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Phase equilibrium and crystallographic structure of clathrate hydrate formed in argon + 2,2-dimethylbutane + water system
Fluid Phase Equilibria 365 (2014) 64–67
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Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid
Phase equilibrium and crystallographic structure of clathrate hydrate formed in argon + 2,2-dimethylbutane + water system Kotaro Murayama a , Satoshi Takeya b , Ryo Ohmura a,∗ a b
Department of Mechanical Engineering, Keio University, Yokohama 223-8522, Japan National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
a r t i c l e
i n f o
Article history: Received 18 October 2013 Received in revised form 11 December 2013 Accepted 16 December 2013 Available online 24 December 2013 Keywords: Clathrate hydrates Structure H Crystallographic structure Noble gases Argon
a b s t r a c t This paper reports the four-phase (Lw –H–V–Lg ) equilibrium conditions for the structure H hydrates formed in argon + 2,2-dimethylbutane. The experiments were performed at temperatures between 275.6 and 284.3 K, and at pressures between 2.91 and 9.41 MPa. The crystallographic structure of the hydrate was identified to be structure H by powder X-ray diffraction measurements. At a given temperature, the equilibrium pressure of the structure H hydrate formed with argon + 2,2-dimethylbutane was lower than that of the structure II pure argon hydrate. The difference in the equilibrium pressure is 8.1–20.9 MPa in the temperature range of 275.6–284.3 K. Among the system using methane, argon, krypton and xenon, the system using argon shows the highest reduction ratio of equilibrium pressures. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Argon is one of the most common noble gases like helium. The volumetric composition of argon in the earth’s atmosphere is approximately 0.9%. Human beings have used argon widely in their industry because of its safety and low cost. Argon is a noble substance, but it is reported that a clathrate hydrate can be formed with argon [1]. New technologies using hydrates formed with argon may be expected. Argon gas has also been found on the other planets and satellites, and argon hydrates may be formed there [2,3]. In planetary geoscience, the thermodynamic stabilities of various clathrate hydrates are required for use in the simulation of atmospheric compositions. Argon is one of the main substances on Mars and Pluto and argon hydrate may exist by forming some structures. Though the thermodynamic stability of the pure argon structure II hydrate was used in the studies for conducting simulations of the atmospheric composition, more accurate simulation can be done if there is thermodynamic stability data of the structure H hydrate formed with argon. In the harvest industry, it is reported that fresh-cut peppers subjected to pressurized argon gas maintain water in the peppers due to the formation of argon hydrate [4]. The information about the stabilities of structure H hydrate formed with argon is
∗ Corresponding author. Tel.: +81 45 566 1813; fax: +81 45 566 1495. E-mail address: [email protected] (R. Ohmura). 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.12.011
needed, because some compounds may exist as a guest substance to form the structure H hydrate with argon in fresh peppers. Clathrate hydrates are ice-like crystalline solid compounds consisting of a network of hydrogen-bonded host water molecules forming cage structures that enclose guest molecules. Depending on the size and shape of the guest substances, water molecules form several different cage structures that interconnect to yield hydrates of different crystallographic structures, such as structure I (sI), structure II (sII), and structure H (sH). The crystal system of the sI and sII hydrates is cubic and that of the sH hydrate is hexagonal. The typical values of the lattice parameters at 100 K are as ˚ and follows: the a axes of sI and sII hydrates are 12 A˚ and 17.2 A, the a and c axes of sH hydrate are 12 A˚ and 10 A˚ [5]. The formation of sH hydrates requires two different guest substances; one is a relatively large molecule guest substance (LMGS), and the other is a small gaseous guest substance, such as methane or xenon. The phase equilibrium conditions of sH hydrates are often milder (i.e., higher temperature and lower pressure) than those of the sI or sII hydrates formed only with small molecule guest substances. The milder phase equilibrium conditions of the sH hydrate formed with argon is advantageous for actual applications. Examples of the applied technologies of the argon sH hydrate are a hydrate-based heat pump [6] and a hydrate engine [7]. Ogawa et al. selected the pair of HFC-32 and cyclopentane as guest substances. HFC-32 is a flammable gas, while argon is an extremely inactive gas. The global warming potential of HFC-32 is also much higher than that of argon. The sH hydrate formed with argon can then be the medium of the
K. Murayama et al. / Fluid Phase Equilibria 365 (2014) 64–67 Table 1 Supplier and purity of the materials used in this study. Chemical
Supplier
Purity
Argon 2,2-Dimethylbutane
Taiyo Nippon Sanso Sigma–Aldrich
>99.9999 >99.0
hydrate heat pump and the hydrate engine in terms of safety and low cost of argon gas. In this study, we targeted argon as a candidate guest gas to form an sH hydrate to be practically applied for the hydraterelated industries because the cost of argon is one-fourth the cost of methane. We selected 2,2-dimethylbutane as the second guest substance used together with argon because the sH hydrates, which have a milder phase equilibrium than the equilibrium of a simple sII hydrate, are often formed with 2,2-dimethylbutane as a LMGS. The phase equilibrium conditions of the sH hydrate formed in methane, krypton or xenon + 2,2-dimethylbutane + water system have previously been reported [8–10]. The equilibrium pressures of these sH hydrates are 10–60% lower than those of the simple methane or xenon sI hydrate and simple Kr sII hydrate. Thus, we expected that the equilibrium pressure of the hydrate formed with argon + 2,2dimethylbutane would be much lower than that of the simple argon hydrate. The solubility of 2,2-dimethylbutane in water is 8.23 × 10−6 and the solubility of water in 2,2-dimethylbutane is 1.53 × 10−4 at 273.1 K [11]. 2,2-dimethylbutane is not miscible with water and forms a separate phase as a liquid guest substance. In this paper, we demonstrate the formation of a sH hydrate with argon + 2,2-dimethylbutane with four-phase (Lw –H–V–Lg ) equilibrium conditions and provide the first phase equilibrium data. The crystallographic structure of this hydrate was identified based on powder X-ray diffraction measurements. 2. Experimental setup and procedure For the experiments presented in this study, the following chemicals were used: deionized and distilled water; pure argon gas (99.9999% certified purity – Taiyo Nippon Sanso, Co., Ltd., Japan); liquid reagent of 2,2-dimethylbutane (99.0% certified purity – Sigma–Aldrich). The deionized and distilled water was laboratorymade. The conductivity of the deionized and distilled water is lower than 300 × 10−4 s/m at 298 K. The purities and suppliers of the fluid samples are specified in Table 1. The apparatus for the phase equilibrium measurements and preparing the crystal samples for the powder X-ray diffraction (PXRD) is schematically illustrated in Fig. 1. The test section for the four-phase equilibrium, water-rich liquid (Lw ), hydrate (H), argon-rich vapor (V) and 2,2-dimethylbutane (Lg ) was a stainless
Fig. 1. Schematic of the experimental apparatus.
65
steel cylindrical vessel with the inner volume of 200 cm3 . A magnetically-driven impeller was inserted into the vessel to agitate the liquid phases in the test section. The temperature of the vessel (T) was controlled at a prescribed value using a PID-controlled heater (TM-1, As One Co., Ltd.) and a cooler placed in the bath. A platinum resistance thermistor was inserted into the vessel to measure the temperature in the test section. The pressure in the vessel (p) was measured by a strain-gauge pressure transducer (PHB-A-10MP, Kyowa Electric Co., Ltd.). The estimated uncertainty of the temperature measurements was ±0.1 K for 223≤ T ≤ 573 K. The estimated uncertainty of the pressure measurements was ±0.005 MPa for p ≤ 10.0 MPa and for 233 ≤ T ≤ 423 K. The present study follows the procedure described by Danesh et al. for measuring the equilibrium temperatures and pressures of the four phases [12]. The vessel was first charged with liquid water and 2,2-dimethylbutane. We used 35 cm3 of liquid water and 15 cm3 of 2,2-dimethylbutane to ensure the presence of the two liquid phases. The volume of 2,2-dimethylbutane is 3/7 of water. This amount of 2,2-dimethylbutane is more than twice the stoichiometric composition of the sH hydrate. In this system, the degree of freedom was determined to be 1 from the Gibbs phase rule. The vessel was then placed in the bath. Gaseous argon was supplied from a high-pressure cylinder through a pressure-regulating valve to the vessel after the air in the vessel was evacuated by a vacuum pump. The motor of the magnetically driven impeller was then turned on and the impeller continued to agitate the system through the experimental run at 300 rpm. After p and T were set at the prescribed values within the pressure and temperature range of 3–10 MPa and 278–290 K, respectively, T was decreased to form a hydrate. The hydrate formation in the vessel was detected by the decrease in p. The temperature T was then increased by steps of 0.1 K. At each temperature step, T was maintained until p did not change by 1 kPa within 12 h. By repetition of this incremental temperature increase, a p versus T plot diagram for each run was obtained and a four-phase equilibrium condition was determined from the diagram. When the hydrate was dissociated by increasing the temperature in the vessel, the pressure in the vessel increased. When all of the hydrate was completely dissociated, then the increase in the pressure became quite small because of the change in the phase equilibrium of the fluids remaining in the test section. This operation was repeated under different initial conditions to obtain the four-phase equilibrium data over the desired temperature–pressure range.
2.1. Powder X-ray diffraction The hydrate crystal sample for the PXRD measurements was prepared with liquid water, argon gas, and 2,2-dimethylbutane using the same type of apparatus for the phase equilibrium measurements. The amount of water and 2,2-dimethylbutane for the sample is the same as for the equilibrium measurements. The temperature inside the vessel was maintained at 276 K. The pressure was set at 6.0 MPa to avoid forming the simple argon sII hydrate that may form at p > 12.2 MPa. Upon hydrate formation, when the pressure decreased to the equilibrium pressure of the sH hydrate formed with argon and 2,2-dimethylbutane, the vessel was recharged with argon gas to 6.0 MPa. This argon charging procedure was repeated until no further pressure reduction was observed. The inside of the vessel was continuously agitated at 300 rpm after the hydrate nucleation. Nearly complete conversion of the water to the hydrate was obtained when no further pressure reduction was observed. After the pressure stabilized, the vessel was subsequently removed from the temperature-controlled bath and immediately immersed in a liquid nitrogen bath. After the temperature in the vessel decreased below 220 K, the vessel was removed from the
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K. Murayama et al. / Fluid Phase Equilibria 365 (2014) 64–67
Table 2 Experimental data for the four-phase (Lw –H–V–Lg ) equilibrium pressure– temperature conditions in a (argon + 2,2-dimethylbutane + water) system.a T (K)
p (MPa)
275.6 275.6 277.8 279.9 282.0 284.3
2.910 2.914 3.845 5.401 7.362 9.410
a
u(T) = 0.1 K; u(p) = 0.005 MPa.
liquid nitrogen pool and quickly disassembled. The lower part of the vessel containing the hydrate sample was then again placed in a liquid nitrogen pool, and the hydrate sample was obtained from the vessel. The hydrate sample was subjected to PXRD measurements after storing in a container kept at a temperature below 100 K. For the PXRD measurements, the hydrate sample was finely powdered in a nitrogen atmosphere at a temperature below 100 K. The finepowdered hydrate samples were top-loaded on a copper (Cu) specimen holder. The PXRD measurements were done using Cu K˛ radiation in a /2 step scan mode with a step width of 0.02◦ (40 kV, 40 mA; Rigaku model Ultima III). Analysis of the lattice constants was done by a full-pattern fitting method using the Rietveld program RIETAN-FP.13 [13].
Fig. 2. Pressure–temperature plot to illustrate phase equilibrium conditions for hydrates formed in (argon + water) and (argon + 2,2-dimethylbutane + water) systems. : (argon + 2,2-dimethylbutane + water) system, : (argon + water) system [1], 䊉: (argon + water) system [15], : (argon + water) system [14].
3. Results and discussion The p–T data of the four-phase (Lw –H–V–Lg ) equilibrium in the (water + argon + 2,2-dimethylbutane) system are listed in Table 2 and plotted in Fig. 2. The equilibrium data of the simple argon hydrate (water + argon) are also shown in Fig. 2 [1,14,15]. The reproducibility of the equilibrium measurements was confirmed by duplicate measurements of the argon + 2,2dimethylbutane + water system at the pressure of 3 MPa. The results of the measurement are also listed in Table 2 and Fig. 2. As can be seen in Table 2, the data (p = 2.910 MPa, T = 275.6 K) is consistent with other data (p = 2.914 MPa, T = 275.6 K) within the mutual uncertainty of u(p) = 0.005 MPa, u(T) = 0.1 K. The procedure for the measurements of the phase equilibrium conditions by Marshall et al. [1] and Holder et al. [14] are the same as the one used in the present study. Mohammadi et al. conducted the measurements using the pressure search method [15]. Each of the four-phase equilibrium conditions obtained in the present study is outside of the pure argon sII hydrate equilibrium line. At a given temperature, the equilibrium pressure of the hydrate formed with argon + 2,2-dimethylbutane is lower than that of the sII hydrate formed with argon. The difference in the equilibrium pressure at 276 K for the hydrate formed with argon + 2,2-dimethylbutane and the sII hydrate formed with argon is approximately 9.0 MPa. Fig. 3 shows the PXRD profiles of the hydrate formed with argon + 2,2-dimethylbutane. The crystallographic structure of the hydrate was identified to be sH by the PXRD measurements. The unit cell parameters of the sH hydrate
Fig. 3. Powder X-ray diffraction patterns for the structure H hydrate at 93 K: argon + 2,2-dimethylbutane hydrate. The hydrate was prepared at p = 3.00 MPa and T = 276.0 K. The tick marks in the lower part represent the calculated peak positions for the structure H hydrate and hexagonal ice.
formed with argon + 2,2-dimethylbutane were determined to be a = 12.165(1) A˚ and c = 9.928(1) A˚ at 93 K. The equilibrium pressures at 276 K and 278 K in the systems of (methane + water) [16], (argon + water) [1,14,15], (krypton + water) [14], (xenon + water) [17], (methane + 2,2dimethylbutane + water) [8], (argon + 2,2-dimethylbutane + water) (present study), (krypton + 2,2-dimethylbutane + water) [9], and (xenon + 2,2-dimethylbutane + water) [10] are compared in Table 3. To quantify the reduction of the equilibrium pressure of the sH
Table 3 The reduction ratio of the equilibrium pressures of the hydrates formed with simple methane, argon, krypton, xenon and formed with each gas + 2,2-dimethylbutane at 276.0 K and 278.0 K. Gaseous guest
Methane [8,16] Argon [1,14,15] Krypton [9,14] Xenon [10,17]
276.0 K
278.0 K
Psimple (MPa)
PsH (MPa)
3.45 12.20 1.92 0.20
1.60 3.50 0.72 0.18
Psimple −PsH Psimple
0.52 0.71 0.63 0.10
Psimple (MPa)
PsH (MPa)
4.22 14.50 2.30 0.25
2.02 4.50 0.85 0.24
Psimple −PsH Psimple
0.52 0.69 0.63 0.04
K. Murayama et al. / Fluid Phase Equilibria 365 (2014) 64–67
hydrate from that of a simple gas hydrate, the reduction ratio, (Psimple − PsH )/Psimple , where Psimple is the equilibrium pressure of the hydrate with each gas, and PsH is the equilibrium pressure of the sH hydrate formed with each gas + 2,2-dimethylbutane, was calculated. The reduction ratios in the methane system are 0.52, those in the argon system are 0.71 and 0.69, those in the krypton system are both 0.63, and those in the xenon system are 0.10 and 0.04. The equilibrium pressure of the hydrate in the xenon system is the lowest, but the reduction ratio is low. The system, which shows the highest reduction ratio, is the system with argon. Although it needs more than 10 MPa at 274 K to produce the pure argon hydrate, the sH hydrates with argon may be formed more easily than the pure argon hydrate because the high pressure reduction was confirmed between the pressure of the sH hydrate formed with argon + 2,2-dimethylbutane and that of the sII pure argon hydrate. 4. Conclusion The four-phase (Lw –H–V–Lg ) equilibrium conditions were measured for the system of (argon + 2,2-dimethylbutane + water) at temperatures from 275.6 K to 284.3 K. The results suggest that the phase equilibrium formed in the system of present study is milder than the equilibrium in the system of pure argon + water. The structure of the hydrate formed with argon + 2,2-dimethylbutane was identified as the structure H by PXRD measurements. The ratio of the difference of the equilibrium pressures between the simple argon hydrate and the structure H hydrate with argon + 2,2-dimethylbutane divided by the equilibrium pressure of the simple argon hydrate is the highest among the systems containing methane, argon, krypton and xenon.
67
Acknowledgements This study was supported by a Keirin-racing-based researchpromotion fund from the JKA Foundation and by JSPS KAKENHI Grant Number 25289045. References [1] D.R. Marshall, S. Sato, R. Kobayashi, AIChE J. 10 (1964) 202–205. [2] J.M. Herri, E. Chassefiere, Planet. Space Sci. 73 (2012) 376–386. [3] O. Mousis, J.I. Lunine, K.E. Mandt, E. Schindhelm, H.A. Weaver, S.A. Sterm, J.H. Waite, R. Gladstone, A. Moudens, Icarus 225 (2013) 856–861. [4] X. Meng, M. Zhang, B. Adhikari, Postharvest Biol. Technol. 71 (2013) 13–20. [5] E.D. Sloan, C.A. Koh, Clathrate Hydrates of Natural Gas, 3rd ed., CRC Press, Boca Raton, FL, 2008. [6] T. Ogawa, T. Ito, K. Watanabe, K. Tahara, R. Hiraoka, J. Ochiai, R. Ohmura, Y.H. Mori, Appl. Therm. Eng. 26 (2006) 2157–2167. [7] S. Obara, T. Yamada, K. Matsumura, S. Takahashi, M. Kawai, B. Rengarajan, Appl. Energy 88 (2011) 4733–4741. [8] R. Ohmura, T. Uchida, S. Takeya, J. Nagao, H. Minagawa, T. Ebinuma, H. Narita, J. Chem. Eng. Data 48 (2003) 1337–1340. [9] R. Ohmura, S. Takeya, T. Maekawa, T. Uchida, J. Chem. Eng. Data 51 (2006) 161–163. [10] T.Y. Makogon, A.P. Mehta, E.D. Sloan, J. Chem. Eng. Data 41 (1996) 315–318. [11] A. Maczynski, D.G. Shaw, M. Goral, B. Wisniewska-Goclowska, A. Skrzecz, I. Owczarek, K. Blazej, M.C. Haulait-Prison, G.T. Hefter, F. Kapuku, Z. Maczynska, C.L. Young, J. Phys. Chem. Ref. Data 34 (2) (2005) 709–753. [12] A. Danesh, B. Tohidi, R.W. Burgass, A.C. Todd, Chem. Eng. Res. Des. 72 (1994) 197–200. [13] F. Izumi, K. Momma, Solid State Phenom. 230 (2007) 15–20. [14] G.D. Holder, Ind. Eng. Chem. Fundam. 19 (1980) 282–286. [15] A.H. Mohammadi, D. Richon, Ind. Eng. Chem. Res. 50 (2011) 11452–11454. [16] T. Nakamura, T. Makino, T. Sugahara, K. Ohgaki, Chem. Eng. Sci. 58 (2003) 269–273. [17] N. Shimada, K. Sugahara, T. Sugahara, K. Ohgaki, Fluid Phase Equilib. 205 (2003) 17–23.
Contents lists available at ScienceDirect
Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid
Phase equilibrium and crystallographic structure of clathrate hydrate formed in argon + 2,2-dimethylbutane + water system Kotaro Murayama a , Satoshi Takeya b , Ryo Ohmura a,∗ a b
Department of Mechanical Engineering, Keio University, Yokohama 223-8522, Japan National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
a r t i c l e
i n f o
Article history: Received 18 October 2013 Received in revised form 11 December 2013 Accepted 16 December 2013 Available online 24 December 2013 Keywords: Clathrate hydrates Structure H Crystallographic structure Noble gases Argon
a b s t r a c t This paper reports the four-phase (Lw –H–V–Lg ) equilibrium conditions for the structure H hydrates formed in argon + 2,2-dimethylbutane. The experiments were performed at temperatures between 275.6 and 284.3 K, and at pressures between 2.91 and 9.41 MPa. The crystallographic structure of the hydrate was identified to be structure H by powder X-ray diffraction measurements. At a given temperature, the equilibrium pressure of the structure H hydrate formed with argon + 2,2-dimethylbutane was lower than that of the structure II pure argon hydrate. The difference in the equilibrium pressure is 8.1–20.9 MPa in the temperature range of 275.6–284.3 K. Among the system using methane, argon, krypton and xenon, the system using argon shows the highest reduction ratio of equilibrium pressures. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Argon is one of the most common noble gases like helium. The volumetric composition of argon in the earth’s atmosphere is approximately 0.9%. Human beings have used argon widely in their industry because of its safety and low cost. Argon is a noble substance, but it is reported that a clathrate hydrate can be formed with argon [1]. New technologies using hydrates formed with argon may be expected. Argon gas has also been found on the other planets and satellites, and argon hydrates may be formed there [2,3]. In planetary geoscience, the thermodynamic stabilities of various clathrate hydrates are required for use in the simulation of atmospheric compositions. Argon is one of the main substances on Mars and Pluto and argon hydrate may exist by forming some structures. Though the thermodynamic stability of the pure argon structure II hydrate was used in the studies for conducting simulations of the atmospheric composition, more accurate simulation can be done if there is thermodynamic stability data of the structure H hydrate formed with argon. In the harvest industry, it is reported that fresh-cut peppers subjected to pressurized argon gas maintain water in the peppers due to the formation of argon hydrate [4]. The information about the stabilities of structure H hydrate formed with argon is
∗ Corresponding author. Tel.: +81 45 566 1813; fax: +81 45 566 1495. E-mail address: [email protected] (R. Ohmura). 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.12.011
needed, because some compounds may exist as a guest substance to form the structure H hydrate with argon in fresh peppers. Clathrate hydrates are ice-like crystalline solid compounds consisting of a network of hydrogen-bonded host water molecules forming cage structures that enclose guest molecules. Depending on the size and shape of the guest substances, water molecules form several different cage structures that interconnect to yield hydrates of different crystallographic structures, such as structure I (sI), structure II (sII), and structure H (sH). The crystal system of the sI and sII hydrates is cubic and that of the sH hydrate is hexagonal. The typical values of the lattice parameters at 100 K are as ˚ and follows: the a axes of sI and sII hydrates are 12 A˚ and 17.2 A, the a and c axes of sH hydrate are 12 A˚ and 10 A˚ [5]. The formation of sH hydrates requires two different guest substances; one is a relatively large molecule guest substance (LMGS), and the other is a small gaseous guest substance, such as methane or xenon. The phase equilibrium conditions of sH hydrates are often milder (i.e., higher temperature and lower pressure) than those of the sI or sII hydrates formed only with small molecule guest substances. The milder phase equilibrium conditions of the sH hydrate formed with argon is advantageous for actual applications. Examples of the applied technologies of the argon sH hydrate are a hydrate-based heat pump [6] and a hydrate engine [7]. Ogawa et al. selected the pair of HFC-32 and cyclopentane as guest substances. HFC-32 is a flammable gas, while argon is an extremely inactive gas. The global warming potential of HFC-32 is also much higher than that of argon. The sH hydrate formed with argon can then be the medium of the
K. Murayama et al. / Fluid Phase Equilibria 365 (2014) 64–67 Table 1 Supplier and purity of the materials used in this study. Chemical
Supplier
Purity
Argon 2,2-Dimethylbutane
Taiyo Nippon Sanso Sigma–Aldrich
>99.9999 >99.0
hydrate heat pump and the hydrate engine in terms of safety and low cost of argon gas. In this study, we targeted argon as a candidate guest gas to form an sH hydrate to be practically applied for the hydraterelated industries because the cost of argon is one-fourth the cost of methane. We selected 2,2-dimethylbutane as the second guest substance used together with argon because the sH hydrates, which have a milder phase equilibrium than the equilibrium of a simple sII hydrate, are often formed with 2,2-dimethylbutane as a LMGS. The phase equilibrium conditions of the sH hydrate formed in methane, krypton or xenon + 2,2-dimethylbutane + water system have previously been reported [8–10]. The equilibrium pressures of these sH hydrates are 10–60% lower than those of the simple methane or xenon sI hydrate and simple Kr sII hydrate. Thus, we expected that the equilibrium pressure of the hydrate formed with argon + 2,2dimethylbutane would be much lower than that of the simple argon hydrate. The solubility of 2,2-dimethylbutane in water is 8.23 × 10−6 and the solubility of water in 2,2-dimethylbutane is 1.53 × 10−4 at 273.1 K [11]. 2,2-dimethylbutane is not miscible with water and forms a separate phase as a liquid guest substance. In this paper, we demonstrate the formation of a sH hydrate with argon + 2,2-dimethylbutane with four-phase (Lw –H–V–Lg ) equilibrium conditions and provide the first phase equilibrium data. The crystallographic structure of this hydrate was identified based on powder X-ray diffraction measurements. 2. Experimental setup and procedure For the experiments presented in this study, the following chemicals were used: deionized and distilled water; pure argon gas (99.9999% certified purity – Taiyo Nippon Sanso, Co., Ltd., Japan); liquid reagent of 2,2-dimethylbutane (99.0% certified purity – Sigma–Aldrich). The deionized and distilled water was laboratorymade. The conductivity of the deionized and distilled water is lower than 300 × 10−4 s/m at 298 K. The purities and suppliers of the fluid samples are specified in Table 1. The apparatus for the phase equilibrium measurements and preparing the crystal samples for the powder X-ray diffraction (PXRD) is schematically illustrated in Fig. 1. The test section for the four-phase equilibrium, water-rich liquid (Lw ), hydrate (H), argon-rich vapor (V) and 2,2-dimethylbutane (Lg ) was a stainless
Fig. 1. Schematic of the experimental apparatus.
65
steel cylindrical vessel with the inner volume of 200 cm3 . A magnetically-driven impeller was inserted into the vessel to agitate the liquid phases in the test section. The temperature of the vessel (T) was controlled at a prescribed value using a PID-controlled heater (TM-1, As One Co., Ltd.) and a cooler placed in the bath. A platinum resistance thermistor was inserted into the vessel to measure the temperature in the test section. The pressure in the vessel (p) was measured by a strain-gauge pressure transducer (PHB-A-10MP, Kyowa Electric Co., Ltd.). The estimated uncertainty of the temperature measurements was ±0.1 K for 223≤ T ≤ 573 K. The estimated uncertainty of the pressure measurements was ±0.005 MPa for p ≤ 10.0 MPa and for 233 ≤ T ≤ 423 K. The present study follows the procedure described by Danesh et al. for measuring the equilibrium temperatures and pressures of the four phases [12]. The vessel was first charged with liquid water and 2,2-dimethylbutane. We used 35 cm3 of liquid water and 15 cm3 of 2,2-dimethylbutane to ensure the presence of the two liquid phases. The volume of 2,2-dimethylbutane is 3/7 of water. This amount of 2,2-dimethylbutane is more than twice the stoichiometric composition of the sH hydrate. In this system, the degree of freedom was determined to be 1 from the Gibbs phase rule. The vessel was then placed in the bath. Gaseous argon was supplied from a high-pressure cylinder through a pressure-regulating valve to the vessel after the air in the vessel was evacuated by a vacuum pump. The motor of the magnetically driven impeller was then turned on and the impeller continued to agitate the system through the experimental run at 300 rpm. After p and T were set at the prescribed values within the pressure and temperature range of 3–10 MPa and 278–290 K, respectively, T was decreased to form a hydrate. The hydrate formation in the vessel was detected by the decrease in p. The temperature T was then increased by steps of 0.1 K. At each temperature step, T was maintained until p did not change by 1 kPa within 12 h. By repetition of this incremental temperature increase, a p versus T plot diagram for each run was obtained and a four-phase equilibrium condition was determined from the diagram. When the hydrate was dissociated by increasing the temperature in the vessel, the pressure in the vessel increased. When all of the hydrate was completely dissociated, then the increase in the pressure became quite small because of the change in the phase equilibrium of the fluids remaining in the test section. This operation was repeated under different initial conditions to obtain the four-phase equilibrium data over the desired temperature–pressure range.
2.1. Powder X-ray diffraction The hydrate crystal sample for the PXRD measurements was prepared with liquid water, argon gas, and 2,2-dimethylbutane using the same type of apparatus for the phase equilibrium measurements. The amount of water and 2,2-dimethylbutane for the sample is the same as for the equilibrium measurements. The temperature inside the vessel was maintained at 276 K. The pressure was set at 6.0 MPa to avoid forming the simple argon sII hydrate that may form at p > 12.2 MPa. Upon hydrate formation, when the pressure decreased to the equilibrium pressure of the sH hydrate formed with argon and 2,2-dimethylbutane, the vessel was recharged with argon gas to 6.0 MPa. This argon charging procedure was repeated until no further pressure reduction was observed. The inside of the vessel was continuously agitated at 300 rpm after the hydrate nucleation. Nearly complete conversion of the water to the hydrate was obtained when no further pressure reduction was observed. After the pressure stabilized, the vessel was subsequently removed from the temperature-controlled bath and immediately immersed in a liquid nitrogen bath. After the temperature in the vessel decreased below 220 K, the vessel was removed from the
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K. Murayama et al. / Fluid Phase Equilibria 365 (2014) 64–67
Table 2 Experimental data for the four-phase (Lw –H–V–Lg ) equilibrium pressure– temperature conditions in a (argon + 2,2-dimethylbutane + water) system.a T (K)
p (MPa)
275.6 275.6 277.8 279.9 282.0 284.3
2.910 2.914 3.845 5.401 7.362 9.410
a
u(T) = 0.1 K; u(p) = 0.005 MPa.
liquid nitrogen pool and quickly disassembled. The lower part of the vessel containing the hydrate sample was then again placed in a liquid nitrogen pool, and the hydrate sample was obtained from the vessel. The hydrate sample was subjected to PXRD measurements after storing in a container kept at a temperature below 100 K. For the PXRD measurements, the hydrate sample was finely powdered in a nitrogen atmosphere at a temperature below 100 K. The finepowdered hydrate samples were top-loaded on a copper (Cu) specimen holder. The PXRD measurements were done using Cu K˛ radiation in a /2 step scan mode with a step width of 0.02◦ (40 kV, 40 mA; Rigaku model Ultima III). Analysis of the lattice constants was done by a full-pattern fitting method using the Rietveld program RIETAN-FP.13 [13].
Fig. 2. Pressure–temperature plot to illustrate phase equilibrium conditions for hydrates formed in (argon + water) and (argon + 2,2-dimethylbutane + water) systems. : (argon + 2,2-dimethylbutane + water) system, : (argon + water) system [1], 䊉: (argon + water) system [15], : (argon + water) system [14].
3. Results and discussion The p–T data of the four-phase (Lw –H–V–Lg ) equilibrium in the (water + argon + 2,2-dimethylbutane) system are listed in Table 2 and plotted in Fig. 2. The equilibrium data of the simple argon hydrate (water + argon) are also shown in Fig. 2 [1,14,15]. The reproducibility of the equilibrium measurements was confirmed by duplicate measurements of the argon + 2,2dimethylbutane + water system at the pressure of 3 MPa. The results of the measurement are also listed in Table 2 and Fig. 2. As can be seen in Table 2, the data (p = 2.910 MPa, T = 275.6 K) is consistent with other data (p = 2.914 MPa, T = 275.6 K) within the mutual uncertainty of u(p) = 0.005 MPa, u(T) = 0.1 K. The procedure for the measurements of the phase equilibrium conditions by Marshall et al. [1] and Holder et al. [14] are the same as the one used in the present study. Mohammadi et al. conducted the measurements using the pressure search method [15]. Each of the four-phase equilibrium conditions obtained in the present study is outside of the pure argon sII hydrate equilibrium line. At a given temperature, the equilibrium pressure of the hydrate formed with argon + 2,2-dimethylbutane is lower than that of the sII hydrate formed with argon. The difference in the equilibrium pressure at 276 K for the hydrate formed with argon + 2,2-dimethylbutane and the sII hydrate formed with argon is approximately 9.0 MPa. Fig. 3 shows the PXRD profiles of the hydrate formed with argon + 2,2-dimethylbutane. The crystallographic structure of the hydrate was identified to be sH by the PXRD measurements. The unit cell parameters of the sH hydrate
Fig. 3. Powder X-ray diffraction patterns for the structure H hydrate at 93 K: argon + 2,2-dimethylbutane hydrate. The hydrate was prepared at p = 3.00 MPa and T = 276.0 K. The tick marks in the lower part represent the calculated peak positions for the structure H hydrate and hexagonal ice.
formed with argon + 2,2-dimethylbutane were determined to be a = 12.165(1) A˚ and c = 9.928(1) A˚ at 93 K. The equilibrium pressures at 276 K and 278 K in the systems of (methane + water) [16], (argon + water) [1,14,15], (krypton + water) [14], (xenon + water) [17], (methane + 2,2dimethylbutane + water) [8], (argon + 2,2-dimethylbutane + water) (present study), (krypton + 2,2-dimethylbutane + water) [9], and (xenon + 2,2-dimethylbutane + water) [10] are compared in Table 3. To quantify the reduction of the equilibrium pressure of the sH
Table 3 The reduction ratio of the equilibrium pressures of the hydrates formed with simple methane, argon, krypton, xenon and formed with each gas + 2,2-dimethylbutane at 276.0 K and 278.0 K. Gaseous guest
Methane [8,16] Argon [1,14,15] Krypton [9,14] Xenon [10,17]
276.0 K
278.0 K
Psimple (MPa)
PsH (MPa)
3.45 12.20 1.92 0.20
1.60 3.50 0.72 0.18
Psimple −PsH Psimple
0.52 0.71 0.63 0.10
Psimple (MPa)
PsH (MPa)
4.22 14.50 2.30 0.25
2.02 4.50 0.85 0.24
Psimple −PsH Psimple
0.52 0.69 0.63 0.04
K. Murayama et al. / Fluid Phase Equilibria 365 (2014) 64–67
hydrate from that of a simple gas hydrate, the reduction ratio, (Psimple − PsH )/Psimple , where Psimple is the equilibrium pressure of the hydrate with each gas, and PsH is the equilibrium pressure of the sH hydrate formed with each gas + 2,2-dimethylbutane, was calculated. The reduction ratios in the methane system are 0.52, those in the argon system are 0.71 and 0.69, those in the krypton system are both 0.63, and those in the xenon system are 0.10 and 0.04. The equilibrium pressure of the hydrate in the xenon system is the lowest, but the reduction ratio is low. The system, which shows the highest reduction ratio, is the system with argon. Although it needs more than 10 MPa at 274 K to produce the pure argon hydrate, the sH hydrates with argon may be formed more easily than the pure argon hydrate because the high pressure reduction was confirmed between the pressure of the sH hydrate formed with argon + 2,2-dimethylbutane and that of the sII pure argon hydrate. 4. Conclusion The four-phase (Lw –H–V–Lg ) equilibrium conditions were measured for the system of (argon + 2,2-dimethylbutane + water) at temperatures from 275.6 K to 284.3 K. The results suggest that the phase equilibrium formed in the system of present study is milder than the equilibrium in the system of pure argon + water. The structure of the hydrate formed with argon + 2,2-dimethylbutane was identified as the structure H by PXRD measurements. The ratio of the difference of the equilibrium pressures between the simple argon hydrate and the structure H hydrate with argon + 2,2-dimethylbutane divided by the equilibrium pressure of the simple argon hydrate is the highest among the systems containing methane, argon, krypton and xenon.
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