(100d) Diffusion Mechanism of CO2 in 13X Zeolite Pellets

A systematic study of the diffusion mechanism of CO₂ in commercial 13X zeolite pellets is presented. In order to gain a complete understanding of the diffusion process of CO₂, kinetic measurements with a Zero Length Column (ZLC) system and a volumetric apparatus have been carried out.

The ZLC experiments were carried out on a single pellet of zeolite 13X at 38 °C at a partial pressure of CO₂ of 0.1 bar, conditions representative of post-combustion capture. Experiments with different carrier gases clearly show that the diffusion process is controlled by the transport inside the macropores. In order to provide a more accurate analysis of the ZLC curves we created an automated tool by linking our adsorption simulator to a Multi-Objective Genetic Algorithm (MOGA). From the fitting of experimental data the kinetic and equilibrium parameters were extracted.

Volumetric measurements using a Quantachrome Autosorb system were carried out at different concentrations. Differently from the ZLC experiments in this case no carrier gas is present. The low pressure measurements show clearly Knudsen diffusion control in the pellet macropores, while in the short time region a contribution from the viscous flow of CO₂ in the uptake cell is also detected. At increasing CO₂ concentrations the transport mechanism shifts from Knudsen diffusion in the macropores to a completely heat limited process. Measurements using two different pellet sizes confirmed that the diffusion process is controlled by the transport in the macropores.

Finally, the diffusivities extracted from the ZLC and the volumetric experiments were used to calculate the tortuosity factor of the 13X pellets. The values of tortuosity obtained from the two experiments are consistent and in excellent agreement with the one estimated from independent mercury porosimetry measurements.