Formation Kinetics of Cyclopentane + Methane Hydrates in Brine

Aug 26, 2015 - *Telephone: +86-20-8705-7037. ... the large cavities (51264) and CH4 is encapsulated in the small cavities (512) of the structure II hy...
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Formation Kinetics of Cyclopentane + Methane Hydrates in Brine Water Systems and Raman Spectroscopic Analysis Qiunan Lv,†,‡ Lu Li,†,‡,§ Xiaosen Li,*,†,‡ and Zhaoyang Chen†,‡ †

Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, and ‡Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 100083, People’s Republic of China ABSTRACT: The kinetics of the formation of cyclopentane (CP) + methane (CH4) hydrates are studied to find the optimum condition for the rapid hydrate formation under the submarine condition (NaCl mass fraction of 0.035, around 277.15 K and 12 MPa). The influences of the CP/liquid phase volume ratio and the volume of the liquid phase on the amount of gas uptake and CH4 consumption rate are determined. The results show that the high volume ratio of CP/liquid phase is favorable for the rapid formation of CP + CH4 hydrates. There is an optimal ratio of the liquid phase volume for obtaining the highest gas consumption. Raman spectrum analyses for CP + CH4 hydrates are carried out to examine the hydrate structure and guest molecule occupation. The results show that CP mainly occupies the large cavities (51264) and CH4 is encapsulated in the small cavities (512) of the structure II hydrates. When the volume ratio of the CP/liquid phase is 0.05, CP with CH4 also occupies the large cavities (51264).

1. INTRODUCTION Natural gas hydrates (NGHs) are non-stoichiometric crystalline inclusion compounds, formed from the reaction between natural gas and water under a certain temperature and pressure.1 Typical natural gas molecules include methane, ethane, propane, and carbon dioxide. Great interests have been concentrated on NGH, because the amount of methane in the form of hydrate below the ocean floor was estimated to be approximately 20 000 trillion m3 in the world.2 Gas hydrate deposits are distributed in the permafrost and deep ocean sediments, where the suitable pressure and temperature conditions exist. It is estimated that the total quantity of gas in hydrates is twice the energy contents of the total fossil fuel reserves.3 Therefore, NGHs are considered to be a potential energy resource for the 21st century.4 Despite its huge gas reserves, how to exploit this large energy resource remains a difficulty. In recent years, research on the NGHs had made some progress. Several methods to exploit NGHs have been put forward, such as (1) thermal stimulation,5−7 in which the hydrate reservoir temperature are increased above the temperature of the hydrate formation using hot water or steam injection to decompose the hydrates, (2) depressurization,8 in which the hydrate reservoir pressure is decreased below the pressure of the equilibrium formation to decompose the hydrates, (3) chemical inhibitor injection,9 in which the chemical additives, such as methanol or glycol, are injected to shift the pressure−temperature equilibrium conditions to decompose the hydrates, and (4) replacement of methane by CO2 hydrate,10 in which liquid CO2 is injected into the hydrate reservoirs to form CO2 hydrate and replace the methane gas. However, each of them has its individual advantages and disadvantages. The gas exploitation method often involves a combination of these dissociation methods.11 The use of hot brine to dissociate solid gas hydrate deposits is considered to be an attractive exploitation scheme because it can reduce the dissociation temperatures and enthalpies of NGHs © 2015 American Chemical Society

and, thus, decrease the requirements of sensible heat and hydrate dissociation energy as well as heat losses along the well in conventional thermal stimulation.12 We proposed a novel technique to prepare hot brine in situ seafloor for marine NGH exploitation.13 In the recovery scheme, some chemical additives are used as the hydrate-forming agents to form the hydrates with the seawater in the submarine conditions, and the effect of desalination and the exothermic effect during the formation of the hydrate raise the salinity and temperature of the remaining seawater, respectively. The remaining hot brine is used to dissociate the NGH by injecting it into the NGH deposits under the seafloor. The major obstacles of the novel technique include the rapid hydration conditions, low hydrate formation rate, and low heat emission. To overcome the obstacles, we need to find a suitable chemical additive to reduce the formation pressure, shorten the induction time, accelerate the hydrate formation rate, and enlarge the heat emission. Some hydrate-forming compounds, such as cyclopentane (CP), can form structure II (SII) hydrate at the lower pressure and higher temperature conditions with the help of gas as the guest substance,14 which may meet the demand for the preparation of hot brine in situ seafloor for marine NGH exploitation. Sun et al.15 found that the equilibrium pressures of CP + methane hydrate are from 0.17 to 7.31 MPa over a temperature range of 282.15−301.90 K in pure water. The pressure range is much lower than the submarine conditions (5− 40 MPa), where the NGH resource exists. The thermodynamics of CP hydrate in CP/water emulsions have previously been investigated using differential scanning calorimetry.16,17 Corak et al.18 conducted some experiments to observe the effect of the subcooling and the amount of the hydrate former on the Received: June 24, 2015 Revised: August 21, 2015 Published: August 26, 2015 6104

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Energy & Fuels formation of CP hydrates in the brine water. The CP−CH4 hydrate rates and apparent small-cavity occupancy levels were deduced from the temperature and pressure measurements in the fresh water. The methane occupancy in the small cavities of the SII hydrates is found to be nearly independent of the methane pressure. The rate of hydrate formation is shown to be limited by the rate of methane mass transfer.19 Zhong et al.20,21 reported that CP was an effective promoter for hydrate formation with recovery of CH4 from the coal mine gas mixture. It was found that the final gas consumption increases with the increase of the CP concentration. The effect of NaCl and NaClO4 on the kinetics of methane enclathration with CP and sodium dodecyl sulfate (SDS) was investigated in a non-stirred batch reactor. These two salts inhibit the hydrate growth. This inhibition effect becomes stronger as the salt concentration increases from 0 to 100 mM.22 In our previous work,23 we studied the phase equilibrium for CP + methane hydrates in brine water (with NaCl mass fractions of 0.035). We discovered that CP−CH4 hydrates form rapidly under certain salinity conditions (NaCl mass fraction of 0.035). Nevertheless, the mechanism of CP promoting the hydrate formation and its cage occupancy values are not quite clear, and they require to be further identified. Raman spectroscopy is a well-known tool used for the solid structural analysis to obtain the information on the crystal structures and composition/cage occupancy values.24,25 In our previous work,23 CP is identified to be an optimum additive for the hydrate formation. Despite the progress made in these studies, most of the studies on CP hydrate are performed in pure water or the solutions containing the additives. The key to prepare the hot brine in situ seafloor is to find the optimum condition for the rapid hydrate formation under the submarine condition (NaCl mass fraction of 0.035, approximately 277.15 K and 12 MPa26). However, there are no kinetic data available for CP + methane hydrates in the brine under the submarine conditions. In this work, main parameters (volume of the liquid phase and volume ratio of CP/liquid phase) affecting gas uptakes are investigated at the submarine pressure and temperature conditions. In addition, Raman spectra for CP + CH4 hydrates are measured to obtain Raman signatures for CH4 molecules with CP as a co-guest in the mixed hydrates and examine guest molecule enclathration.

Figure 1. Schematic of the experimental apparatus. effective volume of 1091 mL are both made of 316 stainless steel. The CR and the SV are immersed in a constant temperature water bath. The SV is employed to precool and measure the amount of the offered gas. The working pressure of the CR is controlled by the proportional− integral−derivative (PID) controller, and the maximum pressure is 30 MPa. A three-paddle helical impeller located at the top of the reactor is installed to agitate the contents in the CR. The temperature in the CR is measured using a Pt1000 thermoprobe (JM6081) with the uncertainty of ±0.05 K. The pressures of the CR and SV are measured by a MBS3000 absolute pressure transducer (range of 0−25 MPa) with the accuracy of ±0.02 MPa. The signals of the pressure and temperature are obtained by a data acquisition system driven by a computer. In this work, we use NaCl aqueous solution as the brine water and the mass fraction of NaCl is 0.035. The CR is cleaned with deionized water and then rinsed with NaCl aqueous solution 3−5 times before experimentation. Subsequently, the NaCl aqueous solution and CP liquid with individual desired volumes are introduced into the CR. Then, the CR is purged with methane gas at least 4 times to remove any residual air or mixed gas. Afterward, the temperature in the water bath is set at the experimental temperature. Once the temperature is stabilized typically within 30 min, methane is charged into the CR from the SV until the pressure reached the desired value. The agitator begins to stir with the rate of 800 revolutions/min, and the experimental data also begin to be recorded once every equal time interval. The time when the agitator begins to stir is defined as t0. The temperature and pressure data in the system are recorded during the experiment. As the gas in the CR is consumed during the hydrate formation, additional methane in the SV is supplied into the CR with a PID controller, to make sure that the pressure in the CR is constant. Once the pressure and temperature are stable and have no change for 30 min, the process of the hydrate formation is considered to be completed and the time is defined as t. The moles of methane gas uptake can be calculated as follows:

2. EXPERIMENTAL SECTION 2.1. Materials. Table 1 shows the purities and suppliers of the chemical reagents used in this work. NaCl with the mass fraction of

Table 1. List of Chemical Reagents Studied in This Work chemical name

molecular formula

CP

C5H10

methane sodium chloride

CH4 NaCl

source Guangzhou Xiya Reagent Co., Ltd. FoshanHuate Gas Co., Ltd. Guangzhou Second Chemical Reagent Factory

mass fraction purity

ΔnH = nH, t − nH,0

0.99

=

0.995 0.995

⎛ PV ⎞ ⎛ PV ⎞ ⎛ PV ⎞ ⎛ PV ⎞ ⎜ ⎟ ⎟ ⎟ ⎟ −⎜ +⎜ −⎜ ⎝ zRT ⎠G,0 ⎝ zRT ⎠G, t ⎝ zRT ⎠SV,0 ⎝ zRT ⎠SV, t

(1)

where subscripts t and 0 represent time t and the initial time, respectively, subscript SV refers to the gas phase in the supply vessel, subscript G refers to the gas phase in the crystallizer, P and T represent the pressure and temperature of the hydrate, respectively, V refers to the volume of SV, R is the universal gas constant (R = 8.314 51 J mol−1 K−1), and z is the compressibility factor of the guest gas (methane) calculated by the Soave−Redlich−Kwong (SRK) equation.5 2.3. Raman Spectroscopy Analysis. After the reaction, the CP− CH4 hydrates are formed. Then, the reactor is removed from the water bath and immediately immersed in the liquid nitrogen container. The excess gas is removed from the reactor, and the reactor is depressurized to the atmospheric pressure. The reactor is opened, and hydrate samples

0.995 was obtained from Guangzhou Second Chemical Reagent Factory, China. Methane with the purity of 0.999 was supplied by Foshan Huate Gas Co., Ltd. CP with the mass fraction of 0.99 was supplied by Guangzhou Xiya Reagent Co., Ltd., China. The deionized water used is produced by an ultrapure water system supplied by Nanjing Ultrapure Water Technology Co., Ltd., China. 2.2. Procedure. The schematic of the experimental apparatus in this work is shown in Figure 1. The high-pressure hydrate crystallizer (CR) with the effective volume of 416 mL and the supply vessel (SV) with the 6105

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Energy & Fuels Table 2. Experimental Conditions and Rresultsa run

CP/liquid phase volume ratio

volume of liquid phase (mL)

temperature (K)

pressure (MPa)

gas uptake (mol)

Rf (mol min−1 m−3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

0.03 0.03 0.03 0.03 0.05 0.05 0.05 0.05 0.10 0.10 0.10 0.10 0.15 0.15 0.15 0.15

180 200 220 240 180 200 220 240 180 200 220 240 180 200 220 240

277.15 277.15 277.15 277.15 277.15 277.15 277.15 277.15 277.15 277.15 277.15 277.15 277.15 277.15 277.15 277.15

12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0

0.211 0.293 0.371 0.320 0.276 0.396 0.461 0.362 0.252 0.307 0.375 0.332 0.265 0.317 0.365 0.271

25.029 38.299 41.284 37.844 31.462 43.526 48.325 40.614 45.185 52.556 49.798 45.509 50.588 58.294 57.754 42.157

Standard uncertainties u are u(T) = ±0.05 K and u(P) = ±0.02 MPa, and mass fraction of aqueous solution u(w) is less than 0.0038. Rf is the CH4 consumption rate for the first 30 min after the nucleation point. a

are quickly collected under the liquid nitrogen temperature (approximately 83 K) and kept in liquid nitrogen for the spectroscopic analysis. Raman spectroscopy (Horiba, LabRAM HR) with a single monochromator of 1800 grooves/mm grating is employed to analyze the gas hydrates. The Raman measurements are carried out using 532 nm (wavelength) radiation from an air-cooled argon ion laser as the excitation source. The Raman measurements are repeatable up to a precision of 0.2 cm−1. The temperature dependence measurements are performed using the Linkam THMS-600 stage. The silicon (Si) crystal standard of 520.7 cm−1 is employed to calibrate the subtractive spectrograph. The Raman spectra of the hydrates are obtained at 243.15 K and atmospheric pressure.

3. RESULTS AND DISCUSSION In this work, a total of 16 experimental runs are carried out at the different conditions, including the volume ratio of CP to the volume of the liquid phase (CP/liquid phase ratio) and the volume of the liquid phase at 12.0 MPa and 277.15 K. Table 2 shows the detailed experimental conditions and results. The following discussions are carried out in detail based on the experimental results. 3.1. Gas Uptake. Figure 2 shows the final CH4 gas uptake change versus the volume of the liquid phase with the different volume ratios of CP/liquid phase at 277.15 K and 12.0 MPa. As shown in Figure 2, the total CH4 consumption first rises and then drops with the increase of the liquid phase volume from 180 to 240 mL at the fixed volume ratio of CP/liquid phase. The peak value of CH4 consumption occurs when the volume of the liquid phase is 220 mL. With the increase of the volume of the liquid, the larger volume of water in the liquid phase is available for the formation of CH4 + CP hydrate. However, when the volume of the liquid phase increases to 240 mL, it means that the volume of the gas phase reduces at the fixed effective volume of the cell in the reactor and the mole of CH4 in gas phase decreases. On the other hand, for the given CP/liquid phase ratio, the increase of the volume of the liquid phase accompanies the corresponding increase of the amount of CP on the top of the NaCl solution. For example, when the volume ratio of CP/liquid phase is 0.05, the moles of CH4 consumption with the volume of the liquid phase of 240 mL is 0.362 and even lower than the gas consumption moles with the volume of the liquid phase of 220 mL. Because the thicker layer of the hydrate occurs at the CP−

Figure 2. Gas uptake change versus the volume of the liquid phase at 277.15 K and 12.0 MPa with different CP/liquid phase ratios.

NaCl solution interface, the layer prevents gas from entering the hydrate phase as the experiment proceeds. The similar phenomenon and explanation can also be found elsewhere.27 Figure 3 shows the final CH4 gas uptake change versus CP/liquid phase ratios with the different volumes of the liquid phase at 277.15 K and 12.0 MPa. As seen from Figure 3, when the volume ratio of CP/liquid phase is 0.05, the value of CH4 consumption is the highest. For the given volume of the liquid phase, when the volume ratio of CP/liquid phase increases from 0.03 to 0.05, the more moles of CP existing in the liquid phase result in the more hydrate formed. Thus, the total mole number of CH4 entrapped in the hydrate phase increases. However, the total moles of CH4 consumption decrease remarkably when the volume ratio of CP/ liquid phase increases from 0.05 to 0.10. This is because hydrates usually form at the liquid−liquid interface when both organic liquid and gas are presented in the systems.28 With the increase of the amount of hydrates, the hydrates accumulate at the gas− liquid interface, which form a thick hydrate layer. The layer dramatically prevents the gas from contacting the NaCl solution and dissolving into the NaCl solution to form the gas hydrate. The degree of the hindrance is bigger with the increase of the 6106

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declined when the volume ratio of CP/liquid phase increases from 0.10 to 0.15. 3.2. Hydrate Formation Process. Panels a−d of Figure 4 show the hydrate formation process. As seen from panels a−d of Figure 4, the CH4 consumptions increase with the increase of the reaction time. When the moles of CH4 consumed do not change any more, the reaction is considered to be completed. The slope of the CH4 consumption curve (the mole number of CH4 versus time) is almost constant during the first 30 min, which is defined as the average rate of the hydrate formation Rf,30 as given in Table 2. Figure 4a gives the comparison of the CH4 consumption for the hydrate formation with the CP/liquid phase volume ratio of 0.03−0.15 in the presence of the 180 mL liquid. As seen from panels a−d of Figure 4, the slope increases with the increase of the CP/liquid phase ratio, indicating that the hydrate formation rate increases correspondingly. Reasons for the above phenomena are mainly attributed to the fact that, for the given volume of the liquid phase, the increase of the CP/liquid phase ratio means more moles of CP existing in the solution for the hydrate formation. Thus, the larger amount of CP results in more gas enclosed to form hydrates and a faster hydrate formation rate. This phenomenon is similar to that reported by Zhong et al.20,21 As shown in Figure 4a, the number of moles of CH4 consumed during hydrate formation increases as the CP/liquid phase volume ratio increases from 0.03 to 0.15 before approximate 3000 s. However, with the increase of the reaction time, the number of moles of CH4 consumed continues increasing only with the CP/liquid phase volume ratio of 0.05 and the total CH4

Figure 3. Gas uptake change versus CP/liquid phase ratios at 277.15 K and 12.0 MPa with different volumes of the liquid phase.

hydrate layer. For example, when the volume of the liquid phase is 240 mL and the volume ratio of CP/liquid phase exceeds 0.10, CP nucleates into a large hydrate block almost immediately after the stirring is started, limiting the amount of gas encaged by the formed hydrate.29 The effect of the hindrance resulting from the increase of the thickness of the hydrate layer reaches its maximum. Therefore, the gas uptake has not increased but

Figure 4. Gas uptake change versus time at 277.15 K and 12.0 MPa with different CP/liquid phase ratios. 6107

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Figure 5. Raman spectra of the CP + CH4 hydrates formed from 240 mL of liquid at 277.15 K and 12.0 MPa: (a) CP/liquid phase volume ratio of 0.03, (b) CP/liquid phase volume ratio of 0.05, (c) CP/liquid phase volume ratio of 0.10, and (d) CP/liquid phase volume ratio of 0.15.

consumption is the largest. A similar phenomenon exists in Figure 4b, with the number of moles of gas consumed during hydrate formation increasing as the CP/liquid phase volume ratio increases from 0.03 to 0.15 before 2400 s. However, the number of moles of CH4 consumed keeps increasing only with the CP/liquid phase volume ratio of 0.05 with the increase of the reaction time. It can also be seen from Figure 4c that the number of moles of CH4 consumed with the CP/liquid phase ratio of 0.05 exceeds that with the CP/liquid phase ratio of 0.10 after 1500 s and later exceeds that with the CP/liquid phase ratio of 0.15 after approximately 2100 s. As seen from Figure 4d, the number of moles of CH4 consumed with the CP/liquid phase ratio of 0.05 exceeds that with the CP/liquid phase ratio of 0.10 after 1200 s. When the CP/liquid phase ratio is 0.15, the hydrate formation rate is the fastest before 750 s. CP has been shown to enhance the hydrate formation rate. The same conclusion was reached by Veluswamy et al.31 More than 87% of the total amount of CH4 consumed is obtained within 1000 s. It means that most of the gas uptake can be achieved within a relatively short time. However, the total moles of CH4 consumed are the least. It is because the increase of the amount of CP results in the decease of the amount of NaCl solution with the fixed volume of the liquid phase. It means that the water participating in the hydrate formation reactions is decreased. On the other hand, the hydrate sharply forms and agglomerates at the interface between water and CP, forming a shell of the hydrate. CH4 must be transported across this shell to sustain the hydrate formation.32,33

The shell of the hydrate prevents more gas from being in contact with the water. The lower gas consumption is due to the masstransfer resistance.34 The effect of the hindrance caused by the hydrate layer becomes bigger with the increase of the volume of the liquid phase. When the volume ratio of CP/liquid phase is 0.05, the total moles of CH4 consumed are the largest with the different volumes of the liquid phase and the reaction time is the longest. This is an interesting phenomenon. However, it cannot be explained from the macroscopic view. The structural characteristics and the composition of the hydrate can be deduced to explain this phenomenon by the Raman spectroscopic measurements of the hydrate samples. 3.3. Structural Verification via Raman Spectroscopy. To confirm the crystal structures and compositions of the hydrates formed from CP and CH4, Raman spectra for the hydrates are measured at 243.15 K. The hydrates are supposed to form structure CP hydrates with CP occupying the large 51264 cavities and CH4 occupying the small 512 cavities.4 Panels a−d of Figure 5 show the respective Raman spectra of the CP−CH4 hydrates formed from the 240 mL liquid at 277.15 K and 12.0 MPa with the different CP/liquid phase volume ratios. The Raman peaks near 895.48−896.7 cm−1 are assigned to the ring breathing of CP.35 The bands at around 2873.48−2878.05 and 2976.66−2985.78 cm−1 can be assigned to C−H symmetric and C−H asymmetric stretching vibrations of CP, respectively. The result is similar to that reported by Lo et al.36 The Raman peak at around 2909.5−2914.2 cm−1 is assigned to the C−H stretching 6108

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together, as shown in panels b−d of Figure 6. The packing density of the hydrates becomes higher. For this reason, it is possible to hypothesize that the hydrate crystals adhere to each other in a stronger framework with 0.15 volume fraction of CP/ liquid phase than those with 0.10, 0.05, and 0.03 volume fractions of the CP/liquid phase. These phenomena confirm that the more water molecules that form hexakaidecahedron (51264) water cages, the more CP molecules make an entrance into the large cavities of the SII hydrates and the stabilization effect on hydrate formation is higher with the increase of the volume of CP in the liquid phase.

vibrations of CH4, which would be due to CH4 enclathrated in small cavities (512) of the SII hydrates. If there is a hydrate of structure I (SI), the Raman peaks of CH4 molecules in the hydrate can be observed at 2904 and 2915 cm−1. The peak at 2904 cm−1 stands for CH4 enclathrated in a small cavity of SI. The peak at 2915 cm−1 stands for CH4 enclathrated in a large cavity of SI. In general, the peak at 2904 cm−1 is larger than the peak at 2915 cm−1.37 Therefore, the Raman peak at around 2909.5−2914.2 cm−1 represents CH4 occupying the small SII cavity (512). The Raman spectra show that both CH4 and CP are enclosed in the hydrate structure. In addition, the C−H vibrational modes of CH4 in the 512 and 51264 cavities of SII are not interfered.38 As seen from Figure 5b, the appearance of the Raman peak near 2902.39 cm−1 indicates that CH4 molecules also occupy the large cavities of the SII hydrates when the CP/ liquid phase volume ratio of 0.05 is used.39,40 The amount of CH4 molecules in the large cavities is very poor. Under this condition, CH4 molecules occupy not only the small cavities of the SII hydrates but also the large cavities. This explains why the biggest gas uptake is obtained when the CP/liquid phase volume ratio is 0.05. As seen from panels a−d of Figure 5, the relative peak intensity of CP increases with the increase of the CP/liquid phase ratio. When the CP/liquid phase ratio is 0.15, the relative peak intensity of CP is the highest. The peak intensity of Raman spectroscopy is proportional to the content of the guest molecules.41 The results show that more water molecules have to form hexakaidecahedron (51264) water cages and more CP molecules make an entrance into the large cavities of the SII hydrates with the increase of the volume of CP in the liquid phase. Panels a−d of Figure 6 show the shapes of CP + CH4 hydrates formed from the 240 mL liquid at 277.15 K and 12.0 MPa with the different CP/liquid phase volume ratios after the reactions. As shown in Figure 6a, the hydrate is in the form of slurry. This is because of the presence of the unreacted NaCl solution in the reactor. With the increase of the CP/liquid phase volume ratio, the hydrates become solid particles and accumulate

4. CONCLUSION Gas uptakes with the different amounts of brine water with NaCl mass fractions of 0.035 in the presence of CP are investigated in this work at the submarine pressure and temperature conditions. The results show that the CP/liquid phase volume ratio and the volume of the liquid phase both have an effect on the formation of CP + methane hydrates. The value of gas uptake rises first and then falls with the increase of the CP/liquid phase volume ratio from 0.03 to 0.15 at the fixed volume of the liquid phase. The biggest gas uptake is obtained when the liquid phase volume is 220 mL with the fixed CP/liquid phase ratio. The Raman peak at around 2909.5−2914.2 cm−1 is assigned to the C−H stretching vibration of CH4. The Raman bands could be ascribed to CH4 enclathrated in the small cavities (512) of the SII hydrates. The band at around 2874 cm−1 is assigned to the C−H stretching mode of CP, which is due to the encaging CP molecules in the large cavities (51264) of the SII hydrates. The appearance of the Raman peak near 2902.39 cm−1 indicates that CH4 molecules occupy not only the small cavities of the SII hydrates but also the large cavities with the CP/liquid phase volume ratio of 0.05. CH4 molecules only occupy the small cavities of the SII hydrates when the CP/liquid phase volume ratio is 0.03, 0.10, and 0.15. This explains why the biggest gas uptake is obtained when the CP/ liquid phase volume ratio is 0.05. The packing density of the hydrate is relevant to the content of the guest molecules. With the increase of the volume of CP in the liquid phase, the more CP molecules make an entrance into the large cavities of the SII hydrates. The packing density of the hydrate is higher.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-20-8705-7037. Fax: 86-20-8703-4664. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Fund for Distinguished Young Scholars of China (51225603), the National Natural Science Foundation of China (51506203), and International S&T Cooperation Program of China (2015DFA61790), which are gratefully acknowledged.

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REFERENCES

(1) Li, X. S.; Wang, Y.; Li, G.; Zhang, Y.; Chen, D. F. Experimental investigation into methane hydrate decomposition during threedimensional thermal huff and puff. Energy Fuels 2011, 25 (4), 1650− 1658. (2) Makogon, Y. F. Natural gas hydratesA promising source of energy. J. Nat. Gas Sci. Eng. 2010, 2, 49−59.

Figure 6. Shapes of CP + CH4 hydrates formed from 240 mL of liquid at 277.15 K and 12.0 MPa: (a) CP/liquid phase volume ratio of 0.03, (b) CP/liquid phase volume ratio of 0.05, (c) CP/liquid phase volume ratio of 0.10, and (d) CP/liquid phase volume ratio of 0.15. 6109

DOI: 10.1021/acs.energyfuels.5b01416 Energy Fuels 2015, 29, 6104−6110

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DOI: 10.1021/acs.energyfuels.5b01416 Energy Fuels 2015, 29, 6104−6110