【作者】 李政；

【导师】 钟栋梁；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2020， 博士

【摘要】 页岩气是一种清洁的非常规天然气,主要成分为甲烷（CH₄）。采用超临界二氧化碳（CO₂）压裂技术开发页岩气能强化开采页岩气并实现CO₂地质封存,但采收的页岩气会与逸出的CO₂混合,因此为了提升页岩气品质,需将CO₂/CH₄混合气高效分离。采用气体水合物方法分离CO₂/CH₄混合气有良好的应用前景,具有反应条件温和、工艺流程简单、经济环保等显著优点。气体水合物是水分子与CH₄、CO₂等气体小分子在低温和高压条件下形成的固态笼型化合物。采用气体水合物法分离气体混合物是根据水分子与气体分子生成水合物难易程度不同的原理实现混合气体分离的新型气体分离技术。目前,国内外对气体水合物法分离CO₂/CH₄混合气的研究从早期的纯水体系发展到热力学促进剂体系,水合反应所需的苛刻操作条件进一步降低,但一级水合反应速率、水合物储气率以及水合物对CO₂气体选择性等关键参数仍无法满足工业应用的需求。为了缓解水合物形成条件、缩短水合反应时间、提高水合物储气率和对CO₂气体的选择性,本文针对以上问题,重点研究了CO₂/CH₄混合气体在不同添加剂体系形成水合物的热力学特性,以及气体水合物法分离CO₂/CH₄混合气的动力学特性与反应机理,从而获得低耗能、快速、高效的气体水合物法分离CO₂/CH₄混合气体系和强化方法,为水合物法分离页岩气中CO₂气体的工业化应用提供理论指导和技术支持。本文的主要研究工作包括:（1）利用自主设计搭建的气体水合物生成/分解实验平台测定了CO₂/CH₄混合气体在热力学促进剂四氢呋喃（THF）和四丁基溴化磷（TBPB）溶液体系生成水合物的热力学条件,发现THF和TBPB都能显著降低CO₂/CH₄混合气体生成水合物的相平衡压力。研究了THF和TBPB溶液浓度对CO₂/CH₄水合物相平衡条件的影响,结果表明THF或TBPB浓度越高（理想化学计量浓度以内）,水合物结构越稳定。（2）基于热力学实验结果,构建了不同的溶液搅拌反应体系,研究了s I型、THF-s II型水合物分离CO₂/CH₄混合气的宏观动力学特性及CO₂分离效率的影响因素。研究发现提高初始反应驱动力能缩短水合物成核时间并加快水合物初始生长速率,但CO₂选择性大幅降低,纯水体系和1.0 mol%THF溶液体系CO₂分离因子分别由8.8±2.0和3.5±0.7降至1.7±0.2和1.9±0.1。在纯水体系生成s I型水合物的气体消耗量（21.5±2.0 mmol gas/mol water）、CO₂回收率（52.3±2.4%）和CO₂分离因子（8.8±2.0）均高于1.0 mol%THF溶液体系的12.9±0.3 mmol gas/mol water、49.9±2.9%和3.5±0.7。尽管THF-s II型水合物有效降低了水合反应的操作压力且反应速率和总体反应时间优于s I型水合物,但对CO₂的选择性较差。（3）针对THF-s II型水合物对CO₂选择性不佳的问题,将TBPB半笼型水合物应用于CO₂/CH₄混合气体的分离,研究发现理想化学计量浓度（33.2 wt%）的TBPB溶液体系水合物生长速率最高（8.6±0.3 mmol gas/mol water/h）,且水合反应时间最短（<2h）,同时将反应的操作温度提升至接近室温的条件（284.2 K）,CO₂分离因子提高至31.1±3.3,表明TBPB半笼型水合物分离CO₂/CH₄混合气体的性能优越,具有很好的应用前景。（4）基于自主设计搭建的高压显微实验平台利用“三步法”研究了TBPB和四丁基溴化铵（TBAB）半笼型水合物在静态体系的微观结晶规律和生长动力学特性,揭示了水合物晶体生长行为对气液传质的作用机理。TBPB水合物晶体在气液界面成核后迅速横向生长导致气液界面被致密的水合物膜所覆盖,进而阻碍了气液传质;TBAB水合物在气液界面生成针状或柱状晶体后发生沉降现象,保留了气液界面,同时在液相堆叠的复杂的多孔水合物结构增加了气液传质通道,提高了水合物储气率。（5）为了解决溶液搅拌反应体系的气液传质阻力较大和能耗高的问题,以13X分子筛和煤颗粒为载体构建了新型吸附-水合反应体系,相比搅拌溶液体系大幅提高了反应速率和储气率,在吸附-水合耦合效应下储气率最高达到51.1±0.9 mmol gas/mol water,且发现通过改变固定床水饱和度、固定床高度等参数,能实现对CO₂/CH₄混合气体分离性能的调控。（6）针对动态分离过程中难以获取水合物相气体组分的难题,基于自行搭建的原位拉曼光谱实验系统结合间接硬建模算法开发了一种原位、在线分析水合物相中CH₄与CO₂物质的量之比的方法,并基于此方法研究了气相、水合物生成前富水液相以及水合物生成后固态水合物相中CH₄与CO₂物质的量之比的时变规律。研究结果表明气体溶解阶段贡献了约72%气相中CH₄与CO₂物质的量之比的增量;水合物浆中固态水合物的含量随原料气中CH₄组分降低而升高;水合物生长初始阶段水合物相中CH₄与CO₂物质的量之比迅速增加,表明CH₄在水合物生长初期大量进入水合物笼型结构;且CO₂分离因子始终遵循富水液相>水合物浆>水合物相的规律。更多还原

【Abstract】 Shale gas is one type of unconventional natural gases.The main component of shale gas is methane（CH₄）.Shale gas exploration by super critical carbon dioxide（CO₂）fracturing can not only enhance the shale gas recovery but also geologically storage CO₂.However,the recovered shale gas are mainly CO₂/CH₄ gas mixtures.The CO₂ gas should be efficiently removed from the CO₂/CH₄ gas mixtures in order to improve the quality of the shale gas.Hydrate-based CO₂/CH₄ separation process is a promising method with considerable merits such as mild operating conditions,simple process flow,low operating cost,less environmental impact,etc.Gas hydrates are solid inclusion compounds formed by water molecules and small gas molecules（such as CH₄,CO₂,etc.）at high-pressure and low temperature conditions.The hydrate-based gas separation process（HBGSP）is a new method on the basis of the fact that the priority for different gas molecules to form gas hydrates with water molecules differs at certain conditions.At present,domestic and foreign research activities of the hydrate-based CO₂ removal from CO₂/CH₄ gas mixtures have developed from the early pure water systems to solution systems containing thermodynamic promoters.Although phase equilibrium conditions for gas hydrates formed from CO₂/CH₄ gas mixtures have been shifted to even milder conditions,key parameters（for example,the rate of hydrate formation,gas storage capacity,CO₂selectivity,etc.）for the one stage HBGSP still need further improvements to meet the industrial demands.In view of the abovementioned issues and in order to further reduce the energy penalty and reaction time for hydrate formation,enhance gas hydrate storage capacity,and improve the CO₂ selectivity of the hydrates,this study is mainly focused on the thermodynamic characteristics of the mixed CO₂/CH₄ hydrates in various reaction systems and kinetic characteristics as well as reaction mechanism for the separation of CO₂/CH₄ gas mixtures via the HBGSP.The purpose of this study is to develop a low energy consumption,fast,and efficient hydrate-based reaction system or enhanced method so as to offer theoretical guidance and technical supports for potential applications of the hydrate-based CO₂ capture from shale gas at an industrial level.The main research work are highlighted as follows:（1）The phase equilibria of CO₂/CH₄ hydrates formed in various concentrations of tetrahydrofuran（THF）or tetra-n-butyl phosphonium bromide（TBPB）solutions were measured and determined by an experimental setup for gas hydrate formation/dissociation,which was independently designed and built.We found that both THF and TBPB can significantly reduce the phase equilibrium pressures of the CO₂/CH₄ mixed hydrates.The hydrate structures turned to be thermodynamically more stable as the increase of THF or TBPB concentrations（within the stoichiometric concentration）.（2）Kinetic characteristics and CO₂ separation efficiency for CO₂ removal from the CO₂/CH₄ gas mixtures by forming s I hydrates or THF-s II hydrates were studied in various reaction systems based on the results obtained from the thermodynamic measurements.A higher initial driving force for hydrate formation resulted in an enhanced rate of hydrate formation at the initial period of hydrate growth but a dramatically reduction of CO₂ separation efficiency.The CO₂ separation factor obtained from the pure water systems and the 1.0 mol%THF solution systems decreased from8.8±2.0 and 3.5±0.7 to 1.7±0.2 and 1.9±0.1,respectively.The normalized gas consumption（21.5±2.0 mmol gas/mol water）,CO₂ recovery（52.3±2.4%）,and CO₂separation factor（8.8±2.0）obtained in the pure water systems were all higher than that obtained in the 1.0 mol%THF solution systems,which were 12.9±0.3 mmol gas/mol water,49.9±2.9%,and 3.5±0.7,respectively.Although the operating pressure was reduced and the hydrate formation rate as well as the entire reaction time was improved in THF-s II hydrates,the CO₂ selectivity was compromised comparing with s I hydrates.（3）Due to the poor CO₂ selectivity of THF-s II hydrates,TBPB semiclathrate hydrate was employed to separate the CO₂/CH₄ gas mixtures in a stirred tank reactor.The highest hydrate formation rate（8.6±0.3 mmol gas/mol water/h）and the optimum reaction time（<2h）were found in the TBPB solutions at the stoichiometric concentration（33.2 wt%）.The CO₂ selectivity factor was improved to as high as31.1±3.3.Moreover,the operating temperature was further improved to near ambient temperature（284.2 K）.These results show a great potential for the application of TBPB semiclathrate hydrate for efficient separation of the CO₂/CH₄ gas mixtures.（4）Based on the self-designed high-pressure morphology apparatus,a"three-step method"was proposed to investigate the crystal growth morphology and kinetic characteristics of TBPB and tetra-n-butyl ammonium bromide（TBAB）semiclathrate hydrates under static conditions.We found that the rapid lateral growth of TBPB hydrate crystals resulted in a blockdown of the gas/liquid interface by the dense hydrate film,which significantly hindered the diffusion of gas molecules towards the liquid phase.While the needle-like or column shaped TBAB hydrate crystals formed near the gas/liquid interface preferred to drop down into the deep bottom of the reactor,leaving the gas/liquid interface free for gas molecules diffusing.Furthermore,the highly porous hydrate structures caused by the sedimentation phenomenon increases the channels for gas/liquid mass transfer.As a result,the gas storage capacity is enhanced in TBAB semiclathrate hydrates.（5）Considering the huge mass transfer resistance and high energy penalty in the stirred tank reactor,novel adsorption-hydration systems in the fixed bed reactor packed with water saturated zeolite 13X or coal particles were employed to enhance the CO₂/CH₄ separation.Compared with the stirred tank reactor,the hydrate formation rate and gas storage capacity obtained in the fixed bed reactor were greatly improved due to the synergistic effect of adsorption and hydrate formation.The highest gas storage capacity reached up to 51.1±0.9 mmol of gas/mol of water.We also found that the performance of CO₂ separation from the CO₂/CH₄ gas mixtures can be improved by adjusting the fixed bed height or water saturations of the fixed beds.（6）A novel method was proposed to evaluate the evolution of the molar ratio of CH₄ and CO₂ in the solid hydrate phase during the hydrate-based CO₂/CH₄ separation process,which is usually challenging in laboratory experiments.It is based on the Raman signals obtained from the self-built in situ Raman spectroscopy experimental system combined with the"Indirect hard modeling"algorithm.With this method,the temporal evolution of molar ratios of CH₄ and CO₂ in the vapor phase,the water-rich liquid phase before hydrate formation,and in the solid hydrate phase during hydrate formation process was investigated.It was found that the gas dissolute stage contributes～72%of the total increasement of the molar ratio of CH₄ and CO₂ in the vapor phase;the hydrate content in the hydrate gel/slurry increases with the decrease of CH₄compositions in the feed gas mixtures;the molar ratio of CH₄ and CO₂ in the solid hydrate phase increased rapidly at the initial stage of hydrate formation,indicating that the hydrate clusters favor CH₄ molecules at initial times of hydrate growth;and the CO₂separation factor obtained in various states always follows the order that the water-rich liquid phase>the hydrate gel>the solid hydrate phase.更多还原