J. Phys. Chem. B 2010, 114, 8199–8206
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Reducing of Nitrous Oxide Emissions Using Ionic Liquids 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: April 26, 2010; ReVised Manuscript ReceiVed: May 5, 2010
This work is focused on the possible capture of nitrous oxide and more precisely protoxide using ionic liquids (ILs). ILs are gaining special attention since 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 five imidazolium-based ILs: 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), 1-butyl-3methylimidazolium thiocyanate ([BMIM][SCN]), 1,3-dimethylimidazolium methylphosphonate ([DMIM][MP]), 1,3-diethoxyimidazolium bis(trifluoromethylsulfonyl)imide ([(ETO)2IM][Tf2N]), and 1,3-dihydroxyimidazolium bis(trifluoromethylsulfonyl)imide ([(OH)2IM][Tf2N]) with N2O at temperatures up to 373 K and pressures up to 300 bar. Experimental data indicate that 44-105 g of N2O can be absorbed per kilograms of IL. Introduction 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, particular attention is devoted to CO2 emission reduction. One approach being considered for capturing CO2 is the use of liquid absorbents designed to selectively solvate CO2.1 Sequestration processes involving dilute aqueous solution of alkanolamines such as monoethanolamine (MEA) are currently used in the removal of acid gases from natural gas. Although these aqueous alkanolamine solutions are industrially effective on CO2 removal, this method presents several drawbacks such as the intensive energy consumption, cost increases, and corrosion problems.2,3 In this regard, it is necessary to find new alternatives for absorption solvents. To this end, 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 pressure. Physical properties of ILs can be modified and adjusted by employing different cation-anion combinations. For all these reasons, a large number of studies on CO2 solubilities in ILs have been performed.4-10 Recently, a few research groups have published measurements of solubilities of other gases in ILs such as CH4,11 H2S,12 and SO2.13 However, up to now, only one study reported N2O solubility in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide at relatively low pressure (below 13 bar).7 Nevertheless, N2O is a potent greenhouse gas with a global warming potential approximately 310 times greater than CO2 when normalized over 100 years.14 This means that 1 kg of N2O released into the atmosphere has a global warming effect equivalent to 310 kg of carbon dioxide over a 100 year period. Moreover, N2O is the major source of NOx in the stratosphere and therefore an important natural regulator of stratospheric ozone.15 While no * Author to whom the correspondence should be addressed. E-mail:
[email protected]. Telephone number: +33 3 83 17 51 31. Fax number: +33 3 83 17 53 95.
governmental legislation has been officially issued, atmospheric N2O concentration continues to rise linearly, approximately 0.26% per year, and reached a concentration of 319 ppb in 2005. N2O concentrations are projected to be 360-460 ppb in 2100, values that are 11-45% higher than current concentrations.15 N2O is produced by both natural and anthropogenic sources. The main N2O emission source is from agriculture activities and represents approximately 62% of total emissions. The industrial sources of N2O include nylon production, nitric acid production, fossil fuel fired power plants, and vehicular emissions. Nitric acid is an inorganic compound used primarily to make synthetic commercial fertilizers. It is also a major component in the production of adipic acid, a feedstock for nylon, and explosives.16 During these synthesis, N2O is formed as an unwanted byproduct and is released from reactor vents into the atmosphere. Although some efforts to reduce N2O emissions from adipic acid production have been already done,17 these technologies can not be applied to other sources such as acid nitric production because of the relatively low concentration of N2O in the gas stream.15 In industry combustion, oxides of nitrogen (N2O and NOx) are formed mainly from the oxidation of molecular nitrogen present in the combustion air and organic nitrogen present in the fuel. Technological options for emission reduction of N2O may be categorized into three groups: reduced emissions from fluidized bed combustion, use of selective catalytic reduction (SCR), and fuel shift and reduction in fossil fuel consumption.18,19 Fluidized bed combustion has a better energy conversion than conventional pulverized fuel combustion, and it has lower NOx emissions due to a lower combustion temperature. However, combustion temperature in a range between 800 and 900 °C enhances N2O emission concentrations of about 30-150 ppmV. Another technology based on the use of selective noncatalytic reduction (SNCR) for reducing NOx emissions requires higher operating temperatures, and it also leads to the formation of N2O. SCR is considered preferable with regards to N2O emission reduction, but the specific cost of NOx abatement of SCR is twice more expensive than the cost of SNCR.18 Finally, use of a nonfossil energy source should further reduce the emissions. All these processes used to reduce N2O emissions require large energy consumption and may
10.1021/jp103734c 2010 American Chemical Society Published on Web 06/02/2010
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Figure 1. Scheme 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.
increase the carbon dioxide emission. Results obtained on the solubility of carbon dioxide in ILs indicate that this class of solvents may be used in friendly environmental processes. Indeed, it was 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 other greenhouse gases in ILs. In this work, we propose to investigate the possible use of ILs as a N2O absorption solvent. The N2O solubilities in five imidazolium-based ILs 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 pressure for a mixture of N2O and IL with a known composition at a fixed temperature. The N2O solubilities in the IL were measured as a function of temperature and pressure. The experimental data for the binary systems of {N2O + ILs} were correlated using the PPR78 model based on the PengRobinson equation of state (PR-EOS). Experimental Procedures Materials or Chemicals. Nitrous oxide was obtained from Messer with a purity of 0.99997 in mass fraction. Five imidazolium-based ILs were used in this study: 1-butyl3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), 1-butyl3-methylimidazolium thiocyanate ([BMIM][SCN]), 1,3-dimethylimidazolium methylphosphonate ([DMIM][MP]), 1,3diethoxyimidazolium bis(trifluoromethylsulfonyl)imide ([(ETO)2IM][Tf2N]), and 1,3-dihydroxyimidazolium bis(trifluoromethylsulfonyl)imide ([(OH)2IM][ Tf2N]). [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 mass fraction), and [(OH)2IM][Tf2N] (purity > 0.98 mass fraction) were purchased from Sigma-Aldrich. Before measurements, the ILs 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 for the ILs using the Karl Fischer technique showed that water contents were from 300 ([BMIM][BF4]) to 700 ppm. Apparatus and Experimental Procedure. Bubble point pressures of the systems {N2O + 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 phase equilibrium measurement was based on a synthetic method that avoids sampling and analyses of the phases. The high-pressure
Revelli et al. 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 1-340 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, N2O is introduced under pressure from an aluminum reservoir tank. Its mass was measured with the precision balance by weighting 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., N2O + synthetic mixture) 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 equilibrium pressure for the fixed temperature. Reproducibility of the pressure measurements is 0.5 bar. Analyses of the ILs 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 1 H and 13C spectra were collected using Bruker Avance 300 MHz using deuterated chloroform as the solvent. For the five ILs investigated in this work, no change of chemical shift was observed. Modeling The PPR78 Model. A simplified version of the PPR78 model was used in order to correlate our data. For clarity, let us recall that the PPR78 model relies on the PR-EOS as published by Peng and Robinson in 1978.20 For a pure component, the PPR78 EOS is
P)
with
{
ai(T) RT V - bi V(V + bi) + bi(V - bi)
R ) 8.314472 J · mol-1 · K-1 RTc,i bi ) Ωb Pc,i Ωb ) 0.0777960739
and
[ ( )]
R2Tc,i2 1 + mi 1 ai ) Ωa Pc,i Ωa ) 0.457235529
T Tc,i
2
(1)
(2)
Reducing of Nitrous Oxide Emissions Using Ionic Liquids
{
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if ωi e 0.491, mi ) 0.37464 + 1.54226ωi-
TABLE 1: Critical Properties and Acentric Factor of Nitrous Oxide and ILs Used in the Modeling
2
0.26992ωi if ωi > 0.491, mi ) 0.379642 + 1.48503ωi-
(3)
0.164423ωi2 + 0.16666ωi3
where P is the pressure, R is the gas constant, T is the temperature, a and b are EOS parameters, V is the molar volume, Tc is the critical temperature, Pc is the critical pressure, and ω is 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:
{
∑ ∑ zizj√aiaj(1 - kij(T))
i)1 ncompd
b)
j)1
(4)
∑ xibi
kij(T) ) ng
∑∑ k)1
l)1
Pc/bar
ω
N 2O [BMIM][BF4] [BMIM][SCN] [DMIM][MP] [(OH)2IM][Tf2N] [(ETO)2IM][Tf2N]
309.57 632.30 1047.40 767.50 1314.10 1310.80
72.45 20.40 19.40 32.60 45.09 28.20
0.1409 0.8489 0.4781 0.4714 0.7267 0.2817
TABLE 2: Bubble Point Data for Various Compositions of Nitrous Oxide in the {N2O + [BMIM][BF4]} System xN2O
T/K
P/bar
T/K
P/bar
T/K
P/bar
0.101
292.75 303.35 312.95 292.85 302.75 312.45 294.75 303.65 312.55 293.35 303.15 312.65 292.45 302.95 313.15
11.4 13.9 15.0 15.9 18.2 21.2 19.3 23.0 26.8 27.3 34.0 40.7 32.7 41.6 51.8
323.45 333.65 343.45 323.75 333.25 343.25 322.75 333.25 342.65 322.95 332.95 343.15 323.25 333.75 344.15
16.7 18.6 20.4 25.4 29.4 36.4 33.7 39.9 44.9 49.0 57.4 67.9 62.9 75.7 89.6
353.25 362.95 372.85 353.25 363.35 373.45 352.65 362.95 372.95 352.85 363.05 373.55 352.45 362.85 372.65
22.8 25.7 28.1 40.8 44.7 48.4 50.6 56.7 63.0 78.0 88.7 100.0 102.1 117.8 133.3
298.15 (Rik - Rjk)(Ril - Rjl)Akl · T
(
2
0.167 0.228
i)1
where zk represents the mole fraction of component “k” in a mixture, and ncompd is 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 method21-23 through the following expression:
ng
Tc/K
ncompd ncompd
a)
1 2
compound
)( ) Bkl
Akl
-1
(
√ai(T) bi
-
√aj(T) bj
)
2
0.33 0.371
considered as a single group. Doing so, the temperaturedependent kij is expressed by
k12(T) )
298.15 A12 · T
(
)( ) B12
-1
A12
2
√ai(T) · aj(T)
(
√a1(T) b1
-
√a1(T) · a2(T)
b2
)
2
b1 · b2
bi · b j
(6)
(5) 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 nitrous oxide were taken from the literature.24 For ILs, experimentally measurable critical points are not available since ILs decompose before reaching their critical point. Therefore, the group contribution’s method proposed by Valderrama et al. was used to estimate these properties.25,26 The predicted IL critical properties as well as those of N2O are listed in Table 1. Because the ionic liquids and N2O 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 PPR78 model in which each molecule is
√a2(T)
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 paper, both of these parameters were determined in order to minimize the following objective function: nbubble
Fobj ) 100
(
)
|∆x| + ∑ 0.5 x|∆x| x 1,exp 2,exp i 1
(7)
with |∆x| ) |x1,exp - x1,cal| ) |x2,exp - x2,cal|. 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 (N2O), and x2 the mole fraction of the heaviest component (it is obvious that x2 ) 1 - x1). Results and Discussion From our knowledge, the solubility of nitrous oxide in ILs is presented for the first time in this article. The (vapor + liquid) equilibrium (VLE) data of nitrous oxide in the five ILs were measured for mole fractions ranging from 0.03 to 0.78 in the temperature range 293-373 K and pressures from 6 to 300 bar. For systems with a fixed overall composition of nitrous oxide
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TABLE 3: Bubble Point Data for Various Compositions of Nitrous Oxide in the {N2O + [BMIM][SCN]} System
TABLE 6: Bubble Point Data for Various Compositions of Nitrous Oxide in the {N2O + [(ETO)2IM][Tf2N]} System
xN2O
T/K
P/bar
T/K
P/bar
T/K
P/bar
xN2O
T/K
P/bar
T/K
P/bar
T/K
P/bar
0.029
293.45 302.75 312.45 293.65 303.05 312.15 293.55 303.25 312.55 293.05 303.45 312.55 293.85 303.85 313.85 322.6 333.3 343.4
10.3 11.4 13.8 19.7 24.0 27.3 23.8 27.2 30.5 35.5 40.8 47.2 46.7 55.5 66.4 83.0 100.8 123.8
323.2 333.6 343.7 323 332.8 342.1 323.2 333.7 344.1 322.8 333.2 343.5 324.1 343.6 353.4 352.9 363.6 373.3
15.7 17.9 20.6 30.8 34.5 37.8 34.7 38.7 44.1 54.1 62.7 72.7 74.8 97.5 108.9 144.6 166.5 184.9
353.1 372.3
23.5 27.9
0.119 0.227
332.65 342.95 352.85 333.05 343.55 353.45 333.05 343.45 353.45 333.15 343.45 353.45 333.35 343.65 353.35 332.25 342.75 352.55 333.25 343.55 353.15 333.45 343.65 353.35 333.55 343.75 353.35 333.25 343.45 353.35 333.05 343.25 352.75 333.45 343.65 353.25 333.45 343.45 353.15 333.35 343.15 352.45 342.95
9.5 10.6 11.8 18.8 21.6 24.2 22.3 25.5 28.6 27.6 31.5 35.4 31.0 35.3 39.5 34.4 39.4 44.4 41.4 47.4 53.3 51.0 58.6 66.1 58.0 66.7 75.6 69.7 80.5 91.9 83.8 98.3 112.9 97.8 116.5 135.4 106.1 127.6 149.8 149.2 179.0 208.7 240.4
13.1
41.3 44.7 48.7 49.5 55.2 60.1 81.7 92.2 100.3 125.4 142.6
6.1 7.2 8.3 12.3 14.3 16.5 14.0 16.5 19.2 17.5 20.7 24.3 19.8 23.3 27.0 21.8 25.9 30.3 26.1 30.8 36.0 31.6 37.6 44.2 35.2 42.2 50.1 42.1 50.4 59.8 48.6 58.8 71.2 54.9 67.0 81.9 57.8 70.8 87.9 63.1 81.8 113.9 98.1 146.1 191.3
362.55
352.8 363 371.6 353.4 363.2 373.2 352.9 363.2 372.7 363.7 373.2
302.95 312.65 322.85 303.05 312.95 323.15 303.15 313.05 323.15 303.05 312.95 323.15 303.05 313.05 323.25 303.05 313.15 323.25 303.05 313.05 323.25 303.05 313.05 323.35 303.05 313.05 323.55 303.05 313.05 323.35 302.75 312.55 323.35 302.85 313.15 323.65 302.75 312.85 323.65 302.05 312.35 323.05 312.65 323.45 333.25
363.15
26.8
363.05
31.7
363.05
39.2
363.15
43.9
362.15
49.4
363.15
59.7
363.35
74.3
363.35
84.9
363.45
104.2
362.55
128.7
362.05
153.4
362.75
171.9
360.65
233.0
0.111 0.125 0.189 0.227 0.294
0.267 0.311 0.351 0.387 0.429
TABLE 4: Bubble Point Data for Various Compositions of Nitrous Oxide in the {N2O + [DMIM][MP]} System xN2O
T/K
P/bar
T/K
P/bar
T/K
P/bar
0.087
293.35 303.65 312.85 292.55 303.55 312.95 292.35 303.75 312.95 293.35 303.65 312.85 312.95 323.05 333.15
21.0 26.0 30.0 25.0 29.0 33.0 30.0 36.0 41.0 48.0 59.0 71.0 80.0 91.0 105.0
323.25 333.15 343.15 323.05 332.95 343.35 323.05 333.15 343.45 323.25 333.15 343.15 343.45 353.05 363.35
34.0 39.0 45.2 37.4 41.9 51.0 50.7 57.9 70.8 81.6 96.7 112.0 119.0 140.0 162.0
353.05 362.85 372.45 353.05 363.15 373.15 353.05 363.35 373.15 353.05 362.85 372.45
52.9 60.1 68.4 60.6 70.5 80.4 89.0 105.3 117.9 132.0 151.6 164.5
0.119 0.175 0.253 0.336
0.483 0.517 0.564 0.613 0.644 0.658 0.694
TABLE 5: Bubble Point Data for Various Compositions of Nitrous Oxide in the {N2O + [(OH)2IM][Tf2N]} System xN2O
T/K
P/bar
T/K
P/bar
T/K
P/bar
0.152
303.05 312.35 323.45 302.85 312.55 323.55 303.05 312.55 323.35 303.25 312.95 323.55 302.95 312.65 322.85 303.75 313.45 323.55 302.95 312.45 323.35 303.05 312.95 323.35
10.2 12.2 14.5 12.6 15.1 17.6 21.3 25.1 29.6 22.7 26.3 30.9 30.3 36.0 42.6 38.5 47.9 57.7 80.7 109.5 147.3 81.1 152.6 204.2
333.15 342.75 352.35 333.35 343.35 352.85 332.95 342.75 352.15 333.45 343.05 352.55 332.75 342.75 352.55 333.95 343.75 353.45 332.95 342.75 352.45 333.05 342.75 352.55
16.7 19.2 21.6 20.1 22.8 25.2 34.1 38.7 43.4 36.1 40.7 45.6 49.5 57.8 65.3 67.9 78.2 88.7 181.5 213.6 240.7 243.2 272.3 301.0
362.45
23.9
0.175 0.278 0.290 0.372 0.437 0.610 0.691
362.75
27.8
362.15
48.5
362.65
50.9
362.45
73.3
363.15
100.1
362.15
267.6
0.781
and IL, bubble point pressures were measured as a function of temperature. The results are listed in Tables 2-6. Dialkylimidazolium-Based ILs. The VLE data for the binary mixtures {[BMIM][BF4] + N2O}, {[BMIM][SCN] + N2O}, and {[DMIM][MP] + N2O} were measured from 293 to 373 K and pressures up to 190 bar. The isotherms above the critical point of pure N2O present VLE throughout the pressure range measured. For temperature below the critical point of N2O, 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. The solubility of N2O 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 N2O solubility by replacing the [SCN] anion by [BF4]. This behavior was already observed with the solubility of carbon dioxide in ILs. Brennecke et al. reported that an increase of the 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
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Figure 2. N2O solubility at 303.15 K in the five ILs: × [DMIM][MP]; ( [BMIM][BF4]; 9 [BMIM][SCN]; b [(OH)2IM][Tf2N]; 2 [(ETO)2IM][Tf2N]. Figure 5. (P, T, x) phase diagram and modeling for the system {N2O + [DMIM][MP]}. T1 ) 303.15 K, T2 ) 323.15 K, T3 ) 343.15 K, T4 ) 363.15 K.
TABLE 7: Values of the Fitted A12 and B12 Parameters and Value of the Corresponding Objective Function for the Five Binary Systems Investigated in This Study system N2O N2O N 2O N 2O N 2O
Figure 3. (P, T, x) phase diagram and modeling for the system {N2O + [BMIM][BF4]}. T1 ) 303.15 K, T2 ) 323.15 K, T3 ) 343.15 K, T4) 363.15 K.
+ + + + +
[BMIM][BF4] [BMIM][SCN] [DMIM][MP] [(OH)2IM][Tf2N] [(ETO)2IM][Tf2N]
A12(MPa)
B12(MPa)
Fobj (%)
153.7 252.0 216.3 -59.9 40.2
-49.1 227.0 -20.0 80.7 184.0
4.25 14.19 9.46 1.94 8.35
temperature-dependent and increases with an increase of temperature. The objective functions for the five binary systems are calculated in order to minimize deviations between experimental and calculated molar fractions (see eq 7). The obtained Fobj values were 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 IL critical properties. However, the satisfactory results indicate that the PPR78 model may be used to determine the phase diagram of ILs 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 Konynenburg as it is shown in Figure 6. This figure represents the global equilibrium phase diagram of the system {N2O+[BMIM][BF4]} as predicted by the PPR78 model. Nevertheless, particular care should be taken when using this
Figure 4. (P, T, x) phase diagram and modeling for the system {N2O + [BMIM][SCN]}. T1 ) 303.15 K, T2 ) 323.15 K, T3 ) 343.15 K, T4 ) 363.15 K.
likely due to their large quadrupole moments, as well as specific interactions between the gas and the anion. The solubility of N2O in [DMIM][MP] (see Figure 2) is very close to that observed with [BMIM][SCN]. In Figures 3-5 are presented the experimental data determined for the binary mixtures {BMIM][BF4] + N2O}, {[BMIM][SCN] + N2O}, and {[DMIM][MP] + N2O}. In the three figures, the experimental 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
Figure 6. Prediction of the global phase equilibrium diagram for the system {N2O + [BMIM][BF4]} using the PPR78 model: + Critical points of the pure substance; O critical end point.
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Revelli et al. TABLE 8: Comparison of the N2O Solubility Expressed in Fraction Molar or N2O with Values Obtained in Terms of Molality
Figure 7. (P, T, x) phase diagram and modeling for the system {N2O + [(OH)2IM][Tf2N]}. T1 ) 303.15 K, T2 ) 323.15 K, T3 ) 343.15 K, T4 ) 363.15 K.
Figure 8. (P, T, x) phase diagram and modeling for the system {N2O + [(ETO)2IM][Tf2N]}. T1 ) 303.15 K, T2 ) 323.15 K, T3 ) 343.15 K, T4 ) 363.15 K.
prediction because the PPR78 parameters have been fitted on our experimental data in a range of temperature from 293 to 373 K. Moreover, the experimental data set measured in this work is not sufficient to determine whether this system is Type III or Type V. Alcohol and Ether Functionalized ILs. The VLE (bubble point) data for the binary mixtures {[(ETO)2IM][Tf2N] + N2O}
ILs
xN2O
-1 molality (molN2O · kgIL )
[BMIM][BF4] [BMIM][SCN] [DMIM][MP] [(OH)2IM][Tf2N] [(ETO)2IM][Tf2N]
0.33 0.15 0.18 0.40 0.51
2.18 0.90 1.14 1.75 2.38
and {[(OH)2IM][ Tf2N] + N2O} were measured from 293 to 373 K and pressures up to 300 bar (Figures 2 and 8). At 303 K, a liquid-liquid-vapor equilibrium was observed at high molar fraction in nitrous oxide. The isotherms above the critical point of pure N2O present VLE 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). With the same anion [Tf2N], the effect of the functionalized imidazolium cation on N2O solubility can be compared at 313 K. As shown in Figure 7, the highest solubility is obtained with the ether-functionalized IL. Using these functionalized ILs significantly improve the N2O solubility in ILs. In both binary mixtures, the PPR78 model predicts a mixture critical point at approximately 258 bar at 303.15 K and 0.95 molar fraction of nitrous oxide. This predicted mixture critical point seems to be an artifact of the equation of state model and the estimated critical properties of the IL. It is probably due to the lack of data representing the strong increase of pressure. Nevertheless, the model provides a good correlation for the solubility data of nitrous oxide in IL in the pressure range of the experiments (
