ARTICLE IN PRESS G Model Fluid Phase Equilibria 277 (2009) 152–161 Contents lists available at ScienceDirect Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid Vapor–liquid equilibrium in the n-butane + methanol system, measurement and modeling from 323.2 to 443.2 K Xavier Courtial a , Chien-Bin Soo a , Christophe Coquelet a , Patrice Paricaud b , Deresh Ramjugernath c , Dominique Richon a,∗ a MINES ParisTech, CEP/TEP - Centre énergétique et procédés, CNRS FRE 2861, 35, Rue Saint Honoré, 77305 Fontainebleau, France b Laboratoire de Chimie et Procédés, ENSTA, ParisTech, 32 Boulevard Victor, 75739 Paris Cedex 15, France c Thermodynamic Research Unit, School of Chemical Engineering, University KwaZulu Natal, Howard College Campus, Durban 4041, South Africa article info Article history: Received 11 September 2008 Received in revised form 1 December 2008 Accepted 4 December 2008 Available online xxx Keywords: Experimental VLE measurement n-Butane Methanol Critical temperature Thermodynamic model abstract New experimental vapor–liquid equilibrium (VLE) data for the n-butane + methanol binary system are reported over a wide temperature range from 323.2 to 443.2 K and pressures up to 5.4 MPa. A static–analytic apparatus, taking advantage of two pneumatic capillary samplers, was used. The phase equilibrium data generated in this work are in relatively good agreement with previous data reported in the literature. Three different thermodynamic models have been used to represent the new experimen- tal data. The first model is the cubic-based Peng–Robinson equation of state (EoS) combined with the Wong–Sandler mixing rules. The two other models are the non-cubic SAFT-VR and PC-SAFT equations of state. Temperature-dependent binary interaction parameters have been adjusted to the new data. The three models accurately represent the new experimental data, but deviations are seen to increase at low temperature. A similar evolution of the binary parameters with respect to temperature is observed for the three models. In particular a discontinuity is observed for the k ij values at temperatures close to the critical point of butane, indicating the effects of fluctuations on the phase equilibria close to critical points. © 2009 Published by Elsevier B.V. 1. Introduction The phase behavior of the system n-butane + methanol is very interesting due to the combination of a non-polar and a polar component forming an azeotrope. The modeling of such binary mixtures is often a challenge due to self-association between the polar methanol molecules, which makes the behavior of the mix- ture highly non-ideal. Several sets of vapor–liquid equilibrium data for the binary mix- ture can be found in the literature. Kretschmer and Wiebe [1] were the first to report measurements of bubble pressures for the mix- ture at 298.15K, although only three points were presented. The authors then used a self-designed apparatus, and thereafter a non- linear empirical equation to relate n-butane mole fractions to total pressure. Petty and Smith [2] obtained bubble and dew pressures for temperatures from 322 to 410K, at liquid and vapor n-butane compositions of 0, 0.25, 0.50, 0.75 and 1. They used an equipment characterized by a mercury compressibility bomb, although there is some disparity between these results and others reported. More consistent data were later published in the works of Leu et al. [3,4], who presented VLE measurements using a variable-volume ∗ Corresponding author. E-mail address: dominique.richon@mines-paristech.fr (D. Richon). vapor–liquid equilibrium cell in the whole range of composition, and temperatures of 273.2, 323.2, 372.5 [3], and 469.9 K [4], with good agreements with two other Refs. [5,6] at 323.2 K. A recent publication by Dell’Era [7] presented data at 364.5 K with a static total pressure apparatus and modeling with Wilson, NRTL and other solution models. In this case, except for the COSMO-RS model, most solution models are quite inaccurate for the mixture, but there is a strong lack in all models in predicting the behavior at the azeotrope. The aim of this paper is to present new isothermal VLE data for the n-butane + methanol binary mixture from 323.2 to 443.2 K, focusing on the phase behavior above and below the n- butane critical temperature. The isotherms reported in this paper were determined using an apparatus based on the static–analytic method, which takes advantage of two ROLSI TM capillary samplers [8] for analysis in a gas chromatograph. Three different models are chosen for the treatment of the new data. The Peng–Robinson (PR) [9] is a cubic equation of state (EoS), well-established in the oil industry for its simplicity and accuracy in describing the behavior of hydrocarbon systems. However, as with most cubic EoS, association interactions between molecules (hydrogen bonding) remain unaccounted for. In recent years, the use of statistical thermodynamics has provided a sounder, more rigorous theoretical basis to fluid models. This has led to the devel- opment of a new family of EoS, which is able to describe associating interactions of polar systems, at the expense of mathematical com- 0378-3812/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.fluid.2008.12.001 FLUID-8083; No. of Pages 10