## 3 Results and Discussion  
### Selection of the Optimal Radial-Flow Type  
Figure 3 gives the change of pressure in the outer flow channel of each radial-flow adsorber within one cycle of a stabilized PSA process. It can be seen that the four radial-flow adsorbers give similar patterns of pressure changes with the order of the value of CF-\(\tau\)\(\rho\)-CF-Z\(>\)CP-Z\(>\)CP-\(\pi\), indicating that the effect of the flow type on the pressure change in the bed during the radial-flow PSA process is small. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Unit** \\ \hline Gas composition & 21\% O\({}_{2}\), 79\% N\({}_{2}\) & - \\ Pressure & 101325 & Pa \\ Temperature & 298 & K \\ Amount of O\({}_{2}\) adsorbed per unit mass of sorbent & 0.0262832 & mol/kg \\ Amount of N\({}_{2}\) adsorbed per unit mass of sorbent & 0.6328067 & mol/kg \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Initial conditions for simulation. \begin{table}
\begin{tabular}{c c} \hline \hline
**adsorbent** & **LiX zeolite (CECA-G5000)** \\ \hline Type & sphere \\ Particle density, \(\rho_{p}\)(kg\(\cdot\)m\({}^{-3}\)) & 1035 \\ Radius, \(r_{p}\)(mm) & 0.8 \\ Thermal conductivity, \(k_{eff}\)(W m\({}^{-2}\) K) & 0.2 [32; 33] \\ Heat capacity of pellet, \(C_{p}\) (J\(\cdot\)kg\({}^{-1}\) K\({}^{-1}\)) & 1010 [34; 35] \\ \hline Adsorption bed & \\ \hline Bed external porosity, \(\epsilon\) & 0.4 \\ Dispersion coefficient, D (m\({}^{2}\) s\({}^{-1}\)) & \(5\times 10^{-5}\)[28] \\ LDF constant for oxygen, \(k_{Q2}\) (s\({}^{-1}\)) & 62.0 [1] \\ LDF constant for nitrogen, \(k_{Q2}\)(s\({}^{-1}\)) & 19.7 [1] \\ Ambient temperature, (K) & 298 \\ \hline Feed & \\ \hline Feed composition & 21\% O\({}_{2}\), 79\% N\({}_{2}\) \\ Feed gas temperature, (K) & 298 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Model parameters. In order to compare the gas distribution of the adsorber in the PSA oxygen generator, a maldistribution factor integrating the surface and corresponding velocity, M\({}_{\text{f}}\), was introduced as in [27; 36]. \[M_{f}=\sqrt{\frac{1}{F_{0}}}\int_{0}^{F_{0}}\bigg{[}\frac{w_{i}-\overline{w}}{ \overline{w}}\bigg{]}^{2}dF \tag{14}\]  
where \(F_{0}\) is the total bed cross section; \(w_{i}\) is the flow velocity in point \(i\); and \(\overline{w}\) is the overall mean velocity, which is further defined as:  
\[\overline{w}=\frac{1}{F_{0}}\int_{0}^{F_{0}}w_{i}dF \tag{15}\]  
Figure 4 shows the variations in M\({}_{\text{f}}\) along with adsorption bed radial length at Ad step in each adsorber. The values of M\({}_{\text{f}}\) in the junction between both the center and outer channel and the adsorbent packing layer is greater than those across the main region of the absorbent packing layer. The concave-type curve shows a higher value of M\({}_{\text{f}}\) for the center channel than for the outer channel. Moreover, the value of M\({}_{\text{f}}\) for the centripetal flow is smaller than that for the centrifugal flow. This is mainly attributed to the large inlet flow corresponding to the large cross-sectional area and the small outer flow after adsorption corresponding to the small cross-sectional area in the centripetal flow type. On the contrary, the centrifugal flow exhibits an opposite situation. In this regard, the centripetal flow mode is much more suitable for PSA oxygen generation. In order to compare the difference in pressure drop between CP-\(\pi\) and CP-Z flows, the variations in the pressure differences between center and outer channels along with axial lengths at the Pu step, which is more sensitive to pressure, are depicted in Figure 5. Both centripetal flow types show similar increasing trends, the value of which is lower for CP-\(\pi\) than for CP-Z. These results indicate better gas distribution in CP-Z flow during the PSA oxygen generation process.