DYNAMIC SIMULATION OF MULTINARY DIFFUSION PROBLEMS RELATED TO SOIL

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Abstract
A dynamic simulation model describing diffusion of gases in multinary gas mixtures was developed to calculate the interdiffusion of gases in complex systems where respiration and denitrification take place. The model is based on the Stefan-Maxwell equations for concentration diffusion of isothermal, isobaric ideal-gas mixtures and applies to a one-phase system in one dimension. To test the correct implementation of the theory in the model, it was used to calculate the mole fraction distribution in a gas layer for some ternary diffusion problems for which analytical solutions to the steady-state situation are known. Agreement between numerical and analytical solutions was within 1%. Subsequently, the model was used to calculate the dynamic behavior of a gas system in which denitrification takes place and acetylene is used to prevent the conversion of nitrous oxide into molecular nitrogen. When a 2% concentration of acetylene was maintained at the surface of the gas layer, and biological activity was positioned at a depth of 0.25 m, these calculations showed the acetylene concentration to reach 1.8%. This value is sufficiently high to inhibit nitrous oxide conversion into molecular nitrogen, but would be equal to 2% when calculated on the basis of Fick''s law. A simplified approach to calculate diffusion in multinary gas mixtures is proposed and tested for the case study of denitrification. It turns out that results of the simplified approach approximate those of the Stefan-Maxwell equations to within 10%. The objectives of this paper are to discuss the model, to compare the results of the numerical and analytical solutions of two ternary diffusion problems, to report the results of the case study of denitrificaton, and to compare these results with those obtained from simplified diffusion theory.