Modelling Nuclei Size Distributions in Wet Granulation using Dimensionless Spray Flux
Wet granulation is a process where assemblies of particles are formed to make a granule. Granules which have improved flow, reduced dust, and other desirable end use properties compared to the original feed powders. In high shear mixers, the powder is agitated in a vessel while liquid is sprayed onto the powder from above. The initial drops form “nuclei” granules, and control of the initial nuclei size distribution has been shown to be crucial to controlling the entire granulation process.
The dimensionless spray flux parameter is a dimensionless group which describes the spray density in the spray zone. At low spray flux, each drop from the spray forms a single granule nucleus, allowing granule size to be controlled by spray droplet size. The spray flux is closely related to the Poisson distribution, and can be used to predict the nuclei size distribution. This technique was used to model the nuclei size distribution at a range of conditions and compare with data generated in previous studies (Hapgood, PhD thesis, UQ, 2000).
The Poisson predictions and experimental data were in reasonable agreement when the nucleation was in the drop controlled regime (low spray flux and low penetration time) but diverged when either the spray flux increased or the drop penetration time increased. Multi-modal experimental results could not be modeled by the Poisson technique used. We conclude that use of spray flux to model nuclei size distributions should be restricted to the drop controlled regime.