Coppens, Marc-Olivier

Rao, Sanjeev M.

English

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The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and ApplicationSimplified theory to predict mixture diffusion in zeolites: Accounting forstrong correlations and examining the role of adsorption thermodynamicsSanjeev M. Rao2, Marc-Olivier Coppens1∗1University College London, London, United Kingdom2Rensselaer Polytechnic Institute, Troy, USA∗m.coppens@ucl.ac.ukThe Maxwell–Stefan (MS) equations have been extensively used to describe mixture diffusion in zeo-lites. Correlation effects can arise due to molecules unsuccessfully attempting to pass each other in thevery narrow pores of the zeolite, e.g., at high loadings of adsorbed molecules. In an extreme situation,diffusion may even become anomalous, giving rise to a single-file phenomenon. These correlation ef-fects are captured in the so-called “exchange terms” within the framework of the MS equations. Theexchange terms are often calculated from pure component data using empirical approaches with differ-ent fits for different combinations of zeolite hosts and molecules. These computations, in turn, can befairly tedious, especially for large multicomponent systems. It is therefore advantageous to be able touse a simplified theory to capture the effect of strong correlations. To this end, the MS equations aresimplified to predict mixture diffusion behavior in the case of strong correlation effects. Intermolecularexchange coefficients are not required. No empirical relations are employed. This simplified modelis used to predict the diffusion behavior in the separation of a CH4-C3H8 mixture through a silicalitemembrane [1].Diffusion in zeolites is also closely coupled with adsorption due to the tightly confining pore space inthe zeolite where the concept of a bulk fluid phase ceases to exist. Adsorption effects are accountedfor in the MS theory through the mixture thermodynamic correction factors, which are derived usingthe pure component adsorption isotherms and theories such as the Ideal Adsorbed Solution Theory(IAST). The calculation of mixture thermodynamic correction factors in the MS equations can alsopose a computational hurdle especially for large systems, and if theories such as the IAST have to beemployed. It is expected that, if a vacant adsorption site is available, the adsorption of a species insidethe zeolite pore should be dictated by the pure component adsorption isotherm alone, and that mixtureadsorption thermodynamics should not play a role, or else correlation effects should be present inadsorption as well, in which case theories such as the IAST cannot be used. To this end, the MSequations are simplified to employ only the pure component thermodynamic correction factors. Thissimplified model was used to predict mixture diffusion for two cases of practical interest [1]: (i) theseparation of a N2-CH4 mixture through a SAPO-34 membrane, and (ii) the alkylation of C6H6 withC2H4 over H-ZSM-5. In all cases, predictions from the simplified models are favorably compared toresults from the more complex MS equations or experiments, indicating that the simplified models canserve as good starting approximations, and can also speed up mixture diffusion calculations in zeolites.References[1] S. Rao, M.-O. Coppens: Predicting mixture diffusion in zeolites: Accounting for strong correla-tions with a simplified theory and examining the role of adsorption thermodynamics. Journal ofPhysical Chemistry C 116, 26816–26821 (2012)

Simplified theory to predict mixture diffusion in zeolites: accounting for strong correlations and examining the role of adsorption thermodynamics

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Simplified theory to predict mixture diffusion in zeolites: accounting for strong correlations and examining the role of adsorption thermodynamics

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