Research Note pubs.acs.org/IECR
Phase Equilibria of Clathrate Hydrates of Methane + n-Propyl Mercaptan or n-Butyl Mercaptan + Water System Amir H. Mohammadi*,†,‡ and Dominique Richon† MINES ParisTech, CEP/TEPCentre Énergétique et Procédés, 35 Rue Saint Honoré, 77305 Fontainebleau, France Thermodynamics Research Unit, School of Chemical Engineering, University of KwaZulu-Natal, Howard College Campus, King George V Avenue, Durban 4041, South Africa
† ‡
ABSTRACT: In this work, we report dissociation pressures for clathrate hydrates of the methane + n-propyl mercaptan + water and methane + n-butyl mercaptan + water systems in the temperature ranges of 282.3−289.9 K and 279.4−287.4 K, respectively. The experimental data were measured using an isochoric pressure search method. The hydrate dissociation data for both measured systems are compared with some selected literature data for the dissociation conditions of methane clathrate hydrates. It is shown that n-propyl mercaptan has a thermodynamic promotion effect on methane clathrate hydrates while n-butyl mercaptan has a negligible effect. Table 1. Purities and Suppliers of Chemicalsa
1. INTRODUCTION Clathrate hydrates, or gas hydrates, are crystalline solids stabilized by the inclusion of suitably sized small molecule(s) inside cavities formed by water molecules through hydrogen bonding.1,2 Clathrate hydrates are known to form generally three typical crystalline structures, namely structure I (sI), structure II (sII) and structure H (sH), where each structure is composed of a certain number of cavities formed by water molecules.1,2 A detailed description of clathrate hydrates is given elsewhere.1,2 Very limited information is available in the literature for clathrate hydrates of mercaptans.2−5 It is believed that methyl mercaptan forms the structure I clathrate hydrate.1,2 So far, we studied clathrate hydrates of methyl mercaptan (or methanethiol) and we argued that other mercaptans likely form clathrate hydrates with a help gas like methane.5 To the best of our knowledge, there is no information on clathrate hydrates of heavy mercaptans. This work aims at studying the dissociation conditions of clathrate hydrates of the methane + n-propyl mercaptan (1propanethiol) + water and methane + n-butyl mercaptan (1butanethiol) + water system in the temperature ranges of 282.3−289.9 K and 279.4−287.4 K, respectively. A previously reported experimental apparatus,6,7 which takes advantages of an isochoric pressure-search method5−10 has been used for performing the measurements. The aforementioned hydrate dissociation data are compared with some selected literature data10−12 for the hydrate dissociation conditions of methane to study thermodynamic promotion effects of n-propyl mercaptan and n-butyl mercaptan.
a
supplier
purity
Messer Griesheim Aldrich Aldrich
0.99995 (mole fraction) 0.99 (mass fraction) >0.99 (mass fraction)
Deionized water was used in all experiments.
resistance thermometers (Pt100) inserted into the vessel are used to measure temperatures and check for their equality within temperature measurement uncertainty, 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 is measured with a DRUCK pressure transducer (Druck, type PTX611 for pressures up to 16 MPa). The pressure measurement uncertainty is estimated to be less than 5 kPa, as a result of calibration against a dead weight balance (Desgranges and Huot, model 520). 2.3. Experimental Method.5−10 The hydrate dissociation conditions were measured with an isochoric pressure search method.5−10 The vessel containing liquids (approximately 0.16 volume fraction of the vessel was filled by water and 0.16 volume fraction by liquid mercaptan) was immersed into the temperature-controlled bath, and the gas was supplied from a cylinder through a pressure-regulating valve into the vessel.6,7 Note that the vessel was evacuated (down to 0.8 kPa for at least 2 h) before introducing any liquid 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.6,7,9 Subsequently, temperature was slowly decreased to a set point temperature (at which hydrate formation is expected) with a cooling rate of 5 K/hour.6,7,9 Hydrate formation in the vessel was observed when a pressure
2. EXPERIMENTAL SECTION 2.1. Chemicals. Table 1 reports the purities and suppliers of the chemicals used in this work. 2.2. Experimental apparatus.6,7 Briefly, the main part of the apparatus is a cylindrical vessel made of Hastelloy, which can withstand pressures up to 20 MPa. The volume of the vessel is approximately 30 cm3. A stirrer installed in the vessel is used to agitate the fluid(s) and hydrate crystals. Two platinum © 2012 American Chemical Society
chemical methane n-propyl mercaptan n-butyl mercaptan
Received: December 19, 2011 Accepted: January 26, 2012 Published: January 26, 2012 3841
dx.doi.org/10.1021/ie202976p | Ind. Eng. Chem. Res. 2012, 51, 3841−3843
Industrial & Engineering Chemistry Research
Research Note
drop at set point temperature was detected by the data acquisition unit.6,7,9 The temperature was then increased with steps of 0.1 K. At every temperature step, the temperature was kept constant with enough time (typically 4 h) to obtain an equilibrium state in the equilibrium cell.6,7,9 Therefore, a pressure−temperature diagram was sketched for each experimental run, from which we determined the hydrate dissociation point.6,7,9 The pressure was gradually increased by increasing the temperature during the dissociation of the hydrate crystals inside the hydrate formation region.6,7,9 However, slighter pressure increase was observed during an increase of temperature outside this region.6,7,9 Consequently, the real hydrate dissociation point can be determined when the slope of the pressure−temperature diagram changes suddenly.6,7,9
3. RESULTS AND DISCUSSION The hydrate dissociation data measured in this work are reported in Table 2, and are plotted in Figures 1 and 2. A Figure 1. Experimental dissociation conditions for clathrate hydrates of the methane + water and methane + n-propyl mercaptan + water systems. Symbols represent experimental data. Methane + water system: (○) ref 10; (◇) ref 11; (Δ) ref 12. Methane + n-propyl mercaptan + water system: (●) this work.
Table 2. Experimental Hydrate Dissociation Conditions for the Methane + n-Propyl Mercaptan + Water and Methane + n-Butyl Mercaptan + Water Systems temperature (K)
pressure (MPa)
Methane + n-Propyl Mercaptan + Water 282.3 4.35 283.6 5.21 284.8 5.94 286.4 7.75 288.0 9.78 289.9 13.05 Methane + n-Butyl Mercaptan + Water 279.4 4.93 280.7 5.62 282.4 6.64 284.2 7.96 286.0 9.91 287.4 11.54
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.1 These figures also show some selected experimental data from the literature for methane clathrate hydrates.10−12 As can be observed in Figure 1, n-propyl mercaptan has a thermodynamic promotion effect on methane clathrate hydrates. Note that hydrate promotion results in a shift of the dissociation conditions of methane hydrates due to the presence of mercaptan in the system to low pressures/high temperatures.6,7 Moreover, a change in the slope of logarithm of hydrate dissociation pressure versus temperature for the methane + npropyl mercaptan + water system is observed in comparison with the slope of logarithm of hydrate dissociation pressure versus temperature for the methane + water system suggesting a change in hydrate structure.1 This change suggests that the clathrate hydrate structure of the methane + n-propyl mercaptan + water system likely is not structure I. In Figure 3, we have compared the hydrate dissociation conditions of the methane + n-propyl mercaptan + water system with the hydrate dissociation conditions of the methane + cyclohexane + water system. As can be seen, logarithm of hydrate dissociation pressure versus temperature for the latter two systems shows a
Figure 2. Experimental dissociation conditions for clathrate hydrates of the methane + water and methane + n-butyl mercaptan + water systems. Symbols represent experimental data. Methane + water system: (○) ref 10; (◇) ref 11; (Δ) ref 12. Methane + n-butyl mercaptan + water system: (●) this work.
parallel behavior. It is known that cyclohexane forms structure II clathrate hydrates with methane.1 It is likely that the clathrate hydrates of the methane + n-propyl mercaptan + water may also form structure II. In this figure, it is also observed that the thermodynamic promotion effect of n-propyl mercaptan on methane clathrate hydrates is lower than the thermodynamic promotion effect of cyclohexane. On the basis of our previous study,16 the low thermodynamic promotion effect of n-propyl mercaptan on methane clathrate hydrates suggests that this effect on simple clathrate hydrates of hydrogen sulfide and carbon dioxide, which form clathrate hydrates at pressures lower than methane hydrates, may also be low. Finally, it is observed in Figure 2 that the hydrate dissociation conditions of 3842
dx.doi.org/10.1021/ie202976p | Ind. Eng. Chem. Res. 2012, 51, 3841−3843
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(2) Davidson, D. W. Water: A Comprehensive Treatise; Plenum: New York, 1973 (quoted in ref 1). (3) von Stackelberg, M. Solid Gas Hydrates. Die Naturwissen. 1949, 36/12, 359−362 (quoted in ref 5). (4) von Stackelberg, M.; Müller, H. R. Feste Gashydrate II. Struktur und Raumchemie. Z. Elektrochem. 1954, 58, 25−39 (quoted in ref 5). (5) Mohammadi, A. H.; Richon. Equilibrium Data of Sulfur Dioxide and Methyl Mercaptan Clathrate Hydrates. J. Chem. Eng. Data 2011, 56, 1666−1668. (6) Mohammadi, A. H.; Richon. Equilibrium Data of Tetrahydropyran + Hydrogen Sulfide and Tetrahydropyran + Methane Clathrate Hydrates. J. Chem. Thermodyn. 2012, 48, 36−38. (7) Mohammadi, A. H.; Richon. Equilibrium Data of Neohexane + Hydrogen Sulfide and Neohexane + Methane Clathrate Hydrates. J. Chem. Eng. Data 2011, 56, 5094−5097. (8) 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. (9) 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. (10) 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). (11) Adisasmito, S.; Frank, R. J.; Sloan, E. D. Hydrates of carbon dioxide and methane mixtures. J. Chem. Eng. Data 1991, 36 (1), 68− 71 (quoted in ref 1). (12) Jhaveri, J.; Robinson, D. B. Hydrates in the methane-nitrogen system. Can. J. Chem. Eng. 1965, 43, 75−78 (Quoted in ref 1.).. (13) Mohammadi, A. H.; Richon. Clathrate Hydrates of Cyclohexane + Hydrogen Sulfide and Cyclohexane + Methane: Experimental Measurements of Dissociation Conditions. J. Chem. Eng. Data 2010, 55, 1053−1055. (14) Sun, Z. G.; Fan, S. S.; Guo, K. H.; Shi, L.; Guo, Y. K.; Wang, R. Z. Gas Hydrate Phase Equilibrium Data of Cyclohexane and Cyclopentane. J. Chem. Eng. Data 2002, 47, 313−315. (15) Tohidi, B.; Danesh, A.; Burgass, R. W.; Todd, A. C. Equilibrium data and thermodynamic modelling of cyclohexane gas hydrates. Chem. Eng. Sci. 1996, 51/2, 159−163 (quoted in ref 1). (16) Mohammadi, A. H.; Richon. Clathrate hydrate dissociation conditions for the methane + cycloheptane/cyclooctane + water and carbon dioxide + cycloheptane/cyclooctane + water systems. Chem. Eng. Sci. 2010, 65, 3356−3361.
Figure 3. Comparison of dissociation conditions of clathrate hydrates of the methane + water, methane + n-propyl mercaptan + water and methane + cyclohexane + water systems. Symbols represent experimental data. Methane + water system: (○) ref 10; (◇) ref 11; (Δ) ref 12. Methane + n-propyl mercaptan + water: (●) this work. Methane + cyclohexane + water: (Δ, gray) ref 13; (◇, gray) ref 14; (○, gray) ref 15.
the methane + n-butyl mercaptan + water system overlap the dissociation conditions of methane clathrate hydrates suggesting n-butyl mercaptan likely does not take part in hydrate formation. However, the final proof for the stable hydrate structure for the latter two mercaptans requires direct measurements by suitable physical techniques (e.g., NMR, Xray, or Raman spectroscopy).
4. CONCLUSIONS This work can be summarized as follows: 1. Dissociation pressures for clathrate hydrates of the methane + n-propyl mercaptan + water and methane + n-butyl mercaptan + water system were reported in the temperature ranges of 282.3−289.9 K and 279.4−287.4 K, respectively. which were measured using an isochoric pressure search method.5−10 2. It was shown that n-propyl mercaptan has thermodynamic promotion effect on methane clathrate hydrates while n-butyl mercaptan has negligible effect. 3. It was argued that clathrate hydrate structure of the methane + n-propyl mercaptan + water system is likely different from structure I. 4. Direct measurements by suitable physical techniques (e.g., NMR, X-ray, or Raman spectroscopy) are required for the final proof of the stable hydrate structure for the latter mercaptans.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Tel.: +(33) 1 64 69 49 70. Fax: +(33) 1 64 69 49 68.
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REFERENCES
(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press, Taylor & Francis Group: Boca Raton, FL, 2008. 3843
dx.doi.org/10.1021/ie202976p | Ind. Eng. Chem. Res. 2012, 51, 3841−3843
