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2.4 Adsorbents Adsorbents play a very important role in the adsorption process and their selection determines the performance of the separation. Several factors are accounted for when selecting an adsorbent for a particular process, such as the multicomponent adsorption equilibrium capacities, selectivity’s, available surface area, among others [15]. The most common adsorbents employed in Pressure Swing Adsorption for hydrogen purification include activated aluminas, silica gels, activated carbons (ACs), and zeolites [16]. Silica Gel and Activated Alumina are both useful desiccants but, while silica gel has high capacities at low temperatures, activated alumina has at high temperatures [6]. Activated carbons exhibit higher CO2 adsorption capacities when compared with the other adsorbents and they are very competitive due to their low cost, high surface area and amenability to pore structure modification, and surface functionalization (Wang et al., 2011) [17]. In the other hand, zeolites are porous crystalline aluminosilicates. The fact that its structure is a combination of SiO4 and AlO4 tetrahedra, connected in several regular arrangements through shared oxygen atoms, forming an open crystal lattice which determines the micropore structure precisely uniform, with no distribution of pore size. This characteristic of zeolites distinguishes them from the other adsorbents [6]. 2.5 PSA basic cycles A Pressure Swing Adsorption process requires an adsorbent that preferentially adsorbs one component from a mixed feed. Two main steps characterize this process: adsorption, during which the preferentially adsorbed species are gathered from the feed, and desorption (or regeneration) step, where species are taken out from the adsorbent [10]. Pressure Swing Adsorption for Hydrogen Purification The basic PSA system proposed by Skartstrom in 1957 still is the basis of the actual PSA processes (Figure 2.2). This specific process entailed two adsorbing beds and each bed was submitted to four different steps: Pressurization, Adsorption (Feed), Countercurrent Blowdown and Countercurrent Purge [10],[18]. When operating, a PSA process produces at least two streams. In the adsorption step, a stream rich in the less adsorbed species is formed and it is usually named as “light product” or raffinate stream. In the desorption step, a stream rich in the strongly adsorbed species is obtained and is often called as “heavy product” or extract stream [19]. Contextualization and state of art 5PDF Image | PRESSURE SWING ADSORPTION FOR THE PURIFICATION OF HYDROGEN
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