Modeling of CO2 Solubility In Several Ionic Liquids Using PC-SAFT

Ionic liquids (ILs) are being widely studied as a class of tunable solvents.  Some ILs show potential for the separation of CO₂ from a variety of gas streams.  For streams with a relatively large partial pressure of CO₂, there is potential for economic separation of CO₂ using ILs that physically absorb CO₂.  Recently, Mejía et al. [1] have measured the solubility, due to physical absorption, of CO₂ in several ILs for which the CO₂ solubility has not been previously measured.  These include five 1-ethyl-3-methylimidazolium [emim] salts, namely those formed with the anions thiocyanate [SCN], hydrogensulfate [HSO₄], methylsulfate [MeSO₄], methanesulfonate [MeSO₃], and diethylphosphate [DEP].  Also studied were ethyl(tributyl)phosphonium diethylphosphate [P₂₄₄₄][DEP], 1-(2-hydroxyethyl)-3-methyl-imidazolium trifluoroacetate [OHemim][TFA], and 1-hexyl-3-methylimidazolium trifluoromethanesulfonate [hmim][OTf].  In this presentation, we will focus on development of an approach for modeling these new results, together with measurements [2] for the pure component IL densities.

An overview of models for gas solubilities in ILs has recently been given by Vega et al. [3].  Though cubic equation-of-state (EOS) models can be used to model CO₂ solubility in ILs (e.g., [4, 5]), we have chosen to use a statistical associating fluid theory (SAFT) model.  Models based on SAFT have become popular in various applications due to its strong theoretical base and due to the ease with which terms can be added to it that account for various types of intermolecular forces.  The strong theoretical base implies a greater predictive capability that would be useful for estimating the properties of untested ILs.  Various studies have been conducted on modeling gas solubilities in ILs using different SAFT-based models, including work by Andreu and Vega [6, 7], Karakatsani et al. [8, 9], and Ji and Adidharma [10, 11].  Some of these models appear to involve an unnecessarily large number of fit parameters, and it is not clear that all of these models accurately reflect the exceedingly low vapor pressure of the pure IL.  For this work, we have chosen to use a basic version of SAFT, namely perturbed chain SAFT (PC-SAFT) [12], which requires a minimal number of fit parameters (three for each pure component, plus a binary interaction parameter).  Parameters for the pure component ILs were fit to pure component density data, but with a constraint that enforces a low volatility; CO₂ parameters were fit to pure component density and vapor pressure data.  Using this approach we were able to successfully model the newly measured CO₂ solubility data, as well as data measured previously for several other ILs.  The model parameters obtained follow patterns that imply a strong predictive power. 

References:

1. Mejía, I.; Stanley K.; Brennecke, J. F.;  On the high pressure solubilities of several ionic liquids and carbon dioxide.  2011, in preparation.

2. Ficke, L. E.; Novak, R. R.; Brennecke, J. F.; Thermodynamic and thermophysical properties of ionic liquid + water systems.  J. Chem. Eng. Data 2010, 55, 4946-4950.

3. Vega, L. F.; Vilaseca, O.; Llovell, F.; Andreu, J. S.;  Modeling ionic liquids and the solubility of gases in them: Recent advances and perspectives.  Fluid Phase Equil. 2010, 294, 15-30.

4. Yokozeki A.; Shiflett M. B.; Binary and Ternary Phase Diagrams of Benzene, Hexafluorobenzene, and Ionic Liquid [emim][Tf2N] Using Equations of State. Ind. Eng. Chem. Res. 2008, 47, 8389-8395

5. Arce P. F.; Robles P. A.; Graber T. A.; Aznar M;  Modeling of High-Pressure Vapor Liquid Equilibrium in Ionic Liquid Plus Gas Systems Using the PRSV Equation of State. Fluid Phase Equil. 2010, 295, 9-16

6. Vega, L. F.; Andreu, J. S.;  The Solubility Behavior of CO₂ in Ionic Liquids by a Simple Model.  J. Phys. Chem. C 2007, 111, 16028-16034.

7. Vega, L. F.; Andreu, J. S.;  Modeling the Solubility Behavior of CO₂, H₂, and Xe in [C_n-mim][Tf₂N] Ionic Liquids.  J. Phys. Chem. B 2008, 112, 15398-15406.

8. Karakatsani, E. K.; Economou, I. G.; Kroon, M. C.; Peters, C. J.; Witkamp, G. J.;  tPC-SAFT Modeling of Gas Solubility in Imidazolium-Based Ionic Liquids.  J. Phys. Chem. C 2007, 111, 15487-15492.

9. Karakatsani, E. K.; Economou, I. G.; Kroon, M. C.; Bermejo M. D.; Peters C. J.; Witkamp G. J.; Equation of State Modeling of the Phase Equilibria of Ionic Liquid Mixtures at Low and High Pressure.  Phys. Chem. Chem. Phys. 2008, 10, 6160-6168.

10. Ji, X.; Adidharma, H.;  Thermodynamic modeling of ionic liquid density with heterosegmented statistical associating fluid theory.  Chem. Eng. Sci. 2009, 64, 1985-1992.

11. Ji, X.; Adidharma, H.;  Thermodynamic modeling of CO₂ solubility in ionic liquid with heterosegmented statistical associating fluid theory.  Fluid Phase Equil. 2010, 293, 141-150.

12. Gross J.;  Sadowski G.;  Perturbed-Chain SAFT:  An Equation of State Based on a Perturbation Theory for Chain Molecules. Ind. Eng. Chem. Res. 2001, 40, 1244-1260.