The Effect of Wall Porosity and Zeolite Film Thickness on the Dynamic Behavior of Adsorbents in the Form of Coated Monoliths

The effects of wall porosity, channel width distribution and zeolite film thickness on the performance of 400 and 1200 cells per square inch (cpsi) cordierite monoliths coated with zeolite X films with thicknesses of 1.5 and 2.5 μm were examined. To investigate the effect of wall porosity and restrict growth of zeolite to the external surface of the monolith channels, the macro pores in the walls of the 1200 cpsi cordierite monoliths were filled with colloidal α-alumina particles. The adsorbents were characterized by Scanning Electron Microscopy (SEM), Mercury Intrusion Porosimetry (MIP) and carbon dioxide breakthrough experiments and a mathematical model describing the diffusion and adsorption in the system was fitted to the data. The model accounted for carbon dioxide uptake by filling the pores in the support by carbon dioxide gas and adsorption of carbon dioxide on cordierite, alumina and zeolite. The model indicates that the uptake of carbon dioxide by adsorption on cordierite is much slower than by pore filling and too slow to influence the very fast breakthrough experiments with monoliths without zeolite film that are over in less than 1 min. It was shown that the pores in the cordierite monolith result in dispersion by pore filling with carbon dioxide gas, not adsorption. The CO₂ adsorption capacity of a 1200 cpsi monolith coated with a 2.5 μm film was 0.13 mmol/cm ³ adsorbent, which should be compared to the adsorption capacity of zeolite X beads, which is about 2.3 mmol/cm ³ adsorbent. To increase adsorption capacity of a non-porous zeolite coated monolith, film thickness could be increased. The model indicated that the film thickness could be increased up to about 10 μm without increasing the dispersion and thereby approach the adsorption capacity for beads. However, simulation of the whole cycle must be performed in order to find the optimum film thickness for a real cyclic process. This work has lead to better understanding of the role of the support porosity and pore size distribution and film thickness for coated monolith adsorbents.