6th International Conference on Gas Hydrates


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   SWAPPING CARBON DIOXIDE FOR COMPLEX GAS HYDRATE STRUCTURES    Youngjune Park, Minjun Cha, Jong-Ho Cha, Kyuchul Shin, and Huen Lee∗ Department of Chemical & Biomolecular Engineering Korea Advanced Institute of Science and Technology 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701 REPUBLIC OF KOREA   Keun-Pil Park, Dae-Gee Huh, Ho-Young Lee, Se-Joon Kim, and Jaehyoung Lee Korea Institute of Geoscience and Mineral Resources 30 Gajeong-dong, Yuseong-gu, Daejeon 305-350 REPUBLIC OF KOREA    ABSTRACT Large amounts of CH4 in the form of solid hydrates are stored on continental margins and in permafrost regions. If these CH4 hydrates could be converted into CO2 hydrates, they would serve double duty as CH4 sources and CO2 storage sites. Herein, we report the swapping phenomena between global warming gas and various structures of natural gas hydrate including sI, sII, and sH through 13C solid-state nuclear magnetic resonance, and FT-Raman spectrometer. The present outcome of 85% CH4 recovery rate in sI CH4 hydrate achieved by the direct use of binary N2 + CO2 guests is quite surprising when compared with the rate of 64 % for a pure CO2 guest attained in the previous approach. The direct use of a mixture of N2 + CO2 eliminates the requirement of a CO2 separation/purification process. In addition, the simultaneously-occurring dual mechanism of CO2 sequestration and CH4 recovery is expected to provide the physicochemical background required for developing a promising large-scale approach with economic feasibility. In the case of sII and sH CH4 hydrates, we observe a spontaneous structure transition to sI during the replacement and a cage-specific distribution of guest molecules. A significant change of the lattice dimension due to structure transformation induces a relative number of small cage sites to reduce, resulting in the considerable increase of CH4 recovery rate. The mutually interactive pattern of targeted guest-cage conjugates possesses important implications on the diverse hydrate- based inclusion phenomena as clearly illustrated in the swapping process between CO2 stream and complex CH4 hydrate structure.  Keywords: gas hydrate, clathrate, CO2 sequestration, methane, swapping phenomenon, NMR   ∗ Corresponding author: Phone: +82 42 869 3917 Fax +82 42 869 3910 E-mail: h_lee@kaist.ac.kr INTRODUCTION There are currently two urgent global issues that should be resolved, global warming effects and future energy sources. In order to effectively control atmospheric CO2 levels, CO2 needs to be sequestered to appropriate sites on a large scale. Several suggested methods that entail injecting CO2 into the ocean involve producing relatively Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. pure CO2 at its source and transporting it to the injection point [1]. In particular, when CO2 is injected in seawater below a certain depth, a solid CO2 hydrate can be formed according to the stability regime [2]. On the other hand, naturally-occurring gas hydrates are deposited on the continental margin and its permafrost regions and are scattered all over the world [3]. The total amount of natural gas hydrate over the world is estimated to be about twice as much as the energy contained in fossil fuel reserves [4, 5]. In order to recover CH4 efficiently, several strategies such as thermal treatment, depressurization, and inhibitor addition into the hydrate layer have been proposed [6]. However, all these methods are based on the decomposition of CH4 hydrate by external stimulation and could potentially trigger catastrophic slope failures [7]. It thus needs to be recognized that the present natural gas production technologies have inherent limitations in terms of their adoption for the effective recovery of natural gas hydrates. As such, the safest and most economically feasible means should be developed with full consideration of environmental impacts. Recently, the replacement technique for recovering CH4 from CH4 hydrate by using CO2 has been suggested as an alternative option for recovering CH4 gas [8, 9]. This swapping process between two gaseous guests is considered to be a favorable approach toward long-term storage of CO2. It also enables the ocean floor to remain stabilized even after recovering the CH4 gas, because CH4 hydrate maintains the same crystalline structure directly after its replacement with CO2. If the CH4 hydrates could be converted into CO2 hydrates, they would serve double duty as CH4 sources and CO2 storage sites. Here, we further extend our investigations to consider the occurrence of CO2 replacement phenomena on sII, and sH hydrate. In this point of view, we present an interesting conclusion reached by inducing a structure transition. A microscopic analysis is conducted in order to examine the real swapping phenomena occurring between CO2 guest molecules and various types of hydrate through spectroscopic identification, including solid-state Nuclear Magnetic Resonance (NMR) spectrometry and FT-Raman spectrometry. More importantly, we also investigate the possibility of direct use of binary N2 and CO2 gas mixture for recovering CH4 from the hydrate phase, which shows a remarkably enhanced recovery rate by means of the cage-specific occupation of guest molecules due to their molecular properties.  RESULTS AND DISCUSSION The recoverable amount of CH4 by replacing sI CH4 hydrate with CO2 could reach around 64% of hydrate composition because CO2 molecules only preferably replace CH4 in large cages, while CH4 molecules in small cages remain almost intact [8]. This swapping process between two gaseous guests is considered to be a favorable way as a long-term storage of CO2 and enables the ocean floor to remain stabilized even after recovering the CH4 gas because sI CH4 hydrate maintains the same crystalline structure directly after its replacement with CO2. We first attempted to examine real swapping phenomenon occurring between binary guest molecules of N2 and CO2 and crystalline sI CH4 hydrate through spectroscopic identification. For CO2 its molecular diameter is the same as the small cage diameter of sI hydrate, and thus only a little degree of distortion in small cages exists to accommodate CO2 molecules. Accordingly, we sufficiently expect that CO2 molecules can be more stably encaged in sI-L under favorable host-guest interaction. On the other hand, N2 is known as one of the smallest hydrate formers and its molecular size almost coincides with CH4. Although N2 itself forms pure sII hydrate with water, the relatively small size of N2 molecules leads to the preference of sI-S over other cages and moreover the stabilization of overall sI hydrate structure when N2 directly participates in forming hydrate.  Figure 1 13C cross-polarization NMR spectra for identifying replaced CO2 molecules in sI CH4 hydrate.  Accordingly, CH4 and N2 are expected to compete for better occupancy to sI-S, while CO2 preferentially occupies only sI-L without any challenge of other guests. Thus, the successful role of these two external guests of N2 and CO2 in extracting original CH4 molecules makes it possible for diverse flue gases to be directly sequestrated into natural gas hydrate deposits.  Figure 2 In-situ Raman spectra of sI CH4 hydrate replaced with N2 + CO2 (80 mol% N2 and 20 mol% CO2) mixture. (a) C-H stretching vibrational modes of CH4 molecules, (b) N-N stretching modes of N2 molecules, (c) C=O stretching and bending vibrational modes of CO2 in clathrate hydrate cages.  To verify several key premises mentioned above we first identified ternary guest distribution in cages through the 13C NMR and Raman spectra. As shown in Figure 1. the NMR spectra provide a clear evidence such that CO2 molecules are distributed only in sI-L. For qualitative description of cage occupancy enforced by N2 molecules, we measured the Raman spectra of the sI CH4 hydrates replaced with N2 + CO2 mixture. Two peaks in Figure 2a representing CH4 in sI-S (2914 cm-1) and CH4 in sI-L (2904 cm-1) continuously decreased during the replacing period of 750 min, but after that no noticeable change occurred in peak intensity. This kinetic pattern can be also confirmed by crosschecking them with the corresponding Raman peaks of N2 and CO2 (Figures 2b and 2c). The quantitative Raman analysis revealed that, 23% of CH4 in hydrate is replaced with N2, while 62% of CH4 is replaced with CO2. Accordingly, approximately 85% of CH4 encaged in saturated CH4 hydrate is recovered and, of course, this recovery rate might be expected to more or less change with variations of external variables such as pressure, temperature and hydrate particle size. The overall kinetic results  lead us to make a clear conclusion that the replacement of sI CH4 hydrate with N2 + CO2 mixture proceeds more effectively in crystalline hydrate than using only pure CO2 because N2 molecules is confirmed to possess the excellent cage-guest interaction in an unusual configuration. Even for simple hydrate systems focused in the present work the unique cage dynamics drawn from spectroscopic evidences might be expected to offer the new insight for better understanding of inclusion phenomena, particularly, host lattice- guest molecule interaction as well as guest-guest replacement mechanism. However, sII and sH hydrates, which are known to be formed by the influence of thermogenic hydrocarbon and mainly includes oil-related C1-C7 hydrocarbons, were discovered at shallow depth in sea floor sediment in a few sites such as the Gulf of Mexico or Cascadian margin [10-12]. Thus, it is also required to verify the swapping phenomena occurring on sII or sH type clathrate hydrate. For sII hydrate, C2H6 is specially selected to form the hydrate with CH4. We note that both CH4 and C2H6 form simple crystalline sI hydrates with water. But, when they are mixed within the limits of specific concentrations, they act as binary guests causing to form the stable sII double hydrate [13]. Figure 3 shows the 13C HPDEC MAS NMR spectra of mixed CH4 + C2H6 hydrates that are replaced with CO2 molecules. Three peaks representing the CH4 in sII-S, CH4 in sII-L and C2H6 in sII-L appeared at chemical shifts of -3.95, -7.7 and 6.4 ppm, respectively. Interestingly, during swapping process the external guest CO2 molecules attack both small and large cages for better occupancy, which causes the structure transition of sII to sI to continuously proceed. Within 24 hours the sII peaks almost disappeared and instead only a very small amount of CH4 in sI-S and sI-L and C2H6 in sI-L was detected at chemical shifts of -4.0, -6.1 and 7.7 ppm, respectively.  Figure 3. The 13C HPDEC MAS NMR spectra of sII CH4 +C2H6 hydrate replaced with CO2.  Figure 4. Relative moles in the sII CH4 + C2H6 hydrate replaced with CO2 measured by gas chromatography.  From structural viewpoint we think that the hydrate lattices are slightly adjusted to accommodate three guests of CH4, C2H6 and CO2 in the highly stabilized hydrate networks. The cage-specific behavior revealed by CO2 can be sufficiently expected according to its molecular dimension over a small cage. Thus, the approaching CO2 competes only with CH4 and C2H6 in sII-L at the initial stage of swapping. CH4 and C2H6 expelled from sII-L provoke losing sustainability of sII phase by getting out of the limit of critical guest concentration. The reestablishment process of guest molecule distribution in the hydrate network causes to alter and ultimately adjust the lattice dimension for structure transition to occur. The effect of a substantial small-cage reduction on CH4 recovery rate was checked by the GC analysis and the results are shown in Figure 4. During the swapping process, the CH4 and C2H6 molecules in hydrate phase continuously decrease until reaching the recovery rate of 92% for CH4 and 99% for C2H6. Both the NMR and GC results imply that most of CH4 molecules in sI-L as well as sI-S were displaced by CO2 molecules. The externally approaching CO2 guests attack and occupy most of the sII-S and sII-L cages accompanying structure transition of sII to sI. We note again that CO2 molecules possess a sufficient enclathration power to be entrapped in sI-S during change of sII to sI, while the CO2 occupancy to sI-S of pure CH4 hydrate is very difficult to occur. The 30% or more CH4 recovery enhancement in sII over 64% in sI is caused by structure transition totally altering the host-guest interactions during swapping. Furthermore, the naturally-occurring sII hydrates contain more amount of CH4 than the laboratory- made sII hydrates used in these experiments and thus the actual limitation of recoverable CH4 in sII hydrate would be higher than the present outcome of 92%. We also examined the swapping capacity of the N2 + CO2 mixture occurring in the mixed sII CH4 + C2H6 hydrate and found that the recovery rates are 95% for CH4 and 93% for C2H6.  Figure 5. The 13C HPDEC MAS NMR spectra of sH CH4 + isopentane hydrate replaced with CO2. In case of sH CH4 hydrate, structure transition was also occurred during the swapping process as shown in Figure 5. Before replacement, isopentane was entrapped in large cages of sH hydrate with CH4 in both small and middle cages. However, external CO2 gas provokes structure transition to sI type hydrate and finally sH phase disappeared. During the replacement, 92% of CH4 was recovered. In addition, by using N2 + CO2 mixture exceeding 90% of recovered CH4 readily achieved.  Type Replaced with CO2 (%) Replaced with N2+CO2 (%) sI (CH4) 64% 85% sII (CH4+C2H6) 92% 95% sH (isopentane+CH4) 92% 90% Table. 1. Recoverable CH4 (mol%) in various types of gas hydrates  CONCLUSION In this study, we investigated the swapping phenomena through flue gas mixtures of N2 and CO2 for efficiently developing gas hydrate in the deep ocean floor. The direct use of N2 + CO2 mixture enhanced CH4 recovery as well as eliminated the CO2 separation/purification process for sequestering CO2. In addition, a spectroscopic analysis reveals that the external N2 molecules attack CH4 molecules already entrapped in sI-S and play a significant role in substantially increasing the CH4 recovery rate. In particular, we performed the replacement experiment for naturally occurring sI, sII, sH hydrate. During the swapping the sII and sH CH4 hydrate, structure transition to sI were observed. The utilization of this natural swapping phenomenon might greatly contribute to realizing both ocean storage of CO2 and CH4 recovery from marine deposits in a large scale.  ACKNOWLEDGMENTS This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the National Research Lab. Program funded by the Ministry of Science and Technology (No. R0A-2005-000-10074-0(2007)), and Gas Hydrate Research and Development Project funded by the Ministry of Commerce, Industry, and Energy of Korea, also partially supported by the Brain Korea 21 Project.  REFERENCES [1] Haugan, P. M., Drange, H. Disposal of SO2 in sea water. Nature 1992;357:318-320 [2] Brewer, P. G., Friederich, G., Peltzer, E. T., Orr, Jr. F. M. Direct experiments on the ocean disposal of fossil fuel CO2. Science 1999;284:943- 945. [3] Milkov, A. V., Sassen, R. Economic geology of offshore gas hydrate accumulations, and provinces. Mar. Petrol. Geol. 2002;19:1-11. [4] Makogon, Y. F. 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