J. of Supercritical Fluids 55 (2010) 671–681 Contents lists available at ScienceDirect The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu Equation of state modeling of the phase equilibria of asymmetric CO 2 + n-alkane binary systems using mixing rules cubic with respect to mole fraction Martín Cismondi a,b,c , Jørgen M. Mollerup d , Marcelo S. Zabaloy a,∗ a Planta Piloto de Ingeniería Química (Universidad Nacional del Sur - CONICET) CC 717, Camino La Carrindanga Km. 7 (8000), Bahia Blanca, Argentina b IDTQ, Facultad de Ciencias Exactas Físicas y Naturales, Universidad Nacional de Córdoba, Av. Velez Sarsfield 1611, Córdoba, Argentina c IVC-SEP, Department of Chemical Engineering, Bygning 229, DTU, DK 2800 Lyngby, Denmark d Prepchrom, Christiansholmsvej 26, 2930 Klampenborg, Denmark article info Article history: Received 1 June 2010 Received in revised form 5 October 2010 Accepted 6 October 2010 Keywords: Equations of state Cubic mixing rules Type III phase behavior Composition dependence Critical lines LLVE High pressure Asymmetric systems Objective function Interaction parameters abstract Both the equation of state (EOS) and the quadratic mixing rules proposed by van der Waals towards the end of the XIX century were enormous contributions to the understanding and modeling of fluids phase behavior. They set the basis for a consistent and useful representation of phase equilibria for a great diversity of mixtures. Nevertheless, the models for representing phase equilibria and physico-chemical properties of asymmetric systems may require more flexible mixing rules than the classical quadratic van der Waals (vdW) mixing rules or their equivalent (with regard to the number of available interaction parameters) in modern equations of state. In particular, the phase equilibria of binary mixtures containing CO 2 and heavy n-alkanes have been studied by an important number of authors and using different types of models, achieving only partially accurate results and realizing the difficulties that these systems showing type III phase behavior (from C14 on) present for predicting or even correlating their phase equilibrium data in wide ranges of temperature and pressure. Cubic mixing rules (CMRs), implemented as a natural extension of the classical quadratic mixing rules, constitute the simplest alternative among different flexible approaches. In addition, they have the advan- tage of allowing correlation of multicomponent data by fitting ternary interaction parameters, while leaving invariant the description of the constituent binary systems. In this work, and after having detected the need for temperature-dependent interaction parameters in a previous study, we implemented an automated parameterization procedure based on characteristic key-points for binary systems showing type III phase behavior. Using the RK-PR EoS coupled to CMRs we present the parameters obtained and results showing for the first time a quite successful complete description of asymmetric CO 2 + n-alkane binary systems, with n-alkane carbon number from 14 to 22. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Before van der Waals, the liquid state of a substance was believed to be formed of atomic complexes, greater in size than the single molecules existing in the gas phase. Johannes van der Waals expressed “...both portions of the isotherm belong to one curve...there would then only be a difference of greater or smaller density in the two states, and thus only a quantitative difference.” [1]. Being such concept, i.e., the continuity between the liquid and vapor states, already a historical contribution, it is remarkable that at the same time van der Waals moved forward and proposed the first model allowing to describe continuously the liquid, vapor and ∗ Corresponding author. Tel.: +54 291 486 1700; fax: +54 291 486 1600. E-mail addresses: mcismondi@efn.uncor.edu (M. Cismondi), mzabaloy@plapiqui.edu.ar (M.S. Zabaloy). supercritical states of pure fluids. Today, more than 130 years later, such model continues to be the root of many present equations of state. In 1890, Van der Waals provided the practical tools for describing simultaneously both vapor–liquid and liquid–liquid phase separation in binary mixtures, by generalizing his equation of state for application to phase separation of binary fluid mixtures. It was a triumph of the Van der Waals mixture equation that it could produce both vapor–liquid and liquid–liquid phase separation of binary mixtures (Levelt Sengers [2] and ref. cited therein). His quadratic mixing rules (QMRs) allowed for a consistent mod- eling of mixtures phase behavior. Based on the van der Waals EOS and QMRs, van Konynenburg and Scott, “generated the first, nearly comprehensive classification of fluid phase equilibria” [3]. Their calculations were mainly devoted to binary systems without dif- ferences in molecular size. They identified five types of fluid phase behavior (I, II, III, IV and V). Studies for size-asymmetric binary sys- 0896-8446/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2010.10.007