Process of reducing water, CO2 and N2O in feed air, which: a first adsorbent such as alumina (25-40% volume) and a second adsorbent such as X zeolite (60-75% volume) are used; the online time of the adsorbent is determined by determining the concentration measured by an analyzer for CO2 concentration at a position within the length of the second adsorbent when a maximum level of N2O is simultaneously obtained at the downstream end of the second adsorbent in the feed direction, wherein the online time is the time taken from commencing passing the feed air to the first and second adsorbents to the measurement by the analyzer of the determined concentration of CO2; at least the second adsorbent is regenerated by heated regeneration gas at a temperature of 140° C.-220° C.; and the molar ratio of the regenerating gas to feed air supplied during one iteration of the cycle is 0.08-0.5.