Reactive Pellet Bed

Use this feature to model packed bed reactors with catalytic pellets. For details, see the section Theory for the Reactive Pellet Bed. By default, subnodes for Porous Media Diffusion, Reactions and Initial Values are added.

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 — (the default), , , , or 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 εpe (dimensionless) to specify the porosity of the pellet. If a size distribution is selected, the porosity and volume fraction of each size are required as input. The bed porosity is taken from the corresponding Porous Matrix feature, where the porosity can also be defined on the base of densities of bed and pellet.

Note that different chemical reactions can be specified for each pellet size if a distribution is specified.

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

•

, assuming that all resistance to mass transfer to/from the pellet is within the pellet and no resistance to pellet-fluid mass transfer is on the bulk fluid side. The concentration in the fluid will thus be equal to that in the pellet pore just at the pellet surface: cpe,i = ci. This constraint also automatically ensures flux continuity between the internal pellet domain and the free fluid domain through so-called reaction forces in the finite element formulation.

•

: The flux of mass across the pellet-fluid interface into the pellet is possibly rate determined on the bulk fluid side, by film resistance. The resistance is expressed in terms of a film mass transfer coefficient, hDi, such that:

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

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

The can be computed from three available expressions: , , and . 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 and . They can be taken or choose the alternative. The expressions of density and dynamic viscosity from Chemistry could be available for both and

In order to add surface species, click the button and enter the species name in the table. Added surface species are available inside all pellet types defined in the section, but not in the bulk fluid.

For each pellet type, specify the , Sb,reac (SI unit: 1/m), corresponding to the surface area, per volume, available for surface reactions.

The extra dimension in the pellet needs to be discretized into elements. Select a — (the default), , or . Enter the Nelem.

To display this section, click the button (

) and select . See the details about the different constraint settings in the section Constraint Reaction Terms in the COMSOL Multiphysics Reference Manual.