Phase Equilibria of CO 2 + n-Alkane Binary Systems in Wide Ranges of Conditions: Development of Predictive Correlations Based on Cubic Mixing Rules Martín Cismondi,* ,†,‡ Sabrina B. Rodríguez-Reartes, † Juan M. Milanesio, † and Marcelo S. Zabaloy* ,† † Planta Piloto de Ingeniería Química, Universidad Nacional del SurCONICET, CC 717, Camino La Carrindanga Km. 7, (8000) Bahia Blanca, Argentina ‡ IDTQ (Grupo Vinculado PLAPIQUI-CONICET), Facultad de Ciencias Exactas Físicas y Naturales, Universidad Nacional de Có rdoba, Av. Velez Sarsfield 1611, Có rdoba, Argentina * S Supporting Information ABSTRACT: The phase equilibria of binary CO 2 + n-alkane mixtures have been studied by an important number of authors, both experimentally and using different types of thermodynamic models. Modeling studies of the phase behavior of such highly nonideal systems have generally achieved only partially accurate results in the correlation of phase equilibrium data when considering wide ranges of temperature, pressure, and n-alkane molecular weight. In this study, a predictive correlation for the phase behavior of CO 2 + n-alkane systems, based on a three-parameter cubic equation of state (EOS), that is, the RK-PR EOS, coupled to cubic mixing rules (CMRs), is developed and tested. CMRs have been shown to be capable of an accurate correlation of the phase equilibria asymmetric CO 2 + n-alkane binary systems, in wide ranges of temperature and pressure, when using system-specific interaction parameters. For developing the predictive correlation a critical review of published experimental data for the series was carried out, covering a total of about 100 references. An important degree of inaccuracy or scatter is often found when comparing data sets from different laboratories, specially for the more asymmetric systems (CO 2 + a long chain n-alkane). Tables of references covering CO 2 + n-alkane systems from C1 to C36 are presented for different types of experimental data, including critical end points (CEPs), critical points, liquid-liquid-vapor (LLV) equilibrium, and isobaric (Txy), isothermal (Pxy), and isoplethic (PT) two-phase equilibrium data sets. Examples of disagreement between different sets of data are presented and discussed. In some cases, a decision concerning the identification of the set that should be regarded as the most reliable, can be based on the experimental method employed, on the purity of the n-alkane, and on the observation of other data for conditions, and/or systems in the series, which are close to those of the data set under scrutiny. Nevertheless, the availability of such information is not enough, in other cases, to assess the quality of a given data set, where we have either different data sets in disagreement or a unique set, for which we are in doubt about its accuracy. In such situation, a predictive correlation for the whole series of binary systems is helpful to make a decision on the possible level of reliability of a given phase equilibrium data set. The present study is useful both to make decisions on conflicts between contradictory phase equilibrium data sets and to predict the phase equilibria of binary systems that have no experimental information available in the literature. 1. INTRODUCTION When considering phase equilibria in mixtures, one very important family of binary systems, both from the technological and academic points of view, is the CO 2 + n-alkane homologue series. The importance of this series, sometimes taken as a ref- erence in the analysis of other nonalkane + CO 2 binary mix- tures, is clear when considering, for example, CO 2 injection as a method for enhanced oil recovery, the recent approaches for CO 2 sequestration in depleted oil wells, and extractions and separations using supercritical CO 2 as solvent. 1,2 Carbon dioxide has a large quadrupole moment 3 while n-alkanes are nonpolar and may have a molecular weight much greater than that of CO 2 . Therefore, most of the binary systems within the CO 2 + n-alkane homologue series are highly asymmetric with regard to both molecular size and energetic interactions. This makes the correlation/prediction of the phase equilibria of the series, over wide ranges of conditions, very difficult. This is a yet not fully solved relevant problem of physical chemistry. Most of the available previous attempts to describe the phase equilibria of CO 2 + n-alkane systems in wide ranges of con- ditions were not completely successful (more details are pro- vided elsewhere 4 ). Models of the equation of state (EOS) type 5 are the proper choice for describing the fluid phase equilibria over a wide range of pressure. This is because EOSs explicitly account for the effect of density on the thermodynamic properties of pure fluids and mixtures. A standard approach is to couple to EOS type models, mixing rules quadratic with respect to mole frac- tion (QMRs). 5 QMRs are double summations in mole frac- tion that make it possible to compute mixture parameters from pure-compound and interaction parameters. QMRs lack the flexibility required to describe highly asymmetric systems such as most of the CO 2 + n-alkane systems. In contrast, cubic Received: August 22, 2011 Revised: March 26, 2012 Accepted: April 2, 2012 Published: April 2, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 6232 dx.doi.org/10.1021/ie2018806 | Ind. Eng. Chem. Res. 2012, 51, 6232-6250