COMSOL 6.2 - The Reaction Engineering Interface

The interface (

), found under the branch (

) when adding a physics interface, is used to model several chemical reactor types and the evolution of chemical reactions over time. The mass balance and energy balance equations describing these systems assume perfect or well-defined mixing in the reactor. Essentially, the physics interface simulates, tank, and plug flow chemical reactors to investigate the behavior over time of a chemical reaction.

The reaction kinetics expressions of a reactor can be exported to a space-dependent model, using the Generate Space-Dependent Model feature. This gives you the power to simulate reacting systems as they depend on fluid flow, mass transfer, and heat transfer — in other words, including space dependencies.

Add physics features from the toolbar, or right-click to select features from the context menu. Many of the fields and nodes described in this section are made available when either a Reaction or a Species (or both) subnode is added to the Model Builder. Because nodes and subnodes are accessible at any time, and any change is updated throughout the model, reactions and species are often defined before the settings described in this section.

All predefined constants and expressions can be overwritten by user-defined expressions. This makes it possible to go beyond the modeling assumptions that are the defaults in this physics interface.

The following is a description of the features and fields available in the window for Reaction Engineering.

The is the default physics interface name.

The 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 field. The first character must be a letter.

The default (for the first physics interface in the model setup) is re.

This section displays the governing equations according to the selection of reactor types in the section.

Select a to define the reaction system. The available reactor types are: , , , , , and . Each reactor type solves a mass balance based on properties typical to the type, such as reactor volume, volumetric production, or flow rate:

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Batch: In reactors no mass enters or leaves the system. This type can account for a variable reactor volume.

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Batch, Constant Volume: Same as Batch but where the reactor volume is assumed to be constant. As this is the situation for most reacting systems, this is the default condition.

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CSTR, Constant Mass/Generic: Continuous stirred tank reactors (CSTR) differ from batch reactors, since these allow species to enter and leave the reactor by means of feed inlet streams and outlet streams. The system’s volume is allowed to change, such as in a car engine cylinder or a balloon. This reactor model can be solved either for a constant mass condition or by selecting a specific outlet flow.

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CSTR, Constant Volume: Same as the CSTR with constant mass/generic reactor but assumes that the volume is constant during operation.

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Semibatch: Semibatch reactors differ from batch reactors in that they allow reactants to enter the reactor by means of one or several feed inlet streams.

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Plug Flow: In the plug flow reactor, the species concentrations and the temperature vary with position. Plug flow in a tubular configuration means that concentration and temperature gradients develop only in the axial direction, but not in the radial direction.

For each , additional settings are shown in Table 2-1. If a surface reaction or species is included, except for the reactor type, the surface reaction area is also a parameter. The parameters and expressions are used in the mass balance equations.

Table 2-1: Reactor Type Parameters.
Reactor Type	Parameters to Define
Batch	Reactor volume Vr
Batch, constant volume	Reactor volume Vr
CSTR, constant mass/generic	Volumetric production rate vp and Volumetric outlet rate v
CSTR, constant volume	Volumetric production rate vp and Reactor volume Vr
Semibatch	Volumetric production rate vp
Plug flow	Volumetric flow rate v

For and reactor types, the (vp) is available. For , the in-built expression is shown. If is selected, the expression can be changed (SI unit: m3/s). For instance, this enables the setting of zero (0) volumetric production rate, which ignores volume changes due to reactions.

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For liquid phase reactions, the expression for the varies with the reaction rate of each species as defined by:

The physics interface automatically inserts the stoichiometric coefficients (νij) and reaction rate expressions for each species (Ri) that depend on j number of reactions of a rate (rj), as defined in the Reaction feature node. Furthermore, the values of the molar mass (Mi) and the species density (ρi) are automatically taken from the Species features.

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For gas phase reactions, the expression for the is similarly given by:

For , select to either or . The selection shows both the expression for (vp) (Equation 2-77 for gas, or Equation 2-76 for liquid phase reactions) and (v) (Equation 2-78). The latter is derived from constant mass flow condition through the reactor:

The mixture density (ρf) of m number of feed inlet streams is determined in the same way as in the section. For both the properties can be edited (SI unit: m3/s). This means that it is possible to completely control the volumetric outlet rate from the CSTR.

For the reactor, the along the reactor is set. The definition computes a variable volumetric flow rate that depends on the molar flow rate of each species (Fi).

The default value for p () is 1 atm in Equation 2-81 and it is set in the section.

This input field sets the Vr — that is, the fluid volume in which chemical reaction takes place. The reactor type can account for a changing volume, thus a time-dependent volume expression can be entered here.

Once a surface reaction, or a surface species, has been added, for all reactor types except , the settings become visible. Here, the area of the surface on which the surface reactions take place can be defined. The surface area can either be defined directly as a parameter, or by defining the .

	Reactor Types in the Reaction Engineering Interface

From the list select to or the energy balance, which in essence determines whether the system is solved for either isothermal or nonisothermal conditions, respectively. The latter introduces the temperature as a variable in the interface.

If is chosen, enter a T for the system.

It is possible to incorporate cooling or heating of the reactor. This is done in (Qext). Enter a negative value to account for cooling or a positive one for heating (SI unit: W). Note that for the Plug Flow reactor type, the (Q) is defined per reactor volume (SI unit: W/m3).

All property parameters and property functions required by the interface can be automatically created by coupling to a system added to the node. To do so click the check box and select an existing Thermodynamic System.

The check box is enabled except if:

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the Thermodynamics node, including one or more systems, is not added under , or

Use the list to specify the state of aggregation of the mixture.

This setting is available when the check box in the section has been selected.

Two settings are always available for the mixture : or . The 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 mixture density is transferred to physics interfaces set up by the Generate Space-Dependent Model feature. The density is compiled for both multicomponent and solute-solvent solutions.

The setting uses the following logic:

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selected for , considers the liquid as ideal and incompressible. The liquid mixture density depends on the density of i number of pure species (ρi) and the species mass fraction (wi).

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set for 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.

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If a Type is set to and the is , the mixture density is the same as the solvent density as defined in in in the Species node. When is , the mixture density is calculated from Equation 2-81 only for the species set as .

The - setting displays the . For all reactor types, except the reactor, select either a pressure computed from the or from any other expression using the option. The and reactor types also have the option to keep the reactor pressure during reaction.

For the Plug Flow reactor, the can be entered in the case of the section having the set to , in which case the alternative is available and a constant pressure fits the conditions.

When the Thermodynamics check box is selected in the section and the species are fully coupled (see the section below), the reactor pressure is set to indicating that it is automatically computed.

The section is activated when the Thermodynamics check box is selected in the section. The species in the Reaction Engineering interface can be matched to species in the Thermodynamic System. This ensures that the arguments in the thermodynamic system functions are correctly defined.

Use the drop-down lists in the 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.

Transport properties are not utilized in the reactor equations available in the Reaction Engineering interface, where perfect mixing is assumed. However, transport properties such as diffusivity, thermal conductivity, and viscosity are of central importance when solving time- and space-dependent models. The physics interface helps to set up detailed expressions of species transport properties and transfers them automatically to the multiphysics model through the Generate Space-Dependent Model feature.

The most general description of a mixture is the one that treats the mixture as a multicomponent solution, where all species interact with each other. A simplified description, but still a common one, assumes that the solution has a solvent that dominates the properties of the solution. The solutes in such a solution interact only with solvent molecules.

Select the check box to enable calculation of mixture transport properties exported from the Reaction Engineering interface.

From the list for each property, select the in-built expression or set a entry. The mixture properties you can transfer for space-dependent models are:

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(cp) (SI unit: J/(kg·K)) (this is available when the Energy Balance is set to )

All species properties needed to compute the mixture properties are entered in the or in the Species node.

	Transport Properties

This section is available when at least one equilibrium reaction has been defined, that is, when a Reaction node incorporates at least one reaction ( is set to ), or when at least one equilibrium reaction has been defined in an Equilibrium Reaction Group.

In the text field, edit, if necessary, the species that depends on the composition of the other species according to the in the Reaction node. To minimize the impact of any numerical errors, it is recommended to set the species with the highest concentration as dependent species. The default species is set to the leftmost species in the .

The check box exists to aid the computation of equilibrium reaction systems. A selected check box ensures that no negative values of concentrations are accepted as solution to the equilibrium condition.

	Example II in Handling of Equilibrium Reactions.

Select the 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 section.

	Activity

This section enables CHEMKIN® import to simulate complex chemical reactions in gas phase.

Two types of CHEMKIN input files can be imported here — and , for thermodynamic properties and transport properties respectively. Properties for either volumetric or surface species are supported. Click to locate the CHEMKIN file to be imported, then click . For Thermo the imported data is directly entered in the fields in the Species section. For Transport the imported data is entered in the Species section.

To display this section, click the button (

) and select .

The check box is not selected by default. When selected, all concentration variables are scaled using the same scale factor in the node. Enabling uniform scaling can decrease solver time for problems involving many concentration variables.