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1.1. Adsorbents 5 τ = k (t-zmax ) (1.8) v where c/c0 is the dimensionless concentration, k (s−1) the mass transfer coefficient, K the dimensionless Henry constant (the slope of the isotherm), ε the void volume, zmax the column length, ξ and τ the dimensionless bed length and time, respectively. For adsorbent particles in form of beads where mass transfer resistance is dominated by the size of the adsorbent particle, and the geometry of the adsorbent is approximated with spheres, the effective diffusivity in the beads can be estimated from [6]: kR2 De,beads= 15 (1.9) where R (m) is the radius of the bead. In this work, thin films of adsorbent materials were developed. For these samples where thin films dominate the mass transfer resistance and the geometry of the adsorbent can be approximated by a slab, the effective diffusivity in the film may be estimated from [6]: where l (m) is the film thickness. 1.1.2 Adsorbent materials kl2 De,film= 3 (1.10) A small amount of adsorbate adsorbs on the surface of an adsorbent in a monolayer or multilayer [7, 8]. Thus, the requirement for significant adsorption capacity restricts the choice of adsorbents to porous materials, with a high surface/volume ratio, with pore sizes ranging from few ̊Ansgtro ̈ms to a few tens of ̊Angstro ̈ms [1]. Some adsorbent materials currently used in adsorption separation processes are briefly described below. Silica gel is a highly porous form of silica with high surface area (ca. 800 m2/g), prepared synthetically from sodium silicate. The material was developed during World War I for use in gas masks, although the material was later substituted by activated carbon. Silica gel is used as a desiccant, for example for drying the air in systems for compressed air, or to avoid degradation of consumer products such as electronics during shipment. Activated alumina is aluminium oxide with a surface area higher than 200 m2/g. As adsorbent it is used in industry for drying warm air or other gas (up to 200◦ C). Activated carbon is a form of carbon which has been processed first by thermal decomposition of carbons with mineral or vegetal origin followed by activation with steam or carbon dioxide at high temperatures, and it has a very high surface area exceeding 500 m2/g. Activated carbon is widely used for air purification in industry. However, due to the low selectivity of activated carbon for molecules of different sizes, carbon molecular sieves have been developed [9], which are widely used for nitrogen separation from oxygen.PDF Image | Structured Zeolite Adsorbents for PSA Applications
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