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CO2-tolerant mixed-conducting multichannel hollow fiber membrane for efficient oxygen separation
Journal of Membrane Science 485 (2015) 79–86
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CO2-tolerant mixed-conducting multichannel hollow fiber membrane for efficient oxygen separation Jiawei Zhu, Shaobin Guo, Zhicheng Zhang, Xin Jiang, Zhengkun Liu, Wanqin Jin n State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 November 2014 Received in revised form 11 February 2015 Accepted 19 February 2015 Available online 2 March 2015
A mixed-conducting multichannel hollow fiber membrane (MCMHF) based on CO2-tolerant SrFe0.8Nb0.2O3–δ (SFN) oxide has been successfully prepared by phase inversion and sintering technique. The morphology, breaking load, oxygen permeability under CO2 atmosphere, thermal cycling performance and the long-term stability of the membrane were investigated in detail. The prepared membrane has a significantly high oxygen permeation flux, which is the highest value among the membranes using pure CO2 as sweep gas. The as-prepared MCMHF membrane has good thermal–mechanical stability in the repeated heating and cooling process. Furthermore, by using pure CO2 as sweep gas, the MCMHF membrane is operated stably for more than 500 h without any degradation of oxygen permeation flux. Our work demonstrates that the MCMHF membrane has a great potential for the practical application in the oxyfuel process for CO2 capture technology. & 2015 Elsevier B.V. All rights reserved.
Keywords: Mixed-conducting membrane Multichannel hollow fiber membrane Oxygen separation CO2-tolerant membrane
1. Introduction Because of the emission of large amount of CO2, power generation from fossil fuels has resulted in global warming [1]. Therefore, the effective measures should be taken to reduce the emission of CO2 into atmosphere to mitigate environmental problems. There are three main routes for mitigation of CO2 emission in power plants: post-combustion process, procombustion process and oxyfuel process [2,3]. Due to the high thermal efficiency and low capital cost, the oxyfuel process has rapidly attracted a lot of attentions [3,4]. The pure oxygen used in the oxyfuel process can be produced by the cryogenic technique. However, mixed-conducting membranes [5–8], which are promising candidates for oxygen separation from air with 100% selectivity, have attracted considerable attentions due to their potential application in oxygen supply to power plant for CO2 capture according to the oxyfuel process [9–11]. Compared to the cryogenic process, oxygen production by mixed-conducting membrane displays significant advantages, such as saving sixty percent of energy consumption and reducing thirty-five percent of cost [12,13]. The mixed-conducting membranes applied in the oxyfuel process should meet the following requirements: high oxygen permeation flux as well as good stability under high concentration
n
Corresponding author. Tel.: þ 86 25 83172266; fax: þ86 25 83172292. E-mail address: [email protected] (W. Jin).
http://dx.doi.org/10.1016/j.memsci.2015.02.034 0376-7388/& 2015 Elsevier B.V. All rights reserved.
CO2 atmosphere and attractive membrane configuration for industrial application. In order to meet these requirements, many research efforts are not only focused on advanced membrane materials with high performance, but also on optimization of the membrane configuration. In general, there are three main types of membrane configurations for mixed-conducting membranes: disk or plate, tube and hollow fiber. In most previous work, disk-shaped membranes were used for the oxyfuel process because of simplified fabrication by a pressing method [14,15]. However, disk-shaped membranes with very small areas have sealing and connection problems at elevated temperature [16]. Tubular membranes, prepared by the paste extrusion method, have small surface/volume ratios and thick membrane walls causing low oxygen productivity and these properties restrict them in practical applications [8,17,18]. Alternatively, hollow fiber membranes fabricated by phase inversion and sintering technique possess a thin wall and an asymmetric structure and they can obtain high oxygen permeation flux [19– 22]. Therefore, the configuration of hollow fiber is considered as the most promising one for the future industrial application in the oxyfuel process. Chen and coworkers [23] reported that the oxygen permeation flux of the Zr0.84Y0.16O1.92–La0.8Sr0.2MnO3 δ (YSZ–LSM) linear single-channel hollow fiber (SHF) membrane was about 0.28 ml min 1 cm 2 at 1223 K sweeping by high concentration CO2 and the membrane showed good stability. Wang et al. [21] stated that the linear SHF membrane had the sealing problem at varying temperatures. When both ends of the
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linear hollow fiber were tightly fixed on the outer host tubes at room temperature, the fiber would be broken due to the expansion at high temperature. Therefore, they proposed a U-shaped hollow fiber membrane configuration to solve this problem for oxyfuel application [24]. However, the SHF membrane for the oxyfuel process may still suffer from low mechanical strength because of thin membrane walls with a lot of finger or sponge-like pores [25,26]. In our recent work [27], a mixed-conducting multichannel hollow fiber (MCMHF) membrane with high oxygen permeation flux and superb mechanical strength was proposed to overcome some drawbacks of conventional SHF membranes. So far, this membrane configuration has not been used for oxygen separation in the oxyfuel process. Therefore, the aim of this work is to develop a MCMHF membrane with high performance to accelerate the industrial application of mixed-conducting hollow fiber membrane for oxygen separation in the oxyfuel process. By using phase inversion and sintering technique, the MCMHF membrane has been prepared and this membrane possessing good mechanical strength can be expected to overcome some drawbacks of SHF membranes. The morphology, breaking load, oxygen permeability under CO2 atmosphere, thermal cycling performance and the long-term stability of the membrane were investigated in detail.
2. Experimental 2.1. Synthesis of powder The SrFe0.8Nb0.2O3–δ (SFN) oxide with excellent CO2 stability and a high and steady oxygen flux under pure CO2 atmosphere [14] was selected to construct the MCMHF membrane. SFN powder was synthesized by the conventional solid-state reaction method. Stoichiometric amounts of SrCO3 (99.9%), Fe2O3 (99.9%) and Nb2O5 (99.9%) were mixed and ball-milled in ethanol for 24 h and then thermally treated at 523 K. The SFN oxide was formed after a further calcination at 1563 K for 10 h in air. And then the SFN powder was grinded and sieved (300 meshes) for preparing MCMHF membrane. 2.2. Preparation of MCMHF membrane The SFN MCMHF membrane was prepared by a combined phase inversion and sintering technique that was reported in our previous report [27]. The spinning suspension was composed of polyethersulfone (PESf), 1-methyl-2-pyrrolidinone (NMP) and SFN powder in the mass ratio of 1:4:8. A tetra-bore spinneret with orifice diameter of 4.8 mm (the diameter of the four bores is 1.2 mm) was used to obtain the precursor. Deionized water was used as the internal and external coagulant. Subsequently, the SFN hollow fiber precursor was dried and sintered at 1613 K in air atmosphere for 10 h. The preparation conditions of obtaining the MCMHF membrane are summarized in Table 1. 2.3. Characterizations The crystal phases of the SFN powder and the SFN MCMHF membrane samples were characterized by X-ray diffraction (XRD, Bruker, model D8 Advance) using Cu Kα radiation. The diffraction patterns were collected at room temperature by step scanning at an increment of 0.051 in the range of 201 r2θ r 801. Morphology and microstructure of the MCMHF membranes were visually observed by using a scanning electron microscope (SEM, Hitachi S-4800, Japan). The mechanical strength of the MCMHF membrane was measured through a three-point bending test performed using a tensile tester (Model CMT6203) provided with a load cell
Table 1 Preparation membranes.
parameters
for
the
Parameter Composition of the spinning suspension SFN powder PESf NMP Spinning temperature Injection rate of internal coagulant Injection rate of suspension Air gap Sintering temperature Sintering time
MCMHF
Value
61.5 wt% 7.7 wt% 30.8 wt% 293 K 20 ml/ min 20 ml/ min 10 cm 1613 K 10 h
for 5 kN. The hollow-fiber samples were fixed in the sample holder with a spec gauge length of 50 mm. The crosshead speed was set at 0.02 cm min 1. The load (Fm) needed to break the hollow fiber was recorded for each sample. The gas tightness of membrane was checked by the nitrogen gas-tight test at room temperature [28]. A MCMHF membrane was first placed into a module which was similar to the membrane module used for oxygen permeation measurements. The nitrogen with an absolute pressure up to 2 atm was feed into lumen side of the membrane. If the membrane was gastight, the permeation of nitrogen was not detected at shell side of the membrane.
2.4. Oxygen permeation measurement The measurement of oxygen permeation fluxes of the dense MCMHF membranes without any defects was performed by using an apparatus shown in our previous work [28]. A MCMHF membrane with the length of about 50 mm was sealed with the two dense alumina tubes. A quartz tube around the two alumina tubes formed the shell side of the membrane. The shell side of the membrane was exposed to air or the mixture of CO2, O2 and N2 and the lumen side was swept by helium or CO2 or the mixture of helium and CO2. The inlet gas flow rates were controlled by mass flow controllers (model D07-19B, Beijing Jianzhong Machine Factory, China). Both sides of the membrane were maintained at the atmospheric pressure. A programmable temperature controller (Model AI-708 PA, Xiamen Yudian automation technology Co., Ltd.) monitored the temperature surrounding the membrane. The composition of effluent streams from the sweep side was analyzed by an on-line gas chromatograph (GC,Shimadzu, model GC-8A, Japan). The leakage of oxygen due to the impact sealing was usually less than 0.5% of the total oxygen permeation flux during the experiments. Assuming that the leakage of nitrogen and oxygen through pores or cracks in accordance with Knudsen diffusion, [21,28] the fluxes of leaked nitrogen and oxygen are related by J nitrogen : J oxygen ¼ ð32=28Þ0:5 79=21 ¼ 4:02. Thus, the oxygen permeation flux was calculated as follows: J O2 ¼ ðC oxygen C nitrogen =4:02Þ Q =A
ð1Þ
where C oxygen and C nitrogen are the concentration of oxygen and nitrogen calculated from the GC measurements, respectively; Q is the flow rate of the permeate gas stream; According to our previous work [27], A (area of the MCMHF membrane) is simply defined as the membrane outer surface area due to the complex structure of the MCMHF membrane and it is about 2.4 cm2 (The effective length of the membrane is about 3.1 cm) in this work.
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3. Results and discussion 3.1. Crystalline structure of the MCMHF membrane The XRD patterns of the as-synthesized SFN powder and the asprepared MCMHF membrane (ground into powder for analysis) are shown in Fig. 1(a) and (b). XRD analysis showed that both the samples of SFN powder and SFN membrane have well-formed perovskite crystalline structure. To test the stability of the SFN membrane
Fig. 1. XRD patterns of SFN samples. (a) Fresh powder; (b) fresh membrane; (c) powder annealed in pure CO2 at1173 K for 400 h; and (d) after long-term operation with pure CO2 as sweep gas at 1173 K for about 500 h.
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material under CO2 atmosphere, we exposed the as-prepared SFN powder to pure CO2 atmosphere at 1173 K and annealed for 400 h. As can be seen in Fig. 1(c), both the phase composition and the perovskite structure of the SFN powder still maintained and no carbonate formation can be found after pure CO2 treatment. Therefore, SFN oxide has high chemical stability under pure CO2 environment. Similar result has been reported by Yi et al. [14]
3.2. Morphology of MCMHF membrane The morphologies of the MCMHF sintered membrane are shown in Fig. 2. The sintered membrane had well-formed tetrabore structure. Fig. 2(a) shows that the outer and inner diameters of the sintered membrane were about 2.5 mm and 0.7 mm, respectively. Thus, the MCMHF membrane can provide a large membrane area per unit packing volume of 1600 m2 m 3. The wall structures of the membrane are displayed in Fig. 2(b). Finger like pores were near the outer and inner wall of the membrane and sponge-like structures appeared at the center wall of the membrane. The formation of the wall structures was tied to the complicated interactions among solvent, non-solvent and binder during the phase inversion process. In detail, the appearance of the wall structures can be attributed to the rapid precipitation that occurred at both the inner and outer walls of the membrane, resulting in formation of finger like pores and slow precipitation giving the sponge-like structure at the center wall of the fiber. The
Fig. 2. SEM images of as-prepared MCMHF membrane. (a) Cross section of the fresh membrane; (b) the wall of the fresh membrane; (c) outer surface of the fresh membrane; (d) inner surface of the fresh membrane; (e) outer surface of the spent membrane; and (f) inner surface of the spent membrane.
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membrane is dense and gas tight because the very small pores in the cross section of the membrane shown in Fig. 2(b) were not connected. The thickness of the membrane dense layer is about 100 μm clearly shown in Fig. 2(b). Fig. 2(c) and (d) gives the outer surface and inner surface of the sintered MCMHF membrane, respectively. The dense structure on the membrane outer was formed upon sintering. Although there are some pores still on the inner surfaces, they do not affect the gas-tightness property of the membrane. 3.3. Mechanical property of the membrane The mechanical strength or breaking load of the mixed conducting hollow fiber membranes is a significant parameter for their practical application. In this work, the breaking load of the MCMHF membrane was measured by a three-point bending test which was a common method to characterize the mechanical strength of the ceramic membrane. By the three-point bending test, we obtained that the as-prepared MCMHF membrane can withstand a big breaking load of 2170.8 N, which about 20 times that of SHF membranes [22,25,26]. For ceramic hollow fiber membranes, the cross section structure may play one of the most important roles for obtaining an excellent value of breaking load. The superb breaking load value of the MCMHF membrane mainly can be attributed to the special crucifix-shape structure shown in Fig. 2(a) which plays a supporting role in bearing the force applied on the MCMHF membrane, although the breaking load of the membrane may have something to do with the fabrication conditions of the membrane and the properties of the membrane material. Therefore, this excellent mechanical strength can make the SFN MCMHF membrane be a promising candidate for industrial application. 3.4. Effect of operating conditions on oxygen permeability of MCMHF membrane In general, the oxygen permeation fluxes of the membrane for oxygen separation in the oxyfuel process are affected by operating conditions such as temperature, air flow rate and CO2 flow rate, the concentration of CO2 in feed gas and sweeping gas, oxygen partial pressure in feed gas and so on. The effects of these operating conditions on the MCMHF membrane were investigated systematically as follows. 3.4.1. Effect of temperature Fig. 3 shows the temperature dependence of the oxygen permeation flux of MCMHF membrane with pure He and CO2 as sweep gas. Each temperature point was held for about 30 min to steady the oxygen permeation flux and the corresponding data points were
Fig. 3. Oxygen permeation fluxes of MCMHF membrane as a function of temperature with pure helium and CO2 as sweep gases (FAir ¼120 ml min 1, FHe or FCO2 ¼80 ml min 1).
recorded at least three times to ensure the accuracy of the experimental data. As known to all, temperature has a significant influence on oxygen permeation flux. The oxygen permeation fluxes were measured in the temperature range of 1073–1173 K. For both the helium and pure CO2 as sweep gases, the oxygen permeation flux increases with increasing temperature, attributed to the improvement of the oxygen bulk diffusion and the oxygen surface reaction rates. When using helium as sweep gas, the oxygen permeation flux increases from 0.52 to 1.24 ml min 1 cm 2 by increasing the temperatures from 1073 to 1173 K. Whereas, when using pure CO2 as sweep gas, the oxygen permeation flux increases from 0.38 to 1.12 ml min 1 cm 2 by increasing the temperatures. Therefore, the experimental finding is that the oxygen permeation flux for using pure CO2 as sweep gas is only slightly lower than that for using helium as sweep gas, which is in good agreement with previous work [14,24]. The reason for this experimental phenomenon is that CO2 has stronger adsorptive interaction with the membrane surface in comparison with helium, because the effect of helium on oxygen exchange reaction is less than that of CO2. For a disk SFN membrane, Yi et al. [14] reported that oxygen permeation fluxes of the membrane are about 0.25 ml min 1 cm 2 at 1173 K with pure CO2 as sweep gas. Nevertheless, the oxygen permeation flux of SFN MCMHF membrane is about 1.12 ml min 1 cm 2 at 1173 K under the similar conditions, which is the highest value among the CO2-stable membranes based on different materials and configurations at 1173 K. The possible reasons could be the following: the wall of the MCMHF membrane is much thinner than that of the symmetric disk membrane. Moreover, MCMHF membrane had a porous-dense asymmetrical microstructure, whereas the symmetric disk membrane reported by Yi et al. was totally dense. 3.4.2. Effect of CO2 concentration in feed gas and sweep gas It was found that the greenhouse gas CO2 in the sweep gas or in the feed gas not only can lower the oxygen permeation flux of the membrane but also has a negative effect on the membrane stability because of carbonate formation. For instance, Arnold et al. [29] reported that the oxygen permeation flux of BSCF membrane decreased about 17% by introducing 5% CO2 into the sweep gas at 1148 K. And they also found that the oxygen permeation flux deceased by increasing the percentage of CO2 mixed in air. Therefore, to investigate the influence of CO2 in both the feed and sweep gases on oxygen permeation flux of the MCMHF membrane is very necessary for CO2 capture by the oxyfuel process. Fig. 4 shows the oxygen permeation flux of SFN membrane as a function of concentration of CO2 in the feed gas at different temperatures. The oxygen partial pressure was always kept at 0.21 atm and the total flux of feed gas was fixed at 180 ml min 1. As shown in Fig. 4, when the concentration of CO2 in the feed gas increases, only a slight decrease in the oxygen permeation flux can be found. Fig. 5 shows the effect of the concentration of CO2 in sweep gas on the oxygen permeation flux at different temperatures. When the concentration of CO2 in the sweep gas increases from 0% to 100%, the oxygen permeation flux only decreases from 1.24 to 1.12 ml min 1 cm 2 at 1173 K. Therefore, the introduction of CO2 in feed or sweep gas has negligible influences on oxygen permeation fluxes of MCMHF membrane. These findings demonstrate that the MCMHF membrane has an excellent tolerance against CO2 atmosphere. 3.4.3. Effect of air flow rate Fig. 6 shows oxygen permeation flux as a function of air flow rate at different operating temperatures using pure CO2 as sweep gas. The flow rate of pure CO2 at the permeate side was fixed at 80 ml min 1. At 1173 K, the oxygen permeation flux increases with increasing air flow rate. It was found that the oxygen permeation
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Fig. 4. Oxygen permeation flux of SFN membrane as a function of concentration of CO2 in the feed gas at different temperatures (FHe ¼ 80 ml min 1).
Fig. 7. Effects of CO2 flow rates on oxygen permeation fluxes (a) and oxygen partial pressure on both sides of the MCMHF membrane (b) (Fair ¼ 120 ml min 1).
Fig. 5. Oxygen permeation flux of MCMHF membrane as a function of concentration of CO2 in the sweep gas at different temperatures (Fair ¼ 120 ml min 1).
Fig. 8. Dependence of the oxygen permeability on the oxygen partial pressure of the feed gas for MCMHF membrane.
Fig. 6. Effect of air flow rates on oxygen permeation fluxes of MCMHF membrane (FCO2 ¼ 80 ml min 1).
flux increased sharply up to an air flow rate of 120 ml min 1, while a further increase of air flow rate only slightly increases the oxygen permeation flux. When the operating temperature is at 1073 K, the oxygen permeation flux almost keeps constant as the air flow rate increases. These results indicate that the oxygen permeation flux is more sensitive to air flow rate at a higher operating temperature. To obtain high oxygen permeation flux of the MCMHF membrane, the feed air should be offered to operation unit abundantly.
3.4.4. Effect of pure CO2 flow rate The influence of the pure CO2 sweep flow rates at the permeate side on the oxygen permeation flux through the MCMHF membrane at different temperatures is shown in Fig. 7(a), while the air flow rate is kept constant of 120 ml min 1. The pure CO2 flow rate varied from 20 to 100 ml min 1 in this experiment. As expected, the oxygen permeation fluxes through the MCMHF membrane increased with increasing the pure CO2 flow rate. The following are the reasons for this phenomenon. As shown in Fig. 7(b), the oxygen partial pressure on the sweep side of the membrane decreases a lot with the increase of CO2 flow rate while the oxygen partial pressure on the feed side hardly changes. Therefore, the high CO2 flow rate increased the difference of oxygen partial pressure on both sides of the membrane. According to the Wagner equation [5], the increase of oxygen partial pressure difference caused by the increased CO2 flow rate can increase the driving force for oxygen transport and then increase the oxygen permeation flux. For instance, at 1173 K, when the CO2 flow rate increases
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from 20 to 100 ml min 1, the oxygen permeation flux increases from 0.76 to 1.15 ml min 1 cm 2.
3.4.5. Effect of oxygen partial pressure in feed gas Because compressed air is occasionally supplied as a feed gas for oxygen separation from air, the effect of oxygen partial pressure on oxygen permeation fluxes of the MCMHF membrane
is a crucial factor for practical application in the oxyfuel process. [30] Fig. 8 shows the effect of oxygen partial pressure at the feed side on oxygen permeation fluxes of the membrane sweeping by pure CO2 at 1173, 1123 and 1073 K. By increasing the oxygen partial pressure at the feed side, the oxygen fluxes dramatically increased because of the increase in the oxygen gradient across the membrane. For example, the oxygen permeation flux increases from 1.12 to 1.97 ml min 1 cm 2 with increase of oxygen partial pressure on the feed side from 0.21 to 0.8 atm at 1173 K. 3.5. Thermal cycling performance of MCMHF membrane
Fig. 9. Thermal cycling performance of MCMHF membrane between 1073 and 1173 K (FAir ¼ 120 ml min 1, FCO2 ¼ 80 ml min 1).
The thermal cycling performance of the membrane is of significance for industrial application in the oxyfuel process for CO2 capture technology because the practical operation process usually contains repeated heating and cooling process. Fig. 9 presents the thermal cycling performance of SFN MCMHF membrane. The three testing temperature points (1073 K, 1123 K and 1173 K) were kept for 50 min to steady the oxygen permeation flux. As can be seen from Fig. 9, the oxygen permeation flux is about 0.38 ml min 1 cm 2 at 1073 K, and it increases to 0.68 ml min 1 cm 2 at 1123 K, and then increases to 1.12 ml min 1 cm 2 at 1173 K. The oxygen permeation flux can be recovered when the temperature decreased to 1073 K. These thermal regeneration cycles can be repeated for five times in this work with no obvious decrease of oxygen permeation flux. This thermal cycling performance demonstrates that the SFN MCMHF membrane has good thermal–mechanical stability in the repeated heating and cooling process. 3.6. Long-term stability of MCMHF membrane
Fig. 10. Long-term oxygen permeation operation of MCMHF membrane with pure He or CO2 as sweep gases and with pure air or air contained 20% CO2 as feed gases at 1173 K.
To evaluate the CO2 stability of SFN MCMHF membrane, the oxygen permeation performance of the membrane was studied over 500 h using pure CO2 as sweep gas. Fig. 10 shows the long-term operation of MCMHF membrane. When helium was used as the sweep gas at 1173 K, the oxygen permeation flux reaches a steady value of 1.24 ml min 1 cm 2 after the two-hour activation process. When pure CO2 was used as sweep gas, the oxygen permeation flux slightly decreased to 1.12 ml min 1 cm 2. There is no obvious change about this value during the operation by using pure CO2 as sweep gas at 1173 K for more than 300 h. To further examine the stability of the
Table 2 Comparison of oxygen permeation fluxes and stabilities of various membranes swept by pure CO2. Membrane materials
Configurations
Thickness (mm)
Temperature (K)
O2 fluxes (ml min 1 cm 2)
Lifetime (h)
References
BaCo0.4Fe0.4Nb0.2O3 δ Ba0.5Sr0.5Fe0.8Zn0.2O3 δ Ba0.5Sr0.5Co0.8Fe0.2O3 δ Ce0.8Sm0.2O1.9–La0.7Ca0.3CrO3 Fe2O3 Ce0.9Gd0.1O2 δ NiFe2O4–Ce0.9Gd0.1O2-δ Ce0.8Sm0.2O2 δ La0.9Sr0.1FeO3 δ Pr0.6Sr0.4FeO3 δ Ce0.9Pr0.1O2 δ (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4 þ δ Ce0.8Sm0.2O1.9–La0.8Sr0.2MnO3 δ. Zr0.84Y0.16O1.92–La0.8Sr0.2MnO3 δ Ce0.9Gd0.1O2 δ–Ba0.5Sr0.5Co0.8Fe0.2O3 δ (Nd0.9La0.1)2Ni0.74Cu0.21Ga0.05O4 þ δ Ce0.8Sm0.2O1.9–SmMn0.5Co0.5O3 Ce0.8Sm0.2O1.9–Sm0.8Ca0.2Mn0.5Co0.5O3 Ce0.85Sm0.15O1.925–Sm0.6Sr0.4Al0.3Fe0.7O3 (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4 þ δ Nd0.6Sr0.4FeO3 δ–Ce0.9Nd0.1O2 δ Pr0.6Sr0.4Fe0.5Co0.5O3-δ Ce0.9Pr0.1O2 δ NiFe2O4 Ce0.8Tb0.2O2 δ La0.6Sr0.4Co0.2Fe0.8O3 δ SrFe0.8Nb0.2O3-δ SrFe0.8Nb0.2O3-δ
Disk Disk Disk Disk Disk Disk Disk Disk Disk Hollow fiber Hollow fiber Disk Disk Disk Disk Disk Hollow fiber Disk Disk Disk Hollow fiber Disk MCMHF
1 1.15 1 1.0 0.5 0.5 1.1 0.6 0.8 0.3 0.16 0.5 0.6 0.5 0.5 0.5 0.2 0.6 0.5 0.68 0.36 1 0.25
1173 1023 1148 1173 1223 1173 1173 1223 1173 1173 1223 1173 1173 1173 1173 1223 1173 1223 1223 1173 1173 1173 1173
0 0 0 0.04 0.08 0.12 0.14 0.18 0.24 0.26 0.29 0.3 0.3 0.36 0.34 0.37 0.38 0.21 0.7 0.11 0.2 0.25 1.05–1.1
150 100 150 150 230 170 250 420 300 300 400 310 4150 400 76 20 210 4500
[31] [32] [29] [33] [34] [10] [9] [35] [36] [37] [23] [38] [39] [11] [11] [40] [24] [41] [42] [43] [44] [14] This work
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membrane in more rigorous condition, the oxygen permeation flux was investigated by using 20% CO2 contained air as feed gas and pure CO2 as sweep gas. When 20% CO2 was introduced into the feed gas, the oxygen permeation flux was about 1.06 ml min 1 cm 2 and maintained constant for more than 200 h. The crystalline structure of the membrane sample after long-term operation was characterized by XRD as shown in Fig. 1(d). The XRD patterns indicated that the SFN membrane sample after long-term operation has pure cubic perovskite structure and no carbonate has been formed. We also studied the membrane surface morphology of the membrane sample after longterm operation by SEM as shown in Fig. 2(e) and (f). In compared with the fresh membrane surface, no significant degradation of the surface microstructure can be observed. All of the above, similar to that reported by Yi et al. [14], indicated that there was no obvious phase structure change of the SFN MCMHF membrane after long-term operation and the membrane exhibited an excellent stability in CO2 environment. Our work demonstrated that the SFN MCMHF membrane can be a competitive candidate for oxygen separation in the oxyfuel process. The oxygen permeation fluxes and stability of various membranes based on different membrane materials and membrane configurations in CO2-contained atmosphere are summarized in Table 2. Comparing with other membranes, the as-prepared MCMHF membrane possesses an outstanding performance in items of oxygen permeability and stability overall. Therefore, we can conclude that the SFN MCMHF membrane is competent for oxyfuel techniques and CO2 capture process.
4. Conclusions By using phase inversion and sintering technique, a CO2tolerant MCMHF membrane with ultra-high oxygen permeation flux and superb mechanical strength and excellent stability under CO2-contianed atmosphere is successfully prepared. The breaking load of the prepared MCMHF membrane is very much higher than that of SHF membranes, which is very helpful to overcome some drawbacks of SHF membranes. The oxygen permeation flux of the MCMHF membrane is higher than that of SFN disk membrane and reaches 1.12 ml min 1 cm 2 at 1173 K, which is the highest value among the membranes based on different materials and configurations reported in the literatures. The thermal cycling performance of MCMHF membranes between 1073 and 1173 K presents that the MCMHF membrane has good thermal–mechanical stability in the thermal cycling process. Furthermore, the MCMHF membrane is operated stably for more than 500 h by using pure CO2 as sweep gas without any degradation of oxygen permeation flux. Our work demonstrates that the MCMHF membrane has a great potential for the practical application in the oxyfuel process for CO2 capture technology.
Acknowledgments This work was supported by the Innovative Research Team Program by the Ministry of Education of China (No. IRT13070) and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] R.S. Haszeldine, Carbon capture and storage: how green can black be? Science 325 (2009) 1647–1652. [2] J.D. Figueroa, T. Fout, S. Plasynski, H. McIlvried, R.D. Srivastava, Advances in CO2 capture technology—the US department of energy's carbon sequestration program, Int. J. Greenh. Gas Control 2 (2008) 9–20.
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
CO2-tolerant mixed-conducting multichannel hollow fiber membrane for efficient oxygen separation Jiawei Zhu, Shaobin Guo, Zhicheng Zhang, Xin Jiang, Zhengkun Liu, Wanqin Jin n State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 November 2014 Received in revised form 11 February 2015 Accepted 19 February 2015 Available online 2 March 2015
A mixed-conducting multichannel hollow fiber membrane (MCMHF) based on CO2-tolerant SrFe0.8Nb0.2O3–δ (SFN) oxide has been successfully prepared by phase inversion and sintering technique. The morphology, breaking load, oxygen permeability under CO2 atmosphere, thermal cycling performance and the long-term stability of the membrane were investigated in detail. The prepared membrane has a significantly high oxygen permeation flux, which is the highest value among the membranes using pure CO2 as sweep gas. The as-prepared MCMHF membrane has good thermal–mechanical stability in the repeated heating and cooling process. Furthermore, by using pure CO2 as sweep gas, the MCMHF membrane is operated stably for more than 500 h without any degradation of oxygen permeation flux. Our work demonstrates that the MCMHF membrane has a great potential for the practical application in the oxyfuel process for CO2 capture technology. & 2015 Elsevier B.V. All rights reserved.
Keywords: Mixed-conducting membrane Multichannel hollow fiber membrane Oxygen separation CO2-tolerant membrane
1. Introduction Because of the emission of large amount of CO2, power generation from fossil fuels has resulted in global warming [1]. Therefore, the effective measures should be taken to reduce the emission of CO2 into atmosphere to mitigate environmental problems. There are three main routes for mitigation of CO2 emission in power plants: post-combustion process, procombustion process and oxyfuel process [2,3]. Due to the high thermal efficiency and low capital cost, the oxyfuel process has rapidly attracted a lot of attentions [3,4]. The pure oxygen used in the oxyfuel process can be produced by the cryogenic technique. However, mixed-conducting membranes [5–8], which are promising candidates for oxygen separation from air with 100% selectivity, have attracted considerable attentions due to their potential application in oxygen supply to power plant for CO2 capture according to the oxyfuel process [9–11]. Compared to the cryogenic process, oxygen production by mixed-conducting membrane displays significant advantages, such as saving sixty percent of energy consumption and reducing thirty-five percent of cost [12,13]. The mixed-conducting membranes applied in the oxyfuel process should meet the following requirements: high oxygen permeation flux as well as good stability under high concentration
n
Corresponding author. Tel.: þ 86 25 83172266; fax: þ86 25 83172292. E-mail address: [email protected] (W. Jin).
http://dx.doi.org/10.1016/j.memsci.2015.02.034 0376-7388/& 2015 Elsevier B.V. All rights reserved.
CO2 atmosphere and attractive membrane configuration for industrial application. In order to meet these requirements, many research efforts are not only focused on advanced membrane materials with high performance, but also on optimization of the membrane configuration. In general, there are three main types of membrane configurations for mixed-conducting membranes: disk or plate, tube and hollow fiber. In most previous work, disk-shaped membranes were used for the oxyfuel process because of simplified fabrication by a pressing method [14,15]. However, disk-shaped membranes with very small areas have sealing and connection problems at elevated temperature [16]. Tubular membranes, prepared by the paste extrusion method, have small surface/volume ratios and thick membrane walls causing low oxygen productivity and these properties restrict them in practical applications [8,17,18]. Alternatively, hollow fiber membranes fabricated by phase inversion and sintering technique possess a thin wall and an asymmetric structure and they can obtain high oxygen permeation flux [19– 22]. Therefore, the configuration of hollow fiber is considered as the most promising one for the future industrial application in the oxyfuel process. Chen and coworkers [23] reported that the oxygen permeation flux of the Zr0.84Y0.16O1.92–La0.8Sr0.2MnO3 δ (YSZ–LSM) linear single-channel hollow fiber (SHF) membrane was about 0.28 ml min 1 cm 2 at 1223 K sweeping by high concentration CO2 and the membrane showed good stability. Wang et al. [21] stated that the linear SHF membrane had the sealing problem at varying temperatures. When both ends of the
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linear hollow fiber were tightly fixed on the outer host tubes at room temperature, the fiber would be broken due to the expansion at high temperature. Therefore, they proposed a U-shaped hollow fiber membrane configuration to solve this problem for oxyfuel application [24]. However, the SHF membrane for the oxyfuel process may still suffer from low mechanical strength because of thin membrane walls with a lot of finger or sponge-like pores [25,26]. In our recent work [27], a mixed-conducting multichannel hollow fiber (MCMHF) membrane with high oxygen permeation flux and superb mechanical strength was proposed to overcome some drawbacks of conventional SHF membranes. So far, this membrane configuration has not been used for oxygen separation in the oxyfuel process. Therefore, the aim of this work is to develop a MCMHF membrane with high performance to accelerate the industrial application of mixed-conducting hollow fiber membrane for oxygen separation in the oxyfuel process. By using phase inversion and sintering technique, the MCMHF membrane has been prepared and this membrane possessing good mechanical strength can be expected to overcome some drawbacks of SHF membranes. The morphology, breaking load, oxygen permeability under CO2 atmosphere, thermal cycling performance and the long-term stability of the membrane were investigated in detail.
2. Experimental 2.1. Synthesis of powder The SrFe0.8Nb0.2O3–δ (SFN) oxide with excellent CO2 stability and a high and steady oxygen flux under pure CO2 atmosphere [14] was selected to construct the MCMHF membrane. SFN powder was synthesized by the conventional solid-state reaction method. Stoichiometric amounts of SrCO3 (99.9%), Fe2O3 (99.9%) and Nb2O5 (99.9%) were mixed and ball-milled in ethanol for 24 h and then thermally treated at 523 K. The SFN oxide was formed after a further calcination at 1563 K for 10 h in air. And then the SFN powder was grinded and sieved (300 meshes) for preparing MCMHF membrane. 2.2. Preparation of MCMHF membrane The SFN MCMHF membrane was prepared by a combined phase inversion and sintering technique that was reported in our previous report [27]. The spinning suspension was composed of polyethersulfone (PESf), 1-methyl-2-pyrrolidinone (NMP) and SFN powder in the mass ratio of 1:4:8. A tetra-bore spinneret with orifice diameter of 4.8 mm (the diameter of the four bores is 1.2 mm) was used to obtain the precursor. Deionized water was used as the internal and external coagulant. Subsequently, the SFN hollow fiber precursor was dried and sintered at 1613 K in air atmosphere for 10 h. The preparation conditions of obtaining the MCMHF membrane are summarized in Table 1. 2.3. Characterizations The crystal phases of the SFN powder and the SFN MCMHF membrane samples were characterized by X-ray diffraction (XRD, Bruker, model D8 Advance) using Cu Kα radiation. The diffraction patterns were collected at room temperature by step scanning at an increment of 0.051 in the range of 201 r2θ r 801. Morphology and microstructure of the MCMHF membranes were visually observed by using a scanning electron microscope (SEM, Hitachi S-4800, Japan). The mechanical strength of the MCMHF membrane was measured through a three-point bending test performed using a tensile tester (Model CMT6203) provided with a load cell
Table 1 Preparation membranes.
parameters
for
the
Parameter Composition of the spinning suspension SFN powder PESf NMP Spinning temperature Injection rate of internal coagulant Injection rate of suspension Air gap Sintering temperature Sintering time
MCMHF
Value
61.5 wt% 7.7 wt% 30.8 wt% 293 K 20 ml/ min 20 ml/ min 10 cm 1613 K 10 h
for 5 kN. The hollow-fiber samples were fixed in the sample holder with a spec gauge length of 50 mm. The crosshead speed was set at 0.02 cm min 1. The load (Fm) needed to break the hollow fiber was recorded for each sample. The gas tightness of membrane was checked by the nitrogen gas-tight test at room temperature [28]. A MCMHF membrane was first placed into a module which was similar to the membrane module used for oxygen permeation measurements. The nitrogen with an absolute pressure up to 2 atm was feed into lumen side of the membrane. If the membrane was gastight, the permeation of nitrogen was not detected at shell side of the membrane.
2.4. Oxygen permeation measurement The measurement of oxygen permeation fluxes of the dense MCMHF membranes without any defects was performed by using an apparatus shown in our previous work [28]. A MCMHF membrane with the length of about 50 mm was sealed with the two dense alumina tubes. A quartz tube around the two alumina tubes formed the shell side of the membrane. The shell side of the membrane was exposed to air or the mixture of CO2, O2 and N2 and the lumen side was swept by helium or CO2 or the mixture of helium and CO2. The inlet gas flow rates were controlled by mass flow controllers (model D07-19B, Beijing Jianzhong Machine Factory, China). Both sides of the membrane were maintained at the atmospheric pressure. A programmable temperature controller (Model AI-708 PA, Xiamen Yudian automation technology Co., Ltd.) monitored the temperature surrounding the membrane. The composition of effluent streams from the sweep side was analyzed by an on-line gas chromatograph (GC,Shimadzu, model GC-8A, Japan). The leakage of oxygen due to the impact sealing was usually less than 0.5% of the total oxygen permeation flux during the experiments. Assuming that the leakage of nitrogen and oxygen through pores or cracks in accordance with Knudsen diffusion, [21,28] the fluxes of leaked nitrogen and oxygen are related by J nitrogen : J oxygen ¼ ð32=28Þ0:5 79=21 ¼ 4:02. Thus, the oxygen permeation flux was calculated as follows: J O2 ¼ ðC oxygen C nitrogen =4:02Þ Q =A
ð1Þ
where C oxygen and C nitrogen are the concentration of oxygen and nitrogen calculated from the GC measurements, respectively; Q is the flow rate of the permeate gas stream; According to our previous work [27], A (area of the MCMHF membrane) is simply defined as the membrane outer surface area due to the complex structure of the MCMHF membrane and it is about 2.4 cm2 (The effective length of the membrane is about 3.1 cm) in this work.
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3. Results and discussion 3.1. Crystalline structure of the MCMHF membrane The XRD patterns of the as-synthesized SFN powder and the asprepared MCMHF membrane (ground into powder for analysis) are shown in Fig. 1(a) and (b). XRD analysis showed that both the samples of SFN powder and SFN membrane have well-formed perovskite crystalline structure. To test the stability of the SFN membrane
Fig. 1. XRD patterns of SFN samples. (a) Fresh powder; (b) fresh membrane; (c) powder annealed in pure CO2 at1173 K for 400 h; and (d) after long-term operation with pure CO2 as sweep gas at 1173 K for about 500 h.
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material under CO2 atmosphere, we exposed the as-prepared SFN powder to pure CO2 atmosphere at 1173 K and annealed for 400 h. As can be seen in Fig. 1(c), both the phase composition and the perovskite structure of the SFN powder still maintained and no carbonate formation can be found after pure CO2 treatment. Therefore, SFN oxide has high chemical stability under pure CO2 environment. Similar result has been reported by Yi et al. [14]
3.2. Morphology of MCMHF membrane The morphologies of the MCMHF sintered membrane are shown in Fig. 2. The sintered membrane had well-formed tetrabore structure. Fig. 2(a) shows that the outer and inner diameters of the sintered membrane were about 2.5 mm and 0.7 mm, respectively. Thus, the MCMHF membrane can provide a large membrane area per unit packing volume of 1600 m2 m 3. The wall structures of the membrane are displayed in Fig. 2(b). Finger like pores were near the outer and inner wall of the membrane and sponge-like structures appeared at the center wall of the membrane. The formation of the wall structures was tied to the complicated interactions among solvent, non-solvent and binder during the phase inversion process. In detail, the appearance of the wall structures can be attributed to the rapid precipitation that occurred at both the inner and outer walls of the membrane, resulting in formation of finger like pores and slow precipitation giving the sponge-like structure at the center wall of the fiber. The
Fig. 2. SEM images of as-prepared MCMHF membrane. (a) Cross section of the fresh membrane; (b) the wall of the fresh membrane; (c) outer surface of the fresh membrane; (d) inner surface of the fresh membrane; (e) outer surface of the spent membrane; and (f) inner surface of the spent membrane.
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membrane is dense and gas tight because the very small pores in the cross section of the membrane shown in Fig. 2(b) were not connected. The thickness of the membrane dense layer is about 100 μm clearly shown in Fig. 2(b). Fig. 2(c) and (d) gives the outer surface and inner surface of the sintered MCMHF membrane, respectively. The dense structure on the membrane outer was formed upon sintering. Although there are some pores still on the inner surfaces, they do not affect the gas-tightness property of the membrane. 3.3. Mechanical property of the membrane The mechanical strength or breaking load of the mixed conducting hollow fiber membranes is a significant parameter for their practical application. In this work, the breaking load of the MCMHF membrane was measured by a three-point bending test which was a common method to characterize the mechanical strength of the ceramic membrane. By the three-point bending test, we obtained that the as-prepared MCMHF membrane can withstand a big breaking load of 2170.8 N, which about 20 times that of SHF membranes [22,25,26]. For ceramic hollow fiber membranes, the cross section structure may play one of the most important roles for obtaining an excellent value of breaking load. The superb breaking load value of the MCMHF membrane mainly can be attributed to the special crucifix-shape structure shown in Fig. 2(a) which plays a supporting role in bearing the force applied on the MCMHF membrane, although the breaking load of the membrane may have something to do with the fabrication conditions of the membrane and the properties of the membrane material. Therefore, this excellent mechanical strength can make the SFN MCMHF membrane be a promising candidate for industrial application. 3.4. Effect of operating conditions on oxygen permeability of MCMHF membrane In general, the oxygen permeation fluxes of the membrane for oxygen separation in the oxyfuel process are affected by operating conditions such as temperature, air flow rate and CO2 flow rate, the concentration of CO2 in feed gas and sweeping gas, oxygen partial pressure in feed gas and so on. The effects of these operating conditions on the MCMHF membrane were investigated systematically as follows. 3.4.1. Effect of temperature Fig. 3 shows the temperature dependence of the oxygen permeation flux of MCMHF membrane with pure He and CO2 as sweep gas. Each temperature point was held for about 30 min to steady the oxygen permeation flux and the corresponding data points were
Fig. 3. Oxygen permeation fluxes of MCMHF membrane as a function of temperature with pure helium and CO2 as sweep gases (FAir ¼120 ml min 1, FHe or FCO2 ¼80 ml min 1).
recorded at least three times to ensure the accuracy of the experimental data. As known to all, temperature has a significant influence on oxygen permeation flux. The oxygen permeation fluxes were measured in the temperature range of 1073–1173 K. For both the helium and pure CO2 as sweep gases, the oxygen permeation flux increases with increasing temperature, attributed to the improvement of the oxygen bulk diffusion and the oxygen surface reaction rates. When using helium as sweep gas, the oxygen permeation flux increases from 0.52 to 1.24 ml min 1 cm 2 by increasing the temperatures from 1073 to 1173 K. Whereas, when using pure CO2 as sweep gas, the oxygen permeation flux increases from 0.38 to 1.12 ml min 1 cm 2 by increasing the temperatures. Therefore, the experimental finding is that the oxygen permeation flux for using pure CO2 as sweep gas is only slightly lower than that for using helium as sweep gas, which is in good agreement with previous work [14,24]. The reason for this experimental phenomenon is that CO2 has stronger adsorptive interaction with the membrane surface in comparison with helium, because the effect of helium on oxygen exchange reaction is less than that of CO2. For a disk SFN membrane, Yi et al. [14] reported that oxygen permeation fluxes of the membrane are about 0.25 ml min 1 cm 2 at 1173 K with pure CO2 as sweep gas. Nevertheless, the oxygen permeation flux of SFN MCMHF membrane is about 1.12 ml min 1 cm 2 at 1173 K under the similar conditions, which is the highest value among the CO2-stable membranes based on different materials and configurations at 1173 K. The possible reasons could be the following: the wall of the MCMHF membrane is much thinner than that of the symmetric disk membrane. Moreover, MCMHF membrane had a porous-dense asymmetrical microstructure, whereas the symmetric disk membrane reported by Yi et al. was totally dense. 3.4.2. Effect of CO2 concentration in feed gas and sweep gas It was found that the greenhouse gas CO2 in the sweep gas or in the feed gas not only can lower the oxygen permeation flux of the membrane but also has a negative effect on the membrane stability because of carbonate formation. For instance, Arnold et al. [29] reported that the oxygen permeation flux of BSCF membrane decreased about 17% by introducing 5% CO2 into the sweep gas at 1148 K. And they also found that the oxygen permeation flux deceased by increasing the percentage of CO2 mixed in air. Therefore, to investigate the influence of CO2 in both the feed and sweep gases on oxygen permeation flux of the MCMHF membrane is very necessary for CO2 capture by the oxyfuel process. Fig. 4 shows the oxygen permeation flux of SFN membrane as a function of concentration of CO2 in the feed gas at different temperatures. The oxygen partial pressure was always kept at 0.21 atm and the total flux of feed gas was fixed at 180 ml min 1. As shown in Fig. 4, when the concentration of CO2 in the feed gas increases, only a slight decrease in the oxygen permeation flux can be found. Fig. 5 shows the effect of the concentration of CO2 in sweep gas on the oxygen permeation flux at different temperatures. When the concentration of CO2 in the sweep gas increases from 0% to 100%, the oxygen permeation flux only decreases from 1.24 to 1.12 ml min 1 cm 2 at 1173 K. Therefore, the introduction of CO2 in feed or sweep gas has negligible influences on oxygen permeation fluxes of MCMHF membrane. These findings demonstrate that the MCMHF membrane has an excellent tolerance against CO2 atmosphere. 3.4.3. Effect of air flow rate Fig. 6 shows oxygen permeation flux as a function of air flow rate at different operating temperatures using pure CO2 as sweep gas. The flow rate of pure CO2 at the permeate side was fixed at 80 ml min 1. At 1173 K, the oxygen permeation flux increases with increasing air flow rate. It was found that the oxygen permeation
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Fig. 4. Oxygen permeation flux of SFN membrane as a function of concentration of CO2 in the feed gas at different temperatures (FHe ¼ 80 ml min 1).
Fig. 7. Effects of CO2 flow rates on oxygen permeation fluxes (a) and oxygen partial pressure on both sides of the MCMHF membrane (b) (Fair ¼ 120 ml min 1).
Fig. 5. Oxygen permeation flux of MCMHF membrane as a function of concentration of CO2 in the sweep gas at different temperatures (Fair ¼ 120 ml min 1).
Fig. 8. Dependence of the oxygen permeability on the oxygen partial pressure of the feed gas for MCMHF membrane.
Fig. 6. Effect of air flow rates on oxygen permeation fluxes of MCMHF membrane (FCO2 ¼ 80 ml min 1).
flux increased sharply up to an air flow rate of 120 ml min 1, while a further increase of air flow rate only slightly increases the oxygen permeation flux. When the operating temperature is at 1073 K, the oxygen permeation flux almost keeps constant as the air flow rate increases. These results indicate that the oxygen permeation flux is more sensitive to air flow rate at a higher operating temperature. To obtain high oxygen permeation flux of the MCMHF membrane, the feed air should be offered to operation unit abundantly.
3.4.4. Effect of pure CO2 flow rate The influence of the pure CO2 sweep flow rates at the permeate side on the oxygen permeation flux through the MCMHF membrane at different temperatures is shown in Fig. 7(a), while the air flow rate is kept constant of 120 ml min 1. The pure CO2 flow rate varied from 20 to 100 ml min 1 in this experiment. As expected, the oxygen permeation fluxes through the MCMHF membrane increased with increasing the pure CO2 flow rate. The following are the reasons for this phenomenon. As shown in Fig. 7(b), the oxygen partial pressure on the sweep side of the membrane decreases a lot with the increase of CO2 flow rate while the oxygen partial pressure on the feed side hardly changes. Therefore, the high CO2 flow rate increased the difference of oxygen partial pressure on both sides of the membrane. According to the Wagner equation [5], the increase of oxygen partial pressure difference caused by the increased CO2 flow rate can increase the driving force for oxygen transport and then increase the oxygen permeation flux. For instance, at 1173 K, when the CO2 flow rate increases
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from 20 to 100 ml min 1, the oxygen permeation flux increases from 0.76 to 1.15 ml min 1 cm 2.
3.4.5. Effect of oxygen partial pressure in feed gas Because compressed air is occasionally supplied as a feed gas for oxygen separation from air, the effect of oxygen partial pressure on oxygen permeation fluxes of the MCMHF membrane
is a crucial factor for practical application in the oxyfuel process. [30] Fig. 8 shows the effect of oxygen partial pressure at the feed side on oxygen permeation fluxes of the membrane sweeping by pure CO2 at 1173, 1123 and 1073 K. By increasing the oxygen partial pressure at the feed side, the oxygen fluxes dramatically increased because of the increase in the oxygen gradient across the membrane. For example, the oxygen permeation flux increases from 1.12 to 1.97 ml min 1 cm 2 with increase of oxygen partial pressure on the feed side from 0.21 to 0.8 atm at 1173 K. 3.5. Thermal cycling performance of MCMHF membrane
Fig. 9. Thermal cycling performance of MCMHF membrane between 1073 and 1173 K (FAir ¼ 120 ml min 1, FCO2 ¼ 80 ml min 1).
The thermal cycling performance of the membrane is of significance for industrial application in the oxyfuel process for CO2 capture technology because the practical operation process usually contains repeated heating and cooling process. Fig. 9 presents the thermal cycling performance of SFN MCMHF membrane. The three testing temperature points (1073 K, 1123 K and 1173 K) were kept for 50 min to steady the oxygen permeation flux. As can be seen from Fig. 9, the oxygen permeation flux is about 0.38 ml min 1 cm 2 at 1073 K, and it increases to 0.68 ml min 1 cm 2 at 1123 K, and then increases to 1.12 ml min 1 cm 2 at 1173 K. The oxygen permeation flux can be recovered when the temperature decreased to 1073 K. These thermal regeneration cycles can be repeated for five times in this work with no obvious decrease of oxygen permeation flux. This thermal cycling performance demonstrates that the SFN MCMHF membrane has good thermal–mechanical stability in the repeated heating and cooling process. 3.6. Long-term stability of MCMHF membrane
Fig. 10. Long-term oxygen permeation operation of MCMHF membrane with pure He or CO2 as sweep gases and with pure air or air contained 20% CO2 as feed gases at 1173 K.
To evaluate the CO2 stability of SFN MCMHF membrane, the oxygen permeation performance of the membrane was studied over 500 h using pure CO2 as sweep gas. Fig. 10 shows the long-term operation of MCMHF membrane. When helium was used as the sweep gas at 1173 K, the oxygen permeation flux reaches a steady value of 1.24 ml min 1 cm 2 after the two-hour activation process. When pure CO2 was used as sweep gas, the oxygen permeation flux slightly decreased to 1.12 ml min 1 cm 2. There is no obvious change about this value during the operation by using pure CO2 as sweep gas at 1173 K for more than 300 h. To further examine the stability of the
Table 2 Comparison of oxygen permeation fluxes and stabilities of various membranes swept by pure CO2. Membrane materials
Configurations
Thickness (mm)
Temperature (K)
O2 fluxes (ml min 1 cm 2)
Lifetime (h)
References
BaCo0.4Fe0.4Nb0.2O3 δ Ba0.5Sr0.5Fe0.8Zn0.2O3 δ Ba0.5Sr0.5Co0.8Fe0.2O3 δ Ce0.8Sm0.2O1.9–La0.7Ca0.3CrO3 Fe2O3 Ce0.9Gd0.1O2 δ NiFe2O4–Ce0.9Gd0.1O2-δ Ce0.8Sm0.2O2 δ La0.9Sr0.1FeO3 δ Pr0.6Sr0.4FeO3 δ Ce0.9Pr0.1O2 δ (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4 þ δ Ce0.8Sm0.2O1.9–La0.8Sr0.2MnO3 δ. Zr0.84Y0.16O1.92–La0.8Sr0.2MnO3 δ Ce0.9Gd0.1O2 δ–Ba0.5Sr0.5Co0.8Fe0.2O3 δ (Nd0.9La0.1)2Ni0.74Cu0.21Ga0.05O4 þ δ Ce0.8Sm0.2O1.9–SmMn0.5Co0.5O3 Ce0.8Sm0.2O1.9–Sm0.8Ca0.2Mn0.5Co0.5O3 Ce0.85Sm0.15O1.925–Sm0.6Sr0.4Al0.3Fe0.7O3 (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4 þ δ Nd0.6Sr0.4FeO3 δ–Ce0.9Nd0.1O2 δ Pr0.6Sr0.4Fe0.5Co0.5O3-δ Ce0.9Pr0.1O2 δ NiFe2O4 Ce0.8Tb0.2O2 δ La0.6Sr0.4Co0.2Fe0.8O3 δ SrFe0.8Nb0.2O3-δ SrFe0.8Nb0.2O3-δ
Disk Disk Disk Disk Disk Disk Disk Disk Disk Hollow fiber Hollow fiber Disk Disk Disk Disk Disk Hollow fiber Disk Disk Disk Hollow fiber Disk MCMHF
1 1.15 1 1.0 0.5 0.5 1.1 0.6 0.8 0.3 0.16 0.5 0.6 0.5 0.5 0.5 0.2 0.6 0.5 0.68 0.36 1 0.25
1173 1023 1148 1173 1223 1173 1173 1223 1173 1173 1223 1173 1173 1173 1173 1223 1173 1223 1223 1173 1173 1173 1173
0 0 0 0.04 0.08 0.12 0.14 0.18 0.24 0.26 0.29 0.3 0.3 0.36 0.34 0.37 0.38 0.21 0.7 0.11 0.2 0.25 1.05–1.1
150 100 150 150 230 170 250 420 300 300 400 310 4150 400 76 20 210 4500
[31] [32] [29] [33] [34] [10] [9] [35] [36] [37] [23] [38] [39] [11] [11] [40] [24] [41] [42] [43] [44] [14] This work
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membrane in more rigorous condition, the oxygen permeation flux was investigated by using 20% CO2 contained air as feed gas and pure CO2 as sweep gas. When 20% CO2 was introduced into the feed gas, the oxygen permeation flux was about 1.06 ml min 1 cm 2 and maintained constant for more than 200 h. The crystalline structure of the membrane sample after long-term operation was characterized by XRD as shown in Fig. 1(d). The XRD patterns indicated that the SFN membrane sample after long-term operation has pure cubic perovskite structure and no carbonate has been formed. We also studied the membrane surface morphology of the membrane sample after longterm operation by SEM as shown in Fig. 2(e) and (f). In compared with the fresh membrane surface, no significant degradation of the surface microstructure can be observed. All of the above, similar to that reported by Yi et al. [14], indicated that there was no obvious phase structure change of the SFN MCMHF membrane after long-term operation and the membrane exhibited an excellent stability in CO2 environment. Our work demonstrated that the SFN MCMHF membrane can be a competitive candidate for oxygen separation in the oxyfuel process. The oxygen permeation fluxes and stability of various membranes based on different membrane materials and membrane configurations in CO2-contained atmosphere are summarized in Table 2. Comparing with other membranes, the as-prepared MCMHF membrane possesses an outstanding performance in items of oxygen permeability and stability overall. Therefore, we can conclude that the SFN MCMHF membrane is competent for oxyfuel techniques and CO2 capture process.
4. Conclusions By using phase inversion and sintering technique, a CO2tolerant MCMHF membrane with ultra-high oxygen permeation flux and superb mechanical strength and excellent stability under CO2-contianed atmosphere is successfully prepared. The breaking load of the prepared MCMHF membrane is very much higher than that of SHF membranes, which is very helpful to overcome some drawbacks of SHF membranes. The oxygen permeation flux of the MCMHF membrane is higher than that of SFN disk membrane and reaches 1.12 ml min 1 cm 2 at 1173 K, which is the highest value among the membranes based on different materials and configurations reported in the literatures. The thermal cycling performance of MCMHF membranes between 1073 and 1173 K presents that the MCMHF membrane has good thermal–mechanical stability in the thermal cycling process. Furthermore, the MCMHF membrane is operated stably for more than 500 h by using pure CO2 as sweep gas without any degradation of oxygen permeation flux. Our work demonstrates that the MCMHF membrane has a great potential for the practical application in the oxyfuel process for CO2 capture technology.
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