Effective macropore diffusivity of carbon dioxide on binderless pellets of Y-type zeolites

Abstract

The adsorption kinetics of carbon dioxide (CO₂) in three cationic forms of binderless pellets of Y-types zeolites (H-Y, Na-Y, and TMA exchanged Na-Y) are studied using the zero-length column (ZLC) technique. The measurements were carried out at \(288.15\,\textrm{K},298.15\,\textrm{K}\) and \({308.15}\,\textrm{K}\) using different flowrates and an initial CO₂ partial pressure of \({0.10} \,\textrm{bar}\)– conditions representative of post-combustion CO₂ capture applications. The mass transport within the adsorbent pellets was described using a 1-D Fickian diffusion model accounting for intra- and inter-crystalline mass transport. For the latter, the parallel pore model formulation was used to explicitly account for the adsorbent’s macropore size distribution in estimating the volume-averaged diffusivity of the gas. Experiments carried out using different carrier gases, namely helium and nitrogen, were used (i) to determine that these systems are macropore diffusion limited and (ii) to simplify the parameter estimation to a single parameter - the macropore tortuosity. The latter (\(\tau =1.3-2.5\)) was in good agreement with independent measurements using MIP (\(\tau \approx 1.7\)). The associated diffusion coefficient, \(D^\textrm{e}_\textrm{mac}\), was found to vary due to differences in the materials’ macropore size distributions and overall porosity. Upon combining the parallel pore model formulation with the temperature dependencies for the pore diffusivities derived from molecular theories of gases, we predict \(D^\textrm{e}_\textrm{mac}\propto {T^b}\) with \(b=[0.78-0.88]\) depending on the macropore size distribution. Notably, for the range of temperature tested in this study, \(D^\textrm{e}_\textrm{mac}\) varies approximately linearly with temperature (\(b\approx 1\))– in contrast to the commonly reported correlation of \(b=1.75\), which may be more appropriate for systems where molecular diffusion dominates and Knudsen diffusion is negligible. The binderless pellets of Y-type zeolites studied exhibit generally higher values for the effective macropore diffusivity of CO₂ compared to previously reported results on commercial FAU zeolites.