NRTL

Non-Random Two-Liquid (NRTL) activity coefficient model for the liquid phase, with binary parameters from the ChemSep databank, an ideal or Peng-Robinson vapor phase and vapor pressures from the compound correlations. For polar and strongly non-ideal liquid mixtures at low to moderate pressure, including partially miscible ones.

DWSIM.Thermodynamics.PropertyPackages.NRTLPropertyPackage
Assembly DWSIM.Thermodynamics.dll · Object ← PropertyPackage ← ActivityCoefficientPropertyPackage ← NRTLPropertyPackage
Name in the flowsheet NRTL · FluentAPI PropertyPackages.NRTL

Scope

An activity coefficient model for the liquid phase of polar and strongly non-ideal mixtures at low to moderate pressure: alcohols, ketones, esters, acids and water, azeotropic and extractive distillation, solvent recovery, and systems with two liquid phases. The liquid fugacity is f_i^L = x_iγ_iP_i^sat times the Poynting factor, with the vapor pressure from the compound correlation; gases above their critical temperature dissolve by Henry's law.

Renon and Prausnitz built the Non-Random Two-Liquid equation on the local composition concept. Unlike the Wilson equation it describes partially miscible liquids, so the same parameters give vapor-liquid and liquid-liquid equilibrium. For each pair,

τ_ij = (A_ij + B_ijT + C_ijT²)/(RT), G_ij = exp(-α_ijτ_ij),

with A in cal/mol and the non-randomness parameter α_ij = α_ji. With the B and C terms at zero this is the classic form τ_ij = a_ij/RT.

Parameters. A₁₂, A₂₁, B, C and α of each pair come from the ChemSep databank and a set for biodiesel systems, shipped with DWSIM. When a pair is missing and AutoEstimateMissingNRTLUNIQUACParameters is on (the default), the package estimates it from UNIFAC activity coefficients; a pair in which one compound boils below 200 K gets near-ideal values. The parameters can be edited in the package editor or set from code with ConfigureNRTL(c => c.WithBinary(c1, c2, a12, a21, alpha12)), and are saved with the flowsheet.

Vapor phase and caloric properties. The vapor is an ideal gas by default; the Peng-Robinson equation can be selected for the vapor fugacity in the package settings (its k_ij are saved as InteractionParameters_PR). By default the enthalpy follows the liquid heat capacity of the compounds from 25 °C, with the vapor reached through the enthalpy of vaporization; the ideal-gas, Lee-Kesler and excess-enthalpy options are in the package settings. Liquid densities come from experimental data, Rackett or COSTALD.

Derivatives. The package provides analytical temperature and composition derivatives of ln γ_i, used by the vapor-fraction flashes and the rigorous column solvers.

Limitations

• Low to moderate pressure: the liquid has no pressure dependence beyond the Poynting factor. Select the Peng-Robinson vapor when the pressure is well above atmospheric.
• The parameters hold within the temperature range of the data they were fitted to. Parameters regressed from vapor-liquid data may predict a liquid-liquid split poorly, or miss it.
• Estimated (UNIFAC) parameters are a fallback; for design work fit the pair to measured data with the Data Regression utility.
• Supercritical gases (H₂, N₂, CH₄, CO₂) dissolve through Henry constants; at high pressure, or for gas-rich mixtures, use an equation of state such as Peng-Robinson.

Example

This code runs on every build of this site, and the output below is what it printed.

Python

fs = (Flowsheet.Create("NRTLExample")
      .WithCompounds("Ethanol", "Water")
      .WithPropertyPackage(PropertyPackages.NRTL))

def bubble_point(tag, x_ethanol):
    # pressure and vapor fraction given: the flash finds the temperature
    return (fs.AddMaterialStream(tag)
            .WithPressure(Q.Bar(1.01325)).WithVaporFraction(0.0)
            .SetCompoundMolarFlow("Ethanol", x_ethanol)
            .SetCompoundMolarFlow("Water", 1.0 - x_ethanol))

dilute = bubble_point("10 % ethanol", 0.10)
azeo = bubble_point("Azeotrope", 0.894)      # measured azeotrope at 1 atm: x = 0.894, 78.15 C

fs.Solve()

pp = list(fs.Inner.PropertyPackages.Values)[0].__implementation__
ip = pp.m_uni.InteractionParameters["Ethanol"]["Water"]
print(f"Binary parameters    = A12 {ip.A12:.2f}, A21 {ip.A21:.2f} cal/mol, alpha {ip.alpha12:.4f}")
for s in (dilute.Object, azeo.Object):
    x = s.Phases[3].Compounds["Ethanol"].MoleFraction
    y = s.Phases[2].Compounds["Ethanol"].MoleFraction
    print(f"x = {x:.3f}: bubble T = {s.GetTemperature() - 273.15:.2f} C, ethanol in vapor y = {y:.4f}")

Output

Binary parameters    = A12 -57.96, A21 1241.74 cal/mol, alpha 0.2937
x = 0.100: bubble T = 86.68 C, ethanol in vapor y = 0.4374
x = 0.894: bubble T = 78.39 C, ethanol in vapor y = 0.8908

DWSIM 10.2.11.0, generated 2026-10-08.

Property methods

How the package calculates each property, as it reports it in PropertyMethodsInfo (the property package editor shows the same list).

Property	Method
Vapor fugacity	Ideal / PR EOS
Liquid fugacity	Activity Coefficient + Poynting + Vapor Pressure / Henry's Constant
Vapor enthalpy, entropy, Cp/Cv	Ideal / Lee-Kesler / Excess / Experimental
Liquid enthalpy, entropy, Cp/Cv	Ideal / Lee-Kesler / Excess / Experimental
Vapor density	Ideal / PR EOS
Liquid density	Experimental / Rackett / COSTALD
Vapor viscosity	Experimental / Lucas / Jossi-Stiel-Thodos
Liquid viscosity	Experimental / Letsou-Stiel
Vapor thermal conductivity	Experimental / Ely-Hanley
Liquid thermal conductivity	Experimental / Latini
Surface tension	Experimental / Brock-Bird
Solid density	Experimental Data / User-Defined
Solid enthalpy, entropy, Cp/Cv	Experimental Solid Cp / From Liquid Phase Enthalpy + Enthalpy of Fusion

Default flash algorithm: Universal.

Configuration saved with the flowsheet

The package writes its settings to the simulation file (SaveData) and reads them back on load (LoadData). The elements below are the ones this package adds to those of every property package, as written for the example.

Element	Content in the example	What it holds
InteractionParameters_PR	empty	kij of the Peng-Robinson equation used for the vapor phase when it is not ideal (Value), pairs of the flowsheet compounds
InteractionParameters_NRTL	1 InteractionParameter entry (Compound1, Compound2, ID1, ID2, A12, A21, B12, B21, C12, C21, alpha12)	NRTL parameters of the pairs of the flowsheet compounds; τij = (Aij + BijT + CijT2)/(RT) with A in cal/mol, and the non-randomness alpha12

First InteractionParameter of InteractionParameters_NRTL in the example

<InteractionParameter Compound1="Ethanol" Compound2="Water" ID1="1102" ID2="1921" A12="-57.9601" A21="1241.7396" B12="0" B21="0" C12="0" C21="0" alpha12="0.2937" />

Settings common to every property package, as saved for the example

Element	Value
Type	DWSIM.Thermodynamics.PropertyPackages.NRTLPropertyPackage
ComponentName	NRTL
ComponentDescription
Tag	NRTL
UseHenryConstants	true
AutoEstimateMissingNRTLUNIQUACParameters	true
UseImmiscibleListForLiquid2InitialEstimates	true
SingleCompoundCheckThreshold	0.99999
OverrideKvalFugCoeff	false
OverrideEnthalpyCalculation	false
OverrideEntropyCalculation	false
LiquidDensityCalculationMode_Subcritical	COSTALD
LiquidDensityCalculationMode_Supercritical	Rackett_and_ExpData
LiquidDensity_CorrectExpDataForPressure	true
LiquidDensity_UsePenelouxVolumeTranslation	true
LiquidViscosityCalculationMode_Subcritical	ExpData
LiquidViscosityCalculationMode_Supercritical	Letsou_Stiel
LiquidViscosity_CorrectExpDataForPressure	true
LiquidViscosity_MixingRule	MoleAverage
VaporPhaseFugacityCalculationMode	Ideal
SolidPhaseFugacityCalculationMethod	FromLiquidFugacity
SolidPhaseFugacity_UseIdealLiquidPhaseFugacity	false
SolidPhaseEnthalpy_UsesCp	false
EnthalpyEntropyCpCvCalculationMode	ExpData
LiquidEnthalpyEntropyCpCvCalculationMode_EOS	EOS
LiquidFugacity_UsePoyntingCorrectionFactor	true
ActivityCoefficientModels_IgnoreMissingInteractionParameters	false
IgnoreVaporFractionLimit	false
IgnoreSalinityLimit	false
CalculateAdditionalMaterialStreamProperties	true
FlashCalculationApproach	NestedLoops
DisplayMissingCompoundPropertiesWarning	false
ForcedSolids	[]
PropertyOverrides	{}
FlashSettings	36 Setting entries

Learn more

• User guide

  ---

  • NRTL model
  • NRTL
  • Data Regression - How To

• Used with

  ---

  • Distillation Column
  • Shortcut Column
  • Separator Vessel
  • Material Stream

• Tutorials

  ---

  • Ethanol Plant (Fermentation)
  • Reverse Osmosis Desalination
  • Simple Flash Drum
  • Distillation Column

• FluentAPI

  ---

  • Property Packages
  • 03 — Distillation (Rigorous, Ethanol/Water)
  • 04 - Bioprocess Pretreatment Train
  • 05 - Anaerobic Digester
  • 07 — Full Kit (LCA + TEA)
  • 11 — Rigorous Distillation (Methanol/Water)

API members

Public members declared by this class. Inherited members are documented on the base classes.

Constructors

NRTLPropertyPackage()

C#VB.NET

public NRTLPropertyPackage()

Public Sub New()

NRTLPropertyPackage(bool)

Parameter	Type	Description
comode	Boolean

C#VB.NET

public NRTLPropertyPackage(bool comode)

Public Sub New(comode As Boolean)

Properties

DisplayDescription

C#VB.NET

public override string DisplayDescription { get; }

Public Overrides ReadOnly Property DisplayDescription As String

DisplayName

C#VB.NET

public override string DisplayName { get; }

Public Overrides ReadOnly Property DisplayName As String

m_uni

C#VB.NET

public NRTL m_uni { get; set; }

Public Property m_uni As NRTL

Popular

C#VB.NET

public override bool Popular { get; }

Public Overrides ReadOnly Property Popular As Boolean

Methods

CheckMissingInteractionParameters(double[])

Parameter	Type	Description
Vx	Double[]

C#VB.NET

public override bool CheckMissingInteractionParameters(double[] Vx)

Public Overrides Function CheckMissingInteractionParameters(Vx As Double()) As Boolean

DisplayEditingForm()

C#VB.NET

public override void DisplayEditingForm()

Public Overrides Sub DisplayEditingForm()

EstimateMissingInteractionParameters(bool)

Parameter	Type	Description
verbose	Boolean

C#VB.NET

public void EstimateMissingInteractionParameters(bool verbose)

Public Sub EstimateMissingInteractionParameters(verbose As Boolean)

GetEditingForm(): Returns the binary interaction parameter editor of this package.

Returns the binary interaction parameter editor of this package.

C#VB.NET

public override object GetEditingForm()

Public Overrides Function GetEditingForm() As Object

GetModel()

C#VB.NET

public override object GetModel()

Public Overrides Function GetModel() As Object

RunPostMaterialStreamSetRoutine()

C#VB.NET

public override void RunPostMaterialStreamSetRoutine()

Public Overrides Sub RunPostMaterialStreamSetRoutine()

Fields

ClassId

C#VB.NET

public const string ClassId = "D42F0157-5750-4c89-A94E-634A04701568"

Public Const ClassId As String = "D42F0157-5750-4c89-A94E-634A04701568"