Thermodynamic characteristics of methane hydrate formation in high-pressure microcalorimeter under different reaction kinetics

To understand the thermodynamic characteristics of CH4 hydrate formation in a high-pressure microcalorimeter under different reaction kinetics, five typical systems including CH4-H2O, CH4-Tetrahydrofuran (THF)-H2O, CH4- Cyclopentane (CP)-H2O, CH4-Methyl cyclohexane (MCH)-H2O, and CH4-tert-Butyl methyl ether (TBME)-H2O are adopted to conduct experiments in this work. The results show that their hydrate formation thermodynamic characteristics depend greatly on the promoter. For the CH4-H2O system, a hydrate crystals film will form quickly at the beginning of the reaction, hindering the mass transfer between gas and liquid, thereby presenting an extremely slow hydrate formation kinetics. After adding the water-soluble promoter that cannot form an oil phase, like THF, a rapid hydrate formation process is observed from the cooling stage. However, if the promoter can form an oil phase (CP, MCH, and TBME), regardless of whether the promoter is soluble in water, only a slow hydrate formation kinetics can be observed. This is because the oil phase can separate the water and gas phase, reducing their mass transfer efficiency, thereby restricting the hydrate growth. Interestingly, a rapid hydrate formation process can be obtained during their heating stage. These indicate that high driving force cannot always lead to a rapid hydrate formation kinetics. It is worth mentioning that a similar hydrate formation and dissociation behavior in high-pressure microcalorimeter is observed for water-insoluble promoters (CP, MCH), indicating the hydrate structure will not significantly affect their thermodynamic behavior except for the in-tensity of the peaks.