Phase Stability of Hydrates of Methane in Tetrahydrofuran Aqueous

Publication Date (Web): October 23, 2014 .... Kinetics of methane hydrate formation in an aqueous solution of thermodynamic promoters (THF and ... sys...
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Phase Stability of Hydrates of Methane in Tetrahydrofuran Aqueous Solution and the Effect of Salt Deepjyoti Mech and Jitendra S. Sangwai* Petroleum Engineering Program, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai, 600 036, India ABSTRACT: Phase equilibrium data are generated for clathrate hydrates of methane in THF aqueous solution for (0.040, 0.016, 0.010, and 0.005) mass fraction and with NaCl ((0.03, 0.05, and 0.10) mass fraction) in combination with THF ((0.010 and 0.005) mass fraction) for the methane hydrate system to study the effect of salt. The pressure−temperature curves for equilibrium points have been generated by employing an isochoric pressure-search method. The phase stability conditions were reported for a wide range of pressure (2.17 MPa to 6.43 MPa) and temperature (276.15 K to 297.70 K). Contending effects of THF and NaCl at various concentrations on the phase stability of the clathrate hydrate of methane have been studied. The inhibition effect of NaCl is limited by the promotion effect of THF for the clathrate hydrate of methane, even though there is a shift in the hydrate equilibrium curve toward the inhibition zone. The inhibition effect shown by salt is more enunciated at higher pressures compared to lower pressures. The promotion effect is found to decrease as the concentration of NaCl is increased. Moreover, the promotion effect of THF at a lower concentration is commendable on NaCl with higher concentration for methane hydrate formation. The values of heat of dissociation of hydrates for the (THF + CH4) system and the (THF + NaCl + CH4) system at different experimental pressure and temperature conditions are calculated using the Clausius−Clayperon equation for the obtained phase equilibrium data and reported. This study shows that the clathrate hydrates of methane in THF and in (THF + NaCl) aqueous system are anticipated to be more stable as compared to the hydrates of pure methane, thus promising their use for formation, storage, and transportation of the hydrates of natural gas in a real environment.

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INTRODUCTION Gas hydrates, or clathrate hydrates, are made up of water molecules (host) and gas molecules (guest) at low temperature and high pressure conditions. The gas or volatile liquid molecules can be methane (CH4), ethane (C2H6), propane (C3H8), or nonhydrocarbon gases such as carbon dioxide (CO2) and nitrogen (N2), etc. In gas hydrates, the water molecules are connected through a hydrogen bonding making suitable structures, such as, structure I (sI), structure II (sII), and structure H (sH), which are composed of a certain number of water molecules.1−17 The sI, sII, and sH structures accommodate gas molecules (as guest molecules) in them as per the size of gas molecules. For example, CH4 and CO2 forms sI hydrate structures, and the higher-end hydrocarbon gases form sII structures. The formation conditions of pressure and temperature for various hydrate structures primarily depend upon the type of guest gas molecules forming them. Methane typically requires higher pressure and temperature conditions for the formation of hydrates.18 Gas hydrate is one of the focal points of research over the past several years because of its role in the plugging of pipelines, in flow assurance issues in upstream oil, and in the gas industry. Several thermodynamic and kinetic inhibitors were discovered to address the issues of flow assurance due to hydrates. In addition, gas hydrates are also observed to be a source of natural gas owing to their vast © 2014 American Chemical Society

presence in the region of permafrost and lying under subsea sediment.18,19 As hydrates can store a large amount of natural gas in a unit volume, they show potential as a solution for natural gas storage.20,21 However, to improve the storage efficacy and economics, the formation conditions for methane hydrate need to be reduced by the uses of thermodynamic promoters. It is known that the tetrahydrofuran (THF) forms structure II in the absence or presence of guest gas molecules. THF are observed to be good thermodynamic promoters by helping to form the hydrate of methane at relatively lower pressure and temperature circumstances suitable for carbon capture, gas separation, storage, and transportation of natural gas, etc.22 The concentration effect of THF on the hydrate formation conditions of CH4 is important for their efficient and optimum use for various applications as mentioned above. Various studies on the phase equilibrium of hydrate of different gases, such as, CH4,2−4,23 CO2,5,6,24 and N2,3,24 etc., using THF as a hydrate promoter were investigated. Table 1 gives a brief idea about the available literature information on the phase stability of THF + CH4 hydrate systems. However, we observe that Received: September 11, 2014 Accepted: October 9, 2014 Published: October 23, 2014 3932

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Table 1. Overview of Phase Equilibrium Studies on the Clathrate Hydrates of Methane in THF Aqueous Solution sr. no. 1 2 3 4

clathrate hydrate system THF THF THF THF

+ + + +

CH4 CH4 CH4 CH4

+ + + +

H2O H2O H2O H2O

THF mass fraction 0.036, 0.1741, 0.3672 0.1026 0.016, 0.0353 0.1908

pressure range, P/MPa 2.05 2.02 0.68 0.33

to to to to

14.04 8.91 5.68 12.5

temperature range, T/K

no. of data points, N

reference

year of publication

289.54 to 306.23 292.16 to 302.46 278.8 to 296.6 277.9 to 305

10, 9, 9 4 8, 9 14

Deugd et al. Seo et al. Mohammadi et al. Lee et al.

200125 200126 200922 201218

solutions were prepared through an analytical balance (Radwag AS-220/X) with an uncertainty mass fraction of ± 0.00004. Experimental Setup. The setup used for the experimental measurement of the gas hydrate equilibrium conditions is shown in Figure 1. The setup consists of a high pressure reactor constructed of SS-316 with a capacity of 250 mL at a working pressure up to 10 MPa. A pressure transducer and a flexible platinum resistance thermometer (Pt-100) is provided and allowed to move freely inside the thermowell which is attached to the reactor. The dead-weight pressure testing equipment is used for the calibration of the pressure transducer for the range 1 MPa to 7 MPa. The pressure and temperature sensors used for the study have and uncertainty of ± 0.005 MPa and ± 0.05 K, respectively. The gas−liquid interface mixing is provided by a high performance magnetic stirrer which is used at 1000 rpm (revolution per minute) to help the mixture reach equilibrium. Sufficient agitation is provided by this speed to ensure the metastability of the liquid phase in the thermodynamic hydrate stability region as well as for reducing the resistance of mass and heat transfer by constraining the rate of hydrate formation and dissociation. A jacket surrounds the reactor to maintain the temperatures of the experimental study with the help of a Thermo Haake water bath (model A 25) which is used to circulate the mixture of water and glycol. The charging and discharging of aqueous solutions of THF and THF + NaCl are done by loosening the clips on the top of the reactor. Experimental Procedure. In this study, the high pressure reactor is properly cleaned with deionized water. To remove the air within the reactor, a vacuum pump is used. This is followed by the filling of the various aqueous solutions (THF and THF + NaCl) of 160 mL in the reactor. The reactor is then pressurized using methane from a methane gas cylinder and it is preflushed twice with CH4 gas at near 0.2 MPa to ensure that no air remains inside the reactor as well as to remove the dissolved gases present in the solution. A magnetic stirrer is used to rotate at 1000 rpm in the reactor system in order to make gas−liquid contact, and the reactor temperature is allowed to decrease to form hydrate with the help of circulation of the water + glycol mixture from the Thermo Haake A25 water bath. The equilibrium conditions for the three phase hydrate−liquid water−vapor (H−Lw−V) system are determined through an isochoric pressure-search method. A sample of isochors is shown in Figure 2 for the case of methane gas hydrate in an aqueous solution of THF + NaCl. Various isochors are generated to obtain the equilibrium values for different initial pressure and temperature conditions and not reported here for the sake of simpleness. As shown in Figure 2, the pressure is decreased (from a to b) as the reactor temperature starts to decrease because of cooling of the reactor mixture. Hydrate formation inside the reactor is shown by a sharp decrease in pressure (from b to c). Point “c” indicates the hydrate has sufficiently formed because the pressure at this point has decreased by about 2 MPa less than the pressure of the nucleation point at “b”. After that the temperature of the

there is still a scope to develop the phase stability information for the THF + CH4 hydrate system at lower concentrations of THF suitable for their application related to natural gas storage and transportation. Salts such as calcium chloride (CaCl2), magnesium chloride (MgCl2), potassium chloride (KCl), sodium chloride (NaCl), etc., were observed to affect the phase stability of hydrates in the presence or absence of thermodynamic promoters.27,28 This may be due to their effect in lowering the activity of aqueous system affecting the phase stability. As the naturally occurring water may contain salt, it is essential to study the effect of these on the phase stability of THF + CH4 hydrate system for their use in a real situation. Sabil et al.29 studied the effect of NaCl on the phase equilibrium of hydrates for the quaternary system of CO2 + THF + NaCl + H2O. They observed that the promotion effect of THF + CO2 hydrate get negated in the presence of salts, particularly at lower concentration of salt. In our recent study,27 it is observed that the salts marginally affect the phase stability of the semiclathrate hydrates of the TBAB + CO2 system as compared to the pure hydrates of CO2. The effect of salts on pure hydrate of methane and other hydrate systems is well-known, their effect on the hydrates of THF require investigations. In this work, we report the data of phase equilibrium for ternary hydrate system of CH4 + THF + H2O aqueous system with various concentrations of THF, particularly at lower concentrations of THF which are not reported in the open literature. In addition, the effect of salt, NaCl, on the phase stability of a quaternary hydrate system containing CH4 + THF + NaCl + H2O hydrate system is investigated for the salt concentration of (0.03, 0.05, and 0.10) mass fraction. The phase stability conditions were reported for wide range of pressure (2.17 MPa to 6.43 MPa) and temperature (276.15 K to 297.70 K). Initially, a few experiments were performed to check the validity of the experimental procedure with the reported literature values and are also reported here along with new experimental results.

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EXPERIMENTAL SECTION Materials. The materials used in this experimental study are listed in Table 2 with respective suppliers and mass fraction purity. With the help of a gravimetric method, the aqueous Table 2. Purities and Suppliers of Materialsa chemical methane tetrahydrofuran (THF) sodium chloride (NaCl) a

supplier Bhuruka Gas Agency, Banglore Sisco Research Laboratories, Mumbai Finar Chemicals Limited, Ahmedabad

purity mass fraction 0.995 0.995 0.999

Deionized water was used during all the experimental runs. 3933

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Figure 1. Schematic diagram of the experimental setup.

Figure 2. Isochoric PT (pressure−temperature) trace for CH4 + THF (aqueous) system in the presence of inhibitor (NaCl).

Figure 3. Experimental equilibrium values for CH4 clathrate hydrates with THF aqueous system. Open symbols, literature works; filled symbols, this work. Experimental points: ◊, pure CH4 hydrate;30 □, 0.016 mass fraction;22 △, 0.036 mass fraction;25 +, 0.1741 mass fraction25 ○, 0.3672 mass fraction;25 ■, 0.04 mass fraction (this work); ◆, 0.016 mass fraction (this work); ▲, 0.005 mass fraction (this work); ●, 0.01 mass fraction (this work).

reactor system is gradually increased at a dissociation rate of 1 K/h (from points c to d) followed with a rate of 0.1 to 0.2 K/h (from points d to e) using ramp profile heating.27,28 With the help of the above procedure, the pressure−temperature diagram is plotted for each experimental run. The equilibrium point is determined from the intersection point between the two-phase contraction line of the fluid mixture (V−L) and the three-phase (H−Lw−V) dissociation line as shown in Figure 2 with an expanded uncertainty of ± 0.061 K and ± 0.019 MPa.

with the literature data showing the reliability of the experimental setup and procedure adopted in this work. With an increase in THF concentration from (0.005 to 0.016) mass fraction for the hydrate of CH4 in the THF aqueous system, a relatively large shift in the equilibrium curves to lower pressure and higher temperature is observed. It is also observed that the equilibrium temperature increases around 5.6 K with the said increases in the THF concentration, while it is a 5.1 K shift for the concentration range from (0.016 to 0.04) mass fraction of THF in the aqueous solution. The data generated for the lower THF concentrations of 0.005 mass fraction and 0.01 mass fraction are thus vital to fill the gap in the phase stability region of the hydrate of CH4 and the aqueous THF system. Such lower concentrations of THF also support their economical use for methane hydrate formation for suitable applications.

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RESULTS AND DISCUSSION The equilibrium pressure and temperature plots for hydrate of methane for various concentrations of THF ((0.040, 0.016, 0.010, and 0.005) mass fraction) and NaCl ((0.03, 0.05, and 0.10) mass fraction) in an aqueous system are provided in Figures 3 and 4, respectively, and listed in Table 4. All the shaded symbols represent the data generated in this work and the unshaded symbols represent the literature data. Additionally, as seen in Figure 3, a comparison has made between the equilibrium values of this work and literature data of different concentrations of THF. The results obtained in this work for 0.016 mass fraction and 0.04 mass fraction is properly matched 3934

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Table 3. Details of Experiments and Number of Data Points N for Various Mass Fractions w of THF and NaCl in an Aqueous Solution for the Range of Equilibrium Pressure P and Temperature T Obtained in This Work for Clathrate Hydrates of CH4 in THF and NaCl in Aqueous Solution wTHFa 0.040 0.016 0.010

wNaCl

T/K

0 (290.45 to 297.70) 0 (288.59 to 291.44) 0 (283.32 to 291.34) 0.03 (283.33 to 288.04) 0.10 (278.50 to 284.45) 0.005 0 (281.65 to 288.70) 0.03 (277.66 to 285.87) 0.05 (279.05 to 285.05) 0.10 (276.15 to 283.05) total no. of experimental runs/data points

P/MPa (2.31 (2.82 (2.19 (3.32 (3.06 (2.33 (2.31 (3.12 (2.17

to to to to to to to to to

6.43) 4.23) 5.96) 5.74) 5.80) 5.84) 5.76) 5.72) 5.72)

N 6 3 5 5 4 5 5 4 5 42

a wTHF values were determined with an expanded uncertainty of Uc(w) = ± 0.00004.

THF with 0.1 mass fraction of NaCl shifts the equilibrium curves more toward the left as compared to the (0.03 and 0.05) mass fractions of NaCl on hydrate formation for the CH4 + H2O + THF hydrate system. The average hydrate suppressing temperature for 0.03 mass fraction and 0.05 mass fraction is around 4.2 K approximately for all pressure conditions, and for 0.10 mass fraction, it is around 5.8 K. The same effect on hydrate suppression is observed for 0.03 mass fraction and 0.05 mass fraction of NaCl in suppressing the hydrate formation for the CH4 + H2O + THF system (with 0.005 mass fraction THF); hence, the equilibrium values are generated for the hydrate of methane for 0.03 mass fraction and 0.10 mass fraction of NaCl in the 0.010 mass fraction THF aqueous system and reported in Figure 4b. It is observed that NaCl with 0.10 mass fraction for the CH4 + H2O + THF system shifts the equilibrium curves toward the left more and gives a hydrate suppression temperature around 7.2 K, whereas for 0.03 mass fraction, it appears around 1.6 K. It is interesting to note that the hydrate suppression temperature for 0.10 mass fraction of NaCl in 0.010 mass fraction is higher as compared to that of the 0.005 mass fraction of the THF aqueous system. However, the effect of inhibition for NaCl with 0.1 mass fraction is more as compared to that for the 0.03 mass fraction and 0.05 mass fraction for both the 0.005 mass fraction and 0.010 mass fraction of THF. Therefore, with an increase in concentration of NaCl, the promotion of hydrate formation with THF is found to decrease. The inhibition effect of salt is found to be greater at higher pressure compared to lower pressure conditions. Moreover, the promotion effect of THF mass concentration for 0.010 mass fraction and 0.005 mass fraction is still substantial even in the presence of NaCl with (0.03, 0.05, and 0.10) mass fraction on the methane hydrate system. So, the effect of THF in promotion is found to reduce the inhibition effect of NaCl with less concentration of THF, even though there is a shift in the methane hydrate equilibrium curve toward the inhibition zone. Heat of dissociation is also determined for hydrates of CH4 with various THF and THF + NaCl aqueous solutions studied in this work. For the successful engineering applications of the hydrate of methane in THF and the THF + NaCl system, the knowledge of the heat of dissociation is required. A direct calorimetric measurement is employed to measure the dissociation enthalpies of gas hydrates, and indirect measure-

Figure 4. Experimental equilibrium values for CH4 clathrate hydrates in THF + NaCl aqueous system. Experimental points: ◊, pure CH4 hydrate;30 (a) ▲, 0.005 mass fraction THF (this work); +, 0.005 mass fraction THF + 0.03 mass fraction NaCl (this work); ∗, 0.005 mass fraction THF + 0.05 mass fraction NaCl (this work); ×, 0.005 mass fraction THF + 0.1 mass fraction NaCl (this work); (b) ●, 0.01 mass fraction THF (this work); ■, 0.01 mass fraction THF + 0.03 mass fraction NaCl (this work); , 0.01 mass fraction THF + 0.1 mass fraction NaCl (this work.

The effects of NaCl on hydrate of CH4 in THF system is investigated and reported in Table 3 in the temperature range of (276 to 289) K and pressures of (2.17 to 5.8) MPa and are shown in Figure 4. A comparison is made between the measured equilibrium data for the quaternary CH4 + H2O + THF + NaCl hydrate system for different concentrations of NaCl ((0.03, 0.05, and 0.10) mass fraction) and ternary CH4 + H2O + THF system for different concentrations of THF (0.010 mass fraction and 0.005 mass fraction). The equilibrium values generated in this work for hydrate of CH4 + H2O + THF with THF concentration of 0.005 mass fraction and 0.010 mass fraction in the aqueous system are compared with the hydrate system of CH4 + H2O + THF + NaCl for different concentrations of NaCl ((0.03, 0.05, and 0.10) mass fraction). From Figure 4a, it is observed that the 0.005 mass fraction of 3935

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Table 4. Equilibrium Values of Three Phase (H−Lw−V) Equilibrium Pressure, P, and Temperature, T, and Heat of Dissociation, ΔHd, for different Mass Fractions, w, of Clathrate Hydrates of CH4 in THF and NaCl in Aqueous Solutiona wTHF

wNaCl

T/K

P/MPa

ΔHd/(kJ/mol)

0.040

0

0.016

0

0.010

0

290.45 292.25 293.85 294.73 296.29 297.70 288.59 289.87 291.44 283.32 285.96 288.89 289.87 291.34 284.50 286.20 287.60 288.50 289.22 278.50 281.15 283.35 284.45 281.65 283.82 286.35 287.50 288.70 277.66 279.81 282.93 284.80 285.87 279.05 281.55 283.93 285.05 276.15 278.05 280.25 281.45 283.05

2.31 3.10 3.88 4.35 5.33 6.43 2.82 3.49 4.23 2.19 3.22 4.38 5.01 5.96 3.32 4.16 4.88 5.37 5.74 3.06 3.97 5.02 5.80 2.33 3.19 4.29 5.16 5.84 2.31 3.09 4.29 5.20 5.76 3.12 4.04 5.06 5.72 2.17 3.11 4.08 4.85 5.72

110.37 108.94 107.63 106.90 105.47 104.01 110.12 108.89 107.63 93.99 92.26 90.54 89.66 88.45 83.86 82.63 70.94 66.17 62.25 59.53 58.56 57.50 56.77 91.58 90.12 88.44 87.17 86.31 82.98 81.71 79.98 78.79 78.13 62.85 61.81 60.77 60.14 115.69 113.44 111.36 109.79 108.25

0.03

0.10

0.005

0

0.03

0.05

0.10

Figure 5. Semilogarithmic plot of pressure vs temperature for the THF + CH4 aqueous system. Experimental points: ■, 0.04 mass fraction (this work); ◆, 0.016 mass fraction (this work); ▲, 0.005 mass fraction (this work); ●, 0.01 mass fraction (this work).

constant, ΔHd is the molar enthalpy of dissociation of methane gas hydrate in (kJ/mol). The dissociation enthalpies of methane hydrate in the presence of THF and THF + NaCl aqueous solutions are determined by the Clausius−Clapeyron equation from the measured equilibrium data shown in Table 4. The Clausius− Clapeyron equation assumed ΔV = VG in the predictions of heat of dissociation. In Table 4, the values of ΔHd were obtained from the Clausius−Clapeyron equation. A plot has been made between ln P versus (1/T) as shown, for example, in Figure 5 for the case of hydrate of CH4 in THF aqueous solution. The ln P versus (1/T) shows a good linear relationship indicating an increase in equilibrium pressure with an increase in equilibrium temperature. From the aforementioned discussion, it can be concluded that the effect of various concentrations of THF on methane hydrate promotion on the phase behavior of CH4 with different aqueous solutions is more pronounced. Simultaneously, the effect of NaCl with higher concentration on hydrates of CH4 + H2O + THF system as compared to lower concentrations of NaCl is more pronounced. The equilibrium data which are generated in this study will be useful for the efficient use of the hydrate of methane in THF in real environments where the formation water containing salts is required to form hydrates.

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CONCLUSIONS Knowledge of the phase stability of the hydrate of methane in THF system in the presence of salt is necessary to design the technologies that will use this system. In this work, phase equilibrium data are generated on clathrate hydrates of methane in THF aqueous solution for (0.040, 0.016, 0.010, and 0.005) mass fraction and with NaCl containing (0.03, 0.05, and 0.10) mass fraction in combination with THF (0.010 mass fraction and 0.005 mass fraction) for the methane hydrate system to study the effect of salt. The contending effects of THF and NaCl at various concentrations on the phase stability of the clathrate hydrate equilibrium of methane have been studied. It is observed that the effect of inhibition shown by NaCl is limited by the promotion effect of THF for the clathrate hydrate of methane, even though the hydrate equilibrium curve shifts toward the inhibition zone. The inhibition effect of salt is more enunciated at higher pressures

T and P were determined with an expanded uncertainty of Uc(T) = ± 0.061 K and Uc(P) = ± 0.019 MPa, respectively. a

ment is done using the Clausius−Clapeyron equation by phase differentiation of equilibrium pressure and temperature data. The Clausius−Clapeyron equation is given below:31 d ln P d

1 T

()

=−

ΔHd zR

(1)

where P is the pressure, T is the temperature, z is the compressibility factor for gas and is determined through the Soave−Redlich−Kwong (SRK) equation, R is the universal gas 3936

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(11) Adisasmito, S.; Frank, R. J.; Sloan, E. D. Hydrates of carbon dioxide and methane mixtures. J. Chem. Eng. Data 1991, 36, 68. (12) Jhaveri, J.; Robinson, D. B. Hydrates in the methane-nitrogen system. Can. J. Chem. Eng. 1965, 43, 75. (13) Carroll, J. J.; Mather, A. E. Phase equilibrium in the system water−hydrogen sulphide: Hydrate-forming conditions. Can. J. Chem. Eng. 1991, 69, 1206. (14) Selleck, F. T.; Carmichael, L. T.; Sage, B. H. Phase behavior inthe hydrogen sulfide−water system. Ind. Eng. Chem. 1952, 44, 2219. (15) Bond, D. C.; Russell, N. B. Effect of antifreeze agents on the formation of hydrogen sulfide hydrate. Pet. Trans. AIME 1949, 179, 192. (16) Ng, H.; Robinson, D. B. Hydrate formation in systems containing methane, ethane, propane, carbon dioxide or hydrogen sulfide in the presence of methanol. Fluid Phase Equilib. 1985, 21, 145. (17) Pinder, K. L. Time-dependent rheology of the tetrahydrofuran hydrogen sulphide gas hydrate slurry. Can. J. Chem. Eng. 1964, 42, 132. (18) Lee, Y. J.; Kawamura, T.; Yamamoto, Y.; Yoon, J. H. Phase equilibrium studies of tetrahydrofuran (THF) + CH4, THF + CO2, CH4 + CO2, and THF + CO2 + CH4 hydrates. J. Chem. Eng. Data 2012, 57, 3543. (19) Makogon, Y. F.; Holditch, S. A.; Makogon, T. Y. Natural gas hydratesA potential energy source for the 21st Century. J. Petrol. Sci. Eng. 2007, 56, 14. (20) Gudmundsson, J. S.; Parlaktuna, M.; Khokhar, A. A. Storing natural gas as frozen hydrate. SPE Prod. Facil. 1994, 9, 69. (21) Seo, Y.; Lee, S.; Cha, I.; Lee, J. D.; Lee, H. Phase equilibria and thermodynamic modeling of ethane and propane hydrates in porous silica gels. J. Phys. Chem. B 2009, 113, 5487. (22) Mohammadi, A. H.; Richon, D. Phase equilibria of clathrate hydrates of tetrahydrofuran + hydrogen sulfide and tetrahydrofuran + methane. J. Ind. Eng. Chem. Res. 2009, 48, 7838. (23) Mohammadi, A. H.; Martınez-Lopez, J. F.; Richon, D. Determining phase diagrams of tetrahydrofuran + methane, carbon dioxide or nitrogen clathrate hydrates using an artificial neural network algorithm. Chem. Eng. Sci. 2010, 65, 6059. (24) Sfaxi, I. B. A.; Durand, I.; Lugo, R.; Mohammadi, A. H.; Richon, D. Hydrate phase equilibria of CO2 + N2 + aqueous solution of THF, TBAB or TBAF system. Int. J. Greenhouse Gas Control 2014, 26, 185. (25) Deugd, R. M. de.; Jager, M. D.; Arons, S J. de. Mixed hydrates of methane and water-soluble hydrocarbons modeling of empirical results. J. Thermodyn. AIChE 2001, 47, 693−704. (26) Seo, Y.-T; Kang, S.-P.; Lee, H. Experimental determination and thermodynamic modeling of methane and nitrogen hydrates in the presence of THF, propyleneoxide, 1,4-dioxane, and acetone. J. Fluid Phase Equilibria. 2001, 189, 99. (27) Godishala, K. K.; Sangwai, J. S.; Sami, N. A.; Das, K. Phase stability of semiclathrate hydrates of carbon dioxide in synthetic sea water. J. Chem. Eng. Data 2013, 58, 1062. (28) Sami, N. A.; Das, K.; Sangwai, J. S.; Balasubramanian, N. Phase equilibria of methane and carbon dioxide clathrate hydrates in the presence of (methanol + MgCl2) and (ethylene glycol + MgCl2) aqueous solutions. J. Chem. Thermodyn. 2013, 65, 198. (29) Sabil, K. M.; Romàn, V. R.; Witcamp, G. J.; Peters, C. J. Experimental observations on the competing effect of tetrahydrofuran and an electrolyte and the strength of hydrate inhibition among metal halides in mixed CO2 hydrate equilibria. J. Chem. Thermodyn. 2010, 42, 400. (30) Gayet, P.; Dicharry, C.; Marion, G.; Graciaa, A.; Lachaise, J.; Nesterov, A. J. Chem. Eng. Sci. 2005, 60, 5751. (31) Gupta, A.; Lachance, J., Jr.; Sloan, E. D.; Koh, C. A. Chem. Eng. Sci. 2008, 63, 5848.

compared to lower pressures. The promotion effect is found to decrease as the concentration of NaCl is increased. Moreover, the promotion effect of THF at lower concentration on NaCl with higher concentration for methane hydrate formation is commendable. The values of enthalpy of dissociation provide information on the heat of dissociation of hydrates for the (THF + CH4) system and the (THF + NaCl + CH4) system at different experimental pressure and temperature conditions. The study in general shows that the clathrate hydrates of methane in THF and in the (THF + NaCl) aqueous system are anticipated to be more stable than the hydrates of pure methane thus promising use for formation, storage, and transportation of the hydrates of natural gas in real environment.

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Tel.: + 91-44-2257-4825. Fax: + 91-44-2257-4802. Funding

Financial support from GAIL India Limited through grant GAIL/NOIDA/C&P/12099/5900000020/AD/63 dated 21.12.2012 is highly appreciated and acknowledged. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Shri. Parashuram Chaurasia, Shri. Parivesh Chug, and Shri Sunil Haldar of GAIL India, Ltd. for valuable insights into the work.

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REFERENCES

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dx.doi.org/10.1021/je500841b | J. Chem. Eng. Data 2014, 59, 3932−3937