Hydrate Equilibrium Conditions for Water, Diethylene Glycol

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Hydrate Equilibrium Conditions for Water, Diethylene Glycol Monoethyl Ether Acetate, and Methane Masato Kida,*,† Mizuho Watanabe,† Yusuke Jin,† Yoshihiro Konno,† Jun Yoneda,‡ Koji Yamamoto,§ and Jiro Nagao*,† †

Methane Hydrate Production Technology Research Group, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), 2-17-2-1 Tsukisamu-Higashi, Toyohiraku, Sapporo, Hokkaido 062-8517, Japan ‡ Methane Hydrate Geo-mechanics Research Group, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan § Japan Oil, Gas and Metals National Corporation (JOGMEC), 1-2-2 Hamada, Mihama-ku, Chiba-shi, Chiba 261-0025, Japan S Supporting Information *

ABSTRACT: This report describes the effects of diethylene glycol monoethyl ether acetate on the thermodynamic stability of methane hydrate. We measured the hydrate equilibrium conditions for water + diethylene glycol monoethyl ether acetate + methane at pressures (3.65 to 9.25) MPa and temperatures of (274.7 to 284.2) K. The addition of diethylene glycol monoethyl ether acetate into the water + methane system shifts the hydrate equilibrium conditions to a lower temperature and higher pressure region, suggesting that diethylene glycol monoethyl ether acetate plays a role of thermodynamic inhibitor for methane hydrate formation. Hydrate crystals formed in the system of water + diethylene glycol monoethyl ether acetate + methane show the same crystallographic characteristics as pure methane hydrate does. Furthermore, we demonstrate that contact with liquid diethylene glycol monoethyl ether acetate on methane hydrate promotes methane hydrate dissociation even if methane hydrate is in a thermodynamically stable condition.

1. INTRODUCTION Gas hydrates are clathrate compounds in which guest gas molecules are incorporated in a host framework comprising hydrogen-bonded water molecules. The thermodynamic stability of gas hydrates varies depending on the temperature, pressure, and hydrate-bound gas composition.1 Light hydrocarbons including methane in natural gas are hydrate formers, which can be trapped as gas hydrates in deep marine/lake sediments2 or permafrost environments in polar2 and high mountain3 regions. Technological research and development for gas production from naturally occurring gas hydrates have been conducted to secure natural gas resources.4−7 In the conventional oil and natural gas processing, a flow line blockage by hydrate formation presents severe difficulties; technology to prevent hydrate formation has been investigated.1 However, gas hydrates are applicable for natural gas storage and transportation,1,8 thermal storage,9 and molecular separation10 because of their potential properties such as high gas capacity, formation/dissociation heat, and gas trapping selectivity. In industrial applications related to gas hydrates, potentially useful chemicals for controlling thermodynamic stability, nucleation, and crystal growth of gas hydrates have been investigated. Hydrate inhibitors have been developed since Hammerschmidt suggested that gas hydrate formation caused blockage in natural gas flow lines in 1934.11 Alcohols or glycols © XXXX American Chemical Society

are known as thermodynamic hydrate inhibitors (THIs), which shift the hydrate equilibrium condition to a lower temperature and higher pressure region.12 Low molecular weight polymers represented by poly N-vinylpyrrolidone (PVP), and poly-Nvinylcaprolactam (PVCap) or antifreeze proteins (AFPs) from cold-water fish such as the winter flounder are called kinetic inhibitors (KHIs).13 They act as a gas hydrate antinucleator.12 Although KHIs and antiagglomerants (AAs) such as some surfactants provide a stable water-in-oil emulsion that allows hydrate formation in a fluid, these chemicals have a delaying hydrate nucleation that prevents agglomeration and crystal growth of gas hydrates.13 However, shortening of the induction time for hydrate formation and gas hydrate crystal growth promotion are expected to be useful for gas hydrate formation for applications such as natural gas storage and transportation.14 Surfactants of some kinds represented by sodium dodecyl sulfate (SDS) are efficient promoters of hydrate formation.14,15 Consequently, suitable chemicals for controlling gas hydrate characteristics have been studied for potential applications of gas hydrates. The search for more useful chemicals is an important task for current and future investigation. Received: July 15, 2016 Accepted: August 23, 2016

A

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Table 1. Chemical Material Studied in This Work

study. Diethylene glycol monoethyl ether acetate (DE acetate) with purity of >99.00%, supplied by Sigma-Aldrich Corp., was used. The DE acetate chemical structure is shown in Table 1. DE acetate aqueous solutions with 0.011 and 0.025 mole fraction were prepared using distilled water. Hereinafter, the mole fraction of DE acetate in aqueous solution is denoted as xDEA. 2.2. Experimental Procedure. The hydrate equilibrium conditions for water + DE acetate + methane were obtained by measuring dissociation temperatures at given pressures. The experimental setup for the equilibrium measurements is the same as that in our earlier study21 of hydrate equilibrium. The experimental system consists of a stainless steel high-pressure reactor (approximately 1.2 × 10−4 m3), equipped with an impeller for mixing the vapor−liquid interface, and a cooling bath (thermo Haake C41P). The temperature (T) of liquid phase in the systems was measured using a calibrated type-T thermocouple. The expanded uncertainty of temperature was ±0.1 K with a confidence level of approximately 95%. The pressure (p) in the reactor was detected using a pressure sensor (AP-14S; Keyence Co.). The uncertainty of the pressure measurements was ±0.05 MPa. Phase equilibrium conditions were measured using the following procedure. First, approximately 50 g of prepared aqueous solution of DE acetate was enclosed in the high-pressure reactor. The air in vapor was thoroughly replaced by the methane used for hydrate equilibrium measurements. Then the reactor was pressurized up to the desired pressure of methane. The system temperature was lowered while stirring at 300 rpm for hydrate formation. Once hydrate formation with pressure drop was almost terminated, to predict its rough temperature for complete hydrate dissociation at a given pressure, the system temperature was increased. Thereafter, the system was cooled again for the reformation of hydrate crystals. After hydrate reformation, the cooling bath was set at the temperature below its complete hydrate dissociation temperature predicted. Then, the set value of the bath temperature was increased in steps of 0.1 K up to more than the predicted temperature. The p−T conditions, which became constant over approximately a half day at each bath temperature, were recorded. The p−T conditions for complete hydrate dissociation, indicating hydrate equilibrium conditions for water + DE acetate + methane, were determined by the inflection point of plot for the p−T conditions measured at each temperature. The reliability of the experimental procedure for determination of hydrate equilibrium conditions was confirmed by measuring the hydrate equilibrium condition for water + methane. Dissociation behavior of methane hydrate crystals when contacting methane hydrates with DE acetate was examined using an experimental setup consisting of a stainless steel highpressure vessel (approximately 1 × 10−5 m3) equipped with a pressure sensor (AP-14S; Keyence Co.), a calibrated type-T thermocouple, and a cooling bath (Haake C40). The synthetic pure methane hydrate and solid DE acetate were sieved from 1 to 2 mm. Then they were mixed at the same weight ratio. First, 1.6 g of the mixture was introduced into the high-pressure

Gas recovery from naturally occurring gas hydrates has been attempted by controlling its thermodynamic hydrate stability.4−7 Hydrate-bound natural gas is producible from gashydrate-bearing sediments by decomposing gas hydrates, either by heating them, decreasing the pressure around them, or by shifting the hydrate stability condition. Depressurization, thermal stimulation, and inhibitor injection have been proposed as a gas recovery method from gas-hydrate-bearing sediments.13 Establishment of methods for gas production from naturally occurring gas hydrates is a major challenge for securing future hydrocarbons. In the past 10 years, some gas production tests using depressurization for gas hydrate-bearing sediments in oceanic and permafrost environments have been conducted.6,7 In 2007 and 2008, gas production tests for gas hydrates accumulating under a permafrost environment were conducted in Mackenzie Delta, Northwest Territories, Canada.6 In 2013, the first gas production test for offshore gas hydrates was conducted at the eastern Nankai Trough area off central Japan, by Japan Oil, Gas and Metals National Corp. (JOGMEC) as a part of MH21, the Research Consortium for Methane Hydrate Resources in Japan, funded by the Ministry of Economy, Trade and Industry (METI) of Japan.7,16 During the 6 days of production testing, the cumulative volume of gas produced reached approximately 120 000 m3.16 Analyses of gases recovered during preparatory surveys for the production test showed that the hydrate-bound gas comprises mainly methane.17,18 In these gas production tests, sand grains were also produced because the gas hydrates were accumulated in unconsolidated sediment layers.16,19 The gas-productionimpeding impact of sand production presents severe difficulties for continuous gas production. To address this issue, a highly reliable production well design is needed. Actually, METI has announced a new plan of a longer-term gas production test from hydrate-bearing marine sediments.16,20 In the next production test, it was decided to adopt a production well with a sand control system using shape memory polymer foam activated by a solvent.20 In addition, METI has announced the use of diethylene glycol monoethyl ether acetate (DE acetate) as the solvent.20 An understanding of the effects of DE acetate on the thermodynamic stability of methane hydrate is important not only from the perspective of assessing effects of the chemical injection into the well system on gas hydrates accumulated in sediments but also that of the search for more useful chemicals to control the thermodynamic stability of gas hydrates. This study measured the hydrate phase equilibrium conditions for water, DE acetate, and methane. In addition, the dissociation behavior of methane hydrate crystals when contacting methane hydrates with DE acetate was examined. Crystallographic properties of hydrate crystals formed in the system for methane, DE acetate, and water were measured using powder X-ray diffraction (PXRD) and solid-state 13C NMR techniques.

2. EXPERIMENTAL SECTION 2.1. Materials. Methane with a purity of >99.9%, supplied by Sumitomo Seika Chemicals Co., Ltd., was used for this B

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literature.23−26 The phase equilibrium condition of methane hydrate was determined to be 6.74 MPa at 282.7 K using the experimental setup and procedures described in the Experimental Section. The hydrate equilibrium p−T condition for water + methane in this study shows good agreement with those reported earlier in literature,23−26 indicating that the experimental setup and procedure in this study is appropriate for determination for the hydrate phase equilibrium conditions. Table 2 shows the obtained hydrate equilibrium conditions for water + DE acetate + methane. Hydrate equilibrium

vessel. The temperature was kept below 173 K. The vapor phase was replaced by methane. Next, the vessel was pressurized by methane up to a pressure above the equilibrium pressure of methane hydrate. The vessel with the sample was set in the cooling bath kept at 243 K. After keeping the sample for approximately 30 min, the dissociation test was conducted at bath temperatures warming from (243 to 293) K at a rate of +0.2 K/min. After the temperature ramping test, to confirm the complete hydrate dissociation during the test, the system was cooled to 253 K at a rate of −0.2 K/min, followed by reheating to 293 K with a rate of +0.2 K/min. For comparison with the pure methane hydrate and DE acetate mixture, dissociation behavior of 0.8 g of the pure methane hydrate was examined using the same experimental procedure. The hydrate formed from methane and DE acetate aqueous solutions with xDEA = 0.011 was recovered from the reactor using liquid nitrogen quenching. Crystallographic properties for the recovered hydrate sample were analyzed using PXRD and solid-state 13C NMR techniques. The PXRD profile was taken at 170 K using a quartz glass capillary cell (2.0 mm diameter, 0.01 mm thick, 20 mm long; Hilgenberg GmbH) with an X-ray diffraction apparatus using Cu Kα radiation (45 kV, 200 mA, SmartLab; Rigaku Corp.). The PXRD profile was measured using a step width of 0.01° and scanning speed of 4.0°/min. The 13C single-pulse magic angle spinning (MAS) NMR spectrum was measured using an NMR spectrometer (100 MHz, Avance III 400, Bruker BioSpin; Bruker Analytik GmbH) at approximately 169 K. The NMR spectrum was acquired under the following conditions: 5.5 μs 13C pulse length (90°), 50 s pulse delay time, 160 acquisitions, and a 3.0 kHz spinning rate at the magic angle. The 13C chemical shift values were referred to an external adamantane peak at 29.472 ppm22 at 298 K.

Table 2. Hydrate Equilibrium p−T Data for Water−DE Acetate−Methane Systems xDEA

Ta/K

pb/MPa

0.011

275.6 278.9 282.0 284.2 274.7 278.1 281.1 283.8

3.65 5.04 7.00 8.96 3.68 5.09 7.02 9.25

0.025

The expanded uncertainty of T value was ±0.1 K with a confidence level of approximately 95%. bUncertainty of p values were ±0.05 MPa.

a

conditions for the water−methane systems including DE acetate in this study are depicted in Figure 2 together with

3. RESULTS AND DISCUSSION The pressure and temperature data obtained in this study for the phase equilibrium condition of methane hydrate are presented in Figure 1 together with the hydrate equilibrium condition for water + methane in several reports of the

Figure 2. Hydrate equilibrium p−T conditions for water−methane systems with the additives of DE acetate and methanol (the molar fraction of methanol in aqueous solution is denoted by xMeOH): ■, xDEA = 0.011; ●, xDEA = 0.025; ◇, xMeOH = 0.0089 (ref 27); △, xMeOH = 0.0250 (ref 27); +, no additives (refs 23−26).

those excluding chemical additives and including methanol in the literature.27 The hydrate equilibrium conditions for the systems including DE acetate with xDEA = 0.011 and 0.025 shift to a lower temperature and higher pressure region than those for the pure methane hydrate system. This fact suggests that DE acetate plays a role as a thermodynamic inhibitor for methane hydrate formation. In addition, the higher concentration of DE acetate in the system shifts the hydrate equilibrium conditions to lower temperature and higher pressure conditions, suggesting that the hydrate inhibition effect on methane hydrate formation is greater with increasing concentration of the DE acetate. Compared with a representative inhibitor, methanol, the hydrate equilibrium

Figure 1. Hydrate dissociation p−T conditions and hydrate equilibrium conditions for a water−methane system: ○, hydrate dissociation p−T conditions (this work); ●, hydrate equilibrium condition (this work); , hydrate equilibrium conditions (ref 23); ×, hydrate equilibrium conditions (ref 24); ∗, hydrate equilibrium conditions (ref 25); +, hydrate equilibrium conditions (ref 26). C

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conditions for DE acetate with xDEA = 0.025 is approximately consistent with those for methanol with the same mole fraction in the literature.27 This consistency suggests that the inhibition effects per unit molecule of both additives into water−methane system are almost mutually equal. Figure 3 shows hydrate dissociation behavior when methane hydrate crystals contact with DE acetate. The pressures change

methane hydrate and DE acetate mixture, the pressure change during the test is described by the red solid line with uppercase alphabets in Figure 3. Pressure changes are shown in alphabetical order (A−F). After starting the test at 243 K (A), the pressure increases linearly to point B because of expansion of the vapor phase in the vessel during temperature ramping with a constant rate of 0.2 K min−1. When the temperature crossed the melting point of DE acetate, no pressure change resulting from hydrate dissociation was observed. This fact suggests that the contact of methane hydrate and solid DE acetate and solid−liquid phase change in DE acetate have no influence on methane hydrate stability under pressure. The rapid increase in pressure commences at point (B) and continues until point (C), indicating methane hydrate dissociation. The onset and end of the hydrate dissociation p−T conditions are, respectively, 275.4 K and 7.15 MPa (B), and 283.5 K and 7.96 MPa (C), which are in the lower temperature and higher pressure region than the hydrate equilibrium conditions for methane and water. This fact suggests that even though p−T conditions are under a thermodynamically stable condition for methane hydrate, methane hydrate contacting with liquid DE acetate dissociates completely, which supports the fact that DE acetate plays a role of thermodynamic inhibitor for methane hydrate formation. These results will be significant for predicting dissociation behavior of hydrate-bearing regions around the production well. Furthermore, the fact shows that DE acetate is a candidate chemical material for controlling the thermodynamic stability of gas hydrate in gas hydrate-related technologies. The PXRD pattern of the hydrate formed in the system including DE acetate with xDEA = 0.011 is presented in Figure 4.

Figure 3. Pressure changes during the temperature ramping test and one cycle of cooling and reheating for the mixture of methane hydrate and solid DE acetate with the same weight ratio. Solid black and red lines respectively present the behaviors for pure methane hydrate and the mixture of methane hydrate and solid DE acetate. Small and capital alphabets respectively denote the orders of pressure change for methane hydrate and the mixture of methane hydrate and solid DE acetate. Solid squares (■) present methane hydrate equilibrium conditions (refs 23−26).

during the hydrate dissociation tests in the order of the lowercase alphabet in Figure 3. The dissociation behavior of pure methane hydrate is shown by the black solid line with lowercase alphabets. First, the pressure increased with a constant gradient (a−b) during temperature ramping at a constant ramping rate of 0.2 K min−1 from 243 K. Then the slope of the pressure increase became greater at 283.7 K and 7.37 MPa (b). The inflection point shows good agreement with the phase equilibrium condition of methane hydrate in reports of the relevant literature.23−26 The methane hydrate dissociation commences at point b. Methane release from the hydrate crystals stops at point c. The linear change in pressure during temperature ramping for the period of a−b results from the gasphase expansion. The pressures increase linearly with a constant rate just as they do in the period of a−b at temperatures higher than 288 K (c−d), indicating complete hydrate dissociation. After the system temperature reaching 293 K, the pressure decreases linearly to point e during cooling at a constant rate of −0.2 K min−1. A sharp temperature spike is observed at point e, suggesting that exothermic ice forming from the supercooled water remains after hydrate dissociation in the high-pressure vessel. Then the pressure decreases with a constant gradient until the system temperature reaches 253 K (e−g). The pressure jump occurs at the point h during reheating at a constant ramping rate of 0.2 K min−1 from 253 K of the system temperature similarly to the behavior at point b. The sample pressure and temperature at point h are, respectively, 284.4 K and 7.92 MPa, showing good agreement with the hydrate equilibrium line for water and methane.23−26 This behavior indicates that methane hydrate crystals form during cooling of the system. However, in the system of

Figure 4. PXRD pattern of the hydrate formed in the water−DE acetate−methane system with xDEA = 0.011. Asterisks denote peaks from hexagonal ice in the sample.

Diffraction peaks are assigned to sI gas hydrate crystals and ice Ih as described in an earlier report.28 There are no diffraction peaks from gas hydrate crystals with sII or sH. Figure S1 shows that no sharp diffraction peaks from solid DE acetate are observed in the solid phases of DE acetate and DE acetate aqueous solution. The lattice constant of hydrate crystals with sI is estimated as 1.1861 nm at approximately 170 K, which shows good agreement with that of sI hydrate crystals in reports of the literature.28 The 13C MAS NMR spectrum of the hydrate formed in the system including DE acetate with xDEA = 0.011 is depicted in Figure 5. The chemical shift values of two main 13C NMR signals at −4.2 and −6.6 ppm show good agreement with those of pure methane hydrate,29 which are assigned to methane molecules incorporated into the 512 and 51262 cavities of the sI hydrate framework. The molecules of DE acetate are too large to be trapped in the cage cavities of gas hydrate. Therefore, the 13 C NMR signals of others in a more downfield region than the hydrate-bound methane signals are attributed to free solid DE D

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Funding

This work was supported by funding from the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium). We thank Mrs. B. Hay, W. Furlan, C. Williams, and N. Carrejo (Baker Hughes Inc.), Mr. Y. Terao (Japan Methane Hydrate Operating Co., Ltd.), and Drs. N. Tenma, and M. Oshima of AIST for their valuable discussions. Notes

The authors declare no competing financial interest.

■

Figure 5. 13C MAS NMR spectrum of the hydrate formed in the water−DE acetate−methane system with xDEA = 0.011. The labels on the spectra (a to g) correspond to the carbons in the structural formula of DE acetate. No NMR signal from the carbon-h was observed.

(1) Sloan, E. D., Jr. Fundamental principles and applications of natural gas hydrates. Nature 2003, 426, 353−359. (2) Milkov, A. V. Molecular and stable isotope compositions of natural gas hydrates: A revised global dataset and basic interpretations in the context of geological settings. Org. Geochem. 2005, 36, 681−702. (3) Wang, P.; Zhang, X.; Zhu, Y.; Li, B.; Huang, X.; Pang, S.; Zhang, S.; Lu, C.; Xiao, R. Effect of permafrost properties on gas hydrate petroleum system in the Qilian Mountains, Qinghai, Northwest China. Environ. Sci. Process Impacts 2014, 16, 2711−2720. (4) Bily, C.; Dick, J. W. L. Naturally Occurring Gas Hydrates in the Mackenzie Delta, N.W.T. Bull. Can. Petrol. Geol. 1974, 22, 340−352. (5) Dallimore, S. R.; Collett, T. S.; Dallimore, S. R.; Collett, T. S. Summary and implications of the Mallik 2002 gas hydrate production research well program. In Scientific Results from the Mallik 2002 Gas Hydrate Production Well Program, Mackenzie Delta, Northwest Territories, Canada. Geol. Surv. Can., Bull. 2005, 585, 1−36. (6) Dallimore, S. R.; Wright, J. F.; Yamamoto, K.; Bellefleur, G.; Dallimore, S. R.; Yamamoto, K.; Wright, J. F.; Bellefleur, G. Proof of concept for gas hydrate production using the depressurization technique, as established by JOGMEC/NRCan/Aurora Mallik 2007−2008 Gas Hydrate Production Research Well Program in Scientific Results from the JOGMEC/NRCan/Aurora Mallik 2007− 2008 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada. Geol. Surv. Can., Bull. 2012, 601, 1−15. (7) The Ministry of Economy, Trade and Industry (METI) of Japan, http://www.meti.go.jp/english/press/2013/0318_03.html (accessed August 18, 2016). (8) Gudmundson, J.; Borrehaug, A.; Monfort, J. P. Frozen Hydrate for Transport of Natural Gas. Proc. 2nd Int. Conf. Natural Gas Hydrates 1996, 415−422. (9) Bi, Y.; Guo, T.; Zhu, T.; Zhang, L.; Chen, L. Influences of additives on the gas hydrate cool storage process in a new gas hydrate cool storage system. Energy Convers. Manage. 2006, 47, 2974−2982. (10) Tajima, H.; Yamasaki, A.; Kiyono, F. Energy consumption estimation for greenhouse gas separation processes by clathrate hydrate formation. Energy 2004, 29, 1713−1729. (11) Hammerschmidt, E. G. Formation of Gas Hydrates in Natural Gas Transmission Lines. Ind. Eng. Chem. 1934, 26, 851−855. (12) Kelland, M. A. History of the Development of Low Dosage Hydrate Inhibitors. Energy Fuels 2006, 20, 825−847. (13) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2007. (14) Zhong, Y.; Rogers, R. E. Surfactant effects on gas hydrate formation. Chem. Eng. Sci. 2000, 55, 4175−4187. (15) Sun, Z.-G.; Ma, R.-S; Wang, R.-Z.; Guo, K.-H.; Fa, S.-S. Experimental Studying of Additives Effects on Gas Storage in Hydrates. Energy Fuels 2003, 17, 1180−1185. (16) Yamamoto, K.; Terao, Y.; Fujii, T.; Ikawa, T.; Seki, M.; Matsuzawa, M.; Kanno, T. Operational overview of the first offshore production test of methane hydrates in the Eastern Nankai Trough. The Offshore Technology Conference, Houston, Texas, USA, 5−8 May 2014, OTC 25243. (17) Kida, M.; Suzuki, K.; Kawamura, T.; Oyama, H.; Nagao, J.; Ebinuma, T.; Narita, H.; Suzuki, H.; Sakagami, H.; Takahashi, N. Characteristics of natural gas hydrates occurring in pore-spaces of

acetate coexisting with methane hydrate. The methane occupancies for the 512 and 51262 cavities can be estimated using the signal intensity ratio of two 13C NMR signals from methane molecules, based on a statistical thermodynamic model of van der Waals and Platteeuw.30,31 Methane occupancies for the 512 and 51262 cavities in sI hydrate crystals formed in the system including DE acetate with xDEA = 0.011 are estimated as 0.81 and 0.98, respectively, showing good agreement with that for pure methane hydrate described in an earlier report.31 This fact suggests that the cavity occupation of methane molecules in the hydrate crystals formed under the water−DE acetate−methane system is the same as that formed in a water−methane system.

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CONCLUSION To assess the effects of diethylene glycol monoethyl ether acetate on methane hydrate, we measured the hydrate equilibrium conditions for water + methane + diethylene glycol monoethyl ether acetate with 0.011 and 0.025 mole fractions at pressures of (3.65 to 9.25) MPa and at temperatures of (274.7 to 284.2) K. The hydrate equilibrium conditions for the systems including diethylene glycol monoethyl ether acetate with 0.011 and 0.025 mole fraction shift to a lower temperature and higher pressure region than those for the pure methane hydrate system, which suggests that diethylene glycol monoethyl ether acetate plays the role of thermodynamic inhibitor for methane hydrate formation. The inhibition effect per unit molecule for diethylene glycol monoethyl ether acetate is approximately the same as that for methanol. The structure I hydrate crystals formed in the system of water + methane + diethylene glycol monoethyl ether acetate show the same cage occupation of methane and lattice parameter as pure methane hydrate, suggesting that the coexistence of diethylene glycol monoethyl ether acetate does not affect the hydrate properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00642. Powder X-ray diffraction patterns (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. E

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