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5.3 Case Studies and Computational Results is stopped in CnB and it starts operating in the atmospheric range. Therefore, we have an atmospheric desorption step for CnB (step 7) in which desorption occurs at around 120 kPa. The desorbed gas is sent to CoB which undergoes a heavy reflux step at further elevated pres- sures (step 3). In the final operating step, the values of α(t) and β(t) both go to 1, indicating no light or heavy product extraction from the system. The feed enters midway through the step for a short duration, and is otherwise at zero. To reflect this, a dotted line is shown for feed during this step in Figure 5.2. The profiles of Pads(t) and Pdes(t) show that the pressure rises in both beds. Since the PSA system gets isolated during this step and a recirculation of the components occurs within the system, we call it a total reflux step (step 4 and step 8). After the total reflux step, the co-current bed follows the steps of the counter-current bed and vice-versa. This completes the cycle. Figure 5.2 together with the gas-phase CO2 concentration profiles in Figure 5.3 illustrate several unconventional, but key, aspects of the cycle. First, the light reflux step at vacuum (step 5) follows the total reflux step at around 950 kPa (step 4). Such a transition in the bed pressure, although not economical, is essential to improve the purity and recovery of CO2 in the final product. During the light reflux step, a large amount of CO2 desorbs in CnB which is then sent to CoB. This is necessary to enrich the adsorbed-phase CO2 concentration towards the heavy end of CoB. From step 1 in Figure 5.3, it can be observed that the CO2 front rises significantly towards the heavy end due to this recycle. Such a significant rise is important to achieve the desired CO2 purity and recovery. Since a large amount of CO2 is desired for this enrichment, and since the duration of the light reflux step is short, the step operates at vacuum and entire desorbed CO2 is sent to the other bed. Second, the light reflux step at vacuum (step 5) precedes the vacuum desorption step (step 6), whereas conventionally it is vice-versa. Since the step duration for step 5 is small, the hydrogen recycle helps in getting more CO2 desorbed in that interval. This hydrogen reflux is obtained from the feed stream going in CoB. Also, since the hydrogen reflux dilutes the product CO2, it is collected during the next vacuum desorption step and not during step 5. Chapter 5. Superstructure Case Study: Pre-combustion CO2 Capture 81PDF Image | Design and Operation of Pressure Swing Adsorption Processes
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