Segregation in silos during filling and discharge
Bulk solids made of particles of different size, shape or density separate when they move. In the silo this happens mainly at the surface cone during filling and in the flow channel during discharge – the composition of the discharge stream then varies although the product filled in was homogeneous.
We detect segregation with measurements on samples from different areas; designing for mass flow limits it during discharge.
Segregation during filling and discharge
Filling produces radial segregation: fine in the centre, coarse at the edge. How it affects the discharge is decided by the flow pattern.
-
Filling At the surface cone, coarse particles roll towards the wall; fine ones stay below the inlet.
-
Discharge in funnel flow The fine material from the centre flows out first, then the coarse material from the edge.
-
Discharge in mass flow The whole cross-section flows at the same time – fines and coarse fraction arrive at the outlet together.
- PercolationFine particles trickle down through the gaps between the coarse ones – at every moving surface, above all at the surface cone.
- Rolling segregationCoarse, round particles roll further outwards on the surface cone; fine ones remain below the inlet.
- Air-current segregationDuring free fall and when material collapses in the flow channel, fine, light particles are carried along by the air stream and deposited elsewhere (see fluidisation).
Consequences of segregation in operation
Product and process
- Varying composition and particle size in the discharge stream
- Deviations from the specification, quality problems in downstream processes
- Varying dosing and process control
Silo and flow behaviour
- Different flow properties of fines and coarse fraction in the silo
- Preferred flow channels, funnel flow or eccentric flow
- Uneven, eccentric loads on the silo wall
Measurements to detect and limit segregation
Samples from different areas of the silo or from the discharge stream at different times are compared. For designing for mass flow, yield locus and wall friction are added.
| Measurement | Parameters | Use |
|---|---|---|
| Sieve analysis | particle size distribution to DIN 66165, fines < 63 µm | differences in composition between the samples |
| Yield locus | flowability ffc, effective angle of internal friction φe | flow behaviour of fines and coarse fraction, mass flow criterion |
| Wall friction | wall friction angle φw | mass flow criterion |
| Angle of repose | angle of repose φr to DIN ISO 4324 or ASTM D6393 | shape of the surface cone during filling |
| Moisture content | moisture of the samples | moisture differences between fines and coarse fraction |
Measures against segregation
Segregation during filling can be reduced; its effect on the discharge is limited above all through the flow pattern.
- Mass flowDesigning the hopper for mass flow so that the whole cross-section is drawn off at the same time.
- FillingLow drop height, concentric filling, distributors or several inlets instead of a high surface cone.
- DischargeA feeder that draws off the whole outlet cross-section evenly.
- Bulk solidNarrower particle size distribution, granulation or a low moisture content that binds the particles.
- ProcessSmaller intermediate storage or a mixer after the silo where homogeneity is critical.
Frequently asked questions about segregation
- Which bulk solids segregate most?
- Free-flowing bulk solids with a wide particle size distribution or with components of different density. Cohesive, fine bulk solids segregate less, because adhesive forces bind the particles together.
- Does mass flow prevent segregation?
- Not completely. Segregation during filling occurs regardless of the flow pattern. During discharge, however, mass flow draws off the whole cross-section at the same time and brings fines and coarse fraction back together at the outlet – considerably more uniformly than funnel flow.
- How can segregation be detected?
- With samples from different areas of the silo or from the discharge stream at different times. Sieve analysis, moisture and flow properties of these samples show how much the composition differs.
- Is there a calculation of segregation?
- There is no standardised calculation. It is assessed via the flow pattern – mass flow or funnel flow from measured wall friction and internal friction – and via comparative measurements on samples from the silo.
Detecting and limiting segregation
Describe your bulk solid and your silo – we put together the right measurements and prepare a non-binding quote for you.
- Comparative measurements on samples from different areas
- Sieve analysis, moisture and flow properties
- Mass flow criterion for your hopper