The removal of CO2 and N2 from natural gas: A review of conventional and emerging process technologies

This article provides an overview of conventional and developing gas processing technologies for CO2 and N2 removal from natural gas. We consider process technologies based on absorption, distillation, adsorption, membrane separation and hydrates. For each technology, we describe the fundamental separation mechanisms involved and the commonly applied process flow schemes designed to produce pipeline quality gas (typically 2% CO2, <3% N2) and gas to feed a cryogenic gas plant (typically 50 ppmv CO2, 1% N2). Amine absorption technologies for CO2 and H2S removal (acid gas treating) are well-established in the natural gas industry. The advantages and disadvantages of the conventional amine- and physical-solvent-based processes for acid gas treating are discussed. The use of CO2 selective membrane technologies for bulk separation of CO2 is increasing in the natural gas industry. Novel low-temperature CO2 removal technologies such as ExxonMobil’s Controlled Freeze Zone™ process and rapid cycle pressure swing adsorption processes are also emerging as alternatives to amine scrubbers in certain applications such as for processing high CO2 concentration gases and for developing remote gas fields. Cryogenic distillation remains the leading N2 rejection technology for large scale (feed rates greater than 15 MMscfd) natural gas and liquefied natural gas plants. However, technologies based on CH4 selective absorption and adsorption, as well as N2 selective pressure swing adsorption technologies, are commercially available for smaller scale gas processing facilities. The review discusses the scope for the development of better performing CO2 selective membranes, N2 selective solvents and N2 selective adsorbents to both improve separation power and the durability of the materials used in novel gas processing technologies.