Nitrogen Sorption in a Transition Metal Complex Solution for N₂ Rejection from Methane

Abstract

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.

A critical review on the practical use of lithium cycle as an upfront nitrogen removal technology from natural gas

“…This method was investigated by Gilbertson et al 78 more than a decade ago, and while the authors report the technical feasibility of using trans ‐Fe(DMeOPrPE) 2 Cl 2 complex for the absorption of N 2 from N 2 /CH 4 mixtures, they do not elaborate on performance parameters and no selectivities/separation efficiencies were provided. On the other hand, Li et al 15 report an N 2 /CH 4 selectivity in the range of 1.7–2.4 by K‐[Ru II (EDTA)] depending on feed temperture and pressure at 30°C and 3–30 bar, respectively. The use of this technology's for nitrogen removal is severely hindered due to lack of interest in this kind of research.…”

“…The literature review outlines numerous processes for UNR, including (1) physical separation technologies, such as adsorption, 6–8 membrane separation, 9–11 hybrid processes, and distillations 12 ; (2) chemical separation technologies, 13–15 including absorption 15 and lithium‐based adsorption 14,16 ; and (3) gas hydrate technology 17–20 . A detailed discussion of all these processes can be seen in our previous publication 5 …”

Prospective of Upfront Nitrogen (N2) Removal in LNG Plants: Technical Communication

“…The utilized solutions managed to have around 5.75 N 2 selectivity over methane. Li et al [65] achieved a N 2 selectivity over methane of around 2 using ruthenium-based TMC at ambient temperature and a pressure of 30 bar. However, Li stated that a ruthenium-based TMC absorption process would be hindered in industrial-scale applications due to its limited availability and its low concentration in an aqueous solution [66].…”