Peng-Robinson failing to solve pure

[Mahaki-Leach]

Idaes fails to solve single components
e.g. build_package("peng-robinson", ["benzene"], ["Liq", "Vap"])

currently assuming state definition change to enthalpy will fix this.

[alma-walmsley]

from pyomo.environ import ConcreteModel, Constraint, Var, SolverFactory, value, assert_optimal_termination
from idaes.core import FlowsheetBlock
from property_packages.build_package import build_package

m = ConcreteModel()
m.fs = FlowsheetBlock(dynamic=False)
m.fs.properties = build_package("peng-robinson", ["water"])
m.fs.state = m.fs.properties.build_state_block([0], defined_state=True)
sb = m.fs.state[0]

sb.flow_mol.fix(1)
sb.pressure.fix(101325)
sb.constrain("enth_mol", -40000)
sb.mole_frac_comp["water"].fix(1)

m.fs.state.initialize(outlvl=1)

opt = SolverFactory('ipopt')

def try_solve(enth_val):
    sb.constraints.del_component("enth_mol")
    sb.constrain("enth_mol", enth_val)
    res = opt.solve(m)
    assert_optimal_termination(res)
    print(f"[enth_mol={enth_val}]: Temperature: {value(sb.temperature)}")

# now try to solve the block with different enthalpy values
# and observe the temperature
try_solve(-40000)
try_solve(-41000)
try_solve(-42000)
try_solve(-43000)
try_solve(-44000)
try_solve(-45000)
try_solve(-46000)

the temperature values we're getting here might suggest a modelling error

[enth_mol=-40000]: Temperature: 1843.1391638994066
[enth_mol=-41000]: Temperature: 1860.375110055527
[enth_mol=-42000]: Temperature: 1851.107709183429
[enth_mol=-43000]: Temperature: 1769.2528300282333
[enth_mol=-44000]: Temperature: 1781.3766245135319
[enth_mol=-45000]: Temperature: 1795.091817831292
[enth_mol=-46000]: Temperature: 1810.0660459820208

[alma-walmsley]

The problems with initialization come at the "Phase equilibrium initialization" step.

before initialization:

Vars:
fs.state[0].flow_mol: 1 fixed
fs.state[0].mole_frac_comp: water: 1.0 fixed
fs.state[0].pressure: 101325 fixed
fs.state[0].temperature: 298.15 fixed
fs.state[0].flow_mol_phase: Liq: 0.99999 unfixed
fs.state[0].flow_mol_phase: Vap: 1e-05 unfixed
fs.state[0].mole_frac_phase_comp: ('Liq', 'water'): 1.0 unfixed
fs.state[0].mole_frac_phase_comp: ('Vap', 'water'): 1.0000005210883731 unfixed
fs.state[0].phase_frac: Liq: 0.99999 unfixed
fs.state[0].phase_frac: Vap: 1e-05 unfixed
fs.state[0]._teq: ('Vap', 'Liq'): 374.51545147861975 unfixed
fs.state[0]._t1_Vap_Liq: 374.51545535191264 unfixed
fs.state[0].temperature_bubble: ('Vap', 'Liq'): 374.51545535191264 unfixed
fs.state[0]._mole_frac_tbub: ('Vap', 'Liq', 'water'): 1.0000005210883731 unfixed
fs.state[0].log_mole_frac_comp: water: -4.073182704905997e-08 unfixed
fs.state[0].log_mole_frac_tbub: ('Vap', 'Liq', 'water'): -4.4723563077377095e-08 unfixed
fs.state[0].temperature_dew: ('Vap', 'Liq'): 374.51545147861975 unfixed
fs.state[0]._mole_frac_tdew: ('Vap', 'Liq', 'water'): 0.9999995664086361 unfixed
fs.state[0].log_mole_frac_tdew: ('Vap', 'Liq', 'water'): -8.029931331992884e-08 unfixed
fs.state[0].log_mole_frac_phase_comp: ('Liq', 'water'): 0.0 unfixed
fs.state[0].log_mole_frac_phase_comp: ('Vap', 'water'): 5.210883731177063e-07 unfixed

--------------------------------
Constraints:
fs.state[0].total_flow_balance: active
fs.state[0].component_flow_balances: water: active
fs.state[0].sum_mole_frac: active
fs.state[0].phase_fraction_constraint: Liq: active
fs.state[0].phase_fraction_constraint: Vap: active
fs.state[0]._t1_constraint_Vap_Liq: active
fs.state[0].eq_temperature_bubble: ('Vap', 'Liq', 'water'): active
fs.state[0].log_mole_frac_comp_eqn: water: active
fs.state[0].log_mole_frac_tbub_eqn: ('Vap', 'Liq', 'water'): active
fs.state[0].eq_mole_frac_tbub: ('Vap', 'Liq'): active
fs.state[0]._teq_constraint_Vap_Liq: active
fs.state[0].eq_temperature_dew: ('Vap', 'Liq', 'water'): active
fs.state[0].log_mole_frac_tdew_eqn: ('Vap', 'Liq', 'water'): active
fs.state[0].eq_mole_frac_tdew: ('Vap', 'Liq'): active
fs.state[0].equilibrium_constraint: ('Vap', 'Liq', 'water'): active
fs.state[0].log_mole_frac_phase_comp_eqn: ('Liq', 'water'): active
fs.state[0].log_mole_frac_phase_comp_eqn: ('Vap', 'water'): active

after:

Vars:
fs.state[0].flow_mol: 1 fixed
fs.state[0].mole_frac_comp: water: 1 fixed
fs.state[0].pressure: 101325 fixed
fs.state[0].temperature: 298.15 fixed
fs.state[0].flow_mol_phase: Liq: 0.011998855736683581 unfixed
fs.state[0].flow_mol_phase: Vap: 0.9880011442633164 unfixed
fs.state[0].mole_frac_phase_comp: ('Liq', 'water'): 1.0000002858083423 unfixed
fs.state[0].mole_frac_phase_comp: ('Vap', 'water'): 0.999999996175808 unfixed
fs.state[0].phase_frac: Liq: 0.011998855736683581 unfixed
fs.state[0].phase_frac: Vap: 0.9880011442633164 unfixed
fs.state[0]._teq: ('Vap', 'Liq'): 374.515200140377 unfixed
fs.state[0]._t1_Vap_Liq: 374.5154503041165 unfixed
fs.state[0].temperature_bubble: ('Vap', 'Liq'): 374.51544997674324 unfixed
fs.state[0]._mole_frac_tbub: ('Vap', 'Liq', 'water'): 1.0000000000033347 unfixed
fs.state[0].log_mole_frac_comp: water: 3.190064691904021e-15 unfixed
fs.state[0].log_mole_frac_tbub: ('Vap', 'Liq', 'water'): 0.0 unfixed
fs.state[0].temperature_dew: ('Vap', 'Liq'): 374.5154499767442 unfixed
fs.state[0]._mole_frac_tdew: ('Vap', 'Liq', 'water'): 0.9999999999966523 unfixed
fs.state[0].log_mole_frac_tdew: ('Vap', 'Liq', 'water'): -6.695188269298816e-12 unfixed
fs.state[0].log_mole_frac_phase_comp: ('Liq', 'water'): 6.290014676962778e-07 unfixed
fs.state[0].log_mole_frac_phase_comp: ('Vap', 'water'): -7.983200736531084e-06 unfixed

--------------------------------
Constraints:
fs.state[0].total_flow_balance: active
fs.state[0].component_flow_balances: water: active
fs.state[0].sum_mole_frac: active
fs.state[0].phase_fraction_constraint: Liq: active
fs.state[0].phase_fraction_constraint: Vap: active
fs.state[0]._t1_constraint_Vap_Liq: active
fs.state[0].eq_temperature_bubble: ('Vap', 'Liq', 'water'): active
fs.state[0].log_mole_frac_comp_eqn: water: active
fs.state[0].log_mole_frac_tbub_eqn: ('Vap', 'Liq', 'water'): active
fs.state[0].eq_mole_frac_tbub: ('Vap', 'Liq'): active
fs.state[0]._teq_constraint_Vap_Liq: active
fs.state[0].eq_temperature_dew: ('Vap', 'Liq', 'water'): active
fs.state[0].log_mole_frac_tdew_eqn: ('Vap', 'Liq', 'water'): active
fs.state[0].eq_mole_frac_tdew: ('Vap', 'Liq'): active
fs.state[0].equilibrium_constraint: ('Vap', 'Liq', 'water'): active
fs.state[0].log_mole_frac_phase_comp_eqn: ('Liq', 'water'): active
fs.state[0].log_mole_frac_phase_comp_eqn: ('Vap', 'water'): active

Notice the change in phase_frac. It gets driven down for some reason. I suspect this is because of the equilibrium_constraint constraint.

[alma-walmsley]

Solving through 373 K proved infeasible. (water)

[alma-walmsley]

Okay this is the weirdest thing. Same T values, different initial values