Recovering CH4 from Natural Gas Hydrates with the Injection of

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Recovering CH4 from Natural Gas Hydrates with the Injection of CO2-N2 Gas Mixtures Xuebing Zhou, De-Qing Liang, Shuai Liang, Li-Zhi Yi, and Fu-Hua Lin Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 02 Feb 2015 Downloaded from http://pubs.acs.org on February 4, 2015

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Recovering CH4 from Natural Gas Hydrates with the Injection of CO2-N2 Gas Mixtures Xuebing Zhou,a,b Deqing Liang,a*Shuai Liang,a Lizhi Yi,a Fuhua Lina a

Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China

b

Graduate University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

Abstract: Conversion of natural gas hydrates into CO2 hydrates is renowned as an innovative and eco-friendly method of both a future energy production and the greenhouse gas control. To increase the total CH4 production rate by using such a method, experiments were carried out by replacing CH4 with gas mixtures containing CO2 and N2 from naturally occurring CH4 hydrates at 273.9 K and a pressure ranging from 2.50 MPa to 6.67 MPa. Raman spectroscopic analysis was performed to examine the structure and gas distributions in hydrate phase. Results showed no structural transition in the hydrate phase. CO2 was found to prefer to replace the CH4 in the large hydrate cages while N2 was found to prefer to replace the CH4 in the small ones. The total CH4 productions were found to increase with addition of N2. The experimental pressure showed little effect on the CH4 recovery rate with pure CO2. However, with addition of N2, the CH4 recovery rate was found to decrease and became dependent on the experimental pressure. The transportation of CO2 molecules in hydrate phase was suggested to be the rate-limiting step while N2 captured in hydrate cages slowed down the diffusion rate of CO2 in hydrate phase, leading to a lower CH4 recovery rate. Keywords: CH4 hydrate, Mixed hydrate, Replacement phenomenon, Nitrogen

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1. Introduction The replacement method for methane (CH4) recovery from natural gas hydrates (NGH) by use of carbon dioxide (CO2) is renowned as a novel technology for NGH exploitation.1-4 By exposing methane hydrates to a CO2-rich circumstance where CO2 hydrate structures can stably exist, the CH4 molecules trapped in hydrate lattice are forced to make an in situ exchange with CO2 molecules.5, 6 Comparing to the traditional NGH exploitation techniques, such as depressurization, thermal stimulation and chemicals injection, CH4-CO2 replacement method retains the original form of NGH reservoir which is helpful to avoid some possible catastrophic geo-mechanical hazards such as earthquakes and landslides during gas exploitation and after. In addition, a natural gas hydrate production technology coupling with the long-term storage of CO2 can be an innovative and efficient method with full consideration of energy shortage and environmental impacts. However, the promotion and application to the replacement method for natural gas hydrate exploitation is still facing some difficulties and challenges. First, the CH4 molecules in hydrate phase cannot be fully replaced by CO2 in one CH4-CO2 replacement reaction.7,

8

Through thermodynamic equilibrium studies on the mixed

CH4-CO2 hydrates, Lee et al.9, 10 found the expected CH4 production levels ranged from 64 to 67% from batch type reactions. CO2 molecules tend to coexist with CH4 molecules in hydrate phase other than to form pure CO2 hydrate. By immersing the CH4 hydrate into the continuous flow of gaseous CO2, Ota et al.11 measured the total CH4 production which was only about 31%, suggesting the actual CH4 production levels were much lower than the theoretical values. Raman and NMR analysis on replacement reactions showed that large gas molecules such as CO2 prefer to replace CH4 in large hydrate cages in CH4 hydrate, leaving CH4 in small cages almost intact.10-12 Meanwhile, the replacement reactions which take place deep inside the hydrate phase may proceed much slower than those on the surface of hydrate bulk, which means it would take significantly long time to reach the so-called equilibrium conditions.13


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Another difficulty lies in the low replacement reaction rate in hydrate phase. Ota et al.14 measured CH4 recovery from CH4 hydrate using pressurized gaseous CO2 and found that 15-17% of CH4 could be covered in 120-150 hours. Later, they applied liquid CO2 instead of gaseous CO2 and obtained about 34% CH4 from CH4 hydrate in 114 hours.15 Zhou et al.16, 17 compared the CH4 recovery rate in hydrate-bearing quartz sands with the injection of gaseous, liquid CO2 and CO2 emulsions respectively and noted that the CO2 diffusion rate in hydrate should be a rate-limiting step during replacements. Yuan et al.18, 19 observed the CH4–CO2 replacements in a three-dimensional middle-size reactor and noted that the newly formed mixed CH4-CO2 hydrates may coat outside the CH4 hydrate particles, causing the subsequent CO2 harder to reach the reaction spots. Molecular simulation studies on CH4-CO2 replacement reactions suggested that only a small portion of hydrate cages was opened without significant impact to the main hydrate structure.20, 21 Tung et al.22 simulated the replacement process at the interface between CH4 hydrate and free CO2 molecules, and found the hydrate structure tend to restore the original state once the gas swapping process was accomplished, which means that the CH4 molecules near the core of hydrate particles have to complete a swapping process at every step on the path to the surface of hydrate particles. The gas diffusion rate in hydrate phase was also found to be significantly lower compared to ice in case of the gas-swapping process.23, 24 Taking these two factors into account, the methods to increase the total CH4 production efficiency should focus on how to get more CH4 molecules that retain in small hydrate cages and shorten the gas diffusion path between the hydrate and gas phase. Nitrogen (N2) molecules which have smaller size than CH4 molecules are proposed to be an alternative for replacing CH4 in small cages.25 Park et al.26 measured the replacement process by injecting the flue gas containing 80 mol% N2 and found the total CH4 production increased up to 85%. Additionally, the hydrate structure may get weakened when N2 diffused through the hydrate phase, causing gas replacements easier to accomplish.27 Considering the potential ability of N2 may be demonstrated during the


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replacement reactions, we anticipated that a small fraction of N2 added in CO2 may facilitate the gas swapping at the reaction spots and avoid the system pressure being too high to control. Therefore, two N2/CO2 mixtures containing 25 mol% and 50 mol% N2 were used in this work to investigate the role of N2 molecules played during the gas replacement process. The hydrate structures and the gas distributions in hydrate phase were examined by Raman spectroscopy before and after the replacements. 2. Experimental 2.1 Apparatus and procedures for macroscopic measurement The CH4 and CO2 gases with a stated purity of 99.9% and the gas mixtures of which the molar ratios of CO2 and N2 are 3:1 and 1:1 respectively, were supplied by Shiyuan Gas Co. (China). The deionized water with the resistivity of 18.2 mΩ/cm was made in the laboratory. The experimental apparatus with an internal volume of 5.3 L was applied by Buchi Co. (Switzerland). The high pressure reactor was equipped with a cooling jacket coated outside that was made of 316 stainless steels, as seen in Fig. 1. The content could be vigorously agitated by an anchor-type stirrer. A resistance thermometer with an accuracy of ±0.1 K was inserted into the reactor to measure the temperature of the gas phase. The pressure transducer with an uncertainty of 0.01 MPa was used to measure the system pressure. The compositions of the gas phase during replacement reaction were analyzed by gas chromatography (Agilent 7890, USA) at regular time intervals. Before the replacement reaction, the pure CH4 hydrate was prepared. The reactor was first rinsed by deionized water twice and evacuated by the vacuum pump. After the injection of about 500 g of deionized water, the reactor was pressurized with CH4 up to 6.2 MPa. Then the reactor was cooled down to 273.9 K, and the stirrer was set at 100 rpm to strengthen the heat and mass transfer during crystallization. The CH4 consumed for hydrate in each experimental run was calculated by use of P-R equation of state (EoS)28 and listed in Table 3 and 6. To monitor the overall saturation of the formed CH4 hydrate, the hydration numbers were calculated during


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CH4 hydrate formation. The hydration number of CH4 hydrate usually varies from 5.75 to 6.70.1 In present work, when hydration number decreased below 6.5, all the liquid water was considered to be fully converted into CH4 hydrate. Then the stirrer was stopped manually while the system was cooled down to about 250 K so that CH4 hydrate could not decompose easily. After that, CH4 remained in the gas phase was quickly released and evacuated. The temperature further decreased to about 230 K due to pressure reduction. Once the absolute pressure decreased down to about 50 kpa, the gas mixtures was immediately injected into the reactor to avoid quick decomposition of CH4 hydrate. And then the temperature was set back to 273.9 K. When the system pressure was 0.2 to 0.5 MPa lower than the desired value, the gas injection stops. The pressure would then slowly rise to the desired value in case of thermal expansion. Notably, the temperature was not allowed to surpass 270 K during the gas injection so that the CH4 hydrate could be considered not affected significantly by gas swapping process in the gas phase. Gas samples were taken at regular intervals and each sample volume was no more than 20ml at atmosphere so that the gas loss due to gas sampling could be ignored. After a given elapsed time, the entire hydrate phase was decomposed at 298 K and the gas compositions in the gas phase were collected and measured by gas chromatography. 2.2 Sample preparations for Raman analysis The hydrate samples for Raman analysis were prepared in a custom-designed high pressure vessel with an internal volume of 155 mL which was convenient for opening and hydrate sampling. To achieve similar results, the gas-water ratio and preparation of CH4 hydrate was generally the same as those in the 5.3 L reactor. For CH4 hydrate formation, approximately 14.6 g of finely grinded ice particles at the temperature of liquid nitrogen was loaded into the reactor. CH4 was injected into the reactor up to 6 MPa at 268.2 K after the vapor phase was evacuated. The replacements were then carried out at 273.9 K after the complete formation of CH4 hydrate. The replacements are assumed to reach the equilibrium in 15-17 days. At last, the gas composition in the vapor phase


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was analyzed by the gas chromatography to measure the CH4 productions, and the hydrate samples were collected and finely grinded in a liquid nitrogen vessel. A Raman spectrometer (Horiba, LabRAM HR) equipped with a multichannel air-cooled CCD detector and Ar+ laser operating at 523 nm with a maximum power of 100 mW was used in present work. The first-order Raman peak of silicon crystal standard, which was assigned as a Raman shift of 520.7 cm-1 at 300 K, was used as an external Raman shift reference. Raman spectra was recorded by placing the crushed hydrate sample into a sapphire sample holder, which was precooled down to the temperature of 243 K and placed in a custom-made box with a 2.0 cm diameter window covered with sapphire glass. Signals were collected at 2 - 3 positions on each sample and reported on the average. 3. Results and Discussion 3.1 Raman spectra analysis On the basis of the difference in the size and shape of the hydrate cages, gas hydrates formed from CH4, N2, CO2 and their mixtures are assumed to be in a form of two possible structures. The unit crystalline of the cubic structure I (sI) hydrate consists of two pentagonal dodecahedra (512) and six tetrakaidecahedra (51262), while the cubic structure (sII) II hydrate has sixteen pendtagonal dodecahedra (512) and eight hexakaidecahedron (51264).29 Whether the gas molecules can be kept in the cage is largely determined by size of the gas molecules and hydrate cages. The CH4 molecules with the diameter of 4.36 Å can be suitable for both small and large cages of sI and sII hydrates, whereas the CO2 molecules, 5.12 Å in diameter, which is a bit bigger than the small cages, prefer to occupy the large cages in sI and sII.30, 31 The N2 molecules, 4.1 Å in diameter, are even smaller than CH4 molecules, leading to the preference of small cages in hydrate structures.25, 26 Although the three phase equilibrium pressure of N2 hydrate is higher than CH4 hydrate, the competences between CH4 and N2 molecules for better occupancy to small hydrate cages are expected to be introduced to the gas swapping process.


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To verify these key premises, the hydrate structures and gas distributions were identified by Raman spectroscopy. Fig. 2 presented the Raman spectra of CH4 in hydrate phase before and after replacement. The pure CH4 hydrate, known to form sI hydrate, had two strong and distinct Raman peaks at 2906 and 2916 cm-1 with a resolution of about 0.3 cm-1 representing the totally symmetric C-H stretching mode of CH4 in the small (512) and large (51262) cages of sI respectively. These results were in agreement with the literatures.31, 32 For pure CH4 hydrate, the integrated intensity ratio of large to small cages (AL/As) was 3.88 as seen in Fig. 2(a). By assuming that the large cages of sI are fully occupied, the hydration number of CH4 hydrate was assigned to be 6.09 which was generally accord with macroscopic calculations listed in Table 1. For the mixed hydrates after replacement, no noticeable change in peak position was observed. However, the CH4 peaks decrease which was expected to be replaced by the exchanging gases. For CH4-CO2 replacement, the peak at 2906 cm-1 dropped significantly while the peak at 2916 cm-1 remained almost unchanged as seen in Fig. 2(b). The AL/As was down to 2.22, which was caused by predominant occupation of CO2 in the large cages. For CH4-N2-CO2 replacement, both peaks decreased notably as seen in Fig. 2(c) and (d). The AL/As were 2.71 and 2.63 by using N2-CO2 mixtures containing 25mol% and 50 mol% N2 respectively. N2 molecules were assumed to expel the CH4 molecules already trapped in small (512) cages. On the other hand, the spectra of CO2 and N2 enclathrated in hydrate cages were shown in Fig. 3 and 4 respectively. The double peaks at 1276 and 1380 cm-1, were assigned to be the molecular vibrations of CO2 trapped in the hydrate cages of sI as seen in Fig. 3. The bands were thought to arise from the Fermi resonance resulting from two vibrational modes of CO2 molecules with nearly the same energy.31, 33 Although CO2 was found to occupy both the large and small cages,34 it was very difficult to quantitatively determine the CO2 in the large and small cages through Raman spectra. The Raman peak at 2326 cm-1 was assigned to be the symmetric N-N vibrations of N2 molecules entrapped in the sI hydrate as seen in Fig. 4. It should be noted that there was no


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difference indicating the occupation of N2 in different cages of sI. In previous studies, N2 molecules in large and small cages could hardly be identified by Raman spectroscopy in either sI or sII hydrate.32, 33 Hondoh et al. noted that the magnitude of interaction between the gas molecules and host water lattice did not differ significantly from cages so that only a single peak was observed even though both cages could be occupied by N2 molecules.35 By analyzing the gas compositions, the CH4 productions could be measured which were listed in Table 1. The CH4 production in CH4-CO2 replacement was 33%, which was generally identical to the result obtained by Ota et al..11 By using N2-CO2 mixtures, the CH4 productions increased by 10-15%. The CH4 production did not get significantly increased when feed concentration of N2 increased from 25 to 50 mol%. Considering the equilibrium conditions of the mixed CH4-N2-CO2 hydrates, no more CH4 could be replaced when CH4 fugacity in gas phase met the equilibrium fugacity in hydrate phase. 3.2 Macroscopic analysis 3.2.1 Replacements with pure CO2 In previous works, the temperature effect on CH4-CO2 swapping phenomenon in hydrate phase had been intensively studied.7, 14 The so-called gas swapping rate constants were found to follow the Arrhenius’ equation and got effectively increased with the increasing temperature. However, viewed as the driving force of the replacement reactions, the pressure effect on the replacement reactions seemed to be rarely concerned. Besides, analysis on the kinetic model of CH4-CO2 replacement reaction indicated that the CO2 consumption rates largely depended on the experimental pressure while the CH4 recovery rate did not. In this work, we took the pressure effect on the replacement reactions into consideration. The experiments were carried out at three different pressure, below, equal and above the CH4 equilibrium pressure at 273.9 K which were listed in Table 2. The material balance in present work summed up the total amount of each gas species in the reactor at the beginning


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and the end of the replacements respectively, and performed as the mole percentage based on the amount of each gas species at the beginnings. The results were found to be in a range of 93-96% for CH4 and 101-103% for CO2 as seen in Table 3. The error was assumed to originate in the gas loss during gas samplings, the calculations in P-R EoS and gas chromatography analysis, which could be accepted in this work. Fig. 5 showed the time evolution of methane recovered from the hydrate phase. All the CH4 recovery curves seemed to follow a same routine and increased linearly with time. In about 132 hours, the CH4 productions from the hydrate phase were 17.5%, 18.3% and 21.8% for run 1, 2 and 3 respectively. By following the calculations used by Ota et al.14, the hydrate surface area was estimated to be 0.016 m2, and the CH4 recovery rates per unit surface could be obtained, as listed in Table 4. The CH4 recovery rate only increased from 0.353 to 0.378 moll m-2 h-1 when pressure increased from 2.64 to 3.51 MPa. The CH4 equilibrium pressure was also found to have the little impact on CH4 recovery rate. We compared the CH4 recovery rate performed by Ota et al.14 at 273.2 K, 3.26 MPa. Their result, around 0.075 mol m-2 h-1, was about one fifth of the current results. Such a difference may be probably due to the error in hydrate surface area estimation. Considering the amount of CH4 hydrate particles adhering to the wall of the reactor may dramatically increase the hydrate surface, it was difficult to accurately estimate the surface area on the bulk hydrate phase. The CO2 uptake curves during the reactions were shown in Fig. 6. The consumption rates of CO2 were a little bit larger than the corresponding CH4 recovery rates. The molar ratio of the consumed CO2 and the recovered CH4 ranged from 1.1 to 1.4 so that CH4 was proved to be replaced with CO2 from hydrate phase. The additional CO2 consumptions were probably due to the difference in hydration number between CH4 hydrate and mixed CH4-CO2 hydrate.25 As seen in Table 4, the increase of CO2 consumption rates were also not evident as the pressure increases from 2.64 to 3.15 MPa. The CO2 consumption rates in run 2 and 3 are only 0.94 and 1.01 times of run 1 respectively. However, given that the CO2 hydrate equilibrium fugacity was 1.30 MPa, the initial


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CO2 consumption rate obtained in run 2 and 3 should be 1.4 and 1.6 times higher respectively than that in run 1 according to the kinetic model proposed by Ota et al..14 Therefore, there were close relationships between the CH4 recovery and CO2 consumption during replacement which shall not be treated independently. 3.2.2 Replacements with mixed N2-CO2 The composition of N2-CO2 gas mixtures and the initial conditions for each experimental run were listed in Table 5. Raman analysis on mixed CH4-N2-CO2 hydrates showed that hydrate would remain as sI when feed concentration of N2 was low. Hydrate structure was not assumed to change during the replacement. To compare with the CH4-CO2 replacement, run 4, 5 and 6 were carried out under the pressure generally identical to run 1, 2 and 3 respectively by use of N2-CO2 mixtures containing 25mol% N2. The pressure chosen was still above the corresponding equilibrium pressure of mixed N2-CO2 hydrate which was assigned to be 1.85 MPa. To further investigate the role of N2 in the replacement reaction, we used the N2-CO2 mixtures containing 50 mol% N2 in run 7 and 8. The material balance before and after the replacement was found to be in a range of 92-101% for CH4, 99-104% for CO2 and 98-101% for N2 which were listed in Table 6. Fig. 7(a) showed the time evolution of CH4 recovered from the hydrate phase using the N2-CO2 mixture containing 25 mol% of N2. All the CH4 recovery curves increased linearly within 140 hours, with the CH4 productions of 9.5%, 12.6% and 17.9% for run 4-6 respectively. The CH4 recovery rates in run 4 and 5 did not vary significantly and reduced to nearly half of the values in run 1 and 2. In run 6, the CH4 recovery rate was dramatically increased, but still no more than the value obtained in run 3, suggesting the existence of N2 slowed down the CH4 recovery rate rather than increased. Therefore, the CH4 recovery rate was assumed to slow down because of the existence of N2 and became dependent on the experimental pressure due to the existence of N2. The pressure effect on CH4 recovery rate became more evident when N2 concentration increased up to 50 mol% as seen in Fig 7(b). In around 144 hours, the CH4 productions were 8.3% and 17.7% for run 7 and 8 respectively.


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Further, when pressure increased from 5.00 to 6.67 MPa, the CH4 recovery rate increased from 0.177 to 0.328 mol m-2 h-1 as seen in Table 7. However, the recovery rates were still below the values of the replacement without N2. In run 7 where the initial fugacity of CO2 was generally equal to that in run 1, the CH4 recovery rate decreased down to nearly half of the value in run 1. In run 8 where the partial pressure of CO2 approached to the liquefaction pressure, the recovery rate was still slightly below the value in run 3. Combining with the results in run 4-7, low concentration of N2 in N2-CO2 mixtures were suggested not to increase the replacement rates and made the replacement rates more dependent on pressure. The N2 and CO2 consumed for the replacements were shown in Fig. 8. The consumption of N2 could be observed macroscopically. The molar ratio of the N2 and CO2 consumed and the CH4 recovered ranges from 1.1 to 1.8 so that the amount of the gas consumed and CH4 recovered were still closely related. In Fig. 8, N2 consumed for replacement was quite low and its total consumptions were positively correlated with its feed concentration and pressure. Fig 9 and 10 showed the time evolution of the molar ratio of CO2 and N2 for runs 4-8. It was clear to see that the ratios were among 6 to 9 by using the mixture containing 25 mol% N2 (runs 4, 5 and 6), and among 8 to 12 by using the mixture containing 50 mol% N2 (runs 7 and 8). We calculated the molar ratios of N2 and CO2 in hydrates formed from N2-CO2 mixtures containing 25 mol% and 50 mol% N2 at 273.9 K by CSMHYD36, which were assigned to be 39.2 and 13.4 respectively. They were much higher than the ratio consumed for replacement which meant the N2-CH4 replacement proceeded faster than CO2-CH4 replacement in hydrate phase. However, considering the stability of hydrate structure, the N2-CH4 swapping process was assumed to mainly occur in the small cages of sI. Notably, N2 molecules did not help to dramatically increase the CH4 recovery rate, but were consumed much faster than CO2 during the replacement. Owning a relative small size, N2 molecules could enter the hydrate cages without significant breakage of the host lattice, making them more acceptable to the original form of


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hydrate structure and easier to swap with the CH4 molecules trapped in hydrate cages.22, 25 Additionally, it would also be suitable for N2 to fill some vacancies inside the hydrate phase where CH4 hydrate crystal was not fully formed, such as the empty hydrate cages or some defects in host lattice.23, 37 In this case, N2 patched up the original hydrate structures rather than induce them to be destroyed. As for CO2 molecules, they were assumed to diffuse slower than N2 molecules and tended to swap the CH4 molecules trapped in the large cages of sI at the interface between the original CH4 hydrate and the mixed CH4-CO2-N2 hydrate.38 After the replacement, partial of the CH4 molecules were swapped by CO2 and N2 molecules so that the hydrates attained to a more stable condition.6, 21, 27 Since the fugacity of N2 was low, N2 molecules could not accomplish the replacement without the existence of CO2 in hydrate phase. The CH4 released from the hydrate cages together with the N2 hindered around may greatly reduce the CO2 concentration at the reaction spots. This may explain why the CH4 recovery rate decreased when N2 participated the replacement reactions. The replacement between CH4 and CO2 in large hydrate cages was still considered to be the rate-limiting step for replacement reactions. 4. Conclusions In this study, the replacement of CH4 by CO2 and mixed N2-CO2 mixtures in methane hydrate was investigated as an innovative method of both recovering the naturally occurring methane hydrate deposits and sequestering a greenhouse gas. In CH4–CO2 replacement, only the CH4 molecules in large cages of sI hydrate were found to be recovered by CO2 molecules from spectroscopic analysis and CH4 recovery rates were not affected by the experimental pressure when CO2 was in gaseous state. However, the CH4 recovery rate became depended on the experimental pressure when N2 was added to the gaseous CO2. Spectroscopic analysis revealed that the N2 molecules helped to recover the CH4 molecules in the small cages of sI hydrate and CH4 productions were therefore increased. However, the CH4 recovery rates of the replacements with N2-CO2 mixtures were not


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found to surpass those with pure CO2. The transportation CO2 in hydrate phase was still supposed to be the rate-limiting step during the replacements. AUTHOR INFORMATION Corresponding Author *Deqing. Liang: E-mail: [email protected]. Tel: +86 20 87057669. Fax: +86 20 87057705 ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51176192), CAS Program (KGZD-EW-301), NOG Program (GHZ2012006003) Notes The author declare no competing financial interest.

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methane hydrate with carbon dioxide. Angewandte Chemie-International Edition 2003, 42, (41), 5048-5051. 10. Lee, S.; Park, S.; Lee, Y.; Seo, Y., Thermodynamic and C-13 NMR spectroscopic verification of methane-carbon dioxide replacement in natural gas hydrates. Chemical Engineering Journal 2013, 225, 636-640. 11. Ota, M.; Saito, T.; Aida, T.; Watanabe, M.; Sato, Y.; Smith, R. L.; Inomata, H., Macro and microscopic CH4-CO2 replacement in CH4 hydrate under pressurized CO2. Aiche Journal 2007, 53, (10), 2715-2721. 12. Beeskow-Strauch, B.; Schicks, J. M., The Driving Forces of Guest Substitution in Gas Hydrates-A Laser Raman Study on CH4-CO2 Exchange in the Presence of Impurities. Energies 2012, 5, (2), 420-437. 13. Lee, B. R.; Koh, C. A.; Sum, A. K., Quantitative measurement and mechanisms for CH4 production from hydrates with the injection of liquid CO2. Physical Chemistry Chemical Physics 2014, 16, (28), 14922-14927. 14. Ota, M.; Abe, Y.; Watanabe, M.; Smith, R. L.; Inomata, H., Methane recovery from methane hydrate using


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Energy & Fuels

pressurized CO2. Fluid Phase Equilibria 2005, 228, 553-559. 15. 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 Conversion and Management 2005, 46, (11-12), 1680-1691. 16. Zhou, X. T.; Fan, S. S.; Liang, D. Q.; Du, J. W., Replacement of methane from quartz sand-bearing hydrate with carbon dioxide-in-water emulsion. Energy & Fuels 2008, 22, (3), 1759-1764. 17. Zhou, X. T.; Fan, S. S.; Liang, D. Q.; Du, J. W., Determination of appropriate condition on replacing methane from hydrate with carbon dioxide. Energy Conversion And Management 2008, 49, (8), 2124-2129. 18. Yuan, Q.; Sun, C. Y.; Yang, X.; Ma, P. C.; Ma, Z. W.; Liu, B.; Ma, Q. L.; Yang, L. Y.; Chen, G. J., Recovery of methane from hydrate reservoir with gaseous carbon dioxide using a three-dimensional middle-size reactor. Energy 2012, 40, (1), 47-58. 19. Yuan, Q.; Sun, C.-Y.; Liu, B.; Wang, X.; Ma, Z.-W.; Ma, Q.-L.; Yang, L.-Y.; Chen, G.-J.; Li, Q.-P.; Li, S.; Zhang, K., Methane recovery from natural gas hydrate in porous sediment using pressurized liquid CO2. Energy Conversion and Management 2013, 67, 257-264. 20. Qi, Y. X.; Ota, M.; Zhang, H., Molecular dynamics simulation of replacement of CH4 in hydrate with CO2. Energy Conversion and Management 2011, 52, (7), 2682-2687. 21. Geng, C. Y.; Wen, H.; Zhou, H., Molecular Simulation of the Potential of Methane Reoccupation during the Replacement of Methane Hydrate by CO2. Journal of Physical Chemistry A 2009, 113, (18), 5463-5469. 22. Tung, Y. T.; Chen, L. J.; Chen, Y. P.; Lin, S. T., In Situ Methane Recovery and Carbon Dioxide Sequestration in Methane Hydrates: A Molecular Dynamics Simulation Study. Journal of Physical Chemistry B 2011, 115, (51), 15295-15302. 23. Liang, S.; Kusalik, P. G., The Mobility of Water Molecules through Gas Hydrates. Journal of the American Chemical Society 2011, 133, (6), 1870-1876.


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24. Davies, S. R.; Sloan, E. D.; Sum, A. K.; Koh, C. A., In Situ Studies of the Mass Transfer Mechanism across a Methane Hydrate Film Using High-Resolution Confocal Raman Spectroscopy. Journal of Physical Chemistry C 2010, 114, (2), 1173-1180. 25. Jung, J. W.; Espinoza, D. N.; Santamarina, J. C., Properties and phenomena relevant to CH4-CO2 replacement in hydrate-bearing sediments. Journal of Geophysical Research-Solid Earth 2010, 115. 26. Park, Y.; Kim, D. Y.; Lee, J. W.; Huh, D. G.; Park, K. P.; Lee, J.; Lee, H., Sequestering carbon dioxide into complex structures of naturally occurring gas hydrates. Proceedings of the National Academy of Sciences of the United States of America 2006, 103, (34), 12690-12694. 27. Dornan, P.; Alavi, S.; Woo, T. K., Free energies of carbon dioxide sequestration and methane recovery in clathrate hydrates. Journal of Chemical Physics 2007, 127, (12). 28. Ricaurte, M.; Torre, J. P.; Asbai, A.; Broseta, D.; Dicharry, C., Experimental Data, Modeling, and Correlation of Carbon Dioxide Solubility in Aqueous Solutions Containing Low Concentrations of Clathrate Hydrate Promoters: Application to CO2-CH4 Gas Mixtures. Industrial & Engineering Chemistry Research 2012, 51, (7), 3157-3169. 29. Sloan, E. D.; Koh, C., Clathrate hydrates of natural gases, 3rd edn.; CRC Press, 2008. 30. Sum, A. K.; Burruss, R. C.; Sloan, E. D., Measurement of clathrate hydrates via Raman spectroscopy. Journal Of Physical Chemistry B 1997, 101, (38), 7371-7377. 31. Qin, J. F.; Kuhs, W. F., Quantitative analysis of gas hydrates using Raman spectroscopy. Aiche Journal 2013, 59, (6), 2155-2167. 32. Seo, Y.; An, S.; Park, J. W.; Kim, B. S.; Komai, T.; Yoon, J. H., Occupation and Release Behavior of Guest Molecules in CH4, CO2, N2, and Acetone Mixture Hydrates: An In Situ Study by Raman Spectroscopy. Industrial & Engineering Chemistry Research 2014, 53, (14), 6179-6184.


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33. Lee, H. H.; Ahn, S. H.; Nam, B. U.; Kim, B. S.; Lee, G. W.; Moon, D.; Shin, H. J.; Han, K. W.; Yoon, J. H., Thermodynamic Stability, Spectroscopic Identification, and Gas Storage Capacity of CO2-CH4-N2 Mixture Gas Hydrates: Implications for Landfill Gas Hydrates. Environmental Science & Technology 2012, 46, (7), 4184-4190. 34. Ripmeester, J. A.; Ratcliffe, C. I., The diverse nature of dodecahedral cages in clathrate hydrates as revealed by Xe-129 and C-13 NMR spectroscopy: CO2 as a small-cage guest. Energy & Fuels 1998, 12, (2), 197-200. 35. Hondoh, T.; Anzai, H.; Goto, A.; Mae, S.; Higashi, A.; Langway, C. C., The Crystallographic Structure of the Natural Air-Hydrate in Greenland Dye-3 Deep Ice Core. Journal of Inclusion Phenomena and Molecular Recognition in Chemistry 1990, 8, (1-2), 17-24. 36. Sloan, E. D., Clathrate hydrates of natural gases, 2nd ed.; Marcel Decker: New York, 1998. 37. Ikeda-Fukazawa, T.; Kawamura, K.; Hondoh, T., Diffusion of nitrogen gas in ice Ih. Chemical Physics Letters 2004, 385, (5-6), 467-471. 38. Demurov, A.; Radhakrishnan, R.; Trout, B. L., Computations of diffusivities in ice and CO2 clathrate hydrates via molecular dynamics and Monte Carlo simulations. Journal of Chemical Physics 2002, 116, (2), 702-709.


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Table 1 Summary of hydrate sample preparation for Raman analysis. The pure CH4 and CO2 hydrates were formed at about 6.0 MPa and 3.0 MPa respectively, 268.2 K to compare with the hydrate samples after replacements. All the replacements are carried out at 273.9 K.

Exchanging gas

Initial

(a) Initial

Hydration

pressure

moles of CH4 in

number of

(b) moles of

CH4 production

CH4 recovered

ratio (b)/(a) (%)

(MPa)

hydrate

CH4 hydrate

CO2

3.47

0.1315

6.17

0.0431

33

CO2 +N2 (25 mol%)

3.57

0.1341

6.05

0.0562

42

CO2 +N2 (50 mol%)

7.17

0.1332

6.09

0.0612

46


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Energy & Fuels

Table 2 Initial conditions for the CH4-CO2 replacements at 273.9 K Temperature

Initial pressure

Water injected

Hydration number of CH4

(K)

(MPa)

(mol)

hydrate

1

273.9

2.64

27.7778

6.25

2

273.9

3.15

27.7811

6.35

3

273.9

3.51

27.8044

6.11

Run


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Table 3 Material balance for CH4 and CO2 before and after the replacements at 273.9 K

Run

(a) Amount of gas before

(b) Amount of gas after

Material balance

replacement (mol)

dissociation (mol)

(b)/(a) (%)

CH4

CO2

CH4

CO2

CH4

CO2

1

4.4416

7.0955

4.1510

7.1573

93

101

2

4.3725

8.9504

4.2062

9.0303

96

101

3

4.5484

10.8858

4.3491

11.2203

96

103


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Table 4 The gas swapping rates in CH4-CO2 replacements Pressure

CH4 recovery rate

CO2 consumption rate

(MPa)

(mol m-2 h-1)

(mol m-2 h-1)

Run 1

2.64

0.353

0.439

Run 2

3.15

0.357

0.413

Run 3

3.51

0.378

0.442


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Table 5 Initial conditions for the replacements between CH4 and CO2-N2 mixtures

Run

Initial pressure

Water injected

Hydration number of CH4

(MPa)

(mol)

hydrate

Temperature (K)

4

274.0

2.60

27.8189

5.91

5

274.0

3.11

27.7922

6.48

6

274.0

3.50

27.8072

5.93

7

273.9

5.00

27.7772

6.35

8

273.9

6.67

27.7783

6.19


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Energy & Fuels

Table 6 Material balance for CH4, CO2 and N2 before and after the replacements at 273.9 K (a) Amount of gas before

(b) Amount of gas after

Material balance

replacement (mol)

dissociation (mol)

(b)/(a) (%)

Run CH4

CO2

N2

CH4

CO2

N2

CH4

CO2

N2

4

4.7101

4.5174

1.5049

4.7285

4.5956

1.4834

101

102

99

5

4.2880

5.8628

2.0177

4.3099

5.7797

1.9479

101

99

97

6

4.6878

6.4732

2.2853

4.3145

6.7041

2.2915

92

104

101

7

4.3757

6.2722

6.2973

4.1149

6.4920

6.1733

94

104

98

8

4.4906

8.7798

8.7587

4.2758

8.9232

8.6036

96

102

99


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Table 7 The gas swapping rates in the replacements between CH4 and N2-CO2 mixtures CH4 recovery rate

Gas consumption rate (mol m-2 h-1)

Run (mol m-2 h-1)

CO2

N2

4

0.189

0.251

0.019

5

0.172

0.214

0.014

6

0.359

0.411

0.029

7

0.177

0.228

0.025

8

0.328

0.398

0.043


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Fig. 1 Schematic diagram of the experimental apparatus: (1) high pressure reactor; (2) magnetic stirrer; (3) thermostatic bath; (4) thermometer; (5) pressure transducer; (6) pressure relief valve; (7) data acquisition; (8) gas cylinder; (9) reducing valve; (10) air booster; (11) vacuum pump; (12) gas sampling valve; (13) gas chromatography; (14-18) valves.


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Fig. 2 Raman spectra of CH4 molecules encaged in sI before and after the replacements.


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Energy & Fuels

Fig. 3 Raman spectra of CO2 molecules encaged in sI.


Energy & Fuels

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Fig. 4 Raman spectra of N2 molecules encaged in sI.


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Fig. 5 The amount of CH4 recovered from CH4 hydrate as a function of time at 273.9 K.


Energy & Fuels

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Fig. 6 The amount of CO2 consumed for hydrate formation as a function of time at 273.9 K.


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Fig. 7 The amount of CH4 recovered from CH4 hydrate as a function of time at 273.9 K: (a) replacements using N2-CO2 mixtures containing 25 mol% N2; (b) replacements using N2-CO2 mixtures containing 50 mol% N2.


Energy & Fuels

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Fig. 8 The amount of CO2 and N2 consumed for hydrate formation as a function of time at 273.9 K: (a) replacements using N2-CO2 mixtures containing 25 mol% N2; (b) replacements using N2-CO2 mixtures containing 50 mol% N2.


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Fig. 9 Time evolution of the ratios of consumed CO2 and N2 during the replacements by using CO2-N2 mixtures containing 25 mol% N2.


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Fig. 10 Time evolution of the ratios of consumed CO2 and N2 during the replacements by using CO2-N2 mixtures containing 50 mol% N2.


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Recovering CH4 from Natural Gas Hydrates with the Injection of

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