Abstract

Agglomeration of small particles in binder granulation is achieved by spraying onto a shearing powder mass, a fluid that, owing to its high viscosity and surface tension, binds the particles together. In an effort to model granulation, the present work describes computer simulations of shear-flows of solid particles, some of which are covered by binder (and are therefore "sticky") while the rest are dry. Two distinct regimes of agglomeration found in a typical granulator are studied: that of granule growth and subsequent granule breakup. During granule growth-simulations, the movement of sticky (binder covered) particles is studied in a constant shear rapid granular flow. From these simulations, final granule size (and shape) and size distributions can be obtained using a pattern-recognition routine. The size distribution results are compared with granulation data from measurements performed by the authors and from the literature.

A second kind of simulation, also using rapid granular flow modeling, follows the rotation and deformation of an "agglomerate" made of the original particles held together by a liquid, viscous binder. Results
from these simulations yield critical values of a dimensionless parameter containing inertia and viscous dissipation (the so-called Stokes number). Below the critical value of the Stokes number, the agglomerates are stable and only rotate in response to shear. Above the critical value they break into pieces. At and around the critical value, they attain a steady elongation. These simulation results allow one to obtain correlations of the stable critical sizes versus the different parameters of the problem.