Dual-layer Hollow Fiber Membranes Research Articles

Enhancing antibacterial performances of PVDF hollow fibers by embedding Ag-loaded zeolites on the membrane outer layer via co-extruding technique

Ag-loaded zeolites were synthesized and embedded into the outer layer of poly(vinylidene fluoride) (PVDF) dual-layer hollow fiber membrane (D) using a dry-jet wet-spinning co-extruding technique. The distribution of the Ag-loaded zeolites in the outer layer was confirmed by field emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDX). It was found that the D contained only 54% of the amount of Ag-loaded zeolites that were distributed throughout the cross section of PVDF single-layer hollow fiber membrane (S), as calculated by thermogravimetric analysis (TGA). However, D showed excellent antibacterial efficiency and resistance to bacterial adhesion because of the higher concentration of Ag+ in the outer layer of the membrane. The surface morphologies, pure water flux, mean pore size, pore size distribution, and thermal stability of both PVDF membranes were also investigated. (C) 2014 Elsevier Ltd. All rights reserved.

Dual layer composite nanofiltration hollow fiber membranes for low-pressure water softening

Nanofiltration (NF) membrane process has become increasingly attractive due to their unique characteristics to selectively remove specific compounds or ions. The most commonly NF membranes are negatively charged which is unsuitable for hardness removal. Therefore, the development of novel NF membranes with a positively charged skin has become a key issue for low pressure water softening.In this study, dual layer microporous hollow fiber membranes were fabricated using Torlon poly(amide-imide) (PAI) as the material of the selective outer layer and polyethersulfone (PES) as the material of the porous support inner layer. A positively charged NF-like selective layer was developed by a simple polyelectrolyte cross-linking using polyallylamine (PAAm). The newly developed (PAI–PAAm)–PES dual layer hollow fiber membranes show a salt water permeability of 15.8 L/m2 h bar and high Mg2+ and Ca2+ rejections of 94.2% and 92.3%, respectively, at operating pressure of 2 bar using a 3000 ppm TDS feed solution. This study provides a simple and effective approach to produce positively charged NF hollow fiber membranes for water softening applications with low energy consumption.

High-performance sulfonated polyimide/polyimide/polyhedral oligosilsesquioxane hybrid membranes for ethanol dehydration applications

A novel miscible polymer blend of copoly(1,5-naphthalene/3,5-benzoic acid-2,2′-bis(3,4-dicarboxyphenyl)) hexafluoropropanedimide 6FDA-NDA/DABA and its sulfonated polyimide has been investigated as a water-selective layer to develop dual-layer hollow fiber membranes for ethanol dehydration via pervaporation. The resultant fibers display an excellent flux of >3.0kg/m2h. This is possibly ascribed to a good distribution of sulfonic groups in the polymer matrix, the increased hydrophilicity and enlarged d-space induced by sulfonic groups, which are verified by polarized light microscopy (PLM), contact angle measurement, sorption tests and XRD. Three different post-treatment approaches, namely, thermal treatment, PDMS coating and modification with polyhedral oligosilsesquioxane (POSS) were employed to improve the membrane selectivity. Their effects on polymer chain packing, dense skin layer thickness and ethanol dehydration performance have been investigated with the aid of the advanced PALS technique. Thermal treatment can densify the selective layer structure while PDMS coating provides a thin shielding layer to protect the selective layer from feed-induced swelling. More importantly, POSS modification displays as the most effective way to enhance the membrane selectivity due to the anti-swelling property of POSS inorganic nanoparticles, mobility restriction of polymer chains and narrowed d-space in polymer structure. Compared to various membranes in the literature, the newly developed hollow fibers in the present study have superior fluxes of >2.0kg/m2h and good separation factor of >200 for ethanol dehydration. This study may provide useful insights for the exploration of new materials and post-modifications for biofuel separations.

The development of high-performance 6FDA-NDA/DABA/POSS/Ultem® dual-layer hollow fibers for ethanol dehydration via pervaporation

A dual-layer hollow fiber membrane consisting of copoly(1,5-naphthalene/3,5-benzoic acid-2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropanedimide) (6FDA-NDA/DABA)/polyhedral oligomeric silsesquioxane (POSS) as the outer functional layer and polyetherimide (Ultem®) as the inner support layer has been developed for ethanol hydration via pervaporation. By using lower dope flow rates and optimizing inner dope concentration, the newly developed hollow fiber membranes have desirable morphology consisting of a porous inner layer with low transport resistance and a macrovoid-free selective layer for a superior separation performance. Doppler broadening energy spectra (DBES) have been employed to analyze the depth profile of morphological evolution and examine the thickness of dense selective layer as a function of spinning conditions. Incorporating a small amount (2wt%) of POSS into the selective layer increases its free volume size and diffusion selectivity and hence improves the pervaporation performance of the hybrid membranes. The diffusion mechanism of permeate molecules through polymer chains containing POSS nanoparticles was proposed and verified via FTIR spectra, XRD curves, positron annihilation lifetime (PAL) characterization and vapor sorption tests. Compared to other hollow fiber membranes, the newly developed hollow fibers have a superior flux of 1.9kg/m2h and a comparable separation factor of 166 for ethanol dehydration. The work may open up a new perspective for the development of next-generation high-performance dual-layer hollow fiber membranes with a POSS embedded selective layer for biofuel separation.

POSS-containing delamination-free dual-layer hollow fiber membranes for forward osmosis and osmotic power generation

Novel mixed matrix hollow fiber membranes composed of a PBI/POSS outer layer and a PAN/PVP inner layer have been developed for forward osmosis and osmotic power generation. It is found that the incorporation of a small amount of POSS nanoparticles into the outer PBI dope has significant influence on both the morphology and the performance of the developed membranes. The addition of POSS and PVP into the outer PBI and inner PAN dopes respectively resulted in an integrally macrovoid-free and delamination-free dual-layer membrane. Increasing POSS concentration in the PBI dope enhances both water and salt permeability across the membranes. A POSS loading of 0.5wt% has been identified through NF and FO tests as the optimal concentration in this study. The membrane with this optimized concentration shows a maximum water flux 31.37LMH at room temperature using 2.0M MgCl2 as the draw solution in the FO process and a maximum power density of 2.47W/m2 in the PRO process at 7bar using 1.0M NaCl as the draw solution. With its unique fully hydrophilic structure, easy processability and cost-effective ultra-thin PBI outer-layer, this membrane may have wide applications in the future.

Understanding of low osmotic efficiency in forward osmosis: Experiments and modeling

This study discloses the critical factors that result in the low osmotic efficiency in forward osmosis (FO). Specifically, dual-layer hollow fiber membranes are prepared with newly-synthesized cellulose acetate propionate (CAP) as the outer active layer and commercial cellulose acetate (CA) as the inner sublayer. By carefully analyzing the hollow fiber cross section images, the porosity of the sublayer is found to be nonuniformly distributed at different locations. Viewing the membrane matrix as three consecutive layers, i.e., the active layer, the sublayer and the interface between them, the draw solute concentration profiles within each layer, the osmotic pressure gradients across each layer, and the transport resistance of each layer are determined. One interesting observation is that the active layer of the CAP-CA hollow fibers creates much larger resistance than the interface and the sublayer, indicating that the low osmotic efficiency (i.e., low water flux) is mainly due to the low water permeability of the active layer while internal concentration polarization (ICP) within the sublayer is less important. For any membranes, the active layer–sublayer interface also creates certain transport resistance. These findings provide a valuable reference for the understanding of FO and the design of advanced FO membranes.

Fabrication of poly(amide-imide)-polyethersulfone dual layer hollow fiber membranes applied in forward osmosis by combined polyelectrolyte cross-linking and depositions

Delamination-free ultrafiltration (UF) dual layer hollow fibers have been successfully fabricated by dry jet-wet spinning technique. Two different polymers of poly(amide-imide) (PAI) and polyethersulfone (PES) were utilized as the selective layer on the outer side and the porous support layer on the inner side, respectively. The dual layer substrates were subsequently modified to develop a nanofiltration (NF)-like skin on the outer layer by polyethyleneimine (PEI) cross-linking followed by multilayer polyelectrolyte depositions. These membranes were characterized by a series of standard protocols in terms of membrane structure, permeability and salt rejection, and were applied in FO process.Experiments reveal that the dual-layer NF hollow fiber modified by polyelectrolyte cross-linking and depositions can achieve pure water permeability of 4.1l/m2.h.bar and a high rejection to MgCl2 up to 97%. In FO process, the dual-layer hollow fibers exhibited a water flux of 39 and 21lm−2h−1 in the orientations of active layer facing draw solution and feed water, respectively, by using 0.5M MgCl2 as draw solution and de-ionized (DI) water as feed at room temperature. It suggests that the combined treatment of polyelectrolyte cross-linking and deposition is an effective way to form a NF-like selective layer for FO dual-layer hollow fiber membranes.

Natural gas purification and olefin/paraffin separation using cross-linkable dual-layer hollow fiber membranes comprising β-Cyclodextrin

In this study, thermally cross-linkable co-polyimide dual-layer hollow fiber membranes grafted with β-Cyclodextrin for separation of CO2/CH4 and propylene/propane have been fabricated. In order to find the best spinning condition, the performance of hollow fiber membranes at various take-up velocities and outer-layer dope flow rates was investigated. The fiber membranes were thermally cross-linked at different temperatures and the performance of the fibers before and after silicon rubber coating was studied using CH4, CO2, propane and propylene. It was observed that permeances of all gases decreased with an increase in take-up velocity and outer-layer dope flow rate. Selectivities of the membrane with respect to the take-up velocity initially increased and after a take-up velocity value of 7.4m/min started to decrease. This up and down trend was attributed to the influence of elongational draw ratio and change in surface porosity of the membrane. Optimum take-up velocity and outer-layer dope flow rate for as-spun fibers were 7.4m/min and 0.5ml/min, respectively. These conditions resulted in CO2/CH4 selectivities of 6.22 and 14.3 before and after silicon rubber coating, respectively. The results demonstrated that thermal treatment improves membrane selectivities and decreases membrane permeances. The enhancement of selectivities can be a result of cross-linking and reduction in permeances due to densification of the hollow fiber membranes. Selectivities of thermally treated fiber membranes at 350°C were slightly higher than those of the precursor fibers and this improvement was more significant for membranes treated at 400°C. This enhancement demonstrates that cross-linking is more severe at 400°C. The best separation performance of the annealed and silicone rubber coated hollow fibers in this study has a CO2 permeance of around 82GPU with a CO2/CH4 ideal selectivity of around 20 and a high C3H6 permeance of around 29GPU with a C3H6/C3H8 ideal selectivity of 15.3. It can also resist CO2 induced plasticization until 25atm. It is believed that, with these gas separation and anti-plasticization properties, the newly developed membranes may have high prospective for natural gas purification and olefin/paraffin separation.

Explorations of delamination and irregular structure in poly(amide-imide)-polyethersulfone dual layer hollow fiber membranes

Dual layer hollow fiber membranes have become increasingly attractive due to their many benefits made possible by one-step fabrication using the non-solvent induced phase inversion method. Obtaining a good lamination between the two layers as well as a regular morphology are critical to make a usable hollow fiber membrane. This study aims to explore the mechanism of the lamination/delamination phenomenon and regular/irregular morphology that can occur in the fabrication of dual layer hollow fiber membrane. Simultaneous extrusion of poly(amide-imide) and polyethersulfone (PES) dope solutions was carried out by using a triple orifice spinneret. Thermodynamic properties and phase separation kinetics of the polymer dopes as well as various spinning parameters were carefully tailored in order to investigate the evolution of the membrane morphology and structure. A series of experiments have confirmed that when the external coagulant, water, has a higher diffusion rate in the outer layer than in the inner layer, the outer layer tends to expand to form large macrovoids and to hold more water at the interface. As a result, the accumulated water probably impedes the adhesion of the two layers, leading to a delamination of two layers. On the other hand, if water has a slower penetration rate through the outer layer dope than the inner layer dope, a good adhesion between these two layers is expected. Under this scenario, since macrovoids formed in the inner layer may lead to the expansion of the inner layer, a distortion of the finger-like structure/macrovoids in the inner and an irregularity of the inner contour may occur. This study provides a solid foundation to develop superior dual layer hollow fiber membranes with an inter-penetrating dual layer structure.

Novel dual-layer hollow fiber membranes applied for forward osmosis process

Abstract A novel dual-layer forward osmosis (FO) hollow fiber membrane has been designed and successfully fabricated by using a triple orifice spinneret. The fiber consists of two layers made from polyamide-imide (PAI) polymer for the outer layer and polyethersulfone (PES) polymer for the porous inner layer. Specifically, after obtaining asymmetric microporous PAI/PES dual-layer hollow fibers via non-solvent induced phase inversion, polyethyleneimine (PEI) polyelectrolyte modification on the outer PAI layer was applied to produce a nanofiltration (NF)-like thin layer, while the PES porous inner layer remained intact as PES is inert to PEI. The membrane morphology, structure and surface property were carefully tailored by adjusting polymer dope composition and spinning conditions. These membranes were subsequently characterized by a series of standard protocols in terms of membrane structure, permeability and salt rejection, and were utilized in FO process. It was found that the resultant dual-layer NF hollow fiber membrane can achieve pure water permeability of 15.9 l m −2 h −1 bar −1 and a high rejection to divalent cations up to 89%. In FO process, the dual-layer hollow fiber exhibited a water flux of 27.5 l m −2 h −1 in the orientation of active layer facing feed water by using 0.5 M MgCl 2 as draw solution and de-ionized (DI) water as feed at room temperature. The newly developed dual layer hollow fibers outperform all the single layer and dual-layer NF hollow fibers reported in the literature for FO applications.

Effects of separation layer thickness on oxygen permeation and mechanical strength of DL-HFMR-ScSZ

It has been demonstrated in our previous studies that in order for greater methane conversion and less coke-formation, a higher oxygen permeation rate through the outer oxygen separation layer of a functional dual-layer ceramic hollow fibre membrane is needed. Besides new membrane materials with higher oxygen permeability, another way of improving oxygen permeation is to reduce the separation layer thickness, although this strategy is limited by the characteristic thickness, Lc, where bulk diffusion and surface oxygen exchange are both important. As a result, a series of La0.80Sr0.20MnO3−δ (LSM)-Scandia(10%)-Stabilized-Zirconia (ScSZ)/ScSZ-NiO functional dual-layer hollow fibres (DL-HF) with an outer oxygen separation layer thickness between approximately 8.0 and 72.4 μm were fabricated in this study, by using the single-step co-extrusion and co-sintering process. The effects of separation layer thickness on oxygen permeation and mechanical strength were investigated. The oxygen permeation of the LSM-ScSZ separation layer is more likely to be controlled by surface exchange at higher temperatures, and changes to mixed control by both bulk diffusion and surface exchange at lower temperatures. A thicker separation layer also results in a thinner catalytic substrate layer, and subsequently decreases the mechanical strength of the dual-layer hollow fibre membrane.

Novel thin-film composite nanofiltration hollow fiber membranes with double repulsion for effective removal of emerging organic matters from water

A novel thin-film composite nanofiltration (NF) membrane was fabricated by interfacial polymerization of hyperbranched polyethyleneimine (HPEI) and isophthaloyl chloride (IPC). For the first time, a dual-layer hollow fiber membrane was employed as the substrate for interfacial polymerization. The substrate possesses a unique cross-section structure comprising a layer full of macrovoid located in the middle and sandwiched by two thin spongy-like layers. Such a sandwich-like sublayer structure provides minimal transport resistance and sufficient mechanical strength for water permeation under high pressures. After interfacial polymerization, the NF membrane possesses a negatively charged substrate and a positively charged selective layer with a mean effective pore radius of 0.36 nm, molecular weight cut off of 500 Da, and pure water permeability of 4.9 l m−2 bar−1 h−1. Due to this double-repulsion effect, together with the steric-hindrance and the solute electro-neutrality effects, the newly developed NF membrane shows superior rejections (over 99%) for both positively and negatively charged dye molecules. By adjusting the pH of cephalexin aqueous solution to modify the ionization states of this zwitterionic molecule, the NF membrane shows high rejections over a wide pH range. The NF membrane may potentially be useful to reduce waste, recycle valuable products and reuse water for textile, pharmaceutical and other industries.

PVDF/Nanosilica Dual-Layer Hollow Fibers with Enhanced Selectivity and Flux as Novel Membranes for Ethanol Recovery

In this work, we have demonstrated the design and engineering of poly(vinylidene fluoride) (PVDF)/nanosilica dual-layer hollow fibers as novel pervaporation membranes for ethanol recovery. The newly developed dual-layer hollow fiber membrane can exhibit a high separation factor of up to 29 with a sustainable high flux of 1.1 kg m–2 h–1, which is equivalent to the separation performance regime of inorganic membranes. Central to this performance achievement is the synergy of (1) desirable membrane morphology, nanopore size, and high surface porosity of a thin-PVDF/nanosilica composite on a fully porous substrate accomplished by the dual-layer coextrusion technology, and (2) optimal operating downstream pressure with the aid of controlled pervaporation transport. The membrane selectivity-downstream pressure dependence of PVDF/nanosilica hybrid membranes is comprehensible via a modified pore-flow model. This study may represent a new class of membranes for ethanol–water separation.

Development and positron annihilation spectroscopy (PAS) characterization of polyamide imide (PAI)–polyethersulfone (PES) based defect-free dual-layer hollow fiber membranes with an ultrathin dense-selective layer for gas separation

► Defect-free dual-layer hollow fiber membranes with ultrathin dense-selective layer. ► O 2 /N 2 selectivity of 7.73 with a dense-selective layer thickness of 63 nm. ► PAS to explore the morphology and predict the gas performance. ► PAS estimate the outer-layer thickness and demonstrate the layered structure. ► PAS fitted data agrees well with the experimental result. Defect-free polymeric dual-layer hollow fiber membranes consisting of an ultrathin dense-selective polyamide imide (PAI) layer and a polyethersulfone (PES) supporting layer have been successfully fabricated in this study for gas separation application. It is observed that a lower outer-layer dope flow rate does not necessarily result in the formation of an ultrathin dense-selective layer upon the PES supporting layer. An optimization in the velocity between the inner-layer and the outer-layer dopes at the exit of the spinneret is essential to minimize additional stresses and defect formation in the outer functional layer. The best gas separation performance of the PAI–PES dual-layer hollow fibers fabricated in this study has an O 2 /N 2 selectivity of 7.73 with a dense-selective layer thickness of 63 nm. Positron annihilation spectroscopy (PAS) has been used for the first time to explore the morphology and predict the gas separation performance of PAI–PES based dual-layer hollow fiber membranes. Doppler broadening energy spectra (DBES) from PAS accurately estimate the outer-layer thickness and demonstrate the existence of the multilayered structure of the dual-layer hollow fiber membranes. Besides, the PAS fitted data reveal that the fiber spun under the optimal condition has the densest selective layer, which agrees well with the highest gas-pair selectivity observed under this condition.

Processing and engineering of pervaporation dehydration of ethylene glycol via dual-layer polybenzimidazole (PBI)/polyetherimide (PEI) membranes

Operating conditions play a significant role in determining the separation performance of a pervaporation process, because they not only manipulate the driving forces to transport permeants but also affect the physicochemical properties of the pervaporation membrane itself. In this study, fundamental governing equations have been derived to correlate separation performance with system operation conditions and intrinsic separation characteristics of the pervaporation membrane. Polybenzimidazole/polyetherimide (PBI/PEI) dual-layer hollow fiber membranes were chosen to study the pervaporation dehydration of ethylene glycol (EG) under different testing protocols. The effects of operational parameters such as operation temperature, permeate pressure, feed composition and operation duration on performance indicators (flux and separation factor, permeance and selectivity) have been investigated. Experimental results show that an increase in operation temperature results in an increase in flux and selectivity, but a decrease in permeance and separation factor. In addition to other factors, decreasing sorption, less EG–water clusters and lower membrane-EG affinity with increasing temperature, play essential roles for the opposite trends. Both flux and permeance decrease with an increase in permeate pressure, while both separation factor and selectivity have an up-and-down trend. An increase in EG composition in the feed from 50 to 90 wt.% results in a lower water flux and permeance, but EG flux and permeance first increase and then decrease. This is due to the combined effect of water-induced membrane swelling and the formation of an EG boundary layer upon the membrane surface. The long-term test up to 33 days proves the membrane durability for EG dehydration. This work may provide useful insights to pervaporation fundamentals, system design and scale up for the EG dehydration.

Functional LSM–ScSZ/NiO–ScSZ dual-layer hollow fibres for partial oxidation of methane

In this study, a functional La 0.80Sr 0.20MnO 3−δ (LSM)–Scandia-Stabilized-Zirconia (ScSZ)/NiO–ScSZ dual-layer hollow fibre has been developed using a single-step co-extrusion and co-sintering process, and has been employed as a dual-layer hollow fibre membrane reactor for partial oxidation of methane. Oxygen permeation rate between 0.49 and 1.82 ml/min and methane conversion between 53.55% and 98.78% have been achieved when operating temperature is elevated from 920 to 1060 °C, together with a significant reduction in coke-deposition. Oxygen permeation through the outer LSM–ScSZ permeation layer (approximately 109.2 μm in thickness) is found to be the controlling step to methane conversion at the operating temperature below 990 °C, above which the excessive oxygen permeation results in formation of CO 2 and H 2O as by-products. The experimental results further suggest that the amount of NiO in the inner NiO–ScSZ layer should be optimised based on the factors such as catalytic activity/stability, porosity and mechanical strength in addition to the sintering behaviour which has to be matched to the outer LSM–ScSZ layer.

Effect of inner-layer thermal conductivity on flux enhancement of dual-layer hollow fiber membranes in direct contact membrane distillation

The thermal conductivity of hydrophobic/hydrophilic hollow fiber membranes is a key factor affecting flux in direct contact membrane distillation processes. In this paper both experimental work and mathematical modeling are performed to investigate the effect of the hydrophilic layer's thermal conductivity on vapor flux. Modeling predicts a significant increase in vapor flux from 31.4 to 78.5 kg m −2 h −1 under the feed and distillate temperatures of 80 and 20 °C, respectively, when the thermal conductivity of the inner hydrophilic layer is varied from 0.2 to 1.4 W m −1 K −1. To investigate this experimentally, graphite particles and multiwall carbon nanotubes (MWNT) were embedded for the first time into the hollow fiber hydrophilic layer to improve its thermal conductivity. It was found that incorporating graphite alone led to only a minor improvement in thermal conductivity. However, by incorporating both graphite and MWNT the thermal conductivity was increased from 0.59 to 1.30 W m −1 K −1. This improvement is attributed to the conducting network formed by the MWNT which bridges the polymer nodules. The improved thermal conductivity led to a significant increase in vapor flux from 41.2 to 66.9 kg m −2 h −1, under the inlet feed and distillate temperatures of 80.4 and 15.3 °C, respectively, in general agreement with the mathematical modeling.

Novel polyamide-imide/cellulose acetate dual-layer hollow fiber membranes for nanofiltration

A novel dual-layer nanofiltration (NF) hollow fiber membrane was fabricated by the simultaneous co-extrusion of Torlon ® polyamide-imide and cellulose acetate dopes through a triple-orifice spinneret in a dry-jet wet phase inversion process. For the first time, the nanopores of dual-layer hollow fiber membranes were molecularly designed by controlling the phase inversion process with the aid of various non-solvent additives into the polymer solutions. Compared to ethanol and 2-propanol, the addition of methanol into the dope led to a significantly decreased pore size but dramatically increased pure water permeability. The improved NF performance may be attributed to (1) a controllable thin selective outer layer; (2) a less resistant interface between the outer and inner layers; and (3) a fully porous substructure with reduced transport resistance. In addition to non-solvent additives, spinneret temperature also plays an important role in designing dual-layer hollow fiber membranes with desirable NF performance. When the spinneret temperature was increased from 25 °C to 50 °C, the mean effective pore radius and the pure water permeability were simultaneously decreased, which was due to the formation of a denser surface skin and a more compact interface between the two layers. In addition to exhibiting a higher rejection of divalent anions than monovalent anions, and a lower rejection of divalent cations at pH 7.0, the newly developed NF dual-layer hollow fiber membranes with methanol as additive has a relatively high pure water permeability of 11.93 l m −2 bar −1 h −1 with a mean effective pore radius of 0.63 nm. These concepts hold great potential for the design of tailor-made NF membranes for various industrial applications.

Polyimide/Polyethersulfone Dual-Layer Hollow Fiber Membranes for Hydrogen Enrichment

We have developed functional poly(1,5-naphthalene-2,2′-bis(3,4-phthalic)hexafluoropropane diimide (6FDA-NDA)/polyethersulfone (PES) dual layer hollow fiber membranes and studied the H2/CO2 separation performance with and without 1,3-diaminopropane (PDA) modification. The effects of air gap on the membrane gas transport properties are investigated and it is observed that the optimal air gap for maximizing the ideal permselectivity is strongly dependent on the kinetic diameters of gas pair molecules. A higher air gap diminishes the population of Knudsen pores in the apparently dense outer skin and induces greater elongational stresses on polymer chains. The latter enhances polymer chain alignment and packing which possibly results in the shift and sharpening of the free volume distribution. The diamine modification of 6FDA-NDA/PES dual layer hollow fiber membranes effectively enhances the H2/CO2 permselectivity and suppresses the CO2-induced plasticization. At higher temperatures, the H2 permeance of the PD...

Pervaporation dehydration of ethylene glycol through polybenzimidazole (PBI)-based membranes. 1. Membrane fabrication

Abstracts Ethylene glycol is an important commodity in chemical industries and dewatering is a critical process in the production and recycle of ethylene glycol. In this work, we have developed dual-layer polybenzimidazole/polyetherimide (PBI/PEI) hollow fiber membranes for ethylene glycol dehydration via pervaporation. Three types of membranes have been prepared; namely, (1) PBI flat dense membranes; (2) PBI single-layer hollow fiber membranes; and (3) PBI/PEI dual-layer hollow fiber membranes. PBI flat dense membranes have the lowest separation performance due to severe swelling. PBI single-layer hollow fiber membranes show better separation performance in terms of permeation flux and separation factor but have very low tensile strains. The dual-layer PBI/PEI hollow fiber membranes have the best separation performance due to (1) unique combination of the superior physicochemical properties of the PBI selective layer and the less swelling characteristics of the PEI supporting layer, and (2) synergistic effects of molecularly designed membrane morphology via dual-layer co-extrusion. The effects of spinning parameters of PBI single-layer and PBI/PEI dual-layer hollow fiber membranes on pervaporation performance have been investigated. A thermal treatment of PBI/PEI dual-layer hollow fiber membranes at 75 °C can significantly enhance the separation performance. Compared with other polymeric membranes, the newly developed PBI/PEI dual-layer hollow fiber membranes have much better separation factors and slightly lower fluxes for the ethylene glycol dehydration. It is believed that the science and engineering of designing PBI/PEI dual-layer hollow fiber membranes with an ultra-thin functional separation layer and a synergic supporting layer may open new perspective for the development of next-generation high-performance multilayer membranes for liquid separations.