DEM Simulation of the Mixture of Dry and Wet Particles inside Rotary Mixer

Industrial particle coating process often involves spraying the coating fluid on the top layer of moving particles inside mixer. It takes time on the order of several minutes or more for all particles to pass through the “spray zone”.  During this transient time the bulk of the granular material inside the mixer contains the mixture of dry particles and sticky wet particles with amount of wet particles continuously increasing while the amount of dry particles is continuously decreasing. The surface frictional properties of dry and wet particles can be very different. We developed a numerical model based on the Discrete Element Method (DEM) to simulate this dry-to-wet transition during coating.  In this model every dry particle is replaced by wet particle if it is inside the cone-shape spray zone. The model is implemented within commercial software STAR-CCM+ version 11.04 developed by CD-adapco. We tested the model in several mixer geometries and analyzed the bulk properties of the solid phase as a function of time. For the simplest geometry case, a rotating drum, we obtained the expected change in dynamic angle of repose in the cases of all-dry and all-wet particles and, in terms of the flow pattern, repose angle, and the frequency of avalanching, we obtained results for all-wet particles comparable with published experimental results [1]. Figure below shows the side-view of dry (grey color) and wet (blue color) particles inside the rotating drum for increasing values of time.

Time = 5.5 s	Time = 19.2 s	Time = 24.5 s	Time = 68 s

We believe that our model is useful for both better understanding of the bulk flow during dry-to-wet transition and its ability to differentiate different mixer designs in industrial processes where the difference in frictional properties between wet and dry particle cannot be neglected.

[1] P. Y. Liu, R. Y. Yang and A. B. Yu. Dynamics of wet particles in rotating drums: Effect of liquid surface tension. Physics of Fluids, 23, 013304, (2011)