In both cases, the driving force for the oxygen permeation is the oxygen chemical potential/partial pressure difference on both sides of the membrane [11,12]. In the case of pure oxygen conducting membranes (materials with only ionic conductivity), an external electrical circuit is needed for the conduction of electrons (Figure 4, left) as these materials are very often used in high temperature fuel cell applications [7]. In the case of pure oxygen conducting membranes (materials with only ionic conductivity), an external electrical circuit is needed for the conduction of electrons (Figure 4, left) as these materials are very often used in high temperature fuel cell applications [7].
However, the properties of the "standard" perovskites can be tuned by introducing metal cations into the membrane structure [50]. This effect would contribute to increasing the concentration of oxygen vacancies and the ionic conductivity of the membrane, resulting in a growth of the oxygen permeation flux. An improvement in oxygen permeability was observed in comparison with self-supported BSCF membranes.
77] observed that when CaO replaces erbium oxide, there is no improvement in oxygen permeability. In Table 2, some physical properties of metals used in two-phase oxygen membranes are summarized. Table 3 summarizes some important parameters of different ceramic-ceramic two-phase membranes presented in the literature.
Factors Affecting Permeation and Stability
On the other hand, their chemical compatibility and expansion at high temperatures must be taken into account, as well as their stability in the presence of certain gas species. The ordered arrangement of oxygen vacancies in the brownmillerite structure, in which one-sixth of the oxygen sites are empty, reduces the oxygen flux. In the phase transformation from perovskite (ABO) to brownmillerite (A), a two-phase region exists at lower oxygen partial pressures (<0.1 atm) and at low temperatures [100].
As the oxygen content of the perovskite phase decreases (i.e. increase in the number of oxygen vacancies), a more ordered brownmillerite phase is formed [8], which has a lower permeation flux due to its vacancy-ordered structure [93]. Below 850◦C, this cubic perovskite changes into an orthorhombic brownmillerite phase [8,11], with a concomitant dramatic decrease in oxygen permeability. In the phase transformation from perovskite (ABO3) to brownmillerite (A2B2O5), a two-phase region exists at lower oxygen partial pressures (<0.1 atm) and at low temperatures [100].
Not only the effect of temperature on the phase transformation, but also the oxygen pressure should be considered to consider the possibility of applying perovskite membranes. One of the most studied cobalt-free membranes is BaFeO3-δ, but it shows low oxygen permeability because it crystallizes in the hexagonal structure, which is less permeable than the cubic structure. Partial substitution at the A site with smaller cations such as Sr, Ca, La and Y can lead to stabilization of the cubic structure.
Membrane surfaces exposed to gases containing small amounts of carbon dioxide (CO2), sulfur compounds (H2S and SO2) or steam (H2O) are often poisoned (depending on the membrane material), which results in a decrease of oxygen flux and, in the worst case, in membrane failure. Thus, the applicability of perovskites for some applications is limited, as they contain alkaline-earth metals that can react in the presence of gases such as CO2 and steam. 124] reported an immediate decrease in oxygen flux for a BSCF membrane (Ba1-xSrxCo1-yFeyO3-δ) when CO2 was used as the purge gas.
Tong's group [53] also found that exposure of BCFZ (BaCo1-x-yFeyZrxO3-δ) membranes to CO2 can lead to a significant decrease in oxygen permeability. Doped cobalt La Ca FeO can withstand the presence of CO in the stream. Undoped cobalt La0.6Ca0.4FeO3-δ can withstand the presence of CO2 in the stream.
Membranes based on BSCF show low stability in the presence of sulfur compounds, as the heat generation of BaS and SrS is very high, ƺ406 kJ mol-1 and ƺ472 kJ mol-1, respectively.
Oxygen Permeation Improvement
Modifying the surface of the membrane can lead to a strong improvement of the oxygen permeability. A very thin layer consisting of nanoparticles is an interesting approach to obtain a meso-structured layer on top of the oxygen-selective layer [153,154]. Supporting a dense membrane layer on a porous substrate can help reduce the thickness of the functional layer without sacrificing the mechanical strength of the membrane.
The thermal and chemical expansion of the selective layer and the support must be as close as possible. There are two options for selecting the support material, including using the same material as used for the selective layer or using a different material but with good chemical and thermal compatibility. However, the low coefficient of thermal expansion of this material leads to the breakdown of the selective layer due to chemical and physical incompatibility with perovskites, which have a much higher chemical and thermal expansion (see Section 4.1) compared to aluminum oxide (Al2O C −1) [167].
For this reason, a thin dense layer is easier to deposit on a porous support of the same material. On the other hand, large oxygen fluxes are required for industrial exploitation of the perm selective oxygen membranes. Moreover, the porosity of the support is another important point, which was observed by Schulze-Küppers et al.
The YSZ-LSCrF composite was then coated with Sm0.2Ce0.8O2 nanoparticles on the inner surfaces of the support and the oxygen permeation flux observed was 1.64×10−6 mol cm−2s−1 at 900◦C [174] . Membrane thickness is controlled by slurry viscosity and blade gap. Green membranes are formed by forcing ceramic slurry through a high-pressure orifice, and the dimensions of the cover determine the geometry of the membrane.
In the coagulation bath, the coagulant exchanges with the solvent of the suspension, resulting in polymer-lean and polymer-rich phases. Oxygen transport resistance through the membrane is reduced due to the micro-channeled structure and the channels provide a large surface area, which will promote the oxygen permeability of the membrane by increasing surface kinetics.
Application of Oxygen Conducting Membranes in Membrane Reactors
A porous membrane was simulated to distribute the oxygen along the axial length of the reactor. It turned out that a good tuning of the catalytic activity and oxygen flow across the membrane is crucial to achieve good OCM performance. The poor stability under OCM conditions, mainly related to the reactivity of CO with MIEC materials, and a better understanding of the membrane-catalyst interaction are the main challenges discussed in the literature.
The poor stability under OCM conditions, mainly related to the reactivity of CO2 with MIEC materials, and a better understanding of the membrane-catalyst interaction are the main challenges discussed in the literature. After 500 hours of a long-term POM test, an XRD analysis showed that the structure of the membrane remained the same. The membrane structure was characterized after the test and, unlike most perovskites, the K2NiF4 structure remained unchanged.
Cross-section ( ) and wall ( ) image of the SFN multi-channel hollow fiber membrane reported by Zhu et al. The characterization analysis of the membrane done after the test showed good tolerance to the POM-reducing atmosphere. However, since the yield of this reaction is already high enough, maximizing the membrane oxygen flux will increase the overall product production.
A summary of the membranes used for Oxidative Dehydrogenation of Ethane (ODHE) found in the literature is provided in Table8. The authors attributed the improvements to the increase in the surface membrane area and the minimization of the successive oxidation reactions. Moreover, in some of these processes, an increase in the overall performance of the process has been achieved.
Another critical aspect is related to the coefficients of thermal expansion of the membrane and sealing materials. The only limitation is related to the membrane force that can be caused due to the temperature gradient that occurs between the reaction zone (slightly above 800 °C) and the enclosure (room temperature).
Conclusions and Future Trends
First, the material selected for sealing must be solid at the operating temperature, which for most processes is between 750 and 950 ◦C. 243], the main drawback of this technique is likely contamination of the MIEC membrane, which leads to a decrease in oxygen permeability through the MIEC material in long-term experiments. First, the material chosen for sealing must be solid at the operating temperature, which for most processes is between 750 and 950 °C.
The ceramic (glass-based) seal can probably react with some materials used to make MIEC membranes. Properties of oxygen permeation and partial oxidation of methane in La0.6Sr0.4CoO3−δ(LSC)-La0.7Sr0.3Ga0.6Fe0.4O3−δ(LSGF) membrane. Contribution of the surface exchange kinetics to the oxygen transport properties in Gd0.1Ce0.9O2−δ-La0.6Sr0.4Co0.2Fe0.8O3−δdual-phase membrane.
Synthesis, oxygen permeation study and membrane performance of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxygen permeable dense ceramic reactor for partial oxidation of methane to syngas. Oxygen permeation and stability study of La0.6Sr0.4Co0.8Ga0.2O3−δ(LSCG) hollow fiber membrane with exposure to CO2, CH4 and He. Influence of CO2 on the oxygen permeability and microstructure of perovskite-type (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ membranes.
Effect of SO2 poisoning on oxygen permeability in La0.6Sr0.4Co0.2Fe0.8O3−δperovskite hollow fiber membranes. Effect of grain size on the oxygen permeability capacity of perovskite (Ba0.5Sr0.5)(Fe0.8Zn0.2)O3−δ membranes. Significant improvement of the oxygen permeability current of tubular Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranes covered with a thin layer of La0.6Sr0.4Ti0.3Fe0.7O3−δ.
Improved oxygen permeation behavior of Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranes in a CO2-containing atmosphere with a Sm0.2Ce0.8O1.9 functional shell. Oxygen transport properties of tubular Ce0.9Gd0.1O1.95-La0.6Sr0.4FeO3−dcomposite asymmetric oxygen permeation membranes supported on magnesium oxide. Effects of manufacturing processes on oxygen permeation of Nb2O5-doped SrCo0.8Fe0.2O3−δmicro-tube membranes.
Oxygen permeation and partial oxidation of methane reaction in Ba0.9Co0.7Fe0.2Nb0.1O3−δoxygen permeation membrane.
