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Powder Flow Testing with the FT4 Powder Rheometer

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Bulk properties are not a direct measurement of flowability or shear, but nevertheless influence process performance and product attributes. The FT4 Powder Rheometer® measures typically three types of bulk powder properties: -

 

Density

 

Density defines the relationship between mass and volume. In principle this seems a simple concept, but the nature of powders means that their packing structure can change easily and significantly. Therefore when defining density, it is essential to ensure the packing state is well known and can be reproduced. This is achieved on the FT4 using a Conditioning cycle. When combined with other features such as the built in balance and Split Vessels, which allow a precise volume to be attained, the Conditioned Bulk Density can be measured with unprecedented levels of accuracy.

Bulk Properties

Compressibility

 

The measurement of Compressibility is achieved by applying increasing levels of compressive force with a piston to a conditioned powder and measuring the change in volume as a function of the applied load. The Vented Piston ensures that air trapped within the powder is able to readily escape, and the high resolution of the position measurement system allows for precise definition of Compressibility, expressed as a percentage change in volume for a given applied normal stress.

It is possible to apply a number of mathematical models to this data, but it is important to consider that in doing so, trends may be exaggerated or reduced. Fitting Mohr stress circles to the yield locus identifies the Major Principle Stress (Sigma 1) and Unconfined Yield Strength (Sigma c), and the ratio of the former to the latter quantifies the Flow Function, FF. Flow Function is a parameter commonly used to rank flowability, with values below 4 denoting poor flow and above 10, good flow.

 

Wall Friction

 

The Wall Friction test provides a measurement of the sliding resistance between the powder and the surface of the process equipment. This is particularly important for understanding discharge behaviour from hoppers, continuity of flow in transfer chutes and tablet ejection forces. It is also useful when investigating whether a powder will adhere to the wall of process equipment and various other surfaces, such as the inside of sachets, capsules and other packaging material.

 

The measurement principle is very similar to the shear cell test, but rather than shearing powder against powder, in this test a coupon of material representing the process equipment wall is sheared against the powder in question. The FT4 Wall Friction accessory allows for a range of coupons to be investigated, and bespoke surfaces can be manufactured if required.

 

Data is typically represented as a plot of shear stress against normal stress, allowing the determination of Wall Friction Angle (phi). The greater the wall friction angle, the higher the resistance between the powder and wall coupon.

 

This data can be utilised in specific studies, as outlined above, but is also required as part of a hopper design exercise.

 

Hopper Design

 

Hoppers are used extensively throughout the processing environment and whilst they are often considered to be simple systems, they are responsible for causing a great deal of process interruption and product quality issues.

 

If a powder possesses properties that are not optimised for the hopper geometry and equipment surface, then flow from the hopper may be variable or even none existent. However, since the pioneering work carried out by Andrew Jenike in the mid twentieth century, it has been possible to utilise data from shear cell and wall friction tests to calculate the critical hopper dimensions to ensure good flow.

 

The FT4 comes with fully automated hopper design software, which takes the results directly from shear cell and wall friction tests and runs the data through the hopper design algorithms. The result is a fully automated hopper design exercise in less than 3 hours.

Today, this hopper design process remains one of the very few fundamental approaches to equipment design, based on an understanding of material properties and the stress regime within the equipment. Unfortunately, such an approach is not available for mixers, feeders, conveyers, dryers, compression processes or any of the other unit operations routinely used in powder processing.

 

Please contact us for more information on the hopper design process, if required.

 

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