Predictive models of the excess Gibbs energy of liquid mixtures are paramount in chemistry research and chemical process design. The UNIQUAC pair-interaction parameters characterize the molar dissolution energy per molecular segment for each species at infinite dilution, which can be theoretically computed from the liquid-liquid interfacial tension. For mutual binary solubility estimation of water-organic systems, the root-mean square error (RMSE) in xo(w) (solubility of organic compound in water-rich phase) for UNIQUAC and modified UNIFAC are 1.283 (37 data points) and 2.690 (30 data points) correspondingly, the RMSE in xw(o) (solubility of water in organic-rich phase) for UNIQUAC and modified UNIFAC are 0.754 and 1.938 respectively. For prediction of solid solute solubility in pure liquid solvents (10 solid solutes, 96 systems, 267 data points), the total RMSE of UNIQUAC and COSMO-SAC are 0.651 and 1.436 respectively; which indicates that this new theoretical computation method of UNIQUAC binary energy parameters seems promising, and the predictive UNIQUAC exhibits better performance than COSMO-SAC in solid-liquid equilibrium (SLE) calculations. This new theoretical method couples excess Gibbs energy models with interfacial thermodynamics.
Shale reservoirs contain complex mineral assemblages, yet the differences in phase behavior of fluids confined within different mineral nanopores remain insufficiently understood. In this study, Monte Carlo simulations combined with molecular dynamics simulations are used to compare the phase behavior of C1-nC4 mixtures confined in organic matter and illite nanopores. The organic surface is modified with oxygen atoms to match the carbon-to-oxygen ratio of kerogen in the studied formation. The results show that, owing to stronger adsorption, the phase envelope contracts more significantly in organic nanopores than in illite pores of the same size, with higher dew point pressures and lower bubble point pressures. Fluid-surface interaction energy analysis indicates that nC4 is preferentially adsorbed in both organic and inorganic nanopores. Organic nanopores exhibit deeper potential energy wells, indicating stronger fluid-surface attraction. The phase behavior of mixtures confined in micropores is further examined. In 2 nm organic pores, the superposition of surface potential fields suppresses vapor-liquid phase transition at 30°C, and the confined fluid exhibits micropore-filling characteristics. In contrast, vapor-liquid phase transition still occurs in illite pores. Stronger fluid-surface interactions promote enrichment of heavy components in the adsorbed phase near the pore wall, while light components are relatively enriched in the free phase. This compositional fractionation enhances the control of light components on dew point behavior, thereby increasing the dew point pressure. At the bubble point, adsorption-layer interactions deepen the potential well of free-phase C1 and reduce its escape tendency, increasing the difficulty of vaporization and lowering the bubble point pressure.
