Modelling of monoliths for adsorption processes
Item statusRestricted Access
Embargo end date11/04/2023
Straight-channel monoliths are a promising means to achieve process intensification of adsorption processes compared to conventional packed beds. Their main benefits are handling of high throughputs and good thermal management. The efficiency of a straight-channel monolith can be assessed via the definition of its height equivalent to a theoretical plate (HETP) and pressure drop. This thesis aims at developing a systematic procedure for the derivation of HETP correlations for industrially relevant straight-channel monoliths and review the pressure drop correlations for monoliths available in the literature. The HETP correlations derived are validated against full 3D numerical simulations of the single representative channel of each straight-channel monolith under analysis. The HETP correlations predict with great accuracy the HETP from numerical simulations. Moreover, simplified reduced order models are developed. The models are able to capture the overall dynamics of the 3D simulations for both isothermal and linear conditions, and non-isothermal and non-linear ones. The reduced order models are fully predictive, and strongly rely on accurate equilibrium and kinetic parameters. Given the relevance of reliable equilibrium and kinetic parameters for the simulation of monoliths, this thesis further investigates how to model multicomponent adsorption on heterogeneous solids, and how to extract kinetic parameters from experiments. The second part of this work presents the multisite rigid adsorbent lattice fluid (multi-RALF) model, a novel thermodynamic theory to model multicomponent adsorption on heterogeneous adsorbents. The parameterisation of multi-RALF is analysed in regard to the azeotropic adsorption of benzene and propene on silicalite and to the adsorption of CO2 on the flexible synthetic zeolite (Na,TEA)-ZSM-25. The former study is carried out using molecular simulations, while the latter uses experimental data. The results of the molecular simulations show that the azeotrope is caused by steric hindrance of benzene in the adsorbent framework. Once correctly parametrised using single component isotherms, multi-RALF can predict the azeotrope of the system. Multi-RALF has been proven to be an effective model for the flexibility of (Na,TEA)-ZSM-25 upon CO2 adsorption, as well. Experimental isotherms of CO2 adsorption on (Na,TEA)-ZSM-25 always present an inflection at a constant adsorbed amount. This has been explained as a gate opening effect. The adsorbent is made of two sites, α and β. The site β becomes accessible only after a critical uptake of 0.6 mol/kg. At this uptake, the cations blocking the site β interact with the adsorbate and move away from the windows of the β site. This effect leads to the inflection in the isotherm and a relaxation of the solid framework with a small breathing effect. The breathing behaviour effect on the system kinetics is then investigated using the zero length column (ZLC) technique. The ZLC curves at different flowrates and temperatures always present a transition between an equilibrium controlled regime at high partial pressures of CO2, while becoming kinetically controlled in the limit of zero-loading. The transition between the two regimes is related to the cation movement, and it is carefully accounted for in a ZLC numerical model which successfully fits the experimental data. Finally, thermal frequency response (TFR) measurements for air separation on the zeolite LiLSX are presented. This is a fast diffusing system, difficult to study with commercial equipment. Hence, the purpose-built dual piston pressure swing adsorption apparatus is used for the experiments. Two models are developed to analyse both single- and multi-component measurements. From the single component model, a tortuosity of 3.3 is regressed, while the multicomponent data show a N2/O2 selectivity of 6. Both values are in accordance with the available literature. TFR has proven to be a powerful technique to handle challenging diffusing systems, able to effectively discriminate the relevant mass and heat transfer time constants as shown from experiments at conditions relevant to process application. The work here presented aimed at providing a reliable methodology for modelling of monoliths and derivation of the relevant equilibrium and kinetic parameters. In future, the combination of the monoliths’ models and multi-RALF could become a robust and reliable tool for the deployment of monoliths for process intensification.