Storage and Flow of Solids

Abstract: Memento, homo, qui pulvis est et pulverem reverteris. Genesis 3 Polvos seran, mas polvo enamorado. Francisco de Quevedo The physics of granular materials in ambient gases is governed by interparticle forces, gas–particle interaction, geometry of particle positions and geometry of particle contacts. At low consolidations these are strongly dependent on the external forces, boundary conditions and on the assembling procedure. For dry fine powders of micron and sub-micron particle size interparticle attractive forces are typically much higher than particle weight, and particles tend to aggregate. Because of this, cohesive powders fracture before breaking, flow and avalanche in coherent blocks much larger than the particle size. Similarly the drag force for micron sized particles is large compared to their weight for velocities as low as 1 mm/s. Due to this extreme sensitivity to interstitial gas flow, powders transit directly from plastic dense flows to fluidization without passing through collisional re...

Abstract: The author was asked by the International Fine Particle Research Institue (IFPRI) to write a cri- tical review on shear testers for IFPRI-members. The re- view was delivered to IFPRI in summer 1999. Following the contract with IFPRI it was not allowed to publish the review elsewhere for at least two years. Granular Matter invited the author to submit the review in the original form; during the refereeing process (besides other chang- es) it turned out that some remarks should be added here for clarification: - Since 1999, the author is not aware of a really new device for testing bulk solid properties, which could lead to a change of the general comments and con- clusions provided in the review. - It was argued, that the review is referring too much to the work of Jenike, while the works of Johanson and Peschl were not adequately cited. Both are excellent en- gineers with a lot of experience, but their basic ideas are not available in published form, and if, they are not set in relation to alternative approaches so that an objec- tive comparison in detail would be a future research issue rather than a topic in this report. - A discussion on the influence of electrostatic char- ges was missing. There hardly is an influence, since the particles are in continuous contact. Only with non-con- ducting plastic particles electrostatic charges could cause problems. But no relevant experiments and results are known. In closed systems, the effect of electric charges is thus mainly neglected, but it is clear that electrostatic forces are eminent in flows with a free surface - an issue not addressed in this review.

Abstract: The flow properties and powder physical properties were measured for 13 food powders. The flow properties were measured using shear cell techniques, and the powder physical properties measured were particle size, moisture, bulk and particle densities. The flowability of the food powders, as characterised by flow index, varied from easy flow to very cohesive. Particle size and moisture content do affect flowability, however there was no strong relationship for trying to relate the flowability of the food powders based solely on these physical properties. There was no relationship between measured powder physical properties and their wall friction characteristics. As a result, surface forces between the powder particles, and between particles and the wall surface play an important role in determining the flow nature of the powders, and this is an area requiring research. Jenike’s mathematical analysis to determine the minimum hopper angle and opening size for mass flow is the engineering standard practice for designing a hopper. Applying this analysis, using values of the measured food powder flow properties, shows that this can occasionally produce some unexpected values for the hopper opening size.

Abstract: The objective of the present work was to carry out a systematic evaluation of flow of pharmaceutical powders and granules using compendial and non-compendial methods. Angle of repose, bulk density, tapped density, Carr’s compressibility index, and Hausner ratios were evaluated. Additionally, flow was characterized using a powder rheometer in which a sensitive force transducer monitors the forces generated as a result of the sample displacement. The critical attributes such as cohesivity index, caking strength, and flow stability were determined for samples. The samples consisted of different grades of magnesium stearate powder including bovine, vegetable, and food grade, physical mixture powder blend consisting of a model formulation, granules prepared by various methods including slugging, high shear granulator, and fluid bed dryer. Lubricant efficiency was also determined for granules lubricated with various concentrations of magnesium stearate. It was observed that the compendial methods were often non-discriminating for minor variations in powder flow. The additional characterization such as cohesivity, and caking strength were helpful in understanding the flow characteristics of pharmaceutical systems. The flow stability test determined that the powders were not affected by the test conditions on the rheometer. The non-compendial tests were discriminating to even minor variations in powder flow.

Abstract: The term powder flowability is used loosely and has generally been more closely associated to the test method used to measure it than the significance to the process. To the formulator, flowability is linked to the product. To the engineer, flowability relates to the process. Relating powder flowability results to actual behavior in the production process is the true reason flowability is measured. This article connects typical powder handling processes to flow property measurements of value to the formulator and the process engineer.