Multicomponent Diffusion

The governing equations that are describing the multicomponent mass transfer are well known, and the theory behind them is extreme developed. But the transport coefficients entering these equations will remain unknown and especially for realistically complex mixtures. Number of models for diffusion coefficients in multicomponent mixtures has been developed, but all these models are mostly based on the experimental arguments.

The better theoretical understanding of the structure of the transport coefficient is required to compensate for deficiency of the existing experimental data. By means of theory of fluctuations around an equilibrium state, the theoretical framework is constructed, which is described by both, the methods of the statistical mechanics and the theory of Markov processes.  There are three factors to describe the rate of diffusion in the mixture; 1, the thermodynamic factor, determined by the structure of the mixture and the system of thermodynamic coordinates in which the diffusion is expressed; 2, the kinetic factor, or the rate of molecular motion; 3, the resistance factor which is determined by the fact that other molecules may serve the obstructions on the way of a given molecule.

Validate the developed model on the large set of the experimental data on the diffusion coefficient in binary mixtures. Different models of penetration lengths in the combination with different equations of state for the mixtures are implanted. Then compare the results with the other models available. The model will show the excellent agreement with the experimental data, and the deviations will be 2-3%.

This experimental program generally involves two directions. The first direction is; the macroporus media is used for the measurement of the diffusion coefficients like that of petroleum reservoir, with the help of X-ray computer tomography scanner. It has been theoretically demonstrated that the diffusion coefficients in the macroporus media must be rescaled when compared to the diffusion coefficients in the free space, the rescaling parameter not dependent on the specific mixture but it is only dependent on geometry of the porous medium. The series of experiments on the diffusion of the salt solutions in poro9us media has been initiated in order to measure the rescaling parameter (the porosity – tortuosity factor).  This experiment have shown the general applicability of the method and produced the values of the porosity- tortuosity factor for a series of samples of the petroleum rock.  The second direction is the diffusion measurements with the applications of the Raman spectography.

Questions:

• Explain multicomponent diffusion?
• What are the two directions of experimental program?