A transient heat and mass transfer model is developed to simulate the desorption step of a temperature vacuum swing cyclic process for CO2 capture from ambient air. The model incorporates binary CO2 and H2O adsorption equilibrium data, heat of adsorption, heat conduction, gas phase diffusion, and considers CO2 adsorption from both dry and moist air. A set of desorption experimental runs was carried out using a packed bed of amine-functionalized nanofibrillated cellulose sorbent material in the temperature range 20-95 degrees C and at 62 mbarabs desorption pressure. Numerically calculated temperature profiles were compared to those obtained experimentally. The simulation results indicate fast gas diffusion and local re-adsorption of CO2 and H2O in colder zones of the reactor. The risk of O-2-induced sorbent degradation is substantially reduced under humid air conditions as opposed to dry conditions, since air is purged out of the reactor by co-desorbed H2O before reaching a critical temperature. Further, the CO2 recovery rates at a given purity level are analyzed for the first time for a direct air capture system and it is found that, for typical process conditions, more than 90% of the captured CO2 can be recovered at >99% purity. (C) 2015 Elsevier B.V. All rights reserved.