The Chemistry Interface
This physics interface can be used to create reaction kinetics and optionally compute transport and thermodynamic properties for direct use in 1D, 2D, or 3D models. This is similar to The Reaction Engineering Interface, except that it does not solve for an ideal reactor model.
The Chemistry (chem) interface () is found under the Chemical Species Transport branch () when adding a physics interface. The Chemistry interface is also created when the Generate Space-Dependent Model feature is used in the Reaction Engineering interface, collecting all mixture variables and properties for use in a space-dependent model.
This physics interface is a tool for generating a set of variables to be used for modeling chemical species and reactions systems. The variables are generated from species and reaction properties and can be divided in two categories:
Many of the fields and nodes described in this section are only made available when either a Reaction or a Species (or both) subnode is added to the Model Builder. All predefined constants and expressions can be overwritten by user-defined expressions. This makes it possible to go beyond the modeling assumptions set as defaults in this physics interface.
The following is a description of the features and fields available on the Settings window for the Chemistry interface.
The Label is the default physics interface name.
The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first physics interface in the model) is chem.
Model Input
This section sets the Temperature, Pressure and Electrode potential (only available with a Battery Design Module, Corrosion Module, Electrochemistry Module, Electrodeposition Module, or Fuel Cell & Electrolyzer Module license) to be used by the current interface. Use the lists to select a temperature, pressure or electrode potential defined and announced by another interface in the model. For example, when a heat transfer interface is also included, the temperature solved for is available in the Temperature list.
For Temperature or Pressure, you can also select Common model input to use a globally available common model input. In all three cases, select User defined to manually define the variable in question.
Mixture Properties
Select Diluted species from the Type list to use the concentration variables from a Transport of Diluted Species interface in the Species Matching section. The same setting should be used for any other interface solving for species concentrations using a diluted species assumption. Select Concentrated species from the Type list to use the mass fractions from a Transport of Concentrated Species interface.
All property parameters and property functions required by the interface can be automatically created by coupling to a system added to the Thermodynamics node. To do so, click the Thermodynamics check box and select an existing Thermodynamic System.
The Thermodynamics check box is enabled when the Thermodynamics node, including one or more systems, is available under Global Definitions. Also, the Chemistry interface needs to include at least one species.
Use the Phase list to specify the state of aggregation of the mixture.
The mixture density settings are only available when you have selected the Calculate mixture properties check box in the Calculate Transport Properties section.
Density always has two settings available: Automatic or User defined. The Thermodynamics options is available when the interface is coupled to a Thermodynamic System, and all interface species has been matched to species in the system. In this case the density is defined by a function automatically added under the thermodynamic system coupled to.
The Automatic setting uses the following logic:
When Phase is set to Liquid, Automatic assumes that the liquid is ideal and incompressible. The liquid mixture density depends on the density of i number of pure species (ρi) and the species weight fraction (wi).
The volume fraction is given by the species concentration (ci) and the molar mass (Mi).
When Phase is set to Gas, Automatic calculates the gas mixture density (ρ) from the concentrations (ci) and molar masses (Mi) of the mixture species, which are automatically taken from Species features.
If the Species Type, for one of the present species, is set to Solvent, and the Mixture is Liquid, the mixture density is the same as the solvent density. The species density is defined in the General parameters section of the corresponding Species node. When Mixture is Gas, the mixture density is calculated from Equation 2-97 only for the species set as Solvent.
Species Matching
Species Concentration Input Table
Use the Species concentration input table to specify the concentrations to be used as arguments in variables generated by the Reaction and the Species features (the reaction rate for example). The species concentrations are also used in the mixture properties (transport and thermodynamic properties).
When Diluted species is selected in the Type list in the Mixture Properties section, enter the names of the dependent variables in the Molar concentration column. When Concentrated species is selected, enter the names of the dependent variables in the Mass fraction column.
Surface Species
Specify the concentration of surface species in the table in this section.
The species present in this section will not be used to define mixture properties.
Solid Species
Specify the concentration of solid species in the table in this section.
The species present in this section will not be used to define mixture properties.
Aqueous Species
Specify the concentration of solid species in the table in this section.
The species present in this section will not be used to define mixture properties.
Thermodynamics Coupling
When coupled to Thermodynamics is enabled, the species in the Chemistry interface can be matched to the species in the Thermodynamic System. This is needed to ensure that arguments for the thermodynamic functions are correctly defined. Use the drop-down 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 the corresponding thermodynamic system. When all species are matched, the interface is considered fully coupled and functions representing mixture properties, such as the density, are also added automatically under the corresponding thermodynamic system.
Equilibrium Reaction Rate Table
This table is only available when equilibrium reactions are present in the interface. Use the Reaction rate column to specify the rate for each equilibrium reaction. By doing so the postprocessing variable for the total reaction rate for each species, of form chem.Rsum_species, will be updated correctly. For a mass transport interface, say Transport of Diluted Species, the reaction rate needed for an equilibrium reaction is typically a dependent variable. In that case, the name of the variable can be found in the Shape Function section of the Equation View of the node.
When the Chemistry interface is created using from the Generate Space-Dependent Model the table is automatically set up in accordance with the added equilibrium reaction nodes.
Calculate Transport Properties
Select the Calculate mixture properties check box to calculate mixture properties that can be picked up in the space-dependent model interfaces. The properties that can be calculated are shown beneath the check box. Consider also if the built-in Automatic expressions fits the model or if User defined expressions are more suitable. In general, the Chemistry interface calculates properties in the same way as the Reaction Engineering interface.
Select the Use activity check box to solve for species activities instead of species concentrations, which is a common approach when non-ideal fluids are modeled.
An activity coefficient other than 1 can be set for each species for the Species node in the Species Concentration/Activity section.
CHEMKIN Import for Species Properties
This section enables CHEMKIN® import to simulate complex chemical reactions in gas phase.
Two types of CHEMKIN input files can be imported here: Thermo and Transport, for thermodynamic properties and transport properties, respectively. Properties for either volumetric or surface species are supported. Click Browse to locate the CHEMKIN file to be imported, then click Import. For Thermo, the imported data is directly entered in the NASA format fields in the Species node’s Thermodynamic Expressions section; for Transport, the imported data is entered in its Transport Expressions section.
Extra Dimension
Select Define variables in extra dimension when the current Chemistry interface is coupled to a feature on an extra dimension. An example of this is when the concentrations in the Concentration input column of the Model Inputs, Concentration correspond to pellet concentrations from a Transport of Diluted Species interface’s Reactive Pellet Bed feature.
When selected, generated variables will be defined using concentrations averaged over the extra dimension. Note that the generated variables are global in order be available on the main geometry, in contrast to variables on the extra dimension which only exist on the corresponding dimension.