Computer simulation of wet granulation

Agglomeration of fine particles in wet granulation is achieved by introduci ng a binder fluid onto a shearing mass of powder. Owing to the viscosity an d the surface tension of the fluid, powder particles are bound together to form larger aggregates. Despite its widespread use in the chemical, pharmac eutical and food industries, little effort has gone into comprehensive mode ling of the overall process from first principles. Modeling is important ho wever, if one needs to estimate a-priori agglomerated granule characteristi cs such as size, shape and density, from knowledge of operating conditions and powder and binder physical and chemical properties. In this work, we present a model of wet granulation that is essentially a c omputer simulation of shear flows of solid particles, some of which are wet (covered by binder and therefore sticky) while the rest are dry. While sim ulations of shear flows of dry solid particles have earlier been reported i n the literature, present work takes this simulation a step further and int roduces a liquid surface layer to some particles in the domain. The additio nal force experienced by two relatively moving particles interacting via th eir binder-covered layers is modeled by using results from lubrication theo ry and Stokesian dynamics. The numerical simulations reflect two distinct regimes of agglomeration: th at of granule growth and that of granule breakup. The granule growth regime takes place in all granulators including low and high shear machines while granule break-up is mainly characteristic of medium and high-shear devices in which agitation as generated by some mechanical means. During granule g rowth-simulations, the movement of sticky (binder covered) particles is stu died in a constant shear, rapid granular flow regime. From these simulation s, final granule size, shape and size distributions were obtained using a p attern-recognition technique. A second kind of simulation, also using rapid granular flow modeling,follow s the deformation and break-up of an agglomerate made from particles held t ogether by a liquid, viscous binder. Results from these simulations yield c ritical values of a dimensionless parameter that contains inertial and visc ous dissipation effects (the so-called Stokes number). Below a critical val ue of the Stokes number, agglomerates are stable and only rotate in respons e to shear while above the critical value they break into several pieces. A round the critical value, they attain a steady elongation. These simulation s allow one to obtain correlations between critical sizes, i.e., granules t hat deform somewhat but do not break, and different parameters of the probl em. (C) 2000 Elsevier Science S.A. All rights reserved.