DEM Study of the Effect of Impeller Design on Mixing Performance in a Vertical Mixer

Vertical mixers have been extensively practiced in a variety of engineering processes, yet the understanding of the relationship between solid transportation and mixing performance is still lacking. In this work, the discrete element method is used to study the particle dynamics and mixing characteristics in a vertical mixer featuring different impellers. After model validation, the effect of impeller design on the solid transportation (e.g., velocity, dispersion, and collision) and mixing performance is discussed. The results show that particles in the dead zone away from the influential region of the impellers tend to be stagnant and hard to be mixed. The two-impeller mixer shows the smallest dead zone ratio and the resulting best mixing performance among all mixers. The particle dispersion coefficient in the horizontal direction (Dx, Dy) shows the largest value, about one magnitude larger than that in the vertical direction (Dz) in all mixers, representing the more significant influence of the impeller on the horizontal particle motion than the vertical one. Dy is slightly larger than Dx due to the asymmetrical installation of impellers. For all mixers, the collision force in the vertical direction is nearly two times of that in the horizontal direction due to the combined effects of gravity and segregation. The present work is expected to shed light on the mixer design and process optimization.