Wet granulation uses liquid binder and various methods of agitation to design granule products with desired properties. The diverse set of granulation processes across industries has caused the development of a variety of mixers. The underlying rate processes that describe the granulation behavior however are more universal. A broadly applicable physically-based description of these rate processes is therefore desirable. The initial interaction of the liquid binder in the powder bed often plays a major role in the overall granulation behavior. This rate processes, nucleation and wetting, is the first opportunity to make design changes that improve control and yield of a granulation process. Typically, even and slow liquid distribution often allows for a narrow nuclei size distribution and eventual granule size distribution. Concentrated and fast liquid distribution leads to breakage-governed mechanical distribution of liquid binder.
In this work gPROMS ModelBuilder is used to develop a multi-dimensional population balance model (PBM). A compartmental approach to segregate rate processes into well-mixed zones is used to better describe liquid distribution, which usually occurs only in one part of a granulator. The configuration used in the case study for this work involves two well-mixed control volumes: a spray zone and a circulation zone. The spray zone represents the region of a granulator where liquid first comes into contact with both primary particles and existing granules. Within the spray zone we chose to model non-uniform liquid distribution according to two characteristic parameters, the dimensionless spray flux and the droplet penetration time. The model for liquid distribution is a modified version of the Poisson distribution model by Hapgood et al. (2004). The nucleation of this spray is also captured with a physically-based model for solid layering onto the wet nuclei granule based on the work by Hounslow et al. (2009).
The experimental validation of this model is carried out in a rotating drum granulator varying the powder and spray properties. Pressurized sprays and droplet controlled systems are used to vary the dimensionless spray flux from the drop controlled to the mechanical dispersion regime. It is found that previous models for the dimension spray flux and description of spray zones may not describe the nuclei size distribution at high spray flux well. The proposed model is a complete and general description for sprays into a mixing bed, from slow to fast nucleation rates, and low to high spray flux.