Chapter 22 Fluidisation of cohesive particles

Abstract

The basic behavior of a fluidized bed is well understood and predictable using well-tested approaches. This understanding is less complete where cohesive behavior is concerned. Cohesion among particles in a fluidized bed may arise through the natural effects of van der Waals forces or through artificially enhanced effects because of the presence of surface liquid or material migration because of sintering. The forces which arise will have static and (in the case of liquid layers) dynamic components. In addition, there may be time-dependent bond formation in the case of sintering and (though not considered here) surface reactions. There is now a widespread agreement that cohesive forces, however caused, of the same order as the single particle weight, will give rise to fluidization characteristics which are similar to those observed in Geldart's “group A.” Much larger ratios of cohesive force to particle weight will prevent fluidization unless mechanical agitation is provided. The mechanism by which cohesion allows a degree of expansion without bubbling remains unclear, but the formation and expansion of cavities in the expanded structure seems likely and their breakdown will occur at the minimum bubbling point.