ADSORBENT PARTICLE-SIZE EFFECTS IN THE SEPARATION OF AIR BY RAPID PRESSURE SWING ADSORPTION

Experimental and theoretical investigations into air separation by rap id pressure swing adsorption over zeolite 5A are presented. These conc entrate on the effect of adsorbent particle size on the separation per formance of the unit undergoing simple cycles consisting of pressurisa tion and depressurisation steps. An optimum particle size for maximum cyclic equilibrium product oxygen purity is shown to exist; this is ac curately predicted by model simulation. Calculations indicate that for beds containing very small particles, a poor separation results from ineffective pressure swing, and for beds containing very large particl es from intraparticle diffusional limitations. For the zeolite 5A adso rbent used in this work, theoretical calculations indicate that the ra te limiting intraparticle diffusion is described by a parallel combina tion of molecular and Knudsen diffusion within the macropores of the a dsorbent particles. Axial dispersion within the bed is also shown to h ave a significant effect upon the cyclic equilibrium value of the cycl e-average product oxygen purity. In addition to measurements at the cy clic equilibrium, temporal profiles of the product oxygen purity durin g the approach to cyclic equilibrium are shown. Under certain operatin g conditions, an overshoot of oxygen purity is found to exist. This be haviour can be attributed to ineffective pressure swing within the pro duct end region of the bed, and thus to the poor utilisation of adsorb ent in this region