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RESEARCH NOTES Phase Equilibria of Methane Hydrates in the Presence of Methanol and/or Ethylene Glycol Aqueous Solutions Amir H. Mohammadi* and Dominique Richon MINES ParisTech, CEP/TEPsCentre E´nerge´tique et Proce´de´s, 35 Rue Saint Honore´, 77305 Fontainebleau, France
In this communication, experimental dissociation data for methane hydrates in the presence of 0.042 mass fraction methanol aqueous solution and/or 0.069 mass fraction ethylene glycol aqueous solution are first reported. The experimental data were generated using an isochoric pressure-search method. The experimental dissociation data for methane hydrates in the presence of 0.042 mass fraction methanol aqueous solution or 0.069 mass fraction ethylene glycol aqueous solution are successfully compared with the predictions of a thermodynamic model to demonstrate its reliability for low concentrations of methanol or ethylene glycol in aqueous solution. The model predictions are then compared with the experimental data for the methanol + ethylene glycol aqueous solution system to study its reliability. We finally report experimental dissociation data for methane hydrates in the presence of 0.1, 0.2, 0.35, and 0.5 mass fraction methanol or ethylene glycol aqueous solutions with the aim of studying the reliability of corresponding experimental data reported in the literature and the predictions of the thermodynamic model. 1. Introduction Gas hydrate formation is a serious problem in natural gas production, transportation, and processing, which can give rise to equipment blockage, operational problems, and safety concerns.1-3 Gas hydrates, or clathrate hydrates, are solid crystalline compounds physically resembling ice, in which small molecules (typically gases) are trapped inside cages of hydrogenbonded water molecules.1-3 To avoid formation of gas hydrates, aqueous solutions of organic inhibitors such as methanol or ethylene glycol are normally used.1-3 To develop and validate thermodynamic models for predicting hydrate stability zones of natural gas, reliable gas hydrate phase equilibrium data for the main components of these gases in the presence/absence of inhibitor aqueous solutions are necessary.3 Although sufficient experimental data have been reported for gas hydrates of these components in the presence of methanol or ethylene glycol aqueous solutions,1 information for gas hydrates of natural gas main components in the presence of mixtures of organic inhibitor aqueous solution is limited.1 Although the latter system has seldom application, it can be used to study the reliability of thermodynamic models. Moreover, information for gas hydrates of natural gas main components in the presence of high concentrations of organic inhibitor aqueous solutions is also limited.1 In this communication, we first report experimental dissociation data for methane hydrates in the presence of 0.042 mass fraction methanol aqueous solution and/or 0.069 mass fraction ethylene glycol aqueous solution, which have been measured using an isochoric pressure-search method.2-4 Comparisons are made between the experimental dissociation data for methane hydrates in the presence of 0.042 mass fraction methanol aqueous solution or 0.069 mass fraction glycol aqueous solution with the predictions of a thermodynamic model5-7 to demonstrate its reliability for low concentrations of methanol or * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +(33) 1 64 69 49 70. Fax: +(33) 1 64 69 49 68.
ethylene glycol in aqueous solution. The model5-7 predictions are then compared with the experimental data for the methanol + ethylene glycol aqueous solution system to study its reliability. Experimental dissociation data for methane hydrates in the presence of 0.1, 0.2, 0.35, and 0.5 mass fraction methanol or ethylene glycol aqueous solutions are finally reported. These data are used to study the reliability of the corresponding experimental data reported in the literature and also the model5-7 predictions. 2. Experimental Section 2.1. Materials. Table 1 reports the purities and suppliers of the materials used in this work. Aqueous solutions were prepared following the gravimetric method using an accurate analytical balance. Consequently, uncertainties on the basis of mole fraction are estimated to be below 0.01. 2.2. Experimental Apparatus.2 Briefly, the main part of the apparatus is a sapphire cylindrical vessel, which can withstand pressures up to 15 MPa. The volume of the vessel is 33.1 cm3. A stirrer was installed in the vessel to agitate the fluids and hydrate crystals inside it. Two platinum resistance thermometers (Pt100) inserted into the vessel were used to measure temperatures and check for equality of temperatures within temperature measurement uncertainties, which is estimated to be less than 0.1 K. This temperature uncertainty estimation comes from calibration against a 25 Ω reference platinum resistance thermometer. The pressure in the vessel was measured with two DRUCK pressure transducers (Druck, type PTX611 for pressure ranges up to (2.5 and 12) MPa, respectively). Pressure measurement uncertainties are estimated to be less than 5 kPa, as a result Table 1. Purities and Suppliers of Materialsa material
supplier
purity
methane methanol ethylene glycol
Messer Griesheim Aldrich Aldrich
99.995 (vol %) 99.9 (%, GC) 99 (%, GC)
a
Deionized water was used in all experiments.
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Table 2. Experimental Dissociation Data for Methane Hydrates in the Presence of Methanol and/or Ethylene Glycol Aqueous Solution T/K
p/MPa
0.042 mass fraction methanol aqueous solution 272.0 273.8 275.3 276.5 277.4 278.6 280.1 281.7 283.2 283.8 285.1
2.76 3.33 3.87 4.34 4.75 5.35 6.31 7.39 8.76 9.38 10.79
0.069 mass fraction ethylene glycol aqueous solution 271.8 273.3 274.1 274.8 276.5 277.9 279.3 280.6 282.4 284.7
2.79 3.24 3.50 3.77 4.40 5.09 5.80 6.63 8.01 10.42
0.042 mass fraction methanol and 0.069 mass fraction ethylene glycol aqueous solution 270.4 271.6 272.3 273.4 274.1 274.7 276.0 277.2 278.5 279.3 280.6 281.3 282.0 282.5 283.1
2.46 2.98 3.19 3.61 3.87 4.31 4.75 5.40 6.13 6.42 7.65 8.32 8.88 9.37 10.24
of calibration against a dead weight balance (Desgranges and Huot, model 520). 2.3. Experimental Method.2 The liquid(aqueous) + hydrate + gas equilibrium conditions were measured with an isochoric pressure-search method.2-4 The vessel containing aqueous solution (approximately 10% by volume of the vessel was filled by aqueous solution) was immersed into the temperaturecontrolled bath, and the gas was supplied from a high-pressure cylinder through a pressure-regulating valve into the vessel. Note that the vessel was evacuated before the introduction of any aqueous solution and gas. After obtaining temperature and pressure stability (far enough from the hydrate formation region), the valve in the line connecting the vessel and the cylinder was closed. Subsequently, temperature was slowly decreased to form the hydrate. Hydrate formation in the vessel was detected by pressure drop. The temperature was then increased with steps of 0.1 K. At every temperature step, the temperature was kept constant for 4 h to achieve an equilibrium state in the vessel. In this way, a pressure-temperature diagram was obtained for each experimental run, from which we determined the hydrate dissociation point.2,3,8 If the temperature is increased in the hydrate-forming region, hydrate crystals partially dissociate, thereby substantially increasing the pressure. If the temperature is increased outside the hydrate region, only a smaller increase in the pressure is observed as a result of the temperature change of the fluids in the vessel.2,3,8 Consequently, the point at which
Figure 1. Experimental and predicted dissociation conditions of methane hydrates in the presence of methanol and/or ethylene glycol aqueous solution. Symbols represent experimental dissociation conditions: (9) pure water;9 (2) pure water;10 (() pure water;11 (O) 0.042 mass fraction methanol aqueous solution, this work; (0) 0.069 mass fraction ethylene glycol aqueous solution, this work; ()) 0.042 mass fraction methanol and 0.069 mass fraction ethylene glycol aqueous solution, this work; (bold solid line) predictions of the thermodynamic model5-7 for the methane + water system; (solid line) predictions of the thermodynamic model5-7 for the methane + 0.069 mass fraction ethylene glycol aqueous solution system; (dashed line) predictions of the thermodynamic model5-7 for the methane + 0.042 mass fraction methanol aqueous solution system; (bold dashed line) predictions of the thermodynamic model5-7 for the methane + 0.042 mass fraction methanol and 0.069 mass fraction ethylene glycol aqueous solution system. Temperature error band: 1 K.
the slope of pressure-temperature data plots changes sharply is considered to be the point at which all hydrate crystals have dissociated and hence reported as the dissociation point.2,3,8 3. Results and Discussion All dissociation data are reported in Tables 2-4 and are plotted in Figures 1-3. A semilogarithmic scale has been used in these figures to show the data consistency, as the logarithm of hydrate dissociation pressure versus temperature has approximately linear behavior. In these figures, we have also shown some selected experimental data from the literature on the dissociation conditions of methane hydrates in the presence of pure water to show the inhibition effects of the aqueous solutions studied in this work. Note that the inhibition effect means shifting hydrate dissociation conditions to high pressures/ low temperatures due to the presence of inhibitor(s) in aqueous solution. In Figure 1, the experimental dissociation data for methane hydrates in the presence of 0.042 methanol aqueous solution or 0.069 mass fraction ethylene glycol aqueous solution are first compared with the predictions of a thermodynamic model5-7 to demonstrate its reliability for low concentrations of methanol or ethylene glycol in aqueous solution. A detailed description of this model is given elsewhere.6,7 The model5-7 is briefly based on the equality of fugacity concept, which uses the Valderrama modification of the Patel-Teja equation of state12 and nondensity dependent mixing rules13 for modeling the fluid phases while the van der Waals and Platteeuw theory14 is used for modeling the hydrate phase. As can be observed, the thermodynamic model5-7 shows good predictions for low concentrations of methanol or ethylene glycol in aqueous solution (less than 1 K deviation). This is partly because of the
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Table 3. Experimental Dissociation Data for Methane Hydrates in the Presence of Methanol Aqueous Solution mass fraction of methanol 0.10
0.20
0.35
0.50
T/K
p/MPa
268.5 272.8 276.1 279.7 281.4 264.6 268.5 272.1 274.8 276.3 254.4 258.7 262.1 265.7 244.1 246.3 250.9 254.6
2.62 3.95 5.52 7.85 9.78 2.95 4.34 6.01 8.08 10.11 2.88 4.58 6.56 9.87 3.26 4.02 6.54 10.11
Table 4. Experimental Dissociation Data for Methane Hydrates in the Presence of Ethylene Glycol Aqueous Solution mass fraction of ethylene glycol 0.10
0.20
0.35
0.50
T/K
p/MPa
271.9 275.5 278.8 281.8 283.4 270.4 273.6 277.3 279.8 268.5 269.9 271.3 274.6 258.7 260.3 262.9 263.7
2.91 4.00 5.99 8.05 9.93 3.06 4.42 6.95 9.41 5.31 6.21 7.44 10.62 5.67 6.95 9.15 10.11
fact that the model5-7 correctly takes into account the interactions between gas-water, gas-methanol/ethylene glycol and water-methanol/ethylene glycol at low concentrations of methanol or ethylene glycol in aqueous solution.6,7 The thermodynamic model5-7 was then applied to predict the dissociation conditions of methane hydrates in the presence of methanol + ethylene glycol aqueous solution. To our knowledge, the experimental data for gas hydrate phase equilibrium in the presence of mixed organic inhibitors aqueous solution have not been reported in the literature. As seen in Figure 1, the model5-7 predictions show deviations (equal or more than 1 K deviation) from the experimental data for the mixed methanol and ethylene glycol aqueous solution system though the concentrations of methanol and ethylene glycol in the aqueous solution are low. This can partly be attributed to the interactions between methanol and ethylene glycol, which are not taken into account correctly in the model.5-7 Improvements are, however, expected by taking into account the above-mentioned interactions. This is a useful remark that is not normally considered in gas hydrate thermodynamic models. On the other hand, as mentioned earlier, the hydrate dissociation data at high concentrations of organic inhibitor in aqueous solutions are rare. In Tables 3 and 4, we report experimental dissociation data for methane hydrates in the presence of 0.1, 0.2, 0.35, and 0.5 mass fractions of methanol or ethylene glycol in aqueous solutions, respectively. These data
Figure 2. Experimental and predicted dissociation conditions of methane hydrates in the presence of methanol aqueous solution. Symbols represent experimental dissociation conditions. Pure water: (•) ref 9; (9) ref 10; (2) ref 11. A 0.1 mass fraction methanol aqueous solution: (*) this work; (+) ref 15; (×) ref 16. A 0.2 mass fraction methanol aqueous solution: ()) this work; (∆) ref 15; (O) ref 16. A 0.35 mass fraction methanol aqueous solution: (0 with shadow), this work; (∆ with shadow), ref 17. A 0.5 mass fraction methanol aqueous solution: (O with shadow) this work; () with shadow) ref 15; (0 with shadow) ref 17; (-) ref 18. (solid lines) Predictions of thermodynamic model.5-7 Temperature error band: 2 K.
Figure 3. Experimental and predicted dissociation conditions of methane hydrates in the presence of ethylene glycol aqueous solution. Symbols represent experimental dissociation conditions. Pure water: (•) ref 9; (9) ref 10; (2) ref 11. A 0.1 mass fraction ethylene glycol aqueous solution: (×) this work; (*) ref 19; (+) ref 17. A 0.2 mass fraction ethylene glycol aqueous solution: (0) this work; ()) ref 19. A 0.35 mass fraction ethylene glycol aqueous solution: () with shadow) this work. A 0.5 mass fraction ethylene glycol aqueous solution: (O with shadow) this work; (∆ with shadow) ref 19; (0 with shadow) ref 17. (solid lines) Predictions of the thermodynamic model5-7 (The model did not converge at concentrations equal to/higher than 0.35 mass fraction ethylene glycol aqueous solution). Temperature error band: 1 K.
are also illustrated in Figures 2 and 3, respectively. The model5-7 predictions and the corresponding literature data are also shown in these figures. As can be observed, the model5-7 predictions are generally acceptable (less than 1 K deviation) at low concentrations (less than 0.2 mass fraction) of organic inhibitor in aqueous solutions but they show deviations (higher than 1 K deviation) at intermediate-high concentrations (higher than 0.2 mass fraction) of methanol or ethylene glycol in
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aqueous solutions. As pointed out earlier, these deviations can be attributed to the molecular interactions in the studied systems, which cannot be taken into account correctly in the model5-7 at intermediate-high concentrations of organic inhibitor in aqueous solutions. Moreover, some experimental data reported in the literature show disagreements at concentrations higher than 0.2 mass fraction of organic inhibitor in aqueous solution as shown in Figures 2 and 3. These disagreements are considerable for the systems containing methanol aqueous solutions. However, the experimental data on the systems containing ethylene glycol aqueous solutions are generally in agreement. This can be partly attributed to the heating rate used to measure dissociation conditions and probably the procedure used to prepare aqueous solutions. In overall, our experimental data, however, are in better agreement (less than 1 K deviation) with the experimental data reported in refs 15 and 19. 4. Conclusions Experimental dissociation data for methane hydrates in the presence of methanol and/or ethylene glycol aqueous solutions with different concentrations of methanol and/or ethylene glycol were reported in this work (Tables 2-4). An isochoric pressuresearch method2-4 was used for performing all the measurements. A thermodynamic model5-7 based on the Valderrama modification of the Patel-Teja equation of state12 + nondensity dependent mixing rules13 for modeling the fluid phases and the van der Waals and Platteeuw theory14 for modeling the hydrate phase was used to predict hydrate dissociation conditions. The thermodynamic model5-7 successfully predicted the dissociation conditions of methane hydrates in the presence of low concentrations of methanol or ethylene glycol in aqueous solution while it showed deviations for mixed methanol and ethylene glycol aqueous solution system and also for high concentrations of methanol or ethylene glycol in aqueous solution. The deviations were partly attributed to the molecular interactions in the studied systems, which cannot be taken into account in the model.5-7 By taking into account the above-mentioned interactions, improvements are expected. Acknowledgment The financial support of Agence Nationale de la Recherche (ANR) is gratefully acknowledged. Literature Cited (1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, Third ed.; CRC Press, Taylor & Francis Group: Boca Raton, 2008.
(2) Mohammadi, A. H.; Richon, D. Equilibrium Data of Carbonyl Sulfide and Hydrogen Sulfide Clathrate Hydrates. J. Chem. Eng. Data 2009, 54, 2338–2340. (3) Mohammadi, A. H.; Kraouti, I.; Richon, D. Methane hydrate phase equilibrium in the presence of NaBr, KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions: Experimental measurements and predictions of dissociation conditions. J. Chem. Thermodyn. 2009, 41, 779–782. (4) Tohidi, 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. (5) Heriot-Watt University Hydrate model (Version 1.1). http://www.pet.hw.ac.uk/research/hydrate/ (Accessed June 2007). (6) Avlonitis, D. Thermodynamics of Gas Hydrate Equilibria. Ph.D. Thesis, Department of Petroleum Engineering, Heriot-Watt University, Edinburgh, UK, 1992. (7) Tohidi-Kalorazi, B. Gas Hydrate Equilibria in the Presence of Electrolyte Solutions. Ph.D. Thesis, Department of Petroleum Engineering, Heriot-Watt University, Edinburgh, UK, 1995. (8) Ohmura, R.; Takeya, S.; Uchida, T.; Ebinuma, T. Clathrate hydrate formed with methane and 2-Propanol: Confirmation of structure II hydrate formation. Ind. Eng. Chem. Res. 2004, 43, 4964–4966. (9) Mohammadi, A. H.; Anderson, R.; Tohidi, B. Carbon Monoxide Clathrate Hydrates: Equilibrium Data and Thermodynamic Modeling. AIChE J. 2005, 51, 2825–2833; quoted in ref 1. (10) Adisasmito, S.; Frank, R. J.; Sloan, E. D. Hydrates of carbon dioxide and methane mixtures. J. Chem. Eng. Data 1991, 36, 68–71; quoted in ref 1. (11) Jhaveri, J.; Robinson, D. B. Hydrates in the methane-nitrogen system. Can. J. Chem. Eng. 1965, 43, 75–78; quoted in ref 1. (12) Valderrama, J. O. A generalized Patel-Teja equation of state for polar and nonpolar fluids and their mixtures. J. Chem. Eng. Jpn. 1990, 23, 87–91. (13) Avlonitis, D.; Danesh, A.; Todd, A. C. Equilibrium data and thermodynamic modelling of cyclohexane gas hydrates. Fluid Phase Equilib. 1994, 94, 181–216. (14) van der Waals, J. H.; Platteeuw, J. C. Clathrate Solutions. AdV. Chem. Phys. 1959, 2, 1–57; quoted in ref 1. (15) Haghighi, H.; Chapoy, A.; Burgess, R.; Mazloum, S.; Tohidi, B. Phase equilibria for petroleum reservoir fluids containing water and aqueous methanol solutions: Experimental measurements and modelling using the CPA equation of state. Fluid Phase Equilib. 2009, 278, 109–116. (16) Ng, H. J.; 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, 145–155; quoted in ref 1. (17) Robinson, D. B.; Ng, H. J. J. Can. Pet. Tech. 1986, 25, 26; quoted in ref 1. (18) Ng, H. J.; Chen, C. J.; Saeterstad, T. Hydrate formation and inhibition in gas condensate and hydrocarbon liquid systems. Fluid Phase Equilib. 1987, 36, 99–106; quoted in ref 1. (19) Haghighi, H.; Chapoy, A.; Burgess, R.; Tohidi, B. Experimental and thermodynamic modelling of systems containing water and ethylene glycol: Application to flow assurance and gas processing. Fluid Phase Equilib. 2009, 276, 24–30.
ReceiVed for reView August 29, 2009 ReVised manuscript receiVed November 17, 2009 Accepted December 2, 2009 IE901357M
