Spatially Resolved Simulations of Heterogeneous Dry Reforming of Methane in Fixed-Bed Reactors

Over the last decades the concentration of “greenhouse” gases has increased dramatically. Besides methane and carbon dioxide disposal and removal a major global interest lies in processes in which these gases react to syngas. The catalytic dry reforming of methane (DRM) is such a process operating typically at 700-1000 °C. Nevertheless, carbon deposition at the catalytic surface is a great drawback [1]. Hence, transient surveys of DRM catalysts, mostly Ni or noble metals, form the basis of reactor dimensioning.

            Detailed fluid dynamic simulations accompany more often and cheaper the design and calculation of chemical reactors. Fixed-bed reactors with small tube-to-particle-diameters are often applied for extremely exothermic or endothermic reactions. In such reactors simplified fluid dynamic models fail due to the strong interplay between chemical kinetics and transport of momentum, heat and mass. Consequentially, the model has to account for spatially resolution and detailed reaction mechanisms.

            In this work we investigated the specially resolved heterogeneous catalysis of the DRM in terms of CFD simulations. A catalytic spherical fixed-bed reactor is simulated with a small tube-to-particle-diameter. The randomly packed bed (D = 16.2 mm, d_P = 4 mm, H = 40 mm) was generated using the discrete element method (DEM) [2], which consists of 113 mono-disperse spheres. A detailed reaction mechanism is implemented involving 42 irreversible reactions with 12 surface-adsorbed species and 6 gas phase species [3]. Additionally, conduction through the spheres was applied. We combined several process parameters, i.e. temperature, inlet velocity and feed composition, to achieve a better understanding of the characteristics of DRM in fixed-bed reactors.

            Not only axial but also strong radial temperature profiles were observed. As a consequence, axial as well as radial concentration profiles occur, respectively. Additionally, carbon deposition can also be noticed, see Figure 1. This highlights the strong temperature dependence of the DRM catalysis and the interplay between chemistry and fluid dynamics. The applied investigation of spatially resolved simulations of fixed-beds allows a deeper understanding of this highly transient and multi-dimensional process.

Figure
1: Spatially resolved simulation of the dry reforming of methane in a small fixed bed

[1]        M. C. J. Bradford, M. A. Vannice, Catalysis Reviews, 1999, 41, 1-42

[2]        T. Eppinger, K. Seidler, M. Kraume, Chemical Engineering Journal, 2011, 166,    324 – 331

[3]        N. E. McGuire, N. P. Sullivan, O. Deutschmann, H. Zhu, R. J. Kee, Applied           Catalysis A: General, 2011, 394, 257 - 265