Segregation by percolation in powders differing by size

Segregation

Every process involving the handling of particulate materials with different properties, i.e. particle size, density, shape or surface roughness can lead to a non-uniform distribution of the particles in the bulk. This phenomenon is called segregation. The British Materials Handling Board asserts that “segregation is the most influential common factor that adversely affects the uniformity of bulk materials “, causing in several cases products out of specification. A broad size distribution is probably the major factor determining the segregation in granular materials.

percolation mechanism
Fig.1 – Percolation of particles differing in size under shear in a gravitational field.
The percolation mechanism

Small particles, due to gravity and to some level of shear, can fall down into the voids created by the largest one; as a result some regions of the bulk enriches in fines while other enriches in coarse particles. This specific mechanism is the percolation (Fig. 1).

The control and the mitigation of percolation is also an important industrial challenge. APTLab has therefore dedicated a three years long research projects to study the problem. This effort finally resulted in a model for the prediction of  percolation in binary mixtures of granular materials discharging from hoppers. The model has been implemented in full scale, 3D industrial geometries.

 

sergegation by percolation in funnel flow regime
Fig. 2 – Size segregation by percolation in funnel flow discharge.
sergegation by percolation in mass flow regime
Fig. 3 – Size segregation by percolation in mass flow discharge.
Prediction of percolation

The model finally allowed to predict variations of composition in the bulk due to percolation during the discharge of funnel (Fig.2) and mass flow (Fig.3) silos. The model take into account the local mixture composition and the local shear rate. An ad hoc rheological model simulates the dense flow of granular material, while the percolation model take into account the segregation occurring during the flow.

Results validation

Numerical simulations have been compared with published experimental data to validate the model. Most noteworthy the model predicts three different stages during the discharge in the two flow regimes (funnel and mass flow in Fig.4 and Fig.5 respectively). Discharge starts with an initial transient and an enrichment of fines at the outlet. Then the flow continues in a pseudo-steady state without segregation.  Finally the discharge stops with a transient characterized by a large separation of fines. This is particularly evident in the funnel flow regime. In contrast mass flow shows a rather constant composition trend.

 

funnel flow prediction of segregation
Fig. 4 – Experiments vs. simulation for mass flow discharge.
mass flow prediction of segregation
Fig. 5 – Experiments vs. simulation for mass flow discharge.

For more details see: D. Bertuola, S. Volpato, P. Canu, A.C. Santomaso, Prediction of segregation in funnel and mass flow discharge (2016) Chemical Engineering Science, 150, 16-25. [DOI: http://dx.doi.org/10.1016/j.ces.2016.04.054, preprint].

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