., Cullinan, J., Georghiades, S., Gilvray, D. and Pope, D.J., 1979, Trans. Inst. Chem. Engrs 57, 269) that adding fine particles increases the elutriation of coarse particles.