Abstract

The interparticle attraction forces acting between particles or a particle and a device wall play an important role in many powder handling processes such as fluidisation, dry dispersion, transportation etc. In macroscopic modelling of these operations, a popular method of accounting for the effect of internal forces within a bulk powder is to measure the cohesion of the particulate material from its yield locus, to be determined by a shear tester. Although widely used in the design of storage bins and hoppers, it is unclear whether results obtained by this technique can fit operations where particles are subjected to the dragging action of a fluid, a condition that cannot be analysed in a shear tester.

An alternative approach, suitable for determining bulk solid cohesion in the presence of gas flow, is the so-called "rain-off" experiment, in which a bed of solids is supported against gravity by a rising air flow. Subsequently, the gas flow rate is slowly reduced, thus the supporting drag force is gradually decreased until particles rain down in thin layers or in lumps. At this stage the cohesion of the solid phase can be measured by determining the balance of forces acting at the moment the powder bed fails.

This paper aims to compare the results obtained by the "rain-off" procedure with data provided by a standard shear tester. Several samples of fine powders of interest for the chemical and pharmaceutical industries are discussed. Results are investigated to evaluate the possible influence of gas flow at various degrees of bulk consolidation.