SIMULATION OF DIFFUSION IN ZEOLITIC STRUCTURES

Using Maxwell-Stefan equations, experimental and computational results of binary diffusion in pore- and cage-type zeolitic structures are de scribed. 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 con centration dependence of the GMS diffusivity for one- and two-componen t diffusion in zeolitic structures is investigated. From the Maxwell-S tefan equations, different models for the Fick diffusion coefficient m atrix 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 apparen t diffusivities as a function of the occupancy are compared to results from Monte Carlo simulations. Second, theoretical results of binary u ptake profiles are compared to experimental results for the system eth ylbenzene/benzene/ZSM-5. For different zeolitic structures, that is, p ore- 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 be nzene and ethylbenzene in ZSM-5 follow predictions of the theoretical single-file diffusion model.