refactor: pass fluid_service to SimplifiedStreamMixer in __init__

- Remove unnecessary @DataClass(frozen=True) decorator
- Convert mix_streams from static method to instance method
- Update Mixer to create SimplifiedStreamMixer instance
- Update tests to use new instance-based API

Author: kjbrak

src/libecalc/domain/process/entities/process_units/mixer/mixer.py

@@ -7,7 +7,7 @@
 class Mixer:
     def __init__(self, number_of_inputs: int, fluid_service: FluidService):
         self.number_of_inputs = number_of_inputs
-        self.fluid_service = fluid_service
+        self._stream_mixer = SimplifiedStreamMixer(fluid_service)
 
     def mix_streams(self, streams: list[FluidStream]) -> FluidStream:
         """
@@ -22,4 +22,4 @@ def mix_streams(self, streams: list[FluidStream]) -> FluidStream:
         if self.number_of_inputs != len(streams):
             raise ValueError("Number of input streams must match the number of inputs defined for the mixer.")
 
-        return SimplifiedStreamMixer.mix_streams(streams=streams, fluid_service=self.fluid_service)
+        return self._stream_mixer.mix_streams(streams=streams)

src/libecalc/domain/process/entities/process_units/simplified_stream_mixer/simplified_stream_mixer.py

@@ -1,5 +1,4 @@
 from collections import defaultdict
-from dataclasses import dataclass
 
 from libecalc.domain.process.value_objects.fluid_stream import FluidService, FluidStream
 from libecalc.domain.process.value_objects.fluid_stream.exceptions import (
@@ -12,7 +11,6 @@
 
 
 # TODO: Use enthalpy balance instead of simplified mixing?
-@dataclass(frozen=True)
 class SimplifiedStreamMixer:
     """Process unit for mixing multiple fluid streams.
 
@@ -24,13 +22,14 @@ class SimplifiedStreamMixer:
     Note: This method is most appropriate when mixing streams with similar temperatures.
     """
 
-    @staticmethod
-    def mix_streams(streams: list[FluidStream], fluid_service: FluidService) -> FluidStream:
+    def __init__(self, fluid_service: FluidService):
+        self._fluid_service = fluid_service
+
+    def mix_streams(self, streams: list[FluidStream]) -> FluidStream:
         """Mix multiple streams using component-wise molar balance.
 
         Args:
             streams: List of streams to mix (must have same EoS model)
-            fluid_service: Service for performing flash operations
 
         Returns:
             A new FluidStream representing the mixed stream
@@ -83,7 +82,7 @@ def mix_streams(streams: list[FluidStream], fluid_service: FluidService) -> Flui
         )
 
         # Get properties at mixed conditions via fluid service
-        mix_props = fluid_service.flash_pt(
+        mix_props = self._fluid_service.flash_pt(
             fluid_model=mix_fluid_model,
             pressure_bara=reference_pressure,
             temperature_kelvin=temperature_mix,

tests/libecalc/domain/process/entities/process_units/test_stream_mixer.py

@@ -44,7 +44,8 @@ def test_mix_medium_and_ultra_rich_compositions(self, medium_composition, ultra_
         ultra_rich_stream = FluidStream(fluid=ultra_rich_fluid, mass_rate_kg_per_h=mass_rate_ultra_rich)
 
         # Mix streams using StreamMixer
-        mixed_stream = SimplifiedStreamMixer.mix_streams([medium_stream, ultra_rich_stream], fluid_service=service)
+        mixer = SimplifiedStreamMixer(fluid_service=service)
+        mixed_stream = mixer.mix_streams([medium_stream, ultra_rich_stream])
 
         # Expected values from mixing medium (30%) and ultra rich (70%) compositions
         expected_values = {
@@ -88,7 +89,8 @@ def test_mix_streams_with_different_conditions(self, medium_composition):
         stream2 = FluidStream(fluid=fluid2, mass_rate_kg_per_h=500.0)
 
         # Mix streams
-        mixed_stream = SimplifiedStreamMixer.mix_streams([stream1, stream2], fluid_service=service)
+        mixer = SimplifiedStreamMixer(fluid_service=service)
+        mixed_stream = mixer.mix_streams([stream1, stream2])
 
         # Verify the mixed stream uses the simplified mass-weighted average temperature and lowest pressure
         expected_temperature = (
@@ -123,4 +125,5 @@ def test_mix_streams_with_different_eos_models(self, medium_composition):
         stream2 = FluidStream(fluid=pr_fluid, mass_rate_kg_per_h=500.0)
 
         with pytest.raises(IncompatibleEoSModelsException):
-            SimplifiedStreamMixer.mix_streams([stream1, stream2], fluid_service=service)
+            mixer = SimplifiedStreamMixer(fluid_service=service)
+            mixer.mix_streams([stream1, stream2])