CFD Method To Couple Three-Dimensional Transport and Reaction inside Catalyst Particles to the Fixed Bed Flow Field Anthony G. Dixon,* ,† M. Ertan Taskin, †,§ Michiel Nijemeisland, ‡ and E. Hugh Stitt ‡ Department of Chemical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, Massachusetts 01609-2280, and Johnson Matthey, P.O. Box 1, Belasis AVenue, Billingham, CleVeland TS23 1LB, United Kingdom A new method is presented to couple the fluid flow in a fixed bed to the transport and reaction inside a catalyst particle, using computational fluid dynamics (CFD). The particle is modeled as solid, allowing no- slip surface flow boundary conditions to be used. Species transport inside the particle is represented by user- defined scalars, and the catalytic reactions are represented by user-defined functions. The new method is validated using standard cases for which exact results are known. Previous work has used a porous representation of the catalyst particle, which results in inaccurate temperature and species profiles due to an artifact of convective flux across the particle-fluid interface. This also gives incorrect values of the particle- to-fluid heat transfer coefficient, compared to standard correlations. Simulation results are presented for methane steam reforming using spherical particles in a wall segment, under tube inlet and midtube conditions, to illustrate the solid particle method. 1. Introduction Fixed bed reactors are commonly used throughout the chemical processing industry. Despite their relatively simple arrangement, in which a fluid flows through a tube packed with catalyst particles, it has proven to be no simple matter to obtain reliable estimates of transport parameters and catalyst particle reaction rates in these reactors. One possible reason is the complex flow path for the fluid, due to the random arrangement of the catalyst particles, which are themselves of many different sizes and shapes, according to the needs of the particular process. A second reason is the extremely strong effects of heat transfer in such reactors, necessitating the use of slim tubes to facilitate heating or cooling at the tube wall, combined with relatively large particles to reduce pressure drop, leading to tubes of low tube-to-particle diameter ratio N. In order to obtain more in-depth information about the interactions between flow, diffusion, heat transfer, and chemical reaction in a fixed bed, we wish to couple the three-dimensional flow around catalyst particles, to a three-dimensional description of transport and chemical reaction within the particles. The classical chemical reaction engineering treatments have usually approached this problem under various simplifying conditions, for example, uniform fluid conditions surrounding the particle or a simplified and more computationally tractable flow such as creeping flow. Often, regular pellet shapes are combined with the simplified flow field so that problems of reduced dimen- sionality may be solved due to symmetry. While these ap- proaches are essential to the practical application of reaction engineering models, in our work we wish to examine situations in which no such symmetry exists, so the problem remains three- dimensional and the interactions between conditions inside the catalyst particle and realistic flow fields around it are the focus. The use of computational fluid dynamics (CFD) as a tool for obtaining detailed 3D flow and scalar transport information in chemical reactors is an obvious approach, which has been extensively used for CSTRs, bubble columns, and fluidized beds. For fixed beds it has been slower to be adopted, due to the apparent intractability of the geometry of the packing. Here, we use the term CFD (sometimes the terms “detailed” and “interstitial” CFD are used) in the context of fixed beds to mean simulation of flow in the interstitial space at the particle scale, not the more conventional pseudocontinuum representation of the packed bed, even with spatially distributed velocity fields. Detailed CFD for fixed beds has been reviewed recently. 1 For beds of large tube-to-particle diameter ratio (N) the unit- cell approach with periodic boundary conditions, of a repre- sentative part of the packing, has yielded some interesting insights. 2 For beds of small N, advances in computational power have made it possible to simulate entire beds 3,4 or reproducibly packed segments of the bed, especially near the tube wall. 5 In particular, operation of fixed beds at high particle Re (>1000) and low N (2 < N < 10) are of interest as highly exothermic partial oxidation reactors and highly endothermic steam reform- ing reactors operate under these conditions. Fixed bed CFD studies of pressure drop were among the first to be carried out, for structured packings, 3 and continue to be developed for random packings, often in conjunction with discrete element methods (DEM). 6 Several studies of particle to fluid heat transfer have been made, 7-12 although many of these are at relatively low Re, as well as work on heat transfer between the packing and the tube wall. 5 Dispersion of chemical species is being included in some CFD simulations of flow in full beds 13-16 to obtain better understanding of dispersion coefficients. Some studies 17 using the lattice Boltzmann method (LBM) for reaction on solid surfaces have appeared, but these have been restricted to low Re values and isothermal conditions. Very recently, simulations using a finite volume CFD code have been made, 18 again with surface reactions only. The develop- ment of general CFD fixed bed methods to include heteroge- neous catalytic chemical reactions does not yet appear to have become a routine capability, due to the need to represent chemical species inside the catalyst particles. * To whom correspondence should be addressed. E-mail: agdixon@ wpi.edu. † Worcester Polytechnic Institute. ‡ Johnson Matthey. § Current address: Artificial Organs Laboratory, Department of Surgery, University of Maryland School of Medicine, MSTF-454 10 S. Pine St., Baltimore, MD 21201. Ind. Eng. Chem. Res. 2010, 49, 9012–9025 9012 10.1021/ie100298q  2010 American Chemical Society Published on Web 09/01/2010