Soave-Redlich-Kwong (SRK)

Soave-Redlich-Kwong cubic equation of state (1972) with the van der Waals one-fluid mixing rules and binary interaction parameters k_ij from the databank. For gases, hydrocarbons and gas processing.

DWSIM.Thermodynamics.PropertyPackages.SRKPropertyPackage
Assembly DWSIM.Thermodynamics.dll · Object ← PropertyPackage ← SRKPropertyPackage
Name in the flowsheet Soave-Redlich-Kwong (SRK) · FluentAPI PropertyPackages.SoaveRedlichKwong

Scope

A cubic equation of state for gases, hydrocarbons and gas processing: natural gas, LPG, refinery gases, compression, and vapor-liquid equilibrium of nonpolar mixtures from low to high pressure. Like Peng-Robinson, it gives the fugacity coefficients of both phases from one equation.

The Soave-Redlich-Kwong (1972) equation is P = RT/(V - b) - a(T)/[V(V + b)], with b = 0.08664 RT_c/P_c and a = 0.42747 (R²T_c²/P_c) α(T), α = [1 + m(1 - T_r^1/2)]², m = 0.48 + 1.574ω - 0.176ω². Mixtures use the van der Waals one-fluid rules with binary interaction parameters k_ij. The liquid takes the smallest root of the cubic in Z and the vapor the largest. Enthalpy and entropy are the ideal-gas values plus the departure functions of the equation.

Parameters. T_c, P_c and ω come from the compound database; the k_ij come from an SRK databank shipped with DWSIM (a separate table from the Peng-Robinson one). A pair without a value has k_ij = 0. The values can be edited in the package editor, or set from code with ConfigureSRK(c => c.WithKij(...)), and are saved with the flowsheet.

Liquid density. By default from the experimental data of the compound or the Rackett equation; the equation-of-state density with the Peneloux volume translation is an option of the package settings.

Derivatives. Analytical temperature and composition derivatives of the fugacity coefficients are available to the vapor-fraction flashes and the rigorous column solvers.

Limitations

• Liquid volumes from the equation itself are poor, as for every two-parameter cubic; keep the default liquid density method unless the volume translation is wanted.
• Polar and associating liquids (water, alcohols, glycols) are outside the model; see NRTL, UNIQUAC or the CPA models of ThermoPack.
• A missing k_ij is zero; propane with n-butane, the pair of the example, has k_ij = 0 in the saved configuration below. Check the pairs with CO₂, H₂S, N₂ and H₂.
• A stream with a single compound (mole fraction above 0.99999) is flashed on the vapor pressure curve of the compound database.

Example

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

Python

fs = (Flowsheet.Create("SRKExample")
      .WithCompounds("Propane", "N-butane")
      .WithPropertyPackage(PropertyPackages.SoaveRedlichKwong))

# temperature and vapor fraction given: the flash finds the pressure
bubble = (fs.AddMaterialStream("Bubble point")
          .WithTemperature(Q.Celsius(25.0)).WithVaporFraction(0.0)
          .SetCompoundMolarFlow("Propane", 0.6)
          .SetCompoundMolarFlow("N-butane", 0.4))
dew = (fs.AddMaterialStream("Dew point")
       .WithTemperature(Q.Celsius(25.0)).WithVaporFraction(1.0)
       .SetCompoundMolarFlow("Propane", 0.6)
       .SetCompoundMolarFlow("N-butane", 0.4))

fs.Solve()

b, d = bubble.Object, dew.Object
print(f"Bubble pressure      = {b.GetPressure() / 1e5:.3f} bar")
print(f"Propane in vapor     = {b.Phases[2].Compounds['Propane'].MoleFraction:.4f} (first bubble, mole fraction)")
print(f"Dew pressure         = {d.GetPressure() / 1e5:.3f} bar")
print(f"Vapor density        = {d.Phases[2].Properties.density:.2f} kg/m3 (Z = {d.Phases[2].Properties.compressibilityFactor:.4f})")
print(f"Liquid density       = {b.Phases[3].Properties.density:.1f} kg/m3")

Output

Bubble pressure      = 6.557 bar
Propane in vapor     = 0.8293 (first bubble, mole fraction)
Dew pressure         = 4.506 bar
Vapor density        = 9.98 kg/m3 (Z = 0.9051)
Liquid density       = 527.8 kg/m3

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	Soave-Redlich-Kwong EOS
Liquid fugacity	Soave-Redlich-Kwong EOS
Vapor enthalpy, entropy, Cp/Cv	Soave-Redlich-Kwong EOS
Liquid enthalpy, entropy, Cp/Cv	Soave-Redlich-Kwong EOS
Vapor density	Soave-Redlich-Kwong EOS
Liquid density	Soave-Redlich-Kwong EOS (+VT) / 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	1 InteractionParameter entry (Compound1, Compound2, Value)	kij (Value) of the pairs of the flowsheet compounds, from the databank or entered in the package editor

First InteractionParameter of InteractionParameters in the example

<InteractionParameter Compound1="Propane" Compound2="N-butane" Value="0" />

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

Element	Value
Type	DWSIM.Thermodynamics.PropertyPackages.SRKPropertyPackage
ComponentName	Soave-Redlich-Kwong (SRK)
ComponentDescription
Tag	Soave-Redlich-Kwong (SRK)
UseHenryConstants	true
AutoEstimateMissingNRTLUNIQUACParameters	true
UseImmiscibleListForLiquid2InitialEstimates	true
SingleCompoundCheckThreshold	0.99999
OverrideKvalFugCoeff	false
OverrideEnthalpyCalculation	false
OverrideEntropyCalculation	false
LiquidDensityCalculationMode_Subcritical	Rackett_and_ExpData
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	LeeKesler
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

  ---

  • Soave-Redlich-Kwong Equation of State
  • Peng-Robinson, Soave-Redlich-Kwong

• Used with

  ---

  • Material Stream
  • Separator Vessel
  • Compressor
  • Absorption Column

API members

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

Constructors

SRKPropertyPackage()

C#VB.NET

public SRKPropertyPackage()

Public Sub New()

SRKPropertyPackage(bool)

Parameter	Type	Description
comode	Boolean

C#VB.NET

public SRKPropertyPackage(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

ImplementsAnalyticalDerivatives

C#VB.NET

public override bool ImplementsAnalyticalDerivatives { get; }

Public Overrides ReadOnly Property ImplementsAnalyticalDerivatives As Boolean

MobileCompatible

C#VB.NET

public override bool MobileCompatible { get; }

Public Overrides ReadOnly Property MobileCompatible As Boolean

Popular

C#VB.NET

public override bool Popular { get; }

Public Overrides ReadOnly Property Popular As Boolean

Methods

AUX_CM(object)

Parameter	Type	Description
Vx	Object

C#VB.NET

public double AUX_CM(object Vx)

Public Function AUX_CM(Vx As Object) As Double

AUX_CM(Phase)

Parameter	Type	Description
Phase	Phase

C#VB.NET

public double AUX_CM(Phase Phase)

Public Function AUX_CM(Phase As Phase) As Double

AUX_VAPDENS(double, double)

Parameter	Type	Description
T	Double
P	Double

C#VB.NET

public override double AUX_VAPDENS(double T, double P)

Public Overrides Function AUX_VAPDENS(T As Double, P As Double) As Double

AUX_Z(double[], double, double, PhaseName)

Parameter	Type	Description
Vx	Double[]
T	Double
P	Double
state	PhaseName

C#VB.NET

public override double AUX_Z(double[] Vx, double T, double P, PhaseName state)

Public Overrides Function AUX_Z(Vx As Double(), T As Double, P As Double, state As PhaseName) As Double

CalcIsothermalCompressibility(IPhase)

Parameter	Type	Description
p	IPhase

C#VB.NET

public override double CalcIsothermalCompressibility(IPhase p)

Public Overrides Function CalcIsothermalCompressibility(p As IPhase) As Double

CalcJouleThomsonCoefficient(IPhase)

Parameter	Type	Description
p	IPhase

C#VB.NET

public override double CalcJouleThomsonCoefficient(IPhase p)

Public Overrides Function CalcJouleThomsonCoefficient(p As IPhase) As Double

ConfigParameters()

C#VB.NET

public override void ConfigParameters()

Public Overrides Sub ConfigParameters()

DisplayEditingForm()

C#VB.NET

public override void DisplayEditingForm()

Public Overrides Sub DisplayEditingForm()

DW_CalcCompPartialVolume(Phase, double, double)

Parameter	Type	Description
phase	Phase
T	Double
P	Double

C#VB.NET

public override void DW_CalcCompPartialVolume(Phase phase, double T, double P)

Public Overrides Sub DW_CalcCompPartialVolume(phase As Phase, T As Double, P As Double)

DW_CalcCp_ISOL(Phase, double, double)

Parameter	Type	Description
Phase1	Phase
T	Double
P	Double

C#VB.NET

public override double DW_CalcCp_ISOL(Phase Phase1, double T, double P)

Public Overrides Function DW_CalcCp_ISOL(Phase1 As Phase, T As Double, P As Double) As Double

DW_CalcCv_ISOL(Phase, double, double)

Parameter	Type	Description
Phase1	Phase
T	Double
P	Double

C#VB.NET

public override double DW_CalcCv_ISOL(Phase Phase1, double T, double P)

Public Overrides Function DW_CalcCv_ISOL(Phase1 As Phase, T As Double, P As Double) As Double

DW_CalcdKdComposition(double[], double[], double, double, State, string): Composition (mole-number) derivative of the equilibrium K-values, d(K_i)/dn_j, with respect to the mole numbers of...

Composition (mole-number) derivative of the equilibrium K-values, d(K_i)/dn_j, with respect to the mole numbers of the phase indicated by withRespectTo (State.Liquid perturbs Vx, State.Vapor perturbs Vy), on a total-moles = 1 basis. Returns an (nc x nc) matrix. Base implementation is a finite difference; EOS/activity packages override it using the analytical d(ln phi)/dn (or d(ln gamma)/dx) building blocks.

Parameter	Type	Description
Vx	Double[]
Vy	Double[]
T	Double
P	Double
withRespectTo	State
type	String

C#VB.NET

public override double[,] DW_CalcdKdComposition(double[] Vx, double[] Vy, double T, double P, State withRespectTo, string type = "LV")

Public Overrides Function DW_CalcdKdComposition(Vx As Double(), Vy As Double(), T As Double, P As Double, withRespectTo As State, type As String = "LV") As Double(,)

DW_CalcdKdT(double[], double[], double, double, string)

Parameter	Type	Description
Vx	Double[]
Vy	Double[]
T	Double
P	Double
type	String

C#VB.NET

public override double[] DW_CalcdKdT(double[] Vx, double[] Vy, double T, double P, string type = "LV")

Public Overrides Function DW_CalcdKdT(Vx As Double(), Vy As Double(), T As Double, P As Double, type As String = "LV") As Double()

DW_CalcdLnFugCoeffdn(double[], double, double, State): Composition (mole-number) derivative of the natural logarithm of the fugacity coefficients, d(ln phi_i)/dn_j, for...

Composition (mole-number) derivative of the natural logarithm of the fugacity coefficients, d(ln phi_i)/dn_j, for the given phase, evaluated on a total-moles = 1 basis. Returns an (nc x nc) matrix. Base implementation is a finite difference in mole numbers; EOS/activity packages override this with the closed-form partial-molar expression.

Parameter	Type	Description
Vx	Double[]
T	Double
P	Double
st	State

C#VB.NET

public override double[,] DW_CalcdLnFugCoeffdn(double[] Vx, double T, double P, State st)

Public Overrides Function DW_CalcdLnFugCoeffdn(Vx As Double(), T As Double, P As Double, st As State) As Double(,)

DW_CalcdLnFugCoeffdT(double[], double, double, State): Temperature derivative of the natural logarithm of the fugacity coefficients, d(ln phi_i)/dT, for the given phase.

Temperature derivative of the natural logarithm of the fugacity coefficients, d(ln phi_i)/dT, for the given phase. Base implementation is a forward finite difference; EOS/activity packages override this with a closed-form expression. Distinct from DW_CalcdFugCoeffdT, which returns d(phi)/dT (not the logarithm).

Parameter	Type	Description
Vx	Double[]
T	Double
P	Double
st	State

C#VB.NET

public override double[] DW_CalcdLnFugCoeffdT(double[] Vx, double T, double P, State st)

Public Overrides Function DW_CalcdLnFugCoeffdT(Vx As Double(), T As Double, P As Double, st As State) As Double()

DW_CalcEnergyFlowMistura_ISOL(double, double)

Parameter	Type	Description
T	Double
P	Double

C#VB.NET

public override double DW_CalcEnergyFlowMistura_ISOL(double T, double P)

Public Overrides Function DW_CalcEnergyFlowMistura_ISOL(T As Double, P As Double) As Double

DW_CalcEnthalpy(Array, double, double, State): Calculates the enthalpy of a mixture.

Calculates the enthalpy of a mixture.

Parameter	Type	Description
Vx	Array	Vector of doubles containing the molar composition of the mixture.
T	Double	Temperature (K)
P	Double	Pressure (Pa)
st	State	State enum indicating the state of the mixture (liquid or vapor).

C#VB.NET

public override double DW_CalcEnthalpy(Array Vx, double T, double P, State st)

Public Overrides Function DW_CalcEnthalpy(Vx As Array, T As Double, P As Double, st As State) As Double

DW_CalcEnthalpyDeparture(Array, double, double, State): Calculates the enthalpy departure of a mixture.

Calculates the enthalpy departure of a mixture.

Parameter	Type	Description
Vx	Array	Vector of doubles containing the molar composition of the mixture.
T	Double	Temperature (K)
P	Double	Pressure (Pa)
st	State	State enum indicating the state of the mixture (liquid or vapor).

C#VB.NET

public override double DW_CalcEnthalpyDeparture(Array Vx, double T, double P, State st)

Public Overrides Function DW_CalcEnthalpyDeparture(Vx As Array, T As Double, P As Double, st As State) As Double

DW_CalcEntropy(Array, double, double, State): Calculates the entropy of a mixture.

Calculates the entropy of a mixture.

Parameter	Type	Description
Vx	Array	Vector of doubles containing the molar composition of the mixture.
T	Double	Temperature (K)
P	Double	Pressure (Pa)
st	State	State enum indicating the state of the mixture (liquid or vapor).

C#VB.NET

public override double DW_CalcEntropy(Array Vx, double T, double P, State st)

Public Overrides Function DW_CalcEntropy(Vx As Array, T As Double, P As Double, st As State) As Double

DW_CalcEntropyDeparture(Array, double, double, State): Calculates the entropy departure of a mixture.

Calculates the entropy departure of a mixture.

Parameter	Type	Description
Vx	Array	Vector of doubles containing the molar composition of the mixture.
T	Double	Temperature (K)
P	Double	Pressure (Pa)
st	State	State enum indicating the state of the mixture (liquid or vapor).

C#VB.NET

public override double DW_CalcEntropyDeparture(Array Vx, double T, double P, State st)

Public Overrides Function DW_CalcEntropyDeparture(Vx As Array, T As Double, P As Double, st As State) As Double

DW_CalcFugCoeff(Array, double, double, State): Calculates fugacity coefficients for the specified composition at the specified conditions.

Calculates fugacity coefficients for the specified composition at the specified conditions.

Parameter	Type	Description
Vx	Array	Vector of doubles containing the molar composition of the mixture.
T	Double	Temperature in K
P	Double	Pressure in Pa
st	State	Mixture state (Liquid or Vapor)

C#VB.NET

public override double[] DW_CalcFugCoeff(Array Vx, double T, double P, State st)

Public Overrides Function DW_CalcFugCoeff(Vx As Array, T As Double, P As Double, st As State) As Double()

DW_CalcK_ISOL(Phase, double, double)

Parameter	Type	Description
Phase1	Phase
T	Double
P	Double

C#VB.NET

public override double DW_CalcK_ISOL(Phase Phase1, double T, double P)

Public Overrides Function DW_CalcK_ISOL(Phase1 As Phase, T As Double, P As Double) As Double

DW_CalcMassaEspecifica_ISOL(Phase, double, double, double)

Parameter	Type	Description
Phase1	Phase
T	Double
P	Double
pvp	Double

C#VB.NET

public override double DW_CalcMassaEspecifica_ISOL(Phase Phase1, double T, double P, double pvp = 0)

Public Overrides Function DW_CalcMassaEspecifica_ISOL(Phase1 As Phase, T As Double, P As Double, pvp As Double = 0) As Double

DW_CalcMM_ISOL(Phase, double, double)

Parameter	Type	Description
Phase1	Phase
T	Double
P	Double

C#VB.NET

public override double DW_CalcMM_ISOL(Phase Phase1, double T, double P)

Public Overrides Function DW_CalcMM_ISOL(Phase1 As Phase, T As Double, P As Double) As Double

DW_CalcOverallProps()

C#VB.NET

public override void DW_CalcOverallProps()

Public Overrides Sub DW_CalcOverallProps()

DW_CalcPhaseProps(Phase)

Parameter	Type	Description
Phase	Phase

C#VB.NET

public override void DW_CalcPhaseProps(Phase Phase)

Public Overrides Sub DW_CalcPhaseProps(Phase As Phase)

DW_CalcProp(string, Phase): Provides a wrapper function for CAPE-OPEN CalcProp/CalcSingleProp functions.

Provides a wrapper function for CAPE-OPEN CalcProp/CalcSingleProp functions.

Parameter	Type	Description
property	String	The property to be calculated.
phase	Phase	The phase where the property must be calculated for.

C#VB.NET

public override void DW_CalcProp(string property, Phase phase)

Public Overrides Sub DW_CalcProp([property] As String, phase As Phase)

DW_CalcPVAP_ISOL(double)

Parameter	Type	Description
T	Double

C#VB.NET

public override double DW_CalcPVAP_ISOL(double T)

Public Overrides Function DW_CalcPVAP_ISOL(T As Double) As Double

DW_CalcTensaoSuperficial_ISOL(Phase, double, double)

Parameter	Type	Description
Phase1	Phase
T	Double
P	Double

C#VB.NET

public override double DW_CalcTensaoSuperficial_ISOL(Phase Phase1, double T, double P)

Public Overrides Function DW_CalcTensaoSuperficial_ISOL(Phase1 As Phase, T As Double, P As Double) As Double

DW_CalcViscosidadeDinamica_ISOL(Phase, double, double)

Parameter	Type	Description
Phase1	Phase
T	Double
P	Double

C#VB.NET

public override double DW_CalcViscosidadeDinamica_ISOL(Phase Phase1, double T, double P)

Public Overrides Function DW_CalcViscosidadeDinamica_ISOL(Phase1 As Phase, T As Double, P As Double) As Double

GetModel()

C#VB.NET

public override object GetModel()

Public Overrides Function GetModel() As Object

IntegralSimpsonCp(double, double, double, string)

Parameter	Type	Description
a	Double
b	Double
Epsilon	Double
subst	String

C#VB.NET

public double IntegralSimpsonCp(double a, double b, double Epsilon, string subst)

Public Function IntegralSimpsonCp(a As Double, b As Double, Epsilon As Double, subst As String) As Double

IntegralSimpsonCp_T(double, double, double, string)

Parameter	Type	Description
a	Double
b	Double
Epsilon	Double
subst	String

C#VB.NET

public double IntegralSimpsonCp_T(double a, double b, double Epsilon, string subst)

Public Function IntegralSimpsonCp_T(a As Double, b As Double, Epsilon As Double, subst As String) As Double

RET_KIJ(string, string)

Parameter	Type	Description
id1	String
id2	String

C#VB.NET

public double RET_KIJ(string id1, string id2)

Public Function RET_KIJ(id1 As String, id2 As String) As Double

RET_VC()

C#VB.NET

public double[] RET_VC()

Public Function RET_VC() As Double()

RET_VKij()

C#VB.NET

public override double[,] RET_VKij()

Public Overrides Function RET_VKij() As Double(,)

RET_VS()

C#VB.NET

public double[] RET_VS()

Public Function RET_VS() As Double()

ReturnCriticalPoints()

C#VB.NET

public ArrayList ReturnCriticalPoints()

Public Function ReturnCriticalPoints() As ArrayList

RunPostMaterialStreamSetRoutine()

C#VB.NET

public override void RunPostMaterialStreamSetRoutine()

Public Overrides Sub RunPostMaterialStreamSetRoutine()

SupportsComponent(ICompoundConstantProperties)

Parameter	Type	Description
comp	ICompoundConstantProperties

C#VB.NET

public override bool SupportsComponent(ICompoundConstantProperties comp)

Public Overrides Function SupportsComponent(comp As ICompoundConstantProperties) As Boolean

Fields

ClassId

C#VB.NET

public const string ClassId = "920D043C-F640-4cca-B301-228E773D4E35"

Public Const ClassId As String = "920D043C-F640-4cca-B301-228E773D4E35"

m_pr

C#VB.NET

public SRK m_pr

Public m_pr As SRK