Direct Measurement of the Four-Phase Equilibrium Coexistence Vapor-Aqueous Solution-Ice-Gas Hydrate in Water-Carbon Dioxide System.

Precise data on the non-variant equilibrium of the four phases (vapor-aqueous solution-ice-gas hydrate) in P - T coordinates are highly desired for developing accurate thermodynamic models and can be used as reference points (similar to the triple point of water). Using the two-component hydrate-forming system CO 2 -H 2 O, we have proposed and validated a new express procedure for determining the temperature and pressure of the lower quadruple point Q 1 . The essence of the method is the direct measurement of these parameters after the successive formation of the gas hydrate and ice phases in the initial two-phase gas-water solution system under intense agitation of the fluids. After relaxation, the system occurs in the same equilibrium state ( T = 271.60 K, P = 1.044 MPa), regardless of the initial parameters and the order of crystallization of the CO 2 hydrate and ice phases. Considering the combined standard uncertainties (±0.023 K, ±0.021 MPa), the determined P and T values agree with the results of other authors obtained by a more sophisticated indirect method. Validating the developed approach for systems with other hydrate-forming gases is of great interest.