Highlighting diffusional coupling effects in zeolite catalyzed reactions by combining the Maxwell-Stefan and Langmuir-Hinshelwood formulations

The combined phenomena of intra-crystalline adsorption, diffusion and reversible chemical reactions inside microporous crystalline zeolite catalyst particles are described by combining the Langmuir-Hinshelwood kinetics with the Maxwell-Stefan (M-S) diffusion formulation. Simulations of transient diffusion and reaction inside catalyst crystallites are performed for a variety of reactions including: alkane isomerization, xylene isomerization, ethylation of benzene and dehydrogenation of ethane. For all reaction systems, the transient diffusion/reaction process exhibits overshoots in the loading of the more mobile guest molecules within the zeolite pores; such overshoots imply the attainment of supra-equilibrium conversions for a limited time span. The origin of the transient overshoots is traceable to the use of chemical potential gradients as the proper driving forces in the M-S formulation; this leads to coupling effects induced by mixture adsorption thermodynamics. Use of the simplified Fick's law for intra-crystalline diffusion, ignoring thermodynamic coupling, does not result in overshoots. Simulations of fixed zeolite bed reactors are performed to demonstrate the significant differences in the productivity predicted by the M-S and Fick models for intracrystalline diffusion.