On the energetics of oxygen separation from air

• The minimum thermodynamic and the value function are introduced and compared. • Minimum theoretical energies are evaluated for the oxygen separation from air. • Cryogenic distillation processes are more optimally designed than adsorption systems. • The minimum separative energy represents a better lower bound for energy consumption. For more than a century, various technologies and innovations have been proposed to reduce the large energy requirements associated with oxygen separation processes from air. However, the traditional approach to assess the process efficiency based on the minimum thermodynamic energy of separation may not be useful when it is desirable to obtain a high purity oxygen stream with a high oxygen recovery. A better lower bound estimate can be found in the minimum separative energy derived from the value function and the theory of isotope separation. Such minimum separative energy is in fact around 1.3–2.5 times greater than the minimum thermodynamic energy considering a wide range of oxygen product purities (90–99.9%) and oxygen product recoveries (50–99%). By comparing performance indicators of recent works on cryogenic distillation and adsorption systems for oxygen separation from air, the associated real specific energies of separation are still greater but much closer to their respective minimum specific separative energies. It is also confirmed that at present cryogenic distillation processes are more optimally designed than adsorption systems.