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 mixture's 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. (C) 2000 Elsevier Science S.A. All rights reserved.