(507g) Axial Dispersion Coefficient of Mixing in Dilute Systems

Mixing of minor granular ingredients is a problem with pedigree. More often than not the properties of the minor ingredients are very different from the bulk material which amplifies the challenge. For example, in the pharmaceutical industry, lubricants, surfactants, or flow aids are added in small quantities and their properties are largely different from excipients. Not only is the homogeneity of these ingredients critical in the final product but there is significant concern about over-mixing since excessive mixing is known to mar critical quality attributes of the final product.

The axial dispersion model is the most commonly model used to understand axial mixing in horizontal rotating drums for flowing and non-flowing systems. The powder bed is treated as a continuum and mixing in the axial direction is quantified by the axial dispersion coefficient based on Fick’s second law.  The focus of this work has been on mixing of dilute systems in rotating drums in which the properties of the materials being mixed are vastly dissimilar from each other.

Materials with variable cohesion indices have been studied and a relation between their flowability and bed behavior (slipping, slumping, rolling, cascading, cataracting and centrifuging) in the rotating drum (for a given set of conditions) has been established. Mustard seeds, poppy seeds, nonpareil beads, glass beads, microcrystalline cellulose and lactose were some of the materials that were studied. Materials were paired based on dissimilarity between at least one of their fundamental properties. For example, microcrystalline cellulose and mustard seeds were paired given their difference in particle size while glass beads were paired with nonpareil beads to exploit their difference in densities. The axial dispersion coefficient was calculated from concentration versus axial position profiles measured for a series of time points. Relationships between axial dispersion coefficients for material pairs have been established.

References:

1. Sherritt R.G.,  Jamal C., Mehrotra A.K. and Behie L.A. Axial Dispersion in the Three-Dimensional Mixing of Particles in a Rotating Drum Reactor. 2003. Chem. Engg. Sci. 58:401-415.

2. Duong N., Arratia P., Muzzio F.J., Lange A., Timmermans J. and Reynolds S. A Homogeneity Study Using NIR Spectroscopy: Tracking Magnesium Stearate in Bohle Bin-Blender. 2003. Drug Dev. Ind. Pharm. 29:679-687.

3. Wrightman C. and Muzzio F.J. Mixing of granular material in a drum mixer undergoing rotational and rocking motions I. Uniform particles. 1998. Powder Tech. 98:113–124.