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4 The process. Separation by adsorption The Pressure Swing Adsorption (PSA) technology is based on a physical binding of gas molecules to adsorbent material. The respective force act- ing between the gas molecules and the adsorb- ent material depends on the gas component, type of adsorbent material, partial pressure of the gas component and operating temperature. A qualitative ranking of the adsorption forces is shown in the figure below. The separation effect is based on differences in binding forces to the adsorbent material. Highly volatile components with low polarity, such as hydrogen, are practically non-adsorbable as opposed to molecules as N2, CO, CO2, hydrocar- bons and water vapour. Consequently, these impurities can be adsorbed from a hydrogen- containing stream and high purity hydrogen is recovered. Qualitative ranking of adsorption forces Adsorption and regeneration The PSA process works at basically constant temperature and uses the effect of alternating pressure and partial pressure to perform adsorp- tion and desorption. Since heating or cooling is not required, short cycles within the range of minutes are achieved. The PSA process conse- quently allows the economical removal of large amounts of impurities. The figure on page 5 illustrates the pressure swing adsorption process. It shows the adsorp- tion isotherms describing the relation between partial pressure of a component and its equili- brium loading on the adsorbent material for a given temperature. Adsorption is carried out at high pressure (and hence high respective partial pressure) typically in the range of 10 to 40 bar until the equilibrium loading is reached. At this point in time, no further adsorption capacity is available and the adsorbent material must be regenerated. This regeneration is done by lowering the pressure to slightly above atmospheric pressure resulting in a respective decrease in equilibrium loading. As a result, the impurities on the adsorbent material are desorbed and the adsorbent material is regenerated. The amount of impurities re- moved from a gas stream within one cycle corresponds to the difference of adsorption to desorption loading. After termination of regeneration, pressure is increased back to adsorption pressure level and the process starts again from the beginning. Pressure swing adsorption plant in Leuna, Germany Hydrogen weak Oxygen Argon Nitrogen Carbon monoxide Methane Carbon dioxide Ethane Ethylene Propane Butane Propylene Ammonia Hydrogen sulfide Mercaptanes BTX Water strongPDF Image | Hydrogen Recovery by PSA
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