Classification of Fluidized Particles

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. 

For Group B particles, there is no particulate fluidization regime. In this case, Umf equals Umb. The bubble size increases with the bed height and bed expansion is moderate. For Group B particle fluidization, there exists no maximum stable bubble size. 

Group D comprises coarse particles (dp 1 mm) which are commonly processed by spouting. When Group D particles are fluidized, the bed expansion is low and the particle mixing is not as good as that for Group A and B particles. 

For demarcation between Group C and A particles, Molerus (1982) proposed a semiem-pirical criterion based on a balance between the van der Waals force and the hydrodynamic forces as given by 

For demarcation between Group A and B particles, an empirical equation with air as the fluidizing gas under ambient conditions was suggested by Geldart (1973)  

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

Goossens WRA. Classification of fluidized particles by Archimedes number. Powder Technol 98 48-53, 1998. 

Figure 9.2 Geldart s classification of fluidization behavior of the particles.

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...

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). 

Classification The separation of fine particles from coarse can be effected by use of a fluidized bed (see Drying ). However, for economic reasons (i.e., initial cost, power requirements for compression of fluidizing gas, etc.), it is doubtful except in special cases if a fluidized-bed classifier would be built for this purpose alone. 

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. 

In principle, the experimental protocol of fluidized bed adsorption does not deviate from packed bed operations, the main difference being the direction of liquid flow. The standard sequence of frontal chromatography, equilibration, sample application, wash, elution, and cleaning (CIP) is performed with an upward direction of flow as shown in Fig. 3. During equilibration of the matrix the stabilization of the fluidized bed occurs, in case of size and/or density distribution of the adsorbent particles the classification within the bed may be detected by visual observation of the bed. As discussed below, bed stability may... 

The transport behavior of fluidized bed systems can be described in light of the properties of the fluidized particles and the flow regimes. In the following, particle and regime classifications along with general components in a fluidized bed are given. 

Many reactors fall in the classification of fluid-solid catalytic units where the catalyst may be retained in a fixed-bed position in the reactor with the reactant flowing through the catalyst bed, or the unit may be operated as a fluidized-bed reactor with the catalyst particles being suspended in the flowing fluid due to motion of the fluid. A third type of reactor is one in which the catalyst particles fall slowly through the fluid by gravity in the form of a so-called moving bed. 

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 ... 

Jet-milling with ultrafine classification of particles can be achieved using a Fluidized Bed Opposed Jet Mill 100 AFG (Alpine, Augsburg, Germany). It is a... 

A full classification of powders according to their behaviour in fluidization is complex because their behaviour depends on many particle properties. There is, however, now a widely accepted classification of powders proposed by Geldart26 which takes the two most important particle properties into account, the particle size and the particle density. Fig. 10a shows this classification, for fluidization by air at ambient conditions. 

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