The effects of spinning conditions on the structure formation and the dimension of the hollow-fiber membranes and their relationship with the permeability in dry–wet spinning technology

Hollow-fiber membranes were prepared by dry–wet spinning under different spinning conditions. The used spinneret was tube-in-orfice type and the membrane material was polysulfone (Udel P-3500). N-methyl-2-pyrrolidinone (NMP) and water were used as solvent and coagulant, respectively. The concentration of the dope solution was 22 wt %. The effects of the three spinning factors—spinning height; extrusion rates of dope solution and inner coagulant; dimensions (inner diameter, outer diameter, and thickness) and permeation properties of the hollow-fiber membranes—were studied. The results were as follows: With changes in the spinning factors, spinning velocity and falling time before the membrane entered the water (coagulant) were changed; consequently, the structures and the dimensions of the hollow-fiber membranes varied with a certain tendency. The permeation properties of the hollow fibers were related very closely to the changes in the fibers' structures and dimensions. © 1995 John Wiley & Sons, Inc.     

Einsatz von perowskitischen Hohlfasermembranen in einem Mikrowellenplasma

For the first time the combination of a separation process with a plasma process was successfully tested. In this case, a mixed‐conducting perovskite membrane separates the oxygen. At 1 kW a permeation of 2.24 mL min^?1cm^?2 could be achieved. Corresponding perovskite membranes have been manufactured as hollow fibers with a very good CO₂ stability. The hollow fibers showed a constant permeation over more than 200 h. Furthermore, a spinning process with a sulphur‐free polymer binder has been established.     

Preparation and oxygen permeation of U‐shaped perovskite hollow‐fiber membranes

The oxygen permeation of dense U‐shaped perovskite hollow‐fiber membranes based on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ prepared by a phase inversion spinning process is reported. The perovskite hollow fibers with totally dense wall were obtained with the outer diameter of 1.147 mm and the inner diameter of 0.691 mm. The dependences of the oxygen permeation on the air flow rate on the shell side, the helium flow rate on the core side, the oxygen partial pressures, and the operating temperatures were experimentally investigated. According to the Wagner theory, it follows that the oxygen transport through the U‐shaped hollow‐fiber membrane is controlled by both surface reaction and bulk diffusion at the temperature ranges of 750–950°C. High oxygen permeation flux of 3.0 ml/(min cm²) was kept for about 250 h at 950°C under the conditions of the air feed flow rate of 150 ml/min and the helium flow rate of 50 ml/min. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Multichannel mixed‐conducting hollow fiber membranes for oxygen separation

A multichannel mixed‐conducting hollow fiber (MMCHF) membrane, 0.5 wt % Nb₂O₅‐doped SrCo_0.8 Fe_0.2O_3‐δ (SCFNb), has been successfully prepared by phase inversion and sintering technique. The crystalline structure, morphology, sintering behavior, breaking load, and oxygen permeability of the MMCHF membrane were studied systematically. The MMCHF membrane with porous‐dense asymmetrical microstructure was obtained with the outer diameter of 2.46 mm and inner tetra‐bore diameter of 0.80 mm. The breaking load of the MMCHF membrane was 3–6 times that of conventional single‐channel mixed‐conducting hollow fiber membrane. The MMCHF membrane showed a high oxygen flux which was about two times that of symmetric capillary membrane at similar conditions as well as a good long‐term stability under low oxygen partial pressure atmosphere. This work proposed a new configuration for the mixed‐conducting membranes, combining advantages of multichannel tubular membrane technology and conventional hollow fibers. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1969–1976, 2014     

Preparation and characterization of polyacrylonitrile-based membranes: Effects of internal coagulant on poly(acrylonitrileco-malefic acid) ultrafiltration hollow fiber membranes

Ultrafiltration hollow-fiber membranes (UHFMs) ofpoly(acrylonitrile-co-malefic acid) (PANCMA) were prepared by a dry-wet phase inversion process. The morphologies of inner surface and cross section for these hollow fibers were inspected with scanning electron microscopy. It was found that, by increasing the amount of solvent DMSO in internal coagulant, the number and size of macrovoid underneath the inner surface decreased. The water flux of the UHFMs also decreased while the bovine serum albumin rejection increased minutely. These results were interpreted based on the ternary phase diagrams for the PANCMA/DMSO/(H₂O+DMSO) system, which was obtained from the experimental cloud point measurements and empirical linearized cloud point relation. It was envisaged that the membrane surface could be further modified by the reaction of acid groups with poly(ethylene glycol).     

Thermo-responsive mixed-matrix hollow fiber membranes

Up to date, preparation of thermo-responsive mixed-matrix membranes (MMM) has only be described as small scale flat membranes or multi-step processes for hollow fiber membranes. In this work, the development of thermo-responsive MMM hollow fibers composed of polyethersulfone as membrane polymer and poly(N-isopropylacrylamide) (PNIPAM) microgel particles via the wet spinning process is presented. PNIPAM particles are synthesized with (NP-S, z_{avg 20°C} = 105 nm) and without (NP-L, z_{avg 20°C} = 250 nm) sodium dodecyl sulfate and their thermo-responsive behavior is characterized by dynamic light scattering. Particle size (NP-S, NP-L), particle content (10%, 15%) and the extrusion pressure in the wet spinning process (1.0–3.0 bar) are investigated as experimental parameters. Reversible thermo-responsive behavior of the hollow fibers is demonstrated by water permeability measurements at different temperatures (20 and 50°C). The largest switching factors (R) are observed for the hollow fibers containing NP-L. For 15% NP-L and 1 bar extrusion pressure, water permeances between 0.5 and 6.0 L m⁻² h⁻¹ bar⁻¹ are observed, corresponding to R = 12 and a dextran (500 kDa) rejection of 91% at 25°C.     

Effect of polymer and additive on the structure and property of porous stainless steel hollow fiber

Porous stainless steel hollow fiber has been widely used due to its high mechanical strength, excellent thermal conductivity and good sealing properties compared with other porous supports. We successfully prepared porous stainless steel hollow fibers using polyacrylonitrile (PAN) as polymer via dry-wet spinning followed by sintering through temperature programming method. The PAN concentration had an obvious impact on the structure and property of porous stainless steel hollow fiber even if it would be burned off during sintering. The results showed that the morphology could be tuned by adjusting the concentration of PAN. With increasing PAN concentration in casting solution for spinning, the viscosity was increased dramatically, resulting in much compact structures with high pure water flux (higher than 3×10⁵ L·m^?2·h^?1·Pa^?1). A more dense structure could be obtained by adding additive polyvinylpyrrolidone (PVP) as viscosity enhancer.     

Fabrication of ultrathin La0.6Sr0.4Co0.2Fe0.8O3-δ hollow fibre membranes for oxygen permeation

An ultrathin La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) hollow fibre membrane for enhanced oxygen permeation flux was fabricated using a wet spinning/sintering method. The membrane exhibits a highly asymmetric structure comprising of a very thin dense outer layer supported by finger-like structures that are fully open on the inner surface. Oxygen permeation measurements were conducted using sweep gas as an operating mode. Effects of operating temperatures and flow rates of the sweep gas on the oxygen permeation fluxes were investigated in details. The highest oxygen permeation flux, i.e. 0.096 cm³/cm² s (5.77 cm³/cm² min) was obtained from the ultrathin hollow fibre membrane at 1323 K (1050 °C) and the sweep gas flow rate of 2.42 cm³/s. The results indicate that the oxygen permeation flux obtained is much higher (4.9-11.2 times) than that obtained from conventional LSCF hollow fibre membranes mainly due to the reduced thickness of the membrane as well as the porous surface on the permeate side. In addition, despite a very thin dense layer, the LSCF hollow fibre membrane possessed a reasonable mechanical strength (113.22 MPa).     

Oxygen separation through U‐shaped hollow fiber membrane using pure CO2 as sweep gas

A number of U‐shaped K₂NiF₄‐type oxide hollow fiber membranes based on (Pr_0.9La_0.1)₂(Ni_0.74Cu_0.21Ga_0.05)O_4+δ (PLNCG) were successfully prepared by a phase inversion spinning process. The PLNCG hollow fiber membranes were then used to investigate the effect of CO₂ concentration in both the sweep gas and the feed air on the oxygen permeation flux. With pure CO₂ as the sweep gas and even 10% CO₂ in the feed air, a steady oxygen permeation flux of 0.9 mL/min^·cm² (STP) is obtained at 975°C during 310 h, and no decline of the oxygen permeation flux is observed. XRD, SEM and EDS characterizations show the spent membrane still maintains the intact microstructure and perfect K₂NiF₄‐type phase structure without carbonate, which indicates that the U‐shaped PLNCG hollow fiber membrane is a very stable membrane under CO₂ atmosphere and has great potential for the practical application in oxyfuel techniques for CO₂ capture and storage.©2011 American Institute of Chemical Engineers AIChE J, 2012     

Integrally skinned polysulfone hollow fiber membranes for pervaporation

From polysulfone as polymer, integrally skinned hollow fiber membranes with a defect-free top layer have been spun. The spinning process described here differs from the traditional dry-wet spinning process where the fiber enters the coagulation bath after passing a certain air gap. In the present process, a specially designed tripple orifice spinneret has been used that allows spinning without contact with the air. This spinneret makes it possible to use two different nonsolvents subsequently. During the contact time with the first nonsolvent, the polymer concentration in the top layer is enhanced, after which the second coagulation bath causes further phase separation and solidification of the ultimate hollow fiber membrane. Top layers of ± 1 μm have been obtained, supported by a porous sublayer. The effect of spinning parameters that might influence the membrane structure and, therefore, the membrane properties, are studied by scanning electron microscopy and pervaporation experiments, using a mixture of 80 wt % acetic acid and 20 wt % water at a temperature of 70°C. Higher fluxes as a result of a lower resistance in the substructure could be obtained by adding glycerol to the spinning dope, by decreasing the polymer concentration, and by adding a certain amount of solvent to the bore liquid. Other parameters studied are the type of the solvent in the spinning dope and the type of the first nonsolvent. © 1994 John Wiley & Sons, Inc.     

Gas permselection properties in silicone-coated asymmetric polyethersulfone membranes

The gas permeation properties of H₂, He, CO₂, O₂, and N₂ through silicone-coated polyethersulfone (PESf) asymmetric hollow-fiber membranes with different structures were investigated as a function of pressure and temperature and compared with those of PESf dense membrane and silicone rubber (PDMS) membrane. The PESf asymmetric hollow-fiber membranes were prepared from spinning solutions containing N-methyl-2-pyrrolidone as a solvent, with ethanol, 1-propanol, or water as a nonsolvent-additive. Water was also used as both an internal and an external coagulant. A thin silicone rubber film was coated on the external surface of dried PESf hollow-fiber membranes. The apparent structure characteristics of the separation layer (thickness, porosity, and mean pore size) of the asymmetric membranes were determined by gas permeation method and their cross-section morphologies were examined with a scanning electron microscope. The results reveal that the gas pressure normalized fluxes of the five gases in the three silicone-coated PESf asymmetric membranes are nearly independent of pressure and did not exhibit the dual-mode behavior. The activation energies of permeation in the silicone-coated asymmetric membranes may be larger or smaller than those of PESf dense membrane, which is controlled by the membrane physical structure (skin layer and sublayer structure). Permselectivities for the gas pairs H₂/N₂, He/N₂, CO₂/N₂, and O₂/N₂ are also presented and their temperature dependency addressed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 837–846, 1997     

Partial oxidation of methane in hollow‐fiber membrane reactors based on alkaline‐earth metal‐free CO2‐tolerant oxide

The U‐shaped alkaline‐earth metal‐free CO₂‐stable oxide hollow‐fiber membranes based on (Pr_0.9La_0.1)₂(Ni_0.74Cu_0.21Ga_0.05)O_4+δ (PLNCG) are prepared by a phase‐inversion spinning process and applied successfully in the partial oxidation of methane (POM) to syngas. The effects of temperature, CH₄ concentration and flow rate of the feed air on CH₄ conversion, CO selectivity, H₂/CO ratio, and oxygen permeation flux through the PLNCG hollow‐fiber membrane are investigated in detail. The oxygen permeation flux arrives at approximately 10.5 mL/min cm² and the CO selectivity is higher than 99.5% with a CH₄ conversion of 97.0% and a H₂/CO ratio of 1.8 during 140 h steady operation. The spent hollow‐fiber membrane still maintains a dense microstructure and the Ruddlesden‐Popper K₂NiF₄‐type structure, which indicates that the U‐shaped alkaline‐earth metal‐free CO₂‐tolerant PLNCG hollow‐fiber membrane reactor can be steadily operated for POM to syngas with good performance. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3587–3595, 2014     

Preparation and characterization of alumina hollow fiber membranes

With the rapid development of membrane technology in water treatment, there is a growing demand for membrane products with high performance. The inorganic hollow fiber membranes are of great interest due to their high resistance to abrasion, chemical/thermal degradation, and higher surface area/volume ratio therefore they can be utilized in the fields of water treatment. In this study, the alumina (Al₂O₃) hollow fiber membranes were prepared by a combined phase-inversion and sintering method. The organic binder solution (dope) containing suspended Al₂O₃ powders was spun to a hollow fiber precursor, which was then sintered at elevated temperatures in order to obtain the Al₂O₃ hollow fiber membrane. The dope solution consisted of polyethersulfone (PES), Nmethyl-2-pyrrolidone (NMP) and polyvinylpyrrolidone (PVP), which were used as polymer binder, solvent and additive, respectively. The prepared Al₂O₃ hollow fiber membranes were characterized by a scanning electron microscope (SEM) and thermal gravimetric analysis (TG). The effects of the sintering temperature and Al₂O₃/PES ratios on the morphological structure, pure water flux, pore size and porosity of the membranes were also investigated extensively. The results showed that the pure water flux, maximum pore size and porosity of the prepared membranes decreased with the increase in Al₂O₃/PES ratios and sintering temperature. When the Al₂O₃/PES ratio reached 9, the pure water flux and maximum pore size were at 2547 L/m²·h and 1.4 μm, respectively. Under 1600dgC of sintering temperature, the pure water flux and maximum pore size reached 2398 L/(m²·h) and 2.3 μm, respectively. The results showed that the alumina hollow fiber membranes we prepared were suitable for the microfiltration process. The morphology investigation also revealed that the prepared Al₂O₃ hollow fiber membrane retained its’asymmetric structure even after the sintering process.     

A CO2‐stable hollow‐fiber membrane with high hydrogen permeation flux

A Mo‐substituted lanthanum tungstate mixed proton‐electron conductor, La_5.5W_0.6Mo_0.4O_11.25?δ (LWM04), was synthesized using solid state reactions. Dense U‐shaped LWM04 hollow‐fiber membranes were successfully prepared using wet‐spinning phase‐inversion and sintering. The stability of LWM04 in a CO₂‐containing atmosphere and the permeation of hydrogen through the LWM04 hollow‐fiber membrane were investigated in detail. A high hydrogen permeation flux of 1.36 mL/min cm² was obtained for the U‐shaped LWM04 hollow‐fiber membranes at 975°C when a mixture of 80% H₂?20% He was used as the feed gas and the sweep side was humidified. Moreover, the hydrogen permeation flux did not significantly decrease over 70 h of operation when fed with a mixture containing 25% CO₂, 50% H₂, and 25% He, indicating that the LWM04 hollow‐fiber membrane has good stability under a CO₂‐containing atmosphere. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1997–2007, 2015     

The effect of heat-treatment on the ultrafiltration performance of polyethersulfone (PES) hollow-fiber membranes

Polyethersulfone (PES) hollow-fiber membranes were prepared by the dry-wet spinning method and then heated in an oven at different temperatures to investigate the effect of heat-treatment on their ultrafiltration performance. It was found that the hollow-fiber membranes shrank by heat treatment, as evidenced by a decrease in flux and an increase in solute separation, although there was no visible change in the hollow-fiber dimension. The best results were obtained when the hollow fibers were heated at 150°C. A further investigation was made on the effect of the heating period, while the temperature was fixed to 150°C. It was found that the best combination of the temperature and the heating period was 150°C and 5 min.     

Fabrication of 6FDA‐durene polyimide asymmetric hollow fibers for gas separation

We have developed defect‐free asymmetric hexafluoro propane diandydride (6FDA) durene polyimide (6FDA‐durene) hollow fibers with a selectivity of 4.2 for O₂/N₂ and a permeance of 33.1 ×10^?6 cm³ (STP)/cm²‐s‐cmHg for O_2. These fibers were spun from a high viscosity in situ imidization dope consisting of 14.7% 6FDA‐durene in a NMP solvent and the inherent viscosities (IV) of this 6FDA‐durene polymer was 0.84 dL/g. Low IV dopes cannot produce defect‐free hollow fibers, indicating a 6FDA‐durene spinning dope with a viscosity in the region of chain entanglement seems to be essential to yield hollow fibers with minimum defects. The effects of spinning parameters such as shear rates within a spinneret and bore fluids as well as air gap on gas separation performance were investigated. Experimental data demonstrate that hollow fibers spun with NMP/H₂O as the bore liquid have higher permeances and selectivities than those spun with glycerol as the bore liquid because the former has a relatively looser inner skin structure than the latter. In addition, the selectivity of hollow fibers spun with NMP/H₂O as the bore liquid changes moderately with shear rate, while the selectivity of hollow fibers spun with glycerol are less sensitive to the change of shear rate. These distinct behaviors are mainly attributed to the different morphologies generated by different bore fluids. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2166–2173, 2001     

Sulfur dioxide non‐dispersive absorption in N,N‐dimethylaniline using a ceramic membrane contactor

BACKGROUND: Removal of sulfur dioxide from gas emissions by selective absorption is a common method to separate and concentrate sulfur dioxide and to reduce air pollution and environmental risks. N,N‐dimethylaniline is an organic solvent used in some industrial applications for its sulfur dioxide affinity, leading to a regenerative process. However, the use of scrubbers and equipment in which direct contact between gas and liquid takes place leads to solvent losses due to evaporation and drops dragging. RESULTS: In this work, an innovative procedure based on non‐dispersive absorption in a ceramic hollow fibre membrane contactor was studied in order to avoid drops dragging. The absorption efficiency ranged between 40 and 50%, showing the technical viability of the process. The sulfur dioxide flux through the membrane has a linear relationship with the concentration of SO₂ in the gas stream and an overall mass transfer coefficient K_overall = (1.10 ± 0.11) × 10^?5 m s^?1 has been obtained. CONCLUSIONS: The mass transfer behaviour of a ceramic hollow fibre membrane contactor for sulfur dioxide non‐dispersive absorption in N,N‐dimethylaniline has been studied. The main resistance is found to be the ceramic membrane and the effective diffusivity has been inferred. The mass transfer model and parameters allow the evaluation of equipment design for technical applications. Copyright © 2008 Society of Chemical Industry     

纺丝条件对PVDF/PVC中空纤维膜性能的影响

采用干-湿法纺丝工艺制备PVDF/PVC共混中空纤维膜,通过对水通量、孔径、截留率等的测试,研究了挤出速率、芯液流量、干纺程对PVDF/PVC中空纤维膜性能及结构的影响,并进行了详细的理论分析。试验结果表明挤出速率与膜通量存在最大值,芯液流量与膜通量及截留率呈线性关系,干纺程的影响效果跟挤出速率类似。可以通过改变纺丝条件来制备性能不同的中空纤维膜。     

A rational asymmetric hollow fiber membrane for oxygen permeation

A typical oxygen permeation hollow fiber membrane fabricated by phase inversion-based extrusion process demonstrates heterogeneous porous microstructures, in which the surface layer with relatively low porosity is used as a separation layer after sintering. It is usually not convenient to control the thickness of separation layer. And a high sintering temperature is needed to densify the separation layer, which in turn could destroy the desired porous microstructures in other portion. This paper studies a novel process to fabricate multilayer asymmetric hollow fiber membrane with a rational design using 67 vol. % Gd_0.2Ce_0.8O_2−δ−33 vol. % La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (GDC-LSCF) as a model material system. The phase inversion-based extrusion process in open literature is employed to fabricate a hollow fiber substrate featuring radially well-aligned microchannels open at the inner surface. Built upon the hollow fiber substrate, a thin dense separation layer and porous surface catalyst layer at shell side are then fabricated through dip-coating and sintering process alternatively. The oxygen permeation flux of the fabricated hollow fiber membrane reaches 2.68 mL/cm²/min at 900°C under Ar/air gradient, the highest performance of the membranes with GDC-LSCF material system in open literature. The innovative fabrication process is able to readily control the thickness of functional layers while decreasing sintering temperatures.     

Preparation,characterization and application of iron (III)-loaded chitosan hollow fiber membranes as a new bio-based As (V) sorbent

Fe (III)-loaded chitosan (CS) hollow fibers (CS-Fe (III) HF) were successfully prepared according to the dry-wet spinning technique. The CS-Fe (III) HFs were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis (TGA). Removal of pentavalent arsenic was studied through biosorption on CS-Fe (III) HF adsorptive membranes. The response surface methodology (RSM) was applied to investigate the influence of the main operating parameters such as contact time, pH, initial As (V) concentration and HFs dosage on the adsorption capacity of As (V). From the Pareto analysis, pH, [As (V)]_o, [CS-Fe (III) HF membranes] and squared effect of [As(V)]_o were found to produce the largest effect on biosorption of As (V). Kinetic studies showed that the pseudo-second-order kinetic model provides the best correlation to the experimental results. Equilibrium data fitted well with the Langmuir model with maximum adsorption capacity of 3,703 μg g^?1. A laboratory scale glass membrane module consisting of three CS-Fe(III) HFs has also been prepared and tested for biosorption of As (V) at a real scale. Permeability of As (V) ions through the CS-Fe (III) HF membranes was 0.145 μmol m^?2 h^?1 bar ^?1.