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 CO2-H2O, we have proposed and validated a new express procedure for determining the temperature and pressure of the lower quadruple point Q1. 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 CO2 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.

Dataset for the dimethyl sulfoxide as a novel thermodynamic inhibitor of carbon dioxide hydrate formation.

The temperatures and pressures of the three-phase equilibrium V-Lw-H (gas - aqueous solution - gas hydrate) were measured in the CO2 - H2O - dimethyl sulfoxide (DMSO) system at concentrations of organic solute in the aqueous phase up to 50 mass%. Measurements of CO2 hydrate equilibrium conditions were carried out using a constant volume autoclave by continuous heating at a rate of 0.1 K/h with simultaneous stirring of fluids by a four-blade agitator at 600 rpm. The equilibrium temperature and pressure of CO2 hydrate were determined for the endpoint of the hydrate dissociation in each experiment. The CO2 gas fugacity was calculated by the equation of state for carbon dioxide for the measured points. The flow regime in the autoclave during the operation of the stirring system was characterized by calculating the Reynolds number using literature data on the viscosity and density of water and DMSO aqueous solutions. We employed regression analysis to approximate the dependences of equilibrium pressure (CO2 gas fugacity) on temperature by two- and three-parameter equations. For each measured point, the value of CO2 hydrate equilibrium temperature suppression ΔTh was computed. The dependences of this quantity on CO2 gas fugacity are considered for all DMSO concentrations. The coefficients of empirical correlation describing ΔTh as a function of the DMSO mass fraction in solution and the equilibrium gas pressure are determined. This article is a co-submission with a paper [1].