Phase Equilibria of CO₂ Hydrate in Aqueous Solutions of N-Methyldiethanolamine

Phase equilibria of carbon dioxide (CO2) hydrate in aqueous solutions of N-methyldiethanolamine (MDEA) at four different mass fractions (5, 10, 15, and 25 wt %) in the temperature range of 274.01 to 281.93 K and pressure range of 1.79 to 4.34 MPa were experimentally investigated by adopting an isochoric pressure search method. The reliability and accuracy of our experimental apparatus and method were examined by comparing the measured data with published ones. The enthalpy of CO2 hydrate dissociation was calculated by the Clausius-Clapeyron equation. The experimental results showed that MDEA acted as a thermodynamic inhibitor which could suppress the equilibrium line of CO2 hydrate to lower temperatures or higher pressures. The thermodynamic inhibition effect was enhanced after increasing the mass fraction. However, it was weaker than the conventional thermodynamic inhibitors of ethylene glycol (EG) and sodium chloride (NaCl). MDEA had synergistic inhibition effects with EG and NaCl, as their mixtures had stronger thermodynamic inhibition performance. The average enthalpies of CO2 hydrate dissociation in the presence of MDEA (5 to 25 wt %) were found to be in the range of 67.562 to 80.776 kJ center dot mol-1, which were very close to that of pure water. The research results have theoretical guidance for the flow assurance of natural gases.