Gas-Phase Diffusion Coefficients

The theory for diffusion in gas phase is well developed. The diffusion flux of a component i(Ni) depends on all of the components. According to the Stefan-Maxwell theory, the diffusion flux and concentration gradient are governed by the matrix relation 

An approximate calculation of the diffusion flux may be based on the concept of effective diffusion coefficients and Pick s law, which yields a simple relation between the diffusion flux and the concentration gradient  

This approximation is valid mainly for components in diluted gases, that is, for low concentrations. The effective diffusion coefficient for a gas-phase component in a porous particle can be calculated from the relation 

The diffusion coefficient (D,) can be divided into two parts one originating from inter-molecular collisions (Dmi) and the other from collisions between the molecules and pore walls, the so-called Knudsen diffusion (DkO- The diffusion coefficient (D,) can be estimated from these two parts from the formula 

Equation A4.4 is strictly valid for equimolar diffusion in a binary solution (Na = —Nb). 

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For prediction of gas phase diffusion coefficients in multicomponent hydi ocarbon/nonKydi ocai bon gas systems, the method of Wilke shown in Eq. (2-154) is used. 

The gas-phase diffusion coefficients are estimated from the molecular weights M of the speciesl0 ... 

This equation shows that the saturation greatly affects the effective gas-phase diffusion coefficients. Hence, flooding effects are characterized by the saturation. 

Equation (105) is the basis for the determination of gas-phase diffusion coefficients and ultra low vapor pressures using the methods proposed by Davis and Ray (1977), Ravindran et al. (1979), and Ray et al. (1979). Additional information can be gained by writing the Chapman-Enskog first approximation for the gas-phase diffusivity (Chapman and Cowling, 1970),... 

Transport of the gas to the surface and the initial interaction. The first step in heterogeneous reactions involving the uptake and reaction of gases into the liquid phase is diffusion of the gas to the interface. At the interface, the gas molecule either bounces off or is taken up at the surface. These steps involve, then, gaseous diffusion, which is determined by the gas-phase diffusion coefficient (Dg) and the gas-surface collision frequency given by kinetic molecular theory. 

Adapted from Schwartz (1984a), and Shi and Seinfeld (1991). ka = particle radius, Dg = gas-phase diffusion coefficient, D, = liquid-phase diffusion coefficient, H = Henry s law constant, a = mass accommodation coefficient, u.w = mean thermal speed, and k = first-order aqueous-phase rate constant. 

Fuller E.N., Schettler P.D., Giddings J.C. (1966) A new method for prediction of binary gas-phase diffusion coefficients. Industrial and Engineering Chemistry 58, 19-27. 

The liquid-phase diffusion coefficients are found with the Nemst-Hartley equation (193), which describes the transport properties in weak electrolyte systems. The gas-phase diffusion coefficients are estimated according to the... 

The gas-phase diffusion coefficients are calculated using the equation given in Ref. [40]. The liquid-phase diffusion coefficients of components at infinite dilution in... 

J. C. Giddings, A new method for prediction of binary gas-phase diffusion coefficients, Ind. 

Most of the plate height terms we have discussed are velocity sensitive. For gases, any term containing the mobile (gas) phase diffusion coefficient Dm (Dg for gases) is also pressure dependent, since Dg varies in inverse proportion to pressure p. For an ideal gas... 

Predict gas phase diffusion coefficient Da i for each component i in the presence of all other components, using Wilke s equation (196). 

Because of volume changes due to the reaction, pressure gradients may occur inside the catalyst pellet. This can give rise to two effects. First, it influences the effective diffusion coefficients, since the gas-phase diffusion coefficients depend on pressure. Second, the pressure gradients affect the concentrations (or more accurately, chemical activities), which determine the reaction rate. Hence pressure gradients must directly influence the effectiveness factor. 

D ,h binary water/hydrogen gas-phase diffusion coefficient... 

Computer Output of Diffusion Coefficients for Liquid and Gas Phases Diffusion Coefficients for Liquids and Gases... 

At ambient temperature and pressure, gas-phase diffusion coefficients ate of the order of 10 i-10 1 frVs (10 S-I0 m2/s). Table 2.3-1 presents some representative values for biomy gas mixtures at 1 aim. Marrero and Mason" have provided an extensive review of expeiimamal values of gas phase binary diffusivities. 

Enter the gas-phase diffusion coefficient, DG, m2/s, and the liquid-phase diffus coefficient, DL, mVs. 

Typical values of gas phase diffusion coefficients are ca. DA binary diffusion coefficients are presented in Table 9.8. 10"5 m2/s. Some values... 

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