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Author's Accepted Manuscript Study of gas hydrate metastability and its decay for hydrate samples containing unreacted supercooled liquid water below the ice melting point using pulse NMR Marat Sh Madygulov, Anatoliy N Nesterov, Alexey M Reshetnikov, Valeriy A Vlasov, Alexey G Zavodovsky www.elsevier.com/locate/ces PII: DOI: Reference: S0009-2509(15)00447-9 http://dx.doi.org/10.1016/j.ces.2015.06.039 CES12442 To appear in: Chemical Engineering Science Received date: 24 April 2015 Revised date: June 2015 Accepted date: 12 June 2015 Cite this article as: Marat Sh Madygulov, Anatoliy N Nesterov, Alexey M Reshetnikov, Valeriy A Vlasov, Alexey G Zavodovsky, Study of gas hydrate metastability and its decay for hydrate samples containing unreacted supercooled liquid water below the ice melting point using pulse NMR, Chemical Engineering Science, http://dx.doi.org/10.1016/j.ces.2015.06.039 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain Title Study of gas hydrate metastability and its decay for hydrate samples containing unreacted supercooled liquid water below the ice melting point using pulse NMR Authors Marat Sh Madygulov a,b , Anatoliy N Nesterov a,b,∗ , Alexey M Reshetnikov a,b , Valeriy A Vlasov a,∗∗, Alexey G Zavodovsky a Affiliations a Institute of the Earth Cryosphere, Siberian Branch of the Russian Academy of Sciences, P.O Box 1230, 625000 Tyumen, Russian Federation b Tyumen State Oil and Gas University, Volodarskogo Str 38, 625000 Tyumen, Russian Federation Corresponding authors ∗ Corresponding author at: Institute of the Earth Cryosphere, SB RAS, P.O Box 1230, 625000 Tyumen, Russian Federation Tel.: +7 3452 68 87 22; fax: +7 3452 68 87 87 ∗∗ Corresponding author E-mail addresses: nesterov@ikz.ru (A.N Nesterov), vlasov.ikz@gmail.com (V.A Vlasov) The behaviour of bulk Freon-12 hydrate samples containing inclusions of unreacted liquid water below the ice melting point at pressures below the hydrate–ice–gas equilibrium pressure has been studied using nuclear magnetic resonance (NMR) spectroscopy The amount of liquid water in the samples was directly measured by transverse relaxation measurements of the hydrogen nucleus of the system studied Evidence of the long-lived existence of gas hydrate as a metastable phase and hydrate dissociation into supercooled water and gas has been presented It was shown that the dissociation of the bulk metastable hydrate into supercooled water and gas was reversible An influence of the phase state of the unreacted water in the samples on the stability of the metastable hydrate and mechanism of hydrate dissociation was revealed: ice crystallization led to the decay of the hydrate metastability and the hydrate dissociation into ice and gas Keywords: Gas hydrate; Metastability; Phase equilibria; NMR; Kinetics; Supercooled water Introduction It was previously shown that gas hydrates dissociated into ice and gas below the ice melting point at an anomalously low rate (Davidson et al., 1986; Stern et al., 2001; Takeya et al., 2001; Yakushev and Istomin, 1992) The phenomenon was named the self-preservation effect (Yakushev and Istomin, 1992) and is also known as the anomalous preservation regime (Stern et al., 2001) The gas hydrate self-preservation effect is of considerable academic interest and can have commercial applications for the temporary storage and transportation of natural gas in the hydrate form (Gudmundsson et al., 1994; Horiguchi et al., 2011; Rehder et al., 2012) The self-preservation effect was studied by numerous researchers including in recent investigations (Falenty et al., 2014; Kida et al., 2014; Mimachi et al., 2014; Nakoryakov and Misyura, 2013; Sato et al., 2013; Stoporev et al., 2014) Nevertheless, the mechanism of this phenomenon is still not fully understood Istomin et al (2006) supposed that gas hydrate dissociation below the ice melting point could proceed through the formation of an intermediate metastable phase of supercooled water In the authors’ opinion, this water forms a continuous ice coating on the hydrate surface that hinders further hydrate dissociation Thermodynamic calculations (Istomin et al 2006) showed that the hydrate dissociated into supercooled water and gas at pressures below the ice– hydrate–gas (I–H–G) equilibrium pressure For the first time, reliable experimental evidence on gas hydrate dissociation into supercooled water and gas below the ice melting point as well as pressure-temperature (p–Т) data on the supercooled water–hydrate–gas (Lsw–H– G) metastable equilibrium were obtained for propane hydrate (Mel’nikov et al., 2003, 2007) Subsequently, the possibility of hydrate dissociation into supercooled water and gas was confirmed for the hydrates of methane, ethane, carbon dioxide, and Freon-12 (CCl2F2) using optical microscopy observations (Melnikov et al., 2009, 2010, 2011), Raman (Ohno et al., 2011) and pulsed NMR (Melnikov et al., 2012, Vlasov et al., 2013) spectroscopy, and differential thermal analysis (DTA) (Drachuk et al., 2015; Melnikov et al., 2015) Hence, we can conclude that the possibility gas hydrates dissociating below the ice melting point into supercooled water and gas is a universal property common to gas hydrates This property follows from the water–gas phase diagram shown in Fig Ice is the stable phase below the I–H–G equilibrium pressure However, the transition into a stable state that is predicted from thermodynamics may be delayed, and the real system behaviour will be determined by the transition kinetics In Fig 1, the Lsw–H–G metastable equilibrium line is in actuality an extension of the liquid water–hydrate–gas (Lw–H–G) equilibrium line into the temperature region below 273 K in the p–T phase diagram (Mel’nikov et al., 2007; Melnikov et al., 2009, 2010, 2011; Ohno et al., 2011; Vlasov et al., 2013) The region in the p–T phase diagram between the I–H–G equilibrium line and the Lsw– H–G metastable equilibrium line is the hydrate metastability region In this region, hydrates may exist as a long-lived phase in the metastable state without dissociation into ice and gas (Melnikov et al., 2010) A metastable state corresponds to a local minimum of the system free energy at given external conditions (Skripov, 1974) The depth of the minimum determines the stability and lifetime of the metastable state; the deeper the minimum is, the more stable is the metastable state and the longer is its lifetime Metastable states often occur in nature, and their lifetimes may change over a wide range and exceed any possible time periods of the investigation of the phases in their metastable state (Brazhkin, 2006) For example, metastable diamond at room temperature can live virtually infinitely Melnikov et al (2010) observed the existence of methane hydrate in the region of its metastability at 268 K without any evidence of the hydrate dissociation into ice and gas over at least 14 days Longer experiments were not performed, but when the pressure was decreased below the pressure of the Lsw–H–G metastable equilibrium, the hydrate dissociated into supercooled water and gas without any delay It should be recognized that the dissociation of a gas hydrate into supercooled water and gas below the ice melting point is a rare case rather than a common rule (Adichtchev et al., 2013; Falenty and Kuhs, 2009) Nevertheless, to gain better insight into the mechanism of the unusual persistence of gas hydrates outside their stable region below the ice melting point, it needs to be ascertained why at times hydrates dissociate into supercooled water and gas and other times they dissociate into ice and gas Recently, Drachuk et al (2015) and Melnikov et al (2015) found that the mechanism of gas hydrates dissociation below the ice melting point depends on the phase state of the residual water that did not transform into a hydrate Using the DTA method, they observed that dispersed hydrates formed from individual water droplets of a size of approximately μm, and their assemblies of up to 40 μm in the dispersion “dry water” always dissociated below 273 K into ice and gas if the hydrate samples contained inclusions of unreacted water in the form of ice If the residual water in the samples was in the form of supercooled water, hydrates could stably exist in the region of their metastability without dissociation into ice or supercooled water and gas at the pressure just below the pressure of the Lsw–H–G metastable equilibrium The purpose of this study is to examine the behaviour of bulk gas hydrates containing residual unreacted water in the hydrate metastability region and obtain more insight into the dissociation mechanism of bulk hydrates below the ice melting point For this, bulk Freon-12 hydrate samples and pulsed NMR were used to provide direct measurements of the amount of liquid water in the samples Experimental section 2.1 Experimental apparatus Fig shows the scheme of the experimental setup used in this work The main elements of the setup were a thin-walled glass reactor with an external diameter of 10 mm and a Bruker Minispec-mq pulsed NMR relaxometer operating at a 1H resonance frequency of 19.65 MHz The maximum allowable working pressure for the glass reactor was 0.3 MPa Because of this, Freon-12 was used to prepare the hydrate samples for the NMR measurements Freon-12 hydrate has the stoichiometric formula CCl2F2·17H2O and can be formed at low pressure In addition, Freon-12 molecules not contain hydrogen atoms, simplifying the 1H NMR analysis The necessary pressure in the reactor was maintained by the supply of additional gas from a cylinder with liquefied Freon-12 or the evacuation of gas from the reactor by a vacuum pump The pressure in the reactor was measured with an accuracy of ±1.5 kPa Freon-12 destroys atmospheric ozone, and because of this, the gas evacuated from the reactor was collected in a receiver for further utilization For NMR measurements, the reactor with the hydrate sample placed in the NMR probe was cooled by a stream of nitrogen gas A Bruker BVT 3000 temperature unit was used to control the temperature of the cooling gas stream with an accuracy of ±0.2 K 2.2 Procedures of hydrate formation and NMR measurements The Freon-12 hydrate formation procedure was described elsewhere (Vlasov et al., 2013) Briefly, powdered ice with a particle size of 0.1–0.2 mm and Freon-12 gas (99.7 mol.%) were used to form a hydrate Approximately 300 mg of powdered ice, weighed on an analytical balance with an accuracy of ±1 mg, was loaded into the reactor After loading with ice, the reactor was immersed into a refrigerated circulator kept at 263 K and was evacuated and charged with Freon12 gas to the required pressure Then, the reactor was slowly heated to 275 K to melt the ice and cooled again to 263 K to freeze the unreacted water The and the Lsw–H–G metastable equilibrium line has been studied It was found that the behaviour of bulk hydrates is the same as that previously observed for dispersed hydrates formed from single water droplets or in the dispersion “dry water” In the region of hydrate metastability, if ice is absent from the samples, the hydrate persistence may be caused by the kinetic difficulty of the ice nucleation but not the self-preservation effect Nucleation of ice in the system leads to the decay of the hydrate metastability and the dissociation of hydrate into ice and gas Therein lies one of the distinctions between metastable hydrates and hydrates that persist due to the self-preservation effect Below the pressure of the Lsw–H–G metastable equilibrium, bulk metastable hydrates dissociate into supercooled water and gas without any delay The dissociation of metastable hydrates into supercooled water and gas is reversible, and hydrates may grow from supercooled water and gas even below the I–H–G equilibrium pressure; however, self-preserved hydrates below the I–H–G equilibrium pressure slowly and irreversibly dissociate into ice and gas This is another distinction between the hydrate metastability and self-preservation 18 Acknowledgements This work was supported by the Presidium of the Russian Academy of Sciences (basic research program “Exploratory Basic Scientific Research for Development of the Arctic Zone of the Russian Federation”), the Siberian Branch of the Russian Academy of Sciences (interdisciplinary project no 144) and the grant of the President of the Russian Federation for leading scientific schools (no NSh-3929.2014.5) One of us (M.Sh Madygulov) is grateful to the Russian Foundation for Basic Research for the 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Mimachi, H., Kinoshita, T., Iwasaki, T., Sano, K., Ohgaki, K., 2013 Self-preservation of methane hydrate revealed immediately below the eutectic temperature of the mother electrolyte solution Chemical Engineering Science 91, 86–89 Skripov, V.P., 1974 Metastable Liquids Wiley, New York 24 Sloan, E.D., Koh, C.A., 2008 Clathrate Hydrates of Natural Gases, third ed CRS Press and Taylor & Francis Group, Boca Raton Stern, L.A., Kirby, S.H., Durham, W.B., 1996 Peculiarities of methane clathrate hydrate formation and solid-state deformation, including possible superheating of water ice Science 273, 1843–1848 Stern, L.A., Circone, S., Kirby, S.H., Durham, W.B., 2001 Anomalous preservation of pure methane hydrate at atm The Journal of Physical Chemistry B 105, 1756–1762 Stoporev, A.S., Manakov, A.Yu., Altunina, L.K., Bogoslovsky, A.V., Strelets, L.A., Aladko, E.Ya., 2014 Unusual self-preservation of methane hydrate in oil suspensions Energy & Fuels 28, 794–802 Takeya, S., Shimada, W., Kamata, Y., Ebinuma, T., Uchida, T., Nagao, J., Narita, H., 2001 In situ X-ray diffraction measurements of the self-preservation effect of CH4 hydrate The Journal of Physical Chemistry A 105, 9756–9759 Vlasov, V.A., 2013 Formation and dissociation of gas hydrate in terms of chemical kinetics Reaction Kinetics, Mechanisms and Catalysis 110, 5–13 25 Vlasov, V.A., Zavodovsky, A.G., Madygulov, M.Sh., Nesterov, A.N., Reshetnikov, A.M., 2013 Pulsed NMR investigation of the supercooled water–gas hydrate–gas metastable equilibrium Russian Journal of Physical Chemistry A 87, 1789–1792 Watanabe, K., Wake, T., 2009 Measurement of unfrozen water content and relative permittivity of frozen unsaturated soil using NMR and TDR Cold Regions Science and Technology 59, 34–41 Yakushev, V.S., Istomin, V.A., 1992 Gas hydrate self-preservation effect, in: Maeno, N., Hondoh, T (Eds.), Physics and Chemistry of Ice Hokkaido University Press, Sapporo, pp 136–140 26 p I–H–G L Q w– H– G H+G Lw + G hydrate metastability G H– – region L sw I + G or Lsw + G T Fig Hydrate metastability region in the p–T phase diagram for a H2O–gas system Q is the quadruple point where liquid water, ice, hydrate and gas coexist at equilibrium Fig Scheme of the setup 27 Echo amplitude, a.u 40 echo time = 0.2 ms number of echoes = 20000 number of scans = 30 20 10 0 100 200 300 400 500 Time, ms Fig CPMG signal for the H2O–Freon-12 system 155 rapid depressurization slow cooling p, kPa 80 Q I–Lw–G A B start hydrate metastability region 266 270 274 278 T, K Fig Diagram of experimental conditions for the NMR study on Freon-12 hydrate metastability in relation to the p–T phase diagram (Vlasov et al., 2013) 28 23 46 msw, mg 40 19 pmeq 34 28 17 p, kPa 21 p 15 msw 22 13 20 40 60 80 100 120 t, Fig Mass of supercooled water msw in the sample during the growth/dissociation of metastable Freon-12 hydrate at 270 K and different pressures in the reactor as a function of time t The pressure of the Lsw–H–G metastable equilibrium pmeq is taken from the paper by Vlasov et al (2013) dmsw/dt, mg/min 2.0 experimental data 1.5 linear approximation 1.0 0.5 0.0 ∆p, kPa Fig Change of the dissociation rate of metastable Freon-12 hydrate with the driving force of the hydrate dissociation at 270 K 29 hydrate growth p eq msw 40 28 ice crystallization 20 30 p p meq p, kPa msw, mg 60 26 24 20 40 60 80 100 t, Fig Mass of supercooled water in the sample and pressure in the reactor during the Freon-12 hydrate growth/dissociation and ice crystallization at 272 K as functions of time The pressure of the I–H–G equilibrium peq and the pressure pmeq are taken from the paper by Vlasov et al (2013) 30 Graphical abstract 31 • Behaviour of metastable bulk gas hydrate samples below 273 K was studied by H NMR • Metastable hydrate can contain residual supercooled water but does not contain ice • Dissociation of metastable hydrate into supercooled water and gas is reversible • Ice crystallization leads to the decay of the hydrate metastability 32 [...]... lies one of the distinctions between metastable hydrates and hydrates that persist due to the self-preservation effect Below the pressure of the Lsw–H–G metastable equilibrium, bulk metastable hydrates dissociate into supercooled water and gas without any delay The dissociation of metastable hydrates into supercooled water and gas is reversible, and hydrates may grow from supercooled water and gas... unreacted 20 residual water on a mechanism of gas hydrates dissociation Journal of Energy Chemistry 24, 309–314 Falenty, A., Kuhs, W.F., 2009 “Self-preservation” of CO2 gas hydrates surface microstructure and ice perfection The Journal of Physical Chemistry B 113, 15975–15988 Falenty, A., Kuhs, W.F., Glockzin, M., Rehder, G., 2014 “Self-preservation” of CH4 hydrates for gas transport technology: Pressure–temperature... into ice and gas without any delay (Drachuk et al., 2015; Melnikov et al., 2015) 3 Results and discussion 3.1 Reversible dissociation of metastable hydrates into supercooled water and gas It was previously shown using optical spectroscopy that metastable hydrates could grow on the surface of single small droplets of supercooled liquid water or dissociate into supercooled water and gas below the pressure... region in the p–T phase diagram between the I–H–G equilibrium line 17 and the Lsw–H–G metastable equilibrium line has been studied It was found that the behaviour of bulk hydrates is the same as that previously observed for dispersed hydrates formed from single water droplets or in the dispersion “dry water” In the region of hydrate metastability, if ice is absent from the samples, the hydrate persistence... Proceedings of the 7th International Conference on Gas Hydrates July 17-21, Edinburgh, Scotland Ishizaki, T., Maruyama, M., Furukawa, Y., Dash, J.G., 1996 Premelting of ice in porous silica glass Journal of Crystal Growth 163, 455–460 21 Istomin, V.A., Yakushev, V.S., Makhonina, N.A., Kwon, V.G., Chuvilin, E.M., 2006 Self-preservation phenomenon of gas hydrates Gas Industry of Russia 4, 16– 27 Kida, M.,... Engineering Science 42, 1645–1653 Makogon, Y.F., 1981 Hydrates of Natural Gas PennWell, Tulsa Mel’nikov, V.P., Nesterov, A.N., Reshetnikov, A.M., 2003 Mechanism of gas hydrate decomposition at a pressure of 0.1 MPa Doklady Earth Sciences 389, 455–458 Mel’nikov, V.P., Nesterov, A.N., Reshetnikov, A.M., 2007 Formation of supercooled water upon dissociation of propane hydrates at T < 273 K Doklady Physical Chemistry... Nesterov, A.N., Reshetnikov, A.M., Zavodovsky, A.G., 2009 Evidence of liquid water formation during methane hydrates dissociation below the ice point Chemical Engineering Science 64, 1160–1166 22 Melnikov, V.P., Nesterov, A.N., Reshetnikov, A.M., Istomin, V.A., Kwon, V.G., 2010 Stability and growth of gas hydrates below the ice–hydrate–gas equilibrium line on the P–T phase diagram Chemical Engineering Science... equality of pressures, temperatures and chemical potentials in all phases From the data presented in Fig 5, it follows that the rates of the dissociation (dmsw/dt) and growth (–dmsw/dt) of metastable hydrates at constant pressure and temperature do not change with time and depend on ∆p = pmeq – p, where p is the pressure in the reactor Here, ∆p is known as the driving force of hydrate formation/dissociation... technique After finishing the hydrate sample preparation, the reactor with the sample was transferred from the refrigerated circulator into the NMR relaxometer The NMR experiments on study of metastable hydrates were carried out at a temperature not lower than 270 K At a lower temperature, the supercooled water in the sample solidified very quickly, transforming into ice, and we were not able to carry... dissociation of metastable hydrate A constant pressure in the reactor was maintained during the hydrate dissociation by the evacuation of gas from the reactor 13 Hydrate dissociation or the growth of metastable hydrates was observed each time below or above the pressure pmeq (Fig 5), respectively, highlighting the reversibility of the dissociation of the metastable hydrate into supercooled water and gas At pressures