Phase equilibria and crystallographic structure of clathrate hydrate formed with carbon dioxide and cyclohexanone

Abstract

This paper reports the phase equilibrium and crystallographic data of the hydrate formed in the CO₂ + cyclohexanone + water system. We measured the phase equilibrium condition and conducted the powder X-ray diffraction measurements. The formation of the structure II hydrate in the system of CO₂ + cyclohexanone + water was observed at the temperature from 270.0 K to 275.6 K, under the pressure from 0.62 MPa to 1.70 MPa.

At 270.0 K – 275.6 K and 0.62 MPa – 1.70 MPa, the structure II hydrate formed, and the phase equilibrium condition alleviated in the system of CO₂ + cyclohexanone + water, while the hydrate formed at 276.5 K - 280.7 K and 2.02 MPa – 3.33 MPa, displayed structure I and cyclohexanone acted as an inhibitor. Equilibrium conditions were measured using the isochoric method, revealing different p-T slopes at high and low temperatures. The high-temperature slope closely resembled that of structure I CO₂ hydrate reported previously. We continuously monitored temperature and pressure during the structural phase transition, resulting in a slope change. To directly determine the hydrate structure, PXRD measurements were conducted on two samples: one from the high-temperature side and the other from the low-temperature side. The sample from the high-temperature side exhibited structure I with a lattice constant of 11.8749 (9) Å at 153 K, while the low-temperature sample displayed structure II with a lattice constant of 17.443(1) Å at 153 K.