ELUTRIATION FROM FLUIDIZED-BEDS .2. DISENGAGEMENT OF PARTICLES FROM GAS IN THE FREEBOARD

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. 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.