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Honeycomb-structured perovskite hollow fibre membranes with ultra-thin densified layer for oxygen separation
Separation and Purification Technology 80 (2011) 396–401
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Short Communication
Honeycomb-structured perovskite hollow fibre membranes with ultra-thin densified layer for oxygen separation Nan Liu a, Xiaoyao Tan a,⇑, Bo Meng a, Shaomin Liu b,⇑ a b
School of Chemical Engineering, Shandong University of Technology, Zibo 255049, China Centre for Advanced Energy Science and Engineering, Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia
a r t i c l e
i n f o
Article history: Received 1 March 2011 Received in revised form 8 April 2011 Accepted 12 April 2011 Available online 17 May 2011 Keywords: Hollow fibre membrane Oxygen permeation Perovskite Microstructure
a b s t r a c t New honeycomb-structured and gas-tight La0.6Sr0.4Co0.2Fe0.8O3-a (LSCF) perovskite hollow fibre membranes have been developed with an ultra-thin dense layer using phase inversion/sintering technique with an EtOH–NMP mixture as the internal coagulant. By varying the rate of internal precipitation the surface morphology of the hollow fibre could be altered to provide a single thin dense layer and high support surface area. These membranes were able to provide significantly higher O2 fluxes than conventional types, with an improvement factor up to 15 at 800 °C, while maintaining membrane quality and mechanical stability. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Oxygen separation using mixed ionic-electronic conducting (MIEC) ceramic membranes have attracted increasing attention in clean energy research, due to its ability to operate at much lower costs when compared with conventional cryogenic technology [1–12]. Most research in this area has looked at disk and tubular membranes fabricated using conventional methods such as static-pressing or ram extrusion [4,13]. Although a multiple planar stack or tubular design can be used to enlarge the membrane area to a plant scale, many engineering problems arise from these designs such as high temperature sealing, connection, pressure resistance and small surface/volume ratios. On the other hand, because of the symmetric structure of these disk or tubular membranes prepared, oxygen ion bulk diffusion is usually the rate-limiting step in air separation process resulting in a low oxygen permeation rate [13]. In contrast to these traditional configurations, asymmetric hollow fibres appear to be a promising alternative. Hollow fibres can be densely packed in small modules, providing the largest membrane area per unit packing volume, while the thin separating layer produced by the asymmetric structure can achieve higher oxygen
⇑ Corresponding authors. Tel.: +86 533 2786292 (X. Tan), tel.: +61 8 92669056 (S. Liu). E-mail addresses: [email protected] (X. Tan), [email protected], [email protected] (S. Liu). 1383-5866/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2011.04.014
flux than other techniques [8,14]. Furthermore, high temperature sealing problems can be avoided by adopting a long hollow fibre module which keeps the sealing ends out of the high temperature zone. Ceramic hollow fibre membranes can be fabricated via a combined phase inversion and high temperature sintering technique. During the extrusion step via phase inversion, water is often used as the internal and external coagulants, producing a hollow fibre membrane with an integral asymmetric structure showed schematically in Fig. 1A [15]. In this structure, three dense layers are integrated with porous layers of the same material, producing many closed or isolated short finger-like pores near the surface layers. In MIEC membranes, surface reactions and bulk ion diffusion provide the greatest resistance to oxygen transport [16]. The isolated pores increase the number of surface reactions required and this in combination with multiple dense layers causes a large resistance to gas transport. Acid etching can help reduce transport resistance by opening the dense layers at the surfaces, however, this process is complicated and time consuming [17,18]. In this report, we look at an improved method for fabrication of honeycomb structured perovskite hollow fibre membranes. By changing the internal coagulant (also called bore liquid in the spinneret), a honeycomb structure with a single dense layer was produced (Fig. 1B) without the need for further acid etching steps. In this work, La0.6Sr0.4Co0.2Fe0.8O3-a (LSCF) perovskite was used as the membrane material due to its high mechanical and phase stability. However, the developed method can be extended to utilise other membranes materials for other applications.
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Fig. 1. Schematic of membrane architectures to be improved [(A) cross sectional view of the original hollow fibre membranes with three dense layers using water as the both coagulants; (B) improved membrane architectures using new method with only one dense layer near the outside surface and honeycomb structure viewed from the inner surface side].
2. Experimental 2.1. Preparation of LSCF powder and hollow fibres La0.6Sr0.4Co0.2Fe0.8O3-a (LSCF6428) perovskite powders were prepared through a sol–gel process, which is described in details elsewhere [19,20]. For spinning hollow fibre membranes, the powder precursor was calcined at 800 °C for 3 h to remove residual carbon and to form the desired structure. It was then ball-milled for 48 h in an agate jar and sieved using 200 mesh for 24 lm sievepore diameter. The hollow fibre membranes were made from the calcined and ball-milled powders using the phase inversion and sintering technique. This is described in detail in a previous report [21]. In this study, the spinning solution consisted of 62.98 wt.% LSCF powders, 6.30 wt.% polyethersulfone (PESf) [(Radel A-300), Ameco Performance, USA], 25.18 wt.% 1-methyl-2-pyrrolidinone (NMP) [AR Grade, >99.8%, Kermel Chem Inc., Tianjin, China] as solvent, and 3.71 wt.% polyvinyl pyrrolidone (PVP, K30) [from Fuchen Chem Inc., Tianjin, AR Grade, Mw = 10,000] and 1.85 wt.% DI water as non-solvent additives. The viscosity of the spinning solution was measured at room temperature to be 55,800 mPa s1 at shear rate of 3 rpm. The bore fluid for spinning was DI water or a mixture of EtOH–NMP with different NMP concentrations while tap water was used as the external coagulant. The hollow fibre precursors were calcined at 1420 °C in ambient non-flowing air atmosphere for 4 h to obtain gas-tight membranes.
membrane were tested for a gas tight seal using gas permeation measurement. The hollow fibres was housed in a quartz tube (18 mm in diameter and 400 mm in length) with a high-temperature silicone sealant (purchased from Tonsan New Materials and Technol. Co., Beijing) able to withstand up to 350 °C. The permeation cell was positioned in a U22 180 mm tubular furnace with an effective heating length of 5 cm. Air was fed on the shell side with helium used a sweep gas on the permeate side. Gas feed flow rates were controlled by mass flow controllers (D08-8B/ZM, Shanxi Chuangwei Instrument Co., Ltd., China) which were calibrated using a soap bubble flow meter in advance. Compositions of the permeate gas were measured online using a gas chromatograph (Agilent 6890N) fitted with a 5 Å molecular sieve column (U3 mm 3 m) and a TCD detector. Highly purified hydrogen was used as carrier gas and the flow rate was set at 40 cm3 min1. GC calibration was performed using a standard gas mixture consisting of 5% oxygen, 5% nitrogen and 90% helium (mole fractions with ±2% accuracy) purchased from Baiyan Gases Ltd., Co., Zibo. All the gas composition measurements were made after 20 min following a temperature change or sweep-gas rate change. At least two measurements were conducted for each experimental condition. During the testing, a small amount of nitrogen (<0.15%) was detected in the permeate stream at low permeation temperature. It was assumed that minor leaking occurred at connecting joints of the permeation cell. In order to compare the permeation property of the hollow fibre membranes with different wall thicknesses, a specific oxygen flux was used, as defined by the product of oxygen flux and membrane thickness. Hence, specific oxygen flux was calculated as,
2.2. Characterisation Viscosity of the spinning solutions were collected using Physica UDS-200 rheometer at shear rates of 3 rpm at 20 °C. Spinning suspension samples were taken and tested immediately prior to fibre spinning. Morphology and macrostructures of the hollow fibre membranes were observed using scanning electron microscopy (SEM) (FEI Sirion 200, Netherlands). Gold sputter coating was performed on the samples under vacuum before the measurements. The porosity and the apparent density of the hollow fibres were measured by the Archimedes method. At least three samples were taken from each hollow fibre membrane. The mechanical strength of the hollow fibres was measured on a three-point bending instrument (Instron Model 5544) with a crosshead speed of 0.5 mm min1. 2.3. Oxygen permeation measurement Oxygen permeation properties of the LSCF6428 hollow fibre membranes were measured in an oxygen permeation cell schematically shown elsewhere [18]. Prior to the assembly, the hollow fibre
J O2 ¼
VðyO2 ð21=78ÞyN2 Þ ðRo Rin Þ Am
ð1Þ
where V is the permeate gas flow rate, cm3(STP) min1; yO2 and yN2 are the oxygen and nitrogen concentrations in the permeate stream, mol.%; Am (cm2) is the effective membrane area calculated by pðRo Rin ÞL Am ¼ 2lnðR in which L is the effective length for oxygen permeo =Rin Þ ation of the hollow fibre membrane. In this work, L was considered to be equal to the constant heating length of the furnace (5 cm). 3. Results and discussion Fig. 2a–c shows SEM micrographs of a conventional LSCF hollow fibre precursor using water as the internal and external coagulants. As expected, near the outer and inner walls of the fibre precursor, short finger-like structures are present while at the centre (marked with red box in Fig. 2a1) sponge-like structures are observed. The 1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
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Fig. 2. Sectional SEM images of the LSCF hollow fibre precursors (a–c) spun using H2O as the internal and external coagulants and the sintered LSCF hollow fibre (A–C) [(a, A) – cross sectional view; (b, B) – internal surface; (c, C) – external surface].
fibre structure seen in Fig. 2a indicates the occurrence of rapid precipitation at both the inner and outer fibre walls. Water is a strong non-solvent for polymers and immediate phase separation. Liquid– liquid demixing takes place on both surfaces after contact with water. This leads to the formation of relatively dense skin layers on both surface sides as shown in Fig. 2b and c. Once the two dense skin layers are formed, the exchange rate between water and solvent (NMP) slows down, leading to a lower phase separation rate causing the formation of large finger-like pores and the thick sponge-like layer. After sintering at 1420 °C, the general morphology of the fibre precursor is maintained (Fig. 2A), however, the two surface layers and the sponge-like layer (Fig. 2B and C) are transformed into densified, gas-tight layers. During densification, the sponge layer results in a much thicker layer (marked with red box in Fig. 2A) than that of the surface layers. To prepare a new ceramic hollow fibre with a single, thin, gas-tight layer, it was therefore necessary to hinder the formation of the sponge and second surface dense layer. The formation of the skin layer is a result of the fast phase separation rate impacted by the choice of internal
and external coagulant. By using a bore liquid containing the NMP to slow down the phase separation rate it is possible to create a new fibre precursor architecture with only one skin layer. In this work, LSCF hollow fibre precursors of varying structure were prepared by varying the NMP contents in water or in ethanol. Experimental results exhibit that ideal fibre precursor with desired structure as discussed can be achieved by using a bore liquid containing 10% ethanol and 90% NMP (by weight). Fig. 3a–c shows the SEM images of the hollow fibre precursor produced using an EtOH–NMP mixture as the internal coagulant. Comparing the cross section with Fig. 2a, it can be seen that the sandwich structure is no longer produced, and only one thin skin layer is near the outer surface. As shown in Fig. 3b, the finger-like pores reach through to the inner surface, with no dense layer to block access, this is quite different from Fig. 2b. When a mixture of EtOH–NMP is used as the internal coagulant, the solvent–nonsolvent exchange rate or the precipitation rate on the inner side of the nascent membrane is retarded, thus the macrovoids are suppressed in the inner layer region. When the NMP content in the
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Fig. 3. Sectional SEM images of the LSCF hollow fibre precursors (a–c) spun using 10% ethanol + 90% NMP as the internal coagulant and 100% H2O as the external coagulant and the sintered LSCF hollow fibre (A–C) [(a, A) – cross sectional view; (b, B) – internal surface; (c, C) – external surface].
EtOH bore liquid reaches a certain level, i.e., 90% by weight, the solvent–nonsolvent exchange rate on the inner surface is so slow that the NMP solvent almost outflows only from the outer surface. As a result, the finger-like pores formed due to the immersion of membrane into water coagulation bath may be extended to the inner surface of the hollow fibres [22]. The influence of internal coagulant composition, however, does not affect the outside surface morphology. This is confirmed by comparing the outer surface of the new hollow fibre (Fig. 3c) to the conventional (Fig. 2c). Fig. 3A–C shows the SEM images of the sintered hollow fibre derived from the EtOH–NMP mixture following sintering under similar conditions to that used in the conventional fibres (Fig. 2). It is observed that after sintering the asymmetric structures of the fibre was maintained. The inner surface kept its porous quality, while the out layer densified to produce a solid skin layer of around 30 lm. As shown in Fig. 3B, a very uniform honeycomb structure is clearly produced at the inner surface, with pore sizes of around 40 lm. Due to the existence of larger pores compared to the original samples from Fig. 2A–C, the porosity of this new membrane has been increased from 18% to 47%; the three-point-bending strength
has been lowered from 155 to 88 Mpa, but it still is sufficient to be routinely assembled into a compact module. Using XRD (Fig. 4), it can be seen that both types of coagulant result in a similar perovskite structure. In this work the NMP/EtOH mixture does not adversely affect the final crystalline structure, as all organics are burnt out during the high temperature sintering stage. Such highly asymmetric structure with one dense layer integrated on porous structure is very favourable for oxygen permeation. When oxygen permeates through the hollow fibre (i.e., Fig. 3A) with the overall wall thickness of 215 lm, only the thin densified skin layer of thickness 30 lm poses the major diffusion resistance and the porous structure provides more surface area for oxygen exchange reactions. Oxygen permeation measurements for all the hollow fibre membranes were conducted using the same air feed flow rate of 250 cm3 min1 in the shell side but with different He sweep gas rates from 70 to 147 cm3 min1 through the lumen side. Fig. 5 presents the oxygen permeation fluxes against temperature for the two hollow fibres. These fluxes are highly temperature dependant, indicating that oxygen ionic transport/surface exchange kinetics
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Intensity (a.u.)
a
b
10
20
30
40
50
60
70
80
2 /degrees Fig. 4. XRD patterns of LSCF hollow fibre membranes from precursors spun using 100% water (a) and mixture of 10%EtOH + 90%NMP (b) as the internal coagulant, respectively (sintered at 1420 °C for 4 h).
1000oC
(b)
950oC
900oC 850oC Oxygen Flux (ml/min/cm2)
are the rate limiting steps similar to conventional hollow fibres. At 1000 °C, the O2 flux of the honeycomb membrane reached a maximum of 4.6 mL cm2 min1, a factor of 13 higher than its flux seen at 800 °C. In addition, this membrane provided significantly improved flux from the conventional membrane, showing 15 times the permeation at 800 °C. At 900 °C with the same sweep gas flowrate, O2 flux of 0.16 mL cm2 min1 from the original sandwichstructured membranes has been improved by a factor of 12 to a value of 1.90 mL cm2 min1. Fig. 6 shows the effects of sweep gas flow rates on O2 fluxes. The flux increment with increasing sweep gas rate can be explained by the enlarged driving force because of lowering the permeate side O2 partial pressure. Again, at other sweep gas flow rates, the new structured membrane showed much improved O2 fluxes at similar operating temperatures when compared to the conventional membranes. For example, at sweep gas flow rate of 74 mL/min, the new membrane achieved an O2 flux of 0.22 mL cm2 min1 at 800 °C with an improvement factor of 14 over the conventional membrane. This enhanced flux can be contributed to two factors: (1) thin effective membrane thickness with a reduced bulk diffusion resistance; (2) porous downstream surface with a much larger membrane area for the surface exchange reactions.
800oC
Helium Sweep Rate (mL/min)
(a) 1000oC
5.0
Oxygen Flux (ml/min/cm2)
b 4.0
950oC
3.0
900oC 850oC
2.0
1.0
0.0 750
Helium Sweep Rate (mL/min)
a 800
850
900
950
1000
800oC
1050
Temperature (oC) Fig. 5. Oxygen fluxes through the conventional LSCF hollow fibres (a) and the honeycomb structured LSCF hollow fibres (b) (measured at air feed flow rate of 250 mL/min in the shell side and He sweep gas rate of 147 mL/min in the hollow fibre lumen).
Fig. 6. Effects of helium sweep rate on the oxygen fluxes through the conventional LSCF hollow fibre (a) and the honeycomb structured LSCF hollow fibre (b) (measured at air feed flow rate of 250 mL/min on the shell side and He sweep gas rates ranged from 74 to 147 mL/min in the hollow fibre lumen).
To achieve high gas permeance, asymmetric structure with thin separating layer is always the preferred choice for researchers. However, the process usually involves many steps particularly for the preparation of molecular sieving membranes. In this regard,
N. Liu et al. / Separation and Purification Technology 80 (2011) 396–401
the method developed in this work may provide some useful information on how to design new process to simplify the traditional complex procedures. The developed mixed conducting ceramic membranes with new honeycomb structure can also find their applications as catalytic membrane reactors for light hydrocarbon oxidation with easy additional catalyst loading inside the relative larger finger-pores via membrane internal surface impregnation.
[4]
[5]
[6]
4. Conclusion
[7]
During the phase inversion technique to prepare inorganic hollow fibres, the amount of internal coagulant used to shape the hollow fibre geometry is very minimal, but it can play a vital role in influencing morphology of the hollow fibre. When a mixture of 10% EtOH and 90% NMP replaced DI water as the internal coagulant, a complete new honeycomb structured hollow fibre membrane with a single thin densified layer was produced. This single dense layer hollow fibre membrane delivered much higher O2 fluxes with improvement factor up to 15 compared to the traditional multi-dense layer sandwiched membrane structure. The techniques reported here are based on the fabrication of gas-tight LSCF hollow fibres, but they are also suitable for the fabrication of thin tubes or hollow fibres of other perovskite or ceramic materials.
[8] [9] [10] [11]
[12]
[13] [14] [15] [16]
Acknowledgements [17]
The authors gratefully acknowledge the research funding provided by the National High Technology Research and Development Program of China (No. 2006AA03Z464), the National Natural Science Foundation of China (NNSFC, No. 20976098) and the Australian Research Council (DP0985578). References [1] S.P.S. Badwal, F.T. Ciacchi, Ceramic membrane technologies for oxygen separation, Adv. Mater. 13 (2001) 993–996. [2] G.J. Stiegel, R.C. Maxwell, Gasification technologies: the path to clean, affordable energy in the 21st century, Fuel Process. Technol. 71 (2001) 79–97. [3] D. Bücker, D. Holmberg, T. Griffin, Techno-economic evaluation of an oxyfuel power plant using mixed conducting membranes, in: Carbon Dioxide Capture
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Short Communication
Honeycomb-structured perovskite hollow fibre membranes with ultra-thin densified layer for oxygen separation Nan Liu a, Xiaoyao Tan a,⇑, Bo Meng a, Shaomin Liu b,⇑ a b
School of Chemical Engineering, Shandong University of Technology, Zibo 255049, China Centre for Advanced Energy Science and Engineering, Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia
a r t i c l e
i n f o
Article history: Received 1 March 2011 Received in revised form 8 April 2011 Accepted 12 April 2011 Available online 17 May 2011 Keywords: Hollow fibre membrane Oxygen permeation Perovskite Microstructure
a b s t r a c t New honeycomb-structured and gas-tight La0.6Sr0.4Co0.2Fe0.8O3-a (LSCF) perovskite hollow fibre membranes have been developed with an ultra-thin dense layer using phase inversion/sintering technique with an EtOH–NMP mixture as the internal coagulant. By varying the rate of internal precipitation the surface morphology of the hollow fibre could be altered to provide a single thin dense layer and high support surface area. These membranes were able to provide significantly higher O2 fluxes than conventional types, with an improvement factor up to 15 at 800 °C, while maintaining membrane quality and mechanical stability. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Oxygen separation using mixed ionic-electronic conducting (MIEC) ceramic membranes have attracted increasing attention in clean energy research, due to its ability to operate at much lower costs when compared with conventional cryogenic technology [1–12]. Most research in this area has looked at disk and tubular membranes fabricated using conventional methods such as static-pressing or ram extrusion [4,13]. Although a multiple planar stack or tubular design can be used to enlarge the membrane area to a plant scale, many engineering problems arise from these designs such as high temperature sealing, connection, pressure resistance and small surface/volume ratios. On the other hand, because of the symmetric structure of these disk or tubular membranes prepared, oxygen ion bulk diffusion is usually the rate-limiting step in air separation process resulting in a low oxygen permeation rate [13]. In contrast to these traditional configurations, asymmetric hollow fibres appear to be a promising alternative. Hollow fibres can be densely packed in small modules, providing the largest membrane area per unit packing volume, while the thin separating layer produced by the asymmetric structure can achieve higher oxygen
⇑ Corresponding authors. Tel.: +86 533 2786292 (X. Tan), tel.: +61 8 92669056 (S. Liu). E-mail addresses: [email protected] (X. Tan), [email protected], [email protected] (S. Liu). 1383-5866/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2011.04.014
flux than other techniques [8,14]. Furthermore, high temperature sealing problems can be avoided by adopting a long hollow fibre module which keeps the sealing ends out of the high temperature zone. Ceramic hollow fibre membranes can be fabricated via a combined phase inversion and high temperature sintering technique. During the extrusion step via phase inversion, water is often used as the internal and external coagulants, producing a hollow fibre membrane with an integral asymmetric structure showed schematically in Fig. 1A [15]. In this structure, three dense layers are integrated with porous layers of the same material, producing many closed or isolated short finger-like pores near the surface layers. In MIEC membranes, surface reactions and bulk ion diffusion provide the greatest resistance to oxygen transport [16]. The isolated pores increase the number of surface reactions required and this in combination with multiple dense layers causes a large resistance to gas transport. Acid etching can help reduce transport resistance by opening the dense layers at the surfaces, however, this process is complicated and time consuming [17,18]. In this report, we look at an improved method for fabrication of honeycomb structured perovskite hollow fibre membranes. By changing the internal coagulant (also called bore liquid in the spinneret), a honeycomb structure with a single dense layer was produced (Fig. 1B) without the need for further acid etching steps. In this work, La0.6Sr0.4Co0.2Fe0.8O3-a (LSCF) perovskite was used as the membrane material due to its high mechanical and phase stability. However, the developed method can be extended to utilise other membranes materials for other applications.
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397
Fig. 1. Schematic of membrane architectures to be improved [(A) cross sectional view of the original hollow fibre membranes with three dense layers using water as the both coagulants; (B) improved membrane architectures using new method with only one dense layer near the outside surface and honeycomb structure viewed from the inner surface side].
2. Experimental 2.1. Preparation of LSCF powder and hollow fibres La0.6Sr0.4Co0.2Fe0.8O3-a (LSCF6428) perovskite powders were prepared through a sol–gel process, which is described in details elsewhere [19,20]. For spinning hollow fibre membranes, the powder precursor was calcined at 800 °C for 3 h to remove residual carbon and to form the desired structure. It was then ball-milled for 48 h in an agate jar and sieved using 200 mesh for 24 lm sievepore diameter. The hollow fibre membranes were made from the calcined and ball-milled powders using the phase inversion and sintering technique. This is described in detail in a previous report [21]. In this study, the spinning solution consisted of 62.98 wt.% LSCF powders, 6.30 wt.% polyethersulfone (PESf) [(Radel A-300), Ameco Performance, USA], 25.18 wt.% 1-methyl-2-pyrrolidinone (NMP) [AR Grade, >99.8%, Kermel Chem Inc., Tianjin, China] as solvent, and 3.71 wt.% polyvinyl pyrrolidone (PVP, K30) [from Fuchen Chem Inc., Tianjin, AR Grade, Mw = 10,000] and 1.85 wt.% DI water as non-solvent additives. The viscosity of the spinning solution was measured at room temperature to be 55,800 mPa s1 at shear rate of 3 rpm. The bore fluid for spinning was DI water or a mixture of EtOH–NMP with different NMP concentrations while tap water was used as the external coagulant. The hollow fibre precursors were calcined at 1420 °C in ambient non-flowing air atmosphere for 4 h to obtain gas-tight membranes.
membrane were tested for a gas tight seal using gas permeation measurement. The hollow fibres was housed in a quartz tube (18 mm in diameter and 400 mm in length) with a high-temperature silicone sealant (purchased from Tonsan New Materials and Technol. Co., Beijing) able to withstand up to 350 °C. The permeation cell was positioned in a U22 180 mm tubular furnace with an effective heating length of 5 cm. Air was fed on the shell side with helium used a sweep gas on the permeate side. Gas feed flow rates were controlled by mass flow controllers (D08-8B/ZM, Shanxi Chuangwei Instrument Co., Ltd., China) which were calibrated using a soap bubble flow meter in advance. Compositions of the permeate gas were measured online using a gas chromatograph (Agilent 6890N) fitted with a 5 Å molecular sieve column (U3 mm 3 m) and a TCD detector. Highly purified hydrogen was used as carrier gas and the flow rate was set at 40 cm3 min1. GC calibration was performed using a standard gas mixture consisting of 5% oxygen, 5% nitrogen and 90% helium (mole fractions with ±2% accuracy) purchased from Baiyan Gases Ltd., Co., Zibo. All the gas composition measurements were made after 20 min following a temperature change or sweep-gas rate change. At least two measurements were conducted for each experimental condition. During the testing, a small amount of nitrogen (<0.15%) was detected in the permeate stream at low permeation temperature. It was assumed that minor leaking occurred at connecting joints of the permeation cell. In order to compare the permeation property of the hollow fibre membranes with different wall thicknesses, a specific oxygen flux was used, as defined by the product of oxygen flux and membrane thickness. Hence, specific oxygen flux was calculated as,
2.2. Characterisation Viscosity of the spinning solutions were collected using Physica UDS-200 rheometer at shear rates of 3 rpm at 20 °C. Spinning suspension samples were taken and tested immediately prior to fibre spinning. Morphology and macrostructures of the hollow fibre membranes were observed using scanning electron microscopy (SEM) (FEI Sirion 200, Netherlands). Gold sputter coating was performed on the samples under vacuum before the measurements. The porosity and the apparent density of the hollow fibres were measured by the Archimedes method. At least three samples were taken from each hollow fibre membrane. The mechanical strength of the hollow fibres was measured on a three-point bending instrument (Instron Model 5544) with a crosshead speed of 0.5 mm min1. 2.3. Oxygen permeation measurement Oxygen permeation properties of the LSCF6428 hollow fibre membranes were measured in an oxygen permeation cell schematically shown elsewhere [18]. Prior to the assembly, the hollow fibre
J O2 ¼
VðyO2 ð21=78ÞyN2 Þ ðRo Rin Þ Am
ð1Þ
where V is the permeate gas flow rate, cm3(STP) min1; yO2 and yN2 are the oxygen and nitrogen concentrations in the permeate stream, mol.%; Am (cm2) is the effective membrane area calculated by pðRo Rin ÞL Am ¼ 2lnðR in which L is the effective length for oxygen permeo =Rin Þ ation of the hollow fibre membrane. In this work, L was considered to be equal to the constant heating length of the furnace (5 cm). 3. Results and discussion Fig. 2a–c shows SEM micrographs of a conventional LSCF hollow fibre precursor using water as the internal and external coagulants. As expected, near the outer and inner walls of the fibre precursor, short finger-like structures are present while at the centre (marked with red box in Fig. 2a1) sponge-like structures are observed. The 1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
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Fig. 2. Sectional SEM images of the LSCF hollow fibre precursors (a–c) spun using H2O as the internal and external coagulants and the sintered LSCF hollow fibre (A–C) [(a, A) – cross sectional view; (b, B) – internal surface; (c, C) – external surface].
fibre structure seen in Fig. 2a indicates the occurrence of rapid precipitation at both the inner and outer fibre walls. Water is a strong non-solvent for polymers and immediate phase separation. Liquid– liquid demixing takes place on both surfaces after contact with water. This leads to the formation of relatively dense skin layers on both surface sides as shown in Fig. 2b and c. Once the two dense skin layers are formed, the exchange rate between water and solvent (NMP) slows down, leading to a lower phase separation rate causing the formation of large finger-like pores and the thick sponge-like layer. After sintering at 1420 °C, the general morphology of the fibre precursor is maintained (Fig. 2A), however, the two surface layers and the sponge-like layer (Fig. 2B and C) are transformed into densified, gas-tight layers. During densification, the sponge layer results in a much thicker layer (marked with red box in Fig. 2A) than that of the surface layers. To prepare a new ceramic hollow fibre with a single, thin, gas-tight layer, it was therefore necessary to hinder the formation of the sponge and second surface dense layer. The formation of the skin layer is a result of the fast phase separation rate impacted by the choice of internal
and external coagulant. By using a bore liquid containing the NMP to slow down the phase separation rate it is possible to create a new fibre precursor architecture with only one skin layer. In this work, LSCF hollow fibre precursors of varying structure were prepared by varying the NMP contents in water or in ethanol. Experimental results exhibit that ideal fibre precursor with desired structure as discussed can be achieved by using a bore liquid containing 10% ethanol and 90% NMP (by weight). Fig. 3a–c shows the SEM images of the hollow fibre precursor produced using an EtOH–NMP mixture as the internal coagulant. Comparing the cross section with Fig. 2a, it can be seen that the sandwich structure is no longer produced, and only one thin skin layer is near the outer surface. As shown in Fig. 3b, the finger-like pores reach through to the inner surface, with no dense layer to block access, this is quite different from Fig. 2b. When a mixture of EtOH–NMP is used as the internal coagulant, the solvent–nonsolvent exchange rate or the precipitation rate on the inner side of the nascent membrane is retarded, thus the macrovoids are suppressed in the inner layer region. When the NMP content in the
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Fig. 3. Sectional SEM images of the LSCF hollow fibre precursors (a–c) spun using 10% ethanol + 90% NMP as the internal coagulant and 100% H2O as the external coagulant and the sintered LSCF hollow fibre (A–C) [(a, A) – cross sectional view; (b, B) – internal surface; (c, C) – external surface].
EtOH bore liquid reaches a certain level, i.e., 90% by weight, the solvent–nonsolvent exchange rate on the inner surface is so slow that the NMP solvent almost outflows only from the outer surface. As a result, the finger-like pores formed due to the immersion of membrane into water coagulation bath may be extended to the inner surface of the hollow fibres [22]. The influence of internal coagulant composition, however, does not affect the outside surface morphology. This is confirmed by comparing the outer surface of the new hollow fibre (Fig. 3c) to the conventional (Fig. 2c). Fig. 3A–C shows the SEM images of the sintered hollow fibre derived from the EtOH–NMP mixture following sintering under similar conditions to that used in the conventional fibres (Fig. 2). It is observed that after sintering the asymmetric structures of the fibre was maintained. The inner surface kept its porous quality, while the out layer densified to produce a solid skin layer of around 30 lm. As shown in Fig. 3B, a very uniform honeycomb structure is clearly produced at the inner surface, with pore sizes of around 40 lm. Due to the existence of larger pores compared to the original samples from Fig. 2A–C, the porosity of this new membrane has been increased from 18% to 47%; the three-point-bending strength
has been lowered from 155 to 88 Mpa, but it still is sufficient to be routinely assembled into a compact module. Using XRD (Fig. 4), it can be seen that both types of coagulant result in a similar perovskite structure. In this work the NMP/EtOH mixture does not adversely affect the final crystalline structure, as all organics are burnt out during the high temperature sintering stage. Such highly asymmetric structure with one dense layer integrated on porous structure is very favourable for oxygen permeation. When oxygen permeates through the hollow fibre (i.e., Fig. 3A) with the overall wall thickness of 215 lm, only the thin densified skin layer of thickness 30 lm poses the major diffusion resistance and the porous structure provides more surface area for oxygen exchange reactions. Oxygen permeation measurements for all the hollow fibre membranes were conducted using the same air feed flow rate of 250 cm3 min1 in the shell side but with different He sweep gas rates from 70 to 147 cm3 min1 through the lumen side. Fig. 5 presents the oxygen permeation fluxes against temperature for the two hollow fibres. These fluxes are highly temperature dependant, indicating that oxygen ionic transport/surface exchange kinetics
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Intensity (a.u.)
a
b
10
20
30
40
50
60
70
80
2 /degrees Fig. 4. XRD patterns of LSCF hollow fibre membranes from precursors spun using 100% water (a) and mixture of 10%EtOH + 90%NMP (b) as the internal coagulant, respectively (sintered at 1420 °C for 4 h).
1000oC
(b)
950oC
900oC 850oC Oxygen Flux (ml/min/cm2)
are the rate limiting steps similar to conventional hollow fibres. At 1000 °C, the O2 flux of the honeycomb membrane reached a maximum of 4.6 mL cm2 min1, a factor of 13 higher than its flux seen at 800 °C. In addition, this membrane provided significantly improved flux from the conventional membrane, showing 15 times the permeation at 800 °C. At 900 °C with the same sweep gas flowrate, O2 flux of 0.16 mL cm2 min1 from the original sandwichstructured membranes has been improved by a factor of 12 to a value of 1.90 mL cm2 min1. Fig. 6 shows the effects of sweep gas flow rates on O2 fluxes. The flux increment with increasing sweep gas rate can be explained by the enlarged driving force because of lowering the permeate side O2 partial pressure. Again, at other sweep gas flow rates, the new structured membrane showed much improved O2 fluxes at similar operating temperatures when compared to the conventional membranes. For example, at sweep gas flow rate of 74 mL/min, the new membrane achieved an O2 flux of 0.22 mL cm2 min1 at 800 °C with an improvement factor of 14 over the conventional membrane. This enhanced flux can be contributed to two factors: (1) thin effective membrane thickness with a reduced bulk diffusion resistance; (2) porous downstream surface with a much larger membrane area for the surface exchange reactions.
800oC
Helium Sweep Rate (mL/min)
(a) 1000oC
5.0
Oxygen Flux (ml/min/cm2)
b 4.0
950oC
3.0
900oC 850oC
2.0
1.0
0.0 750
Helium Sweep Rate (mL/min)
a 800
850
900
950
1000
800oC
1050
Temperature (oC) Fig. 5. Oxygen fluxes through the conventional LSCF hollow fibres (a) and the honeycomb structured LSCF hollow fibres (b) (measured at air feed flow rate of 250 mL/min in the shell side and He sweep gas rate of 147 mL/min in the hollow fibre lumen).
Fig. 6. Effects of helium sweep rate on the oxygen fluxes through the conventional LSCF hollow fibre (a) and the honeycomb structured LSCF hollow fibre (b) (measured at air feed flow rate of 250 mL/min on the shell side and He sweep gas rates ranged from 74 to 147 mL/min in the hollow fibre lumen).
To achieve high gas permeance, asymmetric structure with thin separating layer is always the preferred choice for researchers. However, the process usually involves many steps particularly for the preparation of molecular sieving membranes. In this regard,
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the method developed in this work may provide some useful information on how to design new process to simplify the traditional complex procedures. The developed mixed conducting ceramic membranes with new honeycomb structure can also find their applications as catalytic membrane reactors for light hydrocarbon oxidation with easy additional catalyst loading inside the relative larger finger-pores via membrane internal surface impregnation.
[4]
[5]
[6]
4. Conclusion
[7]
During the phase inversion technique to prepare inorganic hollow fibres, the amount of internal coagulant used to shape the hollow fibre geometry is very minimal, but it can play a vital role in influencing morphology of the hollow fibre. When a mixture of 10% EtOH and 90% NMP replaced DI water as the internal coagulant, a complete new honeycomb structured hollow fibre membrane with a single thin densified layer was produced. This single dense layer hollow fibre membrane delivered much higher O2 fluxes with improvement factor up to 15 compared to the traditional multi-dense layer sandwiched membrane structure. The techniques reported here are based on the fabrication of gas-tight LSCF hollow fibres, but they are also suitable for the fabrication of thin tubes or hollow fibres of other perovskite or ceramic materials.
[8] [9] [10] [11]
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Acknowledgements [17]
The authors gratefully acknowledge the research funding provided by the National High Technology Research and Development Program of China (No. 2006AA03Z464), the National Natural Science Foundation of China (NNSFC, No. 20976098) and the Australian Research Council (DP0985578). References [1] S.P.S. Badwal, F.T. Ciacchi, Ceramic membrane technologies for oxygen separation, Adv. Mater. 13 (2001) 993–996. [2] G.J. Stiegel, R.C. Maxwell, Gasification technologies: the path to clean, affordable energy in the 21st century, Fuel Process. Technol. 71 (2001) 79–97. [3] D. Bücker, D. Holmberg, T. Griffin, Techno-economic evaluation of an oxyfuel power plant using mixed conducting membranes, in: Carbon Dioxide Capture
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