Physical Property Calculations

NeqSim computes phase and mixture properties after thermodynamic initialization. This guide highlights the most used methods and how to choose appropriate models.

Density and Compressibility

• Call fluid.initPhysicalProperties() or fluid.initProperties() after a flash to populate density (getDensity()) and compressibility (getZ()).
• Apply volume-shifted PR/SRK models when liquid density underpredicts lab data.

Viscosity

• getViscosity() on a phase returns dynamic viscosity. Correlations switch automatically based on system type:
  • Corresponding-states for gases and light condensates.
  • Heavy-oil extensions when TBP fractions or high molar masses are present.
  • Association-corrected viscosity for CPA systems.
• Use setMixingRule(7) or CPA fluids when hydrogen bonding impacts rheology (water, glycols).

Thermal Conductivity and Heat Capacity

• getThermalConductivity() provides phase thermal conductivity using dense-gas corrections.
• getCp() and getEnthalpy() support energy balances. Reinitialize (init(3)) if temperature changes substantially between calls.

Surface and Interfacial Tension

• getInterfacialTension(phase1, phase2) calculates tension between phases (e.g., gas-oil, gas-water) using parachor correlations tied to the active EOS.
• Ensure both phases are present after a flash; otherwise, perform a two-phase flash at representative separator conditions first.

Diffusion and Mass Transfer

• getDiffusionCoefficient() is available on phases for estimating film and molecular diffusion coefficients.
• In unit operations, enable mass-transfer calculations to access local Sherwood correlations and film models.

Numerical Tips

• Always flash (TPflash, PHflash, etc.) before requesting properties; raw composition-only systems do not hold valid properties.
• When looping over many states, reuse fluids and call initPhysicalProperties() after each state change to refresh transport properties without repeating equilibrium calculations.