Use this feature to model packed bed reactors with catalytic pellets in porous media. For details, see the section Theory for the Reactive Pellet Bed. By default, the following subnodes are added; Pellet Transport Properties, Reaction Sources (Reactive Pellet Bed), and Initial Values.

The default shape is spherical. Cylinders, flakes, and user-defined shapes can also be selected. A uniform pellet size or a discrete size distribution can be selected. Select a Pellet size distribution — Uniform size (the default), Two sizes, Three sizes, Four sizes, or Five sizes to select up to five different particle sizes.

Depending on the shape selection, equivalent radii or volumes and surface areas will be required as input. Enter a Porosity εpe (dimensionless) to specify the porosity of the pellet. Note that the bed porosity is taken from the corresponding Porous Media Transport Properties feature, where the porosity can also be defined on the base of densities of bed and pellet. Select Pellet temperature Tpe — From physics (the default). The temperature is required to calculate the density to solve the pressure inside the pellet. If a size distribution is selected, the porosity, volume fraction and temperature of each size are required as input. The temperature for all pellets are supposed to be the same when selecting All pellets from list of Individual or all pellets.

Note that if a distribution is specified, the Fluid density, Dynamic viscosity and Permeability in subnode Pellet Transport Properties need to be specified for each pellet, and the same to Reaction Rates in subnode Reaction Sources.

For the coupling of concentration between the pellet internals and the surrounding fluid, two Coupling type options are available:

The Film resistance (mass flux) option computes the inward surface flux, hDi is the mass transfer coefficient (SI unit: m/s) and is calculated with the default Automatic setting from a dimensionless Sherwood number expression or with User defined mass transfer coefficients.

The Active specific surface area (SI unit: m-1) is required to couple the mass transfer between the pellets and the bed fluid. Select either the Automatic setting that calculates the specific surface area from the shape information given above. User defined is also available for explicit surface area specification.

The Sherwood number expression can be computed from three available expressions: Frössling, Rosner, and Garner and Keey. The Frössling equation is the default and probably the most commonly used for packed spheres. All of these are based on the dimensionless Reynolds, Re, and Schmidt, Sc, numbers, which are computed from Density and Dynamic viscosity. The Density can be taken from From material, Density (tcs) (from physics) and User defined, while Dynamic viscosity can be taken either From material or choose the User defined alternative. The expressions of density and dynamic viscosity from Chemistry could be available for both Density and Dynamic viscosity.

The extra dimension in the pellet needs to be discretized into elements. Select a Distribution — Cubic root sequence (the default), Linear, or Square root sequence. Enter the Number of elements Nelem.

To display this section, click the Show button () and select Advanced Physics Options. See the details about the different constraint settings in the section Constraint Reaction Terms in the COMSOL Multiphysics Reference Manual.