Pressure swing adsorption: Effects of incomplete purge

Theoretical and experimental studies of pressure swing adsorption (PSA) cycles for various extents of purging have been conducted. In both the theoretical and experimental portions, pressurization was accomplished with product rather than with feed. A simple, local equilibrium model was employed, and the results were analyzed by the method of characteristics. It was found that for difficult separations (i.e., closely spaced isotherms and/or heavily contaminated feeds), incomplete purging, e.g., as little as 50% complete, yielded recoveries attainable only at much higher pressure ratios with complete purge. Conversely, for easy separations (i.e., widely spaced isotherms and dilutely contaminated feeds), incomplete purging was less attractive, yielding only slight improvement over complete purge. Splitting oxygen from air with zeolite 5A is moderately difficult due to the proximity of the isotherms and due to the predominance of the more strongly adsorbed nitrogen in the feed. This case was also modeled and was studied experimentally. The experiments yielded nearly double the recovery of oxygen for minimal purge compared with that available with complete purge. The productivity of oxygen was relatively high, e.g., 0.0378 m3(O2 at STP)/kg (adsorbent) · h. Those experiments validated the trends predicted theoretically.