(618e) Characterisation of CO2 Adsorption on Monolith Structures

Monoliths are designed to optimise process performances by addressing some of the shortcomings of conventional pelletised materials. Channel shapes and flow patterns are engineered to minimise pressure drop, improve adsorption/desorption kinetics and maximise accessibility to the active material. The complexity of these structures has led to the conception that the main challenges in the field of monoliths are associated to the synthesis and production process while it is often neglected that testing and characterising these structured materials is equally challenging. In fact, monoliths are produced in a variety of shapes and sizes and this, in many cases, prevents the use of most conventional and commercial adsorption techniques. Most of these systems (microbalances and commercial volumetric apparatuses, for example) are generally designed to minimise dead volumes and sample units are meant to house relatively small quantities of powders or pelletised materials. This means that the most practical way to characterise a monolith is to break it down into fragments to be fit into the sample cells of these systems. While tests on monolith fragments can provide essential fundamental information on the material, these alone do not allow to assess the dynamic performance of the fully formed material. For this reason, it is good practice to combine experiments on fragments with tests on the whole monolithic unit.

In this work, we present the characterisation of CO₂ adsorption on a commercial monolith designed as a car catalytic converter. In order to gain a better understanding of the adsorption process, a systematic experimental approach was designed that combined measurements on both fragments and the whole monolithic unit across four different techniques. Experiments on the Zero Length Column (ZLC) (using fragments) were carried out over a wide range of CO₂/H₂O compositions to determine the effect of the presence of H₂O in the adsorption equilibrium and kinetics of CO₂. The results show a clear impact of H₂O on both the kinetics and the equilibrium of CO₂ adsorption on the sample. A detailed analysis of the pure CO₂ adsorption kinetics in the whole monolithic unit was also carried out using a novel low pressure Adsorption Differential Volumetric Apparatus (ADVA) which allows accurate kinetic measurements at different pressure levels up to 1.3 atm. Being a built-in-house system it could be easily modified with a custom-made sample cell to house the monolith sample.

The fundamental equilibrium and kinetic properties obtained from these experiments were then used to predict the breakthrough dynamics of CO₂ and H₂O the whole monolith. A dedicated breakthrough system was used for this experiment with a sample column specifically designed to house the monolith and allow homogenous flow across the unit. The system is equipped with a H₂O delivery system, a humidity probe and a Mass Spectrometer for multicomponent measurements. Experiments were carried out at different flowrates and concentration levels of CO₂ and H₂O. Finally, to gain a deeper understanding of the impact of CO₂ adsorption on the pore structure of the monolith, cryogenic N₂ and Ar experiments were carried on the monolith fragments with and without pre-adsorbed CO₂ using a Quantachrome Autosorb iQ2 system