Semiclathrate hydrates of methane+tetraalkylammonium hydroxides

•TBAOH has a better promotion effect than TBAB on CH4 hydrate.•TBAOH gives only slightly lower methane storage capacity than TBAB.•TPrAOH and TPeAOH have weak inhibition effects on CH4 hydrate. Clathrate hydrate crystallisation is a promising method for the capture, storage and transport of methane, but one of its major technical barriers is the high pressure required for hydrate formation. In search of suitable and cost-effective promoters for hydrate-based gas processing and handling, this paper reports the pressures required for the formation of clathrate hydrates of methane in the presence of one of the following three quaternary ammonium salts with the same anion (OH−) but different carbon chain lengths: tetrapropylammonium hydroxide (TPrAOH), tetrabutylammonium hydroxide (TBAOH), and tetrapentylammonium hydroxide (TPeAOH). The phase boundary between hydrate–liquid–vapor (H-L-V) phases and liquid–vapor (L-V) phases in the temperature range of 281.81–301.91K and pressure range of 4.46–16.05MPa was determined using an isochoric equilibrium step-heating pressure search method. The results show that TBAOH is a strong thermodynamic promoter for methane hydrate formation whereas TPrAOH and TPeAOH are weak hydrate inhibitors. We then compared the phase equilibrium conditions and gas storage capacities of TBAOH and a widely studied hydrate promoter, tetrabutylammoium bromide (TBAB). It was found that at a given dosage, TBAOH substantially outperformed TBAB in thermodynamically promoting methane hydrate formation and with an appropriate driving force TBAOH gave only slightly lower methane storage capacity than TBAB. These features make TBAOH a prospective promoter for hydrate-based methane storage and transport.