Geldart powder classification

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

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

Fig. 63. Geldart s powder classification chart, (a) Geldart s original diagram for air (b) Grace s improvement on Geidart s diagram, 1986.

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. ... 

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. 

It has long been recognized that pressure exerts a strong influence on the bubbling behavior of gas-fluidized beds. The literature up to 1993 was reviewed by Yates (1996), and this is reproduced and extended to 1998 in Table 3. The three principal groups of powders in Geldart s classification will be considered in turn. 

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.

Figure 49 shows a set of bed collapsing curves for a Geldart Group A-A (for Geldart s classification of solid particles, see Geldart, 1972, 1973) binary solids mixture, two closely sized alumina powders, of average particle diameter 104 and 66 microns, respectively. The curve on the extreme left with 0% fines represents the pure coarse component, which is... 

Incorporation of Geldart s classification of powders in relation to fluidisation characteristics (Chapter 6). 

Geldart (1973) classified powders into four groups according to their fluidization properties by air at ambient conditions. This classification is now used widely in all fields of powder technology. 

The following test materials have often been used FCC catalysts, aluminium oxide, silica gel, glass beads, silica or quartz sand, sea sand, coal and coal ash, petroleum coke, metal powders, resin particles, boric acid, and magnesite powder. Mean particle size ranges from 11 /un to 1,041 /rm, and particle density, from 384 kg/m3 to 7,970 kg/m3. According to Geldart s classification (1973), most of these materials belongs to Class A, some to Class B, and a few to Class D or C, as listed in Table II. 

Classiflcations of Fluidization Behavior Geldart [Powder Technology, 7, 285 (1973)] and later Dixon [Pneumatic Conveying, Plastics Conveying and Bulk Storage, Butters (ed.). Applied Science Publishers, 1981] developed a classification of fluidization/aeration behavior from studies of fluidized beds and slugging in vertical tubes. 

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