Dean Brone

Enhanced mixing in double-cone blenders

Experiments were conducted comparing mixing performance in a conventional double-cone blender and in a double-cone blender that was modified by means of a stationary deflector plate in order to enhance axial particle flow. Mixing performance was assessed qualitatively using a transparent mixing vessel to visualize particle mixing patterns and determine the state of homogeneity at the mixtures surface during the entire experiment. Mixing performance was also examined quantitatively by repeatedly vacuuming several layers of beads, taking a digital image of the bed after vacuuming, and using image analysis to subdivide the images into samples and determine the composition of each sample. The effect of operating conditions (rotation rate, vessel fill percentage and total number of revolutions) was examined. Mixing was quantified in terms of the standard deviation of the concentration of a tracer. The evolution of the process was accurately described by a single-parameter model that characterized axial mixing as a first order process with a characteristic rate constant. For double-cone mixers of standard design, under all operating conditions, slow flow of particles through a vertical plane of symmetry at the center of the vessel caused poor mixing performance. Insertion of a deflector plate inclined relative to this plane was very effective in enhancing mixing. The effect of the deflector was to create a convective axial flow across the center of the mixer, increasing the mixing rate by a factor of 25:1.

Using flow perturbations to enhance mixing of dry powders in V-blenders

Experiments were conducted to compare mixing performance in a conventional V-blender and in a V-blender that incorporates perturbations of the particle flow by rocking the mixing vessel during rotation. Mixing was investigated using glass beads with sizes from 40 to 800 μm in vessels of approximately one liter volume. Mixture uniformity was assessed qualitatively using two different methods. One method used a transparent mixing vessel to visualize particle flow patterns and assess the state of homogeneity at the mixtures surface during the entire experiment. The second method involved solidification of the mixture by infiltration with a binder inside disposable aluminum mixing vessels. Using this method, it was possible to assess the state of the entire mixture, including its interior structure, by slicing the solidified structure after completion of each experiment. Mixture uniformity was also assessed quantitatively using image analysis to determine the composition of the solidified samples. In all cases, mixing was greatly enhanced in the rocking V-blender compared to the conventional V-blender.

Mixing dynamics in catalyst impregnation in double-cone blenders

Abstract 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 μm 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.