910
Ind. Eng. Chem. Res. 2008, 47, 910-919
Correlations of Low-Pressure Carbon Dioxide and Hydrocarbon Solubilities in Imidazolium-, Phosphonium-, and Ammonium-Based Room-Temperature Ionic Liquids. Part 2. Using Activation Energy of Viscosity Prem K. Kilaru and Paul Scovazzo* Department of Chemical Engineering, UniVersity of Mississippi, 134 Anderson Hall, UniVersity, Mississippi 38677
This paper provides insights into the dependence of gas solubility on the viscosities of room-temperature ionic liquids (RTILs). Hildebrand solubility parameters are estimated from the activation energy of viscosity via a proportionality constant. The solubility of the gases CO2, ethylene, propylene, 1-butene, and 1,3-butadiene at low pressure and constant temperature are correlated, with the estimated RTIL Hildebrand solubility parameters, into two categories: bis(trifluoromethyl(sulfonyl)imide ([Tf2N]) and non-bis(trifluoromethyl(sulfonyl)imide (non-[Tf2N]) RTILs. The RTILs used in this work are based on 1-alkyl-3-methylimidazolium, quaternary phosphonium, and quaternary ammonium cations. The non-[Tf2N] anions used in the trial set are chloride [Cl], hexafluorophosphate [PF6], trifluoromethanesulfonate [TfO], bis((perflurorethyl)sulfonyl)imide [BETI], dicyanamide [DCA], diethylphosphate [DEP], and dodecylbenzenesulfonate [DBS]. The analysis of the correlation parameters against theory indicates that solute/solvent interactions may be negligible, compared to solvent/solvent and solute/solute interactions, when correlating the relative trend in gas solubilities between RTILs. This even occurs for the non-[Tf2N] anion classification for CO2 solubility that contained anions over a range of electron donor potentials from [Cl] to [BETI], which is in contrast to the widely published statement that the relative CO2 solubility between RTILs is related to anion interactions. Within an anion class, the RTIL solubility parameters decrease as the length of the cation-alkyl chain increases. Overall, the viscosity correlations presented here give better universal correlations for gas solubility than those previously presented in the literature (surface tension, lattice-energy densities, and melting points). Introduction The growing interest in the application of room-temperature ionic liquids (RTILs) as gas-separation media1-3 stems from their exceptional properties, such as negligible vapor pressure, high thermal stability, and tunability of various other properties with the structural changes of the RTILs. Gas solubility is an important design parameter in equilibrium stage- and rate-based separations.4 The prediction of gas solubility in RTILs is a fundamental step toward the development of simulation tools to aid in the process calculations prior to industrial applications. In Part 1 of this work,5 we correlated gas solubility with RTIL surface tension using regular solution theory for RTILs categorized as imidazolium, phosphonium, and ammonium, with the anions chloride [Cl], hexafluorophosphate [PF6], trifluoromethanesulfonate [TfO], bis(trifluoromethyl(sulfonyl))imide [Tf2N], dicyanamide [DCA], and diethylphosphate [DEP]. The predictive models through surface tensions of the RTILs seem to confirm a physical absorption process. This paper further supports the previous discussion about low-pressure isothermal gas absorption in RTILs through solubility parameters calculated from the activation energy of viscosity. The regression constants obtained from the surface tension-gas solubility regular solution model are functions of both the gas and the type of RTILs used in the correlation.5 Unfortunately, a model that encompasses only one family of RTILs does not give flexibility toward estimation of gas solubility in other families of RTILs. Because of this limitation, we focused on developing a more universal model to characterize RTILs across different cation families. * To whom correspondence should be addressed. Tel.: (662)-9155354. Fax: (662)-915-7023. E-mail address:
[email protected].
The property of viscosity is used in many process calculations, ranging from the estimation of mass-transfer coefficients to the calculation of power requirements of pumps. The high viscosities of the RTILs result in high pumping costs. To design economically viable processes, low-viscosity RTILs will be needed for gas absorption media. The viscosities of RTILs with a fixed anion increase with the length of the alkyl chain.6 RTILs with bis(trifluoromethyl(sulfonyl))imide [Tf2N] anions have drastically lower viscosities, compared to other RTILs. For example, replacing chloride [Cl] with the [Tf2N] anion in trihexyl(tetradecyl)phosphonium RTILs reduces the viscosity by ∼80% (see Table 1). In addition, literature studies7,8 have shown that RTILs with the viscosity-reducing [Tf2N] anion have higher gas solubilities; therefore, we hypothesized that viscosity could be a predictor of solubility in RTILs. Theory The vapor-liquid equilibrium of a gas-RTIL system, represented in terms of fugacity of the gaseous solute, is given as
f G2 ) y2φ2P ) x2γ2 f 02
(1)
where y2 is the mole fraction of solute in the gas phase, φ2 the gas-phase fugacity coefficient of the solute, fG2 the fugacity of the solute in the gas mixture, x2 the mole fraction of the solute in the RTIL solution, γ2 the activity coefficient of solute in the solution, and f 02 the fugacity of the solute at a hypothetical liquid state, at solution temperature and pressure. Because RTILs have no measurable vapor pressures, the mole fraction of RTIL in the gaseous phase is approximately zero (y1 ) 0). Assuming
10.1021/ie070836b CCC: $40.75 © 2008 American Chemical Society Published on Web 12/22/2007
Ind. Eng. Chem. Res., Vol. 47, No. 3, 2008 911 Table 1. Physical Properties of RTILs of This Study at 30 °C (the Probable Halide Impurity is Indicated Next to the Reported Value) RTIL
molar volumea (cm3/mol)
viscosityb (cP)
halide contentc (chloride wt %)
252.7 191.7 211.4 294.7 294.0e 332.5 241.3
26d 45f 176e 52f 77e 554e 55d
