Distillation Based CO2 Removal from Natural Gas for Small and Medium Sized Plant

Abstract

The removal of excess CO2 from natural gas to levels as low as 50 ppm is essential for the safe and reliable operation of liquefied natural gas (LNG) transport and delivery systems. Current chemical purification techniques, which are suitable for large processing plants, might not be suitable for small or mid-size plants which are expected to operate in future LNG delivery networks. The feasibility of purification of natural gas (NG) from CO2 down to a concentration of 50 ppm by multi-stage distillation is studied. A three-column distillation system is proposed that can purify NG to lower than 50 ppm concentration of CO2, while avoiding CO2 freezeout. The columns include a 30-stage demethanizer, in which high purity methane is obtained in the distillate by separating the impurities from natural gas including CO2; a 50-stage extractive column where the azeotrope between CO2 and ethane is broken; and a 50-stage solvent recovery column that recovers a mixture of heavy hydrocarbons suitable for recycling as a solvent back into the extractive column. The proposed system avoids CO2 freezeout by utilizing a multi component feed of some heavier hydrocarbons added to natural gas; propane, butane and pentane additives are injected into stage 20 of the demthanizer column alongside the raw feed. Furthermore, arrangements are made to break the CO2-ethane azeotrope, which may occur in the bottoms stream of the demthanizer by administering a solvent stream in the extractive column. The proposed system can operate in a closed loop arrangement where the bottoms stream that leaves the recovery column can be recycled and injected into the extractive column for azeotrope prevention.