Neqsim Phase Envelope

NeqSim PT Phase Envelopes

Overview

Use this skill to generate physically interpretable PT phase envelopes and to modify the Michelsen continuation solver safely. Prefer the structured segment API, verify branch identity from physics, and preserve the caller's thermodynamic system.

Route the Task

• Use @thermo.fluid for envelope generation, plotting, properties, and solver defects.
• Chain to neqsim-eos-regression or @pvt.simulation when matching lab data or tuning EOS parameters.
• Chain to neqsim-flow-assurance for operating-path, hydrate, wax, or pipeline assessments.
• Chain to neqsim-ccs-hydrogen for CO2/H2 impurity envelopes.
• Chain to neqsim-troubleshooting when continuation does not converge or output is incomplete.

Workflow

1. Verify the API and inputs

Read these sources before generating or changing code:

• ThermodynamicOperations.calcPTphaseEnvelope(...) and getEnvelopeSegments()
• PTPhaseEnvelopeMichelsen result getters and continuation settings
• EnvelopeSegment getters and PhaseType
• Neighboring tests in PTPhaseEnvelopeMichelsenTest, PTPhaseEnvelopeRobustnessTest, and PTPhaseEnvelopeSegmentsTest

Validate that temperature is in K, pressure is in bara, component amounts are finite and non-negative, and the mixing rule is set. Choose the EOS from the fluid chemistry, not from the desired plot shape.

2. Handle zero and trace components correctly

The Michelsen Jacobian contains terms divided by overall mole fraction $z_i$. Components with zero or numerically negligible $z_i$ can therefore make the Jacobian singular.

• Current PTPhaseEnvelopeMichelsen excludes components with $z_i < 10^{-12}$ from a private clone after composition initialization.
• Do not remove components from the caller's SystemInterface.
• Do not discard physically meaningful trace impurities at or above the threshold merely to obtain convergence. Their impact can be important for CO2 quality, water behavior, and H2S.
• When changing the threshold or filtering logic, add a regression proving both finite envelope output and unchanged caller composition.

3. Generate the envelope

SystemInterface fluid = new SystemSrkEos(273.15 + 25.0, 50.0);
fluid.addComponent("methane", 0.85);
fluid.addComponent("ethane", 0.10);
fluid.addComponent("propane", 0.05);
fluid.setMixingRule("classic");

ThermodynamicOperations ops = new ThermodynamicOperations(fluid);
ops.calcPTphaseEnvelope(true, 1.0);

List<EnvelopeSegment> segments = ops.getEnvelopeSegments();
double[] cricondenbar = ops.get("cricondenbar");
double[] cricondentherm = ops.get("cricondentherm");

Use calcPTphaseEnvelope(true, lowPressureBara) when the start side and lower pressure matter. Use the no-argument overload for the standard calculation.

4. Consume structured segments by default

Use getEnvelopeSegments() for plotting, JSON export, and AI-generated analysis. Each segment is contiguous and contains no NaN branch separators.

for (EnvelopeSegment segment : ops.getEnvelopeSegments()) {
  double[] temperaturesK = segment.getTemperatures();
  double[] pressuresBara = segment.getPressures();
  EnvelopeSegment.PhaseType storedType = segment.getPhaseType();
  // Plot each segment independently; do not connect separate segments.
}

The legacy ops.get("dewT"), ops.get("dewP"), ops.get("bubT"), and ops.get("bubP") arrays remain supported. They may contain intentional NaN values that mark branch breaks. Never reject an envelope solely because these flat arrays contain NaN; reject infinities and require finite physical points.

5. Classify physical branches

For calcPTphaseEnvelope(true, 1.0), stored dew/bubble labels can be swapped by the historical Michelsen tracing order. This applies to legacy array names and can affect EnvelopeSegment stored PhaseType labels. Do not infer physical identity from a getter or enum name alone.

Classify the physical dew side as the branch containing the cricondentherm, normally the branch with the highest finite temperature. The other side is the physical bubble branch. Preserve segment boundaries while grouping segments by physical side.

finite_a = branch_a_t[np.isfinite(branch_a_t)]
finite_b = branch_b_t[np.isfinite(branch_b_t)]
if finite_a.max() > finite_b.max():
    dew_t, dew_p = branch_a_t, branch_a_p
    bubble_t, bubble_p = branch_b_t, branch_b_p
else:
    dew_t, dew_p = branch_b_t, branch_b_p
    bubble_t, bubble_p = branch_a_t, branch_a_p

If the branches have overlapping maxima or an unusual topology, use the reported cricondentherm point and critical-point continuity instead of relying only on maximum temperature.

6. Validate the result

Require all applicable checks:

1. At least one non-empty structured segment.
2. At least one finite point on each expected physical branch.
3. No infinite temperatures or pressures.
4. Positive pressures and physically plausible temperatures.
5. Cricondentherm equals the maximum finite envelope temperature within numerical tolerance.
6. Cricondenbar equals the maximum finite envelope pressure within numerical tolerance.
7. Bubble and dew branches meet continuously near the critical point.
8. The original fluid still has the same component count and composition.
9. Benchmark key points against lab, literature, or another trusted EOS implementation when results drive an engineering decision.

Do not require every flat-array value to be finite because NaN is the branch-break sentinel.

Solver Change Protocol

When modifying PTPhaseEnvelopeMichelsen or SysNewtonRhapsonPhaseEnvelope:

1. Add a focused regression that fails before the code change.
2. Assert physical outputs, not only assertDoesNotThrow.
3. Cover zero fraction, near-zero fraction, ordinary gas, and a heavy/TBP mixture as relevant.
4. Verify the operation may use a filtered clone while the caller's system is unchanged.
5. Preserve two-pass continuation, critical-point handling, and segment construction.
6. Run the focused regression immediately after the first edit.
7. Run the full Michelsen and robustness test classes.
8. Run Java 8 compilation, Spotless, Checkstyle, and JavaDoc gates.

Focused Windows commands:

.\mvnw.cmd test "-Dtest=PTPhaseEnvelopeMichelsenTest#testMethod"
.\mvnw.cmd test "-Dtest=PTPhaseEnvelopeMichelsenTest,PTPhaseEnvelopeRobustnessTest,PTPhaseEnvelopeSegmentsTest"
.\mvnw.cmd --file pomJava8.xml test "-Dtest=PTPhaseEnvelopeMichelsenTest#testMethod" "-Djacoco.skip=true"
.\mvnw.cmd spotless:apply
.\mvnw.cmd spotless:check

Use explicit Java 8 types. Do not use var, List.of, String.repeat, records, or other Java 9+ APIs. Use Log4j2 rather than System.out or System.err.

Troubleshooting

Always Cross-Check a Lean Gas With a Flash Scan

On a lean gas with a small heavy tail the Michelsen continuation can truncate the dew branch and still return a plausible-looking envelope. Observed on a 90.5 mol% methane gas-cap gas with 0.25 mol% C7+: calcPTphaseEnvelope(true, 1.0) reported a cricondentherm of −66 °C, while a direct TPflash at 6 °C and 45 bara found 0.9 mol% liquid — the true cricondentherm was +41 °C. Nothing in the envelope output flagged the truncation, and the error would have hidden the whole liquid-handling and slugging issue for a subsea tie-back.

Cheap guard: bisect on the phase count at one or two pressures and compare.

def dew_point_temperature(fluid, pressure_bara, t_high=90.0, t_low=-60.0):
    """Upper (retrograde) dew temperature by bisection on the phase count."""
    lo, hi = t_low, t_high
    for _ in range(40):
        mid = 0.5 * (lo + hi)
        w = fluid.clone()
        w.setTemperature(273.15 + mid)
        w.setPressure(pressure_bara)
        ThermodynamicOperations(w).TPflash()
        if int(w.getNumberOfPhases()) > 1:
            lo = mid
        else:
            hi = mid
    return 0.5 * (lo + hi)

If the scan and the envelope disagree by more than a few kelvin, trust the scan and report the envelope as unreliable for that fluid.

Point Dew-Point Flashes on Wet Gas

ThermodynamicOperations.dewPointTemperatureFlash() seeds an aqueous incipient liquid whenever water carries moles, so on a wet gas it returns the water dew point. For a hydrocarbon dew point, clone the fluid and removeComponent("water") first.

Seed the flash at the cricondentherm temperature so it descends onto the upper (physical) dew branch rather than a low-temperature retrograde root:

double[] cct = envOps.get("cricondentherm"); // [T (K), P (bara)]
hcFluid.setPressure(pBara, "bara");
hcFluid.setTemperature(cct[0]);
hcFluid.init(0);
new ThermodynamicOperations(hcFluid).dewPointTemperatureFlash();

Zero-fraction water no longer changes the result: the aqueous seed is gated on ConstantDutyTemperatureFlash.hasSignificantWater, so a component with $z_i = 0$ behaves like an absent one.

Output Convention

Report:

• EOS and mixing rule
• normalized composition and any filtered numerical-zero components
• critical point, cricondenbar, and cricondentherm with K/bara units
• physically classified dew and bubble segments
• operating points or paths overlaid on the envelope when relevant
• convergence or filtering warnings
• benchmark source and deviations for decision-critical work

Known Limitations

• Stored branch labels can differ from physical branch identity for bubble-first tracing.
• Cubic-EOS envelope accuracy depends on heavy-end characterization and binary interaction parameters; numerical convergence does not prove physical accuracy.
• PT phase envelopes do not replace hydrate, wax, solid, electrolyte, or reactive-equilibrium boundaries. Add those analyses through their dedicated skills.