Ind. Eng. Chem. Res. 2005, 44, 7633-7637
7633
Preparation of Oxygen Ion Conducting Ceramic Hollow-Fiber Membranes Shaomin Liu and George R. Gavalas* Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
Hollow-fiber membranes of mixed conducting perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ were prepared using a sequence of extrusion, gelation, and sintering steps. For this purpose, a finely divided mixture of the component oxides was prepared by the modified Pechini technique and suspended in a polymer solution. The suspension was extruded through a spinneret and gelled in water. The resulting fiber was first heated at 800 °C to remove the polymer and then at 1100 °C to form the perovskite and simultaneously sinter the particles to a gastight membrane. The fibers were characterized by scanning electron microscopy and tested for air separation at ambient pressure and temperature between 700 and 950 °C. The maximum oxygen flux measured was 3.9 mL/min/cm2 at 950 °C and 0.022 atm of average permeate oxygen partial pressure. 1. Introduction Ceramic membranes of mixed ionic-electronic conductivity are promising for use in high-temperature air separation and selective oxidation of light hydrocarbons to synthesis gas or oxygenates.1-4 One attractive material for this purpose is the perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), which possesses good mixed ionic (oxygen ion)-electronic conductivity and good structural stability.5,6 Membranes of this perovskite have been previously prepared in the form of disks or tubes of approximately 1 cm diameter by conventional ceramic processing techniques.7 An alternative membrane geometry providing a larger membrane area per unit volume is that of thin tubes or hollow fibers.8 These fibers can be conveniently prepared by “wet spinning” (extrusion of a ceramic-polymer suspension and gelation in water), followed by sintering to the final dense membrane. Regardless of whether the membranes are made by pressing,10 extrusion,11,12 or wet spinning,9,13,14 the preparation involves (i) making the perovskite powder by reacting the individual oxides or carbonates at temperatures in excess of 900 °C, (ii) forming the “green” membrane by pressing or wet spinning the ceramic powder, and (iii) sintering at 1200 °C or higher temperature to the final dense membrane. The first step can be carried out by direct reaction of the oxide powders or by using a solution (e.g., the Pechini) technique to provide enhanced component mixing and more facile reaction i. Either alternative requires two high-temperature steps. In a previous paper, we reported membrane preparation by the above steps i-iii9 using the Pechini technique in step i. In the present brief paper, we describe an alternative processing technique replacing step i by step i′ in which the fine-grained oxide mixture prepared by the Pechini technique is used directly in step ii without first converting it to the perovskite. The mixed oxide powder will be referred to below as the precursor powder. Eliminating one of the two high-temperature steps results in a simpler and lower energy preparation that may be applicable to materials other than the BSCF * To whom correspondence should be addressed. Tel.: (626) 395-4152. Fax: (626) 568-8743. E-mail: gavalas@ cheme.caltech.edu.
perovskite. The preparation by steps i′, ii, and iii is summarized below along with membrane characterization and measurements of oxygen permeation. 2. Experimental Section 2.1. Materials. Ba(NO3)2, Co(NO3)2 , Fe(NO3)3, and Sr(NO3)2 (all >98%, Alfa Aesar), citric acid (>99%, Ajax), and ethylenediaminetetraacetic acid (EDTA; >99.5%, Aldrich) were used as received. Poly(ether sulfone) (PESf; Radel A-300, Solvay Advanced Polymers) and N-methyl-2-pyrrolidone (NMP; EMD Chemicals Inc.) were used for preparation of the suspension. Poly(vinylpyrrolidone) (PVP; Mw ) 1 300 000, Alfa Aesar] was used as an additive. Ag paste (Shanghai, China) was used as the high-temperature sealant. Tap water was used as both the internal and external gelation media (nonsolvent). 2.2. Preparation of the BSCF Perovskite or Precursor Powder. The powder was prepared by the modified Pechini technique. The complexing mixture was prepared by adding EDTA powder into aqueous ammonium hydroxide (28.0-30.0%) under magnetic stirring to form a water-soluble ammonium salt. In a separate beaker, stoichiometric quantities of Ba(NO3)2, Sr(NO3)2, Co(NO3)2, Fe(NO3)3, and citric acid in granular form were dissolved in distilled water. The EDTA solution was then added into the solution of the metal ions and citric acid under stirring. The molar ratio of EDTA, citric acid, and total metal ions in the final solution was 1:2:1. The final mixture was heated at 100 °C for several hours to remove excess water until a viscous gel was obtained. This gel was heated in a furnace at 600 °C for 12 h to obtain a black solid mass called the oxides mixture or precursor powder. All powder preparation steps were carried out in a wellventilated fume hood to prevent inhalation of decomposition gases and particles. 2.3. BSCF Fiber Preparation. The precursor powder was added to the polymer solution, and the mixture was stirred for 24 h to ensure uniform distribution of the particles. The resulting suspension was subsequently degassed at room temperature and transferred to a stainless steel reservoir, which was pressurized with nitrogen to 40 psig. Extrusion was carried out through a tube-in-orifice spinneret with the orifice and inner
10.1021/ie040279i CCC: $30.25 © 2005 American Chemical Society Published on Web 05/20/2005
7634
Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005
Figure 1. XRD of (a) oxides mixture powder, (b) fiber after heat treatment at 800 °C for 15 h, and (c) sintered fiber at 1100 °C for 8 h (O, 3, b, and 0 stand for the peaks of SrCO3, Co3O4, BaSO4, and BSCF perovskite, respectively).
diameters 2.5 and 0.72 mm, respectively. The fibers emerging from the spinneret at 10 m/min were passed through an air gap of 4 cm and immersed in a water bath to complete gelation. After thorough washing in water, the gelled hollow fibers were dried in an oven at 150 °C. The dried hollow fibers were heated in a furnace to 800 °C at about 3 °C/min and maintained at that temperature for 15 h to decompose and remove the polymer. Sintering was then carried out at 1100 °C for 8 h to obtain an impermeable structure. The fibers were finally cooled to room temperature at 2 °C/min.
2.4. Characterization. The powder precursor or the crushed heat treated fibers were examined using an X-ray diffractometer (Scintag Pad V), while the oxidespolymer fibers and the sintered fibers were examined using a scanning electron microscope (Leo 1550 VP field emission SEM). 2.5. Oxygen Permeation Measurements. BSCF fibers were connected on both sides with small-diameter quartz tubes and sealed with Ag paste. To conduct permeation measurements, a fiber was placed directly into a tube furnace. The furnace was heated at a rate of 3 °C/min to 950 °C and maintained at 950 °C for 60 min to soften the Ag seals. The softened Ag provided the required sealing. After cooling to 600 °C and maintaining that temperature for 6 h to stabilize the membrane, the furnace was reheated to the measurement temperature. During measurement, an air flow of 300 mL/min was passed through a small quartz tube located outside of the membrane in the furnace in order to maintain a constant composition along the fiber. The permeate gas was collected from the fiber lumen by a He sweep gas at slightly above atmospheric pressure and conducted to a gas chromatograph (GC; HP 5890II series) for analysis. A washed molecular sieve 5A (6 ft × 1/8 in. × 0.085 in.; 80/100 mesh) was used for the separation of oxygen and nitrogen. The flow rate was measured by a bubble flowmeter downstream of the fiber. Details of the oxygen permeation measurements can also be found elsewhere.9 3. Results and Discussion The precursor powder in step i′ was obtained using the modified Pechini technique. An EDTA-citratemetal (Ba, Sr, Co, and Fe) complex was first made and heated at 600 °C to decompose and oxidize the organics. Figure 1a shows the room-temperature X-ray pattern of the precursor powder after heat treatment at 600 °C. The pattern indicates a mixture of oxides, carbonates, and some amorphous material but does not indicate any perovskite phase. The powder was sieved, and only particles below 90 µm were used in step ii, although the
Figure 2. SEM of the hollow-fiber precursor: (a) lower magnification of the cross section; (b) cross section; (c) external surface. Bar ) 100 µm (a), 100 µm (b), and 10 µm (c).
Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7635
Figure 3. SEM of the hollow fibers (a and b) after polymer removal and (c-e) after sintering at 1100 °C: (a) cross section; (b) external surface; (c) lower magnification of the cross section; (d) cross section; (e) external surface. Bar ) 100 µm (a), 1 µm (b), 100 µm (c), 20 µm (d), and 2 µm (e).
grain size was only about 50 nm, as estimated by applying Scherrer’s equation. In step ii, the precursor powder was mixed with a polymer solution and extruded through a spinneret. Subsequent immersion in water was used for gelation of the polymer and attainment of sufficient strength for subsequent handling. After gelation (phase inversion) in water, the oxide-polymer fibers were dried to be ready for step iii. SEM micrographs of an oxidepolymer fiber prepared from a suspension containing 61.51 precursor powder, 7.67 PESf, 30.69 NMP, and 0.13 PVP, all in wt %, are shown in Figure 2. The fiber outside and inside diameters measured in this figure are 2.16 and 1.32 mm, respectively. Figure 2b shows a porous region near the inner surface and a denser spongelike region near the outer surface, resulting from diffusion and phase separation phenomena occurring upon immersion in water. The micrograph of the outside fiber surface in Figure 2c shows precursor powder (oxide mixture) particles embedded inside the polymer. The final step iii involves heating at progressively higher temperatures. The oxide-polymer fibers were
first heated at 800 °C for 15 h to decompose and remove the polymer and any residual carbon. During this period of heat treatment, the majority of the fiber material was transformed to the perovskite, as shown in Figure 1b. Parts a and b of Figure 3 show SEM micrographs of a fiber after polymer removal. The microstructure of the outer surface (Figure 3b) consists of uniform particles weakly connected to each other, unlike the structure of the oxide-polymer fiber shown in Figure 2c. The fibers were finally sintered at 1100 °C for 8 h. Sintered fibers can be handled without breaking; however, their bending strength was not measured. Sintered fibers were crushed and examined with an X-ray diffractometer. As shown in Figure 1c, the crystalline phases of the sintered fibers are mainly cubic perovskite together with a small amount of BaSO4. BaSO4 is derived from a reaction between barium oxide and sulfur dioxide produced from the PESF polymer in the oxygencontaining atmosphere. SEM micrographs of a BSCF fiber that was sintered at 1100 °C are shown in Figure 3c-e. Figures 2a and 3c show that the fiber outside and inside diameters shrank from 2.16 and 1.32 mm to 1.36
7636
Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005
Figure 4. SEM of the BSCF hollow fiber (a and b) after polymer removal and (c and d) after sintering at 1100 °C: (a and c) cross section; (b and d) external surface. The fiber was made using protocol i-iii. Bar ) 100 µm (a), 10 µm (b), 100 µm (c), and 10 µm (d).
and 0.86 mm, respectively. A comparison of Figures 2b and 3d illustrates that the initial fingerlike voids in the ceramic-polymer fiber were converted to small isolated pores. Further inspection of the microstructures on the outer surface of the sintered fibers shown in Figure 3b,e reveals that small BSCF particles had coalesced into larger particles to form a dense layer. For comparison purposes, a BESF fiber was also made using protocol i-iii. A BSCF powder was prepared by heating the component oxide powder at 900 °C for 5 h. The product BSCF powder was ground to the same particle size, passing a testing sieve (with openings of 90 µm), the same as that used to sieve the oxide mixture in the preparation just described. Subsequently, a suspension consisting of 64.2 BSCF perovskite, 7.1 PESf, 28.5 NMP, and 0.10 PVP, all in wt %, was extruded, gelled, and dried as previously described. After polymer decomposition at 800 °C, the fiber was examined using SEM. Figure 4 shows the cross-sectional area and outside surface and indicates that the BSCF particle arrangement in the second fiber is irregular and the pores inside the membrane are too large to be sintered into a dense structure, even at a temperature of 1190 °C. Because of its lower melting point (1200 °C), the BSCF particles prepared at 900 °C were aggregated into larger particulates, which were difficult to disperse uniformly inside the polymer solution. However, the precursor powder prepared at 600 °C is easy to disperse inside in the polymer solution. When the oxide-polymer fibers were heated above 800 °C, the component oxides reacted to form smaller perovskite particles, making the subsequent densification possible at 1100 °C. BSCF fibers of 7 cm length were tested in air separation at high temperatures. The experimental results for oxygen permeation under different conditions
are shown in Figures 5 and 6. Figure 5 shows that the oxygen flux increases sharply with temperature. At a helium sweep rate of 34.5 mL/min/cm2, the oxygen flux rose from 0.3 to 2.9 mL/min/cm2 as the temperature was increased from 700 to 950 °C, from which an activation energy of 93.6 kJ/mol was calculated. A dense ionicelectronic membrane such as the one tested in this study would be permeable to oxygen only. However, because of leaks through sealing areas, a low nitrogen concentration was detected in the permeate side. For example, at a helium sweep rate of 34.5 mL/min/cm2 and temperatures of 700 and 750 °C, the detected N2 mole fractions at the permeate side were 6% and 0.09%, respectively. Given that the fiber was gastight at room temperature (using epoxy sealing), the nitrogen flux was due to leakage through the high-temperature seal. The N2 flux decreased with an increase in the temperature and became undetectable by a GC at temperatures above 750 °C (the detection limit was 0.004 mL/min/
Figure 5. Effect of the temperature on the oxygen flux through a fiber (He sweep rate, 34.5 mL/min/cm2; effective length, 70 mm; o.d., 1.33 mm; i.d., 0.83 mm).
Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7637
“asymmetric” structures. It should also be mentioned that, although the techniques reported here are based on the fabrication of gastight BSCF hollow fibers, they are also suitable for the fabrication of thin tubes or hollow fibers of other perovskite materials with a general stoichiometric formula AyB1-yMxN1-xO3-a. It may also be possible to process fluorite-type structuredYSZ-based materials in a similar fashion. Acknowledgment We gratefully acknowledge the support provided by the Petroleum Research Fund of the American Chemical Society (Grant 38162-AC5). Figure 6. Effect of the helium sweep rate on the oxygen flux through the fiber of Figure 5.
cm2). Evidently, softening of the Ag at the higher temperatures reduced the seal porosity. Figure 6 shows that the oxygen flux increased with an increase in the helium sweep rate because of lowering of the permeate side pressure of oxygen. For example, at 900 °C, increasing the helium flow rate from 8.3 to 84 mL/min/cm2 raised the oxygen flux from 1.2 to 2.5 mL/min/cm2. The oxygen flux measured in this work compares favorably with the values reported in the literature. For a tubular BSCF membrane, Wang et al. reported an oxygen flux of 0.98 mL/min/cm2 at 850 °C and a 24 mL/min/cm2 sweep rate.7 Under similar conditions, the flux measured here was 1.5 mL/min/cm2. Using a BSCF ceramic disk (of thickness 1.5 mm and membrane area 0.85 cm2), Shao et al. reported an oxygen flux of 1.52 mL/min/cm2 at 950 °C and a 80 mL/ min/cm2 He sweep rate.5 Under similar conditions, the flux measured in this work was 3.8 mL/min/cm2. 4. Conclusion A finely divided mixture of component oxides can be prepared using a wet chemical route followed by heat treatment at 600 °C. Using this precursor powder as the starting material, an oxide-polymer fiber can be prepared by extrusion and gelation. Heat treating the ceramic-polymer fiber at temperatures as low as 1100 °C forms the perovskite and simultaneously sinters the particles to a gastight membrane. In air separation, the maximum oxygen flux measured was 3.9 mL/min/cm2 at 950 °C and at 0.022 atm of average permeate oxygen partial pressure at the permeate side. It should be noted that the final membranes are gastight but not fully densified. As long as there is no connected porosity across the membrane, separation by mixed conduction will take place. In fact, the ideal structure would be a dense skin supported on a highly porous substructure, as is the case of polymeric membranes. At this time, we do not know how to go about preparing such
Literature Cited (1) Shao, Z. P.; Dong, H.; Xiong, G. X.; Gong, Y.; Yang, W. S. Performance of a mixed-conducting ceramic membrane reactor with high oxygen permeability for methane conversion. J. Membr. Sci. 2001, 183, 181. (2) Badwal, S. P. S.; Ciacchi, F. T. Ceramic membrane technologies for oxygen separation. Adv. Mater. 2001, 13, 993. (3) Chen, C. S.; Feng, S. J.; Ran, S.; Zhu, D. C.; Liu, W.; Bouwmeester, H. J. M. Conversion of methane to syngas by a membrane-based oxidation-reforming process. Angew. Chem., Int. Ed. 2003, 42, 5196. (4) Bouwmeester, H. J. Dense ceramic membranes for methane conversion. Catal. Today 2003, 82, 141. (5) Shao, Z. P.; Yang, W. S.; Cong, Y.; Dong, H.; Tong, J. H.; Xiong, G. X. Investigation of the permeation behaviour and stability of a Ba0.5Sr0.5Co0.8Fe0.2O3 oxygen membrane. J. Membr. Sci. 2000, 172, 177. (6) van Veen, A. C.; Rebeilleau, M.; Farrusseng, D.; Mirodatos, C. Studies on the performance stability of mixed conducting BSCFO membranes in medium-temperature oxygen permeation. Chem. Commun. 2003, 32, 32. (7) Wang, H.; Cong, Y.; Yang, W. S. Oxygen permeation study in a tubular Ba0.5Sr0.5Co0.8Fe0.2O3 oxygen permeable membrane. J. Membr. Sci. 2002, 210, 259. (8) Tan, X.; Li, K. Modeling of air separation in a LSCF hollow fiber membrane module. AIChE J. 2002, 48, 1469. (9) Liu, S. M.; Gavalas, G. R. Oxygen selective ceramic hollow fiber membranes. J. Membr. Sci. 2005, 246, 103. (10) Xu, N.; Li, S.; Jin, W.; Shi, J.; Lin, Y. S. Experimental and modeling study on tubular dense membranes for oxygen permeation. AIChE J. 1999, 45, 2519. (11) Lu, Y.; Dixon, A. G.; Moser, W. R.; Ma, Y. H.; Balachandran, U. Oxygen-permeable dense membrane reactors for the oxidative coupling of methane. J. Membr. Sci. 2000, 170, 27. (12) Zhu, D. C.; Xu, X. Y.; Feng, S. J.; Liu, W.; Chen, C. S. La2NiO4 tubular membrane reactor for conversion of methane to syngas. Catal. Today 2003, 82, 151. (13) Liu, S.; Tan, X.; Li, K.; Hughes, R. Preparation and characterisation of SrCe0.95Yb0.05O2.975 hollow fibre membranes. J. Membr. Sci. 2001, 193, 249. (14) Luyten, J.; Buekenhoudt, A.; Adriansens, W.; Cooymans, J.; Weyten, H.; Servaes, F.; Leysen, R. Preparation of LaSrCoFeO3-x membranes. Solid State Ionics 2000, 135, 637.
Received for review November 15, 2004 Accepted March 22, 2005 IE040279I
