Hydrate Formation during Transport of Natural Gas Containing Water and Impurities

The upper limit of water content permitted in a natural gas stream during its pipeline transport without a risk of hydrate formation is a complex issue. We propose a novel absolute-thermodynamic scheme for investigation of different routes to hydrate formation, with ideal gas used as reference state. This makes comparison between different hydrate formation routes transparent and consistent in free energy changes and associated enthalpy change. Pipeline inner walls are typically initially covered by rust. Natural gas hydrate can form directly from water dissolved in natural gas; the other two alternatives will involve water either condensing out to create a liquid water phase, or adsorbing on solid surfaces. Hydrate former guests CO2 and H2S exhibit substantial solubility in water and adsorb together with water onto rust surfaces (taken to be hematite in our study). Natural gas from the North Sea is typically lean in H2S, with separated gas streams containing up to 10% CO2. In contrast with transport of dense CO2, methane will dominate the density dependence at densities below 300 kg/m3. The maximum tolerated water fraction was not found to be sensitive to sour gas concentrations below 10%, with the results clearly more sensitive to H2S than CO2.