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Depressurization (D): the pressure is reduced by putting in contact the column with another at a lower pressure level. Blowdown (BD): the pressure is reduced to the lowest level in order to regenerate the bed. A stream of CO2- rich gas is leaving the column during this step. Purge (Pu): the regeneration is completed by injecting a purging gas into the column, normally counter- currently. This step is again carried out at the lowest pressure of the system and produces a CO2-rich gas stream. Pressurization (P): the pressure is increased by putting in contact the column with another at a higher pressure level. Null (N): the column is left idle. Feed Pressurization (FP): part of the feed gas is used to pressurize the column to the highest pressure level necessary for the adsorption process. The different operating conditions in which the PSA process is supposed to perform in post- and pre- combustions scenarios, necessarily led to different configurations, in terms of number of beds and type of steps. The guiding criterion, for the selection of the process layout, was the necessity of approaching values of CO2 recovery and purity sufficient for a CCS application (i.e., CO2 recovery ≈ 90% and purity ≈ 95%). A multitude of different process configurations exists and may be employed. Given the large number of variables to consider (i.e., type and number of steps, duration of the cycle, adsorbent material, etc.) there is not a well-defined framework to pinpoint the most suitable alternative. In the present work, it was decided to refer to cycle configurations successfully employed in the literature [16, 25]. Minor changes have been done with respect to those cycles, in order to deal with the slightly different operating conditions considered. However, other configurations are possible and may lead to similar good performance. For the post-combustion scenario, a first PSA stage consists in a three-bed five-step cycle, while a second stage consists in a two-bed five-step cycle. The sequence of different steps undergone by a column is shown in Figure 4. For the pre-combustion scenario, the PSA configuration adopted in the present work is a seven-bed twelve-step cycle, where the sequence of different steps undergone by a column is shown in Figure 5. Different boundary conditions have to be established for each step of the PSA cycle. The Danckwerts boundary conditions are applied. They assume no dispersion or radial variation in concentration or temperature either upstream or downstream of the reaction section. Table 11 in the Appendix B reports those boundary conditions. The energy consumption directly related to the PSA process consists in the power necessary to a fan to overcome the pressure drops and the power necessary to the vacuum pump to create an under-atmospheric pressure (when requested from the regeneration process). If a rinse step is implemented, a fan is necessary for feeding the rinse flow rate into the column and overcoming the pressure drops. Furthermore, a gas compression may be applied, with the relative compression power duty. These energy consumptions were evaluated within the PSA model as following: Fan power 1 is fan 1 P fan1 R Tin in fan 1nin fan Pout Compressor power 1 compr R Pin compr 1 n 1 inP in (7) out (8) compr 1 1 Vacuumpower is vacuum 1nin (9) is compr vacuum P vacuum 1 atm vacuum 1R in P out PDF Image | Evaluating Pressure Swing Adsorption as a CO2 separation technique in coal-fired
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