The Many States of Powder Flow

  In my July Blog post I would like to share some thoughts around the importance of considering an holistic approach to powder flow. The traditional approach of focussing purely on the transition from static to flowing is completely inadequate to ensure robust, on specification operation of complex powder processing plants.

A complete understanding of powder flow requires a consideration of both the initiation of flow and the behaviour during flow. The study of flow initiation is very well understood and documented (e.g., Powder Technology, Volume 1, Issue 6, April 1968, Pages 369-373). However, the study of the behaviour during flow is comparatively poorly understood and warrants significant further research.

Figure 1: Common shear cell testers. Image reproduced with kind permission of Freeman Technology

Fig. 1: Common shear cell testers.
Image reproduced with kind permission of Freeman Technology

Both states of powder flow are very important for industry. Thanks to proven shear cell methods (fig. 1), it is relatively simple to measure material properties and to design reliable bins and hoppers. It is indeed critical for materials to flow from storage when desired but it is equally critical that they continue to flow in a predictable and reliable fashion throughout the whole process. In most plants handling powders, the bulk of the unit operations will involve powder in varying states of motion and under different degrees of stress and shear. This will include unit operations such as screw feeders, belt conveyors, mixers, granulators, dryers, screeners and packing operations. In industry, all of these operations and more, cause problems resulting in down time and increased costs due to the dynamic flow behaviour of the powder.

Fig. 2: Build up inside a vibratory air slide
Fig. 2: Build up inside a vibratory air slide

Fig. 2 illustrates powder build up inside a large vibratory air slide handling ≈20 tonnes/hr of a cohesive powder. This is an example of significant velocity gradients within a dynamic flow situation where at the extreme, a “boundary” layer undergoes a transition to a static state. In this example, the undesirable dynamic flow behaviour of the product resulted in significant process down-time to allow for manual cleaning.

How can process engineers and researchers in industry quantify, understand and importantly predict these phenomena in order to adapt equipment designs and/or modify material properties?  The solution lies in a dynamic based methodology to complement existing static studies of the transition to flow.

What are the requirements for a dynamic test method?

  • Simulates a low stress, dynamic flow regime.

    Fig. 3: Revolution Powder Flow tester
    Fig. 3: Revolution Powder Flow tester
  • Is able to measure the work done on a powder as a function of flow rate and also for samples conditioned in certain states (e.g., humidity, consolidated and aerated conditions)
  • Quantifies physical interlocking and the friction forces between particles.

Test equipment that is capable of measuring some of these properties has been developed. One example is from Mercury Scientific who produce a so called Revolution Powder flow tester. However this example is limited to very low stress conditions and is unable to study the impact of aeration.  The FT4 powder tester from Freeman Technology is an interesting example that combines both static and many types of dynamic test into one equipment.

In selecting a powder flow tester, the user must decide what parameters are critical for the application in question and be mindful that a problem involving dynamic flow cannot be properly understood by methods measuring the initiation of flow.

What Next ?

A complete theoretical understanding of how dynamic flow relates to material properties is still in its infancy, however with instruments such as the FT4 an empirical approach can yield much useful information. The challenge is to use more examples of behaviour as shown in fig. 2 to relate observed behaviour in processes to measured dynamic flow properties. In this way, empirical correlations will be developed that are of practical use to industrial engineers responsible for the reliable operation of powder processing plants.