Ind. Eng. Chem. Res. 2006, 45, 6375-6382
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Gaseous Absorption of Fluoromethane, Fluoroethane, and 1,1,2,2-Tetrafluoroethane in 1-Butyl-3-Methylimidazolium Hexafluorophosphate Mark B. Shiflett DuPont Central Research and DeVelopment, Experimental Station, Wilmington, Delaware 19880
A. Yokozeki* DuPont Fluoroproducts Laboratory, Chestnut Run Plaza 711, Wilmington, Delaware 19880
Recently, we have reported solubility studies of several hydrofluorocarbons (HFCs) (from the methane and ethane series) in room-temperature ionic liquids. The present report is a continuation of such a study to complete the methane and ethane series of HFCs in ionic liquid. The solubilities of fluoromethane, fluoroethane, and 1,1,2,2-tetrafluoroethane in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) have been measured with a gravimetric microbalance in a temperature range of 283.15-348.15 K and pressures up to 2 MPa. Solubility data have been correlated successfully with the Non-Random Two-Liquid (NRTL) activity coefficient model. Also, Henry’s law constants for the present gases have been obtained from the solubility data. In this paper, we focus on the thermodynamic properties at infinite dilution. Results are compared and discussed, together with those of other HFCs in our previous report. 1. Introduction The present work is a continuation of our previous studies on solubility of hydrofluorocarbons (HFCs) in ionic liquids.1-3 To complete the solubility study of the methane- and ethaneseries HFCs in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) ionic liquid, except for uncommon HFCs such as R-152 (1,2-difluoroethane) and R-143 (1,1,2-trifluoroethane), the solubilities (PTx) of R-41 (fluoromethane), R-161 (fluoroethane), and R-134 (1,1,2,2-tetrafluoroethane) in [bmim][PF6] have been measured with a gravimetric microbalance4 at pressures up to 2 MPa and at four isotherms (283.15, 298.15, 323.15, and 348.15 K). The PTx data are correlated with the Non-Random Two-Liquid (NRTL) activity coefficient model, as have been done in our previous reports.1-3 In this paper, we focus on the thermodynamic properties at the infinite dilution state. Activity coefficients at infinite dilution (γ∞) and Henry’s law constants (kH) are analyzed for the present systems, as well as those of our previous solubility data of HFCs.1 There are several such studies for various solutes with ionic liquid solvents in the literature.5-10 Here, we discuss the physical meanings and thermodynamic relations of gaseous HFC solubilities in some details, and we try to shed some light on such properties. In addition, thermodynamically derived properties from γ∞and kH, and some interesting correlations among them, will be discussed. 2. Experimental Section 2.1. Samples. R-134 was obtained from DuPont Fluoroproducts (Wilmington, Delaware) with a minimum purity of 99 mol %. R-41 and R-161 were obtained from Speciality Gases of America (Toledo, OH) with minimum purities of 99 and 97 mol %, respectively. The purity of the gases were determined using a gas chromatography (GC) method (Agilent 6890N, Restek Rtx-200 column, 105 m × 0.25 mm). A molecular sieve * To whom correspondence should be addressed. Tel.: (302) 9994576. Fax: (302) 999-5340. E-mail address: akimichi.yokozeki@ usa.dupont.com.
trap was installed to remove any trace amounts of water from the gases prior to entering the microbalance. The [bmim][PF6] was obtained from Fluka, Sigma-Aldrich Chemie GmbH (Buchs, Switzerland, Catalog No. 70956, Lot No. 1152140) with a stated purity of >97%. The initial as-received water content was 0.23 mass % and was measured using Karl Fischer titration (Aqua-Star C3000 and Aqua-Star Coulomat C and A solutions). The as-received chlorine content was measured using two methods. The extractable chlorine content was 4.7 mg/cm3 and was measured by ion chromatography (Dionex AS17 column). The bound chlorine content was 0.79 ppm (by weight) and was measured using a Wickbold torch method. 2.2. Apparatus and Measuring Technique. A detailed description of the experimental equipment and procedure is available in our previous report.4 Therefore, only the basic experimental technique and experimental uncertainties are given here. The gas solubility measurements were made using a gravimetric microbalance (Hiden Isochema Ltd, model IGA 003). Initially, ∼10-70 mg of ionic liquid was loaded into the sample container and heated to 348.15 K under a vacuum of ∼10-9 MPa for 10 h to remove any trace amounts of water or other impurities. For example, [bmim][PF6] with an initial mass of 64.9608 mg was dried with a final dry mass of 64.7383 mg, resulting in a mass loss of 0.34%. The majority of the mass loss is due to the removal of water. Four isotherms were measured at (283.15, 298.15, 323.15, and 348.15) K over a pressure range from ∼0.01-2.0 MPa for R-41, ∼0.01-0.7 MPa for R-161, and ∼0.01-0.35 MPa for R-134. The upper pressure limit for R-41 was 2.0 MPa, which is the maximum operating pressure of the microbalance. The upper pressure limit for R-161 and R-134 was dependent on the saturation pressure in the sample container at ambient temperature (0.83 MPa at 294.15 K for R-161 and 0.46 MPa at 294.15 K for R-134). To ensure sufficient time for gas-liquid equilibrium, the ionic liquid samples were maintained at each pressure set point for a minimum of 3 h and a maximum of 10 h. The instrumental uncertainties in temperature T and pressure P are within 0.1 K and 0.8 kPa, respectively. These uncertainties
10.1021/ie060192s CCC: $33.50 © 2006 American Chemical Society Published on Web 08/05/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 18, 2006
Table 1. Experimental Solubility (T, P, x) Data of R-41, R-134, and R-161 in [bmim][PF6] (1) R-41/(2) [bmim][PF6]
(1) R-134/(2) [bmim][PF6]
temperature, T (K)
pressure, P (MPa)
100x1
283.17 283.14 283.18 283.09 283.14 283.17 283.12 283.16 283.18
0.0100 0.0496 0.0999 0.3992 0.6992 0.9994 1.2994 1.5005 1.9995
298.05 298.18 298.00 298.17 298.08 297.97 298.01 298.05
(1) R-161/(2) [bmim][PF6]
temperature, T (K)
pressure, P (MPa)
100x1
temperature, T (K)
pressure, P (MPa)
100x1
0.2 2.1 4.4 17.2 28.2 37.8 46.3 51.3 63.7
283.13 283.11 283.15 283.15 283.13 283.31
0.0101 0.0502 0.1000 0.1503 0.2002 0.2498
3.1 19.0 37.8 54.2 68.5 78.9
283.18 283.22 283.18 283.06 283.18 283.22
0.0099 0.1000 0.1994 0.2995 0.3992 0.4993
0.9 10.7 22.1 32.9 44.3 57.5
0.0499 0.0996 0.3995 0.6993 0.9993 1.2994 1.5005 1.9992
1.4 3.1 12.6 21.1 28.6 35.3 39.2 48.4
298.17 298.08 298.04 298.08 298.07 298.15 298.05 298.09
0.0105 0.0504 0.1004 0.1503 0.2003 0.2502 0.3000 0.3503
2.4 11.6 22.5 33.0 42.8 52.2 61.1 68.9
297.98 298.05 298.06 298.17 298.20 298.06 297.99 298.08
0.0100 0.1000 0.1999 0.2993 0.3992 0.4994 0.5996 0.6994
0.7 7.3 14.4 21.5 28.3 35.1 42.0 49.6
323.13 323.16 323.14 323.14 323.15 323.15 323.13 323.16
0.0500 0.0999 0.3996 0.6996 0.9993 1.2995 1.5003 1.9993
0.6 1.7 7.9 13.5 18.7 23.5 26.4 33.2
323.12 323.12 323.14 323.16 323.13 323.15 323.12 323.16
0.0105 0.0504 0.1004 0.1504 0.2004 0.2504 0.3002 0.3505
0.6 4.9 10.3 15.5 20.5 25.5 30.2 34.6
323.15 323.18 323.14 323.16 323.13 323.16 323.12 323.16
0.0100 0.0997 0.1996 0.2996 0.3996 0.4999 0.6002 0.7005
0.5 4.1 8.2 12.2 16.0 19.8 23.5 27.0
348.18 348.15 348.16 348.15 348.15 348.15 348.16 348.15 348.16
0.0097 0.0499 0.1001 0.3998 0.6998 0.9997 1.2992 1.5002 1.9993
0.2 0.6 1.4 5.6 9.5 13.1 16.6 18.7 23.8
348.08 348.16 348.14 348.16 348.14 348.16 348.15 348.14
0.0105 0.0505 0.1004 0.1504 0.2004 0.2503 0.3000 0.3500
0.6 2.9 5.8 8.7 11.4 14.1 16.7 19.6
348.16 348.16 348.16 348.13 348.13 348.15 348.15 348.14
0.0100 0.1001 0.1998 0.2996 0.3996 0.4998 0.5998 0.7003
0.3 2.5 5.1 7.6 10.0 12.4 14.7 17.0
do not cause any significant changes in the gas solubility measurement. One of the largest sources of uncertainty in the present experiment is reproducibility. We have examined the reproducibility by repeating the R-134/[bmim][PF6] isotherm at 283.15 K with three different mass loadings (65.2487, 31.0615, 11.4152 mg) at different times (for example, several days apart for the same isotherm). Our best estimate for the present experimental reproducibility, including the sample (ionic liquid) purity effect, has been
