Gas dispersion coefficient

According to bed voidage, which can be calculated from gas velocity and solids circulation rate by using Eqs. (32) to (38), the axial gas dispersion coefficient can thus be easily obtained. 

This equation indicates that axial gas dispersion coefficient increases with gas velocity and solids circulation rate. This dependence of axial dispersion coefficient on gas velocity is at variance in the available literature. 

There exists both axial and radial gas diffusion in the CFB combustor, the radial gas dispersion coefficient being much smaller than the axial. 

Radial and axial dispersion measurements in risers were first reported by van Zoonen [71]. With hydrogen gas as a tracer he found that radial dispersion coefficients ranged between 2.5 and 36 cmVs and axial gas dispersion coefficients were between 4,500 and 14,400 cmVs. 

Tracer techniques are commonly used to determine the gas dispersion coefficients in fluidized bed reactors. The tracer concentration measured at the outlet in response to a pulse or step input of the tracer at the inlet can be used to calculate the dispersion coefficient based on the dispersion models in a form similar to Eq. (11), i.e.,... 

Figure 31 shows the model analysis of the effects of radial gas dispersion coefficient on radial profiles of propylene concentration. The radial mass transfer has a significant effect on the conversion and yield. When the radial Peclet number decreases from 1400 to 200, the conversion of propylene increases by over 10%, and the yield of acrylonitrile increases by about 7%. Since the reaction is first order with respect to propylene, risers are operated under dilute conditions at Pe = 200, so the radial concentration distribution of propylene is uniform and radial mass transfer is not... 

A radial gas dispersion coefficient can be obtained from the steady-state tracer method by fitting measured radial profiles of tracer concentration downstream of the injection point to a two-dimensional... 

The radial gas dispersion coefficient associated with the lean phase can be obtained separately (Werther et al., 1992 Koenigsdorff and Werther, 1995 Mastellone and Arena, 1999b). It has been found to increase with increasing G (Mastellone and Arena, 1999b) or to be insensitive to Gs, and it tends to increase with increasing /g (Werther et al., 1992 Koenigsdorff and Werther, 1995). 

Thakkar A J, Hettema H and Wormer P E S 1992 Ab initio dispersion coefficients for interactions involving rare-gas atoms J. Chem. Phys. 97 3252... 

The recommended correlation for the gas-phase axial-dispersion coefficient is given by Field and Davidson (loc. cit.) ... 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

Worst-case atmospheric conditions occur to maximize (C). This occurs with minimum dispersion coefficients and minimum wind speed u within a stability class. By inspection of Figs. 26-54 and 26-55 and Table 26-28, this occurs with F-stability and u = 2 m/s. At 300 m = 0.3 km, from Figs. 26-54 and 26-55, <3 = 11m and <3 = 5 m. The concentration in ppm is converted to kg/m by application of the ideal gas law. A pressure of 1 atm and temperature of 298 K are assumed. 

In addition, it was concluded that the liquid-phase diffusion coefficient is the major factor influencing the value of the mass-transfer coefficient per unit area. Inasmuch as agitators operate poorly in gas-liquid dispersions, it is impractical to induce turbulence by mechanical means that exceeds gravitational forces. They conclude, therefore, that heat- and mass-transfer coefficients per unit area in gas dispersions are almost completely unaffected by the mechanical power dissipated in the system. Consequently, the total mass-transfer rate in agitated gas-liquid contacting is changed almost entirely in accordance with the interfacial area—a function of the power input. 

Specific interface in gas/liquid systems Mass-transfer coefficient Time-dependent dispersion coefficient Knudscn number Reaction rate constant... 

Crowel576] classified the numerical models for dilute sprays as two-fluid model and discrete elementmodel. The Iwo-fluid model treats the dispersed phase as another fluid with appropriate constitutive relationships for effective viscosity and thermal conductivity. The advantage of the two-fluid model is that the same algorithm used for the gas phase can be applied directly to the droplet phase. The drawback is the lack of information on the dispersion coefficient and the effective... 

Here, the densities of the gaseous and solid fuels are denoted by pg and ps respectively and their specific heats by cpg and cps. D and A are the dispersion coefficient and the effective heat conductivity of the bed, respectively. The gas velocity in the pores is indicated by ug. The reaction source term is indicated with R, the enthalpy of reaction with AH, and the mass based stoichiometric coefficient with u. In Ref. [12] an asymptotic solution is found for high activation energies. Since this approximation is not always valid we solved the equations numerically without further approximations. Tables 8.1 and 8.2 give details of the model. 

The expression for the effective gas phase coefficient that would account for axial dispersion and hence give a proper mass transfer zone length is ... 

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