Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride

The experimental study on phase equilibrium conditions for semi-clathrate hydrates formed in the systems of CH4/N2 gas mixture and tetra-butyl-ammonium fluoride (TBAF) with different ratios (0.210, 0.293 and 0.500 mol%) to water was conducted in this paper. The experiments were carried out under the conditions of 281.15-291.15 K and 0.30-3.70 MPa. The structures of the hydrates were characterized using in-situ Raman spectroscopy at 276.15 K and 2.50 MPa. At a certain given pressure, the equilibrium temperature of the semi-clathrate hydrates formed in the systems with 0.500 mol% TBAF is significantly higher than that of semi-clathrate hydrates formed the systems with 0.210 or 0.293 mol% TBAF, and even higher than that of structure II hydrates formed in the systems with THF-SDS. Moreover, TBAF has more positive influence on enhancing dissociation enthalpies of the hydrates than THF. Due to the quite weak signal of N-N triple bond vibration or/and seldom N2 molecules encaged into the cavities, only CH4 molecules could be determined clearly in the mixed hydrates. In addition, the crystal morphology of the semi-clathrate hydrates is affected by the ratio of TBAF to water.