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Assessing the Benefits of Powder Rheometry

April 18, 2012
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Use of a Powder Rheometer has enhanced powder-testing capabilities at the research center of AZO GmbH + Co. KG, Osterburken, Germany. Gains include not only dynamic powder testing, but also fully automated shear- and bulk-property measurement.

AZO is a specialist supplier of bulk-material handling equipment, with users concentrated in the food, pharmaceutical, bulk chemical and polymer sectors. The FT4 Powder Rheometer from UK-based Freeman Technology has contributed positively to the company’s approach to equipment design and powder processing plant specification.

AZO’s Osterburken site focuses solely on dry materials. Its turnkey solutions for powders and granules can typically include silos and hoppers for storage; filling stations for either intermediate bulk containers (IBCs) or sacks; dosing and weighing systems; and pneumatic conveying plant. The site has a dedicated research facility and test center where trials are conducted on full-scale plant. As many as 500 new materials are assessed in a year, and each one presents a powder-processing challenge.

Anatomy of the Challenge
With its full-scale test facility, AZO can trial and optimize individual powder processing solutions. However, such trials use substantial material amounts and are relatively expensive to run. The research facility identifies optimal processing solutions ahead of any trial, on the basis of experience and testing.

Before purchasing the Powder Rheometer the research teams used two powder-testing systems to guide this identification: an in-house Jenike-style shear cell and a basic powder tester that offered techniques such as angle-of-repose and tapped-density methods using the Carr’s Index.

Shear cells were developed by Jenike in the 1960s to support design methodologies he had proposed for hopper specification. They remain in widespread use today and also are more generally used for assessing powder cohesivity. However, despite the technique’s longevity, its application to hopper specification, as well as the actual testing, remains an expert task often outsourced.

For process development studies, the shear cell was not ideal in terms of either efficiency or applicability. Shear analysis of a single sample was a lengthy process and results were operator dependent. Furthermore, while the data were useful in silo and bin specification, the technique provided little information about other aspects of powder behavior such as fluidization properties highly relevant to equipment design and operation.

Results generated by the powder tester similarly gave insight into a powder’s nature, but in addition to poor repeatability didn’t fully quantify behavior in a way that related to in-process performance. Tests such as angle-of-repose, angle-of-fall and Carr’s Index classify powder flow, across the range from free-flowing to cohesive, but don’t simulate the stresses and flow regimes a powder may be subjected to. Nor do they provide numerical descriptors that can be used to develop process predictive models. These limitations restrict the techniques’ applicability where the aim is to evolve secure design algorithms of the type routinely applied in other areas of design.

The limitations of both instruments, and the need for better powder characterization, were the basis for assessing alternative powder-testing instruments.

Choosing a Powder Tester
The FT4 Powder Rheometer was identified as a potentially suitable solution, providing capabilities such as:
• Automated shear testing with faster measurement times and no operator-to-operator variability
• Dynamic testing methods, to measure a set of parameters suitable for further insight into in-process powder behavior
• Materials measurement in an aerated state — a key feature of dynamic testing
• Use of small sample sizes for testing

To evaluate the instrument, four samples of well-known powders were submitted for comparative testing. The results were convincing, showing clear correlations between measured powder properties and known processing performance.

In terms of measurement practicalities, the new instrument delivers two important benefits. Firstly, it substantially updates and refreshes AZO’s shear-testing capabilities, reducing measurement times by a factor of four. Further, results are no longer influenced by the operator. A lighter analytical workload can now be spread across a greater number of people.

Beyond shear testing, however, the instrument’s wider capabilities are exploited to measure properties that enable new design strategies for core unit operations. The dynamic property of Basic Flowability Energy (BFE) is now the most commonly measured powder parameter. The standard BFE test is complete in less than 20 minutes. AZO engineers find it a fast, sensitive, reliable and differentiating predictor of powder behavior for new material initial assessment (see figure 1). In addition, because of its ability to detect batch-to-batch variability, BFE testing is now offered as a supplementary support service for customers, a complement to in-house quality-control techniques able to more sensitively differentiate between materials. Customers can confirm the suitability of an alternative supplier with a high degree of confidence or verify
product consistency and quality.

Shear analysis remains an important activity within the research center; more so now that it is easier to do and more productive. In addition, aeration/de-aeration testing and permeability measurement have become elements of the toolkit.

Storage, Discharge and Pneumatic Conveying
With automated shear analysis in place, and integrated software to streamline the application of Jenike’s design methodologies, AZO can test powders and design storage solutions more effectively than with the old shear cell. Previously, difficulties of shear testing, coupled with challenges applying the data, inhibited routine use of the Jenike method. Wall friction testing, which quantifies the strength of interactions between a construction material and a powder, is also carried out using the Powder Rheometer, especially for new construction or coatings materials.

For certain materials, such as cohesive powders like titanium dioxide, the Jenike methods break down, reporting values for outlet size, for example, wider than the diameter of the silo. The Jenike method generates suitable hopper dimensions to avoid problems such as channeling and arching, but this isn’t the case when powders are extremely cohesive. Efficient discharge of these powders requires mechanical aids — vibrating devices or air-injection systems. The new testing regimen efficiently identifies these powders.

A further issue is the impact of consolidation over time. The flow properties of a powder stored under a compressing load, even if just its own weight, can be transformed. With the new Powder Rheometer it is possible to directly investigate this behavior and use results to guide operational practice. This includes determining the frequency with which a silo must be emptied, for example, or assessing the need for re-circulation, where powder is routinely discharged and re-loaded to prevent excessive consolidation.

During discharge from a storage vessel, certain powders — especially those with particle sizes between 20 and 100 microns — can become uncontrollable, fluidizing as they draw in air, and flooding from the hopper. For powders that do not flood too easily, dosing screws are one option for controlling discharge. Rotary valves are an alternative for more challenging materials. Which system users adopt is a critical decision.

AZO is exploring whether the rheometer’s ability to directly quantify powder response to air can be exploited to more rapidly identify discharge solutions. It has had some success correlating aeration data with discharge characteristics, but other powder properties also are relevant. Work continues to refine the selection process through the most appropriate application of aeration data alongside other variables. Powder properties are already being used to investigate dosing units’ accuracy and output.

In pneumatic conveying, powders are transported in a fluidized state that can be directly studied via dynamic-powder characterization. Through appropriate testing it’s possible to determine whether a material can be fluidized or not, and to measure the air velocity required to reach fluidization. Such testing supports optimal setting of fluidization operating parameters.

For more detailed sizing of conveying systems, two parameters are especially important: throughput and pressure drop. Tests suggest that a number of dynamic properties and the bulk property of permeability may all be important for calculating these parameters. AZO researchers are working to establish closer correlation between pneumatic conveying performance and powder properties.

The Way Forward
AZO has a database that links different powders with appropriate processing equipment. This database is an ongoing reference resource and in the past was used alongside limited powder-testing capabilities to successfully steer solution development. What was lacking, however, was a more effective method to define and rationalize why certain powders and processes work well together.

The Powder Rheometer has opened up a wealth of new powder-characterization possibilities, leading to more efficient processing-equipment specification. The database is being rapidly populated with BFE values as the company focuses on being able to pinpoint processing solutions from a quick test with a small amount of material. Other dynamic properties also deliver new insight, with aeration energy proving especially interesting.

The nature of powder processing is such that there will always be a requirement both for expert interpretation and full-scale trials. However, by exploiting the full capabilities of the FT4 Powder Rheometer, AZO is gaining greater process understanding and moving toward a more scientific approach based on mathematical models. Such developments may ultimately reap rewards in testing productivity that would dwarf the gains already made.

This article originally appeared in the April 2012 issue of Processing magazine.

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