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Experimental Investigation on CyclopentaneMethane Hydrate Formation Kinetics in Brine Xueru Zang, Qiunan Lv, Xiao-Sen Li, and Gang Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02435 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016
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Energy & Fuels
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Experimental Investigation on Cyclopentane-Methane
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Hydrate Formation Kinetics in Brine
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Xueru Zang†,‡,§,∥, Qiunan Lv†,‡,§, Xiaosen Li*,†,‡,§, Gang Li†,‡,§
∥
4
†
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of Sciences, Guangzhou 510640, PR China
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‡
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Academy of Sciences, Guangzhou 510640, PR China
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§
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PR China
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Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy
Guangdong Key Laboratory of New and Renewable Energy Research and Development, Chinese
Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640,
∥
Nano Science and Technology Institute, University of Science and Technology of China, Suzhou
215123, PR China
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ABSTRACT: Based on the hot brine in situ seafloor prepared for marine NGHs
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exploitation, formation kinetic behavior of cyclopentane (CP)- methane hydrate was studied
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at various depths of ocean water. The effects of the temperature, pressure, gas-liquid ratio and
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ratio of CP/ liquid phase on gas uptake were investigated to reveal the affecting factors of the
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hydrate rapid formation. The experimental results indicated that the driving force played an
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important role in the hydrate formation. In addition, the CP/liquid phase ratio of 5% was
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beneficial to gas uptake. When the conditions of driving force and the CP/liquid phase ratio
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were very favorable for hydrate formation, the gas uptakes slightly changes with the
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gas-liquid ratio. On the contrary, a smaller gas-liquid ratio was conductive to gas uptake.
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1. INTRODUCTION
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Gas hydrates are solid crystals formed from water and small gas molecules at low
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temperature and high pressure. Water molecules are linked through hydrogen bonding and
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they create cavities (host lattice) that can enclose a large variety of molecules (guests). There
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is no chemical bonding between the host water molecules and the enclosed guest molecules.1,2
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The characteristics of gas hydrates lie in that their empty hydrate lattice is just like an efficient
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gas storage, which is evidenced by the fact that 1 m3 methane hydrates can be decomposed to
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160−180 m3 of methane gas. Hydrates have been detected in marine and permafrost region
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throughout the world.3 Natural gas hydrates (NGHs) have the potential to meet the global
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energy needs in the future. Therefore, the NGHs have been attracting considerable attention in
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development of methods for producing natural gas from hydrates.
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However, exploiting NGHs in safety and high efficiency is the key issue.4 The major
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methods of gas production from NGHs reservoirs are depressurization, thermal stimulation
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and chemical injection.5, 6 Each of these methods has its own advantages and disadvantages.
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The depressurization method is demonstrated to be a high energy-efficient when the NGH
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deposits are in conjunction with free-gas zone.7-9 However, it will not likely provide a steady,
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reliable gas production. The faster gas production rate is seen in initial stage, the local
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temperature will continuously drop while NGHs dissociate, and the dissociation of NGHs
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might stop once the local temperature drops to the equilibrium temperature at a certain
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pressure.10 The thermal injection method could effectively avoid the problems that emerged in
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the depressurization process11-14, such as the low rate of gas production, blocking caused by
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secondary hydrate formation or ice formation.15 However, the technical difficulty of thermal
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stimulation method is that the heat would diffuse before it reaches the decomposition zone.
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The heat loss leads to low efficiency of NGHs exploitation system.16 Among the three
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methods, the chemical injection is considered to be a highly effective method for hydrate
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dissociation,17 but it has the defects of large reagent consumption and high cost. And
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large-scale use of chemicals will cause environmental pollution.18 In addition, Ohgaki et al19
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proposed CH4-CO2 replacement in NGHs. This method has the problem that replacement rate
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is extremely low and the CO2 hydrate will cover on the surface of NGHs, which leads to heat
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and mass transfer barriers during the decomposition.
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In this paper, we developed a novel technique to prepare hot brine in situ seafloor for
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marine NGHs exploitation based on hydrate technology.20 The basic idea of this method is
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described as follows: an apparatus is installed into situ seafloor, the apparatus is extended
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from the surface of the sea to the seafloor, by adding hydrate formation agent into the
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apparatus, hydrate forms rapidly at the conditions (pressure and temperature) in the seafloor.
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Due to a lot heat released in the process of hydrate formation, the unreacted brine is
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concentrated and heated to hot brine. Then, the hot brine prepared is expelled from the
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apparatus and injected into NGH deposits for exploitation. The solid hydrates float up to the
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surface by its own buoyancy. Then it decomposes into water and the hydrate formation agent
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at the temperature and pressure of sea surface. The hydrate formation agent can be recycled
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due to its insoluble in water. This method combines thermal stimulation and chemical
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injection and avoids heat loss during transiting the heat carrying medium from the ocean
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surface to the NGH regions in conventional thermal stimulating methods. It is very important
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to develop an economical, efficient technology of hydrate-forming by utilizing appropriate
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hydrate formation agent. The appropriate hydrate formation agent must meet the following
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requirements: non-toxic or low toxic and insoluble with seawater, the density of hydrate
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formation agent is lower than that of brine so as to facilitate its recovery and recycling. Yan et
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al.21 proved that CP is an effective hydrate formation additive that can accelerate gas hydrate
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formation, and the hydrate formation enthalpy of CP - methane is as high as 130.25 kJ·mol−1
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at 285 K. Therefore, in this study, CP is considered as appropriate hydrate formation promoter.
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Recent years, there are many researchers studied the hydrate formation and dissociation
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kinetics in fresh water and NaCl solution or seawater.11,22-27 The investigation into gas hydrate
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formation of methane in a large-size bubble column reactor was carried out in the presence of
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CP11. Kerkar et al.22 investigated the influence of pressure on methane hydrate formation and
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dissociation kinetics in seawater. The kinetics of the CP–methane binary hydrate growth in
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fresh water and brine solutions were studied by Cai.23,
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formation rate in salt water was 2–3 times smaller than that in fresh water at small subcooling
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degree (∆T
