Get NeqSim running in Python in under 5 minutes.

Step 1: Install neqsim-python (1 minute)

pip (Java 17+)

pip install neqsim

conda (OpenJDK included)

conda install -c conda-forge neqsim

The current neqsim-python package requires Python 3.10+. A pip installation also requires Java 17 or newer; install a 64-bit JDK from Adoptium and keep the Python and Java architectures aligned. The Conda package installs OpenJDK automatically, so it does not need a separate Java setup. See the authoritative NeqSim Python installation guide when selecting a runtime.

Step 2: First Flash Calculation (2 minutes)

# Import NeqSim - Direct Java Access via jneqsim gateway
from neqsim import jneqsim

# Create a natural gas at 25°C and 50 bar
# Note: Temperature in KELVIN!
fluid = jneqsim.thermo.system.SystemSrkEos(273.15 + 25, 50.0)

# Add components (mole fractions)
fluid.addComponent("methane", 0.85)
fluid.addComponent("ethane", 0.10)
fluid.addComponent("propane", 0.05)

# IMPORTANT: Always set mixing rule
fluid.setMixingRule("classic")

# Run flash calculation
ops = jneqsim.thermodynamicoperations.ThermodynamicOperations(fluid)
ops.TPflash()

# Initialize properties (required for most property access)
fluid.initProperties()

# Print results
print(f"Number of phases: {fluid.getNumberOfPhases()}")
print(f"Density: {fluid.getDensity('kg/m3'):.2f} kg/m³")
print(f"Z-factor: {fluid.getZ():.4f}")
print(f"Molar mass: {fluid.getMolarMass('kg/mol') * 1000:.2f} g/mol")

# Pretty print full results
fluid.prettyPrint()

Step 3: First Process Simulation (2 minutes)

from neqsim import jneqsim

# Convenient class imports
SystemSrkEos = jneqsim.thermo.system.SystemSrkEos
ProcessSystem = jneqsim.process.processmodel.ProcessSystem
Stream = jneqsim.process.equipment.stream.Stream
Separator = jneqsim.process.equipment.separator.Separator
Compressor = jneqsim.process.equipment.compressor.Compressor

# 1. Create feed fluid (30°C, 50 bara)
fluid = SystemSrkEos(273.15 + 30, 50.0)
fluid.addComponent("methane", 0.70)
fluid.addComponent("ethane", 0.10)
fluid.addComponent("propane", 0.10)
fluid.addComponent("n-butane", 0.05)
fluid.addComponent("n-pentane", 0.05)
fluid.setMixingRule("classic")

# 2. Create process system
process = ProcessSystem()

# 3. Add feed stream
feed = Stream("Feed", fluid)
feed.setFlowRate(10000, "kg/hr")
process.add(feed)

# 4. Add separator
separator = Separator("HP Separator", feed)
process.add(separator)

# 5. Add compressor on gas outlet
compressor = Compressor("Gas Compressor", separator.getGasOutStream())
compressor.setOutletPressure(80.0, "bara")
compressor.setIsentropicEfficiency(0.75)
process.add(compressor)

# 6. Run the process
process.run()

# 7. Print results
print("=== SEPARATOR ===")
print(f"Gas out: {separator.getGasOutStream().getFlowRate('kg/hr'):.1f} kg/hr")
print(f"Liquid out: {separator.getLiquidOutStream().getFlowRate('kg/hr'):.1f} kg/hr")

print("\n=== COMPRESSOR ===")
print(f"Power: {compressor.getPower('kW'):.1f} kW")
print(f"Outlet T: {compressor.getOutletStream().getTemperature() - 273.15:.1f} °C")

Common Import Patterns

from neqsim import jneqsim

# Thermodynamic systems
SystemSrkEos = jneqsim.thermo.system.SystemSrkEos
SystemPrEos = jneqsim.thermo.system.SystemPrEos
SystemSrkCPAstatoil = jneqsim.thermo.system.SystemSrkCPAstatoil

# Process equipment
ProcessSystem = jneqsim.process.processmodel.ProcessSystem
Stream = jneqsim.process.equipment.stream.Stream
Separator = jneqsim.process.equipment.separator.Separator
ThreePhaseSeparator = jneqsim.process.equipment.separator.ThreePhaseSeparator
Compressor = jneqsim.process.equipment.compressor.Compressor
Heater = jneqsim.process.equipment.heatexchanger.Heater
Cooler = jneqsim.process.equipment.heatexchanger.Cooler
ThrottlingValve = jneqsim.process.equipment.valve.ThrottlingValve
Mixer = jneqsim.process.equipment.mixer.Mixer
Splitter = jneqsim.process.equipment.splitter.Splitter

# Pipelines
AdiabaticPipe = jneqsim.process.equipment.pipeline.AdiabaticPipe
PipeBeggsAndBrills = jneqsim.process.equipment.pipeline.PipeBeggsAndBrills

Common Gotchas

Issue Solution
TypeError: unsupported format string Java String needs conversion: str(java_string)
AttributeError: 'NoneType' JVM not started - use from neqsim import jneqsim
Unexpected density Selecting a density unit does not enable Peneloux volume correction; correction is a separate validated model choice
Temperature seems wrong NeqSim uses Kelvin. Convert: T_K = T_C + 273.15
JVMNotFoundException For pip, install Java 17+ and set JAVA_HOME; Conda installs OpenJDK automatically

Java String Conversion

When using Java strings in Python f-strings with format specifiers:

# WRONG - causes TypeError
comp_name = fluid.getComponent(0).getComponentName()
print(f"{comp_name:<12}")  # Error!

# CORRECT - convert to Python string
comp_name = str(fluid.getComponent(0).getComponentName())
print(f"{comp_name:<12}")  # Works!

Working with Units

This standalone example uses an SRK hydrocarbon mixture. Set the stream’s state and flow before running it, then read properties from its current fluid: run() may replace the fluid object. Rerun and initialize properties again after changing temperature, pressure, composition, or flow.

import math
from neqsim import jneqsim

fluid = jneqsim.thermo.system.SystemSrkEos(298.15, 10.0)  # K, bara
fluid.addComponent("methane", 0.85)
fluid.addComponent("ethane", 0.10)
fluid.addComponent("propane", 0.05)
fluid.setMixingRule("classic")

stream = jneqsim.process.equipment.stream.Stream("units example", fluid)
stream.setFlowRate(1000.0, "kg/hr")
stream.setTemperature(30.0, "C")
stream.setPressure(50.0, "bara")       # Absolute pressure, not gauge pressure
stream.run()                          # TP flash at the specified stream state

solved_fluid = stream.getFluid()
solved_fluid.initProperties()          # Initialize caloric and transport properties
temp_kelvin = solved_fluid.getTemperature()
temp_celsius = solved_fluid.getTemperature("C")
pressure = solved_fluid.getPressure("bara")
density = solved_fluid.getDensity("kg/m3")
viscosity = solved_fluid.getViscosity("cP")
enthalpy = solved_fluid.getEnthalpy("kJ/kg")  # Specific enthalpy, not an energy rate

assert math.isclose(temp_kelvin, 303.15, abs_tol=1e-8)
assert math.isclose(temp_celsius, 30.0, abs_tol=1e-8)
assert math.isclose(pressure, 50.0, abs_tol=1e-8)
assert math.isfinite(density) and density > 0.0
assert math.isfinite(viscosity) and viscosity > 0.0
assert math.isfinite(enthalpy)          # Its sign depends on the reference state
print(f"T = {temp_celsius:.2f} C, P = {pressure:.2f} bara")
print(f"Density = {density:.3f} kg/m3, viscosity = {viscosity:.5f} cP")
print(f"Specific enthalpy = {enthalpy:.3f} kJ/kg")

Choosing "kg/m3" changes the density unit, not the density model; Peneloux volume correction is a separate model choice. These checks demonstrate API and unit handling, not experimental validation of this fluid model.

Next Steps

Jupyter Notebook Tips

# Suppress Java logging in notebooks
import logging
logging.getLogger('jpype').setLevel(logging.WARNING)

# Display fluid as table
import pandas as pd

def fluid_to_dataframe(fluid):
    """Convert fluid composition to pandas DataFrame."""
    data = []
    for i in range(fluid.getNumberOfComponents()):
        comp = fluid.getComponent(i)
        data.append({
            'Component': str(comp.getComponentName()),
            'z': comp.getz(),
            'MW': comp.getMolarMass() * 1000
        })
    return pd.DataFrame(data)

df = fluid_to_dataframe(fluid)
display(df)