A Numerical Optimization Study on the Catalytic Dry Reforming of Methane in a Spatially Resolved Fixed-Bed Reactor
Thomas Eppinger1, Nico Jurtz1,Gregor D. Wehinger2, Matthias Kraume2, Ravindra Aglave3
1 CD-adapco, Nuremberg, Germany
2 CD-adapco, Houston,TX
3 Chair of Chemical and Process Engineering, Technische Universität Berlin, Germany Introduction
Fixed bed or packed bed reactors are widely used in the chemical and process industry amongst others for highly exothermic or endothermic catalytic surface reactions. A small tube to particle diameter ratio (D/d-ratio) facilitates a safe thermal performance of these reactors. Many correlations reported in literature for pressure drop and heat transfer are limited in their predictive capability for such packed bed reactors due to a dominant influence of the confining wall. It affects the porosity distribution and as a result the velocity field as well as the species and temperature distribution within the bed. Accurate measurements for development of new correlations can be cumbersome, expensive and many times inaccurate due to the complexity of the geometry. The increasing power of computational hardware in the last decade has enabled virtual experiments: Computational Fluid Dynamics (CFD) can now be used for three dimensional spatially resolved investigations of those reactor types. Aim of this contribution
The current contribution is such an investigation of the catalytic dry reforming of methane (DRM) in a packed bed reactor based on the detailed reaction mechanism proposed by [McGuire et al., 2011]. It will be shown how a fully automated workflow for a three dimensional and spatially resolved packed bed simulation with rigorous optimization methods can lead to a significant increase in terms of maximum yield and space-time yield. Methods
Discrete Element Method (DEM) is used for the generation of a randomly packed bed. A finite volume (FV) code is used for the calculation of the fluid flow, temperature and species field within the packed bed. To avoid the well-known meshing problems near the particle-particle and particle-wall contacts, a local wall flattening approach is used.
These methods have been validated in terms of packing structure, pressure drop, velocity and temperature profiles ([Eppinger et. al., 2011] and [Zobel et. al., 2012]) and were recently used to investigate the effect of different particle shapes on the performance of a packed bed reactor ([Wehinger et al., 2015]).
In this study a detailed surface reaction mechanism [McGuire et al., 2011] is used and solved by DARS-CFD, a code that is developed to solve stiff differential equation systems efficiently. For the optimization of operating parameters the SHERPA algorithm of HEEDS by Red Cedar Technology is used. SHERPA is a hybrid, adaptive optimization algorithm that blends global and local optimization methods to provide the most efficient route to an optimized design. The whole analysis is performed using STAR-CCM+ by CD-adapco which allows an out of the box coupling of the FV code with DARS (using DARS-CFD) and HEEDS (using the Optimate/Optimate+ plug in). Results
The sensitivity of the yield and space-time to parameters like residence time, inlet and wall temperature is calculated. The working parameters are optimized to maximize the yield while taking the constraints such as maximum local temperature into account. The results of the spatially resolved simulation will finally be compared with results of a pseudo-homogenous (unresolved in 3D) simulation and published experimental data. References
N. E. McGuire, N. P. Sullivan, O. Deutschmann, H. Zhu, R. J. Kee Dry reforming of methane in a stagnation-flow reactor using Rh supported on strontium-substituted hexaaluminate, Applied Catalysis A: General. 2011, 257-265
T. Eppinger, K. Seidler, M. Kraume. DEM-CFD simulations of fixed bed reactors with small tube to particle diameter ratios. Chemical Engineering Journal 2011, 166(1): 324-331.
G. D. Wehinger, T. Eppinger, M. Kraume. Evaluating Catalytic Fixed-Bed Reactors for Dry Reforming of Methane with Detailed CFD, Chem. Ing. Tech., in press
N. Zobel, T. Eppinger, F. Behrendt, M. Kraume. Influence of the wall structure on the void fraction distribution in packed beds. Chemical Engineering Science 2012, 212–219.