Phase Equilibria and Dissociation Enthalpies of Methane Hydrate in

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Phase Equilibria and Dissociation Enthalpies of Methane Hydrate in Imidazolium Ionic Liquid Aqueous Solutions Zhen Long, Xuebing Zhou, Xiaodong Shen, Dong-Liang Li, and De-Qing Liang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03480 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 6, 2015

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Industrial & Engineering Chemistry Research

Phase Equilibria and Dissociation Enthalpies of Methane Hydrate in Imidazolium Ionic Liquid Aqueous Solutions Zhen Long1, Xuebing Zhou1,2, Xiaodong Shen1, Dongliang Li1,Deqing Liang1*

1

Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China

2

University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: This paper reports the thermodynamic inhibition effect of seven ionic liquids (ILs) on the methane hydrate formation. The isochoric multi-step heating dissociation pressure search method is applied for experimentally determining the phase boundary between hydrate-liquid-vapor (H-L-V) phases and liquid-vapor (L-V) phases in the pressure and temperature ranges of (3.45 to 13.28) MPa and (274.3 to 287.6) K, respectively. All the studied IL aqueous solutions are used at a mass fraction of 0.1. A comparison of the thermodynamic inhibitory performance of various ILs is carried out and reveals the predominant role of the type of anion of ILs. Considering the difficulty in directly measuring the dissociation enthalpies, the values of △H of methane hydrate are also calculated using the Clausius–Clapeyron equation. It is found that the mean dissociation enthalpies of methane hydrate in the presence of the seven ILs vary from 59.05 to 60.81 kJ·mol-1, and are very close to that in pure water.

Keywords: methane hydrate, phase equilibria, enthalpy of dissociation, hydrate inhibitor, ionic liquid,

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1. INTRODUCTION

Gas hydrates are ice-like crystalline compounds made of small gas molecules (guest), such as methane, ethane, carbon dioxide, or hydrogen sulfide, which are trapped in 3-dimensional lattices formed by hydrogen-bonded water molecules (host).1-3 Large deposits of natural gas (mostly methane) hydrate, which are considered as a possible alternative energy to fossil fuels, are estimated to abundantly exist in deep oceans and permafrost on a global scale.4-6 There are numerous areas of positive applications of gas hydrates, including refrigeration and air conditioning systems,7, 8 separation of close-boiling point compounds,9, 10 water treatment and desalination,11, 12 gas storage and seperation,13, 14, carbon dioxide capture,15, 16 etc. However, gas hydrates are also pointed out to be responsible for blockage of the pipelines, causing huge hazards as well as economic loss to production, transmission, and transportation system.17, 18A well-known example is the oil leakage of deep water drilling well in Gulf of Mexico, where the presence of water and favorable condition of high pressure and low temperature frequently result in the formation of gas hydrates. To mitigate the hydrate formation, the common method is to inject chemical inhibitors: thermodynamic hydrate inhibitor (THI) and kinetic hydrate inhibitor (KHI).19-21 Ethylene glycol (EG) and methanol (MeOH) are widely used as THI to effectively shift the hydrate phase boundary conditions towards lower temperature or higher pressure. Due to large quantities of usage, an expensive cost is inevitably spent, accompanying with environmental issue. For this reason, such KHI as polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap) is proposed for managing the hydrate formed.1, 20, 22 Instead of completely avoiding the hydrate formation, they allow but delay the hydrate formation through slowing down the nucleation/growth rate of gas hydrates until the oil or gas is safely transported to non-hydrate zones. A low concentration of less than


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0.01mass fraction is enough active in under-inhibited KHI systems. The puzzling problem in the applications of KHI as the hydrate control strategy is that KHI will fail in hydrate inhibition under high subcoolings of up to 10 K.18, 20 Recently, ionic liquids (ILs) as a new class of hydrate inhibitors have captured the researches’ great interests. According to the open studies, it is well recognized that ILs are made of organic cations (e.g., imidazolium-, pyrrolidinium-, ammonium-, and phosphonium-) and organic/inorganic anions.23, 24 Most of them are commonly less volatile, non-flammable, and highly thermally and chemically stable, and so on. Additionally, they are often characterized by environmental-friend due to their biodegradable feature, which are possible recovery technique of them.23 The tunability of cation and anion offers them designable structure for satisfying specific demands and various application areas. 24, 25 Xiao et al.26 presented the first discovery that imidazolium-based ILs were effective as THI as well as KHI for methane hydrate. The kinetic inhibition effect of [Emim][BF4] at 0.01 mass fraction was even much more excellent than those of Luvicap and PVCap. Since then, their study on the new class of ILs inhibitors was extended to dialkylimidazolium-based ILs with halide anions.27 ILs containing halide anions were reported to have strong thermodynamic inhibitory effects than ILs containing [BF4]–. Moreover, the thermodynamic inhibition effectiveness of ILs was demonstrated to be closely related to their electrical conductivity and/or the strength of hydrogen bonding with water, which was dependent on the type of cations and anions. Besides, [Emim][Cl] was observed to perform increased inhibition effectiveness with increased pressures and may surpass the thermodynamic inhibition effect of monoethyleneglycol (MEG) at high concentrations.28 Li et al.,29 Partoon et al.,30 and Sabil et al.31 all agreed that hydroxyl group could be beneficial for ILs to increase their electrostatic charges and make ILs possess better thermodynamic inhibition than typical imidazolium-based ILs without functional groups. In addition, Shin et al.32 studied the role of various anions and cations in determining the thermodynamic inhibition effectiveness of ILs. They found the anion was the main factor of improving the IL



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inhibitory capability. Compared to large phase equilibrium temperature shift by common THI (i.e., about 2-2.5 K by EG), the thermodynamic inhibition effect of these ILs at 0.1 mass fraction is yet very limited, which often depress the equilibrium temperature of methane hydrate less than 1.5 K. It is very difficult to practically apply ILs as gas hydrate inhibitors in the oil and gas industry. However, their dual inhibition functionality due to tunable structure provides a competitive potential over other type of hydrate inhibitors. So, knowledge of newly designed ILs as hydrate inhibitors is necessary that significantly contributes to an improved understanding of the inhibition mechanism and vice versa. In this work, the thermodynamic performance of imidazolium-based ILs as inhibitors on the gas hydrates is investigated. The dissociation conditions of methane hydrate in the presence of seven aqueous solutions of ILs are measured at pressures ranging from (3.45 to 13.28) MPa and temperatures ranging from (274.3 to 287.6) K. The seven ILs are 1-ethyl-3-methylimidazium perchlorate [Emim][ClO4], 1-ethyl-3-methylimidazolium thiocyanate [Emim][SCN],

1-ethyl-3-methylimidazolium

acetate

[Emim][Ac],

1-butyl-3-methylimidazolium

acetate

[Bmim][Ac], 1-hydroxyethyl-3-methylimidazolium perchlorate [OH-Emim][ClO4], 1-ethyl-3-methylimidazolium chloride [Emim][Cl] and 1-hydroxyethyl-2,3-dimethylimidazolium chloride [OH-Emmim][Cl]. The dissociation enthalpies are also calculated from the measured phase equilibrium data of methane hydrate in the presence of ILs using Clausius-Clapeyron equation. 2. EXPERIMENTAL SECTION 2.1 Materials. The seven ILs studied in this work are obtained from Lanzhou Institute of Chemical Physics. They are water-soluble and used without further purification. Table 1 shows the chemical structures and purities of these ILs. The mass fractions of ILs used are 0.1. Methane gas with a purity of 0.9999 mole fraction is supplied by South China Special Gases Ltd., CO. Deionized water with a resistivity of 18.25 mΩ·cm-1 is made by an ultrapure water


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system and used for preparing all of the sample solutions. Samples are weighed using an electronic analytical balance with an accuracy of ±0.1 mg. 2.2 Apparatus and Procedure. The apparatus used in this work is the same as that described in our previous

study.33 The apparatus allows the measurement conditions within the pressure range of (0 to 20) MPa with an uncertainty of ±0.02 MPa and the temperature range of (233.15 to 383.15) K with an uncertainty of ±0.1 K. The cylindrical hydrate crystallizer is made of 316 stainless steel and has an effective volume of 175 mL. The samples in the crystallizer are fully mixed through an electrically-driven magnetic stirrer, which is coupled with a magnet mounted outside the cell. The stirring speed is set to 800 rpm. The system temperature is controlled by a programmable air bath with an uncertainty of ±0.1 K and measured by a platinum resistance (Pt100) thermometer. The system pressure, which is measured by a CYB-20S pressure transducer, can be controlled by injection or withdrawal of water using a high-pressure pump. The data of temperature and pressure are real-time recorded and displayed in a computer via an Agilent data acquisition system. A schematic diagram of the experimental apparatus is given in Figure 1. The hydrate dissociation points are determined by an isochoric equilibrium step-heating pressure search method.33-37 The method has been verified to be reliable and employed for obtaining data in many experimental systems.38-42 Briefly speaking, it operates in a three-step process. The first step includes the preparation of the ionic liquid aqueous solution, washing and drying the crystallizer and all connected pipelines, evacuating the crystallizer, and introducing the methane gas into the crystallizer until the desired initial experimental pressure at the room temperature. The following is lowering the temperature and starting the stirrer to facilitate the formation of gas hydrate. At the onset of gas hydrate formation, a sharp pressure drop is often observed, along with an evident temperature increase. With the accumulating gas hydrate formed, the last but important is to stepwise increase the



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temperature to make the gas hydrate dissociate. At every heating step, it often takes enough time (i.e. 6 h to 10 h) to achieve a steady equilibrium state. When the system temperature is increased beyond the hydrate formation region, the pressure versus temperature exhibits an approximately linear trend again. The experiment ends until the system returns to initial stable state. In this way, the dissociation point, where the slope of P-T curve changes sharply, is determined (see Figure 2) and represents the boundary between liquid-vapor (L-V) phases and hydrate-liquid-vapor (H-L-V) phases.33-37 3. RESULTS AND DISCUSSION 3.1 Validation of Apparatus and Procedure. The methane hydrates phase equilibrium conditions in pure water are measured in the pressure range of (3.40 to 13.27) MPa and temperature range of (275.8 to 288.5) K and presented in Table 2 and Figure 3. Also plotted in Figure 2 are those reported from Adisasmito et al. 43 Mohammadi et al.,44 and Keshavarz et al.45 by using isochoric step-heating pressure search method. The data from the present study are observed to agree well with those given in literature. It validates that our experimental apparatus and approach are well established and reliable. 3.2 Hydrate Phase Equilibrium Conditions in the Presence of Ionic liquid. The phase equilibrium data of methane hydrate in the presence of ILs with the mass fraction of 0.1 are measured at pressures from 3.45 to 13.28

MPa. The obtained experimental results are listed in Table 3. First, to study the effect of cations of ILs on their thermodynamic inhibition performance, four cations [Emim]+, [Bmim]+, [OH-Emim]+, and [OH-Emmim]+ are selected for analysis. According to the anion type in each group, groups A-C are formed, represented in [Cation][ClO4], [Cation][Ac], and [Cation][Cl], respectively. The available P-T data for each group are plotted in Figures 4a-c. It is clearly visible that the inhibition effect of ILs containing [Emim]+ is slightly stronger than that of ILs containing [Bmim]+. The same behavior also happens in groups [Cation][BF4],26,


32

[Cation][Br],27

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and [Cation][I].29 Those results suggest that dialkylimidazolium-based ionic liquids with a shorter alkyl in the cation have better thermodynamic inhibition effectiveness than ionic liquids with a longer alkyl. A possible explanation for the finding is that the introduction of longer alkyl chains to the imidazolium cation can increase hydrophobicity of ionic liquids and therefore hinder the formation of hydrogen bonding with water molecules. The comparisons between [OH-Emim][ClO4] and [Emim][ClO4], [OH-Emim][Cl] and [Emim][Cl] verify that the substitution of hydroxyl group in the cation can enhance the ability of the ionic liquid to form hydrogen-bonding with water, thereby contributing to the increased inhibitory effects.29-31 However, the addition of hydroxyl group into the cation [Emim]+ increases the size of cation itself, which may be negative to the incorporation of the ionic liquids into the hydrogen-bonding network of water. Thus, the increase of inhibition strength due to the presence of hydroxyl group is found to be not significant.29, 30 In contrast to the positive effect of hydroxyl group on ILs’ inhibition ability, the introduction of [OH-Emmim][Cl], where the position-2 hydrogen in the imidazolium cation ring is replaced by a methyl group, displays weaker inhibition effect than [Emim][Cl] and [OH-Emim][Cl], as illustrated in Figure 4c. Following are the inhibitory effects of various anions of ionic liquids containing the same cation [Emim]+ on the methane hydrate formation investigated. Figure 5 shows the phase equilibrium data of methane hydrate in the presence of [Emim][ClO4], [Emim][SCN], [Emim][Ac], and [Emim][Cl], which are compared with those in [Emim][HSO4] from Zare et al.,46 [Emim][NO3] from our recent study33 and pure water. Those ionic liquids present similar inhibition behavior that the inhibitory effect increases with the increased pressure. The order of the effectiveness of each anion in thermodynamic methane hydrate inhibition is as follows: [Cl]– > [Ac]– > [NO3]– > [SCN]– > [ClO4]– > [HSO4]–. It appears that the thermodynamic inhibition tendency is closely related to the anion size. Smaller is the size of anion, the ion charge density is higher at the same mass fraction of aqueous solution. This



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can result in stronger electrostatic interaction between IL and water molecules and may make the IL more effectively suppress hydrate formation.32 To better evaluate the thermodynamic inhibition effectiveness of these ILs, a standard statistical method is used to calculate the average shift temperature △Tave as follows: 27, 31, 46

∆Tave =

1 n

n

∑ ∆T

(1)

i

i =1

where n is the number of data points and △Ti is the shift in the hydrate phase equilibrium temperature measured in the presence of the ionic liquid relative to that in pure water at the same pressure. Considering that the experimental data for various ionic liquids are available at different pressures, the dissociation temperatures in the pure water at the same pressures are calculated by the model of CSMGem.1 The results for the ionic liquids studied are listed in Table 4. From Table 4, [Emim][Cl] is the strongest thermodynamic inhibitor among all the studied ionic liquids, shifting the phase equilibrium curve by 1.7-1.9 K. Since the uncertainty of the temperature measurement in our experiments is ±0.1 K, it seems that the inhibition effects of [Emim][SCN], [Emim][ClO4] and [OH-Emim][ClO4] are approximately the same as seen in Table 4. Despite of longer alky chain in the cation, [Bmim][Ac] is found to be more effective than the three ionic liquids mentioned above as inhibitors. In this way, it demonstrates a more significant correlation between the overall inhibition effect and anion type rather than cation type, which is consistent with the results from Shin et al.32 However, it is considered as inconclusive because of insufficient cation and anion types. Another is noticed that ILs as effective inhibitors, are generally synthesized in laboratories. Their use as inhibitors may be expensive in comparison with traditional THIs like methanol and ethylene glycol. Therefore, more researches should be carried out to make their use economical. 3.3 Dissociation Enthalpy of Methane Hydrates. For oil and gas industry, the thermal properties such as enthalpy


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of dissociation and specific heat are important for predicting the heat requirement for hydrate dissociation. It is meanwhile to assess the molar enthalpy of dissociation, which is defined as the enthalpy change when one molar of hydrate is transformed into gas and water. In this work, it is calculated by using Clausius-Clapeyron equation expressed as: 47, 48

d ln P ∆H =− d (1 T ) zR

(2)

where R is the universal gas constant, and z is the compressibility factor of methane gas, which can be determined by the Peng-Robinson (PR) equation of state 49 The left side in Eq. 2 can be solved by plotting the experimental data in the ionic liquids as lnP versus 1/T. A good linear relationship, with the correlation coefficient R2 close to 1, can be seen by observation of Figure 6. The value of d(lnP)/d(1/T) is assumed equal to the slope of the regressed curve. The average dissociation enthalpies of methane hydrate in these ionic liquids and pure water are summarized in Table 5. It is noted that due to small measurement uncertainties of the phase equilibrium data, this can lead to the large errors in the compressibility factor z and d(lnP)/d(1/T). Within the errors in values of △H, the calculated △H value for methane hydrate in pure water agrees well with those reported in literature. Compared with that in pure water systems, there is no significant difference in the molar dissociation enthalpy for ILs-contained systems. The molar dissociation enthalpy of the ILs does not demonstrate an apparent relationship with the thermodynamic inhibition effectiveness of the ILs. The slope of the hydrate dissociation lines (lnP versus 1/T), which is related to the cavity size occupied by the guests, is also found to be similar for the systems with and without ILs. It is assumed that the simple methane hydrate is formed in the presence of ILs aqueous solutions, which do not participate in the cage structures. More important evidences are required throughout our next work. 4. CONCLUSIONS In this work, the measurements of the phase equilibrium data of methane hydrate in the presence of seven ILs are



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carried out by using the isochoric step-heating pressure search method. All the ionic liquids present a thermodynamic inhibition effect to shift the formation condition of methane hydrate to lower temperatures and higher pressures. The depression temperature among the studied ILs is about between 0.84 and 1.69 K. Although some functional group in the cation, i.e., hydroxyl group, can help strengthen the interaction of the ILs with water, the overall inhibitory performance of the ILs is found to be primarily dependent on the type of anion. Taking use of the available experimental data, the calculated average molar dissociation enthalpies of methane hydrate with and without ILs are obtained. Within the statistical errors of △H, the results show a good agreement of our values with literature data in pure water and no much difference of the dissociation enthalpy is observed due to the addition of various ILs. There is also no apparent relationship between the dissociation enthalpy of the ILs with their thermodynamic inhibition effectiveness. AUTHOR INFORMATION Corresponding Author *Phone: +86 20 8705 7669. Fax: +86 20 8705 7669. Email: [email protected] (D.Q. Liang) ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 51376182), and the Chinese Academy of Sciences Key Development Program (No. KGZD-EW-301). Notes The authors declare no competing financial interest. REFERENCES (1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3 rd ed.; CRC Press-Taylor & Francis: New York, 2008.


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(33) Long, Z.; Zhou, X. B; Liang, D. Q; Li, D. L. Experimental study of methane hydrate equilibria in [EMIM]-NO3 aqueous solutions. J. Chem. Eng. Data 2015, 60, 2728-2732. (34) Tohid, B.; Burgass, R. W.; Danesh, A.; Østergaard, K. K.; Todd, A. C. Improving the accuracy of gas hydrate dissociation point measurements. Ann. N.Y. Acad. Sci. 2000, 912, 924-931. (35) Mohammadi, A. H.; Richon, D. Phase equilibria of clathrate hydrates of cyclopentane + hydrogen sulfide and cyclopentane + methane. Ind. Eng. Chem. Res. 2009, 48, 9045-9048. (36) Mohammadi, A. H.; Richon, D. Phase equilibria of hydrogen sulfide clathrate hydrates in the presence of methanol, ethanol, NaCl, KCl, or CaCl2 aqueous solutions. Ind. Eng. Chem. Res. 2009, 48, 7847-7851. (37) Long, Z.; Zha, L.; Liang, D. L.; Li, D. L. Phase equilibria of CO2 hydrate in CaCl2-MgCl2 aqueous solutions. J. Chem. Eng. Data 2014, 59, 2630-2633. (38) Mohammadi, A. H., Richon, D. Phase Equilibria of Clathrate Hydrates of Tetrahydrofuran +Hydrogen Sulfide and Tetrahydrofuran + Methane. Ind. Eng. Chem. Res. 2009, 48, 7838–7841. (39) Du, J.; Li, H.; Wang, L. Phase equilibria and methane enrichment of clathrate hydrates of mine ventilation air + tetrabutylphosphonium bromide. Ind. Eng. Chem. Res. 2014, 53, 8182-8187.


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(40) Du, J.; Wang, L. Equilibrium conditions for semiclathrate hydrates formed with CO2, N2, or CH4 in the presence of tri-n-butylphosphine oxide. Ind. Eng. Chem .Res. 2014, 53, 1234-1241. (41) Najibi, H.; Kamalia, Z.; Mohammadi, A. H. Phase equilibria of carbon dioxide clathrate hydrates in the presence of methanol/ethylene glycol + single salt aqueous solutions: experimental measurement and prediction. Fluid Phase Equilib. 2013, 342, 71-74. (42) Du, J.; Li, H.; Wang, L. Thermodynamic stability conditions, methane enrichment, and gas uptake of ionic clathrate hydrates of mine ventilation air. Chem. Eng. J. 2015, 273, 75-81. (43) Adisasmito, S.; Frank, R. J.; Sloan, E. D. Hydrates of carbon dioxide and methane mixtures. J. Chem. Eng. Data 1991, 36, 66-71. (44) Mohammadi, A. H.; Anderson, R.; Tohidi, B. Carbon monoixde clathrate hydrates: equilibrium data and thermodynamic modeling. AIChE J, 2005, 51, 2825-2833. (45) Keshavarz, L.; Javanmardi, J.; Eslamimanesh, A.; Mohammadi, A. H. Experimental measurement and thermodynamic modeling of methane hydrate dissociation conditions in the presence of aqueous solution of ionic liquid. Fluid Phase Equilib. 2013, 354, 312-318. (46) Zare, M.; Haghtalab, A.; Ahmadi, A. N.; Nazari, K. Experiment and thermodynamic modeling of methane hydrate equilibria in the presence of aqueous imidazolium-based ionic liquid solutions using electrolyte cubic square well equation of state. Fluid Phase Equilib. 2013, 341, 61-69. (47) van der Waals, J. H.; Platteeuw, J. C. Clathrate solutions. Adv. Chem. Phys. 1959, 2, 1. (48) Sloan, E. D.; Fleyfel, F. Hydrate dissociation enthalpy and guest Size. Fluid Phase Equilib. 1992, 76, 123-140. (49) Peng, D. Y.; Robinson, D. B. A new two-constant equation of state. Ind. Eng. Chem. Fundam. 1976, 15, 59-64.



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(50) Handa, Y.P. Compositions, Enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of methane, ethane, and propane, and enthalpy of dissociation of isobutane hydrate, as determined by a heat-flow calorimeter. J. Chem. Thermodyn. 1986, 18, 915-921.


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Table 1. Ionic liquid studied in this work

Symbol

Chemical name

Chemical structure

Mass fraction purity

[Emim][ClO4]

1-ethyl-3-methylimidazolium perchlorate

[OH-Emim][ClO4]

1-hydroxyethyl-3-methylimidazolium

0.99

0.99 perchlorate [Emim][SCN]

1-ethyl-3-methylimidazolium thiocyanate

[Emim][Ac]

1-ethyl-3-methylimidazolium acetate

[Bmim][Ac]

1-butyl-3-methylimidazolium acetate

[Emim][Cl]

1-ethyl-3-methylimidazolium chloride

[OH-Emmim][Cl]

1-hydroxyethyl-2,3-dimethylimidazolium

0.98

0.985

0.985

0.99

0.99 chloride



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Table 2. Experimental dissociation data for methane hydrate in the presence of pure watera T /K

P /MPa

275.8

3.40

279.6

4.97

282.0

6.32

284.4

8.21

288.5

13.27

a

Uncertainties u are u(T)=±0.1 K, u(P)=±0.02 MPa


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Table 3. Experimental dissociation data for methane hydrate in the presence of 0.1 mass fraction of ionic liquid aqueous solutionsa Ionic liquid

T /K

P /MPa

Ionic liquid

T /K

P /MPa

[Emim][ClO4]

275.3

3.54

[Emim][SCN]

275.2

3.52

278.6

4.88

278.6

4.95

281.3

6.45

281.5

6.44

283.4

8.13

283.8

8.51

287.6

13.21

287.5

13.15

275.0

3.45

274.3

3.47

278.6

4.94

277.7

4.85

281.3

6.44

280.3

6.43

283.5

8.34

282.5

8.06

287.4

12.96

286.7

13.17

274.5

3.51

274.7

3.51

277.8

4.95

278.1

4.91

280.3

6.34

280.5

6.27

282.7

8.38

283.2

8.24

286.6

13.08

287.4

13.28

274.9

3.49

278.1

4.94

[OH-Emim][ClO4]

[Emim][Ac]

[Bmim][Ac]

[Emim][Cl]

[OH-Emmim][Cl]



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a

280.5

6.37

283.1

8.41

286.8

13.04

Uncertainties u are u(T)=±0.1 K, u(P)=±0.02 MPa


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Table 4. Average shift temperature △Tave for studied ionic liquid at 0.1 mass fractions Ionic liquid

△Tave /K

[Emim][ClO4]

0.8

[Emim][SCN]

0.9

[OH-Emim][ClO4]

0.9

[Bmim][Ac]

1.4

[Emim][Ac]

1.7

[Emim][Cl]

1.7

[OH-Emmim][Cl]

1.7



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Table 5. Average enthalpies of methane hydrate dissociation for (methane+water) and (methane+ILs) (0.1 mass fraction)-water systema Enthalpy Ionic liquid

P/MPa

T/K

R2

Slope △H/kJ·mol-1

Pure water (Handa et al.)50

1.013

273.15

-

54.19b

-

1.013

273.15

-

56.9c

-

Pure water (this work)

3.40-13.27

275.8-288.5

-8505.42

59.82

0.996

[Emim][ClO4]

3.54-13.21

275.3-287.6

-8476.30

59.46

0.996

[OH-Emim][ClO4]

3.45-12.96

275.0-287.4

-8429.62

59.11

0.996

[Emim][SCN]

3.52-13.15

275.2-287.5

-8429.30

59.05

0.993

[Bmmim][Ac]

3.49-13.04

274.9-286.8

-8685.67

60.81

0.998

[Emim][Ac]

3.51-13.08

274.5-286.6

-8542.60

59.77

0.997

[Emim][Cl]

3.47-13.17

274.3-286.7

-8449.31

59.20

0.997

[OH-Emmim][Cl]

3.51-13.28

274.7-287.4

-8247.01

57.81

0.997

Pure water (Sloan Fleyfel, 1992)

a

48

Uncertainties u are u(T)=±0.1 K, u(P)=±0.02 MPa, u(H)=±3.49 kJ·mol-1; b measured value via calorimetry;

calculated value with the Clausius-Clapeyron equation


c

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Figure 1. Schematic of experimental apparatus. V1-V7, valves; T, resistance thermometer; P, pressure transducer; BC, buffer cell; GC, gas cylinder; M, magnetic stirrer



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Figure 2. A typical experimental P-T diagram for dissociation point determination for methane hydrate in the presence of 0.1 mass fraction ionic liquid [Emim][ClO4] aqueous solution.


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Figure 3. Experimental dissociation conditions for methane hydrate in the presence of pure water.



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Figure 4. Effect of cation of 0.1 mass fraction ionic liquids on the phase equilibrium curves of methane hydrates. (a) group [Cation][ClO4]; (b) group [Cation][Ac]; (c) group [Cation][Cl].


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Figure 5. Phase equilibrium conditions of methane hydrate in the presence of 0.1 mass fraction Emim ionic liquids.



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Figure 6. Semilogarithmic plot of pressure versus reciprocal temperature for methane hydrate in pure water


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For Table of Contents Only


Phase Equilibria and Dissociation Enthalpies of Methane Hydrate in

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