A microlevel-based characterization of granulation phenomena

Abstract

Past granulation research has been essentially restricted to a macroscopic study of the impact of operating variables on granule morphology. While fundamental groundwork regarding agglomeration forces has been laid by pioneers such as Rumpf, little progress towards an a priori characterization of microlevel phenomena in terms of macroscopic process variables has been achieved. The present work centers on this microscale and introduces a classification of granulation mechanisms based on the collisional dissipation of relative particle kinetic energy. The mechanism of granule coalescence is, in part, a function of a dimensionless binder Stokes' number St v, which is a measure of the ratio of granule collisional kinetic energy to the viscous dissipation brought about by interstitial binder. The Stokes' number provides a convenient classification of granulation regimes. For small St v, coalescence hinges on the presence and distribution of binder and is independent of particle kinetic energy and binder viscosity. In this regime, binder viscosity controls the rate of granule consolidation and ultimate granule voidage. For the case where the maximum St v is of the order of St* v, increases in binder viscosity increase coalescence rate as traditionally expected. Here St* v is a critical Stokes' number, being a known function of the volume of binder deposited on the bed. Finally, for large St v, only granule coating is possible. Fluid-bed granulation and defluidization experiments supporting this simple classification of granulation regimes is presented. Implications regarding successful granulation operation are drawn.