Challenges in Handling Fine Powders: Flooding, Flushing and De-aeration

by Lyn Bates


1. Fine powders with low permeability will exhibit fluid like behaviour due to high degree of aeration and subsequent low de-aeration rates. Uncontrolled and unpredictable discharge of fine powders out of process vessels is called flooding or flushing. Fine powders can flow uncontrollably through belt feeders, vibratory feeders or screw feeders since they rely on cohesion and angle of repose to contain the bulk solid. Fine powders also leak through the clearances of a rotary feeder under a surcharge pressure. These features pose major problems in handling fine powders for industrial applications.

2. Excess gas in the interstitial voids partially supports compacting loads and opposes the development of shear strength, allowing the mass to behave similar to a liquid of very low viscosity. In contrast, the same material in a settled condition exhibits extreme flow difficulties due to its poor permeability which inhibits the expansion of the bulk solids.

3. The flooding tendency of a powder depends both on the bulk material characteristics (particle size, size distribution, permeability, particle density) and on the process conditions (strain rate, discharge rate, residence time, surcharge load or pressure, process temperature, flow pattern). Powders with flooding tendency can, therefore, be handled satisfactorily if proper control of process conditions is maintained.

4. Typical mechanisms for flooding are –

a. As the powder from top surface of a rathole (in a funnel flow bin) sloughs off and falls down towards the outlet, it entrains air which causes it to fluidize and flood out of the feeder.
b. The residence time of powder in the central core in a bin with funnel flow pattern can be very short. If incoming material is not sufficiently de-aerated during its transit time to the feeder, it may flood on reaching the exit.
c. If a vessel is filled rapidly with fine powder and discharged without allowing sufficient time to de-aerate, flooding may take place.
d. If the relative velocity between particle and air at the silo outlet is higher than minimum fluidization velocity, the powder is likely to fluidize and flood out.
e. Uncontrolled air injection in hoppers can result in localized fluidization and create surcharge pressures which aggravate the tendency to flood.

5. Recognize the potential behaviour variation by relating a sample to the Geldart diagram. Bulk materials falling into Class ‘C’ are likely to experience radical differences in flow characteristics according to the state of dilatation and slow to de-aerate when fluidised by agitation or excess air entrainment. Materials with Geldart Class “A” are also prone to flooding if sufficient de-aeration time is not adequate.

6. Note that the viscosity of gases increase with temperature, so that the escape of void gas by percolation is impeded by increased viscous drag at higher temperatures, hence the output from dryers, kilns or other hot regions will exacerbate fluidity problems. These may also be sensitive to ambient variation due to weather or site circumstances.

7. A hopper designed for mass flow will not function as such if the internal friction is less than wall friction, the latter not being affected by dilation of the bulk. Even if the stored contents of a hopper are such as to mass flow, the converging section will experience a velocity gradient. The dilated product in the central region will exert a hydrostatic pressure that overcomes the minimum principle stress of the boundary material to progressively penetrate the bed and may eventually flood.

8. There exists a critical strain rate where the material behavior changes from rate-independent transmitted shear stress (powder-like flow) to rate-dependent liquid-like flow. This transition can be sharp or fuzzy but high flow rates increase the flooding risk.

9. Segregation of fines fraction (esp. fraction rich in particles size less than 40 μm) within a silo can create a potential for flooding. Fines fraction lowers the critical strain rate required to change bulk material into a liquid like state. Segregation will also cause variations in bulk density, product flowability and product composition (for multi-component mixtures).

10. Measures available to address flooding problems in industrial applications –

a. Avoid Funnel Flow pattern in a bin/hopper
b. Retain heel of material in bin to avoid immediate discharge from filling.
c. Use tangential entry of material as it is fed pneumatically into a bin/silo. Vertical impact can exacerbate fluidization and segregation.
d. Design to minimize segregation of fines and coarse
e. Allow for sufficient storage/residence time for the material to de-aerate before discharging.
f. Minimize surcharge load or gas pressure on top of the bin; ensure good venting.
g. Avoid uncontrolled air injection as a flow aid. Provide a continuous, limited-volume rate, air bleed in the region adjacent to the hopper outlet to prevent the void volume and pressure reducing to a value that cannot supply the expansion required for flow to take place. Note that the air injection rate required is very small compared with the rate of loss during settlement of a highly aerated mass.

h. Consider the use of static de-aeration devices, such as vertical rods or wires, to enhance deaeration rates. The porosity of packed bed near the wall is slightly higher than in the bulk. A fluidised powder in a deep transparent tube will form 'rivulets' of gas that run up the walls of a container as gas flow reinforces weak escape channels, rather than work through the close packed bed away from the container surface. Place a thin wire in the centre of the container and a small 'volcano' of gas will erupt at the surface as the 'statistical empty space' forms a hole in the bed for air to short circuit the tortuous path through the interstices of a close packed array.

i. The above effect can be enhanced by rotating the vertical rods at a natural frequency to generate vertical holes between the nodes in deep beds to short-circuit the gas exit route. Alternately, fit inclined plates that accumulate rising gas from underneath and provide a region shielded from overpressures up which the gas can travel. Contact pressure of the solids in low and particles are easily displaced by the increasing gas flow. Inverted ’V’ fittings with vent pipes to the headspace to provide a local unconfined surface.

j. Exploit plane flow and extended outlet slots with progressive extraction to enhance storage capacity for additional residence time and reduce hopper flow velocities and velocity gradients during discharge. An 'extended outlet slot with progressive extraction' is a long hopper outlet (typically served by a screw or belt feeder) which generates “live feed” over the total length. This construction allows a 'V' shaped hopper to be employed that provides plane flow for better discharge than a round or square opening. A 'V' shaped hopper also has more capacity as compared to a conical hopper by virtue of the lower wall inclinations permissible by plane flow. More capacity and larger cross section of flow channel means greater residence time and lower flow velocities that are both favourable for avoiding flooding. It should be noted that the maximum allowable working pressure may be limited for a V shaped hopper design due to flat surfaces.

k. Consider a large “Chinaman's Hat Insert” with a relatively narrow annulus, under which is a converging cone where the powder will reliably slide to the final outlet. The flow area of a large, but narrow, annulus can be many times greater than the final outlet because the area increases as the square of the diameter. This will greatly reduce the flow velocity and provide a shallow, unconfined surface under the insert for further de-aeration.

Useful References:

1. R.L. Carr, Chem. Eng. (Feb 1, 1965) 69
2. W. Bruff and A.W. Jenike, Powder Technology, 1 (1967/68) 252-256
3. J.P. Mogan, R.W. Taylor and W.H. Merrill, Powder Technology, 2 (1968/69) 241-243
4. J. R. Johanson and A.W. Jenike, Powder Technology, 5 (1971/72), 133-145
5. P.G. Murfitt and P.L. Bransby, Powder Technology, 27 (1980) 149-162
6. G. I. Tardos, D. Mazzone, R. Pfeffer, D. Degani, A. Nir, Powder Technology, 41 (1985) 135-146
7. D. Geldart and J.C. Williams, Powder Technology, 43 (1985) 181-183
8. J.M. Kirby, Powder Technology, 44 (1985) 69-75
9. D. Geldart and A. C. Y. Wong, Chemical Engineering Science, Volume 40, Issue 4 (1985) 653-661
10. P.J. Lloyd and P.J. Webb, Powder Technology, 51 (1987) 125-133
11. T. Rathbone and R.M. Nedderman, Powder Technology, 51 (1987) 115-124
12. T. Rathbone, R.M. Nedderman and J.F. Davidson, Chemical Engineering Science, Vol. 42, No. 4 (1987) 725-736
13. T.A. Royal and J. W. Carson, Fine Powder Flow Phenomena in Bins, Hoppers and Process Vessels, Presented at Bulk 2000: Bulk Material Handling Towards the Year 2000, London, 1991
14. Y. Tomita, H. Ikeuchi, S. Kuchii and K. Funatsu, J. Rheo., Vol. 38, No. 2 (1994) 231-239
15. S. Jing, H. Li, Powder Technology, 103 (1999) 297-299
16. S. Kuchii and Y. Tomita, Powder Technology, 126 (2002) 275-282
17. L. Bates, “Deaeration of Powders” available from http://www.ajax.co.uk/papers.htm
18. L. Bates, archived article on powderandbulk.com, http://www.powderandbulk.com/pb_serv...ng_powders.htm




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