Bulk Solids Basics: Time Consolidation

Bulk Solids Basics: Flow Properties (Shear Cell)
After years of working with bulk solids heat exchangers, I have come to the realization that bulk solids knowledge is not as widely spread throughout the engineering society as I first thought. So, I’ve taken this opportunity with Bulk Inside to provide a bi-weekly content series that will summarize some things you should know. Have fun reading, and do not hesitate to email me at  Gerald.marinitsch@solexthermal.com. In case you have further questions.

The behavior of bulk solids may change over time when stored and are exposed to compressive stress. With compressive stress, the weight of the particles in a silo or container is enough to change the properties of the bulk over time. This effect is called time consolidation, which can lead to caking.

What exactly happens in the bulk over time? Depending on the bulk’s base material, many different effects have an impact on the time consolidation, including:

  • Physical, such as de-aeration, plastic deformation, or condensation
  • Chemical, such as chemical binding on touching surfaces or crystallization
  • Biological, such as fungal growth

In some of these cases, the bulk density may also change. It is most likely to increase over time, which usually reduces the flowability. The following chart shows how the consolidation stress “s_c” affects the yield strength “s” over time. The different lines show different consolidation times, while T=0 is the yield strength behavior without time consolidation.

This can create an issue if bulk solids are stored over a long time in a silo without movement, especially if time consolidation wasn’t considered in the design of the silo bottom or discharge device.

What can be done to mitigate time consolidation?

Time consolidation in principle cannot be avoided. However, the effect of time consolidation can be reduced by storage conditions such as moisture and silo levels. Another possibility is to move the product via a recycle loop in the silo. In any case, it is important to investigate the time consolidation behavior in mind when sizing a silo, hopper, or discharge device as this can help to avoid troubles in the future.

Bulk solids basics: Time consolidation

Next time, we will look at the angle of repose, which a quite easy to determine figure for bulk solids.

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