(741b) Parametric Study of Powder Flow (quasi-static bed) Down a Vertical Pipe

The dynamics of a bi-disperse powder bed transitioning down a vertical pipe has been studied experimentally and under a discrete element modeling (DEM) framework. The effect of bed height, particle-size ratios, flow-rates, and ingredient flowability has been studied on axial and radial segregation tendencies of a quasi-static (radial and axial cross-sections sections exhibit the same arrangement of components over time but subsequent sections may not) powder bed transitioning down a vertical pipe.

Granular transport is a critical operation in all industries that involve powder handling. Often, the transport is gravity driven in the form of a vertical bed that is quasi-static if the operation is continuous and prolonged. Of interest is the distribution of the granular components along radial and axial cross-sections. For example, in the pharmaceutical industry the composition of the bed is monitored using a suitable spectrometer but penetration depth of the spectrometers is a small fraction of the total bed depth. It is thus critical to understand the distribution of ingredients along the dimensions of the bed to ensure representative sensing. Large difference in particle size or the density of ingredients can also cause segregation which is undesirable for downstream processing.

Although gravity driven powder flow has been extensively studied [1-3], there is a lack of phenomenological understanding of vertical transitioning powder mixtures. The focus of this work is the granular dynamics of a vertical powder bed (mixtures) that is continuously transporting powder but the bed is being replenished by the addition of fresh material at the top. Thus, at steady state, the bed can be considered to be quasi-static.

The radial and axial distribution of components in the bed has been quantified as a function of bed height by using pipes of variable lengths and sampling the throughput of the pipe. Particle size and density ratios have been varied by investigating a gamut of materials such as mustard seeds, poppy seeds, glass beads, nonpareil beads, microcrystalline cellulose and lactose. The total rate of transport was varied by using loss-in-weight screw feeders which fed material into a continuous blender which is responsible for feeding material into the mouth of the vertical pipe. The rate of output is controlled by a pre-calibrated rotary valve fitted at the bottom of the pipe and is set such that the rate of input is equal to the rate of output. Experimental results have been complemented with DEM simulations.

References:

1. Bertho Y., Giogiutti-Dauphine F., Raafat T., Hinch E.J., Herrmann H.J. and Hulin J.P. Powder flow down a vertical pipe: the effect of air flow. 2002. J. Fluid Mech. 459:317-345.

2. Aider J. L., Sommier N., Raafat T. and Hulin, J.P. Experimental study of a granular flow in a vertical pipe: a spatio-temporal analysis. 1999. Phys. Rev. E. 59, 778-786

3. Liss E.D., Conway S.L., Zega J.A., and Glasser B.J. Segregation of powders during gravity flow through vertical pipes. 2004. Pharma Tech.