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Temperature Dependent Gas Solubility of H 2 , CO 2 , SO 2 , H 2 S and their Mixtures in the Ionic Liquid 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C 6 mim + ][Tf 2 N - ]): A Monte Carlo Simulation Study Ramesh Singh, Edward J. Maginn and Joan F. Brennecke Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556 [email protected], [email protected], [email protected] Simulation results are compared against experimental results [9,10,11,12] Predicted solubility of gases are in good agreement with previous simulations and experiments except H 2 S Solubility of CO 2 , SO 2 and H 2 S decreases with increase in temperature whereas H 2 solubility increases with increase in temperature Error bars are very small and not shown for clarity ACKNOWLEDGMENTS RESULTS GCEP Stanford University CRC Notre Dame Predicted pure gas absorption isotherms for CO 2 , SO 2 and H 2 agree very well with experiments Simulations predict higher solubility of H 2 S than experiment [12] Solubility of CO 2 , SO 2 and H 2 S decreases with increase in temperature whereas H 2 solubility increases with increase in temperature Solubility order (from high to low): SO 2 >H 2 S >CO 2 > H 2 Estimated partial molar enthalpies are consistent with values reported by others Results indicate only slight change in the structure of ionic liquid upon absorption Solubility selectivity in [C 6 mim + ][Tf 2 N - ] varies depending on the vapor phase composition Near ideal selectivity for CO 2 / H 2 mixture and non-ideal selectivity for SO 2 / CO 2 and H 2 S/ CO 2 mixtures Solubility selectivity decreases with increase in temperature IONIC LIQUIDS SIMULATION DETAILS CONCLUSIONS MOTIVATION Design an ionic liquid solvent for pre-combustion CO 2 capture Ionic liquids have highly tunable properties: ≈10 6 ILs could be formed by varying the cations or the anions Impossible, or at least very impractical, to synthesize and conduct experiments on all possible ionic liquids Development of computational techniques that can deal with the novel ionic liquid solvents for CO 2 capture is critical Methods must be capable of: Molecular simulation- most promising methods to predict the properties and gain atomistic understanding Describing the thermodynamic behavior of phase equilibria Helping understand the behavior at the atomic level Screening the potential materials for CO 2 capture 1 μm Ionic liquids (ILs) Organic salts (melting points at or below 100 °C ) Melting Point > 800°C Packing of irregular shaped ions Ionic Liquids Packing of regular shaped ions Inorganic salts Melting Point < 100°C Low melting point - asymmetry between the ions prevent formation of stable crystal lattice (“frustrated crystal packing”) Many have unique properties: non-flammable, negligible vapor pressure, high conductivity and excellent thermal and chemical stabilities Isothermal-isobaric Gibbs ensemble Monte Carlo method (NPT-GEMC) [1] [C 6 mim + ] [Tf 2 N - ] Ionic liquid Force fields and parameters Vapor phase (Pure CO 2 ) Liquid phase (IL+ CO 2 ) Solute Phase (β) Ionic liquid Phase (α) T α = T β P α = P β μ α = μ β Random moves (e.g. translation, rotation, volume change, exchange, and regrowth) REFERENCES [C 6 mim + ][Tf 2 N - ]: All atom force field (Wei Shi and Edward J. Maginn, J. Phys. Chem. B, 2008) H 2 : Two site Cracknell model (Cracknell, R. F., Phys. Chem. Chem. Phys. , 3, 2091,2001) CO 2 : TraPPE model (J.J. Potoff and J.I. Siepmann, AIChE J. 47, 1676-1682 ,2001) SO 2 : TraPPE mode (Ganesh Kamath, MaryBeth Ketko, Gary A. Baker, and Jeffrey J. Potoff, J. Chem. Phys. 136, 044514, 2012) H 2 S: Three site model (Shyamal K. Nath, J. Phys. Chem. B, 2003) SIMULATION DENSITY AND PRODUCTION RUNS ABSORPTION ISOTHERMS HENRY’S LAW CONSTANT AND MOLAR ENTHALPY Predicted IL densities agree well with extrapolated experimental densities [7,8] Production runs are well equilibrated The Henry’s law constant is the linear relationship between gas solubility and fugacity (corrected pressure) in the limit of low solute concentration Partial molar enthalpy (ΔH) indicate the strength of interaction between solvent and solute molecules Low pressure absorption isotherms are fitted to straight line in order to get Henry’s law constants Van’t Hoff relationship is used compute the partial molar enthalpy (ΔH) of the gases MIXED GAS SOLUBILITY RADIAL DISTRIBUTION FUNCTION Near ideal selectivity for for CO 2 over H 2 Solubility selectivity in [C 6 mim + ][Tf 2 N - ] varies depending on the vapor phase composition Selectivity decreases from about 63-41 at 333 K to about 4.3-3.3 at 573 K (results not shown) Non-ideal selectivity for SO 2 / CO 2 (selectivity≈11.0, ideal selectivity =12.45 ) and H 2 S/ CO 2 (selectivity≈2.5, ideal selectivity = 3.58 ) mixtures Radial distribution function g(r)-probability of finding a atom/molecule at a distance of r away from a given reference atom/ molecule No change in the ionic structure upon absorption Solute molecules occupy the inter ionic space Henry’s law constant (bar) enthalpy solute 333 K 373 K 413 K 573 K 700 K (kJ/mol) H 2 1496±7.53 (1634) - - 1288±4.23 973±12.23 1.91±0.06 (4.0) CO 2 38.72±2.45 (48.0) 58.62±1.73 99.61± 1.84 261.54±0.73 - -12.81±0.73 (-11.8) SO 2 3.11±0.02 (4.09) 7.00±0.40 13.34±0.70 69.30±0.22 - -20.17± 0.13 (-20.3) H 2 S 10.81±0.29 (21.2) 16.23±0.15 27.75±0.67 97.53±0.07 - -14.91±0.11 (-14.5) 1. Panagiotopoulos, A.Z., Molecular Physics, 1987. 61(4): p. 813-826. 2. Shi, W. and E.J. Maginn, The Journal of Physical Chemistry B, 2008. 112(7): p. 2045-2055. 3. Cracknell, R.F., Physical Chemistry Chemical Physics, 2001. 3(11): p. 2091-2097. 4. Kamath, G., et al., The Journal of Chemical Physics, 2012. 136(4): p. 044514. 5. Chen, B., J.I. Siepmann, and M.L. Klein, The Journal of Physical Chemistry B, 2001. 105(40): p. 9840-9848. 6. Nath, S.K., The Journal of Physical Chemistry B, 2003. 107(35): p. 9498-9504. 7. Widegren, J.A. and J.W. Magee, Journal of Chemical & Engineering Data, 2007. 52(6): p. 2331-2338. 8. Tariq, M., et al., The Journal of Chemical Thermodynamics, 2009. 41(6): p. 790-798. 9. Aki, S.N.V.K., et al., The Journal of Physical Chemistry B, 2004. 108(52): p. 20355-20365. 10. Kumełan, J., et al.,. The Journal of Chemical Thermodynamics, 2006. 38(11): p. 1396-1401. 11. Anderson, J.L., et al., The Journal of Physical Chemistry B, 2006. 110(31): p. 15059-15062. 12. Jalili, A.H., et al., The Journal of Physical Chemistry B, 2012. 116(9): p. 2758-2774. [C 6 mim + ][Tf 2 N - ] y x T (K) P (bar) CO 2 H 2 CO 2 H 2 selectivity ! ! ! /! !" ! 333 30 0.0708(5) 0.9292(5) 0.0917(35) 0.0191(7) 63.0(0.64) 41.794 333 30 0.1342(11) 0.8658(11) 0.1445(15) 0.0144(10) 64.6(4.51) 333 30 0.2140(25) 0.7860(25) 0.1866(97) 0.0127(4) 54.0(1.80) 333 30 0.4124(25) 0.5876(25) 0.2734(48) 0.0082(1) 47.3(1.80) 333 30 0.5622(36) 0.4378(36) 0.3125(57) 0.0060(2) 40.1(0.35) 333 30 0.6918(17) 0.3082(17) 0.3582(27) 0.0036(2) 44.1(4.89)
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