Flow and Mixing of Cohesive Particles in a Vertical Bladed Mixer

Mixing of fine particles is an important operation to obtain products with controlled properties in the pharmaceutical as well as many other industries. Here, the Discrete Element Method (DEM) is used to simulate the mixing behavior of monosized fine particles in a vertically shafted cylindrical bladed mixer. The mixer impeller consists of two rotating blades of rake angle phi fitted to a vertical shaft. The particle cohesion is considered to be due to the van der Waals forces and is changed by varying the Hamaker constant Ha for particles of a given size. The aim is to examine the effects of interparticle cohesion, rake angle, and particle-wall cohesion on the flow and mixing behavior of fine particles. The results suggest that all these factors should be carefully selected for obtaining a good mixing performance. In particular, it is shown that the particle bed may be lifted up and remains above the rotating blades without mixing if the interactions between particles and walls are highly cohesive. At a high shaft speed, blades of 90 degrees rake angle can result in a high mixing rate and a small shaft torque, with mixing mechanisms different from those at other rake angles.