The ribbon blender is perhaps the most widely used convective mixer, being employed to mix pharmaceuticals, chemicals, food, fertilizers, plastics and many other particulate materials.
A common configuration is comprised of inner and outer helical ribbons of opposite pitch. The ribbons are driven by the same shaft within a static U-shaped horizontal trough.
Ribbon blenders can be used in either batch or continuous mode.
The mixing properties of a device, of length 500 mm and width 300 mm, with two dual helical ribbons have been simulated. A total of 100,000 identical particles of diameter 6 mm were considered. The ribbons have a rotational speed of 30 rpm.
Animations of the DEM simulation results below show the time dependence of the particle positions. To obtain a qualitative indication of mixing, the particles are coloured according to their initial positions in the mixer trough.
time = 0 s
time = 5 s
A more quantitative assessment of the mixing properties of this ribbon blender can be obtained through the temporal dependence of an appropriate mixing measure. Here we consider the GMMI measure, defined by the relative position (in x, y or z direction) of the centroid of a chosen group of coloured particles.
The plot below shows the calculated values of the GMMI in the lateral (x) and axial (z) directions. In can be seen that rapid mixing occurs in the lateral direction, with little variation observed after 10 s (corresponding to 5 rotations of the ribbons). The mixing in the axial direction, however, is observed to be significantly slower.