以分子動態模擬探討甲烷及二氧化碳氣體水合物結晶機制及二氧化碳置換甲烷機制

Abstract

我們利用三相(氣體水合物晶相，液態水相，甲烷/二氧化碳氣/液相)系統分子動態模擬，研究影響甲烷及二氧化碳水合物結晶速率(growth rate)因子。三相平衡水合物相圖成功由分子動態模擬預測並與實驗一致，因此我們的分子模擬參數及模型設計，可以正確描述氣體水合物結晶動態行為。影響水合物結晶生成速率主要為(1) 操作與平衡溫度/壓力間產生的驅動力(driving force) (2)氣體水合物在液態水相中的溶解度(solubility)(3)水分子的移動能力(mobility)。從我們的結果發現，甲烷水合物結晶速率隨著壓力(甲烷溶解度)增加而增加。另一方面，水分子的移動能力會受壓力影響或NaCl存在下改變，二氧化碳水合物結晶速率隨著壓力增加而減小，NaCl加入將明顯將低甲烷水合物結晶速率。 我們也利用分子模擬研究原地(in-situ)二氧化碳置換取代甲烷水合物機制，結果顯示，置換取代程序不需要將甲烷水合物溶解(固相中)即可進行。我們以二氧化碳液相直接接觸甲烷水合物的雙相系統進行置換模擬。置換機制會因與液固界面位置距離遠近而不同，分別為(1)甲烷與二氧化碳直接swapping(2)甲烷與二氧化碳先co-occupation，然後甲烷被推擠置換出來。在置換過中，二氧化碳將自發性經由水籠結構裂孔(opening of the cage structure)進入水籠結構進行置換，這些水籠結構裂口主要是由破損水籠結構氫鍵造成，水籠結構氫鍵破損主要原因為(1) 不穩定結構的水籠結構水分子的震盪(fluctuation)(2)氣體分子碰撞(collision)水籠結構(3)氣體分子與水籠結構水分子產生交互作用(interaction)而形成氫鍵鍵結。

The key factors that affect the growth of methane and carbon dioxide hydrates from pure or aqueous NaCl solutions are identified using molecular dynamics simulations. The three-phase molecular models consisting of methane/carbon dioxide gas, liquid water, and solid hydrate phase are used in this study. The melting temperatures of pure methane and carbon dioxide hydrates are found to be in good agreement with experiment over a wide range of pressures. The growth rate of clathrate hydrate is found to be dominated by (1) the temperature and pressure driving forces (2) the solubility of gas in the liquid phase, and (3) the mobility of water molecules. From our simulation results, the growth rate of methane hydrate increases with the pressure (the solubility of gas) below the equilibrium temperature of clathrate hydrate. In addition, the mobility of water is affected by and the pressure and the presence of NaCl. The growth rate of carbon dioxide hydrate decreases with the pressure and the low growth rate of methane hydrate is found when the NaCl is added. The mechanism of in-situ methane recovery and carbon dioxide sequestration in methane hydrate is uncovered using molecular dynamics simulations. Our results suggest that in situ conversion of methane hydrate to carbon dioxide hydrate without melting is possible. The two-phase molecular models consisting of liquid carbon dioxide and solid methane hydrate phase are used in this study. Depending on the distance to the interface, there are two replacement mechanisms (1) the swapping of methane and carbon dioxide molecules (2) co-occupation of methane and carbon dioxide molecules. The carbon dioxide spontaneously enters into the cage through the an opening of broken hydrogen bond. The break of hydrogen bond is caused by the fluctuation of water in the unstable cages, the collision of gas molecules, or the hydrogen bonding interaction between the solute and water.