CO2 capture: Value functions, separative work and process economics

The relationship between the thermodynamic minimum work of separation and the “separative work” from the theory of staged separation processes is examined. These functions show similar patterns of concentration dependence over much of the range but they diverge at the extremes of high and low concentrations with the result that, for separation processes in which a pure product is extracted from a low concentration feed, the “separative work” is substantially greater than the thermodynamic minimum. The implications for CO2 capture, both from stack gas and from the atmosphere, are examined in detail. It is shown that, according to this model, the separative work required for direct air capture (per mole of CO2) is substantially greater than the thermodynamic minimum and may be as much as 100 times greater than the energy required for capture from the stack gas of a coal fired power plant.