Mixing dynamics in catalyst impregnation in double-cone blenders

The mixing of solids was studied in a 10 in. diameter acrylic scaled-down model of a commercial double-cone blender as a method of investigating catalyst impregnation variables. Layers of labeled and unlabeled particles were assembled in specific horizontal and vertical geometries and a Computed Tomography scanner was used to non-destructively image the particle bed in 10 mm slices at 50 mm intervals after different numbers of rotations through the mixing process. Experiments were performed with both 1/16 in, diameter pellets and nominally 100 mu m diameter spherical particles in order to study the effect of particle to vessel diameter ratio. These studies showed that for both material sizes (1) axial mixing (perpendicular to the axis of rotation) was essentially complete within 10 to 20 rotations and that the surface was refreshed after a single rotation; (2) radial mixing (along the axis of rotation) was found to be significantly poorer; (3) filling the vessel 80% rather than 50% full resulted in segregation and therefore very poor mixing. All of these results were quantitatively confirmed in experiments in which vacuuming and image analysis were used to quantify the concentration of particles of a given color throughout the granular bed. These results suggest that, in coating operations-such as catalyst impregnation-the liquid spray distribution must reflect the volume distribution of the solid along the rotation axis in order to avoid uneven distribution of the liquid solution caused by slow axial mixing of the granular bed. (C) 1999 Elsevier Science S.A. All rights reserved.