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Dissociation behavior of “dry water” C3H8 hydrate below ice point: Effect of phase state of unreacted residual water on a mechanism of gas hydrates dissociation
Journal of Energy Chemistry 24(2015)309–314
Dissociation behavior of “dry water” C3H8 hydrate below ice point: Effect of phase state of unreacted residual water on a mechanism of gas hydrates dissociation Andrey O Drachuka , Vladimir P Melnikova,b, Nadezhda S Molokitinaa∗, Anatoliy N Nesterova,b , Lev S Podenkoa , Aleksey M Reshetnikova,b, Andrey Yu Manakovc a. Institute of Earth Cryosphere, SB RAS, 625026, Malygin str. 86, Tyumen, Russian Federation; b. Tyumen State Oil and Gas University, 625000, Volodarsky str. 38, Tyumen, Russian Federation; c. Nikolaev Institute of Inorganic Chemistry, SB RAS, 630090, Lavrentiev av. 3, Novosibirsk, Russian Federation [ Manuscript received December 2, 2014; revised January 21, 2015 ]
Abstract The results on a dissociation behavior of propane hydrates prepared from “dry water” and contained unreacted residual water in the form of ice inclusions or supercooled liquid water (water solution of gas) were presented for temperatures below 273 K. The temperature ramping or pressure release method was used for the dissociation of propane hydrate samples. It was found that the mechanism of gas hydrate dissociation at temperatures below 273 K depended on the phase state of unreacted water in the hydrate sample. Gas hydrates dissociated into ice and gas if the ice inclusions were in the hydrate sample. The samples of propane hydrates with inclusions of unreacted supercooled water only (without ice inclusions) dissociated into supercooled water and gas below the pressure of the supercooled water-hydrate-gas metastable equilibrium. Key words gas hydrates; dry water; supercooled liquid water; ice; hydrate dissociation
1. Introduction “Dry water” is a free-flowing powder produced by mixing conventional water (up to 98 wt%) and nanoparticles of hydrophobized pyrogenic silica in air with a high speed [1]. Water in the powder is in the form of the isolated droplets with a size of several micrometers or their assemblies of up to several tens of micrometers [2]. A stability of such dispersion system is provided by the presence of silica nanoparticles on the surface of water droplets preventing their coalescence. Recently, it was shown that the use of “dry water” for preparation of gas hydrates dramatically increased a rate of hydrate formation and a conversion of water to hydrate compared with bulk water or powdered ice [3−7]. Thus, “dry water” can be considered as a promising system for producing gas hydrates in order to use them in alternative technologies of a transportation and temporary storage of natural gas in the form of gas hydrates. One volume of natural gas hydrates contains more than 160 volumes of gas (under normal conditions). The development and practical application of hydrate technologies
of transportation and storage of natural gas are being actively studied at present [8,9]. The developed hydrate technologies of transportation and storage of natural gases are based on the phenomenon of anomalously low rate of gas hydrate dissociation at temperatures below the ice melting point [10]. This phenomenon is known as the self-preservation effect [11] or the anomalous preservation regime of gas hydrates [12]. It is proposed that the anomalous gas hydrates dissociation at temperatures below 273 K is caused by the formation of an ice coating on the surface of hydrate particles in the initial moment of their dissociation and this ice coating prevents the evolution of gas from hydrate [9−11,13]. Ice is the stable water phase which is formed during the hydrate dissociation at temperatures below 273 K. However, it was shown recently [14−16] that gas hydrates dissociation at temperatures below 273 K can precede through the intermediate stage of the supercooled water formation. Supercooled water [17] or residual ice [18] can also be presented in the gas hydrate samples as unreacted water phase which is not converted into hydrates during their
∗
Corresponding author. Tel: +79-22-0759866; +73-45-2688709; E-mail: [email protected], [email protected] This work was supported by the Council for Grants of the President of the Russian Federation (Grant NSh–3929.2014.5), the Basic Research Programs of the RAS (Program No. 8, and Program the Arctic) and the Siberian Branch of the RAS (Interdisciplinary Project No. 144). Copyright©2015, Science Press and Dalian Institute of Chemical Physics. All rights reserved. doi: 10.1016/S2095-4956(15)60316-3
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formation. It is supposed that unreacted water in gas hydrate samples or water formed during the hydrate dissociation and the phase state of this water (ice or supercooled liquid) will influence the dissociation behavior of gas hydrates below the ice melting point [18,19]. The mechanism of this influence is poorly understood and further examination is needed. This paper presented the results of our study on the dissociation behavior of propane hydrate at temperatures below the ice melting point. The samples were formed from “dry water” and contained residual unreacted water in the form of ice or supercooled liquid. Propane was used as a model hydrate forming gas because it forms the same hydrate crystal structure (s II) like natural gas but a high pressure is not required for the formation of propane hydrates.
[20]. Hydrates were produced in the glass reactor with a volume of about 9 cm3 . The reactor was placed in a cryostat filled with an aqueous solution of ethylene glycol. The temperature was measured inside the reactor in the center of the sample (Ts ) and in the cryostat near the outer surface of the reactor (Tcr ) by two semiconductor sensors with an accuracy of ±0.1 K. The pressure in the reactor was measured by a digital pressure gauge with an accuracy of ±1.5 kPa. We used inhouse hardware and software for data acquisition, processing and storage.
2. Experimental 2.1. Sample preparation To prepare “dry water”, we used distilled water and a hydrophobic fumed silica powder Aerosilr R202 (Evonik Industries AG, Germany) with a specific surface area of 100 m2 ·g−1 (by BET), a bulk density of about 60 g·L−1, and a carbon content of 3.5–5.0 wt%. “Dry water” was prepared by mixing appropriate amounts of water (95 or 90 g) and Aerosil (5 or 10 g) in a Braun VX2050 household blender with a speed of 18700 rpm for 60 s. A similar technique to produce “dry water” was used earlier [1−7]. The resulting “dry water” was a free-flowing powder (Figure 1) with a bulk density of 0.5– 0.6 g·cm−3. It was shown previously [2] that “dry water” prepared in the same way comprised individual water droplets of size about 4 µm and their assemblies of up to 40 µm.
Figure 2. Scheme of the experimental setup
For hydrate formation, about 130 mg “dry water” (0.2 cm3 ) was loaded into the reactor, afterwards the “dry water” was frozen by cooling the reactor with a rate of 0.5 K/min. We judged on freezing of water in the sample from differential thermal analysis (DTA) thermograms. Figure 3 shows an example of the freezing/thawing thermogram of “dry water”. An exothermic peak upon cooling of the sample (Figure 3, peak 1) observed in the temperature range of 265–257 K is due to a heat release during the freezing of water in the sample. As seen from the thermogram, the freezing of “dry water” ended at 257 K. Upon heating, the frozen “dry water” melted at 273 K (Figure 3, an endothermic peak 2).
Figure 1. “Dry water” with 5 wt% Aerosil
Technical propane (C2 H6 , 1.2 mol%; C3 H8 , 95.0 mol%; i-C4 H10 , 2.7 mol%; n-C4 H10 , 1.1 mol%) was used as purchased from a commercial supplier to form gas hydrates. A detailed description of the experimental setup (Figure 2) used in this work for the formation of propane hydrates and in situ study of their dissociation behavior were given elsewhere
Figure 3. Freezing/thawing DTA thermogram of “dry water” with 10 wt% Aerosil. Cooling/heating rate is 0.5 K/min. D T = Ts –Tcr . (1) Crystallization, (2) melting
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After freezing of “dry water”, the reactor was heated to 272 K, evacuated and charged with gas at a pressure up to 400 kPa. Hydrate formation was carried out under the isochoric conditions followed by a reducing of pressure in the reactor. The temperature rise in the reactor relative to its initial value 272 K due to the exothermic nature of the hydrate formation did not exceed 0.2 K. A number of moles of propane consumed during hydrate formation at time t = τ after the charging of gas into the reactor (t = 0) was calculated by the following equation: V P P − D n= R zTs 0 zTs τ
where, P is the pressure in the reactor, Ts is the temperature in the reactor measured in the centre of the sample, R is the universal gas constant, V is gas volume in the reactor, z is the compressibility factor which can be calculated using the PengRobinson equation of state and D n is a number of propane moles consumed during hydrate formation. The preparation of hydrates was continued for 30 h or until total stopping pressure drop in the reactor. Assuming that the composition of the formed propane hydrates is determined by the stoichiometric ratio of C3 H8 ·17H2 O, the conversion of water to hydrate (D h, weight of water in hydrate/initial weight of water in the sample) can be calculated as: 17µ · D n m where, µ is the molar mass of water and m is initial mass of water in a sample of “dry water” loaded into the reactor. The conversion of water to hydrate was about D h = 0.6 for samples prepared from the frozen “dry water” with 5 wt% Aerosil and about 0.85 for the samples with 10 wt% Aerosil. Propane hydrate could not be formed from the freshly prepared “dry water” without its freezing due to a long induction time of propane hydrate nucleation (more than 24 h). The induction time was absent for propane hydrate formed from frozen “dry water”. The hydrate samples prepared from frozen “dry water” contained unreacted water in the form of ice. To obtain hydrate samples with residual water in the form of liquid, the hydrate samples formed from frozen “dry water” were heated under isochoric conditions to 274 K and kept at this temperature for 3 h. For this time, the unreacted ice in the samples melted, and the pressure and temperature in the reactor stabilized. It is known that freezing/thawing of the “dry water” sample leads to its destruction [4]. This is expressed through the coalescence of separated water droplets and formation of the bulk water phase. However in our case, the gas hydrate dispersion system prepared from frozen “dry water” was not destroyed after the thawing of the unreacted ice in the sample. Apparently, the hydrate shell formed on the surface of the ice prevented the coalescence of water drops after the ice melting.
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region of their thermodynamic stability at temperature below 273 K was studied. The samples contained unreacted water in the form of ice or supercooled liquid water inclusions. Either of two general procedures was used to dissociate the gas hydrate [12]: (1) slowly isochoric heating of the hydrate samples with a constant rate known as “temperature ramping” and (2) isothermal decrease of pressure in the reactor below icehydrate-gas equilibrium pressure termed “pressure release”. Data on P -V -T measurements and a differential thermal analysis were used to control the state of the “dry water”—propane system and phase transitions which could be in the system when P -T conditions were changed. Using the transparent glass reactor enabled additional visual monitoring of the sample during the preparation and dissociation of propane hydrates.
D h=
2.2. Measurement method The behavior of propane hydrate samples outside of the
Figure 4. Pressure changing in the reactor (curves 1 and 2) during isochoric heating of the propane hydrate samples contained unreacted ice: (1) 5 wt% Aerosil, D h = 0.6, (2) 10 wt% Aerosil, D h = 0.85, (3) P -T equilibrium conditions of propane hydrate dissociation [21,22], (4) supercooled water-propane hydrate-gas metastable equilibrium [14]. Q is the quadruple point where ice+water+hydrate+gas coexist
3. Results and discussion 3.1. Dissociation of hydrate sample contained unreacted ice The curves of isochoric heating for propane hydrate samples prepared from “dry water” with 5 wt% or 10 wt% Aerosil and contained unreacted ice are shown in Figure 4. After ending of hydrate formation at 400 kPa and 272 K, the samples were cooled in the reactor to a temperature of about 252 K. To avoid the condensation of propane during cooling, pressure in the reactor was lowered to 200 kPa prior to the start of cooling. At 252 K, pressure in the reactor was again lowered to 75 kPa. This pressure was 20 kPa higher than the equilibrium pressure of propane hydrate dissociation into ice and gas at the given temperature [21,22]. It should be noted that the technical propane contained impurities of ethane (1.2 mol%) and butanes (iso, 2.7 mol% and normal, 1.1 mol%) which did not
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change the crystal structure of gas hydrate formed from technical propane and the equilibrium conditions of hydrate dissociation in comparison with those for pure propane hydrate as it followed from the calculation by CSMHYD [22]. After pressure and temperature became stable, the reactor was heated with a rate of 0.02 K/min under isochoric conditions. As it followed from the data presented in the temperature range between 252 K and 255 K (Figure 4) and until the heating curves intersected the ice-hydrate-gas equilibrium line, the pressure rise in the reactor corresponded to the pressure behavior during heating of gas in the reactor. On further heating to 267 K for samples with 5 wt% Aerosil (curve 1) and to 270 K for samples with 10 wt% Aerosil (curve 2), the heating curves coincided with P -T line of the three phase ice-hydrate-gas equilibrium. This means that propane hydrates dissociated into ice and gas when the samples in the reactor were heated above 255 K. In the temperature range between 268 K and 276 K for the samples with 5 wt% Aerosil and between 271 K and 276 K for the samples with 10 wt% Aerosil, it was observed again that a pressure rise in the reactor corresponded to the pressure behavior during heating of gas in the reactor. This clearly demonstrates that the hydrates completely dissociated into ice and gas at temperature below 273 K, and hence the self-preservation effect was absent for propane hydrate formed from “dry water”. It was shown previously that the self-preservation effect was not observed for the bulk propane hydrates [23,24]. Figure 5 shows the pressure changing in the reactor (curve 1) and the differential thermogram (curve 2) for propane hydrate sample containing unreacted ice during reactor depressurization with an average rate about 15 kPa/min. During depressurization, a constant temperature of 267.8 K was maintained in the cryostat. The depressurization rate was regulated with a needle valve by venting gas from the reactor. The slow depressurization did not change the temperature of the sample in the reactor. The endothermic peak was observed in the thermogram when a pressure in the reactor was decreased only below a pressure of 135 kPa which corresponded to the icehydrate-gas equilibrium pressure at 267.8 K (Figure 5, curve 3). The appearance of the endothermic peak at the pressure below the ice-hydrate-gas equilibrium pressure but above the supercooled water-hydrate-gas metastable equilibrium pressure (Figure 5, curve 4) means that the hydrate dissociated into ice and gas. After the beginning of hydrate dissociation, the venting valve was closed. Thereafter the pressure increased initially and then it remained constant but below the ice-hydrate-gas equilibrium pressure. The pressure did not change for the next ten hours under the isothermal conditions. Such pressure behavior in the reactor could be caused by either the stopping of hydrate dissociation due to the selfpreservation or the absence of hydrates in the reactor due to their complete dissociation. To clarify which of two factors determined the pressure behavior in the system, the reactor was heated to 276 K with a rate of 0.5 K/min. During heating, ice melting was evidenced by the appearance of an endothermic peak at 273 K in the DTA-thermogram. However, the additional amount of propane in the gas phase that could appear
due to the hydrate dissociation during the heating was not detected. The pressure increase corresponded to the growth of pressure in the reactor with gas during its heating. Thus, the propane hydrate prepared from frozen “dry water” and contained inclusion of the unreacted ice completely dissociated into ice and gas when pressure was decreased below the ice-hydrate-gas equilibrium pressure, the same as in a case of heating (Figure 4).
Figure 5. Pressure changing in the reactor and the thermogram of the propane hydrate sample contained unreacted ice (D h = 0.85) during depressurization (ambient temperature is 267.8 K): (1) pressure, (2) D T , (3) equilibrium pressure of propane hydrate dissociation into ice and gas at 267.8 K (135 kPa) [21,22], (4) pressure of the supercooled water-propane hydrate-gas metastable equilibrium at 267.8 K (60 kPa) [14]. The moment of depressurization stop is marked by an arrow. Content of Aerosil in the initial “dry water” is 10 wt%
3.2. Dissociation of hydrates contained unreacted supercooled water Figure 6 shows the isochoric heating curve of propane hydrate sample which contained unreacted water at the temperature below 273 K in the form of supercooled water. Before heating, the propane hydrate samples with unreacted water were kept about 3 h at 273.5 K and 250 kPa. Then the reactor was cooled to 267 K with a rate of 0.5 K/min. Thereafter the pressure was lowered to 50 kPa by evacuating gas from the reactor. This pressure was below the equilibrium pressure of the hydrate dissociation into ice and gas, but higher than the pressure of supercooled water-gas-hydrate metastable equilibrium. The phase state of unreacted water in propane hydrate samples and dissociation of hydrate during the cooling and pressure release steps were controlled by the DTAthermograms. The appearance of the exothermic peak at the thermogram was an evidence of water freezing in the sample, whereas endothermic peak pointed to hydrate dissociation. If water crystallization was detected during cooling or gas evacuating, the run was terminated and such sample was not used for further studies. Data for propane hydrate sample contained 10 wt% Aerosil is presented in Figure 6. For the samples contained 5 wt% Aerosil, supercooled water always froze during the pressure release step. Probably, gas bubbles which real-
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ized from water during pressure decreasing caused a dynamic ice nucleation [16]. It was observed that the induction times of the supercooled water crystallization in the hydrate samples with 10 wt% Aerosil which were cooled to 267 K in the hydrate stable region varied from several hours to tens of hours. At this temperature, the unreacted supercooled water in the samples did not freeze either during the pressure release. It allowed us to examine the dissociation of propane hydrate contained the inclusions of unreacted water in the form of supercooled liquid. For hydrate dissociation, the reactor with the hydrate sample at 267 K was evacuated to 50 kPa and heated under isochoric conditions with a rate of 0.02 K/min (Figure 6).
Figure 6. Pressure changing in the reactor during isochoric cooling, pressure release and isochoric heating of the propane hydrate sample contained unreacted water: (1) propane hydrate formed from “dry water” with 10 wt% Aerosil (D h = 0.85), (2) equilibrium conditions of propane hydrate dissociation [21,22], (3) supercooled water-hydrate-gas metastable equilibrium [14]
According to the data in Figure 6, the heating curve of propane hydrate samples (curve 1) coincided with the supercooled water-hydrate-gas equilibrium line (curve 3) in the temperature range of 268−272 K. At further heating to 278 K, the pressure behavior in the reactor corresponded to the pressure changing during the gas heating in the reactor. This means that the hydrates dissociated completely into supercooled water and gas along the supercooled water-hydrate-gas equilibrium line in the temperature range of 268−272 K. Figure 7 presents the data on pressure changing in the reactor and the thermogram for propane hydrate sample during the depressurization from 200 to 40 kPa with a rate of 15 kPa/min at a constant ambient temperature of 267.8 K. The studied hydrate sample contained the inclusions of unreacted water in the form of supercooled liquid. A pressure of 40 kPa was lower than the ice-hydrate-gas equilibrium pressure (135 kPa) [21,22] and also this pressure was lower than the supercooled water-hydrate-gas metastable equilibrium pressure (60 kPa) [14] at 267.8 K. During the pressure decrease, it was observed that the sample temperature remained constant until the pressure was above 60 kPa. Immediately after the pressure reached 60 kPa, the endothermic peak appeared in the thermogram that pointed to hydrate dissocia-
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tion into supercooled water and gas. After stop of gas evacuating (marked by an arrow in Figure 7), the pressure in the reactor increased and reached supercooled water-hydrate-gas metastable equilibrium pressure (60 kPa) after 4 h exposure at 267.8 K (this was not shown in Figure 7). It follows from data presented in Figure 7 that the hydrate sample contained the inclusions of unreacted supercooled water at 267.8 K dissociated during the depressurization as soon as the pressure decreased below the supercooled water-hydrate-gas metastable equilibrium pressure. The sample dissociated into supercooled water and gas.
Figure 7. Pressure changing in the reactor and the thermogram for the propane hydrate sample contained unreacted liquid water (D h = 0.85) during depressurization (ambient temperature is 267.8 K) and the follow keeping at isothermal conditions: (1) pressure, (2) D T , (3) equilibrium pressure of propane hydrate dissociation into ice and gas at 267.8 K, (4) pressure of the supercooled water-propane hydrate-gas metastable equilibrium at 267.8 K [14]. The moment of depressurization stop is marked by an arrow. Content of Aerosil in “dry water” is 10 wt%
4. Conclusions These studies have shown that the dissociation mechanism of gas hydrate samples below the ice melting temperature depends on the phase state of unreacted water in the sample: (1) Gas hydrate dissociates into ice and gas if the sample contains the inclusions of unreacted water in the form of ice. (2) Propane hydrate samples contained inclusions of metastable liquid water below 273 K (but not contained ice inclusions) can remain stable at the pressure below hydrate dissociation into ice and gas but above the pressure of hydrate dissociation into supercooled water and gas. However the gas hydrates dissociate into liquid water and gas at the pressure below the supercooled water-hydrate-gas metastable equilibrium pressure. Data presented in the paper are obtained for gas hydrate formed in a highly dispersed water system which is “dry water”. In future, we are going to obtain evidence of the phase state effect of unreacted residual water on the mechanism of gas hydrate dissociation for bulk gas hydrate.
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Acknowledgements The present work was supported by the Council for Grants of the President of the Russian Federation (grant NSh 3929.2014.5), the Basic Research Programs of the RAS (program no. 8, and program the Arctic) and the Siberian branch of the RAS (interdisciplinary project no. 144).
References [1] Binks B P, Murakami R. Nat Mater, 2006, 5(11): 865 [2] Podenko L S, Nesterov A N, Komisarova N S, Shalamov V V, Reshetnikov A M, Larionov E G. J Appl Spectrosc, 2011, 78(2): 260 [3] Wang W X, Bray C L, Adams D J, Cooper A I. J Am Chem Soc, 2008, 130(35): 11608 [4] Carter B O, Wang W X, Adams D J, Cooper A I. Langmuir, 2010, 26(5): 3186 [5] Hu G W, Ye Y G, Lin C L, Meng Q G, Zhang J, Diao S B. Fuel Process Technol, 2011, 92(8): 1617 [6] Ildyakov A V, Larionov E G, Manakov A Yu, Fomin V M. Gazokhimiya (in Russian), 2011, 17(1): 28 [7] Fan S S, Yang L, Wang Y H, Lang X M, Wen Y G, Lou X. Chem Eng Sci, 2014, 106: 53 [8] Horiguchi K, Watanabe S, Moriya H, Nakai S, Yoshimitsu A, Taoda A. In: Proc of the 7th Int Conf on Gas Hydrates, Edinburgh, 2011 [9] Rehder G, Eckl R, Elfgen M, Falenty A, Hamann R, Kahler N, Kuhs W F, Osterkamp H, Windmeier C. Energies, 2012, 5(7): 2499
[10] Davidson D W, Garg S K, Gough S R, Handa Y P, Ratcliffe C I, Ripmeester J A, Tse J S, Lawson W F. Geochim Cosmochim Acta, 1986, 50(4): 619 [11] Yakushev V S, Istomin V A. In: Maeno N, Hondoh T eds. Physics and Chemistry of Ice, Sapporo: Hokkaido University Press, 1992. 136 [12] Stern L A, Circone S, Kirby S H, Durham W B. J Phys Chem B, 2001, 105(9): 1756 [13] Takeya S, Shimada W, Kamata Y, Ebinuma T, Uchida T, Nagao J, Narita H. J Phys Chem A, 2001, 105(42): 9756 [14] Mel’nikov V P, Nesterov A N, Reshetnikov A M. Dokl Phys Chem, 2007, 417(2): 304 [15] Melnikov V P, Nesterov A N, Reshetnikov A M, Zavodovsky A G. Chem Eng Sci, 2009, 64(6): 1160 [16] Ohno H, Oyabu I, Iizuka Y, Hondoh T, Narita H, Nagao J. J Phys Chem A, 2011, 115(32): 8889 [17] Vlasov V A, Zavodovsky A G, Madygulov M S, Nesterov A N, Reshetnikov A M. Russ J Phys Chem A, 2013, 87(11): 1789 [18] Giavarini C, Maccioni F. Ind Eng Chem Res, 2004, 43(20): 6616 [19] Melnikov V P, Nesterov A N, Reshetnikov A M, Istomin V A, Kwon V G. Chem Eng Sci, 2010, 65(2): 906 [20] Podenko L S, Nesterov A N, Drachuk A O, Molokitina N S, Reshetnikov A M. Russ J Appl Chem, 2013, 86(10): 1509 [21] Istomin V A, Yakushev V S. Gas Hydrates in Natural Conditions. Moscow: Nedra, 1992. 236 (in Russian) [22] Sloan E D. Clathrate Hydrates of Natural Gases. 2nd ed. New York: Marcel Dekker, 1998. 705 [23] Melnikov V P, Nesterov A N, Reshetnikov A M. Gas Industry, Special Issue “Gas Hydrate”(in Russian), 2006: 55 [24] Takeya S, Ripmeester J A. Angew Chem Int Ed, 2008, 47(7): 1276
Dissociation behavior of “dry water” C3H8 hydrate below ice point: Effect of phase state of unreacted residual water on a mechanism of gas hydrates dissociation Andrey O Drachuka , Vladimir P Melnikova,b, Nadezhda S Molokitinaa∗, Anatoliy N Nesterova,b , Lev S Podenkoa , Aleksey M Reshetnikova,b, Andrey Yu Manakovc a. Institute of Earth Cryosphere, SB RAS, 625026, Malygin str. 86, Tyumen, Russian Federation; b. Tyumen State Oil and Gas University, 625000, Volodarsky str. 38, Tyumen, Russian Federation; c. Nikolaev Institute of Inorganic Chemistry, SB RAS, 630090, Lavrentiev av. 3, Novosibirsk, Russian Federation [ Manuscript received December 2, 2014; revised January 21, 2015 ]
Abstract The results on a dissociation behavior of propane hydrates prepared from “dry water” and contained unreacted residual water in the form of ice inclusions or supercooled liquid water (water solution of gas) were presented for temperatures below 273 K. The temperature ramping or pressure release method was used for the dissociation of propane hydrate samples. It was found that the mechanism of gas hydrate dissociation at temperatures below 273 K depended on the phase state of unreacted water in the hydrate sample. Gas hydrates dissociated into ice and gas if the ice inclusions were in the hydrate sample. The samples of propane hydrates with inclusions of unreacted supercooled water only (without ice inclusions) dissociated into supercooled water and gas below the pressure of the supercooled water-hydrate-gas metastable equilibrium. Key words gas hydrates; dry water; supercooled liquid water; ice; hydrate dissociation
1. Introduction “Dry water” is a free-flowing powder produced by mixing conventional water (up to 98 wt%) and nanoparticles of hydrophobized pyrogenic silica in air with a high speed [1]. Water in the powder is in the form of the isolated droplets with a size of several micrometers or their assemblies of up to several tens of micrometers [2]. A stability of such dispersion system is provided by the presence of silica nanoparticles on the surface of water droplets preventing their coalescence. Recently, it was shown that the use of “dry water” for preparation of gas hydrates dramatically increased a rate of hydrate formation and a conversion of water to hydrate compared with bulk water or powdered ice [3−7]. Thus, “dry water” can be considered as a promising system for producing gas hydrates in order to use them in alternative technologies of a transportation and temporary storage of natural gas in the form of gas hydrates. One volume of natural gas hydrates contains more than 160 volumes of gas (under normal conditions). The development and practical application of hydrate technologies
of transportation and storage of natural gas are being actively studied at present [8,9]. The developed hydrate technologies of transportation and storage of natural gases are based on the phenomenon of anomalously low rate of gas hydrate dissociation at temperatures below the ice melting point [10]. This phenomenon is known as the self-preservation effect [11] or the anomalous preservation regime of gas hydrates [12]. It is proposed that the anomalous gas hydrates dissociation at temperatures below 273 K is caused by the formation of an ice coating on the surface of hydrate particles in the initial moment of their dissociation and this ice coating prevents the evolution of gas from hydrate [9−11,13]. Ice is the stable water phase which is formed during the hydrate dissociation at temperatures below 273 K. However, it was shown recently [14−16] that gas hydrates dissociation at temperatures below 273 K can precede through the intermediate stage of the supercooled water formation. Supercooled water [17] or residual ice [18] can also be presented in the gas hydrate samples as unreacted water phase which is not converted into hydrates during their
∗
Corresponding author. Tel: +79-22-0759866; +73-45-2688709; E-mail: [email protected], [email protected] This work was supported by the Council for Grants of the President of the Russian Federation (Grant NSh–3929.2014.5), the Basic Research Programs of the RAS (Program No. 8, and Program the Arctic) and the Siberian Branch of the RAS (Interdisciplinary Project No. 144). Copyright©2015, Science Press and Dalian Institute of Chemical Physics. All rights reserved. doi: 10.1016/S2095-4956(15)60316-3
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formation. It is supposed that unreacted water in gas hydrate samples or water formed during the hydrate dissociation and the phase state of this water (ice or supercooled liquid) will influence the dissociation behavior of gas hydrates below the ice melting point [18,19]. The mechanism of this influence is poorly understood and further examination is needed. This paper presented the results of our study on the dissociation behavior of propane hydrate at temperatures below the ice melting point. The samples were formed from “dry water” and contained residual unreacted water in the form of ice or supercooled liquid. Propane was used as a model hydrate forming gas because it forms the same hydrate crystal structure (s II) like natural gas but a high pressure is not required for the formation of propane hydrates.
[20]. Hydrates were produced in the glass reactor with a volume of about 9 cm3 . The reactor was placed in a cryostat filled with an aqueous solution of ethylene glycol. The temperature was measured inside the reactor in the center of the sample (Ts ) and in the cryostat near the outer surface of the reactor (Tcr ) by two semiconductor sensors with an accuracy of ±0.1 K. The pressure in the reactor was measured by a digital pressure gauge with an accuracy of ±1.5 kPa. We used inhouse hardware and software for data acquisition, processing and storage.
2. Experimental 2.1. Sample preparation To prepare “dry water”, we used distilled water and a hydrophobic fumed silica powder Aerosilr R202 (Evonik Industries AG, Germany) with a specific surface area of 100 m2 ·g−1 (by BET), a bulk density of about 60 g·L−1, and a carbon content of 3.5–5.0 wt%. “Dry water” was prepared by mixing appropriate amounts of water (95 or 90 g) and Aerosil (5 or 10 g) in a Braun VX2050 household blender with a speed of 18700 rpm for 60 s. A similar technique to produce “dry water” was used earlier [1−7]. The resulting “dry water” was a free-flowing powder (Figure 1) with a bulk density of 0.5– 0.6 g·cm−3. It was shown previously [2] that “dry water” prepared in the same way comprised individual water droplets of size about 4 µm and their assemblies of up to 40 µm.
Figure 2. Scheme of the experimental setup
For hydrate formation, about 130 mg “dry water” (0.2 cm3 ) was loaded into the reactor, afterwards the “dry water” was frozen by cooling the reactor with a rate of 0.5 K/min. We judged on freezing of water in the sample from differential thermal analysis (DTA) thermograms. Figure 3 shows an example of the freezing/thawing thermogram of “dry water”. An exothermic peak upon cooling of the sample (Figure 3, peak 1) observed in the temperature range of 265–257 K is due to a heat release during the freezing of water in the sample. As seen from the thermogram, the freezing of “dry water” ended at 257 K. Upon heating, the frozen “dry water” melted at 273 K (Figure 3, an endothermic peak 2).
Figure 1. “Dry water” with 5 wt% Aerosil
Technical propane (C2 H6 , 1.2 mol%; C3 H8 , 95.0 mol%; i-C4 H10 , 2.7 mol%; n-C4 H10 , 1.1 mol%) was used as purchased from a commercial supplier to form gas hydrates. A detailed description of the experimental setup (Figure 2) used in this work for the formation of propane hydrates and in situ study of their dissociation behavior were given elsewhere
Figure 3. Freezing/thawing DTA thermogram of “dry water” with 10 wt% Aerosil. Cooling/heating rate is 0.5 K/min. D T = Ts –Tcr . (1) Crystallization, (2) melting
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After freezing of “dry water”, the reactor was heated to 272 K, evacuated and charged with gas at a pressure up to 400 kPa. Hydrate formation was carried out under the isochoric conditions followed by a reducing of pressure in the reactor. The temperature rise in the reactor relative to its initial value 272 K due to the exothermic nature of the hydrate formation did not exceed 0.2 K. A number of moles of propane consumed during hydrate formation at time t = τ after the charging of gas into the reactor (t = 0) was calculated by the following equation: V P P − D n= R zTs 0 zTs τ
where, P is the pressure in the reactor, Ts is the temperature in the reactor measured in the centre of the sample, R is the universal gas constant, V is gas volume in the reactor, z is the compressibility factor which can be calculated using the PengRobinson equation of state and D n is a number of propane moles consumed during hydrate formation. The preparation of hydrates was continued for 30 h or until total stopping pressure drop in the reactor. Assuming that the composition of the formed propane hydrates is determined by the stoichiometric ratio of C3 H8 ·17H2 O, the conversion of water to hydrate (D h, weight of water in hydrate/initial weight of water in the sample) can be calculated as: 17µ · D n m where, µ is the molar mass of water and m is initial mass of water in a sample of “dry water” loaded into the reactor. The conversion of water to hydrate was about D h = 0.6 for samples prepared from the frozen “dry water” with 5 wt% Aerosil and about 0.85 for the samples with 10 wt% Aerosil. Propane hydrate could not be formed from the freshly prepared “dry water” without its freezing due to a long induction time of propane hydrate nucleation (more than 24 h). The induction time was absent for propane hydrate formed from frozen “dry water”. The hydrate samples prepared from frozen “dry water” contained unreacted water in the form of ice. To obtain hydrate samples with residual water in the form of liquid, the hydrate samples formed from frozen “dry water” were heated under isochoric conditions to 274 K and kept at this temperature for 3 h. For this time, the unreacted ice in the samples melted, and the pressure and temperature in the reactor stabilized. It is known that freezing/thawing of the “dry water” sample leads to its destruction [4]. This is expressed through the coalescence of separated water droplets and formation of the bulk water phase. However in our case, the gas hydrate dispersion system prepared from frozen “dry water” was not destroyed after the thawing of the unreacted ice in the sample. Apparently, the hydrate shell formed on the surface of the ice prevented the coalescence of water drops after the ice melting.
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region of their thermodynamic stability at temperature below 273 K was studied. The samples contained unreacted water in the form of ice or supercooled liquid water inclusions. Either of two general procedures was used to dissociate the gas hydrate [12]: (1) slowly isochoric heating of the hydrate samples with a constant rate known as “temperature ramping” and (2) isothermal decrease of pressure in the reactor below icehydrate-gas equilibrium pressure termed “pressure release”. Data on P -V -T measurements and a differential thermal analysis were used to control the state of the “dry water”—propane system and phase transitions which could be in the system when P -T conditions were changed. Using the transparent glass reactor enabled additional visual monitoring of the sample during the preparation and dissociation of propane hydrates.
D h=
2.2. Measurement method The behavior of propane hydrate samples outside of the
Figure 4. Pressure changing in the reactor (curves 1 and 2) during isochoric heating of the propane hydrate samples contained unreacted ice: (1) 5 wt% Aerosil, D h = 0.6, (2) 10 wt% Aerosil, D h = 0.85, (3) P -T equilibrium conditions of propane hydrate dissociation [21,22], (4) supercooled water-propane hydrate-gas metastable equilibrium [14]. Q is the quadruple point where ice+water+hydrate+gas coexist
3. Results and discussion 3.1. Dissociation of hydrate sample contained unreacted ice The curves of isochoric heating for propane hydrate samples prepared from “dry water” with 5 wt% or 10 wt% Aerosil and contained unreacted ice are shown in Figure 4. After ending of hydrate formation at 400 kPa and 272 K, the samples were cooled in the reactor to a temperature of about 252 K. To avoid the condensation of propane during cooling, pressure in the reactor was lowered to 200 kPa prior to the start of cooling. At 252 K, pressure in the reactor was again lowered to 75 kPa. This pressure was 20 kPa higher than the equilibrium pressure of propane hydrate dissociation into ice and gas at the given temperature [21,22]. It should be noted that the technical propane contained impurities of ethane (1.2 mol%) and butanes (iso, 2.7 mol% and normal, 1.1 mol%) which did not
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change the crystal structure of gas hydrate formed from technical propane and the equilibrium conditions of hydrate dissociation in comparison with those for pure propane hydrate as it followed from the calculation by CSMHYD [22]. After pressure and temperature became stable, the reactor was heated with a rate of 0.02 K/min under isochoric conditions. As it followed from the data presented in the temperature range between 252 K and 255 K (Figure 4) and until the heating curves intersected the ice-hydrate-gas equilibrium line, the pressure rise in the reactor corresponded to the pressure behavior during heating of gas in the reactor. On further heating to 267 K for samples with 5 wt% Aerosil (curve 1) and to 270 K for samples with 10 wt% Aerosil (curve 2), the heating curves coincided with P -T line of the three phase ice-hydrate-gas equilibrium. This means that propane hydrates dissociated into ice and gas when the samples in the reactor were heated above 255 K. In the temperature range between 268 K and 276 K for the samples with 5 wt% Aerosil and between 271 K and 276 K for the samples with 10 wt% Aerosil, it was observed again that a pressure rise in the reactor corresponded to the pressure behavior during heating of gas in the reactor. This clearly demonstrates that the hydrates completely dissociated into ice and gas at temperature below 273 K, and hence the self-preservation effect was absent for propane hydrate formed from “dry water”. It was shown previously that the self-preservation effect was not observed for the bulk propane hydrates [23,24]. Figure 5 shows the pressure changing in the reactor (curve 1) and the differential thermogram (curve 2) for propane hydrate sample containing unreacted ice during reactor depressurization with an average rate about 15 kPa/min. During depressurization, a constant temperature of 267.8 K was maintained in the cryostat. The depressurization rate was regulated with a needle valve by venting gas from the reactor. The slow depressurization did not change the temperature of the sample in the reactor. The endothermic peak was observed in the thermogram when a pressure in the reactor was decreased only below a pressure of 135 kPa which corresponded to the icehydrate-gas equilibrium pressure at 267.8 K (Figure 5, curve 3). The appearance of the endothermic peak at the pressure below the ice-hydrate-gas equilibrium pressure but above the supercooled water-hydrate-gas metastable equilibrium pressure (Figure 5, curve 4) means that the hydrate dissociated into ice and gas. After the beginning of hydrate dissociation, the venting valve was closed. Thereafter the pressure increased initially and then it remained constant but below the ice-hydrate-gas equilibrium pressure. The pressure did not change for the next ten hours under the isothermal conditions. Such pressure behavior in the reactor could be caused by either the stopping of hydrate dissociation due to the selfpreservation or the absence of hydrates in the reactor due to their complete dissociation. To clarify which of two factors determined the pressure behavior in the system, the reactor was heated to 276 K with a rate of 0.5 K/min. During heating, ice melting was evidenced by the appearance of an endothermic peak at 273 K in the DTA-thermogram. However, the additional amount of propane in the gas phase that could appear
due to the hydrate dissociation during the heating was not detected. The pressure increase corresponded to the growth of pressure in the reactor with gas during its heating. Thus, the propane hydrate prepared from frozen “dry water” and contained inclusion of the unreacted ice completely dissociated into ice and gas when pressure was decreased below the ice-hydrate-gas equilibrium pressure, the same as in a case of heating (Figure 4).
Figure 5. Pressure changing in the reactor and the thermogram of the propane hydrate sample contained unreacted ice (D h = 0.85) during depressurization (ambient temperature is 267.8 K): (1) pressure, (2) D T , (3) equilibrium pressure of propane hydrate dissociation into ice and gas at 267.8 K (135 kPa) [21,22], (4) pressure of the supercooled water-propane hydrate-gas metastable equilibrium at 267.8 K (60 kPa) [14]. The moment of depressurization stop is marked by an arrow. Content of Aerosil in the initial “dry water” is 10 wt%
3.2. Dissociation of hydrates contained unreacted supercooled water Figure 6 shows the isochoric heating curve of propane hydrate sample which contained unreacted water at the temperature below 273 K in the form of supercooled water. Before heating, the propane hydrate samples with unreacted water were kept about 3 h at 273.5 K and 250 kPa. Then the reactor was cooled to 267 K with a rate of 0.5 K/min. Thereafter the pressure was lowered to 50 kPa by evacuating gas from the reactor. This pressure was below the equilibrium pressure of the hydrate dissociation into ice and gas, but higher than the pressure of supercooled water-gas-hydrate metastable equilibrium. The phase state of unreacted water in propane hydrate samples and dissociation of hydrate during the cooling and pressure release steps were controlled by the DTAthermograms. The appearance of the exothermic peak at the thermogram was an evidence of water freezing in the sample, whereas endothermic peak pointed to hydrate dissociation. If water crystallization was detected during cooling or gas evacuating, the run was terminated and such sample was not used for further studies. Data for propane hydrate sample contained 10 wt% Aerosil is presented in Figure 6. For the samples contained 5 wt% Aerosil, supercooled water always froze during the pressure release step. Probably, gas bubbles which real-
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ized from water during pressure decreasing caused a dynamic ice nucleation [16]. It was observed that the induction times of the supercooled water crystallization in the hydrate samples with 10 wt% Aerosil which were cooled to 267 K in the hydrate stable region varied from several hours to tens of hours. At this temperature, the unreacted supercooled water in the samples did not freeze either during the pressure release. It allowed us to examine the dissociation of propane hydrate contained the inclusions of unreacted water in the form of supercooled liquid. For hydrate dissociation, the reactor with the hydrate sample at 267 K was evacuated to 50 kPa and heated under isochoric conditions with a rate of 0.02 K/min (Figure 6).
Figure 6. Pressure changing in the reactor during isochoric cooling, pressure release and isochoric heating of the propane hydrate sample contained unreacted water: (1) propane hydrate formed from “dry water” with 10 wt% Aerosil (D h = 0.85), (2) equilibrium conditions of propane hydrate dissociation [21,22], (3) supercooled water-hydrate-gas metastable equilibrium [14]
According to the data in Figure 6, the heating curve of propane hydrate samples (curve 1) coincided with the supercooled water-hydrate-gas equilibrium line (curve 3) in the temperature range of 268−272 K. At further heating to 278 K, the pressure behavior in the reactor corresponded to the pressure changing during the gas heating in the reactor. This means that the hydrates dissociated completely into supercooled water and gas along the supercooled water-hydrate-gas equilibrium line in the temperature range of 268−272 K. Figure 7 presents the data on pressure changing in the reactor and the thermogram for propane hydrate sample during the depressurization from 200 to 40 kPa with a rate of 15 kPa/min at a constant ambient temperature of 267.8 K. The studied hydrate sample contained the inclusions of unreacted water in the form of supercooled liquid. A pressure of 40 kPa was lower than the ice-hydrate-gas equilibrium pressure (135 kPa) [21,22] and also this pressure was lower than the supercooled water-hydrate-gas metastable equilibrium pressure (60 kPa) [14] at 267.8 K. During the pressure decrease, it was observed that the sample temperature remained constant until the pressure was above 60 kPa. Immediately after the pressure reached 60 kPa, the endothermic peak appeared in the thermogram that pointed to hydrate dissocia-
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tion into supercooled water and gas. After stop of gas evacuating (marked by an arrow in Figure 7), the pressure in the reactor increased and reached supercooled water-hydrate-gas metastable equilibrium pressure (60 kPa) after 4 h exposure at 267.8 K (this was not shown in Figure 7). It follows from data presented in Figure 7 that the hydrate sample contained the inclusions of unreacted supercooled water at 267.8 K dissociated during the depressurization as soon as the pressure decreased below the supercooled water-hydrate-gas metastable equilibrium pressure. The sample dissociated into supercooled water and gas.
Figure 7. Pressure changing in the reactor and the thermogram for the propane hydrate sample contained unreacted liquid water (D h = 0.85) during depressurization (ambient temperature is 267.8 K) and the follow keeping at isothermal conditions: (1) pressure, (2) D T , (3) equilibrium pressure of propane hydrate dissociation into ice and gas at 267.8 K, (4) pressure of the supercooled water-propane hydrate-gas metastable equilibrium at 267.8 K [14]. The moment of depressurization stop is marked by an arrow. Content of Aerosil in “dry water” is 10 wt%
4. Conclusions These studies have shown that the dissociation mechanism of gas hydrate samples below the ice melting temperature depends on the phase state of unreacted water in the sample: (1) Gas hydrate dissociates into ice and gas if the sample contains the inclusions of unreacted water in the form of ice. (2) Propane hydrate samples contained inclusions of metastable liquid water below 273 K (but not contained ice inclusions) can remain stable at the pressure below hydrate dissociation into ice and gas but above the pressure of hydrate dissociation into supercooled water and gas. However the gas hydrates dissociate into liquid water and gas at the pressure below the supercooled water-hydrate-gas metastable equilibrium pressure. Data presented in the paper are obtained for gas hydrate formed in a highly dispersed water system which is “dry water”. In future, we are going to obtain evidence of the phase state effect of unreacted residual water on the mechanism of gas hydrate dissociation for bulk gas hydrate.
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Acknowledgements The present work was supported by the Council for Grants of the President of the Russian Federation (grant NSh 3929.2014.5), the Basic Research Programs of the RAS (program no. 8, and program the Arctic) and the Siberian branch of the RAS (interdisciplinary project no. 144).
References [1] Binks B P, Murakami R. Nat Mater, 2006, 5(11): 865 [2] Podenko L S, Nesterov A N, Komisarova N S, Shalamov V V, Reshetnikov A M, Larionov E G. J Appl Spectrosc, 2011, 78(2): 260 [3] Wang W X, Bray C L, Adams D J, Cooper A I. J Am Chem Soc, 2008, 130(35): 11608 [4] Carter B O, Wang W X, Adams D J, Cooper A I. Langmuir, 2010, 26(5): 3186 [5] Hu G W, Ye Y G, Lin C L, Meng Q G, Zhang J, Diao S B. Fuel Process Technol, 2011, 92(8): 1617 [6] Ildyakov A V, Larionov E G, Manakov A Yu, Fomin V M. Gazokhimiya (in Russian), 2011, 17(1): 28 [7] Fan S S, Yang L, Wang Y H, Lang X M, Wen Y G, Lou X. Chem Eng Sci, 2014, 106: 53 [8] Horiguchi K, Watanabe S, Moriya H, Nakai S, Yoshimitsu A, Taoda A. In: Proc of the 7th Int Conf on Gas Hydrates, Edinburgh, 2011 [9] Rehder G, Eckl R, Elfgen M, Falenty A, Hamann R, Kahler N, Kuhs W F, Osterkamp H, Windmeier C. Energies, 2012, 5(7): 2499
[10] Davidson D W, Garg S K, Gough S R, Handa Y P, Ratcliffe C I, Ripmeester J A, Tse J S, Lawson W F. Geochim Cosmochim Acta, 1986, 50(4): 619 [11] Yakushev V S, Istomin V A. In: Maeno N, Hondoh T eds. Physics and Chemistry of Ice, Sapporo: Hokkaido University Press, 1992. 136 [12] Stern L A, Circone S, Kirby S H, Durham W B. J Phys Chem B, 2001, 105(9): 1756 [13] Takeya S, Shimada W, Kamata Y, Ebinuma T, Uchida T, Nagao J, Narita H. J Phys Chem A, 2001, 105(42): 9756 [14] Mel’nikov V P, Nesterov A N, Reshetnikov A M. Dokl Phys Chem, 2007, 417(2): 304 [15] Melnikov V P, Nesterov A N, Reshetnikov A M, Zavodovsky A G. Chem Eng Sci, 2009, 64(6): 1160 [16] Ohno H, Oyabu I, Iizuka Y, Hondoh T, Narita H, Nagao J. J Phys Chem A, 2011, 115(32): 8889 [17] Vlasov V A, Zavodovsky A G, Madygulov M S, Nesterov A N, Reshetnikov A M. Russ J Phys Chem A, 2013, 87(11): 1789 [18] Giavarini C, Maccioni F. Ind Eng Chem Res, 2004, 43(20): 6616 [19] Melnikov V P, Nesterov A N, Reshetnikov A M, Istomin V A, Kwon V G. Chem Eng Sci, 2010, 65(2): 906 [20] Podenko L S, Nesterov A N, Drachuk A O, Molokitina N S, Reshetnikov A M. Russ J Appl Chem, 2013, 86(10): 1509 [21] Istomin V A, Yakushev V S. Gas Hydrates in Natural Conditions. Moscow: Nedra, 1992. 236 (in Russian) [22] Sloan E D. Clathrate Hydrates of Natural Gases. 2nd ed. New York: Marcel Dekker, 1998. 705 [23] Melnikov V P, Nesterov A N, Reshetnikov A M. Gas Industry, Special Issue “Gas Hydrate”(in Russian), 2006: 55 [24] Takeya S, Ripmeester J A. Angew Chem Int Ed, 2008, 47(7): 1276