Measuring permeability and flowability of powders at various packing fractions

Permeability is a key powder property for many industrial applications as it affects the flowability during powder handling and the quality of the final product. Indeed, the ability of air to pass through a powder bed, quantified by permeability, is decisive for die or bag filling, dry powder inhalers, silo discharge, pneumatic transport, fluidized bed, etc. Usually, the permeability is measured on a powder bed subjected to high consolidation stresses and without the possibility to control the packing fraction of the powder. In the present study, we show how GranuPack measurement can be combined with a permeability measurement cell to measure the permeability at low consolidation and for various packing fractions, which correspond to many process conditions. A selection of usual powders (pharmaceutical excipients and abrasives) has been tested and the results highlight how the permeability can be used to obtain additional information about powder behavior. For that, a two levels analysis is proposed: an entry-level based on straightforward parameters like initial and final (after the tapping process) permeability and a more advanced level based on two new metrics. These metrics are the permeability ratio and the rate of variation of permeability. These parameters are directly related to the powder cohesiveness and hence can be used to complement the classical flowability indexes.