Hydrate phase equilibrium condition of the synthetic natural gas with high content of CO2 in the electrolyte solutions containing methanol

It is well known that CO2 is easier to form hydrate because its hydrate phase equilibrium pressure was lower than CH4 at a given temperature. The natural gas containing high content of CO2 increases pipeline blockage risk due to hydrate problem in realistic situations. In industry, the organic inhibitors are often added to pipelines combined with the dissolved salts in the produced water to prevent hydrate formation. In this study, the hydrate phase equilibrium condition of two synthetic natural gas (SNG) samples (with high and low content of CO2) in various systems was measured with the isothermal pressure search method. It was found the adding of 0.1 mass fraction MeOH and 0.075 mass fraction NaCl to pure water lowered the hydrate equilibrium temperature of SNG with high content of CO2 7.61 K at a given pressure. In addition, the hydrate morphology and size was investigated with the particle video microscope and focused beam reflectance measurement probes, respectively. The SNG formed flake-like hydrates in the pure water and salt solution systems, respectively, while powder-like hydrates occurred in the system containing MeOH. It was found that the average chord length of SNG hydrate changes from 205 μm to 53 μm after adding MeOH in the system. Based on the measured phase equilibrium data, the hydrate dissociation enthalpy was calculated with the Clausius-Clapeyron equation. The results indicated that the hydrate dissociation enthalpies of two SNG samples are (67.32 and 63.04)  kJ·mol−1, respectively. The inhibitors (salt and MeOH) have no effect on the hydrate dissociation enthalpy since they would not enter hydrate cavity.