(690c) Comparison of Solid and Porous Approaches to the Resolution of Intra-Particle Gradients in CFD Simulations of Steam Reforming

The application of CFD to packed bed reactor modeling, taking into account the actual packing structure, is on the increase. Many studies have been made on fluid flow, heat transfer and dispersion in either low-N packed tubes or small periodic groups of spherical particles, without reaction. The inclusion of chemical reaction has mainly been limited to gas-phase or surface reactions. Simulations which explicitly include intraparticle gradients of temperature and species along with heterogeneously-catalyzed reaction, coupled to realistic external 3D flow, species and temperature fields, are only recently beginning to appear (Dixon et al., Chem. Eng. Sci., 62 (2007) 4963-4966).

Two approaches to the inclusion of heterogeneous reactions in a discrete-particle CFD simulation are described and compared, using a commercial CFD code. In one, the solid particles are taken as porous, which is represented in the CFD code as a fluid region with extra pressure drop terms, so that temperature and species mass fractions are available inside the particles. The bulk flow in the porous medium is zero or greatly suppressed, and the temperatures and species are coupled naturally across the particle-fluid interface. Care must be taken to define the effective transport properties correctly for the porous regions, in particular the effective diffusion coefficients are constrained to equal the fluid diffusivities. In the other approach, the particles are taken as truly solid, so the flow field is zero inside the particles, the effective transport properties for the particles can be set independently, but the species mass fractions are not explicitly available and must be provided by user-defined scalars and coupled by the user to the corresponding variables in the external flow field.

The validation of the solid-particle methodology is demonstrated on a single particle model, addressing issues of effective diffusivity in the particles, change of moles due to reaction, and computing species fluxes at solid surfaces. Comparisons will be shown of the porous region and solid particle approaches, on both single particles and tube segments, under steam reforming conditions. We will demonstrate problems with the porous region approach with regard to the effective diffusivity and the no-slip condition at the particle surface, which lead to incorrect prediction of particle temperature and species profiles and reaction rates. These problems are eliminated in the solid particle approach.