Oxygen selective ceramic hollow fiber membranes

Journal of Membrane Science 246 (2005) 103–108

Short communication

Oxygen selective ceramic hollow fiber membranes Shaomin Liu, George R. Gavalas∗ Division of Chemistry and Chemical Engineering, California Institute of Technology, Caltech 210-41, Pasadena, CA 91125, USA Received 22 June 2004; received in revised form 9 September 2004; accepted 15 September 2004 Available online 19 November 2004

Abstract Oxygen ion conducting Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ hollow fiber membranes with o.d. 1.15 mm and i.d. 0.71 mm were fabricated using a sequence of extrusion, gelation, coating and sintering steps. The starting ceramic powder was synthesized by combined EDTA–citrate complexing followed by thermal treatment at 900 ◦ C. The powder was then dispersed in a polymer solution, and extruded through a spinerette. After gelation, an additional thin coating of the ceramic powder was applied on the fiber, and sintering was carried out at 1190 ◦ C to obtain the final ceramic membrane. The fibers were characterized by SEM, and tested for air separation at ambient pressure and at temperatures between 700 and 950 ◦ C. The maximum oxygen flux measured was 5.1 mL/min/cm2 at 950 ◦ C. © 2004 Elsevier B.V. All rights reserved. Keywords: Hollow fiber; Inorganic membrane; Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ ; Perovskite; Oxygen separation

1. Introduction Ceramic membranes possessing mixed oxygen ion and electronic conduction have attracted much interest for their potential use in air separation and selective oxidation of light hydrocarbons to synthesis gas and other products [1–3]. Most previous work in this area employed membranes in the form of disks or tubes fabricated using conventional methods. Thin tubes, or hollow fibers, are a promising alternative geometry because of their larger membrane area per unit volume [4,5]. Among mixed conductor materials perovskites (ABO3 ) have been studied extensively, and Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ (BSCF), in particular, exhibits high oxygen ion conductivity and structural stability [6,7]. It was, therefore, chosen as the fiber material in the present study. 2. Experimental 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 ethylene∗

Corresponding author. Tel.: +1 626 395 4152; fax: +1 626 568 8743. E-mail address: [email protected] (G.R. Gavalas).

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.09.028

diaminetetraacetic acid (EDTA) [>99.5%, Aldrich] were used as received. Polyethersulfone (PESf) [Radel A-300, Solvay Advanced Polymers], and N-methyl-2-pyrrolidone (NMP) [EMD Chemicals Inc.] were used for preparing the suspension. Polyvinylpyrrolidone (PVP) [Mw = 1,300,000; Alfa Aesar] was used as an additive. Tap water was used as both the internal and the external gelation medium (nonsolvent). 2.2. Preparation of the BSCF powder The powder was prepared by combined EDTA–citrate complexation. For this purpose EDTA powder was added under magnetic stirring into aqueous ammonium hydroxide (28.0–30.0%) 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 ratios of EDTA, citric acid, and total metal ions in the final solution were 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 250 ◦ C for 12 h to obtain a black solid mass which will be referred

104

S. Liu, G.R. Gavalas / Journal of Membrane Science 246 (2005) 103–108

Fig. 1. Schematic of permeation cell.

to as the powder precursor. The powder precursor was heat-treated at 900 ◦ C for 5 h under airflow of 0.5 L/min to remove residual carbon and form the desired perovskite structure. A portion of the BSCF powder (18 g) was ballmilled for 4 days in 300 mL of ethanol to obtain a suspension of particles of size below 3 ␮m. All powder preparation steps were carried out in a well-ventilated fume hood to prevent inhalation of decomposition gases and particles.

2.4. Characterisation

2.3. BSCF fiber preparation

BSCF fibers were connected on both sides with smalldiameter quartz tubes as shown in Fig. 1, and sealed with Ag paste. A few repetitions of the Ag paste coating and drying were carried out to obtain an adequate seal but a small residual leakage from this seal could not be eliminated. To conduct permeation measurements, a fiber was placed directly into a tube furnace and, in order to maintain constant feed composition along the fiber, an airflow of 310 mL/min was passed near the central part of the furnace through a small quartz tube. The permeate gas was collected from the fiber lumen by passing He sweep gas at slightly above atmospheric pressure and conducted to a 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 pressure drop in the fiber at the highest flowrate was less than 0.1 bar. The flowrate

PESf was slowly added to NMP under stirring to form the polymer solution. The BSCF powder with agglomerate size less than 40 ␮m was then added 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 then pressurized with nitrogen to 40 psig. Extrusion was carried out through a tube-in-orifice spinneret with orifice diameter and inner diameter 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. A coating suspension was separately prepared by dispersing the powder particles previously milled to below 3 ␮m size in ethanol. Dried BSCF–polymer fibers 18 cm long were immersed into the coating solution for a few seconds and then rapidly withdrawn to obtain a coating on the o.d. The coated fibers were dried at room temperature for 20 min. Coating and drying were repeated a total of three times to ensure elimination of surface defects. The coated BSCF–polymer hollow fibers were heated in a furnace under airflow at 100 mL/min. The temperature of the furnace was initially raised to 800 ◦ C at about 3 ◦ C/min and maintained for 10 h at the final temperature to decompose and remove the polymer and any remaining carbon residue. Sintering was then carried out at 1190 ◦ C for 5 h to obtain an impermeable structure. Finally, the fibers were cooled to room temperature at 2 ◦ C/min.

The calcined BSCF powders were examined using an Xray diffractometer (Scintag Pad V), while the BSCF–polymer fibers, the coated fibers, and the sintered fibers were examined using a scanning electron microscope (LEO 1550 VP field emission SEM). 2.5. Oxygen permeation measurements

Fig. 2. XRD pattern of BSCF powder.

S. Liu, G.R. Gavalas / Journal of Membrane Science 246 (2005) 103–108

105

Fig. 3. SEM of a precursor fiber: (a) cross-section; (b) membrane wall; (c) high magnification of cross section near outside surface; (d) outside surface; (e) cross section near outside surface after coating; (f) outside surface after coating.

was measured by an electronic flowmeter downstream of the fiber. The leakage of oxygen through the seal was usually less than 1% of the total oxygen flux, and fractionally decreased with temperature and sweep flowrate. Assuming that leakage of nitrogen and oxygen through pores or cracks is in accordance with Knudsen diffusion, the√ fluxes of leaked N2 and Leak = 32/28 × 0.79 : 0.21 = O2 are related by JNLeak : J O 2 2

4.02. The O2 permeation rate was then calculated as follows:
 CN F JO2 (mL/ min /cm ) = CO − 4.02 S 2

(1)

where CO , CN are the oxygen concentration and nitrogen concentration calculated from the GC measurements, F the flowrate of the sweep stream, and S the hollow fiber

106

S. Liu, G.R. Gavalas / Journal of Membrane Science 246 (2005) 103–108

Fig. 4. SEM of a fiber after sintering at 1190 ◦ C for 5 h: (a) cross-section; (b) membrane wall; (c) outside surface.

membrane area calculated according to S=

πL(Do − Di ) ln(Do /Di )

(2)

In Eq. (2), L, Do , and Di are length, outside diameter, and inside diameter of the fiber, respectively.

3. Results and discussion Fig. 2 shows the room temperature X-ray diffraction pattern of the BSCF powder. It displays a single-phase perovskite with cubic structure. SEM micrographs of a BSCF–polymer fiber and a coated fiber, prepared from a suspension of 70.6 wt.% BSCF, 5.9 wt.% PESf, and 23.5 wt.% NMP are shown in Fig. 3. The fiber o.d. and i.d. obtained from this figure are 2.02 and 1.15 mm. Fig. 3(b) shows porous structures near the center, and denser sponge-like structures near the inside and outer surfaces, resulting from complicated diffusion and phase separation phenomena occurring upon immersion in water. The micrograph of the outside fiber surface in Fig. 3(d) shows BSCF particles embedded in the polymer. Finally, Fig. 3(e) and (f) shows the microstructure of the coated fibers. In the cross section micrograph (Fig. 3e), a uniform 5 ␮m coating layer can be observed,

and Fig. 3f shows small BSCF particles on the fiber surface. Gas-tight BSCF membranes were obtained by sintering the coated BSCF–PESf fibers at 1190 ◦ C for 5 h. SEM micrographs of a dense BSCF fiber are shown in Fig. 4. As can be seen in Fig. 4(a), the o.d. and i.d. of the precursor fiber shrunk from 2.02 and 1.15 mm to 1.15 and 0.71 mm in the final fiber. The shrinkage in the fiber length was about 25%. Comparison of Fig. 3(b) with Fig. 4(b) illustrates that the initial finger-like voids in the coated BSCF–PESf fiber were sintered into small isolated pores. Microstructures on the outer surface of the sintered fiber shown in Fig. 4(c) reveal that the initial small BSCF particles had coalesced into larger particles. Upon testing the final BSCF fibers for gas leakage at room temperature, most fibers obtained by sintering precursors that had not been coated were not gas-tight, but those from coated precursors were gas-tight. Due to the limited uniform heating length (7 cm) of the furnace, BSCF fibers with length below 9 cm (two sealing sides occupy 2 cm length) were chosen for permeation testing. Permeation measurements were conducted at temperatures below the melting point 962 ◦ C of the Ag seal and at different He sweep gas flowrates to vary the oxygen pressure at the permeate side.

S. Liu, G.R. Gavalas / Journal of Membrane Science 246 (2005) 103–108

Fig. 5. Effect of temperature on the oxygen flux through a fiber and on the nitrogen leakage through the Ag sealing (He sweep rate 82 mL/min; effective length 69 mm; o.d. 1.15 mm; i.d. 0.71 mm).

The experimental results for oxygen permeation under different conditions are shown in Figs. 5 and 6. Fig. 5 shows that the oxygen flux increases sharply with temperature. At helium sweep rate 82 mL/min, the oxygen flux rose from 0.69 to 3.83 mL/min/cm2 as the temperature increased from 700 to 950 ◦ C. Fig. 5 also shows the effect of temperature on the nitrogen flux at helium sweep rate 82 mL/min. The flux decreased with increasing temperature from 0.04 mL/min/cm2 at 700 ◦ C to 0.009 mL/min/cm2 at 800 ◦ C and became undetectable by GC at temperatures above 900 ◦ C. Given that the fiber was gas-tight at room temperature (using epoxy sealing) the nitrogen flux was due to leakage through the high temperature seal. Evidently, softening of the Ag at the higher temperatures reduced somewhat the seal porosity. Fig. 6 shows that the oxygen flux increased with increasing helium sweep rate. At 900 ◦ C, for example, increasing the helium flowrate from 18.8 to 217 mL/min raised the oxygen flux from 1.54 to 3.84 mL/min/cm2 . The effect of the sweep flowrate on the oxygen flux has been noted by other researchers [8]. The flux increase is obviously due to the decrease of the oxygen pressure at the permeate side. The strong dependence of the flux on the oxygen pressure can be explained by expressions of the form: Flux = Ak(T )G [p(feed)n − p(perm)n ]


p(feed) Flux = Ak(T ) p(perm)

n

with n > 0

with n < 1

(3)

107

Fig. 6. Effect of helium sweep rate on the oxygen flux through fiber of Fig. 5.

To our knowledge, this is the first report of oxygen permeation though oxygen ion conducting membranes in hollow fiber form. Using a BSCF ceramic disk (with thickness 1.5 mm and membrane area 0.85 cm2 ), Shao et al. [6] reported oxygen flux 1.52 mL/min/cm2 at 950 ◦ C and 80 mL/min/cm2 He sweep rate. Under similar conditions the oxygen flux in this work was 3.83 mL/min/cm2 . A tubular BSCF membrane (o.d. 7.96 mm, i.d. 4.56 mm, length 1.77 cm) was tested by Wang et al. [8] who reported oxygen flux of 0.98 mL/min/cm2 at 850 ◦ C and 24 mL/min/cm2 sweep rate. Under similar conditions the flux measured here was 1.39 mL/ min/cm2 .

4. Conclusions Gas-tight BSCF mixed conducting hollow fibers were fabricated using extrusion from a polymer solution followed by gelation and sintering. The oxygen flux increased with temperature and with the He sweep flowrate indicating strong dependence on the permeate side oxygen pressure.

Acknowledgement The authors gratefully acknowledge the support provided by the Petroleum Research Fund of the American Chemical Society, Grant No. 38162-AC5.

(4)

where the p’s are partial pressures of oxygen at the feed side and permeate side, k(T) a temperature factor (incorporating the mobility), and G a geometric factor. Obviously, the flux also depends on the feed-side oxygen pressure, but this pressure was not varied in our experiments. The surface exchange reaction may also offer resistance to the flux, hence, no attempt was made to fit the data to a model like that of Eq. (3) or (4).

References [1] Z.P. Shao, H. Dong, G.X. Xiong, Y. Gong, W.S. Yang, Performance of a mixed-conducting ceramic membrane reactor with high oxygen permeability for methane conversion, J. Membrane Sci. 183 (2001) 181. [2] S.P.S. Badwal, F.T. Ciacchi, Ceramic membrane technologies for oxygen separation, Adv. Mater. 13 (2001) 993. [3] C.S. Chen, S.J. Feng, S. Ran, D.C. Zhu, W. Liu, H.J.M. Bouwmeester, Conversion of methane to syngas by a membrane-

108

S. Liu, G.R. Gavalas / Journal of Membrane Science 246 (2005) 103–108

based oxidation-reforming process, Angew. Chem. Int. Ed. 42 (2003) 5196. [4] S. Liu, X.Y. Tan, K. Li, R. Hughes, Preparation and characterisation of SrCe0.95 Yb0.05 O2.975 hollow fibre membranes, J. Membrane Sci. 193 (2001) 249. [5] X. Tan, K. Li, Modeling of air separation in a LSCF hollow fiber membrane module, AIChE J. 48 (2002) 1469. [6] Z.P. Shao, W.S. Yang, Y. Cong, H. Dong, J.H. Tong, G.X. Xiong, Investigation of the permeation behaviour and stability of

a Ba0.5 Sr0.5 Co0.8 Fe0.2 O3 oxygen membrane, J. Membrane Sci. 172 (2000) 177. [7] A.C. van Veen, M. Rebeilleau, D. Farrusseng, C. Mirodatos, Studies on the performance stability of mixed conducting BSCFO membranes in medium temperature oxygen permeation, Chem. Commun. 32 (2003) 32. [8] H. Wang, Y. Cong, W.S. Yang, Oxygen permeation study in a tubular Ba0.5 Sr0.5 Co0.8 Fe0.2 O3 oxygen permeable membrane, J. Membrane Sci. 210 (2002) 259.