Kinetics of fluidised bed melt granulation V: Simultaneous modelling of aggregation and breakage

Abstract

The main purpose of this paper is to quantify the aggregation and breakage rates in fluidised bed melt granulation (FBMG) and to subsequently relate them to various experimental conditions. The earlier paper of this series (2004d, Powder Technology 143–144, 65–83) illustrated a sequence of development and verification work on a breakage model for FBMG, based on the population balance modelling work on tracer experimental studies. A new error-weighted integral technique was also developed, which allows simultaneous extraction of the aggregation and breakage selection rate constant, as well as the attrition constant that reveals the relative amount of attrition during FBMG. Further research is conducted here, as the similar modelling strategy is employed to extract the aggregation and breakage kinetics at different operating conditions. A series of plots revealing the influence of operating conditions (binder spray rates, bed temperature, droplet size and fluidising air flow rate) on these extracted constants are therefore established. The aggregation rate constant plots reveal that the particle aggregation rate is dependent on the amount of binder available per unit time, and hence scales directly with the binder spray rate. The aggregation rate is also observed to increase with increased bed temperature when a higher viscosity binder is used, but reveals a maximum aggregation rate for a less viscous binder. The aggregation rate also increases with larger droplet size and lower fluidising air velocity. The breakage selection rate and attrition constant plot both reveal no direct dependence on binder spray rate, due to the separation in time scale over which the granule breakage occurs. The breakage rate and the extent of granule attrition is also found to decrease with increased bed temperature and increased fluidising air velocity. Due to scatter in the data, it is not possible to deduce any sensible trend on the influence of droplet size on its relative breakage rate and attrition.