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Dynamic Response/Characteristics of an Oxygen Swing Adsorption Process to Step Perturbations. Part 1 329 all of the following discussion was cycle time and not absolute time. Due to the batch nature of the process, each bed cycled through several steps but the product was only provided during the feed step. Hence, only the bed and product pressures during the feed step and successive cycles were of interest. Thus, from the standpoint of process control, real time must be replaced by cycle time. Effect of process disturbances on bed and product tank pressure For brevity, only the transient and cyclic steady-state responses for a step change in the feed valve position are shown (refer to Figures 5, 6, 7 and 8), as the system responses for the other perturba- tions showed similar results. An increase in the feed valve position directly increased the bed and product tank pressure as depicted in Figures 5 and 7. This was due to additional moles of gas entering the bed and hence an equivalent increase in the amount of adsorbate and product species (oxygen) available for uptake. This resulted in a rise in the bed pressure and hence an equivalent rise in the product tank pressure. The main observations of note are the fast system response to the valve change (negligible dead time), short time constant (steady state reached within two cycles) and the response appearing to be first order. The model captured the trends exhibited by the pilot plant. These included the rapid response, asymptotic closure to steady state and the magnitude of the change from the initial con- ditions (~4.5 kPa and ~3.9 kPa for the pilot plant bed and tank profiles, respectively, compared with 4.65 kPa and 4.54 kPa for the model). The mismatches observed between the experiment and model were due predominantly to the assumption of constant composition (as discussed previously). This is highlighted during the purge steps (3 and 6) as depicted in Figure 6. The model did not capture the increase in the bed pressure due to the purge effect correctly, as the constant composition profile of the entering purge gas led to a larger amount being adsorbed than occurred experimentally. Consequently, the bed pressure did not rise as much as that observed experimentally. This variance in the bed-pressure Figure 5. Bed 1 pressure history whilst undergoing feed (step 2) for a step change in the feed valve position — compar- ison of plant and model. Change in pressure: pilot plant, ~4.5 kPa; model, 4.65 kPa.PDF Image | Dynamic Response and Characteristics of an Oxygen Vacuum Swing Adsorption
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