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Metastable State of Gas Hydrate During Decomposition: a Novel Phenomenon

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Metastable State of Gas Hydrate During Decomposition: a Novel Phenomenon View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Enlighten Chinese Journal of Chemical Engineering 28 (2020) 949–954 Contents lists available at ScienceDirect Chinese Journal of Chemical Engineering journal homepage: www.elsevier.com/locate/CJChE Review Metastable state of gas hydrate during decomposition: A novel phenomenon Ronghui Sun 1,ZhenFan2,LeiYang1,⁎, Yuanping Li 3, Xin Lü 4, Yang Miao 5 1 Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian 116023, China 2 School of Chemistry, University of Glasgow, Glasgow G12 8QQ, United Kingdom 3 Institute of Science and Technology, Shenzhen Branch, China National Offshore Oil Corporation, Guangzhou 510420, China 4 CNOOC Research Institute Co. Ltd, Beijing 100028, China 5 Xingang Shipbuilding Heavy Industry Co., Ltd., Tianjin 300452, China article info abstract Article history: Natural gas hydrates are solid compounds with cage-like structures formed by gas and water. An intriguing phenom- Received 6 December 2018 enonthatgashydratescandissociateatalowratebelowtheicefreezingpointhasbeenviewedasthemetastability Received in revised form 26 September 2019 of hydrate. The mechanisms of hydrate metastability have been widely studied, and many mechanisms were pro- Accepted 7 February 2020 posed involving the self-preservation effect, supercooled water-gas-hydrate metastable equilibrium, and supersatu- Available online 19 February 2020 rated liquid–gas-hydrate system etc. The metastable state of hydrate could be of crucial significance in the kinetics of hydrate formation and decomposition, heat and mass transfer during gas production processes, and the application Keywords: Hydrates of hydrate-based technique involving desalination, energy storage and transportation, and gas separation and se- Dissociation questration. Few researches have systematically considered this phenomenon, and its mechanism remains unclear. Metastability In this work, various mechanisms and hypothesis explaining the metastable state of gas hydrates were introduced Self-preservation and discussed. Further studies are still required to reveal the intrinsic nature of this metastable state of gas hydrate, Supersaturation and this work could give some implications on the existing theory and current status of relevant efforts. © 2020 The Authors. The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). 1. Introduction can produce 164 volumes of natural gas [12]. Many countries have pro- posed national strategies on gas production from NGH as well as their sig- Natural gas hydrates (NGHs) are non-stoichiometry compounds with nificant environmental aspects. The U.S. have been a leading country in hydrogen-bonded water cages acting as host and gas molecules as guest the development of gas hydrate and have taken legislative approach to se- [1–3]. The guest gas molecules involve methane, ethane, propane, and cure NGH as a crucial energy resource [13]. Japan is one of the countries carbon dioxide, etc., among which methane attracts much attention due that approved a national program on NGH firstly and a three-phase- to its potential in energy source and gas storage [4–6].Therearethreedif- program the “Research Consortium for Methane Hydrate Resources in ferent clathrate structures of gas hydrates, namely two cubic structures s I Japan” was conducted [14]. China started programs on NGH in 2002; and s II, and one hexagonal structure s H. The clathrate structure was the first field test in China was conducted in 2017, producing natural found to be dependent upon the size of the guest molecule [7]. Gas hy- gas for 60 successive days [15]. drates were first observed by Priestley in 1778 and documented by Hydrate has also been widely used in various industry-related tech- Davy in 1811 [8]. Since the year of 1934, the existence of natural gas hy- niques, involving storage and transportation of natural gas, sea water drates was found to be the trigger of blockages in pipelines during the desalination, cold energy storage, gas separation etc. [16–20]. The up- transportation of natural gas [9].Thediscoveryoflargeamountofhydrate scale and commercial application of these techniques suffer from the reservoirs in the permafrost and under the continental shelf during the sluggish formation process, insufficient transformation and limited ca- period from the mid-1960 till present made NGH one of the most impor- pacity. These make the thermodynamics and kinetics of hydrate forma- tant energy resources; to develop methods for commercial production of tion and dissociation of great significance [21]. Three main processes for natural gas from hydrate reservoirs is thus attracting considerable atten- hydrate dissociation (thermal stimulation, depressurization and inhibi- tion worldwide [10,11]. Natural gas hydrates are considered to contain al- tor injection) have been proposed [22–24]. The thermodynamic equilib- most 1/3 of world's mobile organic carbon and each volume of hydrate rium conditions of the hydrates govern the formation and dissociation process of hydrates [4,25,26]; thus, the determination and regulation fi ⁎ Corresponding author. of hydrate phase equilibria have always been a signi cant issue in hy- E-mail address: [email protected] (L. Yang). drate related study [27]. https://doi.org/10.1016/j.cjche.2020.02.003 1004-9541/© 2020 The Authors. The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 950 R. Sun et al. / Chinese Journal of Chemical Engineering 28 (2020) 949–954 In the phase diagram of hydrate, the equilibrium curve can be di- understandings of the role of ice [41]. Then, models were introduced vided into stable zone and unstable zone. Hydrates can only remain a to explain the self-preservation [41,42]. The effect of different factors thermodynamic equilibrated state when the pressure and temperature on self-preservation was investigated [43,44]. locate within the stable zone; otherwise the pressure and temperature • conditions in the unstable zone will trigger hydrate decomposition. The second stage (studies on the intermediate metastable phase However, an intriguing phenomenon has been observed during hydrate of ice): formation and dissociation; hydrate may not be generated even after a long period of time (induction period) in the stable zone [28]; hydrate This stage was from around 1990 till present, during which more at- may also decompose at an anomalously low rate in the unstable zone tentions were placed on the effect of metastable water/ice phase on the [29]. These phenomena are identified as a performance of metastability presence of self-preservation phenomenon. Re-crystallization was con- of hydrate. The metastable state was defined as “adefine equilibrium sidered to take place at the ice-hydrate interface, accompany mutual which is, nevertheless, not the most stable equilibrium at the given tem- hydrate-ice transformation. This phenomenon was described as a meta- perature” [30]. Jan Lubas proposed the “area of hydrate metastability; he stable two-phase hydrate-ice equilibrium [45]. However, queries still considered that the hydrate formation and dissociation curve may not exist on the dominant shielding effect of ice on the gas diffusion from coincide and the area between these curves is “the area of hydrate hydrates [46,47]. The annealing of stacking faults of the ice was not metastability” [31]. According to Skripov, a metastable state corre- well understood until the neutron diffraction and cryo-SEM studies sponds to a local minimum of the system free energy at given external were carried out [48,49]. conditions and the depth of the minimum determines the stability • and lifetime of the metastable state [32]. The third stage (more mechanisms of hydrate metastability): Hydrates may persist in a metastable state for millions of years under the seafloor or in the permafrost [33]. This indicates the signifi- This stage started from 2011 when a systematic and detailed classi- cant geological importance of hydrate stability as the metastable hy- fication of hydrate metastability was introduced by Istomin [50]. The drates may likely decompose upon a slight fluctuation of the ambient formation of supercooled liquid water during dissociation of hydrate conditions, which would possibly trigger a strata failure along the con- was for the first time detected [51–53]. A metastable superheated tinental margins thereby contributing to the greenhouse effect [34,35]. solid phase of methane hydrate was presented when studying the effect On the other hand, the metastability can also be applied in the gas stor- of bubbles on the kinetics of hydrate decomposition [54]. The enrich- age and transportation using hydrates and pressure regulation during ments of guest molecules in the water were not easily observed until gas production from hydrates [16,36–38]. the synchrotron micro-tomography with a sub-micron resolution was The development of researches on hydrate metastability during dis- used [55]. These enrichments provide evidences of the existence of sociation can be classified into three stages, as shown in Table. 1. gas in the form of nanobubbles [57]. • fi The rst stage
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