High flux and CO2 stable La0.6Ca0.4Co0.2Fe0.8O3-δ hollow fiber membranes through internal coagulation bath optimization

La_0.6Ca_0.4Co_0.2Fe_0.8O_3-δ (LCCF) ceramic powder prepared by sol-gel method was used to fabricate LCCF hollow fiber (HF) membranes via a combined phase inversion-sintering technique. Three types of LCCF HF membranes were developed by changing the composition of the internal coagulation bath containing H₂O, EtOH or the mixture of NMP + EtOH. The best one was achieved via the mixture (0.7NMP + 0.3EtOH by weight). At 1000 °C, the oxygen flux reached 6.16 mL min⁻¹ cm⁻² under inert sweep gas; however, it reached 8.83 mL min⁻¹ cm⁻² when reactive CH₄ was used as the sweep gas, mirroring the high capability of oxygen transport through the membrane. The good stability of the developed LCCF membrane was confirmed by a series of long-term operation tests up to 300 h with sweep gas containing CO₂. The findings of this work can advance the applications of LCCF membranes to reactors where CO₂ atmosphere cannot be avoided.     

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 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     

Direct dual layer spinning of aminosilica/Torlon® hollow fiber sorbents with a lumen layer for CO2 separation by rapid temperature swing adsorption

The direct dual layer spinning of Torlon^®/silica hollow fibers with a neat Torlon^® lumen layer is reported here for the first time. The dual layer fibers containing a porous Torlon^®/silica main structure and a dense, pure Torlon^® polymer bore‐side coating provide a simplified, scalable platform from which to construct hollow fiber amine sorbents for postcombustion CO₂ capture. After fiber spinning, an amine infusion process is applied to incorporate PEI into the silica pores. After combining dilute Neoprene treatment followed by poly(aramid)/PDMS treatment, a helium permeance of the fiber sorbents of 2 GPU with a He/N₂ selectivity of 7.4 is achieved. Ten of the optimized amine‐containing hollow fibers are incorporated into a 22‐inch long, 1/2 inch OD shell‐and‐tube module and the module is then exposed on the shell side to simulated flue gas with an inert tracer (14 mol % CO₂, 72 mol % N₂, 14 mol % He [at 100% R.H.]) at 1 atm and 35°C in a RTSA system for preliminary CO₂ sorption experiments. The fibers are found to have a breakthrough and equilibrium CO₂ capacity of 0.8 and 1.2 mmol/g‐ dry fiber sorbent, respectively. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41845.     

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     

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).     

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     

Fabrication of lanthanum-based perovskites membranes on porous alumina hollow fibre (AHF) substrates for oxygen enrichment

In this work, two types of lanthanum-based MIEC perovskite oxides, namely La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and La_0.6Sr_0.4Co_0.2Ni_0.8O_3-δ (LSCNi), were deposited onto porous alumina hollow fibre (AHF) substrates and used for oxygen enrichment. Such structure was developed to shorten oxygen ion diffusion distances in dense membranes and simultaneously leading to higher oxygen flux. The perovskite oxides were prepared using Pechini sol-gel method and deposited via a vacuum-assisted technique. The deposition of lanthanum-based membranes onto the outer and inner sides of the porous AHF has been facilitated through numerous microchannels in the AHF substrates. The effects of operating temperature and argon sweep gas flowrate on oxygen permeation flux of lanthanum-based AHF membrane were investigated. The results revealed that the oxygen permeation flux of LSCF-AHF and LSCNi-AHF increased with operating temperatures due to the improvement of bulk diffusion and surface exchange properties after the lanthanum-based perovskite deposition. Higher oxygen flux was observed for LSCNi-AHF as LSCNi possessed balanced oxygen ionic and electronic conductivities as compared to LSCF membranes. Benefitting from improved oxygen activation and vacancy generation properties after Ni substitution into the B-site ion of LSC perovskite, a dramatic increased oxygen fluxes up to 4.5 mL/min·cm² was observed at 950 °C. The present work demonstrated a feasible method for fabricating oxygen transport membrane (OTM) using porous AHF substrates     

BaTiO3-based thermistor hollow fibers prepared using a phase inversion spinning process for energy efficient gas sorption

Positive temperature coefficient of resistivity (PTCR) hollow fibers that exhibit self-regulating heating characteristics have potential applications in temperature-swing adsorption systems (TSA), such as CO₂ recovery and drying of compressed air. La-doped BaTiO₃ hollow fibers displaying a PTCR effect were produced by phase inverting a casting solution consisting of N-methly-2-Pyrrolidone, polymethyl methacrylate, polyvinylpyrrolidone, BaTiO₃, TiO₂, and La₂O₃ through a spinneret into a coagulating waterbath. This was followed by polymer debinding, high temperature sintering between 1350?1400 °C and annealing in air at 1175 °C to produce hollow fibers of the composition Ba_0.9975La_0.0025TiO₃. Hydrothermal synthesis was implemented to deposit an adsorbent porous zeolite X layer within the hollow fiber lumen, which was confirmed by electron dispersive X-ray spectroscopy and CO₂ adsorption at 0 °C. Hence, these materials can be applied to energy efficient TSA gas separation processes. The results are discussed in terms of hollow fiber microstructure, adsorption characteristics and electrical properties.     

Improved microstructure of asymmetric niobium pentoxide hollow fiber membranes

Asymmetric niobium pentoxide (Nb₂O₅) hollow fiber membranes were prepared by the phase inversion and sintering process at temperatures ranging from 1000 to 1350°C. The effects of extrusion parameters on the morphology and properties of the produced membranes were systematically explored. Asymmetric hollow fibers with regular inner contour were obtained at extrusion flow rates of 15 and 25 ml min⁻¹ of ceramic suspension and internal coagulant, respectively. Hollow fibers sintered at temperatures greater than 1200°C presented modifications in the morphology of Nb₂O₅ grains, which were also evidenced by X-ray diffraction and Raman analyses. Hollow fibers produced with an air gap of 50 mm presented a dense outer sponge-like layer and micro-voids formed from the inner surface. These hollow fibers sintered at 1200°C presented suitable bending resistance and water permeability (24.2 ± 0.60 MPa and 3.00 ± 0.01 L h^-1 m^-2 kPa^-1, respectively). The outer sponge like layer was mitigated when the fibers were produced without air-gap.     

A novel porous‐dense dual‐layer composite membrane reactor with long‐term stability

A porous‐dense dual‐layer composite membrane reactor was proposed. The dual‐layer composite membrane composed of dense 0.5 wt % Nb₂O₅‐doped SrCo_0.8Fe_0.2O_3‐δ (SCFNb) layer and porous Ba_0.3Sr_0.7Fe_0.9Mo_0.1O_3‐δ (BSFM) layer was prepared. The stability of SCFNb membrane reactor was improved significantly by the porous‐dense dual‐layer design philosophy. The porous BSFM surface‐coating layer can effectively reduce the corrosion of the reducing atmosphere to the membrane, whereas the dense SCFNb layer permeated oxygen effectively. Compared with single‐layer dense SCFNb membrane reactor, no degradation of performance was observed in the dual‐layer membrane reactor under partial oxidation of methane during continuously operating for 1500 h at 850°C. At 900°C, oxygen flux of 18.6 mL (STP: Standard Temperature and Pressure) cm^?2 min^?1, hydrogen production of 53.67 mL (STP) cm^?2 min^?1, CH₄ conversion of 99.34% and CO selectivity of about 94% were achieved. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4355–4363, 2013     

Effect of stabilization temperature on gas permeation properties of carbon hollow fiber membrane

Carbon hollow fiber membranes (CHFMs) derived from polymer blend of polyetherimide (PEI) and polyvinylpyrrolidone (PVP) were extensively prepared through stabilization under air atmosphere followed by carbonization under N₂ atmosphere. The effects of the stabilization temperature on the morphological structure, chemical structure, and gas permeation properties were investigated thoroughly by means of scanning electron microscopy, Fourier transform infrared spectroscopy, and single gas permeation system. The experiment results indicate that the transport mechanism of small gas molecules of N₂, CO₂, and CH₄ is dominated by the molecular sieving effect. Based on morphological structure and gas permeation properties, an optimum stabilization condition for the preparation of CHFM derived from PEI/PVP was found at 300°C under air atmosphere. The selectivity of ?55 and 41 for CO₂/CH₄ and CO₂/N₂, respectively, were obtained for CHFMs prepared at stabilization temperature of 300°C. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Influence of Microstructure and Surface Activation of Dual‐Phase Membrane Ce0.8Gd0.2O2−δ–FeCo2O4 on Oxygen Permeation

Dual‐phase oxygen transport membranes are fast‐growing research interest for application in oxyfuel combustion process. One such potential candidate is CGO‐FCO (60 wt% Ce_0.8Gd_0.2O_2?δ–40 wt% FeCo₂O₄) identified to provide good oxygen permeation flux with substantial stability in harsh atmosphere. Dense CGO‐FCO membranes of 1 mm thickness were fabricated by sintering dry pellets pressed from powders synthesized by one‐pot method (modified Pechini process) at 1200°C for 10 h. Microstructure analysis indicates presence of a third orthorhombic perovskite phase in the sintered composite. It was also identified that the spinel phase tends to form an oxygen deficient phase at the grain boundary of spinel and CGO phases. Surface exchange limitation of the membranes was overcome by La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) porous layer coating over the composite. The oxygen permeation flux of the CGO‐FCO screen printed with a porous layer of 10 μm thick LSCF is 0.11 mL/cm² per minute at 850°C with argon as sweep and air as feed gas at the rates of 50 and 250 mL/min.     

Polydimethylsiloxane/postmodified MIL‐53 composite layer coated on asymmetric hollow fiber membrane for improving gas separation performance

Composite layer containing postmodified MIL‐53 (P‐MIL‐53) was exploited to be coated on as‐fabricated asymmetric hollow fiber membrane for improving gas separation performance. The morphology and pore size distribution of P‐MIL‐53 particles were characterized by SEM and N₂ adsorption isotherm. The EDX mapping and FTIR spectra were performed to confirm the presence of P‐MIL‐53 deposited on the outer surface of hollow fiber membranes. The results of pure gas permeation measurement indicated that incorporation of P‐MIL‐53 particles in coating layer could improve permeation properties of hollow fiber membranes. By varying coating times and P‐MIL‐53 content, the membrane coated with PDMS/15%P‐MIL‐53 composite by three times achieved best performance. Compared to pure PDMS coated membrane, CO₂ permeance was enhanced from 29.96 GPU to 40.24 GPU and ideal selectivity of CO₂/N₂ and CO₂/CH₄ also increased from 23.28 and 26.95 to 28.08 and 32.03, respectively. The gas transport through composite membrane was governed by solution‐diffusion mechanism and CO₂ preferential adsorption of P‐MIL‐53 contributed to considerable increase of CO₂ solubility resulting in accelerated permeation rate. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44999.     

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.     

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     

Composite hollow fiber membranes

Composite polysulfone hollow fibers consisting of a polysulfone porous substrate coated with crosslinked polyethyleneimine (PEI) or furan resin are reported. These composite hollow fibers are analogous to the flat-sheet composite membranes known as NS-100 and NS-200. The surface structure of the porous substrate was rigorously studied before and after coating. Scanning electron microscope observations and reverse osmosis transport studies showed that the support fiber must have surface pore diameters of less than 0.2 μm to obtain a durable composite hollow fiber membrane. The curing process would normally follow in situ condensation of the PEI or the cationic polymerization of the furfuryl alcohol. However, since both the dense layer and surface of the porous substrate contract when exposed to the curing temperature (110–150°C), it was found to be profitable to cure the hollow fiber before applying the coating. When tested in a reverse osmosis rig, PEI/TDI-coated polysulfone hollow fiber bundles displayed 98% salt rejection and a flux of 5–7 gfd for a feed solution of 10,000 ppm NaCl at a hydraulic pressure of 400 psi. A new method of depositing furan resin on the polysulfone hollow fiber is described. The furfuryl alcohol is instantaneously polymerized by exposing the alcohol-soaked fiber to a 60% solution of concentrated sulfuric acid. It has been demonstrated that in such a polymerization procedure a dense, semipermeable layer is formed on top of the porous substrate; the resulting composite hollow fiber membrane yields salt rejections higher than 98% when tested under the above reverse osmosis conditions.     

Preparation of polyvinylamine/polysulfone composite hollow‐fiber membranes and their CO2/CH4 separation performance

Fixed‐carrier composite hollow‐fiber membranes were prepared with polyvinylamine (PVAm) as the selective layer and a polysulfone ultrafiltration membrane as the substrate. The effects of the PVAm concentration in the coating solution, the number of coatings, and the crosslinking of glutaraldehyde and sulfuric acid on the CO₂ permeation rate and CO₂/CH₄ selectivity of the composite membranes were investigated. As the PVAm concentration and the number of coatings increased, the CO₂/CH₄ selectivity increased, but the CO₂ permeation rate decreased. The membranes crosslinked by glutaraldehyde or sulfuric acid possessed higher CO₂/CH₄ selectivities but lower CO₂ permeation rates. For the pure feed gas, a composite hollow‐fiber membrane coated with a 2 wt % PVAm solution two times and then crosslinked with glutaraldehyde and an acid solution in sequence had a CO₂ permeation rate of 3.99 × 10^?6 cm³ cm^?2 s^?1 cmHg^?1 and an ideal CO₂/CH₄ selectivity of 206 at a feed gas pressure of 96 cmHg and 298 K. The effect of time on the performance of the membranes was also investigated. The performance stability of the membranes was good during 6 days of testing. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1885–1891, 2006     

New morphological Ba0.5Sr0.5Co0.8Fe0.2O3−α hollow fibre membranes with high oxygen permeation fluxes

Perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?α (BSCF) hollow fibre membranes were fabricated by a combined phase inversion and sintering technique. The membranes were characterised by XRD, SEM and tested for air separation. The membrane possesses a novel morphology consisting of one dense layer and one porous layer. Oxygen permeation fluxes through the obtained hollow fibre membranes were measured in the temperature range 650–950 °C using helium sweep gas rates from 50 to 200 mL min^?1. Experimental results indicated the oxygen permeation flux through the BSCF hollow fibre membrane sintered at 1050 °C was approximately 11.46 mL min^?1 cm^?2 at 950 °C when the helium sweep rate was kept at 200 mL min^?1. The BSCF hollow fibre membrane showed a stable oxygen permeation flux of 8.60 mL min^?1 cm^?2 over the investigated period of 120 h at 900 °C.     

A novel lanthanum strontium cobalt iron composite membrane synthesised through beneficial phase reaction for oxygen separation

Composite ceramic membrane is one of the most attractive concepts which combines the advantages of different phases into a single membrane matrix. Recently, the reported significant increased oxygen surface kinetics on the Perovskite/Ruddlesden-Popper composite system because of the formation of novel and fast oxygen transport paths along the hetero-interface has been implanted into the oxygen permeation membrane system. In this work, a novel La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ-(La_0.5Sr_0.5)₂CoO_4+δ (LSCF-LSC) composite hollow fiber membrane is synthesized with oxygen permeation flux of 4.52 mL min⁻¹ cm⁻² at 950 °C. It presents round 4 times and 2.3 times of that of the single LSCF membrane and LSC-coated LSCF membrane at 900 °C. For better comparison, (La_0.576Sr_0.424)_1.136Co_0.3Fe_0.7O_3-δ (LSCF-new) is prepared based on the composition of LSCF-LSC composite. The enhanced oxygen permeability was further investigated through electrochemical impedance spectroscopy (EIS) measurements. We also confirm that LSCF-LSC shows significantly lower area specific resistance (ASRs) for LSCF-LSC|Ce_0.8Sm_0.2O_1.9 (SDC)|LSCF-LSC symmetrical cell relative to other symmetrical cells. This novel LSCF-LSC composite membrane also presents high CO₂ tolerance, with stable oxygen permeation fluxes round 2.6 mL min⁻¹ cm⁻² at 900 °C for 100 h.