天然气水合物的晶体结构及水合系数和比重

GAS HYDRATE STRUCTURE AND HYDRATION NUMBERS AND ITS DENSITIES

天然气水合物具有巨大的资源前景和极强的环境和灾害效应 ,是目前地学研究中的热点。天然气水合物存在三种基本晶体结构 ,五种大小不同的气体占据空间。不同大小的气体分子进入大小相近的气体占据空间 ,形成稳定的天然气水合物。分子太大和太小的气体则不能形成水合物。天然气水合物的气体充填率常小于 1 .0 ,水合系数大于理想水合系数 ,而且受气体类型、组成、温度和压力等条件影响。气体类型明显影响天然气水合物的比重 ,烃类气体可形成比重小于海水的天然气水合物 ,CO2 的水合物比重可明显高于海水比重。

Recently gas hydrate is of great interest in natural science research because it may represent a future energy resource and a driver of climate change and a hazard in various marine operations and in pipelines. We introduced gas hydrate structures and predicted hydration numbers and densities of gas hydrates by simulating techniques. There are three crystal structures of gas hydrates in nature, structures Ⅰ and Ⅱ which are of cubic crystal system, and structure H which is of hexagonal crystal system. In hydrate structures there are five cavities differing in size and geometry, and guest molecules fill the cavities whose size is similar to guest molecular size. Therefore, stable gas hydrates can be formed. Guest molecules below \{0.35\} nm in diameter become too small to stabilize any hydrate structure, while guest molecules from \{0.7\} to \{0.9\} nm in diameter are too large to fit structures Ⅰ and Ⅱ, but stabilize structure H with small help gases. Guest molecules with diameter from 0.35 to 0.42 nm and from \{0.42\} to \{0.6\} nm fit to structures Ⅱ and Ⅰ respectively, and guest molecules from \{0.6\} to \{0.7\} nm form structure\|Ⅱ gas hydrates. But molecules with diameter more than \{0.9\} nm are too large to fit any hydrate structure.\; The molecular formula of gas hydrate in general is written as Gas·nH\-2O, where n is called the hydration number. If all cavities in gas hydrates are occupied by gas molecules, the ideal unit cell formula of hydrate Ⅰ is 6X·2Y·46H\-2O, the general formula is Gas·\{5.75\}H\-2O,and ideal hydration number is \{5.75\}; the ideal unit cell formula of hydrate Ⅱ is 8X·16Y·136H\-2O, the general formula is Gas·\{5.83\}H\-2O,and the ideal hydration number is 5.83; and the ideal unit cell formula of hydrate H is 1X·3Y·2Z·34H\-2O, the general formula is Gas·\{5.80\}H\-2O, and the ideal hydration number is \{5.80\}. But the filling fraction in hydrates always is less than \{1.0\}, and the hydration number is greater than the ideal hydration number. In pure gas hydrates the gas filling fraction and hydration number change with gas hydrate\|forming temperature and pressure, and the filling fraction and hydration number are different for different pure gas forming hydrates. In mixed gas hydrates the filling fraction and hydration number also are different for different gas contents although the components are the same. Gas hydrate density is influenced by several factors, especially gas type. The hydrate densities of hydrocarbon gases are below \{1.0\}, less than those of sea water, while the hydrate density of carbon dioxide is above \{1.0\}, larger than that of sea water.