Review of pervaporation and vapor permeation process factors affecting the removal of water from industrial solvents

A recent review article (Journal of Chemical Technology & Biotechnology 94 : 343–365 (2019)) identified several commercially-available permselective materials for drying organic solvents with pervaporation (PV) and vapor permeation (V·P) separation processes. The membrane materials included polymeric and inorganic substances exhibiting a range in the performance characteristics: water permeance, water/solvent selectivity, and maximum use temperature. This article provides an overview of the factors affecting the design of PV/V·P processes utilizing these membranes to remove water from common organic solvents. Properties of the specific membrane and of the solvent substantially affect the PV/V·P separation. Equally important is the impact of operating parameters on the overall separation. To study these impacts, simplified process performance equations and detailed spreadsheet calculations were developed for single-pass and recirculating batch PV systems and for single-pass V·P systems. Estimates of membrane area, permeate concentration, solvent recovery, permeate condenser temperatures, and heating requirements were calculated. Process variables included: solvent type, water permeance, water/solvent selectivity, initial and final water concentrations, operating temperature (PV) or feed pressure (V·P), temperature drop due to evaporation (PV) or feed-side pressure drop (V·P), and permeate pressure. The target solvents considered were: acetonitrile, 1-butanol, N,N-dimethyl formamide, ethanol, methanol, methyl isobutyl ketone, methyl tert-butyl ether, tetrahydrofuran, acetone, and 2-propanol. Published 2019. This article is a U.S. Government work and is in the public domain in the USA.     

异质结构g-C3N4@AM层状膜构筑及纳滤性能研究

随着化工行业的迅速发展，利用纳滤膜对有机溶剂进行高效分离受到了越来越多的关注，但有机溶剂纳滤膜通量和选择性之间普遍存在 trade-off 效应的限制。以相对疏水的g-C₃N₄纳米片和亲水的直链淀粉（amylose，AM）为构筑单元，利用双针头静电雾化技术制备了异质结构的g-C₃N₄@AM 层状膜。亲水的直链淀粉促进了极性溶剂的溶解，相对疏水的g-C₃N₄纳米片实现了通道对极性溶剂的低阻力扩散；两者协同，极大地增强了膜对极性溶剂的渗透性能，而不降低分离能力。与纯g-C₃N₄层状膜相比，g-C₃N₄@AM 层状膜对极性溶剂的渗透系数提高了1~2倍，对于尺寸大于 1.5 nm 的染料分子可以实现 99% 以上的截留率。在操作稳定性、压力循环和耐酸碱测试后，膜的渗透性能和截留能力基本保持不变，衰减 < 6%，具有较好的操作稳定性。     

Review: membrane materials for the removal of water from industrial solvents by pervaporation and vapor permeation

Organic solvents are widely used in a variety of industrial sectors. Reclaiming and reusing the solvents may be the most economically and environmentally beneficial option for managing spent solvents. Purifying the solvents to meet reuse specifications can be challenging. For hydrophilic solvents, water must be removed prior to reuse, yet many hydrophilic solvents form hard-to-separate azeotropic mixtures with water. Such mixtures make separation processes energy-intensive and cause economic challenges. The membrane processes pervaporation (PV) and vapor permeation (VP) can be less energy-intensive than distillation-based processes and have proven to be very effective in removing water from azeotropic mixtures. In PV/VP, separation is based on the solution–diffusion interaction between the dense permselective layer of the membrane and the solvent/water mixture. This review provides a state-of-the-science analysis of materials used as the selective layer(s) of PV/VP membranes in removing water from organic solvents. A variety of membrane materials, such as polymeric, inorganic, mixed matrix, and hybrid, have been reported in the literature. A small subset of these is commercially available and highlighted here: poly (vinyl alcohol), polyimides, amorphous perfluoro polymers, NaA zeolites, chabazite zeolites, T-type zeolites, and hybrid silicas. The typical performance characteristics and operating limits of these membranes are discussed. Solvents targeted by the United States Environmental Protection Agency for reclamation are emphasized and ten common solvents are chosen for analysis: acetonitrile, 1-butanol, N,N-dimethyl formamide, ethanol, methanol, methyl isobutyl ketone, methyl tert-butyl ether, tetrahydrofuran, acetone, and 2-propanol. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.     

Graphene oxide membrane for liquid phase organic molecular separation

The selective permeation of organic solvents and water through graphene oxide (GO) membranes has been demonstrated. Water was found to permeate through GO membranes faster than various alcohols. The permeation rates of ethanol, 1-propanol and 2-propanol (IPA) are about 80 times lower than that of water. Taking advantage of the differences in the permeation rates, we separated water from the alcohols and obtained alcohols with high purity. For ethanol and 1-propanol, binary solutions of the alcohol and water were filtered efficiently to produce alcohols with concentration of about 97%. However, the selectivity of the filtration of methanol is significantly lower than those of the other alcohols. To understand the mechanism we followed the structural changes in the GO membranes by X-ray diffraction analysis. From the X-ray diffraction results we speculate that the selectivity of the permeation of water and alcohols is closely related to the molecular sizes of the solvents and their polarity. In order to demonstrate the potential applications of this process for the selective removal of water from aqueous organic mixtures, we performed the separation of water from a bio-oil containing 73% of water. The majority of the water was filtered out resulting in a higher purity bio-oil.     

Pervaporation separation of binary and ternary mixtures with polydimethylsiloxane membranes

This study dealt with the separation of binary water–phenol and water–methanol mixtures and ternary water–phenol–methanol mixtures by pervaporation (PV) with polydimethylsiloxane (PDMS) membranes. The effects of the operating conditions (feed temperature, feed concentration, and feed flow rate) on the separation performance for binary mixtures were investigated. An increase in temperature or concentration increased the total permeation flux and decreased the organic separation factor. In other words, an increase in the temperature or feed organic concentration increased the water flux more significantly than the organic compound flux, which resulted in a separation factor reduction. Also, an increase in the feed flow rate increased the total flux and separation factor because the boundary layer effects diminished. The vapor–liquid equilibrium separation factor (α_VLE) and pervaporation separation factor (α_PV) values for the PDMS membrane were calculated, and this showed that α_PV for the water–phenol mixture was greater than α_VLE. This means that the membrane was highly efficient for the PV separation of phenol from dilute aqueous solutions relative to the separation of methanol. This was due to the fact that phenol has a higher solubility parameter than methanol in silicone membranes. To study the effect of a third component on membrane performance, PV experiments were also carried out with water–phenol–methanol mixtures. The results for total permeation flux and the phenol separation factor for PDMS membranes in contact with water–phenol–methanol ternary mixtures are similar to those in contact with water–phenol binary mixtures. The phenol separation factor of the membrane in contact with the ternary mixture was slightly lower than that in contact with the binary mixture. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Ethene/Ethane (C2H4/C2H6) Separation through an Inorganic–Organic Hybrid Membrane Containing Silver(I) Ions as Olefin Carriers, Using Poly(N-vinylpyrrolidone) as a Mediation Agent

Novel inorganic–organic membranes that contained Ag⁺ ions as olefin carriers were prepared using sol–gel and dip-coating processes. The permeance of the membranes for nitrogen, helium, ethane (C₂H₆), and ethene (C₂H₄) were evaluated using the single-gas permeation method at temperatures of 298, 373, and 423 K. The results showed that the selectivity of the membranes to C₂H₄ against C₂H₆ increased as the measurement temperature increased, because the decomplexation rate of C₂H₄ molecules from Ag⁺ sites is enhanced by increases in the temperature. Fourier transform infrared spectrophotometry of the hybrid membranes and the performance of the membranes at 373 and 423 K indicated that poly( N -vinylpyrrolidone) (PVP) had a role in increasing the flexibility of the inorganic network and also served as a mediation agent to fix Ag⁺ ions in the polymer segments, because of the coordination interaction between the Ag⁺ ions and the PVP.     

Reverse osmosis performance of organosilica membranes and comparison with the pervaporation and gas permeation properties

Hybrid organosilica membranes were successfully prepared using bis(triethoxysilyl)ethane (BTESE) and applied to reverse osmosis (RO) desalination. The organosilica membrane calcined at 300^°C almost completely rejected salts and neutral solutes with low‐molecular‐weight. Increasing the operating pressure led to an increase in water flux and salt rejection, while the flux and rejection decreased as salt concentration increased. The water permeation mechanism differed from the viscous flow mechanism. Observed activation energies for permeation were larger for membranes with a smaller pore size, and were considerably larger than the activation energy for water viscosity. The organosilica membranes exhibited exceptional hydrothermal stability in temperature cycles up to 90^°C. The applicability of the generalized solution‐diffusion (SD) model to RO and pervaporation (PV) desalination processes were examined, and the quantitative differences in water permeance were accurately predicted by the application of generalized transport equations. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1298–1307, 2013     

The preparation and characterization of hydrotalcite micromembranes

The focus in this study is on preparing hydrotalcite (HT) thin micromembranes and on understanding their transport characteristics. HT micromembranes were prepared on silicon wafers and on perforated stainless steel discs, using two different types of HT colloidal suspensions, and were characterized by various surface techniques, as well as with single-gas permeation tests. Electron microscopy indicates that the HT films are compact and dense, and well-adhering on the substrates. For the Si-based membranes, the presence of an intermediate γ-Al₂O₃ layer seemed to improve the characteristics of the deposited HT films. The study of the permeation properties of the HT micromembranes indicated ideal separation factors that exceeded the corresponding Knudsen values. Some of these membranes were also permselective towards CO₂. These HT micromembranes show good promise for application in sensor devices and in membrane microreactors.     

Vapor-phase polymerization of high-performance thin-film composite membranes for nanofiltration

Thin-film composite (TFC) membranes are commendable semipermeable barriers for water treatment. Although conventionally immiscible interfaces between aqueous and organic solutions are widely utilized for obtaining TFC membranes, interfacial polymerization still suffers from the issues of harmful solvents, complex diffusion/reaction of the reactants, and thermodynamic and kinetic instability of interfaces. In this study, vapor-phase polymerization with no requirements for organic solvent and immiscible interface is utilized for processing TFC nanofiltration membranes. Through cross-linking of β-cyclodextrin and piperazine layers by trimesoyl chloride vapor, polyester and polyamide TFC membranes with high cross-linking degree are simply prepared in a scalable and reproducible manner. The prepared TFC membranes exhibit stable nanofiltration and desalination performance for all water, organic solvent, and water–organic mixture systems, with permeance up to an order of magnitude higher than that of commercial membranes.     

Pervaporation characteristics and structure of poly(vinyl alcohol)/poly(ethylene glycol)/tetraethoxysilane hybrid membranes

Poly(vinyl alcohol) (PVA) blended with poly(ethylene glycol) (PEG) was crosslinked with tetraethoxysilane (TEOS) to prepare organic–inorganic PVA/PEG/TEOS hybrid membranes. The membranes were then used for the dehydration of ethanol by pervaporation (PV). The physicochemical structure of the hybrid membranes was studied with Fourier transform infrared spectra (FT‐IR), wide‐angle X‐ray diffraction WXRD, and scanning electron microscopy (SEM). PVA and PEG were crosslinked with TEOS, and the crosslinking density increased with increases in the TEOS content, annealing temperature, and time. The water permselectivity of the hybrid membranes increased with increasing annealing temperature or time; however, the permeation fluxes decreased at the same time. SEM pictures showed that phase separation took place in the hybrid membranes when the TEOS content was greater than 15 wt %. The water permselectivity increased with the addition of TEOS and reached the maximum at 10 wt % TEOS. The water permselectivity decreased, whereas the permeation flux increased, with an increase in the feed water content or feed temperature. The hybrid membrane that was annealed at 130°C for 12 h exhibited high permselectivity with a separation factor of 300 and a permeation flux of 0.046 kg m^?2 h^?1 in PV of 15 wt % water in ethanol. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Permeation properties of hydrogen and water vapor through porous silica membranes at high temperatures

Silica and cobalt‐doped silica membranes that showed a high permeance of 1.8 × 10^?7 mol m^?2 s^?1 Pa^?1 and a H₂/N₂ permeance ratio of ～730, with excellent hydrothermal stability under steam pressure of 300 kPa, were successfully prepared. The permeation mechanism of gas molecules, focusing particularly on hydrogen and water vapor, was investigated in the 300–500°C range and is discussed based on the activation energy of permeation and the selectivity of gaseous molecules. The activation energy of H₂ permeation correlated well with the permeance ratio of He/H₂ for porous silica membranes prepared by sol–gel processing, chemical vapor deposition (CVD), and vitreous glasses, indicating that similar amorphous silica network structures were formed. The permeance ratios of H₂/H₂O were found to range from 5 to 40, that is, hydrogen (kinetic diameter: 0.289 nm) was always more permeable than water (0.265 nm). © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Separation of Ethylene Glycol/Water Mixtures using NaA Zeolite Membranes

Inorganic membranes and particularly zeolite membranes are usually used for the dehydration of organic solvents by pervaporation (PV). This work reports an experimental study on the PV dehydration of ethylene glycol (EG)/water mixtures using commercial nanoporous NaA zeolite membranes. The concentration range investigated (C_EG > 70 wt %) was selected according to existing industrial requirements. The recirculation flow rate was kept at a value of 1.5 L/min. The fluxes and separation factors were monitored as the dehydration proceeded. In addition, the activation energy of permeation (E_a) was calculated. The effect of temperature was investigated in the range 50–70 °C. The results obtained demonstrated the successful performance of the membrane for the dehydration of EG/water mixtures. It was observed that at 70 °C and with 70 wt % initial EG concentration, larger fluxes and separation factors could be obtained, i.e., 0.94 kg m^–2h^–1 and 1177, respectively. The Pervaporation Separation Index (PSI) of the membrane was found to be high compared to that of polymeric membranes.     

Permeation and separation characteristics of acetic acid/water mixtures through poly(vinyl alcohol‐g‐itaconic acid) membranes by pervaporation,evapomeation, and temperature‐difference evapomeation

In this study, itaconic acid (IA) was grafted onto poly(vinyl alcohol) (PVA) with cerium(IV) ammonium nitrate as an initiator at 45°C. The grafted PVA was characterized with Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. IA‐grafted PVA membranes were prepared with a casting method, and the permeation and separation characteristics of acetic acid/water mixtures were investigated with pervaporation (PV), evapomeation (EV) and temperature‐difference evapomeation (TDEV) methods. The effects of the feed composition, operating temperature, and temperature of the membrane surroundings on the permeation rate and separation factor for the acetic acid/water mixtures were studied. The permeation rates in EV were lower than those in PV, whereas the separation factors were higher. With the TDEV method, the permeation rates decreased and the separation factors increased as the temperature of the membrane surroundings decreased. The prepared membranes were also tested in PV, EV, and TDEV to separate the various compositions of the acetic acid/water mixtures (20–90 wt % acetic acid) at 40°C. The highest separation factor, 686, was obtained in TDEV with a 90 wt % acetic acid concentration in the feed. The activation energies of permeation in PV and EV were calculated to be 8.5 and 10.2 kcal/mol, respectively, for a 20 wt % acetic acid solution. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2322–2333, 2004     

Gas permeation through water‐swollen polysaccharide/poly(vinyl alcohol) membranes

The effect of Na‐alginate content on the gas permeation properties of water‐swollen membranes prepared by varying Na‐alginate and poly(vinyl alcohol) (PVA) content in membranes was investigated. The influences of water content and crystallinity of the membranes on the gas permeation performance of the water‐swollen membranes were studied. The gas permeation rate and selectivity of Na‐alginate/PVA water‐swollen membranes were compared with those of the dry membranes. The permeation rates of nitrogen and carbon dioxide through water‐swollen membranes were in the range of 0.4–7.6 × 10^?7 to 3.7–8.5 × 10^?6 cm³ (STP)/cm² s^?1 cmHg^?1, which were 10,000 times higher than those of dry‐state membranes. The permeation rates of mixture gases through water‐swollen Na‐alginate/PVA membranes were found to increase exponentially with the increase of Na‐alginate content, whereas carbon dioxide concentration in permeates was decreased linearly. It was found that the gas permeance of the water‐swollen membranes increased with increasing the Na‐alginate content in the membrane. Gas permeation rates of the water‐swollen Na‐alginate/PVA membranes increased with increasing the water content in the membrane and decreasing the crystallinity of the membrane. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3225–3232, 2004     

Tunable physicochemical properties of PVA nanocomposite membranes for enhanced pervaporation performance

This study reveals the effect of hydrophilic bentonite nanoclay on the pervaporation separation of azeotropic composition of water and 1,4‐dioxane. The permselectivity of the membrane increased with filler concentration and was selective toward water at minimum filler loading. The intrinsic properties such as membrane permeance and selectivity increased with the concentration of hydrophilic bentonite nanoclay and crosslinked poly(vinyl alcohol) (PVA) with 2 wt% nanoclay membranes showed intrinsic selectivity 443 (532% increment than that of pristine membranes) with water permeance 4,675 gas permeation unit which is significantly higher compared to earlier literature. X‐ray diffraction and Transmission electron microscopy showed the well exfoliated and distributed nanoclay structure in the crosslinked PVA matrix. Interaction of PVA with nanoclay and the hydrophilic character of the membranes were characterized by Fourier transform infrared spectra and contact angle analysis, respectively. Interestingly, in this work the membranes exhibited simultaneous increment in both permeation flux and selectivity with filler loading, rather than the usual inverse trend of flux and selectivity. A predictive model of pervaporation was used to explain the pervaporation behavior and it showed good agreement with experimental results for overall pervaporation performance, preferential sorption of water, and hydrophilic‐hydrophobic nature of the membranes. POLYM. ENG. SCI., 58:849–858, 2018. © 2017 Society of Plastics Engineers     

Gas permeation properties for organosilica membranes with different Si/C ratios and evaluation of microporous structures

Organosilica membranes were fabricated using bridged organoalkoxysilanes (bis(triethoxysilyl)methane (BTESM), bis(triethoxysilyl)ethane (BTESE), bis(triethoxysilyl)propane (BTESP), bis(trimethoxysilyl)hexane (BTMSH), bis(triethoxysilyl)benzene (BTESB), and bis(triethoxysilyl)octane (BTESO)) to produce highly permeable molecular sieving membranes. The effect of the organoalkoxysilanes on network pore size and microporous structure was evaluated by examining the molecular size and temperature dependence of gas permeance across a wide range of temperatures. Organosilica membranes showed H₂/N₂ and H₂/CH₄ permeance ratios that ranged from 10 to 150, corresponding to network pore size, and both H₂ selectivity decreased with an increase in the carbon number between 2 Si atoms. Organosilica membranes showed activated diffusion for He and H₂, and a slope of temperature dependence that increased approximate to the increase in the carbon number between 2 Si atoms. The relationship between activation energy and He/H₂ permeance ratio for SiO₂ and organosilica membranes suggested that the molecular sieving can dominate He and H₂ permeation properties via the rigid microporous structure, which was constructed by BTESM and BTESE. With increased in the carbon concentration in silica, polymer chain vibration in organic bridges, which is a kind of solution/diffusion mechanism, can dominate the permeation properties. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4491–4498, 2017     

An improved method for surface modification of porous water purification membranes

The surfaces of polysulfone and polyethersulfone ultrafiltration membranes were coated with polydopamine, yielding hydrophilic membranes that, under constant transmembrane pressure fouling conditions, have previously shown enhanced flux relative to unmodified membranes. When evaluated under constant permeate flux fouling, however, modified membranes exhibited higher transmembrane pressures than their unmodified analogs. This increased transmembrane pressure in the coated membranes was ascribed to the decrease in membrane permeance resulting from applying the polydopamine coating. The membrane permeance could be tuned by varying polydopamine deposition time and, even at the shortest deposition times studied here, a few minutes, a substantial increase in membrane hydrophilicity could be achieved. Therefore, polydopamine was deposited on a membrane of relatively high permeance until the pure water permeance of the modified membrane matched that of a membrane having lower native permeance, permitting a comparison of the fouling performance of a modified and unmodified membrane with the same pure water permeance. This approach was repeated, using a single, high permeance membrane as the base membrane for modification, to produce a family of modified membranes having the same initial pure water permeances as lower permeance, unmodified membranes. When unmodified and modified membranes of the same initial permeance were compared at constant flux fouling conditions, the modified membranes consistently exhibited lower transmembrane pressures and similar organic rejections to the unmodified membranes. Because many porous water purification membranes are operated at constant flux in industrial settings, an interesting methodology for membrane surface modification may be to surface-modify a membrane of high permeance until the desired permeance is achieved, rather than by surface modification of a membrane that natively has the desired water transport characteristics, since the surface modification procedures almost invariably lead to lower pure water permeance.     

Polyphosphazene membrane separations—Review

The further growth of the success of membrane based operations in unusual areas depends on the development of new membranes with tunable properties and or capable to withstand severe thermal, chemical and mechanical environments. Poly(organophosphazene)s (POPs) may give an important contribution for the preparation of such new membranes. In this paper the applications of POP materials in ultrafiltration (UF), nanofiltration (NF), pervaporation (PV), vapor permeation (VP) and gas separation (GS) are reviewed, and some perspectives for future developments are outlined. Interesting results obtained with POP UF membranes indicate that they might be applied in the treatment of organic solvents or aggressive streams or also for the construction of membrane contactors. However, the versatile and tunable properties of POPs can be fully exploited in membranes whose transport and separation mechanisms rely on differences in solubility and mobility of the feed species to be separated (i.e., NI, PV, VP, and GS). POP based NF membranes have been used for water potabilization and the separation of organic dies fromi-PrOH. The research work carried out in industrial and academic laboratories, sponsored also by the US Department of Energy, has probably passed the turning point for the production of commercial PV and VP POP membranes for the separation of organies and or water from liquid and gaseous streams. Good results were obtained in the separation of acidic species (SO₃, H₂S, CO₂) from permanent gases with dense POP membranes. In perspective, the availability of new hybrid POP-inorganic materials makes it possible to bridge the gap between polymer and ceramic membranes, whose appealing capabilities are still to be explored. The outstanding versatile properties of POP can also be used for the fixation of catalytic centres on suitable membranes for the preparation of catalytic membrane reactors.     

A systematic study on the effects of synthesis conditions of polyamide selective layer on the CO2/N2 separation of thin film composite polyamide membranes

Different top layer fabrication methods (amine-first, acid-first, spin coating), organic phase solvents (hexane, heptane, mixed hexane/heptane), acid acceptors (triethylamine, sodium carbonate, sodium hydroxide), and surfactant sodium dodecyl sulfate concentrations (0, 0.05, and 0.1 wt%) were utilized to fabricate thin film composite polyamide membranes for CO₂/N₂ separation. The results, according to an L9 orthogonal array of Taguchi approach, showed that employing acid-first method increases both CO₂ permeance and CO₂/N₂ selectivity of the membranes at a feed gas pressure of 3 bars. On the other hand, sodium hydroxide, and triethylamine should be used, as acid acceptors, to maximize CO₂ permeance and CO₂/N₂ selectivity, respectively. Moreover, the use of hexane solvent and 0 wt% surfactant led to maximum permeance, while, hexane solvent and 0.1 wt% surfactant were needed to reach the highest selectivity. The above level setting of synthesis parameters also resulted in the minimum sensitivity of the fabrication process to the noise factors effects. As shown by the analysis of variance, acid acceptor, and organic solvent types were the most influential parameters on CO₂ permeance and CO₂/N₂ selectivity, respectively. The effects of fabrication method and surfactant concentration, as single factors, on permeation/selectivity responses were also investigated.