Semiclathrate hydrate phase behaviour and structure for CH4 in the presence of tetrabutylammonium fluoride (TBAF)

Abstract

Phase behaviour including phase equilibria and dissociation enthalpies and structure of double semiclathrate hydrates formed with tetrabutylammonium fluoride (TBAF) + CH4 were measured to evaluate the performance of TBAF for hydrate based industrial applications. The phase equilibria and the dissociation enthalpies for TBAF + CH4 semiclathrate hydrate (SCH) with (0.02–0.15) salt mass fraction were obtained by employing the isochoric pressure-search method and calculated from the experimental data with Clausius-Clapeyron equation, respectively. A confocal Raman spectrometer was applied to analyze the structure of (TBAF + CH4) SCHs. Data showed that TBAF strongly enhanced the stability of CH4 hydrates by increasing the equilibrium temperature at a given pressure. The Raman spectra revealed that small 512 cages of TBAF hydrates could trap CH4 molecules. In particular, the Raman spectra of double TBAF + CH4 SCHs, as compared with those of pure TBAF SCHs, indicated that the large cages which already trapped the cation of TBAF also had the possibility of encaging CH4 molecules at low salt concentrations (<0.12 mass fraction).