Question about getDensity()

[bgrimstad]

Hi,

Thank you for sharing a useful software.

As a new user of neqsim-python, I am trying to figure out how to correctly access phase densities.

In the script below, I have made an example where I compare the phase densities accessed via getDensity("kg/m3"), getDensity(), and the density reported by tt.printFrame(fluid). In this case, getDensity(), without a specified unit, differs from the other two. Which unit should one expect from getDensity()?

Also, if I set a flow rate, e.g. as fluid.setTotalFlowRate(1.0, "kg/sec"), the three ways to access the densities give me three different answers. Why is this the case?

• getDensity("kg/m3") gives a gas density of 1.10 and an oil density of 601.00
• getDensity() gives a gas density of 48.49 and an oil density of 523.29
• tt.printFrame(fluid) shows a gas density of 48.66 and an oil density of 601.00

I am using version 3.0.51.

from neqsim.thermo import thermoTools as tt

# Create fluid
composition = {
    "N2": 0.004,
    "CO2": 0.010,
    "methane": 0.700,
    "ethane": 0.120,
    "propane": 0.080,
    "i-butane": 0.020,
    "n-butane": 0.030,
    "nC10": 0.036,
    "water": 0.020,
}

fluid = tt.fluid("srk")
total = float(sum(composition.values()))
for name, frac in composition.items():
    fluid.addComponent(str(name), float(frac) / total)

# Set flow rate
# fluid.setTotalFlowRate(1.0, "kg/sec")

tt.TPflash(fluid, temperature=308, pressure=50, tUnit="K", pUnit="bara")

# Extract densities
densities = {}
densities_default_units = {}
for phase in range(fluid.getNumberOfPhases()):
    phase = fluid.getPhase(phase)
    densities[phase.getType().name()] = phase.getDensity("kg/m3")
    densities_default_units[phase.getType().name()] = phase.getDensity()  # Using default units

print(densities)
print(densities_default_units)

tt.printFrame(fluid)

[henrikhestnes]

@EvenSol Do you have any input on this?:D

[EvenSol]

Thank you for the excellent question! This is a common source of confusion, and I'm glad you asked because it highlights an important implementation detail in NeqSim.. Thanks for reminding me about the issue (I did not see it in first place).

I have added some documentation that I hope it will help:

Read fluid properties

and

Fluid properties.

Understanding Density Methods in NeqSim

Short Answer

Method	Returns	Volume Correction
getDensity("kg/m3")	Corrected density	✅ WITH Peneloux volume translation
getDensity()	EoS density	❌ WITHOUT Peneloux correction
printFrame()	Mixed (see below)	Depends on column

Recommendation: Always use getDensity("kg/m3") to get accurate, volume-corrected density values.

Why the Difference?

Cubic equations of state (like SRK, PR) are known to predict liquid densities poorly - often 5-20% too high. The Peneloux volume translation is a correction that shifts the predicted volume to match experimental data without affecting vapor-liquid equilibrium calculations.

When you call:

• getDensity("kg/m3") - The unit specification triggers the corrected calculation path that includes the Peneloux volume shift
• getDensity() - Returns the raw EoS density without volume correction (legacy behavior)

This is documented in our Reading Fluid Properties Guide.

Why Does Setting Flow Rate Change Things?

When you set fluid.setTotalFlowRate(1.0, "kg/sec"), you're changing from molar basis to mass basis calculations. This affects internal normalizations and can cause slight differences in how properties are computed and reported.

The printFrame() output shows yet another value because it may use a different calculation path internally.

Recommended Approach

from neqsim.thermo import thermoTools as tt

# Create fluid
composition = {
    "N2": 0.004,
    "CO2": 0.010,
    "methane": 0.700,
    "ethane": 0.120,
    "propane": 0.080,
    "i-butane": 0.020,
    "n-butane": 0.030,
    "nC10": 0.036,
    "water": 0.020,
}

fluid = tt.fluid("srk")
total = float(sum(composition.values()))
for name, frac in composition.items():
    fluid.addComponent(str(name), float(frac) / total)

fluid.setMixingRule('classic')

tt.TPflash(fluid, temperature=308, pressure=50, tUnit="K", pUnit="bara")

fluid.initProperties()

# RECOMMENDED: Always specify units for density
for i in range(fluid.getNumberOfPhases()):
    phase = fluid.getPhase(i)
    phase_type = phase.getType().name()
    
    # Use these methods for accurate results:
    density = phase.getDensity("kg/m3")           # WITH Peneloux correction
    molar_volume = phase.getMolarVolume("m3/mol") # WITH Peneloux correction
    
    print(f"{phase_type}: {density:.2f} kg/m3")

General Rule for NeqSim Properties

For most properties, specifying units is recommended:

Property	With Units (Recommended)	Without Units
Density	getDensity("kg/m3")	getDensity() - no Peneloux
Viscosity	getViscosity("kg/msec")	getViscosity() - Pa·s
Enthalpy	getEnthalpy("J/mol")	getEnthalpy() - J/mol
Molar Volume	getMolarVolume("m3/mol")	getMolarVolume() - scaled, no Peneloux

Documentation

We've published comprehensive documentation on this topic:

• Reading Fluid Properties Guide - Complete guide to property access
• Unit Conversion Recipes - All supported unit strings
• Python Quickstart - Common Python gotchas
• Troubleshooting Guide - Common issues and solutions

Summary

1. Always use getDensity("kg/m3") for accurate liquid densities
2. The difference is the Peneloux volume translation correction
3. When setting flow rates, stick to the unit-specified methods for consistency
4. This behavior is consistent with the underlying thermodynamic model design

See Also

• JavaDoc: PhaseInterface.getDensity()
• Peneloux Volume Translation (Wikipedia)

[henrikhestnes]

Thanks, @EvenSol! This clears up the confusion. It is very good to know that the unit string triggers the Peneloux correction while the parameter-less method returns the raw EoS value.

Appreciate the detailed explanation and the new documentation links!

[bgrimstad]

Thank you for the thorough description, @EvenSol.

Closing the issue as I have no follow up questions.