Dual-layer Hollow Fiber Research Articles

Microstructure tailoring of YSZ/Ni-YSZ dual-layer hollow fibers for micro-tubular solid oxide fuel cell application

YSZ/NiO-YSZ dual-layer hollow fibers with a thin YSZ top layer integrated on a porous NiO-YSZ (60:40 in weight) support, have been developed by one step method via a co-spinning-sintering process. Hydrogen reduction was performed to form YSZ/Ni-YSZ micro tube as the half solid oxide fuel cells (SOFCs). The microstructure of the dual-layer hollow fibers was tailored by adding ethanol as non-solvent in the initial mixture dopes for NiO-YSZ anode spinning. LSM cathode containing 20 wt%-YSZ was deposited on the electrolyte surface by dip-coating method to fabricate micro-tubular SOFCs. Experimental results indicate that the dual-layer hollow fibers from the anode dopes containing 15–20 wt% of ethanol possess the desired microstructure with optimized properties, such as the bending strength of 180 MPa, the porosity of 38–35% and the conductivity of 3000 S cm−1 at room temperature. The micro-tubular SOFCs fabricated from such hollow fibers show a maximum power density up to 485 mW cm−2 at 850 °C with 20 mL min−1 of H2 as fuel and 30 mL min−1 air as oxidant, respectively.

Fabrication of Y2O3-stabilized-ZrO2(YSZ)/La0.8Sr0.2MnO3−α–YSZ dual-layer hollow fibers for the cathode-supported micro-tubular solid oxide fuel cells by a co-spinning/co-sintering technique

Y2O3-stabilized-ZrO2(YSZ)/La0.8Sr0.2MnO3-α(LSM)–YSZ dual-layer hollow fibers have been fabricated for micro-tubular solid oxide fuel cells (MT-SOFCs) using a co-spinning/co-sintering technique. The hollow fibers consist of a dense YSZ top layer supported on the porous LSM–YSZ substrate with an asymmetric structure. The electrolyte and the cathode layers are perfectly adhered to each other without observable elemental inter-diffusion. Both the mechanical strength of the hollow fibers and the compactness of the electrolyte layer are improved by increasing the sintering temperature, whereas the effective porosity of the cathode layer also decreases. The suitable sintering temperature of the YSZ/LSM–YSZ dual-layer hollow fibers to construct the electrolyte/cathode half-cells should be controlled between 1350 °C and 1400 °C. An Ni-YSZ layer of about 5 μm in thickness is dip-coated on the outer surface of the hollow fibers to serve as the anode of fuel cells. The resultant micro-tubular SOFCs yield a maximum power density of 290 mW cm−2 at 850 °C using 1.49 × 10−5 mol s−1 hydrogen as fuel and 2.23 × 10−5 mol s−1 air as oxidant, respectively. The output performance of the micro-tubular fuel cells is mainly limited by the high polarization resistance of the cathode.

Dual layer hollow fiber sorbents for trace H2S removal from gas streams

Hollow fiber sorbents are pseudo monolithic materials with potential use in various adsorption based applications. Dual layer hollow fiber sorbents have the potential to allow thermal regeneration without direct contact of the regeneration fluid with the sorbent particles. This paper considers the application of dual layer hollow fiber sorbents for a case involving trace amounts of H2S removal from a simulated gas stream and offers a comparison with single layer hollow fiber sorbents. The effect of spin dope composition and core layer zeolite loading on the gas flux, H2S transient sorption capacity and pore structure are also studied. This work can be used as a guide to develop and optimize dual layer hollow fiber sorbent properties beyond the specific example considered here.

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.

Thermally moderated hollow fiber sorbent modules in rapidly cycled pressure swing adsorption mode for hydrogen purification

We describe thermally moderated multi-layered pseudo-monolithic hollow fiber sorbents entities, which can be packed into compact modules to provide small-footprint, efficient H2 purification/CO2 removal systems for use in on-site steam methane reformer product gas separations. Dual-layer hollow fibers are created via dry-jet, wet-quench spinning with an inner “active” core of cellulose acetate (porous binder) and zeolite NaY (69 wt% zeolite NaY) and an external sheath layer of pure cellulose acetate. The co-spun sheath layer reduces the surface porosity of the fiber and was used as a smooth coating surface for a poly(vinyl-alcohol) post-treatment, which reduced the gas permeance through the fiber sorbent by at least 7 orders of magnitude, essentially creating an impermeable sheath layer. The interstitial volume between the individual fibers was filled with a thermally-moderating paraffin wax. CO2 breakthrough experiments on the hollow fiber sorbent modules with and without paraffin wax revealed that the “passively” cooled paraffin wax module had 12.5% longer breakthrough times than the “non-isothermal” module. The latent heat of fusion/melting of the wax offsets the released latent heat of sorption/desorption of the zeolites. One-hundred rapidly cycled pressure swing adsorption cycles were performed on the “passively” cooled hollow fiber sorbents using 25 vol% CO2/75 vol% He (H2 surrogate) at 60 °C and 113 psia, resulting in a product purity of 99.2% and a product recovery of 88.1% thus achieving process conditions and product quality comparable to conventional pellet processes. Isothermal and non-isothermal dynamic modeling of the hollow fiber sorbent module and a traditional packed bed using gPROMS® indicated that the fiber sorbents have sharper fronts (232% sharper) and longer adsorbate breakthrough times (66% longer), further confirming the applicability of the new fiber sorbent approach for H2 purification.

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.

High performance dual-layer hollow fiber fabricated via novel immiscibility induced phase separation (I2PS) process for dehydration of ethanol

This pioneer study discloses a novel immiscibility induced phase separation (I2PS) process to fabricate high performance dual-layer hollow fiber for dehydration of ethanol via pervaporation. The I2PS process takes the advantages of phase separation phenomena occurring in immiscible blend dopes made of incompatible polymers. The immiscible blend dopes were simultaneously extruded through a triple orifice spinneret and fabricated into dual-layer hollow fibers consisting of an outer protective layer and an inner selective layer. A dense selective layer is formed at the outer-surface of the inner-layer due to the incompatibility between the polymer solutions of both inner and outer-layers as well as the shielding effect of the outer protective layer that provides a sufficient time for the formation of an almost defect-free selective layer during the phase inversion process. It is found that the phase inversion kinetics of the outer-layer dope solutions as well as the nature of outer-layer materials play great roles in determining the morphology of the outer protective layers and subsequently affects the permeance of the resultant hollow fibers. On the other hand, the selectivity of the as-spun fibers was found to be controlled by the outer surface of the inner-layer. The novel dual-layer hollow fiber spun from the combination of cellulose triacetate (CTA) and Ultem® possesses a high water permeance (29.33mol/m2hkPa) or water flux (1.77kg/m2h at 50°C) coupled with reasonable water/ethanol selectivity (824mol/mol) as compared to the pervaporation membranes available in the literatures. Heat treatment may enhance the selectivity of the aforementioned hollow fibers with some sacrifices of permeance. The present study may illuminate an innovative approach for hollow fiber fabrication in the future.

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 lanthanum strontium cobalt ferrite (LSCF) cathode properties on hollow fibre micro-tubular SOFC performances

Single layer La0.6Sr0.4Co0.2Fe0.8O3 hollow fibre (HF) precursors (<1 mm ID) produced by phase inversion (PI) were sintered at 1,200, 1,350 and 1,400 °C. The increase in sintering temperature resulted in microstructural changes in the LSCF fibres, reflected in their electrical conductivities. LSCF-based cathodes with different designs were brushed onto co-extruded nickel–gadolinium-doped ceria (CGO) anode/CGO electrolyte dual-layer HFs (<1 mm ID) fabricated by PI. The effect of cathode layers on the overall performance of the fuel cells (FCs) was assessed using nearly identical anode and electrolyte compositions, thicknesses, and microstructures. Cathode microstructure design caused cells to perform differently producing peak power densities of 0.35–0.7 W cm−2 at 600 °C. Impedance spectroscopy analysis at 600 °C on the FCs produced 0.12–0.24 Ω cm2 confirming the cathode’s structural effect on the overall area-specific resistance of the FCs. The best performing FC with a brush-deposited cathode was compared to a similar FC where cathode was deposited by dip coating; at 600 °C the first produced 0.6 W cm−2 while the second cell 0.7 W cm−2. Co-extruding anodes and electrolytes by using PI and combining dip coating for cathode deposition could lead to the fabrication of FCs with enhanced microstructures and improved performances.

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.

Dual-layer hollow fibres with different anode structures for micro-tubular solid oxide fuel cells

In this study, a high performance micro-tubular solid oxide fuel cell (SOFC) has been developed by depositing a multi-layer cathode onto an improved electrolyte/anode dual-layer hollow fibre fabricated via a single-step co-extrusion/co-sintering technique. The use of 0–20wt.% of ethanol in the inner layer spinning suspension allows the control over the asymmetric structure of the Ni–CGO anode layer, i.e. finger-like voids structure covering about 50–85% of the anode layer thickness with the rest volume occupied by sponge-like structure, and at the same time affects the morphology of the CGO electrolyte layer. The presence of finger-like voids significantly facilitates the fuel gas diffusion inside the anode, and as a result, the maximum power density increases from 1.84Wm−2 to 2.32Wcm−2, when the finger-like voids is increased from 50% to 70% of the asymmetric anode layer. However, further growth of finger-like voids, i.e. 85% of the anode layer, dramatically reduce the number of triple-phase boundary (TPB) region and conductivity in the anode, as well as the gas-tightness property of the electrolyte, which consequently decreases the maximum power density to 0.99Wcm−2. Based on the results obtained, therefore, dual-layer hollow fibres with 50–70% of finger-like voids in the anode layer can be considered as the ideal structure for producing high performance micro-tubular SOFCs.

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.

Morphological architecture of dual-layer hollow fiber for membrane distillation with higher desalination performance

A new strategy to enhance the desalination performance of polyvinylidene fluoride (PVDF) hollow fiber membrane for membrane distillation (MD) via architecture of morphological characteristics is explored in this study. It is proposed that a dual-layer hollow fiber consisting of a fully finger-like macrovoid inner-layer and a sponge-like outer-layer may effectively enhance the permeation flux while maintaining the wetting resistance. Dual-layer fibers with the proposed morphology have been fabricated by the dry-jet wet spinning process via careful choice of dopes composition and coagulation conditions. In addition to high energy efficiency ( EE) of 94%, a superior flux of 98.6 L m −2 h −1 is obtained during the direct contact membrane distillation (DCMD) desalination experiments. Moreover, the liquid entry pressure ( LEP) and long-term DCMD performance test show high wetting resistance and long-term stability. Mathematical modeling has been conducted to investigate the membrane mass transfer properties in terms of temperature profile and apparent diffusivity of the membranes. It is concluded that the enhancement in permeation flux arises from the coupling effect of two mechanisms; namely, a higher driving force and a lower mass transfer resistance, while the later is the major contribution. This work provides an insight on MD fundamentals and strategy to tailor making ideal membranes for DCMD application in desalination industry.

Interfacial resistance of dual-layer asymmetric hollow fiber pervaporation membranes formed by co-extrusion

Interface is critical for dual-layer membranes fabricated by co-extrusion and dry-jet wet spinning. In this work, for the first time, the importance of interface structure in overall membrane transport property was revealed by using Polysulfone (outer layer)/Matrimid (inner) dual-layer hollow fibers as a demonstration system. Due to the dope formulations of the two layers, dense skins came into formation at the shell side of Matrimid inner layer facing the interface. The Matrimid inner layer obtained from the dual-layer hollow fiber with the thinnest Polysulfone outer layer exhibited a flux around 1.0 × 10−3 kg/m2-s and a separation factor of ~800 in tert-butanol dehydration (feed flow rate 30 L/h, temperature 80 °C, permeate pressure 2 mbar, the same for the other tests). An estimation based on resistance model clearly indicated the dominance of Matrimid inner layer in the overall mass transfer of corresponding dual-layer membrane. As for hollow fibers with the thickest Polysulfone outer layer, the bulk substrate comprising the interfacial dense skin of Matrimid inner layer also displayed significant resistance and appreciable selectivity. Conclusively, the function of interface should not be ignored. The rule for the evolution of interface structure requires further exploration for fully understanding and utilizing the composite membrane by co-extrusion and phase inversion approach.

NI/NI‐YSZ Current Collector/Anode Dual Layer Hollow Fibers for Micro‐Tubular Solid Oxide Fuel Cells

AbstractA co‐extrusion technique was employed to fabricate a novel dual layer NiO/NiO‐YSZ hollow fiber (HF) precursor which was then co‐sintered at 1,400 °C and reduced at 700 °C to form, respectively, a meshed porous inner Ni current collector and outer Ni‐YSZ anode layers for SOFC applications. The inner thin and highly porous “mesh‐like” pure Ni layer of approximately 50 μm in thickness functions as a current collector in micro‐tubular solid oxide fuel cell (SOFC), aiming at highly efficient current collection with low fuel diffusion resistance, while the thicker outer Ni‐YSZ layer of 260 μm acts as an anode, providing also major mechanical strength to the dual‐layer HF. Achieved morphology consisted of short finger‐like voids originating from the inner lumen of the HF, and a sponge‐like structure filling most of the Ni‐YSZ anode layer, which is considered to be suitable macrostructure for anode SOFC system. The electrical conductivity of the meshed porous inner Ni layer is measured to be 77.5 × 105 S m–1. This result is significantly higher than previous reported results on single layer Ni‐YSZ HFs, which performs not only as a catalyst for the oxidation reaction, but also as a current collector. These results highlight the advantages of this novel dual‐layer HF design as a new and highly efficient way of collecting current from the lumen of micro‐tubular SOFC.

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.