Revalidate EOS-CG phase assertion in CO2 low-temperature notebook #151

Context

A documentation-only catalog update in draft PR #150 triggered the existing CO2 low-temperature notebook validation workflow. The workflow is master-dependent and failed while executing:

notebooks/thermodynamics/CO2_low_temperature_compression_relief_and_depressurization.ipynb

Failed run: https://github.com/EvenSol/NeqSim-Colab/actions/runs/33525804944

This is not caused by the files in PR #150; the notebook and its workflow are unchanged there.

Reproducible environment

• NeqSim Java source: current equinor/neqsim master at fdf6b227b4240589ebbfb900527b37e667f3efc8
• NeqSim Python bridge: 3.17.0
• Python: 3.12
• Java: 17
• Workflow: .github/workflows/co2-low-temperature-notebook-validation.yml

The notebook was last recorded as passing against NeqSim commit ef31a3ba78a325b5784a33486bb047a264425412.

Failure

All checks passed except:

EOS-CG detects nominal two-phase state at 25 C: False

The assertion expects two phases for the notebook's nominal state at 70 bara and 25 °C with 1 mol% total impurity split as 50% nitrogen, 30% oxygen, and 20% argon. The retained validated notebook reports two phases and gas beta 0.03087 for that state; the current-master execution no longer satisfies getNumberOfPhases() == 2.

Requested investigation

1. Reproduce the state against commits ef31a3b and fdf6b22.
2. Identify whether the phase-count change is an intended EOS-CG correction, a flash/model regression, or an overly brittle notebook acceptance point.
3. If intended, update the notebook's physical interpretation, validation condition, retained outputs, and ledger with an independently justified phase-boundary comparison.
4. If unintended, open/link an actionable equinor/neqsim defect and pin the notebook workflow only as a temporary measure.
5. Restore the existing workflow to green against its declared source policy.

Acceptance criteria

• The expected phase behavior at the exact nominal state is documented with commit provenance.
• The check tests a physically justified property rather than simply weakening the assertion.
• The notebook is cleanly re-executed, rendered, and visually reviewed.
• The CO2 low-temperature workflow passes.