Effect of cohesive forces on fluidized and spouted beds of wet particles

Fluid beds are now being used for processing pasty materials including production of fine powders through drying suspensions in beds of inert particles; coating of tablets or pellets; granulation, etc. In such processes, the fluid bed operation becomes more complex due to the development of cohesive forces resulting from liquid bridges between particles. Such forces can affect gas and solids flow leading to uncontrollable particle agglomeration and to poor gas-solid contact. This work is aimed at analyzing and quantifying the differences of flow behavior in fluidized and spouted beds of wet and dry particles. Experimentally, surface stickiness is induced by application of metered amounts of glycerol. Based on pressure drop vs, fluid flow rate curves, solids circulation rates and bed porosity variations, two types of particle-particle interaction forces are identified and their effect on air-solid flow is quantified as a function of glycerol concentration. Implications of these results in coating, granulation and drying of suspensions in these beds are also discussed. (C) 2000 Elsevier Science B.V. All rights reserved.