Generic property package: Using constraints to define state variables

[alma-walmsley]

Fixes # .

Summary/Motivation:

As part of Ahuora we are using the generic property package framework to build custom property packages.

The code looks something like this:

m.fs.properties = GenericParameterBlock(**configuration)  # FTPx
m.fs.state = m.fs.properties.build_state_block([0], defined_state=True)
sb = m.fs.state[0]
sb.flow_mol.fix(1)  # mol/s
sb.temperature.fix(300)  # K
sb.pressure.fix(100000)  # Pa
sb.mole_frac_comp["benzene"].fix(0.5)
sb.mole_frac_comp["toluene"].fix(0.5)

m.fs.state.initialize()

Suppose that we know the value for flow_mass and want to use that instead. We can do this using a Constraint:

sb.flow_mol.unfix()
sb.flow_mass_constraint = Constraint(rule=sb.flow_mass == 1)  # kg/s

However, this ends up running into issues during initialization:

• The initialization routine starts by fixing all state variables that are not currently fixed. This means flow_mol gets fixed anyway, and the block becomes over-defined. However, we are able to prevent this by setting the kwarg state_vars_fixed=True, which means initialization will skip trying to fix the state variables (and just check for 0 degrees of freedom). So this is not an issue at this stage.
• During the initialization routine, constraints are deactivated at the start and then re-activated during the different initialization steps. The constraint flow_mass_constraint gets deactivated during the "Bubble, dew, and critical point initialization" step (which is fine), but needs to be activated for the "Phase equilibrium initialization" step, to ensure 0 degrees of freedom.

The specific error I get is

idaes.core.util.exceptions.InitializationError: fs.state Unexpected degrees of freedom during initialization at phase equilibrium step: 1.

Changes proposed in this PR:

• This PR proposes an approach to activate the flow_mass_constraint (or other constraints as needed) before the "Phase equilibrium initialization" step. This is done by "tagging" the constraint with defining_state_var = True to indicate that this constraint defines a state variable, and should be activated.

For example,

sb.flow_mol.unfix()
sb.flow_mass_constraint = Constraint(rule=sb.flow_mass == 1)  # kg/s
sb.flow_mass_constraint.defining_state_var = True
m.fs.state.initialize(state_vars_fixed=True)

I added this check to both the _GenericStateBlock and ModularPropertiesInitializer initialize methods. I am not too sure how the ModularPropertiesInitializer class is used (seems to be the same logic) but I assume the code fits in both.

Comments on this approach would be useful.

Legal Acknowledgement

By contributing to this software project, I agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the license terms described in the LICENSE.txt file at the top level of this directory.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[alma-walmsley]

I'm having degree-of-freedom issues with unfixing pressure (for example, using enth_mol and temperature to define pressure) in the "Bubble, dew, and critical point initialization." So this may require some more thought - I'll probably have a closer look into how vars and constraints influence the degrees of freedom.