This example uses 95 mol% CO2 and 5 mol% SO2 at 298.15 K and 50 bara. It requires the initialization-order repair for issue #3702. Earlier implementations formed binary reducing factors before loading the critical properties of SO2 and the other added EOS-CG components. The resulting infinite volume factor caused both density-root attempts to fail, including at 350 K and 10 bara. The repair loads all critical properties before forming those factors. The interaction coefficients, density solver, and rejection of nonconverged ideal-gas fallback results are unchanged.

Executable mixture flash

The two phases have different compositions. Report density and mole fraction for each phase, and reconstruct the feed from the phase mole inventories. A density root evaluated at the overall feed composition alone is not the equilibrium liquid density of a two-phase mixture.

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import neqsim.thermo.phase.PhaseInterface;
import neqsim.thermo.system.SystemEOSCGEos;
import neqsim.thermo.system.SystemInterface;
import neqsim.thermo.util.gerg.NeqSimEOSCG;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

public final class EosCgCo2So2Example {
  private static final Logger logger = LogManager.getLogger(EosCgCo2So2Example.class);

  private EosCgCo2So2Example() {}

  public static void main(String[] args) {
    SystemInterface fluid = new SystemEOSCGEos(298.15, 50.0); // K, bara
    fluid.addComponent("CO2", 0.95); // mol
    fluid.addComponent("SO2", 0.05); // mol
    fluid.createDatabase(true);
    new ThermodynamicOperations(fluid).TPflash();
    fluid.init(3);

    assert fluid.getNumberOfPhases() == 2 : "Expected gas and liquid";
    double recoveredCo2 = 0.0;
    double recoveredSo2 = 0.0;
    for (int i = 0; i < fluid.getNumberOfPhases(); i++) {
      PhaseInterface phase = fluid.getPhase(i);
      double density = phase.getDensity("kg/m3");
      double molarDensity = density / (phase.getMolarMass() * 1000.0); // mol/L
      double pressureKPa = new NeqSimEOSCG(phase).getPressure(molarDensity);
      double co2Fraction = phase.getComponent("CO2").getx();
      double so2Fraction = phase.getComponent("SO2").getx();
      assert Double.isFinite(density) && density > 0.0;
      assert Math.abs(pressureKPa - 5000.0) <= 0.005 : "Density must reproduce pressure";
      recoveredCo2 += phase.getNumberOfMolesInPhase() * co2Fraction;
      recoveredSo2 += phase.getNumberOfMolesInPhase() * so2Fraction;
      logger.info("Phase {}; beta {}; x(CO2) {}; x(SO2) {}; density {} kg/m3; pressure {} kPa",
          phase.getType(), phase.getBeta(), co2Fraction, so2Fraction, density, pressureKPa);
    }
    assert Math.abs(recoveredCo2 - 0.95) < 1.0e-9 : "CO2 balance";
    assert Math.abs(recoveredSo2 - 0.05) < 1.0e-9 : "SO2 balance";
  }
}

Run with Java assertions enabled (-ea). SystemEOSCGEosCO2SO2Test also extracts, compiles, and executes this exact Markdown program with assertions enabled.

Numerical validation boundary

The following are calculated results from NeqSim 3.20.0 with the #3702 repair, rounded for display. Pressure is absolute; beta is the molar phase fraction.

Temperature (K)	Pressure (bara)	Phase	Beta	CO2 mole fraction	Density (kg/m3)
298.15	50.0	Gas	0.98747	0.95153	142.786
298.15	50.0	Liquid	0.01253	0.82958	897.995
300.00	50.0	Gas	1.00000	0.95000	139.220
298.15	48.0	Gas	1.00000	0.95000	132.571
350.00	10.0	Gas	1.00000	0.95000	15.972

Regression checks cover these four operating points, both requested density roots at the original feed composition, pressure closure within the existing relative tolerance of 1e-6, positive heat capacities, normalized phase compositions, component balances, and two-phase fugacity agreement. The binary reducing functions are independently checked against Eqs. (5)-(6) and the CO2/SO2 coefficients in Table 4 of Neumann et al. (2023).

These checks establish numerical consistency at the stated points. They are not an experimental density or phase-equilibrium accuracy benchmark, and they do not qualify a continuous temperature/pressure envelope or arbitrary impurity mixtures. Other compositions and operating ranges require their own convergence and reference-data validation. A failed density solve remains an error; an ideal-gas fallback is not an accepted EOS-CG state.

See the GERG-2008 and EOS-CG guide for model selection and the other introductory examples.