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[RuII(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[RuII(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[RuII(EDTA)] solution was in the range of 30–60 kJ/mol N2, suggesting the regeneration of the K[RuII(EDTA)] solution would require less energy than that required for CO2 scrubbing using MEA aqueous solution. The N2/CH4 selectivity in the K[RuII(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[RuII(EDTA)] aqueous solution exhibit a great potential of the solution for nitrogen rejection from natural gas.