A review on removal of sulfur components from gasoline by pervaporation

Desulfurization of gasoline has gained growing importance because of tighter limits of less than 10 ppm sulfur in gasoline in recent regulations. On the other hand, preserving octane rating in gasoline is the most concern subject of the manufacturers. This review focuses on the desulfurization of gasoline by means of pervaporation (PV) process. The process as a new technology has drawn increasing attention and provided an efficient approach for eco-friend sulfur removal in petrochemical industries due to its high selectivity, feasible economics, and safety. Theoretical aspects in selection of materials for the applied membranes and their modifications are investigated. The various parameters including the type and concentrations of sulfur and hydrocarbon species, feed temperature, feed flow rate, and permeate pressure, which influence the performance of PV are discussed.     

Development of polybenzoxazine membranes for ethanol–water separation via pervaporation

Polybenzoxazine membranes have been successfully synthesized from bisphenol-A, formaldehyde, and three different types of diamines: hexa-methylenediamine (hda), tetraethylenepentamine (tepa), and tetraethylenetriamine (teta) via a facile “quasi-solventless” method. To study the possibility of using polybenzoxazine membranes in a pervaporation system for ethanol–water separation, the sorption and swelling behaviors of these membranes were investigated. When hda was used as a reactant, the resulting polybenzoxazine membranes showed the best service time and interestingly only water permeated the membranes under the studied operation conditions. The total permeation flux was found to be 1.52 kg/m²h and the separation factor was higher than 10,000. Additionally, an increased permeation flux was achieved by raising the temperature of the feed solution and decreasing the membrane thickness. The optimum conditions for this study were 70 °C for the feed mixtures when a 200 μm thick was used.     

Manipulation of confined structure in alcohol-permselective pervaporation membranes

Alcohol-permselectivity pervaporation has been arousing increasingly more attention in bioalcohol production due to the advantages of environmental friendliness, lowenergy consumption and easy couplingwith fermentation process. With the intrinsic feature of larger molecules preferentially permeating and the consequent inferiority in selective diffusion, the development of alcohol-permselectivemembrane is relatively retarded compared with water-permselective membrane. This review presents the prevalent membrane materials utilized for alcohol-permselective pervaporation and emphatically expatiates the representative and important developments in the past five years fromthe aspect of tuning confined structure in membranes. In particular, the diverse structure tuningmethods are described with the classifications of physical structure and chemical structure. The corresponding structure-performance relationships in alcohol-permselective pervaporationmembranes are also analyzed to identify the objective of structure optimization. Furthermore, the tentative perspective on the possible future directions of alcohol-permselective pervaporation membrane is briefly presented.     

Selective removal of acetone and butan-1-ol from water with supported ionic liquid–polydimethylsiloxane membrane by pervaporation

The use of ionic liquid–polymer in supported ultrafiltration membrane in vacuum pervaporation has been verified. The ultrafiltration membranes were impregnated by two ionic liquids (1-ethenyl-3-ethyl-imidazolium hexafluorophosphate and tetrapropylammonium tetracyano-borate) and polydimethylsiloxane. These new and very stable supported ionic liquid–polymer membranes were used for separation of ternary mixtures butan-1-ol–acetone–water by vacuum pervaporation. In comparison with polydimethylsiloxane membranes, the average enrichment factor of butan-1-ol increased in both cases. This higher selectivity shows a good potential for improving pervaporation separation process.     

Permselectivities of aromatic polyamide membranes for aqueous alcohol mixtures in pervaporation

The separation of water/alcohol mixtures was carried out using a series of fluorine-containing aromatic polyamide membranes. Aromatic polyamides were prepared by direct polycondensation of fluorine-containing diamine (2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, BAPPH) and various aromatic diacids. The separation factor toward water increased when the feed ethanol concentration was increased. The solubility of ethanol in aromatic polyamide membrane is higher than that of the water, but the diffusivity of water across the membrane is higher than that of alcohols. A separation factor of 83 and a permeation rate of 262 g/m²h with a 90 wt% feed ethanol concentration at 25 °C was obtained.     

Hydrophilic hollow fiber membranes for water desalination by the pervaporation method

Hydrophilic ion-exchange membranes based on sulfonated polyethylene hollow fibers were manufactured, and their suitability for a water pervaporation process was studied for possible application in water desalination systems. The effects of the following parameters on the average water flux were determined: membrane properties (diameter (0.4–1.8 mm) and wall thickness (0.05–0.18 mm)); charge density (0.6–1.2 meq g⁻¹); and operating conditions (brine inlet temperature (30–68°C), air sweep velocity (0–6 m s⁻¹), and salt concentration in the feed brine (0–3 M)). A water flux of 0.8–3.3 kg m⁻² h⁻¹ was obtained using this type of hollow fiber with an inlet brine temperature of 25–65°C. It was found that, for our application, the optimal specifications for the ion-exchange hollow fibers were an outside diameter of 1.2 mm, a wall thickness of 0.1 mm, and an ion-charge density of about 1.0 meq g⁻¹. This information is required as basic data for the design of a prototype water desalination system based on a pervaporation system that uses this type of ion-exchange hollow fiber membrane.     

Experimental study on physical properties and pervaporation performances of polyimide membranes

Five kinds of polyimide membranes have been synthesized from two dianhydrides (including pyromellitic dianhydride (PMDA) and 3, 3^′4, 4^′-benzophenonetetracarboxylic dianhydride (BTDA)) and three diamines (including 4,4^′-diaminodiphenylether (ODA), 4,4^′-diaminodiphenylmethane (MDA) and phenylenediamine (PDA)) via a two-step method, and the properties of polyimide membranes have been characterized by experimental techniques. The permeation experiments of water/ethanol mixtures through the polyimide membranes were carried out at 318, 328, 338 and 348 K. All polyimide membranes in this paper are water selectivity, and the flux of mixtures through MDA-based polyimide membranes is higher than that of solvents through PDA-based polyimide membranes with the same diahydrides, while the separation factor exhibits the opposite variation order at the same temperature. The total flux and the partial water flux through all the membranes increase with the operating temperature rising, and the relationships between the flux and temperature can be described by Arrhenius equation. According to the Arrhenius equation, the active energies of water/ethanol in PMDA-ODA, PMDA-MDA, BTDA-PDA, BTDA-ODA and BTDA-MDA membranes are 35.1, 52.6, 16.4, 19.8 and 27.1 kJ/mol, respectively, and those for partial water flux in PMDA-ODA, PMDA-MDA, BTDA-ODA and BTDA-MDA are 36.1, 46.8, 19.9 and 27.9 kJ/mol, respectively. The separation factors of mixtures in the polyimide membranes but no PMDA-MDA show the trend of increase with the operating temperature. The partial flux of ethanol increases while the separation factor of mixtures in PMDA-MDA decreases with the operating temperature, and the activation energy for partial ethanol flux in PMDA-MDA is 75.2 kJ/mol. In addition, pervaporation performances were attempted to correlate independently with the fractional free volume (FFV) and mean interchain distance (d-spacing) of polymers. It is shown that lnJ increases with d-spacing, while there is no clear relationship between the flux and FFV.     

Mass transfer model,preparation and applications of zeolite membranes for pervaporation dehydration:A review

Pervaporation (including vapor permeation) is a kind of new membrane separation technology, possessing the advantages of high efficiency, energy saving and convenient operation. It has promising application in the separation and purification of organic solvents. Dehydration is an important step in the production and recovery of organic solvents. Zeolite membranes have attracted wide attention for pervaporation dehydration due to their high separation performance and good thermal/chemical stability. So far, zeolite membranes have been preliminarily industrialized for dehydration of organic solvents. This paper reviews the recent development of zeolitemembranes for pervaporation dehydration, includingmass transfermodels, preparation and applications of zeolite membranes. The review also discusses the current industrial applications of zeolite membranes and their future development in pervaporation.     

Fabrication of graphene oxide composite membranes and their application for pervaporation dehydration of butanol

As a new kind of 2D nanomaterials, graphene oxide (GO) with 2–4 layers was fabricated via a modified Hummers method and used for the preparation of pervaporation (PV) membranes. Such GO membranes were prepared via a facile vacuum-assisted method on anodic aluminium oxide disks and applied for the dehydration of butanol. To obtain GO membranes with high performance, effects of pre-treatments, including high-speed centrifugal treat-ment of GO dispersion and thermal treatment of GO membranes, were investigated. In addition, effects of oper-ation conditions on the performance of GO membranes in the PV process and the stability of GO membranes were also studied. It is of benefit to improve the selectivity of GO membrane by pre-treatment that centrifuges the GO dispersion with 10000 r·min?1 for 40 min, which could purify the GO dispersion by removing the large size GO sheets. As prepared GO membrane showed high separation performance for the butanol/water system. The separation factor was 230, and the permeability was as high as 3.1 kg·m?2·h?1 when the PV temperature was 50 °C and the water content in feed was 10%(by mass). Meanwhile, the membrane still showed good stabil-ity for the dehydration of butanol after running for 1800 min in the PV process. GO membranes are suitable candidates for butanol dehydration via PV process.     

Polysiloxaneimide membranes for removal of VOCs from water by pervaporation

For the separation of volatile organic compounds (VOCs) from water by pervaporation, three polysiloxaneimide (PSI) membranes were prepared by polycondensation of three aromatic dianhydrides of 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA), and pyromellitic dianhydride (PMDA) with a siloxane‐containing diamine. The PSI membranes were characterized using ¹H‐NMR, ATR/IR, DSC, XRD, and a Rame‐Hart goniometer for contact angles. The degrees of sorption and sorption selectivity of the PSI membranes for pure organic compounds and organic aqueous solutions were investigated. The pervaporation properties of the PSI membrane were investigated in connection with the nature of organic aqueous solutions. The effects of feed concentration, feed temperature, permeate pressure, and membrane thickness on pervaporation performance were also investigated. The PSI membranes prepared have high pervaporation selectivity and permeation flux towards hydrophobic organic compounds. The PSI membranes with 150‐μm thickness exhibit a high pervaporation selectivity of 6000–9000 and a high permeation flux of 0.031–0.047 kg/m² h for 0.05 wt % of the toluene/water mixture. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2691–2702, 2000     

Plasticization of chitosan membrane for pervaporation of aqueous ethanol solution

The process of pervaporation in which two components diffuse through a nonporous polymer membrane was modelled when one of the penetrants can exert a plasticization action to the membrane material. Thereat a phenomenological model was employed for describing the plasticization effect on the diffusivities for penetrants in the membrane. The sorption equilibria and permeation fluxes for aqueous ethanol solutions in a chitosan membrane were measured, and the permeation fluxes for water were compared with those predicted by the proposed model. The concentration of sorbed water was linear with its weight fraction (x₄) in the feed solution, whereas the permeation flux of water was. affected by the plasticization action of sorbed water to the polymer. This plasticization effect on the diffusion process can be simulated in terms of the proposed phenomenological model.     

Recovery of pyridine from water by pervaporation using filled and crosslinked EPDM membranes

Organoselective membrane was prepared from ethylene propylene diene monomer (EPDM) rubber. Crosslinked EPDM rubber was filled with 2, 4 and 6 wt% N330 carbon black filler to produce three different filled membranes designated as EPDMCV2, EPDMCV4 and EPDMCV6, respectively. These filled rubber membranes were used for pervaporative recovery of low concentration of pyridine from water. These filled membranes were characterized by crosslink density, SEM, XRD and mechanical properties. Sorption thermodynamics were discussed. Partial permeability, intrinsic membrane selectivity and diffusion coefficients of solvents were also determined. The filled membranes showed much higher pyridine selectivity than most of the membranes reported for similar system.     

PDMS/ZIF-8 coating polymeric hollow fiber substrate for alcohol permselective pervaporation membranes

In order to develop high performance composite membranes for alcohol permselective pervaporation (PV), poly (dimethylsiloxane)/ZIF-8 (PDMS/ZIF-8) coated polymeric hollow fiber membranes were studied in this research. First, PDMS was used for the active layer, and Torlon®, PVDF, Ultem®, and Matrimid® with different porosity were used as support layer for fabrication of hollow fiber composite membranes. The performance of the membranes varied with different hollow fiber substrates was investigated. Pure gas permeance of the hollow fiber was tested to investigate the pore size of all fibers. The effect of support layer on the mass transfer in hydrophobic PV composite membrane was investigated. The results show that proper porosity and pore diameter of the support are demanded to minimize the Knudsen effect. Based on the result, ZIF-8 was introduced to prepare more selective separation layer, in order to improve the PV performance. The PDMS/ZIF-8/Torlon® membrane had a separation factor of 8.9 and a total flux of 847 g·m^-2·h^-1. This hollow fiber PDMS/ZIF-8/Torlon® composite membrane has a great potential in the industrial application.     

A study on the effect of cross-linkers on pervaporation performance of room temperature vulcanised silicone rubber membranes for butanol recovery

As a rising membrane separation technology, pervaporation (PV) has been regarded as one of the most promising separation technology applied in separation of azeotropic mixtures due to its high selectivity and low energy consumption. In a PV process, separation process mainly occurs in the PV membrane, so it is of significance to develop a membrane with high performance. Room temperature vulcanised (RTV) silicone rubber is a kind of membrane material possessing good hydrophobicity and low glass transition temperature. It is widely used in the recovery of organic compounds from their dilute aqueous solutions. In this paper, RTV silicone rubber membranes were fabricated by a typical one-pot method and characterised by Fourier transform infrared spectroscopy, thermal gravimetric analysis, water contacting angle, differential scanning calorimetry, scanning electron microscopy and swelling experiment. The membranes were used in PV recovery of n-butanol from its dilute aqueous solution (about 1?wt-%). The influences of temperature and cross-linkers on the performance of PV were discussed at the same time. In conclusion, RTV silicone rubber membrane cross-linked by vinyltrimethoxysilane (WD-21), which obtained membrane selectivity of 82.9 with butanol permeability of 1.1_?×_?10⁶ Barrer at 50°C, got the best performance.     

Using pervaporation technique to separate water from organics

This paper is focused on how water may be separated from dioxan, butanone and butyric acid under constant operating conditions. The analysis is presented in terms of variations in the permeation flux, individual partial fluxes and separation factors. As in a membrane process, in order to separate a liquid mixture, pervaporation relies on differences in membrane permeability as well as the vapour pressures of the components in the mixture. It is found that the separation factors reach infinity as the total permeate pressure is reduced to the vapour pressure of the organic permeant. The permeation flux increases with increasing water concentration in the feed. A drop in the flux has been observed on addition of an extra component. The PVA membrane confirmed its hydrophilic characteristics by not allowing butyric acid and butanone to permeate. Values of the activity coefficients and diffusion coefficients are reported versus the vapour composition for binary, ternary and multi-component systems. The diffusion coefficient increases with a decrease in mole fraction of the component in the membrane.     

Synthesis,characterization and application of PVA/ionic liquid mixed matrix membranes for pervaporation dehydration of isopropanol

In this study,polyvinyl alcohol (PVA)-ionic liquid (IL) membranes were prepared for the separation of isopropylalcohol (IPA)-water azeotropic mixtures by pervaporation.PVA-IL composite membranes were prepared by simple solvent evaporation method using four ILs,viz.,1-n-butyl-3-methylimidazolium chloride (BMIMCl),1-hexyl-3-methylimidazolium chloride (HMIMCl),1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF4)and 1-octyl-3-methylimidazolium chloride (OMIMCl).Three ILs were used to study the effect of alkyl chain on the pervaporation performance.The study had focused on the effect feed water concentration from 10％-40％and effect of feed temperature from 50-80 ℃.Physiochemical properties of all the membranes were studied using Fourier transform infrared spectroscopy (FTIR),scanning electron microscopy (SEM) and contact angle measurement.The Arrhenius activation energies for permeation were estimated to be in the range 4-12 kJ.mol-1 from the temperature dependent permeation values.     

Preparation of zeolite-coated pervaporation membranes for the integration of reaction and separation

Pervaporation is a promising option to enhance conversion of reversible condensation reactions, generating water as a by-product. In this work, composite catalytic membranes for pervaporation-assisted esterification processes are prepared. Catalytic zeolite H-USY layers have been deposited on silica membranes by dip-coating using TEOS and Ludox AS-40 as binder material. Membrane pre-treatment and the addition of binder to the dip-coat suspension appear to be crucial in the process. Tuning of catalytic layer thickness is possible by varying the number of dip-coat steps. This procedure avoids failure of the coating due to the high stresses, which can occur in thicker coatings during firing. In the pervaporation-assisted esterification reaction the H-USY coated catalytic pervaporation membrane was able to couple catalytic activity and water removal. The catalytic activity is comparable to the activity of the bulk zeolite catalyst. The collected permeate consists mainly of water and the loss of acid, alcohol and ester through the membrane is negligible. The performance of the membrane reactor is mainly limited by reaction kinetics and can be improved by using a more active catalyst.     

Separation and purification of isobutanol from dilute aqueous solutions by a hybrid hydrophobic/hydrophilic pervaporation process

In this study, a hybrid hydrophobic/hydrophilic pervaporation process was employed to separate and purify isobutanol from its dilute aqueous solutions. For this purpose, composite polydimethylsiloxane membranes were initially used for the recovery of isobutanol by hydrophobic pervaporation. Then the hydrophilic pervaporation with a composite polyvinyl alcohol membrane was utilized to separate water from the organic phase of the permeate stream of the hydrophobic pervaporation. The effect of feed flow rate on the performance of pervaporation was investigated. The resistance in series model was also applied to calculate the transport resistances through the composite membranes. It was observed that an enhancement in the feed flow rate led to higher permeation flux and selectivity of the more permeable component, while the flux of the less permeable component was almost constant. Also, the ratio of liquid boundary layer resistance to membrane layer resistance decreased by an increase in the feed flow rate. The isobutanol with a purity of higher than 99 wt.% was produced by the hybrid hydrophobic/hydrophilic pervaporation technique from a 2 wt.% aqueous isobutanol solution.     

Water-selective hybrid membranes with improved interfacial compatibility from mussel-inspired dopamine-modified alginate and covalent organic frameworks

Hybrid membranes combining the merits of both polymer matrices and fillers have drawn extensive attention. The rational design of polymer–filler interface in hybrid membranes is vitally important for reducing the occurrence of void defects. Herein, imine-type covalent organic frameworks(COFs) were selected as the fillers due to their totally organic nature and multi-functionalities. Mussel-inspired dopamine-modified sodium alginate(Alg DA) was synthesized as the polymer matrix. The dopamine modification significantly improves the Alg DA–COF compatibility,which enhances the COF content up to 50 wt% in the hybrid membranes. The improved interfacial compatibility enhances the membrane separation selectivity. Accordingly, when utilized for dehydration of ethanol/water mixed solution(water concentration of 10 wt%), the hybrid membrane reveals high water concentration of ～98.7 wt% in permeate, and stable permeation flux larger than 1500 g·m~(-2)·h~(-1). This work might afford useful insights for fabricating hybrid membranes with high separation selectivity by optimizing the polymer–filler interface.     

Recovery of alcohols from water using polydimethylsiloxane–silica nanocomposite membranes: Characterization and pervaporation performance

Different polydimethylsiloxane (PDMS) nanocomposite membranes were synthesized by incorporating various contents of nanosized silica particles to improve the PDMS pervaporation (PV) performance. A uniform dispersion of silica nanoparticles in the PDMS membranes was obtained. The nanocomposite membranes were characterized morphologically by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results showed that surface roughness increases by incorporating silica, and this decreases absorption of penetrants on the membrane. Swelling studies showed that the presence of silica nanoparticles into the PDMS membranes decreases degree of swelling, which can be attributed to rigidification of the PDMS matrix. Additionally, the results revealed that helium permeability decreases through the nanocomposite membranes, due to the more polymer chains packing. Effects of silica on recovery of isopropanol (IPA) from water mixtures were also investigated. Based on the results, incorporating silica nanoparticles promotes significantly the PDMS membrane selectivity because the polymer chains are rigidified and also the polymer free volume decreases. However, permeation flux decreases as diffusion of the penetrants reduces in the presence of silica nanoparticles within the PDMS membranes. As PV performance depends on operating conditions, effects of feed composition, and temperature were also studied. Moreover, recoveries of IPA, ethanol, and methanol from water mixtures were compared using the PDMS‐silica nanocomposite membranes. The results demonstrated that polarity and solubility of alcohols affect permeation flux and selectivity resulting in the higher permeation flux and selectivity for IPA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012