(44a) Flowability Assessment of Weakly Consolidated Powders

Alexandros Georgios Stavrou¹, Colin Hare¹, Ali Hassanpour², and Chuan-Yu Wu¹

¹ Department of Chemical and Process Engineering, University of Surrey, Guildford, GU2 7XH, UK. a.stavrou@surrey.ac.uk

² School of Chemical and Process Engineering, University of Leeds, Leeds, LS2 9JT, UK

The inability of cohesive powders to flow consistently and reliably is one of the most dominant problems across the powder industries. Numerous techniques have been established to study granular flow, mainly measuring the onset of flow at high consolidation pressures to simulate large scale processes, which is relevant to hopper design. In contrast, many industries, e.g. pharmaceutical, involve processes that expose powders to low stress levels (< 1 kPa), such as dry powder inhalers, feeders for packing and tabletting machines, and filling and dosing of powders in capsules. In addition it is important to understand powder flowability at high strain rates to optimise dynamic processes, such as packaging, conveying and mixing. Ball indentation is a new technique that can be used to study these difficult facets of bulk solids flow [1]. In this technique a consolidated powder bed is penetrated by a spherical indenter. The flowability is determined by calculating hardness from the force-displacement response of the indenter. Hardness can be linked to the unconfined yield strength, which is derived by the commonly used shear testers, via the constraint factor [2].

Indentation tests were carried out at low (0.1-1 kPa) and high stresses, and shear cell measurements at high stresses, using a series of materials. The technique exhibits very good reliability. Some of the materials exhibited a linear increase of hardness with the increase of pre-consolidation stress, whereas others showed a more pronounced increase in the region of low stresses. The constraint factor of all the materials was found to be almost constant at high stresses. By inferring the unconfined yield strength at low stresses from ball indentation measurements, for some materials it was found that shear cell extrapolations led to an overestimation of cohesion. By broadening the range of experimentally tested materials it will be evaluated how single particle shape, agglomerate shape, and surface energy influence the constraint factor, while the constraint factor behaviour at low stresses will be studied using DEM.

References

[1] Hassanpour, A. & Ghadiri, M., 2007, Particle and Particle Systems Characterization, 24(2), 117–123.

[2] Zafar, U., Hare, C., Hassanpour, A., Ghadiri, M., 2017, Powder Technology, 310, 300-306.