【作者】 白冬生；

【导师】 张现仁；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2013， 博士

【摘要】 发达国家的工业化进程大大加速了传统化石能源的消耗。目前,以化石能源消费为主的世界各国都面临着能源减少的挑战。而天然气水合物具有分布广、储量大、能量密度高、绿色清洁等特点,被一致认为是21世纪极具潜力的新型替代能源。由于其作为新能源的潜在应用,对水合物的研究已在全球范围内受到了高度重视。我国已经在南海北部的神狐海域和祁连山南缘的永久冻土带中发现并成功取获了天然气水合物实物样品,水合物作为新能源在我国未来能源战略中的地位将越来越重要。然而,和美国、日本等发达国家相比,我国在水合物方面的研究尚处于起步阶段,在基础研究层面上还存在很大的空白。目前,实验科学在水合物的形成、分解、热力学性质等方面已有相当大的突破,但由于实验方法和手段等的限制,仍无法在微观上给出详细的解释。随着计算机科技的迅速发展,计算机模拟方法已经成为在分子尺度上理解微观机理的强大工具。本论文通过分子动力学模拟方法,从分子尺度上研究了气体水合物的成核与生长过程。具体包括二氧化碳水合物在固体表面的生成机理、客体分子性质对水合物生长机理的影响、以及二氧化碳置换甲烷水合物的置换机理。在自然环境中,水合物的生成往往都发生在固体表面,因而理解水合物在固体表面的生成机理对于一些工程应用,例如以水合物的方式封存C02温室气体,是至关重要的。本论文分别模拟了两相环境和三相环境中CO2水合物在固体表面的成核与生长机理,并探讨了固体表面性质对机理的影响。主要研究结论汇总如下。(1)模拟研究发现在亲水性较强的固体表面上,成核是一个三步机理。(2)成核机理随固体表面亲水性的减弱而逐步演化,最终退化成两步机理。成核机理的改变主要是因为固体表面的亲水性对水分子局部结构和C02分布的影响。随着亲水性的减弱,成核的诱导时间减少,表明水合物的成核过程更易于在弱亲水表面下发生。固体表面的结晶度可以影响水合物的无定形程度。(3)在三相环境中,水合物的成核发生在三相接触线附近,然后沿接触线生长并向CO2相偏移。除了自然界广泛存在的天然气水合物之外,很多无机和有机小分子都可以充当客体分子形成水合物。不同的客体分子形成水合物的机理也是有差异的。客体分子性质对水合物形成机理的影响同样是值得关注和研究的一个方面。本论文模拟了(ε，σ)回空间中各种Lennard—Jones客体分子对水合物生长的影响。模拟研究发现水合物的生长过程总是开始于客体分子在水合物环表面的吸附,同时伴随有客体分子迁移率的降低。势阱深度ε调控着水合物核的生长路径和速率,而分子直径σ调控着水合物的动力学优先结构。(ε，σ)平面的动力学相图说明动力学优先结构基本上和热力学稳定的水合物结构相一致。此外,利用CO2置换天然气水合物中的CH4在工程实践中既具有能源开采价值,又具有环境保护意义,因而是气体水合物研究领域极具前景的一个研究热点。本论文模拟了CO2置换水合物中的CH4分子的置换机理和动力学性质。模拟研究发现,熔化的CH4水合物带有大量的残余环,是造成“记忆效应”的主要原因。水合物残余环可以促进C02水合物的成核过程,另一方面客体分子的化学势也会影响置换过程。在动力学方面,随着置换过程的进行,置换形成的CO2水合物层为进一步置换提供了传质能垒,减缓了置换速率。总体上,置换过程协同受控于客体分子的化学势、“记忆效应”以及质量传递。总体而言,气体水合物的成核与生长过程是一个非常复杂的物理化学过程。本论文的研究工作仅对这一过程在分子尺度上的微观机理进行了一定程度的探讨。然而,考虑到成核与生长的复杂性,分子模拟研究绝不仅限于以上课题。相反,计算机模拟技术的应用给该领域带来了新的机遇,同时也开辟了新的研究方向,从而开创新的进展。更多还原

【Abstract】 The consumption of traditional fossil energies is greatly accelerated by the industrialization process in developed countries. So far, all the countries dominated by the consumption of fossil energies in the world are facing with the challenge of the gradual depletion of energy resource. Natural gas hydrates, with its characteristics of wide distribution, high reserves, high energy density, and cleaning, are unanimously considered as a new potential alternative energy resource in21th century. Due to its potential applications as a new energy resource, studies on hydrates have been globally performed. In China, the samples of natural gas hydrates have been found and exploited successfully in Shenhu sea-area in northern South China Sea and in the permafrost in the southern zone of Qilian Mountains. As a new energy resource, the strategic position of hydrates will become increasingly important in the future in China. However, compared with the developed countries such as America and Japan, the study of gas hydrate in China is still in the initial stage, and there exists still an obvious gap in the fundamental research fields. At present, considerable progress has been achieved in the experimental studies in the formation, decomposition, and the thermodynamic properties of hydrate. But it is still difficult to give an explanation in details in microscopic level, because of the limitation of experimental methods. With the rapid development of computer technologies, computer simulation method has become a powerful tool for understanding the microscopic mechanisms at molecular scale.In this dissertation, molecular dynamics simulation method is applied to study the nucleation and growth process of gas hydrates at molecular level, including the formation mechanism of carbon dioxide hydrates on solid surfaces, the effect of the properties of guest molecules on the mechanism of hydrate growth, and the replacement mechanism of methane hydrates with carbon dioxide.In nature the formation of hydrates often occurs on the solid surface. Therefore, the understanding of the formation mechanism of hydrate on solid surface is of vital importance for some engineering applications, such as the sequestration of CO2greenhouse gas in hydrates. In this dissertation, the nucleation and growth mechanism of CO2hydrates on solid surface in two-phase and three-phase systems were investigated, and the effect of the solid surface properties on the mechanism was explored. The main conclusions of the studies are summarized as follows,(ⅰ) Simulation studies show that hydrate nucleation is a three-step process on the solid surface with a strong hydrophilicity.(ⅱ) The nucleation mechanism was found to vary gradually with the decrease of the hydrophilicity of the solid surfaces, and eventually changes into a two-step mechanism. The change of nucleation mechanism is mainly because of the effect of the hydrophilicity of the solid surface on the local structure of water molecules and the distribution of CO2. As the surface hydrophilicity is weakened, the induction time for hydrate nucleation decreases, indicating that hydrate nucleation takes place more easily on a weak hydrophilic surface. The crystallinity of the solid surface can affect the amorphous degree of hydrates.(ⅲ) In three-phase system, the nucleation of hydrates occurs near the three-phase contact line, and growth along the contact line but developed towards the CO2phase.Except for the natural gas hydrates widely existed in nature, plenty of other small inorganic and organic molecules can also act as guest molecules to form hydrates. The formation mechanism for different guest molecules is also different. Thereby the effect of the properties of guest molecules on the formation mechanism of hydrates is also a respect worthy of study. In this dissertation, the effect of various guest molecules in the (ε,σ) space of the Lennard-Jones potential model on hydrate growth were investigated. Simulation studies show that the hydrate growth process always proceeds with the adsorption of guest molecules on the face of hydrate rings, which reduced the mobility of guest molecules. The well depth of potential ε regulates the pathway and the rate of the growth of hydrate nucleus, whereas the molecular size σ controls the dynamically preferable structure of hydrates. The dynamic-phase diagram on (ε, σ) plane shows that the dynamically preferable structure is basically consistent with the thermodynamically stable hydrate structure.In addition, the replacement of CH4in natural gas hydrate by using CO2is not only of the importance of energy exploitation in chemical engineering, but also the significance in environmental protection. So, it becomes a hot issue on the research field of gas hydrates. In this dissertation, replacement mechanism and kinetic properties of the replacement process of CH4hydrate with CO2were investigated. Simulation studies show that there are plenty of residual rings within the melted CH4hydrate, which is mainly responsible for the "memory effect". Hydrate residual rings can promote the nucleation of CO2hydrate, and on the other hand, the chemical potential of guest molecules can affect the replacement process. In the kinetic aspect, with the replacement process proceeds, the CO2hydrate layer formed during the replacement provides a barrier on mass transfer for the further replacement process, and hence slows down the replacement rate. Generally, the replacement process is controlled cooperatively by the chemical potential of guest molecules,"memory effect", and mass transfer barrier.In summary, the nucleation and growth of gas hydrates are complex physical and chemical processes. The studies in this dissertation focus only on the microscopic mechanisms of this process at the molecular scale. However, in consideration of the complexity of the nucleation and growth processes, the studies with molecular simulations by no means limited to above issues. Instead, application of computer simulation techniques in this field opens new opportunities and research directions, and thus may lead to new advances.更多还原