Effective diffusivity in partially blocked zeolite catalyst

Abstract

Diffusivity in zeolites with partially blocked pores and its effect on reaction rates are studied here. A finite periodic lattice model with randomly blocked bonds is used to investigate the effect of pore blocking on zeolitic diffusion. Percolation theory and the technique of effective medium approximation (EMA) are employed to obtain the theoretical dependence of diffusivity on the fraction of blocked bonds. The prediction agrees well with stochastic computer simulation results. Different methods in measuring effective diffusivity, namely, the Wicke-Kallenbach cell, the transient adsorption experiment, and experiment under reactive conditions, are also studied theoretically and by stochastic computer simulations, and compared with classical continuum theory. It is shown that diffusivities obtained by different methods could have different values, depending upon the blocked fraction. The Wicke-Kallenbach method yields a lower bound for the effective diffusivity, while the transient method gives a time-dependent diffusivity, and the reactive diffusivity depends on the value of Thiele modulus. The physical significance of these differences are explained by the rate processes involved and the tortuous nature of the blocked crystals.