Limitations of the Axially Dispersed Plug-Flow Model in Predicting Breakthrough in Confined Geometries

This paper examines the ability of the axially dispersed plug-flow model to accurately predict breakthrough in adsorbent beds confined by rigid walls. The axially dispersed plug-flow model is used to independently extract mass transfer and axial-dispersion coefficients from breakthrough experiments via centerline and mixed-exit concentration measurements, respectively. Four experimental cases are considered: breakthrough of carbon dioxide (CO₂) and water (H₂O), in two cylindrical beds of zeolite 13X (NaX) each. The extracted axial-dispersion coefficients are compared to predictions from existing correlations which are widely used to predict mechanical dispersion in packed beds. We show that such correlations grossly underpredict the apparent axial dispersion observed in the bed because they do not account for the effects of wall channeling. The relative magnitudes of wall-channeling effects are shown to be a function of the adsorption/adsorbate pair and geometric confinement (i.e., bed size). We show that while the axially dispersed plug-flow model fails to capture all the physics of breakthrough when non-plug-flow conditions prevail in the bed, it can still be used to accurately extract mass transfer coefficients using intrabed concentration measurements.