Creating Fluids in NeqSim

This guide provides comprehensive documentation on how to create and configure thermodynamic fluids in NeqSim, including available equations of state, mixing rules, and best practices.

Basic Fluid Creation
Equations of State Overview
Cubic Equations of State
Advanced Equations of State
Reference Equations (Helmholtz-Based)
Activity Coefficient Models
Electrolyte Models
Mixing Rules
Adding Components
Heavy Fraction Characterization
Complete Examples
Model Selection Guidelines

1. Basic Fluid Creation

Creating a fluid in NeqSim follows a consistent pattern:

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

// 1. Create the fluid with initial temperature (K) and pressure (bara)
SystemInterface fluid = new SystemSrkEos(298.15, 10.0);

// 2. Add components
fluid.addComponent("methane", 0.90);    // name, moles
fluid.addComponent("ethane", 0.05);
fluid.addComponent("propane", 0.05);

// 3. Set up the mixing rule
fluid.setMixingRule("classic");

// 4. Calculate equilibrium, then initialize physical properties
ThermodynamicOperations operations = new ThermodynamicOperations(fluid);
operations.TPflash();
fluid.initProperties();

Constructor Parameters

Most equation-of-state system classes provide a temperature/pressure constructor:

Constructor	Description
`SystemXXX(T, P)`	Temperature in K and absolute pressure in bara
`SystemXXX()`	Model-specific defaults; do not assume one common state
`SystemXXX(T, P, checkForSolids)`	Available on many, but not all, system classes

Use the explicit (T, P) form in reproducible examples. Check the selected class’s JavaDoc before using a default or solid-checking overload.

2. Equations of State Overview

NeqSim provides a wide range of thermodynamic models organized into categories:

Category	Use Cases	Examples
Cubic EoS	General hydrocarbon processing	SRK, PR, PR-1978
CPA (Cubic Plus Association)	Polar/associating fluids (water, glycols, alcohols)	SRK-CPA, PR-CPA
Reference EoS	High-accuracy natural gas, CCS	GERG-2008, EOS-CG
SAFT-based	Complex molecular interactions	PC-SAFT
Activity Coefficient	Non-ideal liquid mixtures	UNIFAC, NRTL
Electrolyte	Aqueous salt solutions	Electrolyte-CPA, Pitzer
Specialized	Specific applications	Soreide-Whitson (sour gas/brine)

3. Cubic Equations of State

3.1 Soave-Redlich-Kwong (SRK) Family

SystemSrkEos

The standard SRK equation of state. Best for general gas and light hydrocarbon applications.

SystemInterface fluid = new SystemSrkEos(300.0, 50.0);
fluid.addComponent("methane", 0.8);
fluid.addComponent("CO2", 0.2);
fluid.setMixingRule("classic");

SystemSrkPenelouxEos

SRK with Peneloux volume correction for improved liquid density predictions.

SystemInterface fluid = new SystemSrkPenelouxEos(300.0, 50.0);

SystemSrkMathiasCopeman

SRK with Mathias-Copeman alpha function for better vapor pressure predictions.

SystemInterface fluid = new SystemSrkMathiasCopeman(300.0, 50.0);

SystemSrkTwuCoonEos

SRK with Twu-Coon alpha function.

SystemInterface fluid = new SystemSrkTwuCoonEos(300.0, 50.0);

3.2 Peng-Robinson (PR) Family

SystemPrEos

Standard Peng-Robinson equation. Widely used for oil and gas applications.

SystemInterface fluid = new SystemPrEos(300.0, 50.0);
fluid.addComponent("methane", 0.7);
fluid.addComponent("n-heptane", 0.3);
fluid.setMixingRule("classic");

The PR equation is expressed as: \(P = \frac{RT}{v - b} - \frac{a \alpha}{v(v + b) + b(v - b)}\)

SystemPrEos1978

Original 1978 Peng-Robinson formulation with modified alpha function.

SystemInterface fluid = new SystemPrEos1978(300.0, 50.0);

SystemPrMathiasCopeman

PR with Mathias-Copeman alpha function for polar components.

SystemInterface fluid = new SystemPrMathiasCopeman(300.0, 50.0);

3.3 Other Cubic EoS

SystemRKEos

Original Redlich-Kwong equation (historical interest, less accurate).

SystemInterface fluid = new SystemRKEos(300.0, 50.0);

SystemTSTEos

Twu-Sim-Tassone equation of state.

SystemInterface fluid = new SystemTSTEos(300.0, 50.0);

4. Advanced Equations of State

4.1 CPA (Cubic Plus Association)

CPA models add an association term to handle hydrogen bonding in polar molecules like water, alcohols, and glycols.

SystemSrkCPAstatoil

The Equinor (formerly Statoil) implementation of SRK-CPA. Recommended for water-hydrocarbon systems.

SystemInterface fluid = new SystemSrkCPAstatoil(300.0, 50.0);
fluid.addComponent("water", 0.1);
fluid.addComponent("methane", 0.85);
fluid.addComponent("MEG", 0.05);  // Mono-ethylene glycol
fluid.setMixingRule(10);  // CPA mixing rule with temperature/composition dependency

SystemSrkCPA / SystemSrkCPAs

Alternative CPA implementations.

SystemInterface fluid = new SystemSrkCPA(300.0, 50.0);
fluid.setMixingRule(7);  // CPA mixing rule

SystemPrCPA

Peng-Robinson with CPA association term.

SystemInterface fluid = new SystemPrCPA(300.0, 50.0);

4.2 PC-SAFT

Perturbed Chain Statistical Associating Fluid Theory. Good for polymers and complex molecules.

SystemInterface fluid = new SystemPCSAFT(300.0, 50.0);
fluid.addComponent("methane", 0.5);
fluid.addComponent("ethane", 0.5);

4.3 UMR-PRU (Universal Mixing Rule)

Peng-Robinson with UNIFAC-based mixing rules for improved predictions.

SystemInterface fluid = new SystemUMRPRUEos(300.0, 50.0);

5. Reference Equations (Helmholtz-Based)

For high-accuracy applications, NeqSim provides reference equations of state based on the Helmholtz free energy:

\[\alpha(\delta, \tau, \bar{x}) = \alpha^0(\delta, \tau, \bar{x}) + \alpha^r(\delta, \tau, \bar{x})\]

5.1 GERG-2008

NeqSim includes a GERG-2008 implementation for the ISO 20765-2 natural-gas reference equation. The current SystemGERG2008Eos source explicitly marks parts of the implementation unfinished. Treat it as a calculation and validation tool, not as sole evidence for custody-transfer or fiscal acceptance.

Mapped GERG-2008 components (21): Methane, Nitrogen, CO2, Ethane, Propane, n-Butane, i-Butane, n-Pentane, i-Pentane, n-Hexane, n-Heptane, n-Octane, n-Nonane, n-Decane, Hydrogen, Oxygen, CO, Water, H2S, Helium, Argon.

import neqsim.thermo.system.SystemGERG2008Eos;

SystemInterface fluid = new SystemGERG2008Eos(288.15, 50.0);
fluid.addComponent("methane", 0.90);
fluid.addComponent("ethane", 0.05);
fluid.addComponent("propane", 0.03);
fluid.addComponent("nitrogen", 0.02);
fluid.createDatabase(true);

// Access the GERG-specific density for comparison with validated references
double density = fluid.getPhase(0).getDensity_GERG2008();

5.2 EOS-CG

Extension of GERG-2008 for CCS (Carbon Capture and Storage) applications. Includes EOS-CG-2021 combustion-gas and amine impurity components.

EOS-CG-2021 components: CO2, H2O, N2, O2, Ar, CO, H2, CH4, H2S, SO2, MEA, DEA, HCl, Cl2, NH3, and MDEA.

import neqsim.thermo.system.SystemEOSCGEos;

SystemInterface fluid = new SystemEOSCGEos(300.0, 100.0);
fluid.addComponent("CO2", 0.95);
fluid.addComponent("nitrogen", 0.03);
fluid.addComponent("oxygen", 0.02);

5.3 Other Reference Equations

Class	Description
`SystemSpanWagnerEos`	Span-Wagner equation for CO2
`SystemLeachmanEos`	Leachman equation for hydrogen
`SystemBWRSEos`	Benedict-Webb-Rubin-Starling
`SystemBnsEos`	Burgoyne-Nielsen-Stanko Peng-Robinson correlation

6. Activity Coefficient Models

For non-ideal liquid mixtures, especially polar and chemical systems:

6.1 UNIFAC

Group contribution method for activity coefficients.

import neqsim.thermo.system.SystemUNIFAC;

SystemInterface fluid = new SystemUNIFAC(300.0, 1.0);
fluid.addComponent("methanol", 0.3);
fluid.addComponent("water", 0.7);

6.2 NRTL

Non-Random Two-Liquid model.

import neqsim.thermo.system.SystemNRTL;

SystemInterface fluid = new SystemNRTL(300.0, 1.0);
fluid.addComponent("ethanol", 0.4);
fluid.addComponent("water", 0.6);

6.3 GE-Wilson

Wilson equation for activity coefficients.

import neqsim.thermo.system.SystemGEWilson;

SystemInterface fluid = new SystemGEWilson(300.0, 1.0);

7. Electrolyte Models

For systems containing salts and ions in aqueous solutions:

7.1 Electrolyte-CPA (Equinor)

import neqsim.thermo.system.SystemElectrolyteCPAstatoil;

SystemInterface fluid = new SystemElectrolyteCPAstatoil(298.15, 1.0);
fluid.addComponent("water", 1.0);
fluid.addComponent("Na+", 0.1);
fluid.addComponent("Cl-", 0.1);

7.2 Søreide-Whitson

Modified PR for sour gas systems and brine.

import neqsim.thermo.system.SystemSoreideWhitson;

SystemSoreideWhitson fluid = new SystemSoreideWhitson(350.0, 200.0);
fluid.addComponent("methane", 0.7);
fluid.addComponent("CO2", 0.15);
fluid.addComponent("H2S", 0.05);
fluid.addComponent("water", 0.1);
fluid.addSalinity(2.0, "mole/sec");  // Salt-equivalent molar flow, not concentration
fluid.setMixingRule(11);  // Soreide-Whitson mixing rule

// Optional refreshed eight-gas drop-in BIPs; LEGACY remains the default
fluid.setSoreideWhitsonParameterization("BURGOYNE_NIELSEN_2026");

The Chabab option is validated against NaCl-brine data at approximately 1-3 mol/kg water, 323-373 K, and pressures up to 230 bar. See Søreide-Whitson Model for the correlation, units, comparison example, and extrapolation limits.

The Burgoyne-Nielsen option covers CO₂, H₂S, methane, nitrogen, hydrogen, ethane, propane, and n-butane water pairs. It is opt-in because the refreshed BIPs change results and the published fit used a specified pure-component property set.

7.3 Pitzer Model

7.3 Electrolyte GE Models and Hybrid VLLE

For electrolyte solutions, SystemPitzer, SystemDesmukhMather and SystemKentEisenberg provide a fixed-role hybrid flash in which gas and hydrocarbon liquid use SRK while the aqueous liquid uses the selected GE model.

import neqsim.thermo.phase.PhaseType;
import neqsim.thermo.system.SystemPitzer;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

SystemPitzer fluid = new SystemPitzer(313.15, 50.0);
fluid.addComponent("methane", 5.0);
fluid.addComponent("n-heptane", 2.0);
fluid.addComponent("water", 55.5);
fluid.addComponent("Na+", 1.0);
fluid.addComponent("Cl-", 1.0);
fluid.setMixingRule("classic");
fluid.setMultiPhaseCheck(true);

new ThermodynamicOperations(fluid).TPflash();

// Material roles are selected from gas (SRK), oil (SRK), and aqueous (Pitzer).
boolean hasAqueousPhase = fluid.hasPhaseType(PhaseType.AQUEOUS);

The creation-order role objects are stable even when active phases are density-ordered or disappear. A later flash reconsiders inactive roles from the current feed and conditions. Neutral non-water species in the Pitzer phase use an aqueous Henry reference; water alone uses the Pitzer osmotic/Raoult solvent convention. Calling chemicalReactionInit() couples aqueous reaction equilibrium to the same fixed gas/oil/aqueous roles. This supports activity-based scale-potential screening after reactive gas-aqueous or gas-oil-aqueous flashes. The result is a saturation ratio; explicit mineral precipitation, solid amounts, solid-phase equilibrium and wax checks are not yet supported by the hybrid strategy.

The solver is not restricted to Pitzer. Desmukh-Mather and Kent-Eisenberg use the same reactive coupling when chemicalReactionInit() and setMultiPhaseCheck(true) are enabled. Other SystemEosGE systems can opt in explicitly:

SystemNRTL fluid = new SystemNRTL(313.15, 50.0);
fluid.addComponent("methane", 5.0);
fluid.addComponent("n-heptane", 2.0);
fluid.addComponent("water", 55.5);
fluid.createDatabase(true);
fluid.setMixingRule("classic");
fluid.enableHybridEosGeFlash();

new ThermodynamicOperations(fluid).TPflash();

enableHybridEosGeFlash() configures topology, not electrolyte parameters. Scale calculations require a GE phase with meaningful activities for all requested aqueous species. Pitzer has the broadest concentrated-brine parameter coverage; the amine models retain their narrower component and validity ranges. SystemDuanSun remains excluded from this topology because its current public API accepts only CO2.

8. Mixing Rules

Mixing rules determine how pure-component parameters are combined for mixtures. Set via setMixingRule():

8.1 Available Mixing Rules

Value	Name	Description
1	`NO`	Classic with all kij = 0 (no interaction)
2	`CLASSIC`	Classic van der Waals with kij from database
3	`CLASSIC_HV`	Huron-Vidal with database parameters
4	`HV`	Huron-Vidal including temperature-dependent HVDijT
5	`WS`	Wong-Sandler (NRTL-based coupling)
7	`CPA_MIX`	Classic with CPA kij from database
8	`CLASSIC_T`	Classic with temperature-dependent kij
9	`CLASSIC_T_CPA`	Classic T-dependent kij for CPA
10	`CLASSIC_TX_CPA`	Classic T and composition dependent kij for CPA
11	`SOREIDE_WHITSON`	Søreide-Whitson mixing rule
12	`CLASSIC_T2`	Alternative temperature-dependent classic

8.2 Setting Mixing Rules

// By integer value
fluid.setMixingRule(2);

// By name (string)
fluid.setMixingRule("classic");
fluid.setMixingRule("HV");
fluid.setMixingRule("WS");

8.3 Mixing Rule Recommendations

Application	Recommended Mixing Rule
Light hydrocarbons	`classic` (2)
CO2-hydrocarbon	`classic` (2) with tuned kij
Polar mixtures	`HV` (4) or `WS` (5)
Water-hydrocarbon (CPA)	`CPA_MIX` (7) or `CLASSIC_TX_CPA` (10)
Sour gas with brine	`SOREIDE_WHITSON` (11)

9. Adding Components

9.1 Basic Component Addition

// Add by name and moles
fluid.addComponent("methane", 0.85);
fluid.addComponent("ethane", 0.10);
fluid.addComponent("propane", 0.05);

// Add with flow rate and unit
fluid.addComponent("methane", 100.0, "kg/hr");
fluid.addComponent("ethane", 50.0, "Sm3/day");

// Add multiple components at once
String[] names = {"methane", "ethane", "propane"};
double[] moles = {0.85, 0.10, 0.05};
fluid.addComponents(names, moles);

9.2 Supported Units

For addComponent(name, value, unit):

Molar: mol/sec, mol/hr
Mass: kg/sec, kg/hr
Volumetric: Sm3/hr, Sm3/day, MSm3/day, Nlitre/min

9.3 Common Component Names

NeqSim uses a database-backed component catalog. Verify exact names in the component list; common examples include:

Hydrocarbons: methane, ethane, propane, i-butane, n-butane, i-pentane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane

Inorganics: nitrogen, oxygen, CO2, H2S, water, hydrogen, helium, argon

Polar/Associating: methanol, ethanol, MEG (mono-ethylene glycol), TEG (tri-ethylene glycol), DEG

Ions: Na+, K+, Ca++, Mg++, Cl-, SO4--, HCO3-

10. Heavy Fraction Characterization

For petroleum fluids, NeqSim supports TBP (True Boiling Point) and plus-fraction characterization.

10.1 TBP Fractions

SystemInterface oil = new SystemSrkEos(350.0, 100.0);

// addTBPfraction(name, molarFlow, molarMass [kg/mol], specificGravity)
oil.addTBPfraction("C7", 0.05, 0.096, 0.738);
oil.addTBPfraction("C8", 0.04, 0.107, 0.765);
oil.addTBPfraction("C9", 0.03, 0.121, 0.781);
oil.addTBPfraction("C10", 0.02, 0.134, 0.792);

oil.setMixingRule("classic");

10.2 Plus Fractions

// Use a numeric label; NeqSim stores the pseudo-component with a _PC suffix.
// addPlusFraction(name, molarFlow, molarMass [kg/mol], specificGravity)
oil.addPlusFraction("C20", 0.10, 0.350, 0.88);

10.3 TBP Characterization Models

NeqSim provides several models for estimating critical properties from TBP data:

// Set TBP model before adding fractions
fluid.getCharacterization().setTBPModel("PedersenSRK");  // Default for SRK
fluid.getCharacterization().setTBPModel("PedersenPR");   // Default for PR
fluid.getCharacterization().setTBPModel("Lee-Kesler");
fluid.getCharacterization().setTBPModel("Twu");
fluid.getCharacterization().setTBPModel("RiaziDaubert");

11. Complete Examples

11.1 Natural Gas Processing

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

public class NaturalGasExample {
    public static void main(String[] args) {
        // Create SRK fluid at pipeline conditions
        SystemInterface gas = new SystemSrkEos(283.15, 70.0);

        // Typical natural gas composition
        gas.addComponent("nitrogen", 0.02);
        gas.addComponent("CO2", 0.01);
        gas.addComponent("methane", 0.85);
        gas.addComponent("ethane", 0.06);
        gas.addComponent("propane", 0.03);
        gas.addComponent("i-butane", 0.01);
        gas.addComponent("n-butane", 0.01);
        gas.addComponent("i-pentane", 0.005);
        gas.addComponent("n-pentane", 0.005);

        gas.setMixingRule("classic");

        // Flash calculation
        ThermodynamicOperations ops = new ThermodynamicOperations(gas);
        ops.TPflash();
        gas.initProperties();

        // Display results
        System.out.println("Density: " + gas.getDensity("kg/m3") + " kg/m3");
        System.out.println("Z-factor: " + gas.getZ());
        System.out.println("Molecular weight: " + gas.getMolarMass() * 1000 + " g/mol");
    }
}

11.2 Water-Hydrocarbon System with CPA

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

public class WaterHydrocarbonExample {
    public static void main(String[] args) {
        // CPA for associating systems
        SystemInterface fluid = new SystemSrkCPAstatoil(323.15, 50.0);

        fluid.addComponent("methane", 0.70);
        fluid.addComponent("ethane", 0.10);
        fluid.addComponent("propane", 0.05);
        fluid.addComponent("water", 0.10);
        fluid.addComponent("MEG", 0.05);

        fluid.setMixingRule(10);  // Temperature and composition dependent CPA

        ThermodynamicOperations ops = new ThermodynamicOperations(fluid);
        ops.TPflash();

        System.out.println("Number of phases: " + fluid.getNumberOfPhases());
        fluid.prettyPrint();
    }
}

11.3 GERG-2008 Density Comparison

import neqsim.thermo.system.SystemGERG2008Eos;
import neqsim.thermo.system.SystemInterface;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

public class GergDensityComparisonExample {
    public static void main(String[] args) {
        // Compare NeqSim's GERG-2008 result with an approved reference
        SystemInterface gas = new SystemGERG2008Eos(288.15, 40.0);

        gas.addComponent("methane", 0.92);
        gas.addComponent("ethane", 0.04);
        gas.addComponent("propane", 0.02);
        gas.addComponent("nitrogen", 0.01);
        gas.addComponent("CO2", 0.01);

        gas.createDatabase(true);

        ThermodynamicOperations ops = new ThermodynamicOperations(gas);
        ops.TPflash();

        // GERG-specific high-accuracy density
        double gergDensity = gas.getPhase(0).getDensity_GERG2008();
        System.out.println("GERG-2008 Density: " + gergDensity + " kg/m3");
    }
}

11.4 Oil Characterization

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

public class OilCharacterizationExample {
    public static void main(String[] args) {
        SystemInterface oil = new SystemPrEos(350.0, 150.0);

        // Light ends
        oil.addComponent("nitrogen", 0.005);
        oil.addComponent("CO2", 0.02);
        oil.addComponent("methane", 0.35);
        oil.addComponent("ethane", 0.08);
        oil.addComponent("propane", 0.06);
        oil.addComponent("i-butane", 0.02);
        oil.addComponent("n-butane", 0.03);
        oil.addComponent("i-pentane", 0.02);
        oil.addComponent("n-pentane", 0.02);
        oil.addComponent("n-hexane", 0.03);

        // TBP fractions (molar flow, molar mass kg/mol, specific gravity)
        oil.addTBPfraction("C7", 0.05, 0.096, 0.738);
        oil.addTBPfraction("C8", 0.04, 0.107, 0.765);
        oil.addTBPfraction("C9", 0.03, 0.121, 0.781);
        oil.addTBPfraction("C10", 0.02, 0.134, 0.792);

        // Use a numeric label; NeqSim stores this as C11_PC.
        oil.addPlusFraction("C11", 0.18, 0.250, 0.85);

        oil.setMixingRule("classic");

        ThermodynamicOperations ops = new ThermodynamicOperations(oil);
        ops.TPflash();
        oil.initProperties();

        oil.prettyPrint();
    }
}

12. Model Selection Guidelines

Quick Reference Table

System Type	Recommended Model	Mixing Rule
Dry natural gas	`SystemSrkEos` or `SystemPrEos`	`classic` (2)
Wet gas / condensate	`SystemPrEos`	`classic` (2)
Black oil	`SystemPrEos` with TBP	`classic` (2)
Water-hydrocarbon	`SystemSrkCPAstatoil`	`CLASSIC_TX_CPA` (10)
Glycol dehydration	`SystemSrkCPAstatoil`	`CPA_MIX` (7)
Sour gas / brine	`SystemSoreideWhitson`	`SOREIDE_WHITSON` (11)
Natural-gas reference-property comparison	`SystemGERG2008Eos`	N/A
CCS / CO2 transport	`SystemEOSCGEos`	N/A
Electrolyte solutions	`SystemElectrolyteCPAstatoil`	N/A
Polar organics	`SystemUNIFAC` or `SystemNRTL`	N/A

Decision Flow

Is a validated natural-gas reference calculation required? → Evaluate GERG-2008 against the applicable composition range and an approved reference
Does the system contain water, glycols, or alcohols? → Evaluate CPA models
Is it a sour gas system with brine? → Use Søreide-Whitson
Is it a standard hydrocarbon system? → Use SRK or PR
Does it contain electrolytes? → Use Electrolyte-CPA or Pitzer
Is it a non-ideal organic mixture? → Use UNIFAC or NRTL

Model-Coverage Trade-offs

Accuracy depends on composition, state, parameters, and validation data; the labels below describe model scope rather than guaranteed error.

Model Type	Typical Cost	Intended Scope
Cubic (SRK/PR)	Low	General hydrocarbon process calculations
CPA	Moderate	Associating mixtures within a validated parameter set
GERG-2008	Higher	Natural-gas reference-property comparisons
UNIFAC	Moderate	Screening non-ideal liquid mixtures with available groups