Article pubs.acs.org/jced
Solubility of CO2 in Binary Mixtures of Room-Temperature Ionic Liquids at High Pressures Zhigang Lei,* Jingli Han, Benfeng Zhang, Qunsheng Li, Jiqin Zhu, and Biaohua Chen State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 266, Beijing, 100029, China ABSTRACT: The solubility data of carbon dioxide (CO2) in a series of ionic liquids (ILs): 1-ethyl-3-methylimidazolium t etrafluoroborate ([EMIM] + [BF 4 ] − ), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]+[BF4]−), 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([OMIM]+[Tf2N]−), and their binary mixtures (i.e., [EMIM]+[BF4]− + [OMIM]+[Tf2N]− and [BMIM]+[BF4]− + [OMIM]+[Tf2N]−) at temperatures (313.2 and 333.2) K and pressures up to 6 MPa were measured by a highpressure view-cell technique. The mixed ILs tested were (0, 20, 50, 80, and 100) wt % [EMIM]+[BF4]− (or [BMIM]+[BF4]−) in [OMIM]+[Tf2N]−. The solubility data of CO2 in pure ILs were correlated with the Peng−Robinson equation of state (PR EOS), whereas the solubility data of CO2 in mixed ILs can be well predicted based on the mole fraction average of [EMIM]+[BF4]− (or [BMIM]+[BF4]−) and [OMIM]+[Tf2N]− over the solubility of CO2 in pure ILs. It was found that the solubility of CO2 in pure or mixed ILs increases with increasing pressure at all temperatures but decreases with increasing temperature. The Henry’s constants follow the order of [BMIM]+[BF4]− + [OMIM]+[Tf2N]− > [EMIM]+[BF4]− + [OMIM]+[Tf2N]− at the same content of [OMIM]+[Tf2N]−. The use of mixed ILs provides the opportunity to tune the solubility and selectivity for capturing CO2 at high pressures. It is the first work for us to present the solubility data of CO2 in binary mixtures of ILs at high pressures for physical absorption.
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INTRODUCTION The interest in ionic liquids (ILs) has grown exponentially in recent years. Among the reasons, some peculiarities such as their negligible vapor pressure and potential as “designer solvents” make ILs a very promising alternative for the replacement of volatile organic solvents in chemical and biochemical processes.1−3 In general, the solubility of CO2 in the same ionic liquid in mole fraction scale is much higher than other gases such as hydrogen, carbon monoxide, nitrogen, etc.4,5 Therefore, besides their use as potential new green solvents or reaction media, ILs also can be used for capturing CO2 from gas mixtures in separation science.6−13 An absorption system (consisting of one absorber and one stripper) is the most common technique for gas purification in which the flue gas is bubbled through the ionic liquid, the solvent absorbs the gas of interest (e.g., CO2) from flue gas, and the solvent is then regenerated in the stripper to use it again in the system. In this case, the very low vapor pressure of ILs makes them further attractive for gas separations as they show almost no solubility in the gaseous phase. Previous studies confirmed that the solubility of CO2 in ILs increases with the increase of the number of carbon atoms in the alkyl chain on the cations, but the selectivity for CO2 relative to other gases often decreases instead.14−17 That is to say, in most cases the factors that are favorable for increasing the solubility and selectivity are not compatible. However in the design and optimization of absorption process, we should consider both solubility and selectivity to determine the © 2012 American Chemical Society
amount of IL and the number of theoretical stages required for a given separation task. Therefore, the mixture of binary ILs can be used as the separating agents to tune the separation ability, which will result in desirable gas solubility and selectivity for capturing CO2. In this sense, the application of mixed ILs that can be easily available from chemical markets opens a new window for tailoring the chemistry and properties of ILs. Unfortunately, although by far there have been a large number of publications on the solubility of CO2 in pure ILs18−23 and the mixture of organic solvent and IL,24−26 the number of publications regarding the investigation of the solubility of CO2 in a binary mixture of ionic liquids is still very scarce. For physical absorption, Finotello et al.27 measured the solubility behaviors of CO2, CH4, and N2 gases in binary mixtures of imidazolium-based ionic liquids consisting of 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]+[Tf2N]−) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]+[BF4]−) only at 40 °C and 1 atm (not at high pressure), and it was found that all mixture data points agree well with the theoretical predictions developed from pure IL solubility data. On the other hand, for chemical absorption, Shiflett and Yokozeki28 measured the solubility of CO2 in a mixture containing almost equimolar amounts of 1ethyl-3-methylimidazolium acetate ([EMIM]+[Ac]−) and 1Received: December 22, 2011 Accepted: June 21, 2012 Published: July 6, 2012 2153
dx.doi.org/10.1021/je300016q | J. Chem. Eng. Data 2012, 57, 2153−2159
Journal of Chemical & Engineering Data
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Figure 1. Relative deviations Δx1/x1 = (xexp − xcal)/xexp between the experimental and calculated solubility in mole fraction of CO2 (1) in [EMIM]+[BF4]− at 313.2 K (a), in [BMIM]+[BF4]− at 313.2 (b) and 333.2 K (c), and in [OMIM]+[Tf2N]− at 313.2 (d) and 333.2 K (e): ●, this work with error bars being the extended uncertainty; △, ref 30; ☆, ref 31; ▽, ref 29.
ethyl-3-methylimidazolium trifluoroacetate ([EMIM]+[TFA]−) at temperatures (298.1, 323.1 and 348.1) K and pressures up to 2 MPa, and it was found that CO2 solubility can be well predicted at a fixed pressure based on the mole fraction average of pure component solubilities. As we know, chemical absorption is suitable for capturing CO2 from flue gas streams with a relatively small partial pressure of CO2, while physical absorption is the most commonly used for capturing CO2 from synthesis gas (or syngas) with high partial pressure because physical solvents do not interact strongly with the gas to be captured, and thus it is expected to minimize the energy required to recovery the separating agent. However, to the best of our knowledge, no solubility data of CO2 in binary mixtures of ILs at high pressures for physical absorption have been reported by far.
This work is an experimental research on the solubility of CO2 in pure ILs, i.e., [EMIM]+[BF4]−, [BMIM]+[BF4]−, and [OMIM] + [Tf 2 N] − , and in their binary mixtures, i.e., [EMIM]+[BF4]− + [OMIM]+[Tf2N]− and [BMIM]+[BF4]− + [OMIM]+[Tf2N]− at temperatures (313.2 and 333.2) K and pressures up to 6 MPa. The solubility data are important in establishing the mathematical models of equilibrium stage (EQ) or nonequilibrium stage (NEQ) for absorption process. In this work, the IL [OMIM]+[Tf2N]− was selected because CO2 solubility is higher in ILs with such an anion as [Tf2N]− which contains a fluoroalkyl group, and an increase in the alkyl chain length on the cation also increases the CO2 solubility marginally.29 The ILs [EMIM]+[BF4]− and [BMIM]+[BF4]− were selected because they exhibit higher selectivity but lower solubility as reported by Finotello et al.27 Therefore, the mixed 2154
dx.doi.org/10.1021/je300016q | J. Chem. Eng. Data 2012, 57, 2153−2159
Journal of Chemical & Engineering Data
Article
this case the total weight was the addition of w1 (weight of CO2), w2 (weight of pure or mixed ILs), and w3 (weight of blank sample bomb) measured by an electronic balance (CPA 1003S, Sartorius) with an uncertainty of 0.001 g. Then, CO2 was released slowly at room temperature to separate it from IL until the weight of pure or mixed ILs plus blank sample bomb, i.e., (w2 + w3), was invariable for about 1 h. Since the weight of sample bomb w3 was predetermined, the amounts of CO2 and pure or mixed ILs contained in the sample bomb can be derived. The estimated uncertainty of solubility measurement in mole fractions was less than 0.006. Each experiment was done at least three times in order to ensure the reproducibility of experimental data.
ILs may combine the advantage of possessing both high solubility and high selectivity for capturing CO2. In addition, it is noted that these ILs are very easier to be bought from chemical markets, and we do not need to tune the cation and anion on a single IL through complicated chemical synthesis so as to get the desirable separation performance. This article is arranged as follows: we first give the relative or absolute deviations between calculated (or reference) and measured values for pure ILs, then list the experimental data as well as those for mixed ILs, and establish the suitable thermodynamic model to correlate the experimental data. Finally, concluding remarks are provided in the last section.
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EXPERIMENTAL SECTION Chemicals. The imidazolium-based ionic liquids [EMIM]+[BF4]− (purity >99 wt %, water content
