Axial dispersion effects with small diameter adsorbent particles

Macropore diffusion is traditionally assumed to control the mass transfer rate in columns packed with zeolite particles in an oxygen production process. While numerous studies have confirmed this assumption for the particle size used in industrial size pressure swing adsorption (PSA) processes, it has not been validated for the much smaller particle size used in rapid PSA (RPSA). Smaller particles improve the mass transfer rate by increasing interfacial area per volume as well as decreasing diffusion distance. Despite this reduction, RPSA simulations often still assume a mass transfer rate solely limited by macropore diffusion. This approach fails to adequately account for the influence of other mass transfer mechanisms whose impact increases due to particle size reduction. This study experimentally demonstrates the dominant mass transfer mechanism is no longer macropore diffusion for the particle size used in RPSA for small scale oxygen production. Depending on the gas velocity, axial dispersion effects either become the limiting mechanism or equally as important as macropore diffusion. It also shows that improperly accounting for axial dispersion effects has a significant impact on the mass transfer coefficient estimation, often measured with breakthrough experiments.