SrCoxFe1–xO3−δ Oxygen Sorbent Usable for High-Temperature

Article pubs.acs.org/IECR

SrCoxFe1−xO3−δ Oxygen Sorbent Usable for High-Temperature Pressure-Swing Adsorption Process Operating at Approximately 300 °C Hiroshi Ikeda,† Akinori Tsuchida,† Jun Morita,‡ and Norio Miura*,† †

Art, Science and Technology Center for Cooperative Research, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan

‡

ABSTRACT: Oxygen sorption/desorption properties of SrCoxFe1−xO3−δ were examined as an oxygen sorbent for a high-temperature pressure-swing adsorption (HT-PSA) process. Perovskite-type structure of the SrCoxFe1−xO3−δ powder samples was observed in the composition range of 0 ≤ x ≤ 0.85, by using XRD measurements. A temperature-programmed-desorption (TPD) measurement revealed that the oxygen desorption temperature in N2 for the perovskite-type SrCoxFe1−xO3−δ samples was lowered with increasing x. Consequently, the oxygen desorption temperature of the SrCo0.85Fe0.15O3−δ sample exhibited around 300 °C, which was the lowest of those of the examined samples. A hightemperature X-ray diffraction (HT-XRD) measurement suggested that the oxygen desorption of SrCo0.85Fe0.15O3−δ in N2 occurred, accompanying the phase transition from perovskite-type structure to brownmillerite structure. Isothermal oxygen sorption/desorption behavior of the SrCoxFe1−xO3−δ samples was examined by means of thermogravimetric analysis (TGA). As a result, it was confirmed that the amount of sorbed oxygen for the SrCo0.85Fe0.15O3−δ sample gave 11.7 cm3 g−1 at 300 °C, which was larger than that (8.6 cm3 g−1) for a benchmark oxygen sorbent (La0.1Sr0.9Co0.9Fe0.1O3−δ). In order to obtain oxygen-enriched air by using the SrCo0.85Fe0.15O3−δ pellet sample as the oxygen sorbent, the oxygen separation from synthetic air was carried out by using a small-scale PSA apparatus equipped with a vacuum pump. It was confirmed that the 45 vol % oxygen-enriched air was obtained even at 300 °C, by using this apparatus.

1. INTRODUCTION A high-temperature pressure-swing adsorption (HT-PSA) process using oxygen-deficient perovskite-type oxides, ABO3−δ, is of much research interest as an oxygen separation (production) technique from air with low cost and low energy. Considerable attention has been paid to explore various kinds of perovskite-type oxides as an oxygen sorbent material for the HT-PSA process. Previous studies have reported that Fe-based perovskite-type oxides can exhibit excellent oxygen sorption/ desorption properties.1−9 Especially, among the examined perovskite-type oxides, the Co−Fe-based perovskite oxides have received attention as not only an oxygen-permeation membrane material but also an oxygen sorbent for the HT-PSA process. For application of an oxygen-permeation membrane for oxygen separation from air, Teraoka et al. have first reported that the ceramic membrane of SrCoxFe1−xO3−δ shows a high rate of oxygen permeation.10 Subsequent studies on the oxygen separation using SrCoxFe1−xO3−δ have been focused on improvement in oxygen permeation property or stability of SrCoxFe1−xO3−δ, by substituting an A-site or B-site cation with other metal ions.11−15 On the other hand, for application of an oxygen sorbent for the HT-PSA process, Lin et al. have first proposed that the perovskite-type oxides such as © XXXX American Chemical Society

LaySr1−yCoxFe1−xO3−δ can be used as an oxygen sorbent for the HT-PSA process.1,16 The perovskite-type oxide can sorb and desorb oxygen largely and reversibly by changing ambient partial pressure of oxygen, resulting in separation of oxygen from air. The subsequent studies on the oxygen sorption/ desorption properties demonstrated that the Co−Fe-based perovskite-type oxides usually gave relatively good behavior.3,5,7,17−19 In addition, fundamental research involving the oxygen nonstoichiometry for SrCoxFe1−xO3−δ, influencing both oxygen membrane-permeation property and oxygen PSA sorption/desorption properties, has been conducted over the past decades.20−25 Although SrCoxFe1−xO3−δ species have often been examined in the oxygen-permeation membrane tests, an oxygen separation test from air using the HT-PSA apparatus loaded with SrCoxFe1−xO3−δ sorbent is limited to our preliminary research report.26 Thus, in this study, we aim to evaluate oxygen sorption/desorption properties of SrCoxFe1−xO3−δ, and we examined the preliminary oxygenReceived: April 4, 2016 Revised: May 9, 2016 Accepted: May 20, 2016

A

DOI: 10.1021/acs.iecr.6b01284 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Schematic view of a PSA apparatus equipped with a small column, an electric furnace, and a vacuum pump.

production test by using a small-scale PSA apparatus, especially operateing at lower temperature like 300 °C.

obtain oxygen-enriched air, the extracted gas in the desorption process was captured in a collection bag.

2. EXPERIMENTAL SECTION SrCoxFe1−xO3−δ (x = 0, 0.2, 0.4, 0.6, 0.7, 0.8, 0.85, 0.9, and 1.0) powder samples were synthesized by means of a metal nitrates decomposition method.6,8,9 Stoichiometric amounts of the metal nitrates, Sr(NO3)2, Co(NO3)2·6H2O, and Fe(NO3)3· 9H2O, were dissolved in an appropriate amount of purified water, followed by homogeneous mixing by using a magnetic stirrer. The obtained solution was heated on a hot-plate stirrer at 250 °C to evaporate water. The resulting dried powder was calcined at 850 °C for 5 h in air, and subsequently sintered at 1200 °C for 6 h in air. The sintered samples were ground into fine powders with a mortar and a pestle. The crystal phases of the powder samples were determined by means of X-ray diffraction (XRD; RINT2100, Rigaku, Cu Kα) measurements. Isothermal oxygen sorption/desorption properties of the prepared SrCoxFe1−xO3−δ powder sample were examined by means of a thermogravimetric analysis (TGA; TG/DTA7200, Hitachi, Japan) in a 500 cm3 min−1 stream of synthetic air (21 vol % O2 + N2 balance) or N2 under an atmospheric pressure at given temperatures. Oxygen temperature-programmed-desorption (O2-TPD) profiles for the prepared samples were obtained by the TG measurements at 100−700 °C with a heating rate of 1 °C/min in N2. In order to examine the phase change of the SrCoxFe1−xO3−δ sample associated with oxygen sorption/ desorption, high-temperature X-ray diffraction (HT-XRD) measurements were carried out in synthetic air or N2 at given temperatures. Oxygen separation from air using the prepared sample was performed by using a small-scale PSA apparatus equipped with an electric furnace, a small-column, and a vacuum pump. The schematic drawing of the apparatus is given in Figure 1. The fabrication process of the perovskite pellet sample was reported in our previous paper.9 The obtained SrCo0.85Fe0.15O3−δ pellet sample of 20 g was loaded into the small column and heated up to a given temperature. The oxygen separation (production) was conducted by repeating sorption and desorption processes alternately at an interval of 1 min. In the sorption process, synthetic air was let to flow from a gas bottle into the column at a flow rate of 2000 cm3 min−1 to sorb oxygen into the pellet sample bulk. In the desorption process, the gas desorbed from the sample was extracted by using the vacuum pump. Pressures in the column in the sorption and desorption processes were 130 and 3−5 kPa, respectively. During the sorption/desorption processes, oxygen concentration in the exhausted gas was monitored by means of a zirconia-based oxygen analyzer. To

3. RESULTS AND DISCUSSION 3.1. Oxygen Sorption/Desorption Properties of SrCoxFe1−xO3−δ. Figure 2 shows the XRD patterns of the

Figure 2. XRD patterns of the prepared SrCoxFe1−xO3−δ (x = 0, 0.2, 0.4, 0.6, 0.7, 0.8, 0.85, 0.9, and 1.0) powder samples measured at room temperature in air.

prepared SrCoxFe1−xO3−δ powder samples at room temperature in air. The diffraction patterns of the samples with composition of 0 ≤ x ≤ 0.85 agreed well with the perovskite-type structure (PDF 39-0954) without any impurity phases. In the case of x = 0.9, however, other diffraction peaks were observed. For x = 1.0, the diffraction peaks were assigned to hexagonal strontium cobalt oxide (SrCoO2.52, PDF 40-1018), which differed from the perovskite-type structure at the composition of 0 ≤ x ≤ 0.85. This result suggested that the prepared SrCoxFe1−xO3−δ samples gave the perovskite-type structure in the composition of 0 ≤ x ≤ 0.85. Thus, we evaluated the oxygen sorption/ desorption properties of the perovskite SrCoxFe 1−xO3−δ samples. Compositional dependence of the oxygen desorption behavior for the SrCoxFe1−xO3−δ samples was examined by means of the O2-TPD measurements. Figure 3 shows the O2TPD profiles for the SrCoxFe1−xO3−δ (0 ≤ x ≤ 0.85) samples at temperatures 100−700 °C. In the oxygen desorption curve of the SrFeO3−δ (x = 0) sample, two peaks at ∼320 and ∼530 °C were observed clearly. According to the previous study on the B

DOI: 10.1021/acs.iecr.6b01284 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 3. O2-TPD profiles of the SrCoxFe1−xO3−δ (x = 0, 0.2, 0.4, 0.6, 0.7, 0.8, 0.85) powder samples.

oxygen desorption behavior of SrFeO3−δ,27 these oxygen desorption peaks can be assigned to the phase transitions of SrFeO3−δ involving change in the oxygen content. The oxygen desorption peak at 320 °C is due to the phase transition from tetragonal perovskite-type structure of Sr8Fe8O23 (SrFeO2.875) to orthorhombic perovskite-type structure of Sr4 Fe4 O 11 (SrFeO2.75). The desorption peak at 530 °C is corresponding to the phase transition from the perovskite-type structure of Sr 4 Fe 4 O 11 to brownmillerite-type structure of Sr2 Fe 2 O 5 (SrFeO2.5). It is clearly seen that, with increasing x in the SrCoxFe1−xO3−δ samples, the oxygen desorption peak at the higher temperature is shifting to the lower temperature side. The oxygen desorption temperature of the SrCo0.85Fe0.15O3−δ sample is the lowest among those of the SrCoxFe1−xO3−δ samples. Also, in the desorption profile of the SrCo0.85Fe0.15O3−δ sample, two peaks overlapped and became a broad peak located at around 150−320 °C. This oxygen desorption behavior of the SrCo0.85Fe0.15O3−δ sample implies that the phase transition from the perovskite-type structure to the brownmillerite structure occurs even at lower temperature like around 300 °C. The phase change of the SrCoxFe1−xO3−δ samples associated with the oxygen desorption in N2 was examined at various temperatures by means of the HT-XRD measurements. Figure 4 shows the HT-XRD patterns in N2 for the SrCo0.85Fe0.15O3−δ sample with comparison of that for SrFeO3−δ. In the case of the SrFeO3−δ sample given in Figure 4a, it is seen that the phase transition from perovskite-type structure to brownmillerite-type structure occurs at 600 °C. In contrast, in the case of the SrCo0.85Fe0.15O3−δ sample given in Figure 4b, the phase transition begins even at 350 °C. The phase-transition temperature of the SrCo0.85Fe0.15O3−δ sample is obviously much lower than that of the SrFeO3−δ sample. Additionally, the phase-transition temperature of each sample is roughly corresponding to the oxygen desorption temperature given in Figure 3. From these results, it was confirmed that the oxygen desorption of the SrCo0.85Fe0.15O3−δ sample occurred at a relatively lower temperature of around 300 °C, accompanying

Figure 4. High-temperature XRD patterns of (a) SrFeO3−δ and (b) SrCo0.85Fe0.15O3−δ, measured in N2 at given temperatures.

the phase transition from perovskite-type structure to brownmillerite-type structure. We further evaluated the isothermal oxygen sorption/ desorption behavior of the SrCo0.85Fe0.15O3−δ sample by means of TGA. Figure 5 shows changes in weight of the samples during oxygen sorption/desorption cycle in air/N2 in the temperature range 100−600 °C. In the TG curve at 100 °C, no change in weight was observed in the air/N2 cycle. This fact suggests that the oxygen sorption/desorption cannot occur below 100 °C. At 200 °C, the sample weight increased by

Figure 5. Time course of weight change for the SrCo0.85Fe0.15O3−δ powder sample at given temperatures in air/N2 stream, measured by TGA. C

DOI: 10.1021/acs.iecr.6b01284 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research changing the gas stream from N2 to air. This increment is corresponding to the oxygen sorption into the bulk of the sample. After changing the gas stream from air to N2, the weight decreased gradually with the elapse of time, due to the oxygen desorption. At 300 °C, the weight change was larger than that at 200 °C. Above 400 °C, however, the weight change associated with oxygen sorption/desorption decreased with increasing temperature. At 600 °C, the sample weight was hardly changed, because the sample did not sorb oxygen in air at all. These results suggest that the reversible oxygen sorption/ desorption in air/N2 can occur at temperatures 200−600 °C. On the basis of the results of the weight-change curves for the SrCo0.85Fe0.15O3−δ sample, we calculated the amount of reversibly sorbed oxygen in air/N2 at a steady state by using the following eq 19 amount of sorbed oxygen (cm 3 g −1) =

Figure 7. Oxygen concentration profile in the PSA column loaded with the SrCo0.85Fe0.15O3−δ pellet sample during the sorption and desorption processes at 300 °C.

Wchange Vm × 100 MO2

300 °C. The oxygen sorption and desorption processes were repeated alternately at an interval of 1 min for each process. In the oxygen desorption process using the vacuum pump, the oxygen concentration increased elapsed time and then reached to a maximum value of 59 vol %. This fact suggests that the SrCo0.85Fe0.15O3−δ pellet sample desorbs oxygen by the reduction of oxygen partial pressure around the pellet sample. In the following oxygen-sorption process, the SrCo0.85Fe0.15O3−δ pellet sample loaded in the column sorbed oxygen from synthetic air supplied. Therefore, the oxygen concentration was once reduced down to 18 vol %. Thereafter, the oxygen concentration increased to the same level as in synthetic air (∼21 vol %). These cycle performances in the sorption/ desorption processes could continue without any degradation during 100 cycles tested. In order to obtain some value of oxygen-enriched air by means of the present small-scale PSA apparatus loaded with the SrCo0.85Fe0.15O3−δ pellet sample, the desorbed gases in 30 desorption processes were captured into a collection bag. As a result, about 4000 cm3 of the oxygen-enriched air (45 vol % O2) was obtained at 300 °C. In addition, 29 vol % of the oxygen-enriched air could be obtained even at 200 °C. The amounts of sorbed oxygen obtained by using the small-scale PSA apparatus are 2.0 cm3 g−1 at 300 °C and 0.5 cm3 g−1 at 200 °C. According to the previous study on the oxygen-permeation membrane by using SrCo0.80Fe0.2O3−δ,28 the lowest temperature at which the oxygen permeation through the ceramic membrane took place was 450 °C. Therefore, the operation temperature of the present PSA process is lower than the lowest operation temperature reported for the permeation membrane process. Meanwhile, the operation of the conventional PSA process using microporous materials such as activated carbon (AC) and zeolite is usually operated at room temperature. For instance, the rapid pressure-swing adsorption process by using low-silica Lithium X (LiLSX) zeolite can produce ∼90 vol % O2 at 30−45 °C.29 However, the oxygen separation factor against N2 for the perovskite-type oxide is much higher than that for AC or zeolite, because the perovskite-type oxide can sorb oxygen into an oxide bulk due to the change in oxygen nonstoichiometry. Although the oxygen concentration obtained by the present experiment was relatively low with comparison to the conventional PSA processes, it might be improved by optimizing operational conditions and apparatus design.

(1)

where Vm is the molar volume of a gas at the standard condition (2.24 × 104 cm3 mol−1), MO2 is molecular weight of oxygen (32 g mol−1), and Wchange is the difference between sample weights in air and N2 (%). Figure 6 denotes the temperature

Figure 6. Temperature dependence of the amount of sorbed oxygen for the SrCo0.85Fe0.15O3−δ powder sample by pressure-swing between air and N2.

dependence of the amount of sorbed oxygen for the SrCo0.85Fe0.15O3−δ sample, estimated from the TGA results by using eq 1. The amount of sorbed oxygen for the sample exhibited the maximum value of 11.7 cm3 g−1 at 300 °C. According to our previous study,27 the amount of sorbed oxygen for La0.1Sr0.9Co0.9Fe0.1O3−δ, which is a benchmark sample for the oxygen sorbent, gives 8.6 cm3 g−1 at the same temperature. This result suggests that the SrCo0.85Fe0.15O3−δ sample has a potential to be used as an oxygen sorbent for the HT-PSA process even at relatively lower temperature than that reported before (500−600 °C).26 3.2. Oxygen Separation from Air by Using a SmallScale PSA Apparatus Loaded with SrCo0.85Fe0.15O3−δ. An actual oxygen separation test from air was conducted by means of the small-scale HT-PSA apparatus loaded with SrCo0.85Fe0.15O3−δ sorbent, as shown in Figure 1. Figure 7 depicts the time course of change in oxygen concentration of the exhausted gas from the column loaded with the SrCo0.85Fe0.15O3−δ pellet sample during the PSA operation at D

DOI: 10.1021/acs.iecr.6b01284 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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(14) Shin, M. J.; Yu, J. H. Oxygen transport of A-site deficient Sr1‑xFe0.5Co0.5O3‑δ (x = 0−0.3) membranes. J. Membr. Sci. 2012, 401, 40. (15) Chen, W.; Zuo, Y. B.; Chen, C. S.; Winnubst, A. J. A. Effect of Zr4+ doping on the oxygen stoichiometry and phase stability of SrCo0.8Fe0.2O3‑δ oxygen separation membrane. Solid State Ionics 2010, 181, 971. (16) Lin, Y. S.; McLean, D. L.; Zeng, Y. High temperature adsorption process. US Patent 6,059,858. 2000. (17) Yin, Q. H.; Kniep, J.; Lin, Y. S. Oxygen sorption and desorption properties of Sr-Co-Fe oxide. Chem. Eng. Sci. 2008, 63, 2211. (18) Guntuka, S.; Banerjee, S.; Farooq, S.; Srinivasan, M. P. A- and Bsite substituted lanthanum cobaltite perovskite as high temperature oxygen sorbent. 1. Thermogravimetric analysis of equilibrium and kinetics. Ind. Eng. Chem. Res. 2008, 47, 154. (19) Starkov, I. A.; Bychkov, S. F.; Matvienko, A. A.; Nemudry, A. P. Oxygen release from SrCo0.8Fe0.2O3‑δ. Inorg. Mater. 2013, 49, 916. (20) Liu, L. M.; Lee, T. H.; Qiu, L.; Yang, Y. L.; Jacobson, A. J. A thermogravimetric study of the phase diagram of strontium cobalt iron oxide, SrCo0.8Fe0.2O3‑δ. Mater. Res. Bull. 1996, 31, 29. (21) Vashuk, V. V.; Kokhanovskii, L. V.; Yushkevich, I. I. Electrical conductivity and oxygen nonstoichiometry of SrCo0.25Fe0.75O3‑δ. Inorg. Mater. 2000, 36, 1043. (22) Kokhanovskii, L. V.; Vashuk, V. V.; Ol’shevskaya, O. P.; Kirilenko, O. I. Oxygen stoichiometry and phase transitions of SrCo1‑xFexO3‑δ. Inorg. Mater. 2001, 37, 730. (23) Prado, F.; Grunbaum, N.; Caneiro, A.; Manthiram, A. Effect of La3+ doping on the perovskite-to-brownmillerite transformation in Sr1‑xLaxCo0.8Fe0.2O3‑δ (0 ≤ x ≤ 0.4). Solid State Ionics 2004, 167, 147. (24) McIntosh, S.; Vente, J. F.; Haije, W. G.; Blank, D. H. A.; Bouwmeester, H. J. M. Structure and oxygen stoichiometry of SrCo0.8Fe0.2O3‑δ and Ba0.5Sr0.5Co0.8Fe0.2O3‑δ. Solid State Ionics 2006, 177, 1737. (25) Huang, X. H.; Pei, L. Z.; Fan, C. G.; Zhang, Q. F. Effect of Fe content on the properties of perovskite-related oxides. Inorg. Mater. 2010, 46, 1225. (26) Fujimine, T.; Momma, K.; Izumi, J.; Miura, N. Evaluation of pilot plant for high temperature PSA-oxygen process using perovskitetype oxide adsorbent. Proceedings of 2015 AIChE Annual Meeting 2015, 255D. (27) Ikeda, H.; Nikata, S.; Hirakawa, E.; Tsuchida, A.; Miura, N. Oxygen sorption/desorption behavior and crystal structural change for SrFeO3‑δ. Chem. Eng. Sci. 2016, 147, 166. (28) Teraoka, Y.; Furukawa, S.; Zhang, H. M.; Yamazoe, N. Oxygen permeability of La1‑xSrxCo1‑yFeyO3 perovskite-type oxides. Nippon Kagaku Kaishi 1988, 1084. (29) Vemula, R. R.; Kothare, M. V.; Sircar, S. Performance of a medical oxygen concentrator using rapid pressure swing adsorption process: Effect of feed air pressure. AIChE J. 2016, 62, 1212.

4. CONCLUSIONS The SrCo0.85Fe0.15O3−δ sample gave a large amount of sorbed oxygen even at 300 °C, because the oxygen desorption occurred at this temperature, with accompanying the phase transition from perovskite-type structure to brownmillerite-type structure. Owing to the unique oxygen sorption/desorption properties of the SrCo0.85Fe0.15O3−δ sample, the oxygenenriched air was obtained even at 200 and 300 °C by using a small-scale PSA apparatus. The HT-PSA process using the SrCo0.85Fe0.15O3−δ sorbent seems to be a potential technique for production of oxygen-enriched air at around 300 °C, while phase stability of the sample in the presence of atmospheric water and carbon dioxide should be examined further.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Tokyo Gas Co., Ltd. REFERENCES

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DOI: 10.1021/acs.iecr.6b01284 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX