Prediction of thermodynamic properties of natural gas mixtures using 10 equations of state including a new cubic two-constant equation of state Kh. Nasrifar ⁎ , O. Bolland Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway Received 27 April 2005; received in revised form 29 September 2005; accepted 14 January 2006 Abstract In this contribution, 10 equations of state (EoSs) are used to predict the thermo-physical properties of natural gas mixtures. One of the EoSs is proposed in this work. This EoS is obtained by matching the critical fugacity coefficient of the EoS to the critical fugacity coefficient of methane. Special attention is given to the supercritical behavior of methane as it is the major component of natural gas mixtures and almost always supercritical at reservoir and surface conditions. As a result, the proposed EoS accurately predicts the supercritical fugacity of methane for wide ranges of temperature and pressure. Using the van der Waals mixing rules with zero binary interaction parameters, the proposed EoS predicts the compressibility factors and speeds of sound data of natural gas mixtures with best accuracy among the other EoSs. The average absolute error was found to be 0.47% for predicting the compressibility factors and 0.70% for the speeds of sound data. The proposed EoS was also used to predict thermal and equilibrium properties. For predicting isobaric heat capacity, Joule–Thomson coefficient, dew points and flash yields of natural gas mixtures, the predictive accuracy of the EoS is comparable to the predictive accuracy of the Redlich–Kwong–Soave (RKS) EoS or one of its variants. For predicting saturated liquid density of LNG mixtures, however, the accuracy of predictions is between the RKS and Peng–Robinson (PR) EoSs. © 2006 Elsevier B.V. All rights reserved. Keywords: Natural gas; Thermodynamic property; Compressibility factor; Speeds of sound; Phase equilibrium; Equation of state 1. Introduction Predicting the thermodynamic properties of natural gas mixtures are important in gas industry—that is in production, processing, storage and transportation. Ac- curate values of natural gas compressibility factors and speeds of sound data are crucial in custody transfer operations. Other thermodynamic properties, e.g., saturated liquid density of liquefied natural gas (LNG) mixtures, are used in the design of liquefaction pro- cesses and storage facilities; Joule–Thomson coeffi- cients are used in throttling processes and dew points are used in pipeline design. There are accurate correlations/equations of state (EoSs) for calculating natural gas properties. McCarty (1982) reported an accurate extended corresponding states (ECS) model for LNG systems. Using ECS models, Estela-Uribe and Trusler (2001) and Estela- Uribe et al. (2004) predicted the compressibility factors, Journal of Petroleum Science and Engineering 51 (2006) 253 – 266 www.elsevier.com/locate/petrol ⁎ Corresponding author. Tel.: +47 735 98462; fax: +47 735 98390. E-mail address: khashayar.nasrifar@ntnu.no (Kh. Nasrifar). 0920-4105/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2006.01.004