Large zeolite particles typically operate under the assumption that the overall mass transfer rate into the particle is controlled by intraparticle diffusion. Since the diffusion path inside the particle is large, it is typically a reasonable estimation for process simulators. While this same assumption is often applied to simulations of rapid pressure swing adsorption (RPSA) processes using much smaller particles (<1mm), it has yet to be determined if it remains a suitable approximation. Since the diffusion path inside the particle has been significantly reduced, assuming intraparticle diffusion controls the mass transfer rate into the particle may no longer be valid. Furthermore, some literature correlations suggest that for small particles, axial dispersion effects significantly increase. The goal of this study is to better understand the significance of axial dispersion for columns of small particles in order to better estimate its effect on RPSA processes for simulation purposes. It is also the aim to evaluate the effectiveness of the mass transfer zone (MTZ) measurement technique for determining mass transfer parmaters relevant to RPSA simulations.
The effect of axial dispersion on MTZ spreading is evaluated through traditional breakthrough experiments. The MTZ length is measured on a column packed with a commercial LiLSX zeolite (~0.5 mm in diameter). Previously we conducted the same experiments using a N2-He mixture, however, for this study a N2-O2 gas mixture was used because of its applicability for air separation. The results from these experiments suggest axial dispersion effects are much more significant than predicted through typical correlations used for larger particles. Mass transfer parameters found from the results are used with an in-house PSA simulator to determine how critical it is to account for the higher level of axial dispersion with small particles. Consequences of not properly estimating the effects of axial dispersion will also be discussed.