Effective Diffusivities in Multicomponent Mixtures

Your objectives in studying this section are to be able to  

Estimate the effective diffusivity of a component in a multicomponent mixture of gases from its binary diffusion coefficients with each of the other constituents of the mixture. 

Estimate the effective diffusivity of a dilute solute in a homogeneous solution of mixed solvents from its infinite dilution binary diffusion coefficients into each of the individual solvents. 

The Maxwell-Stefan equations for diffusion in multicomponent systems can become very complicated, and they are sometimes handled by using an effective dif -fusivity or pseudobinary approach. The effective diffusivity is defined by assuming that the rate of diffusion of component i depends only on its own composition gradient that is, 

In gases, the binary MS diffusivities are normally assumed independent of composition. With this approximation, the effective diffusivity of component i in a multi-component gas mixture can be derived from the Stefan-Maxwell equation to give (Treybal, 1980) 

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