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Morphology control of the perovskite hollow fibre membranes for oxygen separation using different bore fluids
Journal of Membrane Science 378 (2011) 308–318
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Morphology control of the perovskite hollow fibre membranes for oxygen separation using different bore fluids Xiaoyao Tan a,b,∗ , Nan Liu b , Bo Meng b , Shaomin Liu c a b c
School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China 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 6 May 2011 Accepted 9 May 2011 Available online 14 May 2011 Keywords: Hollow fibre membrane Oxygen permeation Perovskite Morphology control
a b s t r a c t La0.6 Sr0.4 Co0.2 Fe0.8 O3−␣ (LSCF6428) perovskite hollow fibre membranes were fabricated through a phase inversion/sintering technique. The effects of the internal coagulant (bore fluid) compositions on the morphology of the resultant hollow fibres were systematically investigated. The bore fluids were specially designed to vary the solvent–nonsolvent exchange rates in the phase inversion stage by mixing different polymer solvent (1-methyl-2-pyrrolidinone–NMP) content inside the water or ethanol internal coagulant. The prepared hollow fibre membranes were characterized with scanning electron microscopy (SEM), porosity and bending strength measurements, gas-tightness examination and oxygen permeation tests at high temperatures. The results indicate that the bore fluid composition has dramatic effects on the microstructure of the resultant hollow fibre membranes. With the gradual increase of NMP concentration in the bore fluids, the membrane morphology evolution from multi-dense layers to one single dense layer has been clearly demonstrated. LSCF6482 hollow fibre membranes with one dense layer near the outside surface integrated with the highly porous structure showed the highest oxygen fluxes with a maximum value of 34.4 mmol m−2 s−1 under air/He gradient at 1000 ◦ C. The relationships between bore fluid composition and membrane morphology change and their significant effects on oxygen permeation have been discussed extensively. This work shows the feasibility of achieving ceramic hollow fibre membranes with different architectures for various purposes in one single step simply by adjusting the bore fluid composition. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Among the efforts to battle climate change, cost-effective oxygen production via mixed ionic–electronic conducting (MIEC) ceramic membrane technology (without the need of cryogenics) is of great interest as most large scale clean energy technologies require oxygen as the feed gas. Typical examples of such MIEC ceramic membranes are the La1−x Srx Co1−y Fey O3−ı (LSCF) perovskites often cited over the last two decades as the target materials. Their potential applications in oxygen production from air or partial oxidation of light hydrocarbons to value-added products are being investigated [1–3]. Such perovskite membranes exhibit 100% oxygen permselectivity under a gradient of oxygen chemical potential across the membrane at a high temperature, because the oxygen transport through the membranes is via the diffusion
∗ Corresponding author at: School of Environmental and Chemical Engineering, Tianjin Polytechnic University, No. 63 Chenglin Road, Hedong District, Tianjin, Shandong 300160, China. Tel.: +86 533 2786292; fax: +86 533 278629. E-mail address: [email protected] (X. Tan). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.05.012
of mobile oxygen vacancies and electronic defects, while simultaneously excluding the transport of any gas-phase species. In the oxygen permeation process, three major steps are included: (i) oxygen exchange reactions on the high oxygen partial pressure side of the membrane surface, including oxygen adsorption, dissociation, reduction and incorporation to yield lattice oxygen, (ii) bulk diffusion of oxygen ions or vacancies and electron holes, and (iii) surface reaction between lattice oxygen and electron holes on the other membrane surface (permeate side) and oxygen desorption. It is clear that the major resistance to oxygen transport results from the surface oxygen exchange reactions and bulk diffusion, and the permeation flux is often jointly controlled by both processes [4,5]. In order to obtain a high oxygen flux through a perovskite membrane, it is necessary not only to decrease the membrane thickness but also to increase the membrane surface area favoring the surface exchange kinetics [6–8]. Therefore, the membrane microstructure or morphology may exert a significant influence on the permeation performance of perovskite membranes. Over the past ten years, ceramic membranes with hollow fibre geometry have been developed via combined phase inversion at room temperature and subsequent sintering at high temperatures.
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This method is flexible and versatile, and can be used to synthesize various inorganic hollow fibre membranes and is thus of great interest to researchers. Compared to other configurations like tubular or disk-shaped membranes, the ceramic hollow fibres exhibit many advantages such as facile high-temperature sealing, thin effective membrane thickness, and, more importantly, larger membrane area per unit packing volume. Therefore, the development of perovskite hollow fibre membranes with higher oxygen flux is a step closer toward large scale applications [9–12]. The microstructure of the hollow fibre ceramic membrane precursor formed during the initial phase inversion process possesses an integral asymmetric structure that consists of dense thin layers supported on porous layers of the same material. The subsequent sintering (calcination/heat treatment) does not alter their overall structure but increase the density of the dense layer making it gas-tight. Therefore, the spinning condition may exert significant influence on the microstructure of the hollow fibre membranes [13–16]. For example, when water is used as both the internal and external coagulant, the resultant hollow fibres usually possess a sandwich structure, i.e., a central dense layer and two surface skin dense layers integrated with two porous layers [9]. As the viscosity of the starting solution or the air gap is varied, the relative thickness of the dense and the porous layers will be altered subsequently [17]. In another case, when a solvent instead of water is used as the internal coagulant, the macrovoids or the finger-like pores in the hollow fibres can be extended straightly to the inner surface, forming a highly asymmetric structure-a single dense layer integrated with a porous support [18–21]. Such a highly asymmetric structure is more favourable for oxygen permeation than the sandwiched structure since it not only has lower diffusion resistance to oxygen ions from the single dense layer but also possesses a larger surface area for oxygen exchange reactions [21]. Compared to multi-step treatment, adjusting the spinning conditions to get the desired membrane structure in one step is a better strategy to improve the properties of the ceramic hollow fibre membranes. To reach this purpose, previous works were focusing on adjusting the viscosity of the spinning dope by changing the compositions of the spinning dope [18–20] or the distance of the air gap between the spinneret and external coagulant level. In shaping the hollow fibre precursors, the internal coagulant utilized is usually in a very low concentration, however, it has a significant effect on the internal morphology of the resultant hollow fibre membranes. Such an important role that the internal coagulant plays is often ignored and so far no systematical investigation of the effects of the internal coagulants on the morphology evolution of the resultant dense ceramic hollow fibre membranes has been carried out. This work has three main objectives, namely (1) systematical investigation of the effects of the internal coagulant (bore fluid) composition on the morphology of the resulted perovskite hollow fibres, (ii) fundamental understanding of the bore fluid’s role in the morphological evolution of the perovskite hollow fibres before and after sintering, and (iii) finding of the optimum membrane microstructure for oxygen separation. To achieve these purposes, eight batches of perovskite hollow fibre precursors were spun using different bore fluid compositions. La0.6 Sr0.4 Co0.2 Fe0.8 O3−˛ (LSCF6428) perovskite oxide was selected as the membrane material due to its high mechanical and phase stability. To guarantee the successful preparation of these series of hollow fibres, up to 4 kg LSCF6428 high quality powder was homemade using the EDTA–citrate sol–gel method. Water and ethanol were used as the internal coagulants in this work since they are easily obtainable, cheap in cost and environmentally friendly. A certain amount of solvent (NMP) was added into the internal coagulant to vary the exchange rate between the solvent in the spinning solution and the non-solvent in the internal coagulant.
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Table 1 Preparation conditions for the LSCF6428 hollow fibre membranes.
Composition of the spinning solution
Flow rate of the bore liquid External coagulant Nitrogen pressure Air gap Spinning rate Sintering temperature Sintering time Bore fluid for the fibre samples
Experimental parameters
Values
LSCF6428
62.98 wt%
PESf NMP PVP DI water 20 mL min−1
6.30 wt% 25.18 wt% 3.71 wt% 1.85 wt%
Tap water 0.1 MPa 0 cm 4.5 m min−1 1420 ◦ C 4h A-0
Pure water
A-50 A-70 A-90 B-0 B-50 B-70 B-90
50 wt% H2 O–50 wt% NMP 30 wt% H2 O–70 wt% NMP 10 wt% H2 O–90 wt% NMP Pure ethanol 50 wt% EtOH–50 wt% NMP 30 wt% EtOH–70 wt% NMP 10 wt% EtOH–90 wt% NMP
2. Experimental 2.1. Preparation of LSCF powder and hollow fibres La0.6 Sr0.4 Co0.2 Fe0.8 O3−˛ (LSCF6428) perovskite powders were prepared through a sol–gel process which was described in detail elsewhere [19,22]. For spinning hollow fibre membranes, the powder precursor was calcined at 800 ◦ C for 3 h to remove the residual carbon and to form the desired structure, ball-milled for 48 h in an agate jar and sieved through a sifter of 200-mesh or 24 m sieve-pore diameter. The hollow fibre membranes were made from the calcined and ball-milled powders by the phase inversion and sintering technique. The detailed preparation procedures were described elsewhere [9]. 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-2pyrrolidinone (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% deionized (DI) water as non-solvent additives. The bore fluid for spinning was a mixture of ethanol–NMP or DI water–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. Other preparation parameters were kept the same for all the hollow fibre membranes, which are summarized in Table 1. For simplification, the hollow fibres prepared using the ethanol–NMP or water–NMP mixture as the bore fluid were marked as A–x or B–x in which A or B stands for water or ethanol and x for the weight percentage of the NMP solvent in the bore fluid. 2.2. Characterization Viscosity of the spinning solutions were measured 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. The viscosity of the spinning solution was measured at room temperature to be around 55,800 mPa s−1 . Morphology and microstructures of the hollow fibre membranes
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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 min−1 . The hollow fibre sample was fixed on the sample holder at a distance of 32 mm. The bending strength, F , was calculated from the following equation: F =
FLR0 4 ) (R04 − Rin
(1)
where F is the measured force at which the fracture takes place, R0 , Rin and L are the outer radius, inner e radius and length (32 mm) of the hollow fibre samples, respectively. For each hollow fibre, three samples were taken for the measurement and the average bending strength value was taken as the mechanical strength of the hollow fibre membrane.
2.3. Oxygen permeation measurement Oxygen permeation properties of the LSCF6428 hollow fibre membranes were investigated in an oxygen permeation cell schematically shown elsewhere [23]. Prior to the assembly, the gastightness of the hollow fibre membrane was tested and confirmed by nitrogen gas permeation measurement at room temperature [24]. The hollow fibres were housed in a quartz tube (18 mm in diameter and 400 mm in length) with a high-temperature silicone sealant (1592, purchased from Tonsan New Materials and Technol. Co., Beijing) that is able to withstand up to 350 ◦ C. The permeation cell was positioned in a ˚22 mm × 180 mm tubular furnace having an effective heating length of 5 cm. Air was fed on the shell side and helium was passed through the fibre lumen to collect the oxygen permeated. Gas feed flow rates were controlled by mass flow controllers (D08-8B/ZM, Shanxi Chuangwei Instrument Co. Ltd., China). Compositions of the permeate gas were measured online using a gas chromatograph (Agilent 6890N) fitted with a 5 A˚ molecular sieve column (˚3 mm × 3 mm) and a TCD detector. Highly purified hydrogen was used as carrier gas and the flow rate was set at 40 cm3 min−1 . 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 permeation, a small amount of nitrogen (<0.15%) was detected in the permeate stream at low permeation temperature. It was assumed that a minor leak occurred at connecting joints of the permeation cell. Therefore, the oxygen flux was calculated by: JO2 =
V (yO2 − (21/78)yN2 ) Am
(2)
where V is the permeate gas flow rate (mol s−1 ), yO2 and yN2 are the oxygen and nitrogen concentrations in the permeate stream (mol%), Am is the effective membrane area calculated by Am = (2(R0 − Rin )L)/ln(R0 /Rin ) in which L is the effective length for oxygen permeation 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 3.1. Morphology of the hollow fibre membranes During the preparation of the series of LSCF6428 hollow fibre membranes, all other processing parameters, with the exception of the bore fluid for spinning hollow fibre precursors, were fixed at the same conditions. Therefore, the change of the microstructure and other membrane properties is mainly attributed to the variation of the bore fluid. Fig. 1 shows the cross-sectional SEM micrographs of the hollow fibre precursors using H2 O–NMP and EtOH–NMP mixtures with different NMP concentrations as bore coagulants, respectively. It can be seen that two major types of microstructures, namely the sponge-like structure and the finger-like structure, are simultaneously present in the hollow fibre precursors, forming an asymmetric morphology of the hollow fibres. The hollow fibre membranes derived using pure water as the bore liquid exhibit a sandwich structure, i.e., a central sponge-like structure between two finger-like structures, as shown in Fig. 1A-0. Noteworthy is that both the inner and the outer surfaces also consist of a spongelike structure in these hollow fibres. When NMP solvent is added in the bore liquid up to 50 wt%, the finger layer on the lumen side gradually diminishes to be no longer observable. The thickness of the sponge layer on the lumen side also decreases with the increase of the NMP content in the bore liquid as can be seen in the figure. However, when EtOH–NMP was used as the bore liquid to replace water, the sandwich structure cannot be formed in the resultant hollow fibres. As shown in Fig. 1B-0, the hollow fibre using pure ethanol as the bore liquid has a similar structure to those using H2 O–NMP mixture as the bore fluid, and again the thickness of the sponge-like structure on the lumen side decreases noticeably as the NMP content in the bore liquid is increased to 50 wt%. In addition, the finger-like pores have become much longer in the B-50 fibre than those in the B-0 membrane. When the NMP content in the bore liquid is further increased to 70 wt% or higher, the sponge-like structure on the lumen side disappears and the fingerlike pores straightly extend to the inner surface, leaving only one thin sponge layer on the outer surface of the hollow fibres, as shown in Fig. 1B-70 and B-90. The size of the finger-like pores close to the inner surface can be up to 30 m, but the pores close to the outer sponge-like surface are much smaller. The change of the structural morphology of the hollow fibre membranes with the bore fluid compositions can be explained as follows. It is well known that the formation of either a finger-like structure or a spongelike structure is determined by the solvent–nonsolvent exchange rates during phase separation when the polymer solution touches coagulant. If the rate of nonsolvent inflow is faster than the rate of solvent outflow, a finger-like structure is formed, while a spongelike membrane is formed when the former is lower than that of the latter [25]. Therefore, when pure water is used as the internal coagulant, the solvent–nonsolvent exchange rate on the inner surface is close to that on the outer surface, leading to the similar porous structures in the inner and outer regions, namely a sandwich structure. However, when a mixture of H2 O–NMP or EtOH–NMP is used as the internal coagulant, the solvent–nonsolvent exchange rate, also the precipitation rate, on the inner side of the nascent membrane may be retarded, thus the macrovoids are suppressed in the inner region. Once the NMP content in the ethanol bore liquid reaches a certain level, i.e., >70 wt%, the solvent–nonsolvent exchange rate on the inner surface may be so slow that the NMP solvent almost outflows only from the outer surface of the nascent membranes. As a result, the finger-like pores formed due to the immersion of the membrane into the water coagulation bath can extend straight to the inner surface of the hollow fibres. Clearly, it can also be inferred that the highly asymmetric structure may be formed if the NMP content in the water bore fluid is suffi-
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Fig. 1. Sectional SEM micrographs of the LSCF6428 hollow fibre precursors spun using H2 O–NMP (A–x) and EtOH–NMP (B–x) mixtures as the bore fluid, respectively. (x represents the NMP weight percentage in the mixture).
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ciently high, e.g., >95 wt%, which was observed in our previous work [24]. Fig. 2 compares the SEM inner surfaces of the hollow fibre precursors using H2 O–NMP and EtOH–NMP mixtures with different NMP concentrations as the bore coagulant. As illustrated in the figure, when the NMP content in the H2 O–NMP bore fluid is lower than 70%, the derived hollow fibres possess dense and smooth inner surfaces. The small ceramic particles are well dispersed and connected with each other by the polymer binder. This suggests that the fine LSCF particles can be dispersed uniformly in the PESf–NMP polymer solution. When the NMP content in the H2 O–NMP bore liquid is increased to 90 wt%, however, the inner surface becomes rough with the presence of many small pores, as shown in Fig. 2A90. When ethanol–NMP mixture is used as the bore fluid, the inner surfaces of the derived hollow fibres are much rougher than those using water as the bore liquid. Furthermore, the pores on the inner surface become larger and larger as the NMP content is increased. As the NMP content in the ethanol bore liquid is increased to 90 wt%, the inner surface has become rather rough with many large holes penetrating deep into the interior of the hollow fibres. These results further confirm that the exchange rate between the ethanol and solvent is much slower than that between the water and NMP solvent. Fig. 3 shows the cross-sectional SEM micrographs of the hollow fibre membranes after sintering at 1420 ◦ C for 4 h. It should be mentioned that the highly asymmetric hollow fibres with open finger-like pores (B-70 and B-90) have to be sintered at above 1400 ◦ C to obtain gas-tight properties. Therefore, all the hollow fibres were sintered at 1420 ◦ C in this work, which is much higher than those employed before [9,19], so that we can focus the research mainly on the effect of the bore fluid. It is observed from these SEMs that the asymmetric structures in the precursors have been well-maintained in the sintered hollow fibre membranes. This implies that the sintering only removed the organic components but has not changed the general morphology of the hollow fibres, although the sintered hollow fibres experienced shrinking by 20% or more in the wall thickness and the fibre length. In addition, although the finger-like pores in both the B-90 and B-70 hollow fibres extend straightly to the inner surfaces, the number and the length of the pores in B-90 are more prevalent and longer than those in the latter. Therefore, an improved oxygen permeation performance can be expected for the B-90 hollow fibre membranes, which were ascertained in the following permeation measurements. The inner surface morphology of the sintered hollow fibre membranes are shown in Fig. 4. Again it is clear that the morphology in the hollow fibre precursors has been preserved well after sintering. For example, small pores exist on the rough inner surface of the A90 hollow fibres and large pores are present on the inner surfaces of the B-90 and B-70 hollow fibre membranes. It should be mentioned that the inner surface of the B-50 hollow fibre membranes is also dense and smooth, although the inner surface of the fibre precursor is more porous than those prepared using pure ethanol as the bore liquid. This implies that only those pores larger than a certain value can be preserved after sintering, and the tiny pores tend to be closed in the sintering process. Therefore, the sponge-like regions in the precursors tend to be densified during the calcination and the finger-like regions can nearly retain the original morphology. Fig. 5 shows the outer surface morphology of the hollow fibre precursors and the sintered hollow fibre membranes using different bore fluids for spinning. It can be observed that the hollow fibre precursors have a smooth outer surface and all the sintered fibres have a dense surface structure. This suggests the bore liquid has hardly any noticeable impact on the outer surface morphology of the resultant hollow fibre membranes. In addition, although the LSCF particles on the outer surfaces have also become larger in size after sintering, the crystal boundaries are much less distinct than
those on the inner surfaces. It suggests that the outer and inner surfaces may possibly undergo a different sintering process that is not clear to us at present. 3.2. Porosity and mechanical strength The porosity and the apparent density of the hollow fibres were measured using the Archimedes method with DI water as the medium. A highly densified LSCF6428 hollow fibre in which no voids and defects are present (confirmed by SEM) was fabricated from a highly viscous suspension for comparison purpose. The density of such hollow fibres was also measured using the Archimedes method to be 6.153 g cm−3 . Therefore, the porosity of the asymmetric hollow fibre membranes can be calculated by: ε=
1−
hf d
× 100%
(3)
where hf and d are the density of the hollow fibre membrane and the perfectly dense membrane, respectively. Fig. 6 shows the porosity of the hollow fibre membranes as a function of NMP content in the bore fluid. It can be seen that the porosity of the hollow fibre membranes increases with the NMP content in the bore liquid. Furthermore, the hollow fibres using ethanol–NMP mixture as the bore fluid possess higher porosity than those H2 O–NMP bore fluid derived membranes, except when no NMP is contained in the bore liquid. The maximum porosity of the B-90 hollow fibre membrane can reach up to 46.92%. The mechanical strength of the hollow fibre membranes using different bore liquids is shown in Fig. 7, in which the bending strength values are plotted against the NMP content in the bore fluid. As shown, the hollow fibres using pure water and pure ethanol as the internal coagulant possess the highest mechanical strength of around 154 MPa. As the NMP content in the bore liquid is increased, the mechanical strength of the hollow fibre membranes decreases noticeably. Furthermore, the EtOH–NMP bore fluid derived hollow fibres generally possess lower mechanical strength than the H2 O–NMP bore fluid derived ones. As we know, the mechanical strength of a perovskite hollow fibre membrane is determined not only by the material properties but also by the membrane structure. Therefore, the hollow fibres with lower porosity are reasonably expected to have higher mechanical strength, as demonstrated in this work. 3.3. Oxygen permeation properties Oxygen permeation measurements for all the hollow fibre membranes were conducted using the same gas flow rates, i.e., 250 cm3 min−1 and 147 cm3 min−1 for the air feed and the He sweep gas, respectively. Fig. 8 plots the oxygen permeation fluxes against temperature for the hollow fibre membranes. As is expected, the oxygen flux increased with increasing temperature for all the hollow fibres since the higher operating temperature facilitates both the oxygen diffusion and the surface exchange reactions. It is also clear that the hollow fibre membrane prepared using the bore fluid with a higher NMP content exhibits better permeation performance than those with a low NMP content. This suggests the structure of the hollow fibre plays an important role in the oxygen permeation through the perovskite membranes. As described above, when pure water is used as the bore fluid, three dense layers and two porous layers are formed in the fibre cross-section, where the macrovoids are enclosed by the dense layers. Consequently, the oxygen permeation through such sandwiched membranes has to undergo six steps of surface exchange reaction, although the effective diffusion path is also shortened. Therefore, the sandwiched hollow fibres exhibit the lowest permeation performance. When some NMP solvent is added to the bore fluid, the porous layer on the lumen side
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Fig. 2. SEM images of the inner surface of the LSCF6428 hollow fibre precursors spun using H2 O–NMP (A–x) and EtOH–NMP (B–x) mixtures as the bore fluid, respectively. (x represents the NMP weight percentage in the mixture).
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Fig. 3. Sectional SEM micrographs of the LSCF6428 hollow fibre membranes prepared using different bore fluids (A: H2 O–NMP, and B: EtOH–NMP; sintering temperature = 1420 ◦ C, sintering time = 4 h).
tends to disappear. As a result, the bulk diffusion path becomes longer while the two surface exchange reaction steps on the lumen side pores are also eliminated. From Fig. 8 it can be seen that the membrane with one porous layer, A-50 exhibits higher oxygen flux than the sandwiched fibre A-0. This implies that the macrovoides enclosed in the membranes may deteriorate the permeation performance. That is to say, the enclosed macrovoides in the fibre wall
are not desired unless the dense layers become much thinner than before. The A-70 fibre performs better than the A-50 membrane because despite their similar structure the effective thickness of the A-70 is smaller than that of the latter. Particularly noteworthy is that the A-90 membrane exhibits higher fluxes than A-70 although its thickness is larger because the inner surface of the A-90 fibre is very porous. This implies that the surface exchange kinetics plays
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315
Fig. 4. Inner surface SEM images of the LSCF6428 hollow fibre membranes using different bore fluids containing NMP solvent (sintering temperature = 1420 ◦ C, sintering time = 4 h).
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Fig. 5. SEM images of the outer surfaces of the LSCF6428 hollow fibre membranes before (left side) and after (right side) sintering at 1420 ◦ C.
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Porosity of the hollow fibre, %
50 40 30 20
EtOH-NMP
H2O-NMP
10 0
0
20
40
60
80
100
NMP content in the bore fluid, wt% Fig. 6. Plot of porosity of the LSCF6428 hollow fibre membranes against NMP content in the bore fluid.
Bending strength, MPa
160 EtOH-NMP
140
H2O-NMP
a key role in the oxygen permeation. On the other hand, it is noted that almost all the hollow fibres prepared using EtOH–NMP as the internal coagulant possess higher permeation performance than those derived from the H2 O-based bore fluid with the exception of Fibre A-90. As can be seen, B-50 performs better than B-0 in permeation flux although it has more macrovoids enclosed in the fibre wall. This may be attributed to the longer finger-like pores, resulting in a noticeable decrease in the effective membrane thickness. When the finger-like pores are opened to the inner surface forming a highly asymmetric structure, the oxygen permeation flux can be improved remarkably. For example, the B-90 fibre exhibits a flux of 14.12 mmol m−2 s−1 (1.89 mL cm−2 min−1 ) at 900 ◦ C which is 2.1 times the flux of the B-0 fibre and 12.1 times the flux of the A-0 membranes at the same operating conditions, respectively. At 1000 ◦ C the oxygen flux through the B-90 hollow fibre even can reach up to 34.4 mmol m−2 s−1 (4.62 mL cm−2 min−1 ), which is much higher than those obtained before [9,19,20]. Such a remarkable improvement of the oxygen permeation flux can be attributed to the reduced bulk diffusion resistance due to the decreased membrane effective thickness and the faster surface exchange reaction kinetics results from the larger membrane area provided by the porous inner surface.
120
4. Conclusion 100
80
0
20
40
60
80
100
NMP content in the bore fluid, wt% Fig. 7. Mechanical strength of the LSCF6428 hollow fibre membranes as a function of the NMP content in the bore fluid.
24 20 16
A-0 A-50 A-70
12
Oxygen permeation flux, mmol m-2 s-1
317
A-90
The morphology of perovskite hollow fibre membranes can be modulated using different bore fluids in the process of spinning. The LSCF6428 hollow fibre membranes prepared using EtOH–NMP (1methyl-2-pyrrolidinone) mixture as the bore liquid exhibit higher oxygen permeation flux than those using H2 O–NMP as the bore liquid because of the improved microstructures. When the NMP content in the ethanol based bore liquid is higher than 70 wt%, a highly asymmetric structure consisting of a dense layer and porous substrate can be formed in the perovskite hollow fibre membranes. Such highly asymmetric hollow fibre membranes have exhibited much higher oxygen permeation flux, i.e., a maximum of 34.4 mmol m−2 s−1 at 1000 ◦ C, but reasonably lower mechanical strength than the other structural hollow fibre membranes. The membrane application reported here is based on oxygen permeation through gas-tight LSCF6428 hollow fibres, but the results of the fundamental studies of the influences of bore fluid composition on membrane morphology change in this work will also provide valuable information for other application purposes.
8
Acknowledgements 4
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).
0 40 B-0 32
24
B-50 B-70
References
B-90 16
8
0 750
850
950
1050
Temperature, °C Fig. 8. Oxygen permeation flux of the LSCF6428 hollow fibre membranes at different temperatures (He sweep flow rate = 147 mL min−1 , air feed rate = 250 mL min−1 )
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[6] K.S. Lee, S. Lee, J.W. Kim, S.K. Woo, Enhancement of oxygen permeation by La0.6 Sr0.4 CoO3−ı coating in La0.7 Sr0.3 Ga0.6 Fe0.4 O3−ı membrane, Desalination 147 (2002) 439. [7] F.M. Figueiredo, V.V. Kharton, A.P. Viskup, J.R. Frade, Surface enhanced oxygen permeation in CaTi1−x Fex O3−ı ceramic membranes, J. Membr. Sci. 236 (2004) 73. [8] Z. Wang, H. Liu, X. Tan, Y. Jin, S. Liu, Improvement of the permeation through perovskite hollow fibre membranes by surface acid-modification, J. Membr. Sci. 345 (2009) 65. [9] X. Tan, Y. Lui, 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. [10] T. Schiestel, M. Kilgus, S. Peter, K.J. Caspary, H. Wang, J. Caro, Hollow fibre perovskite membranes for oxygen separation, J. Membr. Sci. 258 (2005) 1. [11] S. Liu, G.R. Gavalas, Oxygen selective ceramic hollow fibre membranes, J. Membr. Sci. 246 (2005) 103. [12] K. Li, X. Tan, Y. Lui, Single-step fabrication of ceramic hollow fibers for oxygen permeation, J. Membr. Sci. 272 (2006) 1. [13] S. Mok, D.J. Worsfold, A.E. Fouda, T. Matsuura, S. Wang, K. Chan, Study on the effect of spinning conditions and surface treatment on the geometry and performance of polymeric hollow-fibre membranes, J. Membr. Sci. 100 (1995) 183. [14] M. Khayet, The effects of air gap length on the internal and external morphology of hollow fiber membranes, Chem. Eng. Sci. 58 (2003) 3091. [15] F. Tasselli, E. Drioli, Tuning of hollow fiber membrane properties using different bore fluids, J. Membr. Sci. 301 (2007) 11.
[16] M. Khayet, M.C. García-Payo, F.A. Qusay, K.C. Khulbe, C.Y. Feng, T. Matsuura, Effects of gas gap type on structural morphology and performance of hollow fibers, J. Membr. Sci. 311 (2008) 259. [17] B.F.K. Kingsbury, K. Li, A morphological study of ceramic hollow fibre membranes, J. Membr. Sci. 328 (2009) 134. [18] W. Yin, B. Meng, X. Meng, X. Tan, Highly asymmetric YSZ hollow fibre membranes, J. Alloys Compd. 476 (2009) 566. [19] Z. Wang, N. Yang, B. Meng, X. Tan, Preparation and oxygen permeation properties of highly asymmetric La0.6 Sr0.4 Co0.2 Fe0.8 O3−˛ (LSCF) perovskite hollow fibre membranes, Ind. Eng. Chem. Res. 48 (2009) 510. [20] B. Zydorczak, Z. Wu, K. Li, Fabrication of ultrathin La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı hollow fibre membranes for oxygen permeation, Chem. Eng. Sci. 64 (2009) 4383. [21] N. Liu, X. Tan, B. Meng, S. Liu, Honeycomb-structured perovskite hollow fibre membranes with ultra-thin densified layer for oxygen separation, Sep. Purif. Technol. (2011), doi:10.1016/j.seppur.2011.04.014. [22] N. Yang, L. Feng, X. Tan, X. Meng, Synthesis of Lax Sr1−x Coy Fe1−y O3−˛ nano powder by the gel self-combustion process, Chin. J. Inorg. Chem. 21 (2005) 1623. [23] X. Tan, Z. Wang, H. Liu, S. Liu, Enhancement of oxygen permeation through La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı hollow fibre membranes by surface modifications, J. Membr. Sci. 324 (2008) 128. [24] X. Tan, Y. Liu, K. Li, Mixed conducting ceramic hollow fiber membranes for air separation, AIChE J. 51 (2005) 1991. [25] H.A. Tsai, M.J. Hong, G.S. Huang, Y.C. Wang, C.L. Li, K.R. Lee, J.Y. Lai, Effect of DGDE additive on the morphology and pervaporation performances of asymmetric PSf hollow fiber membranes, J. Membr. Sci. 208 (2002) 233.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Morphology control of the perovskite hollow fibre membranes for oxygen separation using different bore fluids Xiaoyao Tan a,b,∗ , Nan Liu b , Bo Meng b , Shaomin Liu c a b c
School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China 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 6 May 2011 Accepted 9 May 2011 Available online 14 May 2011 Keywords: Hollow fibre membrane Oxygen permeation Perovskite Morphology control
a b s t r a c t La0.6 Sr0.4 Co0.2 Fe0.8 O3−␣ (LSCF6428) perovskite hollow fibre membranes were fabricated through a phase inversion/sintering technique. The effects of the internal coagulant (bore fluid) compositions on the morphology of the resultant hollow fibres were systematically investigated. The bore fluids were specially designed to vary the solvent–nonsolvent exchange rates in the phase inversion stage by mixing different polymer solvent (1-methyl-2-pyrrolidinone–NMP) content inside the water or ethanol internal coagulant. The prepared hollow fibre membranes were characterized with scanning electron microscopy (SEM), porosity and bending strength measurements, gas-tightness examination and oxygen permeation tests at high temperatures. The results indicate that the bore fluid composition has dramatic effects on the microstructure of the resultant hollow fibre membranes. With the gradual increase of NMP concentration in the bore fluids, the membrane morphology evolution from multi-dense layers to one single dense layer has been clearly demonstrated. LSCF6482 hollow fibre membranes with one dense layer near the outside surface integrated with the highly porous structure showed the highest oxygen fluxes with a maximum value of 34.4 mmol m−2 s−1 under air/He gradient at 1000 ◦ C. The relationships between bore fluid composition and membrane morphology change and their significant effects on oxygen permeation have been discussed extensively. This work shows the feasibility of achieving ceramic hollow fibre membranes with different architectures for various purposes in one single step simply by adjusting the bore fluid composition. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Among the efforts to battle climate change, cost-effective oxygen production via mixed ionic–electronic conducting (MIEC) ceramic membrane technology (without the need of cryogenics) is of great interest as most large scale clean energy technologies require oxygen as the feed gas. Typical examples of such MIEC ceramic membranes are the La1−x Srx Co1−y Fey O3−ı (LSCF) perovskites often cited over the last two decades as the target materials. Their potential applications in oxygen production from air or partial oxidation of light hydrocarbons to value-added products are being investigated [1–3]. Such perovskite membranes exhibit 100% oxygen permselectivity under a gradient of oxygen chemical potential across the membrane at a high temperature, because the oxygen transport through the membranes is via the diffusion
∗ Corresponding author at: School of Environmental and Chemical Engineering, Tianjin Polytechnic University, No. 63 Chenglin Road, Hedong District, Tianjin, Shandong 300160, China. Tel.: +86 533 2786292; fax: +86 533 278629. E-mail address: [email protected] (X. Tan). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.05.012
of mobile oxygen vacancies and electronic defects, while simultaneously excluding the transport of any gas-phase species. In the oxygen permeation process, three major steps are included: (i) oxygen exchange reactions on the high oxygen partial pressure side of the membrane surface, including oxygen adsorption, dissociation, reduction and incorporation to yield lattice oxygen, (ii) bulk diffusion of oxygen ions or vacancies and electron holes, and (iii) surface reaction between lattice oxygen and electron holes on the other membrane surface (permeate side) and oxygen desorption. It is clear that the major resistance to oxygen transport results from the surface oxygen exchange reactions and bulk diffusion, and the permeation flux is often jointly controlled by both processes [4,5]. In order to obtain a high oxygen flux through a perovskite membrane, it is necessary not only to decrease the membrane thickness but also to increase the membrane surface area favoring the surface exchange kinetics [6–8]. Therefore, the membrane microstructure or morphology may exert a significant influence on the permeation performance of perovskite membranes. Over the past ten years, ceramic membranes with hollow fibre geometry have been developed via combined phase inversion at room temperature and subsequent sintering at high temperatures.
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This method is flexible and versatile, and can be used to synthesize various inorganic hollow fibre membranes and is thus of great interest to researchers. Compared to other configurations like tubular or disk-shaped membranes, the ceramic hollow fibres exhibit many advantages such as facile high-temperature sealing, thin effective membrane thickness, and, more importantly, larger membrane area per unit packing volume. Therefore, the development of perovskite hollow fibre membranes with higher oxygen flux is a step closer toward large scale applications [9–12]. The microstructure of the hollow fibre ceramic membrane precursor formed during the initial phase inversion process possesses an integral asymmetric structure that consists of dense thin layers supported on porous layers of the same material. The subsequent sintering (calcination/heat treatment) does not alter their overall structure but increase the density of the dense layer making it gas-tight. Therefore, the spinning condition may exert significant influence on the microstructure of the hollow fibre membranes [13–16]. For example, when water is used as both the internal and external coagulant, the resultant hollow fibres usually possess a sandwich structure, i.e., a central dense layer and two surface skin dense layers integrated with two porous layers [9]. As the viscosity of the starting solution or the air gap is varied, the relative thickness of the dense and the porous layers will be altered subsequently [17]. In another case, when a solvent instead of water is used as the internal coagulant, the macrovoids or the finger-like pores in the hollow fibres can be extended straightly to the inner surface, forming a highly asymmetric structure-a single dense layer integrated with a porous support [18–21]. Such a highly asymmetric structure is more favourable for oxygen permeation than the sandwiched structure since it not only has lower diffusion resistance to oxygen ions from the single dense layer but also possesses a larger surface area for oxygen exchange reactions [21]. Compared to multi-step treatment, adjusting the spinning conditions to get the desired membrane structure in one step is a better strategy to improve the properties of the ceramic hollow fibre membranes. To reach this purpose, previous works were focusing on adjusting the viscosity of the spinning dope by changing the compositions of the spinning dope [18–20] or the distance of the air gap between the spinneret and external coagulant level. In shaping the hollow fibre precursors, the internal coagulant utilized is usually in a very low concentration, however, it has a significant effect on the internal morphology of the resultant hollow fibre membranes. Such an important role that the internal coagulant plays is often ignored and so far no systematical investigation of the effects of the internal coagulants on the morphology evolution of the resultant dense ceramic hollow fibre membranes has been carried out. This work has three main objectives, namely (1) systematical investigation of the effects of the internal coagulant (bore fluid) composition on the morphology of the resulted perovskite hollow fibres, (ii) fundamental understanding of the bore fluid’s role in the morphological evolution of the perovskite hollow fibres before and after sintering, and (iii) finding of the optimum membrane microstructure for oxygen separation. To achieve these purposes, eight batches of perovskite hollow fibre precursors were spun using different bore fluid compositions. La0.6 Sr0.4 Co0.2 Fe0.8 O3−˛ (LSCF6428) perovskite oxide was selected as the membrane material due to its high mechanical and phase stability. To guarantee the successful preparation of these series of hollow fibres, up to 4 kg LSCF6428 high quality powder was homemade using the EDTA–citrate sol–gel method. Water and ethanol were used as the internal coagulants in this work since they are easily obtainable, cheap in cost and environmentally friendly. A certain amount of solvent (NMP) was added into the internal coagulant to vary the exchange rate between the solvent in the spinning solution and the non-solvent in the internal coagulant.
309
Table 1 Preparation conditions for the LSCF6428 hollow fibre membranes.
Composition of the spinning solution
Flow rate of the bore liquid External coagulant Nitrogen pressure Air gap Spinning rate Sintering temperature Sintering time Bore fluid for the fibre samples
Experimental parameters
Values
LSCF6428
62.98 wt%
PESf NMP PVP DI water 20 mL min−1
6.30 wt% 25.18 wt% 3.71 wt% 1.85 wt%
Tap water 0.1 MPa 0 cm 4.5 m min−1 1420 ◦ C 4h A-0
Pure water
A-50 A-70 A-90 B-0 B-50 B-70 B-90
50 wt% H2 O–50 wt% NMP 30 wt% H2 O–70 wt% NMP 10 wt% H2 O–90 wt% NMP Pure ethanol 50 wt% EtOH–50 wt% NMP 30 wt% EtOH–70 wt% NMP 10 wt% EtOH–90 wt% NMP
2. Experimental 2.1. Preparation of LSCF powder and hollow fibres La0.6 Sr0.4 Co0.2 Fe0.8 O3−˛ (LSCF6428) perovskite powders were prepared through a sol–gel process which was described in detail elsewhere [19,22]. For spinning hollow fibre membranes, the powder precursor was calcined at 800 ◦ C for 3 h to remove the residual carbon and to form the desired structure, ball-milled for 48 h in an agate jar and sieved through a sifter of 200-mesh or 24 m sieve-pore diameter. The hollow fibre membranes were made from the calcined and ball-milled powders by the phase inversion and sintering technique. The detailed preparation procedures were described elsewhere [9]. 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-2pyrrolidinone (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% deionized (DI) water as non-solvent additives. The bore fluid for spinning was a mixture of ethanol–NMP or DI water–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. Other preparation parameters were kept the same for all the hollow fibre membranes, which are summarized in Table 1. For simplification, the hollow fibres prepared using the ethanol–NMP or water–NMP mixture as the bore fluid were marked as A–x or B–x in which A or B stands for water or ethanol and x for the weight percentage of the NMP solvent in the bore fluid. 2.2. Characterization Viscosity of the spinning solutions were measured 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. The viscosity of the spinning solution was measured at room temperature to be around 55,800 mPa s−1 . Morphology and microstructures of the hollow fibre membranes
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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 min−1 . The hollow fibre sample was fixed on the sample holder at a distance of 32 mm. The bending strength, F , was calculated from the following equation: F =
FLR0 4 ) (R04 − Rin
(1)
where F is the measured force at which the fracture takes place, R0 , Rin and L are the outer radius, inner e radius and length (32 mm) of the hollow fibre samples, respectively. For each hollow fibre, three samples were taken for the measurement and the average bending strength value was taken as the mechanical strength of the hollow fibre membrane.
2.3. Oxygen permeation measurement Oxygen permeation properties of the LSCF6428 hollow fibre membranes were investigated in an oxygen permeation cell schematically shown elsewhere [23]. Prior to the assembly, the gastightness of the hollow fibre membrane was tested and confirmed by nitrogen gas permeation measurement at room temperature [24]. The hollow fibres were housed in a quartz tube (18 mm in diameter and 400 mm in length) with a high-temperature silicone sealant (1592, purchased from Tonsan New Materials and Technol. Co., Beijing) that is able to withstand up to 350 ◦ C. The permeation cell was positioned in a ˚22 mm × 180 mm tubular furnace having an effective heating length of 5 cm. Air was fed on the shell side and helium was passed through the fibre lumen to collect the oxygen permeated. Gas feed flow rates were controlled by mass flow controllers (D08-8B/ZM, Shanxi Chuangwei Instrument Co. Ltd., China). Compositions of the permeate gas were measured online using a gas chromatograph (Agilent 6890N) fitted with a 5 A˚ molecular sieve column (˚3 mm × 3 mm) and a TCD detector. Highly purified hydrogen was used as carrier gas and the flow rate was set at 40 cm3 min−1 . 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 permeation, a small amount of nitrogen (<0.15%) was detected in the permeate stream at low permeation temperature. It was assumed that a minor leak occurred at connecting joints of the permeation cell. Therefore, the oxygen flux was calculated by: JO2 =
V (yO2 − (21/78)yN2 ) Am
(2)
where V is the permeate gas flow rate (mol s−1 ), yO2 and yN2 are the oxygen and nitrogen concentrations in the permeate stream (mol%), Am is the effective membrane area calculated by Am = (2(R0 − Rin )L)/ln(R0 /Rin ) in which L is the effective length for oxygen permeation 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 3.1. Morphology of the hollow fibre membranes During the preparation of the series of LSCF6428 hollow fibre membranes, all other processing parameters, with the exception of the bore fluid for spinning hollow fibre precursors, were fixed at the same conditions. Therefore, the change of the microstructure and other membrane properties is mainly attributed to the variation of the bore fluid. Fig. 1 shows the cross-sectional SEM micrographs of the hollow fibre precursors using H2 O–NMP and EtOH–NMP mixtures with different NMP concentrations as bore coagulants, respectively. It can be seen that two major types of microstructures, namely the sponge-like structure and the finger-like structure, are simultaneously present in the hollow fibre precursors, forming an asymmetric morphology of the hollow fibres. The hollow fibre membranes derived using pure water as the bore liquid exhibit a sandwich structure, i.e., a central sponge-like structure between two finger-like structures, as shown in Fig. 1A-0. Noteworthy is that both the inner and the outer surfaces also consist of a spongelike structure in these hollow fibres. When NMP solvent is added in the bore liquid up to 50 wt%, the finger layer on the lumen side gradually diminishes to be no longer observable. The thickness of the sponge layer on the lumen side also decreases with the increase of the NMP content in the bore liquid as can be seen in the figure. However, when EtOH–NMP was used as the bore liquid to replace water, the sandwich structure cannot be formed in the resultant hollow fibres. As shown in Fig. 1B-0, the hollow fibre using pure ethanol as the bore liquid has a similar structure to those using H2 O–NMP mixture as the bore fluid, and again the thickness of the sponge-like structure on the lumen side decreases noticeably as the NMP content in the bore liquid is increased to 50 wt%. In addition, the finger-like pores have become much longer in the B-50 fibre than those in the B-0 membrane. When the NMP content in the bore liquid is further increased to 70 wt% or higher, the sponge-like structure on the lumen side disappears and the fingerlike pores straightly extend to the inner surface, leaving only one thin sponge layer on the outer surface of the hollow fibres, as shown in Fig. 1B-70 and B-90. The size of the finger-like pores close to the inner surface can be up to 30 m, but the pores close to the outer sponge-like surface are much smaller. The change of the structural morphology of the hollow fibre membranes with the bore fluid compositions can be explained as follows. It is well known that the formation of either a finger-like structure or a spongelike structure is determined by the solvent–nonsolvent exchange rates during phase separation when the polymer solution touches coagulant. If the rate of nonsolvent inflow is faster than the rate of solvent outflow, a finger-like structure is formed, while a spongelike membrane is formed when the former is lower than that of the latter [25]. Therefore, when pure water is used as the internal coagulant, the solvent–nonsolvent exchange rate on the inner surface is close to that on the outer surface, leading to the similar porous structures in the inner and outer regions, namely a sandwich structure. However, when a mixture of H2 O–NMP or EtOH–NMP is used as the internal coagulant, the solvent–nonsolvent exchange rate, also the precipitation rate, on the inner side of the nascent membrane may be retarded, thus the macrovoids are suppressed in the inner region. Once the NMP content in the ethanol bore liquid reaches a certain level, i.e., >70 wt%, the solvent–nonsolvent exchange rate on the inner surface may be so slow that the NMP solvent almost outflows only from the outer surface of the nascent membranes. As a result, the finger-like pores formed due to the immersion of the membrane into the water coagulation bath can extend straight to the inner surface of the hollow fibres. Clearly, it can also be inferred that the highly asymmetric structure may be formed if the NMP content in the water bore fluid is suffi-
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311
Fig. 1. Sectional SEM micrographs of the LSCF6428 hollow fibre precursors spun using H2 O–NMP (A–x) and EtOH–NMP (B–x) mixtures as the bore fluid, respectively. (x represents the NMP weight percentage in the mixture).
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ciently high, e.g., >95 wt%, which was observed in our previous work [24]. Fig. 2 compares the SEM inner surfaces of the hollow fibre precursors using H2 O–NMP and EtOH–NMP mixtures with different NMP concentrations as the bore coagulant. As illustrated in the figure, when the NMP content in the H2 O–NMP bore fluid is lower than 70%, the derived hollow fibres possess dense and smooth inner surfaces. The small ceramic particles are well dispersed and connected with each other by the polymer binder. This suggests that the fine LSCF particles can be dispersed uniformly in the PESf–NMP polymer solution. When the NMP content in the H2 O–NMP bore liquid is increased to 90 wt%, however, the inner surface becomes rough with the presence of many small pores, as shown in Fig. 2A90. When ethanol–NMP mixture is used as the bore fluid, the inner surfaces of the derived hollow fibres are much rougher than those using water as the bore liquid. Furthermore, the pores on the inner surface become larger and larger as the NMP content is increased. As the NMP content in the ethanol bore liquid is increased to 90 wt%, the inner surface has become rather rough with many large holes penetrating deep into the interior of the hollow fibres. These results further confirm that the exchange rate between the ethanol and solvent is much slower than that between the water and NMP solvent. Fig. 3 shows the cross-sectional SEM micrographs of the hollow fibre membranes after sintering at 1420 ◦ C for 4 h. It should be mentioned that the highly asymmetric hollow fibres with open finger-like pores (B-70 and B-90) have to be sintered at above 1400 ◦ C to obtain gas-tight properties. Therefore, all the hollow fibres were sintered at 1420 ◦ C in this work, which is much higher than those employed before [9,19], so that we can focus the research mainly on the effect of the bore fluid. It is observed from these SEMs that the asymmetric structures in the precursors have been well-maintained in the sintered hollow fibre membranes. This implies that the sintering only removed the organic components but has not changed the general morphology of the hollow fibres, although the sintered hollow fibres experienced shrinking by 20% or more in the wall thickness and the fibre length. In addition, although the finger-like pores in both the B-90 and B-70 hollow fibres extend straightly to the inner surfaces, the number and the length of the pores in B-90 are more prevalent and longer than those in the latter. Therefore, an improved oxygen permeation performance can be expected for the B-90 hollow fibre membranes, which were ascertained in the following permeation measurements. The inner surface morphology of the sintered hollow fibre membranes are shown in Fig. 4. Again it is clear that the morphology in the hollow fibre precursors has been preserved well after sintering. For example, small pores exist on the rough inner surface of the A90 hollow fibres and large pores are present on the inner surfaces of the B-90 and B-70 hollow fibre membranes. It should be mentioned that the inner surface of the B-50 hollow fibre membranes is also dense and smooth, although the inner surface of the fibre precursor is more porous than those prepared using pure ethanol as the bore liquid. This implies that only those pores larger than a certain value can be preserved after sintering, and the tiny pores tend to be closed in the sintering process. Therefore, the sponge-like regions in the precursors tend to be densified during the calcination and the finger-like regions can nearly retain the original morphology. Fig. 5 shows the outer surface morphology of the hollow fibre precursors and the sintered hollow fibre membranes using different bore fluids for spinning. It can be observed that the hollow fibre precursors have a smooth outer surface and all the sintered fibres have a dense surface structure. This suggests the bore liquid has hardly any noticeable impact on the outer surface morphology of the resultant hollow fibre membranes. In addition, although the LSCF particles on the outer surfaces have also become larger in size after sintering, the crystal boundaries are much less distinct than
those on the inner surfaces. It suggests that the outer and inner surfaces may possibly undergo a different sintering process that is not clear to us at present. 3.2. Porosity and mechanical strength The porosity and the apparent density of the hollow fibres were measured using the Archimedes method with DI water as the medium. A highly densified LSCF6428 hollow fibre in which no voids and defects are present (confirmed by SEM) was fabricated from a highly viscous suspension for comparison purpose. The density of such hollow fibres was also measured using the Archimedes method to be 6.153 g cm−3 . Therefore, the porosity of the asymmetric hollow fibre membranes can be calculated by: ε=
1−
hf d
× 100%
(3)
where hf and d are the density of the hollow fibre membrane and the perfectly dense membrane, respectively. Fig. 6 shows the porosity of the hollow fibre membranes as a function of NMP content in the bore fluid. It can be seen that the porosity of the hollow fibre membranes increases with the NMP content in the bore liquid. Furthermore, the hollow fibres using ethanol–NMP mixture as the bore fluid possess higher porosity than those H2 O–NMP bore fluid derived membranes, except when no NMP is contained in the bore liquid. The maximum porosity of the B-90 hollow fibre membrane can reach up to 46.92%. The mechanical strength of the hollow fibre membranes using different bore liquids is shown in Fig. 7, in which the bending strength values are plotted against the NMP content in the bore fluid. As shown, the hollow fibres using pure water and pure ethanol as the internal coagulant possess the highest mechanical strength of around 154 MPa. As the NMP content in the bore liquid is increased, the mechanical strength of the hollow fibre membranes decreases noticeably. Furthermore, the EtOH–NMP bore fluid derived hollow fibres generally possess lower mechanical strength than the H2 O–NMP bore fluid derived ones. As we know, the mechanical strength of a perovskite hollow fibre membrane is determined not only by the material properties but also by the membrane structure. Therefore, the hollow fibres with lower porosity are reasonably expected to have higher mechanical strength, as demonstrated in this work. 3.3. Oxygen permeation properties Oxygen permeation measurements for all the hollow fibre membranes were conducted using the same gas flow rates, i.e., 250 cm3 min−1 and 147 cm3 min−1 for the air feed and the He sweep gas, respectively. Fig. 8 plots the oxygen permeation fluxes against temperature for the hollow fibre membranes. As is expected, the oxygen flux increased with increasing temperature for all the hollow fibres since the higher operating temperature facilitates both the oxygen diffusion and the surface exchange reactions. It is also clear that the hollow fibre membrane prepared using the bore fluid with a higher NMP content exhibits better permeation performance than those with a low NMP content. This suggests the structure of the hollow fibre plays an important role in the oxygen permeation through the perovskite membranes. As described above, when pure water is used as the bore fluid, three dense layers and two porous layers are formed in the fibre cross-section, where the macrovoids are enclosed by the dense layers. Consequently, the oxygen permeation through such sandwiched membranes has to undergo six steps of surface exchange reaction, although the effective diffusion path is also shortened. Therefore, the sandwiched hollow fibres exhibit the lowest permeation performance. When some NMP solvent is added to the bore fluid, the porous layer on the lumen side
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313
Fig. 2. SEM images of the inner surface of the LSCF6428 hollow fibre precursors spun using H2 O–NMP (A–x) and EtOH–NMP (B–x) mixtures as the bore fluid, respectively. (x represents the NMP weight percentage in the mixture).
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Fig. 3. Sectional SEM micrographs of the LSCF6428 hollow fibre membranes prepared using different bore fluids (A: H2 O–NMP, and B: EtOH–NMP; sintering temperature = 1420 ◦ C, sintering time = 4 h).
tends to disappear. As a result, the bulk diffusion path becomes longer while the two surface exchange reaction steps on the lumen side pores are also eliminated. From Fig. 8 it can be seen that the membrane with one porous layer, A-50 exhibits higher oxygen flux than the sandwiched fibre A-0. This implies that the macrovoides enclosed in the membranes may deteriorate the permeation performance. That is to say, the enclosed macrovoides in the fibre wall
are not desired unless the dense layers become much thinner than before. The A-70 fibre performs better than the A-50 membrane because despite their similar structure the effective thickness of the A-70 is smaller than that of the latter. Particularly noteworthy is that the A-90 membrane exhibits higher fluxes than A-70 although its thickness is larger because the inner surface of the A-90 fibre is very porous. This implies that the surface exchange kinetics plays
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315
Fig. 4. Inner surface SEM images of the LSCF6428 hollow fibre membranes using different bore fluids containing NMP solvent (sintering temperature = 1420 ◦ C, sintering time = 4 h).
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Fig. 5. SEM images of the outer surfaces of the LSCF6428 hollow fibre membranes before (left side) and after (right side) sintering at 1420 ◦ C.
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Porosity of the hollow fibre, %
50 40 30 20
EtOH-NMP
H2O-NMP
10 0
0
20
40
60
80
100
NMP content in the bore fluid, wt% Fig. 6. Plot of porosity of the LSCF6428 hollow fibre membranes against NMP content in the bore fluid.
Bending strength, MPa
160 EtOH-NMP
140
H2O-NMP
a key role in the oxygen permeation. On the other hand, it is noted that almost all the hollow fibres prepared using EtOH–NMP as the internal coagulant possess higher permeation performance than those derived from the H2 O-based bore fluid with the exception of Fibre A-90. As can be seen, B-50 performs better than B-0 in permeation flux although it has more macrovoids enclosed in the fibre wall. This may be attributed to the longer finger-like pores, resulting in a noticeable decrease in the effective membrane thickness. When the finger-like pores are opened to the inner surface forming a highly asymmetric structure, the oxygen permeation flux can be improved remarkably. For example, the B-90 fibre exhibits a flux of 14.12 mmol m−2 s−1 (1.89 mL cm−2 min−1 ) at 900 ◦ C which is 2.1 times the flux of the B-0 fibre and 12.1 times the flux of the A-0 membranes at the same operating conditions, respectively. At 1000 ◦ C the oxygen flux through the B-90 hollow fibre even can reach up to 34.4 mmol m−2 s−1 (4.62 mL cm−2 min−1 ), which is much higher than those obtained before [9,19,20]. Such a remarkable improvement of the oxygen permeation flux can be attributed to the reduced bulk diffusion resistance due to the decreased membrane effective thickness and the faster surface exchange reaction kinetics results from the larger membrane area provided by the porous inner surface.
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4. Conclusion 100
80
0
20
40
60
80
100
NMP content in the bore fluid, wt% Fig. 7. Mechanical strength of the LSCF6428 hollow fibre membranes as a function of the NMP content in the bore fluid.
24 20 16
A-0 A-50 A-70
12
Oxygen permeation flux, mmol m-2 s-1
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A-90
The morphology of perovskite hollow fibre membranes can be modulated using different bore fluids in the process of spinning. The LSCF6428 hollow fibre membranes prepared using EtOH–NMP (1methyl-2-pyrrolidinone) mixture as the bore liquid exhibit higher oxygen permeation flux than those using H2 O–NMP as the bore liquid because of the improved microstructures. When the NMP content in the ethanol based bore liquid is higher than 70 wt%, a highly asymmetric structure consisting of a dense layer and porous substrate can be formed in the perovskite hollow fibre membranes. Such highly asymmetric hollow fibre membranes have exhibited much higher oxygen permeation flux, i.e., a maximum of 34.4 mmol m−2 s−1 at 1000 ◦ C, but reasonably lower mechanical strength than the other structural hollow fibre membranes. The membrane application reported here is based on oxygen permeation through gas-tight LSCF6428 hollow fibres, but the results of the fundamental studies of the influences of bore fluid composition on membrane morphology change in this work will also provide valuable information for other application purposes.
8
Acknowledgements 4
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).
0 40 B-0 32
24
B-50 B-70
References
B-90 16
8
0 750
850
950
1050
Temperature, °C Fig. 8. Oxygen permeation flux of the LSCF6428 hollow fibre membranes at different temperatures (He sweep flow rate = 147 mL min−1 , air feed rate = 250 mL min−1 )
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