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Synthesis Gas Production from Methane with SrFeCo0.5Oy Membrane Reactor Shao J. Feng, Shen Ran, De C. Zhu, Wei Liu, and Chu S. Chen* Laboratory of Advanced Functional Materials and Devices, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China Received September 3, 2003. Revised Manuscript Received November 18, 2003
Partial oxidation of methane to synthesis gas was investigated in a reactor consisting of an oxygen-permeable SrFeCo0.5Oy membrane tube and a Ni/γ-Al2O3 catalyst bed located after the membrane tube. In this reactor, part of methane reacted with oxygen that permeated through the membrane from air, and the resultants (H2O, CO2) and the rest of methane were transported to the catalyst bed where they were converted to syngas. When a reactor of membrane surface area 4.6 cm2 was run at 900 °C with a methane feeding rate of 26.8 mL/min, the throughput conversion of methane was ∼98%, the CO selectivity ∼98%, H2/CO ∼1.8, syngas generation rate 16 mL/min/cm2. Under the reactor conditions, the layered phase Sr4(Fe,Co)6O13 in the membrane gradually decomposed to a perovskite phase SrFe1-xCoxO3-δ with high oxygen permeability and spinel phase [(CoFe)]2CoO4 with catalytic activity toward the oxidation of methane. The Ni-based reforming catalyst exhibited desirable activity and stability in the membrane reactor, which may be attributed to the absence of the “hot spots” in the catalyst.
Introduction Conversion of natural gas to liquid fuels (GTL) through a syngas intermediate (CO/H2) is of great importance to safeguarding the world’s future liquidfuels supply. Currently, syngas is produced by reacting methane, the main component of natural gas, with steam at high temperatures and pressures. As this process is highly endothermic and thus very energy- and capital-intensive, the syngas production makes up ∼60% of the overall cost of GTL.1 An alternative process to produce syngas is the partial oxidation of methane (POM) with pure oxygen in the presence of a catalyst.2-4 The exothermic nature of POM makes it attractive in terms of energy consumption. The other advantage of POM over the steam reforming process is that the asproduced syngas has a lower H2/CO ratio suitable for subsequent conversion to liquid fuels. The main difficulty with POM lies in the consumption of large quantities of expensive pure oxygen that is currently produced through cryogenic air separation process. The development of oxygen-permeable dense ceramic membranes opens up a possibility of overcoming this difficulty, as the membrane-based reactor can integrate the oxygen separation and POM process and thus reduce syngas production cost significantly.1,5,6 * Corresponding author. Tel:+86-551-3602940. Fax: +86-5513601592. E-mail:
[email protected]. (1) Balachandran, U.; Dusek, J. T.; Maiya, P. S.; Ma, B.; Mieville, R. L.; Kleefisch, M. S.; Udovich, C. A. Catal. Today 1997, 36, 265272. (2) Ashcroft, A. T.; Cheetham, A. K.; Foord, J. S.; Green, M. L. H.; Grey, C. P.; Murrell, A. J.; Vernon, P. D. F. Nature 1990, 344, 319321. (3) Hickman, D. A.; Schmidt, L. D. Science 1993, 259, 343-346. (4) Dissanayake, D.; Rosynek, M. P.; Kharas, K. C. C.; Lunsford, J. H. J. Catal. 1991, 132, 117-127.
In studying tubular membrane reactors for POM, two types of configuration have been used with respect to the location of the reforming catalyst in the reactors. One is to place the reforming catalyst adjacent to the wall of the membrane tube,1 the other is to locate the catalyst after the membrane tube.6 These two different configurations may involve different reaction pathways and thus have different requirements on the membrane material and catalyst. In the first configuration, as one side of the membrane is exposed to the mixture of H2 and CO, it would be extremely difficult for membrane materials to maintain stability in such a reducing atmosphere (pO2 ) ∼10-19 atm).7 In the second configuration, as one side of the membrane is exposed to the mixture of CO2 and H2O produced from reaction of methane with permeated oxygen, the corresponding oxygen partial pressure is considerably higher (10-1310-14 atm),6 thus posing less stringent requirements on the stability of the membrane. A reactor comprising SrFeCo0.5Oy (SFC) membrane for syngas production was investigated by Balachandran et al.1 In that study an Rh-based reforming catalyst was packed adjacent to the wall of the membrane tube. In the present work, we used a different reactor configuration in which the Ni/γ-Al2O3 reforming catalyst was placed a few centimeters apart from the membrane tube. This paper is to report the syngas formation with this membrane reactor and examine the evolution of the membrane and catalyst under the reactor conditions. (5) Dyer, P. N.; Richards, R. E.; Russek, S. L.; Taylor, D. M. Solid State Ionics 2000, 134, 21-23. (6) Chen, C. S.; Feng, S. J.; Ran, S.; Zhu, D. C.; Liu, W.; Bouwmeester, H. J. M. Angew. Chem., Int. Ed. 2003, 42, 5196-5198. (7) Hendriksen, P. V.; Larsen, P. H.; Morgensen, M.; Poulsen, F. W.; Wiik, K. Catal. Today 2000, 56, 283-295.
10.1021/ef034048k CCC: $27.50 © 2004 American Chemical Society Published on Web 01/07/2004
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Figure 1. Schematic diagram of a two-stage membrane reactor for conversion of methane to synthesis gas.
Experimental Section Preparation and Characterization of Membrane and Catalyst. The SFC membrane tube was prepared by a solidstate reaction route. Appropriate amounts of SrCO3 (A.R), Fe2O3 (A.R), and Co3O4 (A.R) were mixed by ball-milling for 5 h, and calcined at 850, 1000, and 1150 °C for 12, 10, 10 h, respectively, in air with intermediate balling for 5 h. The calcined powder was admixed with some organic additives, and extruded to form tubes. The tubes were sintered to dense at 1200 °C for 12 h in air. A Ni-based reforming catalyst was prepared by impregnating γ-Al2O3 powder (40-60 mesh) with an appropriate concentration of Ni(NO3)2 solution followed by calcinations at 400 °C in air for 3 h. The resulting catalyst had a nickel loading of 12.5 wt % and a BET-N2 surface area of 144 m2/g. The SFC membrane and catalyst were characterized by X-ray powder diffraction (XRD) (Rigaku D/Max-γA, Japan), using the Ni-filtered Cu KR radiation (λ ) 1.5418 Å). The membrane surface morphology was examined by scanning electron microscopy (SEM) (JEOL-JSM-35C, Japan). Partial Oxidation of Methane and Permeation of Oxygen. The reactor configuration for POM is schematically shown in Figure 1. An SFC membrane tube of inner diameter 0.73 cm and wall thickness 0.09 cm was sealed between two dense alumina tubes using 2 mm thick Pyrex glass sealant rings at 950 °C and then cooled to 900 °C and maintained at that temperature; to ensure the gas-tight seals between the tubes a spring load was applied vertically (not shown in Figure 1 for the sake of simplicity). To improve the flow pattern of the reactor, an alumina cylinder was placed inside the membrane tube (also not shown in Figure 1 for the sake of illustration). A bed of Ni/γ-Al2O3 reforming catalyst (200 mg) was placed 3.5 cm after the membrane tube. The catalyst reduction was performed in situ at 900 °C by feeding a mixture of 10%mol H2/He stream at a rate of 50 mL/min for 2 h. POM was conducted by feeding methane into the tube and exposing the shell of the tube to ambient air. The pressure in the tube is ∼0.03 atm higher than that at the shell of the tube (1.00 atm). The composition of the effluents was analyzed with an on-line gas chromatography (Varian 3400).8
Results Generation of Syngas. A syngas generation experiment using helium-diluted methane as a reactant was performed with a reactor comprising an SFC membrane tube of inner wall surface area 4.3 cm2 at 900 °C, and results are summarized in Figure 2. When CH4 was fed into the reactor at a rate of 9 mL(STP)/min along with a diluting helium at a rate of 100 mL/min, the through(8) Zhu, D. C.; Xu, X. Y.; Feng, S. J.; Liu, W.; Chen, C. S. Catal. Today 2003, 82, 151-156.
Figure 2. Performance of an SFC membrane reactor using diluted methane reactant. Temperature: 900 °C, pressure: 1 atm, methane flow rate: 9 mL/min‚min-1, helium flow rate: 100 mL/min, membrane area (inner wall): 4.3 cm2, catalyst Ni/γ-Al2O3: 200 mg.
Figure 3. Performance of an SFC membrane reactor using concentrated methane reactant. Temperature: 900 °C, pressure: 1 atm, membrane area: 4.6 cm2, catalyst Ni/γ-Al2O3: 200 mg.
put conversion of methane was ∼92%, CO selectivity ∼95%, H2/CO ratio ∼1.8; the equivalent oxygen permeation flux was ∼0.9 mL/min/cm2 membrane surface area. After operating for ∼730 h, CO selectivity dropped to 88%, indicating the deactivation of the catalyst. The performance of this reactor is generally comparable with that using Ar-diluted methane as a reactant.1 To increase the rate of syngas formation, an experiment using concentrated methane as a reactant was performed in a reactor of membrane surface area 4.6 cm2 at 900 °C, and results are shown in Figure 3. When the methane flow rate was jumped to a higher value and kept constant after that, the conversion of methane first decreased, then increased gradually along with an increase in oxygen permeation flux, but CO selectivity remained almost unchanged. For example, by changing the methane flow rate from 23.3 to 26.8 mL/min, within ∼30 h the reactor approached a desirable state with throughput conversion of methane of ∼98%, CO selectivity ∼98%, H2/CO ∼1.8, syngas generation rate ∼16 mL/min/cm2; the equivalent oxygen permeation flux was 3.5 mL/min/cm2, almost four times the value obtained from the experiment using the diluted methane reactant. It was observed that the membrane reactor took a long time to be activated and reach a steady state in
Synthesis Gas Production from Methane
Figure 4. Oxygen permeation flux through an SFC membrane tube of surface area 5.6 cm2 in an air/methane gradient at 900 °C.
the case of using concentrated methane feed. And it was found that the required activation time could be reduced significantly in the presence of syngas, which will be examined in more detail below. Oxygen Permeation. Syngas generation in the twostage membrane reactor proceeds via the a sequence of permeation of oxygen through the membrane, total oxidation of methane to CO2 and H2O with permeated oxygen, and reforming of methane to syngas over the catalyst bed.6 To have a better understanding of the processes at the membrane stage, experiments were conducted in a membrane reactor in which the Ni/γAl2O3 reforming catalyst was replaced with quartz chip. In this case, CO2 and H2O were the predominant species in the effluent, and neither H2 nor CO was detectable. As the permeation of oxygen through the membrane is coupled to the total oxidation of methane, the oxygen permeation flux should be affected by the amount of methane fed into the reactor. Figure 4 shows that the oxygen flux increases with increasing methane flow rate. The other important feature observed is the gradual increase of oxygen permeation flux at the fixed methane flow rate. For example, when the methane rate was set at 42 mL/min, the oxygen permeation flux increased from 1.5 to 3.3 mL/min/cm2 over a period of ∼100 h, but still did not reach the steady state. The gradual increase in oxygen flux at the fixed methane flow rate is likely the manifestation of the change in membrane structure/composition. The oxygen flux was found to increase much faster when methane and syngas were co-fed into the membrane tube (Figure 5). For instance, it took only ∼15 h for oxygen permeation flux to reach a value of 2.4 mL/ min/cm2 in the case of co-feeding of methane and syngas, but at least 100 h would be required in the absence of syngas. It can also been seen from Figure 5 that even after switching off the syngas, the membrane still retained the high oxygen flux. It is likely that the highly reducing nature of syngas facilitates irreversible changes of the membrane. Characterization of SFC Membrane and Catalyst. Figure 6 shows the XRD patterns of the SFC membrane before and after membrane reactor experiments. The pre-experimental SFC membrane consists of three phases: layered Sr4(Fe,Co)6O13, perovskite
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Figure 5. Oxygen permeation flux through an SFC membrane tube of surface area 4.7 cm2 in an air/methane and syngas gradient at 900 °C.
Figure 6. XRD patterns of SFC membrane (a) pre-experimental membrane; (b) shell of used membrane tube exposed to air; (c) inner wall of the used membrane exposed to methane side. “L” in (a), “P” and “S” in (c) refer to layered, perovskite, and spinel phase, respectively.
SrFe1-xCoxO3-δ, and [(CoFe)]2CoO4 spinel. When the membrane is subjected to an air/methane gradient at 900 °C, the amounts of perovskite and spinel phase increase at the expense of the layered phase. Such a change in phase composition is most evident in the region of the membrane in contact with methane, where the perovkite phase has become a dominant component whereas the layered phase now is hardly detectable. Figure 7 shows the surface morphologies of the membrane tube before and after the reactor experiment. The pre-experimental membrane contains mainly plateshaped grains corresponding to a layered phase Sr4(Fe,Co)6O13.9 After being exposed to an air/methane gradient, the inner wall of the membrane tube in contact with methane becomes porous and the plate-shaped grains hardly observable; the outer wall remains dense, but the number of the plate-shaped grains decrease. These changes in surface morphology give evidence of (9) Kim, S.; Yang, Y. L.; Christoffersen, R.; Jacobson, A. J. Solid State Ionics 1998, 109, 187-196.
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Figure 7. SEM Photographs of surfaces of (a) pre-experimental SFC membrane, (b) at the air side and (c) at the methane side of the membrane tube after being exposed to an air/methane gradient for 400 h at 900 °C.
decomposition/transformation of the membrane under the reactor conditions. XRD analysis of the catalyst in a reactor run at 900 °C for 1000 h using a diluted methane feed reveals that the part of catalyst support Al2O3 have transformed from γ- to R-phase. For a catalyst in a membrane reactor operated at the same temperature for 400 h using a concentrated methane feed, the metallic nickel remains a major phase while NiO is a minor phase. Discussion Evolution of SFC Membrane under Reactor Conditions. Under the reactor conditions, the membrane is subjected to a large chemical gradient and high temperatures; obviously, the membrane is not in the equilibrium state, and changes in phase composition and structure of the membrane are expected. It is known from the present study that in an air/methane or air/ methane and syngas gradient, the Sr4(Fe,Co)6O13 layered phase decomposes to the perovskite phase SrFe1-xCoxO3-δ and spinel phase [(CoFe)]2CoO4. The decomposition of the layered phase in air at elevated temperatures has been reported in the literature,9,10 but the extent of the decomposition in their cases is much less than that in our case under the reactor conditions. The decomposition of the layered phase of the membrane into the perovskite oxide can account for the gradual increase in oxygen permeation flux and the activation process of the reactor. The perovskite oxide is known to possess much greater oxygen permeability than the layered parent phase. Armstrong et al. shows that the oxygen permeation rate of the perovskite phase is about 1 order of magnitude higher than that of the layered phase.11 The other decomposition product, spinel oxide Co3O4, is believed to play an essential role in converting methane to syngas with the membrane reactor. This is because that syngas production in the membrane reactor involves a key step, oxidation of methane with the permeated oxygen, and the cobalt spinel oxide is known to be a good catalyst for methane oxidation.12 Moreover, as a consequence of the promoted reaction of oxygen with methane, the oxygen at the permeated side of the (10) Bredesen, R.; Norby, T. Solid State Ionics 2000, 129, 285-297. (11) Armstrong, T.; Prado, F.; Xia, Y.; Manthiram, A. J. Electrochem. Soc. 2000, 147, 435-438. (12) Milt, V. G.; Ulla, M. A.; Lombardo, E. A. Catal. Lett. 2000, 65, 67-73.
membrane is depleted, thus providing an even greater driving force for oxygen permeation. Durability of Reforming Catalyst. Partial oxidation of methane to syngas can be realized with a fixedbed reactor in which CH4 and O2 are co-fed, or, with a membrane reactor in which oxygen is supplied through oxygen-permeable membrane. In the fixed-bed reactor, the performance of the Ni/γ-Al2O3 catalyst has been found to deteriorate rapidly. It was reported that Ni/γAl2O3 catalyst lost catalytic activity after operating for ∼40 h at 750 °C with dilute methane feed.13 But, in the present membrane reactor, the Ni/γ-Al2O3 catalyst not only functioned well over a much longer period (∼730 h) with a diluted methane reactant, but also maintained satisfactory catalytic activity over a long period of time in the case of using concentrated methane reactant. The durability of the catalyst is likely related to the configuration of the reactor and the way oxygen is supplied. In the fixed-bed reactor, in which CH4 and O2 are cofed, the oxidation of methane over Ni-based catalyst generates “hot spots” in the catalyst bed, which brings about adverse changes to the catalyst including the agglomeration of Ni particles, formation of NiAl2O4 and deposition of carbon arising from the pyrolysis of methane. But in the two-stage membrane reactor, the “hot spots” should be not present over the Ni catalyst, because the reforming reaction over the catalyst is endothermic, and the catalyst bed is spatially separated from the exothermic reaction of methane with the permeated oxygen on the membrane surface. In methane reforming, the deposition of carbon has been considered as a key factor for deactivation of the catalyst. And high H2O/methane ratio has been used in commercialized steam reforming process to reduce the carbon deposition, the underlying mechanism being assumed as
2H2O + C f CO2 + 2H2 This decarbonation mechanism may also exist in the present membrane reactor, for the steam is the dominant reforming agent. Conclusions (a) Conversion of methane to syngas can be realized by a two-stage reactor comprising a dense SrFeCo0.5Oy (13) Tsipouriari, V. A.; Zhang, Z.; Verykios, X. E. J. Catal. 1998, 179, 283-291.
Synthesis Gas Production from Methane
membrane tube and Ni/γ-Al2O3 catalyst bed. The conversion of methane proceeds via an oxidation-reforming mechanism. The reactor using the concentrated methane reactant can function well with methane throughput conversion 98% and CO selectivity 98% and H2/CO ratio 1.8. (b) The SrFeCo0.5Oy membrane undergoes a decomposition from the layered phase to perovskite SrFe1-xCoxO3-δ and spinel [(CoFe)]2CoO4 under the reactor conditions. The as-formed perovskite SrFe1-xCoxO3-δ exhibits high oxygen permeability, and
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the spinel [(CoFe)]2CoO4 shows desired catalytic activity toward the total oxidation of methane. (c) The Ni-based catalyst in the two-stage membrane reactor configuration can maintain desirable activity over a long period of time, probably due to the absence of “hot spots” in the catalyst bed. Acknowledgment. Financial support from National Natural Science Foundation of China [50225208] is gratefully acknowledged. EF034048K
