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5.4 Comparison of Cycles from Case I and II 5.4 Comparison of Cycles from Case I and II From the case studies above, we observe that the superstructure optimization can yield entirely different configurations with different objectives. The final configurations obtained match the respective objectives sought in both case studies. The major difference between the optimal cycles is the way they enrich the CO2 concentration towards the heavy end of the bed. Since the objective of case I is to maximize CO2 recovery, the optimizer achieves it by minimizing the feed input through the system, thus attaining the specified lower bound for feed flux. As a result, minimal feed is used and the optimal configuration doesn’t use the high CO2 concentration present in the feed to enhance the adsorbed-phase CO2 concentration. Consequently, we observe the utilization of the heavy reflux step through the entire cycle to achieve desired CO2 purity. In contrast, the optimal VSA cycle in case II utilizes the feed stream for CO2 enrichment. Thus, we infer that a heavy reflux step is not an absolute necessity to obtain heavy component at a high purity when the feed to the PSA system is sufficiently rich in the heavy component. As a result of the CO2 enrichment through feed, although the lower bound on feed flux is 35 kgmol m−2 hr−1 for both cases, the optimal feed flux for case II is almost three times this value. Consequently, it also decreases the specific power consumption for the cycle. In contrast, the optimal cycle in case I doesn’t incorporate any power saving step due to the lack of any constraint on the power consumption in the problem formulation. Thus, unlike case II, we do not observe a pressure equalization step in case I. In fact we observe an uneconomical pressure drop from 950 kPa to 50 kPa when the cycle transitions from step 4 to step 5. To avoid this, an upper bound on the power consumption can be used for case I in future. To deduce multibed cycles for a continuous cycle operation from the optimal two-bed solutions, a coordination of step times will be required which will depend upon whether we need a continuous product H2 collection, or a continuous CO2 removal or a continuous feed to the system. In both case I and case II, H2 is collected for a longer period in the cycle and continuous flow can be maintained through product buffer tanks. Thus, the coordination can Chapter 5. Superstructure Case Study: Pre-combustion CO2 Capture 93PDF Image | Design and Operation of Pressure Swing Adsorption Processes
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