Solubility of mixed gases containing carbon dioxide in ionic liquids: Measurements and predictions
Introduction
Room temperature ionic liquids receive growing attention as alternative green solvents for reactions and separations. Due to negligible vapor pressure, they are considered environmentally favorable. Two-phase systems of ionic liquids and mixed gases including carbon dioxide are of potential importance for extracting components from or to the ionic liquid phase. An increasing number of studies is being reported on equilibria of the ionic liquid and pure gas systems. Kim et al. proposed a group-contribution equation of state method for the calculation of solubilities of carbon dioxide in ionic liquids using available data including their own [1]. New solubility data of carbon dioxide in imidazolium-based ionic liquids not used in the analysis of Kim et al. are now available for anions [PF6] [2], [3], [4], [5], [6], [BF4] [6], [7], [8], [9], and [Tf2N] [9], [10], [11]. Usually Henry's law was applied to low pressures for data reduction. An equation of state approach is useful for modeling phase equilibria containing supercritical gases over a wide pressure range. For example, the cubic equations of state were used to model the high-pressure phase equilibria of gases and imidazolium-based ionic liquids [6], [12]. All these studies are for pure gas and ionic liquid systems and no experimental or modeling studies for mixed gas systems are published as far as the present authors know. Considering the variety of ionic liquids that may be formed, predictive methods for equilibrium properties involving ionic liquids are needed.
In the present study, solubilities of pure carbon dioxide and propane and those of the mixed gases carbon dioxide + nitrogen and carbon dioxide + propane in 1-hexyl-3-methylimidazolium tri(bisfluoromethylsulfonyl)imide commonly known as [C6mim][Tf2N] are experimentally studied at 298.15 K and up to 1 MPa. A predictive method based on a group-contribution lattice fluid equation of state [1], [13] is applied for pure and mixed gases with ionic liquids.
Section snippets
Experiment
The experimental apparatus of Kim et al. [1] for the solubility of pure gases was modified to measure pure and mixed gas solubilities as shown in Fig. 1. The equilibration system consisted of a gas manifold, an equilibrium cell and a gas reservoir whose volume were pre-calibrated and was immersed in a temperature controlled water bath. A pre-weighed amount of the ionic liquid was introduced into the equilibrium cell and the system was evacuated with valves V1, V2, and V3 open. Then the valves
Model and parameters
A group-contribution non-random lattice fluid (NLF) equation of state was previously applied to systems of carbon dioxide and ionic liquids containing [PF6], [BF4], and [Tf2N] anions [1]. Working equations for chemical potential and equation of state are given in the reference. In this model, the segment number, ri, and the segment interaction energy, ɛij, that characterize the physical interactions between molecules are obtained from corresponding temperature dependent group parameters using
Results and discussions
The present solubility data for carbon dioxide in Table 1 were found to smoothly extrapolate to the data of Aki et al. [9] in the high-pressure region as shown in Fig. 3 for the same system and at the same temperature. Equilibrium measurements involving ionic liquids are known to be very sensitive to impurities and moisture content of the sample used. The lattice fluid equation of state with a constant lattice cell volume provides a convenient basis for the group-contribution application to
Conclusion
Solubilities of pure carbon dioxide and propane and those of the mixed gases carbon dioxide + nitrogen and carbon dioxide + propane in 1-hexyl-3-methylimidazolium tri(bisfluoromethylsulfonyl)imide commonly known as [C6mim][Tf2N] were measured at 298.15 K and up to 1 MPa. A group-contribution non-random lattice fluid equation of state was applied to predict the solubilites of pure and mixed gases with ionic liquids. The predicted results were found in reasonably good agreement with experimental data.
Acknowledgement
This work was supported by grant No. R01-2003-000-10300-0 from the Basic Research Program of the Korea Science & Engineering Foundation.
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