The presence of liquids in particulate materials can have a significant effect on their bulk behaviour during processing and handling. It is well recognised that the bulk behaviour of particulate materials is dominated by the interactions between particles. Therefore, a thorough understanding of particle-particle interaction with the presence of liquids is critical in unravelling complex mechanics and physics of wet particulate materials. In the current study, a discrete element method for wet particulate systems was developed, in which a contact model for interactions with pendular liquid bridges between particles of different sizes was implemented. In order to evaluate the accuracy and robustness of the developed DEM, normal elastic impacts of wet particles with a wall were systematically analysed. It was shown that the DEM simulations can accurately reproduce the experimental observations reported in the literature. In addition, the DEM analysis was also in good agreement with the elastohydrodynamic model. It was further demonstrated that the rebound behaviour of wet particles is dominated by the Stokes number. There was a critical Stokes number, below which the particle will stick with the wall. For impacts with a Stokes number higher than the critical Stokes number, the coefficient of restitution increases as the stokes number increases for elastic particles. It was also found that the contact angle and surface tension played an insignificant role in the normal impact of wet particles, while the viscosity of the liquid has a dominant effect on the rebound behaviour.
Rotary tabletting presses are widely used to produce tablets in the pharmaceutical industry. In the tabletting process using a rotary press, rotary die filling is one of critical process steps, as powder behaviour during die filling dictates the quality of final products, such as dosage and weight variations. It is hence of importance to understand powder flow behaviour during rotary die filling. The purpose of this study is to identify the critical process parameters and material attributes that determine the die filling performance. For this purpose, a model rotary die filling system with a paddle feeder was constructed to mimic the powder feeding process in a typical rotary press. Using this model system, the effects of turret speed and paddle speed on die filling behaviour were investigated. Three grades of microcrystalline cellulose powders were considered. It was found that the turret speed has a more pivotal role in controlling the die filling performance than the paddle speed. In addition, it is demonstrated that powder flowability has a great impact on the fill weight variation, and a higher weight variation is induced for the powders with poorer flowability.