Prediction of bulk and interfacial thermodynamic properties of polar mixtures by statistical associating fluid theory



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A Statistical Associating Fluid Theory (SAFT) for multi-component mixtures has been implemented in conjunction with a phase-stability and flash algorithm. The model has been extensively tested for various non-associating and associating mixtures and comparisons are made with the Peng-Robinson equation of state. Both Peng-Robinson and SAFT are equally suitable for simple nonassociating mixtures but SAFT clearly is more accurate when polar mixtures are modeled. The phase behavior of methanol-water-hydrocarbon mixtures is studied with the SAFT equation and the Peng-Robinson equation and comparisons are made with experimental liquid dropout data. The SAFT equation of state is shown to give better predictions for methanol-hydrocarbon and methanol-waterhydrocarbon mixtures over a range of pressures and compositions. The effect of methanol concentration and temperature on dew-point to bubble-point transition of a gas-condensate mixture is studied with the SAFT equation of state. The SAFT equation of state is coupled with the Gradient Theory to calculate the interfacial tension of pure components as well as multi-component mixtures. Pure component interaction parameters (cii) for the calculation of interfacial tension with the SAFT equation have been introduced. A mixing interaction coefficient for interfaces (mij) has been defined to satisfactorily predict the interfacial tension of certain mixtures such as water-methanol and waterethanol systems. The effect of temperature and pressure is studied for a methanewater mixture and it is shown that no further adjustable parameters need to be introduced to accurately predict the interfacial tension over a range of temperatures and pressures. Finally, the SAFT equation of state has also been integrated in to the reservoir simulator (UTCOMP) so as to be able to do flow simulations of complex polar mixtures. The flow simulations with SAFT have also been compared with experimental core flood studies and it is shown that both the PR and the SAFT equation give reasonable agreement with experimental data. However, it is shown that the SAFT based model predictions are slightly better during the methanol injection stage of the experiment. However, significantly larger computational time discourages the use of SAFT for such flow simulations.