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.

Table of Contents

1. Basic Fluid Creation
2. Equations of State Overview
3. Cubic Equations of State
4. Advanced Equations of State
5. Reference Equations (Helmholtz-Based)
6. Activity Coefficient Models
7. Electrolyte Models
8. Mixing Rules
9. Adding Components
10. Heavy Fraction Characterization
11. Complete Examples
12. Model Selection Guidelines

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1. Basic Fluid Creation

Creating a fluid in NeqSim follows a consistent pattern:

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

// 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. Initialize the fluid
fluid.init(0);

Constructor Parameters

All fluid system classes accept these constructor signatures:

Constructor	Description
SystemXXX()	Default: 298.15 K, 1.0 bara
SystemXXX(T, P)	Temperature (K), Pressure (bara)
SystemXXX(T, P, checkForSolids)	With solid phase checking enabled

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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)

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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);

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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);

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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

The ISO 20765-2 standard for natural gas. Highest accuracy for custody transfer and fiscal metering.

Supported 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, 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 GERG-specific properties
double density = fluid.getPhase(0).getDensity_GERG2008();

5.2 EOS-CG

Extension of GERG-2008 for CCS (Carbon Capture and Storage) applications. Includes combustion gas components.

Additional components: SO2, NO, NO2, and others relevant to flue gas.

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	BNS equation of state

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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);

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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");  // Add salinity
fluid.setMixingRule(11);  // Soreide-Whitson mixing rule

7.3 Pitzer Model

For concentrated electrolyte solutions.

import neqsim.thermo.system.SystemPitzer;

SystemInterface fluid = new SystemPitzer(298.15, 1.0);

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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)

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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’s database includes hundreds of components. Common names:

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-

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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);
oil.createDatabase(true);  // Required before adding TBP fractions

// addTBPfraction(name, moles, molarMass [g/mol], density [g/cm3])
oil.addTBPfraction("C7", 0.05, 96.0, 0.738);
oil.addTBPfraction("C8", 0.04, 107.0, 0.765);
oil.addTBPfraction("C9", 0.03, 121.0, 0.781);
oil.addTBPfraction("C10", 0.02, 134.0, 0.792);

oil.setMixingRule("classic");

10.2 Plus Fractions

// addPlusFraction(name, moles, molarMass [g/mol], density [g/cm3])
oil.addPlusFraction("C20+", 0.10, 350.0, 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");

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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 High-Accuracy Fiscal Metering (GERG-2008)

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

public class FiscalMeteringExample {
    public static void main(String[] args) {
        // GERG-2008 for custody transfer accuracy
        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);
        oil.createDatabase(true);
        
        // 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 (moles, MW g/mol, density g/cm3)
        oil.addTBPfraction("C7", 0.05, 96.0, 0.738);
        oil.addTBPfraction("C8", 0.04, 107.0, 0.765);
        oil.addTBPfraction("C9", 0.03, 121.0, 0.781);
        oil.addTBPfraction("C10", 0.02, 134.0, 0.792);
        
        // Plus fraction
        oil.addPlusFraction("C11+", 0.18, 250.0, 0.85);
        
        oil.setMixingRule("classic");
        
        ThermodynamicOperations ops = new ThermodynamicOperations(oil);
        ops.TPflash();
        oil.initProperties();
        
        oil.prettyPrint();
    }
}

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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)
Fiscal metering	SystemGERG2008Eos	N/A
CCS / CO2 transport	SystemEOSCGEos	N/A
Electrolyte solutions	SystemElectrolyteCPAstatoil	N/A
Polar organics	SystemUNIFAC or SystemNRTL	N/A

Decision Flow

1. Is high accuracy required for custody transfer? → Use GERG-2008
2. Does the system contain water, glycols, or alcohols? → Use CPA models
3. Is it a sour gas system with brine? → Use Søreide-Whitson
4. Is it a standard hydrocarbon system? → Use SRK or PR
5. Does it contain electrolytes? → Use Electrolyte-CPA or Pitzer
6. Is it a non-ideal organic mixture? → Use UNIFAC or NRTL

Performance vs. Accuracy Trade-offs

Model Type	Speed	Accuracy	Best For
Cubic (SRK/PR)	Fast	Good	General process simulation
CPA	Medium	Very Good	Polar/associating systems
GERG-2008	Slow	Excellent	Fiscal metering, calibration
UNIFAC	Medium	Good	Chemical process design

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See Also

• Thermodynamic Workflows - Flash calculations and operations
• PVT and Fluid Characterization - Heavy fraction handling
• Mathematical Models - Equation details
• GERG-2008 and EOS-CG - Reference equation details
• Physical Properties - Transport property calculations