Replacement of Methane from Quartz Sand-Bearing Hydrate with

Energy & Fuels 2008, 22, 1759–1764

1759

Replacement of Methane from Quartz Sand-Bearing Hydrate with Carbon Dioxide-in-Water Emulsion Xitang Zhou,†,§ Shuanshi Fan,*,‡ Deqing Liang,† and Jianwei Du† Guangzhou Institute of Energy ConVersion, CAS, Guangzhou 510640, China; Key Laboratory of Enhanced Heat Transfer and Energy ConserVation, South China UniVersity of Technology, MOE, Guangzhou 510640, China; and School of Chemical Engineering, Maoming UniVersity, Maoming 525000, China ReceiVed NoVember 25, 2007. ReVised Manuscript ReceiVed January 17, 2008

The replacement of CH4 from its hydrate in quartz sand with 90:10, 70:30, and 50:50 (wCO2:wH2O) carbon dioxide-in-water (C/W) emulsions and liquid CO2 has been performed in a cell with size of φ 36 × 200 mm. The above emulsions were formed in a new emulsifier, in which the temperature and pressure were 285.2 K and 30 MPa, respectively, and the emulsions were stable for 7–12 h. The results of replacing showed that 13.1–27.1%, 14.1–25.5%, and 14.6–24.3% of CH4 had been displaced from its hydrate with the above emulsions after 24–96 h of replacement, corresponding to about 1.5 times the CH4 replaced with high-pressure liquid CO2. The results also showed that the replacement rate of CH4 with the above emulsions and liquid CO2 decreased from 0.543, 0.587, 0.608, and 0.348 1/h to 0.083, 0.077, 0.069, and 0.063 1/h with the replacement time increased from 24 to 96 h. It has been indicated by this study that the use of CO2 emulsions is advantageous compared to the use of liquid CO2 in replacing CH4 from its hydrate.

1. Introduction Gas hydrates, otherwise known as clathrate hydrates, are nonstoichiometric crystalline compounds in which individual guest molecules, either a gas or volatile liquid, of suitable size and shape are caged inside a network of water molecules. These networks which create cavities for guest molecules to occupy are a result of water–water interactions through hydrogen bonding.1 The world reserves of natural gas trapped in the hydrate state have been estimated to be several times of the known reserves of conventional natural gas.2,3 Therefore, developing methods for commercial production of natural gas from hydrates is attracting considerable attention. Most of the known methods for the decomposition of hydrates, such as depressurization,4 thermal stimulation,5,6 and chemicals injection7–9 are based on shifting the thermodynamic equilibrium of the three-phase system (water–hydrate–gas). * Corresponding author. Fax: +86-20-22236581. E-mail: ssfan@ scut.edu.cn. † Guangzhou Institute of Energy Conversion. ‡ South China University of Technology. § Maoming University. (1) Sloan, E. D. Clathrate hydrates of natural gases, 2nd ed.; Marcel Dekker: New York, 1998. (2) Makogon, Y. F. Hydrates of hydrocarbons; Penn Well: Tulsa, OK, 1997. (3) Buffett, B. A. Clathrate hydrates. Annu. ReV. Earth Planet. Sci. 2000, 28, 477–507. (4) Ji, C.; Ahmadi, G.; Smith, D. H. Natural gas production from hydrate decomposition by depressurization. Chem. Eng. Sci. 2001, 56, 801–5814. (5) Yousif, M. H.; Abass, H. H.; Selim, M. S.; Sloan, E. D. Experimental and theoretical investigation of methane-gas-hydrate dissociation in porous media. SPE ReserVoir Eng. 1991, 69–76. (6) Zhou, X.; Fan, S.; Liang, D.; Li, D.; Dong, F. Decomposition of methane hydrate in quartz sands by injecting hot water. Nat. Gas Ind. (China) 2007, 31, 59–62. (7) Gayet, P.; Dicharry, C.; Marion, G.; Graciaa, A.; Lachaise, J.; Nesterov, A. Experimental determination of methane hydrate dissociation curve up to 55 MPa by using a small amount of surfactant as hydrate promoter. Chem. Eng. Sci. 2005, 60, 5751–5758.

Ebinuma10 and Ohgaki et al.11 proposed a scenario in which CH4 from hydrate is replaced by CO2. Their concept involves injecting CO2 gas, which is then allowed to equilibrate with CH4 hydrate along the three-phase equilibrium boundary.12 Because of the difference in chemical affinity of CO2 versus CH4 in the structure I (sI) hydrate structure, the mole fraction of CH4 would be reduced to approximately 0.48 in the hydrate and rise to a value of 0.7 in the gas phase at equilibrium. Using a Raman spectroscopic method, Uchida et al.13 confirmed the guest molecule swapping reaction at the solid–gas interface. With a high-pressure optical cell, Ota et al.14 measured the dynamics of CH4 replacement in the CH4 hydrate with saturated liquid CO2 at 273.2 K. Based on their experimental data, a kinetic model for calculation of the CH4 remaining in the hydrate (8) Lederhos, J. P.; Long, J. P.; Sum, A.; Christiansen, R. L.; Sloan, E. D. Effective kinetic inhibitors for natural gas hydrates. Chem. Eng. Sci. 1996, 51, 1221–1229. (9) Masoudi, R.; Tohidi, B. Estimating the hydrate stability zone in the presence of salts and/or organic inhibitors using water partial pressure. J. Pet. Sci. Eng. 2005, 46, 23–3. (10) Ebinuma, T. Method for dumping and disposing of carbon dioxide gas and apparatus therefore. US Patent 5,261,490, 1993. (11) Ohgaki, K.; Takano, K.; Sangawa, H.; Sangawa, H.; Matsubara, T.; Nakano, S. Methane exploitation by carbon dioxide from gas hydratessphase equilibria for CO2–CH4 mixed hydrate system. J. Chem. Eng. Jpn. 1996, 29 (3), 478–483. (12) Smith, D. H.; Seshadri, K.; Wilder, J. W. Assessing the thermodynamic feasibility of the conversion of methane hydrate into carbon dioxide hydrate in porous media. First National Conference on Carbon Sequestration, 2001; National Energy Technology Laboratory, Proceedings; http://www. netl.doe.gov/events/01conferences/carbseq/carbseq01.html. Cited 12 Nov 2006. (13) Uchida, T.; Takeya, S.; Ebinuma, T.; Narita, H. Replacing methane with CO2 in clathrate hydrate: observations using Raman spectroscopy. In Proceedings of the Fifth International Conference on Greenhouse Gas Control Technologies; Williams, D. J., Durie, R. A., McMullan, P., Paulson, C. A. J., Smith, A. Y., Eds.; CSIRO Publishing: Collingwood, Australia; 2001; pp 523–527. (14) Ota, M.; Morohashi, K.; Abe, Y.; Watanabe, M.; Smith, R. L.; Inomata, H. Replacement of CH4 in the hydrate by use of liquid CO2. Energy ConVers. Manage. 2005, 46, 1680–1691.

10.1021/ef700705y CCC: $40.75  2008 American Chemical Society Published on Web 03/05/2008

1760 Energy & Fuels, Vol. 22, No. 3, 2008

was developed. However, the results showed that the replacement rate was slow, for only 13.2% of CH4 in gas hydrate was replaced after 43 h. McGrail et al.15 proposed a scenario in which CH4 in gas hydrate is replaced by a carbon dioxide-in-water emulsion and performed some primary experiments, but it is difficult to quantify the replacement mechanism due to insufficient data. Because of intellectual property concerns, the method of preparing CO2 emulsion failed to be found from their paper. CO2 is a harmful greenhouse gas16 that contributes to the depletion of the ozone layer and facilitates the global warming effect. In order to slow down the depletion of the ozone by harmful CO2 emissions, scientists are exploring the feasibility of sequestering CO2.17–19 The method of replacement of CH4 in its hydrate with CO2 combines CH4 production and CO2 sequestration successfully, and is no doubt significant environmentally and economically. The main intention of this study is to investigate the kinetics of replacing CH4 from gas hydrate in porous media using both the CO2 emulsion and also liquid CO2, and to give a comparison between the replacement results of using CO2 emulsion and liquid CO2, as well as to determine the main parameters for the preparation of the CO2 emulsion.

Zhou et al.

Figure 1. Experimental apparatus used for replacing CH4 from its hydrate with CO2: (1) CH4 cylinder; (2, 9) buffer; (3, 8) mass flow meter; (4) pressure gauge; (5) thermocouple; (6) cell; (7, 11) sampling point; (10) tank; (12) CO2 cylinder; (13) CO2 liquefier; (14) plunger pump; (15) CO2 emulsion; (16) cooling bath; (17) vacuum pump; (18) wet gas flow meter; (19) data collector; (20) PC.

2. Experimental Section 2.1. Materials. TMN-6 (octa(ethylene glycol) 2,6,8-trimethyl4-nonyl ether, g90% pure) was purchased from Sigma and used as received. SDS (sodium dodecyl sulfate, g98% pure) was purchased from Guangzhou Chemicals Co., Guangzhou. CH4 (g99.95% pure) and CO2 (g99.5% pure) were purchased from Haowen Gases, Foshan. Quartz sand (20-40 mesh, 46.5% porosity) was collected from the Pearl River, Guangzhou. Distilled water was prepared in this laboratory. 2.2. Apparatus. Figure 1 shows the experimental apparatus for the replacement. The main part of the experimental system is a cell with internal dimensions of φ 36 × 200 mm. The maximum operating pressure is 20 MPa. The cell is made of stainless steel with a PTFE inner sleeve for insulation. An integral cooling jacket was employed to keep the temperature in the cell constant. The system temperature was measured with four thermocouples (T1–T4, 60 mm apart) and data was collected with the data collector. Gases used to fill the cell and released from the cell were measured with mass flow meters and a wet gas flow meter (Qixing Huachuang Ltd.). The gas samples were analyzed with a gas chromatograph (HP6890 GC, Agilent Tech.). A schematic illustration of the experimental apparatus for the formation of the CO2 emulsion is shown in Figure 2. The high-pressure cell is made of stainless steel, which has an inner volume of about 350 mL and can withstand a maximum pressure of 50 MPa. Emulsification was achieved by use of the high-speed stirrer, as well as the circulation through a liquid distributing cell. The pressure control (15) McGrail, B. P.; Zhu, T.; Hunter, R. B.; White, M. D.; Patil, S. L.; Kulkarni, A. S. A new method for enhanced production of gas hydrates with CO2. Gas Hydrates: Energy Resource Potential and Associated Geologic Hazards; AAPG Hedberg Conference, September 12–16, Vancouver, BC, Canada, 2004. (16) Wolsky, A. M.; Daniels, E. J.; Jody, B. J. CO2 capture from the flue gas of conventional fossil-fuel-fired power plants. EnViron. Prog. 13 1994, 3, 214–219. (17) deMontigny, D.; Kritpiphat, W.; Gelowitz, D.; Tontiwachwuthikul, P. Simulations production of electricity, steam, and CO2 from small gasfield cogeneration plants for enhanced oil recovery. Energy ConVers. Manage. 1997, 38, 223–228. (18) Hamelinck, C. N.; Faaij, A. P. C.; Turkenburg, W. C.; van Bergen, F.; Pagnier, H. J. M.; Barzandji, O. H. M. CO2 enhanced coal bed methane production in the Netherlands. Energy 2002, 27, 647–674. (19) Brewer, P. G.; Friederich, C.; Peltzer, E. T.; Orr, F. M. Direct experiments on the ocean disposal of fossil fuel CO2. Science 1999, 284, 943–945.

Figure 2. Experimental apparatus used for CO2 emulsion formation: (1) CO2 cylinder; (2) plunger pump; (3) CO2 liquefier; (4) liquid distributing cell; (5) high-pressure cell; (6) pressure gauge; (7) data collector; (8) PC.

system consists of the plunger pump and a pressure gauge with the accuracy of 0.2 MPa. The system temperature was maintained with an accuracy of 0.1 °C by using the cooling water flowing through the jacket. The system temperature and conductivity were measured with the thermocouple and electrodes (Sensorex Ltd.), and data was recorded automatically using a data collector (Agilent Tech.). 2.3. Procedure. 2.3.1. Preparation of CO2 Emulsion. Known amounts of water, TMN-6, and CO2 were placed into the high-pressure cell shown in Figure 2. After the pressure achieved the required value, the pipeline was switched from injecting CO2 to circulation. Based on their experimental results, Varun et al.20 pointed out that an emulsion containing up to 90% CO2 in water (C/W) can be formed with TMN-6 at a temperature below 45 °C and that the stability of the emulsion increases with pressure. The pressure was controlled to 30 MPa with the plunger pump (Zhijiang Petrochem.), and the temperature was controlled to 285.2 K by running the cooling system with a temperature controller (Julabo, Inc.). As a main running parameter, the agitation time was determined experimentally. 2.3.2. Formation of CH4 Hydrate. The required amount of quartz sand wetted with SDS solution (290 ppm) was first introduced into the cell. CH4 gas was then introduced into the cell from a CH4 buffer. The system was pressurized to 15 MPa with a supercharger. The cooling system was switched on to adjust the temperature and to maintain it at 273.2 ( 0.2 K. The data collection system was started to record the experimental data, such as temperature, pressure, gas flux, and so on. Hydrate formation can be detected by observing the changes of the experimental data. Hydrate formation can be considered completed when the experimental measurements attain constant values. (20) Varun, V. D.; Jasper, L. D.; Won, R.; Keith, P. J. High internal phase CO2-in-water emulsions stabilized with a branched nonionic hydrocarbon surfactant. J. Colloid Interface Sci. 2006, 298, 406–418.

Replacement of Methane from Quartz Sand-Bearing Hydrate

Energy & Fuels, Vol. 22, No. 3, 2008 1761

2.3.3. Replacement of CH4 from gas Hydrate. The cell temperature was adjusted to 271.2 ( 0.2 K, and then the free CH4 in the cell was released and measured by a mass flow meter. Then the cell was purged by pressured CO2, which was measured and kept in the tank for analysis, until the CH4 in the gas phase was completely replaced with CO2, which was confirmed by analyzing the gas samples. The CO2 emulsion or liquid CO2 was introduced into the cell and the cell conditions were controlled to about 281.2 ( 0.2 K and 5 MPa, which, in the P–T phase diagram, is below the equilibrium curve of CH4 hydrate and above that of CO2 hydrate, and CO2 is a supersaturated liquid.21 The replacement of CH4 in the hydrate phase started as soon as the cell was filled by CO2 emulsion or liquid CO2. Gas samples were taken from the cell and the tank, respectively, and were analyzed by GC. After the required time, the replacement was terminated by stopping the cooling system. 2.4. Calculations for CH4 Replacement. In the hydration experiment, all runs were performed within 24 h at 273.2 ( 0.2 K, and the initial pressure was always 15 MPa. The mass of water was 28 g, and the initial molar quantity of CH4 in the hydrate phase i (nCH ) was calculated by the following procedure. After the CH4 4,H hydrate was formed, the cell temperature was adjusted to 271.2 ( 0.2 K, and then the CH4 in the gas phase was released. The mass balance of CH4 before and after hydrate formation was determined by means of the following equations: i i i nCH ) nCH - nCH 4,H 4,Total 4,G

(1)

i i nCH ) nCH + xCH4,Tank nM gas,Tank 4,G 4,Release

(2)

i where nCH is the total number of moles of CH4 in the cell, 4,Total i nCH4,G is the number of moles of CH4 in the gas phase in the cell, i nCH is the number of moles of CH4 released, and nM gas,Tank 4,Release and xCH4,Tank are the number of moles and concentration, respeci tively, of CH4 purged from the cell and kept in a tank. Thus, nCH 4,H was calculated using eqs 1 and 2. i The ratio of nCH4,Re to nCH4,H is defined as the replacement percent η, which is used to describe the dynamics of the replacement.

ηRe )

nCH4,Re i nCH 4,H

× 100%

(3)

Here, nCH4,Re is the number of moles of CH4 replaced in its hydrate. This was determined as follows. Gases in the cell were partially released (and kept in the tank) until the cell pressure was lower than the saturated pressure of CO2. The remaining gases were kept in the cell so as to prevent the hydrates from decomposing. Then, nCH4,Re could be determined using the following equation: nCH4,Re ) xCH4,ReleasenMgas,Release + xCH4,InternMgas,Inter

(4)

Figure 3. Electrical resistance versus emulsification time for different content of CO2.

νInter ) νCell - νSand - νH

(6)

where νCell is the volume of the cell, νSand is the true volume of the quartz sand, and νH is the volume of hydrates in the cell, i.e., the mass of hydrates (including those formed from the initial water and the water introduced by the CO2 emulsion) divided by their mean density. nCH4,H can also be calculated by measuring the number of moles nM gas,End and concentration xCH4,End of the mixed gas in the cell after the replacement has been terminated: nCH4,H ) xCH4,EndnMgas,End - xCH4,InternMgas,Inter

(7)

nCH4,Re is equal to the number of moles of CO2 that entering into nCO2,Re , and can be calculated: i nCO2,Re ) nCH4,Re ) nCH - nCH4,H 4,H

(8)

The number of moles of total CO2 that was used to replace CH4 from its hydrate is obtained by means of the following equation: nCO2,Total ) nMgas,Release + nMgas,Inter

(9)

The replacement is regarded as a multiphase reaction, the rate of which can be defined as the following: rRe )

1

dnCH4,H

i nCH 4,H

dt

(10)

3. Results and Discussion

Here, p and T are the pressure and temperature in the cell after the partial release of gases; z is the compression factor, which is approximately equal to ZCO2 since the CH4 concentration is small; R is the universal gas constant; and νInter is the volume of gases in the cell determined by means of the following equation:

3.1. Time of Emulsification and the Stability of CO2 emulsion. In this study, the concentration of TMN-6 was 5 wt % with respect to the amount of water22 and the electrical resistance was measured with two pairs of electrodes (spaced 50 mm apart). It can be seen from Figure 3 that for the system of 90:10, the electrical resistance increased from about zero to 0.94 MΩ in 90 min, then tended to 0.96 MΩ; for the system of 70:30, the electrical resistance increased from about zero to 0.81 MΩ in 70 min, then tended to 0.83 MΩ; and for the system of 50:50, the electrical resistance increased from about zero to 0.68 MΩ in 50 min, then tended to 0.71 MΩ. The stability of the CO2 emulsion was observed by setting the cell to one side. Figure 4 represents the resistance of the emulsions versus quiescent time at 285.2 K and 30 MPa, which shows that the emulsions ceased to be stable after about 7, 10,

(21) Goel, N. In situ methane hydrate dissociation with carbon dioxide sequestration: Current knowledge and issues. J. Pet. Sci. Eng. 2006, 51, 169–184.

(22) da Rocha, S. R. P.; Psathas, P. A.; Klein, E.; Johnston, K. P. Concentrated CO2-in-water emulsion with nonionic polymeric surfactants. J. Colloid Interface Sci. 2001, 239, 241–253.

where nMgas,Release is the number of moles of released gases measured by the mass flow meter, xCH4,Release is the concentration measured by analyzing the released gases collected in the tank, xCH4,Inter is the concentration of mixed gas kept in the cell measured by analyzing a gas sample, and nM gas,Inter is the number of moles of gases kept in the cell, which was determined using the following equation: nMgas,Inter )

pνInter zRT

(5)

1762 Energy & Fuels, Vol. 22, No. 3, 2008

Zhou et al. Table 1. Amounts of CH4 According to Mass Balance

Figure 4. Electrical resistance versus the quiescent time of three emulsions.

run

i nCH (mmol) 4,Total

i nCH (mmol) 4,G

i nCH (mmol) 4,H

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16

552 556 561 559 553 557 560 558 553 557 560 554 552 559 563 555

308.8 314.7 298.9 300.5 310.6 313.9 297.7 296.4 310.8 312.7 311.9 307.5 312.6 314.9 299.7 298.4

243.2 241.3 262.1 258.5 242.4 243.1 262.3 261.6 242.2 244.3 248.1 246.5 239.4 244.1 263.3 256.6

Table 2. Replacement of CH4 with Liquid CO2 According to Mass Balance nCH4,Re (mmol)

Figure 5. Temperature and pressure versus time of CH4 hydrate formation (run 01).

and 12 h. From Figures 3 and 4, we can see that the emulsion containing less CO2 can formed more easily and that the emulsion containing less CO2 existed more stably. 3.2. Changes of Parameters in CH4 Gas Hydrate Formation. For the convenience of comparing the results of each replacement, the original conditions were kept to be as identical as possible. In this study, 276 g of dry sand and 28 g of SDS solution were introduced into the cell in each batch of CH4 hydrate formation; the original pressure in the cell was 15 MPa, and the system temperature was kept at about 273.5 ( 0.2 K. The filling ratio, hydrate saturation, and the quantity of hydrate were 1.0, 0.37, and 32.13 g, respectively. Figure 5 is a representative graph of the CH4 gas hydrate formation in quartz sand. Figure 5 shows a reduction of the system pressure as a result of the CH4 consumption in the process of gas hydrate formation. The maximum values of temperature were observed because of the heat of formation of the CH4 gas hydrate. It can be seen that CH4 gas hydrate was mostly formed in the first few hours, which may be attributed to promotion of hydrate formation by SDS.23 It can be seen from the amplificatory graph of Figure 5 that there were some differences among T1, T2, T3, and T4, and the maximum values of temperature did not appear synchronously, the reason for which was that the coolant flowed from the left-hand side to the right-hand side (Figure 1). Table 1 summarizes the amounts of CH4 according to mass balance. (23) Zhong, Y.; Rogers, R. E. Surfactant effects on gas hydrate formation. Chem. Eng. Sci. 2000, 55 (19), 4175–4187.

run

time (h)

i nCH 4,H (mmol)

nCH4,H (mmol)

(eq 4)

(eq 7)

error (%)

nCO2,Total (mmol)

η (%)

01 02 03 04

24 48 72 96

243.2 241.3 262.1 258.5

223.5 208.8 217.5 213.5

19.7 34.3 44.6 48.8

19.2 35.6 46.5 47.4

-2.36 3.93 4.24 -2.97

1197 1188 1194 1199

8.1 14.1 17.0 18.6

3.3. Replacement Percent of CH4 with Liquid CO2. In this study, liquid CO2 was first used to replace CH4 from its hydrate, and the replacement results were compared with those of Ota’s to observe the reliability of the study. The replacement times were planned to be 24, 48, 72, and 96 h for liquid CO2 to investigate the relation between ηRe and the replacement time. The time and mass balance of the replacement reaction are listed in Table 2. It is shown by runs 01–04 in Table 2 that at 281.2 ( 0.2 K and 5 MPa, 8.1%-18.6% of the CH4 in its gas hydrate was replaced after 24-96 h of replacement, respectively. The replacement percent of this study and that of Ota’s14 are shown in Figure 6, from which we can see that the results were equivalent approximately. In fact, there are several differences between this study and Ota’s. The first is that this study was performed in porous medium (quartz sand). There was a greater interface between CH4 gas hydrate and CO2 because of the gas hydrate distribution, and enhanced diffusion condition for CO2 using quartz sand. Another is that our study was performed at higher temperature, which kinetically favors the replacement.

Figure 6. Replacement percent of CH4 with liquid CO2 versus time.

Replacement of Methane from Quartz Sand-Bearing Hydrate

Energy & Fuels, Vol. 22, No. 3, 2008 1763

Table 3. Replacement of CH4 with CO2 Emulsions According to Mass Balance nCH4,Re (mmol) i nCH4,H

run

time (h)

(mmol)

nCH4,H (mmol)

eq 4

eq 7

error (%)

nCO2,Total (mmol)

η (%)

05 06 07 08 09 10 11 12 13 14 15 16

24 48 72 96 24 48 72 96 24 48 72 96

242.4 243.1 262.3 261.6 242.2 244.3 248.1 246.5 239.4 244.1 263.3 256.6

210.7 190.2 196.5 189.0 208.0 195.4 188.8 183.6 204.5 197.0 203.3 194.8

31.7 51.1 65.8 69.5 34.2 48.9 59.3 62.9 34.9 47.1 60.0 61.8

32.8 52.3 64.0 72.7 35.1 50.4 61.0 61.2 34.3 48.5 62.2 59.7

3.52 2.40 -2.72 4.66 2.63 3.07 2.87 -2.70 -1.72 2.97 3.67 -3.40

1092 1090 1087 1093 908 910 912 907 720 719 721 717

13.1 21.2 25.1 27.1 14.1 19.9 23.7 25.5 14.6 19.3 22.8 24.3

In porous media, the gas hydrate is easy to decompose and difficult to form because of capillary action.24 The last is that our study was performed in static state and that of Ota’s was performed in a stirring cell which favors the replacement because of the continuous renewal of interface. The kinetic results of this study compare well with those of Ota et al., given the positive and negative effects described above. 3.4. Results of Replacing CH4 from Its Hydrate with CO2 Emulsion. In order to observe the relation between replacement result and the content of CO2 in emulsion, 90:10, 70:30, and 50:50 (wCO2:wH2O) carbon dioxide-in-water (C/W) emulsions were used to replace CH4 from its hydrate, the replacement time and replacement percent of which are listed in Table 3. It is shown that 13.1-27.1%, 14.1–25.5%, and 14.6–24.3% of CH4 had been displaced from its hydrate after 24-96 h of replacement at 281.2 ( 0.2 K and 5 MPa. Being convenient for comparison, the replacement percent of CH4 with CO2 emulsions and liquid CO2 are given in Figure 7. From Table 3 and Figure 7, we can see that it is advantageous to replace CH4 in gas hydrate with these CO2 emulsions compared with liquid CO2. In fact, some of emulsion injected into the cell formed CO2 hydrate and resulted in CH4 hydrate being decomposed, which enhanced the release of CH4 from its hydrate. At the same time, most of the emulsion diffused in all directions, driven by the pressure difference. Using emulsion takes advantage of the physical and thermodynamic properties of mixtures in the H2O–CO2 system, and the controlled multiphase flow of heat and mass transport is favorable to the replacement in porous media. It has not been discovered in this study that TMN-6 affects the decomposition of CH4 hydrate and the formation of CO2 hydrate, which means that it is the excellent diffusion of emulsion, not TMN-6, that enhanced the replacement reaction. In fact, the promotion of surfactants, such as SDS, on CH4 hydrate formation results from their reduction on the interfacial tension between the gaseous CH4 and water, which can promote the formation of water crystal lattice. It is known that the replacement rate is controlled principally by the penetration of CO2 into the interface between CH4 and CO2 hydrate. From Figure 7 we can also see that in the first 24 h, ηRe rises from 13.1% to 14.6% with the original water in emulsion increase from 10% to 50%, but in the latter hours, ηRe drops with the increase of water in emulsion. It can be explained that in the first hours more water means more CO2 hydrate formation and more CH4 hydrate dissociation, and in the latter hours the (24) Kim, D. Y.; Seo, Y.; Lee, J. W.; Bae, H. K.; Lee, H. Thermodynamics and spectroscopic analysis of pure and mixed gas hydrates formed in porous media. 15th Symp. Thermophys. Prop. 2003, 22–27.

Figure 7. Replacement percent of CH4 with CO2 emulsions versus time.

Figure 8. Replacement rate of CH4 with different forms of CO2 versus time.

replacement depends mainly on the diffusion of CO2 and the fugacity difference. According to the defining eq 10, rates, rRe, of the replacement at some time were calculated and are shown in Figure 8. From Figure 8, we can see that the replacement rate of CH4 with the above emulsions and liquid CO2 decreased from 0.543, 0.587, 0.608, and 0.348 1/h to 0.083, 0.077, 0.069, and 0.063 1/h with the replacement time increased from 24 to 96 h. Tables 2 and 3 show that in the experiments the employed CO2 decreases with the increase of water emulsion. In fact, most CO2 here was used to maintain the pressure in the cell and the CO2 consumption of in situ replacement must be much smaller. 4. Conclusions A series of carbon dioxide-in-water emulsions were prepared with 5 wt % TMN-6 at 285.2 K and 30 MPa. By measuring the system electrical resistance, the emulsification time of 90:10, 70:30 and 50:50 (wCO2:wH2O) emulsions were determined to be 90, 70, and 50 min, and their stable time were7, 10, and 12 h, respectively. The kinetics of replacement of CH4 from gas hydrate in quartz sand with CO2 emulsions was investigated and compared with that for replacement with liquid CO2. The replacing results showed that 13.1-27.1%, 14.1–25.5%, 14.6–24.3%, and 8.1–18.6% of CH4 was displaced from its hydrate with the above emulsions and liquid CO2 after 24-96 h of replacement, and the replacement rate of CH4 with the above emulsions and liquid

1764 Energy & Fuels, Vol. 22, No. 3, 2008

CO2 decreased from 0.543, 0.587, 0.608, and 0.348 1/h to 0.083, 0.077, 0.069, and 0.063 1/h with the replacement time increased from 24 to 96 h, showing that the use of a CO2 emulsion has advantage over the use of liquid CO2 in displacing CH4 from its hydrate. It was found that in the first 24 h of replacement, ηRe rises from 13.1% to 14.6% with the original water in emulsion increases from 10% to 50%, but in the latter hours, ηRe drops with the increase of water in emulsion. So, the advantage of the emulsion results from CO2 hydrate formation first, and from the CO2 diffusion second. As a whole, high internal phase CO2 emulsion, such as 90:10, can replace more CH4 from its gas hydrate under the same condition and deserves to be used preferentially. Acknowledgment. The financial support received from the National Natural Science Foundation of China (No. 20490207) and the Natural Science Foundation of Guangdong Province (No. 05200113) is gratefully acknowledged.

Nomenclature niCH4,H ) initial number of moles of CH4 in the hydrate phase, mmol i nCH4,Total ) total number of moles of CH4 in the cell, mmol i nCH4,G ) number of moles of CH4 in gas phase in the cell, mmol i nCH4,Release ) number of moles of release CH4, mmol nMgas,Tank ) number of moles of CH4 ) purged from the cell and kept in a tank, mmol

Zhou et al. xCH4,Tank ) concentration of CH4 purged from the cell and kept in a tank nCH4,Re ) number of moles of CH4 replaced after replacement, mmol nCH4,H ) number of moles of CH4 in hydrate after replacement, mmol ηRe ) replacement percent, % nMgas,Release ) number of moles of released mixed gas, mmol xCH4,Release ) concentration of released mixed gas xCH4,Inter ) concentration of mixed gas kept in the cell nMgas,Inter ) number of moles of gases kept in the cell, mmol nMgas,End ) number of moles of gases after the replacement was terminated, mmol xCH4,End ) concentration of the mixed gas after the replacement was terminated nCO2,Re ) number of moles of CO2 entering into the hydrate, mmol nCO2,Total ) number of moles of total CO2 that was injected into the cell,mmol rRe ) replacement rate, 1/h z ) compression factor R ) universal gas constant, J/(mol K) νCell ) volume of the cell, mL νInter ) volume of gases in the cell, mL νSand ) true volume of the quartz sand, mL νH ) volume of hydrates in the cell, mL EF700705Y