Article pubs.acs.org/est
Enhanced CH4 Recovery Induced via Structural Transformation in the CH4/CO2 Replacement That Occurs in sH Hydrates Yohan Lee, Yunju Kim, and Yongwon Seo* School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea S Supporting Information *
ABSTRACT: The CH4/CO2 replacement that occurs in sH hydrates is investigated, with a primary focus on the enhanced CH4 recovery induced via structural transformation with a CO2 injection. In this study, neohexane (NH) is used as a liquid hydrocarbon guest in the sH hydrates. Direct thermodynamic measurements and spectroscopic identification are investigated to reveal the replacement process for recovering CH4 and simultaneously sequestering CO2 in the sH (CH4 + NH) hydrate. The hydrate phase behavior and the 13C NMR and Raman spectroscopy results of the CH4 + CO2 + NH systems demonstrate that CO2 functions as a coguest of sH hydrates in CH4-rich conditions, and that the structural transition of sH to sI hydrates occurs in CO2-rich conditions. CO2 molecules are found to preferentially occupy the medium 435663 cages of sH hydrates or the large 51262 cages of sI hydrates during the replacement. Due to the favorable structural transition and resulting re-establishment of guest distributions, approximately 88% of the CH4 is recoverable from sH (CH4 + NH) hydrates with a CO2 injection. The hydrate dissociation and subsequent reformation caused by the structural transformation of sH to sI is also confirmed using a high-pressure microdifferential scanning calorimeter through the detection of the significant heat flows generated during the replacement.
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the CH4 production process.23 Furthermore, to mitigate global warming, anthropogenic CO2, which is primarily emitted from fossil-fuel-burning power plants, should be captured and sequestered into the ocean or underground chambers such as depleted gas and oil reservoirs, saline aquifers, and coal seams.24 The natural gas hydrate reservoir, where CH4 can be replaced by injected CO2, can be another potential site for the secure storage of CO2 for a long time. Therefore, the CH4/CO2 replacement in natural gas hydrates, which is thermodynamically spontaneous, has recently been proposed as a promising option for efficient CH 4 recovery and innovative CO 2 sequestration.25−41 Much research on the CH4/CO2 exchange in gas hydrates has been conducted, primarily focusing on kinetic and thermodynamic approaches to the CH4/CO2 replacement that occurs in sI hydrates.25−39 However, little attention has been paid to the CH4/CO2 replacement mechanism involved in sH hydrates.42 The cage-specific guest distributions, preferential partitioning of guest molecules, structural transition, hydrate dissociation and reformation, and complex hydrate behavior involved in the injection of external CO2 molecules into the
INTRODUCTION In the past, gas hydrate formation was considered a serious impediment to natural gas transportation through pipelines.1,2 However, recently it has been found that gas hydrates have many useful applications such as natural gas storage, CO2 capture and sequestration, and desalination.3−20 In particular, naturally occurring gas hydrates are potential future energy sources because large quantities of natural gas hydrates are known to be deposited in continental margins and in permafrost regions.1 Gas hydrates have three different structures (structure I (sI), structure II (sII), and structure H (sH)), which consist of differently sized and shaped cages.1 Most natural gas hydrates exist in the form of sI and sII depending on their inside C3H8 and isobutane concentrations because CH4, which is a significant component, forms sI hydrate, whereas C3H8 and isobutane, which are minor components, form sII hydrates.1 However, recent investigations have revealed that sH hydrates, which are capable of capturing larger liquid hydrocarbon molecules in the presence of help gases such as CH4 and N2 and have only been synthesized in the laboratory, also occur in the natural environment.21 To exploit the CH4 in the natural gas hydrate reservoirs, one can adopt an appropriate method such as depressurization, thermal stimulation, and inhibitor injection, considering the locations and circumstances of the reservoirs.22 However, these conventional methods might cause geological hazards during © XXXX American Chemical Society
Received: April 1, 2015 Revised: June 19, 2015 Accepted: June 24, 2015
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DOI: 10.1021/acs.est.5b01640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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abrupt pressure depression was then observed at the hydrate crystal growth after the nucleation stage. When the pressure depression due to the hydrate formation reached a steady-state condition, the temperature was increased in 0.1 K steps with sufficient time; accordingly, the cell pressure increased with the hydrate dissociation. After all hydrates were dissociated with the increasing temperature, the cell pressure was again slightly increased due to the thermal expansion. The hydrate equilibrium point at a given pressure was determined using the intersection between the hydrate dissociation and thermal expansion lines. To replace CH4 with CO2 in the sH hydrate formed in the silica gel pores, we cooled the reactor with liquid N2 to avoid the sH (CH4 +NH) hydrate dissociation during evacuation, and the CH4 vapor was quickly removed from the vessel. Then, precooled CO2 gas was injected into the reactor for the replacement. The replacement reaction using a high-pressure reactor was conducted at 274.15 K and 3.5 MPa for 72 h. NMR, Raman, and HP μDSC Measurements. After the hydrate formation or replacement was completed, the vessel was submerged in a liquid N2 bath to avoid hydrate dissociation; the vapor phase was then evacuated. The hydrate samples were taken and powdered in a liquid N2 vessel for the 13 C NMR and Raman analyses. A Bruker 400 MHz solid-state NMR spectrometer belonging to the Korea Basic Science Institute (KBSI) was used to identify the structure of the CH4 + CO2 + NH hydrates and to examine the cage occupancy of the guest molecules and guest distributions. The powdered hydrate samples were placed in a 4 mm o.d. Zr rotor that was loaded into a variable-temperature (VT) probe and analyzed at 243.15 K. All 13C NMR spectra were obtained at a Larmor frequency of 100.6 MHz with a magic angle spinning (MAS) between 2 and 4 kHz. A pulse length of 2 μs and a pulse repetition delay of 10 s under proton decoupling were used when a radio frequency field strength of 50 kHz corresponding to 5 μs 90° pulses was used. For the Raman spectroscopy analyses, the powdered hydrate samples were pelletized into cylindrical form (1 cm diameter and 0.3 cm height). Under the atmospheric pressure condition, the Raman spectra were collected using a WITech alpha300R Raman spectrometer (Germany) equipped with a 1800 groove/ mm holographic grating and a thermoelectrically cooled CCD detector. The excitation source was an He−Ne laser, which has a wavelength line of 532 nm and an intensity level of 35 mW. The temperature of the sample was maintained at approximately 123 K during the measurement by controlling the flow rate of the liquid N2 vapor. A high-pressure microdifferential scanning calorimeter (HP μDSC, VII Evo, Setaram Inc.) was used to monitor the heat flow, which can be liberated or absorbed during the replacement. The high-pressure cells of the HP μDSC were designed to function up to 40 MPa in a temperature range of 233.15−393.15 K and to contain 0.5 mL of the sample. The HP μDSC has a resolution of 0.02 μW with a temperature deviation of ±0.2 K. In the DSC experiments, approximately 0.07 g of NH-solution-saturated silica gel was charged into the sample cell, and both the reference and sample cells were flushed with CH4 gas to remove the residual air before proceeding to the conversion of NH solution into CH4 + NH hydrates. A multicycle mode of cooling and heating was adopted to completely convert the solution into gas hydrates. For the CH4/CO2 replacement experiments, to prevent any possible CH4 hydrate dissociation during the vapor phase exchange, we
existing sH hydrates have not been fully understood. Therefore, this study was extended to an exploration of the CH4/CO2 replacement in sH hydrates that have been confirmed to exist in deep sea sediments.21,43 In this study, neohexane (2,2-dimethylbutane, NH), which is the most well known of the sH-forming compounds, was used as a liquid hydrocarbon guest. Both macroscopic and microscopic analyses were conducted to reveal the replacement process for recovering CH4 and simultaneously sequestering CO2 in sH (CH4 + NH) hydrates through direct thermodynamic measurements and spectroscopic identification. The complex phase behavior of the CH4 + CO2 + NH system was measured to examine the possibility of both CH4 and CO2 as coguests in sH hydrates and of a structural transition. We also used 13C NMR and Raman spectroscopy to identify the crystal structure and to elucidate the cage occupancy and guest distributions in the CH4 + CO2 + NH hydrates. Moreover, the heat flows, which might emanate from the hydrate dissociation and reformation during the replacement, were monitored using a high-pressure microdifferential scanning calorimeter (HP μDSC).
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EXPERIMENTAL SECTION Materials and Methods. CH4 (99.95%), CO2 (99.99%), and gas mixtures of CH4 (20%) + CO2 (80%), CH4 (40%) + CO2 (60%), CH4 (60%) + CO2 (40%), and CH4 (80%) + CO2 (20%) were supplied by the MS Gas Co. Neohexane (99%) was supplied by the Sigma-Aldrich Co. Double-distilled and deionized water was used in the experiment. Porous silica gels (Silicycle, Quebec, Canada) with an average diameter of 100 nm and a pore volume of 0.83 mL/g were used for the replacement to maximize the gas−water or gas−solid contact area. All materials were used without further purification. The experimental apparatus for the hydrate phase equilibrium measurements was specifically designed to measure the hydrate dissociation pressures and temperatures accurately. The equilibrium cell was constructed from 316 stainless steel and had an internal volume of approximately 250 cm3. The two sapphire windows at the front and back of the cell allowed the visual observation of the phase transitions that occurred inside the equilibrium cell. The vessel was equipped with a sampling valve (Model 7010, Rheodyne) with a loop volume of 20 μL that was connected to a gas chromatograph (Agilent 7890) through a high-pressure metering pump (Eldex, California, USA). A thermocouple with an accuracy of ±0.1 K for full ranges was inserted into the cell to measure the temperature of the inner content. This thermocouple was calibrated using an ASTM 63C mercury thermometer (Ever Ready Thermometer) with a resolution of ±0.1 K. A pressure transducer (S-10, Wika, Germany) with an uncertainty of 0.01 MPa was used to measure the cell pressure. The pressure transducer was also calibrated using a Heise Bourdon tube pressure gauge (CMM137219, 0−10 MPa range) with a maximum error of ±0.01 MPa in the full range. Hydrate Phase Behavior and Replacement Experiment. The vessel was initially charged with 70 cm3 of the water and NH solution (5.0 mol %) and submerged in a water bath to control the temperature. After the equilibrium cell was pressurized to the desired pressure with the CH4 and CO2 gas, the whole main system was slowly cooled to a temperature lower than the expected equilibrium temperature. Due to thermal contraction, the cell pressure was slightly decreased by decreasing the temperature at a cooling rate of 1 K/h. An B
DOI: 10.1021/acs.est.5b01640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology immediately injected the precooled CO2 gas to both the reference cell and sample cell after the vapor phase evacuation of the sH hydrate. The replacement experiments using a HP μDSC were performed at 274.15 K and 3.5 MPa. A more detailed description of the experimental procedures can be found in our previous papers.44−47
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RESULTS AND DISCUSSION Stability Conditions of the CH4 + CO2 + NH Hydrates. Figure 1 presents the hydrate phase equilibria of the CH4 + CO2 + water and CH4 + CO2 + NH + water systems. The CH4 + NH + water system forms gas hydrates at thermodynamically promoted conditions, which represent lower pressures at a given temperature or higher temperatures at a given pressure in comparison to those of the CH4 + water system. This thermodynamic promotion effect observed in the CH4 + NH + water system indicates that the sH hydrate formation occurred as a result of the NH inclusion in the hydrate lattices. However, the extent of the equilibrium curve shifts of the CH4 + CO2 + NH systems due to the NH enclathration became smaller, and then negligible, as the CO2 concentration increased. In particular, in the CH4 (40%) + CO2 (60%) + NH + water system, a slight thermodynamic promotion was observed in the lower pressure regions, whereas the effect of the NH addition was not observed in the higher pressure regions, as clearly demonstrated in Figure 1b. Both CH4 and CO2 are known to function as helping gases in sH hydrates, but CO2 can effectively function as a sH-helping gas only at temperature ranges significantly below 270 K.1,48−50 Accordingly, in this study, the presence of NH did not affect the equilibrium curve shift of the CO2 + water system, which indicates the exclusion of NH in the hydrate lattice and the formation of sI hydrates. The hydrate phase equilibrium behavior of the CH4 + CO2 + NH systems with respect to the CO2 concentrations implies that CO2 can function as a coguest of sH hydrates with NH at low CO2 concentration ranges, even over 273 K, and that sH hydrates can be transformed into sI hydrates with increases in CO2 concentrations. Structure Identification of the CH4 + CO2 + NH Hydrates. Figure 2 presents a stacked plot of the 13C NMR spectra for the pure CH4, CH4 + NH, and CH4 + CO2 + NH hydrates in the chemical shift range from 0 to −10 ppm, where the CH4 molecules can be detected. The cage-dependent 13C NMR chemical shift for enclathrated CH4 molecules is an effective indicator for determining the structure types of the gas hydrates formed.51 The pure CH4 hydrate exhibited two resonance peaks at −4.3 and −6.6 ppm, which corresponded to the CH4 molecules captured in the small 512 cages and large 51262 cages of the sI hydrate, respectively. The CH4 + NH hydrate exhibited two resonance peaks at −4.5 and −4.9 ppm, which could be assigned to the CH4 molecules enclathrated in the small 512 cages and medium 435663 cages of the sH hydrate, respectively. The CH4 + CO2 + NH hydrates with lower CO2 concentrations exhibited two dominant resonance peaks from the sH hydrates, even though there were two minor resonance peaks from the sI hydrates. The coexistence of both the sI and sH peaks are attributed to the hydrate sampling conditions, in which both sI and sH hydrates can be nucleated, because the hydrate phase equilibrium curves of each CH4 + CO2 + NH system are located very close to those of each CH4 + CO2 system, as seen in Figure 1. However, the CH4 + CO2 + NH hydrates with higher CO2 concentrations exhibited two resonance peaks at −4.3 and −6.6 ppm that were identical to
Figure 1. Hydrate phase equilibria of the (a) CH4 + CO2 + NH + water systems and (b) the CH4 + NH + water, CH4 (40%) + CO2 (60%) + NH + water, and CO2 + NH + water systems.
those from the sI CH4 hydrate; accordingly, they indicated the sI hydrate formation. In particular, the CH4 (40%) + CO2 (60%) + NH hydrate was found to form sH hydrate in a lowC
DOI: 10.1021/acs.est.5b01640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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empty lattice relative to the ice. The value of Δμow for the sH hydrate was reported to be 1187.5 J/mol.54,55 From Figure 2, it can be seen that the intensity ratio of the CH4 molecules in the large 51262 to small 512 cages of the sI CH4 hydrate was 3.27, which corresponds to the chemical formula of CH4·6.03H2O. The intensity ratio of the CH4 molecules in the medium 435663 to small 512 cages of the sH (CH4 + NH) hydrate was 0.67, and the resulting cage occupancies were 0.75 (for θS,CH4), 0.75 (for θM,CH4), and 1.00 (for θL,NH) after considering the relative intensity of NH to CH4, which is equivalent to the chemical formula of 3.75CH4 · 1NH·34H2O. As seen in Figure 3, the intensity ratio of the CH4 molecules in the medium 435663 and small 512 cages (IM/IS) of the sH
Figure 2. 13C NMR spectra of the pure CH4, CH4 + NH, and CH4 + CO2 + NH hydrates. LP is the low pressure of 2.1 MPa, and HP is the high pressure of 6.1 MPa.
pressure condition (2.1 MPa), whereas it was found to form sI hydrate in a high-pressure condition (6.1 MPa). The 13C NMR spectra clearly demonstrated that the CH4-rich NH systems form sH hydrates, whereas the CO2-rich NH systems form sI hydrates, which is consistent with the hydrate phase behavior illustrated in Figure 1. As confirmed by the hydrate phase equilibria and 13C NMR spectra, the CH4 (40%) + CO2 (60%) is a transitional composition from sH to sI for the CH4 + CO2 + NH systems. In the 13C MAS NMR spectra, the intensities for each resonance peak are quantitatively proportional to the amount of guest molecules enclathrated in each cage of the sI and sH hydrates. To determine the occupancies of the CH4 and NH molecules in each cage of the sH hydrate, we combined the relative integrated intensities of the 13C NMR spectra with the following thermodynamic expression, which represents the chemical potential of water in the sH hydrate:52−54 μw (h) − μw (ho) =
Figure 3. IL/IS or IM/IS as a function of CO2 concentration. IL/IS and IM/IS represent the intensity ratio of the 13C NMR resonance peaks of the CH4 molecules in the large 51262 and small 512 cages of sI hydrates or those in the medium 435663 and small 512 cages of sH hydrates.
hydrate decreased with increases in the CO2 concentrations, which clearly demonstrated that CO2 preferentially occupied the medium 435663 cages of the sH hydrate because CO2 is a relatively poor guest when competing with CH4 in occupying the small 512 cages. After the structural transition from sH to sI occurred at a 60% CO2 concentration, the intensity ratio of the CH4 molecules in the large 51262 and small 512 cages (IL/IS) of the sI hydrate remained decreased with increases in CO2 concentrations, which again indicated the preferential occupation of CO2 in the large 51262 cages of sI. Figure 4a shows the Raman spectra of the pure CH4, CH4 + NH, CH4 (80%) + CO2 (20%) + NH, and CH4 (40%) + CO2 (60%) + NH hydrates. The pure CH4 hydrate exhibited two Raman peaks at 2903 and 2914 cm−1, which can be assigned to the CH4 molecules captured in the large 51262 cages and small 512 cages of the sI hydrate, respectively, whereas the CH4 + NH hydrate exhibited one Raman peak at 2911 cm−1 in the range of 2880−2940 cm−1, which can be assigned to the CH4 molecules enclathrated in both the small 512 cages and the medium 435663 cages of the sH hydrate. The Raman spectrum of the CH4 (80%) + CO2 (20%) + NH hydrate exhibited the coexistence of sH and sI, which was also observed in the 13C NMR
RT [3 ln(1 − θS,CH4) + 2 34 ln(1 − θM,CH4) + ln(1 − θL,NH)
Here, μw(ho)indicates the chemical potential of the water molecules of a hypothetical empty lattice and θS, θM, and θL are the fractional occupancies of the small 512, medium 435663, and large 51268 cages of the sH hydrate, respectively. When the hydrate is in an equilibrium state with ice, μw(h) − μw(ho) becomes −Δμow, where Δμow is the chemical potential of the D
DOI: 10.1021/acs.est.5b01640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 4. (a) Raman spectra of the pure CH4, CH4 + NH, and CH4 + CO2 + NH hydrates. (b) Raman spectra of the pure CO2, CO2 + NH, and CH4 + CO2 + NH hydrates. HP is the high pressure of 6.1 MPa.
exclusively at temperatures below 266.4 K. In this study, our experimental results from the phase behavior, 13C NMR, and Raman measurements clearly demonstrate that at CH4-rich concentrations, CO2 functions as a help gas of sH with NH even at the temperatures higher than 273.15 K; it is enclathrated preferentially in the medium 435663 cages of sH hydrates, also implying that CO2 injection into the natural sH hydrate reservoir could induce a structural transformation from the initial sH to sI, as the CH4/CO2 replacement reaction in sH hydrates proceeds. Influence of the Replacement on the Hydrate Structure. A 13C NMR spectrum of the CH4 molecules enclathrated in the CH4 + NH hydrate has already been presented in Figure 2. However, the 13C NMR spectra of all guest molecules, including CH4 and NH before and after the replacement, are presented in Figure 5. The 13C NMR spectrum of the CH4 + NH hydrate before the replacement exhibited four resonance peaks at 36.6, 29.9, 28.9, and 8.4 ppm for the NH molecules enclathrated in the large 51268 cages of sH hydrates as well as two resonance peaks at −4.5 and −4.9 ppm for the CH4 molecules enclathrated in the small 512 cages and the medium 435663 cages, respectively. It should be noted that there were also very small resonance peaks that could be assigned to unclathrated NH molecules; however, two were not pronounced due to their very close appearance and to their merging with neighboring major resonance peaks. After the replacement, resonance peaks corresponding to the CH4 molecules appeared at −4.3 and −6.6 ppm and were identical to those of the sI CH4 hydrate. The four resonance peaks at 36.6, 30.5, 28.8, and 8.9 ppm represent the unclathrated NH
measurement. The CH4 (40%) + CO2 (60%) + NH hydrate in a high-pressure condition (6.1 MPa) exhibited two Raman peaks at 2903 and 2914 cm−1. It should be noted that the area ratio of the two Raman peaks corresponding to the large 51262 and small 512 cages of the CH4 (40%) + CO2 (60%) + NH hydrate was smaller than that of the pure CH4 hydrate, even though the Raman peak positions of the CH4 (40%) + CO2 (60%) + NH hydrate were the same as those of the pure CH4 hydrate. This again indicated that the CH4 + CO2 + NH hydrates underwent a structural transformation from sH to sI at higher CO2 concentrations, and that CO2 predominantly occupied the large 51262 cages of sI hydrates. Figure 4b shows the Raman spectra of the pure CO2, CO2 + NH, CH4 (80%) + CO2 (20%) + NH, and CH4 (40%) + CO2 (60%) + NH hydrates. The pure CO2, CO2 + NH, and CH4 (40%) + CO2 (60%) hydrates, which were found to be sI hydrates, exhibited two Raman peaks for the enclathrated CO2 molecules at 1276 and 1380 cm−1, whereas the CH4 (80%) + CO2 (20%) + NH hydrate, which was found to be an sH hydrate, showed two Raman peaks at 1274 and 1380 cm−1. The Raman spectra for the CO2 molecules in the CH4 + CO2 + NH hydrates clearly indicated that the CO2 molecules were enclathrated in the cages of sH hydrates at the CH4-rich NH systems. Ripmeester and Ratcliffe48 first reported that CO2 could function as a help gas for sH hydrates with NH using ice particles. However, several researchers later revealed that CO2 is not able to form sH hydrates with NH at the temperatures above the freezing point of water.56,57 Then, Shen et al.49 found that the sH hydrate formed with CO2 and NH is stable E
DOI: 10.1021/acs.est.5b01640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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molecules that were excluded from the initial sH hydrate after the structural transition from sH to sI. In Figure 5, the intensity ratio of the CH4 molecules in the large 51262 cages and small 512 cages (IL/IS) of sI was 0.96 after the replacement, which also confirmed the preferential occupation of CO2 molecules in the large 51262 cages of the sI hydrate. For the isostructural replacement with a CO2 injection into the CH4 hydrate (sI to sI), it was reported that CO2 could replace approximately 68% of CH4 in the sI structure.25,31,33 However, in this study, when CO2 was injected into the sH (CH4 + NH) hydrate, a direct composition measurement using a gas chromatograph showed that the concentration of CO2 retrieved from the dissociation of the hydrate phase after the replacement was 88 ± 2% (Figure 6). This significantly higher extent of the replacement for the initial sH hydrate can be attributed to an increased CO2 occupancy in the cages of the sI hydrate during the restructuring process of the gas hydrate
structure from sH into sI. As clearly depicted in Figure 5, the initial sH (CH4 + NH) hydrate was transformed into sI after the CH4/CO2 replacement, and the intensity ratio of the CH4 molecules in the large 51262 cages and small 512 cages (IL/IS) was 0.96. It should be noted that in the previous study,33 the intensity ratio of the CH4 molecules in the large 51262 cages and small 512 cages (IL/IS) for the isostructural replacement (sI to sI) was 1.44, which corresponds to a 68% replacement. The lowered intensity ratio of 0.96 after the replacement that accompanied the structural transformation indicates that the CO2 occupancy in the large 51262 cages was more significantly increased than that in the small 512 cages under the hydrate dissociation and subsequent reconfiguration of the crystal structure, which leads to the re-establishment process of the guest distribution in the hydrate structure. In the isostructural replacement (sI to sI), recent MRI and DSC studies demonstrated that the initial CH4 hydrate was not dissociating into the liquid water or ice and then forming the CH4 + CO2 hydrate through only a partial breakup of the cage structures and subsequent exchange of CH4 and CO2 molecules.28,31,39 However, the structural transformation during the CH4/CO2 replacement that occurred in sH hydrates involves the dissociation of the sH hydrate and the reformation of the sI gas hydrate and alters the cage-filling characteristics of guest molecules and guest distributions in each cage; thus, it results in enhanced CH4 recovery via CO2 injection into the sH hydrates. In this study, the heat flow involved in the CH4/CO2 replacement that occurs in sH hydrates was monitored using a HP μDSC to confirm the dissociation and reformation process of the gas hydrate structures during the replacement. For the CH4/CO2 replacement in sH hydrates, a quick evacuation of the vapor phase of the CH4 + NH hydrate should be followed by an injection of precooled CO2. Any possible hydrate dissociation during the evacuation and injection must be minimized for the accurate quantification of the heat flow involved in the replacement process. As depicted in Figure 7, the heat flow changes in the CH4 gas evacuation and CO2 injection processes in the presence of the initial sI CH4 gas
Figure 6. Comparison of the extent of the replacement (sI → sI vs sH → sI).
Figure 7. Heat flows in the blank run, the CH4/CO2 replacement, and the (CH4 + NH)/CO2 replacement at 274.15 K.
Figure 5. 13C NMR spectra of all guest molecules, including CH4 and NH, before and after replacement.
F
DOI: 10.1021/acs.est.5b01640 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology hydrate (referred to as “CH4/CO2 replacement”) was comparable with that in the absence of the gas hydrate, which is referred to as a “blank run”, indicating that the gas hydrate dissociation during the gas evacuation and injection process for the replacement is almost negligible, and that there is no significant gas hydrate dissociation and reformation during the isostructural replacement as reported in previous studies.31,39 However, a large exothermic peak after the appearance of a significant endothermic peak was observed in the (CH4 + NH)/CO2 replacement, which indicates that the initial sH hydrate dissociation was followed by the formation of sI hydrate as the replacement reaction proceeded. The relatively smaller area for the endothermic peak compared to that for the exothermic peak can be attributed to the fact that a substantial portion of the endothermic peak from the sH hydrate dissociation was offset by the immediate appearance of the exothermic peak from the subsequent formation of the sI hydrate structure. The DSC result also supports the observation that the structural transformation can occur during the CH4/CO2 replacement in sH hydrates. The experimental results obtained in this study provide a better understanding of the complex phase behavior, cagespecific occupation of external guest molecules, and structural characteristics of the mixed gas hydrates for the dual purpose of CH4 recovery and CO2 sequestration, even though the kinetics and the extent of the replacement in the actual natural gas hydrate reservoirs are dependent on various factors such as the sediment pore sizes, mass transfer, hydrate particle size, and marine environment.
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pellet transport using the self-preservation effect: A techno-economic analysis. Energies 2012, 5, 2499−2523. (5) Khokhar, A. A.; Gudmundsson, J. S.; Sloan, E. D. Gas storage in structure H hydrates. Fluid Phase Equilib. 1998, 150, 383−392. (6) Xu, C.-G.; Li, X.-S. Research progress of hydrate-based CO2 separation and capture from gas mixtures. RSC Adv. 2014, 4, 18301− 18316. (7) Linga, P.; Adeyemo, A.; Englezos, P. Medium-pressure clathrate hydrate/membrane hybrid process for postcombustion capture of carbon dioxide. Environ. Sci. Technol. 2007, 42, 315−320. (8) Kumar, A.; Sakpal, T.; Linga, P.; Kumar, R. Enhanced carbon dioxide hydrate formation kinetics in a fixed bed reactor filled with metallic packing. Ind. Eng. Chem. Res. 2014, 53, 9849−9859. (9) Babu, P.; Kumar, R.; Linga, P. Pre-combustion capture of carbon dioxide in a fixed bed reactor using the clathrate hydrate process. Energy 2013, 50, 364−373. (10) Eslamimanesh, A.; Mohammadi, A. H.; Richon, D.; Naidoo, P.; Ramjugernath, D. Application of gas hydrate formation in separation processes: A review of experimental studies. J. Chem. Thermodyn. 2012, 46, 62−71. (11) Park, S.; Lee, S.; Lee, Y.; Lee, Y.; Seo, Y. Hydrate-based precombustion capture of carbon dioxide in the presence of a thermodynamic promoter and porous silica gels. Int. J. Greenhouse Gas Control 2013, 14, 193−199. (12) Park, S.; Lee, S.; Lee, Y.; Seo, Y. CO2 capture from simulated fuel gas mixtures using semiclathrate hydrates formed by quaternary ammonium salts. Environ. Sci. Technol. 2013, 47, 7571−7577. (13) Herslund, P. J.; Thomsen, K.; Abildskov, J.; von Solms, N. Modelling of tetrahydrofuran promoted gas hydrate systems for carbon dioxide capture processes. Fluid Phase Equilib. 2014, 375, 45− 65. (14) Duc, N. H.; Chauvy, F.; Herri, J.-M. CO2 capture by hydrate crystallization−a potential solution for gas emission of steelmaking industry. Energy Convers. Manage. 2007, 48, 1313−1322. (15) Ricaurte, M.; Dicharry, C.; Renaud, X.; Torre, J.-P. Combination of surfactants and organic compounds for boosting CO2 separation from natural gas by clathrate hydrate formation. Fuel 2014, 122, 206− 217. (16) Tajima, H.; Yamasaki, A.; Kiyono, F. Energy consumption estimation for greenhouse gas separation processes by clathrate hydrate formation. Energy 2004, 29, 1713−1729. (17) Dashti, H.; Yew, L. Z.; Lou, X. Recent advances in gas hydratebased CO2 capture. J. Nat. Gas. Chem. 2015, 23, 195−207. (18) Cha, J.-H.; Seol, Y. Increasing gas hydrate formation temperature for desalination of high salinity produced water with secondary guests. ACS Sustainable Chem. Eng. 2013, 1, 1218−1224. (19) Yang, M.; Song, Y.; Jiang, L.; Liu, W.; Dou, B.; Jing, W. Effects of operating mode and pressure on hydrate-based desalination and CO2 capture in porous media. Appl. Energy 2014, 135, 504−511. (20) Han, S.; Shin, J.-Y.; Rhee, Y.-W.; Kang, S.-P. Enhanced efficiency of salt removal from brine for cyclopentane hydrates by washing, centrifuging, and sweating. Desalination 2014, 354, 17−22. (21) Lu, H.; Seo, Y.-T.; Lee, J.-W.; Moudrakovski, I.; Ripmeester, J. A.; Chapman, N. R.; Coffin, R. B.; Gardner, G.; Pohlman, J. Complex gas hydrate from the Cascadia margin. Nature 2007, 445, 303−306. (22) Koh, C. A.; Sum, A. K.; Sloan, E. D. Gas hydrates: Unlocking the energy from icy cages. J. Appl. Phys. 2009, 106, 061101. (23) Gunn, D. A.; Nelder, L. M.; Rochelle, C. A.; Bateman, K.; Jackson, P. D.; Lovell, M. A.; Hobbs, P. R. N.; Long, D.; Rees, J. G.; Schultheiss, P.; Robers, J.; Francis, T. Towards improved ground models for slope instability evaluations through better characterization of sediment-hosted gas-hydrates. Terra Nova 2002, 14, 443−450. (24) Yamasaki, A.; Wakatsuki, M.; Teng, H.; Yanagisawa, Y.; Yamada, K. A new ocean disposal scenario for anthropogenic CO2: CO2 hydrate formation in a submerged crystallizer and its disposal. Energy 2000, 25, 85−96. (25) Lee, H.; Seo, Y.; Seo, Y.-T.; Moudrakovski, I.; Ripmeester, J. A. Recovering methane from solid methane hydrate with carbon dioxide. Angew. Chem., Int. Ed. 2003, 42, 5048−5051.
ASSOCIATED CONTENT
S Supporting Information *
Table S1: hydrate phase equilibrium data for the CH4 + NH, CO2 + NH, and CH4 + CO2 + NH systems. Figure S1: 13C NMR spectra of CH4 and NH before and after replacement (10 to −10 ppm). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.est.5b01640.
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AUTHOR INFORMATION
Corresponding Author
* Tel: +82-52-217-2821. Fax: +82-52-217-2819. E-mail: [email protected]. Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENTS This research was supported by the Midcareer Research Program through the National Research Foundation of Korea (NRF) founded by the Ministry of Science, ICT, and Future Planning (NRF-2014R1A2A1A11049950) and was also supported by the 2015 Research Fund (1.150033.01) of the Ulsan National Institute of Science & Technology (UNIST).
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REFERENCES
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(45) Lee, S.; Cha, I.; Seo, Y. Phase behavior and 13C NMR spectroscopic analysis of the mixed methane plus ethane plus propane hydrates in mesoporous silica gels. J. Phys. Chem. B 2010, 114, 15079− 15084. (46) Lee, S.; Lee, Y.; Park, S.; Kim, Y.; Lee, J. D.; Seo, Y. Thermodynamic and spectroscopic identification of guest gas enclathration in the double tetra-n-butylammonium fluoride semiclathrates. J. Phys. Chem. B 2012, 116, 9075−9081. (47) Lee, Y.; Lee, S.; Lee, J.; Seo, Y. Structure identification and dissociation enthalpy measurements of the CO2 + N2 hydrates for their application to CO2 capture and storage. Chem. Eng. J. (Amsterdam, Neth.) 2014, 246, 20−26. (48) Ripmeester, J. A.; Ratcliffe, C. I. The diverse nature of dodecahedral cages in clathrate hydrates as revealed by 129Xe and 13C NMR spectroscopy: CO2 as a small-cage guest. Energy Fuels 1998, 12, 197−200. (49) Shen, R.; Tezuka, K.; Uchida, T.; Ohmura, R. Hydrate phase equilibrium in the system of (carbon dioxide + 2, 2-dimethylbutane + water) at temperatures below freezing point of water. J. Chem. Thermodyn. 2012, 53, 27−29. (50) Tezuka, K.; Shen, R.; Watanabe, T.; Takeya, S.; Alavi, S.; Ripmeester, J. A.; Ohmura, R. Synthesis and characterization of a structure H hydrate formed with carbon dioxide and 3, 3-dimethyl-2butanone. Chem. Commun. 2013, 49, 505−507. (51) Ripmeester, J. A.; Ratcliffe, C. I. On the contributions of NMR spectroscopy to clathrate science. J. Struct. Chem. 1999, 40, 654−662. (52) van der Waals, J. H.; Platteeuw, J. C. Clathrate Solutions. Adv. Chem. Phys. 1959, 2, 1−58. (53) Ripmeester, J. A.; Ratcliffe, C. I. Low-temperature crosspolarization/magic angle spinning 13C NMR of solid methane hydrates: structure, cage occupancy, and hydration number. J. Phys. Chem. 1988, 92, 337−339. (54) Seo, Y. T.; Lee, H. 13C NMR analysis and gas uptake measurements of pure and mixed gas hydrates: development of natural gas transport and storage method using gas hydrate. Korean J. Chem. Eng. 2003, 20, 1085−1091. (55) Mehta, A. P.; Sloan, E. D. Improved thermodynamic parameters for prediction of structure H hydrate equilibria. AIChE J. 1996, 42, 2036−2046. (56) Servio, P.; Lagers, F.; Peters, C.; Englezos, P. Gas hydrate phase equilibrium in the system methane−carbon dioxide−neohexane and water. Fluid Phase Equilib. 1999, 158−160, 795−880. (57) Uchida, T.; Ohmura, R.; Ikeda, I. Y.; Nagao, J.; Takeya, S.; Hori, A. Phase equilibrium measurements and crystallographic analyses on structure-H type gas hydrate formed from the CH4−CO2−neohexanewater system. J. Phys. Chem. B 2006, 110, 4583−4588. (58) Adisasmito, S.; Frank, R. J.; Sloan, E. D. Hydrates of carbon dioxide and methane mixtures. J. Chem. Eng. Data 1991, 36, 68−71.
(26) Dornan, P.; Alavi, S.; Woo, T. K. Determination of NMR lineshape anisotropy of guest molecules within inclusion complexes from molecular dynamics simulations. J. Chem. Phys. 2007, 127, 124510. (27) Ota, M.; Morohashi, K.; Abe, Y.; Watanabe, M.; Smith, J. R. L.; Inomata, H. Replacement of CH4 in the hydrate by use of liquid CO2. Energy Convers. Manage. 2005, 46, 1680−1691. (28) Ersland, G.; Husebø, J.; Graue, A.; Baldwin, B. A.; Howard, J.; Stevens, J. Measuring gas hydrate formation and exchange with CO2 in Bentheim sandstone using MRI tomography. Chem. Eng. J. (Amsterdam, Neth.) 2010, 158, 25−31. (29) Bai, D.; Zhang, X.; Chen, G.; Wang, W. Replacement mechanism of methane hydrate with carbon dioxide from microsecond molecular dynamics simulations. Energy Environ. Sci. 2012, 5, 7033− 7041. (30) 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, 47−58. (31) Lee, S.; Lee, Y.; Lee, J.; Lee, H.; Seo, Y. Experimental verification of methane−carbon dioxide replacement in natural gas hydrates using a differential scanning calorimeter. Environ. Sci. Technol. 2013, 47, 13184−13190. (32) Falenty, A.; Murshed, M. N.; Salamatin, A. N.; Kuhs, W. F. Gas Replacement in Clathrate Hydrates during CO2 Injection - Kinetics and Micro-Structural Mechanism. Proc. ISOPE Ocean Min. Symp., Szczecin, 2013, 10. (33) Lee, S.; Park, S.; Lee, Y.; Seo, Y. Thermodynamic and 13C NMR spectroscopic verification of methane−carbon dioxide replacement in natural gas hydrates. Chem. Eng. J. (Amsterdam, Neth.) 2013, 225, 636−640. (34) Liu, B.; Pan, H.; Wang, X.; Li, F.; Sun, C.; Chen, G. Evaluation of different CH4-CO2 replacement processes in hydrate-bearing sediments by measuring P-wave velocity. Energies 2013, 6, 6242−6254. (35) Lee, B. R.; Koh, C. A.; Sum, A. K. Quantitative measurement and mechanisms for CH4 production from hydrates with the injection of liquid CO2. Phys. Chem. Chem. Phys. 2014, 16, 14922−14927. (36) Koh, D. Y.; Kang, H.; Kim, D. O.; Park, J.; Cha, M.; Lee, H. Recovery of methane from gas hydrates intercalated within natural sediments using CO2 and a CO2/N2 Gas Mixture. ChemSusChem. 2012, 5, 1443−1438. (37) Qin, J.; Kuhs, W. F. Recovering CH4 from clathrate hydrates with CO2 + N2 gas mixtures: quantitative Raman study. Proc. Int. Conf. Gas Hydrates. Beijing, 2014, 8. (38) Shicks, J. M.; Luzi, M.; Beeskow-Strauch, B. The conversion process of hydrocarbon hydrates into CO2 hydrates and vice versa: Thermodynamic considerations. J. Phys. Chem. A 2011, 115, 13324− 13331. (39) Lee, Y.; Kim, Y.; Lee, J.; Lee, H.; Seo, Y. CH4 recovery and CO2 sequestration using flue gas in natural gas hydrates as revealed by a micro-differential scanning calorimeter. Appl. Energy 2015, 150, 120− 127. (40) 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. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 12690−12694. (41) Falenty, A.; Qin, J.; Salamatin, A. N.; Kuhs, W. F. In-situ replacement kinetics of (sII) ethane-methane hydrates exposed to CO2. Proc. Int. Conf. Gas Hydrates. Beijing, 2014, 8. (42) Shin, K.; Park, Y.; Cha, M.; Park, K.-P.; Huh, D.-G.; Lee, J.; Kim, S.-J.; Lee, H. Swapping phenomena occurring in deep-sea gas hydrates. Energy Fuels 2008, 22, 3160−3163. (43) Lorenson, T. D.; Collett, T. S. Gas content and composition of gas hydrate from sediments of the southeastern North American continental margin. Proc. Ocean Drill. Program: , Sci. Results 2000, 164, 37−46. (44) Seo, Y.; Lee, H. Phase behavior and structure identification of the mixed chlorinated hydrocarbon clathrate hydrates. J. Phys. Chem. B 2002, 106, 9668−9673. H
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Enhanced CH4 Recovery Induced via Structural Transformation in the CH4/CO2 Replacement That Occurs in sH Hydrates Yohan Lee, Yunju Kim, and Yongwon Seo* School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea S Supporting Information *
ABSTRACT: The CH4/CO2 replacement that occurs in sH hydrates is investigated, with a primary focus on the enhanced CH4 recovery induced via structural transformation with a CO2 injection. In this study, neohexane (NH) is used as a liquid hydrocarbon guest in the sH hydrates. Direct thermodynamic measurements and spectroscopic identification are investigated to reveal the replacement process for recovering CH4 and simultaneously sequestering CO2 in the sH (CH4 + NH) hydrate. The hydrate phase behavior and the 13C NMR and Raman spectroscopy results of the CH4 + CO2 + NH systems demonstrate that CO2 functions as a coguest of sH hydrates in CH4-rich conditions, and that the structural transition of sH to sI hydrates occurs in CO2-rich conditions. CO2 molecules are found to preferentially occupy the medium 435663 cages of sH hydrates or the large 51262 cages of sI hydrates during the replacement. Due to the favorable structural transition and resulting re-establishment of guest distributions, approximately 88% of the CH4 is recoverable from sH (CH4 + NH) hydrates with a CO2 injection. The hydrate dissociation and subsequent reformation caused by the structural transformation of sH to sI is also confirmed using a high-pressure microdifferential scanning calorimeter through the detection of the significant heat flows generated during the replacement.
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the CH4 production process.23 Furthermore, to mitigate global warming, anthropogenic CO2, which is primarily emitted from fossil-fuel-burning power plants, should be captured and sequestered into the ocean or underground chambers such as depleted gas and oil reservoirs, saline aquifers, and coal seams.24 The natural gas hydrate reservoir, where CH4 can be replaced by injected CO2, can be another potential site for the secure storage of CO2 for a long time. Therefore, the CH4/CO2 replacement in natural gas hydrates, which is thermodynamically spontaneous, has recently been proposed as a promising option for efficient CH 4 recovery and innovative CO 2 sequestration.25−41 Much research on the CH4/CO2 exchange in gas hydrates has been conducted, primarily focusing on kinetic and thermodynamic approaches to the CH4/CO2 replacement that occurs in sI hydrates.25−39 However, little attention has been paid to the CH4/CO2 replacement mechanism involved in sH hydrates.42 The cage-specific guest distributions, preferential partitioning of guest molecules, structural transition, hydrate dissociation and reformation, and complex hydrate behavior involved in the injection of external CO2 molecules into the
INTRODUCTION In the past, gas hydrate formation was considered a serious impediment to natural gas transportation through pipelines.1,2 However, recently it has been found that gas hydrates have many useful applications such as natural gas storage, CO2 capture and sequestration, and desalination.3−20 In particular, naturally occurring gas hydrates are potential future energy sources because large quantities of natural gas hydrates are known to be deposited in continental margins and in permafrost regions.1 Gas hydrates have three different structures (structure I (sI), structure II (sII), and structure H (sH)), which consist of differently sized and shaped cages.1 Most natural gas hydrates exist in the form of sI and sII depending on their inside C3H8 and isobutane concentrations because CH4, which is a significant component, forms sI hydrate, whereas C3H8 and isobutane, which are minor components, form sII hydrates.1 However, recent investigations have revealed that sH hydrates, which are capable of capturing larger liquid hydrocarbon molecules in the presence of help gases such as CH4 and N2 and have only been synthesized in the laboratory, also occur in the natural environment.21 To exploit the CH4 in the natural gas hydrate reservoirs, one can adopt an appropriate method such as depressurization, thermal stimulation, and inhibitor injection, considering the locations and circumstances of the reservoirs.22 However, these conventional methods might cause geological hazards during © XXXX American Chemical Society
Received: April 1, 2015 Revised: June 19, 2015 Accepted: June 24, 2015
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abrupt pressure depression was then observed at the hydrate crystal growth after the nucleation stage. When the pressure depression due to the hydrate formation reached a steady-state condition, the temperature was increased in 0.1 K steps with sufficient time; accordingly, the cell pressure increased with the hydrate dissociation. After all hydrates were dissociated with the increasing temperature, the cell pressure was again slightly increased due to the thermal expansion. The hydrate equilibrium point at a given pressure was determined using the intersection between the hydrate dissociation and thermal expansion lines. To replace CH4 with CO2 in the sH hydrate formed in the silica gel pores, we cooled the reactor with liquid N2 to avoid the sH (CH4 +NH) hydrate dissociation during evacuation, and the CH4 vapor was quickly removed from the vessel. Then, precooled CO2 gas was injected into the reactor for the replacement. The replacement reaction using a high-pressure reactor was conducted at 274.15 K and 3.5 MPa for 72 h. NMR, Raman, and HP μDSC Measurements. After the hydrate formation or replacement was completed, the vessel was submerged in a liquid N2 bath to avoid hydrate dissociation; the vapor phase was then evacuated. The hydrate samples were taken and powdered in a liquid N2 vessel for the 13 C NMR and Raman analyses. A Bruker 400 MHz solid-state NMR spectrometer belonging to the Korea Basic Science Institute (KBSI) was used to identify the structure of the CH4 + CO2 + NH hydrates and to examine the cage occupancy of the guest molecules and guest distributions. The powdered hydrate samples were placed in a 4 mm o.d. Zr rotor that was loaded into a variable-temperature (VT) probe and analyzed at 243.15 K. All 13C NMR spectra were obtained at a Larmor frequency of 100.6 MHz with a magic angle spinning (MAS) between 2 and 4 kHz. A pulse length of 2 μs and a pulse repetition delay of 10 s under proton decoupling were used when a radio frequency field strength of 50 kHz corresponding to 5 μs 90° pulses was used. For the Raman spectroscopy analyses, the powdered hydrate samples were pelletized into cylindrical form (1 cm diameter and 0.3 cm height). Under the atmospheric pressure condition, the Raman spectra were collected using a WITech alpha300R Raman spectrometer (Germany) equipped with a 1800 groove/ mm holographic grating and a thermoelectrically cooled CCD detector. The excitation source was an He−Ne laser, which has a wavelength line of 532 nm and an intensity level of 35 mW. The temperature of the sample was maintained at approximately 123 K during the measurement by controlling the flow rate of the liquid N2 vapor. A high-pressure microdifferential scanning calorimeter (HP μDSC, VII Evo, Setaram Inc.) was used to monitor the heat flow, which can be liberated or absorbed during the replacement. The high-pressure cells of the HP μDSC were designed to function up to 40 MPa in a temperature range of 233.15−393.15 K and to contain 0.5 mL of the sample. The HP μDSC has a resolution of 0.02 μW with a temperature deviation of ±0.2 K. In the DSC experiments, approximately 0.07 g of NH-solution-saturated silica gel was charged into the sample cell, and both the reference and sample cells were flushed with CH4 gas to remove the residual air before proceeding to the conversion of NH solution into CH4 + NH hydrates. A multicycle mode of cooling and heating was adopted to completely convert the solution into gas hydrates. For the CH4/CO2 replacement experiments, to prevent any possible CH4 hydrate dissociation during the vapor phase exchange, we
existing sH hydrates have not been fully understood. Therefore, this study was extended to an exploration of the CH4/CO2 replacement in sH hydrates that have been confirmed to exist in deep sea sediments.21,43 In this study, neohexane (2,2-dimethylbutane, NH), which is the most well known of the sH-forming compounds, was used as a liquid hydrocarbon guest. Both macroscopic and microscopic analyses were conducted to reveal the replacement process for recovering CH4 and simultaneously sequestering CO2 in sH (CH4 + NH) hydrates through direct thermodynamic measurements and spectroscopic identification. The complex phase behavior of the CH4 + CO2 + NH system was measured to examine the possibility of both CH4 and CO2 as coguests in sH hydrates and of a structural transition. We also used 13C NMR and Raman spectroscopy to identify the crystal structure and to elucidate the cage occupancy and guest distributions in the CH4 + CO2 + NH hydrates. Moreover, the heat flows, which might emanate from the hydrate dissociation and reformation during the replacement, were monitored using a high-pressure microdifferential scanning calorimeter (HP μDSC).
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EXPERIMENTAL SECTION Materials and Methods. CH4 (99.95%), CO2 (99.99%), and gas mixtures of CH4 (20%) + CO2 (80%), CH4 (40%) + CO2 (60%), CH4 (60%) + CO2 (40%), and CH4 (80%) + CO2 (20%) were supplied by the MS Gas Co. Neohexane (99%) was supplied by the Sigma-Aldrich Co. Double-distilled and deionized water was used in the experiment. Porous silica gels (Silicycle, Quebec, Canada) with an average diameter of 100 nm and a pore volume of 0.83 mL/g were used for the replacement to maximize the gas−water or gas−solid contact area. All materials were used without further purification. The experimental apparatus for the hydrate phase equilibrium measurements was specifically designed to measure the hydrate dissociation pressures and temperatures accurately. The equilibrium cell was constructed from 316 stainless steel and had an internal volume of approximately 250 cm3. The two sapphire windows at the front and back of the cell allowed the visual observation of the phase transitions that occurred inside the equilibrium cell. The vessel was equipped with a sampling valve (Model 7010, Rheodyne) with a loop volume of 20 μL that was connected to a gas chromatograph (Agilent 7890) through a high-pressure metering pump (Eldex, California, USA). A thermocouple with an accuracy of ±0.1 K for full ranges was inserted into the cell to measure the temperature of the inner content. This thermocouple was calibrated using an ASTM 63C mercury thermometer (Ever Ready Thermometer) with a resolution of ±0.1 K. A pressure transducer (S-10, Wika, Germany) with an uncertainty of 0.01 MPa was used to measure the cell pressure. The pressure transducer was also calibrated using a Heise Bourdon tube pressure gauge (CMM137219, 0−10 MPa range) with a maximum error of ±0.01 MPa in the full range. Hydrate Phase Behavior and Replacement Experiment. The vessel was initially charged with 70 cm3 of the water and NH solution (5.0 mol %) and submerged in a water bath to control the temperature. After the equilibrium cell was pressurized to the desired pressure with the CH4 and CO2 gas, the whole main system was slowly cooled to a temperature lower than the expected equilibrium temperature. Due to thermal contraction, the cell pressure was slightly decreased by decreasing the temperature at a cooling rate of 1 K/h. An B
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Environmental Science & Technology immediately injected the precooled CO2 gas to both the reference cell and sample cell after the vapor phase evacuation of the sH hydrate. The replacement experiments using a HP μDSC were performed at 274.15 K and 3.5 MPa. A more detailed description of the experimental procedures can be found in our previous papers.44−47
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RESULTS AND DISCUSSION Stability Conditions of the CH4 + CO2 + NH Hydrates. Figure 1 presents the hydrate phase equilibria of the CH4 + CO2 + water and CH4 + CO2 + NH + water systems. The CH4 + NH + water system forms gas hydrates at thermodynamically promoted conditions, which represent lower pressures at a given temperature or higher temperatures at a given pressure in comparison to those of the CH4 + water system. This thermodynamic promotion effect observed in the CH4 + NH + water system indicates that the sH hydrate formation occurred as a result of the NH inclusion in the hydrate lattices. However, the extent of the equilibrium curve shifts of the CH4 + CO2 + NH systems due to the NH enclathration became smaller, and then negligible, as the CO2 concentration increased. In particular, in the CH4 (40%) + CO2 (60%) + NH + water system, a slight thermodynamic promotion was observed in the lower pressure regions, whereas the effect of the NH addition was not observed in the higher pressure regions, as clearly demonstrated in Figure 1b. Both CH4 and CO2 are known to function as helping gases in sH hydrates, but CO2 can effectively function as a sH-helping gas only at temperature ranges significantly below 270 K.1,48−50 Accordingly, in this study, the presence of NH did not affect the equilibrium curve shift of the CO2 + water system, which indicates the exclusion of NH in the hydrate lattice and the formation of sI hydrates. The hydrate phase equilibrium behavior of the CH4 + CO2 + NH systems with respect to the CO2 concentrations implies that CO2 can function as a coguest of sH hydrates with NH at low CO2 concentration ranges, even over 273 K, and that sH hydrates can be transformed into sI hydrates with increases in CO2 concentrations. Structure Identification of the CH4 + CO2 + NH Hydrates. Figure 2 presents a stacked plot of the 13C NMR spectra for the pure CH4, CH4 + NH, and CH4 + CO2 + NH hydrates in the chemical shift range from 0 to −10 ppm, where the CH4 molecules can be detected. The cage-dependent 13C NMR chemical shift for enclathrated CH4 molecules is an effective indicator for determining the structure types of the gas hydrates formed.51 The pure CH4 hydrate exhibited two resonance peaks at −4.3 and −6.6 ppm, which corresponded to the CH4 molecules captured in the small 512 cages and large 51262 cages of the sI hydrate, respectively. The CH4 + NH hydrate exhibited two resonance peaks at −4.5 and −4.9 ppm, which could be assigned to the CH4 molecules enclathrated in the small 512 cages and medium 435663 cages of the sH hydrate, respectively. The CH4 + CO2 + NH hydrates with lower CO2 concentrations exhibited two dominant resonance peaks from the sH hydrates, even though there were two minor resonance peaks from the sI hydrates. The coexistence of both the sI and sH peaks are attributed to the hydrate sampling conditions, in which both sI and sH hydrates can be nucleated, because the hydrate phase equilibrium curves of each CH4 + CO2 + NH system are located very close to those of each CH4 + CO2 system, as seen in Figure 1. However, the CH4 + CO2 + NH hydrates with higher CO2 concentrations exhibited two resonance peaks at −4.3 and −6.6 ppm that were identical to
Figure 1. Hydrate phase equilibria of the (a) CH4 + CO2 + NH + water systems and (b) the CH4 + NH + water, CH4 (40%) + CO2 (60%) + NH + water, and CO2 + NH + water systems.
those from the sI CH4 hydrate; accordingly, they indicated the sI hydrate formation. In particular, the CH4 (40%) + CO2 (60%) + NH hydrate was found to form sH hydrate in a lowC
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empty lattice relative to the ice. The value of Δμow for the sH hydrate was reported to be 1187.5 J/mol.54,55 From Figure 2, it can be seen that the intensity ratio of the CH4 molecules in the large 51262 to small 512 cages of the sI CH4 hydrate was 3.27, which corresponds to the chemical formula of CH4·6.03H2O. The intensity ratio of the CH4 molecules in the medium 435663 to small 512 cages of the sH (CH4 + NH) hydrate was 0.67, and the resulting cage occupancies were 0.75 (for θS,CH4), 0.75 (for θM,CH4), and 1.00 (for θL,NH) after considering the relative intensity of NH to CH4, which is equivalent to the chemical formula of 3.75CH4 · 1NH·34H2O. As seen in Figure 3, the intensity ratio of the CH4 molecules in the medium 435663 and small 512 cages (IM/IS) of the sH
Figure 2. 13C NMR spectra of the pure CH4, CH4 + NH, and CH4 + CO2 + NH hydrates. LP is the low pressure of 2.1 MPa, and HP is the high pressure of 6.1 MPa.
pressure condition (2.1 MPa), whereas it was found to form sI hydrate in a high-pressure condition (6.1 MPa). The 13C NMR spectra clearly demonstrated that the CH4-rich NH systems form sH hydrates, whereas the CO2-rich NH systems form sI hydrates, which is consistent with the hydrate phase behavior illustrated in Figure 1. As confirmed by the hydrate phase equilibria and 13C NMR spectra, the CH4 (40%) + CO2 (60%) is a transitional composition from sH to sI for the CH4 + CO2 + NH systems. In the 13C MAS NMR spectra, the intensities for each resonance peak are quantitatively proportional to the amount of guest molecules enclathrated in each cage of the sI and sH hydrates. To determine the occupancies of the CH4 and NH molecules in each cage of the sH hydrate, we combined the relative integrated intensities of the 13C NMR spectra with the following thermodynamic expression, which represents the chemical potential of water in the sH hydrate:52−54 μw (h) − μw (ho) =
Figure 3. IL/IS or IM/IS as a function of CO2 concentration. IL/IS and IM/IS represent the intensity ratio of the 13C NMR resonance peaks of the CH4 molecules in the large 51262 and small 512 cages of sI hydrates or those in the medium 435663 and small 512 cages of sH hydrates.
hydrate decreased with increases in the CO2 concentrations, which clearly demonstrated that CO2 preferentially occupied the medium 435663 cages of the sH hydrate because CO2 is a relatively poor guest when competing with CH4 in occupying the small 512 cages. After the structural transition from sH to sI occurred at a 60% CO2 concentration, the intensity ratio of the CH4 molecules in the large 51262 and small 512 cages (IL/IS) of the sI hydrate remained decreased with increases in CO2 concentrations, which again indicated the preferential occupation of CO2 in the large 51262 cages of sI. Figure 4a shows the Raman spectra of the pure CH4, CH4 + NH, CH4 (80%) + CO2 (20%) + NH, and CH4 (40%) + CO2 (60%) + NH hydrates. The pure CH4 hydrate exhibited two Raman peaks at 2903 and 2914 cm−1, which can be assigned to the CH4 molecules captured in the large 51262 cages and small 512 cages of the sI hydrate, respectively, whereas the CH4 + NH hydrate exhibited one Raman peak at 2911 cm−1 in the range of 2880−2940 cm−1, which can be assigned to the CH4 molecules enclathrated in both the small 512 cages and the medium 435663 cages of the sH hydrate. The Raman spectrum of the CH4 (80%) + CO2 (20%) + NH hydrate exhibited the coexistence of sH and sI, which was also observed in the 13C NMR
RT [3 ln(1 − θS,CH4) + 2 34 ln(1 − θM,CH4) + ln(1 − θL,NH)
Here, μw(ho)indicates the chemical potential of the water molecules of a hypothetical empty lattice and θS, θM, and θL are the fractional occupancies of the small 512, medium 435663, and large 51268 cages of the sH hydrate, respectively. When the hydrate is in an equilibrium state with ice, μw(h) − μw(ho) becomes −Δμow, where Δμow is the chemical potential of the D
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Figure 4. (a) Raman spectra of the pure CH4, CH4 + NH, and CH4 + CO2 + NH hydrates. (b) Raman spectra of the pure CO2, CO2 + NH, and CH4 + CO2 + NH hydrates. HP is the high pressure of 6.1 MPa.
exclusively at temperatures below 266.4 K. In this study, our experimental results from the phase behavior, 13C NMR, and Raman measurements clearly demonstrate that at CH4-rich concentrations, CO2 functions as a help gas of sH with NH even at the temperatures higher than 273.15 K; it is enclathrated preferentially in the medium 435663 cages of sH hydrates, also implying that CO2 injection into the natural sH hydrate reservoir could induce a structural transformation from the initial sH to sI, as the CH4/CO2 replacement reaction in sH hydrates proceeds. Influence of the Replacement on the Hydrate Structure. A 13C NMR spectrum of the CH4 molecules enclathrated in the CH4 + NH hydrate has already been presented in Figure 2. However, the 13C NMR spectra of all guest molecules, including CH4 and NH before and after the replacement, are presented in Figure 5. The 13C NMR spectrum of the CH4 + NH hydrate before the replacement exhibited four resonance peaks at 36.6, 29.9, 28.9, and 8.4 ppm for the NH molecules enclathrated in the large 51268 cages of sH hydrates as well as two resonance peaks at −4.5 and −4.9 ppm for the CH4 molecules enclathrated in the small 512 cages and the medium 435663 cages, respectively. It should be noted that there were also very small resonance peaks that could be assigned to unclathrated NH molecules; however, two were not pronounced due to their very close appearance and to their merging with neighboring major resonance peaks. After the replacement, resonance peaks corresponding to the CH4 molecules appeared at −4.3 and −6.6 ppm and were identical to those of the sI CH4 hydrate. The four resonance peaks at 36.6, 30.5, 28.8, and 8.9 ppm represent the unclathrated NH
measurement. The CH4 (40%) + CO2 (60%) + NH hydrate in a high-pressure condition (6.1 MPa) exhibited two Raman peaks at 2903 and 2914 cm−1. It should be noted that the area ratio of the two Raman peaks corresponding to the large 51262 and small 512 cages of the CH4 (40%) + CO2 (60%) + NH hydrate was smaller than that of the pure CH4 hydrate, even though the Raman peak positions of the CH4 (40%) + CO2 (60%) + NH hydrate were the same as those of the pure CH4 hydrate. This again indicated that the CH4 + CO2 + NH hydrates underwent a structural transformation from sH to sI at higher CO2 concentrations, and that CO2 predominantly occupied the large 51262 cages of sI hydrates. Figure 4b shows the Raman spectra of the pure CO2, CO2 + NH, CH4 (80%) + CO2 (20%) + NH, and CH4 (40%) + CO2 (60%) + NH hydrates. The pure CO2, CO2 + NH, and CH4 (40%) + CO2 (60%) hydrates, which were found to be sI hydrates, exhibited two Raman peaks for the enclathrated CO2 molecules at 1276 and 1380 cm−1, whereas the CH4 (80%) + CO2 (20%) + NH hydrate, which was found to be an sH hydrate, showed two Raman peaks at 1274 and 1380 cm−1. The Raman spectra for the CO2 molecules in the CH4 + CO2 + NH hydrates clearly indicated that the CO2 molecules were enclathrated in the cages of sH hydrates at the CH4-rich NH systems. Ripmeester and Ratcliffe48 first reported that CO2 could function as a help gas for sH hydrates with NH using ice particles. However, several researchers later revealed that CO2 is not able to form sH hydrates with NH at the temperatures above the freezing point of water.56,57 Then, Shen et al.49 found that the sH hydrate formed with CO2 and NH is stable E
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molecules that were excluded from the initial sH hydrate after the structural transition from sH to sI. In Figure 5, the intensity ratio of the CH4 molecules in the large 51262 cages and small 512 cages (IL/IS) of sI was 0.96 after the replacement, which also confirmed the preferential occupation of CO2 molecules in the large 51262 cages of the sI hydrate. For the isostructural replacement with a CO2 injection into the CH4 hydrate (sI to sI), it was reported that CO2 could replace approximately 68% of CH4 in the sI structure.25,31,33 However, in this study, when CO2 was injected into the sH (CH4 + NH) hydrate, a direct composition measurement using a gas chromatograph showed that the concentration of CO2 retrieved from the dissociation of the hydrate phase after the replacement was 88 ± 2% (Figure 6). This significantly higher extent of the replacement for the initial sH hydrate can be attributed to an increased CO2 occupancy in the cages of the sI hydrate during the restructuring process of the gas hydrate
structure from sH into sI. As clearly depicted in Figure 5, the initial sH (CH4 + NH) hydrate was transformed into sI after the CH4/CO2 replacement, and the intensity ratio of the CH4 molecules in the large 51262 cages and small 512 cages (IL/IS) was 0.96. It should be noted that in the previous study,33 the intensity ratio of the CH4 molecules in the large 51262 cages and small 512 cages (IL/IS) for the isostructural replacement (sI to sI) was 1.44, which corresponds to a 68% replacement. The lowered intensity ratio of 0.96 after the replacement that accompanied the structural transformation indicates that the CO2 occupancy in the large 51262 cages was more significantly increased than that in the small 512 cages under the hydrate dissociation and subsequent reconfiguration of the crystal structure, which leads to the re-establishment process of the guest distribution in the hydrate structure. In the isostructural replacement (sI to sI), recent MRI and DSC studies demonstrated that the initial CH4 hydrate was not dissociating into the liquid water or ice and then forming the CH4 + CO2 hydrate through only a partial breakup of the cage structures and subsequent exchange of CH4 and CO2 molecules.28,31,39 However, the structural transformation during the CH4/CO2 replacement that occurred in sH hydrates involves the dissociation of the sH hydrate and the reformation of the sI gas hydrate and alters the cage-filling characteristics of guest molecules and guest distributions in each cage; thus, it results in enhanced CH4 recovery via CO2 injection into the sH hydrates. In this study, the heat flow involved in the CH4/CO2 replacement that occurs in sH hydrates was monitored using a HP μDSC to confirm the dissociation and reformation process of the gas hydrate structures during the replacement. For the CH4/CO2 replacement in sH hydrates, a quick evacuation of the vapor phase of the CH4 + NH hydrate should be followed by an injection of precooled CO2. Any possible hydrate dissociation during the evacuation and injection must be minimized for the accurate quantification of the heat flow involved in the replacement process. As depicted in Figure 7, the heat flow changes in the CH4 gas evacuation and CO2 injection processes in the presence of the initial sI CH4 gas
Figure 6. Comparison of the extent of the replacement (sI → sI vs sH → sI).
Figure 7. Heat flows in the blank run, the CH4/CO2 replacement, and the (CH4 + NH)/CO2 replacement at 274.15 K.
Figure 5. 13C NMR spectra of all guest molecules, including CH4 and NH, before and after replacement.
F
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ASSOCIATED CONTENT
S Supporting Information *
Table S1: hydrate phase equilibrium data for the CH4 + NH, CO2 + NH, and CH4 + CO2 + NH systems. Figure S1: 13C NMR spectra of CH4 and NH before and after replacement (10 to −10 ppm). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.est.5b01640.
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AUTHOR INFORMATION
Corresponding Author
* Tel: +82-52-217-2821. Fax: +82-52-217-2819. E-mail: [email protected]. Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENTS This research was supported by the Midcareer Research Program through the National Research Foundation of Korea (NRF) founded by the Ministry of Science, ICT, and Future Planning (NRF-2014R1A2A1A11049950) and was also supported by the 2015 Research Fund (1.150033.01) of the Ulsan National Institute of Science & Technology (UNIST).
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