The Prediction Of PVTx And Vapor-Liquid Equilibrium(VLE) Properties Of C-H-O-N-S Fluid Mixtures

Natural gas can be approximated by C-H-O-N-S system,of which major components are CH₄,C₂H₆,C₃H₈,CO₂ and N₂,and minor components are H2 S and SO₂.The temperature-pressure-volume-composition（PVTx）and vapor-liquid phase equilibrium（VLE）properties of the system play an important role for the studies of original,storage and extraction of oil and gas.In addition,the PVTx and VLE properties of S-bearing natural gas mixture are also the basis to determine trapping temperature and trapping pressure of oil-gas inclusions.Experimental studies and computing modeling are the usual tools to determine the PVTx and VLE properties of fluids.An equation of state in form of Helmholtz free energy with good extrapolated ability is used to predict the PVTx and VLE properties of fluid mixtures: A generalized equation of state explicit in dimensionless Helmholtz energy has been developed for the CH₄-C₂H₆-C₃H₈-CO₂-N₂-H₂S-SO₂ fluid mixtures based on thermodynamic models of Lu and Mao（2015）and Lemmon and Jacobsen（1999）and experimental data.The developed equation of state is a function of temperature,density and composition,which has a simple and universal form,with high accuracy and wide range of application.Four mixing parameters in the equation of state are fitted from a large number of PVTx and VLE data.The mixing parameters are fitted only from experimental data of binary systems,which can be used to predict the thermodynamic properties of multi-component fluid mixtures.The calculated results indicate that the equation of state can predict both the PVTx and VLE properties of CH₄-C₂H₆-C₃H₈-CO₂-N₂-H₂S-SO₂ fluid mixtures up to 650 K and 200bar;The accuracy of this equation of state is similar to that of GERG-2008 proposed by Kunz and Wagner（2012）.The whole calculation program is written in international popular Fortran language,which can meet the requirements of corresponding researches.