fix(neqsim): re-flash gas-only system after liquid removal (#1553)

Author: olelod

src/ecalc_neqsim_wrapper/thermo.py

@@ -347,9 +347,11 @@ def _ph_flash(thermodynamic_system: ThermodynamicSystem, use_gerg: bool, enthalp
         return thermodynamic_system
 
     @staticmethod
-    def _remove_liquid(thermodynamic_system: ThermodynamicSystem) -> ThermodynamicSystem:
-        """Remove liquid part of thermodynamic_system, return new NeqsimFluid object with only gas part."""
-        return thermodynamic_system.clone().phaseToSystem("gas")
+    def _remove_liquid(thermodynamic_system: ThermodynamicSystem, use_gerg: bool) -> ThermodynamicSystem:
+        """Return a gas-only thermodynamic system, re-flashed at the current (P, T)."""
+        gas_only_system = thermodynamic_system.clone().phaseToSystem("gas")
+        gas_only_system = NeqsimFluid._tp_flash(thermodynamic_system=gas_only_system, use_gerg=use_gerg)
+        return gas_only_system
 
     def clone_gas_phase(self) -> NeqsimFluid:
         """Clone this fluid, extracting only the gas phase.
@@ -360,7 +362,7 @@ def clone_gas_phase(self) -> NeqsimFluid:
         Returns:
             New NeqsimFluid with gas phase only.
         """
-        gas_only_system = NeqsimFluid._remove_liquid(self._thermodynamic_system)
+        gas_only_system = NeqsimFluid._remove_liquid(self._thermodynamic_system, use_gerg=self._use_gerg)
         return NeqsimFluid(thermodynamic_system=gas_only_system, use_gerg=self._use_gerg)
 
     def copy(self) -> NeqsimFluid:

tests/ecalc_neqsim_wrapper/integration_tests/test_remove_liquid.py

@@ -122,5 +122,49 @@ def test_liquid_takeoff(inlet_fluid, outlet_fluid) -> None:
     assert math.isclose(outlet_fluid.pressure_bara, fluid_after_liquid_takeoff.pressure_bara, rel_tol=0.01)
     assert math.isclose(outlet_fluid.molar_mass, fluid_after_liquid_takeoff.molar_mass, rel_tol=0.01)
     assert math.isclose(
-        outlet_fluid.enthalpy_joule_per_kg, fluid_after_liquid_takeoff.enthalpy_joule_per_kg, rel_tol=0.5
+        outlet_fluid.enthalpy_joule_per_kg, fluid_after_liquid_takeoff.enthalpy_joule_per_kg, rel_tol=0.02
     )
+
+
+# Propane-rich composition that is ~97% liquid at the probe (P, T) below.
+DENSE_LIQUID_COMPOSITION = FluidComposition(
+    water=0.003,
+    nitrogen=2.447,
+    CO2=0.64,
+    methane=41.91,
+    ethane=19.9,
+    propane=24.29,
+    i_butane=3.64,
+    n_butane=5.40,
+    i_pentane=0.83,
+    n_pentane=0.68,
+    n_hexane=0.26,
+).normalized()
+
+
+def test_clone_gas_phase_returns_initialized_properties() -> None:
+    """clone_gas_phase() on a two-phase fluid must return properties matching a clean PT-flash of the gas-only composition."""
+    two_phase = NeqsimFluid.create_thermo_system(
+        composition=DENSE_LIQUID_COMPOSITION,
+        temperature_kelvin=301.45,
+        pressure_bara=91.476,
+    )
+    assert two_phase.vapor_fraction_molar < 0.1, (
+        f"Test precondition: expected mostly-liquid state, got vap={two_phase.vapor_fraction_molar}"
+    )
+
+    gas_only = two_phase.clone_gas_phase()
+
+    assert math.isclose(gas_only.vapor_fraction_molar, 1.0, rel_tol=1e-3)
+    # Uninitialized systems return the sentinel kappa==1.0.
+    assert gas_only.kappa > 1.02, f"Suspicious kappa (uninitialized?): {gas_only.kappa}"
+
+    reference = NeqsimFluid.create_thermo_system(
+        composition=gas_only.composition,
+        temperature_kelvin=gas_only.temperature_kelvin,
+        pressure_bara=gas_only.pressure_bara,
+    )
+    assert math.isclose(gas_only.kappa, reference.kappa, rel_tol=1e-3)
+    assert math.isclose(gas_only.z, reference.z, rel_tol=1e-3)
+    assert math.isclose(gas_only.density, reference.density, rel_tol=1e-3)
+    assert math.isclose(gas_only.enthalpy_joule_per_kg, reference.enthalpy_joule_per_kg, rel_tol=1e-3)