By Robert G. McGregor
Unlike liquids, which, under the influence of gravity, flow easily as they spread out in a horizontal direction, powders exhibit a structure due to internal friction. This structure allows powders to form piles with angles relative to the surface on which they rest.
At ambient conditions, the flow behavior of powders won’t change when subjected to variable shear rate, whereas that of most liquids will. Liquids show limited change in flow behavior under increased pressure. In contrast, when loaded into a hopper, internal pressure can increase the strength of a powder, which affects flowability — i.e., the inter-particle friction increases. In consequence, when powders are loaded into a bin they may not flow as easily, especially through restricted openings like converging hoppers.
What are the most often cited issues with powders used in industrial processing operation?
The classic challenge is the failure of powders to discharge reliably from bins and hoppers into feeders, secondary hoppers, tablet machines, transport vehicles and other type equipment. A discharge failure causes unwanted process interruptions that can lead to the shutdown of equipment and even plants for shorter or longer periods, to correct the flow restrictions or stoppages.
Of course, some plants cope with this situation by instructing an operator to bang on the hopper surface with a hammer to dislodge any hung-up material.
Elsewhere quality-control departments constantly evaluate incoming raw materials in powder form, sourced from multiple suppliers. These shipments may have unacceptable variances that end up affecting process-flow behavior. Incoming inspection tests look to measure variability of particle size, moisture content and basic ingredient make-up. Even then, it’s not a sure thing that proper flow will take place when the powder is loaded into the plant equipment. Many companies rely on an angle-of-repose test to correlate with flowability, but this traditional approach is unfortunately not reliable.
Finally, R&D departments adjust powder-product formulations regularly to satisfy customer demand for improved properties, whether that be better coating action for batters, enhanced taste using unique spice combinations or rapid dissolving of nutraceutical powders when put into solution. New powder formulations won’t necessarily have the same flow properties as those previous, thereby leading to production problems when the process is scaled up to high volume.
How can flow problems be predicted in advance? There is a proven scientific method, called the "flow function test," which can analyze powders for flow behavior in gravity discharge. ASTM D6128 describes a procedure for compressing and shearing powder samples in order to measure the inter-particle friction. The device for performing the procedure is called a "Shear Cell."
Answers to questions
The resulting data produces a "flow function," much like that obtained for liquids when testing with a viscometer to create a "flow curve." The consolidating pressure felt by the powder at the bottom of the bin is plotted on the x-axis and the powder strength is recorded on the y-axis. Analysis of the "flow function," shown in a chart, provides a means for differentiating powder flow behavior according to industrial categories that range from "free-flowing" to "non-flowing."
Powders of highest interest to processors are those in the "cohesive" and "very cohesive" categories because flow issues for these are more pronounced.
Further analysis of flow-function data leads to calculations for hopper-opening dimensions that 1) achieve reliable powder discharge in "mass flow" and 2) predict rathole formation in "core flow." Most processing operations have core-flow behavior due to the cohesive nature of the powder and the bin/hopper design. Mass flow is preferred, if it can be achieved, because it inherently avoids problems with desegregation. The figure below illustrates differences between "mass flow" and "core flow."
Although ASTM D6128 has existed for many years, the instrumentation needed was expensive and required an experienced operator to set up and run the tests. Each data point was generated using a fresh material sample. Therefore the test was lengthy, taking perhaps a half-day minimum to generate all the data points and create the curve seen in the chart. A technical expert was then needed to interpret the results.
An instrument for everyone
More recent advances in electronics and computer science have introduced a new generation of shear cells that make the test method significantly easier to set up and run. An educated generalist can execute the flow-function test and collect the required data in well under an hour. Automated analysis provided by software that comes with the shear cell delivers results in less time than it takes to actually conduct the test. It calculates dimensions needed for a hopper opening that achieves reliable mass flow behavior and makes predictions on rathole diameter in core flow.
A second type of test, called "Wall Friction," evaluates the friction between powder and hopper wall. This enables the calculation of another parameter called "hopper half angle," which is also important for mass flow design.
Increasing demand for use of shear cells in both research and quality-control applications is allowing suppliers to produce instruments in greater quantities and at substantially lower prices. The bottom line for processors is that this well-established scientific method for evaluating powder flowability is both cost-effective and commercially available. There is every reason to investigate shear-cell instrumentation and find out how this proven methodology can benchmark powders for flowability and provide guidance on hopper design modifications for improved operations.
Robert G. McGregor is general manager, global marketing, Brookfield Engineering Laboratories, Inc. He holds M.S. and B.S. degrees in mechanical engineering from MIT in Cambridge, Mass. Reach him at 508.946.6200 ext 7143 or [email protected].
Brookfield Engineering Laboratories has long been considered the World Standard in Viscosity Measurement and Control. From the original Synchro-electric Viscometer to today’s sophisticated DV-III Ultra Rheometer, the Brookfield name is synonymous with quality, dependability and reliability. In recent years, we have added the addition of texture analysis equipment to our line of products. This axial testing capability enhances Brookfield’s existing product line by providing more flexibility in solving the rheological and texture measurement needs of our customers.