BSCF Membranes Research Articles

Diffusion Barriers Minimizing the Strength Degradation of Reactive Air Brazed Ba0.5Sr0.5Co0.8Fe0.2O3-δ Membranes during Aging.

The separation of oxygen from air by means of inorganic ceramic membranes requires gas-tight ceramic-metal joints that enable reliable permeation operation in the oxygen partial pressure gradient at 850 °C. Reactive air brazing is a promising method to solve this challenge. However, reactive air brazed BSCF membranes suffer from a significant strength degradation that is caused by unhindered diffusion from the metal component during aging. In this study, we investigated how diffusion layers applied on the austenitic steel AISI 314 influence the bending strength of BSCF-Ag3CuO-AISI314 joints after aging. Three different approaches were compared as diffusion barriers: (1) aluminizing via pack cementation, (2) spray coating with NiCoCrAlReY, and (3) spray coating with NiCoCrAlReY and an additional 7YSZ top layer. Coated steel components were brazed to bending bars and aged for 1000 h at 850 °C in air prior to four-point bending and subsequent macroscopic as well microscopic analyses. In particular, coating with NiCoCrAlReY showed low-defect microstructures. The characteristic joint strength was raised from 17 MPa to 35 MPa after 1000 h aging at 850 °C. In addition, the dominant delamination fracture between the steel and the mixed oxide layer, observed in the reference series with uncoated steel, could be replaced by mixed and ceramic fractures of higher strength. The effect of residual joint stresses on the crack formation and path is analyzed and discussed. Chromium poisoning could no longer be detected in the BSCF, and interdiffusion through the braze was effectively reduced. Since the strength degradation of reactive air brazed joints is mainly caused by the metallic joining partner, the findings on the effect of the diffusion barriers in BSCF joints might be transferred to numerous other joining systems.

Testing of high performance asymmetric tubular BSCF membranes under pressurized operation – A proof-of-concept study on a 7 tube module

Oxygen transport membranes are a possible low-cost alternative to cryogenic air separation units or pressure swing adsorption for oxygen production, and could facilitate carbon capture by oxy-fuel combustion processes. In this work, a proof-of-concept module with 7 ceramic tubular membranes was designed and tested in a high pressure test stand. To ensure high oxygen production rates, asymmetric membranes with a length of 70 cm consisting of a porous Ba0.5Sr0.5(Co0.8Fe0.2)0.97Zr0.03O3-δ support and a dense Ba0.5Sr0.5(Co0.8Fe0.2)0.97Zr0.03O3-δ thin separation layer were used. The module was successfully operated at up to 5.4 bar for 280 h in a temperature range from 650 °C to 850 °C. Oxygen purities up to 96.5% and flow rates up to 6.2 ml/cm2/min were measured, and a total maximum oxygen production of 227 NLPH was demonstrated.

Reduced Strength Degradation of Reactive Air Brazed Bscf Membranes by Pre-Oxidation of Metallic Components

Reduced Strength Degradation of Reactive Air Brazed Bscf Membranes by Pre-Oxidation of Metallic Components

Characterization of oxygen transport phenomena on BSCF membranes assisted by fluid dynamic simulations including surface exchange

The influence of fluid dynamic conditions on the oxygen transport through mixed ionic-electronic membranes was studied experimentally and numerically. A set of permeation experiments was performed in a wide range of operating conditions combining temperature, driving force and flow rate of air feed and sweep streams. A computational model was built and enabled to systematically evaluate the effect of the fluid dynamic conditions on O2 separation process. This model includes the surface resistance (gas exchange kinetics) and bulk oxygen-ion diffusion. The experimental set was used to obtain the model parameters by following a two-step fitting procedure: a first fitting of a simplified one-dimensional model using genetic algorithms and a subsequent refining of the parameters using the complete model implemented in COMSOL Multiphysics. The high-accuracy model allowed characterizing the O2 transport phenomena and understanding the different factors governing the overall process, e.g. by using different membrane thicknesses, sweep gases or vacuum. Simulations of the fitted model revealed the importance of the sweep effect in the O2 transport across the membrane and enabled to establish rules for both design of permeation experiments and up-scaling.

Preparation and Morphological Property of Co3O4/BaxSr1-xCo0.8Fe0.2O3-δ (x=0.5-0.7) Membranes using Starch as Binder Agent

Ba0.5Sr0.5Co0.8Fe0.2O3-δ (x=0.5-0.7) or BSCF was known as promising material to prepare MIEC membrane for oxygen separation. Several modifications have been performed to improve catalytic activity of BSCF membranes. One of the promising methods was coating of the membrane with catalyst layer such as Co3O4. This work investigates the simple and effective method to coat Co3O4 layer on the surface of BaxSr1-xCo0.8Fe0.2O3-δ (x=0.5-0.7) flat sheet membranes resulting Co3O4/BSCF xx82 membranes. BSCF were prepared from its initial powders by dry pressing method followed by sintering at 1150 °C for 4 hours. The resulted BSCF dense membranes were coated with Co3O4 using starch as binder agent in the form of suspension. The coating suspension consist of 62.5% w/w of starch and 37.5% w/w of Co3O4. The coating suspension was directly coated on the surface of the BSCF dense membranes followed by calcination at 800 °C for 3 hours. The surface morphology and cross section of membranes were observed using Scanning Electron Microscope (SEM) equipped with Energy Dispersive X-ray (EDX). SEM images show that the method succeeds to coat Co3O4 layer on the surface of BSCF membranes. However, the distribution of the Co3O4 was not uniform and some particles form aggregate. The particle size of the membranes was about 0.5-2.0 μm whereas the aggregate size is around 2-5 μm. The presence of Co3O4 layer was also indicated by the increasing of Co intensity in the surface of membranes as analyzed by EDX.

Catalyst Screening for Oxidative Coupling of Methane Integrated in Membrane Reactors

Increased availability of methane from shale gas and stranded gas deposits in the recent years may facilitate the production of ethylene by means of potentially more competitive routes than the state-of-the-art steam cracking processes. One appealing route is the oxidative coupling of methane (OCM), which is considered in this work for the production of ethylene by means of the use of catalytic membrane reactors (CMR) based on Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) ceramic material. In a first approach, a screening of 16 formulations as catalysts for the ethylene-ethane production was conducted on CMR consisted of disk-shaped planar BSCF membranes. At 900 oC, the maximum C2 selectivity was 70%, reached with Ba0.5Sr0.5FeO3 and La0.5Ce0.1Sr0.4Co0.8Fe0.2O3 catalysts. On the other hand, low CH4 conversions (XCH4) resulted in C2 yields below 3%. Operation at 1000 oC significantly shifted XCH4 for all the activated membranes due to the decrease in CH4/O2 ratios, thus obtaining C2 yields close to 9% and productivities of ca. 1.2 ml·min-1·cm-2 with Ce0.9Gd0.1O2 and Ba0.5Sr0.5Co0.8Fe0.2O3 impregnated with Mn-Na2WO4 catalysts. The performance of OCM reaction was also studied in a tubular catalytic membrane reactor. Tubular configuration improved C2 yield by minimizing CH4/O2 ratios up to 1.7, obtaining a maximum of 15.6% at 900 oC with a BSCF capillary membrane activated with a packed bed of 2 wt% Mn / 5 wt% Na2WO4 on SiO2 catalyst.

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.

Application of Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranes in an oxy-fuel combustion reactor

Oxygen separation from air through ceramic ion transport membranes is anticipated to be one of the technologies that can make fossil fuel power plants to operate without carbon dioxide (CO2) emission. Consequently, the level of carbon emission in the atmosphere, which causes global warming, could be contained. The most promising material for such ion transport membranes is the Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), which has been reported to have the highest oxygen permeation flux. However, lack of chemical stability of these membranes in CO2 rich environment is recognized as a major problem. In this study, the performance and stability of BSCF membranes in an oxy-fuel combustor have been evaluated using methane gas. The flow rate of the methane (CH4) gas has been optimized to the value of 0.65 ml/min at 920 °C. Under this flow rate, the BSCF stability has also been evaluated. The study has shown that the BSCF stability depends on the amount of excess-oxygen in the reactor. If all oxygen, generated by the membrane, is consumed during the combustion reaction then BSCF starts to deteriorate and its permeability decreases with time. On the other hand, when the combustion reaction does not consume all oxygen, and a minimum excess-oxygen is retained, the BSCF remains stable with constant oxygen flux. This fact has been established using two BSCF membranes with different thicknesses and same exposed area of 113 mm2. The first membrane has 1.4 mm thickness and its initial oxygen permeability under combustion reaction is 0.83 µmol cm-2 s−1 with an excess of 7.5% oxygen. After 190 h of combustion, the oxygen permeability has dropped to 0.76 µmol cm−2 s−1 (7.9% reduction). The second BSCF membrane has a thickness of 1.0 mm produces 0.91 µmol cm−2 s−1 with a 15% excess oxygen during the combustion reaction. This membrane has shown excellent stability for more than 200 h of complete methane combustion. This is a remarkable result which shows that a carefully controlled fuel volume introduced in a BSCF membrane-reactor provides high oxygen output and excellent stability for a long period of time.

Oxygen permeation and stability of Mo-substituted BSCF membranes

During 200 h of operation at 750 °C the performance of the Ba0.5Sr0.5Co0.8Fe0.2O3-δ membrane decays by 35%, while Ba0.5Sr0.5Co0.5Fe0.125Mo0.375O3-δ shows steady-state permeation and Ba0.5Sr0.5Co0.78Fe0.195Mo0.025O3-δ slows the decay by a factor of two.

New high temperature sealing technique and permeability data for hollow fiber BSCF perovskite membranes

Oxygen permeation through BSCF perovskite hollow fiber membranes has been investigated in a laboratory scale reactor in a temperature range of 850–1000°C at atmospheric pressure. An experimental study was conducted with particular focus on a new sealing technique which was found to provide fully gas-tight conditions with a 100% success rate for the 6 membranes reported in this work (and for 15 membranes in total). An overview of experimental tests on other sealants proposed in the literature is given. A new method for checking the sealing is suggested that allows detecting the possible leakages in all the parts of the membrane module. A loose-end reactor with a shell-and-tube layout is presented which allows accurate measurements of both gas leakages (if any) and product gas composition in a wide range of operating conditions (flow rates on both the permeate and feed side, O2 feed partial pressure, temperature). Permeation fluxes were studied for BSCF membranes with different thicknesses (0.2 and 0.5mm) using He as sweep gas. The O2 flux increased with both increasing the O2 driving force (i.e., the O2 partial pressure difference through the dense ceramic layer) and the reactor temperature. Long term tests (600h) have been carried out proving the high stability of both the membranes used and the newly proposed sealing technique. The presence of both O2 bulk diffusion and surface exchange resistance is suggested to limit the permeation rate of the ceramic hollow fibers for the range of wall thicknesses investigated. Finally, LSCF capillaries have also been successfully sealed allowing to extend the use of the proposed gas tight sealing system to different compositions of hollow fibers perovskite membranes.

A CO2-tolerant nanostructured layer for oxygen transport membranes

Dual-layer membranes with enhanced CO2 tolerance and unprecedented oxygen permeability under CO2-containing sweep gas are reported. Specifically, a SrFe0.8Nb0.2O3−δ/Ba0.5Sr0.5Co0.8Fe0.2O3−δ (SFN/BSCF) dual-layer membrane structure has been successfully prepared by pulsed laser deposition of SFN thin layer onto polished BSCF membranes. The phase structure and microstructure of the SFN/BSCF membrane are characterized by XRD and TEM, respectively. Two distinct phases originated from SFN and BSCF are both obtained, which suggests that the SFN is in high crystallinity under the as-deposited condition and BSCF maintains its original status. TEM images clearly show that SFN nanostructured layer is compactly coating on the BSCF substrate. Oxygen permeation fluxes of 2.721, 2.276, 1.809 and 1.303 mL cm−2 min−1 at 900, 850, 800 and 750 °C are attained for a ∼45 nm nanostructured SFN layer decorated on a 1 mm thick BSCF membrane using air as the feed and He as the sweep gas. These high oxygen permeation fluxes are comparable with the pristine BSCF membrane since SFN membrane is also a promising mixed conductor and the coated layer is extremely thin. Under He sweep gas with 10% CO2, a stable oxygen permeation flux of ∼2.25 mL cm−2 min−1 at 850 °C is maintained for ∼550 min with the SFN/BSCF membrane, while it is only lower than 0.4 mL cm−2 min−1 with the uncoated membrane. The results indicate that both high oxygen flux and stability can be simultaneously achieved with adoption a nanostructured protective layer.

The effect of non-ionic porous domains on supported Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranes for O2 separation

This work investigates the effect of porosity on the performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) membranes for oxygen separation from air. BSCF membranes were sintered as thin dense layers on porous substrate by dry pressing followed by co-firing. The porous substrate was prepared by an optimised mixture of BSCF and 30wt% porogens (Pluronic® F-68) where the porous substrate and dense layer integrity matched well and delivered mechanically stable membranes. The crystal lattice of both BSCF dense layer and porous substrate were essentially the same as verified by X-ray diffraction and oxygen stoichiometry measurements. Pure dense membranes always delivered higher oxygen fluxes than the membranes prepared on porous substrates. Interestingly, the oxygen fluxes at 850°C reduced by 33% due to the porous substrate, and this value correlated well with porosity (35%) and the decreased conductivity (30%). Further, the oxygen flux increased each time that the thickness of the porous substrate was reduced. The pores in the porous substrates were found to have no interconnection. Consequently, this created occlusions of non-ionic domains and resulted in reduced electrical conductivity and oxygen fluxes. Nevertheless, the variations in oxygen fluxes for the porous supported membranes followed geometrical relations associated with porosity and thickness of the porous substrate.

The influence of impurity ions on the permeation and oxygen reduction properties of Ba0.5Sr0.5Co0.8Fe0.2O3−δ perovskite

This study aims to exploit the effects of little impurity ions on the properties and performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) towards the development of membranes for oxygen separation and electrodes for oxygen reduction. The influences of impurity ions (Na+, K+, Mg2+, Ca2+, Crx+, Mnx+, Si4+ and Cl−) on the crystal structure, electrical conductivity, oxygen desorption/permeation behavior, sintering behavior and oxygen reduction activity of BSCF were systematically investigated. Rietveld refinements of the powder X-ray diffraction patterns revealed that most of the impurity ions, except Mg2+ and Si4+, can incorporate into the BSCF lattice without changing the lattice symmetry or forming new phases. The membrane containing the Mg2+ impurity exhibited a reduced grain size, whereas the membrane containing the Cl− impurity exhibits poor sintering ability. The fluctuation in the oxygen permeation flux of the BSCF membranes could be correlated to their different sintering abilities and to the site doped with the impurity ions. The impedance spectra of symmetrical cells with BSCF electrodes containing various impurity ions showed that the small impurity ions did not strongly affect the electrode performance. These results suggested that low purity raw materials could be used to prepare BSCF materials, thus improving their economic attractiveness.

Oxygen permeability and phase stability of Ba0.5Sr0.5Co0.8Fe0.2O3−δ perovskite at intermediate temperatures

The oxygen permeation flux through Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) membranes of different thicknesses (1.5–2.5mm) was measured between 800 and 900°C under vacuum conditions. The vacuum pressure at the permeate side of the membranes was varied from 5mbar to 30mbar. In order to examine the effect of membrane configuration, an asymmetric BSCF membrane with a thin-film layered structure was produced. A significant improvement in oxygen permeability was observed in the asymmetric membrane, which showed an oxygen flux of 32.45mlmin−1cm−2 [STP] at 900°C, much higher than that of dense membranes. The long-term permeation study showed that the membrane properties of the BSCF material undergo gradual deterioration at 800°C due to phase decomposition. The hexagonal phase formed primarily in grain boundaries, and after the saturation of boundary nucleation sites, phase transition also occurred within the matrix. The SEM micrographs of BSCF membranes operating over long periods exhibit more rapid deterioration at the air feed side than at the oxygen-lean side.

Oxygen Permeability of Ba<sub>0.5</sub>Sr<sub>0.5</sub>Co<sub>0.8</sub>Fe<sub>0.2</sub>O<sub>5+δ</sub>Membrane after Surface Modification

The oxygen permeation fluxes of Ba0.5Sr0.5Co0.8Fe0.2O5+(BSCF) membranes coated by RBaCo2O5+ (R= Pr, Nd, Sm, Gd) thin layer were investigated. The oxygen permeation fluxes of BSCF membranes with RBaCo2O5+ (R112)R112 coating layer can be enhanced remarkably. And it is found that the reaction of R112 with BSCF also have effect on the oxygen permeation fluxes. Impurity formed by the reaction of Pr112 with BSCF block oxygen permeation. While NdBaCo2O5+ and SmBaCo2O5+, especially GdBaCo2O5+ have no serious reaction with BSCF, they are desirable materials for coating layer of oxygen permeation membranes.

Effect of microstructure on oxygen permeation of Ba0.5Sr0.5Co0.8Fe0.2O3−δ and SrCo0.8Fe0.2O3−δ membranes

Abstract The effect of grain size on oxygen permeation properties of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and SrCo0.8Fe0.2O3−δ (SCF) membranes was investigated by variation of the dwell time. The membrane microstructure was examined by field-emission scanning microscopy (FE-SEM) and then evaluated using a statistical approach. With longer dwell times the grain growth was stimulated and leaded to grains with a narrower size distribution. The grains of SCF (average size from 11.3 to 19.9 μm) were found to be smaller than those of BSCF (average size from 13.9 to 41.3 μm). The oxygen permeation flux of BSCF membranes was found to be independent of grain size in the range from 24 to 42 μm. However, membranes with smaller grains (13.9 μm) show a decreased oxygen permeation flux. For the SCF membranes a decrease in permeation flux with larger grains was observed for average grain sizes between 11.3 and 19.9 μm. By transmission electron microscopy (TEM) formation of an oxygen ordered SrCo0.8Fe0.2O2.5 brownmillerite by-phase could be observed at the oxygen-depleted sweep side of the membrane.

Development of novel chromium-tolerate barium–chromium oxide coated BSCF membrane for oxygen separation

In this study, effect of barium chromium oxides coated Ba0.5Sr0.5Co0.8Fe0.2O3±δ (BSCF) membrane on oxygen permeation flux was investigated experimentally. Porous barium chromium oxide (BaCrOx) films were deposited by the dip coating method onto BSCF membranes to improve the performance particularly in volatile chromium conditions. We conclude that oxygen permeation in BaCrOx coated BSCF membranes is very high (2.31mL/mincm2) under an air/helium (20mL/min) gradient at 950°C. This finding is in contrast with the well-known unprotected BSCF membranes.

Impact of sulphur contamination on the oxygen transport mechanism through Ba0.5Sr0.5Co0.8Fe0.2O3−δ: Relevant issues in the development of capillary and hollow fibre membrane geometry.

Fabrication of dense perovskite membranes in the form of capillaries or hollow fibres is considered attractive for large-scale oxygen separation applications. For the preparation of such membranes by phase-inversion process polysulphone or polyethersulphone are commonly used as a binder. The decomposition of the sulphur-containing binder during the calcination leads to the formation of sulphates, which negatively affect the oxygen permeation through the membrane. The present work focuses on the comparative analysis of the oxygen transport mechanism through sulphur-free and -containing Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) membranes. The analysis of the thickness dependence of the oxygen permeation fluxes indicated that sulphates decrease the permeation rate mostly due to the partial blocking of the surface oxygen exchange, whilst the bulk ambipolar conductivity remains essentially unchanged. SEM/EDS studies revealed segregation of BaSO4 at the grain boundaries, which might be responsible for the fast oxygen exchange in phase-pure BSCF. The negative impact of sulphur contamination on oxygen permeation was more pronounced at temperatures below 1123K. It has been demonstrated, that, by surface activation, the oxygen flux through sulphur-containing BSCF membranes can be increased to the level of that of sulphur-free membranes.

Experimental investigation and mathematical modeling of oxygen permeation through dense Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) perovskite-type ceramic membranes

Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) perovskite powder was synthesized via EDTA/citrate complexation method. BSCF membranes were formed by pressing powder at 400MPa and sintering at 1100°C for 10h. XRD patterns showed that a high pure powder with cubic structure was obtained. SEM micrographs revealed that the membranes are dense with large grains. Effects of temperature, feed and permeate side oxygen partial pressures, flow rates and membrane thickness on oxygen permeation flux were studied experimentally. A Nernst–Planck based mathematical model, including surface exchange kinetics and bulk diffusion, was developed to predict oxygen permeation flux. Considering non-elementary surface reactions and introducing system hydrodynamics into the model resulted in an excellent agreement (RMSD=0.0617, AAD=0.0487 and R2=0.985) between predicted and measured fluxes. The results showed that oxygen permeation flux increases with temperature, feed side oxygen partial pressure and flow rates, however decreases with permeate side oxygen partial pressure and membrane thickness. Contribution of feed side surface exchange reactions, bulk diffusion and permeate side surface exchange reactions resistances in the total resistance are in the range of 8–32%, 10–81% and 11–59%, respectively. Permeation rate-limiting step was determined using the membrane dimensionless characteristic thickness.

High-purity oxygen production by a dead-end Ba0.5Sr0.5Co0.8Fe0.2O3−δ tube membrane

High-purity oxygen was produced by dead-end Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) tube membranes which were sealed by a reaction-diffusion sintering process. First, phase stability of BSCF membrane in a pure oxygen atmosphere – as it is present on the permeate side of the membrane – was studied at 750 and 950°C, respectively. After the identification of stable operation conditions of BSCF membranes, we studied the oxygen permeation at 950°C using dead-end BSCF tubes (1cm outer diameter, 1mm wall thickness) in (i) a pressure-driven process, (ii) a vacuum process, and (iii) combining both techniques. In all cases, a high oxygen purity of almost 100vol.% can be obtained at operation temperatures≥850°C. The oxygen permeation flux, the oxygen recovery, and the oxygen ionic conductivity were investigated. It was found that – for the same oxygen partial pressure difference – the oxygen permeation flux in the vacuum process is significantly higher than that in the pressure-driven process at all investigated temperatures. Moreover, in all cases, oxygen permeation and oxygen ionic conductivity can be described by the Wagner theory for bulk diffusion of oxygen ions as rate-limiting step with the logarithmic ratio of the oxygen partial pressures on feed and permeate sides as driving force.