High Purity Oxygen Production via BBCN Perovskite Hollow Fiber

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High purity Oxygen production via BBCN perovskite hollow fiber membrane swept by steam Zhigang Wang, Yasotha Kathiraser, Ming Li Ang, and Sibudjing Kawi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01183 • Publication Date (Web): 26 May 2015 Downloaded from http://pubs.acs.org on June 10, 2015

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Industrial & Engineering Chemistry Research

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ACS Paragon Plus Environment


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High purity Oxygen production via BBCN perovskite hollow fiber membrane swept by steam Zhigang Wang, Yasotha Kathiraser, Ming Li Ang and Sibudjing Kawi* Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore, 117576

*To whom correspondence should be addressed

Telephone: (65)65166312; Fax: (65) 6779 1936 Email: [email protected]

(S. Kawi)

Manuscript submitted to Industrial & Engineering Chemistry Research on March 30, 2015.


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Abstract BaBi0.05Co0.8Nb0.15O3-δ (BBCN) ceramic hollow fiber membranes have been fabricated via a phase conversion and sintering technology. Oxygen permeation test was operated under steam as sweep gas and the highest oxygen permeation flux is 11.07 ml cm-2 min-1 with purity of around 99.8% at 950 oC. The stability of the membrane exposure to steam was also investigated by oxygen permeation studies and further characterization by SEM, EDX, ICP, XRD, FTIR and XPS techniques. BBCN hollow fiber membrane still maintained oxygen permeability upon exposure to steam for 325 hours. The results show that despite an initial drop of around 30% in the oxygen permeation for the first 20-30 hours, it stabilized in the remaining time. Interestingly, the oxygen permeation can be in-situ recovered upon using inert sweep gas for 18 hours at high temperature. This study also reveals that the BBCN membrane exhibits good potential for integration with catalyst in a membrane reactor for oxidative steam reforming of hydrocarbon reactions.

Key words: High purity oxygen production, BaBi0.05Co0.8Nb0.15O3-δ (BBCN), ceramic hollow fiber membranes, perovskites, stability in steam, oxygen permeation.



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1. Introduction Oxygen becomes increasingly important due to its wide application in environment protection and chemical industries. Currently, oxygen is mainly generated via cryogenic distillation, pressure swing adsorption （PSA）or polymeric membrane separation. However, cryogenic distillation is a costly and energy intensive process, whilst PSA and polymeric membrane separation are limited by purity, which can only reach a maximum 95-97%1 and 50%2, respectively. Recently, dense ceramic membrane made from mixed ionic and electronic conductors (MIEC) have attracted intense research interest in the past decade owing to their potential applications in air separation which can reduce the high purity oxygen production cost by 30% compared to the conventional method3-13. Especially, ceramic hollow fiber membrane with highly asymmetric morphology shows a great potential for this usage14-24. In detail, such hollow fiber membranes can provide much larger areas per unit volume and are also more easily assembled into a membrane module for scale up compared to disk or tubular membranes14, 25. The oxygen permeation flux through hollow fiber membranes also can be significantly increased by 3~4 times compared to disk or tubular membranes due to much thinner membrane thickness and higher surface area3, 26-29

. The oxygen transport through this type membrane is via oxygen ions instead

of oxygen molecules. This theoretically allows the product to be 100% pure oxygen, as only oxygen can pass through the membrane. However, this mode of oxygen permeation requires oxygen partial gradient as driving force. In general, the use of an


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inert sweep gas to collect the permeated oxygen is a common practice

30-32

and this

leads to a requirement of further downstream separation of the products. Also, the evacuation by high vacuum pumps is another way to supply the oxygen partial pressure gradient25, but it is limited by complicated fabrication procedures of one end closed hollow fiber membrane, the need for high-temperature sealant and also its energy intensive process. Thus, use of steam as sweep gas was proposed to eliminate the need for further separations, as the steam-oxygen mixture collected can be simply cooled to condense the steam to form pure oxygen. Additionally, stable performance in steam atmosphere provides the potential to design a membrane reactor for oxidative steam reforming of hydrocarbon reactions. So far, only a few groups have reported this work33-35. For instance, Costa’s group researched on the oxygen production from BSCF hollow fiber membrane using steam as a sweep gas. Their investigation showed good performance for oxygen permeation with 9.52 ml cm-2 min-1. However, in the long-time stability testing, the permeation rate decreased greatly. The reason attributed was he effect of carbonic acid found in the distilled water used to generate steam. In our work, the effect of carbonic acid in the water will be eliminated. Recently,

our

group

have

reported

ultra-high

oxygen

permeable

BaBi0.05Co0.8Nb0.15O3-δ BBCN hollow fiber membrane with oxygen permeation flux of 14.08 ml cm-2 min-1 at 950 oC swept by He

36

. So far, this value is the highest

permeation obtained under He sweep gas through hollow fiber membranes in comparison with all of the previously reported data under normal atmosphere oxygen



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partial pressure (~21kpa) without any modification. In order to realize the industrial use of this membrane for high purity oxygen production, the inert sweep gas should be replaced by steam. In this work, the oxygen permeation and stability have been tested under steam as sweep gas. Any changes in the membrane structure after the long time permeation study in steam atmosphere were investigated via characterization techniques such as SEM, EDX, ICP, XRD, FTIR and XPS. 2. Experimental 2.1 Oxygen permeation measurement BBCN hollow fiber membranes were fabricated via the phase conversion and sintering technique. Details of preparation and characterization of this hollow fiber membrane can be found in the literature36. The oxygen permeation of BBCN hollow fiber membranes swept by steam was investigated in a permeation apparatus. The detail is shown in Fig. 1. Air was fed on the shell side with a fixed flow rate of 300 ml min-1. Gas feed flow rates were controlled by mass flow controllers (5850E, Brooks), which were calibrated with a bubble flow meter. The mass flow rate of steam was controlled by HPLC pump. The low concentration (200ppm) concentration NaOH aqueous solution was pumped in the heating tube which was heated to 150 oC to form steam. In the permeation experiment, 0.045g/min and 0.09g/min steam with 1.5 ml/min Neon gas as internal standardization gas flow into the lumen side of hollow fiber membranes as sweep gas. The testing temperature range is from 650 oC to 950 oC. The downstream gas first passed through the condenser which was cooled down to 5 oC to condense steam, and


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the outlet gas stream then flowed into a Gas Chromatography (GC 6890N, Agilent) equipped with a Hayesep D100/120 column and analyzed by a TCD detector. The gas flow rate is also measured by bubble flow meter. During the permeation, a small leakage existed since 0.09-0.21% nitrogen in the permeate gas stream on the sweep side was detected depending on the sweep flow rate and temperature. Although the leaking oxygen is much less than the permeated oxygen, it has to be deducted for the calculation of the oxygen permeation flux: 21   J O 2 = V  xO2 - x N 2  /A 78  

(1)

Where JO2 is oxygen permeation flux (ml cm-2 min-1), V is the flow rate of permeate gas stream (ml min-1), xO2 and xN2 are the percentages of oxygen, nitrogen in the effluent, and A=[2π(Ro-Rin)L]/ln(Ro/Rin), in which Ro, Rin, and L are the outer diameter (OD), inner diameter (ID), and effective length for oxygen permeation of the hollow fiber membrane, respectively.

2.2 Characterizations The surface morphology of hollow fiber membranes was ascertained by Scanning Electron Microscope (SEM, Jeol, JSM-6701F). The samples were degassed under vacuum condition to remove impurities. Platinum coating (about 10 nm thickness) was carried out at 20 mA for 50 sec. The purity of steam are dected by ICP 7700 series, and the sample is 10ml liquid condensed by steam. The crystal phase structure of blank membrane and used hollow fiber membranes after permeation test were determined by X-ray diffraction (XRD, Shimadzu XRD-6000 power diffract



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meter) with Cu Kα radiation. Continuous scan mode was used to collect 2θ data from 20o to 80o with a 0.02o sampling pitch and a 2o min-1 scan rate at room temperature. X-ray photoelectron spectroscopy (XPS) was used to obtain information on the binding energies of the Ba 4d, Bi 4f, Co 3p Nb 3d and C 1s in the blank BBCN membrane for comparison with the membranes after permeation under steam as sweep gas. The spectra was obtained via a Kratos AXIS spectrometer with a spatial resolution of 30µm with an Al K α (hυ= 1486.6eV; 1 eV = 1.6302 x 10-19 J) X- ray source. The samples were referenced to the standard calibrated value of the adventitious carbon, C1 s hydrocarbon peak at 284.5 eV prior to fitting the spectrum of samples. Fourier transform infrared spectroscopy (FTIR) analysis was conducted via a Bruker Vertex 70 SN_1253 for the blank and post-reacted (after long-term stability) BBCN powders with a MCT detector, with a scanning range from 4000 to 500 cm-1.

3. Results and discussion 3.1 Elimination of the effect of carbonic acid in water It is known from the literature that the carbonic acid in water might affect the stability of oxygen permeation under steam as sweep gas34. Hence, removing carbonic acid becomes primary point in this study. In our experiment, distilled water was replaced by 200 ppm sodium hydroxide solution to generate steam. Carbon dioxide and carbonic acid in the water can react with sodium hydroxide to form Na2CO3. As sodium carbonate is very stable below 150 oC, this can guarantee the elimination of CO2 in steam. To confirm that the low concentration sodium hydroxide solution does


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not affect the purity of steam, the Inductively Coupled Plasma (ICP) was used to detect sodium concentration in the liquid condensed by the steam. The result shows that an extremely low Na+ concentration of 2ppm is detected in this liquid. Hence, the steam generated from 200 ppm NaOH solution is sufficiently pure without CO2 and other impurities.

3.2 Oxygen permeability and stability of BBCN hollow fiber membranes swept by steam. The BBCN hollow fiber membranes were tested for gas tightness at room temperature by the GC. The oxygen permeation data is obtained by testing the dense ceramic hollow fiber membranes in the steam/air atmosphere at different temperature as shown in Fig. 4. Two mass flow rates of 0.045 g min-1 and 0.09 g min-1 for steam as sweep gas were investigated, with the temperature varied from 650 oC to 950 oC. It can be seen from Fig. 2, the oxygen permeation flux increases with either increasing the operation temperature or increasing the steam flow rate, which is corresponding with the results obtained from BBCN hollow fiber membrane published in the literature using He as sweep gas36. The highest oxygen permeation flux is 11.07 ml cm-2 min-1 at 950 oC with a steam flow rate of 0.09 g min-1. Such a high oxygen permeation value further increases the possibility for industrial application. Furthermore, with a steam flow rate of 0.045 g min-1, the oxygen permeation flux still can reach 7.04 ml cm-2 min-1. Doubling the steam mass flow rate increases the oxygen permeation flux by around 57% at 950 oC, which indicates the importance of partial oxygen pressure gradient for oxygen permeation at high temperature. Meanwhile, the



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difference of oxygen permeation flux for both mass flow rates becomes gradually smaller with decreasing temperature. In addition, the oxygen permeation flux through BBCN hollow fiber membrane swept by steam is almost similar as previous permeation results using inert sweep gas obtained at the same conditions. For example, at 950 oC, the oxygen permeation flux is around 10 ml cm-2 min-1 and 7 ml cm-2 min-1 when swept by 122.5ml/min and 61.3 ml/min of He (equivalent volume flow rate of 0.09 g/min and 0.045 g/min steam at room temperature), respectively36. The stability of oxygen permeation swept by steam is shown in Fig. 3. The permeation experiment was performed at 900°C for 325 hours with steam flow rate of 0.045 g/min while the recovery experiment was conducted in-situ during the stability experiment. In detail, the sweep gas was switched from steam to neon at 975°C for 18 hours. The sweep gas was then switched back to steam with the same flow rate of 0.045 g/min and upon cooling down to 900°C, the oxygen permeation was tested. It can be seen from Fig. 3 that the oxygen permeation flux dropped around 30 % in the initial 30 hours. The oxygen permeation flux then stabilized in the following 150 hours. Subsequently, after 18 hours of regeneration, the oxygen permeation almost recovered to the original value. Similar phenomenon was observed upon resuming the oxygen permeation test; the oxygen permeation flux showed a decrease for a short time initially, before stabilizing for the remaining time of 110 hours. The results show that BBCN hollow fiber membrane after long-time exposure to steam still maintains a stable oxygen permeability and as well as the ability to regenerate. The reasons behind this observed phenomenon will be explained in the next section based on


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analysis by various characterizations.

3.3 Morphology of membrane after long time permeation swept by steam. The morphology of hollow fiber membranes after long time permeation swept by steam was detected by SEM, and the change of elemental composition of membrane was scanned by EDX. It can be seen from Fig. 4, that one sub-layer formed on the inner surface (sweep side) after long time permeation exposure in steam. The thickness of this sub-layer is around 1 µm as can be measured from Fig. 4 a. It can also be seen from Fig. 4 b, that the sub-layer seems to be loose and not a dense layer. The compositions of sub-layer and cross bulk area are detected by EDX and the results are shown in Table 1. It can be seen from Table 1, the composition of cross bulk area mostly maintained the composition of BaBi0.05Co0.8Nb0.15O3-δ (BBCN). However, the composition of elements for sub-layer has varied and a lot of carbon is observed. In addition, according to the proportion of barium, carbon and oxygen, the compound was proposed to be barium carbonate. To confirm this inference, XRD data was analyzed as below.

3.4 Phase structure and FTIR characterization. Both blank BBCN hollow fiber membrane and the membrane after long time permeation swept by steam are crushed and ground into fine powders, which are then scanned by XRD as shown in Fig. 5 a and b. As can be seen from the Fig. 5 a and b, after long time exposure in steam, the structural phase of the membrane post-permeation still maintains the cubic perovskite phase, i.e. no impurity phase can be found in the XRD patterns. Only the peak intensity became weaker compared with



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blank membrane, which indicated the crystal size becomes smaller after long time permeation swept by steam. The question which arises is, why is that the impurity sub-layer observed via SEM image cannot be detected by XRD. The reason is that the 1µm impurity sub-layer is too little compared with the whole bulk membrane when the membrane was ground into fine powders. Hence, the small amount of impurity cannot be detected by XRD. In other words, most of membrane still maintains the cubic perovskite structure. The single sub-layer is difficultly peeled off from the inner surface of membrane by manual method. The membrane after long time permeation were also detected by FTIR mode and compared with blank BBCN perovskite powders. The results are shown in Fig. 6. It can be seen from Fig. 6 that a new absorption peak can be found in the FTIR spectra of the powders after long time exposure in steam. The new peak at around 1450 cm-1 can be interpreted as C =O vibration of carbonate

37-39

, which is inferred as barium

carbonate. Fig. 7 shows the XPS spectra of the full survey, Ba 4d, Bi 4f, Co 3p Nb 3d and C 1s for inner surface of (a) blanks membrane and (b) membrane after long time permeation swept by steam. The quantitative analysis of the binding energy and areas are summarized in Table 2. It can be seen from Fig. 7 that the intensity of XPS peaks for Bi 4f, Co 3p and Nb 3d of membranes after long time permeation swept by steam became much weaker compared with those of blank membranes. Some peaks even cannot be found, such as Bi 4f. Such weak peaks indicated that bismuth, cobalt and niobium almost disappeared from the inner surface of membrane after long time


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permeation swept by steam. The peaks of Ba 4d can be easily found after long time permeation exposed in steam. However, it can be seen from Table 2 that the positions of Ba 4d peaks were shifted from 87.98 and 90.38 eV to 89.21 and 91.75 eV, respectively. The peaks located at 87.98 and 90.38 eV can be attributed to BaO species 40, while the peaks located at 89.21 and 91.75 eV can be attributed to BaCO3 species40. Also, the peaks of C1s are located at 284.57 and 288.57 eV for blank membrane and located at 284.66 and 289.06 eV for the membrane after long time permeation swept by steam. The position of C1s peaks have not changed significantly for both membranes. The peaks located at around 284.5 eV are attributed to C-C or CH bond 41, which could be from the pollution of sample surface and XPS ultrahigh vacuum chamber. The peaks located at around 288.7 eV are attributed to C-O and COO bonds41. It can be seen from Table 2 that the proportion of peak area for high binding energy around 288.7 eV which could be assigned to CO32- , remarkably increased for the membrane after long time test compared to the blank membrane. Hence, the change of both Ba 4d and C 1s spectra indicated that barium carbonate formed on the inner surface. As shown earlier, the carbon dioxide and carbonic acid have been eliminated from the steam. However, the main cause for formation of barium carbonate might be due to the fact that upon exposure to steam, the perovskites structure underwent a slow partial decomposition. In other words, after permeation for prolonged hours under steam, the inner surface of the BBCN hollow fiber might be decomposed and this process only occurred on the inner surface of the membrane, whereby the corrosion is



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limited to only 1 µm. As we know, the decomposed barium oxide easily adsorbs moisture from the atmosphere forming barium hydroxide which reacts readily with CO2 in the air forming barium carbonate. The initial degeneration of oxygen permeation flux during the prolonged permeation with steam as sweep gas is due to the decomposition of the perovskites structure on the inner surface. It can be seen from the stability test that the initial degeneration is very fast before it begin to reach steady state, showing that further decomposition did not take place. This could be attributed to the fact that the initial decomposition of perovskites material on the inner surface immediately affected the oxygen permeation at the initial stage, but the decomposition only took place on the surface and most of the perovskite structure of the membrane can be maintained as observed from XRD image shown in Figure 5b. Therefore, even though the initial stages of the permeation was affected, but it managed to stabilize after a while. Furthermore, the oxygen permeability can be recovered by sweeping with inert gas at high temperature. This could be due to the possibility that the decomposed perovskites phase was able to recover its perovskites structure under treatment with the inert gas atmosphere at high temperature. This easy regenerability of the membrane supports its application for industrial use and gives good potential for further applications.

4. Conclusions High purity oxygen (99.8%) was produced via BBCN ceramic hollow fiber membrane using steam as sweep gas. The highest oxygen permeation flux that can be achieved is 11.07 ml cm-2 min-1 at 950 oC. Throughout the 325-hour stability test in steam


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atmosphere, the oxygen permeation flux decreased by around 30% in the initial 20-30 hours, but maintained its stability for the remaining time. Importantly, the oxygen permeation flux can be almost completely regenerated via Ne sweep on the membrane side within 18 hours at high temperature. The initial degeneration of oxygen permeability for membrane swept by steam is attributed to the partial decomposition of perovskite structure on the inner surface. However, the decomposition is limited to only 1 µm and can be recovered at high temperature. Additionally, the comparable stable performance of this BBCN membrane in steam atmosphere presents tremendous potential for it to be coupled with catalyst in a membrane reactor for oxidative steam reforming of hydrocarbon reactions.

Acknowledgement The authors gratefully thank the National University of Singapore, National Environmental

Agency

(NEA-ETRP

Grant

No.1002114

and

RP

No.

279-000-333-490)) for generously supporting this work. Zhigang Wang sincerely thanks E.T. Saw and U. Oemar for technical support and discussion.

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for oxygen separation using different bore fluids. J. Membr. Sci. 2011, 378, (1–2), 308-318. (24) Yang, N.-T.; Kathiraser, Y.; Kawi, S. A new asymmetric SrCo0.8Fe0.1Ga0.1O3−δ perovskite hollow fiber membrane for stable oxygen permeability under reducing condition. J. Membr. Sci. 2013, 428, (0), 78-85. (25) Tan, X.; Wang, Z.; Meng, B.; Meng, X.; Li, K. Pilot-scale production of oxygen from air using perovskite hollow fibre membranes. J. Membr. Sci. 2010, 352, (1–2), 189-196. (26) Liu, S.; Gavalas, G. R. Oxygen selective ceramic hollow fiber membranes. J. Membr. Sci. 2005, 246, (1), 103-108. (27) Liao, Q.; Zheng, Q.; Xue, J.; Wei, Y.; Wang, H. U-Shaped BaCo0.7Fe0.2Ta0.1O3−δ Hollow-Fiber Membranes with High Permeation for Oxygen Separation. Ind. Eng. Chem. Res. 2012, 51, (46), 15217-15223. (28) Liu,

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Figures and Tables Fig. 1 Schematic graph of oxygen permeation apparatus for hollow fiber membrane swept by steam Fig. 2 The oxygen permeation flux through BBCN ceramic hollow fiber membranes swept by steam with increasing temperature from 650 oC to 950 oC Fig. 3 Oxygen permeation stability through BBCN hollow fiber membranes swept by steam with a mass flow rate of 0.045 g min-1 at 900 oC for 325 hours (▲); in-situ recovering at 975 oC for18 hours during stability test. Fig. 4 SEM and EDX of the BBCN hollow fiber membrane after long time swept by steam; a: cross section of membrane (bulk area), b: inner surface of membrane Fig. 5 X-ray diffraction patterns of BBCN hollow fiber membrane; a: blank membrane, b: membrane after long time permeation swept by steam Fig. 6 FTIR spectra of BBCN powders; a: blank BBCN perovskite powders and b: the powders after long time exposed in steam at 900 oC. Fig. 7 XPS for BBCN hollow fiber membrane inner surface, a: blank membrane, b: membrane after long time permeation swept by steam Table 1 Compositions of elements in the membrane Table 2 XPS binding energy summary of C 1s and Ba 4d elements for inner surface of blank and long-time permeation BBCN hollow fiber membranes


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Fig. 1

Fig. 2



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Fig .3

Fig. 4


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Fig. 5

Fig. 6



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Fig. 7

Table 1. Compositions of elements in the membrane Scanning Area

Element Atomic% Ba

Bi

Co

Nb

O

C

Cross bulk of membrane (a)

16.54%

0.83%

12.05%

3.95%

66.63%

0%

Inner surface of membrane (b)

20.53%

0%

3.91%

0%

54.14%

21.42%


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Table 2. XPS binding energy summary of C 1s and Ba 4d elements for inner surface of blank and long-time permeation BBCN hollow fiber membranes. Spectral Region

C 1s

Ba 4d

Blank membrane

Membrane after permeation

BE (eV)

Peak area (%)

BE (eV)

Peak area (%)

284.57

82.69

284.66

56.99

288.57

17.31

289.06

43.01

87.98

89.21

90.38

91.75


High Purity Oxygen Production via BBCN Perovskite Hollow Fiber

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