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La0.6Sr0.4Co0.8Ni0.2O3−δ hollow fiber membrane reactor: Integrated oxygen separation – CO2 reforming of methane reaction for hydrogen production
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La0.6Sr0.4Co0.8Ni0.2O3Ld hollow fiber membrane reactor: Integrated oxygen separation e CO2 reforming of methane reaction for hydrogen production Nai-Tao Yang, Yasotha Kathiraser, Sibudjing Kawi* Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576, Singapore
article info
abstract
Article history:
An integrated reactor system which combines oxygen permeable La0.6Sr0.4Co0.8Ni0.2O3d
Received 6 November 2012
(LSCN) perovskite ceramic hollow fiber membrane with Ni based catalyst has been suc-
Received in revised form
cessfully developed to produce hydrogen through oxy-CO2 reforming of methane (OCRM).
10 January 2013
Dense La0.6Sr0.4Co0.8Ni0.2O3d hollow fiber membrane was prepared using phase inversion-
Accepted 11 January 2013
sintering method. OCRM reaction was tested from 650 C to 800 C with a quartz reactor packed with 0.5 g Ni/Al2O3 catalyst around the LSCN hollow fiber membrane. CH4 and CO2
Available online xxx
were used as reactants and air as the oxygen source was fed through the bore side of the hollow fiber membrane. In order to gauge the effectiveness of this membrane reactor
Keywords: reforming
Oxy-CO2
of
methane
system, air flow was closed at 800 C and dry reforming of methane (DRM) was tested for comparison. The results show that the oxygen fluxes of LSCN membrane swept by helium
(OCRM) Integrated
catalytic
fiber
are nearly 3 times less than those swept by OCRM reactants. With increasing temperature
perovskite
conversion decreases from 87% to 72% due to the competition reaction with POM. CO
hollow
and oxygen supply, methane conversion in the OCRM reactor reaches 100%, but CO2
membrane reactor La0.6Sr0.4Co0.8Ni0.2O3d membrane
selectivity is as high as nearly 100% at reaction temperatures of 700 Ce800 C while H2
Hydrogen production
selectivity reaches a maximum of 88% at 700 C. At 800 C, when air supply was closed and DRM was conducted for comparison, CO selectivity decreased to 91%, resulting in carbon deposition which was around 4 times more than those obtained under OCRM reaction and H2/CO ratio decreased from 0.93 to 0.74, showing better carbon resistance and higher H2 selectivity of the Ni-based catalyst over the integrated oxygen separation-OCRM reaction across the LSCN hollow fiber membrane reactor. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years, global climate change has been increasingly attributed to greenhouse gas emission from anthropogenic sources. Among the greenhouse gases, carbon dioxide (CO2) and methane (CH4) have the greatest impact [1,2]. Methane is attractive as a promising fuel due to its higher energy density compared to petroleum, and because it emits less carbon
oxides upon combustion and can be converted into hydrogen through reforming reactions by steam, CO2 or oxygen [3e5]. Therefore, dry CO2 reforming of methane (DRM) [6,7] seems to offer an opportunity to transform what would otherwise be two greenhouse gases into a source of green hydrogen. This reaction is particularly interesting when it is carried out with natural gas from fields containing large amounts of CO2 accompanied with methane, without the pre-separation of
* Corresponding author. Tel.: þ65 65162065. E-mail address: [email protected] (S. Kawi). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.01.073
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CO2 from the feed. However, DRM reaction is an endothermic process, hence additional energy is required for the system, leading to increasing cost of operation. Since partial oxidation of methane (POM) is an exothermic reaction [8], therefore a combination reaction of POM with DRM, hereafter called as oxidative-CO2 reforming of methane (OCRM), could reduce the energy requirement and even reach to an autothermal reforming mode by adjusting the ratio of CH4, CO2 and oxygen. Furthermore, introduction of oxygen to the reactor can reduce carbon deposition on the catalyst bed, which is a severe problem during DRM reaction [9,10]. However, one of the major issue facing the OCRM reaction is the high economic, environmental, and safety costs associated with pure oxygen production, storage and transportation [11]. For these purposes, oxygen-permeable ceramic membrane is a promising technique to be applied as the in situ oxygen separator and supplier directly using air. Mixedoxygen ion-electron conducting membranes are oxygen permeable and can produce oxygen from air at elevated temperatures [12e15]. Coincidently, oxygen separation and reforming reaction are both operated at similar high temperature conditions [16]. Therefore oxygen permeable ceramic membranes show a high potential in the application of integrated oxygen separation-reaction process in a single equipment [17e22]. Meanwhile, oxygen species such as O2 or O will react with other reactants mildly on the membrane surface, avoiding the occurrence of combustion reaction and therefore preventing the formation of hot spots on the catalyst bed [23,24]. The dense ceramic membrane allows the conduction of oxygen ions through the lattice of the solid material resulting in 100% selectivity for oxygen regardless of the source of gases [25]. Slade et al. [26] and Wei et al. [27] have previously studied dual CO2 and O2 reforming of methane from the oxygen permeable membrane. However, disk membranes were used in their studies. Compared to the disk membrane, hollow fiber membrane has higher surface area and thinner wall and is easier to be scaled up by assembling in the form of bundle [28,29]. La0.6Sr0.4Co0.8Ni0.2O3d (LSCN) perovskite material has good oxygen reducing properties and has been widely used in intermediate temperature solid oxide fuel cells [30e33]. In this study, LSCN perovskite has been successfully prepared in the form of hollow fiber and tested for its oxygen permeation properties. A catalytic LSCN hollow fiber membrane reactor subsequently integrates oxygen separation coupled with OCRM reaction over Ni/Al2O3 catalyst. By setting up an oxygen-permeable hollow fiber membrane into the OCRM reactor, only pure oxygen is transferred in situ from air side and fed into the Ni/Al2O3 catalyst bed directly. Hence, this integrated separation e reaction membrane reactor system process is able to solve the economic, environmental and safety problems associated with pure oxygen production, transportation, storage and applications.
2.
Experimental
2.1.
Preparation of material and hollow fiber membrane
The oxygen-permeable hollow fiber membrane was La0.6Sr0.4Co0.8Ni0.2O3d (LSCN), which was prepared using
a solegel method as reported in literature [34]. Stoichiometric ratio of La(NO3)3$6H2O, Sr(NO3)2, Co(NO3)2$6H2O, Ni(NO3)2$6H2O were used as the precursors, with citric acid and ethylene glycol as the complexing agents. All of the above materials were sourced from SigmaeAldrich Reagent Co. The sol was dried at 100 C and calcined at 900 C for 5 h, ballmilled for 2 h, and then sieved with a 200-mesh sifter. The hollow fiber membrane was fabricated using a phase inversion-sintering method [34]. LSCN powder, N-methylpyrrolidone (NMP) and polyethersulfone (PES) were mixed in a ratio of 10:5:1 (mass ratio) and stirred together for 72 h to prepare a casting solution. The orifice of the spinneret used for spinning has outer/inner diameters of 3 mm/2 mm. Deionized water was used as internal and external coagulants. The hollow fiber membrane was then calcined at 1300 C for 5 h.
2.2. Characterizations of material and hollow fiber membrane The morphologies of the LSCN powder and LSCN hollow fiber membrane were observed through a field emission scanning electronic microscope (FESEM, Jeol, JSM-6700F). The samples were firstly outgassed under vacuum condition to remove impurities. Platinum coating (about 10 nm thickness) was carried out at 20 mA for 40 s. The crystalline structures of the calcined LSCN powder and hollow fiber membranes (fresh and after OCRM reaction) were determined using a Shimadzu XRD-6000 power diffractometer, with Cu target K-a radiation (l ¼ 0.15406 nm) at 40 kV and 30 mA. The scattering intensities were over an angular range of 20 <2q < 80 with a scanning speed of 2.00 /min.
2.3.
Preparation of catalyst
Ni/Al2O3 (Ni content 10 wt%), which was used as the OCRM catalyst, was prepared using Ni(NO3)2$6H2O, Al(NO3)3$9H2O and citric acid as precursors (SigmaeAldrich Reagent Co.). These precursors were dissolved in deionized water to form a transparent solution, which was then dried at 90 C, calcined at 800 C for 5 h, and subsequently reduced at 800 C for 1 h under H2/He mixture (10% of H2). The Ni/Al2O3 catalyst was kept in a sealed bottle for further use.
2.4.
Oxygen permeability test of LSCN hollow fiber
The oxygen permeability of LSCN hollow fiber was tested with high purified helium (>99.995%) as sweep gas and air as oxygen source from 650 C to 900 C. Helium flows from the shell side between the membrane and a quartz tube (inner/outer diameter of 8/10 mm) with the flow rates from 10 to 100 mL min1. Air flows from the bore side of the hollow fiber membrane with a constant flow rate of 100 mL/min. The shellside gas mixture was injected to a gas chromatograph (GC6890N, Agilent) and analyzed by a TCD detector using a Hayesep D 100/120 column. The oxygen permeation in an OCRM reactor was calculated by equation (1). 1 1 fO2 ¼ fCO þ fH2 O fCO2;in fCO2;out 2 2
(1)
where fO2 is the flow rate of permeated oxygen, and fCO, fH2 O , fCO2;in and fCO2;out are flow rates of CO, water steam, inlet and
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outlet of CO2 during the reaction. Since steam cannot be detected directly from GC since it was collected in a condenser outside the reactor, water produced was calculated by hydrogen balance as equation (2). fH2 O
¼ 2 fCH4;in fCH4;out fH2
(2)
where fCH4;in , fCH4;out and fH2 are the flow rates of CH4 inlet, outlet and flow rate of H2 as analyzed by GC.
2.5.
(6)
where XCH4 , XCO2 , SH2 and SCO are CH4 conversion, CO2 conversion, H2 selectivity and CO selectivity respectively. fCH4;in and fCO2;in are inlet flow rates of CH4 and CO2. fCH4;out , fCO2;out , fH2 and fCO are outlet flow rates of CH4, CO2, H2 and CO from the reactor.
2.6. Characterization of post-reacted LSCN hollow fiber membrane and catalyst
Test of OCRM membrane reactor
Fig. 1 shows the schematic hollow fiber membrane reactor fabricated for the integrated oxygen separation coupled with OCRM reaction. The LSCN hollow fiber was connected by two soft buffer tubes (silicone rubber) to relieve thermal expansion stress. 0.5 g of reduced Ni/Al2O3 catalyst was packed along with the hollow fiber membrane in the shell of a quartz tube (8 mm inner diameter and 10 mm outer diameter). Quartz wool was used to avoid catalyst being blown away. Each side of the quartz tube was fixed with a stainless steel tee joint to avoid gas leakage. The hollow fiber membrane packed with catalyst was put into a tubular furnace and tested at temperatures ranging from 650 C to 800 C. Methane and CO2 were fed in the catalyst bed at 5.00 mL min1. Pure helium (He, >99.995%) with a flow rate of 47.00 mL min1 was used as diluting gas and high purity of neon (Ne, >99.999%) with a flow rate of 3.00 mL min1 was used as an external standard for GC analysis. Purified air was fed in the bore side of LSCN hollow fiber with a constant flow rate of 100 mL/min. The output gases from the shell side (catalyst bed) were firstly condensed to eliminate water prior to GC analysis. The conversion of CO2, CH4 and selectivity of CO, H2 were analyzed based on GC peaks as follows: XCH4 ¼
fCH4;in fCH4;out 100% fCH4;in
(3)
XCO2 ¼
fCO2;in fCO2;out 100% fCO2;in
(4)
fH2 100% SH2 ¼ 2 fCH4;in fCH4;out
f CO 100% SCO ¼ fCO2;in fCO2;out þ fCH4;in fCH4;out
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(5)
The morphology of the reacted LSCN hollow fiber was observed through a field emission scanning electronic microscope (FESEM, Jeol, JSM-6700F). Carbon deposition of the catalyst after OCRM or DRM reaction was tested with Thermal Gravimetric Analysis (TGA, Shimadzu DTG 50). The used catalyst was transferred from the membrane reactor into glass bottles, and immediately covered to prevent exposure with air. About 15 mg catalyst sample was put into the alumina pan and calcined from room temperature to 1000 C with a ramp rate of 10 C/min in an ambient air environment. The crystalline structures of the calcined LSCN powder and hollow fiber were determined using a Shimadzu XRD-6000 power diffractometer, with Cu target K-a radiation (l ¼ 0.15406 nm) at 40 kV and 30 mA. The scattering intensities were over an angular range of 20 <2q < 80 with a scanning speed of 2.00 /min.
3.
Results and discussion
3.1. Structure and oxygen permeability of LSCN hollow fiber membrane Fig. 2 shows the FESEM image of the morphology of the LSCN powder. The particles of the LSCN powder appear to have irregular morphologies, with particle size averaging around 80 nm. Fig. 3 (a) and (b) show the cross section of the LSCN hollow fiber, with the outer and inner diameters of 2.3 mm and 1.7 mm, respectively and a wall thickness of around 250 mm. It has a dense external layer of circa 140 mm in thickness for oxygen separation, and an internal porous layer which is
Fig. 1 e Schematic of the OCRM hollow fiber membrane reactor.
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suitable for gas diffusion. Fig. 3 (c) shows the snapshot of the straight and long LSCN hollow fibers used in this study. Various permeation tests using He, CH4 and CO2 as the sweep gas have been conducted. Fig. 4 shows the oxygen permeation results when ambient air was supplied through the bore side of hollow fiber membrane with a constant flow rate of 100 mL min1, and swept with helium outside of the membrane. Significant oxygen permeation starts to occur only at higher temperatures, circa 800 C because the perovskite membrane becomes activated at higher temperatures [35]. At 800 C, the maximum oxygen flux is only 0.29 mL min1 cm2. With increasing He flow rate, the flux keeps on a nearly constant value. Increasing of sweep gas will increase the driving force for oxygen permeation by lowering the oxygen partial pressure on the permeate side. At the lower temperature region, below 800 C, with an increase in sweep gas flow rate, improvement in the oxygen flux is not observed which shows a surface exchange limiting process. Therefore, since the oxygen lattice is not sufficiently activated thermally, an increase in sweep gas does not cause much effect on the oxygen permeation flux. However, at higher testing temperature such as 850 C and 900 C, the maximum oxygen permeation fluxes increase from 0.39 to 0.69 and 0.59e1.2 mL min1 cm2, respectively, with the He sweep gas flow rate from 10 to 100 mL min1. These phenomena indicate the complex processes for oxygen transport through the mixed membranes. The oxygen flux appears not to have reached the saturated value even under a 100 mL min1 of He flow rate. This behavior follows that of other perovskites whereby oxygen permeation is limited by bulk diffusion at higher temperatures, hence at the elevated temperature regime, effect of sweep gas flow rate plays an important role in increasing the flux. This is not surprising as the transition temperature from the surface-exchange limiting process to the bulk-diffusion limiting process has been reported to be around 900 C for the asymmetric perovskite-type hollow fiber membranes [36]. Fig. 5 shows the oxygen permeation results using methane as the sweep gas following the similar conditions as described above using He as sweep gas. It can be clearly observed that an
Fig. 2 e FESEM image showing the morphology of LSCN powder.
Fig. 3 e Morphology of LSCN hollow fiber membrane: (a) Cross section; (b) Dense layer, and (c) Snapshot of LSCN hollow fibers.
increase in oxygen permeation flux has occurred, whereby the maximum flux already reaches 0.25 mL min1 cm2 at 700 C. At 900 C, under a methane sweep of 100 mL min1, the maximum O2 flux reaches 3 mL min1 cm2, which is nearly 3 times more than those obtained with He as sweep gas. It has been reported by Liu et al. [37] that, after activation by CH4 at 1000 C for 1 h, their oxygen fluxes increased by a factor of 10 at 800 C. According to them, methane decreased the densified region to make it more porous, therefore the ionic resistance of oxygen could be significantly decreased. Likewise, methane sweep is found in this study to be effective to enhance oxygen flux. However, with pure CO2 as the sweep gas, Fig. 6 shows that a significant oxygen permeation was obtained only at 800 C where the flux was circa 0.27 mL min1 cm2 and upon reaching 900 C, the flux was nearly 0.8 mL min1 cm2. This shows a reduction of nearly 1.5 times compared to those obtained using He as the sweep gas. This phenomenon could be attributed to the corrosive nature of CO2, which plays a role in deactivating the membrane permeation properties. Furthermore CO2 readily forms carbonate species with the segregated SrO and CoO phases on the
Fig. 4 e Oxygen permeability of LSCN hollow fiber membrane swept by He.
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Fig. 5 e Oxygen permeability of LSCN hollow fiber membrane swept by CH4.
membrane surface, hence blocking the membrane surface and increasing the resistance to oxygen permeation. The oxygen permeation throughout the reaction is shown in Fig. 7(a) together with the reactant conversions. The average oxygen permeability of the LSCN hollow fiber membrane as a function of temperature during the OCRM reaction - via the catalytic membrane reactor in the presence of Ni-based catalyst - is calculated by equations (1) and (2) and shown individually in Fig. 8. When the temperature is higher than 800 C, the OCRM process is less efficient and has lower H2 selectivity because of the simultaneous occurrence of reverse water gas shift reaction (rWGS) [8]. The oxygen fluxes are found to be around 0.34(650 C), 0.44(700 C), 0.61(750 C) and 0.86(800 C) mL$min1 cm2 respectively. The oxygen permeability during the OCRM reaction turns out to be much more enhanced when compared to those obtained with pure He as the sweep gas. The oxygen fluxes under OCRM condition also reach the values achieved with pure CH4 as the sweep gas. Based on the effect of pure CH4 as the sweep gas (as shown in
Fig. 7 e Oxygen permeability and catalytic performance of an OCRM hollow fiber membrane reactor. (a) Oxygen supply, CH4 and CO2 conversions; (b) H2/CO ratio, H2 and CO selectivity. ( fHe [ 47.00 mL min-1, fNe [ 3.00 mL min-1, fCH4 [ fCO2 [ 5.00 mL min-1, membrane area [ 2 cm2, 0.5 g Ni/Al2O3 catalyst, fair [ 100 mL min-1).
Fig. 5), it can be deduced that during OCRM reaction, the presence of reducing gases such as H2 and CH4 on the catalyst bed side consumes oxygen quickly, thereby forming a much lower oxygen partial pressure on the permeate side of the membrane as compared to those using He sweep gas [19]. Even though CO2, which is present in the OCRM reaction, may deactivate the membrane, however the presence of other reducing gases such as H2, CH4 and CO can react readily with the surface oxygen species to maintain the oxygen flux at reaction conditions.
3.2. Effects of O2 permeation on OCRM catalytic performance
Fig. 6 e Oxygen permeability of LSCN hollow fiber membrane swept by CO2.
Oxygen supply to a hollow fiber membrane OCRM reactor can vary with different reaction conditions, such as temperature, flow rate of reactants and concentration of reducing gases. Therefore the oxygen permeability of the membrane can affect the OCRM reaction, and the OCRM reaction can conversely affect the oxygen flux as well. In this section, the results correlating to the oxygen supply with the catalytic performance during OCRM reaction are discussed. Fig. 7 (a) shows the effect of oxygen membrane flux on the conversion of CH4 and CO2. From 650 C to 700 C, CH4
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Fig. 8 e Oxygen flux of LSCN hollow fiber membrane in an OCRM reactor ( fHe [ 47.00 mL min-1, fNe [ 3.00 mL min-1, fCH4 [ fCO2 [ 5.00 mL min-1, membrane area [ 2 cm2, 0.5 g Ni/Al2O3 catalyst, fair [ 100 mL min-1).
conversion increases from 91% to 96%. But CO2 conversion does not change and keeps constant at 87%. This means that only a small amount of oxygen is supplied from the membrane and is sufficient to react with methane through POM reaction without affecting the CO2 conversion. Equations (7)e(9) shown below summarize the four main reactions occurring in the OCRM reaction:
CO2 þ CH4 /2CO þ 2H2
DH298 ¼ þ273:4 kJ=mol ðDRMÞ
1=2O2 þ CH4 /CO þ 2H2
DH298 ¼ 35:6 kJ=mol ðPOMÞ
H2 þ CO2 /CO þ H2 O DH298 ¼ þ41 kJ=mol ðrWGSÞ
CH4 þ 2O2 /CO2 þ 2H2 O DH298 ¼ 74:8 kJ=mol ðCOMÞ
3.4. Stability, energy saving and carbon resistance of LSCN hollow fiber membrane reactor In order to show the effect of oxygen on the catalytic stability for CO2 reforming reaction, OCRM reaction was conducted at 800 C for 12 h before the air supply was closed for the reactor
(7) (8) (9) (10)
DRM reaction first takes place (Equation (7)), followed by POM reaction (Equation (8)). At 700 C, when more oxygen is introduced to the reaction system, the redundant CH4 is oxidized to generate CO and H2 due to POM reaction, which increases CH4 conversion without decreasing CO2 conversion. At 700 C, POM reaction has little competition with DRM reaction because the temperature is low enough for rWGS reaction (Equation (9)) to occur. However, when the temperature increases from 700 C to 800 C, oxygen supply increases from 0.88(700 C) mL$min1 to 1.72(800 C) mL$min1. Therefore methane is converted completely to 100% at 800 C. At higher temperatures, when amount of oxygen supplied is greater due to the greater permeation, combustion via deep oxidation of methane is likely to occur (Equation (10)). POM reaction shows higher competition in the higher temperatures since CO2 conversion was observed to decrease from 87% at 700 C to 72% at 800 C. Whilst rWGS reaction becomes more active with increasing temperature, H2 is partly consumed, thus leading to lower selectivity of H2.
3.3.
reaction temperature is as low as 650 C, the oxygen supply is low and the activity of catalyst for OCRM is not high enough, therefore both the conversion and selectivity of H2 (84%) and CO (97%) are relatively lower than those obtained at higher temperatures. When the temperature is increased from 650 C to 800 C, the CO selectivity reaches nearly 100%, while the H2 selectivity reaches a peak value of 87.5% at 700 C, and then decreases to 84% (750 C) and 78% (800 C). This result shows that too much oxygen supply to the reaction system is beneficial for the POM reaction and rWGS reaction but detrimental for the DRM reaction because too much oxygen supply consumes part of the generated H2. Therefore, to obtain both high H2 yield and a relatively high CO selectivity, the best temperature range is around 700 Ce750 C, which can yield a maximum H2 flow rate of 8.35 mL min1 and CO selectivity of nearly 100%.
Effect of O2 permeation on selectivity
Fig. 7 (b) shows the selectivity of H2, CO, and H2/CO ratio during the OCRM reaction from 650 C to 800 C. When the
Fig. 9 e Effect of oxygen on the catalytic performance of OCRM with DRM as a comparison (when air was closed) in the LSCN hollow fiber membrane reactor at 800 C (a) oxygen supply, CH4 and CO2 conversions; (b) H2/CO ratio, H2 and CO selectivity. ( fHe [ 47.00 mL min-1, fNe [ 3.00 mL min-1, fCH4 [ fCO2 [ 5.00 mL min-1, membrane area [ 2 cm2, 0.5 g Ni/Al2O3 catalyst, fair [ 100 mL min-1).
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to be operated under DRM mode. 800 C was chosen as the reaction temperature for this comparative study of OCRM and DRM because oxygen permeation at 800 C is sufficiently high to show the obvious differences between DRM and OCRM in the catalytic membrane reactor system. Fig. 9 shows that CO2 and CH4 conversions, H2 and CO selectivity, as well as O2 permeation and H2 yield are stable when the OCRM reaction was performed in the LSCN hollow fiber membrane reactor at 800 C for 12 h. The system was then switched to the DRM mode by closing the air flow in the bore side of the hollow fiber membrane. Fig. 9 (a) and (b) show that without POM as a competing reaction, CH4 conversion decreases from 100% to 67% and CO2 conversion increases from 72% to 78%. This result indicates that, in terms of CO2 utilization, OCRM is not as good as DRM due to the decrease in CO2 conversion. However, the OCRM reaction is excellent for CH4 consumption as complete CH4 conversion can be achieved. Based on the energy balance calculation using Equations (7) and (8), the OCRM reaction needs only as little as 62% of the energy requirement as compared with that required for the pure DRM reaction, assuming that the balance 33% of CH4 can be consumed via POM reaction during the OCRM reaction. Furthermore, from Fig. 9, DRM reaction is shown to be less stable than OCRM, with both conversions of CO2 and CH4 in a declining mode, showing a quick deactivation of the Ni/ Al2O3 catalyst. Since the catalyst bed is still in an environment full of reducing gases at 800 C, the deactivation of catalyst is attributed to the chemical poisoning from carbon deposition on the catalyst bed [38]. Fig. 9 further shows that CO selectivity decreases from nearly 100% to 91%, implying that quite a number of deposited carbon transforms into solid carbon instead of CO. H2 selectivity increases to up to 90% when air is closed suddenly, and then decreases to 75%, showing that the Ni/Al2O3 catalyst in DRM mode can only operate with high activity for shorter duration of less than 3 h due to deactivation resulting from carbon deposition. Meanwhile a much lower H2/CO ratio of 0.74 is obtained in DRM mode compared to OCRM mode because of both lower CH4 conversion and lower H2 selectivity. Therefore in the OCRM membrane reactor, oxygen permeating through the membrane leads to a higher H2/CO ratio, higher catalyst stability, and less energy consumption compared with pure DRM operating mode. Two parallel reactions were also carried out for both OCRM and DRM reactions using 0.5 g Ni/Al2O3 catalyst operated at 800 C for 12 h in order to analyze the amount of carbon deposition on the spent catalysts. Fig. 10 shows the TGA analysis of the amount of carbon formed on the spent catalysts. After operation in an OCRM reactor for 12 h, the total weight loss rate of the Ni/Al2O3 catalyst is only 0.8% (measured from 100 C to 1000 C). This is just about 25% of the carbon amount formed on the catalyst used in the DRM reaction which yields a weight loss of 3.2%. This result confirms that a much lower carbon deposition on the catalyst took place in the OCRM membrane reactor compared to those operating in the DRM mode. The TGA spectrum shows a weight increase at 780 C on the spent catalyst after OCRM reaction. This increase in weight is probably due to the oxidation of metallic Ni present in the catalyst to NiO. Furthermore, there is no weight loss associated with carbon oxidation at temperatures between 400 and 600 C from
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Fig. 10 e TGA graphs of Ni/Al2O3 catalyst after 12 h of (a) OCRM and (b) DRM reaction.
the spent catalyst after OCRM reaction. However, for the spent catalyst after DRM reaction, there are 2 regions of weight loss (i.e. 400e600 C which is associated with the formation of amorphous carbon and above 600 C which is associated with the formation of graphitic carbon). This result shows that oxygen supplied from the LSCN hollow fiber membrane during the OCRM reaction is able to oxidize the amorphous carbon formed at the lower temperatures, hence leading to no change in the TGA profile at the intermediate temperature region between 400 and 600 C. Another interesting phenomenon is that at the lower temperature region (less than 200 C), there is a more drastic weight loss for the spent catalyst after DRM compared to those after OCRM. Generally, the weight loss at the lower temperature region is associated with the loss of water or moisture. Hence, the spent catalyst after
Fig. 11 e XRD graphs of LSCN powder and membranes (a) Powder after sintering at 900 C, (b) LSCN membrane after sintering at 1300 C (c) LSCN membrane after O2 permeation test using He as sweep gas (d) LSCN membrane after OCRM reaction.
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Fig. 12 e External surfaces of LSCN hollow fiber membrane (a) Fresh membrane (b) After OCRM reaction for 12 h.
DRM has higher water content compared to those spent catalyst after OCRM, indicating lower H2 selectivity in the DRM reaction. This is also confirmed by the lower H2/CO ratio for the DRM reaction compared to OCRM reaction as shown in Fig. 9. This means that the oxygen supplied in more proportionate distribution across the LSCN hollow fiber membrane surface enables POM reaction to take place as well, therefore enhancing selectivity for H2 production in the OCRM reaction. It is known that the presence of reducing gases can cause high instability and can cause structural changes in the perovskite structure of the oxygen permeating membrane. In order to investigate this phenomenon, XRD analysis of the LSCN hollow fiber membrane after OCRM reaction was conducted and compared with the perovskite structure of the fresh LSCN hollow fiber membrane. XRD profile of the LSCN hollow fiber membrane after oxygen permeation with He sweep gas was analyzed as well. For reference, the spectrum of the LSCN powder is also displayed as shown in Fig. 11. From the XRD profile, it is obvious that upon formation of hollow fiber membranes, sharp crystallite peaks are formed for the LSCN material. When O2 permeation with both He as sweep gas and during OCRM reaction were compared with the fresh LSCN hollow fiber, it is observed that the crystalline features of the membrane is still intact. However, after OCRM reaction, the bulk perovskite phases have lower crystallinity compared to the fresh LSCN perovskite phase. This result indicates that the presence of reducing gases during the OCRM reaction can slightly reduce the crystallinity of the LSCN perovskite hollow fiber membrane. However, there is no other phase observed in the LSCN membrane after OCRM reaction, indicating that phase segregation did not take place and thus oxygen permeation could still take place for the OCRM reaction. FESEM morphology of the external membrane surface before and after OCRM is shown in Fig. 12. Careful observation indicates that there is presence of sintering phenomena after OCRM reaction. However, the sintering that took place enabled the particles to be more closely packed, instead of having cracks or other surface deformities. This observation does not correlate with the decrease in crystallinity observed from XRD profile in Fig. 11. The XRD profile shows the bulk crystallinity of the perovskite phase, however the FESEM image can only give some indication on the surface properties. Therefore, the FESEM morphology indicates that the presence of the reducing gases during the OCRM reaction did not corrode the
external surface of the hollow fiber membrane. It has been reported that presence of pure methane as sweep gas is known to transform the densified surface layer of the oxygen permeating La0.6Sr0.4Co0.2Fe0.8O3d (LSCF) hollow fiber membrane to be more porous thereby enhancing the oxygen fluxes [37]. However, based on the FESEM image of the LSCN hollow fiber membrane after OCRM reaction, the intactness of the external surface suggest that the presence of CO2 in the reducing gas mixture may negate this surface corrosion phenomenon to take place.
4.
Conclusions
An integrated LSCN catalytic hollow fiber membrane reactor which combines the oxygen permeating hollow fiber membrane with Ni/Al2O3 catalyst to produce hydrogen through oxy-CO2 reforming of CH4 (OCRM) reaction has been successfully developed. The La0.6Sr0.4Co0.8Ni0.2O3d (LSCN) hollow fiber membrane has an oxygen permeability of 0.29 mL min1 cm2 using He as sweep gas when operated at 800 C. However, when it is used as an oxygen supplier in an OCRM reactor, the permeability reaches to 0.86 mL min1 cm2. Due to the oxygen permeation to the catalyst bed through the LSCN hollow fiber membrane, the OCRM reactor shows a very high methane conversion of up to 100%, and a very high CO selectivity of nearly 100%, and a H2/ CO ratio of 0.93. In order to comprehensively evaluate the conversion, selectivity and H2 yield, 700e750 C is a promising temperature region for the operation of OCRM hollow fiber membrane reactor. Compared with the DRM reactor, the conversion of CO2 is lower, but the energy efficiency, catalyst stability, and H2 yield is higher. Furthermore carbon deposition is only 25% in the catalytic OCRM membrane reactor compared to the DRM reactor.
Acknowledgments The authors gratefully thank National University of Singapore, A*STAR and NEA for generously supporting this work (A*STAR SERC Grant No. 092-138-0022 and RP No. 279-000-292305; NEA-ETRP Grant No.1002114 and RP No. 279-000-333-490).
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reference
[1] Bradford MCJ, Vannice MA. CO2 Reforming of CH4. Catal Rev 1999;41:1e42. [2] Lim Y, Lee C-J, Jeong YS, Song IH, Lee CJ, Han C. Optimal design and decision for combined steam reforming process with dry methane reforming to reuse CO2 as a raw material. Ind Eng Chem Res 2012;51:4982e9. [3] Chen YZ, Wang YZ, Xu HY, Xiong GX. Efficient production of hydrogen from natural gas steam reforming in palladium membrane reactor. Appl Catal B-Environ 2008;81:283e94. [4] Wang HH, Tablet C, Schiestel T, Werth S, Caro J. Partial oxidation of methane to syngas in a perovskite hollow fiber membrane reactor. Catal Comm 2006;7:907e12. [5] Sutthiumporn K, Kawi S. Promotional effect of alkaline earth over NieLa2O3catalyst for CO2 reforming of CH4: role of surface oxygen species on H2 production and carbon suppression. Int J Hydrogen Energ 2011;36:14435e46. [6] Barroso Quiroga MM, Castro Luna AE. Kinetic analysis of rate data for dry reforming of methane. Ind Eng Chem Res 2007; 46:5265e70. [7] Dahl JK, Weimer AW, Lewandowski A, Bingham C, Bruetsch F, Steinfeld A. Dry reforming of methane using a solar-thermal aerosol flow reactor. Ind Eng Chem Res 2003;43:5489e95. [8] Oemar U, Hidajat K, Kawi S. Role of catalyst support over PdOeNiO catalysts on catalyst activity and stability for oxyCO2 reforming of methane. Appl Catal A: Gen 2011;402:176e87. [9] Ferreira-Aparicio P, Benito MJ. High performance membrane reactor system for hydrogen production from methane. Ind Eng Chem Res 2005;44:742e8. [10] Ferreira-Aparicio P, Benito M, Kouachi K, Menad S. Catalysis in membrane reformers: a high-performance catalytic system for hydrogen production from methane. J Catal 2005; 231:331e43. [11] Tomishige K, Kanazawa S, Sato M, Ikushima K. Catalyst design of Pt-modified Ni/Al2O3 catalyst with flat temperature profile in methane reforming with CO2 and O2. Catal Lett 2002;84:69e74. [12] Carolan MF, Dyer PN, Labar JM Sr., Thorogood RM. Process for restoring permeance of an oxygen-permeable ion transport membrane utilized to recover oxygen from oxygencontaining gaseous mixtures. US Patent 5240473. 1993. [13] Mazanec TJ. High temperature electroceramic oxygen separation and reaction. Electrochem Soc Interface 1996;5: 46e9. [14] Balachandran U, Ma B, Maiya PS, Mieville RL, Dusek JT, Picciolo JJ, et al. Development of mixed-conducting oxides for gas separation. Solid State Ionics 1998;108:363e70. [15] Qi X, Lin YS, Swartz SL. Electric transport and oxygen permeation properties of lanthanum cobaltite membranes synthesized by different methods. Ind Eng Chem Res 2000;39: 646e53. [16] Dong H, Shao ZP, Xiong GX, Tong JH, Sheng SS, Yang WS. Investigation on POM reaction in a new perovskite membrane reactor. Catal Today 2001;67:3e13. [17] Kniep J, Lin YS. Partial oxidation of methane and oxygen permeation in SrCoFeOx membrane reactor with different catalysts. Ind Eng Chem Res 2011;50:7941e8. [18] Bouwmeester HJM, Burggraaf AJ. Dense ceramic membranes for oxygen separation. Fundamentals of inorganic membrane science and technology. Amsterdam: Elsevier; 1996. [19] Liu Y, Tan X, Li K. Mixed conducting ceramics for catalytic membrane processing. Catal Rev 2006;48:145e98.
4491
[20] Julbe A, Farrusseng D, Guizard C. Limitations and potentials of oxygen transport dense and porous ceramic membranes for oxidation reactions. Catal Today 2005;104:102e13. [21] Foley HC. Chapter 2 catalyst preparation- design and synthesis. Catal Today 1994;22:235e60. [22] Raich BA, Foley HC. Ethanol dehydrogenation with a palladium membrane reactor: an alternative to wacker chemistry. Ind Eng Chem Res 1998;37:3888e95. [23] Tong J, Yang W, Cai R, Zhu B, Lin L. Novel and ideal zirconium-based dense membrane reactors for partial oxidation of methane to syngas. Catal Lett 2002;78:129e37. [24] Bouwmeester HJM. Dense ceramic membranes for methane conversion. Catal Today 2003;82:141e50. [25] Shao ZP, Dong H, Xiong GX, Yang WS. Syngas production using an oxygen-permeating membrane reactor with cofeed of methane and carbon dioxide. Chin Chem Lett 2000;11:631e4. [26] Slade DA, Duncan AM, Nordheden KJ, Stagg-Williams SM. Mixed-conducting oxygen permeable ceramic membranes for the carbon dioxide reforming of methane. Green Chem 2007;9:577e81. [27] Wei YY, Huang L, Tang J, Zhou LY, Li Z, Wang HH. Syngas production in a novel perovskite membrane reactor with Cofeed of CO2. Chin Chem Lett 2011;22:1492e6. [28] Thursfield A, Metcalfe IS. Air separation using a catalytically modified mixed conducting ceramic hollow fiber membrane module. J Membr Sci 2007;288:175e87. [29] Liu H, Tan X, Pang Z, Diniz Da Costa JC, Lu GQ, Liu S. Novel dual structured mixed conducting ceramic hollow fibre membranes. Sep Purif Technol 2008;63:243e7. [30] Hjalmarsson P, Sogaard M, Hagen A, Mogensen M. Structural properties and electrochemical performance of strontiumand nickel-substituted lanthanum cobaltite. Solid State Ionics 2008;179:636e46. [31] Chen J, Liang F, Liu L, Jiang SP, Jian L. Characterization and evaluation of La0.8Sr0.2Co0.8 Ni0.2O3d prepared by a polymerassisted combustion synthesis as a cathode material for intermediate temperature solid oxide fuel cells. Int J Hydrogen Energ 2009;34:6845e51. [32] Xu X, Wang F, Liu Y, Pu J, Chi B, Jian L. Enhanced electrochemical performance of solution impregnated La0.8Sr0.2Co0.8Ni0.2O3d cathode for intermediate temperature solid oxide fuel cells. J Power Sources 2011;196:9365e8. [33] Shaw CKM, Kilner JA, Skinner SJ. Mixed cobalt and nickel containing perovskite oxide for intermediate temperature electrochemical applications. Solid State Ionics 2000;135: 765e9. [34] Meng X, Song J, Yang N, Meng B, Tan X, Ma Z-F, et al. NieBaCe0.95Tb0.05O3d cermet membranes for hydrogen permeation. J Membr Sci 2012;401e402:300e5. [35] Acharya M, Raich BA, Foley HC. Metal-supported carbogenic molecular sieve membranes: synthesis and applications. Ind Eng Chem Res 1997;36:2924e30. [36] Wang Z, Liu H, Tan X, Jin Y, Liu S. Improvement of the oxygen permeation through perovskite hollow fibre membranes by surface acid-modification. J Membr Sci 2009; 345:65e73. [37] Liu H, Pang Z, Tan X, Shao Z, Sunarso J, Ding R, et al. Enhanced oxygen permeation through perovskite hollow fibre membranes by methane activation. Ceram Inter 2009; 35:1435e9. [38] Diez F, Gates BC, Miller JT, Sajkowski DJ, Kukes SG. Deactivation of a NieMo/g-A12O3 catalyst: influence of coke on the hydroprocessing activity. Ind Eng Chem Res 1990;29: 1999e2004.
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La0.6Sr0.4Co0.8Ni0.2O3Ld hollow fiber membrane reactor: Integrated oxygen separation e CO2 reforming of methane reaction for hydrogen production Nai-Tao Yang, Yasotha Kathiraser, Sibudjing Kawi* Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576, Singapore
article info
abstract
Article history:
An integrated reactor system which combines oxygen permeable La0.6Sr0.4Co0.8Ni0.2O3d
Received 6 November 2012
(LSCN) perovskite ceramic hollow fiber membrane with Ni based catalyst has been suc-
Received in revised form
cessfully developed to produce hydrogen through oxy-CO2 reforming of methane (OCRM).
10 January 2013
Dense La0.6Sr0.4Co0.8Ni0.2O3d hollow fiber membrane was prepared using phase inversion-
Accepted 11 January 2013
sintering method. OCRM reaction was tested from 650 C to 800 C with a quartz reactor packed with 0.5 g Ni/Al2O3 catalyst around the LSCN hollow fiber membrane. CH4 and CO2
Available online xxx
were used as reactants and air as the oxygen source was fed through the bore side of the hollow fiber membrane. In order to gauge the effectiveness of this membrane reactor
Keywords: reforming
Oxy-CO2
of
methane
system, air flow was closed at 800 C and dry reforming of methane (DRM) was tested for comparison. The results show that the oxygen fluxes of LSCN membrane swept by helium
(OCRM) Integrated
catalytic
fiber
are nearly 3 times less than those swept by OCRM reactants. With increasing temperature
perovskite
conversion decreases from 87% to 72% due to the competition reaction with POM. CO
hollow
and oxygen supply, methane conversion in the OCRM reactor reaches 100%, but CO2
membrane reactor La0.6Sr0.4Co0.8Ni0.2O3d membrane
selectivity is as high as nearly 100% at reaction temperatures of 700 Ce800 C while H2
Hydrogen production
selectivity reaches a maximum of 88% at 700 C. At 800 C, when air supply was closed and DRM was conducted for comparison, CO selectivity decreased to 91%, resulting in carbon deposition which was around 4 times more than those obtained under OCRM reaction and H2/CO ratio decreased from 0.93 to 0.74, showing better carbon resistance and higher H2 selectivity of the Ni-based catalyst over the integrated oxygen separation-OCRM reaction across the LSCN hollow fiber membrane reactor. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years, global climate change has been increasingly attributed to greenhouse gas emission from anthropogenic sources. Among the greenhouse gases, carbon dioxide (CO2) and methane (CH4) have the greatest impact [1,2]. Methane is attractive as a promising fuel due to its higher energy density compared to petroleum, and because it emits less carbon
oxides upon combustion and can be converted into hydrogen through reforming reactions by steam, CO2 or oxygen [3e5]. Therefore, dry CO2 reforming of methane (DRM) [6,7] seems to offer an opportunity to transform what would otherwise be two greenhouse gases into a source of green hydrogen. This reaction is particularly interesting when it is carried out with natural gas from fields containing large amounts of CO2 accompanied with methane, without the pre-separation of
* Corresponding author. Tel.: þ65 65162065. E-mail address: [email protected] (S. Kawi). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.01.073
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CO2 from the feed. However, DRM reaction is an endothermic process, hence additional energy is required for the system, leading to increasing cost of operation. Since partial oxidation of methane (POM) is an exothermic reaction [8], therefore a combination reaction of POM with DRM, hereafter called as oxidative-CO2 reforming of methane (OCRM), could reduce the energy requirement and even reach to an autothermal reforming mode by adjusting the ratio of CH4, CO2 and oxygen. Furthermore, introduction of oxygen to the reactor can reduce carbon deposition on the catalyst bed, which is a severe problem during DRM reaction [9,10]. However, one of the major issue facing the OCRM reaction is the high economic, environmental, and safety costs associated with pure oxygen production, storage and transportation [11]. For these purposes, oxygen-permeable ceramic membrane is a promising technique to be applied as the in situ oxygen separator and supplier directly using air. Mixedoxygen ion-electron conducting membranes are oxygen permeable and can produce oxygen from air at elevated temperatures [12e15]. Coincidently, oxygen separation and reforming reaction are both operated at similar high temperature conditions [16]. Therefore oxygen permeable ceramic membranes show a high potential in the application of integrated oxygen separation-reaction process in a single equipment [17e22]. Meanwhile, oxygen species such as O2 or O will react with other reactants mildly on the membrane surface, avoiding the occurrence of combustion reaction and therefore preventing the formation of hot spots on the catalyst bed [23,24]. The dense ceramic membrane allows the conduction of oxygen ions through the lattice of the solid material resulting in 100% selectivity for oxygen regardless of the source of gases [25]. Slade et al. [26] and Wei et al. [27] have previously studied dual CO2 and O2 reforming of methane from the oxygen permeable membrane. However, disk membranes were used in their studies. Compared to the disk membrane, hollow fiber membrane has higher surface area and thinner wall and is easier to be scaled up by assembling in the form of bundle [28,29]. La0.6Sr0.4Co0.8Ni0.2O3d (LSCN) perovskite material has good oxygen reducing properties and has been widely used in intermediate temperature solid oxide fuel cells [30e33]. In this study, LSCN perovskite has been successfully prepared in the form of hollow fiber and tested for its oxygen permeation properties. A catalytic LSCN hollow fiber membrane reactor subsequently integrates oxygen separation coupled with OCRM reaction over Ni/Al2O3 catalyst. By setting up an oxygen-permeable hollow fiber membrane into the OCRM reactor, only pure oxygen is transferred in situ from air side and fed into the Ni/Al2O3 catalyst bed directly. Hence, this integrated separation e reaction membrane reactor system process is able to solve the economic, environmental and safety problems associated with pure oxygen production, transportation, storage and applications.
2.
Experimental
2.1.
Preparation of material and hollow fiber membrane
The oxygen-permeable hollow fiber membrane was La0.6Sr0.4Co0.8Ni0.2O3d (LSCN), which was prepared using
a solegel method as reported in literature [34]. Stoichiometric ratio of La(NO3)3$6H2O, Sr(NO3)2, Co(NO3)2$6H2O, Ni(NO3)2$6H2O were used as the precursors, with citric acid and ethylene glycol as the complexing agents. All of the above materials were sourced from SigmaeAldrich Reagent Co. The sol was dried at 100 C and calcined at 900 C for 5 h, ballmilled for 2 h, and then sieved with a 200-mesh sifter. The hollow fiber membrane was fabricated using a phase inversion-sintering method [34]. LSCN powder, N-methylpyrrolidone (NMP) and polyethersulfone (PES) were mixed in a ratio of 10:5:1 (mass ratio) and stirred together for 72 h to prepare a casting solution. The orifice of the spinneret used for spinning has outer/inner diameters of 3 mm/2 mm. Deionized water was used as internal and external coagulants. The hollow fiber membrane was then calcined at 1300 C for 5 h.
2.2. Characterizations of material and hollow fiber membrane The morphologies of the LSCN powder and LSCN hollow fiber membrane were observed through a field emission scanning electronic microscope (FESEM, Jeol, JSM-6700F). The samples were firstly outgassed under vacuum condition to remove impurities. Platinum coating (about 10 nm thickness) was carried out at 20 mA for 40 s. The crystalline structures of the calcined LSCN powder and hollow fiber membranes (fresh and after OCRM reaction) were determined using a Shimadzu XRD-6000 power diffractometer, with Cu target K-a radiation (l ¼ 0.15406 nm) at 40 kV and 30 mA. The scattering intensities were over an angular range of 20 <2q < 80 with a scanning speed of 2.00 /min.
2.3.
Preparation of catalyst
Ni/Al2O3 (Ni content 10 wt%), which was used as the OCRM catalyst, was prepared using Ni(NO3)2$6H2O, Al(NO3)3$9H2O and citric acid as precursors (SigmaeAldrich Reagent Co.). These precursors were dissolved in deionized water to form a transparent solution, which was then dried at 90 C, calcined at 800 C for 5 h, and subsequently reduced at 800 C for 1 h under H2/He mixture (10% of H2). The Ni/Al2O3 catalyst was kept in a sealed bottle for further use.
2.4.
Oxygen permeability test of LSCN hollow fiber
The oxygen permeability of LSCN hollow fiber was tested with high purified helium (>99.995%) as sweep gas and air as oxygen source from 650 C to 900 C. Helium flows from the shell side between the membrane and a quartz tube (inner/outer diameter of 8/10 mm) with the flow rates from 10 to 100 mL min1. Air flows from the bore side of the hollow fiber membrane with a constant flow rate of 100 mL/min. The shellside gas mixture was injected to a gas chromatograph (GC6890N, Agilent) and analyzed by a TCD detector using a Hayesep D 100/120 column. The oxygen permeation in an OCRM reactor was calculated by equation (1). 1 1 fO2 ¼ fCO þ fH2 O fCO2;in fCO2;out 2 2
(1)
where fO2 is the flow rate of permeated oxygen, and fCO, fH2 O , fCO2;in and fCO2;out are flow rates of CO, water steam, inlet and
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outlet of CO2 during the reaction. Since steam cannot be detected directly from GC since it was collected in a condenser outside the reactor, water produced was calculated by hydrogen balance as equation (2). fH2 O
¼ 2 fCH4;in fCH4;out fH2
(2)
where fCH4;in , fCH4;out and fH2 are the flow rates of CH4 inlet, outlet and flow rate of H2 as analyzed by GC.
2.5.
(6)
where XCH4 , XCO2 , SH2 and SCO are CH4 conversion, CO2 conversion, H2 selectivity and CO selectivity respectively. fCH4;in and fCO2;in are inlet flow rates of CH4 and CO2. fCH4;out , fCO2;out , fH2 and fCO are outlet flow rates of CH4, CO2, H2 and CO from the reactor.
2.6. Characterization of post-reacted LSCN hollow fiber membrane and catalyst
Test of OCRM membrane reactor
Fig. 1 shows the schematic hollow fiber membrane reactor fabricated for the integrated oxygen separation coupled with OCRM reaction. The LSCN hollow fiber was connected by two soft buffer tubes (silicone rubber) to relieve thermal expansion stress. 0.5 g of reduced Ni/Al2O3 catalyst was packed along with the hollow fiber membrane in the shell of a quartz tube (8 mm inner diameter and 10 mm outer diameter). Quartz wool was used to avoid catalyst being blown away. Each side of the quartz tube was fixed with a stainless steel tee joint to avoid gas leakage. The hollow fiber membrane packed with catalyst was put into a tubular furnace and tested at temperatures ranging from 650 C to 800 C. Methane and CO2 were fed in the catalyst bed at 5.00 mL min1. Pure helium (He, >99.995%) with a flow rate of 47.00 mL min1 was used as diluting gas and high purity of neon (Ne, >99.999%) with a flow rate of 3.00 mL min1 was used as an external standard for GC analysis. Purified air was fed in the bore side of LSCN hollow fiber with a constant flow rate of 100 mL/min. The output gases from the shell side (catalyst bed) were firstly condensed to eliminate water prior to GC analysis. The conversion of CO2, CH4 and selectivity of CO, H2 were analyzed based on GC peaks as follows: XCH4 ¼
fCH4;in fCH4;out 100% fCH4;in
(3)
XCO2 ¼
fCO2;in fCO2;out 100% fCO2;in
(4)
fH2 100% SH2 ¼ 2 fCH4;in fCH4;out
f CO 100% SCO ¼ fCO2;in fCO2;out þ fCH4;in fCH4;out
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(5)
The morphology of the reacted LSCN hollow fiber was observed through a field emission scanning electronic microscope (FESEM, Jeol, JSM-6700F). Carbon deposition of the catalyst after OCRM or DRM reaction was tested with Thermal Gravimetric Analysis (TGA, Shimadzu DTG 50). The used catalyst was transferred from the membrane reactor into glass bottles, and immediately covered to prevent exposure with air. About 15 mg catalyst sample was put into the alumina pan and calcined from room temperature to 1000 C with a ramp rate of 10 C/min in an ambient air environment. The crystalline structures of the calcined LSCN powder and hollow fiber were determined using a Shimadzu XRD-6000 power diffractometer, with Cu target K-a radiation (l ¼ 0.15406 nm) at 40 kV and 30 mA. The scattering intensities were over an angular range of 20 <2q < 80 with a scanning speed of 2.00 /min.
3.
Results and discussion
3.1. Structure and oxygen permeability of LSCN hollow fiber membrane Fig. 2 shows the FESEM image of the morphology of the LSCN powder. The particles of the LSCN powder appear to have irregular morphologies, with particle size averaging around 80 nm. Fig. 3 (a) and (b) show the cross section of the LSCN hollow fiber, with the outer and inner diameters of 2.3 mm and 1.7 mm, respectively and a wall thickness of around 250 mm. It has a dense external layer of circa 140 mm in thickness for oxygen separation, and an internal porous layer which is
Fig. 1 e Schematic of the OCRM hollow fiber membrane reactor.
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suitable for gas diffusion. Fig. 3 (c) shows the snapshot of the straight and long LSCN hollow fibers used in this study. Various permeation tests using He, CH4 and CO2 as the sweep gas have been conducted. Fig. 4 shows the oxygen permeation results when ambient air was supplied through the bore side of hollow fiber membrane with a constant flow rate of 100 mL min1, and swept with helium outside of the membrane. Significant oxygen permeation starts to occur only at higher temperatures, circa 800 C because the perovskite membrane becomes activated at higher temperatures [35]. At 800 C, the maximum oxygen flux is only 0.29 mL min1 cm2. With increasing He flow rate, the flux keeps on a nearly constant value. Increasing of sweep gas will increase the driving force for oxygen permeation by lowering the oxygen partial pressure on the permeate side. At the lower temperature region, below 800 C, with an increase in sweep gas flow rate, improvement in the oxygen flux is not observed which shows a surface exchange limiting process. Therefore, since the oxygen lattice is not sufficiently activated thermally, an increase in sweep gas does not cause much effect on the oxygen permeation flux. However, at higher testing temperature such as 850 C and 900 C, the maximum oxygen permeation fluxes increase from 0.39 to 0.69 and 0.59e1.2 mL min1 cm2, respectively, with the He sweep gas flow rate from 10 to 100 mL min1. These phenomena indicate the complex processes for oxygen transport through the mixed membranes. The oxygen flux appears not to have reached the saturated value even under a 100 mL min1 of He flow rate. This behavior follows that of other perovskites whereby oxygen permeation is limited by bulk diffusion at higher temperatures, hence at the elevated temperature regime, effect of sweep gas flow rate plays an important role in increasing the flux. This is not surprising as the transition temperature from the surface-exchange limiting process to the bulk-diffusion limiting process has been reported to be around 900 C for the asymmetric perovskite-type hollow fiber membranes [36]. Fig. 5 shows the oxygen permeation results using methane as the sweep gas following the similar conditions as described above using He as sweep gas. It can be clearly observed that an
Fig. 2 e FESEM image showing the morphology of LSCN powder.
Fig. 3 e Morphology of LSCN hollow fiber membrane: (a) Cross section; (b) Dense layer, and (c) Snapshot of LSCN hollow fibers.
increase in oxygen permeation flux has occurred, whereby the maximum flux already reaches 0.25 mL min1 cm2 at 700 C. At 900 C, under a methane sweep of 100 mL min1, the maximum O2 flux reaches 3 mL min1 cm2, which is nearly 3 times more than those obtained with He as sweep gas. It has been reported by Liu et al. [37] that, after activation by CH4 at 1000 C for 1 h, their oxygen fluxes increased by a factor of 10 at 800 C. According to them, methane decreased the densified region to make it more porous, therefore the ionic resistance of oxygen could be significantly decreased. Likewise, methane sweep is found in this study to be effective to enhance oxygen flux. However, with pure CO2 as the sweep gas, Fig. 6 shows that a significant oxygen permeation was obtained only at 800 C where the flux was circa 0.27 mL min1 cm2 and upon reaching 900 C, the flux was nearly 0.8 mL min1 cm2. This shows a reduction of nearly 1.5 times compared to those obtained using He as the sweep gas. This phenomenon could be attributed to the corrosive nature of CO2, which plays a role in deactivating the membrane permeation properties. Furthermore CO2 readily forms carbonate species with the segregated SrO and CoO phases on the
Fig. 4 e Oxygen permeability of LSCN hollow fiber membrane swept by He.
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Fig. 5 e Oxygen permeability of LSCN hollow fiber membrane swept by CH4.
membrane surface, hence blocking the membrane surface and increasing the resistance to oxygen permeation. The oxygen permeation throughout the reaction is shown in Fig. 7(a) together with the reactant conversions. The average oxygen permeability of the LSCN hollow fiber membrane as a function of temperature during the OCRM reaction - via the catalytic membrane reactor in the presence of Ni-based catalyst - is calculated by equations (1) and (2) and shown individually in Fig. 8. When the temperature is higher than 800 C, the OCRM process is less efficient and has lower H2 selectivity because of the simultaneous occurrence of reverse water gas shift reaction (rWGS) [8]. The oxygen fluxes are found to be around 0.34(650 C), 0.44(700 C), 0.61(750 C) and 0.86(800 C) mL$min1 cm2 respectively. The oxygen permeability during the OCRM reaction turns out to be much more enhanced when compared to those obtained with pure He as the sweep gas. The oxygen fluxes under OCRM condition also reach the values achieved with pure CH4 as the sweep gas. Based on the effect of pure CH4 as the sweep gas (as shown in
Fig. 7 e Oxygen permeability and catalytic performance of an OCRM hollow fiber membrane reactor. (a) Oxygen supply, CH4 and CO2 conversions; (b) H2/CO ratio, H2 and CO selectivity. ( fHe [ 47.00 mL min-1, fNe [ 3.00 mL min-1, fCH4 [ fCO2 [ 5.00 mL min-1, membrane area [ 2 cm2, 0.5 g Ni/Al2O3 catalyst, fair [ 100 mL min-1).
Fig. 5), it can be deduced that during OCRM reaction, the presence of reducing gases such as H2 and CH4 on the catalyst bed side consumes oxygen quickly, thereby forming a much lower oxygen partial pressure on the permeate side of the membrane as compared to those using He sweep gas [19]. Even though CO2, which is present in the OCRM reaction, may deactivate the membrane, however the presence of other reducing gases such as H2, CH4 and CO can react readily with the surface oxygen species to maintain the oxygen flux at reaction conditions.
3.2. Effects of O2 permeation on OCRM catalytic performance
Fig. 6 e Oxygen permeability of LSCN hollow fiber membrane swept by CO2.
Oxygen supply to a hollow fiber membrane OCRM reactor can vary with different reaction conditions, such as temperature, flow rate of reactants and concentration of reducing gases. Therefore the oxygen permeability of the membrane can affect the OCRM reaction, and the OCRM reaction can conversely affect the oxygen flux as well. In this section, the results correlating to the oxygen supply with the catalytic performance during OCRM reaction are discussed. Fig. 7 (a) shows the effect of oxygen membrane flux on the conversion of CH4 and CO2. From 650 C to 700 C, CH4
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Fig. 8 e Oxygen flux of LSCN hollow fiber membrane in an OCRM reactor ( fHe [ 47.00 mL min-1, fNe [ 3.00 mL min-1, fCH4 [ fCO2 [ 5.00 mL min-1, membrane area [ 2 cm2, 0.5 g Ni/Al2O3 catalyst, fair [ 100 mL min-1).
conversion increases from 91% to 96%. But CO2 conversion does not change and keeps constant at 87%. This means that only a small amount of oxygen is supplied from the membrane and is sufficient to react with methane through POM reaction without affecting the CO2 conversion. Equations (7)e(9) shown below summarize the four main reactions occurring in the OCRM reaction:
CO2 þ CH4 /2CO þ 2H2
DH298 ¼ þ273:4 kJ=mol ðDRMÞ
1=2O2 þ CH4 /CO þ 2H2
DH298 ¼ 35:6 kJ=mol ðPOMÞ
H2 þ CO2 /CO þ H2 O DH298 ¼ þ41 kJ=mol ðrWGSÞ
CH4 þ 2O2 /CO2 þ 2H2 O DH298 ¼ 74:8 kJ=mol ðCOMÞ
3.4. Stability, energy saving and carbon resistance of LSCN hollow fiber membrane reactor In order to show the effect of oxygen on the catalytic stability for CO2 reforming reaction, OCRM reaction was conducted at 800 C for 12 h before the air supply was closed for the reactor
(7) (8) (9) (10)
DRM reaction first takes place (Equation (7)), followed by POM reaction (Equation (8)). At 700 C, when more oxygen is introduced to the reaction system, the redundant CH4 is oxidized to generate CO and H2 due to POM reaction, which increases CH4 conversion without decreasing CO2 conversion. At 700 C, POM reaction has little competition with DRM reaction because the temperature is low enough for rWGS reaction (Equation (9)) to occur. However, when the temperature increases from 700 C to 800 C, oxygen supply increases from 0.88(700 C) mL$min1 to 1.72(800 C) mL$min1. Therefore methane is converted completely to 100% at 800 C. At higher temperatures, when amount of oxygen supplied is greater due to the greater permeation, combustion via deep oxidation of methane is likely to occur (Equation (10)). POM reaction shows higher competition in the higher temperatures since CO2 conversion was observed to decrease from 87% at 700 C to 72% at 800 C. Whilst rWGS reaction becomes more active with increasing temperature, H2 is partly consumed, thus leading to lower selectivity of H2.
3.3.
reaction temperature is as low as 650 C, the oxygen supply is low and the activity of catalyst for OCRM is not high enough, therefore both the conversion and selectivity of H2 (84%) and CO (97%) are relatively lower than those obtained at higher temperatures. When the temperature is increased from 650 C to 800 C, the CO selectivity reaches nearly 100%, while the H2 selectivity reaches a peak value of 87.5% at 700 C, and then decreases to 84% (750 C) and 78% (800 C). This result shows that too much oxygen supply to the reaction system is beneficial for the POM reaction and rWGS reaction but detrimental for the DRM reaction because too much oxygen supply consumes part of the generated H2. Therefore, to obtain both high H2 yield and a relatively high CO selectivity, the best temperature range is around 700 Ce750 C, which can yield a maximum H2 flow rate of 8.35 mL min1 and CO selectivity of nearly 100%.
Effect of O2 permeation on selectivity
Fig. 7 (b) shows the selectivity of H2, CO, and H2/CO ratio during the OCRM reaction from 650 C to 800 C. When the
Fig. 9 e Effect of oxygen on the catalytic performance of OCRM with DRM as a comparison (when air was closed) in the LSCN hollow fiber membrane reactor at 800 C (a) oxygen supply, CH4 and CO2 conversions; (b) H2/CO ratio, H2 and CO selectivity. ( fHe [ 47.00 mL min-1, fNe [ 3.00 mL min-1, fCH4 [ fCO2 [ 5.00 mL min-1, membrane area [ 2 cm2, 0.5 g Ni/Al2O3 catalyst, fair [ 100 mL min-1).
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to be operated under DRM mode. 800 C was chosen as the reaction temperature for this comparative study of OCRM and DRM because oxygen permeation at 800 C is sufficiently high to show the obvious differences between DRM and OCRM in the catalytic membrane reactor system. Fig. 9 shows that CO2 and CH4 conversions, H2 and CO selectivity, as well as O2 permeation and H2 yield are stable when the OCRM reaction was performed in the LSCN hollow fiber membrane reactor at 800 C for 12 h. The system was then switched to the DRM mode by closing the air flow in the bore side of the hollow fiber membrane. Fig. 9 (a) and (b) show that without POM as a competing reaction, CH4 conversion decreases from 100% to 67% and CO2 conversion increases from 72% to 78%. This result indicates that, in terms of CO2 utilization, OCRM is not as good as DRM due to the decrease in CO2 conversion. However, the OCRM reaction is excellent for CH4 consumption as complete CH4 conversion can be achieved. Based on the energy balance calculation using Equations (7) and (8), the OCRM reaction needs only as little as 62% of the energy requirement as compared with that required for the pure DRM reaction, assuming that the balance 33% of CH4 can be consumed via POM reaction during the OCRM reaction. Furthermore, from Fig. 9, DRM reaction is shown to be less stable than OCRM, with both conversions of CO2 and CH4 in a declining mode, showing a quick deactivation of the Ni/ Al2O3 catalyst. Since the catalyst bed is still in an environment full of reducing gases at 800 C, the deactivation of catalyst is attributed to the chemical poisoning from carbon deposition on the catalyst bed [38]. Fig. 9 further shows that CO selectivity decreases from nearly 100% to 91%, implying that quite a number of deposited carbon transforms into solid carbon instead of CO. H2 selectivity increases to up to 90% when air is closed suddenly, and then decreases to 75%, showing that the Ni/Al2O3 catalyst in DRM mode can only operate with high activity for shorter duration of less than 3 h due to deactivation resulting from carbon deposition. Meanwhile a much lower H2/CO ratio of 0.74 is obtained in DRM mode compared to OCRM mode because of both lower CH4 conversion and lower H2 selectivity. Therefore in the OCRM membrane reactor, oxygen permeating through the membrane leads to a higher H2/CO ratio, higher catalyst stability, and less energy consumption compared with pure DRM operating mode. Two parallel reactions were also carried out for both OCRM and DRM reactions using 0.5 g Ni/Al2O3 catalyst operated at 800 C for 12 h in order to analyze the amount of carbon deposition on the spent catalysts. Fig. 10 shows the TGA analysis of the amount of carbon formed on the spent catalysts. After operation in an OCRM reactor for 12 h, the total weight loss rate of the Ni/Al2O3 catalyst is only 0.8% (measured from 100 C to 1000 C). This is just about 25% of the carbon amount formed on the catalyst used in the DRM reaction which yields a weight loss of 3.2%. This result confirms that a much lower carbon deposition on the catalyst took place in the OCRM membrane reactor compared to those operating in the DRM mode. The TGA spectrum shows a weight increase at 780 C on the spent catalyst after OCRM reaction. This increase in weight is probably due to the oxidation of metallic Ni present in the catalyst to NiO. Furthermore, there is no weight loss associated with carbon oxidation at temperatures between 400 and 600 C from
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Fig. 10 e TGA graphs of Ni/Al2O3 catalyst after 12 h of (a) OCRM and (b) DRM reaction.
the spent catalyst after OCRM reaction. However, for the spent catalyst after DRM reaction, there are 2 regions of weight loss (i.e. 400e600 C which is associated with the formation of amorphous carbon and above 600 C which is associated with the formation of graphitic carbon). This result shows that oxygen supplied from the LSCN hollow fiber membrane during the OCRM reaction is able to oxidize the amorphous carbon formed at the lower temperatures, hence leading to no change in the TGA profile at the intermediate temperature region between 400 and 600 C. Another interesting phenomenon is that at the lower temperature region (less than 200 C), there is a more drastic weight loss for the spent catalyst after DRM compared to those after OCRM. Generally, the weight loss at the lower temperature region is associated with the loss of water or moisture. Hence, the spent catalyst after
Fig. 11 e XRD graphs of LSCN powder and membranes (a) Powder after sintering at 900 C, (b) LSCN membrane after sintering at 1300 C (c) LSCN membrane after O2 permeation test using He as sweep gas (d) LSCN membrane after OCRM reaction.
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Fig. 12 e External surfaces of LSCN hollow fiber membrane (a) Fresh membrane (b) After OCRM reaction for 12 h.
DRM has higher water content compared to those spent catalyst after OCRM, indicating lower H2 selectivity in the DRM reaction. This is also confirmed by the lower H2/CO ratio for the DRM reaction compared to OCRM reaction as shown in Fig. 9. This means that the oxygen supplied in more proportionate distribution across the LSCN hollow fiber membrane surface enables POM reaction to take place as well, therefore enhancing selectivity for H2 production in the OCRM reaction. It is known that the presence of reducing gases can cause high instability and can cause structural changes in the perovskite structure of the oxygen permeating membrane. In order to investigate this phenomenon, XRD analysis of the LSCN hollow fiber membrane after OCRM reaction was conducted and compared with the perovskite structure of the fresh LSCN hollow fiber membrane. XRD profile of the LSCN hollow fiber membrane after oxygen permeation with He sweep gas was analyzed as well. For reference, the spectrum of the LSCN powder is also displayed as shown in Fig. 11. From the XRD profile, it is obvious that upon formation of hollow fiber membranes, sharp crystallite peaks are formed for the LSCN material. When O2 permeation with both He as sweep gas and during OCRM reaction were compared with the fresh LSCN hollow fiber, it is observed that the crystalline features of the membrane is still intact. However, after OCRM reaction, the bulk perovskite phases have lower crystallinity compared to the fresh LSCN perovskite phase. This result indicates that the presence of reducing gases during the OCRM reaction can slightly reduce the crystallinity of the LSCN perovskite hollow fiber membrane. However, there is no other phase observed in the LSCN membrane after OCRM reaction, indicating that phase segregation did not take place and thus oxygen permeation could still take place for the OCRM reaction. FESEM morphology of the external membrane surface before and after OCRM is shown in Fig. 12. Careful observation indicates that there is presence of sintering phenomena after OCRM reaction. However, the sintering that took place enabled the particles to be more closely packed, instead of having cracks or other surface deformities. This observation does not correlate with the decrease in crystallinity observed from XRD profile in Fig. 11. The XRD profile shows the bulk crystallinity of the perovskite phase, however the FESEM image can only give some indication on the surface properties. Therefore, the FESEM morphology indicates that the presence of the reducing gases during the OCRM reaction did not corrode the
external surface of the hollow fiber membrane. It has been reported that presence of pure methane as sweep gas is known to transform the densified surface layer of the oxygen permeating La0.6Sr0.4Co0.2Fe0.8O3d (LSCF) hollow fiber membrane to be more porous thereby enhancing the oxygen fluxes [37]. However, based on the FESEM image of the LSCN hollow fiber membrane after OCRM reaction, the intactness of the external surface suggest that the presence of CO2 in the reducing gas mixture may negate this surface corrosion phenomenon to take place.
4.
Conclusions
An integrated LSCN catalytic hollow fiber membrane reactor which combines the oxygen permeating hollow fiber membrane with Ni/Al2O3 catalyst to produce hydrogen through oxy-CO2 reforming of CH4 (OCRM) reaction has been successfully developed. The La0.6Sr0.4Co0.8Ni0.2O3d (LSCN) hollow fiber membrane has an oxygen permeability of 0.29 mL min1 cm2 using He as sweep gas when operated at 800 C. However, when it is used as an oxygen supplier in an OCRM reactor, the permeability reaches to 0.86 mL min1 cm2. Due to the oxygen permeation to the catalyst bed through the LSCN hollow fiber membrane, the OCRM reactor shows a very high methane conversion of up to 100%, and a very high CO selectivity of nearly 100%, and a H2/ CO ratio of 0.93. In order to comprehensively evaluate the conversion, selectivity and H2 yield, 700e750 C is a promising temperature region for the operation of OCRM hollow fiber membrane reactor. Compared with the DRM reactor, the conversion of CO2 is lower, but the energy efficiency, catalyst stability, and H2 yield is higher. Furthermore carbon deposition is only 25% in the catalytic OCRM membrane reactor compared to the DRM reactor.
Acknowledgments The authors gratefully thank National University of Singapore, A*STAR and NEA for generously supporting this work (A*STAR SERC Grant No. 092-138-0022 and RP No. 279-000-292305; NEA-ETRP Grant No.1002114 and RP No. 279-000-333-490).
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reference
[1] Bradford MCJ, Vannice MA. CO2 Reforming of CH4. Catal Rev 1999;41:1e42. [2] Lim Y, Lee C-J, Jeong YS, Song IH, Lee CJ, Han C. Optimal design and decision for combined steam reforming process with dry methane reforming to reuse CO2 as a raw material. Ind Eng Chem Res 2012;51:4982e9. [3] Chen YZ, Wang YZ, Xu HY, Xiong GX. Efficient production of hydrogen from natural gas steam reforming in palladium membrane reactor. Appl Catal B-Environ 2008;81:283e94. [4] Wang HH, Tablet C, Schiestel T, Werth S, Caro J. Partial oxidation of methane to syngas in a perovskite hollow fiber membrane reactor. Catal Comm 2006;7:907e12. [5] Sutthiumporn K, Kawi S. Promotional effect of alkaline earth over NieLa2O3catalyst for CO2 reforming of CH4: role of surface oxygen species on H2 production and carbon suppression. Int J Hydrogen Energ 2011;36:14435e46. [6] Barroso Quiroga MM, Castro Luna AE. Kinetic analysis of rate data for dry reforming of methane. Ind Eng Chem Res 2007; 46:5265e70. [7] Dahl JK, Weimer AW, Lewandowski A, Bingham C, Bruetsch F, Steinfeld A. Dry reforming of methane using a solar-thermal aerosol flow reactor. Ind Eng Chem Res 2003;43:5489e95. [8] Oemar U, Hidajat K, Kawi S. Role of catalyst support over PdOeNiO catalysts on catalyst activity and stability for oxyCO2 reforming of methane. Appl Catal A: Gen 2011;402:176e87. [9] Ferreira-Aparicio P, Benito MJ. High performance membrane reactor system for hydrogen production from methane. Ind Eng Chem Res 2005;44:742e8. [10] Ferreira-Aparicio P, Benito M, Kouachi K, Menad S. Catalysis in membrane reformers: a high-performance catalytic system for hydrogen production from methane. J Catal 2005; 231:331e43. [11] Tomishige K, Kanazawa S, Sato M, Ikushima K. Catalyst design of Pt-modified Ni/Al2O3 catalyst with flat temperature profile in methane reforming with CO2 and O2. Catal Lett 2002;84:69e74. [12] Carolan MF, Dyer PN, Labar JM Sr., Thorogood RM. Process for restoring permeance of an oxygen-permeable ion transport membrane utilized to recover oxygen from oxygencontaining gaseous mixtures. US Patent 5240473. 1993. [13] Mazanec TJ. High temperature electroceramic oxygen separation and reaction. Electrochem Soc Interface 1996;5: 46e9. [14] Balachandran U, Ma B, Maiya PS, Mieville RL, Dusek JT, Picciolo JJ, et al. Development of mixed-conducting oxides for gas separation. Solid State Ionics 1998;108:363e70. [15] Qi X, Lin YS, Swartz SL. Electric transport and oxygen permeation properties of lanthanum cobaltite membranes synthesized by different methods. Ind Eng Chem Res 2000;39: 646e53. [16] Dong H, Shao ZP, Xiong GX, Tong JH, Sheng SS, Yang WS. Investigation on POM reaction in a new perovskite membrane reactor. Catal Today 2001;67:3e13. [17] Kniep J, Lin YS. Partial oxidation of methane and oxygen permeation in SrCoFeOx membrane reactor with different catalysts. Ind Eng Chem Res 2011;50:7941e8. [18] Bouwmeester HJM, Burggraaf AJ. Dense ceramic membranes for oxygen separation. Fundamentals of inorganic membrane science and technology. Amsterdam: Elsevier; 1996. [19] Liu Y, Tan X, Li K. Mixed conducting ceramics for catalytic membrane processing. Catal Rev 2006;48:145e98.
4491
[20] Julbe A, Farrusseng D, Guizard C. Limitations and potentials of oxygen transport dense and porous ceramic membranes for oxidation reactions. Catal Today 2005;104:102e13. [21] Foley HC. Chapter 2 catalyst preparation- design and synthesis. Catal Today 1994;22:235e60. [22] Raich BA, Foley HC. Ethanol dehydrogenation with a palladium membrane reactor: an alternative to wacker chemistry. Ind Eng Chem Res 1998;37:3888e95. [23] Tong J, Yang W, Cai R, Zhu B, Lin L. Novel and ideal zirconium-based dense membrane reactors for partial oxidation of methane to syngas. Catal Lett 2002;78:129e37. [24] Bouwmeester HJM. Dense ceramic membranes for methane conversion. Catal Today 2003;82:141e50. [25] Shao ZP, Dong H, Xiong GX, Yang WS. Syngas production using an oxygen-permeating membrane reactor with cofeed of methane and carbon dioxide. Chin Chem Lett 2000;11:631e4. [26] Slade DA, Duncan AM, Nordheden KJ, Stagg-Williams SM. Mixed-conducting oxygen permeable ceramic membranes for the carbon dioxide reforming of methane. Green Chem 2007;9:577e81. [27] Wei YY, Huang L, Tang J, Zhou LY, Li Z, Wang HH. Syngas production in a novel perovskite membrane reactor with Cofeed of CO2. Chin Chem Lett 2011;22:1492e6. [28] Thursfield A, Metcalfe IS. Air separation using a catalytically modified mixed conducting ceramic hollow fiber membrane module. J Membr Sci 2007;288:175e87. [29] Liu H, Tan X, Pang Z, Diniz Da Costa JC, Lu GQ, Liu S. Novel dual structured mixed conducting ceramic hollow fibre membranes. Sep Purif Technol 2008;63:243e7. [30] Hjalmarsson P, Sogaard M, Hagen A, Mogensen M. Structural properties and electrochemical performance of strontiumand nickel-substituted lanthanum cobaltite. Solid State Ionics 2008;179:636e46. [31] Chen J, Liang F, Liu L, Jiang SP, Jian L. Characterization and evaluation of La0.8Sr0.2Co0.8 Ni0.2O3d prepared by a polymerassisted combustion synthesis as a cathode material for intermediate temperature solid oxide fuel cells. Int J Hydrogen Energ 2009;34:6845e51. [32] Xu X, Wang F, Liu Y, Pu J, Chi B, Jian L. Enhanced electrochemical performance of solution impregnated La0.8Sr0.2Co0.8Ni0.2O3d cathode for intermediate temperature solid oxide fuel cells. J Power Sources 2011;196:9365e8. [33] Shaw CKM, Kilner JA, Skinner SJ. Mixed cobalt and nickel containing perovskite oxide for intermediate temperature electrochemical applications. Solid State Ionics 2000;135: 765e9. [34] Meng X, Song J, Yang N, Meng B, Tan X, Ma Z-F, et al. NieBaCe0.95Tb0.05O3d cermet membranes for hydrogen permeation. J Membr Sci 2012;401e402:300e5. [35] Acharya M, Raich BA, Foley HC. Metal-supported carbogenic molecular sieve membranes: synthesis and applications. Ind Eng Chem Res 1997;36:2924e30. [36] Wang Z, Liu H, Tan X, Jin Y, Liu S. Improvement of the oxygen permeation through perovskite hollow fibre membranes by surface acid-modification. J Membr Sci 2009; 345:65e73. [37] Liu H, Pang Z, Tan X, Shao Z, Sunarso J, Ding R, et al. Enhanced oxygen permeation through perovskite hollow fibre membranes by methane activation. Ceram Inter 2009; 35:1435e9. [38] Diez F, Gates BC, Miller JT, Sajkowski DJ, Kukes SG. Deactivation of a NieMo/g-A12O3 catalyst: influence of coke on the hydroprocessing activity. Ind Eng Chem Res 1990;29: 1999e2004.