以掃描式熱卡計測量甲基哌啶之甲基位置對氣體水合物融解熱之影響

Abstract

本研究以高壓差式掃描熱卡計測量添加促進劑之氣體水合物其相邊界與融解熱，探討作為結構H型水合物促進劑之甲基哌啶其甲基位置對氣體水合物之促進效果與融解熱之影響，並與結構II型之促進劑四氫呋喃之結果進行比較。四氫呋喃與甲基哌啶促進劑對氪氣水合物由1至10 MPa、甲烷水合物由5至30 MPa之熱力學促進效果順位一致如下：四氫呋喃 > 1-甲基哌啶 > 2-甲基哌啶 ≥ 3-甲基哌啶；而添加四氫呋喃與甲基哌啶促進劑之氣體水合物其融解熱顯示出相同對壓力依賴的趨勢：融解熱在低壓時隨壓力增加而急遽上升，而當壓力高至能驅使幾乎全部的小孔洞被氣體佔據時則變平緩。當融解熱對壓力的依賴性降低時，融解熱之大小排序皆為：四氫呋喃 > 1-甲基哌啶 > 3-甲基哌啶 ≥ 2-甲基哌啶，可看出對甲基哌啶而言，甲基接在氮原子上時能顯著地使促進效果變強、融解熱增加。值得注意的是添加促進劑之氣體水合物在更換氣體種類後僅移動相邊界與融解熱，對促進效果與融解熱之相對排序並無影響，主要還是由添加劑之分子結構與特性所決定。 有鑑於許多研究在量測氣體水合物之融解熱時會在比合成壓力更低之壓力下進行，而非在氣體水合物合成時的壓力下定壓測量，融解壓力對於融解溫度與融解熱之影響應該被仔細檢驗，因此本研究以甲烷四氫呋喃混合水合物與氬氣四氫呋喃混合水合物，於20、30、35 MPa (合成壓力)合成後在3 MPa (融解壓力)下融解，並與定壓下操作之實驗相互比較以探討合成與融解時的壓力對氣體水合物之融解溫度與融解熱有何影響。高壓差式掃描熱卡計之結果顯示當氣體水合物在壓力與合成壓力相同下融解時，融解溫度與融解熱會因孔洞佔有率提升而隨著合成壓力的增加而上升，然而如20 MPa在大多數的小孔洞被氣體佔據後，更高的合成壓力不再提升孔洞佔有率，使融解溫度與融解熱兩者幾乎維持在定值。另一方面，對於在相同高壓(30 MPa)合成之氣體水合物，在3 MPa下融解之融解熱遠低於在30 MPa下定壓融解之融解熱，為解釋此現象，本研究以狀態方程式與焓圖推論出融解熱差值可能源自於系統於不同溫度下擴張和於不同融解壓力下的熱容差，且後者因為液相的熱容遠大於水合物相的熱容，主要貢獻了大部分的融解熱差值。

In this study, the phase boundary and dissociation heat of gas hydrate in the presence of promoter were determined by high pressure differential scanning calorimeter (HPμDSC). The effect of methyl-substituted position in methylpiperidines (MPDs) as structure H hydrate promoter on the promotion capability and dissociation heat of gas hydrate were investigated and compared with the results of tetrahydrofuran (THF) as structure II hydrate promoter. The thermodynamic promotion capability of THF and MPD promoters on krypton hydrate from 1 to 10 MPa and on methane hydrate from 5 to 30 MPa was consistently in the order of THF > 1-MPD > 2-MPD ≥ 3-MPD. The dissociation heat of gas hydrate in the presence of THF and MPD promoters showed the same tendency of pressure-dependent behavior. The dissociation heat increased dramatically with increasing pressure at low pressures and became level off at high pressures when the pressure was high enough to compel gas molecules occupying almost all the small cavities. When the dissociation heat became less pressure-dependent, the dissociation heat was ranked in the order of THF > 1 MPD > 3 MPD ≥ 2 MPD. It can be seen that for the methylpiperidines, the promotion capability and dissociation heat greatly elevated as the methyl group was linked to nitrogen atom. It was also noteworthy that the change of gas type only shifted the phase boundary and the dissociation heat of gas hydrate in the presence of promoter but had no influence on the order of promotion capability and dissociation heat, which were mainly determined by the molecular structure and property of promoters. In view of plenty previous researches measured the dissociation heat of gas hydrate at pressure lower than that gas hydrate synthesized at, instead of measuring at hydrate synthesis pressure isobarically. Effect of dissociation pressure on the dissociation temperature and heat should be carefully examined. Thus, in this study methane + THF mixed hydrate and argon + THF mixed hydrate were synthesized at 20, 30, and 35 MPa (synthesis pressure) then the dissociation temperature and heat were measured while the hydrate dissociated at 3 MPa (dissociation pressure). The HPμDSC results showed that when the gas hydrate dissociated at the same pressure as the synthesis pressure, the dissociation temperature and dissociation heat increased along with increasing synthesis pressure due to an increase in the cage occupancy of gas molecules. However, after most small cages were occupied by gas molecules, i.e., 20 MPa, a higher synthesis pressure no longer increased the cage occupancy, both dissociation heat and dissociation temperature maintained almost constant. On the other hand, for the gas hydrate synthesized at a high pressure (e.g. 30 MPa), the dissociation heat of the gas hydrate dissociating at 3 MPa was far lower than that dissociating isobarically at 30 MPa. To explain this phenomenon, the state function and enthalpy diagram were applied in this study, deducing that the difference of dissociation heat may derive from the system expansion at different temperatures and the heat capacity difference between different hydrate dissociation pressures. And the later one mainly contributed to the difference of dissociation heat because the heat capacity of liquid phase was much higher than that of hydrate phase.