Calculation of vapor–liquid equilibrium and PVTx properties of geological fluid system with SAFT-LJ EOS including multi-polar contribution. Part III. Extension to water–light hydrocarbons systems

Abstract

The SAFT-LJ EOS improved by Sun and Dubessy (2010, 2012) is extended to water–light hydrocarbon systems. Light hydrocarbons (including CH4, C2H6, C3H8 and nC4H10) are modeled as chain molecules without multi-polar moments. The contributions of the shape of molecules and main intermolecular interactions existing in water–light hydrocarbon systems (including repulsive and attractive forces between Lennard-Jones segments, the hydrogen-bonding force and the multi-polar interaction between water molecules) to the residual Helmholtz energy were accounted for by this EOS. The adjustable parameters for the interactions of H2O–CH4, H2O–C2H6, H2O–C3H8, and H2O–nC4H10 pairs were evaluated from mutual solubility data of binary water–hydrocarbon systems at vapor–liquid equilibria. Comparison with the experimental data shows this SAFT-LJ EOS can represent well vapor–liquid (and liquid–liquid) equilibria of binary water–light hydrocarbon systems over a wide P–T range. The accuracy of this EOS for mutual solubilities of methane, ethane, propane and water is within the experimental uncertainty generally. Moreover, the model is able to accurately predict the vapor–liquid equilibria and PVTx properties of multi-component systems composed of water, light hydrocarbon as well as CO2. As we know, this EOS is the first one allowing quantitative calculation of the mutual solubilities of water and light hydrocarbons over a wide P–T range among SAFT-type EOSs. This work indicates that the molecular-based EOS combined with conventional mixing rule can well describe the thermodynamic behavior of highly non-ideal systems such as water–light hydrocarbons mixtures except in the critical region for which long range density fluctuations cannot be taken into account by this analytical model.