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Aeration, Fluidization, Permeability of powders

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Section summary
1. Aeration of powders
2. Permeability of powders

1. Aeration of powder

The percolation of a gas through a powder bed can strongly change its bulk density but also its flowability. The air flow is indeed "areating" the powder, loosening it.

However, all powders do not show the same behavior when submitted to a flow of air. Some powders will expand in an homogeneous way while others will stay more compact with the air gathering in "channels".

How to classify powders according to their aeration behavior ?

The fluidization of powders has been studied by Geldart who has defined 4 groups of products and a graph that allows to estimate, from the material properties, in which group is a powder [Rhodes]. The experiences were done on a fluidized bed.

Geldart classification

The density ρp of the particles used on the graph above is defined as the mass of a particle devided by its volume, including open and closed pores.

The following groups are defined :

  • Group A : aeratable powders. Those powders are retaining air very well and homogeneously. They have a low permeability (see next paragraph) that allow them to retain air over time and stay fluidized. They can be conveyed pneumatically, especially in dense phase.
  • Group B : sand-like powders, the interactions in between particles is low, with a low permeability (see paragraph below) which means that the particles stop being fluidized the instant the air is cut. The bubbles can grow in size and reach the diameter of the fluidizing bed, creating "slugs".
  • Group C : cohesive powders, the gas will not be able to spread evenly in bubbles in the bed of particles but will rather create channels (thus the name channelling). It is possible to anticipate if a powder will be in group C by comparing the loose bulk densities and tapped bulk densities. If the ratio bulk / loose > 1.4, then the powder may be in group C.
  • Group D : spoutable powders, with a behavior similar to group B although the "spouting" state can be reached where a column of gas can be located in the middle of the fluidized bed (it requires however that the air is injected by a single point instead of being distributed on the entire bottom of the bend of particles).

Relation in between aeration and flowability of powders

The fluidization of the powder is reducing the interactions in between particles, which are further one another, separated by air ("aerated"). As a consequence, the powders are more flowable. This fluidization principle is actually used by some discharging aids which are blowing air in hoppers to fluidize bulk solids, it is as well used in fluidized beds where air is distributed at the bottom of a powder bed to make it fluidize. On the other hands, some powders can fluidize too much and flow on an uncontrolled way outside of the silos, it could be the case especially of powders in the group A mentioned above.

2. Permeability of powders

The permeability of a powder is a measure of how easily it is crossed by an air flow. If the powder is not very permeable, then it will require a high air pressure drop to establish a flow through the powder. On the other hand, if it is very permeable, then air goes through it very easily with a minimum pressure drop.

The permeability can be measured by preparing a sample of product, applying a load on top of the sample, to make sure it does not expand (otherwise it would come back to measuring the aeration or fluidization of the powder) and submitting the system to an air flow from the bottom of the sample. The pressure drop is then measured. Comparing the pressure drop induced by different material will allow to define how permeable they are.

The permeability is also linked to air retention capability. A low permeability powder, once fluidize, will actually continue trapping the air for some time and continue to flow even if an air flow stops to be applied. It can be interesting for dense phase conveying as the powder will be less sensitive to process upsets.

Sources

[Rhodes] Principles of Powder Technology, page 123, Rhodes, Wiley, 1990

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