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     

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.     

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     

Selection of oxygen permeation models for different mixed ionic‐electronic conducting membranes

Permeation data of several mixed ionic‐electronic conducting (MIEC) membranes were analyzed by two oxygen permeation models (i.e., Zhu's model and Xu–Thomson's model), respectively, to find a concise method to guide the choice of permeation models. We found that Zhu's model can well fit the permeation data of perovskite‐type membranes, like Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ (BSCF) and BaCe_0.05Fe_0.95O_3‐δ (BCF), and dual‐phase membranes, like 75 wt % Ce_0.85Sm_0.15O_1.925–25 wt % Sm_0.6Sr_0.4Al_0.3Fe_0.7O_3‐δ (SDC‐SSAF), whose oxygen vacancy concentrations are almost independent of the oxygen partial pressure at elevated temperatures. However, Zhu's model was not appropriate for membranes whose oxygen vacancy concentration changed obviously with oxygen partial pressure at elevated temperatures, such as La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ (LSCF) and La_0.7Sr_0.3CoO_3‐δ (LSC). On the contrary, Xu–Thomson's model can fit the data of LSCF and LSC well, but it is inapplicable for BSCF, BCF, and SDC‐SSAF. Therefore, the dependence of oxygen vacancy concentration on oxygen partial pressure was suggested as an index for the selection of the permeation models. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4043–4053, 2017     

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     

Oxy‐fuel combustion for CO2 capture using a CO2‐tolerant oxygen transporting membrane

CO₂ capture via an oxy‐fuel route through the U‐shaped (Pr_0.9La_0.1)₂(Ni_0.74Cu_0.21Ga_0.05)O_4+δ (PLNCG) hollow fiber membrane with 100% CH₄ conversion and 100% CO₂ selectivity for 450 h has been explored for the first time. X‐ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy characterizations of the spent hollow fiber membrane have also been investigated. All these results indicate that PLNCG hollow fiber membrane shows excellent reaction performance and good stability under oxy‐fuel reaction conditions, which will be a potential rounte for reducing CO₂ emissions worldwide. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3856–3862, 2013     

Modeling study of oxygen permeation through an electronically short‐circuited YSZ‐based asymmetric hollow fiber membrane

Here, oxygen fluxes through an electronically short‐circuited asymmetric Ag‐YSZ|YSZ|LSM‐YSZ hollow fiber prepared via a combined spinning and sintering route were tested and correlated to an explicit oxygen permeation model. The average oxygen permeation through such asymmetric hollow fiber with a 27 μm‐thick YSZ dense layer reached 0.52 mL (STP) cm^?2 min^?1 at 1173 K. From the model results, we can obtain the characteristic thickness, the effects of the temperature, and the effect of He sweep gas flow rate to the individual step contribution. The oxygen partial pressure variation in the permeate side, the local oxygen flux, and the three‐different resistance distribution along the axial direction of the asymmetric hollow fiber are theoretically studied; providing guidelines to further improve the membrane performance for oxygen separation. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3491–3500, 2017     

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     

Teflon AF2400/Ultem composite hollow fiber membranes for alcohol dehydration by high‐temperature vapor permeation

High‐temperature vapor permeation has a stringent requirement of membrane stability under harsh feed environments. This work reports the design of Teflon AF2400/Ultem composite hollow fiber (HF) membranes for alcohol dehydration via vapor permeation. Fabrication parameters such as Teflon concentration and coating time were systematically investigated. Interestingly, the fabricated composite HF membranes possess an unusual surface with honeycomb‐like microstructure patterns. Owing to the Teflon protective layer, the newly developed composite HF shows a promising and stable separation performance with a flux of 4265 gm^?2 h^?1 and a separation factor of 383 for 95% isopropanol dehydration at 125°C. The composite HF also performs well under extreme vapor feed compositions from 87 to 99 wt % isopropanol. In addition, it exhibits impressive separation performance for the dehydration of ethanol and n‐butanol. This work may provide useful insights of designing thermal‐stable and high‐performance composite polymeric membranes for vapor permeation. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1747–1757, 2016     

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     

Mixed conducting membranes — Macrostructure related oxygen permeation flux

The influence of the macrostructures of LSCF hollow fiber membranes on oxygen permeation flux was investigated experimentally and theoretically. According to the results, asymmetric membranes perform differently toward oxygen permeation depending on their sintering outcome. If both, outer and inner surfaces of the asymmetric membrane are fully densified, finger‐like pores become dead volume, which has adverse effects, and the oxygen permeation flux is similar to that of the symmetric membrane. This implies that the improved bulk diffusion due to the reduced membrane thickness is compromised by the additional exchange reaction resistance from the enclosed macrovoids in the asymmetric membranes. However, when one surface of the membrane is fully open, then oxygen permeation is greatly enhanced mainly due to the remarkable increase in the effective surface area for surface exchange reactions. In order to evaluate the actual surface exchange area due to the presence of the porous structure within the asymmetric membranes, a correction factor α, has been introduced in the theoretical analysis presented. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Gas permeation performance of cellulose hollow fiber membranes made from the cellulose/N‐methylmorpholine‐N‐oxide/H2O system

Cellulose hollow fiber membranes (CHFM) were prepared using a spinning solution containing N‐methylmorpholine‐N‐oxide as solvent and water as a nonsolvent additive. Water was also used as both the internal and external coagulant. It was demonstrated that the phase separation mechanism of this system was delayed demixing. The CHFM was revealed to be homogeneously dense structure after desiccation. The gas permeation properties of CO₂, N₂, CH₄, and H₂ through CHFM were investigated as a function of membrane water content and operation pressure. The water content of CHFM had crucial influence on gas permeation performance, and the permeation rates of all gases increased sharply with the increase of membrane water content. The permeation rate of CO₂ increased with the increase of operation pressure, which has no significant effect on N₂, H₂, and CH₄. At the end of this article a detailed comparison of gas permeation performance and mechanism between the CHFM and cellulose acetate flat membrane was given. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1873–1880, 2004     

Carbon‐supported perovskite oxides as oxygen reduction reaction catalyst in single chambered microbial fuel cells

BACKGROUND: Pt‐free cathodic catalyst is needed for microbial fuel cells (MFCs). Perovskite‐type oxide could be a substitute for Pt because it has been proved to be a highly active and low‐cost oxygen reduction catalyst in chemical fuel cells. RESULTS: A nano‐sized La_0.4Ca_0.6Co_0.9Fe_0.1O₃ perovskite‐type oxide on a carbon support (LCCF/C) was prepared and tested for its performance and stability (15 cycles) in MFCs. An exchange current density of 7.030 × 10^?5 (A cm^?2) was obtained with fresh LCCF/C cathode and is increased to 7.438 × 10^?5 (A cm^?2) after 15 cycles operating in MFCs. A power density of 405 mW m^?2 was achieved with the LCCF/C cathode at the 2nd cycle which was between those of Pt/C (560 mW m^?2) and C (339 mW m^?2) cathodes. At the end of the 15th cycle, the lowest decay (due to biofouling) rate on the open circuit voltage (2%) and the maximum power density (15%) were observed with LCCF/C cathode compared with those of Pt/C (4%, 17%) and C (22%, 69%) cathodes, respectively. CONCLUSIONS: This study demonstrated that perovskite‐type oxide on carbon support catalysts could be a potential substitute for Pt for cathodic oxygen reduction reaction (ORR) in air‐cathode MFCs. © 2012 Society of Chemical Industry     

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     

Dual‐layer PBI/P84 hollow fibers for pervaporation dehydration of acetone

Acetone dehydration via pervaporation is challenging, because acetone and water have close molecular sizes, and acetone has a much higher vapor pressure than water. Acetone is also a powerful solvent, which dissolves or swells most polymers. We have developed novel polybenzimidazole/BTDA‐TDI/MDI (PBI/P84) dual‐layer hollow fibers for pervaporation dehydration of acetone for industrial and biofuel separations. Both thermal and chemical crosslinking modifications were applied to the membranes to investigate their effectiveness to overcome acetone‐induced swelling. Thermal treatment can effectively enhance separation performance, but performance stability can only be achieved through the crosslinking modification of PBI. Crosslinking by p‐xylene dichloride followed by a thermal treatment above 250°C show significant effectiveness to improve and stabilize pervaporation performance. The fractional free volume of the PBI selective layer reduces from 3.27 to 1.98% and 1.33%, respectively, after thermal treatment and a combination of chemical/thermal crosslinking modifications characterized by positron annihilation spectroscopy. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

The influence of processing parameters on the properties of melt‐spun polypropylene hollow fibers

Isotactic polypropylene hollow fibers were produced by melt spinning. Spinning speeds up to 1880 m/min were used, and sample hollowness (percentage void in cross section) ranged from 0 to 69%. The fiber samples were characterized using dynamic mechanical analysis, birefringence, tensile testing, and differential scanning calorimetry. The hollow fibers were found to have higher crystallinity, orientation, and strength than the analogous solid fibers. In general, the polymer orientation in a hollow fiber was larger than the orientation in a solid fiber, even when the spinning speed for the latter was much larger. For a fixed outer diameter, increasing the hollowness improved fiber properties. However, as hollowness was further increased, fiber properties declined slightly. At a given percentage hollowness, increased spinning speed increased modulus and tenacity. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1759–1772, 2002     

Deformation kinetics of polypropylene hollow fibers in a continuous drawing process

Effects of isothermal drawing conditions on the deformation kinetics and dimensional change of polypropylene (PP) hollow fibers in a continuous drawing process were investigated. The deformation behavior of solid PP polymers during stretching between two rolls in the isothermal bath was analyzed by a simple model describing the continuous drawing process with a constitutive relation that can express a true (stress–strain–strain rate) surface of solid semicrystalline polymers. Necking profiles during drawing can be calculated from this model without any special assumption for neck criterion, and the calculated results predict that the localization of deformation is promoted with the increase of applied draw ratios. It is also found that at 20°C, the neck is observed apparently both from the calculated and experimental results, and the strain‐rate sensitivity parameter is considered to be a critical factor that determines the intensity of the neck geometry. The calculated drawing forces are shown to increase with increasing the applied draw ratio and decreasing the drawing temperature, and these trends were verified by experimental results. The hollowness, defined as the ratio of inner to total cross‐sectional area, increases as it is drawn at 30°C, but decreases as drawn above this temperature compared with that of the undrawn fiber. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1836–1845, 1999     

Nonstoichiometry and Li‐ion transport in lithium zirconate: The role of oxygen vacancies

Understanding Li‐ion migration mechanisms and enhancing Li‐ion transport in Li₂ZrO₃ (LZO) is important to its role as solid absorbent for reversible CO₂ capture at elevated temperatures, as ceramic breeder in nuclear reactors, and as electrode coating in high‐voltage lithium‐ion batteries (LIBs). Although defect engineering is an effective way to tune the properties of ceramics, the defect structure of LZO is largely unknown. This study reports the defect structure and electrical properties of undoped LZO and a series of cation‐doped LZOs: (i) depending on their charge states, cation dopants can control the oxygen vacancy concentration in doped LZOs; (ii) the doped LZOs with higher oxygen vacancy concentrations exhibit better Li⁺ conductivity, and consequently faster high‐temperature CO₂ absorption. In fact, the Fe (II)‐doped LZO shows the highest Li‐ion conductivity reported for LZOs, reaching 3.3 mS/cm at ~300°C that is more than 1 order of magnitude higher than that of the undoped LZO.     

Pushing the limits of high performance dual‐layer hollow fiber fabricated via I2PS process in dehydration of ethanol

The immiscibility induced phase separation (I²PS) process was introduced as a novel method to fabricate hollow fibers with exceptionally high water permeance and reasonably high water/ethanol selectivity in dehydration of ethanol by pervaporation. As a continuation of the previous work, this study discloses the mechanisms to enhance the performance of hollow fibers spun via I²PS by elucidating the material selection at the inner‐layer. Moreover, it revealed the methods to reduce mass‐transport resistance by enhancing surface porosity for both inner and outer surfaces to further improve the permeation flux of the membranes. The continuous performance test demonstrates that the fibers spun from the I²PS possess a stable dehydration performance throughout the monitored period of 300 h. A comparison with pervaporation membranes in the literatures verifies the superiority of the membranes spun via I²PS process with the highest water permeation flux of 9.5 kg/m² h and the permeate water purity of 95.8 wt % at 80°C. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3006–3018, 2013     

Degradation mechanism analysis of Ba0.5Sr0.5Co0.8Fe0.2O3‐δ membranes at intermediate‐low temperatures

The degradation of the permeation flux of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ membranes has typically been attributed to the phase transformation of the material at intermediate temperatures. In this study, the effect of the interfacial oxygen exchange steps was considered to give an overall view of the degradation mechanism. The changes in the interfacial exchange resistances, bulk resistance, and morphologies of the membranes were investigated via physical characterizations and a permeation model. The interfacial oxygen exchange resistances increased more quickly with time than bulk resistance. Meanwhile, BaSO₄ particles were detected on both surfaces of the membranes, and their contents reached maximum at 650°C. However, after the membrane surfaces were coated by Sm_0.5Sr_0.5CoO_3‐δ porous layers, the interfacial oxygen exchange resistances kept constant and the degradation rates were slowed down. The degradation was predominated by the increase of interfacial oxygen exchange resistances induced by the enrichment of BaSO₄ particles on membrane surfaces. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3879–3888, 2015