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Journal of Membrane Science 345 (2009) 47–52

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

peration of perovskite membrane under vacuum and elevated pressures forigh-purity oxygen production

uefeng Zhu, Shumin Sun, You Cong, Weishen Yang ∗

tate Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, PO Box 110, Dalian 116023, China

r t i c l e i n f o

rticle history:eceived 17 April 2009eceived in revised form 6 August 2009ccepted 8 August 2009vailable online 15 August 2009

a b s t r a c t

Oxygen permeation through Ba0.5Sr0.5Co0.8Fe0.2O3−ı (BSCFO) perovskite tubular membranes was inves-tigated under vacuum and elevated pressures conditions. This paper is focused on dependence of oxygenpermeation flux on oxygen recovery, temperature and feed pressure. Oxygen flux decreases with the raiseof oxygen recovery, more quickly when the recovery is larger than about 60%. Elevating the feed pres-

eywords:xygen recoveryxygen productionacuum

sure speeds up the decrease of oxygen fluxes against oxygen recovery. Oxygen permeation flux increaseswith the feed pressure of air, however, the increment decreases gradually. Oxygen permeation flux andrecovery increase with temperature gradually at a constant airflow rate and pressure. The permeationflux reached 9.5 cm3/cm2 min at an oxygen recovery of 48% under the condition of 925 ◦C and vacuumpressure of ∼100 Pa for permeation side and 7 atm for feed side. Long-term operation under vacuum and

that

erovskite membraneSCFO

elevated pressure showssealant.

. Introduction

Dense mixed-conducting perovskite ceramic membranes aref significant interest because of their 100% permselectivity withespect to oxygen [1]. These membranes are investigated due to theotential applications for high-purity oxygen production [2] andany catalytic oxidation reactions at elevated temperatures, such

s partial oxidation of methane to syngas [3–7], oxidative dehy-rogenation of ethane to ethylene [8,9] and partial oxidation ofmmonia to nitric oxide for nitric acid production [10]. The mem-ranes were also suggested to combine with Integrated Gasificationombined Cycle (IGCC) process to increase the efficiency of powereneration and achieve the zero emission of carbon dioxide [11].

During the past decades, many perovskite-type oxy-en permeable materials have been developed, and somef them have high oxygen permeation fluxes. The typi-al materials are based on cobalt-doped perovskite oxides12–14], such as La0.6Sr0.4Co0.2Fe0.8O3−ı, SrCo0.8Fe0.2O3−ı anda0.5Sr0.5Co0.8Fe0.2O3−ı (BSCFO). For example, BSCFO possessingery high oxygen flux (>1.0 cm3/cm2 min for a 1.5 mm disk at

00 ◦C) was investigated as membranes for air separation [14],embrane reactors for hydrocarbons conversion [9] and cathodeaterials for solid oxide fuel cell (SOFC) in recent years [15]. In

iteratures, oxygen fluxes of these materials were tested under

∗ Corresponding author. Tel.: +86 411 84379073; fax: +86 411 84694447.E-mail address: yangws@dicp.ac.cn (W. Yang).URL: http://www.yanggroup.dicp.ac.cn (W. Yang).

376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2009.08.020

oxygen purity larger than 99.4% can be obtained when silver was used as

© 2009 Elsevier B.V. All rights reserved.

ambient pressure with inert gas as the sweeping gas to achievethe oxygen chemical potential. It is improper to use the methodfor pure oxygen production in factories. However, pure oxygencan be obtained by raising the air pressure up than 5 atm tomake the oxygen partial pressure of the air side larger than1 atm, or by pumping the permeated oxygen from permeationside, or combining the both. Several studies related on oxygenpermeation through perovskite membranes at elevated pressureswere reported in recent years [16–19]. Our group investigated theoxygen permeation mechanisms of the disk-type BSCFO mem-branes at high pressure (up to 10 atm) [16]. However, two sidesof the membrane had the same pressure to avoid the break, andhelium was used as the sweeping gas. Ito et al. [19] reported anasymmetrical tubular perovskite membrane with a ∼50 �m denselayer operated at elevated pressure up to 20 atm, and obtained avery high oxygen flux of 9 cm3/cm2 min at an oxygen recovery of20% under the condition of 900 ◦C and 10 atm air.

There are still few reports on the separation of pure oxygenunder elevated air pressure due to the difficulties in sealing of themembrane. According to the Wagner equation, the oxygen flux lin-early increases with the logarithmic value of PH/PL, where PH and PLare the oxygen partial pressure of the feed side and the permeationside, respectively. Therefore, to get a high oxygen flux, a great airpressure is required. Leakage due to the imperfect seal is in direct

ratio to the difference of pressure across the membrane, and largedifference of pressure leads to the difficulties in sealing membranes.Under the same oxygen partial pressure gradient, i.e. the logarith-mic value of PH/PL, there is a greater difference of pressure in thecase of applying a high air pressure without pumping than that of

48 X. Zhu et al. / Journal of Membrane Science 345 (2009) 47–52

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a large difference of pressure. For example, pylex glass usually usedas sealant can seal disks and tubes at 1040 ◦C under ambient pres-sure; however, although the operation temperature is lower than800 ◦C, the sealant is still soft and cannot sustain 1 atm difference

Fig. 1. Schematic picture of apparatus for e

pplying a moderate air pressure with pumping. Therefore, for theame oxygen partial pressure gradient, the later has a less leakagehan the former one.

Usually, the feed gas, air, is far excessive comparing to oxygenermeation flux. Operation costs are important for producing purexygen in large scale; therefore, it is not economical to take a lowxygen recovery. There is an optimum oxygen recovery consideringperation costs and oxygen yield. However, dependence of oxygenermeation flux on oxygen recovery is still not clear for perovskiteembranes.In this work, dense small tubes (OD = 2.38 mm and ID = 1.40 mm)

ade of Ba0.5Sr0.5Co0.8Fe0.2O3−ı (BSCFO) were used for oxygen sep-ration. The feed side of the membrane was under moderate airressure (1–7 atm), and the permeation side was under vacuum∼100 Pa). Relationship between oxygen flux and recovery, opera-ion temperature, air pressure of feed side is especially concerned.urthermore, the seal method is also reported in detail.

. Experimental

BSCFO oxide powder was synthesized by a combined citric acidnd EDTA complexing method. Detail information for the prepa-ation of BSCFO oxide powder can be found in Ref. [14]. Thereparation and sintering process of membrane tube have beeneported in the former publication [20]. In the former research, weound that oxygen flux could be improved by reducing the diame-er of membrane tube as keeping other parameter in constant [21].ere, we prepared green tubes with outer diameter (OD) of 3 mmnd inner diameter (ID) of 2 mm. After sintered at 1180–1220 ◦C,he resultant tubes have an OD of 2.38 mm, an ID of 1.40 mm,nd length up to 150 mm. Only those dense membranes verifiedy permeation experiments at room temperature were used forlevated temperature permeation tests. Membrane area was cal-ulated using the following equation:

= �L(do − di)ln(do/di)

(1)

here do, di and L are OD, ID and length of the BSCFO tube, respec-ively.

ting the BSCFO perovskite membrane tube.

Silver was used as sealant, and the membrane was sealed at963 ◦C. The contribution to oxygen permeation by silver is negligi-ble due to its poor permeability compared with BSCFO membrane.Glass sealant cannot be successful in sealing mixed-conductingmembranes under big difference of pressure at high temperatures,because glass sealant has a very wide range of softening tempera-ture. In the temperature range, the sealant is soft and cannot sustain

Fig. 2. Schematic drawing and photo of sealing the BSCFO open tube with silver assealant.

brane

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X. Zhu et al. / Journal of Mem

f pressure. Silver is different from glass sealant. It solidifies andecomes hard when the temperature is below the melted pointf 963 ◦C. Therefore, silver is a good sealant in elevated pressurexperiments. Usually, one end closed tube used in experiments.or example, reports of Air Products [18], Elton Research [22], and

Fig. 3. SEM pictures of BSCFO tubes before (a–c) and after sintered (d–f) at 122

Science 345 (2009) 47–52 49

Ito et al. [19] were all concerning the one end closed tubes ratherthan open tubes. Since when an elevated pressure applied on mem-branes, the one end closed tubes are easily sealed than the opentubes in experiments. As a result, although the later is facile to beprepared through plastic pressing method, there is no report on

0 ◦C for 3 h. (a and d) top view; (b and e) surface; (c and f) cross-section.

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Dependence of oxygen permeation flux and oxygen recov-ery on feed pressures is illustrated in Fig. 5. Oxygen flux andrecovery increase with the feed pressure gradually, and the incre-ments become small with the elevation of feed pressure. Theincrease of oxygen flux is attributed to the improvement of driving

0 X. Zhu et al. / Journal of Mem

pen tubes operated at high difference of pressure. Here, we reportmethod to seal open tubes under elevated pressure. The setup

s shown in Fig. 1. A quartz cap with one end of ID = 2.5 mm andhe other end of ID = 8.0 mm was used in sealing the top of mem-rane tube. The detailed structure of the sealing is shown in Fig. 2.he figure shows the schematic drawing and photo of sealing theSCFO open tube with silver as sealant. Ceramic sealant (HT767A,uitian Adhesive Enterprise Co. LTD. China) used in the sealing pro-ess, which can solidify at room temperature and not soften at highemperatures, has two functions. One is fixing the membrane tubend quartz cap; the other purpose is to hold the melted silver. Aundle of tubes is also facile to be sealed by using this method.

Operation temperature was controlled by a microprocessoremperature controller (Model Al-708, Xiamen Yuguang Elec-ronics Technology Research Institute, China) through a type Khermocouple. Dried synthesized air (O2: 22.36%) was used as theeed gas with a flow range of 40–360 cm3/min. An oil-free scrollump was connected with the setup to achieve a vacuum pres-ure of ∼100 Pa. Flow rate of air was controlled by a mass flowontroller (Models D07-7A/ZM, Beijing Jianzhong Machine Factory,hina). The pumped effluents were analyzed by a gas chromato-raph (GC, Agilent 6890) equipped with a 3 m-13 X column, andhe total flow rate of the effluents was determined by a bubble flow

eter. Nitrogen in the effluents can be detected by GC due to thelight leakage of seal. Oxygen purity was high up to 99.96% under abar/100 Pa difference of pressure, and it changed a little with thedjustment of operation parameters. The leakages were subtractedhen calculated oxygen permeation fluxes.

Phase structure of the as-prepared powder and the sinteredembrane were examined by X-ray diffraction (XRD, Rigaku/Max-RB, Cu K� radiation) in a 2� range of 20–80◦ with a stepidth of 0.02◦. Morphologies of the green and sintered membranesere observed on a Quanta 200F scanning electron microscopy

SEM).

. Results and discussion

BSCFO powder mixed with organic additives was applied forlastic pressing. After slowly dried under room temperature, thereen tubes were sintered to the dense membrane tubes. The XRDatterns of the as-prepared powder and the crushed tubes afterintered show the same single cubic perovskite structure, whicheveals that organic additives have no effects on phase structuref the membrane. The organic additives contain H, C, O and N. Allhe elements except O can be completely removed during the sin-ering process, so the additives would not destroy the perovskitetructure. However, organic additives containing sulfur were usedn preparing hollow fibers. Sulfates (such as BaSO4 and SrSO4) areormed during heat treatment of green hollow fibers in oxygen-ontaining atmosphere [23,24]. Not liking carbonate counterparts,ulfates would not decompose during the sintering process. There-ore, it is clear that the resultant sulfate destroys the perovskitetructure and has negative influences on permeability of mem-ranes.

Fig. 3 shows the SEM pictures of green and sintered tubes. Thereen tube has an OD of 2.98 mm and ID of 1.87 mm after drying,nd the dimensions decreased to 2.38 mm and 1.40 mm after sin-ered at 1220 ◦C for 3 h. As shown in Fig. 3b and c, the grain sizes ofhe green tube are in the range of 1–3 �m, and grows to 15–70 �mfter sintered (Fig. 3e). There is no large pore for the plastic pressed

ubes from the surface and cross-section of green tubes (Fig. 3bnd c), that is the prerequisite for preparing dense tubes withoutpen pores. Cross-section, as shown in Fig. 3f, is dense withoutpen pores to ensure the gastight when a big difference of pressurepplied on membranes.

Fig. 4. Dependence of oxygen permeation flux on oxygen recovery under differentfeed pressures at 925 ◦C. Length: 4.0 cm, S = 2.3 cm2.

Oxygen recovery is an important parameter for the productionof pure oxygen in large scale. Operation costs cannot be ignoredin economical budget. Although air is very cheap, the heating andcompressing process need enormous energies. Therefore, improv-ing the ratio of air (or oxygen recovery) can save the operationcosts. However, there is few data about the relationship betweenoxygen flux and recovery supporting the design of industrial pro-cess for pure oxygen production. Fig. 4 shows the dependence ofoxygen permeation flux on oxygen recovery under different feedpressures at 925 ◦C. Oxygen flux increases with the feed pressure.For example, the flux is about 4.0 cm3/cm2 min under 1 atm at 30%recovery; the value increases to 8.5 cm3/cm2 min under 4 atm at thesame oxygen recovery. As shown in Fig. 4, oxygen flux decreaseswith the increase of oxygen recovery under all the investigated feedpressures. Under a lower feed pressure, the oxygen flux decreasesmore slowly against oxygen recovery than it under a higher pres-sure does. Under the same feed pressure, the oxygen flux degradesquickly with the increase of oxygen recovery, especially when therecovery is larger than 60%. It can be concluded that the improve-ment of oxygen recovery is at the cost of oxygen flux (or oxygenyield). Thus, there is an optimal oxygen recovery according to thepractical production process.

Fig. 5. Dependence of oxygen permeation flux and oxygen recovery on feed pres-sures at 925 ◦C. Solid symbols: oxygen permeation flux; open symbols: oxygenrecovery. Air flow rate: 200 ml/min for square and 100 ml/min for circle. Length:4.0 cm, S = 2.3 cm2.

brane Science 345 (2009) 47–52 51

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X. Zhu et al. / Journal of Mem

orce across the membrane. The oxygen permeation flux reaches.5 cm3/cm2 min under 48% oxygen recovery at 7 atm and with00 cm3/min air flowing over the feed side. A lower feed flow rate

eads to a higher oxygen recovery and lower oxygen flux. There is aarginal enhancement from 5 to 7 atm for oxygen flux and recov-

ry when 100 cm3/min air flows over the feed side. Increasing theirflow rate can improve the increment during the elevation of feedressure. As the flow rate of air is fixed, there is a depletion of oxy-en along the tubular membrane in the feed side. Besides, withhe increase of feed pressure, the oxygen partial pressure degradesuickly along the tube, and becomes very low at the end of the tube.

n the former modeling studies on tubular BSCFO membranes, weound that oxygen flux increases with air pressure when the airressure is lower than 50 atm [21]. However, among the total incre-ent, there is about 70% achieved by increasing the pressure fromto 10 atm. Elevated operation pressure would bring out many dif-culties in practical production of pure oxygen, such as sealing ofembranes and setups, operation costs, etc.Fig. 6 shows the dependence of oxygen flux and recovery

n temperature. The experiment was conducted under 1 andatm of feed pressure; simultaneously, the effects of air flow rateere also investigated. In the studied temperature range, oxy-

en flux increases with temperature linearly. As expected, withhe enhancement of oxygen flux, oxygen recovery also increasesith temperature. In the experiment, the leakage becomes big dur-

ng slowly cooling the furnace. For example, the concentration ofitrogen was 0.18% at 925 ◦C under 3 bar and 200 cm3/min air;he value increased to 4.8% at 825 ◦C under the same conditions.evertheless, the leakage decreased to ∼0.6% when the operation

◦

emperature increased to 925 C again. We think that this is dueo the mismatch of thermal expansion coefficient between quartz0.59 × 10−6 K−1) and silver (18 × 10−6 K−1). It can be expected thatf a metal or zirconia tube was used in the experiment, the sealing

ould be improved.

ig. 6. Dependence of oxygen permeation flux and oxygen recovery on temper-tures. Solid symbols: 3 atm; open symbols: 1 atm. Airflow rate: 200 ml/min forquare and 100 ml/min for circle. Length: 4.0 cm, S = 2.3 cm2.

Fig. 7. Long-term operation of oxygen purity and oxygen permeation flux of BSCFOtube membrane under vacuum and 3 atm at 925 ◦C. Length: 4.5 cm, S = 2.6 cm2, air-flow rate: 75 cm3/min.

The reliability of mixed-conducting perovskite membrane forpure oxygen production is important. In the former research, wefound BSCFO disk membrane can sustain more than 1000 h oper-ation without degradation under oxygen permeation conditions[14]. Nevertheless, oxygen permeability under great difference ofpressure is not reported up to now for BSCFO materials. Here, weevaluated the BSCFO tube for a 100 h oxygen permeation test withpermeation side under vacuum and feed side under elevated pres-sure (3 bar) at 925 ◦C. Simultaneously, the durability of silver asthe sealant sealed by the aforementioned method was investigated.Oxygen flux and oxygen purity during long-term operation of theBSCFO tubular membrane under vacuum and elevated pressure isshown in Fig. 7. From the figure, it was confirmed that the oxygenpurity is constant and very high up to 99.4% during the operation.This fact indicates that sealing at the interface between the per-ovskite tube and the quartz tube is formed with high reliability(under the requirement of temperature kept constant) as well asthe tube being highly reliable itself. Oxygen permeation flux alsokeeps constant during the 100 h operation.

4. Conclusions

In this work, pure oxygen production under vacuum and ele-vated pressure was investigated. A reliable method for sealing opentubes under large difference of pressure was reported. By using themethod, high oxygen purity up to 99.4% was achieved with oneside under vacuum and the other side under elevated pressure.Relationship between oxygen flux and oxygen recovery, operationtemperature, feed pressure and airflow rate was investigated. Oxy-gen flux decreases with the enhancement of oxygen recovery, anddegrades more quickly when oxygen recovery is larger than 60%.Elevation of feed pressure can increase the driving force across themembrane, and result in the rise of oxygen permeation flux. Whenfeed pressure is raised to a high value, increasing the pressure againcan only marginally enhance the oxygen flux. At a fixed airflow rate,oxygen flux and recovery increase with feed pressure and decreasewith temperature. The long-term operation reveals that the sealingmethod is reliable and the BSCFO membrane is durable under thecondition of vacuum and elevated pressure.

Acknowledgments

The authors gratefully acknowledge the financial support ofNational Science Fund for Distinguished Young Scholars of China(20725313), and the Ministry of Science and Technology of China(Grant no. 2005CB221404).

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