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Dynamic Response/Characteristics of an Oxygen Swing Adsorption Process to Step Perturbations. Part 1 343 Figure 22. Transient temperature response at various axial locations in adsorber bed 1 for a 5% increase in the feed valve position. adsorption/desorption occurred and the temperature shift was entirely due to (slow) convective heating of the bed. At 1.6 m, the response was rapid because movement of the mass-transfer zone led to enhanced temperature changes due to increased adsorption/desorption. Hence, there was no one time scale for the temperature changes in the bed — as discussed earlier, there are multiple time scales involved in O2 VSA responses. Figure 23 shows the change in bed temperature (from its baseline value as shown in Figure 20) at cyclic steady state after a 3% increase in purge valve position. Even though the purity response was similar (compare Figures 16 and 17), the bed temperature response was entirely different. Temperatures in the main portion of the bed had increased by about 1.5 C with little change in temperature of the cold spot. Once again, the temperature swing at the exit of the bed has increased, reflecting the presence of the mass-transfer zone. The transient response of the system to a 3% increase in purge valve is shown in Figure 24. The thermocouple at 0.7 m now shows a significant dead time (absent in the feed valve responses) and a cyclic steady state was approached only after ca. 310 cycles. It is clear that significantly differ- ent transient and steady-state temperatures were attained as a result of different perturbations. Wilson and Webley (2002a) have discussed a simple method for the determination of cyclic steady-state profiles following a change in boundary conditions that can be used to explain some of the effects resulting from these case studies. These complications in thermal transients can however be neglected by the control engineer as it was observed that, although the time-variant thermal energy profiles had not yet attained an asymptotic value, their closure had a negligible impact on the controlled variables (such as oxygen purity, flow and product pressure). This is principally because the cyclic steady-state temperature closure was within 2 C of its initial value and was either not significant enough to cause a difference in the output variables and/or was within the error tolerance of the sensors.PDF Image | Dynamic Response and Characteristics of an Oxygen Vacuum Swing Adsorption
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