Powder Flow: Powder characterization for process designers and engineers

For process designers specifying new plant, the goal is to engineer equipment that will process and handle powders consistently and efficiently, as defined by the design brief. In contrast, engineers working as part of the manufacturing team rarely have the option of changing out equipment, but strive to achieve acceptable operation using the existing plant. While these goals are somewhat different the two groups share a need for detailed process understanding, knowledge of the interplay between powder properties and process equipment and how, in combination, they deliver product with the intended properties and quality. For the particulate handling industries this can be a daunting challenge. To successfully design and operate powder processes, engineers need to determine the conditions to which a specific unit operation will subject the powder, and then measure the powder’s response to each of these environmental conditions.

Consider the example of a blend flowing from a hopper into the feed shoe of a tablet press. Each time the powder level falls below a certain point, the hopper is refilled with a new batch of feed. With certain blends it is noticeable that a short time after this refill, discharge flow becomes erratic, resulting in a plant stoppage. When hopper flow is reinstated the blockage problem translates through to the shoe, with poor flow to the dies giving rise to inconsistent filling. However, not all blends exhibit this behavior.   

Figure 1 – Tablet Press Schematic

Analysis of these simple process steps reveals the conditions imposed on the powder. During discharge, material at the exit of the hopper is being compressed by the weight of powder above it, a force closely related to the powder bed height. When flow from the shoe is stopped, consolidation by vibration from surrounding machinery becomes an issue. Here then the response of the powder to consolidation, by direct compression or vibration, is highly relevant. If consolidation brings about a major change in flow properties then problems are more likely to arise.

Universal powder testers incorporate bulk, shear and dynamic measurement in a single instrument, and for engineers offer an intuitively sensible approach, measuring the powder in motion, and permitting the analysis of samples in a consolidated, conditioned, aerated or even fluidized state. Comparing the flow energies of conditioned samples with those of samples consolidated by compression or tapping gives a consolidation index (CI). Quantifying the response of the powder to consolidation in this way provides the insight necessary to rationalize the processing behavior outlined above.

Comparative studies of the die filling performance of two different powders, A and B, provide a perfect illustration of this point. Sample A has a CI (tapped) of 1.11 while that of sample B is 2.32. This indicates that B, a relatively cohesive material with very fine (4 microns) angular particles, is significantly more affected by vibration than A. Die filling trials confirm that the performance of sample B deteriorates markedly if it is consolidated, as would be expected from the results. Sample A is likely to exhibit much more robust behavior. 

Figure 2 – Examples of Die Filling

In this case, a designer with access to the information provided by the powder tester has options - specify a more accommodating hopper, with more steeply angled walls; pursue a policy for reducing equipment vibration; and/or install kit for rapidly releasing a blocked hopper. This same information leads the manufacturing team to better operational practice with respect to hopper filling and an improved response in the event of blockage. Refilling the hopper more frequently with smaller quantities of feed is likely to be one of the best ways of reducing process upsets. For both groups it is detailed and relevant powder testing that provides the information needed to effectively manipulate either design parameters or operating practice to achieve manufacturing goals.

Author Biography

Tim Freeman, Managing Director, Freeman Technology

Tim Freeman is Managing Director of powder characterisation company Freeman Technology for whom he has worked since the late 1990s. He was instrumental in the design and continuing development of the FT4 Powder Rheometer® and the Uniaxial Powder Tester. Through his work with various professional bodies, and involvement in industry initiatives, Tim is an established contributor to wider developments in powder processing.

Tim has a degree in Mechatronics from the University of Sussex in the UK. He is a mentor on a number of project groups for the Engineering Research Center for Structured Organic Particulate Systems in the US and a frequent contributor to industry conferences in the area of powder characterisation and processing. A past Chair of the American Association of Pharmaceutical Scientists (AAPS) Process Analytical Technology Focus Group Tim is a member of the Editorial Advisory Board of Pharmaceutical Technology and features on the Industry Expert Panel in European Pharmaceutical Review magazine.  Tim is also a committee member of the Particle Technology Special Interest Group at the Institute of Chemical Engineers, Vice-Chair of the D18.24 sub-committee on the Characterisation and Handling of Powders and Bulk Solids at ASTM and a member of the United States Pharmacopeial (USP) General Chapters Physical Analysis Expert Committee (GC-PA EC).

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