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340 C.C.K. Beh and P.A. Webley/Adsorption Science & Technology Vol. 21 No. 4 2003 TABLE 2. 1st Order with Dead Time Model Parameters and Performance Kp (%O2/% valve open) 0.765 0.3167 0.2925 0.69%O2/kPa load p D IAE Feed valve Purge valve Product valve Product load disturbance aUsed for FODT fit. 5% increase from baselinea 5% decrease back to baseline 3% decrease back to baseline 3% increase from baselinea 3% decrease back to baseline 5% decrease back to baseline 10% decrease from baselinea 10% increase back to baseline 7% increase from baseline (cycles) 5 4 6 6 (cycles) (%O2) 2 3.16 3.45 9.55 2 2.80 3.05 11.82 3 2.29 3.66 6.92 3 2.16 vary for a particular set of manipulated and process variables. Retuning or gain scheduling, in this case, would be the appropriate course of action. It is important to note that feed, purge and prod- uct valves all affect the oxygen purity with different process dynamics. Thus, using these valves to control other variables (such as product flow and pressure) would lead to strong coupling between the control loops — the extent of this coupling is investigated in Part 2 of this series. The individual responses shown in Figures 16–19 will now be discussed. The decrease in oxygen purity following an increase in feed (Figure 16) and purge (Figure 17) valve positions arises from differing causes. In the event of an increase in the feed valve position, an increase in the number of moles of gas entering the bed causes movement of the adsorption front further towards the product end of the bed. This shift in the position of the front causes more moles of nitrogen to leave the bed as product and to enter the bed undergoing purge, adding further to the moles of nitrogen ready to be released as product during the feed step of the second bed. Figure 16 shows the product purity dropping from an initial value of 85% and asymptotically converging to a steady value of ca. 81%, 20 cycles after the valve perturbation was made. However, in the case where an increase in the purge valve occurred, movement of the front during the purge step allowed nitrogen to contaminate the top of the bed receiving purge as discussed earlier. The convergence of this condition is faster (15 cycles) with the purity decreasing slightly (87.2% to 86.2% in Figure 17).PDF Image | Dynamic Response and Characteristics of an Oxygen Vacuum Swing Adsorption
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