Powder processing presents widely recognised challenges. Equipment selection and design typically call for specific expert input and even then, over the long term, day-to-day problems with product variability, blockages and stoppages often erode profitability.
Powders usually have a specification defined on the basis of composition and particle size, but successful powder handling and processing requires greater insight into the potential behaviour of the powder than these analyses afford. Powder flowability is a property that is often identified as being useful to measure and there are a wide range of options when it comes to test equipment. This raises the question of how to choose a powder tester that is well matched to a specific, process-related need.
By Tim Freeman, Managing Director, Freeman Technology Ltd
The commercial introduction of a new powder testing technique – uniaxial testing – makes it timely to review the most valuable testers for process support and compare their suitability for specific applications. Here we consider the advantages and limitations of uniaxial testing, biaxial shear cell analysis and dynamic powder testing within this context.
Exploring requirements for powder testing
A robust assessment of the relative merits of alternative powder testing options relies, in the first instance, on understanding why testing is carried out within the process environment.
Powder testers differ substantially in terms of the data they deliver. Matching the capability of each tester and the information they provide to defined process needs is the key to choosing an appropriately specified tester that will yield a good return on investment.
Powder flowability measurements can improve manufacturing efficiency in a number of ways and are routinely implemented in:
- QA/QC – to assess the consistency of a raw material or finished product; to evaluate an alternative supplier; and/or to achieve value added product performance.
- Routine plant operation – to optimise aspects of manufacturing practice, such as hopper fill level and shutdown protocols; to troubleshoot issues with product quality or plant performance.
- Equipment design and specification – to determine whether existing plant can be re-used in the manufacture of a new product and/or to develop an optimal process design.
One of the first issues to consider when selecting a tester is relevance. To be relevant, and consequently useful, a tester must measure a property that correlates directly with performance in process.
It is not unusual to test a powder in QA/QC against a defined specification and find it acceptable, but then encounter problems when the material is introduced into the plant or released to the customer. This situation results from a failure to define a relevant specification i.e. one that relates to how the powder will actually perform during a particular processing stage.
The sensitivity of a powder tester is crucial to its ability to support process optimisation. Certain techniques are inherently more sensitive than others but repeatability and reproducibility are also critical. Where instruments utilise the same technique, the one with higher repeatability and reproducibility will more successfully differentiate samples and, therefore, offer greater insight to advance process understanding.
Features which enhance repeatability and reproducibility include well-defined measurement protocols, precision engineering and a high level of automation. Such features also beneficially impact the third factor that influences the value of a tester, which is practicality. Ease of use and measurement time both affect the ability of a powder tester to provide efficient manufacturing support.
Reviewing the options
The limitations of simple powder testing methods, such as tapped density, flow through an orifice and angle of repose, have led many powder processors to seek alternative techniques to support in this area. Uniaxial testing, on the other hand, is a recently commercialised approach with considerable promise. Understanding what each of these techniques has to offer is crucial for their efficient application.
How does it work and what does it measure?
Uniaxial testing involves measuring the normal stress required to fracture a free-standing consolidated powder column. It delivers values of unconfined yield strength (UYS) as a function of a preconsolidation or major principle stress, σ1, the same terms that are generated less directly by translational/rotational (biaxial) shear testing.
Figure 1: Uniaxial testing involves the fracture of a consolidated powder column and directly measures UYS.
The practicalities of application
Uniaxial testing is simple and fast, with measurement times in the order of just a few minutes. Furthermore, equipment costs are low - automated instruments are around £10,000, and manual versions around half this price. These are crucial advantages for the manufacturing environment.
The primary challenge in implementing uniaxial testing is to construct a uniformly consolidated powder column, and this is especially difficult for more free-flowing powders. Recently commercialised instrumentation for uniaxial testing (Uniaxial Powder Tester, Freeman Technology) has features that directly address this challenge and enable highly repeatable measurement for a wide range of powders.
However, the technique is somewhat limited in terms of the conditions applied during measurement, with relatively high consolidation stresses. It is also less suitable for very free-flowing materials.
Areas of application
Uniaxial testing is arguably the simplest, most inexpensive way of quantifying powder shear strength, with high repeatability. Shear strength relates to how a powder flows, making uniaxial testing a cost-effective choice for the measurement and comparison of powder flowability.
It is likely to prove particularly useful to powder processors who currently rely on test methods that are failing to provide sufficient differentiation, or who currently have no access to powder testing. Uniaxial testing is well-suited to QA/QC for rapid assessment of the consistency of a raw material or finished product, and is an accessible choice for process optimisation/troubleshooting when exploring the causes of problems such as variable fill weight or erratic hopper discharge.
Biaxial shear cell testing
Understanding the technique and what it measures
Biaxial shear cell testing involves measuring the forces required to shear one consolidated powder plane relative to another. Shear stress is measured at a range of applied normal stresses and the resulting data are used to generate UYS values by a process of extrapolation.
Analogous methods are applied to measure wall friction - the friction between a coupon of an actual or potential material of construction and the powder in question. Combining both sets of data enables the calculation of flow function (FF) and flow factor (ff), the two parameters routinely used in hopper design.
Figure 2: All shear cell designs measure the forces required to shear one consolidated powder plane relative to another to quantify certain powder flow properties.
The practicalities of measurement
Since shear cell testing was introduced more than 50 years ago, substantial effort has been expended in refining instrument design. Today’s shear cell testers vary in their level of automation and also in terms of cost, with the most precise instruments enabling effective control of the test conditions applied and highly repeatable measurement.
However, even with the best systems measurement times remain in the order of 10 – 20 minutes and some expertise is required for effective application of the technique and processing of the resulting data.
The fact that testing is carried out using consolidated samples means that the relevance of the data is limited for certain processes; however, this is also the case for uniaxial testing, which demands the application of higher consolidation stresses.
Areas of application
Biaxial shear cell testing was developed specifically to generate parameters for hopper design and this remains a primary application. By measuring powder properties under the moderate to high stress conditions that develop in small to large hoppers respectively, biaxial shear cell testing is useful not only for design, but also for determining whether existing storage is suitable for a new powder product and for troubleshooting erratic or sub-optimal discharge, a perennial problem.
More broadly, shear testing enables the investigation and comparison of how consolidated powders transition from the static to dynamic state. Shear cell analysis is also used more widely to support process optimisation and can be effective within this context providing there is recognition of its limitations with regards to predicting the behaviour of free-flowing powders and performance in a low stress and/or dynamic environment.
Dynamic powder testing
Understanding the technique and what it measures
Dynamic powder testing involves measuring the axial and rotational forces acting on an impeller as it is precisely rotated through a powder sample. The properties measured include Basic Flowability Energy (BFE), which quantifies how easily a powder flows under forcing conditions, and Specific Energy (SE), which defines unconfined flow properties.
Dynamic testing can be applied to consolidated, moderate stress, aerated or even fluidised samples, to comprehensively characterise flow behaviour across a complete range of process relevant conditions.
The practicalities of application
Dynamic test methodologies are well-defined and instrumentation is precision engineered and highly automated. As a result, dynamic powder testers, though relatively expensive, deliver unrivalled sensitivity and versatility.
For example, using dynamic testing it is possible to clearly differentiate samples that would be classified as identical with shear cell testing but would go on to perform differently in a process.
Measurement times are in the region of 5 – 10 minutes for a basic assessment of flowability, but the range of testing available means that longer, more in-depth investigations of the effect of variables - such as flow (or strain) rate, air and moisture content, storage time and electrostatic charge - are routinely implemented.
Testers that offer dynamic powder characterisation also enable shear and bulk property measurement and can be productively used to build a database of powder properties to support the optimal processing of specific powders in a wide range of different unit operations.
Areas of application
Dynamic powder testers provide detailed and valuable information from early product and process development through to QC, but most rapidly deliver an attractive return on investment when other techniques fail to rationalise poor process or product performance.
The ability to measure as many as 25 – 30 different powder properties makes dynamic powder testers unrivalled in terms of troubleshooting capability, while exceptional sensitivity allows advantage for the robust differentiation of raw materials and products.
The ability to directly characterise the response of a powder to air, up to and beyond the point of fluidisation, is a unique benefit that offers enhanced support for the optimisation of, for example, fluidised bed and pneumatic conveying processes.
Figure 3: Dynamic powder testing measures the powder in motion and can be applied to samples in a wide range of different states to simulate numerous process environments.
Making a choice
No one powder tester offers the best cost benefit ratio for every application. The introduction of a new powder tester, the Uniaxial Powder Tester from Freeman Technology, brings greater choice in powder tester selection, and crucially, reduces the price point at which robust and repeatable powder testing is accessible.
However, over the long term, alternative testers can ultimately yield a greater return, providing that the information they produce is relevant to the process need, and therefore of practical and commercial value. Rigorously reviewing what a tester can deliver, against what is required, is the key to an optimal choice.