- Email: [email protected]
New concept on air separation
Journal of Membrane Science 323 (2008) 221–224
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Rapid communication
New concept on air separation Xuefeng Zhu, Shumin Sun, Yufeng He, You Cong, Weishen Yang ∗ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, PO Box 110, 116023 Dalian, China
a r t i c l e
i n f o
Article history: Received 25 April 2008 Received in revised form 21 June 2008 Accepted 24 June 2008 Available online 1 July 2008 Keywords: Air separation Ceramic membrane Oxygen absorbent PSA
a b s t r a c t A new process which combines oxygen permeable membrane with oxygen absorbent was proposed to simultaneously produce pure oxygen and nitrogen from air. In the process, air is fed in the oxygen permeable membrane, and a large part of oxygen permeates through the membrane. Then, the oxygen-depleted air (∼7% O2 ) passes through an oxygen absorption bed to make the residual oxygen absorbed; simultaneously, pure nitrogen is produced at the exit of the absorption bed. After the absorption bed reaches its saturated capacity, the oxygen-depleted air pass through another absorption bed switched by an automatic 3-way valve; at the same time the saturated absorption bed is under a desorption process by vacuum to renew the absorption capacity. The pumped out oxygen has a high-purity due to the oxygen absorbent is 100% selectivity to oxygen. As a result, nearly 100% recoveries of oxygen and nitrogen, and >99.4% oxygen purity and >99.0% nitrogen purity was achieved simultaneously. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Oxygen and nitrogen are used in many industrial processes. Pure oxygen is used in the production of metals, glass and ammonia, and in the integrated gasification combined cycle (IGCC) for power generation [1]. It has been predicted that the total market would grow significantly if pure oxygen could be produced at lower cost. Nitrogen has a wide variety of industrial applications, with special relevance to the chemical, alimentary, electrical and metallurgical industries. The technology used for commercial separation of air varies according to the scale and requirements for oxygen purity. For example, the cryogenic distillation method that was started in 1902 is used for large-scale production of pure oxygen, and the simultaneous production of nitrogen, argon and helium. Unfortunately, high investment costs make it difficult to integrate this procedure with other industrial process, such as IGCC technology. The pressure swing absorption (PSA) method is used for smalltonnage production, and other processes are available to meet the requirement of less than 1 ton of oxygen per day. It is difficult to yield oxygen and nitrogen simultaneously, and to produce highpurity oxygen by the PSA method, due to the similar absorption properties of O2 and Ar on zeolite adsorbents. Membrane technology for gas separation has developed rapidly in recent years, and organic polymeric membranes have been used commercially for oxygen separation; however, the low separation
∗ Corresponding author. Tel.: +86 411 84379073; fax: +86 411 84694447. E-mail address: [email protected] (W. Yang). URL: http://www.yanggroup.dicp.ac.cn (W. Yang). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.06.052
factor of 2–6 limits membrane technology to produce oxygenenriched air rather than pure oxygen. Increasing the purity of oxygen is associated with rapidly increasing production costs [2]. The use of a dense mixed-conducting, perovskite-type ceramic membrane is a new technology for the production of pure oxygen [3–5]. For more than 20 years of research, great efforts have been devoted to the improvement of oxygen permeation flux [6,7], and now the oxygen permeation flux of some membranes meets the demand for large amounts of pure oxygen [8]. For example, the Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−ı (BSCFO) membrane has a high oxygen flux (∼1.5 cm3 cm−2 min−1 ) at 950 ◦ C for a 1.5 mm disk-typed membrane [7]. Higher oxygen flux can be achieved by elevating the feeding air pressure and reducing the thickness of the membrane. An obvious advantage of perovskite membranes is their 100% selectivity for the permeation of oxygen. Pure oxygen can be obtained by perovskite membrane technology, but it is difficult to obtain pure nitrogen. For the large-scale production of pure oxygen, a high level of oxygen recovery is needed to decrease the operation costs; nevertheless, the oxygen permeation flux decreases quickly with improvement of oxygen recovery. As well as the disadvantages mentioned above, all of the technologies have drawbacks that prevent cost-effective production of high-purity oxygen, and they are prohibitively expensive for many potential applications. Here, we describe a new technology for the production of pure oxygen and nitrogen simultaneously by combination of mixedconducting ceramic perovskite membrane and perovskite oxygen absorbent. The basic scheme is shown in Fig. 1. Firstly, air is fed into the perovskite membrane separator, and a large part of the oxygen permeates through the membrane under an oxygen partial pressure gradient. Then, the oxygen-depleted air (∼7% oxygen) passes
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through an oxygen absorption bed filled with perovskite absorbent to adsorb the residual oxygen; pure nitrogen is produced at the exit of the absorption bed. Once the absorption bed is saturated, the oxygen-depleted air is switched by an automatic 3-way valve to pass through another absorption bed. Meanwhile, the saturated absorption bed is switched to a vacuum pump to renew the absorption capacity. The oxygen is pure, due to the 100% selectivity of the perovskite absorbent for oxygen, and a very high oxygen recovery is achieved by combining the permeated and desorbed oxygen, and the recovery of nitrogen also is close to 100%. 2. Experimental BSCFO powder was prepared by a combined EDTA and citric acid method [7]. The preparation and sintering processes used in the preparation of the membrane tube have been described in Ref. [9]. Here, we prepared small tubes with an outer diameter (OD) of 3 mm and inner diameter (ID) of 2 mm. The sintered tubes have an OD of 2.38 mm, an ID of 1.40 mm, and length up to 150 mm. Only membranes that had no open pores were used for the permeation tests. Dried synthesized air (22.36% O2 ) was used as the feeding gas with a flow range of 40–360 cm3 min−1 . The feeding pressures on the shell side ranged from 0.1 MPa to 0.5 MPa (absolute pressure). An oil-free scroll vacuum pump was connected to the tube side (or permeation side) of the membrane to achieve a vacuum pressure of ∼100 Pa. Membrane areas, S, was calculated as S=
L(do − di ) ln(do /di )
(1)
where do , di and L are OD, ID and length of the BSCFO tube, respectively. The membrane was sealed with silver at 963 ◦ C. The pumped effluents were analyzed by a gas chromatograph (GC, Agilent 6890) equipped with a 3 m-13x column, and the total flow-rate of the effluents was determined by a bubble flow-meter. The flow-rates of gases shown in this communication were all tested at 0.1 MPa. Nitrogen in the effluents due to slight leakage of the seal can be detected by GC. The leakages were subtracted when oxygen permeation fluxes were calculated. BSCFO was ground in a ball-mill machine, and the resultant powder was pressed, crushed and sieved to grains with a size of 0.3–0.8 mm. Two quartz tubes filled with 23 g of BSCFO were used as absorption beds. The combination of the membrane permeator and absorption beds is shown schematically in Fig. 1. One difference is that the 3-way valve was replaced by an automatic 4-way valve. To simplify the operation, high-purity helium with a flowrate of 200 cm3 min−1 was used as the purging gas to renew the absorbent. The concentrations of oxygen in the effluents passed through the absorbent beds were analyzed by an oxygen sensor.
Fig. 2. Relationships between oxygen flux, feed pressure and oxygen recovery at different temperatures.
On a large-scale, a vacuum pump can be used to renew the saturated oxygen absorbent, and the desorbed high-purity oxygen can be collected to improve the total oxygen recovery. 3. Results and discussion Mixed-conducting perovskite oxide BSCFO has been widely investigated as a dense ceramic membrane for oxygen separation from air [7], as a catalytic membrane reactor for the conversion of light hydrocarbons [10], and as a highly active cathode material for solid oxide fuel cells (SOFC) [11]. Here, BSCFO was chosen as the perovskite membrane and oxygen absorbent for the separation of oxygen from air. Fig. 2 shows the relationships between oxygen permeation flux, feeding pressure and oxygen recovery at 875 ◦ C and 925 ◦ C. The membrane was operated under vacuum (∼100 Pa) at the permeation side and elevated pressure at the feeding side. Oxygen flux decreases with the increase of oxygen recovery under all the feeding pressures and temperatures investigated, as shown in Fig. 2. Under high feeding pressure, the oxygen flux decreases more quickly against oxygen recovery than it does under a low feeding pressure. Under the same feeding pressure, the oxygen flux decreases quickly with the increase of oxygen recovery, especially when the recovery is larger than 60%. It can be concluded that the improvement of oxygen recovery is at the cost of oxygen flux. Oxygen flux and the level of recovery increase gradually with feeding pressure due to enhancement of the driving force across the membrane; however, the increment decreases with the elevation of feeding pressure. Modeling studies have revealed that oxygen flux and oxygen recovery increase with increased air pres-
Fig. 1. Concept of the combination of perovskite oxygen membrane with oxygen absorbent for air separation.
X. Zhu et al. / Journal of Membrane Science 323 (2008) 221–224
223
sure up to 5 MPa [12]. The increment achieved by increasing the pressure from 0.1 MPa to 1 MPa is about 70%. Therefore, it is not cost-effective to recover all of the oxygen by reducing the oxygen flux or by improving the feeding air pressure to achieve the complete separation of oxygen and nitrogen through mixed-conducting perovskite membranes. Lin and co-workers [13–16] suggested that mixed-conducting perovskite oxides could be used as oxygen absorbent for hightemperature oxygen separation via a pressure-switching circle. The Linde group developed a high-temperature oxygen absorption based technology termed CAR (ceramic autothermal recovery) and its application in an oxyfuel based power plant [17]. The absorption/desorption mechanism is based on the following reversible reactions. Under high oxygen partial pressure: •• 1 O (g) + VO 2 2
→
O× O
+ 2h
•
(2)
Under low oxygen partial pressure: • •• 1 + 2h → VO + O2 (g) (3) 2 • •• where VO , O× are oxygen vacancies occurring in O and h perovskite oxide, lattice oxygen and mobile electronic hole, respectively. Oxygen combined with oxygen vacancy is stored as lattice oxygen during the absorption step, and released to form oxygen vacancy again during the desorption step. This is a chemical absorption/desorption process and the selectivity for oxygen against other gases is 100%. Fig. 3 shows the oxygen absorption/desorption loops of BSCFO absorbent at 470 ◦ C. Highpurity helium at 200 cm3 min−1 was used for oxygen desorption. The figure shows a fast absorption/desorption kinetics, and good repeatability during the oxygen absorption/desorption loops. The oxygen absorption capacity of perovskite absorbents is usually no more than 10 ml g−1 , and it is ∼4.0 ml g−1 for BSCFO at 470 ◦ C. Therefore, it is difficult to use a perovskite oxygen absorbent for oxygen separation due to the poor capacity compared to zeolite counterparts. O× O
However, we can fully utilize the advantages of perovskite as an oxygen membrane and as an absorbent to avoid their disadvantages by combining them together. After most of the oxygen has permeated from the feeding side to the permeation side of the membrane, the resultant oxygen-depleted air has only a low content of oxygen. For example, the oxygen content decreased from 21% to 7.4% at an oxygen recovery of 70%. When the oxygen-depleted air (O2 con-
Fig. 4. Long-term combination operation of perovskite oxygen membrane and perovskite oxygen absorbent.
centration is only 1/3 of air) passed through the perovskite oxygen absorbent, the yield of nitrogen should be about three times of air as feed, although the absorption capacity was same. At the same time, the residual oxygen can be recovered through the PSA loop. As a result, the total oxygen and nitrogen recovery is close to 100%, and the combination gives the perovskite absorbent a larger capacity for the separation of oxygen and nitrogen. Under these conditions, a very high oxygen recovery through the perovskite membrane is not needed. Consequently, the membranes can provide a higher oxygen permeation flux, and the operation is more cost-effective compared to the use of a single membrane. Fig. 4 shows long-term operation of the combined oxygen membrane and absorbent processes. A BSCFO tubular membrane was used for separation of a large part of the oxygen. The membrane was 4.5 cm long and had an area of 2.6 cm2 . Air flowed along the membrane at 75 cm3 min−1 air with a pressure of 0.3 MPa. A vacuum pressure of ∼100 Pa was achieved on the permeation side by using an oil-free scroll pump. Under these conditions, the oxygen flux was 5.0 cm3 cm−2 min−1 and the recovery was 77.5%. Two quartz tubes filled with 23 g of BSCFO powder were used as absorption beds, with switching between beds at 15 min intervals. As shown in Fig. 4, the combined membrane and absorbent system was very stable during 100 h of operation. Oxygen purity was up to 99.4%, and nitrogen purity was 99.0–100.0%. In earlier research we confirmed the 1000 h durability of the BSCFO membrane for oxygen permeation [7]. The reversibility of the BSCFO oxygen absorbent was verified again during 400 times loops in long-term operation. Therefore, the combined system can separate oxygen and nitrogen in one step with close to 100% pure gas recoveries. For industrial application, no extra heat is needed for the absorption bed because the depleted air coming out from membrane permeator has a hightemperature than the absorption bed needed. The combination will improve the efficiency and decrease the operation costs. 4. Conclusions
Fig. 3. Oxygen absorption/desorption loops of BSCFO absorbent at 470 ◦ C.
In summary, the results presented here demonstrated the possibility of combined perovskite membrane and oxygen absorbent technology to separate oxygen and nitrogen in one step. The combination can fully utilize the advantages and overcome the disadvantages associated with the two technologies when they are used alone to separate air. It is important to note that the yield of pure oxygen and nitrogen can be adjusted easily by changing the oxygen recovery of the perovskite oxygen membrane and the loop intervals without varying the membrane and absorbent. Moreover, perovskite oxygen membranes can be combined with carbon
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molecular sieve absorbents for the separation of oxygen and nitrogen. Acknowledgments The authors gratefully acknowledge the financial support of National Science Fund for Distinguished Young Scholars of China (20725313), and the Ministry of Science and Technology of China (Grant No. 2005CB221404). References [1] G. Stiegel, Mixed conducting ceramic membranes for gas separation and reaction, Membr. Technol. 110 (1999) 5. [2] Y. Zhang, Z. Wang, S.L. Ji, Oxygen-enriched Technologies and Their Applications, Chemical Industry Press, Beijing, 2005, 4–20. [3] H.J.M. Bouwmeester, A.J. Burggraaf, in: P.J. Gellings, H.J.M. Bouwmeester (Eds.), CRC Handbook of Solid State Electrochemistry, CRC Press, Boca Raton, 1997, pp. 481–553 (Chapter 14). [4] W.S. Yang, H.H. Wang, X.F. Zhu, L.W. Lin, Development and application of oxygen permeable membrane in selective oxidation of light alkanes, Top. Catal. 155 (2005) 35. [5] Y. Liu, X. Tan, K. Li, Mixed conducting ceramics for catalytic membrane processing, Catal. Rev. 48 (2006) 145. [6] Y. Teraoka, H.M. Zhang, S. Furuka, N. Yamazoe, Oxygen permeation though perovskite-type oxides, Chem. Lett. (1985) 1743.
[7] Z.P. Shao, W.S. Yang, Y. Cong, H. Dong, J.H. Tong, G.X. Xiong, Investigation of the permeation behavior and stability of a Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−␦ oxygen membrane, J. Membr. Sci. 172 (2000) 177. [8] B.C.H. Steele, Oxygen ion conductors and their technological applications, Mater. Sci. Eng. B 13 (1992) 79. [9] H.H. Wang, Y. Cong, W.S. Yang, Oxygen permeation study in a tubular Ba0.5 Sr0.5 Co0.8 Fe0.2 O3-ı oxygen-permeable membrane, J. Membr. Sci. 210 (2002) 259. [10] H.H. Wang, Y. Cong, W.S. Yang, High selectivity of oxidative dehydrogenation of ethane to ethylene in an oxygen permeable membrane reactor, Chem. Commun. 14 (2002) 1468. [11] Z.P. Shao, S.M. Haile, A high-performance cathode for the next generation of solid-oxide fuel cells, Nature 431 (2004) 170. [12] H.H. Wang, R. Wang, D.T. Liang, W.S. Yang, Experimental and modeling studies on Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−ı (BSCF) tubular membranes for air separation, J. Membr. Sci. 243 (2004) 405. [13] Z.H. Yang, Y.S. Lin, Y.X. Zeng, High-temperature sorption process for air separation and oxygen removal, Ind. Eng. Chem. Res. 41 (2002) 2775. [14] Z.H. Yang, Y.S. Lin, Equilibrium of oxygen sorption on perovskite-type lanthanum cobaltite sorbent, AIChE J. 49 (2003) 793. [15] Q.H. Yin, Y.S. Lin, Beneficial effect of order-disorder phase transition on oxygen sorption properties of perovskite-type oxides, Solid State Ionics 178 (2007) 83. [16] M. Bulow, J. Boer, W. Burckhardt, H.U. Guth, H. Ullmann, V.V. Vashook, Oxygen sorbent compositions and methods of using same, US Patent 7,338,549 (2008). [17] K.R. Krishnamurthy, Demonstration of a novel, low-cost oxygen supply process and its integration with oxy-fuel coal-fired boilers, in: Proceedings of the Sixth Annual Conference on Carbon Capture & Sequestration, Pittsburgh, May 2007.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Rapid communication
New concept on air separation Xuefeng Zhu, Shumin Sun, Yufeng He, You Cong, Weishen Yang ∗ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, PO Box 110, 116023 Dalian, China
a r t i c l e
i n f o
Article history: Received 25 April 2008 Received in revised form 21 June 2008 Accepted 24 June 2008 Available online 1 July 2008 Keywords: Air separation Ceramic membrane Oxygen absorbent PSA
a b s t r a c t A new process which combines oxygen permeable membrane with oxygen absorbent was proposed to simultaneously produce pure oxygen and nitrogen from air. In the process, air is fed in the oxygen permeable membrane, and a large part of oxygen permeates through the membrane. Then, the oxygen-depleted air (∼7% O2 ) passes through an oxygen absorption bed to make the residual oxygen absorbed; simultaneously, pure nitrogen is produced at the exit of the absorption bed. After the absorption bed reaches its saturated capacity, the oxygen-depleted air pass through another absorption bed switched by an automatic 3-way valve; at the same time the saturated absorption bed is under a desorption process by vacuum to renew the absorption capacity. The pumped out oxygen has a high-purity due to the oxygen absorbent is 100% selectivity to oxygen. As a result, nearly 100% recoveries of oxygen and nitrogen, and >99.4% oxygen purity and >99.0% nitrogen purity was achieved simultaneously. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Oxygen and nitrogen are used in many industrial processes. Pure oxygen is used in the production of metals, glass and ammonia, and in the integrated gasification combined cycle (IGCC) for power generation [1]. It has been predicted that the total market would grow significantly if pure oxygen could be produced at lower cost. Nitrogen has a wide variety of industrial applications, with special relevance to the chemical, alimentary, electrical and metallurgical industries. The technology used for commercial separation of air varies according to the scale and requirements for oxygen purity. For example, the cryogenic distillation method that was started in 1902 is used for large-scale production of pure oxygen, and the simultaneous production of nitrogen, argon and helium. Unfortunately, high investment costs make it difficult to integrate this procedure with other industrial process, such as IGCC technology. The pressure swing absorption (PSA) method is used for smalltonnage production, and other processes are available to meet the requirement of less than 1 ton of oxygen per day. It is difficult to yield oxygen and nitrogen simultaneously, and to produce highpurity oxygen by the PSA method, due to the similar absorption properties of O2 and Ar on zeolite adsorbents. Membrane technology for gas separation has developed rapidly in recent years, and organic polymeric membranes have been used commercially for oxygen separation; however, the low separation
∗ Corresponding author. Tel.: +86 411 84379073; fax: +86 411 84694447. E-mail address: [email protected] (W. Yang). URL: http://www.yanggroup.dicp.ac.cn (W. Yang). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.06.052
factor of 2–6 limits membrane technology to produce oxygenenriched air rather than pure oxygen. Increasing the purity of oxygen is associated with rapidly increasing production costs [2]. The use of a dense mixed-conducting, perovskite-type ceramic membrane is a new technology for the production of pure oxygen [3–5]. For more than 20 years of research, great efforts have been devoted to the improvement of oxygen permeation flux [6,7], and now the oxygen permeation flux of some membranes meets the demand for large amounts of pure oxygen [8]. For example, the Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−ı (BSCFO) membrane has a high oxygen flux (∼1.5 cm3 cm−2 min−1 ) at 950 ◦ C for a 1.5 mm disk-typed membrane [7]. Higher oxygen flux can be achieved by elevating the feeding air pressure and reducing the thickness of the membrane. An obvious advantage of perovskite membranes is their 100% selectivity for the permeation of oxygen. Pure oxygen can be obtained by perovskite membrane technology, but it is difficult to obtain pure nitrogen. For the large-scale production of pure oxygen, a high level of oxygen recovery is needed to decrease the operation costs; nevertheless, the oxygen permeation flux decreases quickly with improvement of oxygen recovery. As well as the disadvantages mentioned above, all of the technologies have drawbacks that prevent cost-effective production of high-purity oxygen, and they are prohibitively expensive for many potential applications. Here, we describe a new technology for the production of pure oxygen and nitrogen simultaneously by combination of mixedconducting ceramic perovskite membrane and perovskite oxygen absorbent. The basic scheme is shown in Fig. 1. Firstly, air is fed into the perovskite membrane separator, and a large part of the oxygen permeates through the membrane under an oxygen partial pressure gradient. Then, the oxygen-depleted air (∼7% oxygen) passes
222
X. Zhu et al. / Journal of Membrane Science 323 (2008) 221–224
through an oxygen absorption bed filled with perovskite absorbent to adsorb the residual oxygen; pure nitrogen is produced at the exit of the absorption bed. Once the absorption bed is saturated, the oxygen-depleted air is switched by an automatic 3-way valve to pass through another absorption bed. Meanwhile, the saturated absorption bed is switched to a vacuum pump to renew the absorption capacity. The oxygen is pure, due to the 100% selectivity of the perovskite absorbent for oxygen, and a very high oxygen recovery is achieved by combining the permeated and desorbed oxygen, and the recovery of nitrogen also is close to 100%. 2. Experimental BSCFO powder was prepared by a combined EDTA and citric acid method [7]. The preparation and sintering processes used in the preparation of the membrane tube have been described in Ref. [9]. Here, we prepared small tubes with an outer diameter (OD) of 3 mm and inner diameter (ID) of 2 mm. The sintered tubes have an OD of 2.38 mm, an ID of 1.40 mm, and length up to 150 mm. Only membranes that had no open pores were used for the permeation tests. Dried synthesized air (22.36% O2 ) was used as the feeding gas with a flow range of 40–360 cm3 min−1 . The feeding pressures on the shell side ranged from 0.1 MPa to 0.5 MPa (absolute pressure). An oil-free scroll vacuum pump was connected to the tube side (or permeation side) of the membrane to achieve a vacuum pressure of ∼100 Pa. Membrane areas, S, was calculated as S=
L(do − di ) ln(do /di )
(1)
where do , di and L are OD, ID and length of the BSCFO tube, respectively. The membrane was sealed with silver at 963 ◦ C. The pumped effluents were analyzed by a gas chromatograph (GC, Agilent 6890) equipped with a 3 m-13x column, and the total flow-rate of the effluents was determined by a bubble flow-meter. The flow-rates of gases shown in this communication were all tested at 0.1 MPa. Nitrogen in the effluents due to slight leakage of the seal can be detected by GC. The leakages were subtracted when oxygen permeation fluxes were calculated. BSCFO was ground in a ball-mill machine, and the resultant powder was pressed, crushed and sieved to grains with a size of 0.3–0.8 mm. Two quartz tubes filled with 23 g of BSCFO were used as absorption beds. The combination of the membrane permeator and absorption beds is shown schematically in Fig. 1. One difference is that the 3-way valve was replaced by an automatic 4-way valve. To simplify the operation, high-purity helium with a flowrate of 200 cm3 min−1 was used as the purging gas to renew the absorbent. The concentrations of oxygen in the effluents passed through the absorbent beds were analyzed by an oxygen sensor.
Fig. 2. Relationships between oxygen flux, feed pressure and oxygen recovery at different temperatures.
On a large-scale, a vacuum pump can be used to renew the saturated oxygen absorbent, and the desorbed high-purity oxygen can be collected to improve the total oxygen recovery. 3. Results and discussion Mixed-conducting perovskite oxide BSCFO has been widely investigated as a dense ceramic membrane for oxygen separation from air [7], as a catalytic membrane reactor for the conversion of light hydrocarbons [10], and as a highly active cathode material for solid oxide fuel cells (SOFC) [11]. Here, BSCFO was chosen as the perovskite membrane and oxygen absorbent for the separation of oxygen from air. Fig. 2 shows the relationships between oxygen permeation flux, feeding pressure and oxygen recovery at 875 ◦ C and 925 ◦ C. The membrane was operated under vacuum (∼100 Pa) at the permeation side and elevated pressure at the feeding side. Oxygen flux decreases with the increase of oxygen recovery under all the feeding pressures and temperatures investigated, as shown in Fig. 2. Under high feeding pressure, the oxygen flux decreases more quickly against oxygen recovery than it does under a low feeding pressure. Under the same feeding pressure, the oxygen flux decreases quickly with the increase of oxygen recovery, especially when the recovery is larger than 60%. It can be concluded that the improvement of oxygen recovery is at the cost of oxygen flux. Oxygen flux and the level of recovery increase gradually with feeding pressure due to enhancement of the driving force across the membrane; however, the increment decreases with the elevation of feeding pressure. Modeling studies have revealed that oxygen flux and oxygen recovery increase with increased air pres-
Fig. 1. Concept of the combination of perovskite oxygen membrane with oxygen absorbent for air separation.
X. Zhu et al. / Journal of Membrane Science 323 (2008) 221–224
223
sure up to 5 MPa [12]. The increment achieved by increasing the pressure from 0.1 MPa to 1 MPa is about 70%. Therefore, it is not cost-effective to recover all of the oxygen by reducing the oxygen flux or by improving the feeding air pressure to achieve the complete separation of oxygen and nitrogen through mixed-conducting perovskite membranes. Lin and co-workers [13–16] suggested that mixed-conducting perovskite oxides could be used as oxygen absorbent for hightemperature oxygen separation via a pressure-switching circle. The Linde group developed a high-temperature oxygen absorption based technology termed CAR (ceramic autothermal recovery) and its application in an oxyfuel based power plant [17]. The absorption/desorption mechanism is based on the following reversible reactions. Under high oxygen partial pressure: •• 1 O (g) + VO 2 2
→
O× O
+ 2h
•
(2)
Under low oxygen partial pressure: • •• 1 + 2h → VO + O2 (g) (3) 2 • •• where VO , O× are oxygen vacancies occurring in O and h perovskite oxide, lattice oxygen and mobile electronic hole, respectively. Oxygen combined with oxygen vacancy is stored as lattice oxygen during the absorption step, and released to form oxygen vacancy again during the desorption step. This is a chemical absorption/desorption process and the selectivity for oxygen against other gases is 100%. Fig. 3 shows the oxygen absorption/desorption loops of BSCFO absorbent at 470 ◦ C. Highpurity helium at 200 cm3 min−1 was used for oxygen desorption. The figure shows a fast absorption/desorption kinetics, and good repeatability during the oxygen absorption/desorption loops. The oxygen absorption capacity of perovskite absorbents is usually no more than 10 ml g−1 , and it is ∼4.0 ml g−1 for BSCFO at 470 ◦ C. Therefore, it is difficult to use a perovskite oxygen absorbent for oxygen separation due to the poor capacity compared to zeolite counterparts. O× O
However, we can fully utilize the advantages of perovskite as an oxygen membrane and as an absorbent to avoid their disadvantages by combining them together. After most of the oxygen has permeated from the feeding side to the permeation side of the membrane, the resultant oxygen-depleted air has only a low content of oxygen. For example, the oxygen content decreased from 21% to 7.4% at an oxygen recovery of 70%. When the oxygen-depleted air (O2 con-
Fig. 4. Long-term combination operation of perovskite oxygen membrane and perovskite oxygen absorbent.
centration is only 1/3 of air) passed through the perovskite oxygen absorbent, the yield of nitrogen should be about three times of air as feed, although the absorption capacity was same. At the same time, the residual oxygen can be recovered through the PSA loop. As a result, the total oxygen and nitrogen recovery is close to 100%, and the combination gives the perovskite absorbent a larger capacity for the separation of oxygen and nitrogen. Under these conditions, a very high oxygen recovery through the perovskite membrane is not needed. Consequently, the membranes can provide a higher oxygen permeation flux, and the operation is more cost-effective compared to the use of a single membrane. Fig. 4 shows long-term operation of the combined oxygen membrane and absorbent processes. A BSCFO tubular membrane was used for separation of a large part of the oxygen. The membrane was 4.5 cm long and had an area of 2.6 cm2 . Air flowed along the membrane at 75 cm3 min−1 air with a pressure of 0.3 MPa. A vacuum pressure of ∼100 Pa was achieved on the permeation side by using an oil-free scroll pump. Under these conditions, the oxygen flux was 5.0 cm3 cm−2 min−1 and the recovery was 77.5%. Two quartz tubes filled with 23 g of BSCFO powder were used as absorption beds, with switching between beds at 15 min intervals. As shown in Fig. 4, the combined membrane and absorbent system was very stable during 100 h of operation. Oxygen purity was up to 99.4%, and nitrogen purity was 99.0–100.0%. In earlier research we confirmed the 1000 h durability of the BSCFO membrane for oxygen permeation [7]. The reversibility of the BSCFO oxygen absorbent was verified again during 400 times loops in long-term operation. Therefore, the combined system can separate oxygen and nitrogen in one step with close to 100% pure gas recoveries. For industrial application, no extra heat is needed for the absorption bed because the depleted air coming out from membrane permeator has a hightemperature than the absorption bed needed. The combination will improve the efficiency and decrease the operation costs. 4. Conclusions
Fig. 3. Oxygen absorption/desorption loops of BSCFO absorbent at 470 ◦ C.
In summary, the results presented here demonstrated the possibility of combined perovskite membrane and oxygen absorbent technology to separate oxygen and nitrogen in one step. The combination can fully utilize the advantages and overcome the disadvantages associated with the two technologies when they are used alone to separate air. It is important to note that the yield of pure oxygen and nitrogen can be adjusted easily by changing the oxygen recovery of the perovskite oxygen membrane and the loop intervals without varying the membrane and absorbent. Moreover, perovskite oxygen membranes can be combined with carbon
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