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Dynamic powder testing and measurement of bulk powder properties can complement shear cell testing to identify the causes of poor hopper performance in solid-dosage drug manufacturing.
Shear cell testing was developed in the 1960s specifically to deliver design parameters for hopper specification (1). Its introduction marked a major step forward in powder testing, and the associated hopper design methodologies were the first to bring a mathematical approach to powder handling equipment specification. They remain in widespread use, but issues with hopper performance are still commonplace, highlighting the limitations of relying solely on shear cell analysis to design and optimize this crucial unit operation. Dynamic powder testing and the measurement of bulk powder properties can complement shear cell testing, especially when shear cell testing fails to reveal the causes of poor hopper performance, such as erratic discharge and inconsistent flow.
The application of shear cell analysis
The design methodologies developed by Jenike for hopper specification center on identifying the flow/no flow condition that marks the boundary between acceptable and unacceptable hopper discharge (1). Similarly, shear cell analysis involves measurement of the force required to shear one pre-consolidated powder plane relative to another to induce the transition from stasis to flow. Figure 1 shows three common configurations of shear cells. The differences in physical geometry and configuration of the apparatus will influence aspects such as accuracy, repeatability, and applicability, but essentially, all shear cells are intended to measure similar properties of the sample.
Shear cell data is thus most relevant when investigating the performance of powders at the onset of flow, under moderate to high stress. This situation would occur in a large hopper in which the powder is stored under its own considerable weight. However, shear cell testing has a number of limitations that are problematic in developing a broader understanding of powder behavior, including:
These limitations mean that the behavior of many powders, in a variety of unit operations (including small hoppers) can be quite different from what might be predicted on the basis of shear cell data. If a powder is free-flowing or prone to aeration, for example, it may flood from a hopper, flowing far more easily than shear cell data might predict. On the other hand, if a powder is particularly sensitive to the impact of moisture, hopper discharge may cease far more readily than shear cell analysis would indicate, if the processing environment is poorly controlled.
Benefits of dynamic testing
Dynamic powder testing was developed in the 1990s to meet the need to measure powder flowability under process-relevant conditions (2). It involves measurement of the torque and force acting on a helical blade at it rotates through a powder sample along a defined path, as shown in Figure 2a. As the blade traverses through the powder, force and torque, with respect to height, are recorded. These two measurements are used to determine the energy gradient (i.e., work done per mm of travel) as shown in Figure 2b. Total flow energy is the area under the energy gradient curve.
Well-defined methodologies are in place for a range of dynamic properties, including the following:
Other parameters characterize how powders behave in a consolidated, conditioned, aerated, or even fluidized state, all of which can be investigated directly using dynamic methods. Dynamic testing, therefore, has strengths that directly address the limitations of shear cell analysis.
Optimizing hopper performance
Dynamic testing can be used to measure flowability under low stress conditions to simulate the conditions in smaller hoppers. Furthermore, dynamic testing can be deployed to systematically investigate the impact on powder flow behavior of a wide range of parameters, including aeration and moisture, even for relatively free-flowing powders. Such information can be valuable in troubleshooting poor hopper performance.
In addition, there are bulk powder properties that are not routinely measured but have been demonstrated to significantly influence hopper flow. Permeability, for example, is a measure of how readily a powder transmits air through its bed. Poor permeability is known to contribute to low and/or pulsatile flow (3).
In summary, shear, bulk, and dynamic properties are all relevant for the design and operation of hoppers. While shear cell data remains useful for generating fundamental parameters for large-scale hopper design, dynamic testing and permeability measurements can be valuable for optimizing the design and performance of hoppers, particularly smaller vessels routinely used in the pharmaceutical industry. Using these techniques in tandem allows comprehensive hopper optimization.
About the author
Jamie Clayton is operations director at Freeman Technology, 1 Miller Court, Severn Drive, Tewkesbury, Gloucestershire, GL20 8DN, tel: +44 (0) 1684 851551, firstname.lastname@example.org. For further information, read Jamie’s article in Pharmaceutical Technology’s Oct. 2015 issue, Identifying Powder Properties that Define Process Performance.