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Screening adsorbents for a layered adsorbent bed for Hydrogen separation using breakthrough experiments Kovacevic, Stevo B.

Abstract

The aim of the present study was to apply the breakthrough experiment to screen and select adsorbents used for gas purification. Layered adsorbent beds employed in the Pressure Swing Adsorption (PSA) process for hydrogen purification from Steam Methane Reforming and Partial Oxidation product gasses use activated carbon and 5A molecular sieve in the first and second layer, respectively. Few studies on the use of alternative adsorbents have been reported and the present study was aimed at investigating new adsorbents and generating the adsorption data required to assess the adsorbents suitability for hydrogen purification using PSA. The adsorption of CO₂, CO, CH₄ and N₂ on 7 adsorbents, activated alumina AA- 300, CBV 780 zeolite, PCB activated carbon, MHSZ-177 zeolite, 13X, 5A, and VSA6 molecular sieves were investigated using the breakthrough experiment. The first four adsorbents (AA, CBV780, PCB carbon, and MHSZ-177) were considered as candidates for the first adsorbent layer, which is typically an activated carbon. The last three (13X,5A and VSA6) were candidates for the second adsorbent layer which is typically a 5A molecular sieve The first and second moments, height of equivalent theoretical plates, HETP, axial dispersion coefficient, DL, Henry's equilibrium constant, and the adsorbtion isotherm for each adsorbent/sorbate system were extracted from the breakthrough data. These data were used to select the most promising adsorbents for the first and second adsorbent layer. The MHSZ-177 and VSA6, respectively, were found to be the most promising candidates for the first and second adsorbent layer of adsorbent beds. The VSA6 had the sharpest mass transfer zone, MTZ and the longest retention time for all light sorbates and therefore was the most promising candidate for the second adsorbent layer. The MHSZ177 had the highest retention time for CO2 and despite having a broader MTZ than activated carbon it had the higher selectivities for CO2 with regard to light sorbates. This put MHSZ-117 in front of activated carbon as the choice for the first layer. The values for DL calculated from the averaged breakthrough data were in very good agreement with the published values.

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