Manufacturing strategies for asymmetric ceramic membranes for efficient separation of oxygen from air

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Journal of the European Ceramic Society 33 (2013) 1251–1261

Feature article Manufacturing strategies for asymmetric ceramic membranes for efficient separation of oxygen from air S. Baumann, W.A. Meulenberg ∗ , H.P. Buchkremer Forschungszentrum Jülich, Institute of Energy and Climate Research IEK-1, Germany Received 16 November 2012; received in revised form 11 December 2012; accepted 15 December 2012 Available online 20 January 2013

Abstract Membrane technology has the potential to perform separation tasks more efficiently than other technologies. Polymeric membranes, in particular, are widely used in different applications, while ceramic membranes have captured only limited applications so far. This feature article introduces different types of ceramic membranes, focussing on mixed ionic electronic conductors (MIEC) for separating oxygen from air. The transport processes through the membrane are described briefly, before the necessity of supported membrane structures (asymmetric membrane) is outlined. Membrane microstructure is discussed with regard to its influence on membrane properties as are appropriate manufacturing techniques for the targeted microstructure. Existing module concepts are described and their prospects discussed. Finally, an outlook of the research and development required in the near future is given from the authors’ point of view. © 2013 Elsevier Ltd. All rights reserved. Keywords: Oxygen transport membranes; Membrane manufacturing; Asymmetric membrane; Perovskites; Mixed ionic electronic conductors

1. Introduction A membrane is a “structure, having lateral dimensions much greater than its thickness, through which mass transfer may occur under a variety of driving forces”.1 Its functions and potential applications vary widely, particularly in biology and technology. In artificial systems, membrane technology can be integrated into many advanced system concepts for the production and purification of water, liquid energy carriers and chemicals, oxygen generation, low-CO2 -emission power generation, hydrogen technology, fuel cells, solid-state batteries, etc. The wide spectrum of applications requires very different membrane systems to separate solid, liquid, and/or gaseous species from each other. Accordingly, different mass transport mechanisms are used, for example, ionic conductance and viscous flow or the diffusion of gases or liquids. In addition to polymeric membranes, ceramic membranes are promising because of their high mechanical, chemical, and thermal stability.

∗ Corresponding author at: Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research IEK-1, Leo-Brandt-Str, D-52425 Jülich, Germany, Tel.: +49 2461 61 6323; fax: +49 2461 61 2455. E-mail address: [email protected] (W.A. Meulenberg).

0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2012.12.005

Separation processes using inorganic membranes is a growing field with respect to the high efficiency of membrane-based processes, e.g. in energy applications.2 In general there are two different classes of membranes, dense and porous ones. Porous membranes are developed, e.g. for water purification, where the separation tasks are classified with respect to the size of the molecules dm that are to be rejected: microfiltration (dm > 100 nm), ultrafiltration (2 nm < dm < 100 nm), and nanofiltration (dm < 2 nm). In the case of very small molecules, such as salt dissolved in water, reverse osmosis is used, where an applied transmembrane pressure causes a selective movement of the solvent against its osmotic pressure difference.1 Such types of polymeric or inorganic membranes are already commercially available in the water purification,4 chemical and petrochemical industries,5 the food industry,6 and in medical7 and environmental applications.8 In the case of gas separation, the molecules that have to be separated are much smaller (Table 1). Molecular sieve membranes with micropores (free volume in the lattice of an amorphous or crystalline structure) smaller than 1 nm are also being developed based on molecular gas transport. Dense membranes, in contrast, utilize ionic conductance. In solid-state batteries for example, the membrane (electrolyte) separates the solid anode from the solid cathode. Metal ions,

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Table 1 Kinetic diameter dkin of selected gases. Species

He

NH3

H2 O

H2

C2 H2

CO2

O2

SO2

N2

CO

CH4

dkin [Å]

2.60

2.60

2.65

2.89

3.30

3.30

3.46

3.60

3.64

3.76

3.80

e.g. Li+ and Zn2+ , pass through the semipermeable membrane by ionic diffusion, thus allowing electricity to be stored or supplied.9 Solid oxide fuel cells are another energy application in which a ceramic membrane transports ions (oxygen ions or protons) between electrodes. In this case, the membrane separates two gas phases, i.e. air and a fuel, such as hydrogen or methane 10 . A third application utilizing ionic conductance is oxygen separation from air.11 This technology has attracted great interest because it can be used to generate pure oxygen more efficiently than mature technologies, such as air liquefaction or pressure swing adsorption in, e.g., oxyfuel power plants (Table 2).3 In general, membranes should be as thin as possible in order to maximize the flux through the membrane. Due to mechanical requirements, thin membranes have to be supported by a macro-porous support. This inevitably leads to the development of asymmetric membranes, which consist of “two or more structural planes of non-identical morphologies”.1 Defect-free thin films on highly porous supports are essential, as are different porous interlayers and catalytic layers on demand, which in turn requires sophisticated manufacturing. In principle, the membrane process itself does not consume energy although in some cases, e.g. mixed ionic electronic conducting (MIEC) membranes, heat is necessary, which provides an appropriate temperature in order to facilitate transport by diffusion. Moreover, the driving force, i.e. a chemical potential gradient, must be ensured in all cases. Different concepts can be used for this. The permeated gas stream can be swept away using a sweep gas stream, which keeps the partial pressure of the permeating gas sufficiently low, while the feed gas can additionally be compressed in order to enhance the partial pressure. In this so-called four-end concept, the membrane module is connected to four gas streams: feed and retentate, i.e. depleted feed, as well as sweep and permeate, i.e. enriched sweep. This mode is beneficial if certain oxygen content in the sweep is desired, e.g. flue gas enriched in oxygen for oxyfuel combustion or methane partially oxidized to syngas. It can be operated in co-current mode or counter-current mode, which means that the feed and sweep gas streams flow in the same direction or the opposite direction.

Alternatively, for the generation of pure oxygen the permeated gas stream can be collected using a slight low pressure. This is known as the three-end mode because only three gas streams have to be handled, i.e. feed, retentate, and permeate. Again, it may be necessary to compress the feed gas stream. The concept chosen combined with the specific conditions, e.g. pressures applied, determines the driving force, and thus the permeation rate, the energy required for the corresponding turbo machinery, and the mechanical load applied to the membrane by a certain pressure difference across the membrane. This represents an optimization task in engineering because normally a targeted high permeation rate is accompanied by a high mechanical load and high energy consumption, which affects component reliability and cost, respectively. A lower permeation rate allows the mechanical load and energy consumption to be lowered, but the membrane area required for the targeted overall flux increases, which again leads to increased costs. Therefore, each application must be investigated carefully in order to optimize the overall process, which will then be used to define targets for membrane development. This article focuses on the manufacturing of oxygen transport membranes (OTM) in different geometries, all of which are based on mixed ionic–electronic conductors (MIEC). Processing of the membranes will be discussed with respect to design, manufacturability, reproducibility, and scalability. Finally, the authors will present an outlook of the research and development required in the near future. 2. Membrane classes In general, ceramic membranes can be separated in two classes: dense and porous. Different transport mechanisms occur, i.e. ionic and molecular diffusion, Fig. 1. A ceramic microporous membrane is featured by a functional layer with a pore size <2 nm and a thickness of 50–200 nm. Evidently, such a layer has to be supported by a porous structure, which should provide high permeability thus exhibit coarse microstructure. One or more interlayers are necessary to bridge

Table 2 Overview of different asymmetric oxygen transport membranes. Membrane material

Layer thickness (␮m)

jasymmetric (ml min−1 cm−2 )

jasymmetric /jbulk

Lbulk /Lasymmetric

T (◦ C)

La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ La0.5 Sr0.5 CoO3−δ Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ La0.58 Sr0.4 Co0.2 Fe0.8 O3−δ SrCo0.4 Fe0.5 Zr0.1 O3−δ La0.6 Ca0.4 CoO3−δ Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ

6 20 70 200 20 200 10 120

1.2 1.0 5.1 0.2 0.1 1.0 1.5 1.94

3.3 10 3.9 3.3 1.9 1.9 4.4 1.3

33.3 22.0 14.2 10.0 50.0 7.5 100 8.3

850 800 900 800 900 900 900 900

Reprinted with permission from Elsevier.

Source 19 20 21 22 23 24 25 26

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Fig. 1. Transport processes in dense (left) and porous (right) ceramic membranes.

the gap between coarse and fine microstructure of the support and functional layer, respectively. Therefore, a microporous membrane can generally be considered as a graded multilayer porous body with macroporous, mesoporous and microporous layers. According to IUPAC notation, porous materials are classified into three kinds: microporous materials have a pore diameter <2 nm, mesoporous materials have pore diameters between 2 nm and 50 nm, and macroporous materials have a pore diameter >50 nm.12 This classification is consistent with regard to the micro, meso, and macro level in different areas, e.g. social sciences. But, the term microporous or micropore often leads to confusion because of mix-up with the micrometer scale. Therefore, we suggest a redefinition by using nanoporous instead of microporous, which has become more frequent in the last few years, anyway. Microporous membranes are considered by many research groups as potential candidates for separation problems involving small gases, such as H2 , He, N2 , O2 , CO, CO2 , and CH4 . Typically, microporous ceramic membranes are developed as an alternative to polymeric gas separation membranes for industrial applications in which polymers cannot perform well or do not have the required lifetime. Examples include applications with acid contaminants, steam, high temperatures, and high pressures. The main classes of ceramic microporous membranes are amorphous metal oxides, such as pure and doped silica, zirconia, or titania. Moreover, crystalline zeolite membranes and hybrid membranes, i.e. metal organic frameworks (MOFs), are also being developed. However, microporous membranes are not suitable for the separation of pure oxygen from air, i.e. mainly nitrogen, because oxygen and nitrogen are very similar in size and chemical nature. While the resulting limited selectivity enables oxygen enrichment, e.g. by polymer membranes in medical applications, the generation of pure oxygen cannot be achieved. Dense membranes are principally gas tight, but mass transport is realized by ionic diffusion through the crystal lattice accompanied by electrical conductivity for charge compensation. Such MIEC transport either oxygen ions or protons through vacancies in the crystal lattice and thus can be used to separate pure oxygen from air or for hydrogen purification in chemical processes. In oxygen transport membranes (OTMs), diffusion occurs when a neighbouring oxygen ion hops into a vacancy. Without any driving force, i.e. no oxygen partial pressure gradient, this hopping is statistically distributed. When a partial pressure

gradient is applied, the hopping is directed from the highpressure to the low-pressure side, resulting in oxygen ion diffusion. At the side with a high oxygen partial pressure, the remaining vacancies are re-occupied by oxygen ions, which are generated by ionization and dissociation of molecular oxygen in the gas phase. At the side with a low partial pressure, the oxygen is released into the gas phase. In the steady state, oxygen diffusion jO2 through a solid crystal can be described by the Wagner equation13 : jO2 =

1 pO2 RT In σ , amb 16F 2 L pO2

(1)

where R is the gas constant, T is temperature, F is the Faraday constant, σ amb is ambipolar conductivity, L is the membrane thickness, and pO2 and pO2 are the oxygen partial pressure on the oxygen-rich and oxygen-lean side, respectively. The only material parameter in the Wagner equation is ambipolar conductivity, which is composed of the ionic σ i and electronic conductivity σ e : σe · σi σamb = . (2) σe + σi Today the highest permeation rates can be obtained using perovskites ABO3 ,11 particularly Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ (BSCF).14,15 In perovskites, the electronic conductivity is normally significantly higher than the ionic conductivity, which is why the latter is the rate-limiting property. Following the Nernst–Einstein relation (3), σ i is proportional to the oxygen vacancy concentration [VO •• ], as the vacancy diffusion coefficient DV can be considered as constant in a certain range of oxygen partial pressures: σi =

4F 2 [VO •• ]DV , RT Vm

(3)

Vm is the molar volume. It is nearly impossible to simultaneously achieve high permeability and stability in perovskite materials. Taking this into account, the Wagner equation (1) indicates that apart from the process parameters, i.e. temperature and partial pressure conditions, the only method of enhancing the permeation rate is to reduce the membrane thickness L. However, thickness reduction alone is not sufficient for performance maximization because surface exchange reactions between oxygen molecules in the gas phase and the solid membrane can become rate limiting

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Fig. 2. Cross section of an asymmetric membrane assembly consisting of porous support, dense membrane layer, and porous surface activation layer made from Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ .

below a characteristic thickness Lc ,13 which is not implied in the Wagner equation (1). Lc =

D ks

(4)

where D is the oxygen diffusion coefficient and ks is the oxygen surface exchange coefficient. Thus, surface exchange kinetics must be accelerated by catalytic activation if maximum permeation rates are necessary. 3. Asymmetric membrane structures-processing, properties, and prospects In general, the permeation through a membrane is proportional to the ratio of a driving force term to the membrane thickness (Eq. (4)) so that a membrane should be as thin as possible. driving force . (5) jO2 ≈ thickness Consequently, the development of asymmetric membrane structures is appropriate. Reducing the thickness will cause the limiting step for permeation to change as the transport through the membrane layer becomes faster than other processes involved. In MIEC membranes, it is well known that the surface exchange kinetics become rate limiting below a characteristic thickness because the bulk diffusion becomes faster.13 If the permeation rate is very high, concentration polarization also occurs in the gas phase, leading to depletion or enrichment of the permeating species in a layer close to the membrane surface at the feed or permeate side, respectively. This effect is strongly dependent on the fluid dynamics of the gas streams and thus on the module design. Furthermore, a porous mechanical support is required for thin membranes in order to facilitate gas transport to the membrane layer itself. Therefore, the architecture of asymmetric membranes (Fig. 2) consists at least of a macroporous support to ensure mechanical stability, and the dense membrane layer. If required, graded porous interlayer(s) can bridge the gap between the macropores in the support and the dense membrane

layer, which can be composed of or infiltrated with a catalytically active material. On top of the membrane layer, another porous catalytic layer can be applied (Fig. 2). At the feed side catalysis is necessary in order to facilitate the oxygen surface exchange reactions comprising ionization and dissociation of an oxygen molecule to oxygen ions, which subsequently recombine with an oxygen vacancy in the crystal lattice. At the permeate side the reactions occur in the opposite order. In a first step these reactions can be facilitated by simply enlarging the membrane surface by a thin porous layer of the membrane material (Fig. 2). If the surface exchange is still rate limiting a more active catalyst needs to be applied. In asymmetric membranes, an additional potentially ratelimiting effect occurs in contrast to unsupported membranes, namely concentration polarization in the support pores.15 Since these support pores are normally small, convection does not occur, so that the gas transport is realized by diffusion only when a sweep gas is used in the four-end concept. Microstructure optimization has to consider both aspects, i.e. high permeability requiring high porosity, and sufficient mechanical stability requiring limited porosity.16 Furthermore, sealing of the ceramic components to the metallic environment at high operation temperatures is a challenge. Standard reactive metal brazing technology using titanium as an active element cannot be used due to the high vacuum required at high temperatures during brazing.17 These conditions destroy the required perovskite crystal structure. At present, the most promising solution is reactive air brazing (RAB) based on Ag–CuO brazes, which is performed in atmospheric air instead of a high vacuum.18 Reductions in membrane layer thickness make engineering aspects such as processing and thermomechanical properties more and more important, and strongly determine the choice of a support material, which can be either ceramic or metallic. 3.1. Ceramic supports Perovskites – the materials with the highest permeability – exhibit unique expansion behaviour, comprising the usual thermal expansion superposed with a chemical expansion, which originates from the fact that the oxygen vacancy concentration is temperature dependent. The formation of oxygen vacancies is accompanied by a partial reduction in transition metal cations leading to an increased ion radius. Moreover, the electronic repulsion between the cations increases. This leads to the phenomenon that an oxygen vacancy is “larger” than an oxygen ion in the crystal lattice. Since the vacancy concentration increases with temperature above approx. 400–500 ◦ C, the chemical expansion coefficient is positive, thus enhancing the overall expansion coefficient (Fig. 3). A large mismatch in the thermal expansion behaviour of support and membrane layers would inevitably lead to failure in the membrane layer when the membrane assembly is being cooled after sintering. If the thermal expansion coefficient of the membrane layer is higher or lower than that of the support, tensile or compressive stresses, respectively, will occur at room temperature. Since ceramics generally fail under tensile loading, a slight

S. Baumann et al. / Journal of the European Ceramic Society 33 (2013) 1251–1261

Fig. 3. Expansion behaviour of perovskitic A0.68 Sr0.3 Co0.2 Fe0.8 O3−δ with A = Ba, La, Pr, Nd in comparison to yttria stabilized zirconia (8YSZ).

compressive stress in the membrane layer is favourable. On the other hand, if the compressive stresses are too large, this again will lead to failure due to spallation. Due to their unique expansion behaviour, asymmetric perovskite membranes are normally supported by a porous perovskitic body of the same or a very similar composition as the membrane layer material. 3.2. Metallic supports With regard to the expansion behaviour, austenitic steels or nickel-base alloys are suitable as supports due to their high thermal expansion coefficient, which matches well to that of relevant perovskites, i.e. 15–20 10−6 K−1 between room temperature and 1000 ◦ C.25 However, other challenges have to be faced, i.e. chemical reactions and sintering behaviour. In processing and/or operation, the metallic support is exposed to a relatively high oxygen partial pressure leading to oxidation of the surface. The oxide scale is in direct contact with the membrane material, which inevitably leads to a reaction zone. Therefore, compatible materials have to be chosen, i.e. a limited reaction zone is formed that allows oxygen permeation. From SOFC development, it is well known that chromia poisons perovskites26 either by means of direct contact or via evaporation of CrO2 (OH)2 . This limits the material choice to alumina scale Ni- or Co-base alloys with aluminium content higher than 5%.27 With strontium containing perovskites the respective alumina layer forms a thin reaction zone of strontium aluminate. Sr has been shown to be much more mobile than Al,28 so that the reaction zone adheres to the surface of the metal support particles. Therefore, the membrane layer itself is expected to remain active for oxygen exchange, particularly if a porous interlayer of the membrane material is introduced. This is beneficial for the oxygen surface exchange rate as it enlarges the surface area. The porous metallic support can be produced relatively easily via powder metallurgy, which is comparable to the sintering of ceramics. However, while sintering is normally performed

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in hydrogen or sufficient reducing atmospheres, perovskites must be sintered in air due to their reducibility. Sintering under reducing conditions destroys the required crystal lattice and thus co-sintering of the metal and the perovskite is impossible. Therefore, the metal support has to be sintered first and coated afterwards. The challenge associated with classical processing comprising wet chemical coating, such as screen printing or powder spraying, and subsequent sintering is the fact that the metal support does not shrink any more. This leads to tensile stresses in the membrane (inter)layer causing cracks and/or spalling. In addition, the oxidation behaviour of the metal has to be considered. Due to the porous structure, a high surface area is exposed to air during the sintering of the ceramic layers, which limits the tolerable maximum temperature. Due to this, only porous interlayers could be applied successfully on a metal support.29 In order to overcome this problem, using layer deposition techniques before sintering appear most promising. One appropriate coating technology is physical vapour deposition such as magnetron sputtering. However, thin-film coatings require a nearly perfect surface in terms of pore size, defects, and roughness because the maximum tolerable pore/defect size is determined by the layer thickness. Since a metallic support is particularly macroporous enabling high permeability, interlayers with small pores and a narrow pore size distribution are necessary for thin-film coatings. This also increases concentration polarization effects. Thermal spraying, or to be more specific, low-pressure plasma spraying (LPPS), was investigated recently as another potential technique.30,31 A lot of R&D is still necessary, but should these approaches prove successful, they will expand the choice of materials to non-perovskitic ceramic or metallic supports. This would be a breakthrough development for oxygen transport membranes with respect to mechanical stability, joining technology, and cost.

4. Membrane module designs As discussed above, the overall permeation process must be investigated carefully in order to identify the optimum membrane architecture and corresponding manufacturing techniques. In addition to these microstructural aspects, the membrane module design determines external mechanical loads, which again influence the required microstructure and thus the processing techniques. A general evaluation of membrane module concepts was performed by Vente et al.32 Each concept was shown to have advantages and disadvantages so that any individual case must be judged separately based on application requirements and available technology. Different approaches to oxygen transport membrane modules are based on different membrane geometries, i.e. tubular, planar, hollow fibres, and honeycomb structures. Obviously, specific R&D is required to create a favourable design. A selection of the most important activities on module fabrication is described below. Most of the modules are at the proof-of-concept stage except for a consortium in the US led by Air Products, which is already passing the pilot scale.

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Fig. 5. 1 ton/day component (left) and schematic cross section of the membrane assemblies (right).36 Fig. 4. Sketch of a planned module unit with 150 tons/day oxygen production capacity.36

4.1. Planar design Planar membrane structures can be manufactured using multiaxial pressing, extrusion, or tape casting. At present, development concentrates on the latter since pressing has disadvantages for mass production and extrusion limits the lateral dimensions of the components. In contrast, tape casting provides high flexibility in terms of size and the tailoring of flat ceramic components as well as mass production, as demonstrated by the manufacturing of ceramic components in electronics.33 The slurries used are suspensions of ceramic powder in an organic or inorganic solvent with a high amount of organic additives, such as dispersant, binder, and plasticizer. In addition, pore-forming agents, such as graphite or starch, can be added.34 The high content of organic additives makes the dried tapes flexible like a polymer. Thus, the targeted geometry can be easily and reliably cut. Moreover, lamination or sequential tape casting15,34 of different layers can be used to form a multilayer component with subsequent co-sintering. Debindering is normally integrated into the sintering step, whereas a low heating rate is used in the critical temperature region at which organics are burned out. A consortium led by Air Products in the US has developed and scaled up components based on tape-cast asymmetric membranes. A 5 tons/day oxygen generation module is in operation and a 150 tons/day pilot plant is being constructed (Fig. 4).35 The single planar laminates are made from a perovskite of confidential chemical composition and stacked using rings of the same material as spacers. A full ceramic braze is used for sealing, leading to very large stacks, each with a production capacity of up to 1 ton/day oxygen (Fig. 5). The spacers form a quasi tube through which the oxygen is pumped out of the membrane plates, which are loaded with pressurized air from the outside (three-end mode). Such a design does not permit the use of a sweep gas (four-end mode) but it has several advantages for pure oxygen production. The symmetric assembly, i.e. membrane layers outside with central supporting porous structures, is well suited to withstand the mechanical stresses caused by the compressed air. Another advantage of this module concept is the ability to scale it up. The membrane area can be increased “easily” by elongating the vessel

axis, which is much more appropriate than increasing the vessel diameter. However, intensive studies on fluid dynamics are required in order to control the oxygen distribution in the air feed. One major breakthrough was the development of a full ceramic joint37 enabling single components with a very high oxygen production capacity, thus reducing the required joints to the metallic area, which is still a challenge. Moreover, the gas-tightness requirement of this ceramic–metal junction is low because limited air leakage causes no severe impurities in the oxygen product stream. In order to decrease the number of tapes to be laminated, sequential tape casting was developed.34 In this case, the thin membrane layer is cast first. After drying, similar slurry with a certain amount of pore formers is cast on top of it. Since the same solvent is used, the additives in the membrane layer partly redissolve so that the tapes are adhesively joined. Subsequent sintering provides good contact with a low risk of delamination. In the case of a planar module in a four-end mode, manifolding of the different gas streams, i.e. feed, retentate, sweep, and permeate, is a major challenge. Fig. 6 shows how this is solved for solid oxide fuel cells (SOFCs). However, the pressure difference across the cell in an SOFC stack is only around 100 mbar. In the case of oxygen generation with an OTM, however, a pressure difference of several bars is applied in order to ensure the required driving force, i.e. oxygen partial pressure gradient. This gradient cannot be provided by the sweep gas alone, because its oxygen partial pressure is not low enough compared to atmospheric air (pO2 = 0.21 bar). Nonetheless, patents using a low pressure of 50–100 mbar exist, but technical realization has yet to be demonstrated. 4.2. Tubular design The majority of membranes are tubular, particularly those used in filtration processes. Polymeric membranes, in particular, can be easily manufactured in tubes of any size down to hollow fibres and then assembled to form membrane bundles, which fit well in metallic tubular housings. Consequently, different research groups are working on tubular oxygen transport membranes. The first logical choice of a manufacturing technique is extrusion because it is a mature technology for technical ceramics. Extrusion has already been

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Fig. 6. Planar SOFC stack prior to system mounting (left) and respective schematic of the internal structure (right).38

successfully transferred to perovskites.39 One major challenge here is adjusting the plasticity of the extrusion mass. In order to allow the mass be pressed through the extrusion head, a relatively low viscosity is required. However, a sufficiently high viscosity is required during drying in order to maintain the circularity of the tubes. Furthermore, homogeneity in the mass distribution is necessary, as otherwise deformation occurs during sintering. The Fraunhofer Institute of Ceramic Technologies and Systems (IKTS) has developed a transportable demonstration unit based on extruded tubes made of BSCF with a wall thickness of 0.5–1 mm within the framework of a consortium called MEM-BRAIN. The tubes are closed at one end and loaded with ambient air ventilation. The oxygen is pumped from the tubes using a vacuum pump and a low pressure of 20–100 mbar (Fig. 7).

This concept enables easy transportation, which was a design target, and avoids large pressure vessels and compressors. However, in a real module, the vacuum pump would require too much energy and the module would have to be operated with compressed air and a permeate pressure of 0.5–1 bar. This was realized by RWTH Aachen University with a 0.5 ton/day module (OXYCOAL-AC), which was based on similar tubes of BSCF (Fig. 8).40 The tubes are produced via cold isostatic pressing (CIP) instead of extrusion. The main advantage of CIP is a lower risk of deformation during processing particularly due to the absence of a drying step. The technology enables mass production but is normally not taken into account for the production of tubes. The pressure vessel of the module is constructed for pressures up to 20 bar and the oxygen is transported by a slight underpressure from 15 m2 membrane area.

Fig. 7. IKTS demonstration unit (left) and sketch of the cross section (right).

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Fig. 8. OXYCOAL-AC module (left) and schematic cross section.40

The final evaluation and selection of a manufacturing route is ultimately a task for the interested industry with its respective technology experience. One great challenge associated with a tubular membrane module is the gas-tight joining of each single tube to the metallic area. At present, this is solved by cooling the metal base plate of the module to temperatures below 200 ◦ C. Such low temperatures enable easy sealing techniques, such as crimping or glueing. Such processes lead to a significant loss of membrane area because reasonable oxygen flux through the membrane can only be achieved above 600–700 ◦ C. Therefore, sealing is shifted to the hot zone by the use of reactive air brazing, as described above. Another engineering challenge is scale-up because the pressure vessel is difficult to enlarge by increasing its diameter. Reducing the membrane diameter in order to increase the surface-to-volume ratio resulted in the development of ceramic capillaries, which are often referred to as hollow fibres. These structures are manufactured using the phase-inversion spinning technique41 adapted from the development of polymeric hollow fibres. A polymeric capillary is produced in which ceramic powder is dispersed. After sintering, a solid ceramic capillary

remains with a diameter of a few millimetre and wall thicknesses of a few hundred micrometre. Most of the capillaries have large defects known as macrovoids,42 which reduce the mechanical stability of the capillaries dramatically. In some cases, these voids are connected to the surface and are therefore accessible for the gas phase potentially increasing the permeation rate. However, a detailed analysis of whether macrovoids also have positive effects has yet to be performed using a fluid dynamical simulation. Another problem is the reaction of the ceramic powder with the polymers used during the spinning process, e.g. BSCF and sulphur-containing binders. This leads to the formation of BaSO4 reducing the oxygen permeation rate. The two problems, namely macrovoids and sulphate formation, have since been solved by adopting slurry composition41 (Fig. 9), and using sulphur-free binders such as PEI.42 The capillaries induce a strong pressure drop along the length axis due to their small inner diameter. Therefore, capillary length must be limited, which in turn leads to a large number of capillaries in a module, each of which requires sealing. However, high-temperature sealing is still a major challenge. For this reason, Tan et al.44 chose an intermediate temperature silicone

Fig. 9. Different microstructures of capillaries with macrovoids (left)43 and without macrovoids (right).41 Reprinted with permission from Elsevier.

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Fig. 11. Bundle of seven BSCF capillaries in one BSCF tube.45

Fig. 10. Membrane module based on perovskitic capillaries made from LSCF.44 Reprinted with permission from Elsevier.

sealant (up to 350 ◦ C) in order to make bundles of 7 capillaries in quartz glass tubes and built a module using up to 127 of such bundles, namely 889 capillaries (Fig. 10). Very recently Schulz et al. 45 bundled seven capillaries in a single perovskite tube by a full ceramic high temperature sealing similar to that of the planar design described in Section 4.1 (Fig. 11). This strategy avoids intermediate sealing and the use of an additional material, namely quartz glass. 4.3. Honeycomb structures Another strategy to increase the membrane area per volume is the use of honeycomb structures.46 This concept has many advantages, including its robustness despite relatively thin walls. The monoliths can be manufactured via extrusion in mass production. However, once again the manifolds of the required gas streams have prevented realization so far despite a very good design shown in Fig. 12. Minimal deformation of the thin walls during sintering and/or sealing leads to unacceptably high air leakage rates. The narrow manufacturing tolerances required were found to be too ambitious, and a technical realization has

not yet been achieved. At present, there seem to be no R&D activities in this direction. Also multi-channel tubes are being developed recently. The outer dimensions are much smaller, potentially reducing the risk of failure. Reliable manufacturing and reproducibility of the manifold have yet to be investigated in order to address the principal challenges of the honeycombs. 5. Conclusions and outlook The prospect of oxygen transport membranes is positive from the authors’ point of view. Based on extensive materials research starting in the 1970s, a lot of progress has been made in processing these materials to form membrane components in the last decade. There is a certain variety in membrane design and the relevant manufacturing techniques. Future research and development will reveal which of these are most efficient in terms of large-scale production and corresponding costs. In this regard, the different concepts must be distinguished from each other, i.e. three-end mode and four-end mode. At present, the most promising method for the three-end mode appears to be planar membranes in sandwich architecture stacked together using small spacer rings. The pilot stage as constructed by Air Products Inc. in the US, which generates 150 tons/day oxygen, is a remarkable milestone in the development of this technology. Nonetheless, further R&D tasks in manufacturing and process engineering must accompany this development. In the case of tubular membranes, this step is not yet foreseeable in the short

Fig. 12. Honeycomb structure (left) and design of manifold (middle and right).46

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term. To date, the proof-of-concept modules have worked with thick monolithic membrane tubes. These membrane tubes have performance disadvantages but they are easier to fabricate and to seal. Development work is necessary to decrease membrane thickness. This implies the development of asymmetric membranes, i.e. thin membrane layers on a porous support, as well as crossover designs, e.g. capillaries joined into tubes. In particular for the four-end concept, such membrane designs have significant advantages. One important research area is the development of asymmetric membranes using supports composed of a metal or a structural ceramic. This would provide several advantages such as better mechanical and chemical stability, easier joining, and lower cost. Therefore, better understanding of the coating technologies not requiring a subsequent sintering step, e.g. sputtering or thermal spraying, is necessary. Another major issue is still the sealing technology, particularly to the metallic area of the membrane module. Reactive air brazing is the most promising concept to date. Further development is necessary to increase reliability and cyclability, which should be feasible in the short term. However, another challenge is associated with the development of asymmetric membranes. The silver-based braze usually has a very low viscosity at sealing temperature, which leads to infiltration of the support pores so that not enough material remains for sealing. Avoidance strategies must be investigated and materials research, as well as geometrical aspects, i.e. design of the braze point, are essential. There is of course still a major need for development in materials science, which is not covered by this article. The (further) development of high-performance membrane materials with good stability in terms of performance, i.e. low degradation rate, as well as reactivity in reducing (e.g. CH4 ) or reactive (e.g. CO2 and SOx ) atmospheres are essential. References 1. Koros WJ, Ma YH, Shimidzu T. Terminology for membranes and membrane processes (IUPAC Recommendations 1996). Pure Appl Chem 1996;68:1479–89. 2. Meulenberg WA, Voigt I, Kriegel R, Baumann S, Ivanova M, van Gestel T. Inorganic membranes for CO2 separation. In: Stolten D, Scherer V, editors. Efficient carbon capture coal power plants – process engineering for CCS power plants. Weinheim: Wiley-VCH Verlag; 2011. p. 319–50. 3. Czyperek M, Zapp P, Bouwmeester HJM, Modigell M, Peinemann KV, Voigt I, Meulenberg WA, Singheiser L, Stöver D. MEM-BRAIN gas separation membranes for zero-emission fossil power plants. Energy Procedia 2009;1:303–10. 4. Macedonio F, Drioli E. Pressure-driven membrane operations and membrane distillation technology integration for water purification. Desalination 2008;223:396–409. 5. Ravanchi MT, Kaghazchi T, Kargari A. Application of membrane separation processes in petrochemical industry: a review. Desalination 2009;235:199–244. 6. de Morais Coutinho C, Chiu MC, Correa Basso R, Badan Ribeiro AP, Guaraldo Gonc¸alves LA, Viotto LA. State of art of the application of membrane technology to vegetable oils: a review. Food Res Int 2009;42:536–50. 7. Stamatialis DF, Papenburg BJ, Gironés M, Saiful S, Bettahalli SNM, Schmitmeier S, Wessling M. Medical applications of membranes: drug delivery, artificial organs and tissue engineering. J Membr Sci 2008;308:1–34.

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S. Baumann is a senior scientist at the Institute of Energy and Climate Research – IEK-1 Materials Synthesis and Processing, Forschungszentrum Jülich, Germany. He received his diploma degree in metallurgy and materials engineering focussing on glass and ceramics at RWTH Aachen University in 1998. He then joined the Institute of Ceramic Components in Mechanical Engineering at RWTH Aachen University working on different topics in technical ceramics including thermal shock behaviour of novel refractories earning his Ph.D. in 2007. Since 2006, he is working at IEK-1 on materials synthesis and processing of gas separation membranes heading the activities in oxygen transport membranes. Since 2012, Dr. Baumann is appointed as an Adjunct Professor in the Department of Chemical & Materials Engineering at the University of Alberta, Edmonton, Canada.

W.A. Meulenberg is a group leader at the Institute of Energy and Climate Research – IEK-1 Materials Synthesis and Processing, Forschungszentrum Jülich, Germany. He received his diploma degrees both in Materials Science and in Mineralogy at RWTH Aachen University in 1994. He later joined the Institute of Ceramics and Refractory Materials (GHI) at RWTH Aachen University where he received his Ph.D. in 1999. From 1999 until today he is working in different fields of materials development and component manufacturing for energy applications at Forschungszentrum Jülich starting in the development of the Solid Oxide Fuel Cells. From 2001 to 2003 he was a personal scientific assistant for the board of directors at Forschungszentrum Jülich. Since 2003 he became the group leader in the field of ceramic gas separation membranes. Since 2010, Dr. Meulenberg is a guest lecturer at RWTH Aachen University teaching “Functional ceramics – Inorganic Materials for Energy Applications” and an adjunct associate professor at the University of Queensland, Brisbane, Australia.

H.P. Buchkremer is the director (acting) of the Institute of Energy and Climate Research – Materials Synthesis and Processing (IEK-1) at Forschungszentrum Jülich. He obtained his Ph.D. degree in mechanical engineering at RWTH Aachen University in 1982 and served as a guest researcher at MAN, New Technologies Department, Munich, Germany, during the period 1978–1979. In 1979, he joined Forschungszentrum Jülich as Scientific Assistant and became the Section Leader for materials and processes for Solid Oxide Fuel Cells (SOFC) in 1995 acting as deputy institute director from 1996. He has 433 research publications, including more than 100 in reviewed scientific journals and 26 patents to his credit. His present research activities include new materials and processing of solid oxide fuel cells, solid state batteries, ceramic gas separation membranes, and processing of powders for Shape Memory Alloys. In 2009, he received the Erwin Schrödinger Award from the Helmholtz Association of German Research Centres (HGF).