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Powder Flow Testing at Elevated Humidity and Temperature

An Interview With Dr. Erik Miller

Do temperature and humidity affect your powder flow behavior in gravity discharge from a bin? You may think so, but how do you know for sure? Is there a way to run tests that can conclusively prove what the outcome will be? Unfortunately, using your process equipment to find out can be disastrous if you have to shut everything down because of erratic flow or blockages.


Brookfield Responds: Q&A

Powder Question Challenge

11/2016 - Customer Challenge: The problem that we experience during stoppages is failure of powder to discharge smoothly from the hopper once we restart processing. How do we predict this type of problem and its cause? What corrective action can we take?

Brookfield Answers: Powders will continuously settle once placed in a bin or containment vessel. Void spaces between powder particles will gradually fill as air is squeezed out during settling. Rate of settling in the bin is unique to each powder based on particles sizes and shapes. The shear cell method for predicting powder flow behavior during gravity discharge from hoppers also has the ability to characterize change in flow behavior when the powder is dormant in the bin. The Flow Function test using the Brookfield Powder Flow Tester automatically predicts flow behavior when discharge commences after the bin is filled up. The same test method can be repeated at different time intervals after the initial Flow Function test to predict the effect of remaining dormant in the bin.

Powder Question Challenge

10/2015 - Customer Challenge: I am using a Carr Index for gaging my powders in a pharmaceutical application. Can the Powder Flow Tester measure a Carr Index and how accurate will this be to my historical data?

Brookfield Answers: The Carr Index is a popular method used in the pharmaceutical industry (and others) for characterizing powders. It uses a single value and can be expressed as follows: Where ρB is the freely settled bulk density of the powder, and ρT is the tapped bulk density of the powder.

A Carr index greater than 25 is considered to be an indication of poor flowability, and below 15, of good flowability. However, a tapped bulk density can be subjective.

The Powder Flow Tester compresses the powder sample at specific compaction pressures and graphs the actual bulk density value as a function of consolidating stress. This scientific approach is called by the name “Compressibility Index”. It has become a popular method because there is no human operator involved in running the test. Your historical data will appear on the density vs. consolidating stress curve generated by the PFT. The PFT data is likely to show better accuracy and repeatability.

Powder Question Challenge

10/2015 - Customer Challenge: I have materials that sometimes sit for extended periods of time, perhaps up to a week in duration. How can I predict the flowability of a powder that has sat for a period of time?

Brookfield Answers: The Powder Flow Tester supports a test called “Time Consolidation”. This test allows the user to predict the flow properties of powders that remain stationary a period of time.

One choice is the Quick Time Consolidation test that evaluates powder behavior after a 2 hour time period of being stationary. The issue is to determine how flow properties change due to the settling action of the powder’s self weight.

The Standard Time Consolidation evaluates a 12 hour time interval – essentially investigating what happens when powder is left in a bin overnight. It is best practice to run the Quick Time Consolidation test first. If the material is non-flowing after 12 hours there is no need to run a longer test.

Powder Question Challenge

9/2015 - Customer Challenge: What is the largest size particle I can test using the shear cell?

Brookfield Answers: With the large shear cell, the maximum particle size is 1mm; with the small shear cell, the maximum particle size is 250um.

Powder Question Challenge

8/2015 - Customer Challenge: Can I measure particle size with the Powder Flow Tester?

Brookfield Answers: No; the PFT measures the flow of powders through the orifice of a feeder system or against a hopper wall. While the shapes and sizes of the particles contribute to how they flow, the Powder Flow Tester will measure their flowability only and not the size of the particles.

Powder Question Challenge

7/2015 - Customer Challenge: How easy is the Powder Flow Tester to use? Do I need advanced training?

Brookfield Answers: No. Anyone can be trained to prep and run a sample on the Powder Flow Tester in 5 minutes. The PFT is a very simple and straightforward instrument to run.

Powder Question Challenge

6/2015 - Customer Challenge: How quickly can I get my data from the Powder Flow Tester?

Brookfield Answers: A standard Flow Function test can be run in 25 minutes. There are faster tests that are supported for QA/QC testing. There is a 5 point QA/QC test that is run in 16 minutes and a 2 point test that takes 10 minutes. Bulk density is a common QA/QC test and can be run in 90 seconds.

Powder Question Challenge

05/2015 - My Company produces various types of sugar. It seems that the granular sugar discharges from our hopper with relative ease, but the powder sugar is more problematical. Is there a way to quantitatively determine how difficult the powder sugar will be to process?

Brookfield Answers: The shear cell is the only scientific instrument that can quantitatively measure the flow behavior of the sugar particles. The reason is that the shear cell shears the particles against one another to evaluate the inter-particle friction, which is essentially resistance to flow. This data can be used to calculate the dimensional requirements for hopper opening and half angle to ensure continuous flow.

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