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Ion Exchange Membranes for Pervaporation A Patents Review
Abstract
Pervaporation is a newly emerged separation process, known for its higher efficiency, lower energy consumption lower capital cost and so on. The main research field in pervaporation falls into three aspects: 1) the investigation of new membranes of good separation performance; 2) methods for manufacturing pervaporation membrane with high performance; 3) process modeling to find the bottleneck in the separation process. To date, lots of research papers are published and patents concerning the above mentioned research aspects have also been brought into the sight of those skilled in the art. As a branch membrane for pervaporation purpose, an ion exchange membrane, which is prepared from the charged polymers containing either anionic or cationic groups, has an important influence on the development of pervaporation. This paper thus discusses some valuable patents concerning the research of ion exchange membrane for pervaporation. Some basic processes using anionic membrane or cationic membrane, ways for the modification of charged polymers, manufacturing of pervaporation membrane and some insights of the mass transfer behavior are reviewed in this paper and finally a perspective conclusion is made.
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Cation exchange membranes polyethylene sulfonated polystyrene (PESS) were prepared by sulfonation of an interpolymer polyethylene/poly(styrene-co-divinylbenzene) [PE/poly(St-co-DVB)] with 1 or 3 wt% of the cross-linking agent divinylbenzene (DVB). Swelling properties of the PESS ion‐exchange membranes in contact with water-aliphatic alcohols mixtures were investigated. The PESS membranes were loaded with different alkali metal ions as counterions. The obtained data showed that properties of PESS membranes depended strongly on the kind of counterions, the degree of cross‐linking, and the difference in the polarities between water and the organic component of the binary mixture. Results obtained for PESS membranes were compared with data obtained for the Nafion 117 ion-exchange membrane.
The enzymecatalytic esterification reaction of acetic acid and ethanol in [bmim]PF6 ionic liquid was studied to manufacture natural ethyl acetate by a continuous waste-free process. The optimal parameters of the reaction were determined in batch experiments, and then a continuous set-up was constructed. Ethyl acetate and water formed in the bioreactor were removed by a double pervaporation system using hydrophobic and hydrophilic membranes, respectively. Acetic acid and ethanol were added as they were consumed, thus steady state concentrations in the reactor were maintained and continuous operation for 72 hours long was ensured without any activity loss of the enzyme.
Transport of pure propan-1-ol and propan-2-ol through polyethylene membrane during vapor permeation and pervaporation processes was studied at 20°C. The permeation experiments for different concentration up to the saturated vapor pressure were carried out. It was verified experimentally that there is not a significant difference between permeation fluxes of saturated vapor of propanols and their pervaporation fluxes. Also, the amount of sorbed propanol from liquid and from saturated vapor is the same. The influence of sorption of permeating molecules on the diffusion coefficient in membrane was investigated. The assumption of two types of permeating molecules in the rubbery polymer (mobile and immobile) was applied for the description of the mass transport through polyethylene membrane. Concentrations of mobile and immobile molecules in the membrane were estimated.
Polyvinyl alcohol (PVOH) has been chemically modified by crosslink copolymerization of acrylic acid (AA) and hydroxyethylmethacrylate (HEMA) in aqueous solution of PVOH and finally crosslinking PVOH to produce a full interpenetrating network (IPN) membrane termed as PVAH. Accordingly, three such full crosslink IPNs membranes, i.e. PVAHI, PVAHII and PVAHIII containing varied weight ratio of PVOH and copolymer have been synthesized and used for pervaporative separation of methanol from its mixtures with toluene. For comparison, a conventional PVOH membrane crosslinked with glutaraldehyde has also been used for the same pervaporation study. The flux and selectivity of these IPN membranes were found to be much higher than the conventional glutaraldehyde crosslinked PVOH membrane. Among the three membranes, PVAHII with 50 wt% polyAH incorporation showed optimum performance in terms of flux and methanol selectivity.
In this study, the pervaporation behaviour of azeotrope-forming methanol-carbontetrachloride binary mixture at different compositions was investigated at 30°C and 45°C through LDPE membranes with two different melt flow indices for two different thicknesses. Experimental fluxes and selectivities for methanol and carbontetrachloride were determined. Theoretical calculations for fluxes based on a solution-diffusion model as developed by Yeom and Huang using Fujita′s free volume approach and Flory-Huggins thermodynamics showed deviations from the experimental results. Experimental fluxes were decomposed into sorption and diffusion terms to discuss the effects of experimental conditions.
We show that the mass transport through an ion exchange membrane can be predicted provided that an adequate mass transfer model compatible with linear irreversible thermodynamics is used. An experimental example is proposed in order to check the validity of the mass transfer equations through an ion exchange membrane: pervaporation of water from electrolyte solutions containing sodium or potassium chloride through an ion exchange membrane. Fluxes can be predicted with accuracy by either the Stefan-Maxwell or modified Spiegler equation.
The conductivity, electro-osmotic water transport, hydrodynamic permeability, and “self-diffusion” coefficient were measured for a cation exchange membrane in the Ag+ and Na+-forms. Refinements in experimental technique were made. The phenomenological coefficients were calculated and it was shown that some commonly neglected interactions can be important. The suitability of the pseudo-binary expressions was also investigated.
Polyvinyl alcohol (PVOH) has been chemically modified by crosslink copolymerization of acrylic acid (AA) and hydroxyethylmethacrylate (HEMA) in aqueous solution of PVOH and finally crosslinking PVOH to produce a full interpenetrating network (IPN) membrane termed as PVAH. Accordingly, three such full crosslink IPNs, i.e. PVAHI, PVAHII and PVAHIII containing varied weight ratio of PVOH and copolymer along with crosslink PVOH (which has not been modified by copolymerization) have been synthesized and used for pervaporative separation of isopropanol (IPA)–water mixtures. Permeability and water selectivity of these membranes were found to increase with increasing amount of copolymer in PVOH matrix. However, among the three membranes, PVAHIII were found to show highest flux but lower selectivity while PVAHI membrane showed optimum performance in terms of both flux and selectivity.
Interpenetrating network (IPN) membrane of HTPB-based polyurethane urea (PUU) and poly (methyl methacrylate) (PMMA) was developed and used in the pervaporation separation of water-dimethyl formamide (DMF) mixtures of different compositions. From these data the permeation flux, separation selectivity, degree of swelling were calculated at 25, 40 and 60°C. At a higher temperature, the permeation of diffusing molecules through the membrane becomes easier, resulting in an increase of DMF flux. Again with the increase in PMMA content in the membrane the DMF flux and separation selectivity values are increased due to the presence of more polar groups of PMMA. The membrane showed more selectivity towards DMF than water. The highest DMF flux of 0.231 kg/m2.h was obtained for the PUU-PMMA-3 membrane at a temperature of 60°C for 80% DMF in the feed.
Die vorliegende Arbeit behandelt Studien zur Gleichgewichtseinstellung bei der Herstellung eines Acetals durch Pervaporation. Die Abtrennung des Reaktionswassers wurde mit verschiedenen Membranen bei unterschiedlichen Temperaturen untersucht. Besonderes Interesse galt neben den spezifischen Flussdichten für Wasser auch dem Trennverhalten der jeweiligen Membranen hinsichtlich der Schlüsselkomponenten Acetal und Keton aus dem Zulaufgemisch. Es wurde gefunden, dass eine spezifische Flussdichte für Wasser von J0 = 18.5 kg/h/m2 bei einer Temperatur von T = 368.15 K erreicht werden kann. Acetal wurde bei keiner der getesteten Membranen im Permeat festgestellt. Für die geeignete Membran vom Typ B konnte ein Trennfaktor bzw. eine Selektivität bezüglich des Ketons im Zulaufgemisch von SWasser/Keton = 900 ermittelt werden. Eine Simulation einer dreistufigen Reaktionskaskade bestehend aus jeweils einem Reaktor und einer Pervaporationseinheit mit den gemessenen Werten dieser Membran zeigte, dass eine Steigerung des Umsatzes um ca. X = 40 % möglich ist. Dies resultiert in einer Energieeinsparung von 25 % bei der Aufarbeitung des Reaktionsgemisches.
Twelve kinds of polyimide membranes have been prepared using three dianhydrides (including 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA), 3,3‘4,4‘-benzophenonetetracarboxylic dianhydride (BTDA), and 3,3‘4,4‘-biphenyltetracarboxylic dianhydride (ODPA)) and four diamines (including benzidine (BZD), bis(4-aminophenyl)phenyl phosphate (BAPP), 4,4‘-diaminodiphenylmethane (MDA), and 4,4‘-diaminodiphenyl ether (ODA)) via a two-step method. The polyimides were characterized by FT-IR, DSC, and wide-angle X-ray diffraction (WAXD). The permeation experiments of water + ethanol mixtures through 12 polyimide membranes were carried out at 333 K. The temperature dependence of pervaporation performances of ODA-based polyimide membranes is also investigated. The flux of ethanol + water mixtures through the polyimide membranes with the same dianhydrides increases following the order of BZD < ODA < MDA < BAPP. The permeation flux increases with increase in temperature and the relationships between the flux and temperature can be described by the Arrhenius equation. According to the Arrhenius equation, the active energies of water + ethanol mixtures in BTDA + ODA, BPADA + ODA, and ODPA + ODA membranes are (19.3, 26.0, and 30.6) kJ·mol-1, respectively. However, the relationship between the separation factor and the temperature is not so clear. In addition, the natural logarithm of flux J (ln J) increases linearly with the mean interchain distance.
In order to control the surface pore sizes of polyvinylidene fluoride membranes and their distribution, low temperature plasma-induced grafting modifications of PVDF were studied to prepare hydrophobe membranes. By argon (Ar) treating and subsequent grafting reaction, a hydrophobe monomer, styrene, was introduced into the PVDF membrane. Fourier transform infrared attenuated total reflection (FTIR-ATR) was utilized to characterize the chemical and physical changes in the Ar plasma modified membrane. The surface modifications of PVDF membranes were investigated by using scanning electron microscopy (SEM), atomic force microscopy (AFM) and differential scanning calorimeter (DSC). The water permeability and the solute rejection were measured by PVDF membrane modified in different graft conditions. Results demonstrated that the pores in the modified membranes get smaller and the distribution of pores gets narrowed with the increase in grafting reaction duration. Longer graft time caused the water flux of PVDF membrane to decrease from 578 kg/(m2h) to 23 kg/(m2h) and the solute rejection to increase from 73% to 92%.
Pervaporation of a water-ethanol mixture with sulphonated polyethylene membranes of various capacities at various temperatures and for various feed compositions was studied both theoretically and experimentally. A model is proposed that accounts both for the effects of partial immobilization of water in the membrane due to adsorption by the groups and for the obstruction effect of the polymer matrix on the basis of percolation arguments. Analysis of the pervaporation data against the results of previous sorption experiments showed that the model is in fair agreement with the experiment. A number of peculiarities not predicted by the model and not observed for ethanol transport, such as enhanced diffusivity and selectivity and reduced apparent activation energy, were observed concerning the transport of water. These deviations are explained by assuming the existence of a selective facilitated mechanism of water transport. Other possible mechanisms are also discussed.
An crosslinked polyethylene glycol (PEG) membrane was prepared for fluid catalytic cracking (FCC) gasoline desulfurization. Sulfur enrichment factor come to 4.75 and 3.51 for typical FCC gasoline feed with sulfur content of 238.28 and 1227.24 μg/g, respectively. Pervaporation performance of membranes kept stable within the long time run of 500 h, which indicated that crosslinked PEG membranes had the property of resisting pollution. Judging from chromatographic analysis, the membranes were more efficient for thiophene species. Effects of operation conditions including permeate pressure, feed temperature, feed flow rate and feed sulfur content level on the pervaporation performance were investigated. Permeation flux decreased with increasing permeate pressure while increased with the operating temperature increase. Sulfur enrichment factor increased firstly and decreased then when permeate pressure and temperature rose. The peak value occurred at 10.5 mm Hg and 358 K for model compounds feed (378 K for FCC gasoline feed). Arrhenius relationship existed between flux and operating temperature. Both sulfur enrichment factor and flux were shown to increase with increasing feed flow rate. Permeation flux increased while sulfur enrichment factor decreased as the feed sulfur content increased, but the influence of increasing sulfur content on pervaporation performance weakened when sulfur content come to 600 μg/g.
Enzymatic production of an important natural flavor ester, isoamyl acetate, was studied by using immobilized Candida antarctica lipase B in [bmim]PF 6 ionic liquid. The reaction was completed by removing water produced in the reaction by zeolite adsorption, and practically 100% yield was achieved. An integrated system was developed for the separation of the isoamyl alcoholÁwater Áisoamyl acetate ternary azeotropic mixture, combining pervaporation and osmotic distillation. The Sulzer "Pervap 2256" flat sheet hydrophobic pervaporation membrane was found to be 100% selective for isoamyl alcohol.
Selective and transport properties of an anion-exchange hollow-fiber membrane (Pervasiv Ltd., Israel) were determined in the sweeping gas pervaporation process used for the dehydration of different organic solvents. C2– C4 alcohols (i.e. ethanol — EtOH, propanol — nPrOH, isopropanol — iPrOH, n-butanol — nBuOH and isobutanol —iBuOH) and two esters (i.e. methyl acetate — MeAc and ethyl acetate — EtAc) were chosen for investigations. The water content in the feed mixture was in the range 1–8 wt.%. Pervasiv membrane possessed the hydrophilic properties in contact with water–organic mixtures investigated. The separation factor α of Pervasiv membrane was correlated with the dielectric constant of the organic solvent. It was found that α increased exponentially with the decrease of the dielectric constant of organic solvent. Such behaviour was caused not only by the decrease of the flux of the less polar organic component through the membrane but also by the substantial increase of the water flux. The concentration dependence of the permeate fluxes showed that the total flux increased when the amount of water in the feed increased. The molar partial fluxes of organic solvents were much higher for polar solvents (ethanol) when comparing with solvents of lower polarity (e.g. isobutanol, methyl acetate).
According to the solution-diffusion model, the separation of vapors and liquid mixtures by nonporous membranes in sweeping gas pervaporation or vacuum pervaporation is due to the differences in the solubility and the diffusivity of the mixture components in the membrane material. In the case when ion-exchange membrane is used, the membrane ionic groups and counter-ions can play an important role in the mass transfer process.The permeation rate measurements which are based on the sweeping gas pervaporation concept were performed in order to determine the diffusivity of chosen aliphatic alcohol vapors (i.e. ethanol, i-propanol and t-butanol). The experiments were carried out using different sulfonic ion-exchange membranes: Nafion (perfluorinated polyethylene with pendant ether-linked side chains terminated with sulfonated groups), IonClad (irradiation grafted sulfonated styrene monomers onto polytetrafluoroethylene film) and PESS (membrane prepared by sulfonation of the interpenetrating polymer network system polyethylene–poly/styrene-co-divinylbenzene/) loaded with H+, Li+ or K+ ions as counterions. NAFION and IonClad membranes have the same backbone (i.e. polytetrafluoroethylene). The ionic strength of sulfonic groups is identical for IonClad and PESS membranes whereas the NAFION membrane get a much greater ionic strength. The diffusivity of ethanol was greatly affected by the presence of water in the membrane. The diffusivity characteristics in the sulfonated ion-exchange membranes indicated strong interactions between water and alcohol molecules. The increase of alcohols polarity increases their diffusion properties. The diffusion coefficient of t-butanol in investigated membranes increases when lithium is replaced by potassium.
The sorption, diffusion, and pervaporation (PV) properties of benzene/cyclohexane (Bz/Cx) mixtures on cation-exchange membranes containing copper ions (Cu(II)) were investigated. The equilibrium sorption isotherms of pure vapors in the membranes and the partial solubility of binary solutions in the membranes were described using the UNIQUAC model. The τiM and τMi values were 0.978 and 0.591 for Bz, and 0.922 and 0.475 for Cx. The transient regimes of vapor sorption were employed to calculate the concentration-dependent diffusion coefficients. Long’s model sufficiently explained the diffusivity of Bz and Cx in the membranes. The pre-exponential factors were ∼3×10−13 m2/s and the plasticization factors were 3.0 and 3.6 for Bz and Cx, respectively. Excellent agreement was found with the experimental results applying the solubility and diffusivity data to simulate the pervaporation performance (flux and selectivity) using the modified Maxwell–Stefan equation. The membrane containing Cu(II) demonstrates better facilitating capability for Bz transport than that with Na(I), mainly due to its preferential sorption property toward Bz. Replacing Na(I) with Cu(II) into a Neosepta membrane resulted in better separation efficiency and higher Bz flux throughout the entire Bz concentration range.
In this paper the enzymatic manufacture of glycerol monostearate was studied in high polar organic solvents. Water produced during the reaction has a disadvantageous effect on the reaction rate and enzyme activity. This byproduct water was removed and the water content was kept constant using a pervaporation unit which could be integrated on-line with the reaction system. The best yield (90%) after 6 h reaction time was achieved at 40°C, at seven-fold glycerol excess using dioxane as solvent. The enzymatic esterification reaction could be described by the Random Bi–Bi mechanism. A mathematical model have been estimated and experimentally verified by a set of experiments with varying and constant water contents. The experiments both in batch and integrated system (esterification–pervaporation) could be described with good accuracy by the proposed model.
In this study, poly(vinylidene fluoride-co-hexafluoropropene) (P(VDF-co-HFP)) was firstly used as a membrane material in organophilic pervporation for aroma compounds’ recovery. Separation and transport properties of P(VDF-co-HFP) coated polypropene (PP) composite membrane in pervaporation of EtAc–water mixture were investigated. The effects of two process conditions (EtAc feed concentration and operating temperature) on pervaporation parameters (permeation flux and separation factor) were discussed in detail. The study shows that acetone-fabricated membranes have excellent mechanical strength, thermal stability and film-forming properties. In addition, they have ideal pervaporation performances compared to commonly used poly(dimethylsixolane) (PDMS) membrane and poly (ether block amide) (PEBA) membrane. Its separation factor is higher than that of PEBA membrane but lower that of PDMS membrane. The permeate flux is relatively high, which depends on not only the nature of the given membrane material but also the membrane-formed special structure and morphology. Mass transports in the membrane were investigated based on the change of microstructure from DSC analysis during the swelling process. The activation energies for permeation of water (EP,W) and EtAc (EP,E) calculated from the slopes of Arrhenius plots of ln(J) versus 1/T were employed to explain the diffusion process. In general, the sorption process dominates the separation mechanism.