This physics interface can be used to create reaction kinetics and optionally compute mass 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.

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 in 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.

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.

Use this setting to define specify what kind of mixture assumption is used. For Diluted Species the mixture is composed of low concentration solutes present in a solvent. For Concentrated Species a mixture where no single species is considered to be in excess is assumed.

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 molar 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.

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:

Use this list to populate the inputs in the Bulk species table with dependent variables from a certain mass transfer interface. The Species solved for list contains all present interfaces of the Type selected in the Mixture Properties section. For example, when the Type is Diluted species, all Transport of Diluted Species interfaces or variants thereof, such as Transport of Diluted Species in Porous Media interface, are available in the list.

Use the Bulk species table to specify the concentrations to be used as arguments in reaction kinetics variables, for example the reaction rate. Reaction kinetics variables are generated by the species features (Species and Species Group) and by the reaction features (Reaction, Reversible Reaction Group, and Equilibrium Reaction Group).

When a mass transfer interface has been selected in the Species solved for list, use the Molar concentration column to select one of the dependent variables in the selected interface to the corresponding Species in Chemistry. When using a concentrated species interface (defined by the Type list in the Mixture Properties section), instead use the Mass fraction column to select a dependent variable for each species in Chemistry. For a consistent set up, the species molar mass in the Transport of Concentrated Species interface is updated to the one defined in the Chemistry interface.

The Molar concentration and Mass fraction columns also includes a User defined option, in which case a constant, parameter, or variable expression can be entered in the Value column.

When the Thermodynamics is enabled, the species in the Chemistry interface can be coupled 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 Chemistry to a species in the coupled thermodynamic system. For each thermodynamics-coupled species, the required property parameters and functions are added under the corresponding thermodynamic system. When all species are matched, the Chemistry is considered fully coupled and functions representing mixture properties, such as the density, are also added automatically under the same thermodynamic system.

The Surface Species table is available when there is at least one surface species (with “(ads)” as suffix).

The Solid Species table is available when there is at least one solid species (with “(s)” as suffix).

The Aqueous Species table is available when there is at least one aqueous species (with “(aq)” as suffix).

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.

Select the Calculate mixture properties check box (selected as default) 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 fit 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.

This section is available when the Thermodynamics check box is cleared.

Select the Use activity check box to solve for species activities instead of species concentrations, which is a common approach when nonideal 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.

It is available when the Thermodynamics check box is cleared.

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.

Select the check box Define variables for porous pellets when the Chemistry interface is used to model transport and reactions in porous pellets using the Packed Bed feature. In this case an extra dimension, used to represent solid pellets, or particles, is present in each point of the domain. In this extra dimension both bulk and surface species concentrations are solved for. For correct pellet chemistry make sure to input the intraparticle concentrations in the Species Matching section. When Define variables for porous pellets is selected, reaction rates as well as the mixture properties defined by Chemistry become available in the Packed Bed node and its subnodes.

For more information see the Packed Bed feature in the Transport of Diluted Species interface.