Coupling with the Reaction Engineering and the Chemistry Interfaces
Using the Reaction Engineering or Chemistry interfaces, all species property parameters and property functions required by these interfaces can be created automatically by coupling to an existing Thermodynamic System, Predefined System, or External Thermodynamic System under Thermodynamics. Examples of species properties that can be created automatically are molar mass, heat capacity, enthalpy, and entropy for each species. Parameters and functions for these properties are created by the thermodynamic system. The Reaction Engineering and Chemistry interfaces can also be used to define transport properties for the resulting mixture (all species in the interface). When coupled, the following mixture properties can be automatically created: heat capacity, density, thermal conductivity, and dynamic viscosity.
The Reaction Engineering interface and the Chemistry interface are only available in a limited set of add-on products. See https://www.comsol.com/products/specifications/ for more details on availability.
Note that using a thermodynamic system significantly increases the modeling capabilities in the Reaction Engineering and Chemistry interfaces. All ideal and nonideal thermodynamic models, for gases and liquids, are directly available and also automatically updated by editing the settings for the thermodynamic system in use. The Chemistry interface can furthermore be used to make the mixture properties readily available in space-dependent models for modeling of mass transport, heat transfer, or fluid flow.
Reaction Engineering
Mixture Properties
You can couple a Reaction Engineering interface with an existing thermodynamic system in the Reaction Engineering interface’s settings window.
You need to have at least one species defined in the Reaction Engineering interface in order to couple it to a thermodynamic system. You can make this coupling in the Settings window for the Reaction Engineering interface by selecting the Thermodynamics check box in the Mixture Properties section.
Select a thermodynamic system from the Thermodynamic system list. Use the Phase list to select the phase to be used in the Reaction Engineering interface.
Figure 2-24: Coupling Reaction Engineering Interface with Thermodynamics.
Species Matching
The Species Matching section is activated when the Thermodynamics check box is selected in the Mixture Properties section; see above. The species in the Reaction Engineering interface can be matched to a species in the thermodynamic system. This step ensures that the arguments in the thermodynamic system functions are correctly defined.
Use the lists in the From Thermodynamics column to match each species in the interface to a species in the coupled thermodynamic system.
For each species matched, the required property parameters and functions are added under to the corresponding thermodynamic system.
When all species are matched, the interface is considered fully coupled and functions representing mixture properties, such as density, are also added automatically under the corresponding thermodynamic system.
Figure 2-25: Matching the species in Reaction Engineering to those in the corresponding thermodynamic system.
Calculate Transport Properties
When the interface is fully coupled to a thermodynamic system, property functions for the mixture transport properties are created automatically when the Calculate mixture properties check box is selected. The properties calculated by the thermodynamic system display Thermodynamics in the corresponding list; see below.
Figure 2-26: Select the Calculate mixture properties check box when coupled to a thermodynamic system.
Chemistry
Mixture Properties
You can couple a Chemistry interface with an existing thermodynamic system in the Chemistry interface’s settings window. You can make this coupling in the Settings window for the Chemistry interface by selecting the Thermodynamics check box in the Mixture Properties section. You need to have at least one species defined in the Chemistry interface in order to couple it to a thermodynamic system.
Select a thermodynamic system from the Thermodynamic system list. Use the Phase list to select the phase to be used in the Chemistry interface.
Species Matching
The Species Matching section is activated when the Thermodynamics check box is selected in the Mixture Properties section. Here you can match the variables for the concentrations, and by this calculate mixture properties (transport and thermodynamic properties). For information on how to specify the dependent variables to be used, see Species Matching in The Chemistry Interface documentation in the Chemical Reaction Engineering Module User’s Guide.
You can match the species in the Chemistry interface with those in the corresponding thermodynamic system in the Species matching section’s table. Use the lists in the column with the title From Thermodynamics to match each species in the Chemistry interface to a species in the corresponding thermodynamic system. This ensures that the composition function arguments in the thermodynamic system are correctly defined.
For each species matched, the species property parameters and functions required by the Chemistry interface are automatically created and added under the corresponding thermodynamic system.
When all species are matched, the interface is fully coupled to Thermodynamics and functions representing mixture properties, such as the density, are added automatically under the thermodynamic system that the interface is coupled to.
Figure 2-27: Matching concentration variables in a Chemistry interface, as well as species in a coupled Thermodynamic System.
Calculate Transport Properties
When the interface is fully coupled to a thermodynamic system, property functions for the mixture transport properties are created automatically when the Calculate mixture properties check box is selected. The properties calculated by the thermodynamic system display Thermodynamics in the corresponding list; see Figure 2-26.
Mixture Property Definitions When Coupling to Thermodynamics
When all species in a physics interface (Reaction Engineering or Chemistry) are matched to the corresponding species in a thermodynamic system, the mixture properties are calculated based on the composition of the mixture. For example, consider Zmix(T, P, n1, …, nm), which denotes a generic extensive mixture property for a mixture of m number of species. The property function’s arguments are the temperature T, the pressure P, and the number of moles, n, for each species.
When the system is fully matched, the mixture property Zmix is defined as
(2-3)
where is the partial molar of species i:
(2-4)
The definition of partial molar properties can be rewritten using mole fraction derivatives as:
(2-5)
When the system is partially coupled, which means that some but not all species have been coupled, the mixture property is instead calculated assuming ideal mixing:
(2-6)
using the pure species properties Zi = Zmix(T, P, xi =1, 0, …, 0).