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Ind. Eng. Chem. Res. 2004, 43, 4964-4966
RESEARCH NOTES Clathrate Hydrate Formed with Methane and 2-Propanol: Confirmation of Structure II Hydrate Formation Ryo Ohmura,* Satoshi Takeya, Tsutomu Uchida, and Takao Ebinuma Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology (AIST), 2-17-2-1 Tsukisamu-Higashi, Toyohira-ku, Sapporo 062-8517, Japan
This paper reports confirmation of structure II hydrate formation in a methane-2-propanolwater system, which was previously suggested by Østergaard et al. (Ind. Eng. Chem. Res. 2002, 41, 2064-2068) based on a comparison of the phase-equilibrium data with corresponding statistical-thermodynamics predictions. A hydrate crystal sample was prepared with a 16.4 mass % aqueous solution of 2-propanol pressurized with methane and then subjected to a powder X-ray diffraction analysis. The X-ray diffraction pattern thus obtained from the sample indicated that the crystallographic structure of the hydrate was structure II. The pressure-temperature data for aqueous liquid-hydrate-methane-rich vapor three-phase equilibrium in a temperature range from T ) 273 to 283 K are also reported. Introduction Østergaard et al.1 recently demonstrated the formation of clathrate hydrate in systems of methane-2propanol-water and natural gas-2-propanol-water, noting the lack of information on the hydrate formation ability of 2-propanol used in the petroleum industry. They measured phase-equilibrium temperaturepressure conditions in the above systems and compared them to statistical-thermodynamics model predictions in which 2-propanol was assumed to be an inhibitor for hydrate formation, a structure II hydrate former, or a structure H hydrate former. The experimental data were well represented by a model prediction based on the assumption that 2-propanol is a structure II hydrate former, thereby strongly suggesting the formation of structure II hydrate with methane and 2-propanol as guest substances. However, confirmation of structure II hydrate formation with 2-propanol by directly measuring the hydrate phase is required for a final conclusion, as noted by Østergaard et al.1 Therefore, we attempted to determine the crystallographic structure of a hydrate formed in a methane-2-propanol-water system by means of a powder X-ray diffraction technique. We first measured the three-phase-equilibrium data in temperatures from 273 to 283 K to ensure the formation of a hydrate other than a structure I methane hydrate in our experimental system. We then prepared a hydrate sample in the system and subjected the sample to X-ray diffraction analysis. Experimental Section Fluid samples used in the experiments were deionized and distilled liquid water, methane of 99.99 vol % certified purity (Takachiho Chemical Industrial Co., * To whom correspondence should be addressed. Tel.: +81-11-857-8949. Fax: +81-11-857-8471. E-mail: r.ohmura@ aist.go.jp.
Ltd., Tokyo, Japan), and 2-propanol of 99 mass % certified purity (Tokyo Kasei Kogyo Co., Ltd., Tokyo, Japan). A liquid solution of water and 2-propanol mixed at a molar ratio of 17:1 (i.e., 16.4 mass % 2-propanol aqueous solution) was prepared and used in all of the experiments. This molar ratio is stoichiometric for water to 2-propanol molecules in the resulting hydrate if 2-propanol molecules are assumed to fully occupy the 51264 cavities in a structure II hydrate. The experimental apparatuses to measure the temperature-pressure conditions for the three phases, water-rich liquid (Lw), hydrate (H), and methane-rich vapor phase (V), are the same as those used in our previous study.2 The main part of the apparatus is a stainless steel cylinder with a 200 cm3 inner volume. A magnetic stirrer is installed in the vessel through its lid to agitate fluids and hydrate crystals inside the vessel. The vessel is immersed in a temperaturecontrolled bath to maintain the temperature inside the vessel, T, at a prescribed level. Two thermocouples are inserted into the vessel to measure the gas and liquid temperatures. The pressure in the vessel, p, is measured with a strain-gauge pressure transducer (model PH100KB, Kyowa Electric Co., Ltd.). The estimated uncertainties of temperature and pressure measurements are (0.1 K and (0.016 MPa. The equilibrium conditions were measured with the batch, isochoric procedure, described by Danesh et al.3 Each run was initiated by charging the vessel with 50 cm3 of a 16.4 mass % 2-propanol aqueous solution. The vessel containing the liquid was then immersed into the temperature-controlled bath, and T was set at 283 K. The methane gas was supplied from a high-pressure cylinder through a pressure-regulating valve into the evacuated vessel, thereby setting p at a prescribed level between 2 and 6 MPa. After T and p stabilized, the valve in the line connecting the vessel and the high-pressure
10.1021/ie0498089 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/08/2004
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cylinder was closed. Subsequently, T was decreased to form hydrate. If hydrate formation in the vessel was detected by a decrease in p and an increase in T, the temperature of the bath was kept constant for 6 h, thereby keeping T constant. We then increased T incrementally in steps of 0.1 K. At every temperature step, T was kept constant for 6 h to achieve a steady equilibrium state in the vessel. In this way, we obtained a p-T diagram for each experimental run, from which we determined a three-phase-equilibrium point. If the temperature is increased in the hydrate-forming region, hydrate crystals partially dissociate, thereby substantially increasing the pressure. If the temperature is increased in the region without hydrate, 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. Consequently, the point at which the slope of p-T data plots changes sharply is considered to be the point at which all hydrate crystals dissociate and hence as the point of the three-phase equilibrium. This operation was repeated at several different initial pressures to obtain the three-phase-equilibrium data over a temperature range from 273 to 283 K. A hydrate crystal sample was prepared with a 16.4 mass % 2-propanol aqueous solution and methane gas using the same experimental apparatus as that used for the phase-equilibrium measurements. The pressure, p, and temperature, T, were set at p ) 2.50 MPa and T ) 274.0 K, outside structure I methane hydrate formable conditions to avoid the possible formation of structure I methane hydrate. The line connecting the test cell and the high-pressure methane cylinder was opened during the hydrate formation in the test cell to keep p constant by continuously supplying methane gas through a pressure-regulating valve to compensate for pressure reduction in the test cell due to hydrate formation, so that a sufficient amount of hydrate crystals would be stored in the cell. p and T were kept constant for 12 h with continuous agitation in the vessel at 400 rpm after nucleation of the hydrate. The vessel was subsequently taken out of the temperature-controlled bath and then immediately immersed into a liquidnitrogen pool in a stainless steel container. We allowed 20 min for T to decrease below ∼170 K and then disassembled the vessel to remove the hydrate crystals inside. The samples thus prepared were stored in a container kept at a temperature of ∼170 K and subjected to the X-ray diffraction measurements. The procedure for X-ray diffraction measurements was the same as that described previously by Takeya et al.4 and is outlined below. A portion of the stored sample was ground into small particles with the dimensions of 5-100 µm, manipulating a pestle on a mortar cooled from the back by liquid nitrogen to maintain the temperature of the sample crystals below 170 K, thereby avoiding dissociation of the hydrate crystals in the sample. The sample crystals thus ground were put in a quartz glass capillary cell (Hilgenberg; L 2.0 mm, 0.01 mm thickness) that was put on top of the goniometer of the X-ray diffraction apparatus. The X-ray intensity measurement was conducted using Cu KR (40 kV, 250 mA; Rigaku model Rint-2000). The measurement was obtained at 113 K under atmospheric pressure. The temperature of the sample was controlled within (1 K at 113 K by continuously supplying a cold, dry nitrogen gas flow around the capillary sample cell. The capillary
Table 1. Aqueous Liquid + Hydrate + Methane-Rich Vapor Three-Phase-Equilibrium p-T Conditions in a Methane-2-Propanol-Water Systema T/K
p/MPa
T/K
p/MPa
T/K
p/MPa
273.6 274.1
2.045 2.175
276.8 279.1
2.989 3.889
280.9 282.2
4.682 5.461
a
The molar ratio of water and 2-propanol in the system is 17:
1.
Figure 1. Lw + H + V three-phase-equilibrium p-T conditions in a methane-2-propanol-water system: O, present study; 0, Østergaard et al.1 The molar ratio of water and 2-propanol in the system is 17:1. The three-phase-equilibrium conditions in a methane-water system,4 4, are also indicated.
cell was rotated 360° during the measurement to avoid possible effects of preferred orientations of the sample crystals. Results and Discussion The phase-equilibrium conditions in the methane2-propanol-water system are presented in Table 1. The data are included in Figure 1 together with one of the data points reported by Østergaard et al.1 and with those in the methane-water system5 without 2-propanol. The data obtained in the present study were consistent with the data reported by Østergaard et al.1 within the estimated or claimed mutual uncertainties, indicating the occurrence of the same phenomenon in the experimental system used in the present study as that observed by Østergaard et al.1 The equilibrium pressures in the system with 2-propanol are lower by 0.8 MPa at a given temperature than those in the methane-water system. The reduction in the equilibrium pressures in the 2-propanol system compared to those in the methane-water system suggests the formation of a hydrate different from a structure I methane hydrate. The crystallographic structure of the hydrate formed with methane and 2-propanol was determined by X-ray diffraction measurements. Figure 2 depicts the X-ray diffraction profile, measured at 113 K, obtained from the prepared hydrate sample. The profile shown in Figure 2 indicates that the crystallographic structure of the hydrate was structure II with a 1.733 nm lattice constant. This result demonstrates that addition of 2-propanol into a methane-water system alters the crystallographic structure of the hydrate formed in the
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is consistent with the statistical-thermodynamics model prediction of Østergaard et al.1 Acknowledgment This study was supported by the Industrial Technology Research Grant Program in 2003 (Grant 03B64003c) from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Literature Cited Figure 2. X-ray diffraction profile obtained from hydrate samples prepared in a methane-2-propanol-water system at p ) 2.50 MPa and T ) 274.0 K. The molar ratio of water and 2-propanol in the system was 17:1. The measurement was performed at 113 K. The crystallographic structure of the hydrate was determined to be structure II. Asterisks indicate the diffraction peaks of ice.
system, resulting in formation of a structure II hydrate instead of a structure I hydrate. The ice is considered to be transformed from unreacted liquid water in the high-pressure test cell during the cooling procedure. Conclusions The three-phase-equilibrium temperature and pressure conditions for methane, a 2-propanol aqueous solution, and hydrate were measured over temperatures from 273 to 283 K, thereby confirming and complementing the results reported by Østergaard et al.1 The X-ray diffraction measurement performed with a hydrate sample prepared in a methane-2-propanol-water system revealed that the hydrate was structure II, which
(1) Østergaard, K. K.; Tohidi, B.; Anderson, R.; Todd, A. C.; Danesh, A. Can 2-Propanol Form Clathrate Hydrates? Ind. Eng. Chem. Res. 2002, 41, 2064-2068. (2) Ohmura, R.; Uchida, T.; Takeya, S.; Nagao, J. Minagawa, H.; Ebinuma, T.; Narita, H. Phase Equilibrium for Structure-H Hydrates Formed with Methane and each of Pinacolone (3,3dimethyl-2-butanone) and Pinacolyl alcohol (3,3-dimethyl-2butanol). J. Chem. Eng. Data 2003, 48, 1337-1340. (3) Danesh, A.; Tohidi, B.; Burgass, R. W.; Todd, A. C. Hydrate Equilibrium Data of Methyl Cyclopentane with Methane or Nitrogen. Chem. Eng. Res. Des. 1994, 72, 197-200. (4) Takeya, S.; Kamata, Y.; Uchida, T.; Nagao, J.; Ebinuma, T.; Narita, H.; Hori, A.; Hondoh, T. Coexistence of structure I and II hydrate formed from mixture of methane and ethane gases. Can. J. Phys. 2003, 1/2, 479-484. (5) Adisasmisto, S.; Frank, R. J., III; Sloan, E. D., Jr. Hydrates of Carbon Dioxide and Methane Mixtures. J. Chem. Eng. Data 1991, 36, 68-71.
Received for review March 12, 2004 Revised manuscript received June 14, 2004 Accepted June 22, 2004 IE0498089
