On a rapid method to characterize intercrystalline defects in zeolite membranes using pervaporation data (original) (raw)
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Modeling Pervaporation of Ethanol/Water Mixtures within ‘Real' Zeolite NaA Membranes
Industrial & Engineering Chemistry Research, 2008
A modified version of the adsorption-diffusion model derived form the Maxwell-Stefan theory developed in a previous study (Pera-Titus, et al. Catal. Today 2006, 118, 73) is presented in this paper to describe the dehydration behavior of zeolite NaA membranes for pervaporation of ethanol/water mixtures. Compared to the former version, two additional contributions are included in the model: (1) the adsorbed solution theory of Myers and Prausnitz is used instead of the extended Langmuir isotherm to account for binary adsorption equilibria of water and ethanol on zeolite A, and (2) the explicit role of pressure-driven mechanisms in large intercrystalline defects (macrodefects) to permeation is considered. These refinements in the Maxwell-Stefan equations provide a superior description of solvent dehydration using zeolite NaA membranes. The fitted surface diffusivities at 323 K and at zero loading of water and ethanol for weak confinement show values in the order of 10-12 and 10-13 m 2 ‚s-1 , respectively. The former values are 3-4 orders of magnitude higher than those that have been measured from water adsorption kinetics experiments. This difference might be ascribed to a certain role of nanosized grain boundaries between adjacent zeolite A crystals. Grain boundaries might behave as fast diffusion paths or nanoscopic shortcuts due to anisotropy of zeolite layers, resulting in higher apparent water surface diffusivities and lower apparent activation energies for surface diffusion.
Ind. Eng. Chem. …, 2002
Silicalite, ZSM5, and mordenite membranes were prepared on alumina and stainless steel porous tubes by several procedures. The N 2 permeation flux was measured before and after exposure to water, n-octane, or n-butane under different conditions (capillary condensation and adsorption). The reduction of permeation flux that takes place in the presence of adsorbable compounds can be related to the quality and separation properties of a given membrane. Also, a good correlation was observed between the separation selectivity and the time required to reach steady state after exposure of the membrane to quasi-saturation conditions. Both measurements can be used as fast and reliable techniques for membrane characterization.
Fundamentals and applications of pervaporation through zeolite membranes
Journal of Membrane Science, 2004
Zeolite membranes have uniform, molecular-sized pores, and they separate molecules based on differences in the molecules' adsorption and diffusion properties. Zeolite membranes are thus well suited for separating liquid-phase mixtures by pervaporation, and the first commercial application of zeolite membranes has been for dehydrating organic compounds. Because of the large number of zeolites that can be prepared, zeolite membranes have also been used to remove organic compounds from water, separate organic mixtures, and remove water from acid solutions on the laboratory scale. The fundamental aspects of separations by pervaporation through zeolite membranes are reviewed, and examples of the selectivities and fluxes obtained are presented. Some aspects of these separations are similar to gas-phase separations using zeolite membranes, but feed-side coverages are close to saturation during pervaporation, making competitive adsorption and molecule-molecule interactions more important during multicomponent diffusion. Some of the topics that are discussed include: (1) the use of feed fugacities to predict separation selectivities; (2) the effects of coverage, competitive adsorption, heats of adsorption, molecular sizes, temperature, membrane structure, non-zeolite pores, concentration polarization, and support resistance on transport and separations; (3) the ability of one molecule to slow down or speed up another molecule in the zeolite pores, and (4) the techniques used to measure adsorption and diffusion properties. Several possibilities for improving understanding and effectiveness of pervaporation through zeolite membranes are also suggested.
Membrane separation process—Pervaporation through zeolite membrane
Separation and Purification Technology, 2008
Membrane separation process has become one of the emerging technologies that undergo a rapid growth for the past few decades. Pervaporation is one of the membrane separation processes that have gained increasing interest in the chemical and allied industries. It is an effective and energy-efficient technology that carries out separations, which are difficult to achieve by conventional separation processes. Inorganic membrane such as zeolite membranes with uniform, molecular-sized pores offer unique type of pervaporation membrane for a number of separation processes. This review presents the role of zeolite membrane and its progress in the pervaporation process. The fundamental aspects of pervaporation over different types of membranes are reviewed and compared. The focus of this review is on zeolite membrane covering: (a) synthesis of zeolite membranes; (b) membrane characterization; (c) pervaporation studies; (d) its applications in alcohol dehydration, organic/organic separations and acid separations. The transport mechanism during pervaporation is discussed and the issues related with pervaporation are addressed. Innovation and future development of zeolite membrane in pervaporation are also presented.
Adsorption and diffusion properties of zeolite membranes by transient permeation
Desalination, 2002
Adsorption isotherms and diffusion coefficients for light gases and butane isomers were measured for the transport pathways involved in gas permeation through H-ZSM-5 membranes by a transient permeation technique. The permeate responses to step changes in the feed were measured, and the transport was modeled as Maxwell-Stefan diffusion with single-site Langmuir adsorption in the zeolite. Isotherms measured for N 2 , CO 2 , and CH 4 at 295 K were nearly identical to those measured by calorimetry on H-ZSM-5 powders. Isotherms for butane isomers were also similar to isotherms for MFI powders and heats of adsorption and diffusion activation energies were in the ranges reported in the literature. Maxwell-Stefan diffusion coefficients for all gases studied increased slightly with feed partial pressure and were similar to those measured by other macroscopic methods for zeolite membranes and crystals. Effective membrane thicknesses were also determined non-destructively for tubular zeolite membranes by the transient permeation technique.
Synthesis, Evaluation, Modeling and Simulation of Nano-pore NaA Zeolite Membranes
Zeolite membranes have uniform and molecular-sized pores that separate molecules based on the differences in the molecules' adsorption and diffusion properties. Strong electrostatic interaction between ionic sites and water molecules (due to its highly polar nature) makes the zeolite NaA membrane very hydrophilic. Zeolite NaA membranes are thus well suited for the separation of liquid-phase mixtures by pervaporation. In this study, experiments were conducted with various Ethanol–water mixtures (1–20 wt. %) at 25 °C. Total flux for Ethanol–water mixtures was found to vary from 0.331 to 0.229 kg/m 2 .h with increasing Ethanol concentration from 1 to 20 wt.%. Ionic sites of the NaA zeolite matrix play a very important role in water transport through the membrane. These sites act both as water sorption and transport sites. Surface diffusion of water occurs in an activated fashion through these sites. The precise Nano-porous structure of the zeolite cage helps in a partial molecular sieving of the large solvent molecules leading to high separation factors. A comparison between experimental flux and calculated flux using Stephan Maxwell (S.M.) correlation was made and a linear trend was found to exist for water flux through the membrane with Ethanol concentration. A comprehensive model also was proposed for the Ethanol/water pervaporation (PV) by Finite Element Method (FEM). The 2D model was masterfully capable of predicting water concentration distribution within both the membrane and the feed side of the pervaporation membrane module.
Microstructural Optimization of a Zeolite Membrane for Organic Vapor Separation
Science, 2003
A seeded growth method for the fabrication of high-permeance, high-separation-factor zeolite (siliceous [Si 96 O 192 ]-MFI) membranes is reported. The method consists of growing the crystals of an oriented seed layer to a well-intergrown film by avoiding events that lead to a loss of preferred orientation, such as twin overgrowths and random nucleation. Organic polycations are used as zeolite crystal shape modifiers to enhance relative growth rates along the desirable out-of-plane direction. The polycrystalline films are thin (ϳ1 micrometer) with single grains extending along the film thickness and with large in-plane grain size (ϳ1 micrometer). The preferred orientation is such that straight channels with an open diameter of ϳ5.5 angstroms run down the membrane thickness. Comparison with previously reported membranes shows that these microstructurally optimized films have superior performance for the separation of organic mixtures with components that have small differences in size and shape, such as xylene isomers.
2015
The production of liquid transportation fuels such as bioethanol and more recently also biobutanol from renewable resources has received considerable attention. In the production of bio-based alcohols, the separation steps are expensive as the mixtures to be separated are dilute. As an energy-efficient separation technology, pervaporation is considered to be a potential process in biofuel purification. One of the main constraints in the commercialization of pervaporation has been low membrane fluxes, and the consequent high costs due to the high membrane area needed. In order to obtain high fluxes, the membranes should be as thin as possible. In this thesis, the performance of ultrathin zeolite membranes in pervaporation was investigated. Binary ethanol/water and n-butanol/ water mixtures were studied using both hydrophobic and hydrophilic zeolite membranes for alcohol concentration, as well as dehydration. The development of pervaporation membranes and processes has been mainly emp...
A novel approach to fabricate zeolite membranes for pervaporation processes
2015
The conventional methods used in preparing zeolite membranes, such as the secondary growth and in situ crystallization methods, involve long and complex procedures that require the preparation of the zeolite aluminosilicate gel prior to the fabrication process and often result in membranes which contain pin holes. Here we report a simple, cheap, and less time-consuming technique to fabricate zeolite A and mordenite membranes on a porous stainless steel support. In addition, the technique makes it possible to fabricate types of zeolite membranes that have been previously difficult to synthesise. A clinoptilolite membrane was fabricated to demonstrate the ability to manufacture a zeolite membrane from an existing crystalline zeolite (natural or synthetic). All three membranes were subjected to separation tests, (ethanol dehydration, ethanol-cyclohexane and phenol removal from water) to demonstrate the efficacy of membrane synthesis. The fluxes obtained and separation factors which were achieved are comparable with literature values but as with most zeolite membranes there is a trade-off between high flux and separation. Experimental Membrane preparation The overriding aim of the work is to produce a simple and repeatable method of producing zeolite membranes. The method which has been developed can be summarised as
Journal of Membrane Science, 2009
Pervaporation through zeolite membranes involves local heat effects and combined heat and mass transport. The current state-of-the-art Maxwell-Stefan (M-S) models do not take these effects into account. In this study, transport equations for the coupled heat and mass transport through a zeolite membrane are derived from the framework of non-equilibrium thermodynamics (NET). Moreover, the assumption of equilibrium between the adjacent bulk phases at the feed and permeate sides of the zeolite layer is abandoned in favor of local equilibrium. The equations have been used to model pervaporation of water through a 2 m thick NaA type zeolite membrane, deposited on an asymmetric ␣-alumina support, at a feed temperature of 348 K. Assuming a flux of 10 kg m −2 h −1 (0.15 mol m −2 s −1 ), the transport through the zeolite layer, as well as the liquid feed side boundary layer and the support layers is modeled. The activity, fugacity, and temperature profiles are calculated with and without taking coupling effects and surfaces into account. The profiles show distinct differences between the two cases. Including the surface effects leads to discontinuities in the activity and temperature at the membrane interfaces. A significantly higher temperature drop of 1.3 K is calculated across the zeolite, compared to 0.4 K when surface and coupling effects are not accounted for. The calculated decrease in temperature over the zeolite layer is dominated by the surfaces. This could indicate that temperature polarization is, to a large extent, a surface effect. The heat flux induces an extra driving force for mass transport, reducing the activity difference over the membrane. A positive jump in activity is observed at the interfaces, revealing the mass transport across the interfaces is governed by the coupling with the heat flux. The support layers contribute significantly to the total mass transport resistance. (J. Kuhn). and vapor permeation [7-9] and pervaporation processes in zeolite [10] and silica [11] membranes.