Understanding Powder Processing Behavior Using Powder Rheometry

In order to process and manipulate powders, a diverse range of unit operations are utilized. These invariably subject materials to a large variety of conditions – from the low stress, dynamic conditions experienced during fluidization, to the relatively high and static compaction stresses observed in hoppers.

In order to design and monitor the transfer systems and unit operations which make up any given process, a thorough understanding of how a bulk material will behave over a range of conditions, and in various phases of flow – whether in motion, stationary, or about to move – is crucial.

There is still a tendency to identify a simple, single parameter with which to characterize a powder – despite the extreme differences that exist between various operations. Single number characterization, like those categorizing performance from ‘cohesive’ to ‘free flowing’, will probably not be enough to evaluate and predict the performance of materials across a range of processes fully.

The key is to ensure compatibility between processing equipment and the characteristics of the powders it will handle. From the outset, this method requires a comprehensive understanding of the bulk behavior of the materials to give relevant information to process design and development.

FT4 Powder Rheometer®

The FT4 Powder Rheometer is a universal powder tester which supplies reliable, automated, and comprehensive measurement of bulk material characteristics. To increase processing efficiency and aid quality control, this information can be correlated with process experience.

The FT4 specializes in the quantification of dynamic flow properties, it also incorporates a Shear Cell, and the ability to measure bulk properties such as compressibility, density, and permeability, allowing comprehensive characterization of a powder in a context which is process relevant.

Uniaxial Powder Testing for Optimizing High Shear Wet Granulation

Pneumatic Conveyance

Powder is transported in a fluidized state using air flow or a vacuum during dilute phase pneumatic conveying. There are various issues that can arise in this process, like adhesion, choking, or flooding.

Therefore, a powder’s response to the presence of air, and permeability are both likely to be vital properties. Through appropriate testing it is possible to establish if a material can reach a fluidized state, and the air flow needed in order to attain this. This testing supports the ascertainment of optimal operating parameters.

Sensitivity to air can be calculated by employing the FT4’s Aeration test. Flow energy is measured in this test, while air is introduced to the powder bed at increasing velocities. This allows precise and simple identification of when fluidization happens, as shown in the two examples in Figure 1.

Typical Aeration test result.

Figure 1. Typical Aeration test result.

When flow energy reaches a near zero value, fluidization is considered to have happened. In Figure 1 both powders reach a fluidized state, yet for one powder this happens at an air velocity of 4 mm/s, but the other needs a higher velocity of 8 mm/s.

A permeability test directly measures Pressure Drop across the powder bed, calculating a powder’s resistance to the transmission of air. The grey trace in Figure 2, a lower pressure drop, shows higher permeability.

Typical Permeability test result.

Figure 2. Typical Permeability test result.

As air will more readily pass through the powder bed instead of transporting it efficiently, this may be detrimental to a dilute phase pneumatic conveyance process. This information may be utilized for optimizing process conditions and for specifying process parameters in pneumatic transfer systems.

Size Reduction

A reduction in particle size before further processing can be advantageous in some applications. Size reduction for example, can increase the dissolution rate of an active ingredient in a pharmaceutical blend, or prior to a mixing process, it can help enhance homogeneity within the mixture...

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This information has been sourced, reviewed and adapted from materials provided by Freeman Technology.

For more information on this source, please visit Freeman Technology.

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