Dehydration of water-alcohol mixtures by pervaporation and vapor permeation through surface resintering expanded poly(tetrafluoroethylene) membranes

For the purpose of separating aqueous alcohol mixtures by the use of the pervaporation and vapor permeation techniques, a surface resintering expanded poly(tetrafluoroethylene) (e-PTFE), membrane was investigated. The surface properties of the modified e-PTFE membranes were characterized by atomic force microscopy, scanning electron microscopy, and contact angle meter. The X-ray diffraction measurements show that the crystallinity of the e-PTFE membrane decreases with increasing the surface resintering temperature. The surface roughness decreases with the surface resintering temperature increases. The membrane exhibited water selectivity during all process runs. The effects of feed composition, surface resintering temperature, and molar volume of the alcohols on pervaporation and vapor permeation were investigated. Compared with the e-PTFE membrane without surface modified, the e-PTFE membrane with surface resintering treatment effectively improve the separation factor for pervaporation of aqueous alcohol mixtures. The separation performances of e-PTFE membranes in vapor permeation are higher than that in pervaporation.     

Pervaporation dehydration of isopropanol with chitosan membranes

Homogeneous and composite chitosan based membranes were prepared by the solution casting technique. The membranes were investigated for the pervaporation dehydration of isopropanol-water systems. The effects of feed concentration and temperature on the separation performance of the membranes were studied. In terms of the pervaporation separation index (PSI), the composite membrane was more productive than the homogeneous membrane for pervaporation of feed with high isopropanol content. It was observed that permeation increased and the separation factor decreased with the temperature. Modification of the homogeneous chitosan membrane by chemical crosslinking with hexamethylene diisocyanate improved the permselectivity but reduced the permeation rate of the membrane.     

Vapor permeation of ethanol-water mixtures using sodium alginate membranes with crosslinking gradient structure

For the vapor permeation of ethanol-water mixtures, two types of dense sodium alginate (SA) membranes have been prepared: a nascent SA membrane and crosslinked SA membranes with glutaraldehyde (GA). In the vapor permeation of the concentrated ethanol-water mixtures through the SA membranes, the effects of feed temperature, cell temperature and crosslinking density in the membrane were investigated on the membrane performance, and a comparison of vapor permeation process was made with pervaporation. SA membranes having different crosslinking gradients have been fabricated by exposing the nascent membrane to different GA content of reaction solutions. The extent of the gradient was controlled by the exposing time. The permeation performance of the membranes will be discussed with the extent of the gradient. An optimal crosslinking gradient was determined in terms of flux and membrane stability. The separation of ethanol-water mixtures through the membrane with the optimal crosslinking gradient was carried out by vapor permeation and the permeation performance will be discussed, and compared with pervaporation.     

Pervaporation with chitosan membranes II. Blend membranes of chitosan and polyacrylic acid and comparison of homogeneous and composite membrane based on polyelectrolyte complexes of chitosan and polyacrylic acid for the separation of ethanol-water mixtures

Three different types of blend membranes based on chitosan and polyacrylic acid were prepared from homogeneous polymer solution and their performance on the pervaporation separation of water-ethanol mixtures was investigated. It was found that all membranes are highly water-selective. The temperature dependence of membrane permselectivity for the feed solutions of higher water content (>30 wt%) was unusual in that both permeability and separation factor increased with increase in temperature. This phenomenon might be explained from the aspect of activation energy and suggested that the sorption contribution to activation energy of permeation should not always be ignored when strong interaction occurs in the pervaporation membrane system.A comparison of pervaporation performance between composite and homogeneous membranes was also studied. Typical pervaporation results at 30°C for a 95 wt% ethanol aqueous solution were: for the homogeneous membrane, permeation flux = 33 g/m² h, separation factor = 2216; and for the composite membrane, permeation flux = 132 g/m² h, separation factor = 1008. A transport model consisting of dense layer and porous substrate in series was developed to describe the effect of porous substrate on pervaporation performance.     

Pervaporation separation of water/ethanol mixture by sulfonated polysulfone membrane

For dehydrating a water/ethanol mixture by pervaporation, a sulfonated polysulfone membrane was prepared. The separation performance of water and ethanol are shown to strongly depend on the degree of substitution of polysulfone membrane. The degree of substitution increased with increasing chlorosulfonic acid in the casting solution, and the substitution reaction was achieved within 2 h. The water permeation rate and separation factor increased with increasing substitution of polysulfone membrane up to a substitution of 2.0. The effect of sulfonation on separation performance was due to the improvement of hydrophilicity of sulfonated membrane. It was found that the solubility of water/ethanol in the membrane was not the dominant factor for separation but it was rather the diffusion difference in the membrane. The diffusion difference between permeate through sulfonated membrane was the dominant factor for separating the water/ethanol mixture. The high performance of pervaporation membrane can be prepared by sulfonated polysulfone.     

Enhanced Pervaporation Performance of Multi-layer PDMS/PVDF Composite Membrane for Ethanol Recovery from Aqueous Solution

Multi-layer PDMS/PVDF composite membrane with an alternative PDMS/PVDF/non-woven-fiber/PVDF/PDMS configuration was prepared in this paper. The porous PVDF substrate was obtained by casting PVDF solution on both sides of non-woven fiber with immersion precipitation phase inversion method. Polydimethylsiloxane (PDMS) was then cured by phenyltrimethoxylsilane (PTMOS) and coated onto the surface of porous PVDF substrate one layer by the other to obtain multi-layer PDMS/PVDF composite membrane. The multi-layer composite membrane was used for ethanol recovery from aqueous solution by pervaporation, and exhibited enhanced separation performance compared with one side PDMS/PVDF composite membranes, especially in the low ethanol concentration range. The maximum separation factor of multi-layer PDMS/PVDF composite membrane was obtained at 60 °C, and the total flux increased exponentially along with the increase of temperature. The composite membrane gave the best pervaporation performance with a separation factor of 15, permeation rate of 450 g/m²h with a 5 wt.% ethanol concentration at 60 °C.     

Preparation and characterizations of asymmetric sulfonated polysulfone membranes by wet phase inversion method

This paper presents an original approach to prepare the asymmetric sulfonated polysulfone membranes by using wet phase inversion method and their applications for dehydrating a water/ethanol mixture by pervaporation. The separation performances of sulfonated membranes were strongly affected by the degree of sulfonation and the degree of swelling of membranes. The substitution degree of sulfonic group enhanced the permselectivity of sulfonated polysulfone membranes by increasing the hydrophilicity of polymer backbone. Based on the observations of membrane morphology and light transmittance measurements, the degree of sulfonation of polysulfone presented less influence on the membrane formation pathway and the final structure of membrane in wet phase inversion process. It was also found that the sulfonated membranes showed well hydrophilic properties and facilitated water adsorption in the membranes. The sorption and permeation properties also showed that the permselectivity of asymmetric membrane was dominated by the permeate diffusion rather than the permeate sorption in the skin layer. The high separation performance of pervaporation membrane can be achieved by phase inverse method with sulfonated polysulfone.     

Chemical-Modified Nylon 4 Membrane for Pervaporation

Abstract

A hydrophilic polymer membrane was synthesized with 2-hydroxyethyl methacrylate (HEMA) onto a Nylon 4 polymer backbone, PHEMA-g-N4. The membranes were water permselective because of the hydrophilicity, and the water permselectivity increased with increasing the degree of grafting. Permseparation of water was investigated with respect to the feed aqueous alcohol concentration, feed temperature, size of the alcohols, and degree of grafting. The separation factors of this PHEMA-g-N4 membrane were higher than those of the unmodified Nylon 4 membrane for pervaporation of aqueous ethanol solution, while the permeation rate was slightly lower. A separation factor of 98 and a 194 g/m²·h permeation rate could be obtained. Compared with an unmodified Nylon 4 membrane, the PHEMA-g-N4 membrane effectively increased the pervaporation separation index for the water-ethanol mixtures on pervaporation separation.     

具有界面交联结构藻酸钠复合膜的制备与性能

报道了一种具有界面交联结构的新型藻酸钠复合膜及其对醇水和其它有机物水体系的渗透汽化分离性能.该膜的活性层为藻酸钠,支撑层为氨化聚丙烯腈(PAN)多孔膜,在这两层之间存在着界面交联结构.研究了PAN多孔膜的水解时间、进行氨基化的二元胺种类及浓度对复合膜分离性能的影响,用己二胺进行氨基化所得到的复合膜的分离性能明显优于用乙二胺的结果.扫描电镜照片显示水解及氨基化改变了PAN超滤膜的孔结构,这也是影响新型复合膜性能的一个重要原因.     

壳聚糖膜的处理方法与其渗透汽化性能间的关系

对壳聚糖均质膜折脱酸处理、干燥方法与所得膜的渗透汽化性能间的关系进行了研究。结果表明，处理方法的不同直接影响到膜的透过、分离性能。用含３ｗｔ．％ＮａＯＨ的醇水溶液（乙醇／水＝５０／５０（ｗｔ．／ｗｔ．））进行脱酸处理的膜，其α水／乙醇值，在料液温度为５５－７５℃的范围内几乎不变。     

改性壳聚糖膜用于醇水混合液分离的研究(Ⅱ)─壳聚糖－羧甲基纤维素共混物膜

制备了壳聚糖(CS)─羧甲基纤维素(CMC)共混物膜(I-2),对其成膜反应、溶胀度、交联度及拉伸强度进行了研究,结果表明,壳聚糖与羧甲基纤维素共混物在成膜的同时还发生交联反应;当CS/CMC=1时,交联度最大,此时共混物膜不溶于稀醋酸水溶液。首次将此共混物膜用于乙醇/水混合液的分离,该膜具有优良的醇水分离性能,当CS/CMC=1时,渗透通量和分离因子皆达到最大值[J=0.9kg/(m²·h),a=800,90wt%乙醇,45℃],且该膜的分离因子基本上不随温度变化,醇水透过I-2膜的表现活化能△E为32.6kJ/mol.对CS/CMC2+2+2+2+次序递增,分离因子变化次序则刚好相反。     

Permeation and separation of styrene/ethylbenzene mixtures through cross-linked poly(hexamethylene sebacate) membranes

Poly(hexamethylene sebacate) (PHS) which has strong affinity for styrene was selected as membrane material, and the characteristics of permeation and separation for the styrene/ethylbenzene mixtures through these PHS cross-linked with N,N,N′,N′-tetraglycidyl m-xylenediamine(TETRA-DX) membrane by pervaporation were investigated. The cross-linked PHS membranes exhibited a styrene permselectivity for the styrene/ethylbenzene mixtures and the permeation rate increased with increasing styrene in the feed solution. The permselectivity of their membranes was strongly governed by the sorption separation process depending on the difference of the solubility between styrene and ethylbenzene. The molecular weight of PHS had also influence to the separation factor and permeation rate in pervaporation.     

Pervaporation properties of novel alginate composite membranes for dehydration of organic solvents

A novel organic dehydration membrane consisting of aminated polyacrylontrile (PAN) microporous membrane as sublayer, alginate coating as top layer has been prepared and characterized by pervaporation experiment. The influence of hydrolysis and amination of the microporous support layer on selectivity and flux was studied and it was shown that amination of the sublayer improved pervaporation performance of the composite membrane greatly. The counter cation of alginate coatings as dense separating layer also influenced separation properties of the membrane, which was better for K⁺ than for Na⁺. This novel composite membrane with K⁺ as counter ion has a high separation factor of 1116 and a good permeation rate of 350 g/m² h for pervaporation of 90 wt.% ethanol aqueous solution at 70°C, higher separation factors and fluxes for n-PrOH/water, i-PrOH/water, acetone/water and dioxane/water systems. The results show that the separation factor and flux of this membrane increase with raising the operating temperature. At the same time, SEM micrographs show that the hydrolysis and amination of PAN microporous membrane change its pore structure. From the results it can be concluded that pore structure of the sublayer in addition to its chemical structure also make influence of separation properties of the composite membrane.     

聚酰亚胺中空纤维膜用于甲醇/甲基叔丁基醚的分离

甲醇／甲基叔丁基醚混合物的膜法分离，大多采用渗透汽化方法，少有采用蒸汽渗透法。用聚酰亚胺中空纤维膜，对以蒸汽渗透和渗透汽化两种方式分离甲醇／甲基叔丁基醚混合物（甲醇质量分数为0．01－0．30）的效果进行了对比。结果显示，在甲醇质量分数低于0．05时，蒸汽渗透较渗透汽化法的分离性能优越。     

A novel composite chitosan membrane for the separation of alcohol-water mixtures

A novel alcohol dehydration membrane with a three layer structure has been prepared. The top layer is a thin dense film of chitosan (CS), and the support layer is made of microporous polyacrylonitrile (PAN). Between the dense and microporous layer, there is an intermolecular cross-linking layer. This novel composite membrane has a high separation factor of more than 8000 and a good permeation rate of 0.26 kg/m² h for the pervaporation of 90 wt% ethanol aqueous solution at 60°C, 0.8 kg/m² h flux for a n-PrOH/water system and around 1 kg/m² h flux for an i-PrOH/water system using 80 wt% alcohol concentration at 60°C. The separation factor for both cases is more than 10⁵. The separation performance varies with feed composition, operating temperature and conditions of membrane preparation. The results show that the separation factor and flux of this membrane increase with raising the operating temperature. At the same time, the crosslinking layer improves durability of the composite membrane, and the pervaporation performance can be adjusted by changing the structure of the cross-linking layer. The cross section of the composite membrane has been examined by SEM.     

Novel nanocomposite pervaporation membranes composed of poly(vinyl alcohol) and chitosan-wrapped carbon nanotube

Novel nanocomposite membranes (PVA–CNT(CS)) were prepared by incorporating chitosan-wrapped multiwalled carbon nanotube (MWNT) into poly(vinyl alcohol) (PVA). To further explore the intrinsic correlation between pervaporation performance and free volume characteristics, molecular dynamics simulation was first introduced to qualitatively analyze the contribution of carbon nanotube incorporation on improving free volume characteristics of the nanocomposite membranes. Secondly, the pervaporation performance of PVA–CNT(CS) nanocomposite membranes was investigated using permeation flux and separation factor as evaluating parameters. For benzene/cyclohexane (50/50, w/w) mixtures at 323 K, permeation flux and separation factor of pure PVA membrane are only 20.3 g/(m² h) and 9.6, respectively, while the corresponding values of PVA–CNT(CS) (CNT content: 1%) nanocomposite membrane are 65.9 g/(m² h) and 53.4. In order to explain the simultaneous increase of permeation flux and separation factor, as well as to check the calculation reliability of molecular dynamics simulation, positron annihilation lifetime spectroscopy (PALS) analysis was employed.     

Recent advances in sodium alginate-based membranes for dehydration of aqueous ethanol through pervaporation

Sodium alginate (SA) is a progressive material for membrane fabrication. The technological development of SA-based membranes has made a significant contribution to the separation techniques, especially in aqueous organic solutions. The outstanding performance of SA is attributed to its outstanding structural flexibility and hydrophilicity. In view of structural characteristics, SA membranes have immense utilization in the pervaporation separation of organics. Among various organics, dehydration of aqueous ethanol is employed as a standard to check the success of pervaporation (PV) membrane. Because ethanol and water have comparable molecular sizes, thus difficult to extract water from aqueous ethanol mixtures than it is for other organics. A literature survey shows that wide-ranging data are available on the PV performance of SA and its modified membranes. In this context, the present review addresses the recent advances made in SA membranes for enhanced ethanol dehydration performance during the last decade. Available data since 2010 has been compiled for grafted, crosslinked, blend, mixed matrix, and composite hybrid sodium alginate membranes in terms of separation factor, permeation flux, and pervaporation separation index PSI. The data are assessed with reference to the effect of feed composition, membrane selectivity, flux, and swelling behavior.     

Use of pervaporation to separate butanol from dilute aqueous solutions: Effects of operating conditions and concentration polarization

This study deals with the separation of n-butanol from aqueous solutions by pervaporation. The effects of feed concentration, temperature, and membrane thickness on the separation performance were investigated. Over the low feed butanol concentration range (0.03–0.4 wt%) studied, the butanol flux was shown to increase proportionally with an increase in the feed butanol concentration, whereas the water flux was relatively constant. An increase in temperature increased both the butanol and water fluxes, and the increase in butanol flux was more pronounced than water flux, resulting in an increase in separation factor. While the permeation flux could be enhanced by reducing the membrane thickness as expected for all rate-controlled processes, the separation factor was compromised when the membrane became thinner. The effect of membrane thickness on the separation performance was analyzed taking into account the boundary layer effect. This could not be fully attributed to the concentration polarization, which was found not significant enough to dominate the mass transport. A variation in the membrane thickness would vary the local concentration of permeant inside the membrane, thereby affecting the permeation of butanol and water differently. Thus, caution should be exercised in using permeation flux normalized by a given thickness to predict the separation performance of a membrane with a different thickness because the membrane selectivity can be affected by the membrane thickness even in the absence of boundary layer effect.     

Multicomponent pervaporation process for volatile aroma compounds recovery from pomegranate juice

The present work describes the possibility of using pervaporation process to recover the pomegranate aroma compounds from an actual pomegranate juice and a model aroma solution. Four different chemicals representing four major kinds of aroma compounds, namely, 3-methyl butanal, isopentyl acetate, n-hexanol and α-ionone, were utilized in this work. Three POMS membranes and two PDMS membranes were tested for pervaporation and compared for their separation performance. The influence of various operating parameters such as feed flow rate, feed temperature and permeate pressure on the permeation flux and aroma compounds enrichment factor was investigated. Feed flow rate was shown to have no significant effect on both total flux and aroma enrichment factor, whereas feed temperature and permeate pressure had highly significant effects. An increase in feed temperature led to higher flux and enrichment factor. As permeate pressure increased, the flux and enrichment factor of some aroma compounds decreased. Some of the aroma compounds showed higher enrichment factor at higher permeate pressures. Finally, the activation energy of permeation and the membrane permeability for each aroma compound were determined.