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Hydrogen Purification by Pressure Swing Adsorption

Most commonly, hydrogen is produced by the catalytic reforming of natural gas combined with a water gas shift reaction step, from which a hydrogen stream containing a number of impurities such as H2O vapor, CO2, CH4, CO, and N2 is obtained. These impurities must be removed in order to attain the 99.95%+ H2 purity that is normally demanded. In fuel cell applications, the purity demands are as high as 99.99+%. The separation is usually carried out a large scale in PSA units, operating at pressures reaching about 7 MPa, using the Skarstrom, or modified Skarstrom PSA cycle. There are multiple steps, or stages, involved in the operation of each of the beds in the Skarstrom cycle In the simplest case, the four steps in the sequence are as follows. (a) Pressurization (with feed or raffinate product) (b) High pressure adsorption separation with feed, with withdrawal of purified raffinate (c) Depressurization, or “blowdown”, counter-current to the feed (d) Desorption at the lower operating pressure. This is accomplished by evacuation or purging the bed with (a portion) of the purified raffinate product The use of 3-layer adsorbent beds is another important characteristic of the currently employed PSA technology for hydrogen purification. Commonly, the first layer is either alumina or silica that retains the water vapor. Then an activated carbon (AC) layer is used to selectively adsorb CO2. The last layer is a cation-exchanged zeolite (such LTA-5A, with Na+ and Ca++ extra-framework cations, and NaX, also commonly denoted by its trade name 13X, with Na+ extra-framework cations) with enhanced capacity for CO, and N2. CO2 and water vapor are strongly adsorbed in the zeolite and cannot be readily desorbed by decreasing the pressure and start to accumulate in this adsorbent as the cycles proceed. The main task of the alumina and AC layers is to prevent the H2O vapor and CO2 from reaching the zeolite layer. . The modelling details are contained in my paper R. Krishna, Metrics for Evaluation and Screening of Metal-Organic Frameworks for Applications in Mixture Separations, ACS Omega 5 (2020) 16987−17004. https://doi.org/10.1021/acsomega.0c02218 Watch also my video presentations Transient Breakthrough Simulations, and Screening of MOFs at    / @rajamanikrishna250