Temperature-Dependent Structural Transitions in Methane–Ethane

Nov 16, 2014 - A thermodynamic interpretation of the interconversion between structures I and II occurring in methane (CH4) + ethane (C2H6) mixed gas ...
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Temperature-Dependent Structural Transitions in Methane−Ethane Mixed Gas Hydrates Minchul Kwon,† Jong-Won Lee,*,‡ and Huen Lee*,† †

Department of Chemical and Biomolecular Engineering (BK21+ program), KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Department of Environmental Engineering, Kongju National University, 275 Budae-dong, Cheonan, Chungnam 331-717, Republic of Korea S Supporting Information *

ABSTRACT: A thermodynamic interpretation of the interconversion between structures I and II occurring in methane (CH4) + ethane (C2H6) mixed gas hydrates is of great importance from both fundamental and applied perspectives. The present study experimentally confirms the predicted temperature dependence of structural changes in the lower transition region (72−74 mol % of CH4 balanced with C2H6) of the CH4 + C2H6 + H2O system. The measurements of phase equilibria and Raman spectra, at the macroscopic and microscopic levels, respectively, reveal the phase transition point at which the structural rearrangements occur. The isothermal data reported here clearly demonstrate significant changes of transition behavior from sII inhibition to sII promotion in accordance with increased equilibrium temperatures. This solid−solid transition trend may be dictated by the peculiar structural feature of the CH4 + C2H6 mixed gas hydrates on the basis of the comprehensive experimental and theoretical data published previously. The predominance of CH4 over C2H6 in cage occupancy may lead to a change in guest molecules playing a dominant role in determining the preferential hydrate structure.

1. INTRODUCTION Gas hydrates are ice-like solid complexes formed by the enclathration of suitably sized gas molecules (guest) into hydrogen-bonded water frameworks (host) under appropriate conditions (relatively low temperature and high pressure).1 The crystallographic structures of hydrate phases are determined by the arrangement of the host water lattice depending on the size and combination of guest species. In general, several types of polyhedral cavities (cages) are combined to generate three hydrate unit crystals (sI, sII, and sH).2 A thorough analysis of structural features and their associated structural transitions between different structures can provide crucial information on the guest distribution, dissociation enthalpy, and other thermophysical properties. Since the first discovery of clathrate compounds, two main streams of gas hydrate research have emerged as a bridge between academic achievement and practical involvement in energy industries. The first one has focused on a flow assurance issue concerning industrial safety in oil and gas production pipelines, which is initiated by the appearance of hydrate deposits on pipe walls.3 The second deals with the exploitation of natural gas hydrates (NGH) containing a substantial amount of CH4 in permafrost regions and deep−ocean sediments.4 It should be noted that there is significant overlap between these two areas of investigations in terms of the formation of mixed gas hydrates from the major compositions of natural gas (C1− C4). © XXXX American Chemical Society

The determination of NGH structures depends on the molar ratio of CH4 to other light hydrocarbons (C1/C2+) and, more generally, to ethane (C2H6). Naturally occurring gas hydrates in the earth favor sI hydrate, since the CH4 content of hydrate− bound gas in marine sediments dominates over other hydrocarbon molecules.5 However, only small amounts of larger guests, such as propane (C3H8) and isobutane (i-C4H10), can lead to the formation of sII hydrate during the production and transportation of oil and gas.3 Although CH4 and C2H6 form sI hydrates with its pure gases, the crystal structure of their mixed gas can be transformed into the sII hydrate in the CH4 composition ranges from 75 to 99 mol %.1 This indicates that the molecular sizes of light hydrocarbons other than CH4 are more suitable to be incorporated into the larger 51264 cages of sII hydrate instead of 51262 cages of sI hydrate. Indeed, both structures including sH have been recently discovered worldwide through microbial and thermogenic sources of hydrocarbons.6 In particular, the existence of mixed sI/sII layers and intimately associated sI/sII hydrates implies a complex formation mechanism of natural gas hydrates in hydrocarbon seepage environments.7−9 In this mechanism, the formation of sII hydrate is attributed to the presence of C2H6, which is the dominant component of heavy (C2 and larger) hydrocarbons. Received: October 9, 2014 Revised: November 12, 2014

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monitored by the pump controller display (Teledyne, ISCO Dseries). The vapor pressure in the closed system was dominated by the pump controller. The operating temperature was maintained with a circulating bath (Jeio Tech., RW-2040G) with a stability of ±0.05 K. An analog-to-digital converter (ADC) was connected to the computer for pressure and temperature monitoring. The water-free vapor composition in equilibrium was analyzed by a gas chromatograph (Young−Lin, YL 6100 GC) equipped with a thermal conductivity detector (TCD) using a Porapak N column (8 ft length × 1/8 in. outer diameter). The column oven temperature was 140 °C, and the injector and detector were held at 150 °C. High-purity He (N50 quality) was used as the carrier gas at a flow rate of 30 mL/min. Sample injections were performed using a Rheodyne 7413 microscale injection valve containing a 1 μL internal loop. The loading pressure was maintained at 0.50 MPa by a line pressure regulator valve (Chiyoda Seiki, GHN-4, 0−25 MPa). Standard gas mixtures for gas chromatograph calibration were supplied by YG Special Gas (Korea). Autochro 2000 software (Young− Lin) was utilized for data acquisition and analysis. The software was calibrated by an external standard method using two reference gas mixtures (65.27 and 72.04 mol % of CH4 balanced with C2H6, respectively). Raman spectra were collected using a dispersive Raman microscope (Horiba Jobin Yvon) equipped with a CCD detector cooled by liquid nitrogen. An Ar-ion laser with a grating of 1800 grooves/mm was used for 514.53 nm excitation. A confocal hole was set to 1000 μm, and the slit was 100 μm. The intensity of the laser was 20 mW. The sample was placed in a Linkam TMS600 cryostat cooled with liquid nitrogen during Raman scattering measurements. For all measurements, an exposure time of 5 s and an accumulation number of 5 were used. Data were collected with the software LabSpec (Horiba Jobin Yvon). 2.2. Procedures. The experimental apparatus and procedure of this work are based on our previous work.11 An amount of 10 g of liquid water was initially charged into the equilibrium cell. A mechanical agitator was assembled with a pressure vessel to maintain the stirring rate at 150 rpm. The circulating bath maintained the temperature of the aqueous phase inside the blind cell at each temperature. After vacuum operation at the maximum volume of the syringe pump, gas mixtures flowed into the syringe and vessel. The pressure was adjusted to be lower than the predicted dissociation pressure by the gas regulator. Then, it was pressurized and operated in constant pressure mode to induce hydrate nucleation and growth. The formation of solid hydrate crystals in the aqueous phase leads to the consumption of guest mixtures, generating the corresponding exothermic heat. These two changes were detected by the pump controller display and the temperature sensor, respectively. After complete formation, the stirring of the aqueous phase was paused for more than 1 h. Then, heat generation did not occur, and the displacement of the syringe pump remained steady on a milliliter scale. Then, the isobaric condition was lowered to enable the stirring of bulk coexisting LW−H−V phases. At the moment the agitator was restarted, the syringe pump operated in constant flow mode until the internal temperature exceeded each isothermal condition. After that, the vapor volume expansion reduced the pressure in the blind cell. Once the syringe pump operated in refill mode with a constant flow of 100 μL/min, the endothermic dissociation reaction occurred in the aqueous phase containing

Therefore, a further discussion on the guest distribution and structure of natural gas hydrates requires knowledge of the intrinsic properties of the CH4 + C2H6 + H2O system. Above all, the thermodynamic study of the competition for cage occupancy and preferential structure between CH4 and C2H6 will serve to specify the boundary conditions of sI and sII hydrates in nature. The formation of mixed gas hydrates in subsea flowlines and natural sediments occurs throughout a wide range of pressure and temperature conditions. The actual operation of hydratebased processes requires qualitative and quantitative information about the thermodynamic effect on guest inclusion behavior and structural preference of mixed gas hydrates. Although quite a number of reports dealing with structural transitions of mixed hydrate systems have been carried out, there has been no thorough study on their temperature dependence. Thus, as a first step, the present study addresses the effect of thermodynamic variables on the trend of sI−sII transitions in CH4 + C2H6 mixed gas hydrates. To clarify this intrinsic behavior, it is necessary to observe changes in the cage occupancy of binary guests depending on the equilibrium conditions. It was verified experimentally that the cage occupancy ratios (θL/θS) of CH4 for single gas hydrates decreased with an increase in the equilibrium temperature.10 In mixed hydrate systems, the temperature dependence of cage occupancy and structural transition is found to follow the expected trend in the phase diagram. Finally, an exceptional phenomenon of the CH4 + C2H6 hydrates was observed through thermodynamic and spectroscopic analyses, thereby elucidating the theoretical relationship between thermodynamic variables and structural transitions. We emphasize that these investigations will help clarify the temperature-dependent nature of the competition for preferential hydrate structures between multiple guests. Our study establishes a foundation for further research on the guest distribution and structural behavior of individual guest molecules for multicomponent gas hydrates with consideration of the pressure and temperature effects.

2. EXPERIMENTAL SECTION 2.1. Materials and Apparatus. CH4 and C2H6 gases were purchased from Special Gas (Korea) with stated minimum purities of 99.99 and 99.95 mol %, respectively. CH4 + C2H6 mixed gas was supplied by YG Special Gas (Korea) and had a certified standard mixture composition of 72.04 mol % CH4 balanced with C2H6. Ultrahigh-purity water was obtained from a Millipore purification unit. The equilibrium cell was a bolted closure-type high-pressure vessel made of stainless steel. This blind cell was assembled with a vertical magnetic drive agitator. The maximum allowable operating pressure was 16 MPa, and the internal volume was 100 cm3. A four-wire type Pt-100Ω probe was used for the temperature sensing devices with a full scale accuracy of ±0.05%. The temperature was calibrated using an ASTM 63C nitrogen-filled thermometer with 0.1 K precision. The pressure transducer (Druck, PMP5073) had an accuracy to 0.20% of full scale with a range of 0−70 MPa sealed gauge. A digital pressure gauge (Druck, DPI 104) for pressure calibration was accredited as a testing laboratory apparatus by the Korea Laboratory Accreditation Scheme (KOLAS). The total volume of the system was controlled by using a microflow syringe pump (Teledyne, ISCO 500D). The syringe pump was able to operate up to 25.86 MPa with full capacity of 507.37 mL. The syringe volume and flow rate were B

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Table 1. Effect of Pressure and Temperature on the Cage Occupancy of Guest Molecules in Mixed Gas Hydrates system CH4 + CO2 CH4 + CO2 N2 + CO2 CH4 + acetone CH4 + C2H6 + C3H8

thermodynamic relations

range

when the equilibrium p was lowered, the cavities were preferentially occupied by CO2 rather than CH4 overall composition of methane in the hydrate phase increases with increasing temperature CO2 selectivity in the hydrate phase increases at lower hydrate formation temperature fractional occupancy of CH4 and acetone relatively increases and decreases with increasing temperature larger molecules are more concentrated in the hydrate phase with lower decomposition p at the same T

entire vapor composition 43 and 66 mol % of CH4 entire vapor composition 3.33 mol % acetone

isobaric three-phase (LW−H−V) equilibria ab initio quantum mechanics

method

ref 12

isothermal three-phase (H−V) equilibria modeling of empirical results

14

three samples

spectroscopic analysis

16

13

15

Table 2. Effects of Pressure and Temperature on Structural Transitions in Binary Mixed Gas Hydrates

a

system

structures

composition

preferences

size differencesa

method

ref

Ar + CH4 Ar + C2H6 N2 + CH4 N2 + CO2 CO2 + C3H8 C2H6 + C3H8 Xe + c−C3H6

sII−sI sII−sI sII−sI sII−sI sI−sII sI−sII sI−sII−sI

y1 ≈ 0.60 y1 ≈ 0.70 y1 ≈ 0.70 y1 ≈ 0.90 y1 ≈ 0.95 y1 ≈ 0.80 interconversion

sII promotion sII promotion sI promotion sII promotion sI promotion sI promotion sI promotion

0.56 1.7 0.26 1.02 1.16 0.78 1.22

ab initio method ab initio method cell potential cell potential cubic spline fits of data modified Clapeyron equation isothermal three-phase equilibria

17 18 19 19 20 21 22

Molecular diameters obtained from ref 1.

the solid precipitates containing hydrate crystals were dropped directly into liquid nitrogen. After the particles were ground in a mortar and pestle, the powder samples were characterized by Raman spectroscopy. Because such a process necessarily undergoes partial dissociation of the hydrate phase, the samples were confirmed to be unsuitable for X-ray diffraction measurements.

solid hydrates. As this reaction proceeded, the internal temperature of the pressure vessel decreased until it reached a minimum value. Then the external heat of the circulation bath suddenly flowed into the aqueous phase. Such a rapid recovery of temperature implies that the hydrate decomposition no longer appears in the aqueous phase. Despite continuous operation in the refill mode of the syringe pump, the internal temperature of water−rich liquid phase in the blind cell remained unchanged. After the syringe volume reached its full capacity, the pressure and volume were restored to the initial levels. The dissociation pressure of three coexisting phases under the corresponding isothermal condition was determined by the pressure at the minimum internal temperature. From then on, the syringe pump operated to maintain the experimentally determined equilibrium pressure in the isobaric mode at a constant temperature. After at least an hour of stabilization in the equilibrium cell, the composition of the guest mixtures was analyzed by gas chromatography. For the variation in composition of the charged gas mixture, one of the gas components was flowed into the vapor phase by the pressure difference between a pure gas cylinder and the system. The desired composition of the gas mixture in equilibrium was confirmed using gas chromatography. The mean values of yi reported in this work were also averaged over five measured values except for the maximum and minimum compositions. The equilibrium vapor phase composition was identified by gas chromatography with a stability of ±0.02 in mol % composition. The statistical thermodynamics program CSMGem ver. 1.10 (Colorado School of Mines, USA) predicted the phase equilibrium behavior of the gas hydrate systems.1 The mole fractions of water and guest gases were set at 0.30 and 0.70, respectively. The graphical presentation of the obtained results and the analysis of fitting precision were generated using the OriginPro 8.5 software (OriginLab, USA). For Raman studies, sample preparation was performed under near LW−H−V equilibrium conditions. Detailed conditions for each sample are listed in the relevant tables. After the system was vented, the pressure vessel was separated from the measuring apparatus and quenched by liquid nitrogen. Then,

3. RESULTS AND DISCUSSION Before dealing with structural transitions in incipient CH4 + C2H6 hydrates, the general trend of temperature-dependent structural transitions in binary mixed gas hydrates should be taken into account. Also, a knowledge of the cage occupancy behavior of binary gas mixtures could help to explain the tendency. The thermodynamic parameters of mixed gas hydrates consist of the pressure (p), temperature (T), and phase composition of the system, which directly affect the cage occupancy of guest species. Assuming that the vapor composition remains constant, an increase in the equilibrium temperature of the hydrate-forming systems leads to an increase in the dissociation pressure. In this regard, two approaches to guest enclathration can be explained on the basis of the cage occupancy changes in the equilibrium conditions of hydrates above the ice point. First, the higher the equilibrium temperature is, the more compressed the host lattice in the gas hydrate is, leading to preferential partitioning of the smaller guest molecules within the cages. Although an increase in pressure raises the total cage occupancy of binary guest mixtures, the inclusion of larger guest molecules is relatively depressed. Second, from a thermodynamic point of view, the higher the equilibrium pressure is, the more affordable the trapping of smaller guests becomes. In other words, highpressure conditions are favorable to enclathration of smaller guests, even into identically sized cages. Table 1 presents the aforementioned relationship between thermodynamic parameters and cage occupancy of guest mixtures reported in the literature.12−16 This table shows that, with increasing pressure and temperature, the enclathraC

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area shown in Figure 1 as follows. (1) Structure II disappeared over the entire composition range at temperatures exceeding 300 K. This result agrees with the notion that the enclathration of C2H6 molecules in hydrates steadily diminishes with increasing equilibrium pressure and temperature. (2) The structural transition points in the region of extremely dilute C2H6 concentrations, called upper transition, follow the shift pattern discussed above. In other words, more C2H6 molecules are required to form sII hydrate with increasing p and T. (3) However, a significant phase behavior change from sII inhibition to sII promotion appears in the lower transition region. The theoretical prediction of this phenomenon was experimentally verified in the present study on the basis of the premise that the cage occupancy of larger guests in hydrates relatively decreases with increasing p and T. Two experimental approaches, phase equilibrium and Raman measurements, are used to measure the temperature dependence of structural transitions in CH4 + C2H6 hydrates as described in our previous report.11 In that report, we demonstrated that our newly developed method enables an approximate determination of structural transition points with an accuracy comparable to Raman spectroscopy. In addition, the lower transition point of incipient CH4 + C2H6 hydrates at 283.10 K was determined in advance of the present work. Thus, to identify the temperature dependence of sI−sII transition points on the phase diagram, the equilibrium pressure and vapor phase composition (p−yi conditions) of the CH4 + C2H6 + H2O system at 277.60 and 290.10 K were measured. As shown in Figure 2, two isotherms of sI and sII were distinguished, and then the sI−sII transition p−yi conditions were obtained by nonlinear fitting of these points (Table S1 and S2, Supporting Information).31 Table 3 lists all the results of isothermal phase equilibrium measurements in our previous and present studies.11 These transition points can be ascribed to the change in structural behavior from sII inhibition to sII promotion by connecting the coordinates. However, a more detailed analysis of structural transitions needs to be carried out to ensure the accuracy of the experimental data and fitting results. Therefore, this paper reports the analysis of Raman spectra in order to ensure the validity of our thermodynamic results. Figure 1 indicates that the subtle structural change at the lower transition can be investigated with the isocompositional method (vertical scan) rather than the isothermal method (horizontal scan). The molar concentration of CH4 was determined to be 72% of the total, and the temperature range covered was between 276 and 291 K. All of the Raman spectra were obtained with the sample quenched in liquid nitrogen near the equilibrium pressure of each CH4 + C2H6 hydrate as listed in Table 4. The average equilibrium vapor phase composition analyzed by gas chromatography was found to be 72.34 mol % of CH4. This table also includes the number of spectra acquired at each temperature because only trace amounts of hydrate crystals were detected in the quenched samples. All samples were allowed to remain near equilibrium pressure for more than a day to ensure the intrinsic crystal structure of incipient CH4 + C2H6 hydrates. The preparation procedure was based on the method used by Subramanian et al.23 The Raman spectra in Figures 3 and 4 demonstrate the structural conversion of sII into sI and sI into sII with increasing equilibrium temperature at the lower transition. The v3 C−C stretches of C2H6 at frequencies of 1000.9 and 992.9

tion of the larger guest molecules continues to lag behind that of smaller guests as discussed above. It is predicted that thermodynamic, spectroscopic, and theoretical studies on mixed-gas hydrates will enable the analysis of temperaturedependent inclusion of guest mixtures. Furthermore, this relationship may change the equilibrium points at which structural transitions occur as follows. Binary mixed gas hydrates in which two guests have different structures in each pure hydrate possess their own composition ranges of structural transitions as verified experimentally and theoretically.1 Therefore, it is reasonable to propose that the region of a structure favoring the inclusion of larger guests becomes narrower with increasing equilibrium p and T. Several examples of temperature-dependent structural transitions in binary mixed gas hydrates are shown in Table 2.17−22 From this table, it is apparent that all the predicted transition behaviors follow the trends described above, except for the N2 + CH4 hydrate system. This exceptional case can be explained by the molecular size differences. The difference in the molecular sizes of N2 and CH4 is not so large compared to other binary guests listed in Table 2. Thus, it can be inferred that the temperature dependence of structural transitions in mixed gas hydrate systems becomes more distinct with increasing differences in gas molecule sizes. This is because the preferential partitioning of guests into the host cages appears to amplify the effect of temperature on structural changes in mixed gas hydrates. Although the Xe + c-C3H6 system follows the trend at least for the two temperatures of 279.15 and 289.15 K,22 it is worth noting that a detailed investigation of this dependence has yet to be carried out experimentally. Unlike the normal transition behavior observed in the binary systems listed in Table 2, the CH4 + C2H6 + H2O system forms the sII hydrate in the CH4 composition ranges from 75 to 99 mol %, even though the sI hydrates are generated in each pure system. This implies that the formation of both sI and sII is thermodynamically favored in CH4 + C2H6 hydrates under suitable conditions. The structures and interconversion of CH4 + C2H6 hydrates have been investigated extensively since the work of Subramanian et al.23−28 It is already known that C2H6 molecules mainly occupy the large cages of both structures with the aid of CH4 inclusion.23,29 Figure 1 schematically illustrates the predicted sII region in equilibrium as previously reported by Ballard et al.29 and Klauda et al.30 In this work, we address three key aspects of this

Figure 1. Schematic diagram of equilibrium in the CH4 + C2H6 + H2O system as given by Ballard et al.29 and Klauda et al.30 D

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Table 4. Isothermal Measurements of Three-Phase (LW−H− V) Equilibrium Pressure for Incipient CH4 (1) + C2H6 (2) Hydrates (y1 = 0.7243) Prepared for Raman Analysis T (K)

pa (MPa)

pcalcdb (MPa)

ARDc (%)

pqcdd (MPa)

(pqcd − p)/p· 100 (%)

no. of Raman spectra

structure type

276 278 281 284 286 288 291

1.36 1.69 2.28 3.37 4.27 5.50 8.28

1.34 1.68 2.36 3.36 4.29 5.62 8.46

1.49 0.59 3.17 0.36 0.49 2.19 2.21

1.39 1.72 2.31 3.44 4.36 5.60 8.43

2.41 1.84 1.35 2.21 2.01 1.98 1.92

6 7 11 13 21 3 8

sII sI sI sI sII sII sII

a

Experimental determination with vapor volume expansion at a rate of 100 μL/min. bPredictions of corresponding equilibrium pressure using CSMGem. cAbsolute relative deviation (ARD) = 100 × (| pcalcd − p |/pcalcd). dPressure at which sample was preserved and quenched.

large (51262) cavities, two frequencies at 2887.3 and 2942.3 cm−1 are assigned to sII hydrate large (51264) cavities. Also, Figure 4 indicates that the inversion of the relative intensity for the two v1 C−H stretches of CH4 at frequencies of 2904.8 and 2914.9 cm−1 corresponds to the structural transition between sI and sII. This result reflects the abrupt changes in relative numbers of large and small cavities within the hydrate structures. All of the experimental data obtained are plotted in the phase diagram in Figure 5, which is classified into the

Figure 2. (a) Isothermal three-phase (LW−H−V) equilibrium pressure−composition (p−y1) diagram for incipient CH4 (1) + C2H6 (2) hydrates at 277.60 K. (Deaton: Experimental data from the literature.31 This work with nonlinear fitting curve: experimental determination with vapor volume expansion at a rate of 100 μL/min. CSMGem: predictions of distinct phase boundaries with respect to the interconversion of sI and sII.) (b) Isothermal three-phase (LW−H−V) equilibrium pressure−composition (p−y1) diagram for incipient CH4 (1) + C2H6 (2) hydrates at 290.10 K (y1 = equilibrium vapor phase mole fraction of CH4).

Table 3. Experimental Determinations of Structural Transitions by the Isothermal Measurements of Three-Phase (LW−H−V) Equilibrium Pressure for Incipient CH4 (1) + C2H6 (2) Hydrates (y1 = Equilibrium Vapor Phase Mole Fraction of CH4) T (K) 277.6 283.1 290.1 a

y1 0.7269 (sI) to 0.7293 (sII) 0.7345 (sI) to 0.7373 (sII) 0.6950 (sI) to 0.6978 (sII)

p (MPa) 1.5952 (sI) to 1.5071 (sII) 3.0384 (sI) to 3.0960 (sII) 6.9427 (sI) to 7.1670 (sII)

y1a

pa

ref

0.7354

1.6546

0.7390

3.1293

this work 11

0.6940

7.3743

this work

Predictions of transition composition and pressure using CSMGem.

cm−1 in Figure 3 correspond to the large cages in sI (51262) and sII (51264), respectively. While the Raman bands at 2891.2 and 2946.2 cm−1 in Figure 4 are identified as C2H6 in sI hydrate

Figure 3. Raman spectra of the C−C stretching vibration of C2H6 for incipient CH4 (1) + C2H6 (2) hydrates quenched close to LW−H−V equilibrium pressure at temperatures from 276 to 291 K (y1 = 0.7243). E

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behavior of CH4 + C2H6 hydrates in the lower transition was observed in the thermodynamic studies. As mentioned above, the theoretical evidence in this study was based on the predictions of each methodology employed in previous works of Ballard et al.29 and Klauda et al.30 These results showed the same trends in the lower transition and, in particular, the experimental data reported here are in good agreement with the predictions by CSMGem developed by Ballard et al.33 Thus, it is plausible that the driving force for unusual structural behavior in the CH4 + C2H6 hydrate system can be elucidated by means of this statistical thermodynamics program. The outcome of this process may reveal a better understanding of the change in structural preference at the lower transition against the region of extremely dilute C2H6 concentrations. For the reason above, we attempted to simulate the temperature−dependent variation in the cage occupancy of CH4 and C2H6 for identical crystal structures. The mixture compositions were kept constant to estimate the p and T effects on the cage occupancy, and the C2H6 occupancy of the small (512) cavities converging to zero was excluded from this process. As shown in Figure 6, we used CSMGem to calculate the cage occupancy of CH4 + C2H6 hydrates at the loading compositions of 0.725 and 0.745 mol % CH4 to take account of Figure 4. Raman spectra of the C−H region for incipient CH4 (1) + C2H6 (2) hydrates quenched close to LW−H−V equilibrium pressure at temperatures from 276 to 291 K (y1 = 0.7243).

Figure 5. Isothermal three−phase equilibrium T−y1 diagram for incipient CH4 + C2H6 hydrates in the lower transition region (Solid symbol, this work; open symbol, literature data11).

macroscopic and microscopic levels. As shown in Figure 5, the temperature-dependent structures of CH4 + C2H6 hydrates were determined by a combination of spectroscopic analysis and comparison with reported data. It is also remarkable that the accuracy of the transition concentrations in the T−yi diagram is high enough to warrant the use of CSMGem for the CH4 + C2H6 hydrate system. These results clearly reflect the phase behavior change from sII inhibition to sII promotion as theoretically explained by the work of Ballard et al.32 From these results, we can conclude that the peculiar structural

Figure 6. Relationship between the absolute cage occupancy and the equilibrium temperature in the (a) structure I hydrate for the CH4 + C2H6 + H2O system (y1 = 0.725) and in the (b) structure II hydrate for the CH4 + C2H6 + H2O system (y1 = 0.745). F

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structures I and II, respectively. This indicates that the inclusion of CH4 and C2H6 respectively increases and decreases with increasing equilibrium temperature, as expected. However, the present study focused on the fact that the absolute cage occupancy of CH4 in the structure I hydrate surpasses that of C2H6 at temperatures exceeding 281 K. This implies that the guest component playing a major role in thermodynamically driving the sI−sII transition would be changed from C2H6 to CH4 with increasing temperature. In other words, the formation of sII may be inhibited from the ice point up to about 281 K, since the influence of C2H6 on structural transitions in this temperature range is stronger than that of CH4. However, the formation of sII will be promoted at higher temperatures based on the fact that CH4 as well as C2H6 favors structure II owing to the ratio of 1:2 of large-to-small cavities. It is further proposed that this phenomenon appears only in the composition range where CH4 and C2H6 are able to compete with each other as in the lower transition region. Also, a detailed theoretical investigation would be of great value in future studies for elucidating the temperature dependence of structural transitions in CH4 + C2H6 hydrates.

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +82-41-521-9425. Fax: +82-41-552-0380. E-mail: [email protected]. *Tel.: +82-42-350-3917. Fax: +82-42-350-3910. E-mail: h_ [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MEST) (2010-0029176). It was also supported by the Ministry of Knowledge Economy through the “Recovery of Natural Gas Hydrate in Deep-Sea Sediments Using Carbon Dioxide and Nitrogen Injection” project (KIGAM-Gas Hydrate R&D Organization).

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REFERENCES

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4. CONCLUSIONS The purpose of the present study was to investigate the effect of thermodynamic variables on the structural behavior in binary mixed gas hydrates. To achieve this, we performed the data interpretation and experimental verification of the predicted temperature−dependent structural transitions in CH4 + C2H6 hydrates. The first part of this work accounted for the consistent trends in the cage occupancy of binary guest mixtures based on the results reported in other recent studies. It was demonstrated that the combination of different gas components relatively reduces the enclathration of larger guest molecules into certain types of cages with increasing pressure and temperature. Second, we found that the selective inclusion behavior for guest molecules with different molecular size is reflected in structural transitions in mixed gas hydrates, which is evidenced by theoretical rather than experimental studies. On the basis of these two considerations, the anomalous behavior of the CH4 + C2H6 hydrate system in the lower transitions was characterized in detail by thermodynamic and spectroscopic measurements. In particular, the Raman spectra results indicated, with increasing equilibrium temperature, sII−sI−sII transitions of the quenched samples for a mixture composition of 72.43 mol % CH4. Lastly, we attempted to identify the driving force for changing the transition points on the phase diagram by using a statistical program. This phenomenon was presumed to occur through the temperature dependence of the preferential hydrate structure since the concentration difference between CH4 and C2H6 in the mixture is small enough for them to compete with each other for their occupancy. The results obtained in this investigation will contribute not only to better understanding of natural gas hydrate structures, but also to the practical design of hydrate-based chemical technologies across a range of thermodynamic conditions.

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S Supporting Information *

Isothermal measurements of three-phase (LW−H−V) equilibrium for incipient CH4 + C2H6 hydrates at 277.60 and 290.10 K; nonlinear curve fitting results of experimental data. This material is available free of charge via the Internet at http:// pubs.acs.org. G

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