fix(process): scale mass rate when LiquidRemover drops out liquid phase (#1546)

test(process): assert LiquidRemover scales mass rate by gas mass fraction

docs: changelog entry for LiquidRemover mass-rate fix

fix(process): raise on non-positive inlet molar mass in LiquidRemover

test(process): assert LiquidRemover raises on non-positive inlet molar mass

Author: olelod

docs/drafts/next.draft.md

@@ -14,6 +14,7 @@ STP: "flare" column has been added to STP Export - for `FIXED` installations onl
 ## Bug Fixes
 
 - Hardened compressor PH flash handling so invalid thermodynamic states are no longer used in compressor outlet calculations.
+- `LiquidRemover` now scales the outlet mass rate by the gas mass fraction when liquid is dropped, so the removed liquid mass is no longer carried by the gas stream downstream.
 
 ## Breaking changes
 

src/libecalc/process/process_units/liquid_remover.py

@@ -1,6 +1,7 @@
 from typing import Final
 
 from libecalc.process.fluid_stream.constants import ThermodynamicConstants
+from libecalc.process.fluid_stream.exceptions import InvalidStreamException
 from libecalc.process.fluid_stream.fluid_service import FluidService
 from libecalc.process.fluid_stream.fluid_stream import FluidStream
 from libecalc.process.process_pipeline.process_unit import ProcessUnit, ProcessUnitId
@@ -16,7 +17,14 @@ def get_id(self) -> ProcessUnitId:
 
     def propagate_stream(self, inlet_stream: FluidStream) -> FluidStream:
         """
-        Removes liquid from the fluid stream.
+        Removes liquid from the fluid stream. The new stream's mass rate is scaled
+        down by the gas mass fraction so the dropped-out liquid isn't re-injected
+        into the gas phase.
+
+        The removed liquid (mass = inlet.mass_rate * (1 - gas_mass_fraction),
+        composition = inlet - new_fluid) is currently discarded. It could later
+        be exposed as a separate outlet stream — e.g. routed to an oil pump,
+        accounted for in emissions, or reported back to the user.
 
         Args:
             inlet_stream: The fluid stream to be scrubbed.
@@ -26,6 +34,13 @@ def propagate_stream(self, inlet_stream: FluidStream) -> FluidStream:
         """
         if inlet_stream.vapor_fraction_molar < ThermodynamicConstants.PURE_VAPOR_THRESHOLD:
             new_fluid = self._fluid_service.remove_liquid(inlet_stream.fluid)
-            return inlet_stream.with_new_fluid(new_fluid)
+            inlet_molar_mass = inlet_stream.fluid.molar_mass
+            if inlet_molar_mass <= 0.0:
+                raise InvalidStreamException(
+                    f"Cannot remove liquid from a degenerate stream with non-positive molar mass: {inlet_molar_mass}"
+                )
+            gas_mass_fraction = inlet_stream.vapor_fraction_molar * new_fluid.molar_mass / inlet_molar_mass
+            new_mass_rate = inlet_stream.mass_rate_kg_per_h * gas_mass_fraction
+            return inlet_stream.with_new_fluid(new_fluid).with_mass_rate(new_mass_rate)
         else:
             return inlet_stream

tests/libecalc/process/process_units/test_liquid_remover.py

@@ -1,6 +1,12 @@
+from unittest.mock import MagicMock
+
+import pytest
+
 from ecalc_neqsim_wrapper.thermo import STANDARD_PRESSURE_BARA, STANDARD_TEMPERATURE_KELVIN
+from libecalc.process.fluid_stream.exceptions import InvalidStreamException
 from libecalc.process.fluid_stream.fluid_model import EoSModel, FluidComposition, FluidModel
 from libecalc.process.fluid_stream.fluid_stream import FluidStream
+from libecalc.process.process_units.liquid_remover import LiquidRemover
 
 
 def test_liquid_remover_removes_liquid(fluid_service, liquid_remover_factory):
@@ -33,3 +39,48 @@ def test_liquid_remover_removes_liquid(fluid_service, liquid_remover_factory):
 
     assert inlet_stream.vapor_fraction_molar < 1.0
     assert outlet_stream.vapor_fraction_molar == 1.0
+
+    expected_gas_mass_fraction = (
+        inlet_stream.vapor_fraction_molar * outlet_stream.fluid.molar_mass / inlet_stream.fluid.molar_mass
+    )
+    expected_mass_rate = inlet_stream.mass_rate_kg_per_h * expected_gas_mass_fraction
+    assert outlet_stream.mass_rate_kg_per_h < inlet_stream.mass_rate_kg_per_h
+    assert outlet_stream.mass_rate_kg_per_h == expected_mass_rate
+
+
+def test_liquid_remover_passthrough_when_no_liquid(fluid_service, liquid_remover_factory):
+    composition = FluidComposition(
+        nitrogen=3,
+        CO2=1,
+        methane=80,
+        ethane=10,
+        propane=6,
+    )
+    fluid_model = FluidModel(eos_model=EoSModel.SRK, composition=composition)
+    fluid = fluid_service.create_fluid(
+        fluid_model=fluid_model,
+        pressure_bara=STANDARD_PRESSURE_BARA,
+        temperature_kelvin=STANDARD_TEMPERATURE_KELVIN,
+    )
+    inlet_stream = FluidStream.from_standard_rate(
+        standard_rate_m3_per_day=100000,
+        fluid_model=fluid.fluid_model,
+        fluid_properties=fluid.properties,
+    )
+    remover = liquid_remover_factory()
+    outlet_stream = remover.propagate_stream(inlet_stream)
+
+    assert inlet_stream.vapor_fraction_molar == 1.0
+    assert outlet_stream.mass_rate_kg_per_h == inlet_stream.mass_rate_kg_per_h
+
+
+def test_liquid_remover_raises_on_non_positive_inlet_molar_mass():
+    inlet_stream = MagicMock()
+    inlet_stream.vapor_fraction_molar = 0.5
+    inlet_stream.fluid.molar_mass = 0.0
+
+    fluid_service = MagicMock()
+    remover = LiquidRemover(fluid_service=fluid_service)
+
+    with pytest.raises(InvalidStreamException, match="non-positive molar mass"):
+        remover.propagate_stream(inlet_stream)