| Summary: | Three types of gas dispersion in a three-phase fluidized bed have been distinguished. Beds of large, high inertia, particle lead to bubble break-up and a good gas dispersion; conversely small particles give large bubbles; while the third type is an intermediate one whose behaviour depends on the voidage. A model of the interaction between a single bubble and a single particle in a fluidized bed is described. The two-dimensional equations of motion for the particle are solved numerically and the results are presented in dimensionless form as the particle impact velocity on the bubble surface versus the fluidization velocity. These results can be used for prediction of the type of gas dispersion for any bed of specified voidage, made up of particles and a liquid of known properties with low or moderate gas flow rates. Experiments with beds of 2mm diem, glass beads fluidized to various expansions by a liquid paraffin were carried out using two modes of gas injection, continuous and pulsed. High speed cine films were taken of the bubble dispersion emerging from the surface of the bed and analysed frame by frame to give bubble sizes and size distribution when gas was injected continuously. Similar films were taken of pulsed bubbles at the injection zone, rising through the bed and emerging from the surface. Typical sequences of photographs for each set of conditions are presented. Some sequences are of particular interest in showing the mechanism involved in bubble break-up. The experimental observations indicate that when this system is highly fluidized it behaves as a bubble break-up type of bed: conversely at low voidage there is little break-up and a greater tendency for bubble coalescence. The predictions from the theoretical model are shown to be in agreement with these experimental findings.
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