(157b) Using Dem to Identify Mixing Mechanisms In Bladed Mixers

Flow and aggregation of powders is of central interest for applications like powder mixing, granulation or drying in the pharmaceutical industry. Despite intensive research has been carried out in this field, the description of mixing in that type of equipment is still a great challenge. This is because experimental techniques to measure powder flow and mixing quality have still significant limitations. Especially the complex flow in equipment with rotating internals, e.g. bladed mixers, is poorly understood from a theoretical point of view.

By using specialized simulation techniques it is possible to get a deep insight into granular flow and mixing. In our work we have used the discrete element method (DEM) to simulate the flow and mixing in a bladed mixer. The main goal was to investigate the effects of powder moisture and stirrer speed on mixing mechanisms in a wet powder bed. The method is used to explore phenomena that have been observed in the experimental work of Lekhal et al.¹

By using sophisticated particle interaction models we were able to study both dry and wet bulk solid flow behavior. Because DEM is currently limited to a relatively small number of particles, we have used dimensionless quantities to effectively simulate the motion of the powder bed. The cohesive forces have been characterized by the granular Bond number Bo_g and the Collision number Co recently introduced by Li and McCarthy.² The results show that mixing in radial direction is strongly influenced by the powder moisture. However, the integral mixing quality in the mixer seems to be quite unaffected by the water content of the powder. Using the detailed information from our simulations we were able to investigate the mechanism behind this phenomena. Hence, we present the differences in granular diffusion coefficients between dry and wet powders and their effect on mixing pattern.

(1) Lekhal A, Conway SL, Glasser BJ, Khinast JG. Characterization of granular flow of wet solids in a bladed mixer. AIChE Journal. 2006;52:2757-2766.

(2) Li HM, McCarthy JJ. Cohesive particle mixing and segregation under shear. Powder Technology. 2006;164:58-64.