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Ind. Eng. Chem. Res. 2009, 48, 7838–7841
RESEARCH NOTES Phase Equilibria of Clathrate Hydrates of Tetrahydrofuran + Hydrogen Sulfide and Tetrahydrofuran + Methane Amir H. Mohammadi* and Dominique Richon MINES ParisTech, CEP/TEPsCentre Energe´tique et Proce´de´s, 35 Rue Saint Honore´, 77305 Fontainebleau, France
In this work, experimental dissociation data for the clathrate hydrates of tetrahydrofuran + hydrogen sulfide and tetrahydrofuran + methane are reported. The experimental data were generated using an isochoric pressuresearch method. The dissociation data for the tetrahydrofuran + methane clathrate hydrates are compared with the corresponding experimental data reported in the literature, and the acceptable agreement demonstrates the reliability of the experimental method used in our work. Moreover, we extend the literature data for the latter system to a low concentration of tetrahydrofuran in its aqueous solution. As addition of high concentrations of tetrahydrofuran in aqueous solution diminishes its pressure-reducing effect, we therefore measured and report the experimental dissociation data for the tetrahydrofuran + hydrogen sulfide clathrate hydrates at low concentrations of tetrahydrofuran in its aqueous solution. 1. Introduction Clathrate hydrates or gas hydrates are solid crystalline compounds, which are formed through a combination of water and guest molecules, like methane, hydrogen sulfide, etc.1-22 In the clathrate lattice, water molecules form hydrogen-bonded cagelike structures, encapsulating the guest molecule(s).1 Clathrate hydrates can form crystalline structures I (sI), II (sII), or H (sH), where each structure is composed of a certain number of cavities formed by water molecules.1 The presence of large and small guest molecules are normally required for the formation of structure II with heavy molecules or structure H.1 Heavy molecules occupy large cavities while small molecule(s) (called help gas) fills the remaining cavities.1 It has been proven that tetrahydrofuran (THF) can form structure II with or without the presence of a help gas.1 The presence of tetrahydrofuran in its aqueous solution (with low-intermediate concentration) normally reduces hydrate formation pressures of gases in the presence of pure water.1 This effect is called the hydrate promotion effect, which is used in gas separation, storage, and transportation processes using gas hydrate crystallization.1 Hydrate phase equilibrium information is therefore required to design the aforementioned processes. Most phase equilibrium studies for the clathrate hydrates of tetrahydrofuran + gas have been reported for the tetrahydrofuran + methane,2-4 tetrahydrofuran + carbon dioxide,5,6 and tetrahydrofuran + nitrogen3 systems. To our knowledge, there is no phase equilibrium data for the clathrate hydrates of tetrahydrofuran + hydrogen sulfide. However, a rheological study has already been made on clathrate hydrates of the latter system.22 In addition, few studies have been reported for the clathrate hydrates of hydrogen sulfide in the presence of pure water,16-19 water insoluble hydrate promoters,7-9 and inhibitor aqueous solutions.19-21 The aim of this communication is to extend our previous studies on some water-insoluble hydrate promoters like cyclopentane,7 cyclohexane,8 and methyl cyclohexane9 to tetrahydrofuran, which is an organic water-soluble hydrate promoter * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +(33) 1 64 69 49 70. Fax: +(33) 1 64 69 49 68.
as mentioned earlier. We report gas hydrate dissociation data for the tetrahydrofuran + hydrogen sulfide and tetrahydrofuran + methane systems. The experimental data were generated using an isochoric pressure-search method.7-10 As sufficient experimental hydrate dissociation data for the tetrahydrofuran + methane have been reported in the literature,2-4 we first measured and report few experimental hydrate dissociation data for the latter system and compare them with the corresponding literature data to demonstrate the reliability of the experimental method7-10 used in our work. Moreover, as gas hydrate dissociation data for the tetrahydrofuran + methane system are lacking at low concentrations of tetrahydrofuran in aqueous solution, we therefore measured and report few experimental data for the latter system. Finally, we report hydrate dissociation data for the tetrahydrofuran + hydrogen sulfide, for which there is no experimental data in the literature, as mentioned earlier. 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 mole fractions are estimated to be below 0.01. 2.2. Experimental Apparatus.7-9 Briefly, the main part of the apparatus is a sapphire cylindrical vessel, which can withstand pressures higher than 10 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. The vessel was immersed inside a temperature controlled bath to maintain the temperature inside it at a prescribed level. Two platinum resistance thermometers (Pt100) inserted into the vessel were used to measure temperatures and check for equality of temperatures Table 1. Purities and Suppliers of Materialsa material
supplier
purity
hydrogen sulfide methane tetrahydrofuran
Air Liquide Messer Griesheim Aldrich
99.9 (volume %) 99.995 (volume %) 99.5 (%, GC)
a
Deionized water was used in all experiments.
10.1021/ie900774v CCC: $40.75 2009 American Chemical Society Published on Web 07/24/2009
Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009 Table 2. Experimental Dissociation Data for the Tetrahydrofuran + Methane Clathrate Hydrates T/Ka
p/MPab
methane + 1.05 mol % THF in aqueous solution 283.6 285.4 287.1 288.5 289.7 290.5 291.8 294.3 296.6
0.90 1.16 1.48 1.80 2.14 2.40 2.88 4.10 5.68
methane + 0.48 mol % THF in aqueous solution 278.8 280.9 281.3 282.9 285.2 287.7 290.1 292.4
0.68 0.91 0.96 1.20 1.70 2.42 3.38 4.66
a
Uncertainty on temperatures through calibrated platinum resistance thermometers is estimated to be less than 0.1 K. b Uncertainty on pressures through the calibrated pressure transducers is estimated to be less than 5 kPa.
within temperature measurement uncertainties, which is estimated to be less than 0.1 K. This temperature uncertainty estimation comes from careful 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 8) MPa, respectively). Pressure measurement uncertainties are estimated to be less than 5 kPa, as a result of careful calibration against a dead weight balance (Desgranges and Huot, model 520). 2.3. Experimental Method.7-9 The equilibrium conditions were measured with an isochoric pressure search method, as mentioned earlier.7-10 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 cylinder through a pressure-regulating valve into the vessel. Note that the vessel was evacuated before introducing 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, temperature was kept constant with sufficient time to achieve equilibrium state in the vessel. In this way, a pressuretemperature diagram was obtained for each experimental run, from which we determined the hydrate dissociation point.7-9,11 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 change in the phase equilibria of the fluids in the vessel.7-9,11 Consequently, the point at which 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.7-9,11
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Table 3. Experimental Dissociation Data for the Tetrahydrofuran + Hydrogen Sulfide Clathrate Hydrates T/Ka
p/MPab
hydrogen sulfide + 0.252 mol % THF in aqueous solution 286.6 288.1 289.4 290.8 291.5 292.0 292.7 294.1 295.7 296.9 297.6 298.9 300.1 301.8 303.6 305.1
0.122 0.146 0.177 0.217 0.240 0.259 0.287 0.352 0.424 0.530 0.587 0.685 0.847 1.086 1.414 1.850
hydrogen sulfide + 0.767 mol % THF in aqueous solution 290.8 292.5 294.0 294.7 295.6 296.9 298.8 300.4 301.6 302.8 303.9 305.2 306.7 307.4
0.120 0.154 0.193 0.221 0.244 0.293 0.392 0.497 0.575 0.710 0.822 1.013 1.277 1.438
hydrogen sulfide + 1.30 mol % THF in aqueous solution 295.2 297.5 298.3 299.7 300.4 301.3 303.7 304.5 306.0 306.5 308.7 309.3
0.177 0.251 0.290 0.351 0.390 0.447 0.644 0.715 0.893 0.985 1.372 1.555
a Uncertainty on temperatures through calibrated platinum resistance thermometers is estimated to be less than 0.1 K. b Uncertainty on pressures through the calibrated pressure transducers is estimated to be less than 5 kPa.
3. Experimental Results All the experimental data are reported in Tables 2 and 3 and are plotted in Figures 1 and 2. In all the figures, we have also shown the experimental data reported in the literature in the absence of tetrahydrofuran to identify the hydrate promotion effects of tetrahydrofuran. As can be seen, the presence of the latter chemical in the aqueous solutions studied in this work shifts hydrate dissociation conditions in the presence of pure water to low pressures/high temperatures. We first measured the dissociation conditions of the clathrate hydrates of tetrahydrofuran + methane at 1.05 mol % of tetrahydrofuran in aqueous solution. As can be observed in Figure 1, the experimental data measured in this work for the latter system are in good agreement with the corresponding experimental data reported in the literature4 demonstrating the reliability of the experimental method7-10 used in our work. It should be mentioned that Strobel et al.12 have recently shown that the experimental
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Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009
Figure 1. Experimental hydrate dissociation conditions for the methane + tetrahydrofuran + water and methane + water systems. Symbols represent experimental data. Methane + water system: (×) ref 13; (+) ref 14; (*) ref 15. (b) Methane + 1.05 mol % tetrahydrofuran in aqueous solution, this work. (2) Methane + 0.48 mol % tetrahydrofuran in aqueous solution, this work. (4) Methane + 1.07 mol % tetrahydrofuran in aqueous solution, ref 4. (O) Methane + 6 mol % tetrahydrofuran in aqueous solution, ref 2. ()) Methane + 3 mol % tetrahydrofuran in aqueous solution, ref 3. (0) Methane + 5 mol % tetrahydrofuran in aqueous solution, ref 4. (9) Methane + 10.08 mol % tetrahydrofuran in aqueous solution, ref 4. Pressure band: 4.3 MPa; temperature band: 13 K.
Figure 2. Experimental hydrate dissociation conditions for the hydrogen sulfide + tetrahydrofuran + water and hydrogen sulfide + water systems. Symbols represent experimental data. Hydrogen sulfide + water system: (9) ref 19. (() ref 18. (2) ref 17. (b) ref 16. (O) Hydrogen sulfide + 0.252 mol % tetrahydrofuran in aqueous solution, this work. (4) Hydrogen sulfide + 0.767 mol % tetrahydrofuran in aqueous solution, this work. (0) Hydrogen sulfide + 1.30 mol % tetrahydrofuran in aqueous solution, this work. Pressure band: 0.34 MPa. Temperature band: 7.5 K.
dissociation data reported in ref 4 for the tetrahydrofuran + methane clathrate hydrates are in good agreement with their modeling results. On the other hand, as can be observed in Figure 1, the literature data2-4 have been reported generally at high concentrations of tetrahydrofuran in aqueous solution while the experimental data at low concentrations of tetrahydrofuran are lacking. We therefore measured and report some experimental data for the latter clathrate hydrate system at 0.48 mol % of tetrahydrofuran in aqueous solution. Note that it is believed addition of very high concentrations of organic water-soluble hydrate promoters in the aqueous solution diminishes their pressure-reducing effect while concentrations smaller than 5 mol % relative to water lower the equilibrium pressure at a given temperature significantly.4 The equilibrium data for the clathrate hydrates of tetrahydrofuran + hydrogen sulfide are shown in Figure 2. As can be observed, our experimental data have been reported at low
concentrations of tetrahydrofuran in aqueous solution. To our knowledge, these are the only experimental hydrate dissociation data for the tetrahydrofuran + hydrogen sulfide reported in the literature as mentioned earlier. In Figures 1 and 2, the slope of the logarithm of hydrate dissociation pressure versus temperature changes when tetrahydrofuran exists in aqueous solution indicating a change in clathrate hydrate structure due to the presence of tetrahydrofuran in aqueous solution.1 It is well-known that the clathrate hydrates of tetrahydrofuran + methane form sII.1 It is likely that the tetrahydrofuran + hydrogen sulfide clathrate hydrates also form sII. However, the final proof for the stable hydrate structure and also the exact compositions of the latter clathrate hydrate at various temperatures require direct measurements by suitable physical techniques (e.g., NMR, X-ray, or Raman spectroscopy). 4. Conclusions We reported experimental dissociation data for clathrate hydrates of tetrahydrofuran + methane and tetrahydrofuran + hydrogen sulfide (Tables 2 and 3, respectively). An isochoric pressure-search method7-10 was used to perform all the measurements. The measurements were first performed for the tetrahydrofuran + methane clathrate hydrates, and the comparison between the experimental data measured in this work and the corresponding experimental data reported in the literature4 showed acceptable agreement confirming the reliability of the isochoric pressure-search method7-10 used in this work. The experimental dissociation data for the tetrahydrofuran + methane clathrate hydrates were then reported at low concentration of tetrahydrofuran in aqueous solution, where the literature data were lacking. We finally reported novel experimental dissociation data for the tetrahydrofuran + hydrogen sulfide clathrate hydrates, for which there is no phase equilibrium data in the literature. This study showed that the presence of tetrahydrofuran in the aqueous solutions studied in this work shifts hydrate dissociation conditions of methane or hydrogen sulfide in the presence of pure water to low pressures/high temperatures. It was argued that like the tetrahydrofuran + methane clathrate hydrates which form sII, the tetrahydrofuran + hydrogen sulfide clathrate hydrates most likely form sII. However, direct measurements by suitable physical techniques (e.g., NMR, X-ray, or Raman spectroscopy) are required for final confirmation. Acknowledgment The financial support of the 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) Zhang, Q.; Chen, G. J.; Huang, Q.; Sun, C. Y.; Guo, X. Q.; Ma, Q. L. Hydrate Formation Conditions of a Hydrogen + Methane Gas Mixture in Tetrahydrofuran + Water. J. Chem. Eng. Data 2005, 50, 234–236. (3) Seo, Y. T.; Kang, S. P.; Lee, H. Experimental determination and thermodynamic modeling of methane and nitrogen hydrates in the presence of THF, propylene oxide, 1,4-dioxane and acetone. Fluid Phase Equilib. 2001, 189, 99–110. (4) De Deugd, R. M.; Jager, M. D.; De Swaan Arons, J. Mixed Hydrates of Methane and Water-Soluble Hydrocarbons Modeling of Empirical Results. AIChE J. 2001, 47/3, 693–704. (5) Sabil, K. M.; Peters, C. J. Phase Equilibrium Data of Mixed Carbon Dioxide And Tetrahydrofuran Clathrate Hydrate in Aqueous Electrolyte
Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009 Solutions. Proceedings of the 11th International Conference on Properties and Phase Equilibria PPEPPD, Crete, Greece, 2007; quoted in ref 12. (6) Delahaye, A.; Fournaison, L.; Marinhas, S.; Chatti, I.; Petitet, J. P.; Dalmazzone, D.; Fu¨rst, W. Effect of THF on Equilibrium Pressure and Dissociation Enthalpy of CO2 Hydrates Applied to Secondary Refrigeration. Ind. Eng. Chem. Res. 2006, 45, 391–397. (7) Mohammadi, A. H.; Richon, D. Phase Equilibria of Clathrate Hydrates of Cyclopentane + Hydrogen Sulfide and Cyclopentane + Methane. Ind. Eng. Chem. Res., submitted for publication. (8) Mohammadi, A. H.; Richon, D. Clathrate Hydrates of Cyclohexane + Hydrogen Sulfide and Cyclohexane + Methane: Experimental Measurements of Dissociation Conditions. J. Chem. Eng. Data, submitted for publication. (9) Mohammadi, A. H.; Richon, D. Equilibrium Data of Methyl Cyclohexane + Hydrogen Sulfide and Methyl Cyclohexane + Methane Clathrate Hydrates. J. Chem. Eng. Data, Accepted for publication. (10) 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. (11) 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. (12) Strobel, T. A.; Koh, C. A.; Sloan, E. D. Thermodynamic predictions of various tetrahydrofuran and hydrogen clathrate hydrates. Fluid Phase Equilib. 2009, 280, 61–67. (13) 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. (14) 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.
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(15) Jhaveri, J.; Robinson, D. B. Hydrates in the methane-nitrogen system. Can. J. Chem. Eng. 1965, 43, 75–78; quoted in ref 1. (16) Mohammadi, A. H.; Richon, D. Equilibrium Data of Carbonyl Sulfide and Hydrogen Sulfide Clathrate Hydrates. J. Chem. Eng. Data, in press. (17) Carroll, J. J.; Mather, A. E. Phase Equilibrium in the System WaterHydrogen Sulphide: Hydrate-Forming Conditions. Can. J. Chem. Eng. 1991, 69, 1206–1212; quoted in ref 1. (18) Selleck, F. T.; Carmichael, L. T.; Sage, B. H. Phase Behavior in the Hydrogen Sulfide-Water System. Ind. Eng. Chem 1952, 44, 2219–2226; quoted in ref 1. (19) Bond, D. C.; Russell, N. B. Effect of Antifreeze Agents on the Formation of Hydrogen Sulfide Hydrate. Pet. Trans AIME 1949, 179, 192; quoted in ref 1. (20) 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., Accepted for publication. (21) 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, 145–155; quoted in ref 1. (22) Pinder, K. L. Time-dependent rheology of the tetrahydrofuranhydrogen sulphide gas hydrate slurry. Can. J. Chem. Eng. 1964, 42, 132– 138.
ReceiVed for reView May 14, 2009 ReVised manuscript receiVed July 15, 2009 Accepted July 17, 2009 IE900774V
