Fluidized bed granulation—the importance of a drying zone for the particle growth mechanism

In fluidized bed granulation, either surface layering or agglomeration may lead to granule growth. Agglomeration will occur if a wet particle collides with another particle and is bound by a liquid bridge which becomes solidified during the subsequent drying period. In the case of a two-fluid-nozzle spraying upwards into the fluidized bed, wetted particles may be transported pneumatically by the nozzle air above the bed plenum and dry before they fall down in the outer regions of the bed. Hence, a drying zone above the fluidized bed is formed, leading to significantly less agglomeration.