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.
Bed Parameters
Here you can specify the bed porosity, which is the void fraction in the packed bed structure. Select From densities to calculate the porosity from the bed density and the individual pellet density. Select User defined 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 Pellet size distributionUniform 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. 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 Add button and enter the species name in the Surface species table. Added surface species are be available inside all pellet types defined in the Pellet Shape and Size section, but not in the bulk fluid.
For each pellet type, specify the Reactive specific surface area, Sb,reac (SI unit: 1/m), corresponding to the surface area, per volume, available for surface reactions.
Pellet Parameters
Enter a Pellet porosity εpe (dimensionless) to specify the porosity of the pellet internals.
Select Diffusion modelMillington and Quirk model (the default), Bruggeman model, Tortuosity model, or User defined to describe the effective correction of the diffusion coefficient in the pellet. In the case of the Tortuosity model, a value for the tortuosity τpe within the pellet is required.
Enter also the Diffusion coefficient Dpe,c (SI unit: m2/s). If a User defined diffusion model is selected, an Effective diffusion coefficient 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 Coupling type options are available:
Continuous concentration, 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: . 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.
Film resistance (mass flux): 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 Film resistance (mass flux) option computes the inward surface flux, Ni,inward=hDi(ci-cpe,i). 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. Select these to be taken either From material or choose the User defined alternative.
Pellet Discretization
The extra dimension in the pellet needs to be discretized into elements. Select a DistributionCubic root sequence (the default), Linear, or Square root sequence. Enter the Number of elements Nelem.
Constraint Settings
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.
Further Reading
Theory for the Reactive Pellet Bed in the Theory section of this manual.
For an application using the Reactive Pellet Bed feature, see
A Multiscale 3D Packed Bed Reactor: Application Library path Chemical_Reaction_Engineering_Module/Reactors_with_Porous_Catalysts/packed_bed_reactor_3d