Adsorbents for Hydrogen PSA Processes

The selection of adsorbents is critical for determining the overall separation performance of the above-described PSA processes for hydrogen purification. The separation of the impurities from hydrogen by the adsorbents used in these processes is generally based on their thermodynamic selectivities of adsorption over H2. Thus, the multicomponent adsorption equilibrium capacities and selectivities, the multi-component isosteric heats of adsorption, and the multicomponent equilibrium-controlled desorption characteristics of the feed gas impurities under the conditions of operation of the ad(de)sorption steps of the PSA processes are the key properties for the selection of the adsorbents. The adsorbents are generally chosen to have fast kinetics of adsorption. Nonetheless, the impact of improved mass transfer coefficients for adsorption cannot be ignored, especially for rapid PSA (RPSA) cycles. 

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Although PSA is a batchwise process, by using multiple beds in a sequential manner the overall process is operated in a continuous fashion. Each bed may contain layers of different adsorbent materials selective for specific contaminants in the hydrogen gas stream to be purified. Each bed undergoes a sequence of four basic steps in a PSA cycle adsorption, depressurization, purge at low pressure, and repressurization. This sequence of cyclic operations for each bed is shown schematically for a four-bed PSA process in Figure 8.4 (Yang, 1987 Cassidy, 1980 Miller and Stocker, 1999). 

The Sumitomo-BF PSA process uses carbon molecular sieves (CMS) as the selective adsorbent, CMS has a higher capacity of adsorption than zeolites for methane and oxygen, and it is considered to be advantageous for hydrogen purification. If dirty raw gases are fed to this process, minor amounts of heavy hydrocarbon components such as aromatics are likely to cause deterioration of the adsorbents. To remove the heavy hydrocarbons, prefilter columns that contain activated carbon are placed upstream of the main CMS adsorbent beds4. 

Other recent ideas on (i) simultaneous sorption-reaction process concepts using the principles of a novel PSA technology for the production of fuel cell-grade hydrogen and (ii) improving the H2 recovery from existing H2 PSA processes by integrating it with additional PSA units or nanoporous SSF adsorbent membrane systems are reviewed. 

Pressure-swing adsorption. Although adsorption is most often used as a purification process to remove small amounts of material, a number of applications involve separations of gas mixtures with moderate to high concentration of adsorbates. These are called bulk separations, and they often use different operating procedures than for gas purification. Pressure-swing adsorption (PSA) is a bulk separation process that is used for small-scale air separation plants and for concentration of hydrogen in process streams. 

The PSA cycle makes use of the simple fact that the partial pressure of adsorbate in the gas phase can be reduced by lowering the total pressure. Pressure reduction can thus be used to regenerate adsorbent that has been loaded with adsorbate at an elevated pressure. Since it is not necessary to heat or cool the bed between or during the adsorption and desorption steps, very rapid cycling is possible. The process is now widely used for hydrogen purification, air separation, hydrocarbon separation, and air drying, and new applications are under development. 

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