Phase Equilibria and Dissociation Enthalpies of Hydrogen Semi-Clathrate Hydrate with Tetrabutyl Ammonium Nitrate

Abstract

This paper reports the experimentally determined thermodynamic stability conditions for the hydrogen semiclathrate hydrate generated from tetrabutyl ammonium nitrate (TBANO₃) aqueous solutions at two mole fractions, 0.037 and 0.030, corresponding to the stoichiometric composition for TBANO₃·26H₂O and TBANO₃·32H₂O, respectively. The experiments for this three-component TBANO₃ + water + hydrogen system were performed in the temperature range of (281.9 to 284.9) K and pressure range of (9.09 to 31.98) MPa with using a “full view” sapphire cell. An isochoric equilibrium step-heating pressure search method was employed to determine the phase boundary between hydrate–liquid–vapor (H-L-V) phases and liquid–vapor (L-V) phases. The results showed that the semiclathrate hydrate of TBANO₃·26H₂O + H₂ is more stable than that of TBANO₃·32H₂O + H₂, with both of these semiclathrate hydrates being much more stable than pure hydrogen hydrate. The obtained phase equilibria data were analyzed using the Clausius–Clapeyron equation to determine the dissociation enthalpy at the pressure range from (9 to 32) MPa. It was found that the mean dissociation enthalpies for the hydrogen–TBANO₃·26H₂O and hydrogen–TBANO₃·32H₂O clathrate hydrate systems were 322.53 kJ·mol^–1 and 340.23 kJ·mol^–1, respectively