利用分子模擬探討溫度、溶解度及鹽對甲烷水合物成核機制的影響

Abstract

甲烷水合物成核因素主要來自過冷和過飽和效應。為研究純水成核的濃度效應，我們建立含有水分子及甲烷分子的勻相系統和兩相系統，並且模擬壓力在50MPa及四個溫度，分別是250K、270K、275K和280K，藉此探討成核的溫度效應。成核的過程可以分為三階段，首先是成核前的時間稱為誘導時間，當核超過臨界大小時，會有快速成長期，最後系統的自由水分子減少，系統內的Blob會重組形成結晶結構。誘導時間為成核所需消耗的時間可以從系統內的位能和F4參數隨時間的變化獲得，它會隨濃度增加或是溫度降低而變短。成核的最終結構包含了512, 51262, 51263, 51264 其中51263是不存在於sI和sII的結構中的水籠子，但可以成為sI和sII的介面，而且結晶度也會受到濃度和溫度的影響。 氯化鈉不僅是熱力學的抑制劑也是動力學的抑制劑。在我們的模擬結果中發現氯化鈉離子附近的水分子會因帶電離子影響而有結構變化。成核前要排開氯化鈉進行成核所以誘導時間比較長，Blob的快速成長期也因為氯化鈉而速率下降，最後少部分的氯化鈉會形成水籠子，大部分的氯化鈉會析出。當氯化鈉水溶液濃度增加誘導時間會變長而Blob 的成長速率會下降。

The key factors that affect the nucleation of methane hydrates from liquid water and methane gas are supersaturation and supercooling. In order to study the effect of concentration of methane gas in pure water, we built two phase model and homogeneous model containing liquid water and methane gas. Simulation were performed at pressure 50MPa and four temperatures (Tsim=250K 270K 275K 280K) to study the effect of supercooling (ΔTsub= Tm-Tsim) on nucleation. The induction time, determined based on the evolution of the potential energy and a four-body structural order parameter (F4), is found to decrease with increasing supersaturation of methane in water and supercooling. The resulting structure after nucleation and growth is a combination of normal cages 512, 51262, 51263, 51264. Cage 51263 is not native to the sI and sII crystals, but the present at the interface between sI and sII. The crystallinity is also found to be affected by supersaturation and supercooling of the system. Sodium chloride is not only the thermodynamic inhibitor but the kinetic inhibitor. From our simulation results, the induction time increases and the blob growth rate decreases when the NaCl is added. The local structure of water molecules changed by NaCl results in the lower concentration of methane gas in aqueous solution.