Aerated powders fluidization properties

Powders in group A have the most desirable fluidization properties. They expand significantly when fluidized, and take a long time to de-aerate after the gas supply is cut off. This makes them easy to circulate in pneumatic and fluidized systems, but also makes them liable to flood on discharge from hoppers. Interparticle forces are present in group A powders, but are smaller than the hydrodynamic forces. Gas bubbles are limited in size and may break up at high fluidizing velocities. 

In some cases, especially for very fine powders, researchers have looked at other properties to explain or classify product behavior. For example, Gel dart et al. (1984) have found that the ratio of tapped to aerated bulk density provides a good indication of the likely fluidization characteristics of fine and cohesive powders. 

The above-mentioned classification of powders may be useful even when fluidization as such is not of interest this is through the two related properties of bed expansion and rate of de-aeration which are of concern in the filling of containers and in the residence time needed in hoppers to avoid the powder flooding out when the discharge valve is opened. There is a striking difference, for... 

The properties of concern in this section are to do with behaviour of powders in an aerated state this is relevant in gas fluidization, powder transport and handling. Probably the most important tests in this category are those derived from fluidization and the results of such tests are not necessarily restricted to the area of gas fluidization. It should be emphasized that the following notes apply largely to fine powders (i.e. groups A and AC in Geldart s classification). 

The rate of de-aeration of powder under different conditions is an important property in powder handling and processing. The rate of collapse of the powder in a fluidized bed is one way of measuring the rate of de-aeration but it is specific to the operating conditions the rate depends on, for example, whether or not the plenum chamber (space under the distributor) is vented simultaneously with the stopping of the aeration gas flow. 

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