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’)).

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).

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 ‘total’
momentum_balance_type ‘total’
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, Flash units automatically fix 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

FlashData Class

class idaes.models.flash.FlashData(component)[source]

Standard Flash Unit Model Class

build()[source]

Begin building model (pre-DAE transformation).

Parameters:None
Returns:None
post_transform_build()[source]

Continue model construction after DAE transformation.

Parameters:None
Returns:None