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One end-dead perovskite hollow fiber membranes for high-purity oxygen production from ambient air
Chemical Engineering Journal 183 (2012) 473–482
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One end-dead perovskite hollow fiber membranes for high-purity oxygen production from ambient air Yanying Wei, Jun Tang, Lingyi Zhou, Zhong Li, Haihui Wang ∗ School of Chemistry & Chemical Engineering, South China University of Technology, No. 381 Wushan Road, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 15 September 2011 Received in revised form 8 December 2011 Accepted 10 December 2011 Keywords: Membrane Hollow fiber Oxygen permeation Oxygen separation Perovskite
a b s t r a c t The high-purity oxygen production from ambient air through the dense one end-dead perovskite hollow fiber membrane based on Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−ı prepared by a phase inversion spinning process is reported. High oxygen purity up to 99.7 vol% and high oxygen permeation flux of 1.74 ml/min cm2 are obtained after 2-h activation in the initial stage at 950 ◦ C under the vacuum degree of 99.0 kPa on the core side and the ambient air of 1.0 atm on the feed side. The X-ray diffraction (XRD), scanning electron microscope (SEM) and energy spectrum analysis (EDS) show that the carbonate appears in the spent hollow fiber membrane after 24-h oxygen permeation in the ambient air, which leads to the decline of the oxygen permeation flux. However, the pure oxygen can be produced in one step through the one end-dead hollow fiber membrane without any inert gas, so the second separation is not necessary any more and it is beneficial to the industrial application for high-purity oxygen production. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Oxygen is ranking among the top five in the production of commodity chemicals in the world [1]. The technique to obtain cheap and high-purity oxygen is very important in industry. Nowadays, there are three traditional ways to get oxygen. The first one is cryogenic fractionation technology which requires a large-scale plant and high operation cost, although >99.0 vol% oxygen can be obtained. The second method is pressure swing adsorption (PSA) through molecular sieve adsorbents due to the difference in the quadruple moment between oxygen and nitrogen [2], which can give oxygen with a maximum purity of 95.0–97.0 vol%. The third way is the polymeric membranes. However, the maximum oxygen concentration produced by a single-stage system is around 50.0 vol% due to a low separation factor of polymeric membranes [3]. In recent years, the mixed conducting ceramic oxides with oxygen ionic and electronic conductivity provide a promising way for oxygen production [4,5] due to the infinite oxygen selectivity at high temperature. The ceramic membranes also have potential applications in membrane reactors for selective oxidation of hydrocarbons [6–10], as the cathode of solid oxide fuel cell (SOFC) [11] and for oxygen supplying to power stations with CO2 sequestration according to the oxyfuel concept [12–17].
∗ Corresponding author. Tel.: +86 20 87110131; fax: +86 20 87110131. E-mail address: [email protected] (H. Wang). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.12.038
However, inert sweep gas such as helium or argon is used to decrease the oxygen partial pressure on the permeated side of the membrane during oxygen permeation. Therefore, there are only a few percent of oxygen and large amount of inert gas in the effluent gases. As a result, the second separation is needed to get high-purity oxygen. Obviously, this method for oxygen production through the ceramic membrane using sweep gas is not suitable for industrial oxygen production. An excellent method to obtain high-purity oxygen through the dense ceramic membranes is by applying a vacuum pump to pump the permeated oxygen. One end of the membrane is connected to a vacuum pump, and the other end should be gastight itself, i.e. a dead end membrane. Zhu et al. [5] investigated the oxygen permeation through the Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−ı perovskite tubular membranes under vacuum. One end of the tubular membrane was covered with a quartz cap which was filled with Ag paste to keep gastight. Liang et al. [18] also produced high-purity oxygen in a dead-end permeator with BaCox Fey Zr1−x−y O3−ı hollow fiber membranes which were sealed with a gold plug using Au paste on one end of the hollow fiber. Tan et al. [19,20] investigated the scaling-up effect and energy consumption of the membrane system in pilot-scale production of oxygen through hundreds of La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı hollow fiber membranes with pumping the permeated oxygen, but they did not report the detail about the hollow fiber with dead end. During these reports related to the high-purity oxygen production under vacuum, the methods for sealing one end of the tube or hollow fiber membranes are complex and high cost due to the using of noble metals.
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Furthermore, the sealant and the membrane material are not matching very well, which are also not suitable for industrial applications. Obviously, one end-dead membrane is difficult to prepare and the sealing problem is not easy to solve under the vacuum condition. Herein, we propose a novel one end-dead hollow fiber membrane. One end of hollow fiber membrane is closed with the same material during the phase inversion spinning process, and then the whole one end-dead hollow fiber membrane becomes gastight after sintering in one step. It is convenient and low cost to get the high-purity oxygen through the asprepared one end-dead hollow fiber membranes by vacuum. It can also avoid the subsequent sealing problem and mismatching problem between the sealant and the membrane during heating/cooling process or even under permeation conditions. In the present work, the one end-dead hollow fiber membranes based on the perovskite-type Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−ı (BSCF) are prepared. The oxygen purity and the oxygen permeation flux through the one end-dead BSCF hollow fiber membrane are investigated under vacuum at 950 ◦ C.
2. Experimental
Table 1 Preparation conditions for the one end-dead BSCF hollow fiber membranes. Parameter
Value
Compositions of the starting solution BSCF powder PESf, A-300 NMP PVP, K30 Spinning temperature Injection rate of internal coagulant Spinning pressure Air gap Sintering temperature Sintering time Air flow rate for sintering
59.28 wt% 8.01 wt% 31.91 wt% 0.80 wt% 20 ◦ C 2.3 ml/min 0.4 bar 1.0 cm 1150 ◦ C 10 h 60 ml/min
2.2. Characterization The microstructure and morphology of the BSCF hollow fiber membrane precursor, the fresh and spent BSCF hollow fiber membranes were observed by a scanning electron microscope (SEM, JEOL JSM-6490LA). The phase structure of the spent BSCF hollow fiber membrane after the oxygen permeation test under vacuum was characterized by X-ray diffraction (XRD, Bruker-D8 ADVANCE, Cu K␣ radiation). The elemental compositions of the membrane were determined by energy dispersive spectroscopy (EDS) attached to the above-mentioned SEM system.
2.1. Preparation of BSCF powder and hollow fiber membranes The BSCF oxide powder was synthesized by a combined EDTA–citrate complexation, which was described in the previous work [21]. For spinning hollow fiber membranes, the powder was ball-milled for 24 h and then was dried using a spray dryer (Büchi Mini Spray Dryer, B-290) with a nozzle of 1 m. The obtained fine powder was used for the preparation of the one end-dead hollow fiber membranes. The one end-dead BSCF hollow fibers were fabricated using a wet spinning/sintering method as shown in our previous work [22]. The spinning solution was composed of 8.01 wt% polyethersulfone (PESf, A-300, BASF), 31.91 wt% 1methyl-2-pyrrolidinone (NMP, AR Grade, purity >99.8%, Kermel Chem Inc., Tianjin, China), 0.80 wt% polyvinyl pyrrolidone (PVP, K30, Boao Biotech Co., Shanghai, China) and 59.28 wt% BSCF powder. A spinneret with an orifice diameter and inner diameter of 1.5 and 1.0 mm, respectively, was used to obtain the hollow fiber precursors. Deionized water and tap water were used as the internal and external coagulants, respectively. The forming hollow fiber precursors were immersed in a deionized water bath at room temperature to complete the solidification process. Afterward, the BSCF hollow fiber precursors were cut into 0.3-m pieces. After drying in the air at room temperature more than 24 h, one end of the BSCF hollow fiber precursor was coated with the same spinning solution as described above. Then the BSCF hollow fiber precursors with dead-end were immersed in deionized water for phase inversion again. After being closed on one end, the BSCF hollow fiber precursors were sintered at 1150 ◦ C for 10 h under the air flow rate of 60 ml/min to remove the polymer and get gas-tight membrane. In brief, there are two steps to produce the one enddead hollow fiber membrane. In the first step, we got the precursor of the hollow fiber with two open ends. In the second step, one end of the hollow fiber precursor was coated with the spinning solution followed by a phase inversion process. After these two steps, one end of the hollow fiber precursor was closed. Finally, the dense one-end-dead hollow fiber membrane could be formed after a sintering process. Only the dense hollow fiber can be used in the following oxygen permeation test. The preparation conditions of the one end-dead BSCF hollow fiber membranes are summarized in Table 1.
2.3. Oxygen permeation measurement A high-temperature permeation cell for the pure oxygen production through the one end-dead BSCF hollow fiber membrane by vacuum is schematically illustrated in Fig. 1. The hollow fiber membrane was placed in a vertically positioned tubular furnace with diameter of 3 cm and height of 15 cm. The hollow fiber membrane used here has a total length of 9 cm. The open end of the hollow fiber membrane was sealed in a stainless steel tube (˚6 mm) by a commercial ceramic sealant (HT767, Hutian, China) and epoxy resin (E-44, Dongfeng, China) mixed with curing agent (EP, Dongfeng, China). There are two layers in the sealed junction between the one end-dead hollow fiber membrane and the stainless steel tube. Firstly, the mixture of the epoxy resin and the curing agent was added in the stainless steel tube to form a dense block. Then, the ceramic sealant was covered in the left space of the tube, as well as the surface of the junction. The other end of the stainless steel tube was connected to an oil-free dry scroll vacuum pump (SH-110, Varian, USA). The stainless steel tube and the hollow fiber membrane were fixed on an iron support. The one end-dead hollow fiber was exposed to the atmosphere directly in the oven under the ambient air. The downstream vacuum degree of the hollow fiber membrane was kept at 99.0 kPa by applying a vacuum pump in the hollow fiber lumen. Due to the difference of the oxygen partial pressure between the atmosphere side and core side, oxygen in the ambient air will permeate through the hollow fiber membrane to the core side, and the high-purity oxygen can be obtained. During the experiments, the temperature was controlled using a temperature controller (708P-K1, Yudian, China), where a K-type thermocouple positioned in the middle of the furnace was used for the measurement of temperatures. The oxygen concentration of the product stream was analyzed by an online gas chromatograph (GC, Agilent 7890A). The flow rate of the effluent was measured by a soap bubble flow meter. The oxygen permeation flux is calculated as following: JO2 (ml/min cm2 ) = CO2 ×
F S
(1)
where CO2 is the oxygen concentration in the effluent gas detected by the GC measurement, F is the flow rate of the effluent at the
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Fig. 1. Scheme of the oxygen permeation apparatus for the oxygen permeation of the one end-dead hollow fiber membrane at high temperature.
outlet, which is measured by a soap flow meter, and S is the effective hollow fiber membrane area.
3. Results and discussion 3.1. Morphology and microstructure of the hollow fiber membranes Fig. 2 exhibits the photos of the one end-dead BSCF hollow fiber membranes after sintering at 1150 ◦ C for 10 h. Obviously, one end of the hollow fiber membrane is dead, while the other end is open. The dead end of the BSCF hollow fiber membrane can be observed in high magnification as shown in Fig. 2(A). During the preparation, the linear part of the hollow fiber is formed in the first stage of the phase inversion, while the dead end of the hollow fiber is formed in the second stage. However, these two parts connect to each other firmly although they are formed in different stages of phase inversion. Furthermore, the linear part of the hollow fiber and the dead end, as well as the junction of them all become dense and gastight after sintering at 1150 ◦ C for 10 h. The bending strength of the BSCF hollow fiber membrane by three point bending test is around 300 MPa and the tension strength is around of 60 N/mm2 . The thermal expansion coefficient of BSCF is 1.15 × 10−5 K−1 [23]. Fig. 3 shows the SEM micrographs of the one end-dead BSCF hollow fiber precursor. From the side view of the dead end in Fig. 3(A) and (B), it can be seen that the dead end connects to the linear part of the hollow fiber firmly and glossily without any crack. The two parts finally become a whole after their respective phase inversions. The dead end of the hollow fiber looks orbicular from the top view in Fig. 3(C), which is due to the dipping from the spinning solution. Fig. 3(D) shows the outer surface of the dead end, from which it can be seen that the BSCF particles are well-dispersed and sparsely connected to each other by the polymer binder. After sintered at 1150 ◦ C for 10 h, the dead end and the linear part still connect to each other firmly and glossily as shown in Fig. 4. It can be observed that the outer surface of the membrane becomes dense and no open holes can be found although a large amount of organic additives (about 40 wt%) was used during the spinning process.
3.2. High-purity oxygen production through the membrane The oxygen concentration and the oxygen permeation flux at the outlet from the one end-dead BSCF hollow fiber membrane as a function of time are recorded in Fig. 5. The temperature increases gradually with time at the speed of 1.5 ◦ C/min. As can be seen, both the oxygen concentration and the oxygen permeation flux increase with increasing temperature. In the beginning, the oxygen permeation flux increases very slowly, but it rises rapidly after 100 min. It may due to the oxygen vacancy-ordered structure changes to disordered perovskite structure at temperature above 750 ◦ C [24]. The trend of the oxygen concentration at the outlet is similar to that of the oxygen permeation flux. When the temperature reaches 950 ◦ C, the oxygen purity and the oxygen permeation flux did not instantly reach a steady value. Fig. 6 shows the activation in the initial stage of high-purity oxygen production through the one end-dead BSCF hollow fiber membrane when the downstream vacuum degree is kept at 99 kPa at 950 ◦ C. Both the oxygen concentration at the outlet and the oxygen permeation flux increase with time. After 2-h activation, the oxygen permeation flux rises to a steady value of 1.68 ml/min cm2 with the corresponding oxygen concentration of 98.3%. Fig. 7 shows the performance of the high-purity oxygen production through the one end-dead BSCF hollow fiber membrane as a function of time at 950 ◦ C. In the first stage, i.e. from 0 to 600 min, the oxygen permeation flux keeps around 1.74 ml/min cm2 , which is higher than that of the traditional BSCF hollow fiber with two open ends with 20 ml/min He as the sweep gas [25]. The oxygen purity maintains about 99.7% in the first stage. The oxygen concentration in the product stream cannot reach the theoretical value (100%). It is due to the minor leakage of the membrane system such as the sealing joints which lead to the entry of some nitrogen into the product stream [20]. However, it is much higher than that through the conventional oxygen permeable membrane swept by inert gas [22,25,26]. For the present one end-dead hollow fiber membrane system, the oxygen permeation flux and the oxygen purity can reach above 1.74 ml/min cm2 and 99.7%, respectively, when the downstream vacuum degree was kept at 99 kPa at 950 ◦ C. Unfortunately, in the second stage, i.e. from 600 min to
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Fig. 2. Photos of the one end-dead BSCF hollow fiber membranes. (A) Dead end of the sintered hollow fiber membranes and (B) sintered hollow fiber membranes.
1150 min, the oxygen purity in the product stream decreases obviously. 3.3. Post analysis of the spent hollow fiber membrane In order to explain the reasons of the decline of the membrane performance, XRD, SEM and EDS analysis of the spent hollow fiber membrane are performed. Fig. 8 shows the XRD pattern of the spent one end-dead BSCF hollow fiber membrane after high-purity oxygen production from ambient air. Although the perovskite phase (indicated by ‘p’) is still maintained, many other impurities appear in the spent membrane, such as the carbonate, sulfate and other
oxides labeled in Fig. 8. The carbonate was formed due to the reaction between the alkaline earth-metal in BSCF and CO2 in the ambient air. Chen et al. found that the co-presence of CO2 and H2 O species in air has severe effects on the phase composition, microstructure and oxygen permeability of perovskite membrane, compared with the presence of either species alone [27]. This experiment was done in the air humidity of 96%, which enhanced the formation of BaCO3 . The removal of either CO2 or H2 O species from the feed ambient air can improve the stability of the membrane. The sulfate has also been formed on the spent hollow fiber membrane because of the presence of little SO2 in the ambient air [28] or the residual sulfur of the polymer during the spinning/sintering
Fig. 3. SEM micrographs of the one end-dead BSCF hollow fiber precursor. (A) and (B) Side view of dead end; (C) top view of dead end and (D) outer surface of dead end.
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Fig. 4. SEM micrographs of the sintered one end-dead BSCF hollow fiber. (A) and (B) Side view of dead end; (C) top view of dead end and (D) outer surface of dead end.
process. In addition, some metal oxides, such as Fe3 O4 and Co3 O4 have been formed due to the segregation of constituent membrane elements. All these impurity phases will cut down the oxygen permeation flux through the one end-dead BSCF hollow fiber membrane. Fig. 9 shows the SEM pictures of the hollow fiber membrane at the room temperature zone after high-purity oxygen production from ambient air. The symmetric structure of the spent hollow fiber can still be observed like a fresh one. Because there is almost no oxygen permeated through this part and no reaction takes place on the hollow fiber membrane at the room temperature zone. As shown in Fig. 9(C), the cross-section of the hollow fiber membrane at room temperature zone is dense and no open pore can be found here. Fig. 10 shows the SEM micrographs of the hollow fiber membrane which is hold at the high temperature zone after high-purity
oxygen production from ambient air. From the pictures of the top view and the cross section shown in Fig. 10(A) and (B), it can be noted that the symmetric structure disappears. Compared with the SEM micrographs of the spent hollow fiber membrane at the room temperature zone, it is easy to find that the spent hollow fiber membrane at the high temperature zone has been seriously destroyed and a porous layer is observed. Even in the middle of the cross section shown in Fig. 10(C), the membrane becomes loose and porous. The inner surface of the spent hollow fiber membrane at the high temperature zone shown in Fig. 10(D) has the similar micrograph to the cross section. It looks like the original intact membrane has been cracked to many pieces [29], which leads to the sharp decline of the oxygen purity at the outlet of the one end-dead BSCF hollow fiber membrane after 600 min as shown in Fig. 7. The SEM micrographs of the outer surface of the spent hollow fiber membrane are shown in Fig. 11. There are some remarkable
Fig. 5. Oxygen concentration and the oxygen permeation flux as a function of time with increasing temperature.
Fig. 6. Activation in the initial stage of oxygen permeation under the vacuum degree of 99 kPa at 950 ◦ C.
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Y. Wei et al. / Chemical Engineering Journal 183 (2012) 473–482 Table 2 EDS results of “01” and “02” area in Fig. 11(F). Area
Fig. 11(F)-01 Fig. 11(F)-02
Fig. 7. Oxygen purity and oxygen permeation flux through the one end-dead BSCF hollow fiber membrane as a function of time at 950 ◦ C.
changes in surface microstructure of the spent membrane compared with the fresh membrane. Fig. 11(A) shows the dead end of the spent hollow fiber membrane. Compared to the fresh dead end, the spent one still maintains the original “dead end” shape. However, lots of “blocks” can be observed on the outer surface of the membrane as shown in Fig. 11(B)–(F). Similar result was also found by other researchers [19,30]. The original crystal particles and the particle boundary disappear after the 24-h oxygen permeation. Many blocks with a diameter of 5–20 m are observed on the outer surface of the spent hollow fiber membrane. Moreover, some tiny cracks on the outer surface of membrane can be seen in Fig. 11(F). It looks like the original intact membrane has been cracked to many pieces, which is in accordance with the inner surface shown in Fig. 10(D). In addition, a layer with thickness of 20 m can be observed in the cross-section near the outer surface as shown in Fig. 11(G) and (H). The formation of the layer may be related to the CO2 in the ambient air. Many other researchers also observed a similar layer on the surface of the spent membrane after the partial oxidation of methane [31,32]. Table 2 shows the EDS results of the outer surface of the spent hollow fiber membrane. It can be seen that a large percent of carbon exists on the whole outer surface. From the XRD result, it can be expected that the carbon comes from BaCO3 . Because most of
Fig. 8. XRD pattern of the spent one end-dead BSCF hollow fiber membrane after high-purity oxygen production from ambient air.
Atom (%) Ba
Sr
Co
Fe
C
O
S
1.82 7.87
0.36 4.63
14.06 4.87
0 1.42
30.06 17.84
52.29 56.64
1.41 6.73
the perovskite materials are quite sensitive to CO2 , the membranes containing alkaline-earth elements such as Ba and Sr tend to react with acid gas CO2 to form carbonates [33–36]. When the one enddead BSCF hollow fiber membrane was applied in the high-purity oxygen production from ambient air, CO2 in the ambient air could contact with the membrane, which results in the formation of carbonate on the outer surface of the spent hollow fiber membrane. The ratio of Ba to Sr on the outer surface is higher than the original ratio in BSCF. The cobalt content on the outer surface has also increased. It can be speculated that cobalt and barium tend to move towards the membrane surface during oxygen separation at high temperature [19]. Combining with the results of XRD, SEM and EDS analysis, the blocks on the outer surface of the membrane are consisted of BaCO3 , Co3 O4 and some residual sulfur, which leads to the decrease of oxygen purity at the outlet of the one end-dead BSCF hollow fiber membrane after 600-min operation. 3.4. Advantages of the one end-dead hollow fiber membranes Although the stability of the one end-dead BSCF hollow fiber membrane for the oxygen production from ambient air should be improved in future, the advantages of the one end-dead hollow fiber membrane used for high-purity oxygen production under vacuum are obvious. Fig. 12 shows the oxygen production by sweep gas using two ends-open hollow fiber membranes and by vacuum using one end-dead hollow fiber membranes. In the sweepingmodel with the two ends-open hollow fiber membranes, the inert sweep gas such as helium is used to decrease the oxygen partial pressure on the permeated side of the membrane. The total pressure on both the shell side and the core side of the membrane are one atmosphere. Thus, the oxygen is separated through the hollow fiber membrane under the driving force of the oxygen partial pressure gradient. Finally, oxygen with a low concentration and large amount of the inert gas are obtained in the effluent gases. As a result, the second separation is necessary to obtain the high-purity oxygen, which is not suitable for industrial process. However, it is excited to obtain the high-purity oxygen in one step through the one end-dead hollow fiber membrane by applying a vacuum pump in the hollow fiber lumen. In the vacuum-model with the one enddead hollow fiber membrane, the inert sweep gas is not necessary any more. The pressure on the feed side of membrane keeps one atmosphere, while the pressure on the core side of membrane drops to 99 kPa by the vacuum pump. Under a big driving force of oxygen partial pressure difference, high-purity oxygen can be obtained in the effluent gas easily. The second separation is not desired any more, i.e. the pure oxygen can be produced in one step by the one end-dead hollow fiber membrane. Table 3 summarized the oxygen permeation fluxes through the BSCF membranes with different modes, where CO2 and Csweep gas are the oxygen concentration and the sweep gas concentration in the effluent gas. In all the sweeping-modes, the pressures on both shell side and core side of the membrane are one atmosphere, i.e. there is no pressure difference between two sides of the membrane. In order to generate the oxygen permeation driving force of oxygen partial pressure difference for oxygen production, the inert gas is used to sweep on the core side to decrease the oxygen partial pressure on the core side. And the oxygen concentration in the effluent gas is less than 20% in these sweeping-modes. For example, the
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Fig. 9. SEM micrographs of the hollow fiber membrane at the room temperature zone after high-purity oxygen production from ambient air. (A) Top view; (B) cross-section; (C) higher magnification of cross-section and (D) outer surface.
Fig. 10. SEM micrographs of the hollow fiber membrane at the high temperature zone after high-purity oxygen production from ambient air. (A) Top view; (B) cross-section; (C) higher magnification of cross-section and (D) inner surface.
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Fig. 11. SEM micrographs of the dead-end of hollow fiber membrane at the high temperature zone after high-purity oxygen production from ambient air. (A) Side view; (B) outer surface of dead end; (C)–(F) higher magnification of outer surface of dead end; (G) and (H) cross-section near outer surface.
oxygen concentration in the effluent gas is only 12.7% through the two ends-open hollow fiber membrane with the oxygen permeation flux of 1.65 ml/min cm2 when 20 ml/min He was used as the sweep gas. On the contrary, the pressure difference between the two sides of the one end-dead hollow fiber membrane is as high as 0.99 atm due to the vacuum pump without any inert sweep gas. The oxygen partial pressure on the core side here is only
0.00997 atm, which is much lower than that of the conventional two ends-open hollow fiber membrane [22,25]. Therefore, the oxygen permeation driving force of the one end-dead hollow fiber membrane is 0.20 atm, which is much higher than other driving forces in those sweeping-models. As a result, the oxygen purity in the effluent gas is as high as 99.7% in the vacuum-mode with the oxygen permeation flux of 1.74 ml/min cm2 . After comparison
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Fig. 12. Oxygen permeation modes of two ends-open hollow fiber and one end-dead hollow fiber membranes.
Table 3 Comparison of oxygen permeation fluxes through BSCF membranes with different modes. Mode
Sweeping-mode
Vacuum-mode
Geometry of membrane
Disk
Tubular
Linear hollow fiber
U-shaped hollow fiber
One end-dead hollow fiber
Fsweep gas (ml/min) Thickness (mm) Temperature (◦ C) JO2 (ml/min cm2 ) CO2 (%) CHe (%) Reference
30 1.76 950 1.4 4.0 96.0 [21]
60 1.7 875 1.27 5.4 94.6 [26]
20 0.22 950 1.65 12.7 87.3 [25]
20 0.23 950 1.58 12.9 87.1 [22]
– 0.28 950 1.74 99.7 – Present work
of the oxygen purities and the oxygen permeation fluxes through BSCF membranes with different geometries in two modes, it can be noted that high-purity oxygen can be obtained in one step through the one end-dead hollow fiber membrane by vacuum at high temperature, which is also beneficial to the industrial application for high-purity oxygen production.
in future, the one end-dead hollow fiber membrane still exhibits obvious advantages. Under a big oxygen permeation driving force generated by vacuum, the pure oxygen can be produced in one step through the one end-dead hollow fiber membrane without any inert gas. In other words, the second separation is not necessary any more, which is beneficial to the industrial application for high-purity oxygen production.
4. Conclusion Acknowledgments The dense one end-dead perovskite hollow fiber membranes based on Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−ı (BSCF) are prepared by a phase inversion spinning process. It is used for the high-purity oxygen production from ambient air by vacuum. A high oxygen purity up to 99.7 vol % and high oxygen permeation rate of 1.74 ml/min cm2 are obtained after about 2-h activation in the initial stage at 950 ◦ C under the vacuum degree of 99 kPa on the core side and ambient air of 1 atm on the feed side. However, the hollow fiber membrane cracked after 24-h oxygen permeation in the ambient air. The XRD, SEM and EDS results show that carbonate appears in the phase of the spent hollow fiber membrane due to the reaction between the alkaline earth metal in the membrane and CO2 in the ambient air. The segregation of the constituent membrane elements, such as cobalt, is observed after the oxygen permeation. Although the stability of the one end-dead BSCF hollow fiber membrane for the oxygen production from ambient air should be improved
The authors greatly acknowledge the financial support by Natural Science Foundation of China (Nos. 21176087, U0834004 and 20936001), the National Basic Research Program of China (No. 2009CB623406), the Science-Technology Plan of Guangzhou City (2009J1-C511-1) and the Fundamental Research Funds for the Central Universities, SCUT (2009220038). References [1] H.J.M. Bouwmeester, A.J. Burggraff, Dense ceramic membranes for oxygen separation, in: A.J. Burggraff, L. Cot (Eds.), Fundamentals of Inorganic Membrane Science and Technology, Elsevier Science B.V., 1996, p. 435. [2] R.V. Jasra, N.V. Choudary, S.G.T. Baht, Separation of gases by pressure swing adsorption, Sep. Sci. Technol. 16 (1991) 885. [3] S.L. Matson, W.J. Ward, S.G. Kimure, W.R. Browall, Membrane oxygen enrichment: II. Economic assessment, J. Membr. Sci. 29 (1986) 79.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
One end-dead perovskite hollow fiber membranes for high-purity oxygen production from ambient air Yanying Wei, Jun Tang, Lingyi Zhou, Zhong Li, Haihui Wang ∗ School of Chemistry & Chemical Engineering, South China University of Technology, No. 381 Wushan Road, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 15 September 2011 Received in revised form 8 December 2011 Accepted 10 December 2011 Keywords: Membrane Hollow fiber Oxygen permeation Oxygen separation Perovskite
a b s t r a c t The high-purity oxygen production from ambient air through the dense one end-dead perovskite hollow fiber membrane based on Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−ı prepared by a phase inversion spinning process is reported. High oxygen purity up to 99.7 vol% and high oxygen permeation flux of 1.74 ml/min cm2 are obtained after 2-h activation in the initial stage at 950 ◦ C under the vacuum degree of 99.0 kPa on the core side and the ambient air of 1.0 atm on the feed side. The X-ray diffraction (XRD), scanning electron microscope (SEM) and energy spectrum analysis (EDS) show that the carbonate appears in the spent hollow fiber membrane after 24-h oxygen permeation in the ambient air, which leads to the decline of the oxygen permeation flux. However, the pure oxygen can be produced in one step through the one end-dead hollow fiber membrane without any inert gas, so the second separation is not necessary any more and it is beneficial to the industrial application for high-purity oxygen production. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Oxygen is ranking among the top five in the production of commodity chemicals in the world [1]. The technique to obtain cheap and high-purity oxygen is very important in industry. Nowadays, there are three traditional ways to get oxygen. The first one is cryogenic fractionation technology which requires a large-scale plant and high operation cost, although >99.0 vol% oxygen can be obtained. The second method is pressure swing adsorption (PSA) through molecular sieve adsorbents due to the difference in the quadruple moment between oxygen and nitrogen [2], which can give oxygen with a maximum purity of 95.0–97.0 vol%. The third way is the polymeric membranes. However, the maximum oxygen concentration produced by a single-stage system is around 50.0 vol% due to a low separation factor of polymeric membranes [3]. In recent years, the mixed conducting ceramic oxides with oxygen ionic and electronic conductivity provide a promising way for oxygen production [4,5] due to the infinite oxygen selectivity at high temperature. The ceramic membranes also have potential applications in membrane reactors for selective oxidation of hydrocarbons [6–10], as the cathode of solid oxide fuel cell (SOFC) [11] and for oxygen supplying to power stations with CO2 sequestration according to the oxyfuel concept [12–17].
∗ Corresponding author. Tel.: +86 20 87110131; fax: +86 20 87110131. E-mail address: [email protected] (H. Wang). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.12.038
However, inert sweep gas such as helium or argon is used to decrease the oxygen partial pressure on the permeated side of the membrane during oxygen permeation. Therefore, there are only a few percent of oxygen and large amount of inert gas in the effluent gases. As a result, the second separation is needed to get high-purity oxygen. Obviously, this method for oxygen production through the ceramic membrane using sweep gas is not suitable for industrial oxygen production. An excellent method to obtain high-purity oxygen through the dense ceramic membranes is by applying a vacuum pump to pump the permeated oxygen. One end of the membrane is connected to a vacuum pump, and the other end should be gastight itself, i.e. a dead end membrane. Zhu et al. [5] investigated the oxygen permeation through the Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−ı perovskite tubular membranes under vacuum. One end of the tubular membrane was covered with a quartz cap which was filled with Ag paste to keep gastight. Liang et al. [18] also produced high-purity oxygen in a dead-end permeator with BaCox Fey Zr1−x−y O3−ı hollow fiber membranes which were sealed with a gold plug using Au paste on one end of the hollow fiber. Tan et al. [19,20] investigated the scaling-up effect and energy consumption of the membrane system in pilot-scale production of oxygen through hundreds of La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı hollow fiber membranes with pumping the permeated oxygen, but they did not report the detail about the hollow fiber with dead end. During these reports related to the high-purity oxygen production under vacuum, the methods for sealing one end of the tube or hollow fiber membranes are complex and high cost due to the using of noble metals.
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Furthermore, the sealant and the membrane material are not matching very well, which are also not suitable for industrial applications. Obviously, one end-dead membrane is difficult to prepare and the sealing problem is not easy to solve under the vacuum condition. Herein, we propose a novel one end-dead hollow fiber membrane. One end of hollow fiber membrane is closed with the same material during the phase inversion spinning process, and then the whole one end-dead hollow fiber membrane becomes gastight after sintering in one step. It is convenient and low cost to get the high-purity oxygen through the asprepared one end-dead hollow fiber membranes by vacuum. It can also avoid the subsequent sealing problem and mismatching problem between the sealant and the membrane during heating/cooling process or even under permeation conditions. In the present work, the one end-dead hollow fiber membranes based on the perovskite-type Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−ı (BSCF) are prepared. The oxygen purity and the oxygen permeation flux through the one end-dead BSCF hollow fiber membrane are investigated under vacuum at 950 ◦ C.
2. Experimental
Table 1 Preparation conditions for the one end-dead BSCF hollow fiber membranes. Parameter
Value
Compositions of the starting solution BSCF powder PESf, A-300 NMP PVP, K30 Spinning temperature Injection rate of internal coagulant Spinning pressure Air gap Sintering temperature Sintering time Air flow rate for sintering
59.28 wt% 8.01 wt% 31.91 wt% 0.80 wt% 20 ◦ C 2.3 ml/min 0.4 bar 1.0 cm 1150 ◦ C 10 h 60 ml/min
2.2. Characterization The microstructure and morphology of the BSCF hollow fiber membrane precursor, the fresh and spent BSCF hollow fiber membranes were observed by a scanning electron microscope (SEM, JEOL JSM-6490LA). The phase structure of the spent BSCF hollow fiber membrane after the oxygen permeation test under vacuum was characterized by X-ray diffraction (XRD, Bruker-D8 ADVANCE, Cu K␣ radiation). The elemental compositions of the membrane were determined by energy dispersive spectroscopy (EDS) attached to the above-mentioned SEM system.
2.1. Preparation of BSCF powder and hollow fiber membranes The BSCF oxide powder was synthesized by a combined EDTA–citrate complexation, which was described in the previous work [21]. For spinning hollow fiber membranes, the powder was ball-milled for 24 h and then was dried using a spray dryer (Büchi Mini Spray Dryer, B-290) with a nozzle of 1 m. The obtained fine powder was used for the preparation of the one end-dead hollow fiber membranes. The one end-dead BSCF hollow fibers were fabricated using a wet spinning/sintering method as shown in our previous work [22]. The spinning solution was composed of 8.01 wt% polyethersulfone (PESf, A-300, BASF), 31.91 wt% 1methyl-2-pyrrolidinone (NMP, AR Grade, purity >99.8%, Kermel Chem Inc., Tianjin, China), 0.80 wt% polyvinyl pyrrolidone (PVP, K30, Boao Biotech Co., Shanghai, China) and 59.28 wt% BSCF powder. A spinneret with an orifice diameter and inner diameter of 1.5 and 1.0 mm, respectively, was used to obtain the hollow fiber precursors. Deionized water and tap water were used as the internal and external coagulants, respectively. The forming hollow fiber precursors were immersed in a deionized water bath at room temperature to complete the solidification process. Afterward, the BSCF hollow fiber precursors were cut into 0.3-m pieces. After drying in the air at room temperature more than 24 h, one end of the BSCF hollow fiber precursor was coated with the same spinning solution as described above. Then the BSCF hollow fiber precursors with dead-end were immersed in deionized water for phase inversion again. After being closed on one end, the BSCF hollow fiber precursors were sintered at 1150 ◦ C for 10 h under the air flow rate of 60 ml/min to remove the polymer and get gas-tight membrane. In brief, there are two steps to produce the one enddead hollow fiber membrane. In the first step, we got the precursor of the hollow fiber with two open ends. In the second step, one end of the hollow fiber precursor was coated with the spinning solution followed by a phase inversion process. After these two steps, one end of the hollow fiber precursor was closed. Finally, the dense one-end-dead hollow fiber membrane could be formed after a sintering process. Only the dense hollow fiber can be used in the following oxygen permeation test. The preparation conditions of the one end-dead BSCF hollow fiber membranes are summarized in Table 1.
2.3. Oxygen permeation measurement A high-temperature permeation cell for the pure oxygen production through the one end-dead BSCF hollow fiber membrane by vacuum is schematically illustrated in Fig. 1. The hollow fiber membrane was placed in a vertically positioned tubular furnace with diameter of 3 cm and height of 15 cm. The hollow fiber membrane used here has a total length of 9 cm. The open end of the hollow fiber membrane was sealed in a stainless steel tube (˚6 mm) by a commercial ceramic sealant (HT767, Hutian, China) and epoxy resin (E-44, Dongfeng, China) mixed with curing agent (EP, Dongfeng, China). There are two layers in the sealed junction between the one end-dead hollow fiber membrane and the stainless steel tube. Firstly, the mixture of the epoxy resin and the curing agent was added in the stainless steel tube to form a dense block. Then, the ceramic sealant was covered in the left space of the tube, as well as the surface of the junction. The other end of the stainless steel tube was connected to an oil-free dry scroll vacuum pump (SH-110, Varian, USA). The stainless steel tube and the hollow fiber membrane were fixed on an iron support. The one end-dead hollow fiber was exposed to the atmosphere directly in the oven under the ambient air. The downstream vacuum degree of the hollow fiber membrane was kept at 99.0 kPa by applying a vacuum pump in the hollow fiber lumen. Due to the difference of the oxygen partial pressure between the atmosphere side and core side, oxygen in the ambient air will permeate through the hollow fiber membrane to the core side, and the high-purity oxygen can be obtained. During the experiments, the temperature was controlled using a temperature controller (708P-K1, Yudian, China), where a K-type thermocouple positioned in the middle of the furnace was used for the measurement of temperatures. The oxygen concentration of the product stream was analyzed by an online gas chromatograph (GC, Agilent 7890A). The flow rate of the effluent was measured by a soap bubble flow meter. The oxygen permeation flux is calculated as following: JO2 (ml/min cm2 ) = CO2 ×
F S
(1)
where CO2 is the oxygen concentration in the effluent gas detected by the GC measurement, F is the flow rate of the effluent at the
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Fig. 1. Scheme of the oxygen permeation apparatus for the oxygen permeation of the one end-dead hollow fiber membrane at high temperature.
outlet, which is measured by a soap flow meter, and S is the effective hollow fiber membrane area.
3. Results and discussion 3.1. Morphology and microstructure of the hollow fiber membranes Fig. 2 exhibits the photos of the one end-dead BSCF hollow fiber membranes after sintering at 1150 ◦ C for 10 h. Obviously, one end of the hollow fiber membrane is dead, while the other end is open. The dead end of the BSCF hollow fiber membrane can be observed in high magnification as shown in Fig. 2(A). During the preparation, the linear part of the hollow fiber is formed in the first stage of the phase inversion, while the dead end of the hollow fiber is formed in the second stage. However, these two parts connect to each other firmly although they are formed in different stages of phase inversion. Furthermore, the linear part of the hollow fiber and the dead end, as well as the junction of them all become dense and gastight after sintering at 1150 ◦ C for 10 h. The bending strength of the BSCF hollow fiber membrane by three point bending test is around 300 MPa and the tension strength is around of 60 N/mm2 . The thermal expansion coefficient of BSCF is 1.15 × 10−5 K−1 [23]. Fig. 3 shows the SEM micrographs of the one end-dead BSCF hollow fiber precursor. From the side view of the dead end in Fig. 3(A) and (B), it can be seen that the dead end connects to the linear part of the hollow fiber firmly and glossily without any crack. The two parts finally become a whole after their respective phase inversions. The dead end of the hollow fiber looks orbicular from the top view in Fig. 3(C), which is due to the dipping from the spinning solution. Fig. 3(D) shows the outer surface of the dead end, from which it can be seen that the BSCF particles are well-dispersed and sparsely connected to each other by the polymer binder. After sintered at 1150 ◦ C for 10 h, the dead end and the linear part still connect to each other firmly and glossily as shown in Fig. 4. It can be observed that the outer surface of the membrane becomes dense and no open holes can be found although a large amount of organic additives (about 40 wt%) was used during the spinning process.
3.2. High-purity oxygen production through the membrane The oxygen concentration and the oxygen permeation flux at the outlet from the one end-dead BSCF hollow fiber membrane as a function of time are recorded in Fig. 5. The temperature increases gradually with time at the speed of 1.5 ◦ C/min. As can be seen, both the oxygen concentration and the oxygen permeation flux increase with increasing temperature. In the beginning, the oxygen permeation flux increases very slowly, but it rises rapidly after 100 min. It may due to the oxygen vacancy-ordered structure changes to disordered perovskite structure at temperature above 750 ◦ C [24]. The trend of the oxygen concentration at the outlet is similar to that of the oxygen permeation flux. When the temperature reaches 950 ◦ C, the oxygen purity and the oxygen permeation flux did not instantly reach a steady value. Fig. 6 shows the activation in the initial stage of high-purity oxygen production through the one end-dead BSCF hollow fiber membrane when the downstream vacuum degree is kept at 99 kPa at 950 ◦ C. Both the oxygen concentration at the outlet and the oxygen permeation flux increase with time. After 2-h activation, the oxygen permeation flux rises to a steady value of 1.68 ml/min cm2 with the corresponding oxygen concentration of 98.3%. Fig. 7 shows the performance of the high-purity oxygen production through the one end-dead BSCF hollow fiber membrane as a function of time at 950 ◦ C. In the first stage, i.e. from 0 to 600 min, the oxygen permeation flux keeps around 1.74 ml/min cm2 , which is higher than that of the traditional BSCF hollow fiber with two open ends with 20 ml/min He as the sweep gas [25]. The oxygen purity maintains about 99.7% in the first stage. The oxygen concentration in the product stream cannot reach the theoretical value (100%). It is due to the minor leakage of the membrane system such as the sealing joints which lead to the entry of some nitrogen into the product stream [20]. However, it is much higher than that through the conventional oxygen permeable membrane swept by inert gas [22,25,26]. For the present one end-dead hollow fiber membrane system, the oxygen permeation flux and the oxygen purity can reach above 1.74 ml/min cm2 and 99.7%, respectively, when the downstream vacuum degree was kept at 99 kPa at 950 ◦ C. Unfortunately, in the second stage, i.e. from 600 min to
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Fig. 2. Photos of the one end-dead BSCF hollow fiber membranes. (A) Dead end of the sintered hollow fiber membranes and (B) sintered hollow fiber membranes.
1150 min, the oxygen purity in the product stream decreases obviously. 3.3. Post analysis of the spent hollow fiber membrane In order to explain the reasons of the decline of the membrane performance, XRD, SEM and EDS analysis of the spent hollow fiber membrane are performed. Fig. 8 shows the XRD pattern of the spent one end-dead BSCF hollow fiber membrane after high-purity oxygen production from ambient air. Although the perovskite phase (indicated by ‘p’) is still maintained, many other impurities appear in the spent membrane, such as the carbonate, sulfate and other
oxides labeled in Fig. 8. The carbonate was formed due to the reaction between the alkaline earth-metal in BSCF and CO2 in the ambient air. Chen et al. found that the co-presence of CO2 and H2 O species in air has severe effects on the phase composition, microstructure and oxygen permeability of perovskite membrane, compared with the presence of either species alone [27]. This experiment was done in the air humidity of 96%, which enhanced the formation of BaCO3 . The removal of either CO2 or H2 O species from the feed ambient air can improve the stability of the membrane. The sulfate has also been formed on the spent hollow fiber membrane because of the presence of little SO2 in the ambient air [28] or the residual sulfur of the polymer during the spinning/sintering
Fig. 3. SEM micrographs of the one end-dead BSCF hollow fiber precursor. (A) and (B) Side view of dead end; (C) top view of dead end and (D) outer surface of dead end.
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Fig. 4. SEM micrographs of the sintered one end-dead BSCF hollow fiber. (A) and (B) Side view of dead end; (C) top view of dead end and (D) outer surface of dead end.
process. In addition, some metal oxides, such as Fe3 O4 and Co3 O4 have been formed due to the segregation of constituent membrane elements. All these impurity phases will cut down the oxygen permeation flux through the one end-dead BSCF hollow fiber membrane. Fig. 9 shows the SEM pictures of the hollow fiber membrane at the room temperature zone after high-purity oxygen production from ambient air. The symmetric structure of the spent hollow fiber can still be observed like a fresh one. Because there is almost no oxygen permeated through this part and no reaction takes place on the hollow fiber membrane at the room temperature zone. As shown in Fig. 9(C), the cross-section of the hollow fiber membrane at room temperature zone is dense and no open pore can be found here. Fig. 10 shows the SEM micrographs of the hollow fiber membrane which is hold at the high temperature zone after high-purity
oxygen production from ambient air. From the pictures of the top view and the cross section shown in Fig. 10(A) and (B), it can be noted that the symmetric structure disappears. Compared with the SEM micrographs of the spent hollow fiber membrane at the room temperature zone, it is easy to find that the spent hollow fiber membrane at the high temperature zone has been seriously destroyed and a porous layer is observed. Even in the middle of the cross section shown in Fig. 10(C), the membrane becomes loose and porous. The inner surface of the spent hollow fiber membrane at the high temperature zone shown in Fig. 10(D) has the similar micrograph to the cross section. It looks like the original intact membrane has been cracked to many pieces [29], which leads to the sharp decline of the oxygen purity at the outlet of the one end-dead BSCF hollow fiber membrane after 600 min as shown in Fig. 7. The SEM micrographs of the outer surface of the spent hollow fiber membrane are shown in Fig. 11. There are some remarkable
Fig. 5. Oxygen concentration and the oxygen permeation flux as a function of time with increasing temperature.
Fig. 6. Activation in the initial stage of oxygen permeation under the vacuum degree of 99 kPa at 950 ◦ C.
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Y. Wei et al. / Chemical Engineering Journal 183 (2012) 473–482 Table 2 EDS results of “01” and “02” area in Fig. 11(F). Area
Fig. 11(F)-01 Fig. 11(F)-02
Fig. 7. Oxygen purity and oxygen permeation flux through the one end-dead BSCF hollow fiber membrane as a function of time at 950 ◦ C.
changes in surface microstructure of the spent membrane compared with the fresh membrane. Fig. 11(A) shows the dead end of the spent hollow fiber membrane. Compared to the fresh dead end, the spent one still maintains the original “dead end” shape. However, lots of “blocks” can be observed on the outer surface of the membrane as shown in Fig. 11(B)–(F). Similar result was also found by other researchers [19,30]. The original crystal particles and the particle boundary disappear after the 24-h oxygen permeation. Many blocks with a diameter of 5–20 m are observed on the outer surface of the spent hollow fiber membrane. Moreover, some tiny cracks on the outer surface of membrane can be seen in Fig. 11(F). It looks like the original intact membrane has been cracked to many pieces, which is in accordance with the inner surface shown in Fig. 10(D). In addition, a layer with thickness of 20 m can be observed in the cross-section near the outer surface as shown in Fig. 11(G) and (H). The formation of the layer may be related to the CO2 in the ambient air. Many other researchers also observed a similar layer on the surface of the spent membrane after the partial oxidation of methane [31,32]. Table 2 shows the EDS results of the outer surface of the spent hollow fiber membrane. It can be seen that a large percent of carbon exists on the whole outer surface. From the XRD result, it can be expected that the carbon comes from BaCO3 . Because most of
Fig. 8. XRD pattern of the spent one end-dead BSCF hollow fiber membrane after high-purity oxygen production from ambient air.
Atom (%) Ba
Sr
Co
Fe
C
O
S
1.82 7.87
0.36 4.63
14.06 4.87
0 1.42
30.06 17.84
52.29 56.64
1.41 6.73
the perovskite materials are quite sensitive to CO2 , the membranes containing alkaline-earth elements such as Ba and Sr tend to react with acid gas CO2 to form carbonates [33–36]. When the one enddead BSCF hollow fiber membrane was applied in the high-purity oxygen production from ambient air, CO2 in the ambient air could contact with the membrane, which results in the formation of carbonate on the outer surface of the spent hollow fiber membrane. The ratio of Ba to Sr on the outer surface is higher than the original ratio in BSCF. The cobalt content on the outer surface has also increased. It can be speculated that cobalt and barium tend to move towards the membrane surface during oxygen separation at high temperature [19]. Combining with the results of XRD, SEM and EDS analysis, the blocks on the outer surface of the membrane are consisted of BaCO3 , Co3 O4 and some residual sulfur, which leads to the decrease of oxygen purity at the outlet of the one end-dead BSCF hollow fiber membrane after 600-min operation. 3.4. Advantages of the one end-dead hollow fiber membranes Although the stability of the one end-dead BSCF hollow fiber membrane for the oxygen production from ambient air should be improved in future, the advantages of the one end-dead hollow fiber membrane used for high-purity oxygen production under vacuum are obvious. Fig. 12 shows the oxygen production by sweep gas using two ends-open hollow fiber membranes and by vacuum using one end-dead hollow fiber membranes. In the sweepingmodel with the two ends-open hollow fiber membranes, the inert sweep gas such as helium is used to decrease the oxygen partial pressure on the permeated side of the membrane. The total pressure on both the shell side and the core side of the membrane are one atmosphere. Thus, the oxygen is separated through the hollow fiber membrane under the driving force of the oxygen partial pressure gradient. Finally, oxygen with a low concentration and large amount of the inert gas are obtained in the effluent gases. As a result, the second separation is necessary to obtain the high-purity oxygen, which is not suitable for industrial process. However, it is excited to obtain the high-purity oxygen in one step through the one end-dead hollow fiber membrane by applying a vacuum pump in the hollow fiber lumen. In the vacuum-model with the one enddead hollow fiber membrane, the inert sweep gas is not necessary any more. The pressure on the feed side of membrane keeps one atmosphere, while the pressure on the core side of membrane drops to 99 kPa by the vacuum pump. Under a big driving force of oxygen partial pressure difference, high-purity oxygen can be obtained in the effluent gas easily. The second separation is not desired any more, i.e. the pure oxygen can be produced in one step by the one end-dead hollow fiber membrane. Table 3 summarized the oxygen permeation fluxes through the BSCF membranes with different modes, where CO2 and Csweep gas are the oxygen concentration and the sweep gas concentration in the effluent gas. In all the sweeping-modes, the pressures on both shell side and core side of the membrane are one atmosphere, i.e. there is no pressure difference between two sides of the membrane. In order to generate the oxygen permeation driving force of oxygen partial pressure difference for oxygen production, the inert gas is used to sweep on the core side to decrease the oxygen partial pressure on the core side. And the oxygen concentration in the effluent gas is less than 20% in these sweeping-modes. For example, the
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Fig. 9. SEM micrographs of the hollow fiber membrane at the room temperature zone after high-purity oxygen production from ambient air. (A) Top view; (B) cross-section; (C) higher magnification of cross-section and (D) outer surface.
Fig. 10. SEM micrographs of the hollow fiber membrane at the high temperature zone after high-purity oxygen production from ambient air. (A) Top view; (B) cross-section; (C) higher magnification of cross-section and (D) inner surface.
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Fig. 11. SEM micrographs of the dead-end of hollow fiber membrane at the high temperature zone after high-purity oxygen production from ambient air. (A) Side view; (B) outer surface of dead end; (C)–(F) higher magnification of outer surface of dead end; (G) and (H) cross-section near outer surface.
oxygen concentration in the effluent gas is only 12.7% through the two ends-open hollow fiber membrane with the oxygen permeation flux of 1.65 ml/min cm2 when 20 ml/min He was used as the sweep gas. On the contrary, the pressure difference between the two sides of the one end-dead hollow fiber membrane is as high as 0.99 atm due to the vacuum pump without any inert sweep gas. The oxygen partial pressure on the core side here is only
0.00997 atm, which is much lower than that of the conventional two ends-open hollow fiber membrane [22,25]. Therefore, the oxygen permeation driving force of the one end-dead hollow fiber membrane is 0.20 atm, which is much higher than other driving forces in those sweeping-models. As a result, the oxygen purity in the effluent gas is as high as 99.7% in the vacuum-mode with the oxygen permeation flux of 1.74 ml/min cm2 . After comparison
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Fig. 12. Oxygen permeation modes of two ends-open hollow fiber and one end-dead hollow fiber membranes.
Table 3 Comparison of oxygen permeation fluxes through BSCF membranes with different modes. Mode
Sweeping-mode
Vacuum-mode
Geometry of membrane
Disk
Tubular
Linear hollow fiber
U-shaped hollow fiber
One end-dead hollow fiber
Fsweep gas (ml/min) Thickness (mm) Temperature (◦ C) JO2 (ml/min cm2 ) CO2 (%) CHe (%) Reference
30 1.76 950 1.4 4.0 96.0 [21]
60 1.7 875 1.27 5.4 94.6 [26]
20 0.22 950 1.65 12.7 87.3 [25]
20 0.23 950 1.58 12.9 87.1 [22]
– 0.28 950 1.74 99.7 – Present work
of the oxygen purities and the oxygen permeation fluxes through BSCF membranes with different geometries in two modes, it can be noted that high-purity oxygen can be obtained in one step through the one end-dead hollow fiber membrane by vacuum at high temperature, which is also beneficial to the industrial application for high-purity oxygen production.
in future, the one end-dead hollow fiber membrane still exhibits obvious advantages. Under a big oxygen permeation driving force generated by vacuum, the pure oxygen can be produced in one step through the one end-dead hollow fiber membrane without any inert gas. In other words, the second separation is not necessary any more, which is beneficial to the industrial application for high-purity oxygen production.
4. Conclusion Acknowledgments The dense one end-dead perovskite hollow fiber membranes based on Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−ı (BSCF) are prepared by a phase inversion spinning process. It is used for the high-purity oxygen production from ambient air by vacuum. A high oxygen purity up to 99.7 vol % and high oxygen permeation rate of 1.74 ml/min cm2 are obtained after about 2-h activation in the initial stage at 950 ◦ C under the vacuum degree of 99 kPa on the core side and ambient air of 1 atm on the feed side. However, the hollow fiber membrane cracked after 24-h oxygen permeation in the ambient air. The XRD, SEM and EDS results show that carbonate appears in the phase of the spent hollow fiber membrane due to the reaction between the alkaline earth metal in the membrane and CO2 in the ambient air. The segregation of the constituent membrane elements, such as cobalt, is observed after the oxygen permeation. Although the stability of the one end-dead BSCF hollow fiber membrane for the oxygen production from ambient air should be improved
The authors greatly acknowledge the financial support by Natural Science Foundation of China (Nos. 21176087, U0834004 and 20936001), the National Basic Research Program of China (No. 2009CB623406), the Science-Technology Plan of Guangzhou City (2009J1-C511-1) and the Fundamental Research Funds for the Central Universities, SCUT (2009220038). References [1] H.J.M. Bouwmeester, A.J. Burggraff, Dense ceramic membranes for oxygen separation, in: A.J. Burggraff, L. Cot (Eds.), Fundamentals of Inorganic Membrane Science and Technology, Elsevier Science B.V., 1996, p. 435. [2] R.V. Jasra, N.V. Choudary, S.G.T. Baht, Separation of gases by pressure swing adsorption, Sep. Sci. Technol. 16 (1991) 885. [3] S.L. Matson, W.J. Ward, S.G. Kimure, W.R. Browall, Membrane oxygen enrichment: II. Economic assessment, J. Membr. Sci. 29 (1986) 79.
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