6th International Conference on Gas Hydrates


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   DISSOCIATION HEAT OF MIXED-GAS HYDRATE COMPOSED OF METHANE AND ETHANE   Akihiro Hachikubo∗, Ryo Nakagawa, Daisuke Kubota, Hirotoshi Sakagami, Nobuo Takahashi and Hitoshi Shoji  Kitami Institute of Technology 165 Koen-cho, Kitami 090-8507, JAPAN   ABSTRACT Enormous amount of latent heat generates/absorbs at the formation/dissociation process of gas hydrates and controlls their thermal condition themselves. In this paper we investigated the effect of ethane concentration on dissociation heat of mixed-gas (methane and ethane) hydrate. It has been reported by researchers that a structure II gas hydrate appears in appropriate gas composition of methane and ethane. We confirmed by using Raman spectroscopy that our samples had the following three patterns: structure I only, structure II only and mixture of structures I and II. Dissociation heats of the mixed-gas hydrates were within the range between those of pure methane and ethane hydrates and increased with ethane concentration. In most cases two peaks of heat flow appeared and the dissociation process was divided into two parts. This can be understood in the following explanation that (1) the sample contained both crystal structures, and/or (2) ethane-rich gas hydrate formed simultaneously from dissociated gas and showed the second peak of heat flow.  Keywords: gas hydrates, dissociation heat, Raman, crystal structure, methane, ethane   ∗ Corresponding author: Phone: +81 157 26 9522 Fax +81 157 26 9534 E-mail: hachi@mail.kitami-it.ac.jp INTRODUCTION Formation and dissociation processes of natural gas hydrates in permafrost, marine and lake sediments are strongly controlled by their thermal properties. Dissociation heat of gas hydrates can be estimated from phase equilibrium data using the Clausius-Clapeyron equation [1-2], however, this method is applicable for pure gas hydrate and at a temperature of 0˚C. On the other hand, direct calorimetric measurements on gas hydrates using calorimeter have been developed to obtain thermal properties of gas hydrates: dissociation heat and heat capacity. Handa first applied a Tian-Calvet heat-flow calorimeter to obtain dissociation heat and heat capacity of methane and ethane hydrates under low temperature and high pressure conditions where these hydrates are stable [3]. Recently, thermal properties of synthetic gas hydrates using calorimeter have been investigated by researchers [4-6]. Kida reported that natural gas hydrate obtained at Kukuy K-2 in Lake Baikal contains both structures I and II by CP-MAS 13C NMR spectroscopy [7]. A difference of crystal structure may affect dissociation heat of gas hydrate, which determines its own thermodynamic stability. In this study, an effect of gas composition on dissociation heat of methane and ethane mixed-gas hydrate was investigated.  EXPERIMENTAL PROCEDURES  Sample Preparation Samples of mixed-gas (methane and ethane) hydrate were formed in a cylindrical pressure cell made of stainless steel (volume capacity 120 ml) with an agitation system that used in our previous study [8]. Temperature was measured with a platinum thermometer to an accuracy of ±0.1˚C Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. and pressure with a pressure gauge having a resolution of 0.01 MPa. The cell was immersed in a temperature controlled liquid bath and the temperature was maintained with an accuracy of ±0.1˚C. A magnetic stirring device agitated liquid water for enhancement of mixing of gas and liquid phases at a temperature of +1.0˚C. Research-grade methane with a purity of 99.999 mol% and ethane with a purity of 99.9 mol% (Takachiho Chemical Industry Co. Ltd., Tokyo, Japan) were used. 10g of Liquid water that degassed and purified by distillation and deionization placed in the cell and pressurized by the mixed-gases: six different concentrations of methane and ethane. Initial pressure was from 4.6MPa to 5.1MPa, except for an ethane-rich sample to avoid liquefaction of ethane. After the well agitation of gas and water, a nucleation of gas hydrate was occurred by an artificial vibration. The pressure started to decrease immediately and approached a steady- state value. The temperature was maintained for about 24 hours and then both phases of gas and hydrate were sampled. Gas composition of hydrate phase was obtained by a gas chromatograph (GC- 14B; Shimadzu Corp.) equipped with a flame ionization detector and a packed column (Sunpak- S, Shimadzu Corp.).  Raman Spectra Raman spectra of the hydrate samples were obtained with a spectrometer (RMP-210; JASCO Corporation) using a green laser (emitting a 532 nm line and providing 100mW at the sample) as an excitation source. The spectrometer has a single- dispersed monochromator system equipped with 1800 grooves mm-1 holographic diffraction grating device and thermoelectrically cooled CCD detector. The measurements were performed in the range 980 to 1020 cm-1 to check the C-C stretching peaks of ethane [9-10]. The spectra were collected with a 1.2 cm-1 resolution. The Raman scattering line of a polypropylene standard sample (wave number: 1460cm-1) was measured for the routine calibration of the monochromator. The temperature of the samples was maintained at −150˚C by using an isothermal device (10002; Japan High Tech Co., Ltd.).  Calorimetric measurements A diagram of the experimental apparatus is schematically shown in Figure 1. The experimental setup was similar to that used in our previous study [11] except a gas sampling port. The Tian-Calvet type heat-flow calorimeter (BT2.15; Setaram Instrumentation) was used for a thermodynamic measurement of gas hydrate under high pressure and low temperature conditions. The sample cell is a small pressure chamber for the calorimeter (volume capacity: 3.7ml) and durable to a maximum pressure of 50MPa. The hydrate samples were ground up well in liquid nitrogen, put into the cell and set into the pre-cooled calorimeter at −180˚C. The internal pressure was monitored by a pressure gauge (AP-10S; KEYENCE, resolution: 0.1kPa). The other pressure cell (volume capacity: 155.57ml) was connected for a smooth dissociation of the hydrate sample and an expansion of dissociated gas [11- 12]. The reference cell was filled with nitrogen to a pressure of 0.1MPa at room temperature. The calorimeter was then heated from −180˚C to +25˚C at the rate 0.15˚C min-1 to dissociate gas hydrate sample. After the completion of hydrate dissociation and melting ice, the internal pressure was recorded and the water in the cell was weighed. The amount of gas released from the hydrate phase was calculated from the pressure, temperature and volume data. Dissociation heat (kJ mol-1) was calculated by an integration of the peak of heat flow and the amount of dissociated gas. To check the gas composition 1 μl of dissociation gas was sampled every 15 min and ethane concentration was measured by the gas chromatograph. Pressure Cell (155.57 ml) Gas Sampling Port Calorimeter Sample Cell SUS Tube (volume: 8.55ml) Vacuum Pump Thermometer Pressure Gauge Reference Cell Figure 1 Schematic diagram of the calorimeter.  RESULTS AND DISCUSSIONS Figure 2 shows the C-C stretching region of the Raman spectra for ethane trapped in cages of gas hydrates. The ethane concentrations of the hydrate phase are indicated in this graph. Since structure II (sII) gas hydrate appears in an appropriate composition of ethane and a peak caused by the C- C stretch shifts according to the crystal structure [9], we can identify the crystal structure from the Raman band frequencies for ethane. In Figure 2 the peaks of 1000.5 cm-1 and 991.3 cm-1 correspond to sI and sII, respectively, and agree with the previous results obtained by [9-10]. The hydrate samples those ethane concentrations were 0% (pure methane, no peak in this region), 2.7%, 71.0% and 100% (pure ethane) belonged to sI, whereas those of 17.9% and 24.7% belonged to sII. The samples of 12.5% and 41.3% were mixture of sI and sII because both peaks appeared. At the formation process of the mixed-gas hydrate the ethane composition decreased with gas consumption due to the preferential consumption of ethane to the hydrate phase in the batch-type reactor. Pressure and heat flow changes in the dissociation process are plotted against temperature in Figure 3 and Figure 4, respectively. The pressure increased with temperature along with the equilibrium pressure for each gas composition. The pressure curves for pure methane and ethane were always lower than those of equilibrium values [13] due to nonequilibrium condition. In the dissociation processes of 2.7%C2, 12.5%C2, 24.7%C2 and 41.3%C2 samples the air leaked in the sample cell from the outside at the first stage of the dissociation processes. It is unsettled question why the pressure curves for 12.5%C2, 17.9%C2, 24.7%C2 and 41.3%C2 samples agreed each other below −50˚C. Distinctive dissociation started from −130˚C to −100˚C that depended on ethane concentration and heat flow in Figure 4 decreased with temperature since dissociation heat should be supplied from the outside of the cell. Except pure gases and 71.0%C2 samples, two stages of dissociation were found in these graphs. It is possible that the samples of 12.5% and 41.3% had two peaks those corresponded to each crystal structure, but other samples also had the same two peaks. The base lines of the heat flow gradually decreased according to an increase of specific heat of samples. There are not so big differences between the specific heats (J kg-1) of gas hydrate and ice as reported by [3]. After the dissociation of hydrate samples into gas and ice, the ice melted at 0˚C and large amount of heat of fusion absorbed. Very small increases in 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 -200 -150 -100 -50 0 50 Temperature (℃) P re ss ur e (M P a) 0%C2 (100%C1) 2.7%C2 12.5%C2 17.9%C2 24.7%C2 41.3%C2 71.0%C2 100%C2 Falabella (1975) CH4-H Falabella (1975) C2H6-H 980990100010101020 Raman Shift (cm-1) In te ns ity  (a .u .) 0%C2 2.7%C2 12.5%C2 17.9%C2 24.7%C2 41.3%C2 71.0%C2 100%C2 Figure 2 Raman spectra of the C-C stretching mode for C1 and C2 mixed-gas hydrates. Figure 3 Pressure changes plotted against temperature at the dissociation process of the samples. -200 -150 -100 -50 0 50 Temperature (℃) H ea t F lo w 0%C2 2.7%C2 12.5%C2 17.9%C2 24.7%C2 41.3%C2 71.0%C2 100%C2 Figure 4 Heat flow changes plotted against temperature at the dissociation process of the samples. pressure at 0˚C were due to dissociation of gas hydrate survived in ice. Handa described the same phenomena [12] when the sample contained large crystals due to self-preservation effect [14]. Figure 5 shows a time variation of heat flow and ethane concentration of the dissociation gas in the case 41.3%C2. Although the ethane concentration for total dissociation gas must be 41.3%C2, methane-rich part first dissociated and the ethane concentration increased with temperature. In the first peak of dissociation the ethane concentration increased gradually from 5%C2 to 20%C2, and rapidly increased and reached to 70%C2 in the second peak. There are the following two possibilities: (1) At the first stage of gas hydrate formation in the batch-type reactor ethane was preferentially consumed and fixed in the hydrate phase. In the case of 41.3%C2 sample sI hydrate (ethane rich, relatively) first formed and then sII hydrate (less concentration of ethane than sI) formed. In the other case of 12.5%C2 sample sII hydrate (ethane rich, relatively) first formed and then sI hydrate (less concentration of ethane than sII) formed. These two peaks in the heat flow are due to the difference of crystal structures. (2) When the mixed-gas hydrates started to dissociate, new gas hydrate could form from the dissociation gas. If the partial pressure of ethane in the gas phase was enough higher than the equilibrium pressure of pure ethane hydrate, the original hydrate dissociated and the new hydrate, of which ethane concentration seemed very high, formed simultaneously. The secondary hydrates then dissociated at higher temperature and formed the second peaks in Figure 4. Dissociation heat of gas hydrate increased with ethane concentration (Figure 6). Those of pure methane and ethane hydrates agreed fairly well with the results of the previous study [3]. This tendency seems to be primarily due to an increase in hydration number. Ethane concentration effect on the cage occupancies and hydration number was reported by [9, 15] and indicated that the hydration number increases with ethane concentration in the field of sII.  CONCLUDING REMARKS Dissociation heats of methane and ethane mixed- gas hydrate were measured by using a heat flow calorimeter. In most cases two stages of dissociation were found at the dissociation process. We need to confirm in the next step of this study whether the dissociation of mixed-gas hydrate and the secondary formation of ethane-rich hydrate can occur simultaneously or not. Besides this, gas hydrate samples of which ethane concentration has narrow distribution will be needed for the future study to check the effects of crystal structure.  ACKNOWLEDGMENTS This work was supported by funding agencies: Japan Society for the Promotion of Science KAKENHI 18206099, the Ministry of Education, Culture, Sports, Science and Technology KAKENHI 19740323, and Kitami Institute of Technology Presidential Grant.  REFERENCES [1] Deaton WM, Frost EM. Gas hydrates and their relation to the operation of natural-gas pipe lines. U. S. Bureau of Mines Monograph 8, 1946. 14 16 18 20 22 24 26 28 0 20 40 60 80 100 Composition of C2H6 (mol%) D is so ci at io n H ea t ( kJ  m ol  -1 ) This study Handa (1986) Ⅰ ⅠⅡⅠ+Ⅱ Ⅰ+ⅡDis so ci at io n H ea t ( kJ  m ol  -1 ) -50 -40 -30 -20 -10 0 -120 -100 -80 -60 -40 -20 0 20 Temperature (℃) H ea t F lo w  (m W ) 0 20 40 60 80 100 C 2H 6 R at io  (% ) Heat Flow C2H6 Ratio Figure 5 Heat flow and C2 concentration changes plotted against temperature in the case 41.3%C2 sample. Figure 6 Dissociation heats of C1 and C2 mixed- gas hydrates plotted against C2 concentration in these samples. [2] Lason SD. Phase studies of the two-component carbon dioxide-water system, involving the carbon dioxide hydrate. Ph.D. thesis, University of Illinois, Doctoral Dissertation Published No. 15235, University Microfilms, 1955. [3] Handa YP. Compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of methane, ethane, and propane, and enthalpy of dissociation of isobutane hydrate, as determined by a heat-flow calorimeter. J. Chem. Thermodynamics 1986; 18: 915−921. [4] Rueff RM, Sloan ED. Heat capacity and heat of dissociation of methane hydrates. AIChE Journal 1988; 34(9): 1468−1476. [5] Kang SP, Lee H, Ryu BJ. Enthalpies of dissociation of clathrate hydrates of carbon dioxide, nitrogen, (carbon dioxide + nitrogen), and (carbon dioxide + nitrogen + tetrahydrofuran). J. Chem. Thermodynamics 2001; 33: 513−521. [6] Dalmazzone D, Kharrat M, Fouconnier B, Clausse D. Thermodynamic study of several hydrates in dispersed systems using differential scanning calorimetry. In: Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, May 19−23, pp.331−336, 2002. [7] Kida M, Khlystov O, Zemskaya T, Takahashi N. Minami H, Sakagami H, Krylov A, Hachikubo A, Yamashita S, Shoji H, Poort J, Naudts L. Coexistence of structure I and II gas hydrates in Lake Baikal suggesting gas sources from microbial and thermogenic origin. Geophys. Res. Lett. 2006; 33: L24603, doi:10.1029/2006 GL028296. [8] Hachikubo A, Kosaka T, Kida M, Krylov A, Sakagami H, Minami H, Takahashi N, Shoji H. Isotopic fractionation of methane and ethane hydrates between gas and hydrate phases. Geophys. Res. Lett. 2007; 34: L21502, doi:10.1029/2007GL030557. [9] Subramanian S, Kini RA, Dec SF, Sloan ED. Evidence of structure II hydrate formation from methane + ethane mixtures. Chemical Engineering Science 2000; 55: 1981−1999. [10] Uchida T, Takeya S, Kamata Y, Ikeda IY, Nagao J, Ebinuma T, Narita H, Zatsepina O, Buffett BA. Spectroscopic observations and thermodynamic calculations on clathrate hydrates of mixed gas containing methane and ethane: determination of structure, composition and cage occupancy. J. Phys. Chem. B 2002; 106: 12426−12431. [11] Hachikubo A, Watanabe T, Hyakutake K, Abe K, Shoji H. Calorimetric measurements of gas hydrate containing ice. In: Proceedings of the 5th International Conference on Gas Hydrates, Trondheim, June 13−16, pp.1508−1511, 2005. [12] Handa YP. Calorimetric determinations of the compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of xenon and krypton. J. Chem. Thermodynamics 1986; 18: 891−902. [13] Falabella BJ. Ph.D. Thesis, University of Massachusetts, University Microfilms No. 76−5849; Ann Arbor, MI., 1975. [14] Yakushev VS, Istomin VA. Gas-hydrates self-preservation effect. Physics and Chemistry of Ice, eds. by N. Maeno and T. Hondoh, Hokkaido University Press, Sapporo, 1992. p.136−140. [15] Kida M, Sakagami H, Takahashi N, Hachikubo A, Shoji H, Kamata Y, Ebinuma T, Narita H, Takeya S. Estimation of Gas Composition and Cage Occupancies in CH4 − C2H6 Hydrates by CP-MAS 13C NMR Technique. J. Jpn. 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