J. of Supercritical Fluids 64 (2012) 32–38 Contents lists available at SciVerse ScienceDirect The Journal of Supercritical Fluids jou rn al h om epa ge: www.elsevier.com/locate/supflu Gas–liquid equilibrium modeling of mixtures containing supercritical carbon dioxide and an ionic liquid José O. Valderrama a,b,∗ , Felipe Urbina c , Claudio A. Faúndez c a Univ. of La Serena, Fac. of Engineering, Dept. of Mech. Eng., Casilla 554, La Serena, Chile b Centro de Información Tecnológica, Mons. Subercaseaux 667, La Serena, Chile c Univ. de Concepción, Fac. de Cs. Físicas y Matemáticas, Casilla 160-C, Concepción, Chile a r t i c l e i n f o Article history: Received 26 December 2011 Received in revised form 10 February 2012 Accepted 13 February 2012 Keywords: Ionic liquids Equations of state Wong–Sandler mixing rules Kwak–Mansoori Gas–liquid equilibrium a b s t r a c t Gas–liquid equilibrium in mixtures containing supercritical carbon dioxide and an ionic liquid has been modeled using the Peng–Robinson equation of state and the interesting modification proposed by Kwak and Mansoori. The proposal is based on statistical–mechanical arguments and on the fact that mixing rules are for constants of an equation of state and not for functions such as the alpha expression that makes the attractive term temperature dependent. Experimental data for mixtures containing carbon dioxide and ionic liquids were obtained from literature sources and the adjustable parameters of the mixing rules were found by minimizing the errors between predicted and experimental bubble pressure. Results were compared with experimental data and with results of other classical mixing rules, such as the proposal of Wong and Sandler. The deviations in correlating gas–liquid equilibrium found with the model Peng–Robinson/Kwak–Mansoori are lower in general than the results obtained with other models. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Ionic liquids have attracted special attention from the scientific community in recent years and hundreds of studies involving dif- ferent aspects of the properties and applications of ionic liquids have been presented in the literature. Ionic liquids are organic salts with some special characteristics that make them suitable for many applications. Ionic liquids have low melting temperatures, are liquid at room temperature, have thermal stability up to high temperatures; posses high solubility for both polar and non-polar organic and inorganic substances, exhibit interesting solvation and coordination properties that depend on the nature of the cation and/or anion, and have very low vapor pressure [1]. This special characteristic of almost null vapor pressure has transformed ionic liquids into good alternatives as green solvents of potential com- mercial interest [2]. Ionic liquids are typically composed of a large organic cation and an inorganic polyatomic anion. There is virtually no limit in the number of possible ionic liquids that can be synthesized since there are a large number of cations and anions that can be com- bined. Among the many applications of ionic liquids that have been reported in the literature are: media for clean liquid–liquid ∗ Corresponding author at: Univ. of La Serena, Fac. of Engineering, Dept. of Mech. Eng., Casilla 554, La Serena, Chile. E-mail addresses: jvalderr@userena.cl, citchile@entelchile.net (J.O. Valderrama). extraction processes, as solvents for electrochemical applications, recyclable alternatives to aprotic solvents, catalysts for organic and organometallic synthesis, for catalytic cracking of polyethylene, and for radical polymerization, as media for analytical and physi- cal chemistry, and as lubricants, among many other potential uses. Detailed information about the synthesis and application of ionic liquids is available in the literature [2–7]. For the development of new processes using ionic liquids or to continue some of the studies currently investigated the knowl- edge of physical, physicochemical and transport properties and of phase equilibrium behavior is required. Efforts have been done to gathered physicochemical and transport properties of ionic liquids in data bases [8–10]. Also, during the last fifteen years different research groups started to measure mixture data. In particular, phase equilibrium data of mixtures containing a high pressure gas and an ionic liquid have been reported in the literature [11–19]. For example, Pérez-Salado et al. [20] reported experimental data for the solubility of CO 2 in the ionic liquid [bmin] [PF 6 ] for temper- atures from 293 to 393 K and pressures up to about 9.7 MPa. Shariati and Peters [21] reported experimental data for high-pressure phase equilibrium of the binary systems CO 2 + [emin][PF 6 ], CO 2 + ([bmin] [PF 6 ]), and CO 2 + [hmin] [PF 6 ]. Blanchard et al. [11] have shown that supercritical CO 2 extrac- tion is a viable method for solute recovery from an ionic liquid and that phase behavior of these systems represents very unusual biphasic systems. The authors presented high- pressure phase equilibrium data of six CO 2 + ionic liquid mixtures: 0896-8446/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2012.02.007