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

Image Credit: Freeman Technology

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. Image Credit: Freeman Technology

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. Image Credit: Freeman Technology

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.

Yet, different size reduction methods like milling, crushing, or grinding can have a different effect on the physical properties of the resulting particles, like shape and size, and on the associated flow properties in turn.

Despite the advantages which can be associated with decreased particle size, powders with smaller particles usually act in a more cohesive manner. For lighter and smaller particles, compared to the gravitational forces required to initiate and maintain flow inter-particular forces may be relatively strong.

Calculating a powder’s compressibility provides information on the material’s response to being consolidated via an applied load and the packing efficiency of the powder bed, establishing the level of entrained air. Typically, smaller particles lead to a less efficient packing structure, which is normally associated with more cohesive materials.

A powder’s resistance to flow can also be affected by size reduction, with smaller particles of the same morphology usually generating lower flow energy values as they can be displaced more easily by the rotating blade.

Yet, alterations in morphology and surface properties after size reduction can also have an influence on the friction and inter-locking between particles, as can be seen in Figure 3.

Changes in Flow Energy due to size reduction processes

Figure 3. Changes in Flow Energy due to size reduction processes. Image Credit: Freeman Technology

Dispensing

A key operation in a number of industries is the dispensing of powders. This is to ensure the correct volume or mass of material is passed through to the next stage of a process. Understanding powder behavior in this operation can enhance productivity and efficiency significantly.

Powder will pass through an opening and fill a container, mold or die below in a typical dispensing operation. The ease with which powder flows through the opening, and the rate that can be acquired, is vital to creating an efficient process and achieving consistent filling performance.  

Mechanical locking and inter-particular cohesion can establish if particles will become trapped and form bridges when passing through an opening. The effects of these properties can be calculated by measuring the Specific Energy.

Uniaxial Powder Testing for Optimizing High Shear Wet Granulation

Image Credit: Freeman Technology

This is quantified as the FT4 blade moves upwards through the powder bed and the particles can flow in an unconfined state. In this type of process, powders which produce high Specific Energy values are more likely to experience blockages and inhibited flow.

Permeability measurements also supply an insight into the ability of the powder to displace air. Air must pass readily through the powder bulk for efficient filling, not only to prevent flow being compromised at the dispenser outlet, but also allowing air to vacate the container and reduce the formation of air voids.

Better permeability can be advantageous to dispensing operations, in contrast to pneumatic conveyance outlined above.

Conclusions

The FT4’s multivariate test techniques are ideally suited to characterizing a variety of process-relevant properties which will effect and can be correlated to performance in a number of different processes.

These correlations can be employed to construct a design space of powder properties that create good performance, against which both new formulations, or incoming or outgoing batches, can be evaluated to predict downstream behavior.

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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