Quantitative Predictions and Experimental Validation of Liquid-Vapor Interfacial Tension in Binary and Ternary Mixtures of Alkanes Using Molecular Dynamics Simulations.

Liquid-vapor interfacial properties of alkane mixtures present a challenge for experimental determination, especially under conditions relevant to the energy industry processes. Molecular dynamics (MD) simulations can accurately predict interfacial tensions (IFTs) for complex alkane mixtures under virtually any conditions, thereby alleviating the need for difficult and costly experiments. MD simulations with the CHARMM force field and empirical corrections for the IFT and pressure were used to obtain the IFT for three binary mixtures of ethane (with n-pentane, n-hexane, and n-nonane) and a ternary system (ethane/n-butane/n-decane) under a variety of conditions. The results were thoroughly validated against experimental data from the literature, and new original IFT data were collected using the pendant drop method. The simulations are able to reproduce the experimental IFT to better than 0.5 mN/m or 5% on average and within 1 mN/m or 10% in the worst case. IFTs for the studied three binary and ternary alkane mixtures were predicted for wide ranges of conditions with no known experimental data. Finally, using the MD simulation data, the reliability of the widely used empirical parachor model for predicting IFT was reaffirmed, and the significance of the empirical parameters examined to establish an optimal balance between the accuracy and broad applicability of the model.

Liquid-Vapor Interfacial Tension in Alkane Mixtures: Improving Predictive Capabilities of Molecular Dynamics Simulations.

The liquid-vapor interfacial properties of hydrocarbons and their mixtures are important factors in a wide range of industrial processes and applications. Determining these properties experimentally, however, is not only practically demanding, but many important properties, such as phase densities and compositions are not directly experimentally accessible, thus requiring the development of theoretical models. Molecular dynamics (MD) simulations, by contrast, are relatively straightforward even for the most complex of mixtures and directly provide all of the microscopic quantities for the studied systems. We have previously applied MD simulations to study the liquid-vapor equilibria of mixtures of hydrocarbons and CO2 that are particularly relevant to hydrocarbon recovery from geologic formations. In this study, we explore in more detail the robustness of the simulation methods with respect to the choice of the model system parameters, investigate the accuracy of the simulations in determining the key quantities: system pressure and interfacial tension (IFT), and, finally, devise a simple correction for achieving a much closer agreement between the simulated and experimental quantities. We perform extensive MD simulations for three mixtures, propane/n-pentane, propane/n-hexane, and CO2/n-pentane, using model systems from 1000 up to 100 000 molecules, and different simulation box dimensions to test for the sensitivity to finite-size effects. The results show that changing the system size and box dimensions does not significantly impact the accuracy of the simulations. Subsequently, we examine the accuracy of the MD simulations in determining the pressure and IFT for two pure hydrocarbon systems, n-pentane and n-heptane. Finally, we propose a simple linear correction formula for the pressures and IFTs obtained from the MD simulations that closely reproduce the experimental values for single components and mixtures of hydrocarbons. Our results enable the MD simulations to provide more accurate and reliable predictions of the interfacial properties, thereby reducing the need for challenging laboratory experiments.

Molecular Dynamics Simulations of the Vapor-Liquid Equilibria in CO2/n-Pentane, Propane/n-Pentane, and Propane/n-Hexane Binary Mixtures.

Molecular dynamics (MD) simulations were used to study vapor-liquid equilibrium interfacial properties of n-alkane and n-alkane/CO2 mixtures over a wide range of pressure and temperature conditions. The simulation methodology, based on CHARMM molecular mechanics force field with long-range Lennard-Jones potentials, was first validated against experimental interfacial tension (IFT) data for two pure n-alkanes (n-pentane and n-heptane). Subsequently, liquid-vapor equilibria of CO2/n-pentane, propane/n-pentane, and propane/n-hexane mixtures were investigated at temperatures from 296 to 403 K and pressures from 0.2 to 6 MPa. The IFT, liquid and vapor phase densities, and molecular compositions of the liquid and vapor phases and of the interface were analyzed. The calculated mixture IFTs were in excellent agreement with experiments. Likewise, calculated phase densities closely matched values obtained from the equation of state (EOS) fitted to the experimental data. Examination of the density profiles, particularly in the liquid-vapor transition regions, provided a molecular-level rationalization for the observed trends in the IFT as a function of both molecular composition and temperature. Finally, two variants of the empirical parachor model commonly used for predicting the IFT, the Weinaug-Katz and Hugill-Van Welsenes equations, were tested for their accuracy in reproducing the MD simulation results. The IFT prediction accuracies of both equations were nearly identical, implying that the simpler Weinaug-Katz model is sufficient to describe the IFT of the studied systems.