How Does a Bulk Solid Break Down

Introduction

How a powder or bulk solid breaks down during its movement through a factory is of vital interest to a manufacturer. Should a product in granular form break down there can be several outcomes: small particulates that are broken away can be released into the atmosphere as dust; if small particulates are dispersed throughout the bulk solid the flow characteristics can be changed dramatically resulting in large scale problems; the final product will not have the form required to either satisfy the customer’s requirements or perhaps will not have the surface area to volume ratio required for its purpose.

Information from these studies can prove useful to customers so that alterations can be made to formulations or modification to processing severity to minimise the changes that occur in the product.

Analysis of Bulk Sample

A simple quality control or new product development test allows the customer to imitate the wear of process on the granules, in this case, by using a number of attrition cycles on the PFA.

The following results were generated for two types of coffee granule, assessed as a bulk, using the powder flow analyser attrition test.

The products were initially conditioned to remove any user variation in filling technique. Then following one testing cycle (to profile the granule characteristics) ten attrition cycles occurred to cause attrition, followed by one final testing cycle. The difference between the initial and final testing cycle shows how susceptible the bulk solid is to breaking down during processing.

The above curves would show the customer that the profile of the regular coffee granule has clearly changed after the 10 attrition cycles have taken place, whereas the decaffeinated coffee product has not changed as dramatically suggesting that it would resist process attrition better.

Using the Exponent software we are able to analyse the date further:
The Exponent software is used to measure the area under the curve before and after the attrition cycles. It can clearly be seen that the Regular Coffee has changed greatly after the attrition cycles.

Analysis of Individual Granules

In order to improve a bulk solids resistance to attrition it may be necessary to study the individual granule or particle. This analysis will show exactly how the product breaks down and can allow the user to carefully monitor the effect that formulation or manufacturing changes will have on its attrition resistance.

Individual particles and granules can be analysed by carrying out a simple compression test using the system as a laboratory materials tester and carrying out experiments while measuring at 500pps data acquisition rate.

The below graph shows the curves obtained when two different coffee granules are analysed. It can be clearly seen that the decaffeinated coffee granule has a more distinctive snap; it is a harder and has a clear break point at a defined stress. Whereas the regular type of coffee is a much softer more “crumbly” granule that when compressed does not have the same distinctive fracture event but continuously breaks down under compression.

These results would tell the customer that the regular coffee would be more likely to break down during processing. This break down to its granular structure could result in dust produced into the atmosphere, small particulates in the bulk product causing flow problems, and an unwelcome change to the quality of the final product. By making adjustments to the formulation or manufacturing process it is possible to monitor how the physical characteristics are changed and if they will improve the products resistance to attrition.

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