Predicting the phase equilibria of binary mixtures containing carbon dioxide + n-alkanols from a quadrupolar SAFT-VR approach

Abstract

The vapor–liquid equilibria of binary mixtures containing carbon dioxide + n-alkanols is studied in the framework of the statistical associating fluid theory for potentials of variable range (SAFT-VR). The effect of the quadrupole moment of carbon dioxide on the phase equilibria is considered into the original SAFT-VR approach by incorporating an additional contribution to the Helmholtz free energy due to quadrupole–quadrupole interactions. The carbon dioxide is modeled as two spherical segments tangentially bonded with an axial quadrupolar moment. The n-alkanol molecules are represented as chains of spherical monomer segments with three associating sites to describe the hydrogen bonding interactions between the molecules. The association effects are taken into consideration by using the first-order thermodynamic perturbation theory of Wertheim. The attractive interactions between monomer segments and site-site associative interactions have been accounted for by using a square-well intermolecular potential. The pure component parameters for carbon dioxide and n-alkanols (from methanol to 1-decanol) are obtained by fitting the SAFT-VR approach to experimental saturated liquid density and vapor pressure. These optimized parameters are used to determine the vapor–liquid equilibria of the mixtures. The pressure-composition and density-composition projections predicted by SAFT-VR for the carbon dioxide + n-alkanols are in good agreement with the experimental data. A significant improvement in the description of the vapor–liquid equilibria of binary mixtures containing carbon dioxide + n-alkanols is obtained with the quadrupolar contribution compared with the results obtained from the original SAFT-VR. The optimized molecular parameters are rescaled in order to determine critical lines and the types of phase behavior. The critical lines and critical points are predicted by the original SAFT-VR approach and the quadrupolar version of the SAFT-VR in good agreement with experimental data. The inclusion of the quadrupolar interactions, present in the carbon dioxide, avoids the treatment of these interactions in an effective way via the square-well potential of variable range. The results of the present work are useful for further exploration of adsorption-extraction of n-alkanols in supercritical fluids.