Oxygen Permeation Performance Research Articles

Computational thermal fluid enabled multi-physics transport processes analysis of ceramic hollow fiber membrane for oxygen separation

A comprehensive hollow fiber membrane model is developed using an experimental system as a physical base, where Multiphysics transport processes in the membrane are coupled with a large-scale thermal fluid transport processes in the test assembly and furnace. Systematic parametric studies are carried out to understand fundamental mechanisms and membrane performance. Results indicate that oxygen partial pressure at the permeate side asymptotically increases from the inlet toward the outlet of hollow fiber membrane. Longitudinal distribution of oxygen vacancy concentration follows the opposite trend to that of oxygen partial pressure at the permeate side while oxygen flux distribution follows the profile of oxygen vacancy distribution at the permeate surface. High air pressure at the feed side and low gas pressure at the permeate side facilitate to enhance oxygen permeation performance. Surface exchange resistance at the permeate side dominates the total resistance of the membrane while the bulk diffusion resistance accounts for the minimum contribution in substrate-supported thin film hollow fiber membrane. The modeling study may obtain an insightful understanding of fundamental mechanisms and can provide guidance for membrane design and operations to achieve better performance for oxygen production.

Functionally graded La0.6Sr0.4Co0.2Fe0.8O3−δ hollow fiber membrane for oxygen separation: fabrication, upscaling, and stability

La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) hollow fiber membranes are usually fabricated through slurry spinning process followed by one-step sintering. While high oxygen permeation performance can be obtained, the designs lack sufficient mechanical strength, limiting further upscaling for stack and module development. This research studies a functionally graded hollow fiber LSCF membrane with a three-layer structure, i.e., thick LSCF-ZnO substrate/dense thin film LSCF/porous, thin LSCF catalyst layer. The thick substrate provides sufficient mechanical strength to support the membrane while facilitating facile gas diffusion with embedded open microchannels. The dense thin film LSCF separation layer decreases bulk diffusion resistance. The porous thin LSCF catalyst layer increases the reaction area for surface exchange process. The synergetic combination of the three layers enables to improve both mechanical strength and permeation performance. A stack with three parallelly-connected hollow fibers is assembled to demonstrate its potential capability for upscaling. Permeation performance is systematically measured and accelerated long-term stability is conducted for both the single membrane and the stack. The single membrane and the stack obtain oxygen permeation flux of ∼1.9 and 1.1 ml⋅cm−2⋅min−1 at 900 °C, respectively, and demonstrate excellent stability and robustness during a long-term test of ∼400 h and ∼20 thermal cycles between 600 and 900 °C. Membrane samples are characterized before and after the test. Experimental data are analyzed, and fundamental mechanisms are discussed.

Preparation of Textured Polycrystalline La2NiO4+ δ Membranes and Their Oxygen-Transporting Properties

Due to its high chemical and thermal stability in CO2 atmosphere and its anisotropic crystal structure, the Ruddlesden-Popper phase La2NiO4+δ has attracted considerable attention in the research field of oxygen-transporting membranes. The anisotropic properties of La2NiO4+δ can be exploited in textured polycrystalline membranes to control the oxygen diffusion through this material. To fabricate textured La2NiO4+δ membranes, powder mixtures consisting of fine-grained equiaxial La2NiO4+δ matrix particles and large plate-like La2NiO4+δ template particles in different mass ratios were uniaxially pressed and then sintered in air. For this purpose, the anisotropic template particles were synthesized by molten-flux method using NaOH as flux [1]. In the powder mixture, the La2NiO4+δ template particles can be aligned perpendicular or parallel to the pressing direction, depending on the geometry of the pressing tool and the sample preparation. After the sintering process, textured La2NiO4+δ membranes were obtained, which was verified by measuring X-ray diffraction patterns and pole figures. Further X-ray diffraction measurements together with the calculation of the Lotgering orientation factor revealed that an increasing content of the template particles in the ceramic materials leads to a stronger texturing. Scanning electron microscopy micrographs show some individual plate-like La2NiO4+δ grains well embedded in the matrix. Homogeneous distribution of lanthanum, nickel and oxygen in the ceramics was confirmed by energy-dispersive X-ray spectroscopy. Finally, the influence of the template particle content in the La2NiO4+δ membranes on the oxygen permeation performance is discussed. [1] Escobar Cano, G.; Brinkmann, Y.; Zhao, Z.; Kißling, P.A.; Feldhoff, A. Sol–Gel-Process-Based Molten-Flux Synthesis of Plate-like La2NiO4+ δ Particles. Crystals 2022, 12, 1346. https://doi.org/10.3390/cryst12101346

CO2-stable and cobalt-free Ce0.8Sm0.2O2-δ-La0.8Ca0.2Al0.3Fe0.7O3-δ dual-phase hollow fiber membranes for oxygen separation

In this work, 60 wt% Ce0.8Sm0.2O2-δ-40 wt% La0.8Ca0.2Al0.3Fe0.7O3-δ (SDC-LCAF) dual-phase oxygen permeable hollow fiber membrane precursors were prepared via a combined sol–gel and phase inversion technique. Dual-phase SDC-LCAF hollow fiber membrane with compatible fluorite and perovskite phases was obtained after sintering at 1450 °C. The chemical compatibility, thermal expansion, conductivity, oxygen permeation, CO2 tolerance, and short-term regenerative durability regarding the phase structure and composition were systematically studied. The oxygen flux of the SDC-LCAF dual-phase membrane was significantly increased compared to that of LCAF single phase membrane. At 950 °C, the oxygen flux of the SDC-LCAF hollow fiber membrane reached 1.84 mL min−1 cm−2 when both the He sweep and the air feed flow rates were 100 mL min−1, and the dual-phase membrane exhibited stable oxygen permeation fluxes in pure CO2 atmosphere. When pure CO2 was used as the sweep gas in a 225-h short-term stability test, the oxygen permeation fluxes decreased slightly because of the inhibiting effect of CO2 on oxygen surface-exchange reaction. Nevertheless, the ability of the SDC-LCAF membrane to recover the original oxygen permeation performance indicated that the membrane surface did not react irreversibly with CO2. The CO2 stability was not reflected in the LCAF single phase membrane, thus indicating the superior membrane stability following the incorporation of the SDC phase.

Catalyst-modified perovskite hollow fiber membrane for oxidative coupling of methane

The catalytic oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H6 and C2H4) represents one of the most effective ways to convert natural gas to more useful products, which can be performed effectively using La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) perovskite hollow fiber membrane microreactor. In this work, the effects of adding a thin BaCe0.8Gd0.2O3−δ (BCG) catalyst film onto the inner LSCF fiber surface as the OCM catalyst and a porous Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) perovskite layer onto the outer LSCF surface to improve the oxygen permeation were evaluated. Between 700 °C and 1000 °C, methane conversion increased in the order of uncoated, BCG and BSCF-coated, and BCG-coated LSCF hollow fiber while C2-selectivity and C2-yield increased in the order of BCG and BSCF-coated, uncoated, and BCG-coated LSCF hollow fiber. Oxygen permeation flux at the same temperature range, on the other hand, was enhanced in the order of uncoated, BCG-coated, and BCG and BSCF-coated LSCF hollow fiber. This finding demonstrates the complex interplay between oxygen permeation and OCM performance. The BCG and BSCF-coated hollow fiber was also subjected to thermal cycles between 850 °C and 900 °C for up to 175 hours during which the fiber showed minor degradation in oxygen permeation fluxes and relatively stable OCM performance.

Modeling and simulation study of oxygen permeation in La0.8Ca0.2Fe0.95O3-δ-Ag hollow fiber membrane module

Mixed ionic-electronic conducting (MIEC) ceramic membranes present an attractive alternative oxygen separation technology to cryogenic distillation and pressure swing adsorption. The model of Tan and Li has been extensively adopted in the oxygen permeation modeling through MIEC hollow fiber membranes. To evaluate the performance of the La0.8Ca0.2Fe0.95O3-δ-Ag hollow fiber membrane module here, the oxygen permeation is simulated numerically by feeding the air into the shell while passing sweep gas or creating a vacuum in the permeate side under different configuration. The simulation results show that countercurrent flow configuration is preferable in the sweep gas mode given its capability to separate oxygen completely under moderate flow rate of feed air. For vacuum mode, both cocurrent and countercurrent flow configurations result in a similar oxygen permeation performance, which lies between the sweep gas mode with countercurrent and cocurrent flow configurations. When abundant oxygen presence is created in the feed side by elevating its flow rate, the sweep gas mode loses its oxygen separation efficiency and is thus outperformed by the vacuum mode.

Oxygen-permeable ceramic membrane with improved mediate-temperature stability based on partially A-site doped KxSr1-xCo0.8Fe0.2O3-δ

ABSTRACT Practical application of SrCo0.8Fe0.2O3-–δ(SCF) perovskite oxide for oxygen separation is hindered by poor stability. Here, we developed new K x Sr1–x Co0.8Fe0.2O3–δ membrane material by optimizing K+ doping in A-site of prototypical SCF. The partial substitution of Sr2+ with K+ can lead to the enhanced mechanical strength and intermediate-temperature oxygen permeation stability. It is found that the doping content of K+ should be lower than 20% because the excessive doping of K+ in A-site of SrCo0.8Fe0.2O3–δcan destroy their cubic perovskite structure. While x ≥ 0.2, some diffraction peaks different from cubic perovskite can be evidenced by X-ray diffraction and their oxygen permeation flux also decreases rapidly. Among the fabricated membranes, K0.1Sr0.9Co0.8Fe0.2O3–δexhibits the highest oxygen permeation flux (1.5 cm3 min−1 cm−2 at 950°C) under an air/He gradient. More important is that K0.1Sr0.9Co0.8Fe0.2O3–δ displays a stable oxygen permeation performance during over 120 h permeation operation at 750°C.

New bio-inspired design for high-performance and highly robust La0.6Sr0.4Co0.2Fe0.8O3-δ membranes for oxygen permeation

Ceramic-based oxygen permeation membranes (OPM) are considered to be promising for the separation of oxygen from air. However, state-of-art membrane designs are unable to deliver satisfactory performances in terms of permeation flux, mechanical/chemical stability and membrane surface area. In this study, a new bio-inspired design has been successfully introduced in the micro-monolithic membranes made of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) for oxygen separation. By carefully controlling the process parameters of the fabrication and utilizing the hydraulic pressure of internal coagulant, the geometry of channels in the micro-monolith has been converted from a circular shape to a triangle shape with rounded corners. This new bio-inspired, ‘orange-like’ architecture not only reduces the effective oxygen diffusional length down to approximately 50 μm, but also significantly increases the ratio of active region among the overall circumference up to 90%. This new bio-inspired micro-monolithic design displays an excellent oxygen permeation flux of 1.87 ml min−1cm−2 at 950 °C, which is superior to the most reported values from LSCF material systems. In addition, such a design illustrates an excellent mechanical robustness that has long been a bottleneck for LSCF membranes. This work demonstrates a promising solution to tackle the long-existing trade-off between oxygen permeation performance and mechanical reliability.

La0.6Sr0.4Fe0.8Co0.2O3-δ electrophoretic coating for oxygen transport membranes

This work describes the development of electrophoretic deposition method for the elaboration of a porous coating on a dense membrane in order to improve its oxygen permeation performance. The dense membrane material produced from La0.5Sr0.5Fe0.7Ga0.3O3-δ perovskite is coated with La0.6Sr0.4Fe0.8Co0.2O3-δ perovskite layer by electrophoretic deposition method in order to improve the kinetics of oxygen surface exchanges. Then, the oxygen flux through the La0.5Sr0.5Fe0.7Ga0.3O3-δ dense membrane is largely impacted by La0.6Sr0.4Fe0.8Co0.2O3-δ electrophoretic coating, from 0.5 to 2.6 10−3 mol⋅m−2⋅s−1 without and with La0.6Sr0.4Fe0.8Co0.2O3-δ coating after heating at 900 °C, respectively. Besides, a large impact of the thickness of the electrophoretic coating on the oxygen flux through the membranes is observed. The electrophoretic deposition has shown to be a powerful elaboration method of membrane to produce gaseous oxygen with high purity.

High CO2-tolerant and Cobalt-free dual-phase membranes for pure oxygen separation

A series of La0.15Sr0.85FeO3-δ-xLa0.15Ce0.85O2-δ (LSFO-xLDC) and Cu-doped LSFO-70La0.15Ce0.8Cu0.05O2-δ (LSFO-70LDCC) dual-phase membranes were prepared by one-pot method and evaluated systematically as high CO2-tolerant oxygen separation membranes. The results reveal that the stability of the membranes is improved by compositing LDC. Among the studied compositions, the LSFO-70LDC compound shows excellent stability under reducing atmosphere and CO2-containing atmosphere. The oxygen permeability of LSFO-70LDC membrane with 0.6 mm thickness is 0.49 mL cm−2 min−1 at 950 °C. In addition, LSFO-70LDC shows outstanding long-term stability with stable oxygen permeability of 0.30 mL cm−2 min−1 at 900 °C for 100 h. Cu-substitution prominently enhances the oxygen permeation performance of the LSFO-70LDCC dual-phase membrane, which delivers an enhanced oxygen permeability of 0.65 mL cm−2 min−1 at 950 °C. Moreover, the LSFO-70LDCC membrane exhibits excellent long-term stability for 100 h at 900 °C in both pure He and CO2. These results indicate that the developed LSFO-70LDC and LSFO-70LDCC dual-phase membranes are very promising membranes for oxygen separation.

Phase structure and oxygen permeability of BaCe0.15CoxFe0.85−xO3−δ perovskite materials

Cerium-doped perovskite materials, BaCe0.15CoxFe0.85-xO3−δ, were synthesized by a sol-gel method. Phase structure and microstructure of BaCe0.15CoxFe0.85−xO3−δ were investigated using XRD and SEM. The critical doping value of x = 0.15 for cerium ions can be obtained for BaCe0.15CoxFe0.85−xO3−δ. This paper further investigated the effect of cobalt ions on the phase structure of BaCe0.15CoxFe0.85−xO3−δ. As the cobalt doping content is more than 0.6, BaCe0.15CoxFe0.85−xO3−δ deviates from cubic perovskite phase completely. Oxygen permeation test revealed that the oxygen permeability of BaCe0.15CoxFe0.85−xO3−δ series increases with the increase of cobalt doping. Among BaCe0.15CoxFe0.85−xO3−δ materials, BaCe0.15Co0.4Fe0.45O3−δ exhibits cubic perovskite structure and excellent oxygen permeation performance.

Correction to Effect of Ba Content on Oxygen Permeation Performance of BaxSr1–xCo0.8Fe0.2O3-δ (x = 0.2, 0.5, and 0.8) Perovskite-Type Membrane

Correction to Effect of Ba Content on Oxygen Permeation Performance of Ba_<i>x</i>Sr_1–<i>x</i>Co_0.8Fe_0.2O_3-δ (<i>x</i> = 0.2, 0.5, and 0.8) Perovskite-Type Membrane

Further improvements in oxygen permeation properties of La0.6Sr0.4Ti0.3Fe0.7O3−δ coated Ba0.5Sr0.5Co0.8Fe0.2O3−δ hollow fiber membrane

Abstract Oxygen-permeable Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ (BSCF) hollow fiber membranes was prepared by a phase inversion spinning process. In the second stage, La 0.6 Sr 0.4 Ti 0.3 Fe 0.7 O 3−δ (LSTF) was coated on the surface of the prepared BSCF hollow fiber membranes by dip coating method in an attempt to improve not only the oxygen permeation performance but also the long-term chemical stability. The oxygen permeation fluxes were measured in the temperature range of 850–1000 °C in air. The oxygen permeation flux reached to 9.18 mL/min/cm 2 at 1000 °C. In addition, long-term operation test results indicated that the oxygen permeation fluxes remained consistently during the 240 h operation. With the dual aims of reviewing existing work in the topic and understanding the effects of membrane morphology and thickness on the performance of BSCF membrane, the oxygen permeation property of LSTF-coated BSCF hollow fiber membrane with engineered morphology was compared directly with its corresponding uncoated case and then with other configuration types, such as BSCF disk-shaped and tubular BSCF membranes.

Influence of Mo-doping on structure and oxygen permeation properties of SrCo0.8−xFe0.2MoxO3-δ perovskite membranes for oxygen separation

The effect of cobalt substitution by highly charged Mo6+ cations in SrCo0.8Fe0.2O3-δ (SCF) on the structure, microstructure, oxygen permeation performance and stability in the atmosphere containing carbon dioxide was systematically investigated by XRD, HT-XRD, Mossbauer spectroscopy, oxygen release technique and oxygen permeation experiments. The decrease in oxygen stoichiometry of SrCo0.8-xFe0.2MoxO3-δ (SCFM) materials was accompanied by nanostructuring with observed formation of nanosized domains with ordered oxygen vacancies. Equilibrium "3−δ−lg pO2−T" diagrams showed that SCF doping by molybdenum leads to broadening of the P1 (cubic perovskite phase) stability region. The obtained SCFM membrane materials possess improved phase stability in the intermediate temperature range (T < 700°C) and low oxygen partial pressure (pO2 < 0.06atm) compared to the undoped SCF oxide. The observed non-Arrhenius dependence of oxygen fluxes across SCFM disc membranes is related to combined control by bulk diffusion and surface exchange reactions.

Asymmetric La0.6Sr0.4Co0.2Fe0.8O3-δ membrane with reduced concentration polarization prepared by phase-inversion tape casting and warm pressing

A three-layered La0.6Sr0.4Co0.2Fe0.8O3-δ membrane was prepared via a modified phase-inversion tape casting method combined with warm pressing and screen-printing. The membrane comprised a thick porous support with straight and large open finger-like pores, a dense separation layer with reduced thickness of 40µm, and a thin catalytic layer. Oxygen permeation performance was studied under various conditions, and compared with that for a similar membrane, the support of which was fabricated by conventional tape casting and associated with a distinctly different pore structure. Under an air/He gradient, an oxygen flux as high as 1.54ml (STP) cm−2 min−1 was achieved at 900°C for the former membrane, about 2.5 times higher than that for the latter. When pure oxygen was used instead of air as the feed gas, their oxygen permeation fluxes were only slightly differed. The obtained results clearly indicated serious presence of concentration polarization in air feed gas in the membrane made by conventional tape casting, which was markedly reduced in the phase-inversion derived membrane. The significantly improved oxygen permeation performance of the latter membrane could be ascribed to its unique pore structure, which allowed fast gas transport through the porous support.

Effect of enhanced oxygen reduction activity on oxygen permeation of La0.6Sr0.4Co0.2Fe0.8O3−δ membrane decorated by K2NiF4-type oxide

Asymmetric dense La0.6Sr0.4Co0.2Fe0.8O3−δ hollow fiber membranes were prepared by the joint phase inversion and sintering processes. Surface treatment by hydrochloric acid-etching and decoration of dispersed porous (La0.5Sr0.5)2CoO4+δ thin layer, was applied to improve the surface exchange reaction kinetics of oxygen reduction. Oxygen permeation performance of the unmodified and modified membranes was investigated under different operating conditions. Experimental results indicate that both surface treatment strategies effectively accelerate the oxygen permeation rate of the resultant membranes by the improved surface exchange kinetics; in particular, the oxygen fluxes were enlarged from 0.036 to 1.021 ml min−1 cm−2 of the bare membrane to 0.201–1.311 ml min−1 cm−2 of the decorated membrane by (La0.5Sr0.5)2CoO4+δ with a minimum improvement percentage of 28% when operation temperature varied from 700 to 1000 °C with helium flow rate of 150 ml min−1. Furthermore, better flux enhancement of the surface-modified membrane was observed at lower temperature regimes than at higher temperatures indicates at lower temperatures the surface exchange reaction kinetics play a larger role in controlling the overall oxygen transport through the membrane; in other words, the relative limiting effect of the ionic bulk diffusion becomes more noticeable at higher temperatures. In addition to the significant oxygen flux improvement, the surface decorated membrane also displays high permeation stability under the investigated operating conditions.

Enhanced High Oxygen Permeation of Mixed-Conducting Multichannel Hollow Fiber Membrane via Surface Modified Porous Layer

The oxygen permeation performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) mixed-conducting multichannel hollow fiber (MCMHF) membranes was improved by surface modification via spin-spraying of a La0.6Sr0.4CoO3−δ (LSC) porous layer. At 1173 K, the oxygen permeation flux of the modified membranes was clearly enhanced and reached 9.68 mL·cm–2·min–1, which is a remarkable high value in the field of mixed-conducting oxygen permeation membrane processes. Theoretical calculations demonstrated that the oxygen transport resistance, especially the surface exchange resistance, obviously decreased as a result of the modified LSC porous layer. Moreover, the process of oxygen permeation through the modified membrane was controlled by both bulk diffusion and the surface oxygen exchange reaction, whereas the oxygen permeation of the unmodified membrane was dominantly controlled by the surface oxygen exchange reaction. The modified MCMHF membranes showed generally stable oxygen permeation fluxes over 100 h at 1173 K.

Influence of CO2 on Oxygen Surface Exchange Kinetics of Mixed-Conducting Ba0.5Sr0.5Co0.8Fe0.2O3—δ Oxide

The poisoning effect of CO2 on the oxygen surface exchange kinetics of BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3—δ) is investigated with a novel pulse isotopic exchange technique. The surface exchange rate of BSCF severely decreases after in situ exposure to CO2, which is ascribed to carbonate formation on the material surface. The detrimental effect of CO2 starts at a low temperature of 375 °C and concentration as low as 1%, and becomes more pronounced at higher temperatures. Degradation of the surface exchange kinetics is associated with a rapid loss of oxygen permeation performance of BSCF in CO2.

Influence of different sintering techniques on microstructure and phase composition of oxygen-transporting ceramic

The membrane microstructure and phase composition of Ruddlesden–Popper-type La2NiO4+δ ceramics, which were prepared by field-assisted sintering technique/spark plasma sintering (FAST/SPS) process or by conventional pressing and pressureless sintering were investigated. As starting material, a La2NiO4+δ powder, with an average particle size of 0.2μm was used. The grain-size distribution of the resulting membranes varied from 0.015μm2 for FAST/SPS sintered ceramic to 6.11μm2 for pressureless sintered membrane. The microstructure analysis of membranes was performed by transmission and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. Phase transition from orthorhombic to tetragonal crystallographic structure in FAST/SPS material was investigated by temperature-dependent X-ray powder diffraction. The effect of sintering technique and grain size on the oxygen permeation performance of the membranes is discussed with respect to impurities in the material.

Oxygen permeation through dense La0.1Sr0.9Co0.8Fe0.2O3−δ perovskite membranes: Catalytic effect of porous La0.1Sr0.9Co0.8Fe0.2O3−δ layers

The oxygen permeation performance of a number of La0.1Sr0.9Co0.8Fe0.2O3−δ (LSCF1982)-based membranes, consisting of dense LSCF1982 layer with/without porous LSCF1982 layer, was analyzed on the basis of the thickness of the dense layer and catalytic effect of the porous layer. A 0.27mm thick dense membrane gives oxygen permeation flux (JO2) of 2.33sccmmin−1cm−2 at 900°C, which is increased to 3.55sccmmin−1cm−2 on applying a porous layer of LSCF1982 onto the dense membrane. The membrane gives a stable flux for 300h. The flux was further improved by reducing the thickness of the dense LSCF1982 layer and at 950°C a flux of 4.47sccmmin−1cm−2 is obtained with 0.012mm thick membrane.