Fluid Phase Equilibria 309 (2011) 179–189 Contents lists available at ScienceDirect Fluid Phase Equilibria j our na l ho me page: www.elsevier.com/locate/fluid Thermodynamic modeling of hydrogen sulfide solubility in ionic liquids using modified SAFT-VR and PC-SAFT equations of state Mahboubeh Rahmati-Rostami a , Bahman Behzadi b , Cyrus Ghotbi a,∗ a Department of Chemical and Petroleum Engineering, Sharif University of Technology, P.O. Box 11365-9465, Tehran, Iran b Thermodynamics Research Laboratory, School of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran 16846-13114, Iran a r t i c l e i n f o Article history: Received 2 April 2011 Received in revised form 15 July 2011 Accepted 19 July 2011 Available online 26 July 2011 Keywords: Ionic liquids SAFT-VR PC-SAFT Square-well potential Dipole–dipole interactions Solubility a b s t r a c t Equations of state based on the statistical associating fluid theory for potentials of variable range (SAFT- VR) and the perturbed chain statistical associating fluid theory (PC-SAFT) have been used to model the PVT behavior of ionic liquids and the solubility of H 2 S in six imidazolium-based ionic liquids. The studied sys- tems included [bmim][PF6], [hmim][PF6], [bmim][BF4], [hmim][BF4], [bmim][NTF2] and [hmim][NTF2] at various temperatures and pressures. For pure components, parameters of the models have been obtained by fitting the models to experi- mental data on liquid densities; the average relative deviation between the calculated and experimental densities for ionic liquids is less than 2.42% in the PC-SAFT model and 5.44% in the SAFT-VR approach, the latter which incorporates the square-well potential for short-range interactions. In both models an addi- tional term has been added to account for dipole–dipole interactions between solute molecules resulting from the permanent charges on the chain molecules of the solvents. The model parameters have also been correlated as functions of the molecular weight of the solvents. For binary mixtures of ionic liq- uids and H 2 S, the association interactions between H 2 S molecules and between the ionic liquids and H 2 S molecules have also been taken into account in both approaches, using binary interaction coefficients. The results show an average deviation of less than 5% in the calculation of the mole fraction of H 2 S in the ionic liquids. The effect of inclusion of the polar term has been studied for binary systems in both models. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Ionic liquids (ILs) are found to be potentially suitable solvents for the chemical process industry, with the prospect of being used as environmentally friendly solvents, or as reaction and separation media. They are molten salts with melting points close to room temperature [1]. Their most remarkable property is that their vapor pressure is negligibly small, which results in ILs being practically nonvolatile and odorless, and they are generally non-flammable [2], resulting in a considerable decrease of solvent loss and envi- ronmental harm when used in separation processes, as well as increased safety in handling and processing. Other characteristics of ILs include a wide liquid temperature range, high thermal and electrochemical stability, high ionic conductivity and fair proper- ties as solvents [3,4]. Different ILs can be considered for a variety of applications by altering the anion–cation combination in the molecular structure. Thus, knowledge about the phase behavior and other properties of ∗ Corresponding author. Tel.: +98 21 66005819; fax: +98 21 66022853. E-mail address: ghotbi@sharif.edu (C. Ghotbi). mixtures containing ILs becomes important in the design of sepa- ration and extraction processes. The solubility of gases in ILs is one of the most important properties for the design and operation of many chemical processes. One of the foreseeable industrial appli- cations in the field of natural gas processing is the replacement of traditional organic solvents, usually alkanolamine solutions which are presently used for the absorption of H 2 S and CO 2 from sour gases, by suitable ILs. During the past two decades many studies have been reported on the thermodynamic modeling of gas solubility in ionic liquids (cf. [5–11]). Recently, the tPC-PSAFT equation of state (EOS) has been used by Kroon et al. [5] for the determination of the solubil- ity of CO 2 in several ILs at pressures up to 100 MPa and for mole fractions up to 0.75 for CO 2 ; good agreement was obtained with experimental data. Wang et al. [6] used a square-well chain-fluid equation of state to correlate the solubilities of gases such as CO 2 , C 3 H 6 , C 3 H 8 , C 4 H 10 in various ILs; it is shown that the solubility of CO 2 in ionic liquids can be correlated using only one temperature- independent adjustable interaction parameter. Andreu and Vega [7] used the soft-SAFT EOS to model the solubility of carbon dioxide in imidazolium-based ILs; for the pure components, a correla- tion of the molecular model parameters of the ILs as a function 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.07.013