User-Defined Species
Use this feature to specify the properties of a new species not available in the included COMSOL database. This feature can also be used to edit a database species, by using an existing species as a template, to create a new one. A User-Defined Species can be included in a Thermodynamic System feature, or Predefined System feature, together with predefined database species and other user-defined species.
The data needed for a user-defined species includes both material properties, such as the molar mass and the vapor pressure, and properties for specific thermodynamic models or transport models (see Thermodynamic Models and Theory). For instance, in order to use the UNIFAC thermodynamic model, you need to define UNIFAC groups for the new species.
Note that the thermodynamics calculator includes measures to handle missing species properties, sometimes by applying approximations. For instance, if the Wilson volume is not available, the liquid volume at normal boiling point is used instead. If data for this is also missing, it is estimated from the saturated liquid density correlation.
When creating a user-defined species it is recommended to add the following common material properties:
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Molecular mass
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Critical properties
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Acentric factor
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Absolute entropy
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Standard enthalpy of formation
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Ideal gas heat capacity
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Vapor pressure
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Heat of vaporization
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Saturated liquid density
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Transport properties
Example Model
For an example of using a User-Defined Species see this application example:
Dissociation in a Tubular Reactor: Application Library path Chemical_Reaction_Engineering_Module/Thermodynamics/dissociation
The User-Defined Species Wizard
To create a User-Defined Species, start the User-Defined Species Wizard in one of the following ways:
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Right-click the Global Definitions node, select Thermodynamics and choose User-Defined Species.
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If the model already includes the Thermodynamics node, right-click it and choose User-Defined Species.
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Using the Reaction Engineering interface, or Chemistry interface, choose User-Defined Species from the Thermodynamics section of the Reaction Engineering, or Chemistry toolbar.
The User-Defined Species Wizard contains the following steps:
1
Enter Name and Formula
2
Enter Parameters
3
Define Properties
Enter Name and Formula
Enter the Name, CAS number, and the Chemical formula for the new species.
Figure 6-29: User-Defined Species Wizard; Enter Name and Formula.
If you want to edit a species in the COMSOL database, select the Use species from database as template check box. Then locate the species to edit in the list. The filter text field can be used to search among the available species.
Figure 6-30: User-Defined Species Wizard; edit an available species by selecting Use species from database as template.
Click the Next button () to proceed to the next step.
Enter Parameters
Define the necessary parameter for the new species.
Constants
Use this table to define material constants such as molar mass, critical temperature, and standard enthalpy of formation.
Figure 6-31: User-Defined Species Wizard; Enter material constants.
Structure information
If you want to use the UNIFAC model, specify the structure formula, the UNIFAC groups and their number occurrences.
Figure 6-32: User-Defined Species Wizard; Specify the structure information in the Enter Parameters step.
Model parameters
Specify the parameters for the thermodynamic models and transport models in use.
Figure 6-33: User-Defined Species Wizard; Specify model parameters.
Click the Next button () to proceed to the next step.
Define Properties
The last step in the wizard is to add temperature dependent properties for the new species. All temperature dependent properties are defined using cubic polynomials on the form
(6-7)
Each property can consist of an arbitrary number of temperature intervals, each using the above form. Click the Add button () under the table for a specific property to add an interval.
The following temperature dependent properties are necessary in order to compute the basic set of mixture properties when including the User-Defined Species in a thermodynamic system:
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Ideal gas heat capacity
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Liquid density
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Vapor pressure
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Heat of vaporization
Note that some thermodynamic properties, such as the enthalpy and entropy, of a species or mixture, are estimated from the ideal gas heat capacity and depends on the thermodynamic model applied for the system (see Thermodynamic Properties Definitions).
Click the Finish button () in the top of the Settings window to finalize the species and add it to the User-Defined Species node under Thermodynamics.
Figure 6-34: User-Defined Species Wizard; specify temperature-dependent properties.
Settings
Selecting a species node under User-Defined Species shows the Settings window including the definitions of all species properties. Properties are categorized into sections as seen in Figure 6-35 below. All properties can be edited by expanding the corresponding section.
When a species properties has been changed, any Thermodynamic System that includes the species needs to be updated for the changes to take effect (see Update From Species).
Figure 6-35: The Settings window for a User-Defined Species.