Thermodynamic efficiency of membrane separation of dilute gas: Estimation for CO2 direct air capture application

• The current research activities focus on the throughput of CO 2 separation and ignore energy efficiency. The estimation of energy efficiency specific to direct air capture (DAC) has been awaited. • The theoretical limits of energy (exergy) efficiency of separating very dilute gas corresponding to DAC are calculated as a function of the membrane's CO 2 separation ratio r . Vacuum swing and continuous pumping schemes are treated. • Efficiency critically depends on the separation ratio r : 0.13 % for r = 10, 2.0 % for r = 100, and 49.9 % for r = 5000 for concentrating 400 ppm CO 2 . • The energy efficiency of reported CO 2 separation simulations and experiments has been thoroughly reviewed, and the candidates for achieving high efficiency are presented. Gas separation technology is crucial for addressing environmental issues like CO 2 capture to mitigate climate change. While membrane separation is often cited for its efficiency, accurate estimations are scarce. We present estimations based on classical thermodynamics for very lean CO 2 composition (400 ppm), revealing rich details in simple systems and deriving guiding principles. Our main conclusion emphasizes the critical necessity of a high membrane separation ratio, and we discuss candidates for achieving this goal.