Mixing dynamics in catalyst impregnation in double-cone blenders

The mixing of solids was studied in a 10 in. diameter acrylic scaled-down m odel of a commercial double-cone blender as a method of investigating catal yst impregnation variables. Layers of labeled and unlabeled particles were assembled in specific horizontal and vertical geometries and a Computed Tom ography scanner was used to non-destructively image the particle bed in 10 mm slices at 50 mm intervals after different numbers of rotations through t he mixing process. Experiments were performed with both 1/16 in, diameter p ellets and nominally 100 mu m diameter spherical particles in order to stud y the effect of particle to vessel diameter ratio. These studies showed tha t for both material sizes (1) axial mixing (perpendicular to the axis of ro tation) was essentially complete within 10 to 20 rotations and that the sur face was refreshed after a single rotation; (2) radial mixing (along the ax is of rotation) was found to be significantly poorer; (3) filling the vesse l 80% rather than 50% full resulted in segregation and therefore very poor mixing. All of these results were quantitatively confirmed in experiments i n which vacuuming and image analysis were used to quantify the concentratio n of particles of a given color throughout the granular bed. These results suggest that, in coating operations-such as catalyst impregnation-the liqui d spray distribution must reflect the volume distribution of the solid alon g the rotation axis in order to avoid uneven distribution of the liquid sol ution caused by slow axial mixing of the granular bed. (C) 1999 Elsevier Sc ience S.A. All rights reserved.