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TC1017

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F Tablets & Capsules October 2017 11 powder flow Using fundamental powder properties to optimize flowability Greg Mehos, Mike Eggleston, Tristan Trautman, Matthew Freeman, and Nicole Stevens-Murphy University of Rhode Island Many methods of measuring powder flowability aren't predic- tive and can't be used to select or design the optimal hopper or bin. Shear cells offer a better approach. This article details how to use them to quantify flow behavior and to design vessels that ensure reliable flow. ormulators have a choice of tests to quantify the flowability of powders, including angle of repose, Carr's compressibility index, Hausner ratio, and the time required to discharge powder through an orifice. Unfortunately, none of these methods is predictive, because they don't simulate actual conditions. At best, they can be used to rank the flowability of similar pow- ders. The most useful test method is one that measures a powder's fundamental properties under consolidation stresses that simulate those anticipated when the powder is handled and measures fundamental properties. The fundamental solids flow properties are cohesive strength, internal friction, compressibility, wall friction, and per- meability. By acquiring these data from tests conducted on small samples, it's possible to replicate the conditions within larger-scale systems. This means that you can pre- dict flow behavior from fundamental principles. Methods for measuring fundamental flow properties Shear cell testers measure the cohesive strength, inter- nal friction, compressibility, and wall friction of a pow- der. To do so, a sample is placed in a cell and then pre- sheared, i.e., consolidated by exerting a normal stress σ and then shearing it until the measured shear stress τ is steady. This establishes a state of consolidation in the sample that replicates the pressures that would be experi- enced at a particular position in a storage vessel or con- tainer. Next, the shear step is conducted, in which the vertical compacting load is replaced with a reduced load, and the sample is again sheared until it fails. The pre- shear and shear steps are repeated at the same consolida- tion level for a number of reduced normal stresses, and the yield locus is then determined by plotting the failure shear stress against normal stress. The flow properties of the powder are determined from the yield locus (Figure 1). The semicircles in this plot are called Mohr's circles. They allow us to determine the normal and shear stresses within conveniently chosen frames of reference. The major consolidation stress is determined from a Mohr's circle that intersects the point of steady-state flow (σ ss , τ ss ) and is tangent to the yield locus. The larger point of intersection of the Mohr's circle with the horizontal axis is the major consolidating stress σ 1 . The effective angle of friction δ is determined from the angle formed when a line passing through the origin is tangent to the Mohr's circle. The unconfined yield strength f c is deter- mined by a Mohr's circle that is tangent to the yield locus and passes through the origin. Because the sample vol- ume is recorded, the powder's bulk density is also mea- Figure 1 Construction of a yield locus and determination of the effective angle of friction , kinematic angle of internal friction , major consolidation stress 1 , minor consolidation stress 2 , and unconfined yield strength f c Consolidation stress Shear stress τ ss 0 f c σ 2 σ ss σ 1

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