Gases diffusion coefficient for

Diffusion of gases is fast relative to diffusion in the aqueous phase i.e., step 1 is fast relative to step 3. Thus diffusion coefficients for gases at 1 atm pressure are 0.1-1 cm2 s l, whereas in liquids they are 10 5 cm2 s"1 for small molecules. As discussed in detail by Schwartz and Freiberg (1981), gas-phase diffusion, in most (but not all) cases, will not be the slowest (i.e., rate-determining) step. 

It is of interest that similar ideas have been applied in the conception of a flow system for measuring the diffusion coefficients for gases in porous or microporous solids. Ruthven and Eic (20,21) use a zero-length column (ZLC) to suppress concentration gradients along the bed in the gas phase. As in the differential reactor described previously, a high gas flow rate is used so that the fixed bed acts as if it were very short. A preadsorbed adsorbate is removed by an inert gas stream. The diffusion inside the solid is very close to the classical solution for zero concentration on the surface, but the small concentration actually present in the gas leaving the bed (column) can be measured accurately. 

This relation is useful in determining the diffusion coefficient for gases at dif ferem temperatures and pressures from a knowledge of the diffusion coefficient at a specified temperature and pressure. More general but complicated relations that account for the effects of molecular collisions are also available. The diffusion coefficients of some gases in air at 1 atm pressure are given in Table 14-1 at various temperatures. 

The diffusion coefficient for gases in mbbers has an Arrhenius form ... 

The diffusion coefficients for gases were calculated from the Eyring equation, and A and Ea were determined from experimental results of Wise and Houghton (1966) for N2, O2, and Ar, and from Jahne and Dietrich (1987) for the rest. R = 0.008 314 510 (kj mol K ). 

Examples illustrating the use of Eq. 4.1.1 and 4.1.2 are given by Reid et al. (1977, 1987) and by Danner and Daubert (1983). The same authors describe methods for estimating Fick diffusion coefficients for gases at high pressure. 

Many diffusion coefficients for gases are on the order of 10 cm Vs. Diffusion coefficients can also be defined for liquid and solid phases. Although the kinetic theory of gases does not apply directly to these phases, there are some conceptual similarities. However, diffusion coefficients for condensed phases are much lower than for gases, especially for solids at normal temperatures. Diffusion coefficients for solids are typically in the range of 10 to 10 cmVs. 

Reyes and Jensen evaluated the effective diffusion coefficient for gases in porous media. In the molecular regime, where pore sizes f(r) are much larger than molecular diameters, L(r) is equal to a constant value, Dq, on open lattice bonds and equal to 0 on blocked bonds. The density function for use in evaluating equations 32 and 33 is therefore ... 

Wilke, C.R. and Lee, C.Y. 1955. Estimation of diffusion coefficients for gases and vapors. Industrial and Engineering Chemistry dl, 1253-1257. 

Typical diffusion coefficients for gases are shown in Table 7.4-1. These values are not experimental, but are calculated from the Chapman-Enskog theory (see Section 5.1) and from Table 7.1-1. The first two rows in the table show how the values of D12 and >21 are larger as the solution becomes concentrated. The second and third rows refer to the same solution but with a different species chosen as the solute. The difference in the diffusion coefficients illustrates why ternary diffusion coefficients can be difficult to interpret. The final three rows are other characteristic situations. 

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