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

Experimental determination of CCl4 hydrate phase equlibria up to high pressures

A number of hydrate phase boundaries of the binary system of tetrachloromethane (CCl4) + water were measured experimentally at several temperatures and from low pressures up to 89.25 MPa. These hydrate phase boundaries included hydrate–ice–vapor, hydrate–liquid CCl4–vapor, hydrate–water–vapor, hydrate–solid CCl4–liquid CCl4, hydrate–solid CCl4–water, and hydrate–liquid CCl4–water. From the points of intersections of the different boundaries, the three quadruple points of hydrate–ice–water–vapor, hydrate–liquid CCl4–water–vapor, and hydrate–solid CCl4–liquid CCl4–water were determined as (273.18 K, 4.72 kPa), (273.71 K, 5.25 kPa), and (273.35 K, 55.94 MPa), respectivel

Stable methane hydrate above 2 GPa and the source of Titan's atmospheric methane

Methane hydrate is thought to have been the dominant methane-containing phase in the nebula from which Saturn, Uranus, Neptune and their major moons formed1. It accordingly plays an important role in formation models of Titan, Saturn's largest moon. Current understanding1, 2 assumes that methane hydrate dissociates into ice and free methane in the pressure range 1\ufffd2 GPa (10\ufffd20 kbar), consistent with some theoretical3 and experimental4, 5 studies. But such pressure-induced dissociation would have led to the early loss of methane from Titan's interior to its atmosphere, where it would rapidly have been destroyed by photochemical processes6, 7. This is difficult to reconcile with the observed presence of significant amounts of methane in Titan's present atmosphere. Here we report neutron and synchrotron X-ray diffraction studies that determine the thermodynamic behaviour of methane hydrate at pressures up to 10 GPa. We find structural transitions at about 1 and 2 GPa to new hydrate phases which remain stable to at least 10 GPa. This implies that the methane in the primordial core of Titan remained in stable hydrate phases throughout differentiation, eventually forming a layer of methane clathrate approximately 100 km thick within the ice mantle. This layer is a plausible source for the continuing replenishment of Titan's atmospheric methane.NRC publication: Ye