Measurement of Diffusion Coefficients in binary and ternary mixtures by the Open Ended Capillary Technique(ABSTRACT)

摘要

Multicompnent diffusion and thermal diffusion play a key role in a variety of natural systems and industrial processes, for instance to get a better understanding of the gravitational and thermal effects in hydrocarbon reservoirs and the interactions of light components with asphalthenes[l]. The SoDiUM experiment (Soret and Diffusion Under Microgravity) has been proposed to minimize the effects of convection and obtain accurate measurements. The system C_(12)H_(26) (an alkane) - IBB (a one ring component) - THN (a two ring component) has been chosen as it is representative of a hydrocarbon reservoir mixture.With an aim of determining the optimal experimental conditions to choose for SoDiUM, ground measurements of isothermal and thermal diffusion have to be performed. For isothermal diffusion, the OEC technique was selected because it allows measurements in ternary mixtures.In an OEC experiment, "capillary" tubes with an open end are filled with a homogeneous solution of mass fraction w_(1,0), are immersed in an "infinite" bath (i.e., of a very large volume compared to that of a tube) of mass fraction w_(1,∞) .Diffusion occurs until the equilibrium is reached, i.e., until w_1 =w_(1,∞) everywhere. The solution inside the tubes is more concentrated in the heavier component and denser, and therefore flow due to natural convection is avoided as the solution inside the tubes diffuses. The composition inside the "capillary" tubes can, in first approximation, be described with a one dimensional Fick law. An approximated analytical expression can be derived for the mean composition in the tubes which can be measured versus time to allow the estimation of the diffusion coefficients.The results of a series of experiments on the binaries ethanol-water , C12H26-THN, C12H26-IBB, THN-IBB and on the ternary C12H26-THN-IBB (representing the main families of components of light fractions [2]) at various compositions are presented. The latter, compared with data found in the literature (when available) and discussed.^