Article pubs.acs.org/jced
Enclathration of 2,3,3,3-Tetrafluoropropene into Structure-II Hydrate with Methane Yoshito Katsuta,† Shunsuke Hashimoto,†,§ Teruo Kido,‡ Takeshi Sugahara,*,† and Kazunari Ohgaki† †
Division of Chemical Engineering, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan ‡ Environmental Technology Laboratory, Daikin Industries, Ltd., 1304 Kanaoka, Kita-ku, Sakai, Osaka 591-8511, Japan ABSTRACT: The enclathration of 2,3,3,3-tetrafluoropropene (HFO-1234yf) has been investigated. In the binary system of HFO-1234yf+H2O, simple HFO-1234yf hydrate is not formed even at 50 MPa and 274.15 K. The addition of methane (CH4) as a help molecule results in the structure-II CH4+HFO-1234yf mixed-gas hydrate formation. With difluoromethane (HFC-32), instead of CH4, no HFC32+HFO-1234yf mixed-gas hydrate is formed. Isothermal phase equilibria for the CH4+HFO-1234yf+H2O ternary system were measured at 279.15 K. Based on the extrapolation of two three-phase {(hydrate, aqueous, and gas phases) and (aqueous, HFO-1234yf-rich liquid, and gas phases)} equilibrium curves, the quadruple point of hydrate, aqueous, HFO-1234yf-rich liquid, and gas phases was determined.
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INTRODUCTION Gas hydrate is an inclusion compound and it consists of host water molecules and guest species. Hydrogen-bonded water molecules construct hydrate lattices with cavities, in which guest species are enclathrated. Generally, the van der Waals force interacts between the guest species and water molecules. The unit cell of structure-I {space group, Pm3n (s-I)} is composed of 2 small cages {pentagonal dodecahedron (512, Scage)} and 6 large cages {tetrakaidecahedron (51262, M-cage)}, and that of the structure-II {space group, Fd3m (s-II)} hydrate is composed of 16 S-cages and 8 large cages {hexakaidexahedron (51264, L-cage)}.1 The hydrate structure mainly depends on the size and shape of guest species in addition to temperature and pressure. Hydrofluorocarbons (HFCs) used as a heat medium have large vaporization enthalpy, although HFCs have high global warming potential (GWP). 1,1,1,2-Tetrafluoroethane (HFC134a) is used as a conditioner refrigerant. The GWP of HFC134a (1430 for a 100 year time horizon) is quite large, and the European Union has gradually regulated refrigerants with GWP over 150 in automobile air conditioners since 2011. Therefore, it is necessary to find alternatives to HFC-134a. The GWP of 2,3,3,3-tetrafluoropropene (HFO-1234yf) is 4 (for a 100 year time horizon).2 Moreover, the properties (saturated vapor pressure, density, enthalpy of vaporization, and surface tension) are quite similar to HFC-134a.3 Therefore, HFO-1234yf is expected as an alternative for HFC-134a. Recently, many researchers have studied the utilization of gas hydrates as a thermal storage medium because of their large latent heat.4 The dissociation enthalpy of HFC hydrates is larger than the vaporization enthalpy of HFC fluids.4 The equilibrium pressures of some HFC hydrates are relatively lower (less than 1 MPa) than those of conventional hydrocarbon gas hydrates. The usage of HFC hydrates is expected to enhance cooling processes more efficiently. © XXXX American Chemical Society
However, the existence of gas hydrates enclathrating HFO1234yf has not been reported yet. The purpose of the present study is to investigate the enclathration of HFO-1234yf into hydrate cages. Moreover, the hydrate structure was determined by use of powder X-ray diffraction (PXRD).
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EXPERIMENTAL SECTION The present experimental apparatus for the isothermal phase equilibrium measurement is the same as reported previously.5 The equilibrium temperature was measured by a thermistor probe (Takara, model D-632). The probe was calibrated with a Pt resistance thermometer defined by ITS-90. The programming thermocontroller (EYELA, model NCB-3100) adjusted the cell temperature. The equilibrium pressure was measured by a pressure gauge (Valcom, model VPRT) calibrated by a RUSKA quartz Bourdon tube gauge. The distilled water introduced into the high-pressure glass cell was sufficiently degassed. A gas mixture including CH4 and HFO-1234yf with a given composition was introduced into the cell up to a desired pressure. The contents were agitated by use of a mixing bar that was moved up-and-down by magnetic attraction from outside of the cell. In the measurement of the three-phase equilibrium relations, the content was cooled to generate hydrates or to condensate the gas mixtures of CH4 and HFO-1234yf, respectively. After the formation of hydrate or HFO-1234yf-rich liquid phases, the temperature was kept constant to establish the three-phase equilibrium state of {(hydrate, aqueous, and gas phases) or (aqueous, HFO-1234yfrich liquid, and gas phases)}. The phase behavior was observed directly through the glass wall under the transmitted light. After Received: May 18, 2013 Accepted: July 9, 2013
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dx.doi.org/10.1021/je400481r | J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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the system reached three-phase equilibrium, a small amount of gas phase was taken from the cell for the composition analysis by use of TCD gas chromatography (TCD-GC, model Shimadzu GC-7AG). A small amount of hydrate phase separated with aqueous phase was taken at 253 K after the hydrate was quenched, because only the solid hydrate sample cannot be directly taken from the cell. The hydrate sample was taken into a gas sampler precooled at 77 K. The air in the gas sampler was evacuated at 77 K to keep the hydrate. After that, the hydrate sample was allowed to dissociate at room temperature. The composition of the mixed gas released from the hydrate phase was also measured by the same TCD-GC. Once the hydrate sample for the PXRD measurement was taken from the cell at 253 K, the sample was grained in the mortar immersed in liquid nitrogen. The PXRD pattern was measured at 173 K and atmospheric pressure by use of a diffractometer (Rigaku, model Ultima IV) with a Rigaku D/teX ultra high-speed position sensitive detector and Cu Kα X-ray (40 kV, 50 mA). The PXRD measurements were performed in the stepscan mode with scan rate of 4 deg·min−1 and step size of 0.02°. The PXRD pattern indexing and cell refinement were obtained by use of the Chekcell6 and PowderX7 programs and the initial lattice parameters1 for the refinement. The materials used in the present study were summarized in Table 1. All of them were used without further purification.
Figure 1. PXRD profile of CH4+HFO-1234yf mixed-gas hydrate prepared at p = 4.25 MPa and T = 279.15 K (recorded at 173 K). The mole fraction of HFO-1234yf is 0.20. The vertical lines are derived from structure-II (top) and ice Ih (bottom).
Table 1. Information on the Chemicals Used in the Present Study chemical name 2,3,3,3tetrafluoropropene difluoromethane methane water
source
mole fraction purity
Daikin Ind., Ltd.
> 0.9995
Daikin Ind., Ltd. Liquid Gas Ltd. Wako Pure Chemicals Ind., Ltd.
> 0.9999 > 0.9999 > 0.9999
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RESULTS AND DISCUSSION First, to investigate the possibility of the simple HFO-1234yf hydrate formation, the liquefied HFO-1234yf was pressurized with distilled water at the isothermal condition 274.15 K. In this experiment, we used another high-pressure cell with sapphire windows.8 A ruby ball was introduced into the cell to intermittently agitate the liquid−liquid interfaces inside the cell by the vibration from the outside. After a week at 50 MPa under the isothermal condition of 274.15 K, the simple HFO1234yf hydrate was not formed. To further investigate the enclathration of the HFO-1234yf molecule under the coexistence of small molecules (so-called help molecules), CH4 and difluoromethane (HFC-32) were used. Under the existence of CH4, the CH4+HFO-1234yf mixedgas hydrate is formed at a pressure lower than the equilibrium pressure of simple CH4 hydrate at 279.15 K. The PXRD pattern of the CH4+HFO-1234yf mixed-gas hydrate prepared at 4.25 MPa and 279.15 K is shown in Figure 1. The mole fraction of HFO-1234yf in the hydrate is 0.20 on a water free basis. The PXRD pattern clearly reveals that the CH4+HFO1234yf mixed-gas hydrate is s-II. The lattice parameter of a = (1.727 ± 0.003) nm is similar to the literature value.1 Isothermal pressure (p)−composition (y, z) diagram in the ternary system of CH4(1), HFO-1234yf(2), and water at 279.15 K is shown in Figure 2. The horizontal axis in Figure 2
Figure 2. Isothermal phase equilibria in the ternary system of CH4(1)+HFO-1234yf(2)+water at 279.15 K. The symbols of circle and square stand for the equilibrium relations in the gas and hydrate phases, respectively. The dashed-dotted line represents the phase relation at the quadruple point, which is located at (0.92 ± 0.01) MPa and 279.15 K. The dotted line stands for the estimated equilibrium relation in HFO-1234yf-rich liquid phase calculated by NIST Standard Reference Database, REFPROP.10
indicates the equilibrium composition of HFO-1234yf on a water free basis. Tables 2 and 3 summarize the equilibrium phase relations of p−y and p−z, respectively. The symbols y and z stand for the mole fraction of HFO-1234yf in gas and hydrate phases on a water free basis, respectively. The equilibrium pressure of the s-II CH4+HFO-1234yf mixed-gas hydrate becomes lower with an increase in the composition of HFO-1234yf. As mentioned previously, the simple HFO1234yf hydrate is not formed. Therefore, there should be a pressure limit where the CH4+HFO-1234yf mixed-gas hydrate is thermodynamically stable. Based on the extrapolation of two three-phase equilibrium curves of {(hydrate, aqueous, and gas phases) and (aqueous, HFO-1234yf-rich liquid, and gas phases)}, the quadruple point of hydrate, aqueous, HFO1234yf-rich liquid, and gas phases is located at y2 = (0.465 ± 0.008), z2 = (0.51 ± 0.02), p = (0.92 ± 0.01) MPa, and T = B
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Table 2. Isothermal Phase Equilibrium Data (Pressure p, Mole Fraction y in the Gas Phase on a Water Free Basis) in the Ternary System of CH4 (1), HFO-1234yf (2), and Water at 279.15 Ka
Table 4. Isothermal Phase Equilibrium Data (Pressure p, Mole Fraction y in the Gas Phase on a Water Free Basis) in the Ternary System of HFC-32 (1), HFO-1234yf (2), and Water at 283.15 Ka
p/MPa
y2
p/MPa
y2
4.78 4.72 4.51 3.69 3.05 2.48 1.91 1.68 1.27 1.18 1.14 1.06 1.00 0.98 0.93 0.91b 0.68b 0.52b 0.40b
0.000 0.001 0.005 0.010 0.017 0.031 0.060 0.083 0.133 0.168 0.188 0.234 0.310 0.336 0.457 0.474b 0.572b 0.734b 1.000b
0.453 0.485 0.551 0.588 0.621
0.000 0.061 0.154 0.201 0.243
a
coefficient of HFC-32 in the binary and ternary systems (that is, without and with HFO-1234yf) was calculated by NIST Standard Reference Database, REFPROP.10,11 In the HFC32+HFO-1234yf+water ternary system, the fugacity of HFC-32 in the gas phase equilibrated with hydrate and aqueous phases is almost constant and equal to that in the simple HFC-32 hydrate system at the temperature. This result means that HFO-1234yf is not enclathrated into the hydrate cage in the presence of HFC-32 and HFO-1234yf behaves just like a diluent gas for the simple HFC-32 hydrate formation. The difference between HFC-32 and CH4 would be caused by the S-cage occupancy of HFC-32 lower than that of CH4.
a
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Standard uncertainties u are u(p) = 0.01 MPa and u(y) = 0.003. Coexisted phase other than gas phase is HFO-1234yf-rich liquid phase, not CH4+HFO-1234yf mixed-gas hydrate phase.
b
CONCLUSION In the presence of CH4 as a help gas, HFO-1234yf is enclathrated into the L-cage of the s-II CH4+HFO-1234yf mixed-gas hydrate. The pressure limit in the s-II CH4+HFO1234yf mixed-gas hydrate system, which is identical to the quadruple point of hydrate, aqueous, HFO-1234yf-rich liquid, and gas phases, was determined as (0.92 ± 0.01) MPa at 279.15 K. HFO-1234yf forms no clathrate hydrate without a help gas. HFC-32 does not play the role of the help gas in the HFO1234yf enclathration.
Table 3. Isothermal Phase Equilibrium Data (Pressure p, Mole Fraction z in the Hydrate Phase on a Water Free Basis) in the Ternary System of CH4 (1), HFO-1234yf (2), and Water at 279.15 Ka
a
p/MPa
z2
4.78 4.51 3.53 2.71 2.27 2.15 2.02 0.95 0.93
0.00 0.20 0.22 0.34 0.41 0.38 0.43 0.48 0.53
Standard uncertainties u are u(p) = 0.01 MPa and u(y) = 0.003.
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AUTHOR INFORMATION
Corresponding Author
*Tel./Fax.: +81-6-6850-6293. E-mail:
[email protected]. osaka-u.ac.jp. Present Address §
Thermal Management Lab. Toyota Central R&D Laboratories., Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan.
Standard uncertainties u are u(p) = 0.01 MPa and u(z) = 0.01.
Notes
The authors declare no competing financial interest.
279.15 K. At a pressure lower than the quadruple point, no CH4+HFO-1234yf mixed-gas hydrate exists. With HFC-32 instead of CH4, the HFC-32+HFO-1234yf mixed-gas hydrate was prepared from gas mixtures at 283.15 K. Table 4 summarizes the three-phase equilibrium pressure and the composition in the gas phase equilibrated with hydrate and aqueous phases in the ternary system of HFC-32(1)+HFO1234yf(2)+water. The three-phase equilibrium pressure of the formed hydrate is larger than that of the simple HFC-32 hydrate and increases monotonically with the increase in the composition of HFO-1234yf. The results imply that simple HFC-32 hydrate is generated without HFO-1234yf. To evaluate the contribution of HFO-1234yf, the equilibrium fugacity of HFC-32 in the HFC-32+HFO-1234yf mixed gas was calculated at the whole equilibrium pressure−composition relations obtained in the present study. The thermodynamic analysis has been reported in previous report.9 The fugacity
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ACKNOWLEDGMENTS We acknowledge the material supports from the Daikin Industries, Ltd. We also acknowledge the scientific supports from the “Gas-Hydrate Analyzing System (GHAS)” of the Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University and Rigaku Corporation (for the PXRD measurement at low temperatures). We thank Dr. H. Sato (Osaka University), and Mr. K. Nagao (Rigaku Corporation) for the valuable discussion and suggestions.
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