Oxygen Permeation through U-Shaped K2NiF4-Type Oxide Hollow

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Oxygen Permeation through U-Shaped K2NiF4-Type Oxide Hollow-Fiber Membranes Yanying Wei, Jun Tang, Lingyi Zhou, Zhong Li, and Haihui Wang* School of Chemistry & Chemical Engineering, South China University of Technology, No. 381 Wushan Road, Guangzhou 510640, China ABSTRACT: The first oxygen permeation data for a dense hollow-fiber membrane based on a K2NiF4-type oxide are reported. The U-shaped (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4+δ (PLNCG) hollow-fiber membranes were prepared by a phase-inversion spinning process. The dependences of the oxygen permeation on the feed air flow rate, sweep helium flow rate, oxygen partial pressure on the shell side, and operating temperature were experimentally investigated. The effects of the bulk diffusion and the surface exchange on the oxygen permeation flux through the U-shaped PLNCG hollow-fiber membranes are also discussed. During around 320 h of operation, a steady oxygen permeation flux of 1.0 mL/(min 3 cm2) was obtained at 975 °C under conditions of an air feed flow rate of 180 mL/min and a helium sweep flow rate of 55 mL/min. XRD and SEM analyses of the spent hollow-fiber membrane showed the good stability of the U-shaped PLNCG hollow-fiber membranes.

’ INTRODUCTION Recently, conducting ceramic oxides with oxygen mixed ionicelectronic conductivity (MIEC) have gained increasing attention because of their potential applications in natural gas conversion,16 solid oxide fuel cells,7,8 and power stations with CO2 sequestration according to the oxyfuel concept.9,10 Most studies on MIEC materials for these applications have been concentrated on perovskite-type oxides because of their high oxygen permeation rates and variety of compositions. Following the first report of high oxygen permeability in La1xSrxCo1yFeyO3δ perovskite in the late 1980s,11 perovskite oxides and related intergrowth phases in the Sr(La)FeCoO system have been intensively studied.1216 However, it is still necessary to improve the stability of these materials to meet the requirements of industrial applications. On the other hand, oxides with a K2NiF4-type structure are increasingly attracting significant interest because of their promising oxygen permeability, moderate thermal and chemical expansion, and stability. 17 K2NiF4 has a perovskite-related structure consisting of perovskite (ABO 3 ) layers alternating with rock-salt (AO) layers, where excess oxygen can be incorporated into the rock-salt layers in the form of interstitial species.1820 Therefore, K2NiF4-based materials have high concentrations of excess oxygen in the lattice, and the ionic transport occurs predominantly by an interstitial migration mechanism.21,22 To improve the oxygen permeability of oxides with K2NiF4-type structures, hollow-fiber membranes prepared by a spinning/sintering process are a good choice. First, the oxygen permeability will be improved because of the thin wall of the hollow fibers. Second, these hollow-fiber membranes usually have a structure consisting of a dense layer and a porous layer, which are formed in the spinning process, and the porous layer is beneficial to oxygen surface exchange. Furthermore, hollow-fiber membranes will meet the needs of the industrial applications because of their large membrane areas per unit volume compared to conventional disk and tubular geometries. r 2011 American Chemical Society

However, significant research efforts have recently been focused on perovskite hollow-fiber membranes. The preparation, microstructure, oxygen permeability, catalytic performance, and even permeation modeling of perovskite hollow-fiber membrane have been widely investigated.2339 All of these hollow-fiber studies have focused on perovskite oxides such as Ba0.5Sr0.5Co0.8Fe0.2O3δ (BSCF), La0.6Sr0.4Co0.2Fe0.8O3δ, BaCoxFeyZrzO3δ (x + y + z = 1), SrCe0.95Yb0.05O3δ,25,40 SrCo0.9Nb0.1O3δ,41 SrCo0.9Sc0.1O3δ,42 and Sr(Co0.8Fe0.2)0.8Ti0.2O3δ.28 No hollow-fiber membranes based on oxides with a K2NiF4type structure have been prepared by a spinning/sintering process. In the present work, hollow-fiber membranes based on the K2NiF4-type material (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4+δ (PLNCG)43 were prepared. It was found that the oxygen permeability of Pr2NiO4-based oxides is significantly higher than that of La2NiO4-based ones44,45 and that Pr2NiO4+δ exhibits a higher range of variations in the oxygen nonstoichiometry46 because of smaller size of the A-site cations and, possibly, the presence of a minor fraction of tetravalent Pr4+.47 Pr2NiO4+δ was reported to exhibit the highest oxygen tracer diffusion coefficients in the Ln2NiO4+δ series (Ln = La, Pr, Nd).48 Furthermore, the excess oxygen introduced in Pr2NiO4 doped with Cu and Ga and might contribute to the high oxygen permeation.49 It was also found that the oxygen permeability is considerably increased by the addition of Ga into Pr2(Ni0.75Cu0.25)O4+δ.49 Doping of La atoms at the Pr site improves the phase stability at high temperatures such that decomposition into Pr4Ni3O10- and Pr2O3based phases50 does not occur.43 Therefore, we chose this chemical composition because it exhibits a high oxygen permeation rate as described by Yashima et al.43 The oxygen permeation, oxygen-transport mechanism, and stability of PLNCG Received: June 17, 2011 Accepted: October 5, 2011 Revised: September 4, 2011 Published: October 05, 2011 12727

dx.doi.org/10.1021/ie201298x | Ind. Eng. Chem. Res. 2011, 50, 12727–12734

Industrial & Engineering Chemistry Research

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Table 1. Preparation Conditions for the U-Shaped PLNCG Hollow-Fiber Membranes parameter

value

composition of the starting solution PLNCG powder PESf, A-300

52.60 wt % 9.29 wt %

NMP

37.18 wt %

PVP, K30

0.93 wt %

spinning temperature

25 °C

injection rate of internal coagulant

2.8 mL/min

spinning pressure

0.05 bar

air gap

0.5 cm

sintering temperature sintering time

1300 °C 3h

air flow rate for sintering

60 mL/min

hollow-fiber membranes were investigated in detail. According to our previous work,51 U-shaped hollow-fiber membranes can avoid membrane breakage due to expansion or shrinkage at varying temperatures. Therefore, U-shaped hollow-fiber membranes were chosen in this study.

’ EXPERIMENTAL SECTION The solgel route based on citric acid and ethylenediaminetetraacetic acid (EDTA) as complexing and gelation agents was adapted to prepare the powder. In brief, Ga was dissolved in nitric acid first, and proper amounts of Pr(NO3)3 3 6H2O, La(NO3)3 3 6H2O, Ni(CH3COO)2 3 4H2O, and Cu(NO3)2 3 3H2O were dissolved in water, after which citric acid, EDTA, and NH3 3 H2O were added. The mixture was then evaporated at 150 °C under constant stirring to obtain a dark-green gel. Afterward, the gel was ignited to flame to obtain the precursor. The precursor was ground and calcined at temperatures up to 950 °C for 10 h at a heating rate of 2 °C/min to remove the residual carbon and form the desired K2NiF4-type structure. For the spinning of hollow-fiber membranes, the powder was ballmilled for 24 h and then dried using a spray dryer (B€uchi Mini Spray Dryer, B-290) with a nozzle of 1 μm. The obtained fine powder was used for the preparation of the U-shaped hollowfiber membranes. U-shaped PLNCG hollow fibers were fabricated using a wet spinning/sintering technology.51 The spinning solution was composed of 9.29 wt % poly(ethersulfone) (PESf, A-300, BASF), 37.18 wt % 1-methyl-2-pyrrolidinone (NMP, AR-grade, purity > 99.8%, Kermel Chem Inc., Tianjin, China), 0.93 wt % poly(vinyl pyrrolidone) (PVP, K30, Boao Biotech Co., Shanghai, China), and 52.60 wt % PLNCG powder. A spinneret with orifice and inner diameters of 1.5 and 1.0 mm, respectively, was used to obtain the hollow-fiber precursors. Deionized water and tap water were used as the internal and external coagulants, respectively. Afterward, the PLNCG hollow-fiber precursors were sintered at 1300 °C for 3 h at an air flow rate of 60 mL/min to remove the polymers and obtain gastight membranes. The preparation conditions are summarized in Table 1. The phase structures of the as-prepared PLNCG powder and hollow fibers were characterized by X-ray diffraction (XRD, Bruker-D8 ADVANCE, Cu Kα radiation). The microstructure and morphology of the U-shaped PLNCG hollow-fiber precursors and the sintered PLNCG fibers were observed by scanning

Figure 1. Oxygen permeation apparatus for the oxygen permeation of the U-shaped PLNCG hollow-fiber membrane at high temperature.

electron microscopy (SEM, JEOL JSM-6490LA). The oxygen permeation fluxes through the U-shaped PLNCG hollow-fiber membranes were investigated in a high-temperature permeation cell, as shown in Figure 1. For each experiment, a U-shaped PLNCG hollow-fiber membrane was sealed in a corundum tube with two channels by a commercial ceramic sealant (HT767, Hutian, China). Air or a mixture of nitrogen and oxygen was fed to the shell side, and helium was swept on the core side to collect the oxygen that permeated through the membrane. The gas flow rates were controlled by mass flow controllers (MFCs, Seven Star D08-4F/ZM) calibrated using a soap bubble flow meter. The composition of the permeated gas was measured by online gas chromatography (GC, Agilent 7890) with thermal conductivity detection. The leakage of the oxygen due to imperfect sealing at high temperatures was less than 0.5% during all experiments. The effective area of the U-shaped PLNCG hollow-fiber membranes used here was 1.3 cm2. Details of the calculation of the oxygen permeation flux can be found in our previous work.51

’ RESULTS AND DISCUSSION SEM images of the PLNCG hollow-fiber membrane sintered at 1300 °C for 3 h are presented in Figure 2. Fingerlike structures can be observed in the bulk of the membrane from Figure 2B, which are beneficial for improving the surface exchange rate during oxygen permeation.52 Before a U-shaped hollow-fiber membrane was used for oxygen permeation, its gas tightness was tested at room temperature. No nitrogen was detected on the shell side even when the nitrogen partial pressure on the core side reached 0.5 MPa, which indicates that the hollow-fiber membranes were gastight. Figure 3 shows a plot of the oxygen permeation flux through a U-shaped PLNCG hollow-fiber membrane as a function of operating temperature at a constant sweep gas flow rate of 55 mL/min 3 cm2. The oxygen permeation flux of 0.96 mL/min 3 cm2 through the U-shaped hollow-fiber membrane at 975 °C is nearly twice the 0.52 mL/min 3 cm2 value obtained for a disk membrane with a thickness of 0.5 mm. The oxygen permeability through the 12728

dx.doi.org/10.1021/ie201298x |Ind. Eng. Chem. Res. 2011, 50, 12727–12734


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Figure 2. SEM micrographs of the sintered PLNCG hollow-fiber membrane: (A) cross section, (B) wall of the sintered hollow fiber.

Figure 3. Oxygen permeation flux and Arrhenius plots of the oxygen permeation flux of the dense PLNCG hollow-fiber membrane. Conditions: temperature varied from 700 to 975 °C, Fair = 180 mL/min, FHe = 55 mL/min.

hollow-fiber membrane with thin walls is much better than that through the disk membrane. The minimal thickness of the hollow fibers decreases the ionic bulk diffusion resistance, and the micropores in the hollow-fiber membranes help to increase the oxygen exchange rate, which leads to a higher oxygen permeation flux, especially at high temperature. In the corresponding Arrhenius plots, two straight lines were observed, giving an apparent activation energy of 49.8 kJ/mol at high temperatures of 875975 °C and 117.2 kJ/mol at low temperatures of 700875 °C for the K2NiF4-type PLNCG hollow-fiber membrane. From the subsequent study of the limiting step of oxygen permeation through the PLNCG hollow-fiber membrane, it can be noted that surface exchange and bulk diffusion are two factors contributing to the transport resistance depending on the operating temperature. As temperature increases, the surface exchange is improved, and bulk diffusion becomes increasingly important in the rate-limiting step, in agreement with the previous data.53 Figure 4 shows the effect of the air flow rate on the shell side on the oxygen permeation fluxes through the U-shaped PLNCG hollow-fiber membrane at different temperatures. In this experiment, the helium flow rate on the core side was kept at 55 mL/min. As shown in Figure 4A, the oxygen permeation flux increased with the air flow rate only when the air flow rate was below 100 mL/ min. Once the air flow rate was higher than 100 mL/min, the oxygen permeation flux no longer increased, especially at low temperatures. It also can be observed in Figure 4B that the oxygen permeation fluxes through the PLNCG hollow-fiber membranes

Figure 4. Effect of air feed flow rates on the oxygen permeation flux through the U-shaped PLNCG hollow-fiber membrane at different temperatures. Conditions: FHe = 55 mL/min.

changed only slightly with varying air flow rate. The difference in the oxygen permeation fluxes at different air flow rates increased with increasing temperature, which indicates that the oxygen permeation flux is more sensitive to the air flow rate at higher temperatures. Figure 5 presents the oxygen permeation flux through the U-shaped PLNCG hollow-fiber membrane as a function of the helium flow rate on the core side at various temperatures. As shown in Figure 5A, at the indicated operating temperatures, the oxygen permeation fluxes through the U-shaped PLNCG hollow-fiber membrane increased with increasing helium flow rate because higher helium flow rates dilute the permeated oxygen 12729



Figure 5. Effect of helium sweep flow rates on the oxygen permeation flux through the U-shaped PLNCG hollow-fiber membrane at different temperatures. Conditions: Fair = 180 mL/min.

Figure 6. Model of the oxygen permeation through the PLNCG hollow-fiber membrane.

and lower the oxygen partial pressure on the core side (P2). As shown in Figure 6, the oxygen partial pressure on the core-side membrane surface (P20 ) can be decreased at the same time. Therefore, the oxygen permeation flux increased with increasing helium flow rate on the core side as a result of the increasing permeation driving force. For example, when the helium flow rate on the core side was increased from 20 to 115 mL/min, the oxygen partial pressure at the exit of the core side decreased from 0.0448 to 0.0107 atm at 975 °C. As a result, the oxygen permeation flux rose from

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Figure 7. Oxygen permeation fluxes through the U-shaped PLNCG hollow-fiber membrane as a function of the oxygen partial pressure on the shell side. Conditions: FN2+O2 = 150 mL/min, FHe = 55 mL/min.

0.74 to 1.01 mL/min 3 cm2 at 975 °C. Similar results for perovskite systems were also found by other researchers.42,51 However, the effect of the helium flow rate on the oxygen permeation flux became insensitive at lower temperatures (i.e., below 900 °C). It can also be noted from Figure 5B that the difference in the corresponding permeated oxygen fluxes under different helium flow rates in the high temperature range of 900975 °C were greater than those in the low temperature range of 825900 °C. Figure 6 shows a model of oxygen permeation through a PLNCG hollow-fiber membrane. There is a stranded layer near each interface between the gas phase and membrane surface because of the diffusion resistance. When the air flow rate is increased, even as the oxygen partial pressure on the shell side (P1) remains at 0.21 atm, the stranded layer near the air-side membrane surface becomes thinner, and the oxygen partial pressure near the air-side membrane surface (P10 ) increases. Thus, ΔP1 decreases, and the oxygen permeation driving force (ΔP00 ) increases, leading to a higher oxygen permeation flux, as shown in in Figure 4. When the helium flow rate is increased, the stranded layer near the Heside membrane surface becomes thinner as well. Furthermore, the oxygen partial pressure near the He-side membrane surface (P20 ) decreases sharply because the higher helium flow rate dilutes the permeated oxygen. Therefore, the oxygen permeation driving force (ΔP00 ) increases, which also leads to a higher oxygen permeation flux, as shown in Figure 5. 12730



Figure 8. Oxygen permeation fluxes of the U-shaped hollow-fiber membrane against (P1/P0)n (P2/P0)n at different temperatures. Conditions: P1 varied from 0.1 to 1 atm, P2 varied from 0.0009 to 0.0386 atm, inner diameter = 0.587 mm, outer diameter = 0.979 mm, membrane area = 1.2 cm2.

Figure 7 shows the oxygen permeation fluxes through a PLNCG hollow-fiber membrane with different combinations of temperatures and oxygen partial pressures on the shell side (P1). The total flow rate of oxygen and nitrogen on the shell side was 150 mL/min, and different P1 values were obtained by adjusting the ratio of nitrogen and oxygen. Figure 7A shows the oxygen permeation flux in the temperature range from 800 to 975 °C as a function of P1. As shown in Figure 7A, the oxygen permeation flux increased with increasing P1 because of the increase in the oxygen gradient across the membrane; that is, the oxygen permeation driving force (ΔP00 ) increased because of the increase in P10 . For instance, the oxygen permeation flux under an oxygen partial pressure on the shell side of 1.0 atm was 1.78 mL/min 3 cm2, which is nearly 3 times that under a 0.1 atm oxygen partial pressure on the shell side at 975 °C. The temperature dependence of the oxygen permeation flux at various oxygen partial pressures on the shell side is presented in Figure 7B. The oxygen permeation flux increased with increasing temperature. It can be noted from Figure 7B that the gaps in the corresponding permeated oxygen fluxes at different P1 values in the high temperature range of 900975 °C are larger than those in the low temperature range of 825900 °C. Similarly to the effects on the oxygen permeation flux of the air flow rate on the shell side and the helium flow rate on the core side, the effect of the oxygen partial pressure on the shell side on the oxygen permeation flux was more sensitive at high temperatures than that at low temperatures. Oxygen exchange between the oxide surface and the gas phase is known to play a significant role in the oxygen permeation process through K2NiF4-structure membranes.18,19,21,22 To clarify the limiting step during oxygen permeation through the U-shaped PLNCG hollow-fiber membrane, the oxygen permeation flux as a function of different oxygen driving forces at various temperatures was investigated. A linearized plot of JO2 µ (P1n P2n), with n as a fit parameter deduced from Wagner theory,5456 is shown in Figure 8, and the details of derivation can be found in our previous work.51 From the value of n, the rate-limiting step of oxygen permeation can be identified. Generally, for 0 < n < 0.5, the oxygen permeation is influenced by both the surface reaction and the bulk diffusion.

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Figure 9. Oxygen permeation flux through PLNCG hollow-fiber membrane as a function of time at 975 °C. Conditions: Fair = 180 mL/min, FHe = 55 mL/min.

Figure 10. XRD patterns of PLNCG powder, fresh hollow fiber, and spent membrane after 320 h of oxygen permeation.

The n value derived from experimental data is between 0.23 and 0.37 in the temperature range of 800975 °C, which suggests that the oxygen permeation flux through the U-shaped PLNCG hollow-fiber membrane is controlled by both surface reaction and bulk diffusion at temperatures of 800975 °C. Compared to the n values for other perovskite systems, such as BSCF in our previous work,51 the n value in the K2NiF4-type system is slightly higher at the same temperature, which indicates that surface exchange plays a more important role in oxygen permeation through K2NiF4-type membrane systems. For example, the experimentally determined coefficient n is 0.24 at 950 °C for a PLNCG hollow-fiber membrane, whereas the n value is only 0.19 at 950 °C for a BSCF hollow-fiber membrane. At the same temperature of 850 °C, the n values are 0.3 and 0.24 for the PLNCG and BSCF systems, respectively. Moreover, it can be noted that the same coefficient n of 0.24 can be obtained at 950 °C for the PLNCG hollow-fiber membrane, whereas it is obtained at 850 °C for BSCF, which indicates that the proportion of surface exchange and bulk diffusion in the K2NiF4-type system 12731



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Figure 11. SEM micrographs of the spent PLNCG hollow-fiber membrane: (A) inner and (B) outer surfaces of the fresh hollow-fiber membrane and (C) inner and (D) outer surfaces of the spent hollow-fiber membrane.

at 950 °C is similar to that in the perovskite system at 850 °C. Therefore, it can be concluded that the surface exchange in the K2NiF4-type system contributes more than that in the perovskite system at the same temperature. A promising oxygen-permeable membrane should exhibit not only good oxygen permeability, but also stable oxygen permeation fluxes. Figure 9 shows the long-term behavior of the oxygen permeation flux through a U-shaped PLNCG hollow-fiber membrane at 975 °C. A steady oxygen permeation flux of 1.0 mL/min 3 cm2 was obtained at 975 °C during around 320 h of operation, and no decrease of the oxygen permeation flux was found. Figure 10 shows the phase structures of PLNCG powder prepared by combined EDTAcitrate complexation, fresh sintered hollow-fiber membrane, and spent hollow-fiber membrane after 320 h of oxygen permeation with He as the sweep gas (Figure 9). X-ray diffraction results indicate that the phase of PLNCG powder is pure K2NiF4 structure. The PLNCG hollowfiber membrane sintered at 1300 °C maintained the pure K2NiF4 structure as well. Furthermore, after the 320-h oxygen permeation operation, the K2NiF4 structure of PLNCG was also maintained, which indicates that the U-shaped PLNCG hollow-fiber membrane exhibited excellent structure stability under air/helium conditions. Figure 11 presents SEM images of the inner and outer surfaces of fresh and spent PLNCG hollow-fiber membranes. Panels A and B of Figure 11 show the inner and outer surfaces, respectively, of the fresh hollow fibers after sintering. The particles on both the inner and outer surfaces in this K2NiF4-type PLNCG system appear to be aciculate. It can be noted that the PLNCG particles connect to each other firmly. Panels C and D of Figure 11 show the inner and outer surfaces, respectively, of the spent PLNCG hollow fibers after 320 h of oxygen permeation. The impurity on the outer surface of the spent hollow-fiber membrane exposed to air contains Al and Si, which was detected by energy dispersive X-ray spectroscopy (EDS) and is from the decomposition of the ceramic sealant. However, both the inner and outer surfaces of the membrane remained intact, and the

PLNCG particles were still connected to each other firmly, which suggests that U-shaped PLNCG hollow-fiber membranes have good oxygen permeation stability.

’ CONCLUSIONS U-shaped PLNCG hollow-fiber membranes were prepared by a phase-inversion spinning process. Although the oxygen permeation flux increased with increasing air and helium flow rates, the influence was much weaker than that of a perovskite membrane. From the temperature and oxygen-concentrationgradient dependences of the oxygen permeation flux, it follows that both surface reaction and bulk diffusion are rate-determining steps for oxygen permeation in the temperature range of 800975 °C. As the temperature decreased, the oxygen permeation flux decreased, and the surface exchange reaction played a more important role during oxygen permeation than at high temperatures. A steady oxygen permeation flux of 1.0 mL/min 3 cm2 was obtained at 975 °C during ∼320 h of oxygen permeation. XRD and SEM characterizations indicated that the spent hollowfiber membranes still maintain the perfect K2NiF4-type phase structure. These results indicate that the U-shaped PLNCG hollow-fiber membrane exhibits good oxygen permeability and stability. Compared to the conventional disk and tubular geometries, the oxygen permeability of the hollow-fiber membranes was improved because of thin walls and a structure consisting of a dense layer and a porous layer. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: +86-20-87110131. Fax: +86-2087110131.

’ ACKNOWLEDGMENT The authors acknowledge inancial support from the Natural Science Foundation of China (Nos. 21176087, U0834004, 12732


Industrial & Engineering Chemistry Research 20936001); the National Basic Research Program of China (No. 2009CB623406); the Science-Technology Plan of Guangzhou City (No. 2009J1-C511-1); and the Fundamental Research Funds for the Central Universities, SCUT (Nos. 2009220038, 20112G0011).

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