Article pubs.acs.org/jced
Formation Pressure of Ethylene Hydrates in Carbon Pores at NearCritical Temperatures Jia Liu,† Hongrui Pan,‡ Yaping Zhou,‡ and Li Zhou*,† †
School of Chemical Engineering and Technology and ‡Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, China ABSTRACT: Equilibrium data of ethylene sorption on activated carbon in the presence of water are measured for near-critical temperatures. The isotherms indicate the formation of hydrates. Hydrate is a compressed state of gases and receives interest in gas storage or separation, and the formation in carbon pores may reach complete conversion and show better dynamic behavior. It is presently shown that the hydrate formation pressure at above-critical temperatures observed in carbon pores is considerably less than that reported for water media. Both the molar ratio of water to ethylene at equilibrium and the ratio of formation enthalpies for sub- and supercritical temperatures indicate that the ethylene molecules cannot stay in the small cages of clathrates when the temperature increased to above critical.
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INTRODUCTION Many gases have been found to form hydrates under appropriate conditions, and different potential applications of hydrates have been studied for natural gas in recent years. Gas hydrates have three types of structure: Type-I, Type-II, and Type-H. Details are described in literature.1 It is commonly considered that the size and shape of gas molecules determine the structure of hydrates. For example, CH4, C2H6, and CO2 form Type-I hydrates,2−4 and N2 forms Type-II hydrates.5 Ethylene is an important industrial gas; therefore, the potential application of ethylene hydrates for storage or separation motivated the present study. Ethylene is also a special gas that it forms hydrates over a relatively wide range of temperature covering the critical temperature of 282.6 K. Therefore, the formation equilibrium and the formation dynamics of ethylene hydrates attracted research interest; however, that reported in literature is for water media.6−13 Ethylene only forms Type-I hydrates, and the ethylene molecule occupies both the large and the small cages at same time.8 However, the effect of solid surface on the formation of hydrates, especially in a space of nanodimension, has not been previously considered. Our studies on the formation of methane hydrates in the pore space of activated carbons14−19 found that the formation equilibrium and the formation dynamics were considerably and positively affected by the interaction of fluid molecules with carbon surface. Therefore, studies on the formation equilibrium of ethylene hydrates in carbon pores were carried out.
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used in a previous study on methane storage in wet activated carbon,19 and preparation of the carbon was reported there. The adsorption isotherm of N2 on carbon sample at 77 K was collected on Micromeritics ASAP 2020 and is shown in Figure 1, which based on the BET specific surface area was evaluated as 3459 m2·g−1. The pore volume was 2.0 cm3·g−1, estimated based on the amount adsorbed at a relative pressure of 0.98. In the preparation of activated carbon, attention was paid to the adaptability of pore size to hydrate dimensions, and the pore
Figure 1. Adsorption isotherm of N2 collected on the tested activated carbon at 77 K.
EXPERIMENTAL SECTION
Received: June 7, 2012 Accepted: July 18, 2012 Published: August 7, 2012
The ethylene used in experiments was of 99.95 mol % purity. The activated carbon used in experiments was just same as that © 2012 American Chemical Society
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size distribution of the carbon was determined by the NLDFT (nonlocal density function theory) method20 and is shown in Figure 2. Distilled water was preadsorbed in carbon samples
Figure 3. Comparison of ethylene isotherms on dry and wet (Rw = 2.0) activated carbon at 275 K. Figure 2. Pore size distribution of the tested carbon obtained with the NLDFT method.
before collecting equilibrium data. The water content in wet carbons was indexed as Rw, the weight ratio of water to dry carbon, and the amount of ethylene fixed in wet carbon was shown on the basis of dry carbon. The quantity of water loaded on carbon at Rw = 2.0 was just about to fully fill the pore volume of the tested carbon according to previous studies,19 and Rw = 2.0 was maintained in experiments. The experiments were carried out on a volumetric setup that was used to measure the adsorption equilibrium and the formation equilibrium of methane hydrates, the details of which were previously described.14,21 Measurement precision is determined by the errors embedded in the values of temperature, pressure, and the compressibility factor. A pressure transmitter, model PAA-23/8465.1-200 manufactured by Keller Druckmesstechnik, Switzerland, was used to measure pressure. The deviation from linearity in the whole range of 20 MPa is less than 0.05 %. The volume of the reference cell was determined by a titration method, and that of the adsorption cell was determined by helium expansion. The amplitude of temperature fluctuation is less than ± 0.1 K. The compressibility factor was evaluated by a virial equation of state. The relative error of the measurement varies from 2 % to 3 % depending on equilibrium temperature.22
Figure 4. Two sets of ethylene isotherms on wet carbon with Rw = 2 for sub- and supercritical temperatures.
pressures increase with temperature. The isotherms seem classified into two groups, subcritical and supercritical. Although 281 K is a little lower and 283 K is a little higher than the critical temperature (282.6 K), the sorption at 281 K and 283 K seems to close the supercritical behavior. Differentiation of these isotherms yields a maximum for each differential curve as shown in Figure 5 for 275 K as an example. Because the sorption amount increased drastically when
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RESULTS AND DISCUSSION Isotherm Featured on Wet Carbon. A typical isotherm of ethylene on wet carbon is shown in Figure 3 for 275 K. The adsorption isotherm of ethylene on dry carbon is also shown in the figure for comparison. As is shown, the adsorbed amount of ethylene on the dry carbon slowed down its increase in pressure at about 1 MPa, and condensation occurs at a pressure of about 4.0 MPa. However, a sudden increase was observed on the isotherm for wet carbon at about 0.6 MPa, and the isotherm leveled off at about 2.0 MPa. The big difference between two isotherms was considered as the result of forming hydrates in the wet carbon. The formation equilibrium was reached when the isotherm leveled off. Isotherms for Each Side of the Critical Temperature. Isotherms were collected on the wet carbon for both sub- and supercritical temperatures. As shown in Figure 4, all isotherms indicate the formation of hydrates, though the level-off
Figure 5. Determination of hydrate formation pressure through differentiation. 2550
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ratio of water to ethylene is x = 5.75 when all cages were occupied by ethylene molecules and x < 5.75 at temperatures lower than 278 K when ethylene molecules were also adsorbed in-between clathrates and pore surfaces. However, the thermal vibration strength of ethylene molecules remarkably increased at above-critical temperatures, and the molecules cannot remain in the S-cages; therefore, x increased gradually with temperature until reached 7.66 at 303 K when the ethylene molecules could only stay in the L-cages as was the case shown in Figure 7. Enthalpy Change of Hydrate Formation. The enthalpy change of hydrate formation in carbon pores (ΔH, kJ·mol−1) can be determined through the Clausius−Clapeyron equation:25
hydrate began to form, the equilibrium pressure corresponding to the maximum is considered as the formation pressure, pf, of hydrates. The formation pressures obtained as such were drawn against temperature and shown in Figure 6. A big difference is
⎡ d ln f ⎤ −ΔH = −R ⎢ ⎥ ⎣ d(1/T ) ⎦n
(1)
where f is the fugacity (MPa), T is the temperature (K), and R is the gas constant. Because f = Φpf and
Figure 6. Dependence of formation pressure of ethylene hydrates on temperatures. 1, Data of Ohgaki et al. for water media;8 2, data of the present work for carbon pores.
ln Φ =
∫0
p
(z − 1)dp /p
(2)
where Φ is the fugacity coefficient and z is the compressibility factor. Therefore, the f values corresponding to the abovedetermined formation pressures, pf, can be determined for different temperatures. The plot of ln f versus 1/T is shown in Figure 8. The slope of the plot yields the value of formation
shown for the formation of hydrates in water media8 and in carbon pores at supercritical temperatures, though an agreement was shown for subcritical temperatures. The formation pressure observed in carbon pores is considerably less than that reported for the formation in water media at supercritical temperatures. It is believed that the interactions between water molecules and the surface atoms of carbon favor the formation of clathrate structure, and the hydrates of supercritical ethylene are much easier to form in carbon pores than in water media. Another exposition may be found in the fact that the pressure in micropores can be much higher than the surrounding pressure.23,24 Therefore, the recorded ambient formation pressure is lower than the real pressure in micropores. The molar ratio of water to ethylene at equilibrium was calculated as x = Rw/(n·18), where n is the sorption amount of ethylene (mmol·g−1), and the dependence of x on temperature is shown in Figure 7 for Rw = 2 and a pressure of 3.5 MPa. It is clearly shown that an inflection point appeared at 278 K to 281 K where both the uptake quantity of ethylene and the molar ratio, x, experienced a drastic change. It is known from the hydrate structure that there are 2 S-cages and 6 L-cages in the Type-I clathrate formed with 46 water molecules. The molar
Figure 8. Determination of formation enthalpies of ethylene hydrates at sub- and supercritical temperatures.
enthalpy of hydrates. It is shown that the plot composes of two sections joining at the critical temperature (282.6 K), and two formation enthalpies of hydrates were yielded: −50.3 kJ·mol−1 and −16.0 kJ·mol−1 for the sub- and the supercritical temperatures, respectively. The ratio of two enthalpies is 3.14, about same as the ratio of L-cage number to S-cage number, 3.0, which provides another evidence for the fact that ethylene molecules cannot stay in the S-cages at supercritical temperatures.
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CONCLUSION It is shown with the formation equilibrium data of ethylene hydrates in carbon pores that more than double ethylene was taken up by wet than by dry carbon at about 2 MPa due to hydrate formation. Ethylene hydrates can also form at supercritical temperatures, and the formation pressure in carbon pores is much less than that in water media. However,
Figure 7. Dependence of the sorbed amount and x on temperature (at Rw = 2 MPa and 3.5 MPa). 2551
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(18) Zhou, L.; Liu, J.; Su, W.; Sun, Y.; Zhou, Y. P. Progress in Studies of Natural Gas Storage with Wet Adsorbents. Energy Fuels 2010, 24, 3789−3795. (19) Liu, J.; Zhou, Y. P.; Sun, Y.; Su, W.; Zhou, L. Methane storage in wet carbon of tailored pore sizes. Carbon 2011, 49, 3731−3736. (20) Ravikovitch, P. I.; Vishnyakov, A.; Russo, R.; Neimark, A. V. Unified Approach to Pore Size Characterization of Microporous Carbonaceous Materials from N2, Ar, and CO2 Adsorption Isotherms. Langmuir 2000, 16, 2311−2320. (21) Zhou, L.; Zhou, Y. P.; Bai, S. P.; Yang, B. Studies on the Transition Behavior of Physical Adsorption from the Sub- to the Supercritical Region: Experiments on Silica Gel. J. Colloid Interface Sci. 2002, 253, 9−15. (22) Bai, S. P. Studies on Adsorption Behavior of CO2 on Porous Solids near the Critical Temperature. Ph.D. Dissertation, School of Chemical Engineering & Technology, Tianjin University, Tianjin, China, December 2002. (23) Bae, Y. S.; Lee, C. H. Sorption kinetics of eight gases on a carbon molecular sieve at elevated pressure. Carbon 2005, 43, 95−107. (24) Kaneko, K. Molecular assembly formation in a solid nanospace. Colloids Surf., A 1996, 109, 319−333. (25) Zhou, L.; Liu, X. W.; Sun, Y.; Li, J. W.; Zhou, Y. P. Methane sorption in ordered mesoporous silica SBA-15 in the presence of water. J. Phys. Chem. B 2005, 109, 22710−22714.
the uptake amount of ethylene decreases remarkably at supercritical temperatures due to the fact that ethylene molecules cannot occupy the S-cages, which was confirmed by both changes in the x value and in the formation enthalpy of hydrates.
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AUTHOR INFORMATION
Corresponding Author
*Phone and fax: +86 22 8789 1466. E-mail:
[email protected]. Funding
This work is financially supported by the National Natural Science Foundation of China (Grant No. 21076142). Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Sloan, E. D., Jr. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker: New York, 1998; p 33. (2) Stackelberg, M. V. Feste Gashydrate. Naturwissenschaften 1949, 36, 327−333. (3) Nakano, S.; Moritoki, M.; Ohgaki, K. High-Pressure Phase Equilibrium and Raman Microprobe Spectroscopic Studies on the CO2 Hydrate System. J. Chem. Eng. Data 1998, 43, 807−810. (4) Nakano, S.; Moritoki, M.; Ohgaki, K. High-Pressure Phase Equilibrium and Raman Microprobe Spectroscopic Studies on the Methane Hydrate System. J. Chem. Eng. Data 1999, 44, 254−257. (5) Dyadin, Y. A.; Larionov, E. G.; Aladko, E. Y.; Zhurko, F. V. Clathrate nitrogen hydrates at pressures of up to 15 kbar. Dokl. Phys. Chem. 2001, 378, 159−161. (6) Bansal, V.; Christiansen, R. L.; Sloan, E. D., Jr. Influence of guest vapor liquid critical point on hydrate formation conditions. Am. Inst. Chem. Eng. J. 1993, 39, 1735−1737. (7) Sugahara, T.; Makino, T.; Ohgaki, K. Isothermal phase equilibria for the methane + ethylene mixed gas hydrate system. Fluid Phase Equilib. 2003, 206, 117−126. (8) Sugahara, T.; Morita, K.; Ohgaki, K. Stability boundaries and small hydrate-cage occupancy of ethylene hydrate system. Chem. Eng. Sci. 2000, 55, 6015−6020. (9) Zhang, L. W.; Chen, G. J.; Sun, C. Y. The partition coefficients of ethylene between vapor and hydrate phase for methane + ethane + water and methane + ethylene + THF + water systems. Fluid Phase Equilib. 2006, 245, 134−139. (10) Zhang, L. W.; Huang, Q.; Sun, C. Y. Hydrate formation conditions of methane + ethylene + tetrahydrofuran + water systems. J. Chem. Eng. Data 2006, 51, 419−422. (11) Ma, C. F.; Chen, G. J.; Wang, F.; Sun, C. Y.; Guo, T. M. Hydrate formation of (CH4 + C2H4) and (CH4 + C3H6) gas mixtures. Fluid Phase Equilib. 2001, 41−47. (12) Snell, L. E.; Otto, F. D.; Robinson, D. B. Hydrates in systems containing methane, ethylene, propylene, and water. Am. Inst. Chem. Eng. J. 1961, 7, 482−485. (13) Anthony, R. G.; McKetta, J. J. Phase equilibrium in the ethyleneethane-water system. J. Chem. Eng. Data 1967, 12, 21−28. (14) Zhou, L.; Sun, Y.; Zhou, Y. P. Enhancement of the Methane Storage on Activated Carbon by Pre-adsorbed Water. Am. Inst. Chem. Eng. J. 2002, 48, 2412−2416. (15) Perrin, A.; Celzard, A.; Marêché, J. F.; Furdin, G. Methane storage within dry and wet active carbons: a comparative study. Energy Fuels 2003, 175, 1283−1291. (16) Zhou, Y. P.; Dai, M.; Zhou, L.; Sun, Y.; Su, W. Storage of Methane on Wet Activated Carbon: Influence of Pore Size Distribution. Carbon 2004, 42, 1855−1858. (17) Yan, L. J.; Chen, G. J.; Pang, W. X.; Liu, J. Experimental and Modeling Study on Hydrate Formation in Wet Activated Carbon. J. Phys. Chem. B 2005, 109, 6025−6030. 2552
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