Low-pressure performance evaluation of CO2, H2O and CH4 on Li-LSX as a superior adsorbent for air prepurification

• Discovered the best sorbent, Li-LSX zeolite, for air prepurification via low-pressure isotherms.
• Measured adsorption isotherms at ultra-low pressures down to ppm level.
• The relative merit of different sorbents is correctly interpreted by simple theoretical calculations.

High-volume production of oxygen and nitrogen from atmospheric air is accomplished by cryogenic distillation. Prior to feeding to the air separation unit, the ambient air must be pre-purified by the removal of trace impurities such as CO2, H2O vapor, and light hydrocarbons (e.g., CH4) to their tolerable limits of 1.0 ppm, 0.1 ppm, and to a few ppb, respectively. In this study, the adsorption characteristics of a synthetic zeolite, Li-LSX (where LSX denotes low-silica type-X zeolite where Si/Al=1), for the removal of trace amounts of CO2, CH4, and H2O vapor impurities were analyzed and compared with the conventional synthetic 13X zeolite that has been used for air prepurification as well as the ion-exchanged K-LSX and Ca-LSX zeolites. Isotherms to very low partial pressures (to a few ppm of 1 atm) were measured. The Tὸth, Langmuir–Freundlich (L–F), and the Dubinin–Astakhov (D–A) isotherm models were used to correlate the low pressure experimental data and the Tὸth model was found to be the better fit especially in the low pressure range. The superior adsorption properties of Li-LSX for air prepurification are demonstrated in this work.