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RESEARCH NOTES Tubular Dense Perovskite Type Membranes. Preparation, Sealing, and Oxygen Permeation Properties Shiguang Li, Hong Qi, Nanping Xu,* and Jun Shi Membrane Science & Technology Research Center, Nanjing University of Chemical Technology, Nanjing 210009, P. R. China
Tubular dense perovskite type membranes were prepared by isostatic pressing and plastic extrusion. The resulting tubular La0.6Sr0.4Co0.2Fe0.8O3-δ perovskite type membrane prepared by plastic extrusion (designated as PE-LSCF) has a lower density and oxygen permeation flux compared with that prepared by isostatic pressing (designated as IP-LSCF). A ceramic binder developed by our research center provided reliable sealing for the tubular dense membrane at high temperature. The oxygen permeation flux increases with increasing temperature, and the value is about 0.13 cm3/cm2 min (STP) at 1123 K. The activation energy for oxygen permeation is 168 kJ/mol at the temperature range of 1073-1173 K. X-ray diffraction analysis for the membranes over 110 h of operation indicated that SrSO4, CoSO4, SrO, Co2O3, and La2O3 were formed on the surfaces of the tubular membrane, especially for the tubular PE-LSCF membrane, because of interaction with trace SO2 in the air and the helium and segregation of surface elements. Introduction Over the past several years, perovskite type oxides (ABO3) have been investigated because of their mixed conducting behavior.1-16 Major potential applications of perovskite type dense membranes in chemical and petroleum industries are as a separator for oxygen from air or as membrane reactors for partial oxidative reactions. The dense membrane technology for air separation is economically attractive when integrated with a hot-turbine system.17 Membrane reactor applications have been focused on the partial oxidation of hydrocarbons, e.g., upgrading natural gas to ethylene and ethane (C2 products) or syngas (CO + H2). Most past work in dense membranes has been focused on disk-shaped perovskite type membranes, and the effective area for experimentation was rather limited. There has been almost a dearth of data on tubular geometry because of the fabrication challenges. Balachandran et al.18 appeared to be the only group which reported results on a tubular nonperovskite type SrFeCo0.5O3-δ membrane by plastic extrusion. Two shape-forming methods, i.e., isostatic pressing16,19 and plastic extrusion,18 can be used for preparation of dense tubular membranes. The former can apply uniformly distributed hydrostatic pressure from all directions and achieve a higher and more uniform green density. The latter can prepare green samples of different sizes and geometrical shapes by using different dies conveniently, but the processes of drying and calcination are rigorous because of low density and high deformation. Another important problem associated with application of a dense membrane is sealing of tubes at high * Corresponding author. Tel.: 0086-025-3319580. Fax: 0086025-3300345. E-mail:
[email protected].
temperature. The packing between the membrane and the separator housing must be leak-proof and flexible, withstand thermal shock at high temperatures as required, and match with the membrane as well as the housing material in the thermal expansion coefficient. The reported materials have been focused on glass rings,20,21 precious metal rings,1,13 and ceramic binders2 for sealing of disk-shaped membranes. However, some of them are gas-leap or have possible interfacial reaction with the membrane. The purpose of this study was to investigate the feasible route of preparation, reliable sealing, and oxygen permeation properties of the tubular dense membrane. Two shape-forming methods were evaluated for preparation of perovskite type dense membranes. Three sealing methods and modules were studied in detail. Because the composition La0.6Sr0.4Co0.2Fe0.8O3-δ has high oxygen permeability and good chemical stability among lanthanum cobaltite materials as reported by Xu and Thomson,22 it was selected in this work to investigate oxygen permeation properties of tubular membranes prepared by different methods. Experimental Section Tubular Dense Membrane Preparation and Characterization. The solid-state reaction was used to prepare perovskite type powders including La0.2Sr0.8Co0.2Fe0.8O3-δ, La0.6Sr0.4Co0.2Fe0.8O3-δ, and SrCo0.5FeO3-δ. The detailed process has been indicated in the previous study.16,19 The plastic extrusion and the isostatic pressing were used to prepare green dense tubular membranes. In the process of the isostatic pressing, poly(vinyl acetate) (PVA; 10 wt % in water) was added to increase the fluidity of the powders. Then the powders
10.1021/ie990446s CCC: $18.00 © 1999 American Chemical Society Published on Web 12/06/1999
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 5029 Table 1. Tubular Dense Membranes Prepared by Isostatic Pressing membranes
sintering temp (°C)
La0.2Sr0.8Co0.2Fe0.8O3-δ
1350
La0.6Sr0.4Co0.2Fe0.8O3-δ
1250
SrCo0.5FeO3-δ
1200
quality dense and crack-free dense and crack-free crack
theoretical density (%) 92.3 95.2 93.0
Table 2. Tubular Dense Membranes Prepared by Plastic Extrusion membranes
sintering temp (°C)
La0.2Sr0.8Co0.2Fe0.8O3-δ
1350
La0.6Sr0.4Co0.2Fe0.8O3-δ
1250
SrCo0.5FeO3-δ
1200
quality dense and crack-free dense and crack-free dense and crack-free
theoretical density (%) 82.4 81.7 84.5
were sifted with particle sizes between 40 and 60 mesh. Green tubes were prepared by loading, pressing at the pressure of 25 MPa, and ejecting. In the process of plastic extrusion, powders were mixed with alcohol as the solvent, PVA (10 wt % in water) as the binder, methylbenzene as the dispersant, and glycerine as the plasticizer. These additives were purchased from the Second Chemical Industry of Shanghai with a purity of 99.9%. The mixture was then ball-milled in pure water for 24 h, and the resulting slip was cast into a thin tape. Solvent was removed from the tape by evaporation in air at room temperature for a period of 30 min. The resulting material was further blended in a mixer to obtain a workable plastic mass, which was extruded through a steel die with an adjustable insert to form a green ceramic tube, which was further dried slowly to remove the binder and other volatile organic compounds. The green tubes prepared by the two methods were sintered in air at different temperatures, as shown in Tables 1 and 2, for 5 h with heating and cooling rates of 3 and 2 K/min, respectively. The membrane surface morphology was examined by a scanning electron microscopy (SEM, JEOL JSM-6300). The phase development of the sintered membranes was studied by X-ray diffraction (XRD) analysis (Rigaku D/MAX-rB diffractometer) with Cu KR radiation. The density of the resulting membranes was determined by the Archimedes method. Sealing of Tubular Membranes. Three seal methods including a Cu(OH)2 binder, a Pyrex glass ring, and a ceramic binder were attempted and studied in detail in order to find the best one. The Cu(OH)2 binder (Nanjing Inorganic Chemical Factory) was used together with phosphoric acid. The Pyrex glass ring was manufactured by Nanjing Experimental Instrument Factory. The ceramic binder was developed by our research center. It contains extra-fine alumina and some of the prepared perovskite type oxide. As will be indicated in the subsequent section, the ceramic binder was found to be best and was used for oxygen permeation measurement. Tubular Membrane Reactor Setup. A tubular membrane separator shown in Figure 1 was used for the measurement of oxygen permeation. Two dense alumina tubes (L 8.5 mm i.d.) supported the tubular dense membrane. The ceramic binder was used to seal both ends of the membrane with the walls of these alumina tubes. A quartz tube (L 16 mm i.d.) surround-
Figure 1. Oxygen separator.
ing the two alumina tubes formed the shell side of the separator, which was surrounded by a tubular furnace, and the temperature was measured by a type K thermocouple encased in an alumina tube. A microprocessor temperature controller (model 708PA, Xiamen Yuguang Electronics Technology Research Institute, China) was used to control the temperature to within (1 °C of the set points. The oxygen permeation rates through the dense tubular membranes were measured on the permeation apparatus given in our previous study.16 Air was introduced into the shell side of the tubular membrane. Helium as the sweep gas was fed to the tube side of the tubular membrane. Both upstream and downstream were maintained at atmospheric pressure. The effluent streams were analyzed by a gas chromatograph (model Shimabzu GC-7A), which was equipped with a 2 m 5A molecular sieve operated at 40 °C with H2 as the carrier gas. The amount of oxygen passing through the membrane was calculated from the flow rate and the oxygen concentrations of the effluents. Results and Discussion Qualities and Properties of the Prepared Tubular Dense Membranes. The quality and the percent of the theoretical density of the sintered tubular membranes are listed in Tables 1 and 2. As shown, compositions containing lanthanum can be successfully prepared by isostatic pressing (IP); the reason is not immediately clear, but it may be the particular effect on the element of lanthanum. In contrast, all compositions could be prepared into dense and crack-free tubular dense membranes by plastic extrusion (PE), indicating that solvent, binder, dispersant, and plasticizer have played important roles during the tubular membrane shape forming and calcination. The resulting dense tubes prepared by the two methods are straight and of uniform wall thickness (1.5 mm). Tables 1 and 2 also indicate that the resulting PE samples have lower density when compared with the IP samples, which attribute to the low form-shape pressure and large organic additives. Figures 2 and 3 show the surface and cross-sectional SEM photographs of the tubular IP-LSCF and PE-LSCF membranes. As shown in Figures 2a and 3a, ceramic grains with clear grain boundaries are visible in the figures. Although closed porosity can be seen in the cross-sectional photographs, nitrogen permeation rate measurement proved that open porosity did not exist. Compared with Figures 2b and 3b, the cross-sectional images also indicate that the structure of the tubular PE-LSCF membrane is looser than that of the tubular IP-LSCF membrane, which is coincident with the membrane density values listed in Tables 1 and 2. XRD analysis indicated that both IP-LSCF and PE-LSCF membranes are of perovskite structure.
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Figure 2. SEM photographs of the fresh tubular IP-LSCF membrane: (a) surface; (b) cross section.
Figure 3. SEM photographs of the fresh tubular PE-LSCF membrane: (a) surface; (b) cross section. Table 3. Tubular Dense Membrane Sealing Results sealing method
condition
sealing module
results
Cu(OH)2 binder
room temperature
Pyrex glass ring
930 °C for 0.5 h
ceramic binder
950 °C for 0.5 h
a, b, c a b c a, b, c
gas leak when temperature is high than 550 °C gas leak for the spreading of glass gas leak for the spreading of glass gas-tight but unsuitable for long-term operation gas-tight and suitable for long-term operation
Tubular Dense Membrane Sealing Results. The available sealing is determined by the status of the sealing material together with modules shown in Figure 4. For the same sealing material, the possibility of available sealing is in the order b < a < c according to the force situation of the sealing. Table 3 provides the experimental results of the sealing study. As shown, using the Pyrex glass ring with the module c and the ceramic binder with all modules could obtain a gas-tight seal. However, when using the Pyrex glass, the undesired spreading of glass leads to the interfacial reaction between it and the perovskite type membrane. This decreases the effective membrane area. In contrast, the thermal expansion coefficient of the ceramic binder is
closer to the tubular perovskite type membrane, and the undesired spreading of the sealing was not found. Although there is a slight interaction between the ceramic binder and the perovskite type membrane, the ceramic binder was proven by many experiments to be the best material that provided reliable gas-tight sealing suitable for long-term operation at high temperature. Oxygen Permeation Properties. Oxygen permeation over the tubular IP-LSCF and PE-LSCF membranes under air/helium gradients at the whole temperature range requires about 1 h to reach the steady state. Figure 5 gives the steady-state oxygen permeation fluxes of the two kinds of membranes as a function of temperature, when the oxygen partial pressure in the
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Figure 4. Tubular dense membrane sealing modules.
Figure 5. Temperature dependence of the oxygen permeation flux of the tubular membranes (P′O2 ) 0.21 atm, P′′O2 ) 1 × 10-3 atm): (b) IP-LSCF; (9) PE-LSCF.
feed (P′O2) and permeate side (P′′O2) are 0.21 and 1 × 10-3 atm, respectively. As shown, the oxygen fluxes become substantial above 1073 K because of the orderdisorder transition of the oxygen vacancy.15,23 The PE sample shows consistently slightly lower values of oxygen permeation flux than those of the IP sample. As we know, oxygen permeation fluxes are controlled by
oxygen-ion and electron transport rates through dense perovskite type membranes. The tubular IP-LSCF and PE-LSCF membranes are of the same wall thickness (1.5 mm). The considerable lower density of the PELSCF membrane makes a large tortuous transport path for the oxygen ion and electron. These slight decreases of both conductivities further lead to slightly lower oxygen permeation fluxes. The oxygen flux increases with increasing temperature, and the value is about 0.13 cm3/cm2 min (STP) at 1123 K. Regression analysis indicates that the activation energy is 168 kJ/mol in the temperature range of 1073-1173 K, which coincided with the result of the disk-shaped La0.6Sr0.4Co0.2Fe0.8O3-δ membrane reported by Xu and Thomson.22 Figures 6 and 7 show surface and cross-sectional SEM photographs of the tubular IP-LSCF and PE-LSCF membranes over 110 h of oxygen permeation. Compared with Figures 2b and 3b, there were hardly any changes in the cross section of the tubular membranes. In contrast to Figures 2a and 3a, grain boundaries are not visible on the membrane surfaces after the oxygen permeation operation. XRD analysis of the IP-LSCF membrane after oxygen permeation as shown in Figure 8 indicates that SrSO4 and CoSO4 are present on the membrane surface exposed to air, and only SrSO4 is present on the helium surface. However, XRD analysis of the PE-LSCF membrane after the oxygen permeation as shown in Figure 9 indicates that SrSO4 and CoSO4 are present on both surfaces. ten Elshof 7 also found the presence of SrSO4 on the membrane surface after oxidative coupling of methane in a mixed-conducting perovskite membrane reactor, but he did not explain the
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Figure 6. SEM photographs of the tubular IP-LSCF membrane after oxygen permeation: (a) surface; (b) cross section.
Figure 7. SEM photographs of the tubular PE-LSCF membrane after oxygen permeation: (a) surface; (b) cross section.
Figure 8. XRD patterns of the tubular IP-LSCF membrane after 110 h of oxygen permeation: (a) the helium surface; (b) the air surface. P: Perovskite. 1: SrSO4 (002). 2: Co2O3 (002). 3: SrSO4 (220). 4: CoSO4(131). 5: SrO (220). 6: SrO (311). 7: La2O3 (662).
phenomenon. In this study, the origin of SrSO4 and CoSO4 was attributed to the interaction of a trace amount (
