CPA Water, Ice and Hydrate Boundaries

ThermodynamicOperations.cpaWaterPhaseBoundaryTemperatures() provides one calculation for three water-related equilibrium boundaries at the supplied absolute pressure. It requires a configured SystemSrkCPAstatoil fluid with numeric mixing rule 10 and positive water content. The calculation uses independent copies and preserves the supplied fluid.

Agent-facing MCP operation

For a new JSON-defined fluid, use the existing run_flow_assurance capability with analysis: "waterPhaseBoundaries". Read its current input schema and run validateInput before invoking the connected tool. The analysis defaults to Statoil CPA and numeric mixing rule 10; an explicitly incompatible model is rejected. Absolute pressure_bara is required.

This payload is executed by FlowAssuranceRunnerTest.testCpaWaterPhaseBoundaries:

{
  "analysis": "waterPhaseBoundaries",
  "model": "CPA",
  "temperature_C": 20.0,
  "pressure_bara": 50.0,
  "components": {
    "methane": 0.89991,
    "ethane": 0.079992,
    "propane": 0.019998,
    "water": 0.0001
  }
}

The component map is the complete normalized wet-gas composition: it represents a 90/8/2 dry methane/ethane/propane gas with 100 ppmv wet-basis water. For new characterized oil inputs, use the schema’s typed tbpFractions and plusFraction definitions rather than inventing component names. Existing tuned or characterized Java fluids should use the facade directly, as shown below, so reconstructing a JSON fluid does not discard their fitted model basis.

The response has top-level status equal to success, partial or error. For error, report the returned diagnostic and any retained partialData; do not assume the normal data object exists. Successful and partial responses contain data.liquidWater, data.ice and data.hydrate results, each with status, converged, temperature_C, residual, waterActivity, phaseReference and diagnostic. Failed or inapplicable numerical values are JSON null, never a usable last-trial temperature. Inspect all three outcomes even when the overall status is partial, and report the retained explanation and failure reasons.

Meaning of the results

Result	Equilibrium reference	Interpretation
`getLiquidWater()`	An incipient water-rich liquid, including dissolved components	A fixed-feed-gas dew point; below freezing its liquid reference is metastable
`getIce()`	Pure ice Ih against the equilibrium fluid	A freezing/frost boundary with its actual fluid reference identified
`getHydrate()`	A water/guest-gas clathrate against the equilibrium fluid	Includes the water activity of supplied aqueous inhibitors and the fluid-phase split

The operations are separate onset calculations. Once water, ice or hydrate actually forms, the remaining water and guest inventories change. The three temperatures therefore do not describe successive points on one finite-inventory cooling path. The highest applicable equilibrium boundary can identify the first thermodynamically permitted phase during cooling, but does not predict nucleation delay or a growth rate.

The residuals are dimensionless but have boundary-specific definitions:

Boundary	Returned residual
Liquid water	Logarithm of gas-water fugacity divided by incipient-liquid-water fugacity
Ice	The freezing solver’s logarithmic overall-composition/multiphase fugacity residual
Hydrate	One minus hydrate-water fugacity divided by fluid-water fugacity

Compare each residual with its own convergence criterion; equal residual numbers do not imply equal temperature uncertainty between the three models.

Executable Java 8 setup

This synthetic gas has a dry composition of 90% methane, 8% ethane and 2% propane, with 100 ppmv water on a wet-gas basis, at 50 bara.

import neqsim.thermo.system.SystemInterface;
import neqsim.thermo.system.SystemSrkCPAstatoil;
import neqsim.thermodynamicoperations.ThermodynamicOperations;
import neqsim.thermodynamicoperations.WaterPhaseBoundaryResult;

SystemInterface gas = new SystemSrkCPAstatoil(293.15, 50.0);
double waterMoleFraction = 100.0e-6;
double dryMoleFraction = 1.0 - waterMoleFraction;
gas.addComponent("methane", 0.90 * dryMoleFraction);
gas.addComponent("ethane", 0.08 * dryMoleFraction);
gas.addComponent("propane", 0.02 * dryMoleFraction);
gas.addComponent("water", waterMoleFraction);
gas.setMixingRule(10);

WaterPhaseBoundaryResult boundaries =
    new ThermodynamicOperations(gas).cpaWaterPhaseBoundaryTemperatures();

double liquidWaterC = boundaries.getLiquidWater().getTemperatureC();
double iceC = boundaries.getIce().getTemperatureC();
double hydrateC = boundaries.getHydrate().getTemperatureC();

Before using a temperature, inspect the corresponding result’s getStatus() and isConverged(), followed by getResidual(), getPhaseReference(), getWaterActivity() and getDiagnostic(). A failed or inapplicable result has no usable onset temperature. The top-level result retains getPressureBara(), getModel(), getMixingRule(), getWaterMoleFraction() and getExplanation().

The configured-fluid and caller-preservation contracts are exercised by CpaWaterPhaseBoundaryCalculatorTest. Solver regressions separately exercise warm/cold starts, water content, pressure, inhibitors, phase inventory and final equilibrium residuals. These checks establish the stated numerical contracts; they do not establish experimental accuracy for every composition.

Inhibitors, oil and multiple phases

Retain oil characterization, tuned parameters, the actual water inventory and all inhibitor components in the CPA fluid. Use setMultiPhaseCheck(true) when multiple liquid phases can occur. Specify whether an inhibitor concentration is an aqueous-phase mass fraction, an overall mole fraction or another basis. Replacing a wet oil/gas/water inventory with an arbitrary gas ppmv changes the engineering problem.

The hydrate calculation uses the water activity of the actual fluid, including supplied MEG or methanol, and updates its fluid-phase split during the temperature solve. A free-water hydrate curve is a different input basis from trace-water gas. The combined calculation does not inject water, remove oil or remove inhibitors to produce a result.

The liquid-water dew-point operation is a fixed-feed-gas operation with a water-rich incipient liquid. If the supplied equilibrium fluid already contains oil or aqueous liquid, this boundary is NOT_APPLICABLE; its diagnostic explains why. The independently evaluated ice and hydrate results remain available. Read getPhaseReference() to distinguish gas-only from coexisting-fluid references. Do not describe an equilibrium-fluid freezing result as a fixed-gas-composition frost point.

Ice pressure model and qualification

The ice calculation retains the existing fusion-enthalpy and constant heat-capacity-difference approximation relative to the same CPA pure-liquid water reference. Its water pressure correction uses the ice-Ih melting-curve slope at the triple point from IAPWS R14-08(2011), equation 1. The triple-point values are 273.16 K and 611.657 Pa; pressure in bara must be converted to pascals in the pressure-volume contribution.

The solid-minus-liquid volume difference is obtained from the Clapeyron relation at that reference state. It restores the expected decrease in pure-water melting temperature with increasing pressure without relying on the bundled empirical water density records. This remains a constant-volume fusion/Cp approximation, not a complete IAPWS ice equation of state or a general high-pressure ice-polymorph model. The pressure correction is checked against the IAPWS ice-Ih melting curve over 1–100 bara; that comparison does not extend the model to other ice polymorphs or establish the experimental accuracy of a natural-gas frost point. Retain the tested range and independent reference evidence with engineering results.

Agent use

For water dew point with CPA or vannduggpunkt med CPA, the neqsim-cpa-water-boundaries skill asks the gas-quality and thermo-fluid agents to report all three boundaries by default, their convergence status and their physical differences. Existing characterized fluids stay intact; unsupported input models require explicit configuration rather than an unsafe conversion from component names.

GERG-water/ISO 18453 is a separate model comparison. Below its coefficient switch it uses an ice reference, and above that switch it uses liquid water. Label that reference explicitly when comparing a GERG result with a CPA liquid-water or ice result.