The energy requirements for two CO2 separation processes (VSA and membrane), calculated from detailed numerical simulations, are shown to be qualitatively consistent with the trends predicted from value function theory. At low feed concentrations the work of separation increases strongly, in contrast to the thermodynamic minimum work which shows only a very gradual increase. This has important implications in relation to processes such as direct air capture of CO2 in which a concentrated product is separated from a very dilute feed. The present results suggest that the energy requirements for such processes may be much greater than that has been generally assumed. (C) 2015 Elsevier Ltd. All rights reserved.