6th International Conference on Gas Hydrates

DISSOCIATION AND SPECIFIC HEATS OF GAS HYDRATES UNDER SUBMARINE AND SUBLACUSTRINE ENVIRONMENTS Nakagawa, Ryo; Hachikubo, Akihiro; Shoji, Hitoshi 2008

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   DISSOCIATION AND SPECIFIC HEATS OF GAS HYDRATES UNDER SUBMARINE AND SUBLACUSTRINE ENVIRONMENTS   Ryo Nakagawa*, Akihiro Hachikubo and Hitoshi Shoji  Kitami Institute of Technology 165 Koen-cho, Kitami 090-8507, JAPAN   ABSTRACT Dissociation and specific heats of synthetic methane and ethane hydrates were measured under high-pressure condition by using a heat-flow type calorimeter to understand thermodynamic properties of gas hydrates under submarine/sublacustrine environments. Ice powder was put into the sample cell and pressurized by methane and ethane up to 5MPa and 2MPa, respectively. After the completion of gas hydrate formation, samples were heated from 263K to 288K at the rate of 0.01 K min-1. Large negative peaks of heat flow corresponded to the dissociation of gas hydrates were detected in a temperature range 279-282K at a pressure of 5MPa for methane hydrate and 283-286K at 2MPa for ethane hydrate, respectively. We also obtained the specific heats of gas hydrates in the range 264-276K for methane and 264-282K for ethane under pressure.  Keywords: gas hydrates, dissociation heat, specific heat, methane, ethane                                                     ∗ Corresponding author: Phone: +81 157 26 9533 Fax +81 157 26 9534 E-mail: mcv07011@std.kitami-it.ac.jp INTRODUCTION Gas hydrates that are stable at low temperature and high-pressure conditions exist in submarine and sublacustrine sediments. From the viewpoint of maintenance process of gas hydrates in the sediments, thermodynamic properties (e.g. latent heat, specific heat, etc.) play important roles. Recently, natural gas hydrates have been found in the lake-bottom sediments at Lake Baikal, Russia [1-7]. Although the main component of the natural gas hydrate is methane, it was confirmed that natural gas hydrate discovered in Kukuy K-2 mud volcano of Lake Baikal contained high concentration (up to 15%) of ethane [5-8]. Gas hydrates release / absorb large latent heat at their formation / dissociation processes, respectively. To understand the thermal environment of gas hydrate bearing sediments, thermodynamic properties of gas hydrates are required quantitatively. Dissociation heat (latent heat) of gas hydrates can be estimated from phase equilibrium data using the Clausius-Clapeyron equation [9-10]. This method was improved and it was proposed that a technique employing the Clapeyron equation is preferred to the use of Clausius-Clapeyron equation for determining dissociation heat of gas hydrates [11]. In the 1980s direct calorimetric measurements on gas hydrates using a heat-flow type calorimeter have been developed to obtain such thermodynamic properties of gas hydrates [12-14]. Enthalpies of dissociation of methane and ethane hydrates were measured under low temperature and heat capacities of them were also measured in the temperature range from 85K to 270K and from 85K to 260K, respectively [14]. However, dissociation heats (from gas hydrate to gas and water) were obtained by an indirect method: summation of fusion heat of ice and dissociation heat from hydrate to gas and ice. In this study, dissociation and specific heats of synthetic (pure) methane and ethane hydrates were measured in the temperature range from 263K to 283K under high pressure by using a heat-flow type calorimeter to understand the stability of natural gas hydrates under submarine and sublacustrine environments. Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  EXPERIMENTAL METHODS  Sample materials Gas hydrate samples were formed from ice powder (grain size: less than 0.05mm) and methane or ethane in a pressure cell. Fine ice powder was made from water purified by distillation and deionization (GSH-210; ADVANTEC), and obtained by shaving an ice sample using a microtome. Research grade methane with a purity of 99.999 mol% and ethane with a purity of 99.9 mol% were obtained from Takachiho Chemical Industry Co., Ltd.   Experimental apparatus  A diagram of the calorimeter is schematically shown in Figure 1. The heat-flow calorimeter of Tian-Calvet type (BT2.15; Setaram Instrumentation) was used for thermodynamic measurements of gas hydrates under high-pressure and low-temperature conditions. The sample cell is a small pressure chamber for the calorimeter (volume capacity of 3.7ml) and durable to a maximum pressure of 50MPa. The internal pressure was monitored by a pressure gauge (AP-14S; KEYENCE Corporation, resolution: 0.01MPa). The heat-flow calorimeter was located in a cold room (temperature: 255K). After the preparation of the sample cell it was connected with a pressure gauge, a vacuum pump and the other pressure chamber (volume capacity of 155.25ml) by a manifold. The system was put in an insulation box to keep constant temperature. The other pressure chamber was connected for a smooth dissociation of the hydrate sample and an expansion of dissociated gas [13]. The other cell for reference was filled with pure nitrogen to a pressure of 0.1MPa at room temperature.  Experimental procedure Fine ice powder (about 1.5g) was put into the sample cell in the cold room (temperature: 255K) and the air inside the cell was vacuumed. The cell was set in the calorimeter and pressurized by methane / ethane up to 5MPa / 2MPa, respectively. The calorimeter was then heated from 263K to 278K at the rate of 0.01 K min-1 to melt the ice powder and enhance gas hydrate formation. The sample was cooled to 263K to check residual liquid water and heated again from 263K to 288K at the same rate of 0.01 K min-1. After the completion of hydrate dissociation, the internal pressure was recorded and the water in the cell was weighed. The amount of gas released from the hydrate was calculated from the pressure, temperature and volume data by using an equation of state [15]. Dissociation heat was calculated by an integration of the peak of heat flow and the amount of dissociated gas. On the other hand, specific heat of gas hydrate was calculated from the heat flow data in the ramp mode (0.01 K min-1). Each sample run was followed by a blank run (both cells were empty) with the same temperature controlling method. Specific heat of gas hydrate Cp (J kg-1 K-1) was calculated as  ZmEECp '−=                                                 (1)  where E and E’ (W) are the heat flows during the sample and the blank runs, respectively, Z (K s-1) is the rate of increase of temperature, and m (kg) is the amount of gas hydrate sample.  RESULTS AND DISCUSSION Experimental runs were executed three times for methane hydrate and two times for ethane hydrate. Peaks of heat flow did not appear at 273K when the sample was cooled and heated again, hence we can say that the hydrate samples did not contain any residual ice or liquid water. Figure 2 shows an example of thermographs when gas hydrate samples dissociated at the last stage of the calorimetry. Large negative peaks of heat flow Figure 1 Schematic diagram of the calorimetric system. PressurechamberVacuumpumpPressuregaugeThermometerCalorimeterThermopileInsulation box(298±0.6K)SamplecellReference   cell  those corresponded to the dissociation heat of gas hydrate were detected in a temperature range 279-282K at a pressure of about 5MPa for methane hydrate and 283-286K at 2MPa for ethane hydrate, respectively. Dissociation heat (kJ mol-1) was calculated by an integration of the peak of heat flow (kJ) and the amount of dissociated gas (mol); those from methane and ethane hydrates to water and gases were 55.3 (kJ mol-1) and 71.1 (kJ mol-1), respectively, which agreed well with 54.19±0.28 (kJ mol-1) and 71.80±0.38 (kJ mol-1) obtained by the previous study [14]. Figure 3 shows the results of specific heat plotted against temperature. While Handa obtained the specific heat of hydrates in a temperature range 85-270K for methane and 85-260K for ethane, respectively, we obtained them in the range 264-276K for methane and 264-282K for ethane. The specific heats (kJ kg-1 K-1) of methane and ethane hydrates were fitted to the following equations:  215.2013.0 += TCp     (methane hydrate)   (2) 070.2019.0 += TCp     (ethane hydrate)      (3)  where T is the temperature (°C). Though these results seemed to be slightly larger than those of the previous study [14], agree with them within the error of the data.  CONCLUDING REMARKS We obtained the dissociation and specific heats of methane and ethane hydrates by using heat flow calorimeter. The condition of temperature seemed to cover the bottom water temperatures in nature, but the pressure condition was not enough. It is possible to execute the same measurements under higher pressure (up to 20MPa, which corresponds to a water depth of 2000m) in future.  ACKNOWLEDGMENTS This work was supported by funding agencies: Japan Society for the Promotion of Science KAKENHI 18206099, the Ministry of Education, -120-90-60-30030263 268 273 278 283 288TemperatureHeat flow (mW)(a)-120-90-60-30030263 268 273 278 283 288TemperatureHeat flow (mW)(b)Heat flow (mW)Heat flow (mW)Figure 2 Examples of heat flow plotted against temperature. (a) methane hydrate, (b) ethane hydrate. 0123263 268 273 278 283Temperature (K)Specific heat (kJ kg-1 K-1) (a)0123263 268 273 278 283Temperature (K)Specific heat (kJ kg-1 K-1) (b)Specific heat (kJ kg-1 K-1)Specific heat (kJ kg-1 K-1)Figure 3 Specific heat of gas hydrates plotted against temperature. (a) methane hydrate, (b) ethane hydrate. Broken line: previous study by [14]. Culture, Sports, Science and Technology KAKENHI 15760640 and 19740323, and Kitami Institute of Technology Presidential Grant.  REFERENCES [1] Van Rensbergen P, De Batist M, Klerkx J, Hus R, Poort J, Vanneste M, Granin N, Khlystov O, Krinitsky P. Sublacustrine mud volcanoes and methane seeps caused by dissociation of gas hydrates in Lake Baikal. Geology 2002; 30: 631–634. [2] Klerkx J, Zemskaya TI, Matveeva TV, Khlystov OM, Grachev MA, Namsaraev BB, Dagurova OP, Golobokova LP, Vorobiev SS, Pogodaeva TP, Granin NG, Kalmychkov GV, Ponomarchuk VA, Shoji H, Mazurenko LL, Kaulio VV, Soloviev VA. Methane hydrates in surface layer of deep-water sediments in Lake Baikal (in Russian). Dokl. Akad. Nauk 2003; 393: 822–826. [3] Matveeva TV, Mazurenko LL, Soloviev VA, Klerkx J, Kaulio VV,  Prasolov EM. Gas hydrate accumulation in the subsurface sediments of Lake Baikal (eastern Siberia). Geo Mar. Lett. 2003; 23: 289–299. [4] De Batist M, Naudts L, Criel W, Klerkx J, Poort J, Van Rensbergen P, Granin N, Chensky A, Gnatovsky R. Mud volcanoes, gas seeps and gas hydrates in Lake Baikal—A review. paper presented at Fourth Vereshchagin Baikal Conference, Limnol. Inst. of the Siberian Branch of the Russ. Acad. of Sci., Irkutsk, Russia, 26 Sept. to 1 Oct., 2005. [5] Kalmychkov GV, Egorov AV, Khlystov OM. Isotope characteristics of Baikal methane (Russia, eastern Siberia). paper presented at Fourth Vereshchagin Baikal Conference, Limnol. Inst. of the Siberian Branch of the Russ. Acad. of Sci., Irkutsk, Russia, 26 Sept. to 1 Oct., 2005. [6] Khlystov OM. New finds of gas hydrates in Lake Baikal. paper presented at Fourth Vereshchagin Baikal Conference, Limnol. Inst. of the Siberian Branch of the Russ. Acad. of Sci., Irkutsk, Russia, 26 Sept. to 1 Oct., 2005. [7] Khlystov OM. New findings of gas hydrates in the Baikal bottom sediments. Russ. Geol. Geophys. 2006; 47: 979–981. [8] 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. [9] 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. [10] 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. [11] Anderson GK. Enthalpy of dissociation and hydration number of carbon dioxide hydrate from the Clapeyron equation. J. Chem. Thermo-dynamics 2003; 35: 1171-1183. [12] Handa YP, Hawkins RE, Murray JJ. Calibration and testing of a Tian-Calvet heat-flow calorimeter Enthalpies of fusion and heat capacities for ice and tetrahydrofuran hydrate in the range 85 to 270K. J. Chem. Thermodynamics 1984; 16: 623-632. [13] Handa YP. Calorimetric determinations of the compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270K for clathrate hydrates of xenon and krypton. J. Chem. Thermodynamics 1986; 18: 891-902. [14] 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. [15] 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.  

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