Integrated ethane recovery and cryogenic carbon capture in a dual mixed refrigerant natural gas liquefaction process

Abstract

Carbon capture by direct cryogenic distillation in conventional natural gas liquefaction plants is quite difficult due to the easy sublimation characteristics of CO2. The freeze–out temperature of CH4–CO2–C2H6 ternary mixtures is analyzed in this study, and the results show that with the azeotropic properties of ethane-CO2 mixtures, CO2 in natural gas can be reduced to below 50 ppm by cryogenic distillation. Based on thermodynamic analysis, a novel dual mixed refrigerant (DMR) liquefaction process integrated with ethane recovery and carbon capture is proposed for natural gas that is rich in ethane. CO2 is first captured from natural gas along with ethane by cryogenic distillation, and then further separated from the CO2–C2H6 mixture by extractive distillation to obtain ethane. The proposed process is simulated in HYSYS and optimized by a method combining a genetic algorithm (GA) and stepwise search. Energy analysis results show that when the ethane content is 2–20 mol%, the specific power consumption corresponding to the maximum allowable CO2 content with ethane recovery rate and purity both reaching 99.5 % is around 0.40 kWh/Nm3 (NG). In addition, the comparative analysis results show that the proposed process has better performance than chemical absorption in both energy consumption and cleaner production aspects.