Powder processing. Storage and flow in silos

The storage of divided solids is a common operation in many industries. It's hard to find a product, whether granular or powdered, that hasn't been stored in a silo or hopper at some stage in its production process. Rational silo sizing for reliable emptying was developed and published in the 1960s. Since then, the subject has been the subject of constant interest, and major technological advances have been made. Despite this, for many people, a silo is still regarded as an ordinary container that requires no special dimensioning precautions. This leads to a number of problems when storing and/or emptying industrial silos. It's not uncommon these days to have to use a hammer to get the stored solids out! Similarly, it's no less rare to experience flow problems or, more precisely, "clogging" in silos, necessitating staff intervention to empty tons of caked product with a shovel! On the one hand, these remedial interventions can irreparably damage the installations, and on the other, they can create roughness by deforming the installations, creating new points of attachment for future clogging. Furthermore, the majority of accidents involving personnel occur during work on clogged silos. The economic impact of these interventions is also considerable, since they lead to untimely production stoppages and require recycling or even rejection of the caked product. In addition, there are much more drastic accidents, such as silo fires and explosions.

In previous issues of [J 2 251] and [J 2 252] , we presented the basic concepts associated with powder rheology.

The aim of this dossier is to introduce process engineers to the physical principles governing the flow of powders in silos, and to outline the problems encountered during flow, as well as the existing tools, both computational and technological, that can be used to avoid or resolve a number of them.