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4.5 Comparison of Cycles from Case II and III obtained in both case II and case III depend heavily on the heavy reflux step to enrich gas- phase CO2 concentration towards the heavy end of the adsorber bed. Both case studies employ this step to enhance CO2 purity in the final product, and run it for a substantial 60-65% of the total cycle duration. This not only asserts that such a step is vital for producing heavy product at a high purity, but also proves that the superstructure approach yields intuitive and meaningful configurations since all of the literature studies have included heavy reflux step to boost CO2 purity in the final product. Besides heavy reflux step, both cycles in case II and III are similar in terms of the optimal cycle duration, which is close to 40 minutes for both cases. Moreover, both cycles do not collect CO2 product when a light reflux stream is present as it necessarily dilutes CO2 product, and both employ vacuum for almost entire duration of the cycle to improve CO2 recovery. Barring heavy reflux step and other minor similarities, optimal cycles obtained in case II and III are quite different from each other, especially in the power consumption aspect. For case II, Pads is at its upper bound while Pdes is at its lower limit for almost entire cycle. Hence, the total power consumption is quite high for case II. In contrast, optimal profile for Pads takes a lower value for the most part of the cycle and attains upper bound only for a short duration. This leads to substantial power savings. One of the key differences between cycles of case II and III is the pressure equalization step. This leads to additional power savings as it avoids uneconomical pressure drop when cycle transitions from step 4 to step 5 in case III. However, lack of a pressure equalization step causes such a pressure drop in case II between step 3 and 4 when pressure drops significantly from 450 kPa to 90 kPa. Only reason for such a contrast in the optimal solution is the absence of any constraint on power consumption in the problem formulation for case II. To avoid this, an upper bound on the power consumption can be used for case II in future. Besides power aspect, cycles in case II and III differ in steps 3 and 6. Case III doesn’t incorporate a total reflux step unlike case II. Instead, that step is still a combination of light and heavy reflux steps in case III with the presence of external feed and nitrogen removal. Chapter 4. Superstructure Case Study: Post-combustion CO2 Capture 69PDF Image | Design and Operation of Pressure Swing Adsorption Processes
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