# Research on Pure Hydrogen Production Using a Fuel-Processing System Combined with a PSA System
## 10 PSA Simulation  
The PSA system was simulated by the Aspen Adoption(r) simulator. In the simulation, the Peng-Robinson equation of state was employed for the calculation of the thermodynamic properties of gases, and the Langmuir isotherm was applied for the calculation of the adsorption equilibrium. The pressure change in the bed was calculated using the Ergun equation [47]. Product gas streams at a reformer temperature between 600 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C in the fuel-processing system and at a system pressure between 1 atm and 9 atm were pressurized and supplied to the PSA system to purify H\({}_{2}\). During simulation, isothermal assumption was applied for the adsorption bed [48; 49]. The overall process simulation of the PSA system is shown in Figure 9. Two adsorption beds were working in turns during the purification process, and impurities were removed from the inlet gas. For simulation, one-bed PSA simulation was conducted to determine the swing time under the two-bed condition. The switching time was determined at the time where the hydrogen concentration was 99.97% in the outlet stream of bed 1. An example of the simulation result of bed 1 is shown in Figure 10. In Figure 10, we can see that the swing time at which the concentration of hydrogen is 99.97% in the outlet stream is 16 s and is a function of the size of the adsorber and the properties of the adsorbent. Figure 8: Fuel-processing system yield at various T and P for 10 kW H\({}_{2}\) production. Two-bed PSA process simulation was also conducted, and the cycle of the PSA process is the following:  
Figure 10: One-bed simulation example for determine switching time. Figure 9: PSA flow sheet. 1. Bed 1 is filled with the inlet stream of PSA while bed 2 is blown down. II. Impurities are adsorbed by bed 1 due to inlet gas continuously supplied to bed 1, while adsorbed impurities in bed 2 are desorbed and released from bed 2. After impurities are adsorbed, the hydrogen concentration at the outlet stream of bed 1 would be 99.97%. III. After bed 1 is completely filled with impurities, the outlet stream of bed 1 is introduced to bed 2 to make the pressures of both beds equal. IV. Bed 2 is filled with the inlet stream of PSA, while bed 1 is blown out. V. Go to (i) and repeat the process after swinging bed 1 with 2. VI. The cycle of the whole process is summarized in Table 5.  
For the simulation, a cylindrical bed was used, the length of the bed was 1 m, the diameter was 0.037 m, the adsorbent was assumed to be spherical, and the diameter was 0.0015 m. Langmuir parameters and bed parameters used in the simulation are shown in Table 3[31]. The results of calculating the concentration change at the bed outlet after going through the above process are shown in Figure 11. As the adsorption process progresses during operation, the hydrogen concentration at the outlet of the adsorption bed decreases. The adsorption bed switches when the hydrogen concentration reaches 99.97%. Figure 11: Two-bed system outlet composition. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Step** & **Bed 1** & **Bed 2** \\ \hline
1 & Pressure & Blow down \\
2 & Adsorption & Purge \\
3 & Depressurize & Pressure equalization \\
4 & Blow down & Pressurize \\
5 & Purge & Adsorption \\
6 & Pressure equalization & Depressive \\ \hline \hline \end{tabular}
\end{table} 
Table 5: PSA system cycle steps. The hydrogen-purification efficiency of the PSA system was defined as the ratio between the enthalpy of hydrogen at the PSA outlet stream to the enthalpy of hydrogen at the PSA inlet stream, as shown in Equation (9), and the result is summarized in Figure 12[42; 43]. \[\eta_{2}=\frac{LHV_{H_{2}}\times n_{H_{2}}(PSA\;product\;gas)}{LHV_{H_{2}}\times n _{H_{2}}(PSA\;feed\;gas)} \tag{9}\]  
As shown in Figure 12, the efficiency of the PSA system was constant at about 55% at reformer temperatures of about 850 \({}^{\circ}\)C and higher.