High-Temperature Oxygen Sorption in a Fixed Bed Packed with

Effects of Oxygen Vacancy Order–Disorder Phase Transition on Air Separation by Perovskite Sorbents. Han-Chun Wu and Y. S. Lin. Industrial & Engineer...
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Ind. Eng. Chem. Res. 2003, 42, 4376-4381

SEPARATIONS High-Temperature Oxygen Sorption in a Fixed Bed Packed with Perovskite-Type Ceramic Sorbents Z. H. Yang and Y. S. Lin* Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0171

Oxygen sorption and desorption on two perovskite-type ceramics, La0.1Sr0.9Co0.5Fe0.5O3-δ and La0.1Sr0.9Co0.9Fe0.1O3-δ, were studied in a fixed-bed system. The perovskite-type ceramic packed fixed bed gives very sharp oxygen sorption breakthrough curves with long breakthrough time in the range of 400-900 °C as a result of a favorable oxygen sorption isotherm, high oxygen sorption capacity, and fast oxygen sorption rate for the sorbents. The perovskite-type ceramic sorbents can be reversibly regenerated and exhibit good structural and chemical stability against sorption and regeneration cycles. These results suggest that the perovskite-type ceramic sorbents can be used in fixed-bed processes for effective removal of oxygen from gas streams using a reducing gas sorbent regeneration. The perovskite-type ceramic sorbents can also adsorb a considerable amount of oxygen in 200-300 °C but at a much slower rate. The desorption breakthrough curves are characterized by long tails due to slower desorption rate constants (compared to sorption) and highly favorable oxygen sorption isotherms for these two materials. Process conditions or sorbent material composition should be further optimized or improved in order to develop a fixed-bed process packed with this group of sorbents for air separation. 1. Introduction Air can be separated into its component parts by several techniques, such as cryogenic distillation, membrane separation, and adsorption. Adsorption technology for air separation is highly suitable for a variety of applications, e.g., portable nitrogen generators. The particular adsorbent used in air separation depends, in general, upon the product that is sought. It is usually preferred to use an adsorbent that will adsorb the unwanted components from the gas being separated, so that the desired product can be obtained as the nonadsorbed, high-purity product. Thus, when oxygen is the desired product, nitrogen-selective adsorbents, such as Li-X zeolite or Ca-A zeolite, are used, and when nitrogen is the desired product, it is more efficient to use an oxygen-selective adsorbent, such as carbon molecular sieve (CMS). The X and A zeolites preferably adsorb nitrogen over oxygen. CMS adsorbs both oxygen and nitrogen. However, it adsorbs oxygen at a considerably faster rate than it adsorbs nitrogen. Therefore, it can be efficiently used for oxygen-nitrogen separations carried out by pressure-swing adsorption (PSA) processes having very short cycles. The adsorption is generally carried out at temperatures below 50 °C because the best results are obtained at low temperatures. It is sometimes desirable to conduct adsorption processes at temperatures above 100 °C by PSA or temperature-swing adsorption techniques. Recently, we reported interesting oxygen sorption properties of perovskite-type ceramic oxides1 and suggested their potential applications as adsorbents for * To whom correspondence should be addressed. Tel.: +1 (513)5562761.Fax: +1(513)5563473.E-mail: [email protected].

Figure 1. Comparison of the oxygen and nitrogen sorption equilibria on zeolite Li-X (25 °C) and LSCF perovskite-type ceramics (500 °C).

oxygen removal and air separation at elevated temperatures.1,2 Infinitely large oxygen selectivity and high sorption capacity are the major characteristics of this new type of sorbent. In our recent work, two perovskite-type ceramics, La0.1Sr0.9Co0.5Fe0.5O3-δ (LSCF-1) and La0.1Sr0.9Co0.9Fe0.1O3-δ (LSCF-2), were selected as candidate sorbent materials for oxygen sorption. Oxygen and nitrogen adsorption equilibria of these two perovskite-type ceramics were studied by the thermogravimetric analysis (TGA) method.1 Figure 1 compares the oxygen and nitrogen sorption isotherms of a LSCF sorbent with those on zeolite Li-X.1,3 Zeolite Li-X shows preferable

10.1021/ie030313d CCC: $25.00 © 2003 American Chemical Society Published on Web 08/14/2003

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sorption of nitrogen and gives an equilibrium selectivity (RN2/O2) of around 10 at room temperature. The LSCF sorbent gives a relatively large sorption capacity for oxygen and zero sorption for nitrogen at 500 °C. This monosorption property for LSCF gives infinitely large selectivity for oxygen over nitrogen, argon, or any other gas in air. The unique adsorption properties of the perovskite-type ceramics are a result of reversible variation of their oxygen nonstoichiometry with a change of the surrounding oxygen partial pressures in a certain range of oxygen partial pressure and temperature. We recently determined the oxygen nonstoichiometry, which is related to the oxygen sorption amount, of these two perovskite-type ceramics in the temperature range of 300-900 °C and PO2 range of 1.3 × 10-4-0.209 atm.4 The initial oxygen nonstoichiometries of these two materials were determined by hydrogen (5%) reduction at 850 °C, in which perovskite-type materials La1-xSrxCo1-yFeyO3-δ were assumed to be reduced to metal oxides (La2O3 and SrO) and metals (Fe and Co). Oxygen nonstoichiometries under other experimental conditions were then determined from equilibrium weight loss between fresh and oxygen-desorbed samples under those conditions. The details are given elsewhere.1 Both materials exhibit a large change of oxygen nonstoichiometry in the studied temperature and PO2 ranges. On the basis of a simple defect model,5-7 we also developed a semiempirical equation to describe the isotherms of the oxygen nonstoichiometry (or oxygen sorption capacity) of these perovskite-type ceramics.8 The oxygen sorption equilibrium properties of the single-phased perovskite-type ceramics were well characterized in these studies. The previous studies conducted in our group indicate that novel sorption processes could be developed based on this new type of adsorbent. These sorption processes may find applications in trace oxygen removal, production of an oxygen-enriched gas stream, and air separation. To gain a good understanding of the performance of the perovskite-type ceramic sorbents in practical sorption processes, we conducted a kinetic study on sorption and regeneration in a fixed bed packed with the LSCF sorbents. This study was conducted to identify major performance characteristics of the fixed bed packed with this group of ceramic sorbents. The results would also suggest directions for further improvement of the perovskite-type ceramic adsorbents. The present paper reports the results of the fixed-bed study. 2. Experimental Section Perovskite-type oxide ceramics LSCF-1 and LSCF-2 were prepared by the liquid citrate method. This method has advantages in preparing samples with high surface area and precise stoichiometry. Metal nitrate precursors and 50% excessive citric acid were dissolved in deionized water according to the stoichiometry. The system was under heating and stirring during the polymerization and condensation reactions, at 100-105 and 105-110 °C, respectively. Viscous gellike products were obtained at the end of condensation. After drying at 110 °C for 24 h, the gel was heated in air to 400 °C, at which selfignition occurred to burn out the organics. The resulting oxide powders were sintered at 1250 °C for 25 h, with a ramping rate of 60 °C/h, and then cooled at 60 °C/h to room temperature in order to obtain the perovskite structure confirmed by X-ray diffraction analysis. The final perovskite-type ceramic powder used in the fixedbed study had an average aggregate size of 180 µm. The

Table 1. Typical Operation Conditions for Fixed-Bed Experiments parameter

value

adsorbent packed particle size of sorbent bed length PO2 of the adsorption feed PO2 of the desorption feed flow rate of the gas feed operation temperature

3.66 g (LSCF-1), 3.80 g (LSCF-2) 180 µm 5.8 cm 0.209 atm 1.3 × 10-4 atm 4.88 mL/min 600 °C

oxygen nonstoichiometries of fresh samples were 0.159 and 0.178, respectively, for LSCF-1 and LSCF-2. This indicated that both materials are highly oxygendeficient even at room temperature and in air. The experimental setup for the fixed-bed experiments included a gas delivery system, an adsorber column, an oxygen analyzer (Ceramatec, model 1100), and a computer data acquisition system, as detailed in our previous publication.1 The adsorber was a dense alumina tube of 6 mm i.d. and 9 mm o.d. loaded with about 3.5 g of the perovskite LSCF powder. Air or helium (99.99% purity), at 1 atm, was used as the feed gas for sorption and regeneration, respectively. The oxygen concentration in the effluent of the adsorber was determined by the oxygen sensor as a function of time after the feed gas was switched from helium to air. The desorption process was conducted by sweeping He through the bed at the same temperature. The dead volume was measured at 13.1 mL, and the breakthrough curves presented in the paper were corrected for the dead-volume time. Typical experimental conditions are summarized in Table 1. 3. Results and Discussion Figure 2 gives oxygen sorption breakthrough curves from the fixed bed packed respectively with LSCF-1 and LSCF-2 powders at the feed flow rate of 4.88 mL/min in the high-temperature range (g400 °C). Prior to adsorption, the fixed bed was purged with helium (PO2 ) 1.3 × 10-4 atm) at the same temperature as that of sorption for over 48 h for complete oxygen desorption. The breakthrough curves are characterized with a sharp front and long breakthrough time (100-200-fold of the resident time of the fixed bed). For example, for LSCF-1 at 400 °C, no outgoing oxygen was detected by the oxygen sensor before 1710 s, and therefore high-purity nitrogen (>98%) can be easily collected as a product. After 1825 s, the sorbent was saturated with oxygen and the outlet oxygen concentration equaled the concentration of the feed stream, 20.9%. The breakthrough times for LSCF-2 are longer than those for LSCF-1 in the whole temperature range investigated. The sorption capacity can be calculated from a breakthrough curve by the following mass balance equation considering the pressure drop and volumetric flow rate change in the effluent due to the composition change:

Q)

1 RTms

∫0t (FiXiPi - Fo,tXo,tPo) dt f

(1)

where

Fo,t )

Fi(1 - Xi) 1 - Xo,t

(2)

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Figure 3. Oxygen sorption breakthrough curves of LSCF-2 below 300 °C.

Figure 2. Breakthrough curves of LSCF-1 (A) and LSCF-2 (B) at different temperatures. Table 2. Comparison of the Sorption Capacities Obtained from Breakthrough and TGA Measurementsa LSCF-1 T (°C) 400 500 600 700 800 900 a

LSCF-2

qBR (mmol/g)

qTGA (mmol/g)

0.349 0.345 0.316 0.281 0.248

0.351 0.358 0.319 0.287 0.241

qBR (mmol/g)

qTGA (mmol/g)

0.438 0.370 0.300 0.242 0.196

0.456 0.384 0.300 0.247 0.191

PO2 or PN2 (atm).

in which ms is the amount of sorbent packed, F, X, and P are the volumetric flow rate, oxygen molar percentage, and total pressure, respectively, and subscripts i and o stand for the inlet and outlet, respectively. The calculated oxygen sorption capacities are listed in Table 2. It should be noted that the sorption capacity is related to the change of oxygen nonstoichiometries of LSCF sorbents between the state that it is in equilibrium with the feed gas for sorption (PO2 ) 0.209 atm) and the state that it is in equilibrium with helium. As shown, in the

temperatures above 400 °C, the measured sorption capacity decreases with increasing temperature. In a previous study, oxygen sorption capacities of these two materials were obtained by TGA from the weight change of the LSCF sample after the surrounding gas was switched from helium to air at a given temperature.1 The sorption capacity data measured by the TGA method are compared in Table 2 with those obtained from the breakthrough curves. As seen from the table, the oxygen sorption capacities measured by these two methods agree well. This confirms the reliability of the fixed-bed sorption experiments. Oxygen sorption breakthrough curves for LSCF-2 at the feed flow rate of 4.88 mL/min in the low-temperature range (e300 °C) are shown in Figure 3. As seen from the figure, the oxygen sorption capacity is rather small at temperatures equal to or lower than 250 °C. With the decrease of temperature, the sorption breakthrough time decreases and therefore the sorption capacity decreases too. This temperature dependency of the sorption capacity is different from the sorption data at the higher temperatures. Furthermore, the sorption rates are much lower in the low-temperature range as compared to those at higher temperatures (g400 °C). The above different temperature dependencies of the oxygen sorption capacity in the two temperature ranges can be explained by the following two possibilities. For oxygen sorption on the perovskite-type ceramics, at equilibrium it is oxygen nonstoichiometry, δ in ABO3-δ, and not the sorption capacity that is a function of the oxygen partial pressure and temperature, i.e., δ ) f(PO2,T). During the fixed-bed experiments, air (PO2 ) 0.209 atm) and helium (PO2 ∼ 10-4 atm, due to impurity oxygen) were respectively used as the feeds in the sorption and regeneration steps. As mentioned before, the oxygen sorption capacity calculated from the oxygen sorption breakthrough curve corresponds to the difference of the oxygen in the perovskite-type sorbent between the state at PO2 ) 0.209 atm and that at PO2 ) 10-4 atm, which is

q)

1 [δ(10-4atm,T) - δ(0.209atm,T)] 2M hw

(3)

where M h w is the average molecular weight of the sorbent during the sorption process. In most cases the oxygen

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Figure 5. Sorption breakthrough curves at different flow rates.

Figure 4. Schematic illustration of the relationship between oxygen nonstoichiometry and oxygen sorption capacity and their temperature dependencies.

nonstoichiometry always increases with increasing temperature. However, eq 3 shows that it is possible for the sorption capacity to increase and, after reaching a maximum, decrease with increasing temperature, as schematically shown in Figure 4. Though equilibrium data of the low-temperature range are not available at this time, a similar observation was found on LSCF-1 in the high-temperature range. As can be seen from Table 2, in the temperature range of 400-800 °C the maximum oxygen sorption capacity determined by TGA measurement is at 500 °C. The fixed-bed study showed that the sorption capacity at 400 °C is slightly larger than that at 500 °C. The second possibility is related to the different natures of oxygen in the perovskite-type oxides in the different temperature ranges. Teraoka et al. reported that there are two types of oxygen in perovskite-type oxides.9 One is R-oxygen, which is accommodated in the oxygen vacancies formed by the partial substitution of A-site cations, such as the Sr substitution of La in the two perovskite-type oxides studied in this work. The other is β-oxygen, which is attributed to the reduction of the B-site cation to lower oxidation states. In their temperature-programmed desorption experiments, Srdoped LaCoO3 exhibits a plateaulike peak starting at lower temperature (∼400 K), followed by a sharp peak at higher temperature (∼1100 K). The former is referred to as R-oxygen desorption, and in this case, the 4+ B-site cation is reduced to the trivalent state. The latter is β-oxygen desorption, in which the trivalent cation is further reduced to a divalent one. In regards to the relatively low sorption capacities in the low-temperature range, we believe that the extent of R-oxygen desorption is highly dependent on temperature, which was also suggested by the plateaulike peak in Teraoka et al.’s study.9 In the present study, a very small amount of R-oxygen was desorbed at a temperature below 250 °C, while a distinctive increase can be observed as the temperature increases from 250 to 300 °C. For the two materials of the investigation, when equilibrated with helium with a PO2 of 1.3 × 10-4 atm

Figure 6. Sorption and desorption breakthrough curves of LSCF-1 at 800 °C.

at low temperature, β-oxygen is unlikely desorbed. Therefore, the sorption process in this temperature range is mainly due to R-oxygen sorption, which is a slow process. At temperatures higher than 400 °C, sorptions of both R- and β-oxygen take place when the gas feed is switched from PO2 of 1.3 × 10-4 to 0.209 atm. The incorporation of these two processes enhances the apparent sorption rate because β sorption is believed to be a fast process. Figure 5 shows the effects of the feed flow rate on oxygen breakthrough curves of LSCF-2 at 800 °C. The breakthrough curves are plotted in dimensionless time. As shown, the average dimensionless breakthrough time increases slightly with increasing flow rate. This is because the feed total pressure (Pi in eq 1) increases with increasing feed flow rate. As Pi increases, the oxygen sorption capacity measured by the fixed-bed adsorber increases, as shown by eq 1. In terms of the steepness of the breakthrough curve, it is noticed that the sharpness decreases with increasing flow rate because of the dynamics of the fixed-bed process. This is consistent with the breakthrough behavior of a fixed bed packed with other conventional adsorbents.10 In the present study, helium was used as the sweeping gas for desorption. Sorption and desorption were operated at the same temperature. Figure 6 compares sorption and desorption breakthrough curves from the fixed bed packed with LSCF-1 at 800 °C. One feature

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Figure 7. Comparison of the breakthrough curves of LSCF-1 at 600 °C after different desorption times.

Figure 8. Comparison of the sorption breakthrough curves of LSCF-1: 1st and 10th runs.

of the desorption breakthrough curve is its “long tail”. The sorption process was completed within 1400 s. However, the desorption process was rather slow. After 2000 s, the outlet oxygen concentration was still higher than 1%. To examine the effect of the desorption time on the extent of regeneration, different regeneration times were tested and sorption breakthrough curves after each regeneration were measured for comparison. Sample LSCF-1 was regenerated in helium at 600 °C for 5.5, 18, and 38 h and the sorption breakthrough curves after the desorption are shown in Figure 7. As seen from Figure 7, the breakthrough time (or the oxygen sorption capacity) increases with increasing desorption time. After 5.5 h of desorption, only about 75% of the adsorbed oxygen was desorbed from the perovskite-type structure; 94% of adsorbed oxygen was desorbed after 18 h of regeneration. After 38 h, the sample was fully regenerated and the sorption capacity calculated from this breakthrough curve agrees well with that from TGA measurement. A further increase of the desorption time did not change the sorption breakthrough curve. The desorption breakthrough curves become sharper as the temperature increases. These data indicate that at 600 °C helium should be passed through the fixed bed for at least 2 days in order to fully regenerate the perovskite-type sorbents packed in the fixed bed. The time required for full regeneration would be longer at lower temperatures. The long-tailing oxygen desorption breakthrough curve as compared to sharp oxygen sorption breakthrough can be explained in terms of both sorption equilibrium and kinetics. It is known that the shape of a breakthrough curve is determined by the shape of the oxygen sorption isotherm.11 Generally, there are three different kinds of sorption isotherms: favorable, linear, and unfavorable. For the linear sorption isotherm case, sorption and desorption breakthrough curves become mirror images. These two perovskite-type materials exhibit highly favorable oxygen sorption isotherms, as shown in Figure 1. For this kind of system, the concentration front approaches constant-pattern form in the sorption process while desorption curves tend to approach dispersive-pattern behavior. As a result, a pronounced asymmetry develops between the sorption and desorption breakthrough curves. For the perovskite-type ceramics, the oxygen sorption/ desorption rate constant also depends on the oxygen

partial pressure in the fluid phase.12 In the oxygen sorption step, the perovskite-type ceramic is exposed to a higher oxygen partial pressure (after being switched from helium). The oxygen desorption occurs when the perovskite-type ceramic is exposed to a low oxygen partial pressure (switched from air). The batch experiments showed that the oxygen sorption rate constant is about 1 order of magnitude higher than the oxygen desorption rate12 because of the difference in the oxygen partial pressures. This difference in the rate constant also explains the difference in the oxygen sorption and desorption breakthrough curves from the fixed bed packed with the perovskite-type ceramics. To examine the stability of this new type of sorbent material, sorption breakthrough curves were measured for up to 10 cycles of oxygen sorption and desorption in the fixed bed. Desorption was conducted at the same temperature as adsorption by sweeping He for 48 h or longer. Figure 8 compares the oxygen breakthrough curve in the 10th run with that obtained on the fresh run at 600 °C under the same conditions. As shown, the breakthrough curves are almost identical. Integration of the breakthrough curves shows that after 10 cycles of sorption and desorption the loss of the sorption capacity is less than 4%. This difference could possibly result from baseline shifting of the mass flowmeter because the two breakthrough curves were obtained 1 month apart. This comparison indicates the good structural and chemical stability of the perovskite-type ceramics as adsorbents against oxygen sorption and desorption cycles. Despite the unfavorable oxygen desorption breakthrough curves, an efficient fixed-bed process for trace oxygen removal or production of nitrogen can be designed with the perovskite-type sorbents by taking advantage of the highly favorable oxygen sorption isotherm, large oxygen sorption capacity, and fast sorption rate, which give a sharp oxygen breakthrough curve. In this case, the regeneration of the sorbent can be done by passing a reducing gas, such as methane, through the adsorber after oxygen sorption. Such a regeneration step is generally fairly fast and reversible, and it involves chemical reactions between the oxygen in the lattice of the perovskite-type ceramic sorbent and the reducing gas.12,13 For air separation, the desorption time should be shortened in order to improve the efficiency of the separation process.

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Several approaches can be followed to shorten the desorption time or improve the desorption efficiency. One is to regenerate the sorbent at a higher temperature than the sorption temperature. Because of fast desorption rates at higher temperatures, a shorter desorption period is expected. This suggests that the temperature-swing operation is preferred over the pressure-swing operation for this type of ceramic adsorbent. In some practical applications, full regeneration is not necessary. Therefore, partial regeneration of the adsorbed sorbent would significantly shorten the desorption time. As discussed above, the type of sorption isotherm determines the easiness of sorption and desorption processes. Therefore, the most effective way to improve the desorption process for this type of sorbent material is the selection of material composition which would give a higher desorption rate and a smaller nonlinear oxygen sorption isotherm. The oxygen sorption isotherm for the perovskite-type ceramics is determined by the dependence of oxygen nonstoichiometry on the oxygen concentration in the gas phase. Therefore, by examination of the relationship between oxygen nonstoichiometry and oxygen partial pressure, a sorbent material with a better desorption performance can be found. For many perovskite-type oxides, oxygen nonstoichiometry δ is related to the oxygen partial pressure P by an empirical equation δ ) APn or a semiempirical equation δ ) (3KPn)/(1 + KPn).7 In both equations, the power n is negative and is material-dependent. It is obvious that the more close to 0 n is, the less nonlinearity of the sorption isotherm. To find materials with n close to 0, different materials need to be examined. This could be done by doping different metal cations on the A or B site of the perovskite structure and measuring the oxygen nonstoichiometry with different oxygen partial pressures at various temperatures. It should be pointed out that a larger n (close to 0) would also give less oxygen sorption capacity. Therefore, a compromise should be made between them. Theoretically, preexponential constant A or K does not affect the nonlinearity of the sorption isotherm; therefore, a system with large preexponential constant A or K and large n will give an acceptable sorption capacity and desorption performance. 4. Conclusions Several conclusions can be drawn from the fixed-bed experiments with LSCF-1 and LSCF-2 perovskite-type ceramic sorbents. At temperatures above 400 °C, the fixed beds packed with both sorbents give very sharp oxygen sorption breakthrough curves with long breakthrough times. The oxygen sorption capacities measured from sorption breakthrough curves agree well with those measured by TGA. At temperatures below 300 °C, the perovskite-type ceramic sorbents also adsorb a considerable amount of oxygen but at a very slow rate.

The oxygen desorption process is slow with a long-tailing desorption breakthrough curve because of the highly favorable oxygen sorption isotherm and low desorption rate constant for these sorbents. These sorbents exhibit good reversibility and stability for oxygen sorption and desorption. The results suggest the potential to use the perovskite-type ceramic sorbents in a fixed-bed process for effective trace oxygen removal or production of nitrogen, with the use of a reducing gas in the regeneration step. Process conditions or the sorbent material composition should be further optimized or improved in order to develop a fixed-bed process with this group of sorbents for air separation. Acknowledgment The authors acknowledge the support from the NSF (Grant CTS-0132694) and the BOC Group on this project. Literature Cited (1) Yang, Z. H.; Lin, Y. S.; Zeng, Y. High-Temperature Sorption Process for Air Separation and Oxygen Removal. Ind. Eng. Chem. Res. 2002, 41, 2775. (2) Lin, Y. S.; McLean, D. L.; Zeng, Y. High-Temperature Adsorption Process. U.S. Patent 6,059,858, 2000. (3) Rege, S. U.; Yang, R. T. Limit for Air Separation by Adsorption with LiX Zeolite. Ind. Eng. Chem. Res. 1997, 36, 5358. (4) Yang, Z. H.; Lin, Y. S. Equilibrium of Oxygen Sorption on Perovskite-Type Ceramic Sorbent La1-xSrxCo1-yFeyO3-δ. AIChE J. 2003, 49, 793. (5) van Roosmalen, J. A. M.; Cordfunke, E. H. P. A New Defect Model to Describe the Oxygen Deficiency in Perovskite-Type Oxides. J. Solid State Chem. 1991, 93, 212. (6) van Roosmalen, J. A. M.; Cordfunke, E. H. P. The Defect Chemistry of LaMnO3-δ. 4. Defect Model for LaMnO3-δ. J. Solid State Chem. 1994, 110, 109. (7) van Hassel, B. A.; Kawada, T.; Sakai, N.; Yokokawa, H.; Dokiya, M.; Bouwmeester, H. J. M. Oxygen Permeation Modeling of Perovskites. Solid State Ionics 1993, 66, 295. (8) Yang, Z. H.; Lin, Y. S. A Semiempirical Equation for Oxygen Nonstoichiometry of Perovskite-Type Ceramics. Solid State Ionics 2002, 150, 245. (9) Teraoka, Y.; Yoshimatsu, M.; Yamazoe, N.; Seiyama, T. Oxygen-Sorptive Properties and Defect of Perovskite-Type Oxides. Chem. Lett. 1984, 893. (10) Yang, R. T. Gas Separation by Adsorption Processes; Imperial College Press: River Ridge, 1997. (11) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; VCH: New York, 1994. (12) Zeng, Y.; Lin, Y. S. A Transient TGA Study on Oxygen Permeation Properties of Perovskite Type Ceramic Membrane. Solid State Ionics 1998, 110, 209. (13) Lin, Y. S.; Zeng, Y. Catalytic Properties of Oxygen Semipermeable Perovskite Type Ceramic Membrane Materials for Oxidative Coupling of Methane. J. Catal. 1996, 164, 220.

Received for review April 10, 2003 Revised manuscript received July 7, 2003 Accepted July 9, 2003 IE030313D