Sorbents for air prepurification in air separation

Air fed to air separation units such as cryogenic distillation columns needs to be prepurified; that is, the concentration levels of air-borne impurities such as water vapor, CO₂, and light hydrocarbons need to be brought down below the tolerable limits. This process is commonly carried out by using adsorptive methods such as pressure swing adsorption (PSA) or temperature swing adsorption (TSA). This work deals with the study of adsorption characteristics of two conventional microporous adsorbents, namely 13X zeolite molecular sieves, and activated γ-Al₂O₃, and three non-conventional adsorbents, namely a natural zeolite (clinoptilolite), and its K⁺ - and Ca²⁺-ion exchanged forms. A noteworthy feature of this work is the measurement of adsorption isotherms at very low partial pressures of the adsorbate gas (to a few ppm). The relative merits of these adsorbents for the removal of trace amounts of water vapor, CO₂, and hydrocarbons such as CH₄, C₂H₄, and C₂H₆ are discussed. The isotherm data for 13X zeolite and γ-Al₂O₃ has been fit to the Langmuir-Freundlich, Toth, and Dubinin-Astakhov (DA, or potential theory) isotherm models. It has been found that the potential theory model is the most suitable one for description of low pressure or concentration data. The origin of the better fit by potential theory is that its corresponding energy distribution function follows a quasi-Gaussian distribution with a broadening at high adsorption energies, and the high-energy sites are important for adsoprtion at low pressures or concentrations. Finally, the possibility of using H₂O adsorption isotherm to evaluate the pore size distribution by the Horvath-Kawazoe approach is discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.