Preparation and Oxygen Permeation Properties of Highly Asymmetric

Nov 19, 2008 - Preparation and Oxygen Permeation Properties of Highly Asymmetric La0.6Sr0.4Co0.2Fe0.8O3−α Perovskite Hollow-Fiber Membranes ... Fax...
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Ind. Eng. Chem. Res. 2009, 48, 510–516

SEPARATIONS Preparation and Oxygen Permeation Properties of Highly Asymmetric La0.6Sr0.4Co0.2Fe0.8O3-r Perovskite Hollow-Fiber Membranes Zhigang Wang, Naitao Yang, Bo Meng, and Xiaoyao Tan* School of Chemical Engineering, Shandong UniVersity of Technology, Zibo, China 255049

K. Li Department of Chemical Engineering and Technology, Imperial College London, South Kensington, London SW7 2AZ, United Kingdom

La0.6Sr0.4Co0.2Fe0.8O3-R (LSCF) perovskite powders having a submicrometer size were synthesized by a sol-gel autocombustion method. From these powders, the gastight LSCF hollow-fiber membranes with a highly asymmetric structure comprising a dense layer of thickness ca. 88 µm integrated with a porous substrate were fabricated in a single step through a phase inversion/sintering technique with a mixture of N-methyl2-pyrrolidone (NMP) and water as internal coagulant. Oxygen permeation fluxes through the obtained hollowfiber membranes were measured under air/He gradients at different temperatures. The results indicate that the highly asymmetric hollow-fiber membranes possess an oxygen permeation flux of 0.11-2.19 mL cm-2 min-1 in the temperature range of 650-1000 °C, which is 2.6-10.5 times higher than that of the sandwich-structured hollow-fiber membranes. Oxygen permeation in the hollow-fiber membranes is limited primarily by the surface exchange reactions at lower temperatures, but ionic bulk diffusion will have a rate-limiting effect at temperatures higher than 900 °C. 1. Introduction Dense ceramic membranes made from mixed ionic and electronic conductors (MIEC) have attracted considerable attention in the past decade due to their potential applications in oxygen production and partial oxidation of hydrocarbons to value-added products.1-4 Perovskite oxides with the general formula ABO3-δ are the most promising membrane materials due to their high ionic/electronic conductivity and stability.5-7 But the oxygen permeability of the derived membranes has to be further improved considerably in order to put the ceramic oxygen-permeable membranes into practical applications. For this purpose, considerable efforts have been made on the development of new perovskite materials with high oxygenpermeation properties.8 Besides, since the oxygen permeation through a dense perovskite membrane is controlled by the oxygen diffusion in the membrane as well as the surface oxygen exchange kinetics on either or both sides of the membrane,9,10 the oxygen permeability of the membranes can be improved either by reducing membrane thickness or by coating a porous catalytic layers on either or both sides of the membrane, leading to an enhancement of surface oxygen exchange kinetics.11-16 Recently, perovskite hollow-fiber membranes fabricated via a phase inversion /sintering process have attracted increasing interest due to their large membrane area per unit volume compared to the conventional disk and tubular forms typically used.17-21 The obtained hollow-fiber membranes usually possess an asymmetric structure comprising a central dense layer and two porous layers, which is formed during the spinning process. However, since water is usually used as both the internal and external coagulants, not only the central sector and the outside * To whom correspondence should be addressed. Telephone: (86) 533-2786292. Fax:(86) 533-278629. E-mail: [email protected].

surface of the membrane are dense but also the inner surface of the hollow fibers is more or less dense (so-called sandwich structure).17,18 Consequently, the area for oxygen exchange reactions is restricted to membrane external surfaces. Therefore, surface modifications have to be carried out in order to further improve the surface oxygen exchange kinetics. For example, the oxygen permeation rate through La0.6Sr0.4Co0.2Fe0.8O3-R hollow-fiber membranes could be improved remarkably by an acid treatment where the membrane surfaces were etched into a porous structure.22 In this study, gastight La0.6Sr0.4Co0.2Fe0.8O3-R hollow-fiber membranes with a highly asymmetric structure consisting of a dense layer with a porous substrate of the same material were fabricated by using slightly diluted solvent as internal coagulant during the casting. Oxygen permeation properties of the hollowfiber membranes were measured under an air/He gradient at different temperatures. 2. Experimental Section 2.1. Materials. Sr(NO3)2 (AR grade), La(NO3)3 · 6H2O (AR grade), Co(NO3)3 · 6H2O (>99%), and Fe(NO3)3 · 9H2O (AR grade) were purchased from Kermel Chem Inc., Tianjin, China, and were used as the metallic precursors for the preparation of La0.6Sr0.4Co0.2Fe0.8O3-R (LSCF) powders. Citric acid (>99%, Ajax) and ethylene glycol (AR grade, Longjili, Tianjin) were used as the complexing agents. Nitric acid and ammonium hydroxide were used to adjust the pH of the starting solution. Poly(ether sulfone, PESf (Radel A-300, Ameco Performance), and N-methyl-2-pyrrolidone (NMP) [AR grade, >99.8%, Kermel Chem Inc., Tianjin, China] were used to prepare the spinning suspension. NMP aqueous solution and tap water were used as the internal and external coagulants, respectively.

10.1021/ie8010462 CCC: $40.75  2009 American Chemical Society Published on Web 11/19/2008

Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 511 Table 1. Preparation Conditions for LSCF Hollow-Fiber Membranes experimental parameters

values

composition of spinning solution

LSCF, 61.49 wt %; PESf, 7.61 wt %; NMP, 30.9 wt % spinning temperature 28 °C internal coagulant 94 wt % NMP + 6 wt % water external coagulant tap water injection rate of internal coagulant 8.5 mL min-1 nitrogen pressure (absolute) 0.15 MPa air gap 0 cm sintering temperature 1300 °C sintering time 5h

2.2. Preparation of LSCF Powders. LSCF powders were prepared through a sol-gel combustion process.23 Stoichiometric amounts of Sr(NO3)2, La(NO3)3 · 6H2O, Co(NO3)3 · 6H2O, and Fe(NO3)3 · 9H2O were dissolved in distilled water. Citric acid and ethylene glycol in quantities of 3 times the desired LSCF product were then added under magnetic stirring until they were dissolved completely. The pH of the mixture was adjusted to be 3∼4 by use of nitric acid and ammonium hydroxide to avoid precipitation. Subsequently the solution was stirred at 70 °C on a hot plate for 5∼10 h to form a transparent brown sol. Further heating was conducted under continuous stirring until a viscous gel was formed. As the temperature was increased to around 300 °C, autocombustion took place to form a fluffy black powder (LSCF powder precursor). Under an air flow, the powder precursor was calcined at 800 °C for 3 h to remove the residual carbon and form the desired structure. For spinning LSCF hollow-fiber membranes, the resultant powders were ball-milled for 48 h, followed by sieving through a sifter of 200-mesh or 24 µm sieve-pore diameter. 2.3. Preparation of LSCF Hollow-Fiber Membranes. LSCF hollow-fiber membranes were prepared from the calcined and ball-milled powders by the phase inversion and sintering technique. Detailed preparation procedures were described elsewhere.17 In this study, the spinning suspension consists of 61.49 wt % LSCF powders, 7.61 wt % PESf, and 30.90 wt % NMP. The internal coagulant used contains 94 wt % NMP in water, while tap water was used as the external coagulant. The membrane precursors were calcined at 1300 °C in ambient nonflowing air atmosphere for 5 h. The parameters employed for preparing the LSCF hollow-fiber membranes are summarized in Table 1. 2.4. Oxygen Permeation Measurements. Oxygen permeation properties of the LSCF hollow-fiber membranes were investigated in an oxygen permeation cell schematically shown elsewhere.22 The dense hollow-fiber membrane was housed in a quartz tube (18 mm in diameter and 400 mm in length) with a high-temperature silicone sealant (1592, purchased from Tonsan New Materials and Technology Co, Beijing) that is able to withstand up to 350 °C. The permeation cell was positioned in a # 22 × 180 mm tubular furnace having an effective heating length of 50 mm. Air was fed on the shell side and helium was passed through the fiber lumen to collect the oxygen permeate. Gas feed flow rates were controlled by mass flow controllers (D08-8B/ZM, Shanxi Chuangwei Instrument Co. Ltd.), which were calibrated with a soap bubble flow meter. The effluent flow rates were also measured by the soap bubble flow meter. Compositions of the permeate gas were measured online by a gas chromatograph (Agilent 6890N) fitted with a 5 Å molecular sieve column (# 3 mm × 3 m) and a TCD detector. Highly purified hydrogen was used as the carrier gas and the flow rate was set at 40 mL min-1. GC calibration was performed with a standard gas mixture consisting of 5% oxygen, 5% nitrogen,

and 90% helium (mole fractions with (2% accuracy) purchased from Baiyan Gases Ltd. Co., Zibo, China. All the gas composition measurements were made after 20 min following a temperature change or sweep-gas rate change. At least two measurements were conducted for each experimental condition. During the permeation, a small leakage existed since 0.1-1.8% nitrogen in the permeate gas stream on the sweep side was detected depending on the sweep flow rate and temperature. Although the leaking oxygen is much less than the permeated oxygen (about 0.1∼3%), it has to be deducted for the calculation of the oxygen permeation flux:

(

JO2 ) V xO2 -

21 x ⁄A 78 N2 m

)

(1)

where V is the flow rate of the permeate gas stream in milliliters per minute, xO2 and xN2 are the percentages of oxygen and nitrogen in the effluent, and Am is the effective membrane area in square centimeters. Am ) [2π(Ro - Rin)L]/ln (Ro/Rin), in which Ro, Rin, and L are the outer diameter (OD), inner diameter (ID), and effective length for oxygen permeation of the hollow-fiber membrane, respectively. 2.5. Membrane Characterization. Morphology and microstructures of the LSCF powder and the hollow fibers were ascertained by scanning electron microscopy (SEM) (FEI Sirion200). Gold sputter coating was performed on the samples under vacuum before the measurements. Crystal phases of the LSCF powder and the hollow-fiber membrane were determined by X-ray diffraction (XRD) (Bruker D8 Advance, Germany) with Cu KR radiation (λ ) 0.154 04 nm). The hollow fibers were ground into fine powders prior to the XRD measurements. Continuous scan mode was used to collect 2θ data from 10° to 80° with a 0.02° sampling pitch and a 2° min-1 scan rate. The X-ray tube voltage and current were set at 40 kV and 30 mA, respectively. The surface specific area of the LSCF powders was measured with Micromeritics ASAP 2020 analyzer by the multipoint Brunauer-Emmett-Teller (BET) adsorption technique. The particle size distribution of the final powders for the hollow-fiber spinning was measured in an ultrasonically dispersed water suspension by a laser particle size analyzer (90PLUS).24 3. Results and Discussion 3.1. LSCF Powders. Morphology of the LSCF powders prepared by the sol-gel combustion method is shown in Figure 1. As can be seen from Figure 1a, the combustion product has a fluffy porous structure. After sintering at 800 °C for 3 h, some fine particles have been fused into larger ones as shown in Figure 1b. However, the agglomerates can be broken into smaller particles again by the ball-milling treatment. BET analysis indicates that the combustion product and the sintered and ballmilled LSCF powders have specific surface areas of 26.9 and 10.8 m2 g-1, respectively. This decrease in the specific surface area after sintering also accounts for the fusion taken place during the sintering process. Figure 2 shows the particle size distribution of the sintered and ball-milled LSCF powders measured in the ultrasonically dispersed water suspension. It indicates that the final powders for hollow-fiber spinning have particle sizes ranging from 0.1 to 7.11 µm. The characteristic parameters of the powders given by the Winner 2000 analysis software are D10) 0.14 µm, D50 ) 0.35 µm, D90 ) 2.2 µm, Dav ) 0.8 µm and S/V (ratio of surface area to volume) ) 1.98 × 105 cm2 cm-3. Figure 3 shows the XRD patterns of the LSCF powders. As can be seen from the figure, the perovskite phase has been

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Figure 1. Morphology of LSCF powders: (a) gel-combustion product; (b) sintered and ball-milled product for membrane spinning.

Figure 2. Particle size distribution of LSCF powders for membrane spinning.

Figure 3. XRD patterns of LSCF powders and membranes (p, perovskite phase; 3, impurity phase): (a) gel-combustion generated powders; (b) calcined powders at 800 °C for 3 h; (c) sintered membrane at 1300 °C for 5 h.

basically formed after the gel combustion but there also exists a minor amount of impurity phase (Figure 3a). However, after the combustion powders were calcined at 800 °C for 3 h, the impurity phase has disappeared to form the pure perovskite structure (Figure 3b). For comparison, the XRD pattern of the LSCF hollow-fiber membranes after a heat treatment at 1300 °C for 5 h is also depicted (Figure 3c). No changes in the crystalline structure of the LSCF material have been observed after the powders were formed into hollow-fiber membranes by spinning and sintering. This indicates that the perovskite structure has been preserved perfectly during the spinning and sintering process. However, the intensity of the corresponding characteristic peaks of the perovskite phase in the sintered LSCF hollow fibers are slightly higher than those in the original LSCF powders. This suggests that the crystals in the hollow fibers have become larger due to the high-temperature sintering. 3.2. LSCF Hollow-Fiber Membranes. SEM micrographs of the LSCF hollow-fiber precursors prepared by the wet spinning process from a mixture of 94 wt % NMP and 6 wt % water as internal coagulant are shown in Figure 4. The inner diameter (ID) and the wall thickness of the prepared hollow-

fiber precursors were measured from micrographs in panels a and b to be 1.56 and 0.43 mm, respectively. Since the outer diameter of the precursor is beyond the observing window of the SEM apparatus, the whole hollow-fiber precursor cannot be displayed in Figure 4a. Nevertheless, the outer diameter can still be estimated to be around 2.43 mm by adding the wall thickness to the ID of the precursors. As can be seen from the figure, an asymmetric structure comprising a microporous layer and a macroporous sublayer has been formed during the spinning process, which may be attributed to the different precipitation rates occurring within the nascent membranes.17 When the mixture of NMP and water is used as the internal coagulant, the precipitation rate on the inner side of nascent membranes is much lower than that on the outer side due to the contact of large quantities of external coagulant (water). It is well-known that rapid precipitation usually leads to the formation of microporous structure while slow precipitation on the inner side gives rise to the macro voids.25,26 Therefore, the inner surface is more porous than the outer surface of the hollow-fiber membrane precursors, as shown in Figure 4c,d. In addition, it also can be seen that the LSCF particles are well dispersed and loosely connected to each other by the polymer binder. Figure 5 shows the micrographs of the hollow-fiber membranes sintered at 1300 °C for 5 h. After sintering, the OD/ID of the fibers shrank from 2.43/1.56 to 1.87/1.25 mm, respectively, as obtained from Figure 5a. It indicates that obvious shrinkage of the hollow fibers has taken place in the sintering process due to the removal of organic binder and the combining and sintering of the inorganic particles. In addition, the shrinkage may be calculated to be around 19.9∼23%, which is much smaller than that reported before for the sandwiched hollowfiber membranes.17 This may be possibly attributed to the fact that the LSCF powders for the membrane preparation in this work had been partly sintered prior to spinning. However, the asymmetric structure has been well preserved in the sintered hollow-fiber membranes, as shown in Figure 5b. Therefore, it implies that the sintering only removed the organic components but has not changed the general structure of the hollow fibers. The microstructures of the two layers can be observed more clearly in Figure 5c,d. It shows that the sintered LSCF hollowfiber membranes consist of an inside porous layer with thickness ca. 299 µm and an outside dense layer with thickness ca. 88 µm. In the outside layer, all the LSCF particles have been molten into an integrated body, making the membrane perfectly dense, as shown in Figure 5d. The gastight property of the sintered hollow-fiber membranes was further tested by a gas permeation measurement. Figure 5 panels e and f show the inner surface and the outer surface of the hollow-fiber membranes. It can be seen that the inner surface is porous but the LSCF particles on the outer surface are closely connected with each other into a dense structure. Furthermore, the LSCF particles on the outer

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Figure 4. SEM micrographs of LSCF hollow-fiber precursors: (a) cross sectional; (b) fiber wall; (c) inner surface; (d) outer surface.

surface have grown up into larger ones due to the hightemperature sintering (Figure 5f). However, the grain boundaries on the outer surface are much less distinct than those on the membranes made from the spray-pyrolysis derived powders.17 This indicates that the sol-gel-derived LSCF powders have different sintering properties from those prepared by the spraypyrolysis process, which may also finally influence the microstructure of the resultant hollow fibers. 3.3. Oxygen Permeation through the Hollow-Fiber Membranes. Figure 6 plots the oxygen permeation flux and the oxygen concentration in the downstream effluent against temperature at a given sweep gas flow rate (51.9 mL min-1) and air feed flow rate (180 mL min-1). As can be seen, the oxygen permeation flux and the downstream oxygen concentration have increased from 0.11 to 2.19 mL cm-2 min-1 and from 0.95% to 12.55%, respectively, as the temperature is increased from 650 to 1000 °C. This means that the operating temperature plays a more important role in oxygen permeation than the concentration driving force for LSCF membranes. For comparison, the oxygen permeation fluxes through the sandwich-structured hollow-fiber membranes under the same operating conditions are also plotted in the figure.17 It can be seen that the highly asymmetric hollow-fiber membranes prepared in this work exhibit 2.6-10.5 times the oxygen permeation fluxes of the sandwich-structured membranes depending on operating temperature. For example, the oxygen flux at 800 °C increases from 0.071 mL cm-2 min-1 in the sandwich-structured hollow-fiber membrane to 0.74 mL cm-2 min-1 in the highly asymmetric hollow-fiber membranes. Moreover, if compared under the same oxygen concentration gradient across the membrane, the improvement of oxygen flux by the highly asymmetric hollowfiber membranes over the sandwich-structured fibers would be much higher than the data given above. Such a remarkable improvement of the oxygen permeation flux may be the result of the following three factors: (1) the effective membrane thickness is decreased, leading to a reduced bulk diffusion resistance; (2) the downstream surface is porous, giving a much larger membrane area for the surface exchange reaction; (3) the

grain boundaries in the LSCF membrane are weakened, resulting in the reduced boundary resistance to ionic transportation. Figure 7 shows the oxygen permeation fluxes and oxygen concentrations in the downstream effluent as a function of sweep gas flow rate at different temperatures, where the air feed flow rate is fixed at 180 cm3 min-1. As is expected, the oxygen concentration in the permeate side decreases with increasing sweep gas flow rate at a fixed operating temperature (Figure 7a). Since the oxygen concentration on the shell side (or upstream) was almost unchanged because the amount of oxygen in the air feed is much more than that permeated through the membrane, the lowered oxygen permeate concentration on the lumen side (or downstream) would give rise to a larger driving force for oxygen permeation. As a result, the oxygen permeation flux increases with increasing sweep gas flow rate, as shown in Figure 7b. When the transport resistance from surface exchange reactions and bulk diffusion are considered, the local oxygen permeation rate through a mixed ionic-electronic conducting hollow-fiber membrane such as LSCF perovskite in which the electronic conductivity overwhelms the oxygen ionic conductivity can be given by18,27,28 JO2 )

kr (pO2′)0.5 - (pO′′2)0.5

[

]

Rm ′′ 0.5 2kf(Ro - Rin) ′ ′′ 0.5 Rm ′ 0.5 (p ) + (pO2pO2) + (pO2 ) Ro O2 DV Rin

(2)

where pO2′ and pO2′′ are the oxygen partial pressures on the outer and the inner surface of the hollow-fiber membrane, respectively; Rm is the algorithm radius [Rm ) (Ro - Rin)/ln (Ro/Rin), in which Ro and Rin are respectively the outer and the inner radius of the fiber in centimeters]; DV is the diffusion coefficient of oxygen vacancy; and kf and kr are, respectively, the forward and reverse reaction rate constants for the surface exchange reaction: kf/kr 1 O2 + VO¨ T OO× + 2h˙ (3) 2 where the charge defects are defined by use of the Kro¨ger-Vink

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Figure 5. SEM micrographs of LSCF hollow-fiber membranes sintered at 1300 °C for 5 h: (a) cross sectional; (b) fiber wall; (c) inner porous layer; (d) outer dense layer; (e) inner surface; (f) outer surface.

Figure 6. Effect of temperature on oxygen permeation through the duallayer structured LSCF hollow-fiber membranes (He feed flow rate ) 51.9 mL min-1; air feed flow rate ) 180 mL min-1).

notation. That is, OO× stands for lattice oxygen, VO•• stands for oxygen vacancy, and h•i stands for electron hole. It can be seen from eq 2 that the total permeation resistance is composed of three parts: (1) exchange reaction at the outer membrane surface, (2) bulk diffusion, and (3) exchange reaction at the inner membrane surface (lumen side). For the step of a negligible resistance, it can be directly removed from the equation. For example, when the membrane has a thickness far less than the critical thickness (Lc), which is defined as the membrane thickness at which the oxygen permeation resistance

Figure 7. Effect of sweep gas flow rate on oxygen permeation through dual-layer structured LSCF hollow-fiber membranes at different temperatures.

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by bulk diffusion equals that by the surface exchange reactions, the resistance by bulk diffusion can be negligible and the surface exchange reactions become the rate-limiting step. In this case, eq 2 then reduces to JO2 )

kr[(pO′2)0.5 - (pO2′′)0.5] Rm Rm p ′′ 0.5 + (pO′)2 0.5 Ro ( O2) Rin

(4)

With the assumptions of (1) isothermal operation, (2) constant oxygen concentration on the air side, (3) ideal gas behavior, (4) plug flow, and (5) negligible air leakage, the mass conservation equations in the fiber lumen can be written as

( )

′′ d pO2V ) 2πRmJO2 dl RT

(po - pO′′2)V RT

) FHe

(5) (6)

Figure 8. Comparison of modeling results with experimental data. (s) Modeling results from Xu’s kinetic parameters at (a) 1000, (b) 950, (c) 900, (d) 850, or (e) 800 °C. (- - -) Modeling results from regressed kinetic parameters at (a′) 1000, (b′) 950, (c′) 900, (d′) 850, or (e′) 800 °C.

with the boundary conditions l ) 0 and pO′′2 ) 0

(7)

where po is the overall pressure in fiber lumen, FHe is the molar feed flow rate of sweep gas, R is the ideal gas constant, and T is the temperature. By use of the kinetic parameters obtained by Xu and Thomson10 on the LSCF disk membranes, the oxygen fluxes in the hollow-fiber membranes at different conditions can be calculated by solving governing equations 5-7, as shown in Figure 8. As can be seen, all the experimental fluxes for the fiber membrane surpass the modeling results at the same conditions. For example, the experimental oxygen fluxes at 850 °C have already reached the modeling fluxes at 1000 °C. This indicates that the present LSCF hollow-fiber membranes have much better performance for oxygen permeation than Xu’s disk membranes and the actual kinetic parameters for these fiber membranes must be higher than those obtained by Xu and Thomson.10 It is because the hollow-fiber membranes possess a highly asymmetric structure that the effective thickness for oxygen permeation is smaller than the apparent wall thickness of the hollow fibers and the resistance of the porous layer to oxygen transportation may be negligible. Furthermore, due to the existence of the porous layer, the membrane area for surface exchange reactions is actually increased remarkably, resulting in much higher surface exchange kinetics. It is known that the oxygen permeation process in the perovskite membranes tends to be rate-controlled by the surface exchange kinetics at lower temperatures.29,30 Under this condition, the surface exchange reaction constant kr can be evaluated by fitting the experimental data with the least-squares method, as shown in Figure 9 where kr is plotted against temperature by the Arrhenius equation. It can be seen that kr for the present fiber membranes is higher than that obtained by Xu and Thomson10 but exhibits a lower activation energy, 116 kJ mol-1 compared to their activation energy (241 kJ mol-1),10 which was calculated from the slope of the Arrhenius line at lower temperatures (dashed line in Figure 9). At lower temperatures (i.e., 700 °C), kr for the fiber membranes may be 2 orders of magnitude higher than the data of Xu and Thomson,10 but as temperature is increased to 1000 °C, the two kr values would become close, namely, 4.86 × 10-6 mol cm-2 s-1 for the present hollow-fiber membranes and 2.60 × 10-6 mol cm-2 s-1 for their disk membranes. Generally speaking, the activation energy for oxygen permeation depends on the property of membrane

Figure 9. Arrhenius relationship between surface exchange reaction rate kr and temperature

material and also may be changed with the change in ratecontrolling process. The activation energy for surface-exchange kinetics is higher than that for bulk diffusion. The lower activation energy of the present hollow-fiber membranes indicates that the surface of the present hollow-fiber membrane is more active than the disk membranes of Xu and Thomson.10 But it is higher than the values for the bulk diffusion controlling process.29 On the other hand, when the temperature is higher than 900 °C, the kr value for the fiber membranes would negatively deviate from the Arrhenius line fitted by the lowtemperature values, and the activation energy tends to be decreased. This implies the bulk diffusion has a rate-limiting effect on the oxygen permeation process. Therefore, the bulk diffusion resistance to oxygen permeation cannot be neglected any longer at higher temperatures, although the effective membrane thickness of the hollow fiber is as low as 88 µm. 4. Conclusions La0.6Sr0.4Co0.2Fe0.8O3-R (LSCF) perovskite powders of submicrometer size were synthesized by the sol-gel autocombustion method. Through the phase inversion/sintering technique with a mixture of 94 wt % NMP and 6 wt % water as the internal coagulant, gastight LSCF hollow-fiber membranes with a highly asymmetric structure, comprising a dense outside layer of ca. 88 µm thickness and a porous inside layer of ca. 299 µm thickness, were fabricated in a single step. The dense hollowfiber membranes possess 2.6-10.5 times the oxygen permeation fluxes of the sandwich-structured hollow-fiber membranes in the temperature range of 650-1000 °C. Oxygen permeation through the LSCF hollow-fiber membranes is primarily controlled by the surface exchange kinetics at lower temperatures,

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but the ionic bulk diffusion will have a rate-limiting effect when the temperature is higher than 900 °C. Acknowledgment We gratefully acknowledge the research funding provided by the National High Technology Research and Development Program of China (2006AA03Z464) and the National Natural Science Foundation of China (NNSFC, 20076025). Nomenclature Am ) membrane area ) [2π(Ro - Rin)L]/ln (Ro/Rin), cm2 DV ) dffective diffusivity of oxygen vacancy, cm2 s-1 FHe ) molar feed flow rate of sweep gas kf ) forward surface reaction rate constant of eq 3, cm Pa-0.5 s-1 kr ) reverse surface reaction rate constant of eq 3, mol cm-2 s-1 JO2 ) oxygen permeation flux, mL cm-2 min-1 or mol cm-2 min-1 L ) effective length for oxygen permeation of hollow-fiber membrane, cm pO2′ and pO2′′ ) oxygen partial pressure, upstream and downstream, respectively, Pa po ) overall pressure in fiber lumen, Pa R ) ideal gas constant, 8.314 J mol-1 K-1 Rin ) inner radius of hollow fiber, cm Rm ) log-mean radius of hollow fiber ) (Ro-Rin)/ln (Ro/Rin), cm Ro ) outer radius of hollow fiber, cm T ) temperature, K V ) flow rate of permeate gas stream on sweep side, cm3 min-1 xO2 ) oxygen concentration in sweep gas effluent, % xN2 ) nitrogen concentration in sweep gas effluent, %

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ReceiVed for reView July 6, 2008 ReVised manuscript receiVed October 7, 2008 Accepted October 17, 2008 IE8010462