## 3 Results and Discussion
### Comparison of Oxygen Distributions between CP-\(\pi\) and CF-\(\pi\) Adsorbers  
Figure 6 shows the distribution of oxygen purity (mole fraction of oxygen on the gas phase) in the adsorbent bed after the first cycle of the PSA oxygen generation in the CP-\(\pi\) adsorber. During the Pr step, the oxygen purity in the bed can reach up to 62.1%. Along the centripetal flow direction, the oxygen purity gradually increased, and the oxygen purity in front of the center area is much closer to the one in the center channel. After the Ad step, the oxygen purity can reach up to 76.5%,  
Figure 4: Variations of maldistribution factor integrating the surface and corresponding velocity (M\({}_{\rm f}\)) along with adsorption bed radial length at Ad step in each adsorber. Figure 5: Variations in the pressure differences between center and outer channels along with axial lengths of CP-\(\pi\) and CP-\(Z\) adsorbers at the Pu step. demonstrating the enrichment of oxygen. The Cb step leads to the abundant desorption of nitrogen from adsorbents, resulting in decline in oxygen purity to 8.96% at the inlet end of the bed. Owing to the closedown of the center collector pipes, the highest oxygen purity in the adsorper remained at 59%. After the purge process with highly concentrated oxygen, the oxygen purity in the outlet of the bed is about 40%. Figure 7 shows the distribution of oxygen purity in the adsorbent bed after the first cycle of the PSA oxygen generation in the CF-\(\pi\) adsorber. During the pressurization step, the highest oxygen purity is 39.3% which is closer to the outlet of the adsorbent bed in comparison to the CP-\(\pi\) adsorber. After the Ad step, the highest oxygen purity in the bed is only 49.5%, mainly located at the dead space of the outlet area. The oxygen product concentration can only be enriched to 38%, which is much lower than in the CP-\(\pi\) adsorber. At the end of the Cb step, the oxygen purity in the inlet of the adsorbent bed decreases to 9.52%. After the Pu step, the oxygen purity in the major region is around 10%, indicating that a stronger desorption is needed. Figures 6 and 7 show that the highest oxygen purity in the bed is remarkably higher for CP-\(\pi\) flow than for CF-\(\pi\) flow. After the Pr step, the mass transfer area in the bed is close to the outlet of the bed for the CF-\(\pi\) adsorber. Nevertheless, for the CP-\(\pi\) adsorber, a larger area of adsorbent bed that is not adsorbed still exists in the outlet of the bed, and the desorption of nitrogen after the Pu step is more complete. Figure 8a,b show the oxygen purity profiles at the axial center position along with radial length after the first cycle for both adsorbers, which are similar with results from 1-D simulations [22; 37] and 2-D axial flow simulations [28]. It is clearly observed that the oxygen puri  
Figure 6: Oxygen distribution in the CP-\(\pi\) adsorber at the end of steps (**a**) Pr, (**b**) Ad, (**c**) Cb, and (**d**) Pu. Figure 7: Oxygen distribution in the CF-\(\pi\) adsorber at the end of steps (**a**) Pr, (**b**) Ad, (**c**) Cb, and (**d**) Pu. outlet than in the inlet of the bed. During the Pr and Ad steps, oxygen is enriched along the direction to the outlet of the adsorption bed while during the Cb and Pu steps, an amount of nitrogen desorbed and flowed out of the adsorber. After the Ad and Pu steps, the oxygen purity at the outlet of the adsorption bed of CP-\(\pi\) and CF-\(\pi\) is 66% and 36%, and 45% and 23%, respectively, suggesting a better oxygen enrichment and nitrogen desorption ability of the CP-\(\pi\) adsorber, and thus, more concentrated oxygen can be achieved in the next cycle [1]. The PSA oxygen generation process is a variable mass flow process. The majority of nitrogen in the air is adsorbed by the bed when air flows through the bed, and the flow rate gradually decreases along the flow direction [28]. In the current CP-\(\pi\) adsorber, the cross-sectional area of the inlet is 35 times larger than that of the outlet.