Ind. Eng. Chem. Res. 2008, 47, 2377-2385
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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 pervaporation3). 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 * To whom correspondence should be addressed. Tel.: +54 342 4536861. E-mail address:
[email protected].
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 aforementioned properties and the microscopic mechanisms. Furthermore, the large discrepancies that often exist between diffusivities determined through different experimental techniques8,9 contribute to the difficulty of diffusion prediction. It is generally accepted that the generalized Maxwell-Stefan formulation10 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 phenomena as well as adsorption processes, facilitating the prediction of multicomponent transport, based on pure component Maxwell-Stefan diffusivities and mixture adsorption isotherms.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 membrane 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 membrane used in this study consisted of an MFI-zeolite layer on a stainless steel tubular support (Mott Metalurgical, 10 mm outer
10.1021/ie071296l CCC: $40.75 © 2008 American Chemical Society Published on Web 02/29/2008
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diameter (o.d.) and 7 mm inner diameter (i.d.)). The average pore size of the substrate was 0.2 µm. This was cut into 30mm-long pieces; one end of the porous support then was welded to a nonporous stainless steel tube and the other end was sealed with a nonporous stopper. The ZSM-5 membranes were prepared by casting layers of silicalite seed crystals (ca. 250 nm). The seeding was conducted as follows: the support was dipped in the colloidal solution and vertically withdrawn at a slow rate; it then was dried for 30 min at room temperature and for 2 h at 90 °C. This procedure was repeated twice. The membrane was synthesized via secondary growth on the outer surface of the substrate. The seeded support was subsequently subject to three hydrothermal reaction steps. The first one was conducted at 165 °C for 8 h with a starting hydrogel that had a Si/Al ratio of 100 with the following composition: 21SiO2: 1TPABr:3NaOH:0.105Al2O3:987H2O.13 The other two steps were performed with a starting hydrogel that had a Si/Al ratio of 14, with a molar composition of 28SiO2:1.204TPABr: 3.4NaOH:1Al2O3:1288H2O.14 The first synthesis with an Si/Al ratio of 100 was conducted to facilitate the formation of a continuous and homogeneous layer of zeolite on top of the support, considering that the zeolitization rate decreases when the aluminum content on the hydrogel increases.15 The synthesis steps were repeated until the membrane was nonpermeable to N2, because of the presence of the template in the pores of the zeolite crystals. This confirms the formation of a continuous zeolite layer on top of the support. The permeable area of the membrane was 8.7 × 10-4 m2, and the thickness of the composite was ∼15-20 µm, as estimated by scanning electron microscopy (SEM). More details on the surface seeding and preparation of the clear solutions for secondary growth are given in our earlier publication.16 After hydrothermal synthesis, the membrane was washed with deionized water, dried at 80 °C for 24 h, and then tested for N2 permeation. When the membrane was nonpermeable to N2, the template in the zeolite pores was removed by heating in air at 470 °C for 3 h with a heating rate of 0.3 °C/min and a cooling rate of 0.5 °C/min. The cation exchange of the Na-ZSM-5 (Si/Al 14) membrane was conducted using barium nitrate. The Na membrane was immersed in a 0.1 M Ba(NO3)2 solution at 80 °C for 24 h. After the ion-exchange treatment, the tubes were thoroughly washed with distilled water and dried in air at ambient temperature for 2 h, and then at 80 °C for 24 h. To reach 100% substitution of Na+ by Ba2+, the exchange process was repeated three times. The extent of the cation exchange was determined by atomic absorption spectroscopy (AAS) in the powder sample exchanged under the same conditions as the membrane. No difference was observed in either permeation fluxes or separation factors measured after the second and the third exchange. 2.2. Gas Transport. 2.2.1. N2 Permeation and Thermal Stability. The N2 permeation data was the first tool used to verify the formation of a continuous film on top of the support surface and to detect the presence of extra-zeolitic pores in the membrane. The composite should be nonpermeable to N2, because of the presence of the template in the pores of the zeolite before calcination. It was measured at 25 °C and a transmembrane pressure of 80 kPa. The pressure difference across the membrane was controlled using a pressure regulator (Bronkhorst, Model P-502C). All the permeation data are reported with a (95% confidence interval. Permporosimetry data were used to determine the contribution of the zeolitic and nonzeolitic (defect) pores to the total permeation flow through the membrane. In this case, nitrogen was used as the non-adsorbed permanent gas
and the p-xylene was used as the condensable vapor. The experiments were performed at 25 °C. The pressure difference across the membrane was kept at 60 kPa, and the permeate side was kept at atmospheric pressure. Using this method, the relative contribution of unselective pores (defects) to the permeance through the membrane can be determined, thus allowing calculation of the Knudsen flux contribution. The thermal stability of the Ba-ZSM-5 (Si/Al 14) membrane was investigated. The composite was subject to a sequence of heating and cooling cycles between 30 and 400 °C and the durability was evaluated by xylenes and N2 permeance experiments. First, the mixture fluxes of the xylene isomers were measured in the 150-400 °C temperature range. The membrane then was cooled to ambient temperature. In the second and third cycles, the membrane was subject to a xylene isomerization reaction, up to 370 and 390 °C, respectively. 2.2.2. Xylene Separation. The vapor permeation experiments were conducted using a shell-and-tube membrane module.17 Before testing, the MFI membrane was pretreated at 300 °C under a nitrogen flow for 2 h, then cooled to 150 °C and maintained at this temperature for another 4 h. This treatment secured the desorption of gases that would affect the permeation measurements. The zeolite side of the tube was flushed with a N2 carrier stream that previously passed through the xylene saturator maintained at 35 °C, while the inside of the tube was flushed with N2 as a sweep gas. To prevent condensation of the organics and ensure correct xylene vapor pressure values, all the system lines were maintained at 150 °C, using heating tape. The feed, permeate, and retentate streams were analyzed with a Shimadzu Model GC-9A gas chromatograph that was equipped with a flame-ionization detector and a packed column containing Bentona 34% and SP-1200 5% (Supelco). The experimental error in the molar fraction determination was
