Flow and Mixing of Cohesive Particles in a Vertical Bladed Mixer

Abstract

Mixing of fine particles is an important operation to obtain products with controlled properties in the pharmaceutical as well as many other industries. Here, the Discrete Element Method (DEM) is used to simulate the mixing behavior of monosized fine particles in a vertically shafted cylindrical bladed mixer. The mixer impeller consists of two rotating blades of rake angle ϕ fitted to a vertical shaft. The particle cohesion is considered to be due to the van der Waals forces and is changed by varying the Hamaker constant Ha for particles of a given size. The aim is to examine the effects of interparticle cohesion, rake angle, and particle-wall cohesion on the flow and mixing behavior of fine particles. The results suggest that all these factors should be carefully selected for obtaining a good mixing performance. In particular, it is shown that the particle bed may be lifted up and remains above the rotating blades without mixing if the interactions between particles and walls are highly cohesive. At a high shaft speed, blades of 90°rake angle can result in a high mixing rate and a small shaft torque, with mixing mechanisms different from those at other rake angles.

Modeling granular material blending in a rotating drum using a finite element method and advection‐diffusion equation multiscale model

“…For example, recent DEM simulations by Fan et al and Khola and Wassgren produced expressions for particle diffusion coefficients and segregation rates using simple heap flow and shear cell geometries, respectively . Other studies have investigated the effects of particle size, shape and cohesion, also in simple geometries . Unfortunately, due to computational limitations, DEM simulations are not well suited for quantitative blending predictions at scales of industrial interest, at least using realistic particle sizes.…”

The effect of interparticle cohesion on powder mixing in a ribbon mixer

“…Conversely, such effects have recently been studied for other types of mixers, using the discrete element method (DEM) . For example, in the case of a rotating drum, the effect has been investigated for both uniform and binary particle systems, with the cohesion between particles being represented by a square‐well potential .…”

“…In addition, the mixing quality was found to be a function of the rotation speed, with higher speeds resulting in better mixing. Further, in the case of a vertically shafted bladed‐cylindrical mixer, the cohesive effect has been investigated by simulations for uniform particles of 0.1‐mm diameter, with the interparticle cohesion being represented by the van der Waals force . The study found that the mixing can deteriorate at high interparticle cohesion and rotation speed due to the lifting of the particle bed, particularly if the blade rake angle and particles‐to‐wall cohesion are large.…”

“…The previous studies show that DEM is one possible method to investigate the effect of interparticle cohesion on mixing behavior in a horizontal ribbon mixer. In this regard, Bertrand et al have demonstrated that DEM can be applied to a complex mixer where an impeller consisting of only a single spiral conveys particles axially.…”

Consolidated states of dilation in granular material: Experimental investigation and comparison of rheological properties

“…Even though, these processes were tried and tested with respect to intrinsic material properties, they invariably require methodologies which identify the limitations of vessel geometry. It should be noted that qualitative flow characteristics should be carefully selected for good mixing performance [20,21].…”