Thermodynamic Models in NeqSim

This document provides a comprehensive overview of the thermodynamic models available in NeqSim, their theoretical foundations, and practical guidance on when and how to use each model. Models are classified into categories based on their mathematical formulation and application domain.

Introduction
Model Categories Overview
Cubic Equations of State
CPA (Cubic Plus Association) Models
Reference Equations (Helmholtz-Based)
Activity Coefficient (GE) Models
Electrolyte Models
Mixing Rules
Auto-Select Model Feature
Model Selection Guidelines
Complete Reference Tables

1. Introduction

NeqSim (Non-Equilibrium Simulator) provides a rich library of thermodynamic models for simulating phase equilibria, physical properties, and process operations. Choosing the appropriate thermodynamic model is crucial for obtaining accurate results in process simulation.

General Workflow

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

// 1. Choose and instantiate the thermodynamic model
SystemInterface fluid = new SystemSrkEos(298.15, 10.0);  // T [K], P [bara]

// 2. Add components
fluid.addComponent("methane", 0.90);
fluid.addComponent("ethane", 0.10);

// 3. Set an appropriate mixing rule
fluid.setMixingRule("classic");

// 4. Initialize and perform calculations
fluid.init(0);

2. Model Categories Overview

Category	Mathematical Basis	Key Use Cases	Examples
Cubic EoS	$P = \frac{RT}{v-b} - \frac{a(T)}{(v+\epsilon b)(v+\sigma b)}$	General hydrocarbons, gas processing	SRK, PR
CPA	Cubic EoS + Association term	Polar/associating fluids (water, glycols, alcohols)	SRK-CPA, PR-CPA
Reference EoS	Helmholtz free energy explicit	High-accuracy metering, CCS	GERG-2008, EOS-CG
Activity Coefficient	Excess Gibbs energy ($G^E$)	Non-ideal liquid mixtures	UNIFAC, NRTL
Electrolyte	CPA + Electrostatic contributions	Aqueous salt solutions	Electrolyte-CPA, Pitzer
Specialized	Modified equations	Specific applications	Søreide-Whitson

3. Cubic Equations of State

3.1 Theoretical Foundation

Cubic equations of state express pressure as a function of temperature and molar volume in a cubic polynomial form:

\[P = \frac{RT}{v - b} - \frac{a(T)}{(v + \epsilon b)(v + \sigma b)}\]

Where:

$P$ = pressure
$T$ = temperature
$v$ = molar volume
$R$ = gas constant
$a(T)$ = temperature-dependent energy parameter (attraction)
$b$ = co-volume parameter (repulsion)
$\epsilon, \sigma$ = EoS-specific constants

The energy parameter typically uses an alpha function:

\[a(T) = a_c \cdot \alpha(T_r, \omega)\]

Where $T_r = T/T_c$ is the reduced temperature and $\omega$ is the acentric factor.

3.2 Soave-Redlich-Kwong (SRK) Family

For SRK: $\epsilon = 0$, $\sigma = 1$

\[P = \frac{RT}{v - b} - \frac{a(T)}{v(v + b)}\]

Class	Description	Best For
`SystemSrkEos`	Standard SRK	General gas/light hydrocarbon
`SystemSrkPenelouxEos`	SRK with Peneloux volume correction	Improved liquid density
`SystemSrkMathiasCopeman`	SRK with Mathias-Copeman alpha function	Better vapor pressure
`SystemSrkTwuCoonEos`	SRK with Twu-Coon alpha function	Polar components
`SystemSrkSchwartzentruberEos`	SRK-Schwartzentruber	Polar/associated fluids

Example: Standard SRK

SystemInterface fluid = new SystemSrkEos(300.0, 50.0);
fluid.addComponent("methane", 0.85);
fluid.addComponent("CO2", 0.10);
fluid.addComponent("nitrogen", 0.05);
fluid.setMixingRule("classic");

3.3 Peng-Robinson (PR) Family

For PR: $\epsilon = 1 - \sqrt{2}$, $\sigma = 1 + \sqrt{2}$

\[P = \frac{RT}{v - b} - \frac{a(T)}{v(v + b) + b(v - b)}\]

Class	Description	Best For
`SystemPrEos`	Standard PR	General oil & gas
`SystemPrEos1978`	Original 1978 formulation	Classic applications
`SystemPrMathiasCopeman`	PR with Mathias-Copeman alpha	Polar components
`SystemPrDanesh`	Modified PR	Heavy oil systems
`SystemPrEosvolcor`	PR with volume correction	Liquid density
`SystemPrEosDelft1998`	Delft 1998 modification	Gas condensates

Example: Peng-Robinson

SystemInterface fluid = new SystemPrEos(350.0, 150.0);
fluid.addComponent("methane", 0.70);
fluid.addComponent("n-heptane", 0.20);
fluid.addComponent("n-decane", 0.10);
fluid.setMixingRule("classic");

3.4 Other Cubic EoS

Class	Description
`SystemRKEos`	Original Redlich-Kwong (historical)
`SystemTSTEos`	Twu-Sim-Tassone equation
`SystemBWRSEos`	Benedict-Webb-Rubin-Starling (extended virial)

4. CPA (Cubic Plus Association) Models

4.1 Theoretical Foundation

CPA (Cubic Plus Association) extends cubic EoS to handle hydrogen bonding in polar molecules like water, alcohols, and glycols. The pressure is expressed as:

\[P = P_{\text{cubic}} + P_{\text{association}}\]

The association term accounts for hydrogen bonding:

\[P_{\text{association}} = -\frac{1}{2} RT \rho \sum_i x_i \sum_{A_i} \left( 1 - X_{A_i} \right) \frac{\partial \ln g}{\partial v}\]

Where:

$X_{A_i}$ = fraction of site A on molecule $i$ not bonded to other active sites
$g$ = radial distribution function
Sites can be of type 4C (water, glycols), 2B (alcohols), or other schemes

The non-bonded site fraction $X_{A_i}$ is determined by solving:

\[X_{A_i} = \frac{1}{1 + \rho \sum_j x_j \sum_{B_j} X_{B_j} \Delta^{A_i B_j}}\]

Where $\Delta^{A_i B_j}$ is the association strength between site A on molecule $i$ and site B on molecule $j$.

4.2 Available CPA Models

Class	Description	Mixing Rule
`SystemSrkCPAstatoil`	Recommended Equinor SRK-CPA implementation	10
`SystemSrkCPA`	Standard SRK-CPA	7
`SystemSrkCPAs`	Alternative SRK-CPA	7
`SystemPrCPA`	Peng-Robinson with CPA	7
`SystemUMRCPAEoS`	UMR-CPA with UNIFAC	-

4.3 Association Schemes

Scheme	Sites	Examples
4C	2 donors + 2 acceptors	Water, glycols
2B	1 donor + 1 acceptor	Alcohols
CR-1	Cross-association	Water-MEG, Water-methanol

4.4 Example: Water-Hydrocarbon System

import neqsim.thermo.system.SystemSrkCPAstatoil;

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);  // Mono-ethylene glycol

// Use mixing rule 10 (recommended for CPA)
fluid.setMixingRule(10);  // CLASSIC_TX_CPA

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

5. Reference Equations (Helmholtz-Based)

5.1 Theoretical Foundation

Reference equations of state are explicit in the dimensionless Helmholtz free energy $\alpha$:

\[\alpha(\delta, \tau, \bar{x}) = \frac{a(\rho, T, \bar{x})}{RT} = \alpha^0(\delta, \tau, \bar{x}) + \alpha^r(\delta, \tau, \bar{x})\]

Where:

$\delta = \rho / \rho_r$ = reduced density
$\tau = T_r / T$ = inverse reduced temperature
$\alpha^0$ = ideal gas contribution
$\alpha^r$ = residual contribution

The residual contribution is fitted to high-accuracy experimental data:

\[\alpha^r(\delta, \tau, \bar{x}) = \sum_{i=1}^{N} x_i \alpha_{0i}^r(\delta, \tau) + \sum_{i=1}^{N-1} \sum_{j=i+1}^{N} x_i x_j F_{ij} \alpha_{ij}^r(\delta, \tau)\]

These models provide superior accuracy for density, speed of sound, and heat capacity compared to cubic EoS.

5.2 GERG-2008

Standard: ISO 20765-2
Application: Natural gas custody transfer, fiscal metering
Accuracy: ±0.1% in density for typical natural gas

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", 0.92);
fluid.addComponent("ethane", 0.04);
fluid.addComponent("propane", 0.02);
fluid.addComponent("nitrogen", 0.01);
fluid.addComponent("CO2", 0.01);
fluid.createDatabase(true);

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

// Access GERG-specific high-accuracy density
double gergDensity = fluid.getPhase(0).getDensity_GERG2008();

5.3 GERG-2008-H2 (Hydrogen Enhanced)

Extension for hydrogen-rich blends with improved H2 binary parameters:

SystemGERG2008Eos fluid = new SystemGERG2008Eos(300.0, 50.0);
fluid.addComponent("methane", 0.7);
fluid.addComponent("hydrogen", 0.3);

// Enable hydrogen-enhanced model
fluid.useHydrogenEnhancedModel();

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

5.4 EOS-CG

Application: Carbon Capture and Storage (CCS), combustion gases
Extension: Includes SO2, NO, NO2, HCl, Cl2, COS in addition to GERG-2008 components

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("SO2", 0.02);
fluid.createDatabase(true);

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

double density = fluid.getPhase(0).getDensity_EOSCG();

5.5 Other Reference Equations

Class	Description	Application
`SystemSpanWagnerEos`	Span-Wagner equation	Pure CO2
`SystemLeachmanEos`	Leachman equation	Pure hydrogen
`SystemVegaEos`	Vega equation	Specialized applications
`SystemAmmoniaEos`	Ammonia-specific	Ammonia systems

6. Activity Coefficient (GE) Models

6.1 Theoretical Foundation

Activity coefficient models describe non-ideal liquid behavior through the excess Gibbs energy:

\[G^E = RT \sum_i x_i \ln \gamma_i\]

Where $\gamma_i$ is the activity coefficient of component $i$.

6.2 UNIFAC (Universal Functional Activity Coefficient)

Group contribution method that predicts activity coefficients from molecular group interactions:

\[\ln \gamma_i = \ln \gamma_i^C + \ln \gamma_i^R\]

Where the combinatorial ($C$) and residual ($R$) contributions are calculated from group properties.

import neqsim.thermo.system.SystemUNIFAC;

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

6.3 NRTL (Non-Random Two-Liquid)

Local composition model with binary interaction parameters:

\[\ln \gamma_i = \frac{\sum_j x_j \tau_{ji} G_{ji}}{\sum_k x_k G_{ki}} + \sum_j \frac{x_j G_{ij}}{\sum_k x_k G_{kj}} \left( \tau_{ij} - \frac{\sum_m x_m \tau_{mj} G_{mj}}{\sum_k x_k G_{kj}} \right)\]

import neqsim.thermo.system.SystemNRTL;

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

6.4 Other GE Models

Class	Description
`SystemGEWilson`	Wilson equation
`SystemUNIFACpsrk`	UNIFAC with PSRK parameters
`SystemUMRPRUEos`	Peng-Robinson with UNIFAC mixing
`SystemUMRPRUMCEos`	UMR-PRU with Mathias-Copeman

7. Electrolyte Models

7.1 Theoretical Foundation

Electrolyte models extend CPA with electrostatic contributions for aqueous salt solutions:

\[A^{res} = A^{CPA} + A^{elec}\]

The electrostatic contribution typically includes:

\[A^{elec} = A^{MSA} + A^{Born} + A^{SR}\]

Where:

$A^{MSA}$ = Mean Spherical Approximation (ion-ion screening)
$A^{Born}$ = Born solvation term (ion-solvent)
$A^{SR}$ = Short-range interactions (specific ion effects)

Born Solvation Term:

\[\frac{A^{Born}}{RT} = -\frac{e^2 N_A}{8\pi\varepsilon_0 k_B T} \sum_i n_i \frac{z_i^2}{\sigma_i} \left(1 - \frac{1}{\varepsilon_r}\right)\]

7.2 Electrolyte-CPA (Equinor)

Recommended for: Aqueous electrolyte solutions with associating solvents

import neqsim.thermo.system.SystemElectrolyteCPAstatoil;

SystemInterface system = new SystemElectrolyteCPAstatoil(298.15, 1.01325);
system.addComponent("water", 55.5);
system.addComponent("Na+", 1.0);
system.addComponent("Cl-", 1.0);
system.chemicalReactionInit();  // Enable pH and speciation
system.createDatabase(true);
system.setMixingRule(10);  // Required for electrolyte CPA

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

// Access activity coefficients
double meanGamma = system.getPhase(0).getMeanIonicActivityCoefficient("Na+", "Cl-");

7.3 Søreide-Whitson Model

Application: Sour gas systems with brine

Uses salinity-dependent binary interaction parameters for CO2-water, H2S-water, and hydrocarbon-water systems:

\[k_{ij,\text{CO}_2-\text{water}} = -0.31092(1 + 0.156 S^{0.75}) + 0.236(1 + 0.178 S^{0.98}) T_r - 21.26 e^{-6.72^{T_r} - S}\]

import neqsim.thermo.system.SystemSoreideWhitson;

SystemSoreideWhitson fluid = new SystemSoreideWhitson(350.0, 200.0);
fluid.addComponent("methane", 0.70);
fluid.addComponent("CO2", 0.15);
fluid.addComponent("H2S", 0.05);
fluid.addComponent("water", 0.10);
fluid.addSalinity(2.0, "mole/sec");  // Add NaCl salinity
fluid.setMixingRule(11);  // Søreide-Whitson mixing rule

7.4 Other Electrolyte Models

Class	Description	Use Case
`SystemPitzer`	Pitzer model	Concentrated brines
`SystemDesmukhMather`	Desmukh-Mather	Amine systems
`SystemKentEisenberg`	Kent-Eisenberg	CO2/H2S in amines
`SystemDuanSun`	Duan-Sun	CO2 solubility in brine
`SystemFurstElectrolyteEos`	Fürst electrolyte EoS	General electrolytes

8. Mixing Rules

8.1 Overview

Mixing rules determine how pure-component parameters are combined for mixtures. The choice of mixing rule is as important as the choice of equation of state.

8.2 Classic Mixing Rules

van der Waals One-Fluid Mixing Rule:

\[a_{mix} = \sum_i \sum_j x_i x_j a_{ij}\] \[b_{mix} = \sum_i x_i b_i\]

With combining rule:

\[a_{ij} = \sqrt{a_i a_j}(1 - k_{ij})\]

Type	Name	Description
1	`NO`	All $k_{ij} = 0$ (no interactions)
2	`CLASSIC`	Classic with database $k_{ij}$
8	`CLASSIC_T`	Temperature-dependent: $k_{ij}(T) = k_{ij,0} + k_{ij,T} \cdot T$
12	`CLASSIC_T2`	Inverse T-dependency: $k_{ij}(T) = k_{ij,0} + k_{ij,T}/T$

8.3 Huron-Vidal Mixing Rules

Combines cubic EoS with activity coefficient models at infinite pressure:

\[a_{mix} = b_{mix} \left( \sum_i x_i \frac{a_i}{b_i} - \frac{G^E}{\Lambda} \right)\]

Where $\Lambda \approx 0.693$ for SRK.

Type	Name	Description
3	`CLASSIC_HV`	Huron-Vidal with database NRTL parameters
4	`HV`	HV with temperature-dependent parameters

Usage with GE model:

fluid.setMixingRule("HV", "UNIFAC_UMRPRU");
fluid.setMixingRule("HV", "NRTL");

8.4 Wong-Sandler Mixing Rule

Matches both second virial coefficient and excess Gibbs energy:

\[b_{mix} = \frac{\sum_i \sum_j x_i x_j \left( b - \frac{a}{RT} \right)_{ij}}{1 - \frac{A_\infty^E}{CRT} - \sum_i x_i \frac{a_i}{RTb_i}}\]

Type	Name	Description
5	`WS`	Wong-Sandler (NRTL-based)

fluid.setMixingRule("WS", "NRTL");

8.5 CPA Mixing Rules

Type	Name	Description
7	`CPA_MIX`	Classic for CPA systems
9	`CLASSIC_T_CPA`	Temperature-dependent CPA
10	`CLASSIC_TX_CPA`	Recommended T and x dependent for CPA

Mixing Rule 10 Auto-Selection:

Mixing rule 10 automatically selects the appropriate sub-type:

Condition	Sub-Type	Description
Symmetric $k_{ij}$, no T-dependency	`classic-CPA`	Simple symmetric
Symmetric $k_{ij}$, with T-dependency	`classic-CPA_T`	Temperature-dependent
Asymmetric $k_{ij}$ ($k_{ij} \neq k_{ji}$)	`classic-CPA_Tx`	Full asymmetric

8.6 Søreide-Whitson Mixing Rule

Type	Name	Description
11	`SOREIDE_WHITSON`	Salinity-dependent for sour gas/brine

8.7 Setting Mixing Rules

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

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

// By enum
fluid.setMixingRule(EosMixingRuleType.CLASSIC);

// With GE model (for HV/WS)
fluid.setMixingRule("HV", "UNIFAC_UMRPRU");
fluid.setMixingRule("WS", "NRTL");

8.8 Setting Custom kij Values

EosMixingRulesInterface mixRule = fluid.getPhase(0).getMixingRule();

// Set symmetric kij
mixRule.setBinaryInteractionParameter(0, 1, 0.12);

// Set asymmetric kij (for CPA)
mixRule.setBinaryInteractionParameterij(0, 1, 0.08);  // kij
mixRule.setBinaryInteractionParameterji(0, 1, 0.12);  // kji

// Set temperature-dependent kij
mixRule.setBinaryInteractionParameter(0, 1, 0.10);      // kij0
mixRule.setBinaryInteractionParameterT1(0, 1, 0.001);   // kijT

9. Auto-Select Model Feature

9.1 Overview

NeqSim provides an autoSelectModel() method that automatically chooses an appropriate thermodynamic model based on the components in your fluid. This is useful for quick setups or when you’re unsure which model to use.

9.2 Auto-Selection Logic

The autoSelectModel() method follows this decision tree:

public SystemInterface autoSelectModel() {
    if (hasComponent("MDEA") && hasComponent("water") && hasComponent("CO2")) {
        return setModel("Electrolyte-ScRK-EOS");  // Amine systems
    } 
    else if (hasComponent("water") || hasComponent("methanol") || 
             hasComponent("MEG") || hasComponent("TEG") || 
             hasComponent("ethanol") || hasComponent("DEG")) {
        if (hasComponent("Na+") || hasComponent("K+") || 
            hasComponent("Br-") || hasComponent("Mg++") || 
            hasComponent("Cl-") || hasComponent("Ca++") || 
            hasComponent("Fe++") || hasComponent("SO4--")) {
            return setModel("Electrolyte-CPA-EOS-statoil");  // Electrolytes
        } else {
            return setModel("CPAs-SRK-EOS-statoil");  // Polar/associating
        }
    }
    else if (hasComponent("water")) {
        return setModel("ScRK-EOS");  // Water present
    }
    else if (hasComponent("mercury")) {
        return setModel("SRK-TwuCoon-Statoil-EOS");  // Mercury
    }
    else {
        return setModel("SRK-EOS");  // Default: standard SRK
    }
}

9.3 Model Selection Summary

Components Present	Selected Model
MDEA + water + CO2	Electrolyte-ScRK-EOS
Water/glycols + ions	Electrolyte-CPA-EOS-statoil
Water/glycols (no ions)	CPAs-SRK-EOS-statoil
Only water (no glycols)	ScRK-EOS
Mercury	SRK-TwuCoon-Statoil-EOS
Default (hydrocarbons only)	SRK-EOS

9.4 Usage

import neqsim.thermo.Fluid;

// Method 1: Using Fluid builder with autoSelectModel
Fluid fluidBuilder = new Fluid();
fluidBuilder.setAutoSelectModel(true);
SystemInterface fluid = fluidBuilder.create("black oil with water");

// Method 2: Calling autoSelectModel() on existing fluid
SystemInterface gas = new SystemSrkEos(300.0, 50.0);
gas.addComponent("methane", 0.80);
gas.addComponent("water", 0.15);
gas.addComponent("MEG", 0.05);
gas.createDatabase(true);

// Auto-select will switch to CPA model
SystemInterface optimizedFluid = gas.autoSelectModel();

9.5 Auto-Select Mixing Rule

There is also an autoSelectMixingRule() method that selects an appropriate mixing rule based on the model type:

fluid.autoSelectMixingRule();  // Automatically sets appropriate mixing rule

10. Model Selection Guidelines

10.1 Quick Reference Table

System Type	Recommended Model	Mixing Rule	Notes
Dry natural gas	`SystemSrkEos`	`classic` (2)	Simple, fast
Wet gas / condensate	`SystemPrEos`	`classic` (2)	Better for C7+
Black oil with TBP	`SystemPrEos`	`classic` (2)	With characterization
Water-hydrocarbon	`SystemSrkCPAstatoil`	`CLASSIC_TX_CPA` (10)	Handles association
Glycol dehydration	`SystemSrkCPAstatoil`	`CPA_MIX` (7) or (10)	MEG, TEG, DEG
Sour gas with brine	`SystemSoreideWhitson`	`SOREIDE_WHITSON` (11)	Salinity-dependent
Amine gas treating	`SystemSrkEos`	`HV` (4)	With NRTL
Fiscal metering	`SystemGERG2008Eos`	N/A	ISO 20765-2
CCS / CO2 transport	`SystemEOSCGEos`	N/A	Flue gas components
Electrolyte solutions	`SystemElectrolyteCPAstatoil`	(10)	With chemicalReactionInit
Polar organics	`SystemUNIFAC`	N/A	Group contribution
High-pressure polar	`SystemPrEos`	`WS` (5)	Wong-Sandler

10.2 Decision Flow

1. Is high accuracy required for custody transfer?
   └─ YES → Use GERG-2008 or EOS-CG

2. Does the system contain electrolytes (Na+, Cl-, etc.)?
   └─ YES → Use Electrolyte-CPA

3. Does the system contain water, glycols, or alcohols?
   └─ YES → Use CPA models (SystemSrkCPAstatoil)

4. Is it a sour gas system with brine?
   └─ YES → Use Søreide-Whitson

5. Is it a non-ideal organic mixture?
   └─ YES → Use UNIFAC or NRTL with HV/WS mixing

6. Is it a standard hydrocarbon system?
   └─ YES → Use SRK or PR with classic mixing

10.3 Performance vs. Accuracy Trade-offs

Model Type	Speed	Accuracy	Memory
Cubic (SRK/PR)	⚡⚡⚡ Fast	★★★ Good	Low
CPA	⚡⚡ Medium	★★★★ Very Good	Medium
GERG-2008	⚡ Slow	★★★★★ Excellent	High
UNIFAC	⚡⚡ Medium	★★★ Good	Medium
Electrolyte-CPA	⚡ Slow	★★★★ Very Good	High

11. Complete Reference Tables

11.1 All System Classes

Class	Category	Description
`SystemSrkEos`	Cubic	Standard SRK
`SystemSrkPenelouxEos`	Cubic	SRK with volume correction
`SystemSrkMathiasCopeman`	Cubic	SRK with MC alpha function
`SystemSrkTwuCoonEos`	Cubic	SRK with Twu-Coon alpha
`SystemSrkTwuCoonParamEos`	Cubic	SRK Twu-Coon parameterized
`SystemSrkTwuCoonStatoilEos`	Cubic	SRK Twu-Coon Equinor version
`SystemSrkSchwartzentruberEos`	Cubic	SRK-Schwartzentruber
`SystemSrkEosvolcor`	Cubic	SRK with volume correction
`SystemPrEos`	Cubic	Standard PR
`SystemPrEos1978`	Cubic	Original 1978 PR
`SystemPrMathiasCopeman`	Cubic	PR with MC alpha
`SystemPrDanesh`	Cubic	Modified PR
`SystemPrEosvolcor`	Cubic	PR with volume correction
`SystemPrEosDelft1998`	Cubic	Delft 1998 PR
`SystemPrGassemEos`	Cubic	Gassem PR
`SystemRKEos`	Cubic	Original RK
`SystemTSTEos`	Cubic	Twu-Sim-Tassone
`SystemBWRSEos`	Cubic	Benedict-Webb-Rubin-Starling
`SystemCSPsrkEos`	Cubic	CSP-SRK
`SystemPsrkEos`	Cubic	PSRK
`SystemSrkCPA`	CPA	Standard SRK-CPA
`SystemSrkCPAs`	CPA	Alternative SRK-CPA
`SystemSrkCPAstatoil`	CPA	Recommended Equinor SRK-CPA
`SystemPrCPA`	CPA	PR-CPA
`SystemUMRCPAEoS`	CPA	UMR-CPA
`SystemPCSAFT`	SAFT	PC-SAFT
`SystemPCSAFTa`	SAFT	PC-SAFT variant
`SystemGERG2008Eos`	Reference	GERG-2008 (ISO 20765-2)
`SystemGERG2004Eos`	Reference	GERG-2004 (older version)
`SystemGERGwaterEos`	Reference	GERG with water
`SystemEOSCGEos`	Reference	EOS-CG for CCS
`SystemSpanWagnerEos`	Reference	Span-Wagner for CO2
`SystemLeachmanEos`	Reference	Leachman for H2
`SystemVegaEos`	Reference	Vega equation
`SystemAmmoniaEos`	Reference	Ammonia-specific
`SystemBnsEos`	Reference	BNS equation
`SystemUNIFAC`	GE	UNIFAC
`SystemUNIFACpsrk`	GE	UNIFAC-PSRK
`SystemNRTL`	GE	NRTL
`SystemGEWilson`	GE	Wilson
`SystemUMRPRUEos`	GE-EoS	UMR-PRU
`SystemUMRPRUMCEos`	GE-EoS	UMR-PRU-MC
`SystemElectrolyteCPA`	Electrolyte	Electrolyte CPA
`SystemElectrolyteCPAstatoil`	Electrolyte	Recommended Equinor Electrolyte CPA
`SystemElectrolyteCPAMM`	Electrolyte	Electrolyte CPA-MM
`SystemFurstElectrolyteEos`	Electrolyte	Fürst electrolyte
`SystemFurstElectrolyteEosMod2004`	Electrolyte	Modified Fürst (2004)
`SystemSoreideWhitson`	Electrolyte	Søreide-Whitson for sour gas
`SystemPitzer`	Electrolyte	Pitzer model
`SystemDesmukhMather`	Electrolyte	Desmukh-Mather
`SystemKentEisenberg`	Electrolyte	Kent-Eisenberg
`SystemDuanSun`	Electrolyte	Duan-Sun
`SystemIdealGas`	Ideal	Ideal gas law
`SystemWaterIF97`	Water	IF-97 for steam

11.2 All Mixing Rules

Type	Name	String Key	Best For
1	NO	`"NO"`	Ideal mixtures
2	CLASSIC	`"classic"`	General hydrocarbons
3	CLASSIC_HV	`"CLASSIC_HV"`	Polar mixtures
4	HV	`"HV"`	Alcohol-water-HC
5	WS	`"WS"`	High-pressure polar
7	CPA_MIX	`"CPA_MIX"`	CPA systems
8	CLASSIC_T	`"CLASSIC_T"`	T-dependent kij
9	CLASSIC_T_CPA	`"CLASSIC_T_CPA"`	CPA with T-dependency
10	CLASSIC_TX_CPA	`"CLASSIC_TX_CPA"`	Recommended for CPA
11	SOREIDE_WHITSON	`"SOREIDE_WHITSON"`	Sour gas, brine
12	CLASSIC_T2	`"CLASSIC_T2"`	Inverse T-dependency

Thermodynamic Models in NeqSim

Table of Contents

1. Introduction

General Workflow

2. Model Categories Overview

3. Cubic Equations of State

3.1 Theoretical Foundation

3.2 Soave-Redlich-Kwong (SRK) Family

3.3 Peng-Robinson (PR) Family

3.4 Other Cubic EoS

4. CPA (Cubic Plus Association) Models

4.1 Theoretical Foundation

4.2 Available CPA Models

4.3 Association Schemes

4.4 Example: Water-Hydrocarbon System

5. Reference Equations (Helmholtz-Based)

5.1 Theoretical Foundation

5.2 GERG-2008

5.3 GERG-2008-H2 (Hydrogen Enhanced)

5.4 EOS-CG

5.5 Other Reference Equations

6. Activity Coefficient (GE) Models

6.1 Theoretical Foundation

6.2 UNIFAC (Universal Functional Activity Coefficient)

6.3 NRTL (Non-Random Two-Liquid)

6.4 Other GE Models

7. Electrolyte Models

7.1 Theoretical Foundation

7.2 Electrolyte-CPA (Equinor)

7.3 Søreide-Whitson Model

7.4 Other Electrolyte Models

8. Mixing Rules

8.1 Overview

8.2 Classic Mixing Rules

8.3 Huron-Vidal Mixing Rules

8.4 Wong-Sandler Mixing Rule

8.5 CPA Mixing Rules

8.6 Søreide-Whitson Mixing Rule

8.7 Setting Mixing Rules

8.8 Setting Custom kij Values

9. Auto-Select Model Feature

9.1 Overview

9.2 Auto-Selection Logic

9.3 Model Selection Summary

9.4 Usage

9.5 Auto-Select Mixing Rule

10. Model Selection Guidelines

10.1 Quick Reference Table

10.2 Decision Flow

10.3 Performance vs. Accuracy Trade-offs

11. Complete Reference Tables

11.1 All System Classes

11.2 All Mixing Rules

See Also