Phase Equilibria and Dissociation Enthalpies of Tri-n-butylphosphine Oxide Semiclathrate Hydrates Incorporated with CH4, CO2, and H-2

Abstract

We investigated the phase equilibrium boundary of tri-n-butylphosphine oxide (TBPO) semiclathrate hydrates incorporated with CH4, CO2, and H-2. TBPO aqueous solutions with a molality (m) of (1.61 and 1.98) mol.kg(-1) were used for hydrate formation, which corresponded to the clathrate structures of TBPO. 34.5H(2)O and 28H(2)O, respectively. The phase boundary at both concentrations was shifted to the promotion region represented by lower pressures and higher temperatures, compared to each simple gas hydrate. In particular, TBPO + CO2 double hydrate presented mild hydrate stabilization conditions of <1 MPa at (280 to 285) K. Additionally, the dissociation enthalpy (Delta H-d) calculated from the phase boundary curves for the TBPO + CO2 double hydrates was almost the same as that for tetra-n-butylammonium bromide (TBAB) + CO2 double hydrate (219.5 kJ.mol(-1) for m = 1.61 mol.kg(-1) and 211.6 kj.mol(-1) for m = 1.98 mol.kg(-1)). These results demonstrate that the TBPO + CO2 double hydrate could be used as refrigerants for storage and transportation.