天然氣水合物合成實驗研究:冰晶法生長機制及醇類催化效應探討

Abstract

本研究成功地架設了國內第一套可記錄系統溫壓變化歷程的氣體水合物合成及處理儲存設備。以固態冰晶顆粒與高壓甲烷氣體為材料，在不擾動冰晶標本裝填時所設計的外型及其內部組構條件下，合成出大型塊狀（100公克以上）的飽和甲烷水合物（甲烷飽和度達99%以上）。另外也可搭配混合不同種類的沉積物，製作出產狀與結構各異的水合物標本，適合後續的研究所需。 以「冰晶法」製作飽和的多晶質甲烷水合物標本，其流程主要可以分為「加壓」以及「加熱」兩個階段。本研究首先探討第一個加壓階段，即當系統內僅導入高壓甲烷氣體，而尚未進入第二個階段前（藉由外部加熱以強迫所有的冰晶皆轉換成飽和的甲烷水合物），不同的實驗條件組合（系統設定之初始溫度、壓力及冰晶標本裝填組構）會對冰晶轉換速率與總生產量具有何種影響。 本研究首度證實，僅在瞬間將高壓甲烷氣體導入反應釜內，而標本保持於靜態的條件下，不必進行第2階段的外部加熱，在孔隙率66%的冰晶標本中，最高便有12%的冰晶轉換成甲烷水合物。且在此階段的甲烷水合物生成量與系統設定的初始溫度呈反比，設定的溫度愈低，水合物生成量反而愈多。另在緩慢加壓與標本混雜石英砂等不同的實驗中，會得到更快的水合物生成速率與最高達25%的甲烷水合物生成量。研究也發現，當在僅加入高壓甲烷氣體的第1階段中能有較多的水合物生成，則後續僅須經由較低的加溫溫度與較短的加溫時程，便可以完成幾近飽和的甲烷水合物標本製作，節省大量製備飽和甲烷水合物標本所需的時間，也可降低能源消耗。 經由對影響生成效率的參數探討及電子顯微鏡觀察，本研究所提出的「冰晶法」合成原理及其模式能夠合理地解釋為何所合成的甲烷水合物具有不同的膠結度，以及即使經過第二階段的加溫流程，標本卻不致因固態冰晶融解產生變形或重心改變。而一旦瞭解並掌握住各實驗參數對合成模式的影響效應後，只要適當地設計製作流程與調整溫壓與孔隙率等變化，便可合成出具有不同組構特性的甲烷水合物標本，適合後續不同的實驗工作需求。 過去雖有不少針對減緩或促進液態水中天然氣水合物生成速率而進行的添加劑研究，但卻尚未有關於如何增進在固態冰晶中合成氣體水合物效率的報導。本研究發現若在系統中添加少量的揮發性醇類氣體（甲醇、乙醇與正丙醇），則可大幅地增進甲烷及二氧化碳水合物形成的總生產量。其中在接近冰點的高溫環境下，以氣態乙醇催化效率最佳，最高有接近91% 的冰晶在不需加熱的靜態情況下便能轉換成水合物，大幅增進製備甲烷水合物及二氧化碳水合物的效益。此項發現在未來天然氣的輸運與儲存上應有相當的應用價值。 本研究初步認為微量氣態醇類的存在之所以能增加水合物的生成總量，應該是因為其能防止完整包覆冰晶外層的水合物薄膜在反應初期快速成形，且一併改變所生成的水合物外形，讓後續高壓氣體仍得以通過已生成的水合物膜，持續與內層冰晶的水分子接觸作用。也因此由總體觀之，雖然醇類為廣泛使用作為防止天然氣水合物成形的抑制劑，但是加入微量醇類反而有增進總生成量的效果。醇類的催化程度與系統設定初始溫度的關係，可能與其蒸氣壓的相關，且催化效果最佳的蒸氣壓約在10mmHg。

This study has successfully set up Taiwan’s first gas hydrate synthesizing apparatus which has the capability for recording experimental temperature and pressure. A large (at least 100 grams) methane-saturated (more than 99%) solid hydrate sample can be produced by simply introducing highly pressurized methane gas into the reactor that contains an ice-seed sample (Otherwise called an Ice Seed Method experiment). The appearance and texture of manufactured hydrates remain the same as those of the original ice-seed samples. Also, by this system, we can prepare a variety of samples mixed with different sediments for meeting the needs of follow-up studies. The process of the “Ice Seed Method” can be divided into two main stages, 'Pressurization' and 'Heating'. Firstly, we investigated the influence of different experimental parameters (Initial temperature, pressure, and fabric of ice-seed sample, etc.) on ice-to-hydrate converting rates and total hydrate yields during the pressurization stage. This study has shown for the first time that before the second heating stage, a certain amount of gas hydrate (For example, 12% of ice can be converted to hydrate within 47 min from an ice-seed sample with a porosity of 66%) can be produced after introducing highly pressurized methane gas into the reactor while the texture of sample remains constant. We found that the amount of methane hydrates produced is inversely proportional to the initial temperature; the lower the temperature, the larger the amount of formed hydrate. The converting rates can be even faster in experiments with a slow pressurization procedure or samples mixed with sediments. A 25% conversion can be achieved in a slowly pressurized run within 1042 min. We also found that if there are more hydrates formed during the pressurization stage, the cost of energy and time can be cut down dramatically during the succeeding heating stage for the purpose of making a completely saturated methane hydrate sample. By analyzing experimental data along with some SEM work, this study proposed a simple schematic model to reasonably explain some unique characteristics of the “Ice Seed Method”, such as why hydrate samples can be manufactured with different degrees of cementation, or why the appearance and texture of samples can be maintained after the heating stage. By adjusting some experimental parameters properly, hydrate samples with different features can be manufactured by the guideline obtained from this study. Although there were some previous researches concentrated on how to promote or retard the formation of gas hydrates in a liquid system by adding different additives, no report about how to improve the conversion efficiency of the “Ice Seed Method” can be found. This study found out that the total amount of methane or carbon dioxide hydrates formed during the “Pressurization” stage can substantially increase as some alcohol vapor (methanol, ethanol and 1-propanol) is added into the system. The one that has the best promoting effect is ethanol while the initial system temperature is set at 270.2K. Nearly 91% of ice seed can be converted into methane hydrates during pressurization stage. The discovery may have considerable practical value for the transportation and storage of natural gas. The preliminary hypothesis for this promoting effect is that the presence of these trace gaseous alcohol is able to slow down the formation rate of hydrates and prevent the generation of impervious hydrate film covering the ice core in the early stage. Instead, the formed hydrates are permeable because of a different texture so the inner ice can keep converting into hydrates by continuously interacting with methane molecules. Maybe the catalytic efficiency has a relationship with the alcohol vapor pressure which changes with the temperature. According to our experimental data, the best catalytic efficiency for each kind of alcohol may be achieved at different temperatures when the vapor pressures are all around 10mmHg.