以掃描式熱卡計測量含抑制劑與促進劑的甲烷水合物之融解熱與相平衡

Abstract

本研究使用高壓掃描式熱卡計來測量在定壓環境範圍5.00 ~ 35.00 MPa時，含不同碳鏈長的離子液體的甲烷水合物的相平衡溫度。本研究實驗的離子液體為不同碳鏈長的氯化1-支鍊-3-甲基-咪唑氰胺系列離子液體，分別是支碳鏈長為2,6和10的氯化1-乙基-3-甲基-咪唑氰胺、氯化1-己基-3-甲基-咪唑氰胺和氯化1-癸基-3-甲基-咪唑氰胺。選擇這三個離子液體的目的是為了系統性的研究碳鏈長對於離子液體作為甲烷水合物添加劑的影響。研究結果顯示這三個離子液體都是甲烷水合物的熱力學抑制劑，而抑制能力強弱為: 氯化1-乙基-3-甲基-咪唑氰胺 > 氯化1-己基-3-甲基-咪唑氰胺 > 氯化1-癸基-3-甲基-咪唑氰胺。也就是說碳鏈長越短，抑制效果能力越高。除此之外，本研究用一個熱力學預測模型，成功預測添加了離子液體的甲烷水合物的「氣-液-水合物」三相平衡相平衡邊界。這個預測模型是用Peng-Robinson-Stryjek-Vera(PRSV)狀態方程式搭配COSMO-SAC 活性係數模型與the first order modified Huron-Vidal 混合定律去計算水在氣相與液相的逸壓，而水在水合物相的逸壓則使用凡德瓦(van der Waals)和Platteeuw的模型搭配一個以壓力為變數的朗繆爾吸附常數去計算。這個預測模型的相平衡溫度結果與實驗值的平均誤差為0.54%。 本研究亦建立一個高壓掃描式熱卡計的標準實驗流程，來計算與測量含促進劑甲烷水合物的融解熱與融解溫度，實驗的水合物促進劑包含:四氫氟喃、2,5-二氫氟喃、1,3-二氧六環與環戊醇。在這些藥品中，四氫氟喃是最強的甲烷水合物熱力學促進劑，而環戊醇則是最弱的。而含四氫氟喃的甲烷水合物融解熱根據測量，不管在任何壓力與濃度都幾乎相同不變，約163.8 千焦耳/莫耳促進劑，這個結果與根據分子動力學模擬計算得的結果吻合。但是需要注意的是兩者卻又都小於以克勞修斯-克拉佩龍方程式計算得的含四氫氟喃甲烷水合物融解熱結果。

A high pressure differential scanning calorimeter (HP uDSC) was used to determine the dissociation temperature of methane hydrate in the presence of ionic liquid 1–alkyl–3–methylimidazolium chloride under a constant pressure ranging from 5 to 35 MPa. A homologous series of 1–alkyl–3–methylimidazolium chloride with different alkyl chain lengths, including 1-ethyl-3-methylimidazolium chloride, 1-hexyl-3-methylimidazolium chloride and 1-decyl-3-methylimidazolium chloride, were chosen to examine the chain length effect on the dissociation temperature for methane hydrates. All these ionic liquids have inhibition effect on methane hydrate formation. Moreover, the shorter the alkyl chain length is, the stronger the inhibition effect is. That is, the inhibition effect of these ionic liquids on the methane hydrate formation is in the order of 1-ethyl-3-methylimidazolium chloride > 1-hexyl-3-methylimidazolium chloride > 1-decyl-3-methylimidazolium chloride. The three-phase vapor-liquid-hydrate equilibrium condition of methane hydrate in the presence of 1–alkyl–3–methylimidazolium chloride was successfully described by a predictive thermodynamic model. The Peng-Robinson-Stryjek-Vera equation of state incorporated with COSMO-SAC activity coefficient model and the first order modified Huron-Vidal mixing rule were applied to evaluate the fugacity of vapor and liquid phase. An explicit pressure dependence of the Langmuir adsorption constant in the modified van der Waals and Platteeuw model was applied to determine the fugacity of hydrate phase. The absolute average relative deviation in predicted dissociation temperature from the predictive thermodynamic model was 0.54%. A special operation procedure of the HP uDSC was proposed to determine the dissociation enthalpy and temperature of hydrates in the presence of promoters: tetrahydrofuran, 2,5-dihydrofuran, cyclopentanol and 1,3-dioxane. Among these four promoters, tetrahydrofuran has the strongest promotion capability and cyclopentanol is the weakest one. The dissociation enthalpy of methane hydrates in the presence of tetrahydrofuran is 163.8 kJ/mol promoter and independent of pressures and concentrations of promoter, consistent with the results of molecular dynamic simulations. Note that the dissociation enthalpy of the HP uDSC is consistently smaller than that calculated by using Clausius-Clapeyron equation.