(656b) Revisiting the Measurement of Powder Permeability Under Applied Load

This paper presents an approach to measuring permeability in deep powder beds. Bulk powder permeability is the relationship between interstitial gas velocity and the resulting bed pressure drop as typically governed by Darcy’s law [1] at low Reynold’s number. The permeability of fine powders can have a significant impact on material handling. For example, a powder’s permeability can determine its deaeration time, propensity toward flooding, and rate limiting flow from a hopper.

These analyses often require a knowledge of the permeability as a function of the local bulk density and applied stress, [2] as related by the powder’s compressibility relationship. This is easy to determine if low aspect ratio measurements of permeability are appropriately conducted, as the applied column load is approximately equal to the experienced powder stress. However, this is not possible in tall columns or if the wall friction is incorrectly mobilized; the local powder stress may differ significantly from the applied column load due to wall friction and related Janssen phenomena. This is a complex function of wall friction and surcharge due to the applied load as well as powder above the reference measurement. [3]

This paper addresses these challenges of permeability analysis with supporting experimental measurements in tall columns for polymer, food, pharmaceutical excipient, and glass powders.

References:

[1] Darcy, H. (1856). Les fontaines publiques de la ville de Dijon. Paris: Dalmont.

[2] Ennis, B.J. et al. (2008) Section 21: Solids-Solids Operations & Processing. Perry’s Chemical Engineers’ Handbook. Perry, R. H., & Green, D. W. (eds.), New York: McGraw-Hill.

[3] Nedderman, R. M. (1992). Statics and kinematics of granular materials. Cambridge, Eng.: Cambridge University Press. 217-227.