Molecular Dynamics Simulation Study Of The Nucleation And Growth Of Gas Hydrates

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.