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 Reactions and are added.


	This feature is only available in a limited set of add-on products. See https://www.comsol.com/products/specifications/ for more details on availability.

Bed Parameters

Here you can specify the bed porosity, which is the void fraction in the packed bed structure. Select to calculate the porosity from the bed density and the individual pellet density. Select to specify the porosity directly.

Pellet Shape and Size

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. If a size distribution is selected, the volume percentage of each size is required as input.

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

Surface Species

In order to add surface species, click the button and enter the species name in the table. Added surface species are be 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.

Pellet Parameters

Enter a εpe (dimensionless) to specify the porosity of the pellet internals.

Select — (the default), , , or to describe the effective correction of the diffusion coefficient in the pellet. In the case of the , a value for the tortuosity τpe within the pellet is required.

Enter also the Dpe,c (SI unit: m2/s). If a diffusion model is selected, an Dpeff,c (SI unit: m2/s) is entered. The default value is 1·10-9 m2/s in both cases.

Pellet-Fluid Surface

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, Ni,inward = hD,i(ci − cpe,i). 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 . Select these to be taken either or choose the alternative.

Pellet Discretization

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

Constraint Settings

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.