Modelling Carbon Dioxide Solubility in Ionic Liquids Soheil Mortazavi-Manesh, 1 Marco Satyro 2 and Robert A. Marriott 3 * 1. Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4 2. Virtual Materials Group, Inc., 222-1829 Ranchlands Blvd. NW, Calgary, Alberta, Canada, T3G 2A7 3. Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4 Solubility information for CO 2 in different ionic liquids, ILs, in part can potentially be used to select a specific IL for the separation of CO 2 from hydrocarbon fluids. Unfortunately, not all CO 2 –IL systems have been experimentally described at similar temperatures and pressures; therefore, a direct comparison of performance by process simulation is not always possible. In the extreme cases, the design of a CO 2 separation process may require predicting the CO 2 –IL equilibria for which there are no available solubility data. To address the need for this information, a semi-empirical correlation was developed to estimate the dissolution of CO 2 in CO 2 –IL solvent systems. The theoretical COSMO–RS calculation method was used to calculate the chemical potential of CO 2 in a wide variety of ILs and the Soave–Redlich–Kwong equation was used to calculate the fugacity coefficient of the CO 2 vapour phase. The model was correlated with available literature data, yielding an average error of AAR = 23% and small bias. Keywords: ionic liquids, carbon dioxide, COSMO–RS, gas treatment, chemical potential INTRODUCTION M any hydrocarbon fluids are produced from reservoirs with significant concentrations of acid gases such as CO 2 , where the CO 2 generally must be removed from the raw fluids to meet the heating value specifications for sales gas. Traditionally, various aqueous amine solutions have been used to preferentially absorb CO 2 from raw gas feed stock; how- ever, there is an increasing interest in exploring the advantages of replacing aqueous amines with other selective solvents, such as Ionic Liquids, ILs. In order to conceptually explore the potential of Ils for acid gas separation, process simulation requires that the physical properties and phase equilibria be measured or estimated and incorporated into a thermodynamic model suitable for flash calculations. ILs are molten salts consisting of large organic cations and organic or inorganic anions. The large organic moieties allow the ionic compounds to remain liquid at or near room temperature. As with most ionic compounds, ILs exhibit very low vapour pres- sures making them good candidates for gas conditioning with minimal process and environmental waste during warm regener- ation conditions, that is, less solvent loss and constant recycle of solvent to the absorption system with negligible solvent make-up requirements. Due to the large range of possible organic function- alisation, there is the opportunity to further tune their intrinsic thermo-physical properties by changing their chemical structure at the synthesis stage or systematically choosing the best existing IL. Experimental data for CO 2 –IL equilibria are not available for all the existing ILs and potentially new ILs; therefore, correlations can be used to synthesise the available data in a thermodynamic model that then can be used to target, simulate and optimise IL candidates. Being able to estimate their performance by using molecular simulation allows one to screen for the best candidates and/or preferred chemical functional groups. In this study, we report the development of a semi-empirical model using a theoretically determined chemical potential to esti- mate the solubility of CO 2 in a wide range of ILs. The availability of (i) new theoretical tools (Klamt, 2005) for the estimation of activity coefficients in complex polar liquid, (ii) limited experi- mental solubility data and (iii) the desire to study the conceptual ∗ Author to whom correspondence may be addressed. E-mail address: rob.marriott@ucalgary.ca Can. J. Chem. Eng. 9999:1–7, 2012 © 2012 Canadian Society for Chemical Engineering DOI 10.1002/cjce.21687 Published online in Wiley Online Library (wileyonlinelibrary.com). | VOLUME 9999, 2012 | | THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING | 1 |