Professional Articles

Shear Cell Measurement

Assessing the Flowability of Bulk Solids

Shear cell measurements to assess the flowability are important to characterise powder or particulate materials. The following Technical Note presents a newly developed system specifically designed to determine the relevant figures for a material which is stored and already compacted in a hopper. Correctly assessing the flowability of bulk solids is an important aspect of characterisation which can be used to avoid blockages and poor flow that disrupts chemical production. The process engineer has a number of tests that can be used to assess the likely flowability of the materials. Important aspects include bulk density - this provides the driving force for gravity flow for filling a hopper or screw feeder, for example, and wall friction – this determines the resistance to slip on a contact surface, for any given contact load, for example, the way in which a material slips against a hopper wall, mixer blade and a screw auger flight. Another is shear strength - the feature that inhibits deformation of the bulk material and if significant can lead to the formation of stable rat holes over hopper outlets. This is also important when considering how compounds disperse when being mixed. If the combined materials produce a powder that is slightly cohesive this can be better than a free flowing material which might de-mix (segregate) during handling into the next processing stage. Whilst friction and bulk density measurements are relatively easy to obtain the feature of shear strength is sometimes more involved. Although Jenike [1] pioneered a method of powder testing and reliable hopper design, the method is laborious and not widely applied. A Vertical Shear Cell (VSC) developed by Ajax Equipment offers a more straightforward approach to simultaneously examining compaction behaviour and strength. Analysis of the test results can be used to determine the hopper outlet size needed to achieve reliable flow. The VSC aims to replicate the conditions of a material after filling a hopper whilst the Jenike method is a good model for conditions during flow.