Geldart

Geldanamycins Geldanazines Geldanoxazines Geldart group Gel dyeing Gel electrophoresis Gelfoam... 

Geldart a group Powder Average particle size, (, )J.m Particle density, p, kg/m Angles Internal friction, deg of Repose, deg Sphericity, f... 

Particle Regimes. In 1973, particles were classified with respect to how they fluidize in air at ambient conditions into Geldart groups (6) (Fig. 4). Particles that formed bubbles immediately after the gas superficial velocity exceeded were designated as Group B particles. For these particles, the... 

Fig. 4. Geldart group particle classification diagram for air at ambient conditions (6). Group A consists of fine particles B, coarse particles C, cohesive,...

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). 

D. Geldart, ed.. Gas Eluidi tion Eechnolog ]oEii Wiley Sons, Inc., Chichester, U.K., 1986. 

For details beyond the scope of this subsection, reference should be made to Kunii and Levenspiel, Fluidization Engineering, 2d ed., But-terworth Heinemann, Boston, 1991 Pell, Gas Fluidization, Elsevier, New York, 1990 D. Geldart (ed.). Gas Fluidization Technology, Whey, New York, 1986 and the vast number of papers published in periodicals, transcripts of symposia, and the American Institute of Chemical Engineers symposium series. 

Types of Solids Geldart [Fowder TechnoL, 7, 285-292 (1973)] has characterized four groups of solids that exhibit different properties when fluidized with a gas. Figure 17-j shows the division oi the classes as a function of mean particle size, d,, Im, and densitv difference, (p, — P/ ), g/cm, where p, = particle density and p = fluid density... 

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

Particulate Fluidization Fluid beds of Geldart class A powders that are operated at gas velocities above the minimum fluidizing velocity (L/, y) but belowthe minimum bubbhngvelocity (L/, i) are said to be particulately fluidized. As the gas velocity is increased above L/, y, the bed further expands. Decreasing (p, — Py), d and/or increasing increases the spread between L/, yand U, b until at some point, usually at high pressure, the bed is fully particulately fluidized. Richardson and Zald [Trans. Inst. Chem. Eng., 32, 35 (1954)] showed that U/U = E , where /i is a function of system properties, = void fraction, U = superficial fluid velocity, and Uj = theoretical superficial velocity from the Richardson and Zald plot when = 1. 

All the foregoing pertains to sohds of approximately the same physical characteristics. There is evidence that sohds of widely different characleristics wih classify one from the other at certain gas flow rates [Geldart, Baeyens, Pope, and van de Wijer, Powder Technol., 30(2), 195 (1981)]. Two fluidized beds, one on top of the other, may be formed, or a lower static bed with a fluidized bed above may result. The latter frequently occurs when agglomeration takes place because of either fusion in the bed or poor dispersion of sticl feed solids. 

The L valve is shown schematicaUy in Fig. 17-20. It can act as a seal and as a sohds-flow control valve. However, control of sohds rate is only practical for solids that deaerate quickly (Geldart B and D). The... 

The phenomenon is represented by Figs. 17-40 and 17-41 for Geldart-type A and B solids, respectively (see beginning of Sec. 17). The initial efficiency of a particle size cut is found on the chart, and the parametric hue is followed to the proper overall solids loading. The efficiency for that cut size is then read from the graph. 

FIG. 17-40 Effect of inlet loading on collection efficiency for Geldart Group A and Group C particles. (Couttesy of PSRI, Chicago.)... 

The bulk density of a powder is calculated by dividing its mass by the volume occupied by the powder (Abdullah Geldart, 1999). Tapped bulk density, or simply tapped density, is the maximum packing density of a powder achieved under the influence of well-defined, externally applied forces (Oliveira et al., 2010). Because the volume includes the spaces between particles as well as the envelope volumes of the particles themselves, the bulk and tapped density of a powder are highly dependent on how the particles are packed. This fact is related to the morphology of its particles and such parameters are able to predict the powder flow properties and its compressibility. 

So far, some researchers have analyzed particle fluidization behaviors in a RFB, however, they have not well studied yet, since particle fluidization behaviors are very complicated. In this study, fundamental particle fluidization behaviors of Geldart s group B particle in a RFB were numerically analyzed by using a Discrete Element Method (DEM)- Computational Fluid Dynamics (CFD) coupling model [3]. First of all, visualization of particle fluidization behaviors in a RFB was conducted. Relationship between bed pressure drop and gas velocity was also investigated by the numerical simulation. In addition, fluctuations of bed pressure drop and particle mixing behaviors of radial direction were numerically analyzed. 

Vignale, G., Rasolt, M., Geldart, D. J. W., 1990, Magnetic Fields and Density Functional Theory , Adv. Quantum Chem., 21, 235. 

Pacey, M.S., Dirlam, J.P., Geldart, R.W. et al. (1998) Novel erythromycins from a recombinant Saccharopolyspora erythraea strain NRRL 2338 pIGl. I. Fermentation, isolation and biological activity. The Journal of Antibiotics, 51, 1029. 

Geldart (1970) showed a substantial distinction between bubble sizes in two dimensional and three dimensional beds. He used 128 pm river sand in a 30.8 cm round bed and a 68 1.27 cm rectangular cross section bed. The results, shown in Fig. 12, show that the bubbles in the three dimensional bed are larger. There were differences in the visible bubble flow rate at the same superficial velocity. Geldart ascribes the differences in bubble diameter to differences invisible bubble flow rate as well as to out-of-line coalescence in the three dimensional bed. 

Figure 12. Variation of bubble diameter and concentration with initial bed height / - uG = 6 cm/sec for two dimensional and three dimensional beds. (From Geldart, 1970/71.)...

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. 

---