Pervaporation Flux Research Articles

Extraction of organic components from wastewater by pervaporation

In the run of the last decades the coming up of new technologies as well as new needs has increased the interest in wastewater treatment. This study concerns the synthetic membrane development; synthesis of polymers for membrane preparation and the extraction of volatile organic components in aqueous media by pervaporation through polydimethylsiloxane, which is the most frequently used polymer material for the active layer of pervaporation for the extraction of organic compounds from wastewater. This elastomer has some special characteristics: high hydrophobicity, high permeability, good thermal and mechanical resistance and easy manufacture of the active layer on asymmetric substrate. The elastomer is a cross-linked material in which the degree of cross-linking and the structure of the network theoretically depends uniquely on the chemical structure. In this work we study the influence of some parameters of the preparation procedure on the membrane properties. Sorption of ethyl acetate, an aroma model, in pure state and in aqueous solution of different concentrating was performed and analysed. The curing was also carried out at different temperatures. The membrane physical properties (density, shore hardness and elasticity) were determined. Finally, in order to predict quantitatively the solvent contents in the membrane at the upstream face in pervaporation flux, we used floury, Huggins approach which appears to be valid in its simplified form.

Effects of Zeolite Membrane Structure on the Separation of 1,3-Propanediol from Glycerol and Glucose by Pervaporation

Seven types of zeolite membranes were synthesized on the inside surfaces of alumina and stainless steel supports. The medium-pore membranes (MFI, MEL, and a MFI + MEL composite) were prepared by using organic templates, whereas the larger pore membranes (MOR, FAU) were prepared by a template-free method. These membranes were used in pervaporation to separate 1,3-propanediol from glycerol and glucose in an aqueous solution. All membranes had 1,3-propanediol/glycerol selectivities higher than 27, and zeolite pore size did not have a large influence on membrane selectivity, which was mainly due to preferential adsorption of 1,3-propanediol. The 1,3-propanediol/glucose selectivities were higher than 1600 for all membranes because the selectivities were mainly controlled by molecular sieving. Zeolite structure had a significant effect on the pervaporation flux; the larger pore membranes had higher fluxes. At 308 K, the X-type membrane had the largest 1,3-propanediol flux of 62 g/(m2 h), with a 1,3-propanediol/glycerol selectivity of 59.

Prediction of sorption and flux of solvents through PDMS membrane

Prediction method of permeation flux and sorption characteristics in pervaporation through a polydimethylsiloxane (PDMS) membrane was suggested. The amount of sorption and permeation flux of chloroform, toluene, methanol, and n-butanol were calculated using this method and compared with experimental data. The calculated values of permeation flux and the amount of sorption of good solvents, i.e. toluene and chloroform, were in fairly good agreement with the experimental data. The lower the density of PDMS membrane, the more the permeation flux and sorption quantity. However, the experimental data of poor solvents, i.e. methanol and n-butanol, were quite different from the calculated values. It is shown that the prediction method suggested in this study might be useful for predicting the sorption and permeation flux through the membrane. The prediction method might especially be helpful to design the pervaporation membrane system, as it does not need adjustable parameter or experimental data.

Separation of 1,3-Propanediol from Aqueous Solutions Using Pervaporation through an X-type Zeolite Membrane

1,3-Propanediol was separated from aqueous solutions by pervaporation through an X-type zeolite membrane on a γ-Al2O3 support. Binary, ternary, and quaternary mixtures and a cell-free fermentation broth were used. At 308 K, the total fluxes for the quaternary solution and the fermentation broth were 2.1 and 1.2 kg/(m2 h), respectively. Pervaporation fluxes of all components increased with temperature, but the water flux increased at the fastest rate. From 308 to 328 K, the 1,3-propanediol/glycerol selectivity for the quaternary model solution increased from 59 to 67; it increased from 61 to 110 for the broth. The 1,3-propanediol/glucose selectivity increased from 1900 to 2200. The 1,3-propanediol/glycerol selectivity was mainly controlled by preferential adsorption, and the 1,3-propanediol/glucose selectivity was mainly controlled by differences in diffusion rates. The membrane was stable during the pervaporation of the quaternary mixture for at least one week. In contrast, after 60 h, the broth started t...

Mathematical Modeling of the Pervaporative Separation of Methanol−Methylterbutyl Ether Mixtures

This work reports the kinetic modeling of the pervaporative separation of methanol−MTBE mixtures. Using a commercial membrane, Pervap 2256, that presented high selectivity toward methanol permeation, the influence of the operating variables feed composition in the range 1−20 wt % MeOH, feed temperature between 30 and 50 °C, and permeate pressure between 1 and 20 mmHg on the pervaporation flux was experimentally analyzed in a laboratory setup working under pseudo-steady-state conditions. A mathematical model based on the generalized Fick's law and the assumption that transport through the membrane is the rate-limiting step was developed in order to describe the PV flux of both components. The prediction of the flux of methanol needed of a concentration-dependent diffusion coefficient, whereas a simple model with concentration-independent diffusivity was sufficient for the description of the MTBE flux. Finally, the influence of the temperature on the partial fluxes was described through an Arrhenius-type ex...

New Insights into Pervaporation Mass Transport under Increasing Downstream Pressure Conditions: Critical Role of Inert Gas Entrance

The influence of the total downstream pressure on the pervaporation flux of a pure compound through a dense polymer membrane has been the subject to controversial debates recently. Experimental arguments in favor of either the solution−diffusion model or the newly proposed pore-flow model are alternatively reported on different systems. To critically reexamine this debate, an experimental study under controlled downstream conditions has been performed for pure methanol and pure 2-propanol pervaporation through a poly(dimethylsiloxane) film, the latter having been previously reported to follow pore-flow model predictions. It is shown that a rational analysis of the effects of the downstream pressure on the results can be achieved according to the classical solution−diffusion model, provided that the influence of air leaks in the installation is properly taken into account. Based on this observation, the implications of an inert gas contribution, generally speaking, on pervaporation operation are discussed.

Permeation behaviour in cellulose triacetate dense membrane during pervaporation separation of methanol/methyltert-butyl ether mixture

The pervaporation separation of methanol/methyl tert-butyl ether through cellulose triacetate dense membranes has been carried out under different feed compositions, permeation temperatures and temperature cycles to investigate the permeation behaviour of the membrane during pervaporation process. The experimental data indicate that the plasticization effect has a decisive influence on pervaporation flux, permselectivity and permeation activation energy. The results show that the penetrants of different feeds are transported through the membrane by different pathways with the influence of plasticization effect at different temperatures. It has also been observed that plasticization had great effect on the membrane swelling and sorption selectivity. Furthermore, DSC results show that the membranes retain the influence of the plasticization effect after pervaporation separations are tested. © 2000 Society of Chemical Industry

Polycrystalline MFI zeolite membranes: xylene pervaporation and its implication on membrane microstructure

This paper reports experimental results on pervaporation of pure xylene isomers and their binary mixtures through α-alumina-supported zeolite MFI membranes. The membranes were permeable for all xylene isomers. Intensive fouling of the zeolite membrane was observed during the pervaporation experiments due to strong interaction of xylene molecules with the zeolite pores via π-complexation. The pervaporation flux of pure xylene isomers at 26°C in first 10 h experiment decreases in the order: m-xylene> p-xylene> o-xylene. No separation of binary p-/ m-xylene and p-/ o-xylene mixtures could be obtained in the investigated temperature range (26–75°C). A three-type pore microstructure for the zeolite membrane is proposed to explain the pervaporation and helium permeation data. The zeolite membrane layer contains both zeolitic pores, microporous nonzeolitic pores nonselective for xylene isomers, and a very small amount of larger defect pores. The microporous nonzeolitic pores are formed possibly in the step of template removal. Presence of the microporous nonzeolitic pores and fouling by chemisorbed xylenes are responsible for the nonseparative properties of xylene pervaporation through the polycrystalline MFI membrane observed in this work.

A Pseudophase-Change Solution-Diffusion Model for Pervaporation. II. Binary Mixture Permeation

ABSTRACT On the basis of solution-diffusion mechanism and incorporation of a pseudophase-change phenomenon, the pseudophase-change solution-diffusion (PPCSD) model is developed for pervaporation of a binary mixture through a membrane system. Flux equations of individual components thus derived clearly describe the role of coupling flux in pervaporation and show that it depends mainly on the solubility and mobility of the other component in the membrane. Good agreement is found between the theoretical and experimental results (from literature values for the hexane-heptane-polyethylene system). In addition to the solubility and mobility of individual component, the pervaporation performance of a membrane is influenced by the effect of plasticization and coupling flux, especially when strong interaction, occur between the permeants and the membrane. A parameter study indicates that both plasticizing and coupling effects enhance the permeation rate and usually lead to a decrease in membrane permselectivity.

A Pseudophase-Change Solution-Diffusion Model for Pervaporation. I. Single Component Permeation

ABSTRACT A pseudophase-change solution-diffusion (PPCSD) model, which assumes the pervaporation process is a combination of liquid permeation mechanism and vapor permeation mechanism in series, is proposed. A pseudophase change of permeant is located at the interface between these two mechanisms. Using the assumptions of thermodynamic equilibrium, simple concentration-dependent solubility, and diffusiyity, we derive a set of analytical equations to express the pervaporation flux, the lengths of the liquid and vapor permeation mechanisms. The PPCSD model was tested for its validity and showed good agreement with literature data in terms of the effect of feed pressure and permeate pressure. The length of the permeation zone that behaves as a liquid permeation mechanism as well as the pressure and concentration profiles within the membrane also can be calculated. Comparison among three different membrane separation processes (namely, pervaporation, vapor permeation, and liquid permeation) showed that the permeation flux is nearly the same for pervaporation and vapor permeation processes under the assumption of thermodynamic equilibrium. On the other hand, the permeation flux for the liquid permeation process is always lower unless a certain high feed pressure is reached

Tubular-type pervaporation module with zeolite NaA membrane

Zeolite NaA membranes were prepared on the surfaces of porous tubular supports composed of mullite, α-alumina and/or cristobalite using the hydrothermal synthesis. The zeolite NaA crystals and the surface morphologies of the membranes were characterized by X-ray diffraction and SEM. EPMA analysis revealed that the zeolite composite membranes consisted of three layers — the zeolite layer, the intermediate layer and the mullite support. According to pervaporation measurements using H 2O EtOH mixture, the permselectivity increased in the alumina content of the substrate and reached a constant at >70 wt% Al 2O 3. Zeolite NaA membrane showed very high water-selective permeation through all the ethanol concentrations. The tubular-type module with ca. 0.5 m 2 membrane area was composed of 17 pieces of zeolite membrane using 65 wt% alumina content substrate, which has high pervaporation performance and can be purchased at a low price. The efficiency of the module on permeation rate increased in proportion to the feed flow rate and exhibited 100% at Reynolds number >80. Pervaporation flux and separation factor of the module for 5 wt% water/95 wt% ethanol mixture at 95°C was ca. 2.35 kg m −2 h −1 and was greater than 5 000, respectively.

Improved measurement device for vapor permeation and pervaporation

A precision measurement method of vapor permeability through polymer membranes was developed and tested for cellulose acetate membranes. An apparatus with a differential transformer and a float on the liquid source could precisely measure the vapor permeation rate as well as pervaporation flux by sensing the level of the source liquid. The permeation rates with vapor feed and liquid feed of water, alcohols, and organic solvents were measured for silicone rubber and cellulose acetate, cellulose triacetate, and water-soluble cellulose acetate membranes. The measured vapor permeability directly predicted the pervaporation flux through silicone rubber. In the case of swollen material due to the feed liquid, the vapor permeability was not a controlling property of the pervaporation flux, but became an analysis base for it. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 433–438, 1997

Estimation of activation energy for permeation in pervaporation processes

The effect of operating temperature on permeation flux in pervaporation was analyzed. In the standard pervaporation processes, both the permeability coefficient of a membrane and the driving force for mass transport are influenced by temperature. The so-called “activation energy” of permeation, E J, conventionally obtained from ln J vs. 1 T plot is a compounded parameter characterizing the overall temperature dependence of permeation flux. The activation energy, E P, characterizing temperature dependence of membrane permeability should be evaluated from the log of driving force-normalized flux vs. 1 T data. When the transmembrane driving force is expressed in terms of partial pressure, E P can be evaluated from ln ( J Δp ) vs. 1 T plot. A rule of thumb for estimating E P is to subtract the molar heat of vaporazation from E J.

Transport resistances during pervaporation through a composite membrane: experiments and model calculations

For an investigation of transport resistances of a GFT PVA-PAN Pervap 1000 type membrane the pervaporation flux of pure water was measured as a function of downstream pressure, with the membrane oriented in normal and in reverse position ( =PVA layer facing the feed or the permeate). Assuming equilibrium across all interfaces, it was possible to break up the total resistance into (a) a support layer resistance part, and (b) an active layer resistance part. (a) Supprot layer resistance part=resistance of the support excluding its upstream skin: A model of liquid water transported by capillary forces through small pores, and of vapor by Poiseuille and Knudsen flow and eventually by surface diffusion along pore walls is presented for describing the 3 transport regimes observed experimentally. The support layer was found to consume up to 80% of the total resistance. (b) Active layer resistance part=resistances of the PVA layer and the slightly porous PAN skin: 3 models describing the influence of the PAN skin are discussed and judged in detail. Then the skin resistance is estimated and used to derive the permeability of water in PVA as a function of water activity or ‘equivalent partial pressure’ EPP in the polymer which is then compared to vapor permeation, vapor sorption and diffusivity data published by Heintz and Lichtenthaler. Strong discrepancies are found and discussed in terms of possible interface resistances, of free and bound water and of cluster formation which would mean that in cases like this diffusivities can be derived from stationary state pervaporation but not from non-stationary state kinetic sorption experiments.

Ethanol production in a continuous fermentation/membrane pervaporation system

The productivity of ethanol fermentation processes, predominantly based on batch operation in the U.S. fuel ethanol industry, could be improved by adoption of continuous processing technology. In this study, a conventional yeast fermentation was coupled to a flat-plate membrane pervaporation unit to recover continuously an enriched ethanol stream from the fermentation broth. The process employed a concentrated dextrose feed stream controlled by the flow rate of permeate from the pervaporation unit via liquid-level control in the fermentor. The pervaporation module contained 0.1 m2 commercially available polydimethylsiloxane membrane and consistently produced a permeate of 20%–23% (w/w) ethanol while maintaining a level of 4%–6% ethanol in a stirred-tank fermentor. The system exhibited excellent operational stability. During continuous operation with cell densities of 15–23 g/l, ethanol productivities of 4.9–7.8 gl−1 h−1 were achieved utilizing feed streams of 269–619 g/l glucose. Pervaporation flux and ethanol selectivities were 0.31–0.79 lm−2 h−1 and 1.8–6.5 respectively.

Prediction of flux and selectivity in pervaporation through a membrane

Flux and selectivity in pervaporation through a membrane can be predicted via the solution-diffusion mechanism by the use of solubility and diffusivity models. The solubility model developed takes into account the free energy contributions in the mixture system of multicomponent penetrants and a polymer from combinatorial-entropy, free-volume, interactional-enthalpy, and elastic factors. The diffusivity model developed is the hybrid model that combines the key features of the free-volume and molecular models. In the hybrid model, penetrant molecular thickness and polymer molecular parameters are used to determine the term equivalent to the preexponential factor of the free-volume model, and the free-volume expression is used to relate penetrant diffusivity to penetrant size and the free volume of the mixture system including concentration contributions from the multicomponent penetrants. Both the solubility and diffusivity models have been verified with the sorption of single-component aromatic vapors in a semicrystalline polyethylene film. Based on the single-component solubility and diffusivity model parameters determined from the sorption, the prediction of flux and selectivity is in line with experimental data obtained from the perva-poration of a mixture of toluene, p-xylene, and mesitylene through the polyethylene film at various temperatures and different permeate pressures. The effects of temperature on flux and selectivity are investigated via both the model prediction and experiments. Also predicted are the effects of permeate vapor pressure on flux and selectivity, and the concentration and diffusivity profiles across the membrane.

Separation of MTBE-Methanol Mixtures by Pervaporation

Abstract Membranes made of a polymer blend of poly(acrylic acid) and poly(vinyl alcohol) were evaluated for the separation of methanol from methyl tert-butyl ether (MTBE) by pervaporation. The influence of the blend composition and the feed composition on the pervaporation performance were investigated. Methanol permeates preferentially through all tested blend membranes, and the selectivity increases with increasing poly(vinyl alcohol) content in the blends. However, a flux decrease is observed with increasing poly(vinyl alcohol) content. With increasing feed temperature the flux increases, and the selectivity remains constant. In addition, the influence of crosslinking on the permselectivity was investigated. The pervaporation flux decreases with increasing crosslinking density, but the selectivity is enhanced. This is due to a more rapid decrease in the component flux of MTBE compared to that of methanol.

Effect of Upstream Pressure on the Flux of Single Permeant in Pervaporation

The effect of the upstream pressure on the pervaporation flux of water was studied both theoretically and experimentally. The increase in the pervaporation flux with an increase in the upstream pressure was predicted using newly developed transport equations that allow the pressure change across the membrane and include both liquid and vapor phase transport. The conventional solution-diffusion model, on the other hand, fails to predict the effect of the upstream pressure, particularly when an assumption is made that the pressure of the permeant inside the membrane is constant across the membrane and is equal to the upstream pressure. The pervaporation flux of water through a polydimethysiloxane membrane and an aromatic polyamide membrane was found, experimentally, to increase with an increase in the upstream pressure.

Composite membranes of poly(1‐trimethylsilyl‐1‐propyne) and poly(dimethyl siloxane) and their pervaporation properties for ethanol–water mixture

AbstractA poly(1‐trimethylsilyl‐1‐propyne) (PTMSP) membrane was systematically modified to prevent flux decline over time by incorporating poly(dimethyl siloxane) (PDMS) in three different ways: (1) semi‐interpenetrating polymer network (I series), (2) PDMS sorption (S series), and (3) PDMS sorption and crosslinking (X series). The PTMSP and PDMS phases were partially mixed in the I series, which was confirmed by the measurement of density and glass transition temperature. The flux and separation factor in pervaporation of an ethanol–water mixture decrease with time for the I series, analogous to the behavior of pure PTMSP. However, the flux and separation factor remained steady with time in the case of the S and X series. The sorption method appears to be a good means for maintaining a time‐unvarying flux and separation factor at a minimum expense. © 1994 John Wiley & Sons, Inc.

High‐performances membranes for pervaporation I. Poly(vinyl alcohol)–poly(N‐vinyl pyrrolidone) blends

AbstractDense membranes were prepared from poly(vinyl alcohol)–poly(N‐vinyl pyrrolidone) (PVA–PVP) blends of different compositions and studied in swelling and dehydration by pervaporation of three organic solvents contaminated by 5 wt% water. The swelling generally increases with the PVP content. No extraction occurs in water–tetrahydrofuran (THF) and water–methyl ethyl ketone (MEK) mixtures. In ethanol containing 10 wt% of water, there is no extraction for blends containing less than 40 wt% PVP and an increasing extraction beyond this PVP content. The pervaporation flux of the water–ethanol mixture increases drastically at the same threshold whereas the water permselectivity falls to a low level. The values of the diffusion and permeability coefficients determined from transient permeation of the test water–ethanol mixture exhibit a similar sudden increase at the same PVP content threshold. This singular behavior of the blend membranes is interpreted by a strong affinity of the PVP component to ethanol, combined with a disappearance of crystallites in the blend at this threshold. Consequently the amorphous membrane can swell freely according to the affinity of the PVP component, leading to the observed behavior.