A model for the spray zone in early-stage fluidized bed granulation

The formation of granules in the spray zone of a top-spray fluid-bed granulation process using a known and uniform drop-size is studied experimentally and theoretically. Experimental results indicate a multi-modal size distribution of spherically-shaped granules. A model is presented to explain this based on the formation of nuclei, and the rapid coalescence of some of these nuclei with droplets or other nuclei in the spray zone of the bed. The model is based on a Bernoulli process and predicts the number frequency of granules to which n droplets have contributed. The model predictions are validated with experimental results for granule size distribution, and the nucleus residence time in the spray zone required to bring model predictions and experimental results into line is shown to be physically plausible by comparing with PEPT tracking of an agglomerate-like tracer particle. (c) 2006 American Institute of Chemical Engineers AIChE J, 52: 2736-2741, 2006.