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2394 Luca Riboldi and Olav Bolland / Energy Procedia 114 (2017) 2390 – 2400 showed promising results [23]. As reported in the post-combustion section, additional research effort needs to be undertaken to assure the applicability of this family of adsorbents, especially with regard to particle shaping. However, the possibility to tune their structure and chemical composition in order to obtain desired properties makes these materials important candidates for pre-combustion CO2 capture processes. The wide range of operating conditions and syngas compositions can be effectively addressed with tailor-made materials. For processes at elevated temperature (up to around 673 K [4]) the benchmark adsorbents seem to be potassium promoted hydrotalcites (K- HTC) [36,37]. These materials are positively affected by the presence of water [38], suitable for sour processes [39] and can be operated with a low steam feed. However, hydrotalcites generally display lower adsorption capacity than other common adsorbents. When an additional objective is concentrating H2 to very high purities, multi-layer bed structures are recommended [40,41]. The basic arrangement involves a first layer of ACs followed by a second layer of zeolites. According to their characteristics, the ACs remove the bulk CO2 and CH4 content, while the zeolites purify the gas from the remaining traces of CO and N2. Additional layers can be added to further improve the separation performance (e.g. a silica gel/alumina layer for H2O). 3.2. PSA process configurations A single stage PSA process is normally considered sufficient for CO2 separation in pre-combustion applications. On the other hand, complex PSA designs normally apply, involving many columns working in parallel and a complex cycle scheduling. This is mainly due to the large number of components present in the gas mixture and to the consequent introduction of non-standard PSA process steps. For example, a large number of pressure equalization steps increases the separation performance of the cycle but increases also its complexity. Normally a maximum of 4 equalization steps is envisaged [42]. An alternative approach could be to use a two-stage system or a dual PSA concept [43]. However, the literature generally focuses on single stage PSA designs [10,14,42]. Table 3 reports characteristics and performance of some PSA processes for separating CO2 from a shifted syngas. One may notice that the energy consumptions involved are significantly lower than in the post-combustion cases. Pre-combustion cases are characterized by relatively high pressures at the inlet of the gas separation unit, which give a margin to operate the pressure swing process without using vacuum pressure levels. The avoidance of vacuum drastically reduces the energy demand of the process. The only energy consumption directly associated to the PSA process is that supplied to the fans to overcome the pressure drop in the column. The performance taken from [10] differs from the other cases because the system considered includes an additional double flash purification process downstream the PSA. This process increases the overall H2 recovery and CO2 purity, in order to comply with the requested process specifications. Table 3. Performances of various PSA arrangements for CO2 capture from shifted syngas. The nomenclature used refers to CO2 mole fraction in the feed gas stream (yCO2), feed pressure (PF) and regeneration pressure (PR), CO2 purity (θCO2), CO2 recovery (RCO2), activated carbon (AC) and supported magnesium oxide sorbent (MgO/C). No. PSA stages 1 1 1 Cycle configuration 7-bed 12-step 6-bed 10-step 8-bed 11-step Adsorbent AC AC MgO/C yCO2 38 % 40 % 31 % PF/PR (bar) 38.8/1.0 34.0/1.0 40/1.0 θCO2 81.6 % 93.1 % 93.0 % RCO2 96.2 % 90.3 % 92.7 % Energy (kJ/kgCO2) Ref. 0.5 [10] - [42] - [14] Some studies investigated optimal cycles for sorption enhanced water-gas shift (SEWGS) processes. These processes combine the WGS reaction with the CO2 separation pushing the reaction towards the product formation, hence increasing the H2 production. The cycles are similar to common pre-combustion PSA cycles. A difference is the utilization of steam for the purge step and the introduction of a rinse step, again using steam. Accordingly, the steam to carbon feed ratio (S/C) in the purge and rinse steps is an important optimization parameter. Table 4 outlines the main characteristics and results of the cycles presented in the literature.PDF Image | Pressure Swing Adsorption (PSA) as CO2 Capture Technology
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