23 Modeling Issues in Zeolite Applications Rajamani Krishna University of Amsterdam, Amsterdam, The Netherlands I. INTRODUCTION Zeolitic materials are used as sorbents and catalysts in a variety of processes within the chemical, petroleum, petrochemical, and food industries. Zeolite crystals are incorporated into binders (such as amorphous aluminosilicate) and perhaps a diluent (typically a clay mineral), and used in the form of powder (in fluidized beds) or pellets (in fixed beds). Alternatively, zeolite crystals are coated onto a porous membrane support and used in (catalytic) membrane permeation devices. Zeolite-based processes are carried out either under steady-state, unsteady-state, or cyclic conditions. Fixed-bed adsorbers are typically operated under transient conditions. Zeolite membrane processes typically operate under steady-state conditions. Simulated moving-bed adsorbers operate under cyclic conditions. While many of the transport issues can be understood from the standpoint of classical diffusion and flow, special attention needs to be paid to the proper description of mixture sorption and diffusion in zeolites. The purpose of this chapter is to highlight the special features of zeolite sorption and diffusion by means of several illustrative examples of practical importance. II. TRANSIENT UPTAKE OF A SINGLE COMPONENT WITHIN A ZEOLITE Let us begin by considering the case of a batch adsorber in which zeolite particles are brought into contact with a fluid phase containing a component species i that diffuses into the particle (of diameter d p ) into which the zeolite crystals are embedded; see Fig. 1. There are three steps in the intraparticle diffusion process. 1. Component i in the bulk fluid phase surrounding the particle has first to diffuse across the stagnant layer, of thickness y f , surrounding the particle. The stagnant ‘‘film’’ thickness y f is determined by the fluid–particle hydrodynamics. Higher Reynolds numbers will lead to smaller y f values and, consequently, lower film diffusion resistance. 2. Next, component i diffuses into the macropores. For a fluid at least four resistances contribute to transport in the macropore. These are Knudsen diffusion (a transport process when the fluid is essentially gaseous in nature, where molecular collisions between the diffusing species and the pore walls predominate); surface diffusion (diffusive motion of adsorbed species over the Copyright © 2003 Marcel Dekker, Inc.