Equilibrium Conditions for Semiclathrate Hydrates Formed with CO2, N2, or CH4 in the Presence of Tri-n-butylphosphine Oxide

Abstract

We measured the thermodynamic stability conditions for the N2, CO2, or CH4 semiclathrate hydrate formed from the aqueous solution of tri-n-butylphosphine oxide (TBPO) at 26 wt %, corresponding to the stoichiometric composition for TBPO·34.5H2O. 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 N2, CO2, and CH4 hydrates. At a given temperature, the semiclathrate hydrate of 26 wt % TBPO solution + CH4 was more stable than that of 26 wt % TBPO solution + CO2, which in turn was more stable than that of 26 wt % TBPO solution + N2. 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 + N2, 26 wt % TBPO solution + CO2, and 26 wt % TBPO solution + CH4 were 177.75, 206.23, and 159.00 kJ·mol–1, respectively.