Equilibrium conditions for semi-clathrate hydrates formed with CO2, N2 or CH4 in the presence of tri-n-butylphosphine oxide

We measured the thermodynamic stability conditions for the N, CO, or CH semiclathrate hydrate formed from the aqueous solution of tri-n-butylphosphine oxide (TBPO) at 26 wt %, corresponding to the stoichiometric composition for TBPO·34.5HO. The measurements were performed in the temperature range 283.71-300.34 K and pressure range 0.35-19.43 MPa with the use of an isochoric equilibrium step-heating pressure-search method. The results showed that the presence of TBPO made these semiclathrate hydrates much more stable than the corresponding pure N , CO, and CH hydrates. At a given temperature, the semiclathrate hydrate of 26 wt % TBPO solution + CH was more stable than that of 26 wt % TBPO solution + CO, which in turn was more stable than that of 26 wt % TBPO solution + N. We analyzed the phase equilibrium data using the Clausius-Clapeyron equation and found that, in the pressure range 0-20 MPa, the mean dissociation enthalpies for the semiclathrate hydrate systems of 26 wt % TBPO solution + N, 26 wt % TBPO solution + CO, and 26 wt % TBPO solution + CH were 177.75, 206.23, and 159.00 kJ·mol, respectively