Xylene Permeation Transport through Composite Ba-ZSM-5/SS Tubular Membranes: Modeling the Steady-State Permeation Ana M. Tarditi, Eduardo A. Lombardo, and Adolfo M. Avila* Instituto de InVestigaciones en Cata´ lisis y Petroquı ´mica (FIQ, UNL-CONICET), Santiago del Estero 2829, 3000 Santa Fe, Argentina Steady-state single-component and ternary mixture xylene permeation fluxes through Ba-ZSM-5/SS composite membranes were studied, as a function of temperature and pressure. The single p-xylene flux has a weak maximum, relative to temperature (100-400 °C). The flux magnitude and its maximum location are dependent on the extent of Ba-exchange. The o- and m-xylene fluxes steadily increase with temperature. The single permeation behavior is well-described by a model based on the contribution of different transport mechanisms: Knudsen flux, surface diffusion, and activated gas translation diffusion. The comparisons between either the mixture permeation results or the pressure effect experiments and the simulated data reflect the existing adsorbate-framework interactions that are not easily contemplated by a macroscopic model. 1. Introduction The dimensions of zeolite pores are uniform and close to the molecular dimensions of small hydrocarbons. Therefore, highly selective separations can be achieved based on the molecular sieving and the adsorption-diffusion properties of zeolites. Therefore, zeolite membranes are capable of separating mixtures that are difficult or impossible to separate via other means (e.g., xylene isomers by distillation, 1,2 azeotropic mixtures by per- vaporation 3 ). Molecules diffuse through the pores via various mechanisms. Zeolites can be shape-selective; however, when the interactions between the surface and the diffusing molecules are important, adsorption or surface diffusion can dominate the transport. It is well-established in the literature that p-xylene adsorbs selectively on MFI from mixtures of the three isomers. 4 Therefore, not only the molecular sieving ability of the zeolites but also their adsorption properties are important in the separation of the xylene isomers. In fact, it has been reported that the p-xylene flux presents a maximum, as a function of temperature. The interplay between adsorption and diffusion may be responsible for this maximum. 2,5 One of the most common assumptions, and a very critical one, regarding most zeolite membrane modeling is that the membrane is defect-free. However, most composite membranes are not perfect. Permeation then occurs through the zeolitic channels and nonzeolitic pores (by Knudsen diffusion and/or viscous flow). The separation performance of zeolite membranes is significantly dependent on the membrane quality, because the intercrystalline space and pinholes reduce the membrane selectivity. The quantification of these extra-zeolitic channels is then basic information that is needed to model the transport mechanism through zeolite membranes. 6 Thus, with the purpose of characterizing MFI-zeolite membranes, the permeation behavior of single xylene isomers and their mixtures have been studied, as a function of temperature, and, in fewer cases, the effect of pressure across the membrane also has been explored. 5,7 Moreover, note that there are no studies in the literature reporting the xylene ternary mixture behavior through MFI-zeolite membranes based on macroscopic gas transport models. This is important, to understand not only the transport behavior but also the performance of the composites. In addition, a model description would assist in the comparison of experimental data obtained using membranes with different characteristics and under varying operational conditions. The mass transport of different molecules within the zeolite channels is not only influenced by their adsorption and diffusion characteristics but also by other factors, such as the pore diameters, the structure of the pore walls, the interactions between the surface atoms and the diffusing molecules, the configuration of the diffusing molecules, the way the channels are interconnected, and adsorbate-adsorbate interactions. The quantitative prediction of diffusion rates inside the zeolites with modeling techniques is often difficult to relate to the aforemen- tioned properties and the microscopic mechanisms. Furthermore, the large discrepancies that often exist between diffusivities determined through different experimental techniques 8,9 con- tribute to the difficulty of diffusion prediction. It is generally accepted that the generalized Maxwell-Stefan formulation 10 offers the most convenient and the nearest quantitative prediction of multicomponent transport through zeolite membranes. 11 The strength of this model lies in the fact that it intrinsically encompasses intracrystalline diffusion phe- nomena as well as adsorption processes, facilitating the predic- tion of multicomponent transport, based on pure component Maxwell-Stefan diffusivities and mixture adsorption iso- therms. 12 The objective of this study is to describe the fluxes of single xylenes and their mixtures through Ba-ZSM-5/SS composites, according to a steady-state permeation model that is based on the participation of different parallel fluxes, thus contributing to elucidate the xylene transport mechanism. To this end, the effect of temperature and pressure gradients across the mem- brane on the xylene permeation was studied. With this purpose, the membrane was synthesized by secondary growth on the outer surface of the tubular support. Its thermal stability up to 400 °C was carefully investigated, to ensure the validity of the data obtained. 2. Experimental Section 2.1. Membrane Preparation. The tubular composite mem- brane used in this study consisted of an MFI-zeolite layer on a stainless steel tubular support (Mott Metalurgical, 10 mm outer * To whom correspondence should be addressed. Tel.: +54 342 4536861. E-mail address: aavila@fiqus.unl.edu.ar. 2377 Ind. Eng. Chem. Res. 2008, 47, 2377-2385 10.1021/ie071296l CCC: $40.75 © 2008 American Chemical Society Published on Web 02/29/2008