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J. Chem. Thermodynamics 40 (2008) 84–95 www.elsevier.com/locate/jct

A three-parameter cubic equation of state for prediction of thermodynamic properties of fluids
A.H. Farrokh-Niae a, H. Moddarress
a

a,*

, M. Mohsen-Nia

b

Department of Chemical Engineering, Amir Kabir University of Technology, Tehran, Iran b Thermodynamics ResearchLaboratory, Kashan University, Kashan, Iran Received 11 April 2007; received in revised form 17 May 2007; accepted 23 May 2007 Available online 14 June 2007

Abstract A new cubic three-parameter equation of state has been proposed for PVT and VLE calculations of simple, high polar and associating fluids. The parameters are temperature dependent in sub-critical region, but temperature independent insuper-critical region. The results for 42 simple and 14 associative pure compounds indicate that the calculated saturation properties and volumetric properties over the whole temperature range, up to high pressures, by the proposed equation of state (EOS), were in better agreement with the experimental data, compared with those obtained by the five well-known EOSs (P–R, P–T, Adachi et al., Yu–Lu,and M4). Two derivative properties, molar enthalpy and heat capacity of water and ammonia have been calculated, and demonstrated the thermodynamic consistency of the EOS parameters. Also VLE calculations have been performed for 41 binary mixtures of different type of fluids, including those of interest in petroleum industry. The results indicated the high capability of the proposed EOS forcalculating the thermodynamic properties of pure and fluid mixtures. Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Cubic equation of state; Saturation properties; Enthalpy and heat capacity; (Vapor + liquid) equilibria; Petroleum fluids

1. Introduction Equation of state (EOS) is an important and effective tool for calculations of thermodynamic properties and phase equilibrium of pure and fluidmixtures. The accurate and simple EOSs are widely used for theoretical and practical studies in chemical process design, petroleum industry, reservoir fluids, etc. Since the first cubic EOS was set out by van der Waals in 1873, hundreds EOSs have been proposed so far, in the three main categories of 1 – Cubic EOSs, 2 – Virial Expansion EOSs, and 3 – Non-cubic or

Abbreviations: AAD%, average absolutedeviation percent; EOS, equation of state; M4, Mohsen-Nia et al. EOS [12]; P–R, Peng and Robinson EOS [17]; P–T, Patel and Teja EOS [18]; PVT, pressure, volume, and temperature; VLE, (vapor + liquid) equilibria. * Corresponding author. Tel.: +98 021 64543176; fax: +98 021 66405847. E-mail address: hmodares@aut.ac.ir (H. Moddarress). 0021-9614/$ - see front matter Ó 2007 Elsevier Ltd. All rightsreserved. doi:10.1016/j.jct.2007.05.012

Complex EOSs. There are several review papers published on historical development of the EOSs [1–4]. Among various types of EOSs, the cubic EOSs are simple, flexible to handle and also reliable (according to its accuracy) in different practical applications. The main factors in developing a cubic EOS are its accuracy, flexibility (or simplicity) and generality.Logically there should be a balance between these factors for the applicability of any EOS [5]. Although in semi-empirical cubic EOSs, the parameters may not have sound theoretical basis, they are widely used by chemical engineers due to their simplicity. However, the accuracy of the results remains as the main concern. There has been considerable progress in the development of equations of statebased on the molecular theory of statistical mechanics [6–9], however, due to their complexity it is still worth while to make improvements on semi-empirical equations of state of industrial interest. The two- and three-parameter cubic EOSs and their modifications, predict poor volumetric behaviour (liquid

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