Powder Flow: Powder characterization for equipment manufacturers

Equipment manufacturers share with process engineers the task of specifying plant for optimal performance. For processes involving powders this is a challenge, with fundamental compatibility between plant and material being the key to success. Designing or modifying equipment to suit the characteristics of a powder provides a firm basis for reliable and successful operation, building quality into the manufacturing process, in accordance with the concept of QbD. Here and in a subsequent editorial we examine ways of using universal powder testers such as the FT4 Powder Rheometer from Freeman Technology to achieve these goals.

It is too simplistic to label powders ‘good’ or ‘bad’ when in fact the response they exhibit depends on the demands of any given process step. For example, spherical spray dried lactose with a relatively large particle size may flow freely from a hopper, but perform badly in a force feeder, as the particles ‘lock up’ and the powder bed resists movement with significant strength.  This may subsequently result in attrition, with changes to particle size and morphology.

Developing a relevant and comprehensive understanding of the nature of a powder is the first step towards achieving a good match between formulation and plant. Combining a suite of complementary measurement techniques that allow real insight into powder behavior, universal powder testers deliver the necessary information. Quantifying powders in terms of well-defined shear, dynamic and bulk properties allows the building of a database that can be used to learn how to design equipment that will work well with different materials. And conversely how to develop specifications for powders that will suit a particular piece of plant.

In this context, Processability Index is a useful concept. Consider an equipment manufacturer developing a new tablet press, or working to enhance the capabilities of an existing unit. Various tableting blends are run through a prototype, pilot scale or modified press and the results vary significantly. Formulation A processes easily at high turret speeds, giving tablets of excellent quality while with formulation C, in contrast, no acceptable tablets are produced. Formulation B lies between these two extremes - if the press is carefully operated at moderate speed, then successful manufacture is possible. Ranking these powders in terms of processability, creating an index, we could assign formulation A a score of nine, B five and C two.

Tablet Press

Correlating these scores with measured powder properties identifies the optimum values for the parameters that dictate performance in the press. These would typically include, for example, aerated flow energy (cohesion), compressibility, permeability and shear strength. If an equipment manufacturer develops a specification of this type for each unit then establishing the best option for a customer with a new formulation is straightforward. Testing the new material and comparing the results with the defined specifications identifies the best solution.

A similar approach is valuable when adapting or changing plant to accommodate a certain formulation. In this case analysis and comparison will reveal whether or not a powder will process well, and if not, which properties deviate most markedly from the ideal. This helps to pinpoint where and why problems are likely to occur, promoting informed equipment modification.

Robust and reliable powder processing is achieved by designing and/or specifying plant that works with, rather than against, the powder. Failure to attain a good match results in an inherently sub-optimal plant that is likely to suffer poor operability throughout its lifetime, unless modified. The insight provided by universal powder testers helps equipment manufacturers and process designers to properly understand the nature of a process material, enhancing their ability to establish an optimal manufacturing solution from the outset.

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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