Air-prepurification by pressure swing adsorption using single/layered beds

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, CO,, and light hydrocarbons need to be brought down below the tolerable limits which are in the ppm or sub-ppm levels. This process is commonly carried out by using adsorptive methods such as pressure swing adsorption (PSA) or temperature swing adsorption (TSA). In this work, the adsorption characteristics of a natural zeolite (chabazite) for the trace removal of water vapor, CO,, and a hydrocarbon (CH,) were studied and compared with those of two conventional microporous adsorbents, namely 13X (NaX) zeolite molecular sieves, and activated alumina (gamma -Al2O3). The low pressure isotherm data was fit to the Dubinin-Astakhov (DA, or potential theory) isotherm model and was extended to multicomponent mixtures using the maximum available micropore volume model of Doong and Yang (Ind. Eng. Chem. Res. 27(4) (1988) 630). An equilibrium based non-isothermal model was used to simulate the performance of the above sorbents for a typical four-step Skarstrom-type PSA cycle for the simultaneous removal of H2O and CO2 from feed N-2. Two types of bed configurations were considered: single beds containing only one type of sorbent, and layered beds with contiguous layers of two different sorbents (alumina/13X zeolite). The relative amounts of sorbents required for the layered bed were optimized for particular bed operating conditions. (C) 2001 Elsevier Science Ltd. All rights reserved.