Preparation and Oxygen Permeation of La0. 6Sr0. 4Co0. 2Fe0. 8O3

Article pubs.acs.org/IECR

Preparation and Oxygen Permeation of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) Perovskite-Type Membranes: Experimental Study and Mathematical Modeling Amir Atabak Asadi,† Amir Behrouzifar,† Mona Iravaninia,† Toraj Mohammadi,*,† and Afshin Pak‡ †

Research Centre for Membrane Separation Processes, Chemical Engineering Department, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran ‡ Engineering Department of Oil & Gas Special Projects, Iranian Centeral Oil Field Company, Tehran, Iran ABSTRACT: La0.6Sr0.4Co0.2Fe0.8O3−δ nanopowder, synthesized via an autocombustion technique, was pressed into disk-shaped membranes. Results of permeation experiments revealed that oxygen permeation flux increases as temperature, feed side oxygen partial pressure, and feed and sweep gas flow rates increase, while it decreases with membrane thickness and permeate side oxygen partial pressure. A Nernst−Planck based mathematical model, including surface exchange kinetics and bulk diffusion, was developed to predict oxygen permeation flux. Considering nonelementary surface reactions and introducing system hydrodynamics into the model resulted in an excellent agreement (RMSD = 0.0344, AAD = 0.0274 and R2 = 0.9960) between predicted and measured fluxes. Feed side surface exchange reactions, bulk diffusion, and permeate side surface exchange reaction resistances are in the range of Rex′ = 7 × 103 to 9 × 106, Rdiff = 1 × 105 to 2 × 107, and Rex″ = 1 × 104 to 2 × 107 (s/m), respectively. The permeation rate-limiting step was determined using the membrane dimensionless characteristic thickness.

1. INTRODUCTION Oxygen production from the air is of great importance in a variety of industries. Ultrapure oxygen can be produced by mixed ionic and electronic conducting (MIEC) ceramic membranes at low cost and high efficiency.1 Among MIEC materials, perovskite type materials such as La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) have become of great interest due to their high oxygen permeation fluxes and excellent stabilities.2 Oxygen permeation flux through perovskite membranes is mainly affected by perovskite composition,3−5 powder preparation route,6 and also membrane shaping and sintering conditions,7−12 including sintering temperature and dwell time, heating and cooling rates, and even the furnace atmosphere.13 Not only do operating conditions such as temperature, upstream oxygen concentration, feed and permeate side pressures, feed and sweep gases and their flow rates, and feed impurities even at very low concentrations influence oxygen permeability of the membrane,14−16 but features such as thickness and age also affect the oxygen permeation rate through the membrane.17−19 Developing a mathematical model based on experiments is worthwhile, not only to avoid wasting money and time for more data gathering but also to predict oxygen permeability through the membrane under specified conditions. Therefore, many researchers attempted to model oxygen permeation through perovskite membranes.20−25 The main weakness of these models is assuming the surface reactions of oxygen as an elementary oxidation and reduction reaction, besides neglecting the effects of flow rates, which both lead to a diversion from accurate oxygen permeability evaluation. In this paper, oxygen permeability of the La0.6Sr0.4Co0.2Fe0.8O3−δ membranes was studied experimentally. The influence of temperature, feed side oxygen partial pressure, and feed and sweep gas flow rates on oxygen permeation through four membranes with different thicknesses was investigated. After that, a © 2012 American Chemical Society

mathematical model, which relates oxygen permeation flux to temperature, oxygen partial pressure upstream and downstream, membrane thickness, and feed and sweep gas flow rates, was developed. Finally, to sum up the results and evaluate the proposed model, experimental data were compared with the values predicted by the model. With the aid of improvements, the mathematical model predictions showed excellent agreement with the experimental data. Moreover, the characteristic thickness of the membranes and the contribution of different resistances to oxygen permeation were calculated, and the effects of all operating parameters were discussed briefly.

2. EXPERIMENTAL SECTION 2.1. Materials. Required materials for the synthesis of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) powder include metallic nitrates (La(NO3)3·6H2O, Sr(NO3)2, Co(NO3)2·6H2O, and Fe(NO3)3·9H2O); ammonia solution; EDTA acid; citric acid; and ammonium nitrate, which were supplied by Merck Co. with a purity of higher than 99.9%. 2.2. Preparation of LSCF Powder. The La0.6Sr0.4Co0.2Fe0.8O3−δ membranes were prepared via an autocombustion method, based on the results obtained in our previous work.26 First, the required amount of EDTA acid is dissolved in an ammonium solution. Second, the stoichiometric amounts of metal nitrates are dissolved into deionized water separately, and after that, these solutions are mixed to obtain a solution of metal nitrates. Citric acid is then introduced into this solution. The molar ratio of EDTA acid to citric acid to total metallic Received: Revised: Accepted: Published: 3069

October 23, 2011 January 14, 2012 January 19, 2012 January 19, 2012 dx.doi.org/10.1021/ie202434k | Ind. Eng. Chem. Res. 2012, 51, 3069−3080

Industrial & Engineering Chemistry Research

Article

cations is set to 1:2:1. In the fourth step, a necessary amount of NH4NO3 is added to the mixed solution. Then, the pH of the solution is fixed at 6 by adding a required amount of ammonia solution. It is fair to mention that the mixture of ammonia, EDTA, and citric acid forms a buffer solution with a pH close to 6. EDTA and citric acid play the role of a chelating agent and result in the stability of the four metallic cations in the solution. Both of these two chelating agents have some electron donor atoms that can join with a metallic atom through each of them and avoid partial segregation of the metal cations. As the mixture is stirred over a hot plate at 90 °C, a gel is obtained after about 5 h. Along the stirring period, the solution pH is kept constant at 6 by adding an ammonia solution. Finally, the obtained gel is transferred to a preheated furnace at 250 °C for the autocombustion process. The autocombustion reaction is summarized in eq 1. It is clear that oxygen is consumed in this process. For the complete combustion reaction, the oxygen line is connected to the furnace.27,28

Figure 1. Membrane module.

(GC-2001M, Sanayeh Teif Gostar Co., Iran) equipped with a thermal conductivity detector (TCD) and a 2 × 5 Å molecular sieve column was used to analyze the permeated stream. Argon (99.9%) was used as the carrier gas of the GC. The GC was calibrated by defined concentrations of oxygen, nitrogen, and helium before permeation tests were started. A schematic of the setup used for oxygen permeation experiments is shown in Figure 2.

0.6La(NO3)3 + 0.4Sr(NO3)2 + 0.2Co(NO3)2 + 0.8Fe(NO3)3 + 2C10H16N2O8 + 4C6H8O7 1 + 25NH 4NO3 + (37.8 − δ)O2 2 → La 0.6Sr0.4Co0.2Fe0.8O3 −δ + 29.7N2 + 82H2O + 44CO2

(1)

After cooling the furnace to room temperature at rate of 1 °C/min, the obtained powder, which is very fine and porous, is transferred to an alumina crucible and heated at 1000 °C under static air for 5 h at heating and cooling rates of 1 °C/min. This calcination process burns the organic materials out from the powder structure, and the final LSCF perovskite powder is obtained. 2.3. Fabrication of the Ceramic Membrane Disk. The obtained LSCF powder was shaped to green disk membranes of various thicknesses by hydraulic pressing under 2000 bar in a steel mold with an inner diameter of 1.75 mm. Finally, the raw disks were sintered at 1200 °C for 8 h, with heating and cooling rates of 1 °C/min to obtain the necessary relative density and gas-tightness. These are the best values of shaping and sintering parameters obtained for the LSCF powder.26 The membrane thicknesses were controlled to be 0.25, 0.5, 1.0, and 1.5 mm after sintering. 2.4. Module and Setup for High-Temperature Permeation Tests. To measure oxygen permeation flux through the membranes under different operating conditions, a hightemperature membrane module was used. As shown in Figure 1, this module had four lines for feed, retained flow, sweep gas, and permeated flow. Two steel spirals were used to raise feed and sweep gas temperatures up to the required set points. This module was designed with maximum symmetry. Both sides of the membrane had radial flow patterns from the membrane edge to its center. For achieving a pure product, membrane module sealing is very important. Since oxygen permeation flux tests are performed at elevated temperatures, common sealings cannot be used. To avoid leakage, high temperature glue (Fire Sealant 1200 °C, Den Braven Sealants BV, Netherlands) was used. In almost all of the experiments carried out, no sealing failure was observed. To study oxygen permeation flux, the membrane module was placed inside an electric furnace. To control flow rates, mass flow control (MFC) valves were used. A gas chromatograph

Figure 2. Apparatus used for oxygen permeation measurement (GR, gas regulator; FV, flow valve; MC, mixing chamber; MFC, mass flow controller; MM, membrane module; EF, electric furnace; GC, gas chromatography; TCD, thermal conductivity detector; CCS, computerized control system).

2.5. Powder and Membrane Characterization. The phase structure of the LSCF powder was determined by X-ray diffraction (XRD, Philips PW3710, Netherlands) using Cu Kα radiation and a Ni filter with 2θ varying from 0 to 90°, a scan rate of 1°/min, and 2θ intervals of 0.02°. Scanning electron microscopy (SEM, FESEM-S4160, Hitachi, Japan) was also used to examine the morphology of the LSCF powder and to observe the surface and cross-section of the sintered membranes. Densities of the LSCF powders were measured using a gas pycnometer (AccuPyc 1330, Micrometrics, USA). The relative density of the sintered membranes was defined as (ρ/ρ0) × 100%, where ρ0 was the powder density measured by the gas pycnometer and ρ was the actual density measured using the Archimedes method. 2.6. Experimental Design and Oxygen Permeation. In this study, effects of temperature, feed side oxygen partial pressure, feed and sweep gas flow rates, and membrane thickness were investigated. Except for the temperature, which 3070

dx.doi.org/10.1021/ie202434k | Ind. Eng. Chem. Res. 2012, 51, 3069−3080


Article

temperature. An Arrhenius type equation is one of the most common ones used to describe the temperature dependency of these parameters:

had seven levels, other parameters had four levels. These levels are summarized in Table 1. Table 1. The Experimental Parameters and Their Levels for Oxygen Permeation Measurements parameter

levels

temperature 700, 750, 800, 850, 900, 950, and 1000 feed side oxygen partial pressure 0.21, 1/3, 1/2, 2/3, and 1 feed flow rate 50, 100, 200, and 400 sweep gas flow rate 10, 20, 40, and 80 membrane thickness 0.25, 0.5, 1.0, and 1.5

°C atm cm3/min cm3/min mm

(2)

2

⎛ E ⎞ k r = k r0 exp⎜ − r ⎟ ⎝ RT ⎠

(6c)

kf /kr 1

× + 2h• ←⎯⎯⎯→ OO

2

(7a)

•• O2 + V O

(7b)

In the derivation of eq 5, it is assumed that these reactions are elementary with the following rates of reaction:22 0.5 JO = k f p′O C′ − k r 2 V 2

(8a)

0.5 JO = k r − k f p″O C″ 2 V

(8b)

2

16

However, these reactions are not necessarily elementary and involve many substeps, such as oxygen adsorption, dissociation, charge transfer, bulk diffusion, recombination, and desorption.31 The overall oxygen transport rate may be limited by any of these sequential steps. As mentioned before, it is reasonable to consider the electron hole concentration constant at both membrane surfaces, so the reverse surface reaction rate of eq 7a and the forward reaction rates of eq 7b are zero-order. But, on the other hand, it is reasonable to assume nonelementary rates of reaction with respect to oxygen partial pressure and oxygen vacancy concentration. Therefore, eqs 8a and 8b can be rewritten as follows:

Equation 4 can be derived with the two sets of simplifications made to eq 3: 1. Linear approximation, neglecting cross-phenomenological coefficients and deviations from the Nernst−Einstein and gradient of activity coefficient29 2. No external current and also no local velocity of the inert marker:30

(4)

n C′m − k JO = k f p′O r 2 V 2

(9a)

n C″m JO = k r − k f p″O 2 V

(9b)

2

However, eq 4 is still very complicated. By considering more simplifications,22 eq 5 can be derived, which describes oxygen permeation through ion-conducting membranes with a high ratio of electronic to ionic conductivity, like perovskites: kr 0.5 0.5 (1/p″O − 1/p′O ) 2 2 kf JO = 1 2L 1 2 + + 0.5 0.5 DV k f p ′O k f p ″O

(6b)

kf /kr × 1 •• ← O2 + V O ⎯⎯⎯→ OO + 2h• 2

3. MODEL DEVELOPMENT 3.1. Introductory Model. The flux of charged species in any ionic solution including mixed conductors is described by the Nernst−Planck equation. In mixed conductors, the oxygen ion, oxygen vacancy, proton, electron and electron hole are charged species:29 σ Ji ⃗ = − 2 i 2 ∇⃗μ̃i + Civ ⃗ zi F (3)

⎡ ⎤ 1 − ti zi t j ⎢ ⎥ ⃗ ⃗ ∇C i − ∑ ∇C j ⎥ Ji ⃗ = − DiCi⎢ z C ⎢⎣ Ci ⎥⎦ j≠i j j

⎛ E ⎞ k f = k f0 exp⎜ − f ⎟ ⎝ RT ⎠

However, the oxygen permeation flux estimation using eq 5 reveals that this equation is not accurate. To improve its precision, three more correction terms can be introduced into the equation. The deviation from the elementary reactions assumption can be taken into account, and also the effects of feed and sweep gas flow rates can be considered. 3.2. Effects of Nonelementary Surface Reactions. The following reversible reactions take place on the membrane surface on the feed and permeate sides, respectively:

In this equation, JO2 (cm /(cm × min)) is the oxygen permeation flux and COF 2 and CNF 2 are the concentrations of oxygen and nitrogen in the feed stream, respectively. COP 2and CNP 2 are measured concentrations of oxygen and nitrogen in the permeate stream, respectively. F is the flow rate of the gas on the permeate side and S is the membrane surface area (0.785 cm2). 3

(6a)

dimension

Different flow rates of synthetic air and helium, both at atmospheric pressure, were used as feed and sweep gas streams, respectively. The oxygen permeation flux was calculated using eq 2: F ⎤ ⎡ CO ⎛ 28 ⎞1/2 ⎥ F P P 2 ⎜ ⎟ J(O ) = ⎢CO C − × × × N2 F ⎢ 2 2 ⎝ 32 ⎠ ⎥ S CN ⎦ ⎣ 2

⎛ E ⎞ 0 DV = DV exp⎜ − D ⎟ ⎝ RT ⎠

However, considering nonelementary rates of reaction for oxygen vacancy, this leads to an implicit equation for oxygen permeation flux, which is unsuitable. Thus, supposing an elementary order for oxygen vacancy and a nonelementary order for oxygen partial pressure leads to the following equations for the feed and permeate sides of the membrane, respectively:

(5)

n C′ − k JO = k f p′O r 2 V 2

(10a)

Oxygen vacancy diffusion coefficient and both forward and reverse reaction rate constants are strong functions of the

n C″ JO = k r − k f p″O 2 V 2

(10b)

2

2

3071



Article

denominator is the sum of the three resistances as summarized in Table 2. Using the definition of resistance and eq 14, it can

Combining eqs 10a and 10b with eq 4 and using assumptions stated above results in eq 11, which can be used instead of eq 5: kr n n (1/p″O − 1/p′O ) 2 2 kf JO = 1 2L 1 2 + n + n k f p ′O D k p V f ″O2 2

Table 2. Resistances and Oxygen Permeation Flux under Different Limiting Conditions limiting case

(11)

3.3. Effect of Air and Sweep Gas Flow Rates. Equation 11 is more accurate than eq 5 in the prediction of oxygen permeation flux through the LSCF membranes; however, there is still a gap between the experimental data and the model predictions. Experiments illustrate that the feed and sweep gas flow rates have considerable effects on the oxygen permeation flux, but in eq 11, there is no term to reflect these effects. To derive a much more precise model, it is crucial to take into account effects of all parameters which affect oxygen permeation flux. Therefore, the effects of feed and sweep gas flow rates should be taken into account. Thus, modified oxygen partial pressures, which take the feed and sweep gas flow rates into account, can be defined for each side of the membrane. It is clear that oxygen partial pressure at each side of the membrane is affected by the gas flow rate. A modified oxygen partial pressure, consisting of oxygen partial pressure and a correction term which is a function of the Reynolds number (Re), can be introduced.16 The model is capable of predicting oxygen permeation flux more accurately, but this is not good enough. Therefore, by examining several different functionalities of the Re number, finally the following equations are proposed as the best functions for the modified oxygen partial pressures: p′*O2 = (a′ + b′Re′cf ′)−1p′O2

(12a)

p″*O2 = (a″ + b″Re″cp″)−1p″O2

(12b)

bulk diffusion

q′ πν′λ′

Re″ =

q″ πν″λ″

n

k f p′*O2

2L DV

⎤ ⎡⎛ p′* ⎞n O2 ⎟ JO = k r⎢⎢⎜⎜ − 1⎥⎥ ⎟ 2 * ⎦ ⎣⎝ p″O2 ⎠

JO =

1 R″ex = k f p″*On2

kr (1/p″*On2 − 1/p′*On2 ) kf

2L /D V

⎡ ⎛ p″* ⎞n⎤ O2 ⎟ ⎥ JO = k r⎢⎢1 − ⎜⎜ ⎟ ⎥ 2 * p ′ ⎝ O2 ⎠ ⎦ ⎣

be concluded that if any step has a limiting rate, its resistance dominates the total resistance, so oxygen permeation across the membrane is mainly affected by that resistance. The oxygen permeation fluxes for each limiting case are summarized in the last column of Table 2. 3.5. Characteristic Thickness. As is known, oxygen permeation flux increases with decreasing membrane thickness, but only up to a certain thickness, which is named the characteristic thickness (Lc). For a membrane with its Lc, oxygen permeation flux is controlled by both surface exchange kinetics and bulk diffusion. For a membrane with a thickness smaller than Lc, oxygen permeation flux cannot be improved by making it thinner. To understand the effects of membrane thickness, the characteristic thickness of the LSCF membranes should be calculated. The membrane characteristic thickness can be defined as follows:32 Lc =

DV ks

(15)

In this relation, ks is an average surface exchange coefficient and is given by 1 1 = (R′ex + R″ex ) ks 2

(13b)

Substitution of these modified oxygen partial pressures in eq 11 results in eq 14. The results confirm that the three correction terms are extremely prosperous in improving the accuracy of oxygen permeation flux prediction through the LSCF membranes. The model predictions are in excellent agreement with the experimental data. n

oxygen permeation flux

1

2

(13a)

(16)

The combination of equations presented in the second column of Table 2 with eqs 15 and 16 results in the following relation for the membrane characteristic thickness: ⎛ ⎞ 1⎜ 1 1 ⎟ DV Lc = ⎜ n + n ⎟ 2⎜ * k p″*O2 ⎟⎠ f ⎝ p′O2

n

kr (1/p″*O2 − 1/p′*O2 ) kf JO = 1 2L 1 2 + n + n DV * k f p ′O2 k f p ″*O2

R′ex =

R diff =

surface exchange on the permeate side

Re numbers for both sides of the membrane are defined as follows:

Re′ =

resistance

surface exchange on the feed side

(17)

To more easily determine the situations in which oxygen permeation flux is controlled by bulk diffusion or surface exchange kinetics, it is reasonable to define the dimensionless membrane characteristic thickness (Lcr).

(14)

3.4. Contribution of Resistances. On the basis of the oxygen transfer mechanism through MIEC ceramic materials, the total oxygen transfer resistance is considered to be made up of three main different resistances: resistances of surface exchange kinetics on both membrane sides and bulk diffusion resistance. Flux is the ratio of the driving force to the resistance. Thus, in eq 14, the numerator is the driving force, and the denominator is the total resistance. As can be seen, the

Lcr =

R diff L = Lc R′ex + R″ex

(18)

If the value of Lcr is greater than unity, the bulk diffusion mechanism is the rate-limiting step of the oxygen transport process. Otherwise, surface exchange reactions control oxygen permeation. 3072



Article

Figure 3. XRD pattern of the LSCF powder synthesized via the autocombustion method.

Figure 4. SEM micrograph of the LSCF nanopowder prepared via the autocombustion method. (a) 2000 and (b) 60 000 times magnified.

4. RESULTS AND DISCUSSION

leads to the larger surface area, better sintering properties, and higher actual densities. 4.1.2. Membrane. SEM micrographs of the membrane surface and cross-section are shown in Figure 5. As these micrographs reveal, the membrane surface is completely dense and crack-free. This ensures the membrane gas tightness, and it is due to the fine and small powder particles. In addition, high pressing pressure ensures complete powder particles compression. Selecting proper sintering conditions such as adequate time to grow the grains and also a sufficiently high temperature to supply the required energy results in a satisfactory membrane microstructure. The relative densities of the four membranes are in the range of 95% to 97%, which is favorable for pure oxygen production. These high relative densities reveal that the membranes are dense, and there is a negligible number of holes present in the membrane body. Many more SEM micrographs, illustrating effects of shaping and sintering conditions, were reported elsewhere with further analysis. 4.2. Model Validation. To fit the developed model parameters, a nonlinear regression method can be used. Root mean square deviation (RMSD) is the objective function for error minimization. Furthermore, to illustrate the accuracy of the developed model, average absolute deviation (AAD) and the square of the Pearson product moment correlation coefficient (R2) can be calculated. To determine the effectiveness of the correction terms, model constants were optimized using four

4.1. Characterization. 4.1.1. Powder. The XRD pattern of the LSCF powder is shown in Figure 3. The presence of sharp peaks confirms the formation of the crystalline single phase compound. The autocombustion method features a short synthesis time because of its spontaneous burning process. Organic additives are burned out extremely quickly in preparation of the primary powder, and this results in a very porous bulk product with separated powder particles. Thus, the milling process is not necessary, and hence, impurities associated with the milling process are eliminated. Using XRD data, lattice constant of the LSCF powder can be calculated to be 3.862402 Å, and there is excellent agreement with those reported by Shao et al.11 SEM micrographs of the LSCF powder are shown in Figure 4. The results of particle size distribution calculation reveal that the synthesized powder has a narrow particle size distribution, and its average value is around 80 nm. Extrafine powder is obtained, due to the escape of combustion products from the primary gel in the burn out step. This fine powder results in more compaction during pressing of green membranes, and consequently, the relative density of the membranes increases. In addition, for dense ceramic membrane applications, smaller powder particle size is more favorable. The smaller particle size 3073



Article

Figure 5. SEM micrograph of (a) surface and (b) cross-section of the LSCF membrane, pressed under 2000 bar, sintered at 1200 °C for 8 h.

Table 3. Statistical Parameters Used for Model Validation statistical parameter

abbreviation

root-mean-square deviation

formula

RMSD

1 N average absolute deviation

AAD

1 N R2

square of the Pearson product moment correlation coefficient

Table 4. Statistical Parameters of the Models Considering None, One, and Both of Non-Elementary Surface Reactions and Feed and Sweep Gas Flow Rates statistical parameters model model model model

1 2 3 4

RMSD

AAD

R2

0.1442 0.1267 0.0778 0.0344

0.1082 0.1008 0.0644 0.0274

0.8777 0.8955 0.9763 0.9960

i=1

∑ N

1−

calcd JO ,i 2

exptl

JO , i 2

⎛ exptl exptl,ave calcd calcd,ave ∑iN (JO , i − JO )(JO , i − JO ) ⎜ 2 2 2 2 ⎜ exptl exptl,ave calcd − calcd,ave 2 ⎜ ∑i (J )2 ∑iN (JO JO ) N O2 , i − JO2 ⎝ 2, i 2

⎞2 ⎟ ⎟ ⎟ ⎠

of the model (RMSD increases 0.0175) compared with the effects of feed and sweep gas flow rates (RMSD increases 0.0664). It is worthwhile to mention that the effects of nonelementary reactions were introduced into the model just by one parameter (n); however, the effects of feed and sweep gas flow rates were introduced by six parameters (a′, b′, c′, a″, b″, and c″). The improvement of RMSD per added constant is 0.0175 and 0.0111 for the effects of nonelementary reactions and feed and sweep gas flow rates, respectively. Therefore, it can be concluded that considering nonelementary surface reactions has more of an effect on improvement of the developed model accuracy. However, when the three crucial correction terms are added to the model, its accuracy improves significantly (RMSD = 0.0344, AAD = 0.0274, and R2 = 0.996). Figure 6 illustrates the accuracy of the primary and the modified models by comparing the predicted oxygen permeation fluxes with experimental data. Clearly, the error is small, and the model is able to correlate the experimental data satisfactorily. 4.3. Model Constants. The temperature dependence of the oxygen vacancy diffusion coefficient and both forward and reverse reaction rate constants are considered to be Arrhenius type functions. The values of activation energies and preexponential coefficients and also constants of the correction terms are summarized in Table 5. 4.4. Oxygen Permeation: Experimental and Mathematical Modeling Results. Figures 7−11 reveal the effects of temperature, membrane thickness, feed and sweep gas flow rates, and feed and permeate side oxygen partial pressures on oxygen permeation flux and permeation resistances. The effects of these variables on oxygen permeation flux are discussed, on the basis of experimental studies and mathematical modeling results.

different models. First, eq 5 (model 1) was used for estimation of the oxygen permeation flux through the LSCF membranes, and the results showed significant errors. After that, oxygen permeation flux was estimated using eq 11 (model 2) to illustrate to what extent considering nonelementary surface reactions could effectively improve the model accuracy. To determine the contribution of introducing feed and sweep gas flow rates in the model, oxygen permeation flux was calculated using the model, which just took the effects of feed and sweep gas flow rates into account. The equation of this model was similar to eq 14, but surface reactions were considered to be elementary; i.e., n was held constant and equal to 0.5 (model 3). Finally, the model, which took the effects of both nonelementary surface reactions and feed and sweep gas flow rates into account (model 4), was used to estimate oxygen permeation flux. Table 3 reveals the calculated statistical parameters used as criteria of the model accuracy. Values of these statistical parameters are summarized in Table 4.

model

2 calcd ⎞ JO ⎜ 2, i ⎟ ∑ ⎜⎜1 − exptl ⎟⎟ JO , i ⎠ N ⎝ 2

i=1 ⎛

As can be seen in Table 4, considering the effects of nonelementary surface reactions results in less improvement in accuracy 3074



Article

Figure 6. Comparison between the accuracy of (a) the primary and (b) the modified models.

Table 5. Model Parameters parameter

value

dimension

DV0 kf0 kr0 ED Ef Er n a′ b′ c′ a″ b″ c″

1.183 × 10−5 5.029 × 10+5 2.852 × 10+8 8.845 × 10+4 2.271 × 10+5 2.100 × 10+5 2.050 × 10−1 7.951 × 10+2 −7.644 × 10+2 7.136 × 10−3 −8.846 1.199 × 10+1 −2.143 × 10−2

m2/s m/atmn s mol/m2 s J/mol J/mol J/mol

4.4.1. Effect of Temperature. Parts a of Figures 7−11 illustrate the effects of temperature on the oxygen permeation flux. Enhancement of the oxygen permeation flux is clearly observed by increasing the temperature. In addition, the oxygen diffusion coefficient increases with temperature. As temperature rises, kf and kr increase, and as the activation energy of the former is greater than that of the latter, kf is more sensitive to temperature. The forward reactions define oxygen adsorption on both membrane surfaces while the reverse reaction introduces oxygen desorption. As the oxygen transport direction is from the feed side to the permeate side, the forward rate of the reaction is dominant compared with the reverse one at the feed side, but in the other side, the reverse rate of reaction is dominant. On each side of the membrane, rf is a function of the temperature, pO*2 , and CV on that side. As the temperature rises, rf increases with the temperature. However, rr is an ascending function of only the temperature. Both rf and rr increase with the temperature. In the feed side, rf is greater and more sensitive to temperature compared with rr, while on the permeate side, the opposite is true. Thus, by increasing the temperature, the difference between rf and rr increases, and consequently oxygen permeation flux enhances. Resistance of the surface reaction on each side of the membrane is affected by the variation of both kf and pO*2 of that side with the temperature. As mentioned, DV and kf increase as the temperature rises, resulting in the reduction of all resistances. This is illustrated in parts b of Figures 7−11. However, as the activation energy of diffusion is about 1 order of magnitude smaller than that of forward surface reaction, the

Figure 7. Effects of feed side oxygen partial pressure on (a) oxygen permeation flux and (b) feed side surface exchange reaction resistance (s/m).

resistances of surface reaction decreases more rapidly compared with bulk diffusion. At lower temperatures (e.g., 700 °C), DV and kf are small and grow rapidly with the temperature. Consequently, at lower temperatures, the resistances change more significantly. As calculations reveal, average contribution of the diffusion resistance in total resistance increases from 27.3% to 96.1% as the temperature rises from 700 to 1000 °C. So, as the temperature rises, the contribution of diffusion resistance in total resistance becomes dominant, and thus oxygen permeation tends to be limited by diffusion. p′O*2 depends on temperature only in terms of Re. As the temperature rises, viscosity increases, so Re decreases. As mentioned above, the correction term increases with Re and thus p′O*2 decreases with the temperature. To sum up, the 3075



Article

Figure 9. Effects of feed flow rate on (a) oxygen permeation flux and (b) feed side surface exchange reaction resistance (s/m).

Figure 8. Effects of permeate side oxygen partial pressure on (a) oxygen permeation flux and (b) permeate side surface exchange reaction resistance (s/m).

temperature has two different effects on R′ex, but as its effect on kf is more dominant, R′ex decreases with the temperature. The temperature dependency of Re and p″O*2 results in the variation of p″O*2 with the temperature. As mentioned, increasing the temperature decreases Re, and thus the permeate side correction term decreases. At elevated temperatures, oxygen permeation flux increases, and this results in higher p″O2. Since the effect of oxygen partial pressure is more dominant, p″O*2 increases with the temperature. Both kf and p″O*2 increase with the temperature, and consequently, as the temperature rises, R″ex decreases. 4.4.2. Effect of Feed Side Oxygen Partial Pressure. The effects of feed side oxygen partial pressure on the oxygen permeation are shown in Figure 7a at different temperatures, keeping the feed and sweep gas flow rates constant at 200 and 40 cm3/min, respectively. The membrane thickness is 1.0 mm. As expected, increasing p′O2 enhances the feed side surface reaction rate and increases the permeation driving force. However, this increment is nonlinear. In other words, as the feed side oxygen partial pressure is doubled, the probability of oxygen molecule adsorption into the membrane surface and thus the surface reaction rate increases less than twice. This is due to the limiting numbers of active reaction sites which are inherently constant. As a result, the oxygen permeability increases less than twice. Figure 7a reveals that by doubling p′O2, JO2 increases about 15%. As Figure 7b reveals that, by increasing p′O2, R′ex reduces, while Rdiff is not affected. In addition, the reduction of R′ex and

Figure 10. Effects of sweep gas flow rate on (a) oxygen permeation flux and (b) permeate side surface exchange reaction resistance (s/m). 3076



Article

4.4.4. Effect of Feed Flow Rate. The effects of feed flow rate on oxygen permeability are presented in Figure 9a. Sweep gas flow rates were controlled to be constant at 40 cm3/min, and synthetic air containing 21% oxygen was used as the feed. The membrane thickness was 1.0 mm. It is clear that the oxygen permeation flux more significantly enhances with an increase in the feed flow rate but only up to 200 cm3/min; however, with a further increase, the oxygen permeation flux increases slightly. The feed side approaches well-mixed conditions as the feed flow rate increases due to the gas phase turbulence increment. Since the well-mixed conditions are established asymptotically, the flux improvement rate decreases as the flow rate increases. Hydrodynamics of the feed flow rate do not significantly affect the oxygen permeation, and thus their effects are reasonably negligible. Li et al. studied effects of air flow rates on the oxygen permeation behavior of La0.6Sr0.4Co0.2Fe0.8O3−δ and reported similar behavior.33 As shown in Figure 9b, Re increases with an increase in the feed flow rate, and consequently, p′O*2 increases. This results in a reduction of R′ex. Rdiff does not vary with the feed flow rate, and R″ex decreases as the feed flow rate increases. This reduction in R″ex is an indirect effect and is due to the increasing p″O2 because of the oxygen permeation flux improvement. The contribution of R′ex in total resistance decreases with an increase in the feed flow rate, while that of the bulk diffusion resistance increases. The contribution of R″ex in the total resistance at a lower temperature increases with an increase in the feed flow rate, while it decreases at higher temperatures. 4.4.5. Effect of Sweep Gas Flow Rate. Figure 10a represents effects of sweep gas flow rate on the oxygen permeation flux. A greater sweep gas flow rate results in the reduction of p″O*2 , and this is a driving force behind the increase in oxygen permeation. Consequently, increasing the sweep gas flow rate leads to an oxygen permeation flux enhancement. The trend of oxygen permeation flux as a function of the sweep gas flow rate is the same as the experimental results reported by several researchers.15,34−36 However, the reported fluxes differ from each other due to different membrane thicknesses and operating conditions and even the powder and membrane preparation methods. The equations in Table 2 reveal that only R″ex is affected by the sweep gas flow rate. As the sweep gas flow rate increases, two reverse phenomena arise. The first is increasing Re, which increases the correction term, and the second is a reduction of p″O2, which reduces p″O*2 . As the second phenomenon is dominant, p″O*2 decreases, while the sweep gas flow rate increases. Consequently, as shown in Figure 10b, R″ex and thus the total resistance of the oxygen transfer through the membrane increase with an increase in the sweep gas flow rate. In addition, the oxygen permeation driving force increases and because its effect on the permeation flux is dominant compared with R″ex, the permeation flux enhances, as mentioned before. The contribution of R″ex in the total resistance increases, while the contributions of R′ex and Rdiff decrease with increasing sweep gas flow rate. 4.4.6. Effect of Membrane Thickness. The effects of membrane thickness on oxygen permeation flux are revealed in Figure 11a. As seen in Figure 11b, by increasing the membrane thickness, the bulk diffusion resistance increases, and thus, the oxygen permeation flux decreases. As Rdiff increases, p″O2 reduces, and consequently R″ex increases. R′ex is not affected by the membrane thickness. The ratio of Rdiff over

Figure 11. Effects of membrane thickness on (a) oxygen permeation flux and (b) bulk diffusion resistance (s/m).

consequently the incrementation of the oxygen permeation flux result in a higher p″O2; thus, R″ex decreases very slightly. The contribution of R′ex to the total resistance decreases as the oxygen partial pressure increases, while that of Rdiff increases. The contribution of R″ex to the total resistance increases with an increase in the oxygen partial pressure at low temperatures but decreases at high temperatures. 4.4.3. Effect of Permeate Side Oxygen Partial Pressure. The effects of permeate side oxygen partial pressure on the oxygen permeability are shown in Figure 8a at different temperatures, keeping the feed and sweep gas flow rates constant at 200 and 40 cm3/min, respectively, and synthetic air containing 21% oxygen was used as a feed. The membrane thickness is 1.0 mm. As observed, the oxygen permeation flux decreases as the oxygen partial pressure at the permeate side of the membrane increases. First, this phenomenon is due to the reduction of the oxygen permeation driving force. Second, by increasing p″O2, the rate of the forward reaction on the permeate side increases while the rate of the reverse reaction is constant. Consequently, the increment of p″O2 diminishes the oxygen permeation flux. It is clear that p″O2 affects neither R′ex nor Rdiff. However, as shown in Figure 8b, R″ex decreases as p″O2 increases. The ratio of R″ex/R′ex is equal to the ratio of (p′O*2 /p″O*2 )n. As p′O*2 is larger than p″O*2 , R″ex is greater than R′ex The results confirm this statement and reveal that R″ex is 1.17 to 2.77 times larger than R′ex The calculations show that p″O*2 affects the ratio of the permeate side surface reaction resistance to the total resistance, reversely, while those of the bulk diffusion and the feed side oxygen exchange reaction increase with p″O*2 . 3077



Article

Figure 12. Effects of (a) feed side oxygen partial pressure, (b) permeate side oxygen partial pressure, (c) feed flow rate, (d) sweep gas flow rate, and (e) membrane thickness, on dimensionless characteristic thickness.

the oxygen partial pressures of feed and permeate sides, and the feed and sweep gas flow rates. Figure 12 shows the functionality of Lcr versus these variables. Clearly, Lcr increases with the temperature. But at elevated temperatures, Lcr increases more rapidly than at lower temperatures. The surface exchange reaction and also the bulk diffusion resistances decrease as the temperature rises. As mentioned before, the surface reaction resistances are stronger functions of the temperature with respect to the bulk diffusion resistance. Thus, R′ex and R″ex decrease more rapidly compared with Rdiff. Consequently, as the temperature rises, Lcr tends to become greater, and this means the oxygen permeation through the membrane tends to be limited by the bulk diffusion. The calculations show that, in most cases, Lcr is greater than unity. This indicates that the main resistance to the oxygen permeation is the bulk diffusion

the total resistance reduces as the membrane thickness decreases. Thus, as the membrane becomes thinner, the oxygen diffusion limiting features diminish. Meanwhile, contributions of the other two oxygen exchange resistances in the total resistance increase. This behavior continues until the membrane thickness approaches a specific value which is the characteristic thickness of the membrane. As this condition is established, the surface exchange reactions and the bulk diffusion contribute to each other to limit the oxygen permeation flux. In membranes which are thinner than their characteristic thickness, the bulk diffusion is not the ratelimiting step for the permeation. 4.5. Dimensionless Characteristic Thickness. Equations 17 and 18 illustrate that the dimensionless characteristic thickness of the membranes is a function of the temperature, 3078



Article

dimensionless characteristic thickness, greater than unity in most cases, showed that the bulk diffusion resistance is the main oxygen permeation resistance.

resistance, and oxygen permeation flux is improved by decreasing the membrane thickness. As mentioned before, by increasing the feed side oxygen partial pressure, both R′ex and R″ex decrease, while Rdiff does not vary. This results in a rising Lcr as p′O2 increases. The functionality of Lr with p″O2 is almost the same as that with p′O2. However, p″O2 only affects R″ex and does not affect R′ex. As stated previously, R″ex decreases with p″O2, and thus, Lcr increases with it. To sum up, these behaviors illustrate that by increasing the feed and permeate side oxygen partial pressures, the oxygen permeation is limited by the bulk diffusion. As mentioned previously, the feed flow rate increment results in a reduction of R′ex and R″ex, while Rdiff is constant. Thus, Lcr increases with the feed flow rate. However, R″ex increases with the sweep gas flow rate, while R′ex and Rdiff do not vary. Thus, increasing the sweep gas flow rate influences Lcr reversely. Rdiff and R″ex rise as the membrane thickness increases, but R′ex is constant. The membrane thickness affects Rdiff more significantly than R″ex, and thus, Lcr increases with the membrane thickness.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +98 21 77240496. Fax: +98 21 77240495. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

5. CONCLUSION The La0.6Sr0.4Co0.2Fe0.8O3−δ membranes were synthesized, and the effects of operating conditions on oxygen permeation flux were investigated. A mathematical model was developed to study oxygen permeation through the LSCF membranes. The distinctive features of the developed model are summarized as follows: 1. extremely accurate prediction of the oxygen permeation flux in the domain of operating parameters (RMSD = 0.0344, AAD = 0.0274, and R2 = 0.9960). 2. explicit functionality of the developed model from operating conditions 3. incorporation of both bulk diffusion and nonelementary surface reactions into the model 4. taking the effects of system hydrodynamics, flow rates, and properties of entering fluids on the oxygen permeation flux into account with the aid of a defined Reynolds number in the modified oxygen partial pressure definition 5. capability of the model to calculate permeation resistances 6. definition and calculation of dimensionless characteristic thickness as a criterion for determination of the limiting permeation step. Both feed and permeate side surface reactions and bulk diffusion resistances were taken into account in the developed model. As the temperature elevates, all three resistances decrease. For a membrane with a thickness of 1.0 mm, while the feed side oxygen partial pressure is 21% and the feed and sweep gas flow rates are kept constant at 200 and 40 cm3/min, respectively, increasing the temperature from 700 to 1000 °C increases the contribution of bulk diffusion resistance from 28.2% to 96.6%. Meanwhile, feed and permeate side surface reactions decrease from 23.2% to 1.5% and 48.6% to 1.9%, respectively. A dimensionless characteristic thickness was defined to be used in determination of the permeation-controlling step. It increases with the temperature, and this means the oxygen permeation through the membrane tends to be limited by bulk diffusion at higher temperatures. The dimensionless characteristic thickness increases with increasing oxygen partial pressure on both sides of the membrane and also the feed side flow rate; however, it decreases with the sweep gas flow rate. The

NOMENCLATURE a, b, c, n = constant of correction terms C = molar concentration D = diffusion coefficient E = activation energy F = Faraday constant J = molar flux density k = surface exchange rate constant L = thickness pO2 = oxygen partial pressure q = volumetric flow rate R̅ = universal gas constant R = resistance ti = transference number T = temperature u = fluid velocity zi = valence

Greek Letters

λ = distance between the air entrance and membrane surface μ̃ = electrochemical potential ν = kinematic viscosity ρ = density σ = conductivity ϕ = electric potential

Subscripts and Superscripts

■

c = characteristic cr = characteristic, dimensionless diff = diffusion ex = surface exchange reaction f = forward reaction g = gas phase h = electron hole O = oxygen ion or atom O2 = oxygen molecule r = reverse reaction s = surface phase V = vacancy ′ = feed side ″ = permeate side × = zero charge • = single positive charge

REFERENCES

(1) Suresh, B.; Schlag, S.; Yokose, K.; Ping, Y. Air separation gases: CEH Marketing Research Report. Chem. Econ. Handbook 2008. (2) Sunarso, J.; Baumann, S.; Serra, J. M.; Meulenberg, W. A.; Liu, S.; Lin, Y. S.; Diniz da Costa, J. C. Mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen separation. J. Membr. Sci. 2008, 320 (1−2), 13−41. (3) Li, S.; Jin, W.; Huang, P.; Xu, N.; Shi, J.; Lin, Y. S.; Hu, M. Z. C.; Payzant, E. A. Comparison of oxygen permeability and stability of

3079



Article

perovskite type La0.2A0.8Co0.2Fe0.8O3−δ (A = Sr, Ba, Ca) membranes. Ind. Eng. Chem. Res. 1999, 38 (8), 2963−2972. (4) Zhao, J.; Zhang, K.; Gao, D.; Shao, Z.; Liu, S. Optimization of BaxSr1‑xCo0.9Nb0.1O3‑δ perovskite as oxygen semi-permeable membranes by compositional tailoring. Sep. Purif. Technol. 2010, 71 (2), 152−159. (5) Zeng, P.; Shao, Z.; Liu, S.; Xu, Z. P. Influence of M cations on structural, thermal and electrical properties of new oxygen selective membranes based on SrCo0.95M0.05O3−δ perovskite. Sep. Purif. Technol. 2009, 67 (3), 304−311. (6) Yang, L.; Wu, Z.; Jin, W.; Xu, N. Structure and oxygen permeability of BaCo0.4Fe0.6−xZrxO3−δ oxide: Effect of the synthesis method. Ind. Eng. Chem. Res. 2004, 43 (11), 2747−2752. (7) Tan, X.; Wang, Z.; Li, K. Effects of sintering on the properties of La0.6Sr0.4Co0.2Fe0.8O3−dδ perovskite hollow fiber membranes. Ind. Eng. Chem. Res. 2010, 49 (6), 2895−2901. (8) Mosadeghkhah, A.; Alaee, M. A.; Mohammadi, T. Effect of sintering temperature and dwell time and pressing pressure on Ba0.5Sr0.5Co0.8Fe0.2O3−δ perovskite-type membranes. Mater. Des. 2007, 28, 1699−1706. (9) Möbius, A.; Henriques, D.; Markus, T. Sintering behaviour of La1−xSrxCo0.2Fe0.8O3−δ (0.3 ≤ x ≤ 0.8) mixed conducting materials. J. Eur. Ceram. Soc. 2009, 29 (13), 2831−2839. (10) Tan, X.; Wang, Z.; Li, K. Effects of Sintering on the Properties of La0.6Sr0.4Co0.2Fe0.8O3−δ Perovskite Hollow Fiber Membranes. Ind. Eng. Chem. Res. 2010, 49 (6), 2895−2901. (11) Zeng, P.; Ran, R.; Chen, Z.; Gu, H.; Shao, Z.; da Costa, J. C. D.; Liu, S. Significant effects of sintering temperature on the performance of La0.6Sr0.4Co0.2Fe0.8O3−δ oxygen selective membranes. J. Membr. Sci. 2007, 302 (1−2), 171−179. (12) Wang, H.; Tablet, C.; Feldhoff, A.; Caro, J. Investigation of phase structure, sintering, and permeability of perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranes. J. Membr. Sci. 2005, 262 (1−2), 20−26. (13) Chaim, R.; Levin, M.; Shlayer, A.; Estournes, C. Sintering and densification of nanocrystalline ceramic oxide powders: A review. Adv. Appl. Ceram. 2008, 107 (3), 159−169. (14) Liang, F.; Jiang, H.; Schiestel, T.; Caro, J. High-purity oxygen production from air using perovskite hollow fiber membranes. Ind. Eng. Chem. Res. 2010, 49 (19), 9377−9384. (15) Wang, Z.; Yang, N.; Meng, B.; Tan, X.; Li, K. Preparation and oxygen permeation properties of highly asymmetric La0.6Sr0.4Co0.2Fe0.8O3−α perovskite hollow-fiber membranes. Ind. Eng. Chem. Res. 2009, 48 (1), 510−516. (16) Ghadimi, A.; Alaee, M. A.; Behrouzifar, A.; Asadi, A. A.; Mohammadi, T. Oxygen permeation of BaxSr1−xCo0.8Fe0.2O3−δ perovskite-type membrane: Experimental and modeling. Desalination 2011, 270 (1−3), 64−75. (17) Li, J.-l.; Zeng, Q.; Liu, T.; Chen, C.-s. Oxygen permeability and CO2-tolerance of Sr(Co0.8Fe0.2)0.8Ti0.2O3−δ hollow fiber membrane. Sep. Purif. Technol. 2011, 77 (1), 76−79. (18) Leo, A.; Liu, S.; Diniz da Costa, J. C. Production of pure oxygen from BSCF hollow fiber membranes using steam sweep. Sep. Purif. Technol. 2011, 78 (2), 220−227. (19) Li, Q.; Zhu, X.; He, Y.; Yang, W. Oxygen permeability and stability of BaCe0.1Co0.4Fe0.5O3−δ oxygen permeable membrane. Sep. Purif. Technol. 2010, 73 (1), 38−43. (20) Kim, S.; Yang, Y. L.; Jacobson, A. J.; Abeles, B. Diffusion and surface exchange coefficients in mixed ionic electronic conducting oxides from the pressure dependence of oxygen permeation. Solid State Ionics 1998, 106 (3−4), 189−195. (21) Li, S.; Jin, W.; Xu, N.; Shi, J. Synthesis and oxygen permeation properties of La0.2Sr0.8Co0.2Fe0.8O3−δ membranes. Solid State Ionics 1999, 124 (1−2), 161−170. (22) Xu, S. J.; Thomson, W. J. Oxygen permeation rates through ionconducting perovskite membranes. Chem. Eng. Sci. 1999, 54 (17), 3839−3850.

(23) Yang, Z.; Lin, Y. S. A semi-empirical equation for oxygen nonstoichiometry of perovskite-type ceramics. Solid State Ionics 2002, 150 (3−4), 245−254. (24) Tan, X.; Li, K. Modeling of air separation in a LSCF hollowfiber membrane module. AIChE J. 2002, 48 (7), 1469−1477. (25) Tan, X.; Liu, Y.; Li, K. Mixed conducting ceramic hollow-fiber membranes for air separation. AIChE J. 2005, 51 (7), 1991−2000. (26) Asadi, A. A.; Behrouzifar, A.; Mohammadi, T.; Pak, A., Effects of Nano Powder Synthesis Methods, Shaping and Sintering Conditions on Microstructure and Oxygen Permeation of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) Perovskite-type Membranes. High Temp. Mater. Proc. 2011, DOI: 10.1515/HTMP.2011.128 (27) Ge, L.; Zhou, W.; Ran, R.; Shao, Z.; Liu, S. Facile autocombustion synthesis of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) perovskite via a modified complexing sol-gel process with NH4NO3 as combustion aid. J. Alloys Compd. 2008, 450 (1−2), 338−347. (28) Ge, L.; Ran, R.; Zhou, W.; Shao, Z.; Liu, S.; Jin, W.; Xu, N. Facile auto-combustion synthesis for oxygen separation membrane application. J. Membr. Sci. 2009, 329 (1−2), 219−227. (29) Kontturi, K.; Murtomäki, L.; Manzanares, J. A. Ionic Transport Processes: In Electrochemistry and Membrane Science; Oxford University Press Inc.: New York, 2008. (30) Goto, K. S. Solid State Electrochemistry and Its Applications to Sensors and Electronic Devices; Elsevier: Amsterdam, 1998. (31) Shao, Z.; Xiong, G.; Dong, H.; Yang, W.; Lin, L. Synthesis, oxygen permeation study and membrane performance of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxygen-permeable dense ceramic reactor for partial oxidation of methane to syngas. Sep. Purif. Technol. 2001, 25, 97−116. (32) Bouwmeester, H. J. M.; Burggraaf, A. J. Dense ceramic membranes for oxygen separation. In Fundamentals of Inorganic Membrane Science and Technology; Burggraaf, A. J., Cot, L., Eds.; Elsevier Science B.V.: Amsterdam, 1996. (33) Li, S.; Jin, W.; Huang, P.; Xu, N.; Shi, J.; Lin, Y. S. Tubular lanthanum cobaltite perovskite type membrane for oxygen permeation. J. Membr. Sci. 2000, 166 (1), 51−61. (34) Wang, Z.; Liu, H.; Tan, X.; Jin, Y.; Liu, S. Improvement of the oxygen permeation through perovskite hollow fibre membranes by surface acid-modification. J. Membr. Sci. 2009, 345 (1−2), 65−73. (35) Tan, X.; Wang, Z.; Liu, H.; Liu, S. Enhancement of oxygen permeation through La0.6Sr0.4Co0.2Fe0.8O3−δ hollow fibre membranes by surface modifications. J. Membr. Sci. 2008, 324 (1−2), 128−135. (36) Liu, H.; Pang, Z.; Tan, X.; Shao, Z.; Sunarso, J.; Ding, R.; Liu, S. Enhanced oxygen permeation through perovskite hollow fibre membranes by methane activation. Ceram. Int. 2009, 35 (4), 1435− 1439.

3080


Preparation and Oxygen Permeation of La0. 6Sr0. 4Co0. 2Fe0. 8O3

Recommend Documents