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Ind. Eng. Chem. Res. 1997, 36, 5180-5188
Kinetic and Reaction Engineering Studies of Dry Reforming of Methane over a Ni/La/Al2O3 Catalyst Unni Olsbye,*,† Thomas Wurzel,‡ and Leslaw Mleczko‡ Sintef Applied Chemistry, P.O. Box 124 Blindern, N-0314 Oslo, Norway, and Lehrstuhl fu¨ r Technische Chemie, Ruhr-Universita¨ t Bochum, D-44780 Bochum, Germany,
Kinetic studies of CO2 reforming of methane over a highly active Ni/La/R-Al2O3 catalyst were performed in an atmospheric microcatalytic fixed-bed reactor. The reaction temperature was varied between 700 and 900 °C, while partial pressures of CO2 and CH4 ranged from 16 to 40 kPa. From these measurements kinetic parameters were determined; the activation energy amounted to 90 kJ/mol. The rate of CO2 reforming was described by applying a LangmuirHinshelwood rate equation. The developed kinetics was interpreted with a two-phase model of a fluidized bed. The predictions for a bubbling-bed reactor operated with an undiluted feed (CH4:CO2 ) 1:1) at 800 °C showed that, on an industrial scale, significantly longer contact times (Hmf ) 7.8 m, mcat/VSTP ) 31.8 g‚s‚mL-1) are necessary for achieving thermodynamic equilibrium (XCH4 ) 88.2%, XCO2 ) 93.6%). The performance of the reactor was strongly influenced by the interphase gas exchange: the highest space time yields were obtained for small particles (dp ) 80 µm). 1. Introduction In the last decade, carbon dioxide reforming of methane (eq 1) has attained wide research interest. The reaction can be applied for producing syngas with a low H2:CO ratio suitable for the synthesis of oxygenated chemicals (Edwards, 1995). The reaction also serves for the production of high-purity CO (Teuner, 1985; Kurz and Teuner, 1990). Moreover, two molecules which contribute significantly to the greenhouse effect are converted into value-added products (Rostrup-Nielsen, 1994). Finally, the reaction has already been studied as a storage for solar energy (Wang et al., 1996; Gadalla and Sommer, 1989). However, until now the title reaction had no commercial application by itself (Edwards and Maitra, 1994), but it is used downstream of the steam-reforming process to reduce the H2:CO ratio (Teuner, 1987).
CH4 + CO2 ) 2CO + 2H2 ∆RH°298 K ) +247.9 kJ/mol (1) Syngas yields near the thermodynamic equilibrium in laboratory-scale fixed-bed reactors (e.g., Vernon et al., 1991; Richardson and Paripatyadar, 1990) were achieved by applying supported metal catalysts (for a review see Wang et al., 1996). The main obstacle with respect to the commercialization of the process is given by the severe catalyst deactivation due to carbon deposition during reaction. This limits an application of fixed-bed reactors for this reaction (Rostrup-Nielsen and Bak Hansen, 1993; Seshan et al., 1994; Swaan et al., 1994; Au et al., 1994). Against this background fluidized-bed reactors have been proposed for performing CO2 reforming of methane and studied (Blom et al., 1994; Mleczko et al., 1997b). The experimental results obtained in the laboratory * Author to whom correspondence is addressed. Telephone: +47 22 06 79 01. Fax: +47 22 06 73 50. E-mail: Unni.Olsbye@ chem.sintef.no. † Sintef Applied Chemistry. ‡ Ruhr-Universita ¨ t Bochum. Telephone: +49 234 700 4102. Fax: +49 234 709 4115. E-mail:
[email protected]. S0888-5885(97)00246-7 CCC: $14.00
scale have confirmed the superior performance of this reactor type with respect to syngas yields and less pronounced catalyst deactivation compared to fixed-bed reactors. With respect to the selection of a suitable catalyst, supported noble metals and Ni/R-Al2O3 were proposed. For application in the fluidized-bed reactor not only catalytic performance but also mechanical stability play an important role. It has previously been shown that La/R-Al2O3 has a superior mechanical strength compared to pure R-Al2O3, which makes it particularly suited for fluidized-bed applications (Blom et al., 1994). It has further been shown that the Ni/ La/R-Al2O3 catalyst used in the present work has a superior lifetime stability compared to Ni/R-Al2O3 when used in the CO2 reforming of methane in a fluidizedbed reactor (Blom et al., 1994; Slagtern et al., 1997). The work reported in this paper is aimed at the elucidation of the potential of an industrial fluidizedbed reactor for the CO2-reforming reaction over the Ni/ La/R-Al2O3 catalyst. As a basis for the simulations, kinetic measurements were performed in a microcatalytic fixed-bed reactor. The reaction engineering kinetics that was developed from this study was combined with a model of a fluidized bed. By means of reaction engineering simulations, the influence of the hydrodynamic conditions on the reactor performance was analyzed and main design parameters of the reactor were determined. 2. Experimental Procedures and Modeling 2.1. Experimental Procedures. Catalyst Preparation. Preparation of 0.15 wt % Ni/1.7 wt % La/Al2O3 was performed by modifying θ-Al2O3 (Condea, Scc-a5/ 90, 45-90 µm) with lanthanum nitrate (Alfa, ultrapure) by using the incipient wetness technique. The carrier was heated to 1350 °C (2 °C/min) in dry air and kept at this temperature for 12 h. The resulting carrier had a BET surface area of 2.5 m2/g. Ni(NO3)2‚6H2O was impregnated, also by using the incipient wetness technique. The catalyst was calcined at 900 °C in air for 12 h. Microcatalytic Reactor and Experimental Procedure. Kinetic experiments were carried out at © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5181 Table 1. Range of Feed Gas Compositions Applied in the Kinetic Experiments standard dp,CH4 dp,CO2 dp,CO dp,H2
pCH4, kPa
pCO2, kPa
pCO, kPa
pH2, kPa
pHe, kPa
40 16-40 40 40 40
40 40 16-40 40 40
0 0 0 0-5 0
0 0 0 0 0-5
20 44-20 44-20 20-15 20-15
atmospheric pressure in a tulip-shaped fixed-bed quartz reactor with inner diameter 8 mm, using the 0.15% Ni/ 1.7% La/Al2O3 catalyst (0.05 g, 45-90 µm) diluted with quartz chips (0.9 g,
