Flash Unit¶
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.
Degrees of Freedom¶
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.
Model Structure¶
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 | flow_mass_phase_comp |
| Energy state | |
| temperature | temperature |
| enth_mol | enth_mol_phase |
| enth_mol_phase | enth_mol_phase |
| enth_mass | enth_mass_phase |
| enth_mass_phase | enth_mass_phase |
| Pressure state | |
| pressure | pressure |
Construction Arguments¶
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 |
Additional Constraints¶
Flash units write no additional Constraints beyond those written by the ControlVolume0DBlock and the Separator block.
Variables¶
| Name | Notes |
|---|---|
| heat_duty | Reference to control_volume.heat |
| deltaP | Reference to control_volume.deltaP |
Flash Class¶
-
class
idaes.unit_models.flash.Flash(*args, **kwargs)¶ Parameters: - rule (function) – A rule function or None. Default rule calls build().
- concrete (bool) – If True, make this a toplevel model. Default - False.
- ctype (str) – Pyomo ctype of the block. Default - “Block”
- default (dict) –
Default ProcessBlockData config
- Keys
- dynamic
- Indicates whether this model will be dynamic or not, default = False. Flash units do not support dynamic behavior.
- has_holdup
- Indicates whether holdup terms should be constructed or not. default - False. Flash units do not have defined volume, thus this must be False.
- material_balance_type
- Indicates what type of mass balance should be constructed, default - MaterialBalanceType.componentPhase. Valid values: { MaterialBalanceType.none - exclude material balances, MaterialBalanceType.componentPhase - use phase component balances, MaterialBalanceType.componentTotal - use total component balances, MaterialBalanceType.elementTotal - use total element balances, MaterialBalanceType.total - use total material balance.}
- energy_balance_type
- Indicates what type of energy balance should be constructed, default - EnergyBalanceType.enthalpyTotal. Valid values: { EnergyBalanceType.none - exclude energy balances, EnergyBalanceType.enthalpyTotal - single enthalpy balance for material, EnergyBalanceType.enthalpyPhase - enthalpy balances for each phase, EnergyBalanceType.energyTotal - single energy balance for material, EnergyBalanceType.energyPhase - energy balances for each phase.}
- momentum_balance_type
- Indicates what type of momentum balance should be constructed, default - MomentumBalanceType.pressureTotal. Valid values: { MomentumBalanceType.none - exclude momentum balances, MomentumBalanceType.pressureTotal - single pressure balance for material, MomentumBalanceType.pressurePhase - pressure balances for each phase, MomentumBalanceType.momentumTotal - single momentum balance for material, MomentumBalanceType.momentumPhase - momentum balances for each phase.}
- energy_split_basis
- Argument indicating basis to use for splitting energy this is not used for when ideal_separation == True. default - EnergySplittingType.equal_temperature. Valid values: { EnergySplittingType.equal_temperature - outlet temperatures equal inlet EnergySplittingType.equal_molar_enthalpy - oulet molar enthalpies equal inlet, EnergySplittingType.enthalpy_split - apply split fractions to enthalpy flows.}
- ideal_separation
- Argument indicating whether ideal splitting should be used. Ideal splitting assumes perfect separation of material, and attempts to avoid duplication of StateBlocks by directly partitioning outlet flows to ports, default - True. Valid values: { True - use ideal splitting methods. Cannot be combined with has_phase_equilibrium = True, False - use explicit splitting equations with split fractions.}
- has_heat_transfer
- Indicates whether terms for heat transfer should be constructed, default - False. Valid values: { True - include heat transfer terms, False - exclude heat transfer terms.}
- has_pressure_change
- Indicates whether terms for pressure change should be constructed, default - True. Valid values: { True - include pressure change terms, False - exclude pressure change terms.}
- property_package
- Property parameter object used to define property calculations, default - useDefault. Valid values: { useDefault - use default package from parent model or flowsheet, PropertyParameterObject - a PropertyParameterBlock object.}
- property_package_args
- A ConfigBlock with arguments to be passed to a property block(s) and used when constructing these, default - None. Valid values: { see property package for documentation.}
- initialize (dict) – ProcessBlockData config for individual elements. Keys are BlockData indexes and values are dictionaries described under the “default” argument above.
- idx_map (function) – Function to take the index of a BlockData element and return the index in the initialize dict from which to read arguments. This can be provided to overide the default behavior of matching the BlockData index exactly to the index in initialize.
Returns: (Flash) New instance