Use the workflow below when a calculation needs a characterized fluid, a defined equilibrium specification, and reusable results. Temperature is in K and pressure is absolute bara unless an API call supplies another unit explicitly.

Build and flash a characterized fluid

This complete example adds ordinary database components and one TBP pseudo-component, loads the mixture interaction parameters, performs a TP flash, and initializes properties:

import neqsim.thermo.system.SystemInterface;
import neqsim.thermo.system.SystemPrEos;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

public final class ThermodynamicWorkflowExample {
  private ThermodynamicWorkflowExample() {}

  public static void main(String[] args) {
    SystemInterface fluid = new SystemPrEos(313.15, 80.0);
    fluid.addComponent("methane", 0.85);
    fluid.addComponent("ethane", 0.05);

    // name, moles, molar mass [kg/mol], specific gravity [-]
    fluid.addTBPfraction("C10", 0.10, 0.134, 0.792);

    // Rebuild the temporary component/interaction tables after the component list is complete.
    fluid.createDatabase(true);
    fluid.setMixingRule("classic");

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

    System.out.printf("T = %.2f K%n", fluid.getTemperature("K"));
    System.out.printf("P = %.2f bara%n", fluid.getPressure("bara"));
    System.out.printf("phases = %d%n", fluid.getNumberOfPhases());
    System.out.printf("density = %.3f kg/m3%n", fluid.getDensity("kg/m3"));
    System.out.printf("molar mass = %.3f g/mol%n", fluid.getMolarMass() * 1000.0);
  }
}

addTBPfraction(name, moles, molarMass, density) expects molar mass in kg/mol and specific gravity (numerically equal to g/cm3) in its fourth argument. Values above 1.5 in the density position are interpreted as kg/m3 and divided by 1000. Use addPlusFraction(...) for an unresolved plus fraction; a TBP cut and a plus fraction do not have the same characterization semantics.

createDatabase(true) rebuilds NeqSim’s temporary component and interaction tables for the current component list. It does not switch on a separate database service and it does not infer the molar mass or density of a TBP fraction.

Choose the thermodynamic model and mixing rule separately

The system class selects the thermodynamic model. The mixing rule controls how mixture parameters are combined. In the example, SystemPrEos selects Peng-Robinson and setMixingRule("classic") selects the database-backed classic rule (EosMixingRuleType.CLASSIC, legacy value 2).

Prefer named mixing rules over raw legacy integers. The maintained fluid-creation guide lists the current names, numeric compatibility values, and model-specific recommendations. In particular, legacy value 1 is the no-interaction rule with all binary interaction parameters set to zero; it is not the database-backed classic rule.

Select an equilibrium specification

Create one ThermodynamicOperations instance for the fluid being calculated. Operations update that fluid’s state.

Engineering specification	Source-verified operation	Important basis
Temperature and pressure	operations.TPflash()	Set T and absolute P on the fluid first
Pressure and enthalpy	operations.PHflash(hSpec, "J/kg")	Set absolute P first and state the enthalpy unit
Pressure and entropy	operations.PSflash(sSpec, "J/kgK")	Set absolute P first and state the entropy unit
Dew-point temperature	operations.dewPointTemperatureFlash()	Uses the current composition and pressure; updates T
Bubble-point pressure	operations.bubblePointPressureFlash()	Uses the current composition and temperature; updates P
PT phase envelope	operations.calcPTphaseEnvelope()	Use the dedicated result accessors and inspect convergence

The one-argument PH and PS overloads use total extensive specifications. Prefer the unit-aware overloads in user-facing workflows so a molar, mass, or total basis cannot be confused. Saturation and phase-envelope calculations can fail or become supercritical for some compositions and starting states; handle exceptions and validate the returned state. See the phase-envelope guide for envelope-specific setup and result handling.

Chemical and phase equilibrium require their dedicated model setup and operations; there is no general ThermodynamicOperations.calcChemicalEquilibrium() entry point. Start with the reactive-flash guide for supported reactive workflows.

Clone independent states for sweeps

Clone a configured and flashed fluid before changing a sweep condition. The clone owns an independent thermodynamic state; the original remains at its prior temperature and pressure.

SystemInterface sweepCase = fluid.clone();
sweepCase.setTemperature(280.0, "K");
sweepCase.setPressure(10.0, "bara");

ThermodynamicOperations sweepOperations = new ThermodynamicOperations(sweepCase);
sweepOperations.TPflash();
sweepCase.initProperties();

Cloning is not JSON export. If a workflow needs persistent or transferable state, select and validate a serialization format explicitly rather than treating a clone as a stored artifact.

Read, diagnose, and validate results

• Call initProperties() after the flash before reading density or transport properties.
• Prefer phase names or PhaseType and check phase existence; phase indexes can change after a new equilibrium calculation.
• Use prettyPrint() for a human-readable state table. Do not assume it reports every binary interaction parameter.
• Keep pressure basis and every enthalpy, entropy, density, and flow unit explicit.
• Validate the quantities relevant to the experiment or process: density and compressibility, saturation pressure/temperature, CCE or differential-liberation volumes, phase amounts and compositions, and material/energy closure. Molar mass and Z-factor alone do not validate a characterized petroleum fluid.
• Compare changed-state sweeps against the untouched original to detect accidental state reuse.

For the complete property initialization and phase-access contract, continue with Reading Fluid Properties.