867 ISSN 1070-4272, Russian Journal of Applied Chemistry, 2013, Vol. 86, No. 6, pp. 867−878. © Pleiades Publishing, Ltd., 2013. Original English Text © Mahmood Farzaneh Gord, Mehdi Roozbahani, Hamid Reza Rahbari, Seyed Javad Haghighat Hosseini, 2013, published in Zhurnal Prikladnoi Khimii, 2013, Vol. 86, No. 6, pp. 926−937. PHYSICOCHEMICAL STUDIES OF TECHNOLOGIES Modeling Thermodynamic Properties of Natural Gas Mixtures Using Perturbed-Chain Statistical Associating Fluid Theory Mahmood Farzaneh Gord a , Mehdi Roozbahani a , Hamid Reza Rahbari a , and Seyed Javad Haghighat Hosseini b a Faculty of Mechanical Engineering, Shahrood University of Technology, Shahrood, Iran b National Iran Gas Company, Semnan Province, Iran e-mail: javadsjhh@yahoo.com, mehdi.roozbahany@yahoo.com, rahbarihamidreza@yahoo.com, mahmood.farzaneh@yahoo.co.uk Received March 13, 3013 Abstract—Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) was employed for predicting ther- modynamic properties of natural gas mixture. Thermodynamic properties like density, isobaric and isochoric heat capacity, enthalpy, entropy, and internal energy were calculated with the PC-SAFT. Results are validated against experimental data for natural gas and mixtures similar to natural gas. The validation show that the Average Absolute Deviation (AAD) for density is 1.10% for binary mixture and 1.08% for mixtures similar to natural gas. Also AAD value for enthalpy is 1.42%, for internal energy, 0.77, for entropy, 0.43, for isochoric heat capacity, 1.26%, and for isobaric heat capacity, 2.66%. Results show PC-SAFT to be able to predict all the thermodynamics properties of natural gas and mixtures similar to natural gas with high accuracy in a wide range of temperature and pressure. DOI: 10.1134/S1070427213060153 1. INTRODUCTION Natural gas is a complex mixture containing 21 elements, and a mixture of hydrocarbons with several inorganic compounds. According to the IPCC report (fourth assessment report) of 2004, natural gas produced about 5.3 billion tons of carbon dioxide, while the figures for coal and crude oil are around 10.6 and 10.2 billion tons [1]. So natural gas is the cleanest fossil fuel after hydrogen, and it plays an important role in the world energy today [2]. Natural gas is the clean and cheap source of energy, which is now used for domestic and industrial applications. Applications of the natural gas are dramatically increasing day by day. To deliver natural gas to consumers, it is passed through various processes: The compression, liquefac- tion, pressure drop are some examples. To properly ana- lyze these processes, we need to calculate the thermo- dynamic properties of natural gas with high accuracy. Since natural gas is a complex mixture of several in- gredients, its properties depend on the properties of its ingredients. There are two ways to calculate the thermodynamic properties: 1. Laboratory techniques and 2. Application of equations of state (EOS). Laboratory techniques have a high accuracy, but some of the thermodynamic properties like entropy, enthalpy, and internal energy are not measurable. The cost and difficulty of performance are other negative reasons, which resulted in reducing the use of laboratory techniques. An EOS is a thermodynamic equation describing the state of matter under a given set of physical conditions. It is a constitutive equation which provides a mathematical link between two or more conditions associated with the matter, like temperature, pressure, volume, internal or Helmholtz energy. There are many EOSs used in science and industry with respect to the application. All thermodynamic properties can be calculated using EOS with acceptable accuracy and within a limited time. Because of the mentioned reasons, researchers are always looking for the new