Nitrogen sorption in a transition metal complex solution for N₂ rejection from methane

Nitrogen is a ubiquitous impurity in natural gas that has to be removed for the production of liquefied natural gas (LNG). The most widely used technology for N₂ rejection, cryogenic distillation, is a capital and energy intensive process. In this work, a nitrogen selective K[Ru^II(EDTA)] aqueous solution was prepared and tested for nitrogen sorption with the aim of reducing the N₂ rejection costs by using a continuous recirculation absorption process analogous to the acid gas removal process in LNG production. The overall equilibrium amount of N₂ sorption in the K[Ru^II(EDTA)] solution was obtained at 20 °C (0.075 mol N₂/L solution at 2860 kPa), 30 °C (0.061 mol N₂/L solution at 2873 kPa), and 40 °C (0.052 mol N₂/L solution at 3049 kPa) using a custom-built volumetric sorption measurement apparatus. The corresponding specific N₂ sorption amounts were 0.54 mol N₂/mol Ru at 20 °C and 2860 kPa; 0.43 mol N₂/mol Ru at 30 °C and 2873 kPa; and 0.34 mol N₂/mol Ru at 40 °C and 3049 kPa. These specific N₂ sorption amounts exhibited similar values to the specific loading of carbon dioxide in monoethanolamine (MEA) which is one of the most widely used chemicals for acid gas removal in LNG production industry. The heat of N₂ absorption in the K[Ru^II(EDTA)] solution was in the range of 30-60 kJ/mol N₂, suggesting the regeneration of the K[Ru^II(EDTA)] solution would require less energy than that required for CO₂ scrubbing using MEA aqueous solution. The N₂/CH₄ selectivity in the K[Ru^II(EDTA)] aqueous solution is in the range of 1.7 to 2.4 depending on the pressure of the gas, which is the highest N₂/CH₄ selectivity known for a liquid based N₂ and CH₄ separation system. The comparable specific N₂ sorption capacity to CO₂-amine system and the high N₂/CH₄ selectivity of the K[Ru^II(EDTA)] aqueous solution exhibit a great potential of the solution for nitrogen rejection from natural gas.