利用分子模擬探討異丁醇對氣體水合物生長機制之影響

Abstract

搭配適當的促進劑，擴大氣體水合物穩定存在的區域，氣體水合物將有機會取代高成本、高風險的氣體加壓方法，作為貯存與運輸天然氣的替代方案。本研究利用分子動態模擬研究實驗上已知的熱力學促進劑：第三丁醇對甲烷水合物生長機制的影響。經力場的調整，本研究可模擬出與實驗結果相符合的三相平衡圖，利用此分子模擬參數，搭配氣(甲烷)-液(水與第三丁醇)-固(氣體水合物晶相)三相系統，進一步觀察水分子與第三丁醇分子間的互動。本研究模擬加入第三丁醇前後甲烷水合物的生長情形，發現水合物籠狀結構的形成可分為兩個步驟：首先疏水的客體分子(甲烷或第三丁醇)受破碎的水合物籠狀結構吸引進入其結構中心，接著，客體分子的存在將有效穩定該孔隙，促使周圍水分子排列成完整的籠狀結構。由於兩種常見的氣體水合物結構(結構一型sI，結構二型sII)在幾合上有許多相似的排列方式，第三丁醇有很高的機會吸附在空間較小的結構一型的大籠子(51262)的位置，阻礙了甲烷水合物的生長，因此雖然第三丁醇在熱力學上是甲烷水合物的促進劑，在動力學方面卻是甲烷水合物的抑制劑。

Conventional approach for transport and storage of natural gas by pressurization is energy costly and also potentially risky. Gas hydrates are an alternative form of handling natural gas at lowered pressures. The addition of a suitable promoter can further stabilize the hydrate structure and shift the hydrate formation condition to an even lower pressure. It has been reported that tert-butaonol (TBA) is a thermodynamic promoter of methane hydrates. In this work we use molecular dynamics simulations to elucidate how tert-Butanol effects the growth of gas hydrate, and compare the similarity and difference between the growth of gas hydrates with and without tert-Butanol. Three-phase models comprising liquid, gas and solid hydrate phases are used. Both thermodynamic and kinetic properties are studied, it is found that a tert-Butanol molecule can stabilize the large cage in structure II (sII) hydrate (51264), while its size is too large to be enclosed by a large cage in structure I (sI) hydrate (51262). The encapsulation of TBA in to the large cage of sII hydrate is found to be a two-step process: first, a TBA molecule is adsorbed into an incomplete-cage; second, the TBA stabilizes the cage and attracts surrounding water molecules to form an entire cage. The similar geometry of sI and sII often leads to adsorption of tert-Butanol to the incorrect position at the hydrate-liquid interface, which hinders the growth of hydrate. As a consequence, while TBA is a thermodynamic promoter, it is a kinetic inhibitor.