RAINING OF PARTICLES FROM AN EMULSION-GAS INTERFACE IN A FLUIDIZED-BED

This paper deals with the raining of particles from an interface betwe en a dense fluidized phase and a gas phase with the fluidized phase up permost. Such interfaces occur at the upper surfaces of gas bubbles an d slugs in fluidized beds. Particle rain in these cases would enhance contact between gas and particles within the bubbles and slugs. The ri se velocities of single square-nosed slugs injected in incipiently flu idized beds of different diameters were measured. Relatively small col umns of internal diameters of 0.0125, 0.019 and 0.0254 m were employed in the experiments; In such beds, square-nosed slugs are formed which span the entire cross-section of the beds and rise entirely due to ra ining of particles from their top surfaces. Since the upper surface of such slugs is flat, their motion can be analyzed using the one-dimens ional hydrodynamic theory. Glass ballotini and sand of different sizes were used as bed particles. Comparison of theory and experiment has e nabled the determination of the dimensionless gradient diffusivity cha racterizing the motion of particles induced by a gradient in the void fraction. The results confirm the scaling proposed by Batchelor (1988) . The use of the calculated gradient diffusivity in the criterion for stability of a gas fluidized bed predicts the systems under considerat ion to be always unstable.