Article pubs.acs.org/est
Experimental Verification of Methane−Carbon Dioxide Replacement in Natural Gas Hydrates Using a Differential Scanning Calorimeter Seungmin Lee,† Yohan Lee,‡ Jaehyoung Lee,§ Huen Lee,∥ and Yongwon Seo*,‡ †
Offshore Plant Resources R&D Center, Korea Institute of Industrial Technology, Busan 618-230, Republic of Korea School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea § Petroleum & Marine Resources Division, Korea Institute of Geoscience & Mineral Resources (KIGAM), Daejeon 305-350, Republic of Korea ∥ Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea ‡
S Supporting Information *
ABSTRACT: The methane (CH4) − carbon dioxide (CO2) swapping phenomenon in naturally occurring gas hydrates is regarded as an attractive method of CO2 sequestration and CH4 recovery. In this study, a high pressure microdifferential scanning calorimeter (HP μ-DSC) was used to monitor and quantify the CH4 − CO2 replacement in the gas hydrate structure. The HP μ-DSC provided reliable measurements of the hydrate dissociation equilibrium and hydrate heat of dissociation for the pure and mixed gas hydrates. The hydrate dissociation equilibrium data obtained from the endothermic thermograms of the replaced gas hydrates indicate that at least 60% of CH4 is recoverable after reaction with CO2, which is consistent with the result obtained via direct dissociation of the replaced gas hydrates. The heat of dissociation values of the CH4 + CO2 hydrates were between that of the pure CH4 hydrate and that of the pure CO2 hydrate, and the values increased as the CO2 compositions in the hydrate phase increased. By monitoring the heat flows from the HP μ-DSC, it was found that the noticeable dissociation or formation of a gas hydrate was not detected during the CH4 − CO2 replacement process, which indicates that a substantial portion of CH4 hydrate does not dissociate into liquid water or ice and then forms the CH4 + CO2 hydrate. This study provides the first experimental evidence using a DSC to reveal that the conversion of the CH4 hydrate to the CH4 + CO2 hydrate occurs without significant hydrate dissociation.
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Therefore, the CH4 − CO2 swapping phenomenon between these two guests can be used for stable long-term CO2 storage. Recent studies have included thermodynamic, spectroscopic, and kinetic approaches to reveal the CH4 − CO2 replacement mechanism.7−20 However, little attention has been paid to the thermal properties of the mixed CH4 + CO2 hydrates and their direct relation to the extent of CH4 − CO2 replacement. Among the thermal properties, the heat of dissociation (ΔHd) of the gas hydrates is a key parameter in predicting the heat flow through the hydrate-bearing sediments and the amount of heat required to dissociate the hydrate into water and gas. In the CH4 − CO2 replacement process, the ΔHd is also essential in understanding the dissociation behavior of the mixed CH4 + CO2 hydrates and developing an accurate model for predicting and simulating the CH4 − CO2 replacement because the latent heat is expected to be generated or absorbed during the formation, dissociation, and
INTRODUCTION Large quantities of natural gas hydrates, which are primarily composed of CH4, are found in permafrost regions and deep ocean sediments, and they are regarded as future energy resources.1 CO2, which is primarily produced from fossil fuel combustion, is captured and potentially sequestered as solid CO2 hydrates in the deep ocean.2−6 Recently, the CH4 − CO2 replacement in naturally occurring gas hydrates has been suggested as an attractive method of both CO2 sequestration and CH4 recovery.7−19 The replacement of CH4 with CO2 in gas hydrates is a thermodynamically spontaneous reaction20 and does not accompany the potential geo-mechanical hazards that might occur during the CH4 production process.21 Accordingly, the CH4 − CO2 swapping that occurs in deep-sea natural gas hydrates can be an alternative option for both the innovative storage of CO2 and the efficient recovery of CH4. Gas hydrates have three distinct crystal structures [structure I (sI), structure II (sII), and structure H (sH)], which consist of various cages with different sizes and shapes.1 Both CH4 and CO2 form sI hydrates, but the CO2 hydrate forms at much milder pressure and temperature conditions than the CH4 hydrate.1 © 2013 American Chemical Society
Received: Revised: Accepted: Published: 13184
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replacement processes of the mixed gas hydrates. In addition, the ΔHd values of the mixed CH4 + CO2 hydrates are also important in evaluating and predicting the extent of the CH4 − CO2 replacement because the ΔHd values of the mixed CH4 + CO2 hydrates are expected to be dependent on the cage occupancies and compositions of each guest in the hydrate phase. In this study, in order to reveal the CH4 − CO2 swapping mechanism and to explore the influence of an additional guest on the thermal behavior of the CH4 hydrate, accurate ΔHd values of the mixed CH4 + CO2 hydrates were measured, thermodynamic phase equilibrium conditions before and after the replacement were determined, and the heat flow change during the CH4 − CO2 replacement process was monitored using a high pressure microdifferential scanning calorimeter (HP μ-DSC).
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EXPERIMENTAL SECTION CH4 (99.95%), CO2 (99.99%), and gas mixtures of CH4 + CO2 (20, 40, 60, and 80%) were supplied by PSG Gas Co. (Republic of Korea). Double-distilled and deionized water was used in the experiment. The experimental apparatus was specially designed to measure the hydrate dissociation pressure and temperatures and was used to measure the three-phase equilibrium (hydrate (H)−liquid water (LW)−vapor (V)). The high pressure vessel made from 316 stainless steel had an internal volume of approximately 200 cm3 and was initially charged with 90 cm3 of water. The cell content was vigorously agitated using an impellertype stirrer. An isochoric pressure search method (PVT) with step heating was adopted and the H− LW−V equilibrium points were determined as the intersection between the hydrate dissociation and the thermal expansion lines. A schematic diagram of the experimental apparatus and more detailed descriptions of the experimental procedure for the PVT hydrate phase equilibrium measurements have been given in previous papers.22−24 A high pressure microdifferential scanning calorimeter (HP μ-DSC, VII Evo, Setaram Inc., France) was used to measure the heat of dissociation and the dissociation equilibrium temperature of the gas hydrates. The HP μ-DSC can be operated in a temperature range of 233.15 to 393.15 K, and it was calibrated with naphthalene whose melting point is 353.38 K. The high pressure cells were designed to function up to 40.0 MPa and to contain 0.5 cm3 of the sample. The HP μ-DSC has a resolution of 0.02 μW and a temperature deviation of ±0.2 K. The schematic of this system is presented in Figure 1. The pressure of the sample cell was measured using a pressure transducer (S-10, Wika, Germany) with an accuracy of ±0.25% (range: 0 to 10.0 MPa). Approximately 0.02 g of water was charged to the sample cell and after the complete conversion of water to ice via cooling, the accurate amount of charged water was calculated back and confirmed from the endothermic thermogram of ice melting using an ice melting enthalpy of 333.33 J/g. Then, the DSC cells were flushed with the test gas at least three times to remove any residual air and the sample cell was pressurized to the desired pressure. In the DSC experiment, due to the absence of agitation, the conversion of water to hydrate was generally too low even in high subcooling conditions. Therefore, a multicycle mode of cooling− heating was adopted in order to achieve complete conversion of the water to the hydrate. In this method, each cycle was composed of cooling to 243 K (1.0 K/min) followed by heating (0.5 K/min) to a temperature lower than the dissociation temperature of gas hydrates and higher than the ice melting temperature. The heating−cooling cycles and final hydrate dissociation for the CH4 + CO2 (40%) mixture in the HP μ-DSC is
Figure 1. Schematic diagram of DSC apparatus and picture of the high pressure DSC cell.
illustrated in Figure 2. As the cooling−heating cycles are repeated, both the exothermic peaks from the ice formation and the endothermic peaks from the ice melting continuously decreased, which indicates a correspondingly continuous increase in the conversion of ice or water to hydrate. The cycles were continued until the ice formation and melting was not detected. During the last cycle, the sample was heated to 313.15 K with a heating rate of 0.5 K/min and the dissociation enthalpy of each hydrate system was calculated from the integrated area of the last endothermic peak. In addition, the onset temperature of the endothermic peak was taken as the dissociation temperature of each hydrate system at a specified pressure. In the CH4 − CO2 replacement experiment, porous silica gels with an average diameter of 100 nm (Silicycle, Canada) were used to enhance the gas−water or gas−solid contact area. The pores of the silica gels were saturated with water. For the CH4 − CO2 replacement, the vapor phase was evacuated after complete formation of the CH4 hydrate and then, precooled CO2 gas was introduced into the sample cell. The CH4 − CO2 replacement experiments were conducted at three different temperatures of 263.15, 268.15, and 272.15 K, and the vapor phase was pressurized using CO2 to a pressure higher than the CH4 hydrate dissociation pressures and lower than the CO2 liquefaction pressures at a specified temperature. To avoid CH4 hydrate dissociation during CH4 evacuation and CO2 injection in the sample cell of the HP μ-DSC, all of the experiments for the replacement were performed at the temperatures below 273.15 K. A more detailed description for preparing the water-saturated silica gels and the CH4 − CO2 replacement procedure has been presented in previous papers.25−27
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RESULTS AND DISCUSSION Figure 3 presents the dissociation thermograms of the CH4 + CO2 (40%) hydrates at three different pressure conditions. Significant endothermic peaks were not seen from the ice melting, which indicates the almost complete conversion of water to hydrate as a result of the repeated cooling−heating cycles. At each pressure condition, the onset temperature of the endothermic peak, which was taken as the hydrate dissociation equilibrium temperature, was marked in each thermogram. Dalmazzone et al.28 demonstrated that the DSC can be used 13185
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Figure 2. Changes in heat flow and temperature of the CH4 + CO2 (40%) hydrate during cooling−heating cycles. (a) heat flow change during the whole cooling−heating cycles; (b) temperature change during the whole cooling−heating cycles; (c) heat flow change during the first cooling−heating cycle; and (d) temperature change during the first cooling−heating cycle.
effectively to determine the dissociation equilibrium curves of the gas hydrates, which are generally measured using a classical technique such as an isochoric pressure search method (PVT). In Figure 4, the hydrate dissociation equilibrium data obtained from the DSC method were compared with those
obtained from the classical PVT method and also with the calculated values. The equilibrium criteria of the hydrateforming mixture are based on the equality of fugacities of the specified component i in all phases that coexist simultaneously, 13186
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the PVT results and the calculation values. This confirms that the DSC can provide accurate H−LW−V equilibrium points for the CH4 + CO2 hydrate systems, and it also implies that the onset temperatures, as well as the hydrate heat of dissociation measured after the CH4 − CO2 replacement using the DSC, can offer an approximate estimation of the extent of the CO2 replacement in the CH4 gas hydrate. The equilibrium line and the heat of dissociation value of the pure CO2 hydrate are different from those of the pure CH4 hydrate and, accordingly, both equilibrium conditions and heat of dissociation values of the CH4 + CO2 hydrates after replacement are expected to be located between those of the pure CH4 hydrate and those of the pure CO2 hydrate. Figure 5 presents the dissociation thermogram of the pure CH4 hydrate before replacement and also the thermogram of the
Figure 3. Dissociation thermograms of the CH4 + CO2 (40%) hydrate at three different pressure conditions.
Figure 5. Dissociation thermograms of the pure CH4 hydrate before swapping (9.16 MPa) and the CH4 + CO2 hydrate after swapping at 268.15 K (2.67 MPa).
CH4 + CO2 hydrate after replacement at 268.15 K. The CH4 − CO2 replacement was performed at three different temperatures of 263.15, 268.15, and 272.15 K, and the CO2 injection was conducted at the pressure conditions just below the CO2 liquefaction pressures at each temperature condition. At each pressure condition, the dissociation equilibrium conditions of the CH4 + CO2 hydrates after replacement were obtained from the onset temperatures of the endothermic peaks shown in Figure 5 and are depicted in Figure 6 with those of the pure CH4, CH4 + CO2 (60%), CH4 + CO2 (80%), and pure CO2 hydrates. The initial CH4 hydrate before the replacement converts to the mixed CH4 + CO2 hydrate as the CH4 − CO2 replacement reaction proceeds. Therefore, a comparison of the dissociation equilibrium conditions of the replaced hydrates after the CH4 − CO2 replacement reaction with those of the various CH4 + CO2 hydrates could provide an approximate estimation of the extent of replacement. As can be seen in Figure 6, the three dissociation equilibrium points of the CH4 + CO2 hydrates after the CO2 replacement were located just on the H−LW−V line of the CH4 + CO2 (60%) hydrate. Therefore, it can be reasonably expected from Figure 6 that the CO2 composition in the hydrate phase
Figure 4. Comparison of hydrate dissociation equilibrium data of the CH4 + CO2 (40%) + water mixture (DSC method vs PVT method vs calculation).
fi ̂
H
L V = fi ̂ = fi ̂ ( = fwI )
(1)
where H denotes for the hydrate phase, L for the water-rich liquid phase, V for the vapor phase, and I for the ice phase. For the hydrate equilibrium calculations, the fugacities were calculated using the Soave−Redlich−Kwong equation of state (SRK−EOS) incorporated with the modified Huron-Vidal second order mixing rule for fluid phases and using statistical thermodynamics for the hydrate phase at the same pressure. More details of the model description and the parameters used for the model have been given in previous papers.29,30 As shown in Figure 4, the DSC results were in good agreement with both 13187
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Table 1. Dissociation Enthalpies of the CH4 + CO2 Hydrates with Respect to the CO2 Compositions CO2 composition in hydrate phase (%)
ΔHd (kJ/mol gas)
hydration number
0 15 30 35.6 40 58.2 71 74.8 88.3 100
54.1 ± 0.2 53.4 53 54.8 ± 0.2 57.23 55.2 ± 0.1 62.82 55.6 ± 0.2 56.1 ± 0.1 57.1 ± 0.1
6.00
6.11 6.15 6.19 6.23 6.30
this work Rydzy et al.31 Rydzy et al.31 this work Kwon et al.32 this work Kwon et al.32 this work this work this work
hydrate phase. The ΔHd values of the CH4 + CO2 hydrates were obtained by combining the integration of each endothermic heat flow peak with the hydration number calculated from cage occupancies of CH4 and CO2 using the thermodynamic model. The ΔHd of the CH4 + CO2 hydrates increased with increases in CO2 composition in the hydrate phase. This result supports the previous reports that the ΔHd of the mixed hydrates increases with an increase in the actual composition of the larger guest molecules occupying the large 51262 cages of sI hydrates.31 CO2 has a larger molecular size than CH4 and, thus, in the CH4 + CO2 hydrates, CO2 is more desirable guest than CH4 for occupying the large 51262 cages of sI hydrates, which results in a relatively larger ΔHd value for the CO2 hydrate.1 As seen in Figure 7 and Table 1, the ΔHd values of the CH4 + CO2 hydrates presented in this study are in good agreement with the reference data and are located between those of Ridzy et al.31 measured using a DSC and those of Kwon et al.32 estimated using the Clausius− Clapeyron equation. In addition, the ΔHd values of pure CH4 and pure CO2 hydrates were measured to be 54.1 ± 0.2 and 57.1 ± 0.1 kJ/mol gas, respectively, which are also in good agreement with the reference values of 52−57 kJ/mol gas for the pure CH4 hydrate and 57−65 kJ/mol gas for the pure CO2 hydrate.33−40 By integrating the endothermic peaks of dissociation thermograms after the CO2 replacement shown in Figure 5 and considering the CO2 replacement of 68 ± 2%, the average ΔHd value of the CH4 + CO2 hydrates after the CO2 replacement was found to be 55.5 ± 0.2 kJ/mol gas, which appears to be a reasonable value considering the data in Figure 7 and Table1. Accurate information on the dissociation enthalpies of the CH4 + CO2 hydrates is required in order to fully understand the CH4 − CO2 swapping mechanism in gas hydrates. In addition, the changes in the ΔHd values with changing CO2 compositions can provide fundamental information in revealing the hydrate cage occupancy behavior of guest molecules with different molecular sizes. In this study, the CH4 − CO2 replacement in gas hydrates was monitored using a HP μ-DSC. For the CH4 − CO2 replacement, a vapor phase evacuation of the formed CH4 hydrate should be followed by a quick injection of the precooled CO2, and any possible CH4 hydrate dissociation during this process should be avoided for the accurate replacement experiment. Figure 8 presents the heat flow in the CH4 evacuation and CO2 injection with and without CH4 hydrate as well as that in the CH4 hydrate dissociation under atmospheric pressure at 268.15 K. Actually, a small amount of the heat flow only from gas evacuation and injection in the empty DSC cell was detected. However, the heat flow change in the gas evacuation and injection with gas hydrates, which refers to a replacement, is almost the same as that in the gas
Figure 6. Dissociation equilibrium conditions of the CH4 + CO2 hydrates.
after the replacement is similar to that of the CH4 + CO2 (60%) hydrate and that, accordingly, at least 60% of CH4 was replaced by CO2 considering the relative CO2 enrichment in the hydrate phase. This approximate estimation of the extent of CO2 replacement is consistent with the previous results obtained from the combination of the 13C NMR spectra and pressure−composition diagram.7,8,17 In addition, the actual extent of replacement was measured accurately from the direct dissociation of the hydrate phase after replacement through a gas chromatography and was found to be 68 ± 2%. Figure 7 and Table 1 present the dissociation enthalpies of the CH4 + CO2 hydrates with respect to the CO2 compositions in the
Figure 7. Dissociation enthalpies of the CH4 + CO2 hydrates with respect to the CO2 composition in the hydrate phase. 13188
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peaks were not detected during the replacement within the resolution and sensitivity of the HP μ-DSC, which indicates that the CH4 − CO2 replacement occurred without significant hydrate dissociation or formation. This result is consistent with the recent MRI experiment in which a free water phase was not detected during the CH4 − CO2 replacement process.15,16 Therefore, the experimental results from both the DSC and MRI strongly suggest that during the replacement process, a substantial portion of CH4 hydrate is not dissociating into liquid water or ice and then forming the CH4 + CO2 hydrate. During the CH4 − CO2 replacement reaction, there might be a partial breakup of some cage structures for the exchange of CH4 and CO2 molecules, but the notable heat flow from this was not detected in the DSC because of a relatively slow replacement reaction over a long period of time and a very small amount of hydrate samples in the DSC cell. However, this is the first experimental evidence using a DSC to reveal that CH4 can be replaced by CO2 in the gas hydrate structure without significant hydrate dissociations.
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ASSOCIATED CONTENT
S Supporting Information *
Table of hydrate phase equilibrium data for the CH4 + CO2 + water mixtures. This information is available free of charge via the Internet at http://pubs.acs.org/.
Figure 8. Heat flow in the blank run, CH4 − CO2 replacement at 268.15 K, and CH4 hydrate dissociation under atmospheric pressure at 268.15 K.
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evacuation and injection without gas hydrates, which refers to a blank run, and is significantly lower than that in the CH4 hydrate dissociation. This clearly demonstrates that there is no gas hydrate dissociation during the gas evacuation and injection process for the replacement. Figure 9 shows a thermogram of the CH4 − CO2 replacement for 72 h at 268.15 K. It was reported that the CH4 − CO2
AUTHOR INFORMATION
Corresponding Author
*Phone: +82-52-217-2821; fax: +82-52-217-2819; e-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2012R1A1B6002494) and also by the Future Creativity and Innovation project (2012) of the UNIST (1.130015.01).
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Figure 9. Thermograms of the CH4 − CO2 replacement for 72 h at 268.15 K.
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Experimental Verification of Methane−Carbon Dioxide Replacement in Natural Gas Hydrates Using a Differential Scanning Calorimeter Seungmin Lee,† Yohan Lee,‡ Jaehyoung Lee,§ Huen Lee,∥ and Yongwon Seo*,‡ †
Offshore Plant Resources R&D Center, Korea Institute of Industrial Technology, Busan 618-230, Republic of Korea School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea § Petroleum & Marine Resources Division, Korea Institute of Geoscience & Mineral Resources (KIGAM), Daejeon 305-350, Republic of Korea ∥ Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea ‡
S Supporting Information *
ABSTRACT: The methane (CH4) − carbon dioxide (CO2) swapping phenomenon in naturally occurring gas hydrates is regarded as an attractive method of CO2 sequestration and CH4 recovery. In this study, a high pressure microdifferential scanning calorimeter (HP μ-DSC) was used to monitor and quantify the CH4 − CO2 replacement in the gas hydrate structure. The HP μ-DSC provided reliable measurements of the hydrate dissociation equilibrium and hydrate heat of dissociation for the pure and mixed gas hydrates. The hydrate dissociation equilibrium data obtained from the endothermic thermograms of the replaced gas hydrates indicate that at least 60% of CH4 is recoverable after reaction with CO2, which is consistent with the result obtained via direct dissociation of the replaced gas hydrates. The heat of dissociation values of the CH4 + CO2 hydrates were between that of the pure CH4 hydrate and that of the pure CO2 hydrate, and the values increased as the CO2 compositions in the hydrate phase increased. By monitoring the heat flows from the HP μ-DSC, it was found that the noticeable dissociation or formation of a gas hydrate was not detected during the CH4 − CO2 replacement process, which indicates that a substantial portion of CH4 hydrate does not dissociate into liquid water or ice and then forms the CH4 + CO2 hydrate. This study provides the first experimental evidence using a DSC to reveal that the conversion of the CH4 hydrate to the CH4 + CO2 hydrate occurs without significant hydrate dissociation.
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Therefore, the CH4 − CO2 swapping phenomenon between these two guests can be used for stable long-term CO2 storage. Recent studies have included thermodynamic, spectroscopic, and kinetic approaches to reveal the CH4 − CO2 replacement mechanism.7−20 However, little attention has been paid to the thermal properties of the mixed CH4 + CO2 hydrates and their direct relation to the extent of CH4 − CO2 replacement. Among the thermal properties, the heat of dissociation (ΔHd) of the gas hydrates is a key parameter in predicting the heat flow through the hydrate-bearing sediments and the amount of heat required to dissociate the hydrate into water and gas. In the CH4 − CO2 replacement process, the ΔHd is also essential in understanding the dissociation behavior of the mixed CH4 + CO2 hydrates and developing an accurate model for predicting and simulating the CH4 − CO2 replacement because the latent heat is expected to be generated or absorbed during the formation, dissociation, and
INTRODUCTION Large quantities of natural gas hydrates, which are primarily composed of CH4, are found in permafrost regions and deep ocean sediments, and they are regarded as future energy resources.1 CO2, which is primarily produced from fossil fuel combustion, is captured and potentially sequestered as solid CO2 hydrates in the deep ocean.2−6 Recently, the CH4 − CO2 replacement in naturally occurring gas hydrates has been suggested as an attractive method of both CO2 sequestration and CH4 recovery.7−19 The replacement of CH4 with CO2 in gas hydrates is a thermodynamically spontaneous reaction20 and does not accompany the potential geo-mechanical hazards that might occur during the CH4 production process.21 Accordingly, the CH4 − CO2 swapping that occurs in deep-sea natural gas hydrates can be an alternative option for both the innovative storage of CO2 and the efficient recovery of CH4. Gas hydrates have three distinct crystal structures [structure I (sI), structure II (sII), and structure H (sH)], which consist of various cages with different sizes and shapes.1 Both CH4 and CO2 form sI hydrates, but the CO2 hydrate forms at much milder pressure and temperature conditions than the CH4 hydrate.1 © 2013 American Chemical Society
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replacement processes of the mixed gas hydrates. In addition, the ΔHd values of the mixed CH4 + CO2 hydrates are also important in evaluating and predicting the extent of the CH4 − CO2 replacement because the ΔHd values of the mixed CH4 + CO2 hydrates are expected to be dependent on the cage occupancies and compositions of each guest in the hydrate phase. In this study, in order to reveal the CH4 − CO2 swapping mechanism and to explore the influence of an additional guest on the thermal behavior of the CH4 hydrate, accurate ΔHd values of the mixed CH4 + CO2 hydrates were measured, thermodynamic phase equilibrium conditions before and after the replacement were determined, and the heat flow change during the CH4 − CO2 replacement process was monitored using a high pressure microdifferential scanning calorimeter (HP μ-DSC).
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EXPERIMENTAL SECTION CH4 (99.95%), CO2 (99.99%), and gas mixtures of CH4 + CO2 (20, 40, 60, and 80%) were supplied by PSG Gas Co. (Republic of Korea). Double-distilled and deionized water was used in the experiment. The experimental apparatus was specially designed to measure the hydrate dissociation pressure and temperatures and was used to measure the three-phase equilibrium (hydrate (H)−liquid water (LW)−vapor (V)). The high pressure vessel made from 316 stainless steel had an internal volume of approximately 200 cm3 and was initially charged with 90 cm3 of water. The cell content was vigorously agitated using an impellertype stirrer. An isochoric pressure search method (PVT) with step heating was adopted and the H− LW−V equilibrium points were determined as the intersection between the hydrate dissociation and the thermal expansion lines. A schematic diagram of the experimental apparatus and more detailed descriptions of the experimental procedure for the PVT hydrate phase equilibrium measurements have been given in previous papers.22−24 A high pressure microdifferential scanning calorimeter (HP μ-DSC, VII Evo, Setaram Inc., France) was used to measure the heat of dissociation and the dissociation equilibrium temperature of the gas hydrates. The HP μ-DSC can be operated in a temperature range of 233.15 to 393.15 K, and it was calibrated with naphthalene whose melting point is 353.38 K. The high pressure cells were designed to function up to 40.0 MPa and to contain 0.5 cm3 of the sample. The HP μ-DSC has a resolution of 0.02 μW and a temperature deviation of ±0.2 K. The schematic of this system is presented in Figure 1. The pressure of the sample cell was measured using a pressure transducer (S-10, Wika, Germany) with an accuracy of ±0.25% (range: 0 to 10.0 MPa). Approximately 0.02 g of water was charged to the sample cell and after the complete conversion of water to ice via cooling, the accurate amount of charged water was calculated back and confirmed from the endothermic thermogram of ice melting using an ice melting enthalpy of 333.33 J/g. Then, the DSC cells were flushed with the test gas at least three times to remove any residual air and the sample cell was pressurized to the desired pressure. In the DSC experiment, due to the absence of agitation, the conversion of water to hydrate was generally too low even in high subcooling conditions. Therefore, a multicycle mode of cooling− heating was adopted in order to achieve complete conversion of the water to the hydrate. In this method, each cycle was composed of cooling to 243 K (1.0 K/min) followed by heating (0.5 K/min) to a temperature lower than the dissociation temperature of gas hydrates and higher than the ice melting temperature. The heating−cooling cycles and final hydrate dissociation for the CH4 + CO2 (40%) mixture in the HP μ-DSC is
Figure 1. Schematic diagram of DSC apparatus and picture of the high pressure DSC cell.
illustrated in Figure 2. As the cooling−heating cycles are repeated, both the exothermic peaks from the ice formation and the endothermic peaks from the ice melting continuously decreased, which indicates a correspondingly continuous increase in the conversion of ice or water to hydrate. The cycles were continued until the ice formation and melting was not detected. During the last cycle, the sample was heated to 313.15 K with a heating rate of 0.5 K/min and the dissociation enthalpy of each hydrate system was calculated from the integrated area of the last endothermic peak. In addition, the onset temperature of the endothermic peak was taken as the dissociation temperature of each hydrate system at a specified pressure. In the CH4 − CO2 replacement experiment, porous silica gels with an average diameter of 100 nm (Silicycle, Canada) were used to enhance the gas−water or gas−solid contact area. The pores of the silica gels were saturated with water. For the CH4 − CO2 replacement, the vapor phase was evacuated after complete formation of the CH4 hydrate and then, precooled CO2 gas was introduced into the sample cell. The CH4 − CO2 replacement experiments were conducted at three different temperatures of 263.15, 268.15, and 272.15 K, and the vapor phase was pressurized using CO2 to a pressure higher than the CH4 hydrate dissociation pressures and lower than the CO2 liquefaction pressures at a specified temperature. To avoid CH4 hydrate dissociation during CH4 evacuation and CO2 injection in the sample cell of the HP μ-DSC, all of the experiments for the replacement were performed at the temperatures below 273.15 K. A more detailed description for preparing the water-saturated silica gels and the CH4 − CO2 replacement procedure has been presented in previous papers.25−27
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RESULTS AND DISCUSSION Figure 3 presents the dissociation thermograms of the CH4 + CO2 (40%) hydrates at three different pressure conditions. Significant endothermic peaks were not seen from the ice melting, which indicates the almost complete conversion of water to hydrate as a result of the repeated cooling−heating cycles. At each pressure condition, the onset temperature of the endothermic peak, which was taken as the hydrate dissociation equilibrium temperature, was marked in each thermogram. Dalmazzone et al.28 demonstrated that the DSC can be used 13185
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Figure 2. Changes in heat flow and temperature of the CH4 + CO2 (40%) hydrate during cooling−heating cycles. (a) heat flow change during the whole cooling−heating cycles; (b) temperature change during the whole cooling−heating cycles; (c) heat flow change during the first cooling−heating cycle; and (d) temperature change during the first cooling−heating cycle.
effectively to determine the dissociation equilibrium curves of the gas hydrates, which are generally measured using a classical technique such as an isochoric pressure search method (PVT). In Figure 4, the hydrate dissociation equilibrium data obtained from the DSC method were compared with those
obtained from the classical PVT method and also with the calculated values. The equilibrium criteria of the hydrateforming mixture are based on the equality of fugacities of the specified component i in all phases that coexist simultaneously, 13186
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the PVT results and the calculation values. This confirms that the DSC can provide accurate H−LW−V equilibrium points for the CH4 + CO2 hydrate systems, and it also implies that the onset temperatures, as well as the hydrate heat of dissociation measured after the CH4 − CO2 replacement using the DSC, can offer an approximate estimation of the extent of the CO2 replacement in the CH4 gas hydrate. The equilibrium line and the heat of dissociation value of the pure CO2 hydrate are different from those of the pure CH4 hydrate and, accordingly, both equilibrium conditions and heat of dissociation values of the CH4 + CO2 hydrates after replacement are expected to be located between those of the pure CH4 hydrate and those of the pure CO2 hydrate. Figure 5 presents the dissociation thermogram of the pure CH4 hydrate before replacement and also the thermogram of the
Figure 3. Dissociation thermograms of the CH4 + CO2 (40%) hydrate at three different pressure conditions.
Figure 5. Dissociation thermograms of the pure CH4 hydrate before swapping (9.16 MPa) and the CH4 + CO2 hydrate after swapping at 268.15 K (2.67 MPa).
CH4 + CO2 hydrate after replacement at 268.15 K. The CH4 − CO2 replacement was performed at three different temperatures of 263.15, 268.15, and 272.15 K, and the CO2 injection was conducted at the pressure conditions just below the CO2 liquefaction pressures at each temperature condition. At each pressure condition, the dissociation equilibrium conditions of the CH4 + CO2 hydrates after replacement were obtained from the onset temperatures of the endothermic peaks shown in Figure 5 and are depicted in Figure 6 with those of the pure CH4, CH4 + CO2 (60%), CH4 + CO2 (80%), and pure CO2 hydrates. The initial CH4 hydrate before the replacement converts to the mixed CH4 + CO2 hydrate as the CH4 − CO2 replacement reaction proceeds. Therefore, a comparison of the dissociation equilibrium conditions of the replaced hydrates after the CH4 − CO2 replacement reaction with those of the various CH4 + CO2 hydrates could provide an approximate estimation of the extent of replacement. As can be seen in Figure 6, the three dissociation equilibrium points of the CH4 + CO2 hydrates after the CO2 replacement were located just on the H−LW−V line of the CH4 + CO2 (60%) hydrate. Therefore, it can be reasonably expected from Figure 6 that the CO2 composition in the hydrate phase
Figure 4. Comparison of hydrate dissociation equilibrium data of the CH4 + CO2 (40%) + water mixture (DSC method vs PVT method vs calculation).
fi ̂
H
L V = fi ̂ = fi ̂ ( = fwI )
(1)
where H denotes for the hydrate phase, L for the water-rich liquid phase, V for the vapor phase, and I for the ice phase. For the hydrate equilibrium calculations, the fugacities were calculated using the Soave−Redlich−Kwong equation of state (SRK−EOS) incorporated with the modified Huron-Vidal second order mixing rule for fluid phases and using statistical thermodynamics for the hydrate phase at the same pressure. More details of the model description and the parameters used for the model have been given in previous papers.29,30 As shown in Figure 4, the DSC results were in good agreement with both 13187
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Table 1. Dissociation Enthalpies of the CH4 + CO2 Hydrates with Respect to the CO2 Compositions CO2 composition in hydrate phase (%)
ΔHd (kJ/mol gas)
hydration number
0 15 30 35.6 40 58.2 71 74.8 88.3 100
54.1 ± 0.2 53.4 53 54.8 ± 0.2 57.23 55.2 ± 0.1 62.82 55.6 ± 0.2 56.1 ± 0.1 57.1 ± 0.1
6.00
6.11 6.15 6.19 6.23 6.30
this work Rydzy et al.31 Rydzy et al.31 this work Kwon et al.32 this work Kwon et al.32 this work this work this work
hydrate phase. The ΔHd values of the CH4 + CO2 hydrates were obtained by combining the integration of each endothermic heat flow peak with the hydration number calculated from cage occupancies of CH4 and CO2 using the thermodynamic model. The ΔHd of the CH4 + CO2 hydrates increased with increases in CO2 composition in the hydrate phase. This result supports the previous reports that the ΔHd of the mixed hydrates increases with an increase in the actual composition of the larger guest molecules occupying the large 51262 cages of sI hydrates.31 CO2 has a larger molecular size than CH4 and, thus, in the CH4 + CO2 hydrates, CO2 is more desirable guest than CH4 for occupying the large 51262 cages of sI hydrates, which results in a relatively larger ΔHd value for the CO2 hydrate.1 As seen in Figure 7 and Table 1, the ΔHd values of the CH4 + CO2 hydrates presented in this study are in good agreement with the reference data and are located between those of Ridzy et al.31 measured using a DSC and those of Kwon et al.32 estimated using the Clausius− Clapeyron equation. In addition, the ΔHd values of pure CH4 and pure CO2 hydrates were measured to be 54.1 ± 0.2 and 57.1 ± 0.1 kJ/mol gas, respectively, which are also in good agreement with the reference values of 52−57 kJ/mol gas for the pure CH4 hydrate and 57−65 kJ/mol gas for the pure CO2 hydrate.33−40 By integrating the endothermic peaks of dissociation thermograms after the CO2 replacement shown in Figure 5 and considering the CO2 replacement of 68 ± 2%, the average ΔHd value of the CH4 + CO2 hydrates after the CO2 replacement was found to be 55.5 ± 0.2 kJ/mol gas, which appears to be a reasonable value considering the data in Figure 7 and Table1. Accurate information on the dissociation enthalpies of the CH4 + CO2 hydrates is required in order to fully understand the CH4 − CO2 swapping mechanism in gas hydrates. In addition, the changes in the ΔHd values with changing CO2 compositions can provide fundamental information in revealing the hydrate cage occupancy behavior of guest molecules with different molecular sizes. In this study, the CH4 − CO2 replacement in gas hydrates was monitored using a HP μ-DSC. For the CH4 − CO2 replacement, a vapor phase evacuation of the formed CH4 hydrate should be followed by a quick injection of the precooled CO2, and any possible CH4 hydrate dissociation during this process should be avoided for the accurate replacement experiment. Figure 8 presents the heat flow in the CH4 evacuation and CO2 injection with and without CH4 hydrate as well as that in the CH4 hydrate dissociation under atmospheric pressure at 268.15 K. Actually, a small amount of the heat flow only from gas evacuation and injection in the empty DSC cell was detected. However, the heat flow change in the gas evacuation and injection with gas hydrates, which refers to a replacement, is almost the same as that in the gas
Figure 6. Dissociation equilibrium conditions of the CH4 + CO2 hydrates.
after the replacement is similar to that of the CH4 + CO2 (60%) hydrate and that, accordingly, at least 60% of CH4 was replaced by CO2 considering the relative CO2 enrichment in the hydrate phase. This approximate estimation of the extent of CO2 replacement is consistent with the previous results obtained from the combination of the 13C NMR spectra and pressure−composition diagram.7,8,17 In addition, the actual extent of replacement was measured accurately from the direct dissociation of the hydrate phase after replacement through a gas chromatography and was found to be 68 ± 2%. Figure 7 and Table 1 present the dissociation enthalpies of the CH4 + CO2 hydrates with respect to the CO2 compositions in the
Figure 7. Dissociation enthalpies of the CH4 + CO2 hydrates with respect to the CO2 composition in the hydrate phase. 13188
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peaks were not detected during the replacement within the resolution and sensitivity of the HP μ-DSC, which indicates that the CH4 − CO2 replacement occurred without significant hydrate dissociation or formation. This result is consistent with the recent MRI experiment in which a free water phase was not detected during the CH4 − CO2 replacement process.15,16 Therefore, the experimental results from both the DSC and MRI strongly suggest that during the replacement process, a substantial portion of CH4 hydrate is not dissociating into liquid water or ice and then forming the CH4 + CO2 hydrate. During the CH4 − CO2 replacement reaction, there might be a partial breakup of some cage structures for the exchange of CH4 and CO2 molecules, but the notable heat flow from this was not detected in the DSC because of a relatively slow replacement reaction over a long period of time and a very small amount of hydrate samples in the DSC cell. However, this is the first experimental evidence using a DSC to reveal that CH4 can be replaced by CO2 in the gas hydrate structure without significant hydrate dissociations.
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ASSOCIATED CONTENT
S Supporting Information *
Table of hydrate phase equilibrium data for the CH4 + CO2 + water mixtures. This information is available free of charge via the Internet at http://pubs.acs.org/.
Figure 8. Heat flow in the blank run, CH4 − CO2 replacement at 268.15 K, and CH4 hydrate dissociation under atmospheric pressure at 268.15 K.
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evacuation and injection without gas hydrates, which refers to a blank run, and is significantly lower than that in the CH4 hydrate dissociation. This clearly demonstrates that there is no gas hydrate dissociation during the gas evacuation and injection process for the replacement. Figure 9 shows a thermogram of the CH4 − CO2 replacement for 72 h at 268.15 K. It was reported that the CH4 − CO2
AUTHOR INFORMATION
Corresponding Author
*Phone: +82-52-217-2821; fax: +82-52-217-2819; e-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2012R1A1B6002494) and also by the Future Creativity and Innovation project (2012) of the UNIST (1.130015.01).
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Figure 9. Thermograms of the CH4 − CO2 replacement for 72 h at 268.15 K.
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