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cations represents a valuable measure for tuning the volume available for adsorption, as well as the selectivity. Consequently, the adsorption capacity of CO2 in zeolite X increases with decreasing ion size and increasing charge density, in the order Cs+, Rb+, K+, Na+, Li+ [11]. Low silica zeolite X in the Li+ form (Li-LSX) exhibits a higher selectivity with respect to N2 than zeolite NaX , due to the higher polarizing power of the small Li+ cation and is therefore the most widely used zeolite adsorbent in air separation processes [10]. The adsorption capacity may also be controlled by tuning the Si/Al ratio of the zeolite. In PSA processes, there is a trade-off between pressure drop and mass- and heat-transfer limitations [12]. The adsorbents in industrial PSA processes are usually in the form of beads, pellets or extrudates. Larger adsorbent particles results in lower pressure drop, but increased mass- and heat-transfer limitation due to the long mass and heat transfer path along the radius of the particles resulting in a reduced performance of the process [38, 51]. Thus, it is of great importance to develop novel structured adsorbents, which may exhibit higher mass transfer coefficients, higher throughputs, and lower pressure drop, and represent a competitive alternative to adsorbents in form of beads/pellets [13-15, 48-51]. High mass transfer coefficients may be achieved by using thin films of an adsorbent material grown on structured supports, with the advantage of having a short diffusion path. Although the adsorbent loading is often small compared to traditional adsorbents, high throughputs may be achieved by increasing the cycle frequency and still maintain a low pressure drop whereas using structured supports with defined geometry [51]. In this context, monolithic adsorbents are of great interest, and a few reports on the use of monolith structures in adsorption processes have been published [13-15, 51]. Monoliths are structured materials with parallel channels, available with various cell densities and cell shapes. Low pressure drop, uniform flow distribution, thermal stability and unproblematic scale-up are some of the advantages with these materials [13, 15]. The mineral cordierite, 2MgO x 2Al2O3 x 5SiO2, is often used as the main material in monoliths. Cordierite monoliths with tailored 63PDF Image | Structured Zeolite Adsorbents for PSA Applications
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