Derivation of parameters for a two compartment population balance model of Wurster fluidised bed granulation

A common technology for granulation processes is the fluidised bed bottom-spray. This technology can be improved by the use of an internal riser, a so-called Wurster tube. The internal riser has advantages in terms of system stability and enhanced control of particle growth. To mathematically describe the particle growth in such an equipment population balance models (PBM) are applied. These models can be extended by subdivision into a spray compartment and a drying compartment. Important parameters in the compartment models are the compartment size and the particle residence time. In this work, residence time distributions in the riser spray compartment are obtained by a simple gas–particle flow model. The model validity is proven by Particle Image Velocimetry (PIV) and image analysis measurements in a flat test facility. By using an extended PBM it is shown that the particle size distribution (PSD) of the product is not only influenced by the existence of two compartments, but also by the particle size dependent residence time in the spray compartment, which causes additional broadening and skewness. The influence of riser design on particle motion, circulation patterns and product PSD is discussed on the basis of experimental and computational results.