(11a) An Experimental Investigation of Nucleation Phenomenon in a Static Powder Bed

Wet granulation is a particle agglomeration process where fine particles agglomerate together with a granulating liquid to form larger aggregates. Despite its wide application in industries such as pharmaceuticals, chemical plants, food processing, detergent production, and agriculture, the scientific understanding of the process is very limited. One of the primary reasons is that wet granulation is a complex process that involves several of tightly coupled sub-processes, which exist in both dry and wet states. Over the years, researchers have established qualitative descriptions of the sub-processes that control the wet granulation behavior. However, the quantitative understanding, or the ability to predict the outcome of wet granulation, is far from complete. Consequences are high manufacturing cost, high recycle ratio, dependency on empirical correlations, inability to predict the effects of scale up on the final products, and inability to analyze or understand reasons for manufacturing failures [1], which ultimately translates into high product cost. This warrants a continued endeavor of both theoretical and experimental investigations of wet granulation.

One of the least understood areas in wet granulation is nucleation. It is generally described as the process where the granulating liquid fist comes in contact with the powder bed and forms highly saturated initial agglomerates called nuclei. Although nucleation is thought to occur immediately upon the initial liquid-powder bed interaction, only few studies have investigated nucleation at such time regime [2, 3]. We do not know how nucleation kinetic or nucleus size is influenced by various impact conditions that may exist in wet granulation.

In this study, influences of the liquid drop impact velocity and liquid's physical properties on nucleation kinetics and nucleus size were investigated. Drops of water and glycerin/water mixtures were released from various heights onto a bed of glass beads. The moment of impact and formation of nucleus were recorded using a high speed video camera with a frame rate of up to 5000 frames per second. The high speed video reveals that, when a liquid drop impacts the glass beads, a nucleus is formed as the liquid drop initially spreads over the glass beads and retracts into a spherical shape while the glass beads are pulled into the liquid. The surface of nucleus appears to be highly saturated with liquid and the shape continues to oscillate even after the initial formation, which only takes a few milliseconds to occur. Once the oscillation stabilizes, the degree of surface saturation changes as the liquid in the nucleus is drained through the bottom of the nucleus via capillary action until equilibrium is reached. The shape and the size of nuclei, as well as the time it takes to reach the equilibrium, are found to be highly dependent on the impact velocity and liquid viscosity.

A time dependent nucleation model has been developed based on the capillary penetration of liquid into a porous bed. It correctly predicts the effect of liquid viscosity on the nucleus size. In order for the model to be completely analytical, we need a mathematical model that can predict the spreading profile of an impacting liquid drop on an unconsolidated powder bed surface. Such model is not available in the literature. Using experimental data for the spreading profile, the nucleation model predicts the nucleus size within a reasonable accuracy.

References

1. Iveson, S.M., Litster, J.D., Hapgood, K.P., and Ennis, B.J., ?Nucleation, growth and breakage phenomena in agitated wet granulation processes: a review,? Powder Technology, Vol. 117, pp. 3-39, 2001.

2. Agland, S. and Iveson, S.M., ?The impact of liquid drops on powder bed surfaces,? CHEMECA 99, New Castle, Australia, 1999.

3. Hapgood, K.P., PhD Thesis, University of Queensland, Brisbane, Australia, 2000.