Effects and control of humidity and particle mixing in fluid-bed granulation

The novel technique of spraying binder liquid in pulses of short duration on a bubbling fluidized bed was used to study the effect liquid distribution, mixing, and relative humidity has on granule growth. Two important mixing zones in the fluid-bed granulation process are identified. First, the wetting zone, where the spraying rate relative to the surface renewal rate determines the wetting of the granules and powder. The sur;face renewal has to be sufficient to prevent overwetting of the surface, which would result in defluidization of the bed. Second, the bulk zone, where the turnover rate of granules relative to the drying rate may determine the secondary growth rate of granules. The granule growth is influenced by the adsorption of moisture on the fluidized particles governed by the relative humidity of the interstitial gas, due to enhancement of interparticle forces and reduces the mixing intensity. Spraying with pulses has a great potential for controlling the granulation process.