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 holdup.properties_out[t].temperature. Setting the temperature in one of the outlet streams directly results in a much harder problme ot solve, and may be degenerate in some cases.
Model Structure¶
The core Flash unit model consists of a single Holdup0D (named holdup) with one Inlet Port (named inlet) and one Outlet Port (named outlet, default with two indexes (‘Vap’ and ‘Liq’)). The Flash model supports two methods for splitting the material stream between the two outlets; either by utilizing an IDAES OutletSplitter Block with fixed split fractions, or by directly linking the vapor flow to the “Vap” outlet and the liquid flow to the “Liq” outlet.
Using an OutletSplitter block allows for a generic formulation that can handle more than just vapor-liquid equilibrium and allows for non-ideal splitting (with some entrainment of phases), but results in a larger problem with additonal variables and constraints. The direct splitting option results in a smaller problem size, but is limited to a small set of problem formulations.
Direct outlet splitting requires that the state variables used by the assoicated property package meet specific requirements in order that the Flash model can find the necessary information for splitting the outlet flows. To support direct outlet 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.
- inlet_list - list of names to be passed to the build_inlets method (default = None).
- num_inlets - number of inlets argument to be passed to the build_inlets method (default = None).
- outlet_list - list of names to be passed to the build_outlets method (default = [‘Vap’, ‘Liq’]).
- num_outlets - number of outlets argument to be passed to the build_outlets method (default = None).
- outlet_type - whether to use a genetic OutletSplitter (‘generic’, default) or to use direct outlet splitting (‘direct’).
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 |
---|---|
material_balance_type | ‘component_phase’ |
energy_balance_type | ‘enthalpy_total’ |
momentum_balance_type | ‘pressure’ |
dynamic | False |
include_holdup | False |
has_rate_reactions | False |
has_equilibrium_reactions | False |
has_phase_equilibrium | True |
has_mass_transfer | False |
has_heat_transfer | True |
has_work_transfer | False |
has_pressure_change | True |
Additional Constraints¶
Flash units write no additional Constraints beyond those written by the Holdup Block.
However, if an OutletSplitter is used, the Flash unit automatically fixes the split fractions of the outlets such that all the liquid phase goes to the outlet named “Liq” and all the vapor phase goes to the outlet named “Vap”. This is done as follows for all t in the time domain:
- split_fraction(t, “Liq”, “Liq”).fix(1.0)
- split_fraction(t, “Vap”, “Vap”).fix(1.0)
Variables¶
Flash units add one additional Variable beyond those created by the Holdup Block.
Name | Notes |
---|---|
split_fraction | Reference to holdup.outlet_splitter.split_fraction |