Gas-solid two-phase jets

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. 

Momentum Dissipation of a Gas-Solid Two-Phase Jet. Gas velocity profiles in a gas-solid two-phase jet inside a fluidized bed were determined at five different horizontal planes perpendicular to jet direction using a pitot tube (Yang and Keaims, 1980). The velocity profiles were integrated graphically, and gas entrainment into a jet was found to occur primarily at the base of the jet. 

For concentric jets and gas-solid two-phase jets, the jet momentum flux term pf(73 can be evaluated as follows, as suggested by Yang et al. (1984b) ... 

The solids particle velocity in the gas-solid two-phase jet can be calculated as shown in Eq. (27), assuming that the slip velocity between the gas and the solid particles equals the terminal velocity of a single particle. It should be noted that calculation of jet momentum flux by Eq. (26) for concentric jets and for gas-solid two-phase jets is only an approximation. It involves an implicit assumption that the momentum transfer between the concentric jets is very fast, essentially complete at the jet nozzle. This assumption seems to work out fine. No further refinement is necessary at this time. For a high velocity ratio between the concentric jets, some modification may be necessary. 

Solid Entrainment Rate into Gas and Gas-Solid Two-Phase Jets. 

A mathematical model for solid entrainment into a permanent flamelike jet in a fluidized bed was proposed by Yang and Keaims (1982). The model was supplemented by particle velocity data obtained by following movies frame by frame in a motion analyzer. The experiments were performed at three nominal jet velocities (35, 48, and 63 m/s) and with solid loadings ranging from 0 to 2.75. The particle entrainment velocity into the jet was found to increase with increases in distance from the jet nozzle, to increase with increases in jet velocity, and to decrease with increases in solid loading in the gas-solid, two-phase jet. 

Yang, W. C., and Keaims, D. L., Solid Entrainment Rate into Gas and Gas-Solid, Two-Phase Jets in a Fluidized Bed, Powder Technol., 33 89 (1982)... 

Gas-Solid Two-Phase Jet in a Fluidized Bed. In Fluidization. Ed. Davidson and Keaims. Cambridge Cambridge University Press. 

When solids are fed pneumatieally into operating fluidized beds, they generate jets as well. In this application, they are gas-solids two-phase jets. The gas and solids entrainment into the jets and the extent of the jet... 

Yang WC, Keairns DL. Momentum dissipation of and gas entrainment into a gas-solid two-phase jet in a fluidized bed. In Fluidization (Grace JR, Matsen JM, eds.). New York Plenum Press, 1980a, pp 305-314. 

Yang WC, Keaims DL. Solid entrainment rate into gas and gas-solid two-phase jets in a fluidized bed. Powder Technol 33 89 94, 1982a. 

The gas jets can also carry solids and are referred to as gas-solid two-phase jets in this discussion. 

Many experimental studies have been carried out to investigate the turbulence characteristics in gas-liquid and gas-solid two-phase jets under various flow conditions [31-35]. In single-phase jets, turbulence is produced mainly by entrainment of surrounding fluid into the jets, [29,30] while in two-phase jets, turbulence production takes place also in the wake of bubbles or solid particles in addition to turbulence production due to the entrainment typical of single-phase jets. The size and shape of bubbles or soUd particles are considered to affect the structure of turbulence [36,37]. The effects of the size of bubbles or solid particles on the modulation of turbulence characteristics have been reported by many researchers [35-38]. However, the information on the effect of bubble shape is quite limited. 

On closer examination, it was found that the differences in the above correlations were mainly due to their different gas induction characteristics. These differences were eliminated when the solid suspension criterion was redefined in the form of the critical gas rate at which the two-phase jet reached the vessel bottom as shown in the last stage of Figure 8.16 denoted by the subscript s. . This approach gave a unified correlation independent of the nozzle diameter, diffuser type, etc. 

Fig. 12.2. 2-D calculations of axial radial profiles for the parameters of a two-phase Ar jet at the cross-section positioned 1.5-mm downstream from the nozzle outlet. The solid, dotted, and broken lines represent argon gas pressures of 60, 40, and 20 bar, respectively a the mean cluster concentration, nciust b the gas-phase concentration nat c the mean cluster radius (r) d the mean distance between clusters (d)...

Reasonably, the corrosion form is typical at relatively high velocities between the material surface and flie fluid, and it is particularly intensive in cases of two-phase or multiphase flow, i.e. hquid-gas and liquid-solid particle flow. Components often liable to erosion corrosion are propellers, pumps, turbine parts, valves, heat exchanger tubes, nozzles, bends, and equipment exposed to liquid sputter or jets. Most sensitive materials are those normally protected by corrosion products with inferior strength and adhesion to flie substrate, e.g. lead, copper and its alloys, steel, and under some conditions aluminium/aluminium alloys. Stainless steel, titanium... 

Laderach, Widmer, and Einsele, 1978 the cyclone reactor, Dawson, 1974 the cycle tube cyclone reactor, Liepe et al., 1978). The plunging jet reactor developed by Vogelbusch, Vienna, in collaboration with the Engineering Center, Bohlen G., East Germany, is another type of special design that works with a two-phase pump and a foam-like gas/liquid mixture (Schreier, 1975 Steiner et al., 1977). The last type of bioreactor to be mentioned is the thin-film type. Here the liquid and/or solid phase is in the form of a thin layer, and this promotes the reaction. 

Most of the studies referred to in the previous discussion used two-phase (gas-liquid) systems. The considerations are substantially similar when the liquid jet ejector is to be used as a three-phase catalytic reactor, particularly because the catalyst loading commonly used in a venturi loop system is relatively low. In terms of mass transfer, besides gas-liquid mass transfer, solid-liquid mass transfer step assumes great importance. As discussed in Chapters 7A and 7B, factors relating to dispersion of the gas phase, suspension of solids, and concentration profile of the catalyst phase need to be addressed in the case of a three-phase reactor. 

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