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4.4 Case Studies and Computational Results around 450 kPa while CnB operates at around 85 kPa during this step. Thus, we have a low- pressure adsorption step with a heavy reflux for CoB (step 1) and a low-vacuum desorption step for CnB (step 4) with desorbed CO2 being sent as a heavy reflux from CnB to CoB. After this step, both top and bottom reflux disappear, while φ(t) indicates continuation of the feed to CoB. CoB gets pressurized to the upper bound of 600 kPa and N2 is withdrawn at a high pressure, while CO2 is extracted from CnB at a vacuum of 50 kPa. This suggests a pressurization and high pressure adsorption step for CoB (step 2) and a high vacuum desorption step for CnB (step 5). The feed fraction φ(t) drops at the beginning of this step to facilitate CoB pressurization. We observe a drop in the CO2 concentration in CoB (see step 2 in Figure 4.3) because of its low concentration in feed. Also, because of the application of vacuum, the gas-phase CO2 concentration decreases sharply for both step 4 and 5, as evident in Figure 4.3. Further, the pressures in the beds are held at their same respective levels, while α(t) becomes 1, β(t) approaches 1, and the feed is stopped completely. Because there is no feed nor product at this time, we have a total reflux step (step 3 and 6), in which both the beds are connected to each other and a recirculation of the components occurs within the system. A small amount of N2 is withdrawn at the beginning of this step, and is shown as a dotted line in Figure 4.2. A decrease in Pads(t) and an increase in Pdes(t) towards the end of this step halts this recirculation. After the total reflux step, the co-current bed follows the steps of the counter-current bed and vice-versa. This completes the cycle. From Figure 4.2 together with Figure 4.3, we observe a couple of key aspects of this cycle. First, we observe an extensive use of heavy reflux in the cycle (step 1 and 3) to enrich gas- phase CO2 concentration towards heavy end of CoB to ensure high-purity CO2 production. During both steps 1 and 3, desorbed CO2 from CnB is sent as a heavy reflux to CoB which enriches the adsorbed CO2 concentration towards the heavy end of CoB. This is evident from the gas-phase CO2 concentration profile for steps 1 and 3 in Figure 4.3. Second, we observe a completely novel total reflux step in the cycle. Moreover, this is the longest step and runs for almost 60% of the total cycle duration. This is a vital step to improve CO2 purity as it eschews Chapter 4. Superstructure Case Study: Post-combustion CO2 Capture 59PDF Image | Design and Operation of Pressure Swing Adsorption Processes
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