Phase Equilibrium for Structure-H Hydrates Formed with Methane and Methyl-Substituted Cyclic Ether

Abstract

Clathrate hydrate formation in a (methane + either 3-methyltetrahydropyran or 2-methyltetrahydrofuran + water) system is demonstrated. The first data of the quadruple (water + structure-H hydrate + either 3-methyltetrahydropyran or 2-methyltetrahydrofuran + methane) equilibrium pressure-temperature conditions are measured over temperatures from 273 to 286 K. In the 3-methyltetrahydropyran system, the equilibrium pressures are lower by 1.6-2 MPa than those of the structure-I methane hydrate formed in the methane + water system at given temperatures. In the 2-methyltetrahydrofuran system, equilibrium pressures at temperatures below 278 K are lower than those for the structure-I methane hydrate, and at temperatures above 278 K, they are higher. These phase equilibria suggest the formation of hydrates other than structure-I methane hydrates in the two systems. The crystallographic structures of the hydrates are determined to be structure H by means of X-ray diffraction as expected from considerations of sizes and shapes of the 3-methyltetrahydropyran and 2-methyltetrahydrofuran molecules.

Thermodynamic Stability of Structure H Hydrates Based on the Molecular Properties of Large Guest Molecules

“…To study correlations between the molecular size and thermodynamic stability, we use equilibrium pressure data of structure-H hydrates formed with methane and 20 different LMGSs, as reported in previous studies [8][9][10][11][12][13][14][15] and calculated from interpolation of the data at 276 K. The calculations of different criteria for LMGSs molecular size were performed using the Gaussian03 program. These involved to first optimize the geometry of the LMGSs with the hybrid density functional B3LYP method and aug-cc-pVDZ basis sets, followed by a calculation of the molecular volume and the longest distance between two carbon atoms in a large molecule guest substance.…”

“…For example, at 276 K, the equilibrium pressure for the structure-H hydrate formed with methane and 2-methylbutane is 2.9 MPa [8]; while the formation pressure at 276 K is reduced to 1.6 MPa for the structure-H formed with methane and 2,2-dimethylbutane [9]. Although a number of phase-equilibrium data of structure-H hydrates with various LMGSs have been previously reported [5,[8][9][10][11][12][13][14][15], there is no comprehensive understanding on the correlation between the thermodynamic stability of structure-H hydrates and the chemical species of the LMGSs. As for the structure I and II hydrates formed with a small molecule guests modeled with the Lennard-Jones potential, there seems a tendency that the hydrate stability increases with guest molecules having optimum molecular size and greater molar mass [16].…”

Thermodynamic promotion of carbon dioxide–clathrate hydrate formation by tetrahydrofuran, cyclopentane and their mixtures

“…Tohidi et al (1997) had already shown this by measuring the promoting effect on the dissociation pressures of methane or nitrogen hydrates by adding cyclopentane to binary systems of water and gas. Ohmura et al (2005) measured hydrate dissociation pressures for two methyl-substituted cyclic ethers (2-methyltetrahydrofuran and 3-methyltetrahydropyran) with methane. These ethers were soluble in water to some extent, but not fully miscible with water, like THF.…”

Combination of surfactants and organic compounds for boosting CO2 separation from natural gas by clathrate hydrate formation

“…To the best of the author's knowledge, no data is available in the literature for the system CH 4 -CO 2 -(m-THF). However, a number of studies carried out with only m-THF and CH 4 have demonstrated that the corresponding hydrate equilibrium curve (obtained with 2.9 mol% of m-THF and CH 4 ) is close to that of pure methane hydrate [28,29]. In addition, a study of the CH 4 -CO 2 -neohexane system based on CO 2 concentrations in gas close to that used in our work shows that the equilibrium conditions of the (sH) structure formed in presence of neohexane undergo little impact compared to the CO 2 -CH 4 system which forms (sI) [45].…”

“…THF, DIOX and CP are well known to form hydrates of structure (sII), where the 5 12 cavities are empty and the 5 12 6 4 cavities are occupied by the organic molecules [3]. In addition, a number of studies carried out with m-THF and CH 4 [28,29] have demonstrated that the hydrate formed was of structure (sH). It is supposed here that m-THF forms also a structure (sH) in the presence of CO 2 or a CO 4 -CH 4 gas mixture.…”