Investigation of the Soret effect in binary, ternary and quaternary hydrocarbon mixtures: New expressions for thermodiffusion factors in quaternary mixtures

Thermodiffusion or the Soret effect in binary, ternary and quaternary hydrocarbon mixtures is studied. Using the linear non-equilibrium thermodynamic (LNET) approach, expressions are derived for the estimation of the thermodiffusion coefficients or factors in quaternary mixtures using different approaches. New series of experimental data for binary, ternary and quaternary mixtures are used to evaluate the performance of the Firoozabadi, Kempers (Haase) and our two developed expressions based on the LNET approach. Results show that the model expressions are generally more effective for binary mixtures. In binary mixtures, the Firoozabadi model is preferred, whereas in ternary and quaternary mixtures, our developed expressions based on the LNET and the LNET–Kempers combined approach are superior. Overall, we may conclude that the LNET approach, extended by the authors to ternary and quaternary mixtures, is a reliable model for less-complex systems, such as the linear hydrocarbon mixtures. The LNET approach is also flexible in that provided the required modifications are made, it may be applicable to more complex systems, such as associating mixtures, as well. It is also argued that a thermodiffusion model should be independent of the mass transfer frame of reference, as thermodiffusion coefficients are calculated at the steady state condition, where all fluxes are zero.