Drop Motion on Partially Wetted Powder Surfaces
Wet granulation is a process where assemblies of particles are formed to make a “granule” with improved flow, reduced dust, and other desirable properties. In high shear mixer granulators, the powder is agitated in a vessel while liquid is sprayed onto the powder from above. The initial drops form “nuclei” granules and their formation has been described by dimensionless spray flux and a nucleation regime map. However, the approach assumes dry powder entering the spray zone. Industrial granulation processes commonly add 20-50wt% fluid, and the partially wetted powder recirculates many times through the spray zone. The effect of partially wetted powder on the nucleation process is not currently known.
To investigate, drop nucleation experiments were performed using a syringe and a small powder bed. After allowing an initial drop to penetrate into the powder, a second drop was added at a controlled separation distance and time, and the nucleation behaviour was observed. Experiments were performed with glass ballotini and sucrose powders, nucleated with binders over a range of viscosities. A strong correlation between drop penetration time and drop motion on the powder bed surface was observed. For powder-liquid combinations with fast penetration times, the wetting behaviour was only slightly affected by the presence of the previous drop. However, systems with long penetration times showed significant lateral drop motion due to Laplace pressure differences and formed one single nucleus. Implications for design and control of industrial granulation processes are discussed.