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342 C.C.K. Beh and P.A. Webley/Adsorption Science & Technology Vol. 21 No. 4 2003 the layers was clearly observed (at 0.3 m) — the causes of this cold spot have been discussed extensively (Wilson and Webley 2002b). The temperature swing between feed and evacuation was 4–6 C in the main section of the bed (0.4–1.2 m) but only 1–1.5 C at the top of the bed. This was expected since the oxygen-rich gas was kept at the product end of the bed and little adsorption/desorption occurs here. Upon receiving perturbations in the valve positions, the bed flows and pressure change and, as a result, the temperature profiles change. The final cyclic steady-state temperature profile was established in such a way that the overall energy balance was satisfied at each point in the bed as well as the boundary conditions at each end of the bed. Figure 21 shows the change in bed temperature (from its baseline value as shown in Figure 20) at cyclic steady state after a 5% increase in feed valve position. The maximum change in temper- ature was small (2 C) with very little change occurring between 0.8 m and 1.6 m, indicating little change in adsorptive behaviour in this section of the bed. As shown earlier, the oxygen product purity dropped for this perturbation, indicating a movement of the mass-transfer zone towards the end of the bed. This movement was distinguished as an increase in temperature near the top of the bed (1.6 m onwards). The slightly higher main bed temperatures led to heating of the prelayer on desorption, thus slightly elevating the temperature of the prelayer (0–0.4 m). It is interesting to note that the temperature swing at the top of the bed which was 1–1.5 C before the perturbation now increased to 2–3.5 C, reflecting the movement of the mass-transfer zone and subsequent heat generation via nitrogen adsorption. While Figure 21 shows the final cyclic steady-state temper- ature, Figure 22 shows the transient approach of the system towards cyclic steady state at three points in the bed: 0.3 m, 0.7 m and 1.6 m, respectively. The feed valve perturbation was made at cycle 47. All three thermocouples (0.3 m, 0.7 m and 1.6 m) showed an immediate initial response (most identified by the thermocouple at 1.6 m) fol- lowed by a more gradual approach to cyclic steady state. The temperature at 0.3 m (in the prelayer) took 250 cycles to reach cyclic steady state; that at 0.7 m was essentially unchanged; and the response at 1.6 m was over after two to three cycles. The varying response time for the dif- ferent locations was a result of the different causes of the temperature change. In the prelayer, no Figure 21. Cyclic steady-state change in adsorbent bed temperature as a result of a 5% increase in feed valve position.PDF Image | Dynamic Response and Characteristics of an Oxygen Vacuum Swing Adsorption
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