(578f) Detailed Catalytic Fixed-Bed Reactor Simulations: The Dry Reforming of Methane

The increasing greenhouse gas concentrations have risen global concern about current technological practices. Hence, chemical engineers are interested in processes including and converting such gases. The dry reforming of methane (DRM) represents such process where the greenhouse gases methaneand carbon dioxidereact to syngas, which can be used for example as a feedstock for liquid fuels. DRM has to be carried out with an appropriate catalyst, mostly nickel or noble metals. However, coke formation is the major obstacle of this promising process. Hence, transient surveys of DRM reactor configurations form the basis of reactor dimensioning.

Still today, fixed-bed reactors are the most common device for catalytic reactions. Highly endothermic or exothermic reactions are performed in reactors with small tube-to-particle-diameter ratios N, since heat transfer, high gas velocities and reasonable pressure drops are engineering issues. In all cases randomly packed beds are characterized by inhomogeneous structures. Especially for small N the inhomogeneities become dominant resulting in significant wall effects, local back flows and large axial as well as radial gradients. Consequently, conventional descriptions, based on plug flow and pseudo-homogeneous kinetics, are questionable for these fixed-bed configurations. The strong interplay between velocity, temperature and species distribution makes the fixed-bed reactor a very interesting and likewise challenging device for chemical engineers. This reactor includes several time and length scales and calls for a multiscale modeling. As a consequence, an adequate description of catalytic fixed-beds should include the modeling with full computational fluid dynamics (CFD) in combination with detailed chemical reaction models (also called micro kinetics).

In this work we investigated spatially resolved heterogeneous catalysis of the dry reforming of methane in a fixed-bed reactor by combining full CFD simulations with a detailed reaction mechanism. A catalytic fixed-bed reactor with spherical and non-spherical solid particles and a small tube-to-particle-diameter ratio was numerically simulated. The randomly packed bed was generated using the discrete element method (DEM), described in [1]. A detailed reaction mechanism for the DRM on rhodium is implemented involving 42 irreversible reactions with 12 surface-adsorbed species and 6 gas phase species [2]. Additionally, conduction through the particles was applied. We combined several process parameters, i.e. temperature, inlet velocity and feed composition. With the help of the detailed reaction mechanism regions can be detected where coking takes place.

With this study the strong interplay between catalytic reactions and the surrounding flow in fixed-bed reactors was demonstrated. Furthermore, the feasibility was shown to model catalytic fixed-bed reactors in an adequate multiscale way. Consequentially, resolved simulations can contribute to a better understanding and therefore to a better choice of multiscale chemical reactors. Finally, it is an example how to minimize the dependency on empiricism for the calculation of multiscale reaction devices.

References:

[1] T. Eppinger, K. Seidler & M. Kraume (2011) Chemical Engineering Journal, 166, 324 –331

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