Pervaporation Flux Research Articles

Organic−Inorganic Hybrid Membranes: Solving the Trade-off Phenomenon Between Permeation Flux and Selectivity in Pervaporation

To solve the trade-off phenomenon existing between permeation flux and selectivity of membranes in pervaporation, organic−inorganic hybrid membranes composed of chitosan and TiO2 were prepared usin...

Pervaporation dehydration of alcohol–water mixtures: Optimization for permeate flux and selectivity by central composite rotatable design

A central composite rotatable design (CCRD) of response surface methodology was used to analyze pervaporation performance of homogeneous poly(vinyl alcohol) (PVA) membranes. A regression model was developed for the pervaporation flux and selectivity as a function of the operating conditions: temperature, concentration and flow-rate. Dehydration experiments were performed on two different alcohol–water systems: isopropanol–water (IPA–water) and ethanol–water (Et–water) mixtures around their azeotropic concentrations. Based on preliminary experiments and CCRD design, the ranges of values of the operating conditions were selected: temperature 33–67 °C, feed flow-rate 46–114 L/h, and concentration 83–92 wt% for IPA and 93–98 wt% for Et in feed mixtures. A total of 20 pervaporation experiments were conducted for each alcohol–water system. Judged by the lack-of-fit criterion, the analysis of variance (ANOVA) showed the regression model to be adequate. From the regression analysis, the flux and selectivity were expressed with quadratic equations of temperature, feed concentration and flow-rate. The predicted flux and selectivity from the regression model were presented in 3D surface plots. For both alcohol–water systems, quadratic terms of temperature and feed alcohol concentration showed significant ( p < 0.0001) influence on the flux and selectivity. A strong interaction effect of temperature and concentration was observed on the selectivity for the Et–water system. However, the interaction of flow-rate with temperature or concentration was found to be less significant. In order to optimize the pervaporation flux and selectivity of azeotropic alcohol–water mixtures, the desirability function approach was applied to analyze the regression model equations by commercial software. For the azeotropic IPA–water mixture (87.5 wt% IPA), the optimized dehydration variables were found to be 50.5 °C and 93.7 L/h for temperature and flow-rate, respectively. For the azeotropic Et–water mixture (95.5 wt% Et), the optimized temperature and flow-rate were found to be 57 °C and 89.2 L/h, respectively. Compared with experiments performed at optimized temperature and flow-rate, the predicted flux and selectivity of the azeotropic mixtures showed errors to be within 3–6%.

Separating a water-propanol mixture using PDMS pervaporation membranes

Recovering and purifying organic solvents during chemical and pharmaceutical synthesis has great economic and environmental importance. Water-alcohol mixture pervaporation was investigated using a pervaporation cell and hydrophobic membranes. This work studied polydimethylsiloxane (PDMS) membrane performance and hydrophobic membranes for removing propanol from aqueous mixtures. PDMS is recognised as being alcohol permselective during pervaporation. It was also observed that water was transferred through a hydrophobic membrane as water’s molecular size is smaller than that of propanol. A laboratory-scale pervaporation unit was used for studying this membrane’s separation characteristic in terms of pervaporation flux and selectivity for feeds containing up to water mass and 30°C-50°C. Total propanol/water flux was observed to vary as operating temperature increased. Although PDMS membranes presented good characteristics for separating water/propanol mixtures, the separation factor and pervaporation flow decreased as water content in the feed increased. The tested membrane was found to be very efficient for water concentrations of less than 0.3, corresponding to total flux transfer maximum.

Characterizing Nonzeolitic Pores in MFI Membranes

Methods that use n-hexane (n-hexane permporosimetry and n-hexane/2,2-dimethylybutane (DMB) separation) are shown to not be effective for characterizing MFI zeolite membranes because n-hexane adsorption swells MFI crystals and shrinks the size of nonzeolitic pores. Measurements on a membrane in which 30% of its helium flux at 300 K was through nonzeolitic pores demonstrate that benzene permporosimetry and isooctane vapor permeation as a function of feed activity provide better characterizations. Isooctane condensed in nonzeolitic pores at high activities, and this was used to estimate the sizes of those pores. The average nonzeolitic pore size in this membrane decreased from approximately 3.0 to 1.5 nm as the temperature increased from 300 to 348 K, apparently due to thermal expansion of MFI crystals. Benzene permporosimetry yielded dramatically different results from n-hexane permporosimetry because benzene does not swell the MFI crystals significantly. Single-component pervaporation fluxes as a function ...

Separation of benzene/cyclohexane mixtures by pervaporation using PEBA membranes

Separation of the benzene/cyclohexane mixture is industrially significant. The aim of the study is to disrupt the azeotropic behaviour of the benzene/cyclohexane mixtures by pervaporation using Pebax membranes. To overcome the difficulty of the separation benzene/cyclohexane mixtures by conventional techniques, pervaporation should be used. For this purpose, sorption and pervaporation of benzene/cyclohexane mixtures was studied by using different grades of Pebax (poly(ether-block-amide)) polymer which has soft segments as ether and a hard segment as an amide. It was examined that increasing hardness of the polymeric membrane resulted decreasing pervaporation flux and increasing pervaporation selectivity. Swelling degrees of membranes are decreased from softer to harder grades. Benzene/cyclohexane mixture composition and temperature effects on sorption and pervaporation characteristics are determined at 30, 40, 50°C. Increasing concentration of benzene resulted increasing flux and decreasing selectivity. Fluxes were increased and selectivity decreased with increasing temperature.

Zeolite K-LTL-loaded sodium alginate mixed matrix membranes for pervaporation dehydration of aqueous–organic mixtures

In this work, zeolite K-LTL-loaded sodium alginate (NaAlg)-mixed matrix membranes were prepared by solution casting and cross-linked with glutaraldehyde. The pervaporation (PV) dehydration of isopropanol, 1,4-dioxane and tetrahydrofuran (THF) was tested at 30, 40, 50, 60 and 70 °C as a function of membrane thickness and feed compositions. Activation parameters for permeation were evaluated from the temperature-dependent pervaporation flux data. The results showed a simultaneous enhancement in flux and selectivity at azeotropic compositions of the mixtures due to the addition of K-LTL particles of different silica–alumina ratios in NaAlg matrix. Flux and selectivity values to water were higher for water–1,4-dioxane azeotrope than those of water–isopropanol and water–THF mixtures. Pervaporation results were discussed in terms of sorption–diffusion principles. Molecular sieving effect created due to uniform distribution of K-LTL zeolite particles and its hydrophilic nature in addition to its interaction with hydrophilic NaAlg is responsible to appreciably increase the membrane performance than pristine cross-linked NaAlg membrane. Thermodynamic treatment of sorption process was investigated typically for water + 1,4-dioxane mixtures based on Flory–Huggins theory to explain the PV performance. Based on these results, permeance and driving force mechanisms were elucidated. The present membranes could withstand the repetitive cyclic PV runs on the laboratory level module.

Flexible nanostructure of MFI zeolite membranes

Pervaporation and vapor permeation of hydrocarbons were used to characterize three MFI zeolite membranes that had significant fluxes through non-zeolitic pores. The single-component pervaporation fluxes of n-hexane were only 5–55% of the fluxes of larger molecules (2,2-dimethylbutane (DMB), isooctane, and 1,3,5-trimethylbenzene (TMB)) at ∼300 K, all of which did not adsorb in the MFI pores under the conditions used. In spite of this behavior, these membranes were selective for C 6 isomer separation, and the n-hexane/DMB vapor-phase separation factor was 3200 for one membrane at low hydrocarbon partial pressures. The pervaporation behavior was consistent with the explanation that MFI membrane structures are flexible; MFI crystals increase in size when n-hexane adsorbs, and this expansion dramatically decreases the non-zeolitic pore sizes. Thus, less than 0.8 mol% n-hexane reduced DMB and TMB pervaporation fluxes to ∼5% of their single-component fluxes for one membrane. At high vapor concentrations, molecules condensed in the non-zeolitic pores, which had almost no selectivity, and this capillary condensation decreased the n-hexane/DMB separation factor. Non-zeolitic pores were shown to increase in size after a membrane was calcined at 1000 K.

Prediction of the pervaporation fluxes when the downstream pressure is changed for a blended PVA-PAA membrane with the total solvent volume fraction model (TSVF)

Un modele simplifie de solution-diffusion, dit modele du volume fractionnaire total de solvant (modele TSVF), a ete recemment developpe en pervaporation pour decrire le phenomene de transfert de permeat a travers des membranes fortement selectives. Dans ce modele, le coefficient de diffusion de chaque espece dans la membrane depend de toute la fraction de volume occupee par le solvant dans le polymere et non du gonflement partiel de chaque solvant. Dans cet article, nous appliquons ce modele a la prediction des flux de pervaporation d'umelange « eau-ethanol » a 60°C et a pression aval variable a travers une membrane constituee d'un melange de PVA(90)-PAA(10).

Pervaporation properties of fluoroalkylsilane (FAS) grafted ceramic membranes

Changing the hydrophilic character of ceramic membranes into hydrophobic ones is nowadays of particular interest. The originally hydrophilic character of ceramic membranes can be changed by grafting hydrophobic molecules on the surface. Fluoroalkylsilanes (FAS) are a group of compounds which can be efficiently used to create the hydrophobic character of different surfaces. The grafting process can be performed by a reaction between -OH surface groups of the ceramics and ethoxy groups (-O-Et) presented in organosilane compounds. The aim of this work was to prepare FAS-modified pervaporation ceramic membranes by grafting alumina and/or titania ceramic nanofiltration membranes with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (C8) molecules and to determine the properties of such modified hydrophobic ceramic membranes in the pervaporation process used for the desalination of different salt solutions and for the removal of organic solvents of different hydrophobicity [i.e., ethanol (EtOH) and methyl t-butyl ether (MTBE)] from water. It was found that transport and selective properties of grafted membranes depended on the type of ceramics and the grafting conditions. In general, FAS grafted titania membranes were more permeable and more selective than alumina ones. In the case of desalination it was found that salt rejection was practically 100%; there was no salt found in the permeate. Pervaporation flux in contact with an aqueous salt solution was dependent on the kind of salt, its concentration and feed temperature. FAS grafted membranes prepared from titania showed much higher permeation fluxes compared with alumina ones. Both membranes showed high selectivity towards the organic component of water–organic mixtures. The selectivity factor of FAS-TiO 2 (grafted in 0.1 M C8), FAS-TiO 2 (grafted in 0.01 M C8) and FAS-Al 2O 3 (grafted in 0.01 M C8) membranes in contact with water–MTBE (2 wt.% MTBE) solution was 95, 45 and 23, respectively. The C8 molecules grafted on the titania ceramics underwent conformation change from a tangled form into the straightened one when the C8 environment changed from pure water to a water–organic or water–electrolyte solution. The concentration range in which this conformation change occurred was dependent on the hydrophobicity of organic solvent molecules presented in the solution. In the case of more hydrophobic MTBE molecules the tangled–straightened form change of FAS molecules occurred in a concentration range 0–0.1 mol. % MTBE.

Synthesis and characterization of porous polyurethaneurea membranes for pervaporative separation of 4-nitrophenol from aqueous solution

Hydroxyterminated polybutadiene (HTPB) based porous polyurethaneurea (PUU) membranes were prepared. The porosity was developed by incorporation of lithium chloride into polymer matrix with subsequent leaching of the same in hot water. Scanning electron microscopic analysis of the prepared membrane surfaces was performed. The pervaporation performance of the synthesized membrane was studied with aqueous 4-nitrophenol solution as feed. The effects of various parameters on 4-nitrophenol separation factor and total as well as 4-nitrophenol flux were studied. Polyurethaneurea membrane was found to permeate 4-nitrophenol selectively with high separation factors for the organic component. Pore size and number of pores on the surface of the membrane were calculated from SEM image of the membranes. Effects of pore size and porosity on pervaporation flux were also investigated.

Effect of the hard segment domains on the permeation and separation ability of the polyurethane-based membranes in benzene/cyclohexane separation by pervaporation

A series of membrane materials made of the poly(oxytetramethylene) (PTMO)-based polyurethanes was studied in pervaporation separation of benzene/cyclohexane mixtures. Polyurethanes with a domain morphology synthesised by using different aromatic diols as chain extenders were investigated and their transport properties were compared to those of pure copolymers of 2,4-tolylene diisocyanate (TDI) and PTMO. The chain extended polyurethanes (PUs) showed generally better permselectivity towards benzene and lower pervaporation flux than the respective TDI-based ones. They also exhibited different transport behaviour with respect to the variations in the feed mixture composition that was attributed to the hard segments capability to control membrane swelling. The general trend of increasing permselectivity accompanied by decreasing permeability was observed for both PUs with increasing hard segment length and PUs with decreasing soft segment length. However, the effect of the hard segment aromatic content was found to be more complex and to depend on the length of hard segments. These results were discussed with respect to the microphase-separated structure of segmented PUs and the differences in its nature being a consequence of the PU molecular structure. It was also postulated that the microphase-separated structure may undergo transformation in contact with a feed mixture, the evidence of which was an abrupt change in the transport parameters of the membranes applied in pervaporation of the mixtures with a higher benzene concentration. For those systems, the role of the diffusion step in determining the separation efficiency was strongly diminished.

Effect of residual sugars in fermentation broth on pervaporation flux and selectivity for ethanol

The effect of 5, 10 and 15 g/L glucose or 5 and 10 g/L xylose on the efficiency of ethanol pervaporation through a polydimethyl-siloxane membrane was studied using a 2% w/w aqueous ethanol solution. Membrane total flux and ethanol selectivity were measured at solution feed temperatures of 40, 50 and 60°C and permeate-side pressures of 1, 10, 20 and 40 mm Hg. Glucose at 15 g/L decreased total flux by 8% at a temperature of 60°C and a pressure of 40 mm Hg while ethanol selectivity was decreased by 15.3%. Xylose had practically no effect on total flux and a slight effect on selectivity which interacted with temperature.

Application of poly(ethylene terephthalate)-graft-polystyrene membranes in pervaporation

Gamma radiation-induced grafting of styrene (ST) onto poly(ethylene terephthalate) (PET) films was used to prepare poly(ethylene terephtalate)/polystyrene (PET- g-PST) membranes having various degrees of grafting. Various characterization techniques were used to study both surface and bulk properties of the un-grafted PET and the grafted PET- g-PST membranes. The pervaporation performance of the membranes was investigated using a 50:50% v/v methanol/toluene mixture at 25°C. The effects of the degree of grafting on the sorption behaviour, the pervaporation flux and the permeation selectivity of the membranes were studied. The pervaporation performance of the membranes has been explained using the solubility parameter approach. All the investigated membranes were found to be toluene selective. However, the grafted PET- g-PST membranes exhibit better pervaporation performance than the un-grafted PET membrane as indicated by their high toluene selectivity. Among all PET- g-PST membranes, 35% grafted membrane possesses the most appealing pervaporation properties.

Separation of water and o-chlorophenol by pervaporation using HTPB-based polyurethaneurea membranes and application of modified Maxwell–Stefan equation

Hydroxyterminated polybutadiene (HTPB) was reacted with 2,4-toluylene diisocyanate (TDI) followed by the addition of 4,4′-diaminodiphenyl sulfone (DADPS) (diamine chain extender) to prepare polyurethaneurea (PUUSD). The prepared polyurethaneurea was cast over glass petridish to obtain polymer membrane. The pervaporation (PV) performance of the synthesized membrane was studied with aqueous o-chlorophenol solution as feed. The polyurethaneurea membranes were found to be selective towards aromatic component with separation factors >250. The total flux as well as those of the individual components was measured. The effects of various parameters, such as feed composition and temperature, membrane thickness on permeate flux and separation factor were determined. The activation energy for o-chlorophenol permeation, calculated from Arrhenius plot, indicated that an increase in hard segment content imposed a restriction to the transport of organic component. The dependencies of the pervaporation fluxes and the concentration of organic component in permeate on its concentration in the feed were established with the calculation based on modified Maxwell–Stefan equation. A very close agreement between experimental and theoretical fluxes was observed.

Modelling of the pervaporative flux through hydrophilic membranes

AbstractThis work presents a simplified model of wide applicability for the determination of the pervaporation flux through hydrophilic membranes, provided that the adsorption isotherm of the permeating species onto the pervaporation membrane has a linear shape. The model predicts the pervaporation flux as a function of the activity of the penetrant in the liquid phase and the operation temperature. Experimental results obtained working with two polymeric membranes (CMC‐CF‐23 and Pervap 2256) applied to the dehydration of tetrahydrofuran (THF) and to the separation of methanol from alcohol–ether mixtures, respectively, have been satisfactorily correlated and the characterising parameters have been obtained. Furthermore, the model has been also tested against results obtained with two ceramic membranes, Pervap SMS and zeolite NaA, applied to the dehydration of ketonic mixtures and of tetrahydrofuran respectively. Copyright © 2005 Society of Chemical Industry

A Predictive Model for Permeation Flux in a Membrane Reactor: Aspects of Esterification

:A polymeric membrane (PERVAP® 2201) was used to study permselectivity and flux of water from esterification mixtures at different temperatures (30°C-90°C) and compositions. The pervaporation flux of water from binary mixtures such as butyl acetate/H2O, butanol/H2O, and acetic acid/H2O was accurately predicted by using a solution-diffusion model with concentration-dependent diffusion coefficients. Furthermore, on the basis of a lower activation energy of diffusion and higher (lux for water, we concluded that butyl acetate production can be enhanced significantly by inserting a reactor membrane (PERVAP® 2201)in the esterification process.

Pervaporation of Binary and Ternary Mixtures of Water with Methanol and/or Ethanol

This article explores water, methanol, and ethanol transport through hydrophilic membranes, by measuring the flux and separation factor in pervaporation as a function of the feed composition for binary water/methanol and water/ethanol mixtures. Additionally, the influence of adding a third component to the feed is examined. Because the solution‐diffusion model is considered to be the basic reference model for the description of transport through pervaporation membranes, it was used for understanding differences in transport characteristics. It was found that the feed composition has a large influence on flux and selectivity of all components. The results show that for a given feed concentration, the permeability is different for binary and ternary mixtures. Permeability thus depends also on the other feed components and their concentration, which contradicts the assumptions of the solution‐diffusion model. The deviations from the ideal are explained qualitatively, and may be attributed to the swelling behavior of the membrane, and to differences in size and polarity of the components. Because of the low polarity (and the larger size) of ethanol, ethanol permeation is only possible in the presence of enough water. Furthermore, it was observed for both alcohols that the water–methanol or water–ethanol separation factor in the ternary mixtures was (much) higher than in the binary mixtures, indicating that the presence of a second alcohol in the feed decreases the permeation of both alcohols.

Using pervaporation data in the calculation of vapour permeation hollow-fibre modules for aroma recovery

Taking into account the close similarity between pervaporation and vapour permeation techniques, a method for employing pervaporation flux data in the modelling of vapour permeation modules is proposed. It is based on the use of fugacity gradients across the membrane, instead of concentration ones, as the driving force for mass transfer. This procedure is interesting, for instance, in the case of aroma recovery systems, for which there is much more experimental data on pervaporation available in the literature than on vapour permeation. In order to illustrate the application of this method, pervaporation experiments for an isotropic PDMS membrane were conducted with aqueous solutions of ethyl acetate (113-2474 mg/L), and the results obtained were used to enable simulation of different vapour permeation hollow-fibre modules for recovering ethyl acetate from diluted air streams.

Studies on pervaporative characteristics of bacterial cellulose membrane

Cellulose membrane produced by bacterial Acetobacter xylinum was deproteinated and investigated for pervaporative separation of aqueous organic mixtures. The permeate flux, selectivity, pervaporation separation index (PSI), solubility and degree of sorption were studied as a function of increasing concentration of organics in the feed. The membrane was found highly selective to water; highest selectivity [ α p = 186] was obtained for a mixture of trihydric alcohol viz. glycerol (Gly) with 40% (v/v) water. The binary system of monohydric alcohol viz. ethanol (EtOH) and water (40% (v/v)) showed the lowest selectivity [ α p = 12] but the highest pervaporative flux of 614 gm −2 h −1 at 35 °C which further increased to 1429 gm −2 h −1 at 75 °C. However, selectivity decreased to 1.3 with the increase in temperature. The pervaporation behaviour was interpreted in terms of sorption and diffusivity of the organics, which in turn, was influenced by the extent of their hydrogen bonding with the cellulose units in the membrane and the plasticization induced by the permeating water present in the binary mixture. Substantially high pervaporative separation index (PSI) of the order of 10 3–10 4 gm −2 h −1 and comparatively lower energy of activation (Ej) of 10 kJ/mol is indicative of the pervaporative potential of the bacterial cellulose membrane (BCM) in the separation of aqueous binary mixtures.

Improved dense catalytically active polymer membranes of different configuration to separate and react organics simultaneously by pervaporation

To form membranes with catalytic effectiveness in hydrogenation nano-sized palladium clusters were incorporated in polymeric membranes made from poly(ether- b-amide) (PEBA) mainly by a solution-casting method. Several configurations of the Pd in the membrane were tested to optimise the catalytic activity: homogeneously distributed in PEBA, homogeneously distributed in PEBA/poly(vinylpyrrolidone) (PVP) blend, homogeneously distributed in PEBA/PVP blend with silica filler, two-layer configuration, and surface coated PEBA-membranes. The membranes were characterised by SEM, XRD and pervaporation flux density. The catalytic activity was tested in the pervaporative membrane reactor by hydrodechlorination of chlorophenol and chlorobenzene in diluted aqueous solution saturated by hydrogen at 30 °C. The produced hydrochloric acid is proportional to the reaction and detected exclusively in the feed. The decrease of the feed-pH was used to monitor the reaction progress of chlorophenol hydrodehalogenation online. Alternatively, the hydrodehalogenation of chlorobenzene was traced by head space gas chromatography. An essential improvement of the homogeneous PEBA-membranes resulted in blending with PVP and adding silica filler. The calculated activity per gram of Pd in the hydrodechlorination of 4-chlorophenol increased about four times and an about two times higher conversion was detected, both at an even lower Pd content. The two-layered membranes at similar overall Pd content demonstrated about two times higher activity and conversion per time to related membranes with homogeneously partitioned Pd-nanoclusters. Surface coated membranes with a porous Pd-layer interconnected to the PEBA-membrane showed only at high Pd content a performance related to simple homogeneous membranes.