However, in reality, there may be a trade-off between density and CO2 LDF constant. The optimal adsorbent for a TVSA process should have the lowest heat capacity possible in the range studied, if not lower. It should also have a CO2 isotherm with a gradient in the low-pressure region which is not steeper than that of Lewatit® VP OC 1065. In contrast, for an S-TVSA process, the CO2 isotherm should have as steep a gradient as possible in the low-pressure region to maximise the CO2 adsorption capacity under ambient conditions. Email: jpy1@hw.ac.uk RCCS, Heriot-Watt University