12908
J. Phys. Chem. B 2010, 114, 12908–12913
High Carbon Dioxide Solubilities in Imidazolium-Based Ionic Liquids and in Poly(ethylene glycol) Dimethyl Ether Anne-Laure Revelli, Fabrice Mutelet,* and Jean-Noe¨l Jaubert Laboratoire Re´actions et Ge´nie des Proce´de´s, CNRS (UPR3349), Nancy-UniVersite´, 1 rue GrandVille, BP 20451 54001 NANCY, FRANCE ReceiVed: June 23, 2010; ReVised Manuscript ReceiVed: August 30, 2010
This work is focused on the possible capture of carbon dioxide using ionic liquids (ILs). Such solvents are gaining special attention because the efficiency of many processes can be enhanced by the judicious manipulation of their properties. The absorption of greenhouse gases can be enhanced by the basic character of the IL. In this work, these characteristics are evaluated through the study of the gas-liquid equilibrium of four imidazolium-based ILs: 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4], 1-butyl-3-methylimidazolium thiocyanate [BMIM][SCN], 1,3-dimethylimidazolium methylphosphonate [DMIM][MP], and 1,3-diethoxyimidazolium bis(trifluoromethylsulfonyl)imide [(ETO)2IM][Tf2N] with CO2 at temperatures up to 373 K and pressures up to 300 bar. Solubility of carbon dioxide in poly(ethylene glycol) dimethyl ether, component of selexol, was also measured to evaluate the capture’s efficiency of ionic liquids. Experimental data indicate that 67 to 123 g of CO2 can be absorbed per kg of ionic liquid and 198 g per kg of poly(ethylene glycol) dimethyl ether. Introduction The increase in demand for energy worldwide has aided the search for alternative sources of primary energy even in remote parts of the globe. The major alternative source with less environmental impact discovered some decades ago is energy from natural gas. Natural gas at its geological conditions in some deposits contains some contaminants such as CO2, H2S, and CO, which constitute great environmental hazards upon reaching the atmosphere and also hinder natural gas processes. Recent concerns over global warming due to greenhouse gas emissions from fossil fuel combustion have led to the development of technologies to reduce and capture these gases. The main greenhouse gases in the earth’s atmosphere are water vapor, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Currently, a particular attention is devoted to the CO2 emission reduction. There are a number of different separation technologies that can be applied to carbon dioxide capture including solvent, membrane, and adsorbent based processes. One approach being considered for capturing CO2 is the use of liquid absorbents designed to selectively solvate CO2.1 The alkanolamines are the most generally accepted and widely used of the various available solvents for removal of CO2 from natural gas streams. The reactivity and availability at low cost of this family of compounds, especially monoethanolamine and diethanolamine, have caused the solvent to achieve a pinnacle position in the gas processing industry. Although these aqueous alkanolamine solutions are industrially effective for CO2 removal, this method presents several drawbacks such as the intensive energy consumption, cost increases, and corrosion problems.2,3 Monoethanolamine is particurlarly reactive and can absorb CO2 and H2S simultaneously. However, monoethanolamine reacts with organic sulfur compounds such as carbonyl sulphide, carbon disulfide, and mercaptans. The relatively high vapor pressure of monoethanolamine causes important losses * To whom correspondence should be addressed: E-mail: mutelet@ ensic.inpl-nancy.fr, Tel: +33 3 83 17 51 31, Fax: +33 3 83 17 51 52.
compared with other amines. In the presence of acid gases, significant corrosion may occur at points where the concentration and temperature of acid gas is high. The primary amines are the most corrosive. Therefore, the application of a primary solution requires the use of corrosion inhibitors and the unit may have to be made of special steels.4 Physical solvents such as selexol, N-methyl-2-pyrrolidone (purisol), methanol (rectisol), and propylene carbonate (fluor solvent) are becoming popular as gas-treating solvents, especially for coal gasification applications. The selexol process uses Union Carbide selexol solvent, a physical solvent made of poly(ethylene glycol) dimethyl ether [CH3(CH2CH2O)nCH3] where n is between 3 and 9. Regeneration of the solvent is by air stripping, and it requires no reboiler’s heat. Moreover, selexol process allows for construction of mostly carbon steel due to its nonaqueous and inert chemical characteristics. The main drawback of selexol is its high affinity to heavy hydrocarbon, which will be removed with CO2 and essentially result in hydrocarbon losses. Moreover, the process is more efficient at high operating pressure.5 The hybrid separation processes combining the properties of physical and chemical solvent for effective and selective removal of acid gas from natural gas has been developed.6-10 One of the successful hybrid separation processes used in the oil and gas industry is a mixture containing sulfolane (tetrahydrothiophene 1-1 dioxide: a physical solvent), water and either diisopropanolamine or methyldiethanolamine (two chemical solvents) (sulfinol process). The dual functionality and capacity of physical and chemical solvent mixture of sulfinol make the solvent more efficient. Moreover, the process requires low energy and has a high acid gas loading. Nevertheless, a higher coabsorption of heavy hydrocarbon is observed than that in chemical process absorption. Another class of solvents namely ionic liquids (ILs) seem to be good candidates for capturing greenhouse gases because they have some advantages in comparison to other solvents. Indeed, these liquids have good thermal stability and negligible vapor
10.1021/jp1057989 2010 American Chemical Society Published on Web 09/20/2010
High Carbon Dioxide Solubilities pressure. Physical properties of ILs may be modified and adjusted by employing different cation-anion combinations.11-13 For all these reasons, a large number of studies on CO2 solubilities in ILs have been performed.14-18 Recently, a few research groups have published measurements of solubilities of other gases in ILs such as CH4, H2S, SO2, and N2O.19-23 Results obtained on the solubility of carbon dioxide in ionic liquids indicate that this class of solvents may be used in friendly environmental processes. Indeed, it is observed that CO2 has a good solubility at low temperature and that the gas could be easily removed from the solvent. The cleaned IL could be reinjected in the process. Therefore, it seems particularly interesting to study the phase behavior of CO2 in ionic liquids. In this work, we propose to investigate the possible use of ionic liquids as CO2 absorption solvent. The CO2 solubility in four imidazolium based ionic liquids were measured using a high-pressure equilibrium apparatus equipped with a variable volume view cell. The solubility determination was based on the measurement of bubble pressures for a mixture of CO2 and ionic liquid with a known composition at a fixed temperature. The CO2 solubilities in the ionic liquid were measured as a function of temperature and pressure. Solubility of carbon dioxide in poly(ethylene glycol) dimethyl ether, a component of selexol, was also measured to evaluate the capture’s efficiency of ionic liquids. The experimental data for the binary systems of {CO2 + ionic liquids} were correlated using the PPR78 model based on the Peng-Robinson equation of state. Experimental Procedures Materials or Chemicals. Carbon dioxide was purchased from Messer with a purity of 0.999999 in mass fraction. Four imidazolium-based ILs were used in this study: 1-butyl3-methylimidazolium tetrafluoroborate [BMIM][BF4], 1-butyl-3methylimidazolium thiocyanate [BMIM][SCN], 1,3-dimethylimidazolium methylphosphonate [DMIM][MP], and 1,3-diethoxyimidazolium bis(trifluoromethylsulfonyl)imide [(ETO)2IM][Tf2N]. The solubility of CO2 was also measured in a poly(ethylene glycol) dimethyl ether (PEG where n ) 5). [BMIM][BF4] and [DMIM][MP] were supplied by Solvionic with a minimum of purity in mass fraction of 0.995 and 0.98, respectively. [BMIM][SCN] (purity > 0.95 mass fraction), [(ETO)2IM][Tf2N] (purity > 0.98 j n ) 250 g · mol-1) were purchased from mass fraction), and PEG (M Sigma-Aldrich. Before each measurement, the ionic liquids were purified by subjecting the liquid under vacuum for approximately 12 h to remove possible traces of solvents and moisture. Analysis for the water content of the ionic liquids using the Karl Fischer technique showed that water contents was from 300 to 700 ppm. Apparatus and Experimental Procedure. Bubble point pressures of the systems {CO2 + IL} were obtained using a high-pressure variable-volume visual cell (Top Industrie, S.A.) as shown in Figure 1. The technique used to carry out phaseequilibrium measurement was based on a synthetic method, which avoids sampling and analyses of the phases. The highpressure cell was equipped with a moving piston and a sapphire window allowing a visual observation of the equilibrium cell. The window allows following the phase transition of the binary mixture with pressure and temperature using a video camera and a monitor. The mixture was permanently homogenized thanks to a small magnetic bar and an external magnetic stirrer. The temperature inside the cell is kept constant using a thermostatted bath and is measured by a platinum resistance thermometer PT-100 with an accuracy of (0.1 K. The pressure is measured by a piezoresistive calibrated pressure sensor (KULITE HEM 375, working in the full scale range of 1 to
J. Phys. Chem. B, Vol. 114, No. 40, 2010 12909
Figure 1. Schema of the VLE apparatus. 1, thermostated bath; 2, analytical balance; 3, vacuum pump; 4, piston; 5, temperature probe (Pt100); 6, magnetic stirrer; 7, light source; 8: calibrated pressure sensor (0 < P < 340 bar); 9, sapphire window; 10, video camera; 11, monitor.
340 bar) directly placed inside the cell to minimize dead volumes with an accuracy of 0.1 bar. First, the equilibrium cell is loaded with a fixed amount of IL and its exact mass is determined using an analytical balance (SARTORIUS) with a resolution of (0.001 g. Then, CO2 is introduced under pressure from an aluminum reservoir tank. Its mass was measured with the precision balance by weighing the reservoir tank before and after the gas introduction. After these filling operations, precise mole fractions of the compounds contained in the cell (i.e., CO2 + solute) and molar fractions of each compound could be calculated. When the desired temperature cell is reached, the pressure was slowly increased until the system becomes a one-phase system. The pressure at which the last bubble disappears represents the bubble-point pressure for the fixed temperature. Reproducibility of the pressure measurements is 0.5 bar. Analyses of the ionic liquids before and after gas solubility measurements have been performed by NMR to confirm that no degradation of the IL takes place during the measurements. The 1H and 13C spectra were collected using a Bruker Avance 300 MHz apparatus and using deuterated chloroform as solvent. For the four ILs investigated in this work, no change of chemical shift was observed. Modeling. PPR78 Model. A simplified version of the PPR78 model was used to correlate our data. For clarity, let us recall that the PPR78 model relies on the PengsRobinson equation of state (PR-EOS) as published by Peng and Robinson in 1978.24 For a pure component, the PR78 EOS is:
P)
with:
{
ai(T) RT V-bi V(V + bi) + bi(V-bi)
(1)
R ) 8.314472 J · mol-1 · K-1 RTc,i bi ) Ωb Pc,i Ωb ) 0.0777960739
and:
[ ( )]
2 R2Tc,i 1 + mi 1 ai ) Ωa Pc,i Ωa ) 0.457235529
T Tc,i
2
(2)
12910
{
J. Phys. Chem. B, Vol. 114, No. 40, 2010
Revelli et al.
if ωi e 0.491mi ) 0.37464 + 1.54226ωi - 0.26992ωi2 if ωi > 0.491mi ) 0.379642 + 1.48503ωi -
TABLE 1: Critical Properties and Acentric Factor of Carbon Dioxide and Ionic Liquids Used in the Modeling
0.164423ωi2 + 0.016666ωi3
(3)
where P is the pressure, R the gas constant, T the temperature, a and b are EOS parameters, V the molar volume, Tc the critical temperature, Pc the critical pressure, and ω the acentric factor. To apply the PPR78 EOS to mixtures, mixing rules are used to calculate the values of a and b of the mixtures. Classical mixing rules are used in this study:
{
ω
CO2 [BMIM][BF4] [BMIM][SCN] [DMIM][MP] [(ETO)2IM][Tf2N]
304.12 632.30 1047.40 767.50 1310.80
73.74 20.40 19.40 32.60 28.20
0.2250 0.8489 0.4781 0.4714 0.2817
considered as a single group. Doing so, the temperaturedependent kij is expressed by:
∑ ∑
i)1 ncompd
b)
j)1
k12(T) )
zizj√aiaj(1 - kij(T))
(
298.15 T
)
(
B12
-1)
A12
(4)
2
∑ xibi
-
(
√a1(T) - √a2(T) b1
b2
√a1(T)a2(T)
)
2
b1b2
(6)
i)1
For a given binary system, it is thus enough to fit the two parameters A12 and B12 on the available experimental data. For the five binary systems investigated in this article, these both parameters were determined to minimize the following objective function: nbubble
Fobj ) 100
(
)
|∆x| + ∑ 0.5 x|∆x| x 1,exp 2,exp i 1
(7)
with
kij(T) ) 1 2
Pc/bar
A12
where zk represents the mole fraction of component k in a mixture, and ncompd the number of components in the mixture. In eq 4, the summations are over all chemical species. kij(T), whose choice is difficult even for the simplest systems, is the so-called binary interaction parameter characterizing molecular interactions between molecules i and j. When i equals j, kij is zero. In the PPR78 model (predictive, 1978 PR EOS), kij, which depends on temperature, is calculated by a group contribution method25-27 through the following expression:
-
Tc/K
ncompd ncompd
a)
ng
compound
ng
∑ ∑ (R - R )(R - R )A ( 298.15 T )
(
ik
jk
il
jl
kl
k)1 l)1
2
Bkl Akl
-1)
-
(
)
|∆x| ) |x1,exp - x1,cal | ) |x2,exp - x2,cal |
(5)
nbubble is the number of experimental bubble points for a given binary system. x1 is the mole fraction in the liquid phase of the most volatile component (CO2) and x2 the mole fraction of the heaviest component (it is obvious that x2 ) 1 - x1).
√ai(T) - √aj(T) bi
bj
2
√ai(T)aj(T) bibj
In eq 5, T is the temperature. ai and bi are simply calculated by eq 4. ng is the number of different groups defined by the method. Rik is the fraction of molecule i occupied by group k (occurrence of group k in molecule i divided by the total number of groups present in molecule i). Akl ) Alk and Bkl ) Blk (where k and l are two different groups) are constant parameters (Akk ) Bkk ) 0). As can be seen, to calculate the kij parameter at a selected temperature between two molecules i and j, it is necessary to know: the critical temperature of both components (Tci, Tcj), the critical pressure of both components (Pci, Pcj), the acentric factor of each component (ωi, ωj) and the decomposition of each molecule into elementary groups (Rik, Rjk). The critical properties and acentric factor of carbon dioxide were taken from the literature.28 For ionic liquids, experimentally measurable critical points are not available since ILs decompose before reaching their critical point. Therefore, the group contribution method proposed by Valderrama et al. was used to estimate these properties.29,30 The predicted ionic liquid critical properties as well as those of CO2 are listed in Table 1. Because the ionic liquid groups are not defined, it is not possible to use the PPR78 model to predict the VLE data measured in this study. It is however possible to use a simplified version of the PPR78 model in which each molecule is
Results and Discussion The (vapor + liquid) equilibrium data of carbon dioxide in the four ILs and in poly(ethylene glycol) dimethyl ether were measured for mole fractions ranging from 0.13 to 0.81 in the temperature range 293-383 K and pressures from 10 to 330 bar. For systems with a fixed overall composition of carbon dioxide and ionic liquid, bubble point pressures were measured as a function of temperature. The results are listed in Tables 2-6. Dialkylimidazolium-Based Ionic Liquids. The (vapor + liquid) equilibrium data for the binary mixtures {1-butyl-3methyl-imidazolium tetrafluoroborate [BMIM][BF4] + CO2}, {1-butyl-3-methyl-imidazolium thiocyanate [BMIM][SCN] + CO2},and{1,3-dimethylimidazoliummethylphosphonate[DMIM][MP] + CO2} but also { poly(ethylene glycol) dimethylether} were measured from 293 to 383 K and pressures up to 330 bar. The isotherms above the critical point of pure CO2 present (vapor + liquid) equilibrium (VLE) throughout the pressure range measured. For temperature below the critical point of CO2, a region of VLE exists at low pressures followed by a pressure at which (vapor + liquid + liquid) is observed. Most likely, the type of fluid-phase behavior is type III according to the classification of Scott and Van Konynenburg.
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J. Phys. Chem. B, Vol. 114, No. 40, 2010 12911
TABLE 2: Bubble Point Data for Various Compositions of Carbon Dioxide in the {CO2 + [BMIM][BF4]} System
TABLE 4: Bubble Point Data for Various Compositions of Carbon Dioxide in the {CO2 + [DMIM][MP]} System
xCO2
T/K
P/bar
T/K
P/bar
T/K
P/bar
xCO2
T/K
P/bar
T/K
P/bar
T/K
P/bar
0.141
293.25 303.35 313.25 323.15 293.75 303.45 313.25 323.05 293.45 303.75 313.45 322.65 293.65 303.15 314.05 322.95 293.15 303.85 313.85 324.15 293.25 303.15 313.35 323.55
10.5 12.0 13.9 15.8 19.0 23.3 28.2 34.0 25.2 32.0 38.9 47.2 41.0 49.3 63.5 75.3 47.2 59.8 74.3 89.7 73.0 81.0 105.0 141.0
333.15 343.15 353.25 363.15 333.05 343.15 353.15 362.95 332.85 342.75 352.25 363.15 333.95 343.75 354.25 362.85 333.35 343.45 352.05 357.15 333.15 343.15 353.15 363.15
17.8 20.3 24.1 27.2 40.0 46.9 54.2 62.1 56.1 64.3 79.1 90.0 91.0 104.9 125.0 141.5 105.9 125.7 145.0 154.0 162.0 188.5 223.3 246.0
373.15 383.15
30.3 38.0
0.162
343.65 353.15 363.15 343.15 352.75 363.15 342.65 353.55 363.15 343.15 353.15
49.0 55.5 61.7 92.5 110.1 125.9 132.0 165.0 188.0 213.0 250.0
66.5
373.15
146.0
71.5 80.5
34.0 38.5 44.0 52.0 65.0 78.5 70.0 90.0 115.0 95.0 125.0 178.0
373.15
373.45 383.15
313.45 323.35 333.65 313.15 323.35 333.15 313.45 323.05 333.25 313.35 323.75 332.65
372.95
212.0
0.266
0.350
0.458
0.500
0.610
0.282 0.369 373.15 383.15
98.0 117.0
373.15 382.95
160.0 185.0
363.15 373.15 383.15
171.0 198.0 235.0
T/K
P/bar
T/K
P/bar
T/K
P/bar
0.126
292.35 303.25 312.95 323.15 293.65 303.15 312.75 323.15 293.05 302.55 312.45 324.65 293.35 303.35 313.15 323.65 302.95 312.75 322.75 313.65 323.15 333.15
10.5 13.1 16.0 19.2 20.0 24.5 28.6 32.0 28.5 32.5 37.0 43.8 43.5 48.0 52.5 58.0 55.0 59.0 65.0 99.0 128.0 167.0
333.75 343.55 353.55 363.35 333.15 342.55 352.85 363.15 333.65 343.75 352.95 363.15 333.55 343.45 353.15 362.45 332.95 344.15 353.15 344.15 353.15 361.45
23.1 26.8 30.5 35.5 36.1 39.9 44.5 49.9 49.5 56.1 62.0 68.5 64.0 71.0 78.0 88.0 73.0 80.0 88.0 205.0 242.0 275.0
372.95 381.75
40.8 44.6
373.35 382.65
56.0 61.7
373.15 381.95
74.0 79.8
0.296
0.337 0.430
xCO2
T/K
P/bar
T/K
P/bar
T/K
P/bar
0.396
303.65 313.35 324.15 302.85 313.35 324.15 302.95 313.35 324.15 302.95 313.25 323.95 302.95 312.95 323.85 303.15 313.05 322.15
21.5 25.7 31.2 36.0 44.7 54.6 46.7 57.7 71.1 60.7 79.0 103.4 107.8 166.6 223.3 213.4 282.6 328.6
333.85 343.65 353.25 334.05 343.55 353.25 333.85 343.85 353.25 333.55 343.25 352.85 333.85 343.55
36.0 41.1 46.6 64.8 75.1 85.9 84.5 99.8 115.1 131.7 163.2 193.3 271.6 312.4
363.15
52.5
363.05
97.7
362.95
132.4
362.85
224.2
0.626
xCO2
0.254
TABLE 5: Bubble Point Data for Various Compositions of Carbon Dioxide in the {CO2 + [(ETO)2IM][Tf2N]} System
0.554
TABLE 3: Bubble Point Data for Various Compositions of Carbon Dioxide in the {CO2 + [BMIM][SCN]} System
0.199
0.475
0.712 0.778 0.812
372.55 383.15
96.0 104.0
361.45 373.15 384.15 373.15
96.7 106.7 121.1 315.0
TABLE 6: Bubble Point Data for Various Compositions of Carbon Dioxide in the {CO2 + Poly(ethylene glycol) dimethylether} System xCO2
T/K
P/bar
T/K
P/bar
T/K
P/bar
0.210
293.35 303.25 311.95 293.55 303.25 312.95 294.15 304.35 313.25 293.95 303.05 312.55 292.85 303.15 312.25 292.95 303.25 312.95
11.5 13.4 15.0 14.2 16.8 19.8 18.5 23.1 27.1 21.5 27.6 32.7 28.4 35.5 42.0 33.0 41.0 49.8
323.55 333.25 343.15 322.95 333.35 342.95 322.95 333.15 342.45 322.95 333.25 343.45 323.05 333.05 343.25 322.95 333.25 344.25
17.6 21.0 23.5 24.4 26.5 29.1 31.0 35.2 39.0 38.1 43.8 50.1 48.8 56.8 64.8 58.9 69.0 80.6
353.45 363.05 373.35 352.95 363.15 373.35 362.45 373.35
25.6 28.1 31.7 31.5 33.6 37.0 47.5 52.0
353.15 362.05 373.15 353.35 363.65 373.45 353.45 363.55 373.55
56.2 60.1 64.4 73.0 81.5 88.5 90.6 101.0 112.0
0.286
The solubility of CO2 in [BMIM][BF4] and [BMIM][SCN] is shown in Figure 2 at 313 K, and the results indicate that there is a further increase in CO2 solubility by replacing [SCN] anion by [BF4]. This behavior was already observed with the solubility of nitrous oxide in ionic liquids.23 Brennecke et al. have reported that an increase of number of fluorine atoms on the anion increases the CO2 solubility.27 It was demonstrated that the relatively high solubility of carbon dioxide and nitrous oxide is likely due to their large quadrupole moments, as well as specific interactions between the gas and the anion. The solubility of CO2 in [DMIM][MP] (Figure 2) is very close to that observed with [BMIM][SCN]. The experimental data determined for the three binary mixtures {BMIM][BF4] + CO2}, {[BMIM][SCN] + CO2}, and {[DMIM][MP] + CO2} are presented in Figures 3-5. In these figures, the experimental
0.384 0.445 0.550 0.630
data are correlated with the simplified version of the PPR78 model. The fitted A12 and B12 parameters for each binary system and the corresponding value of the objective function are given in Table 7. The interaction parameter kij of the PPR78 model is temperature dependent and increases with an increase in temperature. The objective functions for the three binary systems are calculated to minimize deviations between experimental and calculated molar fractions (eq 7). The obtained Fobj values were
12912
J. Phys. Chem. B, Vol. 114, No. 40, 2010
Figure 2. CO2 solubility at 303.15 K in the five ILs. ×, [DMIM][MP]; [, [BMIM][BF4]; 9, [BMIM][SCN]; ∆, [(ETO)2IM][Tf2N]; 2, PEG.
Revelli et al.
Figure 5. Experimental and predicted phase behavior of the system {CO2+[DMIM][MP]}. T1 ) 313.35 K, T2 ) 333.25 K, T3 ) 353.15 K, and T4 ) 373.15 K.
TABLE 7: Values of the Fitted A12 and B12 Parameters and Values of the Corresponding Objective Function for the Five Binary Systems Investigated in This Study system CO2+ CO2+ CO2+ CO2+
Figure 3. Experimental and Predicted phase behavior of the system {CO2+[BMIM][BF4]}. T1 ) 303.15 K, T2 ) 323.15 K, T3 ) 343.15 K, T4 ) 363.15 K, and T5 ) 383.15 K.
[BMIM][BF4] [DMIM][MP] [BMIM][SCN] [(ETO)2IM][Tf2N]
A12 (MPa)
B12 (MPa)
Fobj (%)
132.4 121.9 163.6 45.6
-13.0 -75.9 221.8 256.7
4.02 5.71 3.75 13.70
Konynenburg. Nevertheless, experimental data set measured in this work is not sufficient to determine whether this system is Type III or Type V. Ether Functionalized Ionic Liquids. The (vapor + liquid) equilibrium (bubble point) data for the binary mixtures {1,3-diethoxyimidazolium bis(trifluoromethylsulfonyl)imide [(ETO)2IM][Tf2N] + CO2} was measured from 293 to 373 K and pressures up to 300 bar (see Figure 6). At 303 K, a liquid-liquid-vapor equilibrium is observed at high carbon dioxide molar fraction. The isotherms above the critical point of pure CO2 present (vapor + liquid) equilibrium throughout the pressure range measured. Most likely, the type of fluid-phase behavior is type III according to the classification of Scott and Van Konynenburg (this conclusion is supported by the PPR78 model). Using functionalized ionic liquids significantly improves the CO2 solubility in ionic liquids. For this binary mixture, the PPR78 model predicts a mixture critical point at approximately
Figure 4. Experimental and predicted phase behavior of the system {CO2+[BMIM][SCN]}. T1 ) 302.95 K, T2 ) 323.15 K, T3 ) 343.45 K, and T4 ) 363.15 K.
between 2 and 15% as shown in Table 7. The largest deviations between experimental and calculated data are observed near the miscibility gap. This may be explained by the estimation of the ionic liquids critical properties. However, the satisfactory results indicate that the PPR78 model may be used to determine the phase diagram of ionic liquids with greenhouse gases such as nitrous oxide or carbon dioxide. The PPR78 model predicts a type III system according to the classification of Scott and Van
Figure 6. Experimental and predicted phase behavior of the system {CO2 + [(ETO)2IM][Tf2N]}. T1 ) 303.15 K, T2 ) 323.15 K, T3 ) 343.15 K, and T4 ) 363.15 K.
High Carbon Dioxide Solubilities
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TABLE 8: Comparison of the CO2 Solubility Expressed in Molar Fraction or CO2 with Values Obtained in Terms of Molality ionic liquids
xCO2
poly(ethylene glycol) dimethyl ether 0.53 [BMIM][BF4] 0.37 [BMIM][SCN] 0.25 [DMIM][MP] 0.25 [BMIM][Tf2N] 0.50 [BMIM][PF6] 0.40 [(ETO)2IM][Tf2N] 0.55
-1 molality (molCO2 · kgIL )
4.51 2.60 1.69 1.53 2.39 2.27 2.79
350 bar at 303.15K and 0.92 molar fraction of carbon dioxide. This predicted mixture critical point seems to be an artifact of the equation of state model and the estimated critical properties of the ionic liquid. It is probably due to the lack of data representing the strong increase of pressure. The model provides a good correlation for the solubility data of CO2 in IL in the pressure range of the experiments (
