Enhancement of oxygen permeation through U-shaped K2NiF4-type oxide hollow fiber membranes by surface modifications

Separation and Purification Technology 110 (2013) 74–80

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Enhancement of oxygen permeation through U-shaped K2NiF4-type oxide hollow fiber membranes by surface modifications Yanying Wei, Qing Liao, Zhong Li, Haihui Wang ⇑ School of Chemistry & Chemical Engineering, South China University of Technology, No. 381 Wushan Road, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 10 November 2012 Received in revised form 3 January 2013 Accepted 1 March 2013 Available online 14 March 2013 Keywords: Oxygen permeation Hollow fiber membrane Mixed conductor K2NiF4-type structure Surface modification

a b s t r a c t The U-shaped (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4+d (PLNCG) hollow fiber membranes are prepared by a phase inversion spinning process. The membranes are modified by acid etching followed by a porous catalytic layer coating of La0.8Sr0.2CoO3 d (LSC) aiming to improve the surface exchange kinetics, thus increasing the oxygen permeation flux. The oxygen permeation fluxes through the unmodified/acid etched/acid etched followed by LSC coated PLNCG hollow fiber membranes are measured under air/He gradients. At 800 °C, the oxygen permeation flux through the unmodified membrane is 0.06 ml/min cm2, while it rises to 0.21 ml/min cm2 after surface modification. The surface modified hollow fiber membrane exhibits noticeably higher oxygen permeation flux than that through the unmodified membrane, especially at relatively low temperatures, which indicates the membrane surface modification is beneficial for improving the oxygen permeation. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction In recent decades, the mixed conducting ceramic oxides with oxygen ionic and electronic conductivity (MIEC) have gained increasing attention due to their potential applications in the oxygen separation [1,2], conversion of natural gas [3–7] the cathode of solid oxide fuel cell [8,9] and oxygen supplying to power stations with CO2 sequestration according to the oxy-fuel concept [10– 12]. There are two basic requirements for the MIEC membranes in industrial applications. Firstly, the MIEC membranes should have a good stability under various atmospheres at elevated temperatures for a fairly long time, especially under some harsh reducing atmosphere, such as CO, H2, CH4 and CO2. Secondly, the MIEC membranes should exhibit remarkable high oxygen permeation fluxes. In recent years, many efforts have been focused on improvement of the oxygen permeation of these MIEC membranes. Under an oxygen partial pressure gradient at elevated temperature, oxygen permeation through the MIEC membrane takes place by five major steps: (i) the oxygen molecule in air diffusing from the gas phase to the membrane surface; (ii) oxygen exchange reactions on the high oxygen partial pressure side of the membrane surface, including oxygen adsorption, dissociation and incorporation to form lattice oxygen; (iii) bulk diffusion of oxygen ions or vacancies and electron holes; (iv) surface reaction between lattice oxygen and electron holes on the other membrane surface (permeate side) ⇑ Corresponding author. Tel./fax: +86 20 87110131. E-mail address: [email protected] (H. Wang). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.03.006

and oxygen desorption; (v) recombination of the oxygen molecule which leaves the membrane surface to the permeate stream. It is clear that the major resistance of oxygen permeation results from surface reactions and bulk diffusion. If the rate controlling step of the oxygen transporting is only the bulk diffusion, the oxygen permeation flux can be increased by reducing the membrane thickness. Various coating technologies can be used for preparation of the super thin dense MIEC membrane supported on porous substrate [1,13,14]. Hollow fiber membrane with wall thickness between 200 and 300 lm is also a good choice to decrease the bulk diffusion resistance and improve the oxygen permeation flux [15–19]. As is known, when the membrane becomes sufficiently thin, the surface reaction begins to control the rate of oxygen permeation [20]. Under this circumstance, further increase of the oxygen permeation rate can be achieved by improving the surface exchange kinetics. It be realized (i) by increasing the effective membrane surface area, such as coating the membrane surface with porous layer [21,22], roughening the membrane surface by abrading [23,24], acid-modification [25] and methane-modification [26]; (ii) by coating the membrane surface with materials of superior oxygen exchange properties as catalyst to promote a faster oxygen exchange, such as the noble metals [27–33]. In our previous work (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4+d (PLNCG) is proved to be a good K2NiF4-type oxide for oxygen permeation with an excellent chemical stability in the long-term operation, especially under CO2 atmosphere, which promotes the practical application in oxy-fuel combustion for CO2 capture of MIEC membranes [34,35]]. However, the oxygen permeability through PLNCG hollow fiber membrane should be further

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improved. The oxygen permeation process through the hollow fiber membranes may be largely controlled by the surface exchange reactions at the gas/membrane interfaces [31,32]. Therefore, enhanced oxygen permeation fluxes through the hollow fiber membranes can be expected by surface modifications. Herein, La0.8Sr0.2CoO3 d (LSC) was chosen as the catalytic material to modify the PLNCG hollow fiber membrane surface due to its good catalytic activity and good stability under air [36]. In this paper, the PLNCG hollow fiber membranes are modified by acid etching followed by LSC porous layer aimed to increase the oxygen permeation flux. If the surface modified hollow fiber membrane is used for the oxyfuel combustion, etc., only the inner surface of the hollow fiber membrane is exposed to the harsh atmosphere, while the outer surface of the membrane is still exposed to air. Considering the good stability of LSC under air and good stability of PLNCG under even harsh conditions, herein, only the outer surface of the hollow fiber membrane was coated with LSC. The effects of the membrane outer surface modification on the oxygen permeation flux have been investigated.

2. Experimental 2.1. Preparation of powder The sol–gel route based on citric acid and EDTA as complexing and gelation agents has been adapted to prepare the PLNCG and LSC powder [34]. Taking PLNCG as an example, Ga was dissolved in nitric acid first, and proper amounts of Pr(NO3)3
6H2O, La(NO3)3
6H2O, Ni(CH3COO)2
4H2O, Cu(NO3)2
3H2O were dissolved in water followed by the addition of citric acid, EDTA and NH3
H2O. The mixture was then evaporated at 150 °C under constant stirring to obtain a dark-green gel. The gel was firstly ignited to flame. After combustion, precursor-powder can be achieved, which does not present desired phase structure. Therefore, the precursor-powder was ground and calcined at temperatures up to 950 °C for 10 h with a heating rate of 2 °C/min to remove the residual carbon and form the desired K2NiF4-type structure. For spinning hollow fiber membranes, the powder was ball-milled for 24 h and then was dried using a spray dryer (Büchi Mini Spray Dryer, B-290) with a nozzle of 1 lm.

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2.2. Preparation of hollow fiber membrane The U-shaped PLNCG hollow fibers were fabricated using a wet spinning/sintering technology [37]. The spinning solution was composed of 9.29 wt.% polyethersulfone (PESf, A-300, BASF), 37.18 wt.% 1-methyl-2-pyrrolidinone (NMP, AR Grade, purity > 99.8%, Kermel Chem Inc., Tianjin, China), 0.93 wt.% polyvinyl pyrrolidone (PVP, K30, Boao biotech co., Shanghai, China) and 52.60 wt.% PLNCG powder. A spinneret with an orifice diameter and inner diameter of 1.5 and 1.0 mm, respectively, was used to obtain the hollow fiber precursors. Deionized water and tap water were used as the internal and external coagulants, respectively. Afterward, the PLNCG hollow fiber precursors were sintered at 1300 °C for 3 h with the air flow rate of 60 ml/min to remove the polymer and get gas-tight membrane. Membrane surface modification was carried out by two steps, which is shown in Fig. 1. Firstly, the U-shaped PLNCG hollow fiber membrane (Fig. 1A) was modified by acid etching aimed to get a large effective membrane surface area (Fig. 1B). In briefly, the Ushaped hollow fiber membrane was immersed in 18 vol.% HCl solution for 5 min with slow stir. The etched hollow fiber was rinsed with deionized water for three times and dried at room temperature. Secondly, the coating slurry was prepared by mixing of LSC and ethanol in an agate mortar. The LSC slurry was brushed onto the outer surface of the acid-modified U-shaped PLNCG hollow fiber membrane, as shown in Fig. 1C. The surface coating process was repeated for twice after the coating layer dried at room temperature. There is no annealing process after the acid-treatment or coating the catalysis layer. Finally, the U-shaped PLNCG hollow fiber membrane after surface acid-modification and LSC porous layer-modification (Fig. 1D) was prepared for oxygen permeation.

2.3. Characterizations and measurements of oxygen permeation The phase structure of the as-prepared PLNCG powder, LSC powder and the modified hollow fibers were characterized by Xray diffraction (XRD, Bruker-D8 ADVANCE, Cu Ka radiation). The microstructure and morphology of the surface modified PLNCG hollow fiber membranes were observed by a scanning electron microscope (SEM, JEOL JSM-6490LA). The element compositions

Fig. 1. Schematic diagram of the surface modification of PLNCG hollow fiber membrane.

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of the membranes were determined by energy dispersive spectroscopy (EDS). The oxygen permeation fluxes through the modified and unmodified U-shaped PLNCG hollow fiber membranes were investigated in a high-temperature permeation cell [38]. Before the modified hollow fiber membrane is used for the oxygen permeation, the gas tightness is tested at room temperature. No nitrogen is detected on the shell side even when the nitrogen partial pressure on the core side reached 0.5 MPa, which indicates that the hollow fiber membranes are gastight. The U-shaped PLNCG hollow fiber membrane was sealed in a corundum tube with two channels by a commercial ceramic sealant (HT767, Hutian, China). Air or a mixture of nitrogen and oxygen was fed to the shell side while helium swept on the core side to collect the permeated oxygen through the membrane. The gas flow rates were controlled by mass flow controllers (MFC, Seven Star D08-4F/ZM) calibrated using a soap bubble flow meter. The composition of the permeated gas was measured using an online gas chromatograph (GC, Agilent 7890) with a TCD detector. The leakage of the oxygen due to the imperfect sealing at high temperatures was less than 0.5% during all the experiments. The calculation of oxygen permeation flux can be found in our previous work [37]. 3. Results and discussion Fig. 2 shows the XRD patterns of LSC powder, unmodified/acid etched/acid etched followed by LSC-coated PLNCG hollow fiber membranes. The XRD patterns show strong diffraction peaks assigned to the orthorhombic phase of LSC mainly with 2h of 23.2, 33.1, 40.6, 59.3, 69.4, 79.1 for the lattice planes of 110, 112, 022, 220, 312, 224 and 116, respectively. The unmodified PLNCG hollow fiber membranes shows good K2NiF4-type structure mainly with 2h of 24.1, 31.6, 32.8, 44.0, 47.1 and 57.8 for the lattice planes of 111, 113, 200, 204, 220 and 313, respectively. Comparing with the XRD pattern of the unmodified membrane, it can be seen that some peaks assigned to CuCl have occurred on the XRD curve of the HCl-modified hollow fiber membrane. The membrane surface composition before and after acid-treatment have also been checked through EDS characterization, which shows that a small amount of Cl remains on the membrane surface after the acid etching. Wang et al. [25] found that acid etching didn’t lead to the formation of new phases, but only changed the microstructure on hollow fiber surface. It is because La0.6Sr0.4Co0.2Fe0.8O3 d membrane was used and such oxide can react with HCl solution to form soluble chloride salts. After acid etching and LSC porous layer coating,

CuCl

acid etched followed by LSC-caoted LSC powder acid etched unmodified

20

40

60

2theta /

80

o

Fig. 2. XRD patterns of LSC powder, unmodified/acid etched/acid etched followed by LSC-coated PLNCG hollow fiber membranes.

the modified PLNCG hollow fiber membrane exhibits both the perovskite structure from LSC and K2NiF4-type structure from PLNCG. The SEM micrographs of the PLNCG hollow fiber membrane before and after modification are shown in Fig. 3. The unmodified membranes are shown in Fig. 3A and B, which present intact surface. From Fig. 3C and D, it can be found that the membrane surface has been etched after acid modification, which gives more surface area. Fig. 3E–H shows the microstructures of the membrane after acid etching followed by LSC coating. The thickness of LSC coating layer is about 40–70 lm obtained from Fig. 3F. Different from the dense PLNCG hollow fiber membrane sintered at 1300 °C, the coated LSC layer is still highly porous, but is integrated with the PLNCG membrane bulk perfectly, as shown in Fig. 3G. Tan et al. [29] found that a good integration between the membrane bulk and the porous layer is of high importance to minimize the boundary resistance to transportation of oxygen ions and exchange reactions. Fortunately, the LSC porous layer was firmly attached to the dense PLNCG membrane, which will be beneficial for improving the oxygen permeation flux. Furthermore, the interface with a zigzag shape between the dense PLNCG membrane and porous LSC layer can be observed, which is resulted from the surface acidmodification. PLNCG oxides can react with HCl solution intensively to form chloride salts. The U-shaped PLNCG hollow fiber membrane can be thoroughly destroyed by the acid reaction with 18 vol.% HCl solution for 10 min. In order to keep the gas tightness of the hollow fiber membranes, the acid etching is limited to a short reaction time (such as 5 min), as investigated by Wang et al. [25]. After the acid etching, not only does the rough outer surface of the PLNCG membrane provide larger effective membrane area, but also is good for the contact of LSC porous layer. Fig. 3H shows the LSC porous layer coated on the outer surface of PLNCG hollow fiber membrane. The layer is composed of LSC particles with diameters between 0.5 and 1.0 lm. Such a porous catalytic layer will be helpful to reduce the surface oxygen exchange resistance and enhance the oxygen permeation. The oxygen permeation of the surface modified PLNCG hollow fiber membrane under different conditions is investigated. Fig. 4 compares the oxygen permeation fluxes through the unmodified/ acid etched/acid etched followed by LSC coated hollow fiber membranes under different temperatures. Compared with the unmodified membrane, the acid etched one shows only a little higher oxygen permeation flux. However, the acid etched followed by LSC coated hollow fiber membrane exhibits noticeably higher oxygen permeation flux than that of the unmodified membrane, especially at relatively low temperatures. For example, at 800 °C, the oxygen permeation flux through the unmodified membrane is 0.06 ml/min cm2 while it rises to 0.21 ml/min cm2 after surface modification. At an even lower temperature, 750 °C, the oxygen permeation flux is 0.19 ml/min cm2, which is about 19 times higher than that through the unmodified membrane. It is indicated that the acid etching can help to give a larger membrane area with rough surface and it will promote the contacting of the coating layer. The key factor for enhancing the oxygen permeation flux is the porous catalytic LSC layer coating on the membrane surface. The fact that the oxygen permeation flux can be improved by coating a porous catalytic layer through increasing the effective surface area and promoting a faster oxygen exchange rate indicates that surface reaction plays an important role in the determination of oxygen permeation rate. Fig. 5 shows the enhancement factor of the oxygen permeation flux through the modified PLNCG hollow fiber membrane as a function of operating temperature. Obviously, the enhancement factor increases with decreasing temperature. It is because the surface exchange reaction played a more important role during oxygen permeation at relatively low temperatures [38] and the relative limiting effect of the bulk diffusion gradually

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Fig. 3. SEM micrographs of the PLNCG hollow fiber membranes before and after surface modification. (A) cross-section and (B) outer surface of the unmodified PLNCG membrane; (C) cross-section and (D) outer surface of the acid etched PLNCG membrane; (E–G) the cross-section of the acid etched followed by LSC-coated membrane; (H) LSC porous layer coated on the outer surface of PLNCG hollow fiber membrane.

becomes less important as the temperature further decreases. It can be found the enhancement factor of acid etching followed by LSC coating is 3.6 at 800 °C and it rises to 10.3 at 750 °C. The oxygen permeation flux through the modified membrane can even improved with the enhancement factor of 25.3 at 700 °C. Fig. 6 compares the oxygen permeation fluxes through the unmodified/acid etched/acid etched followed by LSC coated hollow fiber membranes as a function of helium sweep flow rates at 825 °C. Generally, the oxygen permeation fluxes through the membrane increased with increasing helium flow rate. Because higher helium flow rates dilute the permeated oxygen and lower the oxy-

gen partial pressure on the core side, i.e. increase the permeation driving force. However, the effect of the helium flow rate on the oxygen permeation flux through PLNCG hollow fiber membrane becomes insensitive when the temperature is below 900 °C [38]. Even so, the improvement of the oxygen permeation flux after surface modification is still obvious. The enhancement factor of acid etching followed by LSC coating keeps around 2.3 at 825 °C, which is in accordance with the result in Fig. 5. The oxygen permeation fluxes through the unmodified/acid etched/acid etched followed by LSC coated hollow fiber membranes as a function of oxygen partial pressure on the shell side

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0.30

.

0.5

2

unmodified etched etched-coated

Oxygen permeation flux / ml/min cm

.

Oxygen permeation flux / ml/min cm

2

0.6

0.4 0.3 0.2 0.1

etched-coated membrane

0.25 0.20

etched membrane

0.15 0.10

unmodified membrane

0.05

0.0 0.00

700

750

800

850

900

20

40

60

80

100

120

Helium flow rate / ml/min

o

Temperature / C Fig. 4. Comparison of oxygen permeation fluxes through the unmodified/acid etched/acid etched followed by LSC-coated PLNCG hollow fiber membranes under different temperatures. Conditions: temperature varied from 700 to 900 °C, Fair = 180 ml/min, FHe = 55 ml/min.

Fig. 6. Comparison of oxygen permeation fluxes through the unmodified/acid etched/acid etched followed by LSC-coated PLNCG hollow fiber membranes as a function of helium sweep flow rates. Conditions: T = 825 °C, Fair = 180 ml/min.

o

Enhancement factor

15 10 5 0

850 C- etched-coated o 850 C- etched o 850 C- unmodified o 800 C- etched-coated o 800 C- etched o 800 C- unmodified

2

20

750

0.60

0.45

0.30

0.15

0.00

enhancement factor = 1

700

.

etched etched-coated

Oxygen permeation flux / ml/min cm

25

800

850

900

o

Temperature / C Fig. 5. Enhancement factor of the oxygen permeation flux through the surfacemodified PLNCG hollow fiber membrane as a function of operating temperature.

at different temperatures are shown in Fig. 7. The total flow rate of the oxygen and nitrogen on the shell side is 120 ml/min and different oxygen partial pressure on the shell side is obtained by adjusting the ratio of nitrogen and oxygen. Both of the oxygen permeation fluxes through the unmodified and modified membranes increase with increasing oxygen partial pressure on the shell side because of the increased oxygen gradient across the membrane. As is known, there are two ways to increase the oxygen gradient, increasing the feed air pressure by a compressor, or reducing the permeate pressure by using a vacuum pump. In fact the latter has a larger impact, as reducing the pressure of the permeate flux will result in higher oxygen fluxes for a given air feed pressure. From Fig. 7, the positive effect of the surface modification on the oxygen permeation flux is more remarkable at relatively low temperature, which is in accordance with the results in Figs. 4 and 5. Furthermore, the oxygen permeation flux through the modified membrane is more sensitive to the oxygen partial pressure on the shell side. It is because the driving force, the oxygen partial pressure gradient across the membrane, has a larger influence on

0.0

0.2

0.4

0.6

0.8

1.0

Oxygen partial pressure on the shell side / atm Fig. 7. Comparison of oxygen permeation fluxes through the unmodified/acid etched/acid etched followed by LSC-coated PLNCG hollow fiber membranes as a function of oxygen partial pressure on the shell side at different temperatures. Conditions: T = 800 or 850 °C, FN2+O2 = 120 ml/min, FHe = 55 ml/min.

the bulk diffusion than that on the surface oxygen exchange [38]. After the surface modification, the surface reaction resistance is reduced. However, the surface kinetic is still controlling the oxygen ionic transport, which is still the limiting transport condition even after surface modification. Fig. 8 shows the apparent activation energy of these hollow fiber membranes. Three straight lines are found which give the apparent activation energy of 112.74, 94.13 and 44.80 kJ/mol for the unmodified, acid etched, acid etched followed by LSC coated PLNCG hollow fiber membranes, respectively, at the temperature range of 700–900 °C. As is known, the apparent activation energy of the oxygen permeation is consisted of two parts: (i) the activation energy of the surface oxygen exchange, and (ii) the activation energy of the bulk diffusion. After the surface modification, the resistance of the surface reaction is reduced. As a result, the apparent activation energy of the modified PLNCG hollow fiber membrane is decreased from 112.74 kJ/mol to 44.80 kJ/mol, which indicates the improvement of the oxygen permeability.

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with rough surface and it will promote the contacting of the coating layer. The key factor for enhancing the oxygen permeation flux is the porous catalytic LSC layer coating on the membrane surface. However, both of these surface modifications are beneficial for improving the oxygen permeation. It has to be admitted that the membrane cannot be used for industrial application based on such low oxygen permeation fluxes measured under air/He gradient, even after surface modification. However, in the reaction of oxyfuel combustion or partial oxidation of methane (POM), etc., the sweep helium gas will be replaced by methane (or some other fuel, hydrocarbons) which can provide even lower oxygen partial pressure on one side of membrane and resulted in even higher oxygen permeation flux.

-6.5 apparent

Ea,etched-coated = 44.80 kJ/mol

apparent

Ea,etched = 94.13 kJ/mol

apparent

Ea,unmodified = 112.74 kJ/mol

2

2 log (JO ) / mol/s.cm

-7.0

-7.5

-8.0

79

-8.5

Acknowledgements

-9.0

0.84

0.88

0.92

0.96

1000/T / K

1.00

1.04

-1

Fig. 8. Comparison of the apparent activation energy of the unmodified/acid etched/acid etched followed by LSC-coated PLNCG hollow fiber membranes.

The authors greatly acknowledge the financial support by National Science Fund for Distinguished Young Scholars of China (No. 21225625), by Natural Science Foundation of China (Nos. 21176087 and 20936001), and by Educational Commission of Guangdong Province for Talent Introduction.

0.3

.

Oxygen permeation flux / ml/min cm

2

References

0.2

0.1

0.0 0

20

40

60

80

100

Time / h Fig. 9. Time-dependence of the oxygen permeation flux through the acid etched followed by LSC-coated PLNCG hollow fiber membrane. Conditions: T = 800 °C, Fair = 180 ml/min, FHe = 55 ml/min.

A promising oxygen permeable membrane should not only possess good oxygen permeation flux, but it should also have good stability. The oxygen permeation flux through the acid etched followed by LSC coated PLNCG hollow fiber membrane as a function of time at 800 °C is shown in Fig. 9. During the first 50 h of operation, the membrane exhibits an initial reduction in the oxygen permeation flux. It is resulted from the poor phase stability of LaxSryCoO3 d (x + y = 1) [39] and the loss of Co occurred at 800 °C. Even so, after 50 h, the oxygen permeation flux keeps stable with the value around 0.18 ml/min cm2, which is about three times higher than the value of the unmodified membrane. 4. Conclusion The U-shaped (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4+d (PLNCG) hollow fiber membranes are modified by acid etching followed by La0.8Sr0.2CoO3 d (LSC) porous layer coating aimed to increase the oxygen permeation flux. The surface modified hollow fiber membrane exhibits noticeably higher oxygen permeation flux than that of the unmodified membrane, especially at relatively low temperatures. The acid etching can help to give a larger membrane area

[1] J.F. Vente, W.G. Haije, Z.S. Rak, Performance of functional perovskite membranes for oxygen production, J. Membr. Sci. 276 (2006) 178–184. [2] B. Meng, Z. Wang, Y. Liu, X. Tan, J.C. Diniz da Costa, S. Liu, Preparation and oxygen permeation properties of SrCo0.9Nb0.1O3 d hollow fibre membranes, Sep. Purif. Technol. 78 (2011) 175–180. [3] H.Q. Jiang, H.H. Wang, S. Werth, T. Schiestel, J. Caro, Simultaneous production of hydrogen and synthesis gas by combining water splitting with partial oxidation of methane in a hollow fiber membrane reactor, Angew. Chem. Int. Ed. 47 (2008) 9341–9344. [4] Z. Shao, G. Xiong, H. Dong, W. Yang, L. Lin, Synthesis, oxygen permeation study and membrane performance of a Ba0.5Sr0.5Co0.8Fe0.2O3 d oxygen-permeable dense ceramic reactor for partial oxidation of methane to syngas, Sep. Purif. Technol. 25 (2001) 97–116. [5] C.S. Chen, S.J. Feng, S. Ran, D.C. Zhu, W. Liu, H.J.M. Bouwmeester, Conversion of methane to syngas by a membrane-based oxidation-reforming process, Angew. Chem. Int. Ed. 42 (2003) 5196–5198. [6] F.T. Akin, Y.S. Lin, Selective oxidation of ethane to ethylene in a dense tubular membrane reactor, J. Membr. Sci. 209 (2002) 457–467. [7] X. Tan, L. Shi, G. Hao, B. Meng, S. Liu, La0.7Sr0.3FeO3 d perovskite hollow fiber membranes for oxygen permeation and methane conversion, Sep. Purif. Technol. 96 (2012) 89–97. [8] K. Zhang, L. Ge, R. Ran, Z.P. Shao, S.M. Liu, Synthesis, characterization and evaluation of cation-ordered LnBaCo2O5+d as materials of oxygen permeation membranes and cathodes of SOFCs, Acta Mater. 56 (2008) 4876–4889. [9] A.Y. Yan, M.J. Cheng, Y.L. Dong, W.S. Yang, V. Maragou, S.Q. Song, P. Tsiakaras, Investigation of a Ba0.5Sr0.5Co0.8Fe0.2O3 d based cathode IT-SOFC I. The-effect of CO2 on the cell performance, Appl. Catal. B: Environ. 66 (2006) 64–71. [10] J.Y. Ren, Y.Q. Fan, F.N. Egolfopoulos, T.T. Tsotsis, Membrane-based reactive separations for power generation applications: oxygen lancing, Chem. Eng. Sci. 58 (2003) 1043–1052. [11] Y.Q. Fan, J.Y. Ren, W. Onstot, J. Pasale, T.T. Tsotsis, F.N. Egolfopoulos, Reactor and technical feasibility aspects of a CO2 decomposition-based power generation cycle, utilizing a high-temperature membrane reactor, Ind. Eng. Chem. Res. 42 (2003) 2618–2626. [12] J. Li, Q. Zeng, T. Liu, C. Chen, Oxygen permeability and CO2-tolerance of Sr(Co0.8Fe0.2)0.8Ti0.2O3 d hollow fiber membrane, Sep. Purif. Technol. 77 (2011) 76–79. [13] X.Y. Tan, Y.T. Liu, K. Li, Mixed conducting ceramic hollow-fiber membranes for air separation, AIChE J. 51 (2005) 1991–2000. [14] C. Li, G. Yu, N. Yang, Supported dense oxygen permeating membrane of mixed conductor La2Ni0.8Fe0.2O4+d prepared by sol–gel method, Sep. Purif. Technol. 32 (2003) 335–339. [15] X.Y. Tan, Y.T. Liu, K. Li, Preparation of LSCF ceramic hollow-fiber membranes for oxygen production by a phase-inversion/sintering technique, Ind. Eng. Chem. Res. 44 (2005) 61–66. [16] S.M. Liu, G.R. Gavalas, Preparation of oxygen ion conducting ceramic hollowfiber membranes, Ind. Eng. Chem. Res. 44 (2005) 7633–7637. [17] T. Schiestel, M. Kilgus, S. Peter, K.J. Caspary, H.H. Wang, J. Caro, Hollow fibre perovskite membranes for oxygen separation, J. Membr. Sci. 258 (2005) 1–4. [18] M. Trunec, Fabrication of zirconia- and ceria-based thin-wall tubes by thermoplastic extrusion, J. Eur. Ceram. Soc. 24 (2004) 645–651. [19] J. Luyten, A. Buekenhoudt, W. Adriansens, J. Cooymans, H. Weyten, F. Servaes, R. Leysen, Preparation of LaSrCoFeO3 d membranes, Solid State Ionics 135 (2000) 637–642.

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[20] H.J.M. Bouwmeester, H. Kruidhof, A.J. Burggraaf, Importance of the surface exchange kinetics as rate limiting step in oxygen permeation through mixedconducting oxides, Solid State Ionics 72 (PART 2) (1994) 185–194. [21] S. Lee, K.S. Lee, S.K. Woo, J.W. Kim, T. Ishihara, D.K. Kim, Oxygen-permeating property of LaSrBFeO3 (B = Co, Ga) perovskite membrane surface-modified by LaSrCoO3, Solid State Ionics 158 (2003) 287–296. [22] K. Watanabe, M. Yuasa, T. Kida, K. Shimanoe, Y. Teraoka, N. Yamazoe, Preparation of oxygen evolution layer/La0.6Ca0.4CoO3 dense membrane/ porous support asymmetric structure for high-performance oxygen permeation, Solid State Ionics 179 (2008) 1377–1381. [23] T. Kida, S. Ninomiya, K. Watanabe, N. Yamazoe, K. Shimanoe, High oxygen permeation in Ba0.95La0.05FeO3 d membranes with surface modification, Appl. Mater. Interfaces 2 (2010) 2849–2853. [24] H. Kusaba, Y. Shibata, K. Sasaki, Y. Teraoka, Surface effect on oxygen permeation through dense membrane of mixed-conductive LSCF perovskitetype oxide, Solid State Ionics 177 (2006) 2249–2253. [25] Z. Wang, H. Liu, X. Tan, Y. Jin, S. Liu, Improvement of the oxygen permeation through perovskite hollow fibre membranes by surface acid modification, J. Membr. Sci. 345 (2009) 65–73. [26] H. Liu, Z. Pang, X. Tan, Z. Shao, J. Sunarso, R. Ding, S. Liu, Enhanced oxygen permeation through perovskite hollow fibre membranes by methane activation, Ceram. Int. 35 (2009) 1435–1439. [27] A. Leo, S. Smart, S. Liu, J.C. Diniz da Costa, High performance perovskite hollow fibres for oxygen separation, J. Membr. Sci. 368 (2011) 64–68. [28] C. Yacou, J. Sunarso, C.X.C. Lin, S. Smart, S. Liu, J.C. Diniz da Costa, Palladium surface modified La0.6Sr0.4Co0.2Fe0.8O3 d hollow fibres for oxygen separation, J. Membr. Sci. 380 (2011) 223–231. [29] X.Y. Tan, Z.G. Wang, H. Liu, S.M. Liu, Enhancement of oxygen permeation through La0.6Sr0.4Co0.2Fe0.8O3 d hollow fibre membranes by surface modifications, J. Membr. Sci. 324 (2008) 128–135.

[30] A. Leo, S. Liu, J.C. Diniz da Costa, The enhancement of oxygen flux on Ba0.5Sr0.5Co0.8Fe0.2O3 d (BSCF) hollow fibers using silver surface modification, J. Membr. Sci. 340 (2009) 148–153. [31] A. Leo, S. Liu, J.C. Diniz da Costa, Z. Shao, Oxygen permeation through perovskite membranes and the improvement of oxygen flux by surface modification, Sci. Technol. Adv. Mater. 7 (2006) 819–825. [32] A. Thursfield, I.S. Metcalfe, Air separation using a catalytically modified mixed conducting ceramic hollow fibre membrane module, J. Membr. Sci. 288 (2007) 175–187. [33] F.M. Figueiredo, V.V. Kharton, A.P. Viskup, J.R. Frade, Surface enhanced oxygen permeation in CaTi1 xFexO3 d ceramic membranes, J. Membr. Sci. 236 (2004) 73–80. [34] J. Tang, Y.Y. Wei, L.Y. Zhou, Z. Li, H.H. Wang, Oxygen permeation through a CO2-tolerant mixed conducting oxide (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4+d, AIChE J. 58 (2012) 2473–2478. [35] Y.Y. Wei, J. Tang, L.Y. Zhou, J. Xue, Z. Li, H.H. Wang, Oxygen separation through U-shaped hollow fiber membrane using pure CO2 as sweep gas, AIChE J. 58 (2012) 2856–2864. [36] L. Zeng, Y. Liu, B. Duan, P. Liu, H. Wang, A. Shui, Preparation and catalytic performance of La0.8Sr0.2CoO3 supported on the mullite fiber ceramic, Front. Chem. Eng. China 1(4) (2007) 372–376. [37] Y.Y. Wei, H.F. Liu, J. Xue, Z. Li, H.H. Wang, Preparation and oxygen permeation of U-shaped perovskite hollow fiber membranes, AIChE J. 57 (2011) 975–984. [38] Y.Y. Wei, J. Tang, L.Y. Zhou, Z. Li, H.H. Wang, Oxygen permeation through Ushaped K2NiF4-type oxide hollow-fiber membranes, Ind. Eng. Chem. Res. 50 (2011) 12727–12734. [39] Z.P. Shao, G.X. Xiong, J.H. Tong, H. Dong, W.S. Yang, Ba effect in doped Sr(Co0.8Fe0.2)O3 d on the phase structure and oxygen permeation properties of the dense ceramic membranes, Sep. Purif. Technol. 25 (2001) 419–429.