Fluidized beds Geldart classification

Tasirin SM, Geldart D. Separation performance of batch fluidized bed air classification. Powder Handling and Processing 12 39-46, 2000. 

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). 

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 ... 

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. 

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. 

The time required to remove a given amount of moisture increases as the square of the particle diameter for a Geldart s type B particle, all other conditions being equal. For type A particles the effect is much smaller. Geldart s classification of powders is discussed in Chapter 3, Bubbling Fluidized Beds. 

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. 

According to Geldart s classification, these particles are of type B. Minimum fluidization with ambient air occurs with void fraction of 8n,f = 0.45. In examples below, we consider the operation of this bed with a superficial gas velocity of /g = 5.0 m/s and a particle mass flux of = 40 or 60 kg/m s. 

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