The IDAES Flash model represents a unit operation where a single stream undergoes a flash separation into two phases. The Flash model supports mutile types of flash operations, including pressure changes and addition or removal of heat.
Flash units generally have 2 degrees of freedom.
Typical fixed variables are:
- heat duty or outlet temperature (see note),
- pressure change or outlet pressure.
Note: When setting the outlet temeprature of a Flash unit, it is best to set control_volume.properties_out[t].temperature. Setting the temperature in one of the outlet streams directly results in a much harder problme to solve, and may be degenerate or unbounded in some cases.
The core Flash unit model consists of a single ControlVolume0DBlock (named control_volume) with one Inlet Port (named inlet) connected to a Separator unit model with two outlet Ports named 'vap_outlet' and 'liq_outlet'. The Flash model utilizes the separator unit model in IDAES to split the outlets by phase flows to the liquid and vapor outlets respectively.
The Separator unit model supports both direct splitting of state variables and writting of full splitting constraints via the ideal_separation construction argument. Full details on the Separator unit model can be found in the documentation for that unit. To support direct splitting, the property package must use one of a specified set of state variables and support a certain set of property calacuations, as outlined in the table below.
| State Variables | Required Properties |
|---|---|
| Material flow and composition | |
| ------------------------------------- | ----------------------------------- |
| flow_mol & mole_frac | flow_mol_phase & mole_frac_phase |
| flow_mol_phase & mole_frac_phase | flow_mol_phase & mole_frac_phase |
| flow_mol_comp | flow_mol_phase_comp |
| flow_mol_phase_comp | flow_mol_phase_comp |
| flow_mass & mass_frac | flow_mass_phase & mass_frac_phase |
| flow_mass_phase & mass_frac_phase | flow_mass_phase & mass_frac_phase |
| flow_mass_comp | flow_mass_phase_comp |
flow_mass_phase_comp Energy state |
flow_mass_phase_comp |
| ------------------------------------- | ----------------------------------- |
| temperature | temperature |
| enth_mol | enth_mol_phase |
| enth_mol_phase | enth_mol_phase |
| enth_mass | enth_mass_phase |
enth_mass_phase Pressure state |
enth_mass_phase |
| ------------------------------------- | ----------------------------------- |
| pressure | pressure |
Flash units have the following construction arguments:
- property_package - property package to use when constructing Property Blocks (default = 'use_parent_value'). This is provided as a Property Parameter Block by the Flowsheet when creating the model. If a value is not provided, the Holdup Block will try to use the default property package if one is defined.
- property_package_args - set of arguments to be passed to the Property Blocks when they are created.
Additionally, Flash units have the following construction arguments which are passed to the Holdup Block for determining which terms to construct in the balance equations.
| Argument | Default Value |
|---|---|
| dynamic | False |
| include_holdup | False |
| material_balance_type | MaterialBalanceType.componentPhase |
| energy_balance_type | EnergyBalanceType.enthalpyTotal |
| momentum_balance_type | MomentumBalanceType.pressureTotal |
| has_phase_equilibrium | True |
| has_heat_transfer | True |
| has_pressure_change | True |
Finally, Flash units also have the following arguments which are passed to the Separator block for determining how to split to two-phase mixture.
| Argument | Default Value |
|---|---|
| ideal_separation | True |
| energy_split_basis | EnergySplittingType.equal_temperature |
Flash units write no additional Constraints beyond those written by the ControlVolume0DBlock and the Separator block.
| Name | Notes |
|---|---|
| heat_duty | Reference to control_volume.heat |
| deltaP | Reference to control_volume.deltaP |
idaes.unit_models.flash
Flash
FlashData