Rheological Techniques for Improved Powder Measurements

Interview conducted by Stuart MilneNov 24 2016

Denis Schütz, Lead Scientist Powder Flow for Anton Paar, talks to AZoM about the importance of measuring powder flow and how the MCR powder rheometer from Anton Paar is ideally suited for various powder measurement applications.

SM: What internal and external influences impact powder flow?

DS: The easier question in this case would be: what does not influence powder flow? Internal influences like shape size and distribution of the particles influence powder flow in a dramatic and unpredictable fashion. The same is true for external influences like temperature and moisture. Overall, there are over 200 influencing factors that potentially can change the way a powder behaves. Therefore it is essential to measure the flow behavior of powder to ensure a smooth process.

SM: Why is powder flow important and how does this differ between various powder types?

DS: Powders are ubiquitous in industrial processes. Simply put, almost every producing industry uses powder in some form, and where something is as widespread, problems will arise. With proper process characterization and continuous monitoring these issues can be avoided, leading to less downtime and a higher, more consistent quality of the final product.

SM: What are the main challenges when designing measurement equipment for bulk solids?

DS: The main challenge is that powder “remembers.” For example, tapping any powder on the desk will change the way the powder behaves and this often carries over into the measurement process. This, however, is just one of the possible operator or storage influences that can be a challenge to overcome. Our measurement technique overcomes those by using one of two strategies: We either fluidize the powder to erase “powder memory” or we apply a large normal force to more or less “overwrite” it. Together with the large amount of automation in our measurement protocols, this gives us the opportunity to reach extremely high repeatability with our instruments.

SM: How does fluidization influence the measurement process?

DS: In two ways: On one hand the process of fluidization can be used to prepare the samples. On the other hand it also allows us to characterize the powder in this fluid state with proven rheological methods, very similar to what is used on liquids.

SM: Powder measurements are essential in many industries. The food and beverage industry rely heavily on these types of measurements as a leading quality indicator. The coffee industry is no exception. What benefits does the MCR powder rheometer offer when measuring the cohesion strength of coffee creamer samples for example?

DS: The ideal procedure would be to perform an easy screening test for any batch of product - a cohesion strength analysis. This serves as a good QC step that will give you an overview in two minutes whether or not anything inside your powder has changed. Further investigations would include, for instance an in-depth analysis of the wall friction coefficient. This value is directly connected to the hopper angle. Another method would be to examine the fluidization, de-fluidization and fluidized bed rheology behavior, which is connected to the pneumatic transport.

SM: Why is it so important to perform fluidization bed measurements and why are rheological measurements ideally suited for these measurements?

DS: Fluidization is a widespread industrial process, and in the past has often been overlooked. Our system gives both the process designer as well as the quality control engineer the power to exactly measure the behavior of powder in the condition it is exposed to during the fluidization process. In whatever condition the powder is: fluidized to pressed, moist to dry, in inert gas or air; the MCR powder rheometer can be considered as a complete toolbox and not just as a single purpose device.

SM: What are the major advantages of using your MCR powder rheometer?

DS: The two most important advantages are accuracy and versatility. The accurate EC motor, which is able to resolve torques down to 1 nNm, together with the unique sample preparation allows us to detect the slightest changes within a powder.

Also, if your process includes some liquid or even solid components, there is no need to buy two instruments since the range of rheometer accessories enables much more than just powder rheological measurements.

SM: How do you plan on developing these systems further for improved powder measurements?

DS: The next obvious steps are to add temperature control and the possibility to control moisture content in the air. But we are also developing heavily into the direction of alternative measurement geometries that would prevent channelling in a fluidized bed and therefore make more cohesive powders measurable in the fluidized bed rheology measurement process.

SM: Where can our readers learn more?

DS: Further information on our system, the MCR powder rheometer, can be found at www.anton-paar.com. In the document finder, a selection of application reports can be found to learn more about certain applications.

To gain fundamental knowledge I recommend the book “Powders and Bulk Solids” by Dietmar Schulze for consolidated (unfluidized) powders and the seminal “Fluidization Engineering” by Kunii und Levenspiel.

About Denis Schütz

Denis Schütz is a graduate of Graz University and PhD graduate of The Technical University of Graz.

He has been studying powders and processing for 10 years and developed the powder cell as the applied scientist.

After release of the instrument he now serves as Lead Scientist Powder Flow for Anton Paar.