Pressure swing adsorption (PSA) and vacuum swing adsorption (VSA) cycles are alternatives to thermal cycles for gas systems. They are particularly useful for more concentrated feeds and/or adsorbates that are not strongly adsorbed. Figure 18-11A shows the steps in the basic Skarstrom cycle (named after Charles Skarstrom, the inventor of the process) for PSA (Ruthven et al., 1994; Wankat, 1986). Usually, the desired product is the pure product gas after adsorbate removal. Typical applications include drying gases, purifying hydrogen, and producing oxygen or nitrogen from air.