Gas fluidization

Gas—solids fluidization is the levitation of a bed of solid particles by a gas. Intense soflds mixing and good gas—soflds contact create an isothermal system having good mass transfer (qv). The gas-fluidized bed is ideal for many chemical reactions, drying (qv), mixing, and heat-transfer appHcations. Soflds can also be fluidized by a Hquid or by gas and Hquid combined. Liquid and gas—Hquid fluidization appHcations are growing in number, but gas—soHds fluidization appHcations dominate the fluidization field. This article discusses gas—soHds fluidization. 

The basic concepts of a gas-fluidized bed are illustrated in Figure 1. Gas velocity in fluidized beds is normally expressed as a superficial velocity, U, the gas velocity through the vessel assuming that the vessel is empty. At a low gas velocity, the soHds do not move. This constitutes a packed bed. As the gas velocity is increased, the pressure drop increases until the drag plus the buoyancy forces on the particle overcome its weight and any interparticle forces. At this point, the bed is said to be minimally fluidized, and this gas velocity is termed the minimum fluidization velocity, The bed expands slightly at this condition, and the particles are free to move about (Fig. lb). As the velocity is increased further, bubbles can form. The soHds movement is more turbulent, and the bed expands to accommodate the volume of the bubbles. 

Interparticle Forces. Interparticle forces are often neglected in the fluidization Hterature, although in many cases these forces are stronger than the hydrodynamic ones used in most correlations. The most common interparticle forces encountered in gas fluidized beds are van der Waals, electrostatic, and capillary. 

Fig. 10. Formation, growth, and splitting of bubbles in gas-fluidized beds, where (—) is theory (e > 0.8) and (----) is experiment.

Particle-to-fluid heat-transfer coefficients in gas fluidized beds are predicted by the relation (Zenz and Othmer, op. cit.)... 

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. 

Reaction conditions are generally mild, but they differ from one process to another. In the newer Unipol process (Eigure 12-1) used to produce both HDPE and LLDPE, the reaction occurs in the gas phase. Ethylene and the comonomers (propene, 1-butene, etc.) are fed to the reactor containing a fluidized bed of growing polymer particles. Operation temperature and pressure are approximately 100°C and 20 atmospheres. A single-stage centrifugal compressor circulates unreacted ethylene. The circulated gas fluidizes the bed and removes some of the exothermic reaction heat. The product from the reactor is mixed with additives and then pelletized. New modifications for gas-phase processes have been reviewed by Sinclair. ... 

Hajek et al. [173] have reported a detailed kinetic study of the solid phase decomposition of the ammonium salts of terephthalic and iso-phthalic acids in an inert-gas fluidized bed (373—473 K). Simultaneous release of both NH3 molecules occurred in the diammonium salts, without dehydration or amide formation. Reactant crystallites maintained their external shape and size during decomposition, the rate obeying the contracting volume equation [eqn. (7), n = 3]. For reaction at 423 K of material having particle sizes 0.25—0.40 mm, the rate coefficients for decompositions of diammonium terephthalate, monoammonium tere-phthalate and diammonium isophthalate were in the ratio 7.4 1.0 134 and values of E (in the same sequence) were 87,108 and 99 kJ mole-1. 

Literature references and the measurement of the contact time distribution in a large, cold-flow model of a gas-fluidized bed are reported in... 

The reactor is a gas-fluidized bed for which the fractional tubularity model is usually appropriate. 

Mass transfer coefficient between the emulsion and bubble phases in a gas fluidized bed 11.45... 

Effect of Uniformity of Gas Distribution on Fluidization Characteristics in Conical Gas Fluidized Beds... 

Experiments were carried out in a conical shape gas fluidized bed (0.1 m-i.d. x 0.6 m-high) that made of a transparent acryl column with an apex angle of 20°. The details of the conical fluidized beds can be found elsewhere [3]. Air velocity (Ug = 0-1.4 m/s) were measured by a flowmeter. The particle used in this study was 1.0 mm glass beads with a density of 2,500... 

In contrast, the high-temperature reactor operates at -350 °C and 25 bar, using a gas-fluidized bed reactor of either the circulating or the normal type. The high-tem-perature process is mainly used to produce gasoline and chemicals, such as alpha olefins, and the low temperature process to produce waxes. 

R. W. Mair, R. L. Walsworth 2005, (Study of gas-fluidization dynamics with laser-polarized 129Xe), Magn. Reson. /mag. 23, 203-207 (b) R. Wang 2005, (Study of gas flow dynamics in porous and granular media with laser-polarized 129Xe NMR), Ph.D. Thesis, Department of Nuclear Engineering, Massachusetts Institute of Technology, February 2005. 

Baeyans, J., and Geldart, D., Predictive Calculations of Flow Parameters in Gas Fluidized Bed and Fluidization Behavior of Various Powders, Proc. Int. Symp. on Fluidization anditsAppl., p. 263 (1973)... 

DeGroot, J. H., Scaling-up of Gas-fluidized Bed Reactors, Proc. of the Int. Symp. on Fluidization, (A. A. H. Drinkenburg, ed.), Netherlands University Press, Amsterdam (1967)... 

Rietema, K., and Piepers, H. W., The Effect of Interparticle Forces on the Stability of Gas-fluidized Beds--I. Experimental Evidence, Chem Eng. Sci., 45 1627 (1990)... 

Roy, R., and Davidson, J. F., Similarity between Gas-fluidized Beds at Elevated Temperature and Pressure, Fluidization VI, Engineering Foundation, New York (1989)... 

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