title: “Phase Models and Phase-Level Properties” description: “Current NeqSim phase-model API boundaries, phase fractions, initialization, phase-type detection, and safe property access.” keywords: “NeqSim, phase model, PhaseInterface, PhaseType, phase fraction, volume fraction, physical properties, hydrate, asphaltene” —

NeqSim represents each equilibrium phase with a PhaseInterface, while the containing SystemInterface owns the phase list, phase-type lookup, and phase-fraction conversions. Most users should create and flash a thermodynamic system, then inspect the returned interfaces. Concrete classes such as PhaseGasEos are implementation details; casting to them makes an example model-dependent.

API ownership at a glance

Task Current API Meaning
Enumerate phases fluid.getNumberOfPhases(), fluid.getPhase(index) Inspect every phase returned by the latest calculation
Detect a type fluid.hasPhaseType(PhaseType.GAS) Check before type-specific access
Retrieve a type fluid.getPhase(PhaseType.GAS) Obtain the phase without a concrete-class cast
Molar phase fraction phase.getBeta() or fluid.getBeta(index) Fraction of total system moles in that phase
Volume fraction fluid.getVolumeFraction(index) Fraction of total system volume
Mass fraction fluid.getWtFraction(index) Fraction of total system mass
Thermodynamic properties phase.getDensity(unit), getEnthalpy(unit), getCp(unit), getZ() Phase-level values after thermodynamic initialization
Transport properties phase.getViscosity(unit), getThermalConductivity(unit) Require physical-property initialization

There is no PhaseInterface.getBetaV(). A molar phase fraction is not interchangeable with a volume or mass fraction, especially when gas and liquid densities differ greatly.

Complete phase-inspection example

This Java 8 example uses only public interfaces and logs through Log4j2.

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import neqsim.thermo.phase.PhaseInterface;
import neqsim.thermo.system.SystemInterface;
import neqsim.thermo.system.SystemSrkEos;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

public final class PhaseInspectionExample {
  private static final Logger logger =
      LogManager.getLogger(PhaseInspectionExample.class);

  private PhaseInspectionExample() {}

  public static void main(String[] args) {
    SystemInterface fluid = new SystemSrkEos(300.0, 50.0);
    fluid.addComponent("methane", 0.80);
    fluid.addComponent("ethane", 0.10);
    fluid.addComponent("propane", 0.05);
    fluid.addComponent("n-pentane", 0.05);
    fluid.setMixingRule("classic");

    ThermodynamicOperations operations = new ThermodynamicOperations(fluid);
    operations.TPflash();
    fluid.initProperties();

    for (int phaseIndex = 0;
        phaseIndex < fluid.getNumberOfPhases();
        phaseIndex++) {
      PhaseInterface phase = fluid.getPhase(phaseIndex);

      logger.info(
          "phase={} type={} moleFraction={} volumeFraction={} massFraction={}",
          phaseIndex,
          phase.getType(),
          phase.getBeta(),
          fluid.getVolumeFraction(phaseIndex),
          fluid.getWtFraction(phaseIndex));
      logger.info(
          "density={} kg/m3, z={}, viscosity={} cP, conductivity={} W/mK",
          phase.getDensity("kg/m3"),
          phase.getZ(),
          phase.getViscosity("cP"),
          phase.getThermalConductivity("W/mK"));
    }
  }
}

The focused documentation regression test executes the same thermodynamic and property calls. The example intentionally reports calculated values instead of promising a fixed phase count or fixed numbers: phase appearance and properties depend on model, composition, temperature, pressure, and enabled phase checks.

Initialization sequence

Use the following sequence for phase-equilibrium and property work:

  1. Define composition, temperature, pressure, thermodynamic model, and mixing rule.
  2. Run the appropriate operation, such as TPflash().
  3. Call fluid.initProperties() when transport properties are needed.
  4. Inspect getNumberOfPhases() and phase types before retrieving a specific phase.
  5. Re-run the flash and initialization after changing state or composition.

TPflash() determines the equilibrium phase split. initProperties() initializes the thermodynamic state and the selected physical-property models. Diffusion is not exposed as a zero-argument method on PhaseInterface; use the initialized physical-properties API described in the physical-properties guide.

Phase types and safe lookup

PhaseType currently defines these stable enum names and string descriptors:

Enum Descriptor
LIQUID liquid
GAS gas
OIL oil
AQUEOUS aqueous
HYDRATE gas hydrate
WAX wax
SOLID solid
SOLIDCOMPLEX solidComplex
ASPHALTENE asphaltene
LIQUID_ASPHALTENE asphaltene liquid

Prefer enum lookup because it avoids descriptor spelling errors:

if (fluid.hasPhaseType(PhaseType.GAS)) {
  PhaseInterface gas = fluid.getPhase(PhaseType.GAS);
  double gasDensity = gas.getDensity("kg/m3");
}

Add this import when using the fragment:

import neqsim.thermo.phase.PhaseType;

The legacy numeric values exposed by PhaseType.getValue() are deprecated and should not be used as a public modeling contract.

Fractions, composition, and components

For phase index p:

Fugacity is available from the phase or through a component’s fugacity coefficient. For activity-coefficient models, use the phase-level getActivityCoefficient(componentIndex) method. ComponentInterface.getActivity() and a generic PhaseInterface.getIonicStrength() are not current public APIs.

Thermodynamic and physical properties

The common phase interface provides:

Always state units in engineering-facing output. Methods without a unit argument may use NeqSim’s internal conventions and are less clear in examples.

Generic phase objects do not expose getdZdT(), getdZdP(), excess enthalpy/entropy/volume, or component fugacity-derivative getters under the names previously shown on this page. Do not build workflows around those names. For equilibrium and state-function calculations, use the supported operations in the thermodynamic operations guide.

Aqueous phases

Check for an aqueous phase before access:

if (fluid.hasPhaseType(PhaseType.AQUEOUS)) {
  PhaseInterface aqueous = fluid.getPhase(PhaseType.AQUEOUS);
  double pH = aqueous.getpH();
  double waterActivityCoefficient =
      aqueous.getActivityCoefficient(aqueous.getComponent("water").getIndex());
}

The pH and activity result is model-dependent. Use an electrolyte-capable model and appropriate chemical-reaction setup when ionic speciation matters; a cubic-EOS water phase alone does not imply a validated electrolyte calculation.

Hydrates, wax, solids, and asphaltenes

Additional phases appear only when the corresponding model and calculation are enabled.

Do not infer a formation temperature from a phase object. Formation-temperature and stability calculations are separate operations with their own model assumptions and convergence behavior.

Stability and model-specific APIs

SystemInterface.checkStability() is the generic system-level stability entry point. It does not make nonexistent methods such as isPhaseStable() available on the system or phase interfaces. Advanced derivatives and residual/excess properties vary by model and should be verified against the exact interface or implementation used.