Kinetics of fluidised bed melt granulation I: The effect of process variables

This paper is the first of a series to study the influence of operating conditions on the kinetics of fluidised bed melt granulation. First, we identify the rate processes responsible for the net growth in granule size in a top-sprayed fluidised bed granulator and propose a sequence of events based on these rate processes. The overall kinetics during the process is identified to be a combination of particle aggregation, binder solidification and granule breakage. By conducting experiments in a small-scale modified commercial fluidised bed granulator, the influence of various operating conditions (binder spray rate, bed temperature, atomising pressure, fluidising air velocity) on the granule growth behaviour was examined. The results indicate the granule growth rate to be directly dependent on the relative amount of binder sprayed into the bed, which essentially determines the speed of the aggregation process. The overall granule growth rate is observed to increase relatively with increased bed temperature for a more viscous PEG4000, while a maximum growth is seen for a lower viscosity PEG1500. A larger droplet size was also seen to have increased the overall growth rate, even though a smaller droplet seems to be able to induce a faster initial growth. The results also reveal the increase in fluidising air velocity to reduce the overall granule growth rate. The final granule size distribution was also observed to become narrower with increased bed temperature and fluidising air velocity. These observations are effectively explained using the proposed sequence of rate events.