A novel two-stage reactor process for catalytic oxidation of methane to synthesis gas

Studies in Surface Science and Catalysis J.J. Spivey,E. Iglesiaand T.H. Fleisch(Editors) 9 2001 Elsevier Science B.V. All rights reserved.

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A novel two-stage reactor process for catalytic oxidation of methane to synthesis gas Shikong Shen," Zhiyong Pan, Chaoyang Dong, Qiying Jiang, Zhaobin Zhang, Changchun Yu Key Laboratory of Catalysis CNPC, University of Petroleum-Beijing, Beijing 102200, China A novel process for catalytic oxidation of methane to synthesis gas, which consists of two consecutive fixed-bed reactors with oxygen or air separately introduced into the reactors, was investigated. The first reactor, packed with a combustion catalyst such as a perovskite type oxide or a supported noble metal catalysts, is used for catalytic combustion of methane at low initial temperature (350---400 ~ and the second reactor, filled with a partial oxidation catalyst such as a Ni-based catalyst, is used for the partial oxidation of methane to syngas. A portion of methane (ca. 6---8 %) is oxidized to CO2 and H20 in the first reactor, in which the reactants are heated to the temperature required for methane partial oxidation (>700~ The remaining oxygen (ca.75 %) is introduced between the exit of first reactor and the inlet of second reactor. In the second reactor, the exotherrnic partial oxidation of methane and the endothermic reforming reactions of CO2 and H20 provide thermal balance. Adiabatic operation can be achieved in this way. The ratio of methane to oxygen is not within the explosion limit in either reactor. 1. INTRODUCTION Interest in the conversion of natural gas to liquid hydrocarbons (GTL) by Fischer-Tropsch synthesis has grown significantly over the last decade [1]. Significant improvement in this technology is still needed to strengthen its economic competitiveness. Most research and development work has been focused on the syngas production step, which accounts for about 60% of the total investment in conventional processes. Reducing the cost of syngas production would have great beneficial effects on the overall economics of GTL process. Catalytic partial oxidation of methane (CPOM) to syngas is a slightly exothermic, highly selective, and energy efficient process [2]. This process produces syngas directly with a nearideal ratio of hydrogen to carbon monoxide for F-T synthesis. However, CPOM processes have not been practiced commercially. Two of the major engineering problems are the high temperature gradient and the risk of explosion with premixed CH4/O2 mixtures. In fluidized bed reactors the heat transfer is more rapid than in fixed beds because of the back mixing, which ensures a more uniform temperature and a safer operation. A technology for syngas production by contacting methane with limited amounts of steam and oxygen in a catalytic fluidized bed reactor has been developed by Exxon [ 3 ] . Alternative catalyst bed configurations, such as dual-bed or mixed-catalyst bed reactors have been examined by Ma * Corresponding author, fax: +86-10-69744849, E-mail: [email protected]

100 and Trimm [4] for the purpose of determining the possible advantages of different configurations of the combustion (Pt) and reforming (Ni) catalysts. They found that the dualbed system was inferior to the single bed system containing two mixed catalysts. Optimal performance, which is 60-65% conversion of methane with 80-85% selectivity, was obtained when both catalysts were on the same support. In the present work, an alternative approach was investigated. This novel process consists of two consecutive fixed-bed reactors in which oxygen or air is introduced separately into the two reactors. The first reactor, which is packed with a combustion catalyst such as a supported noble metal or perovskite type oxide catalyst, is used for the catalytic combustion of methane at a low initial temperature (350-~400 ~ The second reactor, which is filled with a partial oxidation catalyst such as a Ni-based catalyst, is used for the partial oxidation of methane to syngas. A portion of methane is oxidized to CO2 and H20 in the first reactor, in which the reactants are simultaneously heated to the temperature required for methane partial oxidation (>700~ The remaining oxygen is introduced between the exit of first reactor and the inlet of second reactor. In the second reactor, the exothermic partial oxidation of methane and the endothermic reforming reactions of CO2 and H20 provide thermal balance. In this way, adiabatic operation can be achieved. The ratio of methane to oxygen is not within the explosion limit in either reactor. In this paper, the effects of reaction conditions on the catalytic oxidation of methane to syngas in the two-stage reactor are investigated. The experimental results show that catalytic oxidation of methane to syngas can be accomplished using two consecutive fixed-bed reactors and feeding O2 or air separately. 2. EXPERIMENT

2.1. Catalysts preparation 2.1.1. Catalysts for combustion of methane A supported noble metal/)'-A1203 catalyst (0.4 wt% Pd and 0.2 wt% Pt) was prepared by impregnation of 7-A1203 support (from Condea, 7-A1203 spheres of 2.Smm diameter, 210m/g) with a given amount of mixed aqueous solution of PdC12 and HEPtCI2. Then the catalyst was dried at 80 ~ for 10 hr and calcined at 730 ~ for 10 hr. Lanthanum-based perovskite type oxide catalysts were prepared by the addition of citric acid to a given amount of mixed aqueous solution of the nitrates having a ratio of NO3 /C3H4(OH)(COOH)3 = 1. The resulting solution was slowly evaporated until a vitreous material was obtained, subsequently decomposed at 300 ~ for 2 hr, and then calcined at 800 ~ for 5 hr. Finally the catalyst was ground, pressed into pellets, and crushed to 10-20 mesh particles. 2.1.2. Catalysts for catalytic oxidation of methane A La203 promoted 7 wt% Ni/MgAI204-AI203 catalyst (atomic ratio of Ni/La/Mg =100/63/63.) was prepared by a two-step impregnation process according to [4]. First, the ~/A1203 support (from Condea, )'-A1203 spheres of 2.Smm diameter, 210m2/g) was impregnated with an aqueous solution of magnesium nitrate, dried at 360 and calcined at 900 ~ for 10 hr to form MgA1204 spinel compound on the surface of 7-A1203; second, the MgA1204-A1203 was impregnated with a mixed aqueous solution of nickel nitrate and lanthanum nitrate, then dried at 90 ~ and calcined at 700 ~ for 6 hr.

101 2.2. Reaction system The schematic of two-stage reactor is shown in Fig.1. Both of the tubular reactors are made of HK-40 stainless steel with a 380 mm long ceramic lining of 20 mm inner diameter, in which a thermocouple is placed to measure the temperature in the center of reactor. The front combustion reactor is filled with 5g La-based perovskite oxide catalyst, while the rear partial oxidation reactor is filled with 5g La203-Ni/MgA1204-A1203 catalysts. The combustion catalyst bed is diluted with l mm ceramic spheres (in double volumes) to improve thermal conductivity. The feed and products were analyzed on an on-line gas chromatograph equipped with a 1M Porapak N column and a 1.5M 5A Molecular Sieve column using TCD detector.

3. RESULTS AND DISCUSSION 3.1. Evaluation of catalysts for combustion of methane under lean oxygen condition To keep the adiabatic temperature rise in the combustion reactor below 800~ the evaluation of catalyst was carried out under lean oxygen condition (the ratios of CH4 to 02 = 8" 1--6"1). Table 1 shows the results for various catalysts for the combustion of methane. The ignition temperature for the perovskite catalyst is higher than that for the Pd-Pt/7-A1203 catalyst. Although the Pd-Pt/•-A1203 catalyst exhibits higher activity, some partial oxidation products, such as CO and H2, in addition to CO2 and H20, were formed due a to lack of oxygen. To test the catalytic performance of the catalyst at high temperature and lean oxygen concentrations, the catalyst was also tested at 700 ~ The results in Table 1 show that a large amount of CO and H2 is formed over the Pd-Pt/7-A1203 catalyst at a reaction temperature of 700 ~ while only a small amount of CO is formed over perovskite catalysts under the same reaction conditions. It is well know that supported Pd, Pt catalysts have high activity for partial oxidation of methane, while perovskite type oxides catalysts have high activity for deep oxidation of methane. Therefore the La07Cao.3Fe0.3Mn0.703 catalyst was selected for combustion of methane. The experiment data in Fig.2 show that when the reaction pressure is raised to over 1.6 MPa, CO disappears in the products over La0.7Cao.3Feo.3Mno.703 catalyst, which implies that high pressure favors the deep oxidation of methane. The test of 100 hr

Table 1. Evaluation results of various catalysts for combustion of methane*. Catalysts

Pd-Pt/7-A1203 Pd-Pt/7-AI203 La0.7Ca0.3MnO3 La0.7Ca0.3MnO3 La0.7Ca0.3Fe0.3Mn0.703 La0.7Ca0.3Fe0.3Mn0.703

Ignition Temp. ~

Conv. of CH4, %

CO2

325 (477)** 700 (750) 360 (522) 700 (817) 360 (521) 700(820)

6.68 10.68 6.25 6.43 6.25 6.39

82 34 100 89 100 93

Selectivity, % H20 CO 98 43 100 100 100 100

*Reaction condition: CH4/02 =8, GHSV-30,000 hr l and 0.1 M Pa. ** In bracket shows highest temperature of the catalyst bed.

18 66 0 11 0 7

H2 2 57 0 0 0 0

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Fig. 1. Schematic of the two-stage Fig.2.Effect of reaction pressure on the combustion of CH4 at 700 ~ and GHSV=3 9105 h ] on stream at 700 ~ GHSV=30000/h, CH4/O2=8 and 0.1 MPa show that the activity of La0.7Cao.3Feo.3Mno.703 catalyst is stable. The carbon deposit on the used catalyst is 0.8 wt% after 100 hr run. These results show that the La0.7Ca0.3Fe0.3Mno.703 catalyst is more suitable for the complete oxidation of methane at high temperature and lean oxygen conditions than other catalysts. 3.2. Evaluation of catalysts for partial oxidation of methane to syngas

The La203 promoted 7 wt% Ni/MgA1204-AI203 catalyst prepared in our laboratory has excellent activity for the partial oxidation of methane to syngas [5]. The catalyst support, A1203, reacts with the MgO at high temperature forming MgA1204 spinel compound on the surface of A1203, which hinders the formation of NiA1204 at the POM reaction conditions and favors the dispersion of Ni on the catalyst surface. The NiAlzO4 spinel is known to be inactive for the partial oxidation of methane, and is very difficult to reduce into active metal Ni.

40.0

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Time on stream (h) Fig. 4. The Stability of La203-Ni/MgAI204 for partial oxidation of methane The effect of La promoter on carbon deposition has been investigated using an isotopic switch method. Fig. 3 shows that the addition of La decreases the activity of Ni-based catalysts for CH4 decomposition and CO disproportionation. Carbon deposition is probably caused by the accumulation of adsorbed carbon atoms from these reactions, so La203promoted Ni/A1203 exhibits resistance to carbon deposition. The stability test in Fig. 4 shows that the catalyst activity is stable during the 100 hr on stream at 700 ~ 0.1 MPa, GHSV=70000/h and CH4/O2=2. The carbon deposited on the used catalyst after 100 run is 1.1wt%. The results of BET and XRD show that the catalyst maintains structural stability, The result of X-ray fluorescence indicates that Ni is not lost in the reaction. 3.3. Effects of reaction conditions on two-stage oxidation of methane to syngas

5g La0.TCa0.3Fe0.3Mn0.703 perovskite catalyst and 5g Ni/La203/MgA1204-A1203 catalyst were loaded in the first and second reactor respectively. Because partial oxidation with air to produce synthesis gas for F-T process without a recycle loop reduces capital and

(a). Conversionof CH4 vs. temperature and pressure.

(b). Selectivityof CO vs. temperature and pressure.

104

(c). Selectivityof H2 vs. temperatureand pressure. Fig. 5. Effects of temperature and pressure on two-stage oxidation of methane to syngas production costs of GTL [6], air was used as the oxygen resource in these experiments. The reaction conditions of the first reactor were fixed at air/CH4 =5/8 and the furnace temperature was 400~ The 02 to CI-I4 ratio in the feed gas reaches 0.5 by supplying 75 % air to the second reactor. Fig. 5 shows the effects of furnace temperature and pressure on the results of two-stage oxidation of methane to syngas at GHSV=10,000 h l. The experimental data are quite close to the thermodynamic equilibrium. The temperature difference between the furnace and the center of second reactor is much lower than that in the single fixed bed reactor. Methane conversion of 90% and CO and H2 selectivity of 94% are obtained at 880 ~ and 1.0 MPa, while the temperature difference between the furnace and center of second reactor is less than 30 ~ Further increase of GHSV will lead to a decrease in methane conversion and selectivity to CO and H2. The probable reason is the slow reaction rate of steam and CO2 with methane. In summary, the two-stage reactor reduces the possibility of and magnitude of hot spots created during catalytic oxidation of methane, while allowing safer operation. ACKNOWLEDGEMENTS Financial support by the China National Petroleum Corporation is greatly acknowledged. REFERENCES

1. 2. 3. 4. 5. 6.

M.J. Corke, Oil & Gas Journal, Sep. 21, 1998, 71. S.S. Bharadwaj, L. D. Schmidt, Fuel Processing Technol., 42 (1995) 109. B. Eisenberg, R. A. Fiato, T. G. Kaufman, R. F. Bauman, Chemtech, Oct. 1999, 32. L. Ma and D. L. Trimm, Appl. Catal., A 138 (1996) 265. C. Zhang, C. Yu, S. Shen, CUIHUA XUEBAO (Chinese J. Catal.), 21 (2000) 14. A.K. Rhodes, Oil & Gas Journal, Dec. 30, 1996, 85.