SCALE-DOWN OF A HIGH-SHEAR PELLETIZATION PROCESS - FLOW PROFILE AND GROWTH-KINETICS

For the predictive modelling of the high-shear pelletisation process i t is necessary to have a better understanding of the underlying mechan isms. Therefore, pelletisation experiments were carried out with micro crystalline cellulose and lactose in a coffee grinder (small-scale) an d in a Gral 10 ('large'-scale). A toroidal flow-pattern was observed a nd described for both apparatus. There was no effect of increased impe ller speed on the velocity of pellets in the torus; only a change in a ngle with the tangential direction was observed. Moreover, the size of pellets decreased rapidly to a new value. This size increased again w hen the impeller speed was reduced to the initial (low) value. This su ggested the existence of fast break-up and growth, resulting in a dyna mic equilibrium depending on the impeller speed. Tracer experiments we re used to investigate the turnover limes of different sieve fractions . First order conversion rate constants for the pellets of different s ieve fractions, calculated from colour concentrations at several proce ssing times, were equal for both apparatus when plotted against the di mensionless diameter of the pellets (d/d(50)). These results are a str ong indication for the usefulness of small-scale experiments to retrie ve a rapid insight into relevant mechanisms of growth of high-shear pe lletisation. (C) 1998 Elsevier Science B.V. All rights reserved.