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Raman spectroscopic studies on methane + tetrafluoromethane mixed-gas hydrate system
Fluid Phase Equilibria 251 (2007) 145–148
Raman spectroscopic studies on methane + tetrafluoromethane mixed-gas hydrate system Yuuki Kunita, Takashi Makino, Takeshi Sugahara, Kazunari Ohgaki ∗ Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japan Received 2 June 2006; received in revised form 20 November 2006; accepted 5 December 2006 Available online 10 December 2006
Abstract Raman spectra of intramolecular vibration mode for each guest species in the methane + tetrafluoromethane (CF4 ) mixed-gas hydrate crystal have been measured at 291.1 K. Both of pure guest species generate the structure-I hydrate in the present pressure ranges. Isothermal phaseequilibrium curve exhibits two discontinuous points around the equilibrium methane compositions (water-free) in the gas phase of 0.3 and 0.8. At the above points, the Raman spectra of both guest molecules have been drastically changed. One of the most important findings is that the crystal of methane + tetrafluoromethane mixed-gas hydrate shows the structural phase-transition (from the structure-I to the structure-II and back to the structure-I) caused by composition changes. © 2006 Elsevier B.V. All rights reserved. Keywords: Gas hydrate; Raman spectroscopy; Structural transition; Solid–fluid equilibria; Mixture
1. Introduction There are two well-known unit-cell structures of gas hydrates stabilized by the enclathrated guest molecules. One is the structure-I (s-I), which is composed of two different hydrate cages of pentagonal dodecahedron (S-cage) and tetrakaidecahedron (M-cage). The other is the structure-II (s-II), which has the hydrate cage of hexakaidecahedron (L-cage) instead of M-cage. The existence of size-different cages produces some characteristic and interesting phenomena in the mixed-gas hydrate systems. One of the characteristic and interesting phenomena is the structural phase-transition to the s-II caused by the composition change of guest mixtures, while each pure guest species generates s-I hydrates. The structural phase-transition has been classically reported in the H2 S + CH3 CHF2 mixed-gas hydrate system [1,2]. Recently, the mixed-gas hydrates including CH4 have been investigated from the viewpoint of the effective utilization of natural-gas. The structural phase-transitions occur in the CH4 + C2 H6 [3–5] and CH4 + cyclopropane (c-C3 H6 ) [6] mixed-gas hydrate systems, while the s-I precedes in the
∗
Corresponding author. Tel.: +81 6 6850 6290; fax: +81 6 6850 6290. E-mail address: [email protected] (K. Ohgaki).
0378-3812/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2006.12.004
CH4 + C2 H4 [7,8] mixed-gas hydrate system. Mooijer-van den Heuvel et al. [9] have reported that structural phase-transition is expected to occur in the CH4 + CF4 mixed-gas hydrate system from phase-equilibrium measurements and model calculations. In the present study, the existence of structural phase-transition in the CH4 + CF4 mixed-gas hydrate system has been investigated in detail with Raman spectroscopy. 2. Experimental Experimental apparatus for Raman spectroscopy used in the present study is the same as the previous one [7]. Equilibrium temperatures were measured within a reproducibility of 0.02 K using a thermistor probe calibrated by a Pt resistance thermometer (25 ). The thermistor probe was inserted into a hole in wall of high-pressure cell. Equilibrium pressures were measured with an uncertainty of 0.02 MPa. Mixtures of CH4 and CF4 were prepared at desired compositions. The mixture was introduced into an evacuated high-pressure optical cell. The contents were pressurized up to a desired pressure by supplying distilled water successively. The contents were cooled and agitated with using an enclosed ruby ball to generate the CH4 + CF4 mixed-gas hydrate. Temperature was controlled by circulating thermostated water through a
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jacket of the cell. The ruby ball was vibrated by low-frequency vibration from outside. After the CH4 + CF4 mixed-gas hydrate was generated, the temperature was changed gradually to prepare single crystals of mixed-gas hydrate under three-phase coexisting state. The threephase equilibrium state with the existence of single crystal was established to keep the temperature constant for more than 1 day. They were analyzed by in situ Raman spectroscopy by use of a laser Raman microprobe spectrometer with a multichannel CCD detector. Ar ion laser beam (wavelength: 514.5 nm, generation power: 100 mW) condensed to 2 m in spot diameter was irradiated onto them from the object lens. Raman spectra of gas and hydrate phases were obtained separately. Raman spectra considered to be bimodal were analyzed by Voigt equation. The spectral resolution was about 1 cm−1 . The CCD detector was maintained at 140 K by liquid nitrogen for heat-noise reduction. Integration time was varied from 10 to 300 s, depending on intensities of Raman scattering. Equilibrium compositions of gas and hydrate phases were determined by direct comparison of equilibrium Raman-peak area ratios of unknown samples with that of reference mixed-gas samples. The reference samples were prepared by weighing on a precision balance (uncertainty of 0.1 mg, Max. 3 kg, Cho balance). CH4 (research grade of purity 99.99 mol%) and CF4 (research grade of purity 99.999 mol%) were purchased from Takachiho Trading Co., Ltd. and Neriki Gas Co. Ltd., respectively. The distilled water was obtained from Yashima Pure Chemicals Co., Ltd. All of them were used without further purification. 3. Results and discussion
Fig. 1. Raman spectra of the intramolecular C–F stretching vibration mode for CF4 in the mixed-gas hydrate phase at 291.1 K.
Raman spectra of the intramolecular C–F stretching vibration of CF4 in the CH4 + CF4 mixed-gas hydrates are shown in Fig. 1. This vibration mode has the strongest intensity of four vibration modes of CF4 . The symbols y and z stand for the equilibrium CH4 composition in the gas and mixed-gas hydrate phases. Raman peaks of C–F stretching vibration are detected at 908 cm−1 in the composition range of y = 0.04–0.27 (hereafter, range A). These peaks agree well with that of the pure CF4 hydrate system [10] in the pressure range of present study. Therefore, the CH4 + CF4 mixed-gas hydrate in range A is considered to be the s-I hydrate. The single peak of 908 cm−1 corresponds to the CF4 molecule in the M-cage, because the CF4 molecule cannot occupy the S-cage [10]. Raman peaks in the composition range of y = 0.32–0.67 (hereafter, range B) have lower Raman shift (906 cm−1 ) than that of range A, that is, the hydrate cage occupied by the CF4 molecule is larger than the M-cage of s-I hydrate. In the gas phase, Raman shift of C–F stretching vibration mode of CF4 is 909 cm−1 . The above findings reveal that the CH4 + CF4 mixtures are deduced to generate the s-II hydrate in range B and the CF4 molecule is speculated to occupy the L-cage of s-II hydrate. Raman peaks show blue shift in the composition range of y = 0.90–0.95 (hereafter, range C) and get back to a similar position of range A. That is, the CH4 + CF4 mixed-gas hydrate changes from the s-I hydrate (range A) to the s-II hydrate (range B) and back to the s-I hydrate (range C) with increase of CH4 composition. The CF4 molecule occupies only the large cages
(the M- or L-cage), because the single Raman peak is held in the whole composition range. The literature [10] reported that the CF4 molecule could occupy the S-cage by pressurization over 70 MPa. The Raman peak of CF4 encaged in the S-cage was detected 918 cm−1 at 150 MPa and its pressure dependence is small (0.5 cm−1 /100 MPa). Any peak was not detected around that position in the present study. Raman spectra of the intramolecular C–H stretching vibration of CH4 in the CH4 + CF4 mixed-gas hydrate are shown in Fig. 2. Raman peak of the C–H stretching vibration mode in the CH4 + CF4 mixed-gas hydrate phase is doublet in the whole composition range, while the single Raman peak is detected at 2917 cm−1 in the gas phase. The lower peak (2904 cm−1 ) in ranges A and C results from the CH4 molecule in the M-cage, and the higher peak (2914 cm−1 ) corresponds to the CH4 molecule in the S-cage. The Raman spectrum in range B changes drastically from that of ranges A and C. The Raman peak of CH4 in the S-cage shows much stronger intensity than that of L-cage. In addition, the peak (2903 cm−1 ) in range B is 1 cm−1 lower than that of ranges A and C. The Raman peak of CH4 is less sensitive to the size between L-cage and M-cage, because the size of M-cage is large enough for CH4 . Fig. 3 shows the peak area ratio of CH4 in the large cage to the S-cage. The open and solid circles stand for the area ratio of s-I and s-II hydrates, respectively. The ratio increases mono-
Y. Kunita et al. / Fluid Phase Equilibria 251 (2007) 145–148
147
Fig. 4. Isothermal phase equilibria for the CH4 + CF4 mixed-gas hydrate system at 291.1 K. Table 1 Isothermal phase equilibria for the CH4 + CF4 mixed-gas hydrate system at 291.1 K
Fig. 2. Raman spectra of the intramolecular C–H stretching vibration mode for CH4 in the mixed-gas hydrate phase at 291.1 K.
tonically in proportion to the CH4 composition in range A. In range B, the ratio drastically changes and the amount of CH4 in the S-cage is much larger than that of the large cage. This distinction between ranges A and B corresponds to the ratio of large cages to S-cages of the s-II hydrate (1:2) whereas the s-I hydrate (3:1). The ratio increases suddenly at the next transi-
p (MPa)
yCH4
zCH4
The s-I range (range C) 18.5 18.7 18.8
1.0 0.95 0.90
1.0 0.97 0.94
The s-II range (range B) 19.2 20.2 25.7
0.67 0.47 0.32
0.77 0.68 0.65
The s-I range (range A) 27.0 31.3 35.7 49.2 90.0
0.27 0.18 0.12 0.04 0.00
0.49 0.44 0.39 0.27 0.00
tion point between ranges B and C. This discontinuous change of cage occupancy also supports the existence of two structural phase-transition points. Isothermal phase equilibria for the CH4 + CF4 mixed-gas hydrate system at 291.1 K are shown in Fig. 4 and summarized in Table 1. The Raman-peak areas of the C–H symmetric stretching vibration of CH4 and the C–F symmetric stretching vibration of CF4 are used to evaluate the equilibrium compositions of gas and hydrate phases. Equilibrium curves have two discontinuous points that correspond to two hydrate-structural transition points around (1) p = 26 MPa, y = 0.3 and (2) p = 19 MPa, y = 0.8. 4. Conclusions
Fig. 3. Peak area ratio of CH4 in the CH4 + CF4 mixed-gas hydrate crystals under the three-phase coexistence at 291.1 K.
The single crystals of CH4 + CF4 mixed-gas hydrate were analyzed by use of Raman spectroscopy in order to elucidate structural phase-transition. Both Raman spectra of CH4 and CF4 detected in the mixed-gas hydrate phase change drastically within the equilibrium CH4 composition of 0.3–0.8 in the gas phase. These results reveal that the structure of mixed gas-
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hydrate is transformed from the structure-I to the structure-II and back to the structure-I with increase of CH4 composition. Acknowledgements The authors are grateful to the Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University for the scientific support by “Gas-Hydrate Analyzing System (GHAS).” One of the authors (T.M.) expresses his special thanks for the center of excellence (21COE) program “Creation of Integrated EcoChemistry of Osaka University”. List of symbols A Raman-peak area p pressure (Pa) T temperature (K) y composition of CH4 in the mixed-gas phase z composition of CH4 in the mixed-gas hydrate phase
References [1] M. von Stackelberg, W. Jahns, Z. Elektrochem. 58 (1954) 162– 164. [2] J.H. van der Waals, J.C. Platteeuw, Adv. Chem. Phys. 2 (1959) 1– 57. [3] S. Subramanian, R.A. Kini, S.F. Dec, E.D. Sloan Jr., Chem. Eng. Sci. 55 (2000) 1981–1999. [4] S. Subramanian, A.L. Ballard, R.A. Kini, S.F. Dec, E.D. Sloan Jr., Chem. Eng. Sci. 55 (2000) 5763–5771. [5] A.L. Ballard, E.D. Sloan Jr., Chem. Eng. Sci. 55 (2000) 5773–5782. [6] T. Makino, M. Tongu, T. Sugahara, K. Ohgaki, Fluid Phase Equilib. 233 (2005) 129–133. [7] T. Sugahara, T. Makino, K. Ohgaki, Fluid Phase Equilib. 206 (2003) 117–126. [8] C.-F. Ma, G.-J. Chen, F. Wang, C.-Y. Sun, T.-M. Guo, Fluid Phase Equilib. 191 (2001) 41–47. [9] M.M. Mooijer-van den Heuvel, C.J. Peters, J. de Swaan Arons, Fluid Phase Equilib. 172 (2000) 73–91. [10] K. Sugahara, M. Yoshida, T. Sugahara, K. Ohgaki, J. Chem. Eng. Data 49 (2004) 326–329.
Raman spectroscopic studies on methane + tetrafluoromethane mixed-gas hydrate system Yuuki Kunita, Takashi Makino, Takeshi Sugahara, Kazunari Ohgaki ∗ Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japan Received 2 June 2006; received in revised form 20 November 2006; accepted 5 December 2006 Available online 10 December 2006
Abstract Raman spectra of intramolecular vibration mode for each guest species in the methane + tetrafluoromethane (CF4 ) mixed-gas hydrate crystal have been measured at 291.1 K. Both of pure guest species generate the structure-I hydrate in the present pressure ranges. Isothermal phaseequilibrium curve exhibits two discontinuous points around the equilibrium methane compositions (water-free) in the gas phase of 0.3 and 0.8. At the above points, the Raman spectra of both guest molecules have been drastically changed. One of the most important findings is that the crystal of methane + tetrafluoromethane mixed-gas hydrate shows the structural phase-transition (from the structure-I to the structure-II and back to the structure-I) caused by composition changes. © 2006 Elsevier B.V. All rights reserved. Keywords: Gas hydrate; Raman spectroscopy; Structural transition; Solid–fluid equilibria; Mixture
1. Introduction There are two well-known unit-cell structures of gas hydrates stabilized by the enclathrated guest molecules. One is the structure-I (s-I), which is composed of two different hydrate cages of pentagonal dodecahedron (S-cage) and tetrakaidecahedron (M-cage). The other is the structure-II (s-II), which has the hydrate cage of hexakaidecahedron (L-cage) instead of M-cage. The existence of size-different cages produces some characteristic and interesting phenomena in the mixed-gas hydrate systems. One of the characteristic and interesting phenomena is the structural phase-transition to the s-II caused by the composition change of guest mixtures, while each pure guest species generates s-I hydrates. The structural phase-transition has been classically reported in the H2 S + CH3 CHF2 mixed-gas hydrate system [1,2]. Recently, the mixed-gas hydrates including CH4 have been investigated from the viewpoint of the effective utilization of natural-gas. The structural phase-transitions occur in the CH4 + C2 H6 [3–5] and CH4 + cyclopropane (c-C3 H6 ) [6] mixed-gas hydrate systems, while the s-I precedes in the
∗
Corresponding author. Tel.: +81 6 6850 6290; fax: +81 6 6850 6290. E-mail address: [email protected] (K. Ohgaki).
0378-3812/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2006.12.004
CH4 + C2 H4 [7,8] mixed-gas hydrate system. Mooijer-van den Heuvel et al. [9] have reported that structural phase-transition is expected to occur in the CH4 + CF4 mixed-gas hydrate system from phase-equilibrium measurements and model calculations. In the present study, the existence of structural phase-transition in the CH4 + CF4 mixed-gas hydrate system has been investigated in detail with Raman spectroscopy. 2. Experimental Experimental apparatus for Raman spectroscopy used in the present study is the same as the previous one [7]. Equilibrium temperatures were measured within a reproducibility of 0.02 K using a thermistor probe calibrated by a Pt resistance thermometer (25 ). The thermistor probe was inserted into a hole in wall of high-pressure cell. Equilibrium pressures were measured with an uncertainty of 0.02 MPa. Mixtures of CH4 and CF4 were prepared at desired compositions. The mixture was introduced into an evacuated high-pressure optical cell. The contents were pressurized up to a desired pressure by supplying distilled water successively. The contents were cooled and agitated with using an enclosed ruby ball to generate the CH4 + CF4 mixed-gas hydrate. Temperature was controlled by circulating thermostated water through a
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Y. Kunita et al. / Fluid Phase Equilibria 251 (2007) 145–148
jacket of the cell. The ruby ball was vibrated by low-frequency vibration from outside. After the CH4 + CF4 mixed-gas hydrate was generated, the temperature was changed gradually to prepare single crystals of mixed-gas hydrate under three-phase coexisting state. The threephase equilibrium state with the existence of single crystal was established to keep the temperature constant for more than 1 day. They were analyzed by in situ Raman spectroscopy by use of a laser Raman microprobe spectrometer with a multichannel CCD detector. Ar ion laser beam (wavelength: 514.5 nm, generation power: 100 mW) condensed to 2 m in spot diameter was irradiated onto them from the object lens. Raman spectra of gas and hydrate phases were obtained separately. Raman spectra considered to be bimodal were analyzed by Voigt equation. The spectral resolution was about 1 cm−1 . The CCD detector was maintained at 140 K by liquid nitrogen for heat-noise reduction. Integration time was varied from 10 to 300 s, depending on intensities of Raman scattering. Equilibrium compositions of gas and hydrate phases were determined by direct comparison of equilibrium Raman-peak area ratios of unknown samples with that of reference mixed-gas samples. The reference samples were prepared by weighing on a precision balance (uncertainty of 0.1 mg, Max. 3 kg, Cho balance). CH4 (research grade of purity 99.99 mol%) and CF4 (research grade of purity 99.999 mol%) were purchased from Takachiho Trading Co., Ltd. and Neriki Gas Co. Ltd., respectively. The distilled water was obtained from Yashima Pure Chemicals Co., Ltd. All of them were used without further purification. 3. Results and discussion
Fig. 1. Raman spectra of the intramolecular C–F stretching vibration mode for CF4 in the mixed-gas hydrate phase at 291.1 K.
Raman spectra of the intramolecular C–F stretching vibration of CF4 in the CH4 + CF4 mixed-gas hydrates are shown in Fig. 1. This vibration mode has the strongest intensity of four vibration modes of CF4 . The symbols y and z stand for the equilibrium CH4 composition in the gas and mixed-gas hydrate phases. Raman peaks of C–F stretching vibration are detected at 908 cm−1 in the composition range of y = 0.04–0.27 (hereafter, range A). These peaks agree well with that of the pure CF4 hydrate system [10] in the pressure range of present study. Therefore, the CH4 + CF4 mixed-gas hydrate in range A is considered to be the s-I hydrate. The single peak of 908 cm−1 corresponds to the CF4 molecule in the M-cage, because the CF4 molecule cannot occupy the S-cage [10]. Raman peaks in the composition range of y = 0.32–0.67 (hereafter, range B) have lower Raman shift (906 cm−1 ) than that of range A, that is, the hydrate cage occupied by the CF4 molecule is larger than the M-cage of s-I hydrate. In the gas phase, Raman shift of C–F stretching vibration mode of CF4 is 909 cm−1 . The above findings reveal that the CH4 + CF4 mixtures are deduced to generate the s-II hydrate in range B and the CF4 molecule is speculated to occupy the L-cage of s-II hydrate. Raman peaks show blue shift in the composition range of y = 0.90–0.95 (hereafter, range C) and get back to a similar position of range A. That is, the CH4 + CF4 mixed-gas hydrate changes from the s-I hydrate (range A) to the s-II hydrate (range B) and back to the s-I hydrate (range C) with increase of CH4 composition. The CF4 molecule occupies only the large cages
(the M- or L-cage), because the single Raman peak is held in the whole composition range. The literature [10] reported that the CF4 molecule could occupy the S-cage by pressurization over 70 MPa. The Raman peak of CF4 encaged in the S-cage was detected 918 cm−1 at 150 MPa and its pressure dependence is small (0.5 cm−1 /100 MPa). Any peak was not detected around that position in the present study. Raman spectra of the intramolecular C–H stretching vibration of CH4 in the CH4 + CF4 mixed-gas hydrate are shown in Fig. 2. Raman peak of the C–H stretching vibration mode in the CH4 + CF4 mixed-gas hydrate phase is doublet in the whole composition range, while the single Raman peak is detected at 2917 cm−1 in the gas phase. The lower peak (2904 cm−1 ) in ranges A and C results from the CH4 molecule in the M-cage, and the higher peak (2914 cm−1 ) corresponds to the CH4 molecule in the S-cage. The Raman spectrum in range B changes drastically from that of ranges A and C. The Raman peak of CH4 in the S-cage shows much stronger intensity than that of L-cage. In addition, the peak (2903 cm−1 ) in range B is 1 cm−1 lower than that of ranges A and C. The Raman peak of CH4 is less sensitive to the size between L-cage and M-cage, because the size of M-cage is large enough for CH4 . Fig. 3 shows the peak area ratio of CH4 in the large cage to the S-cage. The open and solid circles stand for the area ratio of s-I and s-II hydrates, respectively. The ratio increases mono-
Y. Kunita et al. / Fluid Phase Equilibria 251 (2007) 145–148
147
Fig. 4. Isothermal phase equilibria for the CH4 + CF4 mixed-gas hydrate system at 291.1 K. Table 1 Isothermal phase equilibria for the CH4 + CF4 mixed-gas hydrate system at 291.1 K
Fig. 2. Raman spectra of the intramolecular C–H stretching vibration mode for CH4 in the mixed-gas hydrate phase at 291.1 K.
tonically in proportion to the CH4 composition in range A. In range B, the ratio drastically changes and the amount of CH4 in the S-cage is much larger than that of the large cage. This distinction between ranges A and B corresponds to the ratio of large cages to S-cages of the s-II hydrate (1:2) whereas the s-I hydrate (3:1). The ratio increases suddenly at the next transi-
p (MPa)
yCH4
zCH4
The s-I range (range C) 18.5 18.7 18.8
1.0 0.95 0.90
1.0 0.97 0.94
The s-II range (range B) 19.2 20.2 25.7
0.67 0.47 0.32
0.77 0.68 0.65
The s-I range (range A) 27.0 31.3 35.7 49.2 90.0
0.27 0.18 0.12 0.04 0.00
0.49 0.44 0.39 0.27 0.00
tion point between ranges B and C. This discontinuous change of cage occupancy also supports the existence of two structural phase-transition points. Isothermal phase equilibria for the CH4 + CF4 mixed-gas hydrate system at 291.1 K are shown in Fig. 4 and summarized in Table 1. The Raman-peak areas of the C–H symmetric stretching vibration of CH4 and the C–F symmetric stretching vibration of CF4 are used to evaluate the equilibrium compositions of gas and hydrate phases. Equilibrium curves have two discontinuous points that correspond to two hydrate-structural transition points around (1) p = 26 MPa, y = 0.3 and (2) p = 19 MPa, y = 0.8. 4. Conclusions
Fig. 3. Peak area ratio of CH4 in the CH4 + CF4 mixed-gas hydrate crystals under the three-phase coexistence at 291.1 K.
The single crystals of CH4 + CF4 mixed-gas hydrate were analyzed by use of Raman spectroscopy in order to elucidate structural phase-transition. Both Raman spectra of CH4 and CF4 detected in the mixed-gas hydrate phase change drastically within the equilibrium CH4 composition of 0.3–0.8 in the gas phase. These results reveal that the structure of mixed gas-
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hydrate is transformed from the structure-I to the structure-II and back to the structure-I with increase of CH4 composition. Acknowledgements The authors are grateful to the Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University for the scientific support by “Gas-Hydrate Analyzing System (GHAS).” One of the authors (T.M.) expresses his special thanks for the center of excellence (21COE) program “Creation of Integrated EcoChemistry of Osaka University”. List of symbols A Raman-peak area p pressure (Pa) T temperature (K) y composition of CH4 in the mixed-gas phase z composition of CH4 in the mixed-gas hydrate phase
References [1] M. von Stackelberg, W. Jahns, Z. Elektrochem. 58 (1954) 162– 164. [2] J.H. van der Waals, J.C. Platteeuw, Adv. Chem. Phys. 2 (1959) 1– 57. [3] S. Subramanian, R.A. Kini, S.F. Dec, E.D. Sloan Jr., Chem. Eng. Sci. 55 (2000) 1981–1999. [4] S. Subramanian, A.L. Ballard, R.A. Kini, S.F. Dec, E.D. Sloan Jr., Chem. Eng. Sci. 55 (2000) 5763–5771. [5] A.L. Ballard, E.D. Sloan Jr., Chem. Eng. Sci. 55 (2000) 5773–5782. [6] T. Makino, M. Tongu, T. Sugahara, K. Ohgaki, Fluid Phase Equilib. 233 (2005) 129–133. [7] T. Sugahara, T. Makino, K. Ohgaki, Fluid Phase Equilib. 206 (2003) 117–126. [8] C.-F. Ma, G.-J. Chen, F. Wang, C.-Y. Sun, T.-M. Guo, Fluid Phase Equilib. 191 (2001) 41–47. [9] M.M. Mooijer-van den Heuvel, C.J. Peters, J. de Swaan Arons, Fluid Phase Equilib. 172 (2000) 73–91. [10] K. Sugahara, M. Yoshida, T. Sugahara, K. Ohgaki, J. Chem. Eng. Data 49 (2004) 326–329.