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Theoretical Investigation of Multicomponent Volatile Organic Vapor Diffusion: Steady-State Fluxes

Land, Air, and Water Resources Dep., Univ. of California, Davis, CA 95616.

^* Corresponding author.

ABSTRACT

Liquid mixtures of volatile organic chemicals (VOC) contaminating many underground sites give rise to multicomponent mixtures of their vapors. Previous investigation has shown that in an isoharic, isothermal multicomponent vapor system, Fick's law estimates of the diffusive flux for each species should not be used when the effective binary diffusion coefficients used are sufficiently different. Our theoretical investigation of a multicomponent, semihypothetical "gasoline" mixture containing relatively volatile aromatic constituents plus CCl₄ as an additional component shows that the multicomponent effect also can be observed when the binary diffusion coefficients are not substantially different, but when the total mole fraction of the diffusing species is sufficiently high. The importance of both diffusion coefficients and total mole fraction when modeling total diffusive fluxes requires that simulations be performed for any given vapor mixture. For our "gasoline" mixture, an approximate total mole fraction of 0.05 was found to lead to underestimation of single species fluxes by Fick's law of up to 5%. Simulations of steady-state fluxes further indicate that O₂ and N₂ do not need to be treated differently and can be combined to form one species, "air." Likewise various VOC species with almost identical diffusion coefficients can be grouped to form one representative class. Our investigation of diffusion in nonuniform, multinary mixtures of gases also included pressure diffusion. Pressure diffusion causes heavier (lighter) molecules to diffuse toward the higher (lower) pressure regions of a closed system. The contribution of pressure diffusion to the total diffusive flow under the natural gravitational pressure gradient was calculated to be significant only when the simulated soil column was more than 100 m long. It does not need to be included in multicomponent diffusion formulations involving shorter transport lengths.