Investigation of Membrane Systems Efficiency in Methane Recovery or Nitrogen Removal from Waste Gas of LNG Projects by Mathematical Modeling

Removal of nitrogen from natural gas has been important because of defects such as reduction of flame temperature, thermal value of fuel, increasein costs of transfer pipeline and pressure improving installations. Cryogenic distillation is mostly used for this purpose; however other technologies suchas membrane separation and pressure swing adsorption have been suggested due to high capital investment costs. The Process and economicrationales of these separation methods depend on parameters such as contamination amount in feed, pressure and flow rate of feed and necessary puritygrade of product. In this study, Investigation and design of membrane systems were done by an approximate modeling technique that proposed forspiral-wound membrane for reduction of nitrogen from the waste gas, form 33 mol. % to less than 20 mol. %, were evaluated. Designs were performed based on two types of membrane, methane or nitrogen selective membranes. The obtained results demonstrated that two-step membrane system with methane selective membrane at low flow rate (10 MMscfd) seemed more suitable from process andeconomical view point. Maximum recovery and purity percentages for methane were 90% and 80%, respectively but this system in high flow ratebecause of high capital investment costs is an uneconomical process. Generally, in high flow rate and with nitrogen content more than 15 mol. %,cryogenic distillation is the only economic process by which the obtainable purity percentage for methane could be more than 99%.