(62b) Dimensionless Spray Flux - Effect of Rewetting

Wet granulation is a process where assemblies of particles are formed to make a ?granule?. Granules which have improved flow, reduced dust, more uniform composition and other desirable end use properties compared to the original feed powders.

In high shear mixers used by the pharmaceutical, food and detergent industries, the powder is agitated in a vessel while liquid is sprayed onto the powder from above. The initial drops form ?nuclei? granules. The dimensionless spray flux parameter can be used to describe the spray density and initial nuclei size distribution of nuclei formed in the spray zone. The drop controlled nucleation regime occurs at low spray flux, when each drop from the spray forms a single granule nucleus. However, the approach assumes dry powder entering the spray zone. Industrial granulation processes commonly add 30-50% water to the granulator, and the effects of wet powder on the nucleation process are not currently known.

The aim of this project is to answer the following question: Is there is any difference between the outcomes of the following two scenarios? 1. Two overlapping drops simultaneously hit the powder bed surface. 2. One drop hits the powder bed surface and overlaps with a nuclei formed on a previous pass through the spray zone.

To investigate, single drop nucleation experiments were performed using a syringe and a small powder bed. Nuclei formed from two adjacent drops at varying time lags between are excavated and photographed. Experiments were performed with several size ranges of ballotini and lactose powders, nucleated with binders over a range of viscosities. The effect of separation distance between the drops was also investigated. This project has important implications for design and control of industrial granulation processes.