Powder Flow: Powder Characterization Techniques for Predicting Segregation

The theme of the series of editorials has been material characterization for specific processes or unit operations. This contribution, on the topic of segregation, fits into this category, even though, unlike the other processes discussed, segregation tends to be unplanned and/or undesirable -  or indeed both! In the manufacture of pharmaceuticals, segregation in a feed hopper or feed frame, or during conveying/transport, can have a catastrophic effect on content uniformity. It may also impact processability.

Dynamic powder testing, using a powder rheometer, is one way to evaluate the tendency of a material to segregate. In addition, the technique quantifies the impact of segregation on flow behaviour, a key determinant of processability.

Segregation generally means the separation of one group from another. In powder processing it usually refers to the physical division of a homogeneous particle population, most commonly on the basis of size, although other parameters such as particle density can also be important. Segregation proceeds via two different mechanisms depending on whether it is promoted by vibration or aeration of the sample. Understanding both effects is important for the successful identification and resolution of a problem.

Gently shaking a sample often encourages larger particles to rise to the surface, an easily observable effect. As the powder is agitated finer material travels down filling inter-particulate gaps and forcing larger particles upwards. Segregation of this type is largely driven by differences in particle size.

On the other hand, with an aerated sample, especially one close to the point of fluidization, larger and/or heavier particles can sink to the bottom of the sample leaving fines disproportionately distributed in the upper layers. Here, the bulk of the powder behaves like a fluid through which heavier particles fall in the same way as they would through a lower density liquid.  With samples containing similarly sized particles of different density it will be those that are heavier, denser, that proceed downwards.

Figure 1 – Examples of Segregation

With dynamic powder characterization the flow energy of a sample is measured by recording the force and torque acting on a blade as it rotates through the material in a defined pattern. For segregation the technique has two intrinsic benefits.

Firstly, it is possible to submit the sample to controlled segregation cycles, well-defined low stress agitation for a set period of time. Determining the extent and rate of change of flow energy as a function of number of segregation cycles assesses the tendency of the sample to segregate. Generally speaking, with samples that segregate easily flow energy will change significantly and quickly as the number of segregation cycles increases.

Secondly, dynamic measurements can be carried out after the sample has been aerated or even fluidized. This makes it possible to apply a test methodology that measures if and how the powder tends to segregate as a function of aeration, meaning that both segregation mechanisms can be investigated.

Segregation is a major issue for pharmaceutical manufacturers, with heterogeneous distribution of an active ingredient the most pressing driver for its avoidance. Dynamic powder characterization enables the investigation of both the tendency towards, and the mechanisms of, segregation providing useful information to enable formulators and process engineers to mitigate this effect.   

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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