As a high-performance adsorbent, carbon molecular sieves are widely used in many fields around us. Let us take a closer look at their adsorption characteristics regarding nitrogen and oxygen.
Carbon molecular sieves can adsorb both oxygen and nitrogen from the air; the amount adsorbed increases with pressure, and at any given pressure, there is no significant difference in the equilibrium adsorption capacity between the two gases. Consequently, it is difficult to effectively separate oxygen and nitrogen based solely on pressure changes. However, by considering the rate of adsorption, the characteristics of oxygen and nitrogen adsorption can be effectively distinguished. Oxygen molecules have a smaller diameter than nitrogen molecules and diffuse hundreds of times faster; thus, the adsorption rate for oxygen is very rapid, reaching over 90% within approximately one minute. At this stage, nitrogen adsorption is only about 5%; therefore, the adsorbed gas is predominantly oxygen, while the unadsorbed portion is mostly nitrogen.
Thus, if the adsorption time is limited to one minute, preliminary separation of oxygen and nitrogen can be achieved. In other words, adsorption and desorption are driven by pressure differentials: adsorption occurs as pressure rises, and desorption occurs as pressure falls. The differentiation between oxygen and nitrogen is managed by controlling the adsorption time; by keeping the duration very short, oxygen is fully adsorbed while the process is halted before significant nitrogen adsorption occurs. Therefore, in pressure swing adsorption (PSA) nitrogen generation, the pressure is cycled, and the cycle time is controlled to within one minute.