Thermodynamic System
A thermodynamic system is used to describe properties of pure species and mixtures of chemical compounds for liquids, gases, liquid-vapor equilibria, and liquid-liquid equilibria. It specifies the available species and the phases (states of aggregation) that are present in the modeled system. It also defines and evaluates the functions for thermodynamic and transport properties of the chemical system, that is, the species and mixture properties for liquids, gases and phase equilibria.
To create a Thermodynamic System, start the Thermodynamic System Wizard in one of the following ways:
•
Right-click the Global Definitions node, select Thermodynamics and choose Thermodynamic System.
•
Using the Reaction Engineering interface, choose Thermodynamic System from the Thermodynamics section of the Reaction Engineering toolbar.
•
Correspondingly, using the Chemistry interface, choose Thermodynamic System from the Thermodynamics section of the Chemistry toolbar.
The Thermodynamic System Wizard contains the following steps:
1
Select System
2
Select Species
3
Select Thermodynamic Model
Select System
Use the Select System step in the wizard to define the phases in the modeled system. You can select Gas, Liquid, Vapor-liquid, Vapor-liquid-liquid, or Liquid-liquid. The names of the phases in the Selected system table can be changed by editing the element in the Name column. Click the Next button () to proceed to the next step in the wizard.
Figure 2-3: Thermodynamic System Wizard: Select System.
Select Species
Use the Species Type filter text field to search among the available species in the COMSOL database or the User-Defined Species. Species can be searched for by typing the name, CAS number, or the chemical formula. Using a formula, all species whose chemical formula includes a combination of the given atoms are listed. For example, when searching for ammonia NH3 or H3N can be used.
Select one or more species in the list and click the Add Selected button () to add species to the Selected species table. You can also press the Enter key to add either the first species in the list, or the species that have been selected. To remove species from the Selected species table, use either the Remove Selected button () or the Remove All button (), both located below the table. Click the Next button () to proceed to the next step in the wizard.
Figure 2-4: Thermodynamic System Wizard: Select Species.
Select Thermodynamic Model
The Thermodynamic model controls the relations between the intensive thermodynamic properties of the system such as pressure, specific volume, and temperature. It also controls how extensive properties like enthalpy, entropy, or the heat capacity of the system are computed. Furthermore, the Thermodynamic model specifies the default models used to compute transport properties.
The following Thermodynamic Models are available for all phases:
•
Peng–Robinson
•
Peng–Robinson (Twu)
•
Soave–Redlich–Kwong
•
Soave–Redlich–Kwong (Graboski–Daubert)
For a system including only gas phase, or for liquid phase systems with thermodynamic models requiring an equation of state, the Ideal Gas Law is added to the available models in the Gas phase model list or Equation of state list.
When the system includes one or more liquid phases, the following additional models are also available (in the Liquid phase model list):
•
Chao–Seader (Grayson–Streed)
•
Wilson
•
NRTL
•
UNIFAC VLE
•
UNIQUAC
•
Regular Solution
•
Extended Regular Solution
•
Ideal Solution
These models correspond to activity models.
To calculate properties of water and steam, use the Water (IAPWS) model. This is available for systems that include a single water species.
For recommendations regarding the appropriate thermodynamic model, based on the available phases and the system pressure level, see the section Selecting the Right Thermodynamic Model.
Select the Advanced options check box in order to manually control the models used for thermodynamic properties, transport properties, and surface tension. When this check box is selected, all available property models are shown in the property model table. The available property models are dependent on the phases available in the thermodynamic system.
When only a gas phase is present, models are available for the following properties:
•
Gas diffusivity
•
Gas thermal conductivity
•
Gas viscosity
For a single-phase liquid, or for a two-phase liquid-liquid system, there are models for the following properties:
•
Liquid diffusivity
•
Liquid diffusivity at infinite dilution
•
Liquid thermal conductivity
•
Liquid viscosity
•
Liquid volume
For systems with both vapor and liquid phases, all of the above properties are available, as well as Gas-liquid surface tension.
Figure 2-5: Thermodynamic System Wizard: Advanced thermodynamic model options available for a Vapor-liquid system.
Click the Finish button () to exit the Thermodynamic System Wizard and add the corresponding system under the Thermodynamics node. Note that the default node label reflects the available phase in the system. For example, when creating a vapor-liquid system, a node labeled Vapor-Liquid System is added. The name of the system can be changed using the Label text field in the Settings window for the system.
Settings
Selecting a Thermodynamic System or Predefined System node shows its Settings window with a description of the package content and the available properties. Editing the text in the Label text field changes the name of the system. Below follows a description of the thermodynamic system settings.
Figure 2-6: Thermodynamic System Settings.
Species:
Lists the species included in the thermodynamic system. You can change the list of species by right-clicking the Thermodynamic System node and selecting Add or Remove Species.
Phases:
Lists the phases included in the thermodynamic system. You can add or remove phases, such as gas, liquid or liquid-vapor, by right-clicking the Thermodynamic System node and selecting Define System.
Species Property:
Lists the parameters and functions that describe thermodynamics and transport properties for pure species. Such functions may describe density, heat capacity, thermal conductivity, or other thermodynamic and transport properties. For more information, see Species Property.
Mixture Property:
Lists the available mixture property functions. Note that you have to have a mixture model defined in order to use these functions. You can define a mixture property by right-clicking the Thermodynamic System node and selecting Mixture Property.
Thermodynamic Model:
When a liquid is present, use the Liquid phase model list to select the thermodynamic model for this phase.
When a gas is present, use the Gas phase model list to select the thermodynamic model for this phase. If both gas and liquid phases are available, and the Liquid phase model is set to an equation of state, then the Gas phase model is set to the same model automatically.
Property Models:
Displays the settings for the available individual property models. The property models can be changed by selecting different values from the combo box in the Model column. For example, for Gas thermal conductivity you can select Kinetic theory or Ideal from the corresponding combo box. You can also select the property model in the last step of the thermodynamic system wizard by selecting the Advanced option check box, see Select Thermodynamic Model.
Binary Interaction Parameters:
This section can be used to inspect and edit binary interaction parameters. Select a pair of species (first and second column, Species 1 and Species 2) as shown in Figure 2-7 to edit the corresponding binary interaction parameter currently in use. In cases that the thermodynamic model has more than one type of binary interaction parameter, each type of the interaction parameter will be shown in a column in the table.
Figure 2-7: Entering the Binary Interaction Parameters for the Soave–Redlich–Kwong model.
When a binary interaction parameter is not available in the database for a pair of species, the parameter value is set to zero (default value).
Modifying an Existing System
It is possible to modify an existing thermodynamic system, for example by changing the included species or phases. To do so, right-click the Thermodynamic System node and select Add or Remove Species, or Define System from the context menu. Both of these options are available for systems created using Thermodynamic System. For a Predefined System, the option Add or Remove Species is available for Dry Air and Moist Air. For a Water-steam system, neither of the two options are available.
When the properties of a User-Defined Species have been changed, select Update From Species for the changes to take effect in the thermodynamic system.
Figure 2-8: Modifying the species and phases of a thermodynamic system.
Add or Remove Species
Selecting Add or Remove Species takes you to the Select Species step in the Thermodynamic System Wizard. Use the Add Species button () to add a species to the Selected Species list. To remove a species, select it in the list and use the Remove Species button ().
Make sure to click the Finish button () in order for the changes to take effect.
Note that adding or removing species in a thermodynamic system that is currently coupled to a Reaction Engineering interface or a Chemistry interface breaks this coupling. See the Coupling with the Reaction Engineering and the Chemistry Interfaces for how to update this coupling.
Define System
Selecting Define System takes you to the Select System step in the Thermodynamic System Wizard. You can select the desired system from the list.
Make sure to click the Finish button () button in order for the changes to take effect.
Note that changing the phase(s) in a thermodynamic system that is currently coupled to a Reaction Engineering interface or a Chemistry interface breaks this coupling. See the Coupling with the Reaction Engineering and the Chemistry Interfaces section for how this coupling can be updated.
Update From Species
This option is available in a Thermodynamic System that includes a User-Defined Species. Use it to update the system following a change in the species properties.
Warning Information
A sanity check is always performed when a Thermodynamic System is created. If any problems are found, a Warning Information node listing the problems is added under the package. One example when this occurs is when a parameter required for the thermodynamic model, typically a binary interaction parameter, is not available in the database.