Geldarts classification

Graphical representation of classification of powders according to Geldart. 

The limit of behavior between A and B powders is, therefore, represented by 

In the compacted (or consolidated) state the elasticity module of a powder has a value approximately equal to the value the cohesion constant. The fluidization number for a powder at its consolidated state can be, therefore, modified as 

Equation 2.4 is, actually, the product of the inverse of the cohesion number and another dimensionless group known as the Archimedes number Ar, and thus 

Fluidized bed behaviour is affected not only by gas velocity but also by particle size and density. Based upon observations of fluidized bed behaviour in air at ambient conditions and at velocities below 

Large particles with a mean diameter greater than 600 Um and a density above 4000 kg m are classed as group D and whereas in Geldart B beds the bubbles tend to rise faster than the gas in the dense phase, the reverse is true of bubbles in Geldart D particle beds. Bubbles tend to coalesce horizontally rather than vertically and reach a large 

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Figure 10. Fluidization regimes, adapted from Grace (1986) by Kunii and Levenspiel (1991) particles labeled by Geldart Classifications A, B, D.

First, Geldart (1973) and Geldart and Abrahamson (1978) looked at how different kinds of solids behaved when fluidized, and came up with the following simple classification of solids which we now call the Geldart classification, thus Geldart A, B, C, D. These are shown and described in Fig. 20.2. 

The solids used correspond to group D and B of Geldart classification [22,23] and they are set out in Table 1. 

Pharmaceutical products such as tablets usually consist of active pharmaceutical ingredients. In the pharmaceutical industry, the API particle size has tended to become smaller in recent years and the needle shape has become very common. These small API particles usually belong to Group C according Geldart classification.2 3 It is well known that the needle-shaped Group C particles are very cohesive. They do not flow easily and mix poorly with other particles. 

FIGURE 12.2 The Geldart classification of particles for air at ambient conditions. Region A Range of properties for well-behaved FCC catalyst (from Kunii and Levenspiel, 1991). 

The general flow regime diagram shown i Fig 10.2 illustrates the progression of changes in behavior of a bed of solids as the gas velocity is progressively increased. The letters A, B, C and D refer to the Geldart classification of solids. 

Terminal velocities can then be found directly from eqs (20) and (21). Based on this method, the terminal fall velocities in air of nonspherical particles in Groups A, B, and D of the Geldart classification have been calculated as functions of temperature and pressure (Table 2 and Figs. 3 and 4). 

The main interest here has been their effect on solids carryover and heat transfer (see below). Some work on transition velocities between flow regimes has been reported, however. Thus Cai et al. (1989) studied the effect of operating temperature (50 to 500°C) and pressure (0.1 to 0.8 MPa) on the transition from bubbling to turbulent fluidization of eight powders in Groups A and B of the Geldart classification fluidized in a column 150 mm in diameter and 3.8 m in height. 

Both Molerus (1982) and Rietema (1984) extended the Geldart classification to a dimensionless correlation for bulk powder behaviour (Figure 1.8). A more sophisticated interpretation which takes into account particle interaction as well as particle forces has been proposed by Molerus (1980). 

Fig. 10.2 Flow regime map of gas-soUd contacting, a Characteristics of turbulent flow regime, b Characteristics of spouted beds, bubbling fluidized beds, fast fluidized beds and pneumatic transport regimes. In the figure notation the ordinate u = U p / n(pp — pg)g) is a dimensionless gas velocity, the abscissa d = dp[pg pp — Pg)glp ] a dimensionless particle size, the terminal velocity of a particle falling through the gas (m/s), and Umf the gas velocity at minimum fluidization (m/s). Letters A, B, C and D refer to the Geldart classification of solid particles. Reprinted from [49] with permission from Elsevier...

Gibilaro, L.G., Di Felice, R. and Foscolo, P.U. (1988). On the minimum bubbling voidage and the Geldart classification for gas-fluidized beds. Powder Technol., 56, 21. 

Figure 10.4 shows illustrative examples of this relation for the four powder groups - A, B,C and Z) - of the Geldart classification for ambient air fluidization. Figure 10.1. 

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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