利用分子模擬探討添加劑四氫呋喃對甲烷水合物長晶與成核機制的影響

Abstract

四氫呋喃是一種已知的甲烷水合物強力熱力學促進劑。於本研究中我們利用分子動力學模擬首先再現了四氫呋喃水合物與甲烷＋四氫呋喃水合物的熔點以及四氫呋喃於水中的溶解度後，進行了四氫呋喃水合物與甲烷＋四氫呋喃水合物的長晶與成核機制分析並從中討論甲烷與四氫呋喃分別在長晶及成核機制中扮演的角色。首先於四氫呋喃水合物的長晶分析中，我們發現在液態四氫呋喃濃度約0.3~0.8倍(取決於系統溫度)於水合物相時有最大長晶速度。這說明了四氫呋喃的長晶取決於兩樣競爭的因素：低四氫呋喃濃度時為四氫呋喃分子吸附到生長介面之速率所控制；高四氫呋喃濃度時為四氫呋喃分子吸附到生長介面之速率所控制。在甲烷＋四氫呋喃水合物的長晶分析中，相較於甲烷水合物而言，四氫呋喃的加入會使得水合物有從結構I轉變到結構II的現象。相較於四氫呋喃水合物而言，甲烷的加入增加了四氫呋喃水合物的熔點，並隨壓力增加而有更強促進效果。甲烷＋四氫呋喃水合物的長晶於10 MPa與290 K下有最快速率。其生長速率取決於兩項因素：較適合低溫下進行的甲烷吸附於生長界面速率(與甲烷水合物相同)與較適合高溫下進行的四氫呋喃於生長界面上的重新排列(與四氫呋喃水合物相同)。在高於290 K(約比四氫呋喃水合物溫度高10 K)時，生長界面上需有足夠的甲烷才可使長晶順利進行；低於290 K時，界面上的甲烷則不太對長晶具有影響力。 我們也對甲烷、四氫呋喃、甲烷＋四氫呋喃水合物之成核反應進行分析。本研究中水合物的成核支持並驗證了Blob假說的可行性。比較本研究不同系統之後顯示，水溶液中甲烷分子的存在會提供許多標準水籠子而四氫呋喃分子的存在則因有較高濃度的客體分子而加速Blob的生成(成核誘導時間下降)。

Tetrahydrofuran (THF) is an effective promoter of methane hydrates. In this work, the stability limit (aqueous-hydrate coexisting condition) of THF/CH4+THF hydrates and the THF solubility in water were reproduced using molecular dynamics (MD) simulations. After that, the growth/nucleation mechanism of THF/CH4+THF hydrates were investigated and discussed the roles of CH4 and THF in these reaction. In the research of THF hydrate growth, the rate of growth of THF hydrates is found to exhibits a maximum value when the liquid phase THF concentration is about 0.3 to 0.8 times (depending on temperature) of the THF concentration in the hydrate phase. The maximum growth rate of THF hydrate is a result of two competing effects: the adsorption of THF molecules to the growing interface, which is the limiting step at low THF concentrations, and the desorption/rearrangement of THF molecules at the interface, limiting step at high THF concentrations. The dominating factors for the growth of CH4+THF mixed hydrates are analyzed and the results are compared with the growth of single guest CH4 and THF hydrates. While CH4 hydrate has a type I crystalline structure, the presence of THF in the aqueous phase results in the growth the type II structure hydrate. Compared to THF hydrates, the presence of CH4 in the system enhances the dissociation temperature (increasing with the pressure). The growth rate of CH4+THF mixed exhibits a maximum value at about 290 K at 10 MPa. The growth rate is found to be determined by two competing factors: the adsorption of CH4 at the solid-liquid interface (such as CH4 hydrate growth), which is enhanced with decreasing temperature, and the migration of THF to the proper site at the interface (such as THF hydrate growth), which is enhance with increasing temperature. Above 290 K, which is about 10 K higher than the dissociation temperature of pure THF hydrate, the growth of cage can proceed only when sufficient amount of CH4 is adsorbed at the interface. Below 290 K, the growth is not much affected by the presence of CH4. The nucleation of methane, tetrahydrofuran (THF), and methane+tetrahydrofuran hydrates were also investigated in this work. Our simulation results of the nucleation of CH4 hydrates and CH4+THF hydrates supported the blob hypothesis (BH). Comparing with different systems, the CH4 molecules in the aqueous solution were found to supply the regular cages and the THF molecules were found to enhance the formation of blobs (decrease the induction time) by supplying enough guests.