Drying

Airborne processing is optimal for drying due to its efficient heat/mass transfer. Originally fluid bed drying was used to replace the cumbersome and time consuming tray drying, which sped the process up by about 20 times as fast. Tray drying did not permis a uniform residual granule moisture, because this method presented an unpredictable combination of drying by conduction, convection and radiation. When fluid bed drying all particles are permanently suspended and mixed; allowing an efficient convection drying only. Nevertheless, the inlet air temperature should be chosen in order for drying tio be effected in a thermodynamic equilibrium, i.e. the hot air only removes as much moisture from the surface of the granules as can simultaneously travel outwards through the capillaries of the granules. Should the process air temperature be too high, the capillaries can easily close on the surface of the granules. From that moment onwards, convection drying will change to diffusion drying, resulting in extensively extended processing times.

These principles apply to any fluid bed drying, irrespectively, if drying high-shear granules or spray agglomerates, if batch or continuously processing. It has to however be considered that if effecting spray granulation, agglomeration or coating, one does not only remove a defined moisture content to a defined final residual value, but that drying also occurs during the spray process itself, i.e. that too high process air temperatures may also result in unsufficient liquid bridges, porous granule structures or frosty looking films. On the other hand, too low temperatures may result in lumps or a high percentage of agglomerates when filmcoating.