Diffusion coefficient in a mixture

The glob ll diffusion coefficient dJ of the species Cj in the mixture of the species Cj can be calculated from the binary diffusion coefficients Dj of Cj in Cj k j  

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It is possible to use alternative formulations considering mole fractions rather than mass fractions. For most cases, mass fraction formulations will be adequate. An estimation of the diffusion coefficient (of component k) in a multicomponent mixture Dkm) however, is not straightforward. For mixtures of ideal gases, the diffusion coefficient in a mixture can be estimated as (Hines and Maddox, 1985)... 

The diffusion coefficient in a mixture is obviously affected by the other species present, and to account for this a binary diffusion coefficient f>i2 is specifically related to the diffusivity of species 1 into 2 can be used. Since diffusion of species 1 into 2 is identical to species 2 into 1, D12 = f>2i. In cases where the density is low and diffusion flux is primarily a result of self-interaction, the species self-diffusion coefficient (Djj = D) is used, and the effect of interaction with other species is not included. Later in this chapter methods for calculation of diffusion coefficients are given. 

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