Pyrazine Analogues on Clathrate Hydrates in Methane–WaterSystems

This paper reports the change in equilibrium pressure–temperature (pT) boundaries in a methane (CH4)–water system by adding three pyrazine analogues: 2-methylpyrazine, 2,3-dimethylpyrazine, and 2,3,5-trimethylpyrazine. The 2-methylpyrazine, 2,3-dimethylpyrazine, and 2,3,5-trimethylpyrazine showed no effect on the clathrate hydrate structure that formed in the CH4–water system upon analysis via Raman spectroscopy. Raman spectra revealed that clathrate crystals that formed in the CH4–pyrazine analogue–water system were of a structure-I CH4 hydrate. Phase equilibrium pT boundaries in the three CH4–pyrazine analogue–water systems shifted to a higher p and lower T condition than that for the CH4–water system with no additive. The higher p/lower T shift confirmed that 2-methylpyrazine, 2,3-dimethylpyrazine, and 2,3,5-trimethylpyrazine act as thermodynamic inhibitors for hydrate formation. The order of ΔT, a lower T shift from the CH4–water system, was as follows: 2-methylpyrazine–water > 2,3-dimethylpyrazine > 2,3,5-trimethylpyrazine. This order is in agreement with the number of −CH3 groups in the molecules.