Gas Jets in Fluidized Beds

Gas jets in fluidized beds were reviewed by Massimilla (1985). A more recent review is by Roach (1993) who also developed models to differentiate three jet flow regimes jetting, bubbling and the transition. However, most of the data were from jets smaller than 25 mm. The discussion here will emphasize primarily large jets, up to 0.4 m in diameter, and operation at high temperatures and high pressures. The gas jets can also carry solids and are referred to as gas-solid two-phase jets in this discussion. 

Massimilla, L. (1985). Gas Jets in Fluidized Beds. In Fluidization, 2nd ed. Ed. Davidson, Clift and Harrison. London Academic Press. 

Clift, R. Grace, J.R. Weber, M.E. Bubbles, Drops and Particles Academic Press New York, 1978. Massimilla, L. Gas jets in fluidized beds. In Fluidization, 2nd Ed. Davidson, J.F., Clift, R., Harrison, D., Eds. Academic Press London, 1985. Mori, S. Wen, C.Y. Estimation of bubble diameter in gaseous fluidized beds. AIChE J. 1975, 21, 109. 

L. Massimilla, Gas Jets in Fluidized Beds, in Fluidization, 2d ed., Davidson, Gift, and Harrison eds., Academic Press, London, 1985. 

Filla M, Massimilla L, Vaccaro S. Gas jets in fluidized beds and spouts a comparison of experimental behavior and models. Can J Chem Eng 61 370-376, 1983a. 

Massimilla L 1985 Gas jet in fluidized beds, in Fluidization (eds Davidson JF, Clift R and Harrison D), Academic Press, London, 133-72... 

The jets in fluidized beds have the following properties the jet penetration depth, the jet expansion angle (or the jet half angle), gas and solids entrainment, initial bubble size, and frequency issuing from the jets. They will be discussed now. 

Figure 20 Schlitchting t5 pe and Tollmien type similarities for gas velocity profiles in jets in fluidized beds.

Fluid and particle entrainment into vertical jets in fluidized beds was also studied by Merry (1976) by using a two-dimensional bed of lead shot fluidized by water. Although the system employed is liquid-solid, the appearance of the jet and the motion of particles are very similar to those observed for gas-solid systems. The author considered that the results were equally applicable for gas-solid systems. 

Clift R, Filla M, Massimilla L. Gas and particle motion in jets in fluidized beds. Intern J Multiphase Flow 2 549-561, 1976. 

Donadono A, Massimilla L. Mechanisms of momentum and heat transfer between gas jets and fluidized beds. In Fluidization. (Davidson JF, Keairns DL, eds.) Cambridge Cambridge University Press, 1978, pp 375 380. 

Figure 9 (a) Three-dimensional view of 3D solid concentration with a horizontal gas jet in the fluidized bed (b) 3D voxel-volume-averaged solid phase velocity vector map in the Y-Z plane of the fluidized bed (c) 3D voxel-volume-averaged solid phase vector map (Wang et al., 2008) (see Plate 12 in Color Plate Section at the end of this book). 

A major factor in fluidized bed behavior is the interaction between the gas flow from individual orifices and the particle and gas mixture within the bed. The jet penetration and the subsequent bubble formation have an important influence upon solids and gas mixing and, ultimately, upon the usefulness of the bed for reactor purposes. While flow visualization data are available at ambient pressures and temperatures, the natures of jet penetration and bubble development at high pressures and temperatures are not easily measured. Typical data on bubble size and bubble velocity at ambient conditions are shown, represented by the small size symbols, in Figure 2. It is well known that bubble volume can be correlated as a function of gas volumetric flow rate ( ) and that bubble velocity is related to the size of the bubble radius ( ). Such semi-empirical correlations are indicated as solid lines in that figure. 

Massimilla L, Donsi G, Migliaccio N. The dispersion of gas jets in two-dimensional fluidized beds of coarse solids. AIChE Symp Ser 77(205) 17 27, 1981. 

Vaccaro S. Analysis of the variables controlling gas jet expansion angles in fluidized beds. Powder Technol... 

Gas distributors of fluidized beds are often designed as perforated or nozzle plates. Since a minimum pressure drop is required to obtain a uniform gas distribution over the bed s cross-sectional area, the open surface area is rather small, and the gas jets issuing from the distributor holes are at high velocity. Particles are entrained by these jets, accelerated to high velocities, and impacted onto the fluidized bed suspension at the end of the jets, resulting in particle degradation similar to that in jet grinding processes (Kutyavina and Baskakov, 1972). 

Spouted beds are used for coarse particles that do not fluidize well. A single, high velocity gas jet is introduced under the center of a static particulate bed. This jet entrains and conveys a stream of particles up through the bed into the vessel freeboard where the jet expands, loses velocity, and allows the particles to be disentrained. The particles fall back into the bed and gradually move downward with the peripheral mass until reentrained. Particle-gas mixing is less uniform than in a fluid bed. 

In the above situation, the walls cave in from the sides, cutting off the void and presenting a new surface to the incoming gas. This sequence is illustrated in Figure 32. The size of the initial bubble resulting from a detached void is typically on the order of about half the penetration depth of the gas jet. Bubbles or gas voids rise in a fluidized bed by being displaced with an inflow of solids from their perimeters. 

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