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6 Introduction Adsorbents based on zeolites are a special class of microporous inorganic adsorbents [10, 11] and will be described in detail later in this thesis. 1.2 Pressure and vacuum swing adsorption Pressure swing adsorption (PSA) is a widely used technology for gas purification [1, 12]. The technology was introduced commercially in the 1960s by Skarstrom, and today PSA is used extensively in the production and purification of oxygen, nitrogen, and hydrogen for industrial use. PSA processes are based on the capacity of adsorbents, to selectively adsorb and desorb a gas as the pressure is raised and lowered. PSA is a cyclic process where a number of connected columns containing adsorbent material undergo successive pressurization and depressurization steps in order to produce a stream of purified product gas. Conventional PSA systems used today in industry typically operate at low cycle speeds of 0.05-0.5 cycles/minute since higher cycle speeds would cause the adsorbent beads to fluidize in the vessel, causing the adsorbent beads to wear. At higher cycle speeds, the high pressure drop in the adsorbent bed also become troublesome. Based on the total cycle time, it is possible to distinguish among PSA (hundreds of s/cycle), rapid pressure swing adsorption (RPSA, total cycle time less than 20 sec.) and ultra rapid pressure swing adsorption (URPSA, total cycle time less than 10 sec.) [13]. In vacuum swing adsorption (VSA), low pressure is used to desorb the adsorbed species, when a high recovery of the preferentially adsorbed species is needed. Successful PSA and VSA operations require a careful choice of the materials and process conditions in order to obtain high selectivity and capacity with no breakthrough of the adsorbed species in the bed. The overall performance of a PSA or VSA process depends both on thermodynamic and kinetic factors. The columns used in PSA and VSA processes should ideally fulfill the following cri- teria: • Low capital cost, • Mechanical resistance and durability, • Efficient heat exchange with the environment, • Good temperature control, • Flexible design for the use of different adsorbents. The adsorbent should ideally meet the following requirements: • High adsorption selectivity toward one of the components in the gas mixture, • High volumetric working capacity (loading per adsorbent unit volume),PDF Image | Structured Zeolite Adsorbents for PSA Applications
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