This interface (
), found under the Chemical Species Transport branch (
), is used to model reactors filled with catalytic pellets. The bed pellets are porous, forming a bimodal porosity in the reactor. In addition to transport in the pore space between the pellets, interparticle transport and reactions can be studied in detail. Both heterogeneous reactions and volumetric reactions can be studied within the pellets, as well as volumetric reactions in the pore space. The pellets are available in each point of the bed domain, and are defined using an extra dimension describing the pellet radius. The interface is the same as the
Transport of Concentrated Species interface, but it has a
Packed Bed node added by default. All other features for transport and reactions in porous media are also always available.
The main feature of the interface is the Packed Bed node which adds the governing equations for the mass fractions of all bulk species, and the equations for surface species. It has two subnodes,
Fluid and
Pellets, which in turn are used to define the physical properties corresponding to each phase. The latter node is also used to define volumetric reactions, as well as heterogeneous reactions, occurring within the porous pellets. The
Packed Bed node requires the use of a
Porous Material (described in the
COMSOL Multiphysics Reference Manual) for defining phase-specific properties, and for sharing them among different physics interfaces. A packed bed containing several pellet types can be modeled by adding multiple pellet nodes to the porous material.
When this physics interface is added, the following default nodes are also added to the Model Builder —
Packed Bed,
No Flux (the default boundary condition), and
Initial Values. Then, from the
Physics toolbar, add other nodes that implement, for example, boundary conditions, reaction rate expressions, and species sources. You can also right-click
Transport of Concentrated Species in Packed Beds to select physics features from the context menu.