Characteristics of the Stability and Decomposition of Metastable Gas Hydrates
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ICAL THERMODYNAMICS AND THERMOCHEMISTRY
Characteristics of the Stability and Decomposition of Metastable Gas Hydrates A. G. Zavodovskiia,*, M. Sh. Madygulova, and A. M. Reshetnikova a Institute
of the Earth’s Cryosphere, Tyumen Scientific Center, Siberian Branch, Russian Academy of Sciences, Tyumen, 625026 Russia *e-mail: [email protected]
Received November 19, 2019; revised March 11, 2020; accepted March 17, 2020
Abstract—Results are presented from studying the metastable states of Freon-12 gas hydrate via pulsed NMR in the range of thermodynamic parameters limited by the line of ice–hydrate–gas and supercooled water– hydrate–gas equilibrium in the P–T phase diagram. The dependence is found for the lifetime of metastable Freon-12 gas hydrate on a sample’s temperature and degree of hydrate formation. New data on the conditions of the stability and decomposition of the metastable states of the given gas hydrate are obtained using samples with 100% conversion of water to the former. It is found that the crystallization of water formed upon the dissociation of Freon-12 gas hydrate into supercooled water and gas alters the mechanism of dissociation. A scheme for the formation of water/ice upon the dissociation of gas hydrates is proposed to explain the experimental data. Keywords: gas hydrates, metastable states, lifetime, dissociation, NMR, supercooled water DOI: 10.1134/S0036024420100337
INTRODUCTION The development of new and improved technologies for conserving natural gas hydrates drives the search for different mechanisms of slowing the process of their decomposition. The effect of self-conservation [1, 2], associated with the formation of an ice coating on the surface of a dissociating gas hydrate at temperatures below 273 K [3], is currently the one most studied. The nature of this effect and the mechanisms of the dissociation of different gas hydrates were examined thoroughly in [4, 5]. Compared to the ice that forms upon the dissociation of a gas hydrate into ice and gas, better shielding properties are displayed by the ice that forms as a result of the crystallization of supercooled water on the surface of a gas hydrate [6, 7]. It can form in particular upon the decomposition of a gas hydrate into water and gas at temperatures below 273 K and pressures below the line of supercooled water–hydrate–gas phase equilibrium, as has been confirmed for gas hydrates with CS-I and CS-II structures [7–14] by different experimental means, including NMR [11–13]. A delay in the dissociation of gas hydrates with the formation of ice and gas in the region of the states limited by the lines of ice–hydrate–gas and supercooled water–hydrate–gas equilibrium in the P–T phase diagram was observed during the above studies. It was found that the gas hydrates were in this case in the metastable states achieved when there is water inside a
sample in the form of a supercooled fluid that does not transform into gas hydrate. When ice is present in a sample, gas hydrates dissociate in turn in the region of metastable states to form i
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