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

---

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.

Neutral-gas dissolution also requires a qualified Henry-law reference and, for brines, separately qualified Pitzer neutral-ion interactions. See Henry-law reference states and aqueous gas-solubility evidence for the implemented temperature law, derivative contract, current coefficient audit, source matrix and adoption gates.

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.

For imported Pitzer datasets, check both interaction coverage and scientific qualification. Coverage answers whether the active binary, same-sign, ternary, and neutral topology is explicit; qualification answers which systems and observables have independent evidence:

SystemPitzer qualifiedBrine = new SystemPitzer(298.15, 1.01325);
qualifiedBrine.addComponent("water", 55.508);
qualifiedBrine.addComponent("Na+", 0.5);
qualifiedBrine.addComponent("K+", 0.5);
qualifiedBrine.addComponent("Cl-", 1.0);
qualifiedBrine.init(0);
qualifiedBrine.applyPhreeqcSodiumPotassiumChlorideParameters();

PitzerParameterQualification evidence = qualifiedBrine.getPitzerParameterQualification();

// Property-specific publication gate: complete interaction coverage plus independent
// evidence for the requested observable. A VLE request would fail for this subset.
qualifiedBrine.requirePitzerDatasetValidationFor(
    PitzerParameterQualification.ValidationTarget.AQUEOUS_ACTIVITY_COEFFICIENTS);

// Dataset qualification does not prove that this exact state is inside its evidence envelope.
boolean insideRange = PitzerParameterDatasets.isWithinSodiumPotassiumChlorideValidationRange(
    qualifiedBrine.getTemperature(),
    0.5,  // Na+ molality, mol/kg water
    0.5,  // K+ molality, mol/kg water
    1.0); // Cl- molality, mol/kg water

The accessor completes lazy parameter selection and interaction-coverage auditing but does not run a flash. The target gate is opt-in and executes only when called, so neutral models and ordinary Pitzer property calculations do no new work. It accepts only a completely qualified named dataset with independent evidence for the requested property; callers must still apply the use-case-specific range helper for the current temperature and molality. The legacy requireCompletePitzerDatasetQualification() gate remains available, but its overall level alone must not be treated as VLE, reaction, or mineral evidence.

The complete PHREEQC catalog is intentionally reported as partially validated: CaCl2 and MgCl2 binaries have held-out activity evidence, while exact mixed Ca-Mg-Cl-SO4 activity and mineral precipitation remain separate gates. Process equipment can carry hybrid EOS-Pitzer states and calculate phase density, enthalpy, and heat capacity, but a finite saturation ratio is not yet a mineral-amount or precipitation-complementarity result.

---

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");

10.1.1 Defining a Cut by Boiling Point

A gas chromatograph reports a retention time, so a boiling point is often better known than a density. Two alternatives take a boiling point instead of the missing property:

// molar mass known, specific gravity unknown
oil.addTBPfraction2("C10", 0.02, 0.142, 447.3);   // name, moles, molarMass [kg/mol], Tb [K]

// specific gravity known, molar mass unknown
oil.addTBPfraction3("C10", 0.02, 0.734, 447.3);   // name, moles, specificGravity [-], Tb [K]

addTBPfraction2 obtains the specific gravity from the Riazi-Daubert (1980) correlation $M = 4.5673\times10^{-5}\,T_b^{2.1962}\,SG^{-1.0164}$ (with $T_b$ in degrees Rankine and $M$ in g/mol) inverted analytically for $SG$. addTBPfraction3 inverts the selected TBP model for molar mass, which is the better conditioned direction. In both cases the supplied boiling point is then used by the TBP model itself, so it reaches the critical properties for the boiling-point based models (Lee-Kesler, Twu, Cavett).

Molar mass and boiling point are not independent. In the correlation, a relative error in molar mass carries almost one-for-one into specific gravity, and a relative error in boiling point is amplified by about 2.2. A pair that implies a specific gravity outside 0.5 to 1.3 is rejected rather than silently accepted:

oil.addTBPfraction2("C10", 0.02, 0.129, 560.0);  // throws: a 129 g/mol cut cannot boil at 560 K

If the paraffin, naphthene and aromatic character of the cut is known, the Watson characterization factor is a markedly more accurate route to the density, because $K_w$ encodes exactly that character:

double sg = oil.calculateDensityFromBoilingPointAndWatsonK(447.3, 12.66);  // Tb [K], Kw [-]
oil.addTBPfraction("C10", 0.02, 0.142, sg);

Indicative values are about 12.7 for paraffinic cuts, 11.0 for naphthenic and 10.0 for aromatic. Against pure components this route reproduces the specific gravity to better than 1 %, where the molar-mass route is typically 5 % out for paraffins.

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

1. Is a validated natural-gas reference calculation required? → Evaluate GERG-2008 against the applicable composition range and an approved reference
2. Does the system contain water, glycols, or alcohols? → Evaluate 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

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

---

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