Powders fluidization, classification

FIG, 17-1 Powder-classification diagram for fluidization by air (ambient conditions). [From Geldart, Powder TecbnoL, 7, 285-292 (1973).]... 

For group B and D particles, nearly all the excess gas velocity (U — U,nj) flows as bubbles tnrough the bed. The flow of bubbles controls particle mixing, attrition, and elutriation. Therefore, ehitriation and attrition rates are proportional to excess gas velocity. Readers should refer to Sec. 17 for important information and correlations on Gel-dart s powder classification, minimum fluidization velocity, bubble growth and bed expansion, and elutriation. 

Powder Classification Techniques. The Geldart (1973) fluidization, and Dixon (1981) slugging classifications have been found useful in explaining ... 

ICM classification (Yang et al, 1985) according to the bed-collapsing curve of a fluidized powder, in which three characteristic stages have been identified ... 

Other classification systems are used less frequently. Carr " also devised a system to classify materials as to their floodability. He defines the floodability of a material as its tendency to flow like a liquid because of the natural fluidization of a mass of particles by air. In order to so classify a material, the flowability is determined utilizing the method just described. This value is equivalent to a measurement Carr calls the angle of fall, angle of difference, and dispersibility. Though referred to in any of the papers mentioned here, this system is much less utilized then the flowability measurements. Geldart reported on a characterization system of powders according to their ability to aerate and later Molerus modified this system. In a more recent symposium this method of powder classification was examined. ... 

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). 

Equation (5) is the demarcation between Group B and D powders. The powder classification diagram for the fluidization by air at ambient conditions was presented by Geldart, as shown in Fig. 2. 

We now examine the relation of predicted Smb values to fluidization quality. This turns out to provide considerably more insight into bed behaviour than has hitherto been appreciated, generalizing reported conclusions concerning the influence of measured Smb determinations over limited regions of applicability. To illustrate this relation we first consider the empirical Geldart (1973) powder classification, which has been briefly referred to in the previous chapter. 

The Geldart empirical powder-classification map for fluidization by ambient air... 

Figure 10.1 Powder classification map for fluidization by ambient air. Heavy lines, empirically determined boundaries of Geldart light broken lines, boundary predictions of the particle bed model.

Although we have dealt here solely with predictions for ambient air fluidization, for which validation by means of the counterpart empirical relations may be readily confirmed, the procedures outlined are quite generally applicable. The immediate conclusion is that predicted e b values provide a continuous measure of fluidization quality across the whole spectrum of behaviour corresponding to the Geldart powder classification map. However, when it comes to the general situation of fluidization by any fluid, this measure turns out to be by no means complete. To appreciate this point it becomes necessary to examine in more detail the perturbation wave relations that delivered the Smb predictions in the first place. This will then lead to more comprehensive predictive criteria for fluidization quality in general. 

Figure 10.4 Amplitude growth rates for short wavelengths A 0, ambient air fluidization illustrative examples for the Geldart powder classification groups ...

Figure 13.1 Generalized powder classification for fluidization by any fluid -showing the Geldart classification boundaries (A, B, C and D) and regions corresponding to ambient air and water fluidization.

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