Simulation of granular mixing in a static mixer by the discrete element method

Mixing of solids in static mixers with different geometries was studied using discrete element method (DEM) simulations. The DEM code was validated by comparing the simulation results with experimental data for a static mixer with 180° Kenics elements. Two different types of blending elements, Kenics and LPD, were considered and compared. Effects of the number of blending elements, the angle of twist of Kenics elements, the slope angle of LPD elements and particle to tube diameter ratio were studied on efficiency, mechanism and time of mixing. Flow patterns of solids in the mixers were investigated with the aid of the velocity field of particles, granular temperature and velocity gradient. It was shown that the average solids flow rate decreased from 101.8 g/s to 54.2 g/s when number of blending elements increased from 2 to 5. On the other hand, the mixing efficiency improved with increasing the number of elements from two to four at a constant number of passes. The results of simulations revealed that the Kenics elements with the twist angle of 150° and 180° and LPD elements with the angle of slope of 60° have the best performance in terms of mixing quality and mixing time. It was also found that larger values of tube diameter (smaller d/D) lead to a better performance of static mixers. Hybrid arrangement of elements (a combination of Kenics and LPD elements in one mixer) significantly weakens the performance of the static mixer.