气体水合物形成的热力学与动力学研究进展

气体水合物形成过程中涉及复杂的热力学和动力学问题.本文对水合物热力学理论模型、水合物生成动力学机理等方面的研究成果和最新进展进行了综述.热力学方面重点介绍了基于等温吸附理论 (van der Waals-Platteeuw模型)和基于双过程水合物生成机理(Chen-Guo模型)的相平衡热力学模型,同时介绍水合物结构及其转变方面的最新研究成果.动力学方面介绍了成簇成核、界面成核等成核机理模型以及成核后的水合物生长机理.另外还述及了目前水合物热力学和动力学研究中所涉及的微观、亚微观和宏观测量方法.针对目前水合物热力学和动力学研究中存在的问题,对未来的发展方向和重点提出了建议.     

喷雾反应器中二氧化碳水合物的生长实验研究

在喷雾反应器中进行了CO2水合物生成实验,观察了CO2水合物的生长过程,以结晶过饱和度作为生长驱动力研究了CO2水合物的生长特性. 结果表明,CO2水合物的生成量随生长驱动力增加而增加;反应温度越低,CO2水合物的生长量越大. CO2水合物生长速率为0.531~9.53 mmol/s,与静态反应装置中一致,表明喷雾反应器促进CO2水合物快速生长的作用有待进一步提高. CO2水合物储气率较低,为14~24 m3/m3,主要原因是生长驱动力偏小,水合反应不完全.     

螺旋内槽管内天然气-水-表面活性剂体系的水合物生成动力学计算

针对多组分气体（天然气）-水-表面活性剂体系在螺旋内槽管内的水合物生成过程,首先采用CFD方法结合群体平衡模型（PBM）,基于溶质渗透模型和Kolmogorov各向同性湍流理论对螺旋内槽管内气液传质系数进行了模拟;其次基于Kashchiev和Firoozabadi的经典水合物成核和生长理论,将其体系从单组分-水系统扩展到多组分气体（天然气-水-十二烷基硫酸钠）系统,同时结合经典结晶理论利用传质系数对水合物生长模型进行了修正,建立了适用于螺旋内槽管流动体系内天然气水合物生成动力学模型。通过模拟计算,获得不同水合物生产条件下天然气在水中的平均传质系数;进而利用Microsoft Visual C++编程计算得到不同条件下水合物生成动力学数据,在考察范围内,天然气水合物的成核速率随着反应体系有效表面能的增大而锐减,而水合物生成驱动力和生长速率未受影响,同时水合物生长速率随着流速和反应压力的增大及温度的降低而增大,成核速率随着压力的增大和温度的降低而增大。     

螺旋内槽管内天然气-水-表面活性剂体系的水合物生成动力学计算

针对多组分气体(天然气)-水-表面活性剂体系在螺旋内槽管内的水合物生成过程,首先采用CFD方法结合群体平衡模型(PBM),基于溶质渗透模型和Kolmogorov各向同性湍流理论对螺旋内槽管内气液传质系数进行了模拟;其次基于Kashchiev和Firoozabadi的经典水合物成核和生长理论,将其体系从单组分-水系统扩展到多组分气体(天然气-水-十二烷基硫酸钠)系统,同时结合经典结晶理论利用传质系数对水合物生长模型进行了修正,建立了适用于螺旋内槽管流动体系内天然气水合物生成动力学模型。通过模拟计算,获得不同水合物生产条件下天然气在水中的平均传质系数;进而利用Microsoft Visual C++编程计算得到不同条件下水合物生成动力学数据,在考察范围内,天然气水合物的成核速率随着反应体系有效表面能的增大而锐减,而水合物生成驱动力和生长速率未受影响,同时水合物生长速率随着流速和反应压力的增大及温度的降低而增大,成核速率随着压力的增大和温度的降低而增大。     

初始压力对冰冻石英砂中CO2水合物生成特性的影响

利用冰冻石英砂模拟冻土水合物的赋存条件，研究了压力对二氧化碳水合物生成特性的影响，在300 mL高压水合物反应釜中于271 K下进行了多组CO2液化压力以上及以下的霰状冰粉包裹的石英砂中水合物生成实验。结果表明，充入的CO2未液化时，初始压力越大，水合反应速率越快，压力越早达稳定状态；充入压力达液化压力后，注入的CO2越多，水合反应速率越快。压力作为水合反应的驱动力，压力越高水合物生成越多，冰的最终转化率越高。采用CO2置换冻土区中甲烷水合物时，控制压力低于液化压力或注入过量的CO2，置换效果更好。     

悬垂水滴表面天然气水合物的生长特性

针对天然气水合物的喷雾法制备,设计并搭建了悬垂水滴显微实验装置,研究了不同温度和压力条件下天然气水合物在悬垂水滴表面的生长形态,建立了水滴表面水合物膜的生长模型,并通过实验研究了驱动力、过冷度、水合反应速率、气体扩散速率及十二烷基硫酸钠(SDS)浓度对水合物膜生长的影响. 结果表明,水合物膜的生长速率随过冷度的增加而增加,实验条件下测得水滴表面水合物膜的生长率为0.24~0.39 mm/s. 水滴表面水合物膜的生长为内扩散控制,水合物的活化能为13.01 kJ/mol. 当SDS浓度为550 mg/L时,界面反应速率常数有最大值0.0027 m/s.     

CO2置换天然气水合物中CH4的研究进展

在全面综述国内外天然气水合物开采技术研究现状的基础上,重点介绍了国内外CO2置换开采天然气水合物的研究进展,分别从热力学和动力学角度分析了置换反应过程的可行性. 现有的研究主要集中在自由饱和水体系,而自然界中天然气水合物主要赋存于海底沉积层和永久冻土层中,需结合天然气水合物赋存的自然环境条件,研究其动力学过程,揭示置换反应过程的内在机理. 最后指出了CO2置换开采天然气水合物尚存在的问题和今后的研究方向.     

多孔材料中甲烷水合物生成的传热数值模拟研究

水合物储运(NGH)是近几年发展起来的天然气储运技术,已具备实现工业化的潜力。但水合物的生长是传质传热控制的反应,因此在放大实验中存在诸多不确定因素。针对该问题,对水合物反应器中多孔材料内甲烷水合物生成传热过程建立了基于化学反应动力学和多孔材料内传质传热的甲烷水合物生成传热数学模型,可用于计算反应器内水合物生成分布和热量分布,指导水合反应器的设计和优化。通过模拟与实验数据对比验证了该模型的可靠性,并对使用了不同热导率填料的水合反应过程进行数值模拟。结果显示,模拟值与实验值的绝对平均相对误差小于6%,生成传热模型准确性高;在水合反应过程中,热量传递是影响水合物生成速率的关键因素之一。导热不良时,易在水合物生成中心部分形成局部过热,对水合物生长造成热抑制。在进行水合物生成放大实验时,应特别注意反应器内部的热量控制。     

多孔材料中甲烷水合物生成的传热数值模拟研究

水合物储运(NGH)是近几年发展起来的天然气储运技术,已具备实现工业化的潜力。但水合物的生长是传质传热控制的反应,因此在放大实验中存在诸多不确定因素。针对该问题,对水合物反应器中多孔材料内甲烷水合物生成传热过程建立了基于化学反应动力学和多孔材料内传质传热的甲烷水合物生成传热数学模型,可用于计算反应器内水合物生成分布和热量分布,指导水合反应器的设计和优化。通过模拟与实验数据对比验证了该模型的可靠性,并对使用了不同热导率填料的水合反应过程进行数值模拟。结果显示,模拟值与实验值的绝对平均相对误差小于6%,生成传热模型准确性高;在水合反应过程中,热量传递是影响水合物生成速率的关键因素之一。导热不良时,易在水合物生成中心部分形成局部过热,对水合物生长造成热抑制。在进行水合物生成放大实验时,应特别注意反应器内部的热量控制。     

气体水合物的研究概述

气体水合物是能源开采以及化学工业生产过程中常见的物质。本文综述了水合物的形成机理和分子结构,对水合物的分析、合成与分解的研究,以及水合物的应用与防治等。在形成机理方面主要介绍了水合物的成核过程和生长过程的模型,在水合物的分析方面,主要介绍了对水合物组成和水合物结构的分析方法。     

Hydrate formation from liquid CO2 in a glass beads bed

CO₂ sequestration in marine sediments as solid hydrates is a potential way to capture and store anthropogenic CO₂. In this study, hydrate formation from liquid CO₂ in marine sediments was simulated in a glass beads bed, and the factors affecting the kinetics of hydrate formation were investigated. The results indicated that the rapid initial hydrate formation with a high driving force always increases the mass transfer resistance, which slows down hydrate growth. The final ratio of water conversion is higher under conditions of low temperature and higher pressure. A smaller particle size is conductive to initial CO₂ hydrate growth, but the water conversion ratio in a bed with larger particles is slightly higher. Compared with other factors, the change in water saturation has an obvious effect on the final water conversion. To inhibit the initial hydrate formation during the injection process, in this paper, a kinetic inhibitor is proposed for pre-injection into marine sediments. This work shows that at a low pressure, a low-concentration inhibitor has an obvious inhibition effect on hydrate growth. However, at a high pressure, it is necessary to increase the concentration of inhibitor to produce an obvious inhibition effect.     

Modelling of hydrate formation kinetics: State-of-the-art and future directions

Clathrate hydrates have drawn considerable attention in the last few years due to a variety of possible applications, including transportation and storage of natural gas and carbon dioxide sequestration in the ocean. These applications require the development of effective hydrate formation reactors, which, in turn, asks for a comprehensive understanding of hydrate formation kinetics. Contrary to hydrate thermodynamics, hydrate kinetics are still poorly understood. In the present contribution, a critical review of the literature on hydrate kinetics is provided, with special emphasis upon modelling efforts, essential to the design of any reactor. The main features of the models available in the literature for hydrate nucleation and growth are discussed, together with their limitations. In addition, suggestions for further developments are presented.     

Experimental and theoretical investigations on the carbon dioxide gas hydrate formation kinetics at the onset of turbidity regarding CO2 capture and sequestration processes

The carbon dioxide gas hydrate formation kinetics at the onset of turbidity is experimentally and theoretically investigated. It is shown that the time-dependent heterogeneous nucleation and growth kinetics are simultaneously governing the hydrate formation process at the onset of turbidity. A new approach is also presented for determination of gas hydrate-liquid interfacial tension. The CO₂ hydrate-liquid interfacial tension according to the suggested heterogeneous nucleation mechanism is found to be about 12.7 mJ/m². The overall average absolute deviation between predicted and measured CO₂ molar consumption is about 0.61%, indicating the excellent accuracy of the proposed model for studying the hydrate-based CO₂ capture and sequestration processes over wide ranges of pressures and temperatures.     

水合物生成和分解动力学研究现状

从微观动力学和宏观动力学的角度对水合物生成和分解动力学研究文献进行了分类、评述,着重点是水合物生成的微观机理和水合物晶体生长的宏观动力学.关于水合物生成的微观动力学,着重介绍3个比较完整的水合物生成机理以及相应的机理模型.关于水合物生成的宏观动力学,着重阐述加拿大Bishnoi实验室所取得的研究成果以及日本学者结合CO_2水合物处置法开展的CO_2水合物生成动力学研究工作.关于水合物分解,主要讨论水合物在受热、降压和抑制剂存在时的分解动力学.基于水合物生成和分解动力学的研究现状,指出了今后研究的8个重要方向.     

A new hydration kinetics model of composite cementitious materials,Part 2: Physical effect of SCMs

This paper is the second part of an overall research project that aims to model the hydration kinetics of composite cementitious materials. The new hydration kinetic model of Portland cement was constructed in the first part of this research project. The aim of this part is to characterize the physical effect of SCMs by modifying some kinetic parameters of the hydration kinetic model of cement. In this paper, the filler effect of SCMs is attributed to the “dilution effect”, “nucleation effect” and “accelerated dissolution effect”. The dilution effect is considered by introducing the actual water/cement ratio into the kinetic model. The nucleation surface area, nucleation and growth rates are modified to account for the nucleation effect and accelerated dissolution effect of fillers. The simulative fit results show that the extra contribution of SCMs due to their filler effect on hydration of cement at early ages has an upper limit: at most 20% extra nucleation surface area, 30% extra nucleation ratio and 40% extra growth ratio of calcium silicate hydrate (CSH). The new modified kinetic model and the value-taking method of the newly introduced parameters are verified by simulative fitting to the kinetic data from reference. The apparent activation energies of the nucleation rate, growth rate, and diffusion rate are approximately 35 kJ/mol, 36 kJ/mol and 38.5 kJ/mol, which are slightly smaller than those of the pure Portland cement reported in the first part of this research project.     

Phase equilibria and kinetic behavior of co<Subscript>2</Subscript> hydrate in electrolyte and porous media solutions: application to ocean sequestration of CO<Subscript>2</Subscript>

Understanding the phase behavior and formation kinetics of CO₂ hydrate is essential for developing the sequestration process of CO₂ into the deep ocean and its feasibility. Three-phase equilibria of solid hydrate, liquid water, and vapor were determined for aqueous mixtures containing CO₂ and NaCl/clay to examine the effect of both ocean electrolytes and sediments on hydrate stability. Due to the capillary effect by clay pores and inhibition effect by NaCl the corresponding hydrate formation pressure appeared to be a little higher than that required for simple and pure hydrate at specified temperature. In addition, the hydrate formation kinetics of carbon dioxide in pure water and aqueous NaCl solutions with or without clay mineral were also measured at various conditions. The formation kinetic behavior was found to be strongly influenced by pressure, temperature and electrolyte concentration. A simplified kinetic model having two adjustable parameters was proposed and the estimated results agreed well with the experimental data. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Modeling and simulation of cement hydration kinetics and microstructure development

Efforts to model and simulate the highly complex cement hydration process over the past 40 years are reviewed, covering different modeling approaches such as single particle models, mathematical nucleation and growth models, and vector and lattice-based approaches to simulating microstructure development. Particular attention is given to promising developments that have taken place in the past few years. Recent applications of molecular-scale simulation methods to understanding the structure and formation of calcium–silicate–hydrate phases, and to understanding the process of dissolution of cement minerals in water are also discussed, as these topics are highly relevant to the future development of more complete and fundamental hydration models.