Fluidized drum granulation: studies of agglomerate formation

Abstract

Agglomerate formation in fluidized drum granulation was studied experimentally using a 0.3 m diameter laboratory fluidized drum granular. Glass ballotini of various sizes were coated molten paraffin wax. The effect of particle size (1.5 to 8 mm), liquid spray rate (17 to 43 g/min) and drum speed (20 to 40 rpm) on the fraction of agglomerates formed was measured. The formation of agglomerates was controlled by collisions between wetted granules in the falling particle curtain in or near the spray zone. There is a sharp transition between high fractions of agglomerates (non-inertial regime) and almost no agglomerates (coating regime) as particle size increases. This transition point increases strongly with increasing liquid spray rate but is only a slight function of drum speed. These results agree, qualitatively and quantitatively, with the granulation regime analysis proposed by Ennis. Tardos and Pfeffer, Powder Technol., 65 (1991) 257 which is based on the balance between inertial energy and viscous dissipation during a collision between two wetted granules. Analysis of the data shows the characteristics collision velocity is low, of the order of 0.02 m s −1, and this is consistent with agglomerates forming in the falling particle curtain near the spray zone. A better understanding of particle-particle and particle-binder interactions in the spray zone is needed to extend the theoretical analysis and develop design and scale-up rules.