Geldart’s classification

Whereas Geldart s classification relates fluidized-bed behavior to the average particle size in a bed, particle feed sizes maybe quite different. For example, in fluidized-bed coal (qv) combustion, large coal particles are fed to a bed made up mostly of smaller limestone particles (see Coal conversion processes). 

It is not clear where cohesive forces will become important. The use of very dense particles (for the models of the one atmospheric bed) will cause a shift of the boundary of cohesive influence as given, for example, by Geldart s classification. However, adequate experimental data is still lacking with such dense fine particles to definitely set the limits of cohesive influence. 

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). 

Figure 9.1. Geldart s classification of fluidized particles (from Geldart, 1973).

Particles can be classified into four groups (i.e., Groups A, B, C, and D) based on their fluidization behavior [Geldart, 1973], This classification, known as Geldart s classification, is shown in Fig. 9.1, where particles are classified in terms of the density difference between the particles and the gas (pp — p), and the average particle diameter dp. 

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. 

Intensive studies have been carried out on the ascending bubble diameters in free fluidized beds (C5, K27, R14, R16, W9). Various correlations for estimating bubble diameters have appeared (M36, R13, W9). However, the particles utilized in these experiments belong to group B of Geldart s classification. For this type of particle, bubble diameters are expressed as a function of bed diameter, of distance of the bubble above the distributor, of initial bubble diameter, and of physical properties of the fluidized particles. Mori and Wen (M36) emphasized the former three factors and proposed the equation ... 

FIG. 21-28 Geldart s classification of aeration behavior with Dixon and Gel-dart boundaries. From Mason, Ph.D. thesis, Thornes Polytechnic, London, 1991, with permission.)... 

Materials in pneumatic conveying are classified into many more groups than those of importance in fluidization but Geldart s classification is gaining recognition in that area too. The usual classification is as follows ... 

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). 

Solids that exhibit some region of particulate fluidization, are called type A (aeratable) in Geldart s classification system [2], while coarser solids... 

Behavior of bubbles for a typical fluidized Group B powder (Geldart s classification). 

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