Mixing performance of ribbon mixers : effects of operational parameters

Ribbon mixers are widely used in practice because they are capable of providing high speed convective mixing. Here, the discrete element method (DEM) is used to investigate the effects of impeller speed and fill level on the mixing behaviors of mixtures of particles with different cohesion in two-bladed and four-bladed ribbon mixers, each having a horizontal cylindrical vessel. The mixing behaviors are characterized by a particle-scale mixing index. Simulations show that the mixing rate increases with the impeller speed for both the cohesive and non-cohesive particle mixtures up to a certain speed, beyond which it shows a reduction. The mixing rate becomes poorer at higher impeller speeds for mixing of cohesive particles in the 2-bladed mixer. Inspection of velocity fields shows that many localized recirculation flows exist when the mixing non-cohesive particles, preventing the overall mixing. By contrast, when mixing cohesive particles, there exist circumferential flow about the shaft axis and convective flow in the horizontal axial direction, improving the particle mixing. The mixing rate deteriorated with an increase of the fill level in both the two-bladed and four-bladed mixers. The mixing rate of the particles is higher in the four-bladed mixer compared to the two-bladed mixer. With the increase of fill level, the particle flow changes successively from the sliding type of flow to recirculation flow and then to cascading flow for non-cohesive particles. The four-bladed mixer performs better for mixing at high fill levels and stronger cohesion, consolidating its advantage for mixing cohesive particles.