Simulation of diffusion in zeolitic structures

Abstract

AbstractUsing Maxwell‐Stefan equations, experimental and computational results of binary diffusion in pore‐ and cage‐type zeolitic structures are described. In the generalized Maxwell‐Stefan (GMS) formulation, the Fick diffusivity is written as the product of two separate contributions, the GMS or corrected diffusivity and the thermodynamic factor. The concentration dependence of the GMS diffusivity for one‐ and two‐component diffusion in zeolitic structures is investigated. From the Maxwell ‐ Stefan equations, different models for the Fick diffusion coefficient matrix for the description of binary mass transport in molecular sieve materials are derived. Various models used predict binary diffusion in zeolitic structures. First, theoretical predictions of binary apparent diffusivities as a function of the occupancy are compared to results from Monte Carlo simulations. Second, theoretical results of binary uptake profiles are compared to experimental results for the system ethylbenzene/benzene/ZSM‐5. For different zeolitic structures, that is, pore‐ and cage‐type structures, results of the Monte Carlo simulations agree well with the theoretical predictions. In cage‐type structures, the effect of counterexchange between sorbed molecules is demonstrated. Experimental results of transient uptake profiles of a mixture of benzene and ethylbenzene in ZSM‐5 follow predictions of the theoretical single‐file diffusion model.