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.     

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.     

Maleic Anhydride Crosslinked Alginate-Chitosan Blend Membranes for Pervaporation of Ethylene Glycol-Water Mixtures

Calcium alginate-chitosan (CA/CS) blended membranes were prepared and crosslinked with maleic anhydride (MA) for the pervaporation (PV) separation of ethylene glycol (EG)/water mixtures at 30°C. The structure and properties of blend membranes were studied with the aid of FTIR, XRD, TGA, and SEM. The effect of experimental parameters such as feed composition, membrane thickness, and permeate pressure on separation performance of the MA crosslinked membranes were determined in terms of flux, selectivity, and pervaporation separation index. Sorption studies were carried out to evaluate the extent of interaction and degree of swelling of the blend membranes in pure, as well as in binary mixtures. The experimental results suggested that the crosslinked membrane (M-CA/CS) exhibited a good selectivity of 302 at a normalized flux of 0.38 kg.m^{? 2}.h^{? 1}.10 μ m at 30°C for 96.88 wt% EG aqueous solution.     

Synthesis of polyoxyethylene grafting nylon 6 and the selective separation of cyclohexane/cyclohexanone/cyclohexanol mixture through its membranes

A polymer membrane having polyoxyethylene grafting nylon 6 was prepared by reacting of nylon 6 and ethylene oxide. The chemical compositions of the polyoxyethylene grafting nylon 6 were determined by ¹H NMR. Degree of substitution for amide group, x, and degree of polymerization for polyoxyethylene, n, in bulk polymerization at 80°C for 4–9.5 h were evaluated: x = 0.32 ± 0.01–0.56 ± 0.02 and n = 2.8 ± 0.1–6.0 ± 0.3. The polyoxyethylene grafting nylon 6 membrane showed a selective separation of cyclohexanol from a cyclohexane/cyclohexanone/cyclohexanol mixture by a pervaporation technique. The FTIR and flux analyses verified that the selectivity for cyclohexanol was attributed to the hydrogen-bonding interaction between hydroxyl group in cyclohexanol and the hydroxyl group in polyoxyethylene grafting chain. The pervaporation and an adsorption experiment of cyclohexanol through the present membrane showed that hydroxyl group in graft chain acted as a carrier for cyclohexanol.     

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.     

Comparison between the pervaporation and vapor permeation performances of polycarbonate membranes

The pervaporation and vapor permeation performance of symmetrical and asymmetrical polycarbonate membranes, prepared via a dry-phase inversion and wet-phase inversion methods, respectively, were studied by measuring the permeation rate and separation factor. It was found that the polymer concentration effect on the pervaporation performance for the symmetrical polycarbonate membrane was lower than that for the asymmetrical polycarbonate membrane. Compared with pervaporation, vapor permeation has a significantly increased separation factor with a decreased permeation rate for the symmetrical polycarbonate membrane. Water molecules preferentially dissolve into the symmetrical polycarbonate membrane and diffuse easily through the membrane.     

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。     

Anti-trade-off in dehydration of ethanol by novel PVA/APTEOS hybrid membranes

Novel organic–inorganic hybrid membranes were prepared through sol–gel reaction of poly(vinyl alcohol) (PVA) with γ-aminopropyl-triethoxysilane (APTEOS) for pervaporation (PV) separation of ethanol/water mixtures. The membranes were characterized by FTIR, EDX, WXRD and PALS. The amorphous region of the hybrid membranes increased with increasing APTEOS content, and both the free volume and the hydrophilicity of the hybrid membranes increased when APTEOS content was less than 5 wt%. The swelling degree of the hybrid membranes has been restrained in an aqueous solution owing to the formation of hydrogen and covalent bonds in the membrane matrix. Permeation flux increased remarkably with APTEOS content increasing, and water permselectivity increased at the same time, the trade-off between the permeation flux and water permselectivity of the hybrid membranes was broken. The sorption selectivity increased with increasing temperature, and decreased with increasing water content. In addition, the diffusion selectivity and diffusion coefficient of the permeants through the hybrid membranes were investigated. The hybrid membrane containing 5 wt% APTEOS has highest separation factor of 536.7 at 50 °C and permeation flux of 0.0355 kg m⁻² h⁻¹ in PV separation of 5 wt% water in the feed.     

Dehydration of tetrafluoropropanol (TFP) by pervaporation via novel PBI/BTDA-TDI/MDI co-polyimide (P84) dual-layer hollow fiber membranes

A novel PBI/P84 co-polyimide dual-layer hollow fiber membrane has been specifically fabricated through the dry-jet wet phase inversion process, for the first time, for the dehydration pervaporation of tetrafluoropropanol (TFP). Polybenzimidazole (PBI) was chosen as the outer selective layer because of its superior hydrophilic nature and excellent solvent-resistance together with robust thermal stability, while P84 co-polyimide was employed as the inner supporting layer because of its good solvent-resistance and thermal stability. The PBI/P84 membrane exhibits superior water selectivity and relatively high permeation flux. At 60 °C, the PBI/P84 dual-layer hollow fiber membrane shows a permeation flux of 332 g/(m² h) and a separation factor of 1990 for a feed solution containing of 85 wt% TFP. The preferential water sorption and the significant diffusivity difference between TFP and water are the main causes of high separation factor. However, an increase in feed temperature will greatly increase the permeation flux but seriously decrease the water selectivity. The activation energy data verify that water can preferentially permeate the PBI membrane due to the strong water affinity of PBI and a much smaller molecular size of water.     

SELECTIVE SEPARATION OF WATER-ETHANOL MIXTURES THROUGH COPOLYMERIC MEMBRANES: Ⅰ. ACRYLIC ACID AND ACRYLONITRILE COPOLYMER AND ITS IONIZED MEMBRANES

The copolymer of acrylic acid and acrylonitrile has been synthesized and pervaporation properties of the copolymeric membranes have been investigated. In order to elucidate the influence of membrane-permeate interaction on the pervaporation of water-ethanol mixtures and to prepare much improved membranes, the membranes have been treated with alkali metal, alkali earth metal and transition metal salt aqueous solutions. The treated membranes (ionized membranes) exhibited higher separation factors than the untreated membranes. The separation factors of various alkali metal cation membranes decreased in the following order : Li~+>Na~+>K~+, and the permeation rates showed an opposite tendency. The dependence of pervaporation behavior on the copolymer composition ,feed concentration and operating temperature have been studied with both ionized and non-ionized membranes. The apparent activation energies of water and ethanol permeation were calculated.     

Development of polyelectrolyte complexes of chitosan and phosphotungstic acid as pervaporation membranes for dehydration of isopropanol

Using a solution technique, chitosan-based polyelectrolyte complexes (PECs) were developed as pervaporation membranes by incorporating phosphotungstic acid (PTA). The resulting membranes were characterized by Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). Membranes were tested for their ability to separate water–isopropanol mixtures by pervaporation in the temperature range of 30–50 °C. The experimental results demonstrated that both flux and selectivity were increased simultaneously with increasing PTA content in the membrane. The permeation flux of pure chitosan membrane was increased dramatically from 4.13 to 11.70 × 10⁻² kg/m² h and correspondingly its separation factor was increased from 4490 to 11,241 and then decreased to 7490 at 30 °C for 10 mass% of water in the feed. The total flux and flux of water were found to be almost overlapping particularly for PECs membranes, suggesting that these could be used effectively to break the azeotropic point of water–isopropanol mixtures. From the temperature dependency of diffusion and permeation values, the Arrhenius activation parameters were estimated and discussed in the context of membranes efficiency. The pure chitosan and a small amount of PTA-incorporated PECs membranes exhibited positive heat of sorption while other PECs membranes exhibited negative heat of sorption, giving exothermic contribution.     

Pervaporation separation of water/ethanol mixture by TGN/PSF blending membrane

The separation performance of plasticizer/polysulfone (TGN/PSF) pervaporation membrane was studied. The optimum amount of plasticizer (TGN) in PSF membranes improved the diffusion selectivity of water to ethanol, which was due to the increase in the permeate diffusion rate difference between water to ethanol molecules. On the other hand, the solubility selectivity of water to ethanol in PSF membrane showed a minor change with increasing the plasticizer content in TGN/PSF membrane. The feed ethanol concentration showed a significant influence on the degree of swelling as well as the separation performance of TGN/PSF membrane. It was found that the dominant factor of permeate transport through membranes was the diffusion rate difference, especially at high ethanol concentrations in feed. This study indicated that a good separation performance could be achieved at high ethanol concentrations in feed. This investigation also proves that the flexible polymer chain mobility, which was due to both the addition of TGN in the membrane and the swelling effect of the membrane at the high ethanol concentration in feed solution, strongly influences the separation properties of TGN/PSF membrane.     

聚环氧乙烷（PEO）／壳聚糖（CS）共混膜的醇－水渗透蒸发性能研究

研究了ＥｔＯＨ－Ｈ２Ｏ，ｎ－ＰｒＯＨ－Ｈ２Ｏ，ｉ－ＰｒＯＨ－Ｈ２Ｏ体系在ＣＳ膜和ＰＥＯ／ＣＳ共混膜中的渗透蒸发性能。讨论了料液温度、料液浓度、共混膜组成对分离性能的影响，结果发现ＰＥＯ的掺入能大大提高ｃｓ膜的渗透通量；而分离因子下降。同时从膜材料的聚集态结构出发对相关的渗透蒸发行为进行了讨论。对于ＰＥＯ／ＣＳ共混膜，膜内自由体积的大小是影响分离性能的主要因素，小分子在膜中的渗透蒸发行为主要是由扩散过程控制的。本文还研究了ＰＥＯ的掺入对壳聚塘膜强度的影响以及利用ＤＳＣ谱研究ＰＥＯ掺入后壳聚糖膜聚集态结构的变化。     

Pervaporation separation of aqueous chlorophenols by a novel polyurethane urea–poly (methyl methacrylate) interpenetrating network membrane

A novel polymer membrane system consisting of interpenetrating network (IPN) of hydroxy terminated polybutadiene (HTPB) based polyurethane urea (PUU)–poly (methyl methacrylate) (PMMA) has been designed and developed as highly permselective membrane for pervaporation separation of toxic p-chlorophenol and 2,4-dichlorophenol from their dilute aqueous solutions. It was observed that 3 ppm 2,4-dichlorophenol in water could be reduced to 0.3 ppm 2,4-dichlorophenol using a PUU–PMMA IPN membrane of 28 cm² area and 150 μm thickness. This membrane has shown high selectivity towards p-chlorophenol and 2,4-dichlorophenol at very low concentration in feed. Feed concentration of p-chlorophenol was varied from 1000 to 7000 ppm and that of 2,4-dichlorophenol was varied from 3 to 4000 ppm. Fifty seven percent 2,4-dichlorophenol in permeate was obtained from 3 ppm concentration in feed compared to 87% 2,4-dichlorophenol in permeate from 1000 ppm in feed. Pervaporation studies were carried out by varying the temperature of feed, membrane thickness and PMMA content in the membrane. The results of this investigation have revealed that these membranes would be suitable for separation of chlorophenols from industrial effluents.     

Simultaneous Spray Self‐Assembly of Highly Loaded ZIF‐8–PDMS Nanohybrid Membranes Exhibiting Exceptionally High Biobutanol‐Permselective Pervaporation

The ability to obtain a maximum loading of inorganic nanoparticles while maintaining uniform dispersion in the polymer is the key to the fabrication of mixed‐matrix membranes with high pervaporation performance in bioalcohol recovery from aqueous solution. Herein, we report the simultaneous spray self‐assembly of a zeolitic imidazolate framework (ZIF)–polymer suspension and a cross‐linker/catalyst solution as a method for the fabrication of a well‐dispersed ZIF‐8–PDMS nanohybrid membrane with an extremely high loading. The ZIF‐8–PDMS membrane showed excellent biobutanol‐permselective pervaporation performance. When the ZIF‐8 loading was increased to 40 wt %, the total flux and separation factor could reach 4846.2 g m⁻² h⁻¹ and 81.6, respectively, in the recovery of n‐butanol from 1.0 wt % aqueous solution (80 °C). This new method is expected to have serious implications for the preparation of defect‐free mixed‐matrix membranes for many applications.     

Modeling and simulation of butanol separation from aqueous solutions using pervaporation

A study was conducted to separate butanol from an aqueous solution using pervaporation. A specially designed and manufactured cell was used to separate the butanol from butanol/water solutions of different butanol concentrations (6-8-11-16-20-50) g/l. A 250 cm³ butanol mixture at 33 °C was used to feed the cell, while the pressure of permeation side was about 0 bar. Results revealed that butanol concentration changes non-linearly during the first 3 h, and then proceeds linearly. The percentage of butanol removal increases with increasing feed concentration. The permeability of the used membrane was determined experimentally. A resistance in series model was used to simulate the pervaporation step. The butanol concentration in the feed during the pervaporation step was predicted by using the developed model. There is a fair agreement between butanol concentration in feeding tank of pervaporation cell both experimentally and predicted from the developed model.     

Pervaporation of water from aqueous sulfuric acid at elevated temperatures using Nafion membranes

The concentration of sulfuric acid by pervaporation has been studied using Nafion-112^® and Nafion-117^® membranes, which have been characterized in terms of flux, permeability, and separation factor at 100 and 120 °C. Feed acid concentrations investigated ranged from 40 to over 80 wt%. In general, water fluxes ranged from 100 to 8000 g/m² h, depending on feed acid concentration and separation factors as high as 10⁴ were observed. Membrane stability was probed using dynamic mechanical analysis that revealed an increase in the temperature at which the α transition is observed, which corresponds to the glass transition (T_g) of the hydrophilic domain, upon use, suggesting embrittlement of the polymer structure. Further studies showed that the embrittlement was due to an interaction with the acid and was not induced by the operating temperature.     

Homogeneous polyimide/cyclodextrin composite membranes for pervaporation dehydration of isopropanol

Composite membranes with a sub-nanoscale homogeneous distribution of CD toroids in the Matrimid matrix were developed for dehydration of aqueous isopropanol. The composite membranes demonstrated separation factor far surpassing that of the neat Matrimid dense membrane. The heart of this innovation is the utilization of a CD derivative, ethylenediamine-β-cyclodextrin (EDA-β-CD), where the amine of CD could react with the imide of Matrimid and efficiently immobilize the CD rings during membrane formation. The superior separation properties for membranes embedded with 2–5% EDA-β-CD were attributed to the additional water channels created by the hydrophilic outer surface of CD and its interactions with the polymer matrix. FT-IR, density measurements and XRD have confirmed these hypotheses. Nevertheless, the separation factor exhibited an increasing then decreasing trend as a function of CD content and the opposite trend was observed with permeation flux. Investigation on the effect of feed water concentration showed that the neat Matrimid membrane possessed almost constant performance, but the Matrimid/EDA-β-CD (0.05) composite membrane exhibited an obvious increase in permeability and a decrease in selectivity at high water content. Even though the composite membrane swelled more at higher water content due to the intensified hydrophilicity ascribed to the introduction of CD structure, it always had much better separation factor. In addition, the Matrimid mixed matrix membranes embedded with 2–5% EDA-β-CD held reasonably tensile strength and modulus. The newly developed mixed matrix membrane approach may open up a new way to prepare next-generation high-performance asymmetric pervaporation membranes for isopropanol separation.     

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.     

A facile route for fabricating novel polyelectrolyte complex membrane with high pervaporation performance in isopropanol dehydration

Polyelectrolyte complexes (PECs) of sodium carboxymethyl cellulose (CMCNa) and poly(diallyldimethylammonium chloride) (PDDA) were prepared in dilute hydrochloric acid (HCl) aqueous solution and obtained in its solid form. Element analysis and FT-IR showed that the composition of PECs could effectively be tuned by the concentration of HCl in parent polyelectrolyte solution. The PECs were then dissolved in 0.1 mol/L aqueous NaOH and subsequently cast onto polysulfone ultra-filtration membrane. This composite membrane, which has a unique homogeneous PECs separation layer, was subjected to pervaporation test for the first time and gave a performance of J = 3.0 kg/m² h, α = 960 for 10 wt% water–isopropanol feed at 75 °C. Meanwhile, performance of the PECs membrane displays good stability and unique dependence on feed temperature. These findings, together with its ultra-high performance, are primarily explained by the structure characteristic of PECs.