Sorption and diffusion of chlorinated hydrocarbons in silicalite-filled PDMS membranes (original) (raw)
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Journal of Membrane Science, 1997
The paper describes the investigation of the effect of three different types of hydrophobic zeolites (ultrastable zeolite type Y, pentasyl type zeolite (ZSM-5), and ALPO-5 type zeolite) on the pervaporation properties of zeolite-filled polydimethylsiloxane membranes. The procedure of obtaining membranes of various thicknesses varying in zeolite content was mastered. The physicochemical properties of the utilized zeolites were investigated and determined. The effect of zeolite type and concentration, as well as the membrane thickness and pervaporation temperature on the membrane permeability and selectivity were investigated. The physicochemical properties of the zeolite used, primarily the degree of hydrophobicity, as well as the sorption capacity for ethanol, the specific pore volume, specific area and mean crystallite size of the zeolite, significantly influence the membrane's pervaporation properties. An increase in the zeolite content results in an increase of both membrane permeability and selectivity, while an increase in the pervaporation temperature results in an increase of the permeability and a decrease of selectivity, as opposed to the effect of membrane thickness.
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.
Pervaporation behaviour of chlorinated hydrocarbons through organophilic membranes
Desalination, 1993
The pervaporation behaviour of chlorinated hydrocarbons (Cl-HC's) was studied in detail. The permeability of different Cl-HC's through a hydrophobic membrane was considered in relation to their molecular structure and some of their physicochemical properties. The hydrocarbons selected for this study differed in: the number and position of chlorine atoms, the chain length, the degree of branching and the presence of a double bond.
Sorption-Induced Diffusion-Selective Separation of Hydrocarbon Isomers Using Silicalite
The Journal of Physical Chemistry A, 1998
In this paper we demonstrate a new principle for separation of linear and branched (2-methyl)alkanes, in the five to seven carbon atom range, by means of permeation through a silicalite membrane. The permeation selectivity relies on subtle interplay between sorption and diffusion. The required sorption isotherms for the pure components and mixtures are generated using configurational-bias Monte Carlo (CBMC) simulations. The CBMC simulations of the mixture isotherm show a curious maximum in the loading of 2-methyl alkane; this loading decreases to almost zero with increased pressures. The high sorption selectivity for the linear alkane is due to entropic effects; the linear alkane has a higher "packing" efficiency than the branched alkane within the zeolite structure. Calculations for a 50-50 mixture of n-hexane (n-C 6 ) and 2-methylpentane (2MP), for example, show that the higher sorption selectivity for the linear alkane has the effect of enhancing the flux of n-C 6 through the silicalite membrane by up to a factor of 60 above that of 2MP. Experimental evidence to support our new separation principle is provided by permeation data of Funke et al. 2
Journal of Membrane Science, 2001
Faujasite type zeolite membranes were synthesized on porous ceramic alumina supports by using direct (in situ) and secondary (seeded) growth methods. In the secondary growth method a seed layer of ZSM-2 nanocrystals (prepared according to a report by Schoeman et al. J. Colloid Interface Sci. 1995, 170, 449-456) was deposited on the surface of the support before the hydrothermal growth. For both in situ and secondary growth, the mixture composition was 4.17 Na 2 O:1.0 Al 2 O 3 :10 TEA (triethanol ammonium):1.87 SiO 2 :460 H 2 O. X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron microprobe analysis (EPMA), indicate well intergrown 5-30 m thick FAU films with Si/Al ∼1-1.5. The separation of saturated/unsaturated hydrocarbon mixtures is demonstrated over a range of temperatures (40-160 • C). The mixtures examined (and the corresponding equimolar mixture separation factors) are benzene/cyclohexane (160), benzene/n-hexane (144), toluene/n-heptane (45), propylene/propane (6.2), and ethylene/methane (8.4). In all cases, the membranes are unsaturated hydrocarbon permselective. With equimolar feed mixtures (5 kPa/5 kPa benzene/cyclohexane) and in the temperature range 65-160 • C, the membranes exhibit separation factor of 20-160 with the benzene flux in the range 10 −4 -10 −3 mol m −2 s −1 . Decreasing the total feed partial pressure (0.31/0.31 kPa benzene/cyclohexane) reduces both separation factor (12) and benzene flux. Similar trend is observed when the benzene/cyclohexane ratio in the feed mixture (0.5/9.5 kPa benzene/cyclohexane) is reduced. A sorption diffusion model based on the Stefan-Maxwell formulation has also been employed to show that the benzene/cyclohexane separation can mainly be attributed to differences of their adsorption properties.
Role of Adsorption in the Permeation of CH4 and CO2 through a Silicalite-1 Membrane
Industrial & Engineering Chemistry Research, 2006
The role of adsorption in the single and binary permeation of CH 4 and CO 2 through a silicalite-1 membrane has been investigated. Adsorption on the zeolite is favorable for CO 2 , resulting in selectivity for CO 2 in the permeation. The generalized Maxwell-Stefan (GMS) equations, in combination with the ideal adsorbed solution theory (IAST), were used to model their binary permeation. It is found that the use of accurate adsorption data is of utmost importance for extracting transport properties from the single-component permeation as well as for modeling multicomponent permeation. The GMS model qualitatively and quantitatively predicts the temperature-dependent properties of the mixture permeation, while it slightly deviates from the experimental observation on the pressure dependence of the mixture selectivity. This deviation is ascribed to intercrystalline or surface barriers for the larger molecule CH 4 in the membrane as well as to the overprediction of the CO 2 loading in the zeolite by the IAST.
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.
Sorption and permeation of acetic acid through zeolite filled membrane
Journal of Membrane Science, 1995
Separation of acetic acid from water using silicalite filled PDMS membrane has been studied. The selectivity of the membrane for acetic acid has been found to increase with (i) increasing wt% of silicalite in the membrane and (ii) increasing hydrophobicity of the silicalite. The flux of acetic acid is however, found to behave in an opposite manner. An attempt has been made to propose a Dual Mode model for permeation in filled membranes and explain the results of this investigation as well as those available in the literature in the light of this model.
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.