High Temperature Oxygen Separation Research Articles

Highly stable dual-phase Ce0.9Pr0.1O2-δ-Pr0.6Sr0.4Fe1-xTixO3-δ oxygen transport membranes

Mixed ionic-electronic conducting (MIEC) oxygen transport membranes (OTMs) could be used for high-temperature oxygen separation, solid oxide fuel cells (SOFC), and membrane reactors. However, many high-performance OTM materials are chemically unstable in high-temperature CO2 and reducing atmospheres. Here, we proposed a series of 60 wt%Ce0.9Pr0.1O2-δ-40 wt%Pr0.6Sr0.4Fe1-xTixO3-δ (CPO-PSFTxO, x = 0.05, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1) OTMs through Ti doping in Fe-based dual-phase membranes. A small amount of Ti doping increased the oxygen vacancy concentration and simultaneously enhanced the structural stability at high temperatures. The optimal CPO-PSFT0.1O membrane could maintain oxygen permeation fluxes of 0.512 mL min−1 cm−2 and 0.306 mL min−1 cm−2 over 150 h under He and CO2 sweeping at 1000 °C. Furthermore, co-doping with Cu improved oxygen permeability by more than 50 % while maintaining performance stability. Simultaneously, CPO-PSFT0.1O had excellent CO2 resistance and could maintain structural stability in 5 % H2/Ar at 800 °C. This material has potential applications in constructing membrane reactors containing reducing atmospheres and high-temperature SOFC.

High flux and CO2-resistance of La0.6Ca0.4Co1–xFexO3−δ oxygen-transporting membranes

Most of the currently used perovskite-based oxygen-transporting membranes have insufficient resistance towards CO2 and high material costs that potentially limit their commercial applications. In the present work, a highly CO2-tolerant oxygen permeation membrane based on La0.6Ca0.4Co1–xFexO3−δ (x = 0, 0.3, 0.5, 0.7, 1) was designed and prepared by a scalable reverse co-precipitation method. The oxygen permeation flux through the dense membranes was evaluated and found to be highly dependent on the Co/Fe ratio. La0.6Ca0.4Co0.3Fe0.7O3−δ possessed the highest permeation flux among the investigated samples, achieving 0.76 ml min−1 cm−2 under an Air/He gradient and 0.5 ml min−1 cm−2 under an Air/CO2 gradient at 1173 K for a 1 mm thick membrane. A combination study of first principles calculations and experimental measurements was conducted to advance the understanding of Co/Fe ratio effects on the oxygen migration behavior in La0.6Ca0.4Co1–xFexO3−δ. The observed oxygen permeability is three times higher than that reported in literature under similar conditions. The presented results demonstrate that this highly CO2-tolerant membrane is a promising candidate for high temperature oxygen separation applications.

Process Intensification via Membrane Reactors, the DEMCAMER Project

This paper reports the findings of a FP7 project (DEMCAMER) that developed materials (catalysts and membranes) and new processes for four industrially relevant reaction processes. In this project, active, stable, and selective catalysts were developed for the reaction systems of interest and their production scaled up to kg scale (TRL5 (TRL: Technology Readiness Level)). Simultaneously, new membranes for gas separation were developed; in particular, dense supported thin palladium-based membranes for hydrogen separation from reactive mixtures. These membranes were successfully scaled up to TRL4 and used in various lab-scale reactors for water gas shift (WGS), using both packed bed and fluidized bed reactors, and Fischer-Tropsch (FTS) using packed bed reactors and in prototype reactors for WGS and FTS. Mixed ionic-electronic conducting membranes in capillary form were also developed for high temperature oxygen separation from air. These membranes can be used for both Autothermal Reforming (ATR) and Oxidative Coupling of Methane (OCM) reaction systems to increase the efficiency and the yield of the processes. The production of these membranes was scaled up to TRL3–4. The project also developed adequate sealing techniques to be able to integrate the different membranes in lab-scale and prototype reactors.

Two-step fabrication of BaCo0.7Fe0.2Nb0.1O3−δ asymmetric oxygen permeable membrane by dip coating

Asymmetric membranes for high temperature oxygen separation with identical materials for dense layers and porous supports have been extensively reported in recent years. This paper presents a versatile method for manufacturing such asymmetric membrane, including pre-sintering, dip coating and co-firing. The porous support was elaborated through trial of different pore-forming agents and the amount. The thickness of top layer was controlled by adjusting the concentration of slurries. Several test approaches were employed to detect the detailed parameters of porous supports, such as SEM, mercury intrusion porosimetry and nitrogen permeation measurement. High performance support with excellent permeability and sufficient mechanical strength (43.84MPa) was fabricated successfully. A stable pore structure with open porosity of 33.26% was achieved. The prepared asymmetric membrane with thickness of 20μm for dense layer was subjected to oxygen permeation test and the result reached as high as 4.51ml(STP)cm–2min–1. The results suggest that the oxygen permeability was limited by both bulk diffusion and surface exchange kinetics.

High temperature oxygen separation for the sulphur family of thermochemical cycles – Part II: Sulphur poisoning and membrane performance recovery

The effect of sulphur dioxide (SO2) present in the cathode feed stream on the performance of an 8 mol% yttria-stabilized zirconia/Platinum (8YSZ/Pt) cell was studied. SO2 concentration of 30% in oxygen (5 ml/min) and helium (30 ml/min) was exposed to the cell applying an external DC voltage of 1.5 V at 850 °C. Experiments showed that the cell performance, inferred from current and oxygen permeate flux changes, had two types of degradation. The first was rapid, giving a sharp fall (type I degradation) and the second much more gradual (type II degradation). The presence of Platinum sulphide (PtS) was observed after the SO2 exposure period using X-ray diffraction (XRD), scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDX) and Raman spectroscopy. The cell performance was partially recovered after a 2 min DC voltage pulse treatment. The results of XRD, EDX and Raman analysis indicated that the recovery may be attributed to the removal of the sulphide layer from the cathode surface after the voltage pulse treatment. The greatest recovery of cell performance was obtained after a 2 min pulse of 4.5 V. Preliminary investigations on the effect of a much shorter, 1 s voltage pulse on performance recovery were conducted and recovery was found to increase up to the highest voltage tested, 7 V.

Modeling of the performance of BSCF capillary membranes in four-end and three-end integration mode

Abstract Owing to their high surface-to-volume ratio, there has been an increasing research interest in mixed ionic–electronic conducting (MIEC) capillary membranes for large-scale high temperature oxygen separation applications. They offer an energy-efficient solution for high temperature combustion processes in oxy-fuel and pre-combustion CO 2 capture technologies used in fossil fuel power plants. In order to assess the effectiveness of these membranes in power plant applications, the impact of the geometry of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3− δ (BSCF) capillaries on their performance in the three-end and four-end integration modes has been investigated and thoroughly discussed. The model's parameters were derived from four-end mode lab-scale experiments using gas-tight, macrovoid free and sulfur-free BSCF capillary membranes that were prepared by a phase-inversion spinning technique. The results of this modeling study revealed that in the four-end mode higher average oxygen fluxes and smaller total membrane areas can be obtained than in the three-end mode. This is due to the higher pO 2 gradient across the membrane wall.

High temperature oxygen separation for the sulphur family of thermochemical cycles - part I: Membrane selection and flux testing

This work investigates the feasibility of applying high temperature oxygen separation to the sulphuric acid decomposition process, a common step in the Sulphur Iodine and Hybrid Sulphur thermochemical cycles for massive scale hydrogen production. Process simulations indicate that the decomposition yield can be increased from 62.3% to 90.1% by using a catalytic membrane reactor. Equilibrium calculations show yttria-stabilised zirconia (YSZ), combined with platinum electrodes, to be a candidate membrane for the separation process. Bespoke experimental apparatus was designed and commissioned in order to investigate the oxygen flux through YSZ membranes in the presence of sulphur dioxide, during external voltage application. Results from an experiment at 900 °C show that oxygen permeation through YSZ membranes occurs under these conditions. X-ray diffraction and impedance spectroscopy are used to investigate the reason for the oxygen flux decrease with time.

Development, performance and stability of sulfur-free, macrovoid-free BSCF capillaries for high temperature oxygen separation from air

Capture and storage of CO 2 (CCS) from fossil-fuel power plants is vital in order to counteract a pending anthropogenic global warming. High temperature oxygen transport perovskite membranes can fulfill an important role in the separation of oxygen from air needed in the oxy-fuel technologies for CCS. In this study we present the development, performance and stability of gastight, macrovoid-free and sulfur-free Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ (BSCF) mixed conductor capillary membranes prepared by phase-inversion spinning and sintering. A sulfur-free phase-inversion polymer was chosen in order to obtain a phase-pure BSCF crystal phase. Special attention was given to the polymer solution and ceramic spinning suspension in order to avoid macrovoids and achieve gastight membranes. The sulfur-free BSCF capillaries showed an average 4-point bending strength of 64 ± 8 MPa and a maximum oxygen flux of ∼5.3 Nml/(cm 2 min) at 950 °C for an argon sweep flow rate of 125 Nml/min. The comparison of the performance of sulfur-free and sulfur-containing BSCF capillaries with similar dimensions revealed a profound impact of the sulfur contamination on both the oxygen flux and the activation energy of the overall oxygen transport mechanism. Both long-term oxygen permeation at different temperatures and post-operation analysis of a sulfur-free BSCF capillary were performed and discussed.

The role of A‐site ion nonstoichiometry in the oxygen absorption properties of Sr1+xCo0.8Fe0.2O3 oxides

AbstractSr1+xCo0.8Fe0.2O3 (−0.2 ≤ x ≤ 0.1) oxides have been synthesized and investigated as potential oxygen absorbents for high temperature oxygen separation processes. It was found that the A‐site ion deficient Sr0.95Co0.8Fe0.2O3 and Sr0.9Co0.8Fe0.2O3 oxides have larger oxygen absorption capacities, and slightly higher oxygen desorption rates. However, the A‐site ion excess Sr1.05Co0.8Fe0.2O3 and Sr1.1Co0.8Fe0.2O3 exhibited much higher oxygen desorption rates, but smaller oxygen absorption capacities. The oxygen absorption capacities were further verified using oxygen temperature‐programmed desorption technique, and the oxygen desorption rates were well described by a pseudo‐second‐order kinetics model. In addition, the long‐term stability of Sr0.9Co0.8Fe0.2O3 was investigated for 280 h (1120 loops) with switching between oxygen absorption and desorption. The excellent stability was confirmed by X‐ray diffraction patterns which suggested that the cyclic processes of oxygen absorption and desorption took place between the perovskite structure with disordered oxygen vacancies and the brownmillerite structure with ordered oxygen vacancies. © 2010 American Institute of Chemical Engineers AIChE J, 2011

High-temperature study of SrFe1 − x Mo x O3−z perovskites

The structure of high-temperature SrFe1 − xMoxO3 − z (0 ≤ x ≤ 0.5) phases was studied. Such studies are necessary to understand the mechanism of oxygen transport in membrane materials used for high-temperature oxygen separation.

Improving oxygen permeability in SrFeCo 0.5O x asymmetric membranes by modifying support-layer porous structure

Disk-type SrFeCo 0.5O x asymmetric membranes were prepared for high-temperature oxygen separation using both a previously reported solid state technique in which ethyl cellulose is introduced and then removed by calcination to produce a porous support layer and a modified technique in which carbon fiber is added to the ethyl cellulose. The oxygen permeabilities of the resulting SrFeCo 0.5O x asymmetric membranes were measured at 973–1173 K. The addition of carbon fiber to ethyl cellulose resulted in the improvement of gas diffusivity in the porous support layer, which increased the flux of oxygen through the modified asymmetric membrane.

Stabilized bismuth oxide-noble metal mixed conducting composites as high temperature oxygen separation membranes

Oxygen concentration cells, with dense dual phase composite membranes made from erbia-stabilized bismuth oxide and a noble metal (Au, Ag), were investigated in the temperature range 650–850°C under controlled oxygen partial pressure gradients. An electrochemical treatment was applied to interpretation of the oxygen permeation data. It is found that the composite membranes exhibit high oxygen permeability relative to the single phase bismuth oxide, since oxygen ions and electrons are allowed to transport through the oxide and metal phase, respectively. The oxygen permeability of the silver-containing composite is at least one order higher than that of the gold-containing one, which can be explained by the fact that silver has a higher catalytic activity than gold for the surface oxygen exchange reaction and thus less limitations are exerted on the overall oxygen transport.