概要信息：

Property Methods and Calculations A-1
A-1
A Property Methods and 
Calculations
A.1  Selecting Property Methods ....................................................................... 3
A.2  Property Methods ........................................................................................ 8
A.2.1  Equations of State ................................................................................... 8
A.2.2  Activity Models....................................................................................... 15
A.2.3  Activity Model Vapour Phase Options.................................................... 33
A.2.4  Semi-Empirical Methods........................................................................ 34
A.2.5  Vapour Pressure Property Packages..................................................... 35
A.2.6  Miscellaneous - Special Application Methods........................................ 38
A.3  Enthalpy and Entropy Departure Calculations ....................................... 41
A.3.1  Equations of State ................................................................................. 42
A.3.2  Activity Models....................................................................................... 44
A.3.3  Lee-Kesler Option.................................................................................. 45
A.4  Physical and Transport Properties .......................................................... 48
A.4.1  Liquid Density ........................................................................................ 48
A.4.2  Vapour Density ...................................................................................... 49
A.4.3  Viscosity ................................................................................................ 49
A.4.4  Liquid Phase Mixing Rules for Viscosity ................................................ 51
A.4.5  Thermal Conductivity............................................................................. 52
A.4.6  Surface Tension ..................................................................................... 54
A.4.7  Heat Capacity ........................................................................................ 54
A.5  Volumetric Flow Rate Calculations .......................................................... 55
A.5.1  Available Flow Rates ............................................................................. 55
A.5.2  Liquid and Vapour Density Basis ........................................................... 56
A.5.3  Formulation of Flow Rate Calculations .................................................. 58
A.5.4  Volumetric Flow Rates as Specifications ............................................... 59
A-2 
A-2
A.6  Flash Calculations..................................................................................... 60
A.6.1  T-P Flash Calculation............................................................................. 61
A.6.2  Vapour Fraction Flash............................................................................ 61
A.6.3  Enthalpy Flash ....................................................................................... 63
A.6.4  Entropy Flash......................................................................................... 63
A.6.5  Handling of Water .................................................................................. 63
A.6.6  Solids..................................................................................................... 65
A.6.7  Stream Information ................................................................................ 66
A.7  References ................................................................................................. 67
Property Methods and Calculations A-3
A-3
Introduction
This appendix is organized such that the detailed calculations that 
occur within the Simulation Basis Manager and within the Flowsheet 
are explained in a logical manner.
• In the first section, an overview of property method selection is 
presented. Various process systems and their recommended 
property methods are listed.
• Detailed information is provided concerning each individual 
property method available in HYSYS. This section is further 
subdivided into equations of state, activity models, Chao-
Seader based semi-empirical methods, vapour pressure 
models and miscellaneous methods.
• Following the detailed property method discussion is the 
section concerning enthalpy and entropy departure 
calculations. The enthalpy and entropy options available within 
HYSYS are largely dependent upon your choice of a property 
method.
• The physical and transport properties are covered in detail. 
The methods used by HYSYS in calculating liquid density, 
vapour density, viscosity, thermal conductivity and surface 
tension are listed.
• HYSYS handles volume flow calculations in a unique way. To 
highlight the methods involved in calculating volumes, a 
separate section has been provided.
• The next section ties all of the previous information together. 
Within HYSYS, the Flash calculation uses the equations of the 
selected property method, as well as the physical and transport 
property functions to determine all property values for 
Flowsheet streams. Once a flash calculation has been 
performed on an object, all of its thermodynamic, physical and 
transport properties are defined. The flash calculation in 
HYSYS does not require initial guesses or the specification of 
flash type to assist in its convergence.
• A list of References is included at the end of the Appendix.
A.1 Selecting Property 
Methods
The property packages available in HYSYS allow you to predict 
properties of mixtures ranging from well defined light hydrocarbon 
systems to complex oil mixtures and highly non-ideal (non-electrolyte) 
chemical systems. HYSYS provides enhanced equations of state (PR 
and PRSV) for rigorous treatment of hydrocarbon systems; semi-
empirical and vapour pressure models for the heavier hydrocarbon 
systems; steam correlations for accurate steam property predictions; 
A-4 Selecting Property Methods
A-4
and activity coefficient models for chemical systems. All of these 
equations have their own inherent limitations and you are encouraged 
to become more familiar with the application of each equation.
The following table lists some typical systems and recommended 
correlations. However, when in doubt of the accuracy or application of 
one of the property packages, contact Hyprotech to receive additional 
validation material or our best estimate of its accuracy.
Type of System
Recommended Property 
Method
TEG Dehydration PR
Sour Water PR, Sour PR
Cryogenic Gas Processing PR, PRSV
Air Separation PR, PRSV
Atm Crude Towers PR, PR Options, GS
Vacuum Towers PR, PR Options, GS (<10 
mm Hg), Braun K10, Esso 
K
Ethylene Towers Lee Kesler Plocker
High H2 Systems PR, ZJ or GS (see T/P 
limits)
Reservoir Systems PR, PR Options
Steam Systems Steam Package, CS or GS
Hydrate Inhibition PR
Chemical systems Activity Models, PRSV
HF Alkylation PRSV, NRTL (Contact 
Hyprotech)
TEG Dehydration with 
Aromatics
PR (Contact Hyprotech)
Hydrocarbon systems 
where H2O solubility in HC 
is important
Kabadi Danner
Systems with select gases 
and light hydrocarbons
MBWR
Property Methods and Calculations A-5
A-5
For oil, gas and petrochemical applications, the Peng-Robinson EOS 
(PR) is generally the recommended property package. Hyprotech’s 
enhancements to this equation of state enable it to be accurate for a 
variety of systems over a wide range of conditions. It rigorously solves 
any single, two-phase or three-phase system with a high degree of 
efficiency and reliability, and is applicable over a wide range of 
conditions, as shown in the following table.
The PR equation of state has been enhanced to yield accurate phase 
equilibrium calculations for systems ranging from low temperature 
cryogenic systems to high temperature, high pressure reservoir 
systems. The same equation of state satisfactorily predicts component 
distributions for heavy oil systems, aqueous glycol and CH3OH 
systems, and acid gas/sour water systems, although specific sour water 
models (Sour PR and Sour SRK) are available for more specialized 
treatment. Our high recommendation for the PR equation of state is 
largely due to the preferential attention that has been given to it  by 
Hyprotech. Although the Soave-Redlich-Kwong (SRK) equation will 
also provide comparable results to the PR in many cases, it has been 
found that its range of application is significantly limited and it is not as 
reliable for non-ideal systems. For example, it should not be used for 
systems with CH3OH or glycols.
As an alternate, the PRSV equation of state should also be considered. It 
can handle the same systems as the PR equation with equivalent, or 
better accuracy, plus it is more suitable for handling moderately non-
ideal systems.
The advantage of the PRSV equation is that not only does it have the 
potential to more accurately predict the phase behaviour of 
hydrocarbon systems, particularly for systems composed of dissimilar 
components, but it can also be extended to handle non-ideal systems 
with accuracies that rival traditional activity coefficient models. The 
only compromise is increased computational time and the additional 
interaction parameter that is required for the equation.
The PR and PRSV equations of state perform rigorous three-phase flash 
calculations for aqueous systems containing H2O, CH3OH or glycols, as 
well as systems containing other hydrocarbons or non-hydrocarbons in 
the second liquid phase. For SRK, H2O is the only component that will 
initiate an aqueous phase. The Chao-Seader (CS) and Grayson-Streed 
Method Temp (°F) Temp (°C) Pressure (psia) Pressure (kPa)
PR > -456 > -271 < 15,000 < 100,000
SRK > -225 > -143 <  5,000 < 35,000
NOTE: The range of 
applicability in many cases is 
more indicative of the 
availability of good data 
rather than on the actual 
limitations.
A-6 Selecting Property Methods
A-6
(GS) packages can also be used for three-phase flashes, but are 
restricted to the use of pure H2O for the second liquid phase.
The PR can also be used for crude systems, which have traditionally 
been modelled with dual model thermodynamic packages (an activity 
model representing the liquid phase behaviour, and an equation of 
state or the ideal gas law for the vapour phase properties). These earlier 
models are suspect for systems with large amounts of light ends or 
when approaching critical regions. Also, the dual model system leads to 
internal inconsistencies. The proprietary enhancements to the PR and 
SRK methods allow these EOSs to correctly represent vacuum 
conditions and heavy components (a problem with traditional EOS 
methods), as well as handle the light ends and high-pressure systems. 
Activity Models, which handle highly non-ideal systems, are much 
more empirical in nature when compared to the property predictions 
in the hydrocarbon industry. Polar or non-ideal chemical systems have 
traditionally been handled using dual model approaches. In this type of 
approach, an equation of state is used for predicting the vapour 
fugacity coefficients and an activity coefficient model is used for the 
liquid phase. Since the experimental data for activity model parameters 
are fitted for a specific range, these property methods cannot be used 
as reliably for generalized application. 
The CS and GS methods, though limited in scope, may be preferred in 
some instances. For example, they are recommended for problems 
containing mainly liquid or vapour H2O because they include special 
correlations that accurately represent the steam tables. The Chao 
Seader method can be used for light hydrocarbon mixtures, if desired. 
The Grayson-Streed correlation is recommended for use with systems 
having a high concentration of H2 because of the special treatment 
given H2 in the development of the model. This correlation may also be 
slightly more accurate in the simulation of vacuum towers.
The Vapour Pressure K models, Antoine, BraunK10 and EssoK models, 
are designed to handle heavier hydrocarbon systems at lower 
pressures. These equations have traditionally been applied for heavier 
hydrocarbon fractionation systems and consequently provide a good 
means of comparison against rigorous models. They should not be 
considered for VLE predictions for systems operating at high pressures 
or systems with significant quantities of light hydrocarbons.
Property Methods and Calculations A-7
A-7
The Property Package methods in HYSYS are divided into basic 
categories, as shown in the following table. With each of the property 
methods listed are the available methods of VLE and Enthalpy/Entropy 
calculation.
Please refer to Section A.3 - Enthalpy and Entropy Departure 
Calculations, for a description of Enthalpy and Entropy calculations. 
Property Method VLE Calculation
Enthalpy/Entropy 
Calculation
Equations of State
PR PR PR
PR LK ENTH PR Lee-Kesler
SRK SRK SRK
SRK LK ENTH SRK Lee-Kesler
Kabadi Danner Kabadi Danner SRK
Lee Kesler Plocker Lee Kesler Plocker Lee Kesler
PRSV PRSV PRSV
PRSV LK PRSV Lee-Kesler
Sour PR PR & API-Sour PR
SOUR SRK SRK & API-Sour SRK
Zudkevitch-Joffee Zudkevitch-Joffee Lee-Kesler
Activity Models
Liquid
Chien Null Chien Null Cavett
Extended and General 
NRTL
NRTL Cavett
Margules Margules Cavett
NRTL NRTL Cavett
UNIQUAC UNIQUAC Cavett
van Laar van Laar Cavett
Wilson Wilson Cavett
Vapour
Ideal Gas Ideal Ideal Gas
RK RK RK
Virial Virial Virial
Peng Robinson Peng Robinson Peng Robinson
SRK SRK SRK
Semi-Empirical Models
Chao-Seader CS-RK Lee-Kesler
Grayson-Streed GS-RK Lee-Kesler
A-8 Property Methods
A-8
A.2 Property Methods
Details of each individual property method available in HYSYS will be 
provided in this section, including equations of state, activity models, 
Chao-Seader based empirical methods, vapour pressure models and 
miscellaneous methods.
A.2.1 Equations of State
HYSYS currently offers the enhanced Peng-Robinson1 (PR), and Soave-
Redlich-Kwong2 (SRK) equations of state. In addition, HYSYS offers 
several methods which are modifications of these property packages, 
including PRSV, Zudkevitch Joffee (ZJ) and Kabadi Danner (KD). Lee 
Kesler Plocker3 (LKP) is an adaptation of the Lee Kesler equation for 
mixtures, which itself was modified from the BWR equation. Of these, 
the Peng-Robinson equation of state supports the widest range of 
operating conditions and the greatest variety of systems. The Peng-
Robinson and Soave-Redlich-Kwong equations of state (EOS) generate 
all required equilibrium and thermodynamic properties directly. 
Although the forms of these EOS methods are common with other 
commercial simulators, they have been significantly enhanced by 
Hyprotech to extend their range of applicability. 
The Peng-Robinson property package options are PR, Sour PR, and 
PRSV. Soave-Redlich-Kwong equation of state options are the SRK, 
Sour SRK, KD and ZJ.
Vapour Pressure Models
Mod Antoine Mod Antoine-Ideal Gas Lee-Kesler
Braun K10 Braun K10-Ideal Gas Lee-Kesler
Esso K Esso-Ideal Gas Lee-Kesler
Miscellaneous - Special Application Methods
Amines Mod Kent Eisenberg 
(L), PR (V)
Curve Fit
Steam Packages
ASME Steam ASME Steam Tables ASME Steam Tables
NBS Steam NBS/NRC Steam 
Tables
NBS/NRC Steam 
Tables
MBWR Modified BWR Modified BWR
Property Method VLE Calculation
Enthalpy/Entropy 
Calculation
It is important to note that the 
properties predicted by HYSYS’ 
PR and SRK equations of state 
will not necessarily agree with 
those predicted by the PR and 
SRK of other commercial 
simulators.
Property Methods and Calculations A-9
A-9
PR and SRK
The PR and SRK packages contain enhanced binary interaction 
parameters for all library hydrocarbon-hydrocarbon pairs (a 
combination of fitted and generated interaction parameters), as well as 
for most hydrocarbon-nonhydrocarbon binaries.
For non-library or hydrocarbon pseudo components, HC-HC 
interaction parameters will be generated automatically by HYSYS for 
improved VLE property predictions.
The PR equation of state applies a functionality to some specific 
component-component interaction parameters. Key components 
receiving special treatment include He, H2, N2, CO2, H2S, H2O, CH3OH, 
EG and TEG. For further information on application of equations of 
state for specific components, please contact Hyprotech.
The following page provides a comparison of the formulations used in 
HYSYS for the PR and SRK equations of state. 
Note: The PR or SRK EOS 
should not be used for non-
ideal chemicals such as 
alcohols, acids or other 
components. They are more 
accurately handled by the 
Activity Models (highly non-
ideal) or the PRSV EOS 
(moderately non-ideal).
Soave Redlich Kwong Peng Robinson
  
where
b=
  
bi=
  
a=
  
ai=
  
aci=
  
αi
0.5 =
  
P
RT
V b–
------------
a
V V b+( )
---------------------–=
Z
3
Z
2
– A B– B
2
–( )Z AB–+ 0=
P
RT
V b–
------------
a
V V b+( ) b V b–( )+
------------------------------------------------–=
Z
3
1 B–( )Z
2
A 2B– 3B
2
–( )Z AB B
2
– B
3
–( )–+ + 0=
xibi
i 1=
N
∑ xibi
i 1=
N
∑
0.08664
RTci
Pci
---------- 0.077796
RTci
Pci
----------
xixj aiaj( )0.5
1 kij–( )
j 1=
N
∑
i 1=
N
∑ xixj aiaj( )0.5
1 kij–( )
j 1=
N
∑
i 1=
N
∑
aciαi aciαi
0.42748
RTci( )2
Pci
------------------ 0.457235
RTci( )2
Pci
------------------
1 mi 1 Tri
0.5
–( )+ 1 mi 1 Tri
0.5
–( )+
A-10 Property Methods
A-10
Kabadi Danner
This KD4 model is a modification of the original SRK equation of State, 
enhanced to improve the vapour-liquid-liquid equilibria calculations 
for H2O-hydrocarbon systems, particularly in the dilute regions.
The model is an improvement over previous attempts which were 
limited in the region of validity. The modification is based on an 
asymmetric mixing rule, whereby the interaction in the water phase 
(with its strong H2 bonding) is calculated based on both the interaction 
between the hydrocarbons and the H2O, and on the perturbation by 
hydrocarbon on the H2O-H2O interaction (due to its structure).
Lee Kesler Plöcker Equation
The Lee Kesler Plöcker equation is an accurate general method for non-
polar substances and mixtures. Plöcker et al.3 applied the Lee Kesler 
equation to mixtures, which itself was modified from the BWR 
equation.
mi=
  
When an acentric factor > 0.49 is present HYSYS uses 
following corrected form:
A=
  
B=
  
Soave Redlich Kwong Peng Robinson
0.48 1.574ωi 0.176ωi
2
–+ 0.37464 1.54226ωi 0.26992ωi
2
–+
0.379642 1.48503 0.164423 1.016666ωi–( )ωi–( )ωi+
aP
RT( )2
--------------
aP
RT( )2
--------------
bP
RT
------
bP
RT
------
The Lee Kesler Plöcker 
equation does not use the 
COSTALD correlation in 
computing liquid density. This 
may result in differences when 
comparing results between 
equation of states.
(A.1)z z
o( ) ω
ω r( )--------- z
r( )
z
o( )
–( )+=
Property Methods and Calculations A-11
A-11
The compressibility factors are determined as follows:
 where:
Mixing rules for pseudocritical properties are as follows:
where:
(A.2)
(A.3)
z
pv
RT
------
prvr
Tr
--------- z Tr vr Ak, ,( )= = =
z 1 B
vr
----
C
vr
2
----
D
vr
5
----
C4
Tr
3
vr
2
---------- β γ
vr
2
----+
γ–
vr
2
-----exp+ + + +=
vr
pcv
RTc
--------=
C c1
c2
Tr
----–
c3
Tr
2
-----+=
ω o( )
0=
B b1
b2
Tr
----–
b3
Tr
2
-----–
b4
Tr
3
-----–=
D d1
d2
Tr
----–=
ω r( )
0.3978=
(A.4)Tcm
1
Vcm
η---------
 
 
 
xixjvcij
j
∑
i
∑=
Tcij
Tci
Tcj
( )1 2⁄
= Tcii
Tci
= Tcjj
Tcj
=
vcm
xixjvcij
j
∑
i
∑= vcij
1
8
-- vci
1 3⁄
vcj
1 3⁄
+( )
3
=
vci
zci
RTci
pci
----------= zci
0.2905 0.085ωi–=
pcm
zcm
RTcm
vcm
-----------= zcm
0.2905 0.085ωm–=
ωm xiωi
i
∑=
A-12 Property Methods
A-12
Peng-Robinson Stryjek-Vera
The Peng-Robinson Stryjek-Vera (PRSV) equation of state is a two-fold 
modification of the PR equation of state that extends the application of 
the original PR method for moderately non-ideal systems. It has been 
shown to match vapour pressures curves of pure components and 
mixtures more accurately than the PR method, especially at low vapour 
pressures. 
It has been successfully extended to handle non-ideal systems giving 
results as good as those obtained using excess Gibbs energy functions 
like the Wilson, NRTL or UNIQUAC equations.
One of the proposed modifications to the PR equation of state by 
Stryjek and Vera was an expanded alpha, "α", term that became a 
function of acentricity and an empirical parameter, κi, used for fitting 
pure component vapour pressures. 
where: κ1i = characteristic pure component parameter
ωi = acentric factor
The adjustable κ1i term allows for a much closer fit of the pure 
component vapour pressure curves. This term has been regressed 
against the pure component vapour pressure for all components in 
HYSYS’ library.
For pseudo components that have been generated to represent oil 
fractions, HYSYS will automatically regress the κ1i term for each pseudo 
component against the Lee-Kesler vapour pressure curves. For 
individual user-added hypothetical components, κ1i terms can either 
be entered or they will automatically be regressed against the Lee-
Kesler, Gomez-Thodos or Reidel correlations.
The second modification consists of a new set of mixing rules for 
mixtures. Conventional mixing rules are used for the volume and 
energy parameters in mixtures, but the mixing rule for the cross term, 
aij, is modified to adopt a composition dependent form. Although two 
different mixing rules were proposed in the original paper, HYSYS has 
(A.5)
αi 1 κi 1 Tr
0.5
–( )+[ ]
2
=
κi κ0i κ+
1i
1 Tri
0.5
+( ) 0.7 Tri
0.5
–( )=
κ0i 0.378893 1.4897153ωi 0.17131848ωi
2
– 0.0196554ωi
3
+ +=
Note that if kij =kji , the mixing 
rules reduce to the standard 
PR equation of state. 
Property Methods and Calculations A-13
A-13
incorporated only the Margules expression for the cross term.
where:  
Although only a limited number of binary pairs have been regressed for 
this equation, our limited experience suggests that the PRSV can be 
used to model moderately non-ideal systems such as H2O-alcohol 
systems, some hydrocarbon-alcohol systems. You can also model 
hydrocarbon systems with improved accuracy. Also, due to PRSV’s 
better vapour pressure predictions, improved heat of vaporization 
predictions should be expected.
Sour Water Options
The Sour option is available for both the PR and SRK equations of state. 
The Sour PR option combines the PR equation of state and Wilson’s 
API-Sour Model for handling sour water systems, while Sour SRK 
utilizes the SRK equation of state with the Wilson model. 
The Sour options use the appropriate equation of state for calculating 
the fugacities of the vapour and liquid hydrocarbon phases as well as 
the enthalpy for all three phases. The K-values for the aqueous phase 
are calculated using Wilson’s API-Sour method. This option uses 
Wilson’s model to account for the ionization of the H2S, CO2 and NH3 
in the aqueous water phase. The aqueous model employs a 
modification of Van Krevelen’s original model with many of the key 
limitations removed. More details of the model are available in the 
original API publication 955 titled "A New Correlation of NH3, CO2, and 
H2S Volatility Data from Aqueous Sour Water Systems".
The original model is applicable for temperatures between 20°C (68°F) 
and 140°C (285°F), and pressures up to 50 psi. Use of either the PR or 
SRK equation of state to correct vapour phase non idealities extends 
this range, but due to lack of experimental data, exact ranges cannot be 
specified. The acceptable pressure ranges for HYSYS' model vary 
depending upon the concentration of the acid gases and H2O. The 
method performs well when the H2O partial pressure is below 100 psi. 
This option may be applied to sour water strippers, hydrotreater loops, 
crude columns or any process containing hydrocarbons, acid gases and 
H2O. If the aqueous phase is not present, the method produces 
(A.6)aij aiiajj( )0.5
1.0 xikij– xjkji–( )=
kij kji≠
Note that different values can 
be entered for each of the 
binary interaction 
parameters.
It is important to note that 
because the method performs 
an ion balance for each K-
value calculation, the flash 
calculation is much slower 
than the standard EOS.
A-14 Property Methods
A-14
identical results to the EOS, (PR or SRK depending on which option you 
have chosen). 
Zudkevitch Joffee
The Zudkevitch Joffee model is a modification of the Redlich Kwong 
equation of state. This model has been enhanced for better prediction 
of vapour liquid equilibria for hydrocarbon systems, and systems 
containing H2. The major advantage of this model over the previous 
version of the RK equation is the improved capability of predicting pure 
component equilibria, and the simplification of the method for 
determining the required coefficients for the equation. 
Enthalpy calculations for this model will be performed using the Lee 
Kesler model. 
EOS Enthalpy Calculation
With any the Equation of State options except ZJ and LKP, you can 
specify whether the Enthalpy will be calculated by either the Equation 
of State method or the Lee Kesler method. The ZJ and LKP must use the 
Lee Kesler method in Enthalpy calculations. Selection of an enthalpy 
method is done via radio buttons in the Enthalpy Method group.
Selecting the Lee Kesler Enthalpy option results in a combined 
property package employing the appropriate equation of state (either 
PR or SRK) for vapour-liquid equilibrium calculations and the Lee-
Kesler equation for calculation of enthalpies and entropies (for 
differences between EOS and LK methods, refer to the Section A.3 - 
Enthalpy and Entropy Departure Calculations).
The LK method yields comparable results to HYSYS’ standard 
equations of state and has identical ranges of applicability. As such, this 
option with PR has a slightly greater range of applicability than with 
SRK.
 Figure A.1
The Lee-Kesler enthalpies may 
be slightly more accurate for 
heavy hydrocarbon systems, 
but require more computer 
resources because a separate 
model must be solved.
Property Methods and Calculations A-15
A-15
Zero Kij Option
HYSYS automatically generates hydrocarbon-hydrocarbon interaction 
parameters when values are unknown if the Estimate HC-HC/Set Non 
HC-HC to 0.0 radio button is selected. The Set All to 0.0 radio button 
turns off the automatic calculation of any estimated interaction 
coefficients between hydrocarbons. All binary interaction parameters 
that are obtained from the pure component library will remain.
The Set All to 0.0 option may prove useful when trying to match results 
from other commercial simulators which may not supply interaction 
parameters for higher molecular weight hydrocarbons.
A.2.2 Activity Models
Although equation of state models have proven to be very reliable in 
predicting properties of most hydrocarbon based fluids over a large 
range of operating conditions, their application has been limited to 
primarily non-polar or slightly polar components. Polar or non-ideal 
chemical systems have traditionally been handled using dual model 
approaches. In this approach, an equation of state is used for 
predicting the vapour fugacity coefficients (normally ideal gas 
assumption or the Redlich Kwong, Peng-Robinson or SRK equations of 
state, although a Virial equation of state is available for specific 
applications) and an activity coefficient model is used for the liquid 
phase. Although there is considerable research being conducted to 
extend equation of state applications into the chemical arena (e.g., the 
PRSV equation), the state of the art of property predictions for 
chemical systems is still governed mainly by Activity Models. 
Activity Models are much more empirical in nature when compared to 
the property predictions (equations of state) typically used in the 
hydrocarbon industry. For example, they cannot be used as reliably as 
the equations of state for generalized application or extrapolating into 
untested operating conditions. Their tuning parameters should be 
fitted against a representative sample of experimental data and their 
application should be limited to moderate pressures. Consequently, 
more caution should be exercised when selecting these models for your 
simulation. 
 Figure A.2
This option is set on the 
Binary Coeffs tab of the Fluid 
Package property view.
A-16 Property Methods
A-16
The phase separation or equilibrium ratio Ki for component i, defined 
in terms of the vapour phase fugacity coefficient and the liquid phase 
activity coefficient is calculated from the following expression:
where: γi = liquid phase activity coefficient of component i
 fi° = standard state fugacity of component i
 P = system pressure
 φi  = vapour phase fugacity coefficient of component i
Although for ideal solutions the activity coefficient is unity, for most 
chemical (non-ideal) systems this approximation is incorrect. 
Dissimilar chemicals normally exhibit not only large deviations from an 
ideal solution, but the deviation is also found to be a strong function of 
the composition. To account for this non-ideality, activity models were 
developed to predict the activity coefficients of the components in the 
liquid phase. The derived correlations were based on the excess Gibbs 
energy function, which is defined as the observed Gibbs energy of a 
mixture in excess of what it would be if the solution behaved ideally, at 
the same temperature and pressure. 
For a multi-component mixture consisting of ni moles of component i, 
the total excess Gibbs free energy is represented by the following 
expression:
where: γi is the activity coefficient for component i
Activity Models produce the 
best results when they are 
applied in the operating 
region for which the 
interaction parameters were 
regressed.
(A.7)
Ki
yi
xi
---=
γi fi°
Pφi
---------=
(A.8)G
E
RT ni γiln( )∑=
Property Methods and Calculations A-17
A-17
The individual activity coefficients for any system can be obtained from 
a derived expression for excess Gibbs energy function coupled with the 
Gibbs-Duhem equation. The early models (Margules, van Laar) provide 
an empirical representation of the excess function that limits their 
application. The newer models such as Wilson, NRTL and UNIQUAC 
utilize the local composition concept and provide an improvement in 
their general application and reliability. All of these models involve the 
concept of binary interaction parameters and require that they be fitted 
to experimental data.
Since the Margules and van Laar models are less complex than the 
Wilson, NRTL and UNIQUAC models, they require less CPU time for 
solving flash calculations. However, these are older and more 
empirically based models and generally give poorer results for strongly 
non-ideal mixtures such as alcohol-hydrocarbon systems, particularly 
for dilute regions. The Chien-Null model provides the ability to 
incorporate the different activity models within a consistent 
thermodynamic framework. Each binary can be represented by the 
model which best predicts its behaviour. The following table briefly 
summarizes recommended models for different applications (for a 
more detailed review, refer to the texts "The Properties of Gases & 
Liquids"8 and "Molecular Thermodynamics of Fluid Phase Equilibria" 9). 
A = Applicable; N/A = Not Applicable;? = Questionable; G = Good;             
LA = Limited Application
Vapour phase non-ideality can be taken into account for each activity 
model by selecting the Redlich-Kwong, Peng-Robinson or SRK 
equations of state as the vapour phase model. When one of the 
equations of state is used for the vapour phase, the standard form of the 
Poynting correction factor is always used for liquid phase correction. If 
dimerization will occur in the vapour phase, the Virial equation of state 
should be selected as the vapour phase model. 
APPLICATION Margules van Laar Wilson NRTL UNIQUAC
Binary Systems A A A  A A
Multicomponent Systems LA LA A  A A
Azeotropic Systems A A A  A A
Liquid-Liquid Equilibria A A N/A  A A
Dilute Systems ? ? A  A A
Self-Associating Systems ? ? A  A A
Polymers N/A N/A N/A  N/A A
Extrapolation ? ? G G  G
A-18 Property Methods
A-18
The binary parameters required for the activity models have been 
regressed based on the VLE data collected from DECHEMA, Chemistry 
Data Series3. There are over 16,000 fitted binary pairs in the HYSYS 
library. The structures of all library components applicable for the 
UNIFAC VLE estimation are also in the library. The Poynting correction 
for the liquid phase is ignored if ideal solution behaviour is assumed. 
If you are using the built-in binary parameters, the ideal gas model 
should be used. All activity models, with the exception of the Wilson 
equation, can automatically calculate three phases given the correct set 
of energy parameters. The vapour pressures used in the calculation of 
the standard state fugacity are based on the pure component 
coefficients in HYSYS’ library using the modified form of the Antoine 
equation.
When your selected components exhibit dimerization in the vapour 
phase, the Virial option should be selected as the vapour phase model. 
HYSYS contains fitted parameters for many carboxylic acids, and can 
estimate values from pure component properties if the necessary 
parameters are not available. Please refer to Section A.2.3 - Activity 
Model Vapour Phase Options for a detailed description of the Virial 
option.
General Remarks
The dual model approach for solving chemical systems with activity 
models cannot be used with the same degree of flexibility and reliability 
that the equations of state can be used for hydrocarbon systems. 
However, some checks can be devised to ensure a good confidence 
level in property predictions:
• Check the property package selected for applicability for the 
system considered and see how well it matches the pure 
component vapour pressures. Although the predicted pure 
component vapour pressures should normally be acceptable, 
the parameters have been fitted over a large temperature 
range. Improved accuracies can be attained by regressing the 
parameters over the desired temperature range.
• The automatic UNIFAC generation of energy parameters in 
HYSYS is a very useful tool and is available for all activity 
models. However, it must be used with caution. The standard 
fitted values in HYSYS will likely produce a better fit for the 
binary system than the parameters generated by UNIFAC. As 
a general rule, use the UNIFAC generated parameters only as 
a last resort.
Please note that all of the 
binary parameters in HYSYS’ 
library have been regressed 
using an ideal gas model for 
the vapour phase.
Note that HYSYS’ internally 
stored binary parameters have 
NOT been regressed against 
three phase equilibrium data.
Property Methods and Calculations A-19
A-19
• Always use experimental data to regress the energy 
parameters when possible. The energy parameters in HYSYS 
have been regressed from experimental data, however, 
improved fits are still possible by fitting the parameters for the 
narrow operating ranges anticipated. The regressed 
parameters are based on data taken at atmospheric pressures. 
Exercise caution when extrapolating to higher or lower 
pressure (vacuum) applications. 
• Check the accuracy of the model for azeotropic systems. 
Additional fitting may be required to match the azeotrope with 
acceptable accuracy. Check not only for the temperature, but 
for the composition as well.
• If three phase behaviour is suspected, additional fitting of the 
parameters may be required to reliably reproduce the VLLE 
equilibrium conditions.
• An improvement in matching equilibrium data can be attained 
by including a temperature dependency of the energy 
parameters. However, depending on the validity or range of fit, 
this can lead to misleading results when extrapolating beyond 
the fitted temperature range.
By default, HYSYS regresses ONLY the aij parameters while the bij 
parameters are set to zero, i.e., the aij term is assumed to be 
temperature independent. A temperature dependency can be 
incorporated by supplying a value for the bij term. The matrix for the bij 
values are displayed by choosing the Bij radio button to switch matrices 
(note the zero or blank entries for all the binary pairs).
When using the NRTL, General NRTL or Extended NRTL equations, 
more than two matrices are available. In general, the second matrix is 
the Bij matrix, and the third matrix is the αij parameter where αij = αji. 
Any component pair with an aij value will have an associated α value.
Immiscible
This option is included for modelling the solubility of solutes in two 
coexisting liquid phases that are relatively immiscible with one 
another, such as a H2O-hydrocarbon system. In this system, the 
hydrocarbon components (solutes) are relatively insoluble in the water 
phase (solvent) whereas the solubility of the H2O in the hydrocarbon 
phase can become more significant. The limited mutual solubility 
behaviour can be taken into account when using any activity model 
with the exception of Wilson.
This feature can be implemented for any single component pair by 
using the Immiscible radio button. Component i will be insoluble with 
component j, based on the highlighted cell location. Alternatively, you 
Please note that the activities 
for the unknown binaries are 
generated at pre-selected 
compositions and the 
supplied UNIFAC reference 
temperature.
The Wilson equation does not 
support LLE equilibrium.
A-20 Property Methods
A-20
can have all j components treated as insoluble with component i. 
HYSYS will replace the standard binary parameters with those 
regressed specifically for matching the solubilities of the solutes in both 
phases. Note that both the aij and bij parameters are regressed with 
this option. These parameters were regressed from the mutual 
solubility data of n-C5, n-C6, n-C7, and n-C8 in H2O over a temperature 
range of 313 K to 473 K.
The solubility of H2O in the hydrocarbon phase and the solubility of the 
hydrocarbons in the water phase will be calculated based on the fitted 
binary parameters regressed from the solubility data referenced above.
Chien-Null
The Chien Null model provides a consistent framework for applying 
existing activity models on a binary by binary basis. In this manner, the 
Chien Null model allows you to select the best activity model for each 
pair in the case.
The Chien Null model allows 3 sets of coefficients for each component 
pair, accessible via the A, B and C coefficient matrices. Please refer to 
the following sections for an explanation of the terms for each of the 
models.
Chien Null Form
The Chien-Null generalized multi-component equation can be 
expressed as:
Each of the parameters in this equation are defined specifically for each 
of the applicable activity methods. 
(A.9)
2 Γi
L
ln
Aj i,  xj
j
∑ 
 
 
Aj i,  xj
j
∑ 
 
 
Aj i,  xj
j
∑ 
 
 
Aj i,  xj
j
∑ 
 
 
------------------------------------------------------- xk
Aj k,   xj
j
∑ 
 
 
Rj k,   xj
j
∑ 
 
 
Sj k,  xj
j
∑ 
 
 
Vj i,  xj
j
∑ 
 
 
------------------------------------------------------------  ⋅
k
∑+=
Ai k,  
Aj k,  xj
j
∑
----------------------
Ri k,  
Rj k,  xj
j
∑
----------------------
Si k,  
Sj k,  xj
j
∑
---------------------–
Vi k,  
Vj k,  xj
j
∑
----------------------–+
Property Methods and Calculations A-21
A-21
Description of Terms
The Regular Solution equation uses the following:
δi is the solubility parameter in (cal/cm3)½ and vi
L is the saturated 
liquid volume in cm3/mol calculated from:
The van Laar, Margules and Scatchard Hamer use the following:
For the van Laar, Margules and Scatchard Hamer equations:
where: T must be in K. 
Note that this equation is of a different form than the original van Laar 
and Margules equations in HYSYS, which utilized an a + bT 
relationship. However, since HYSYS only contains aij values, the 
difference should not cause problems.
(A.10)Ai j,
vi
L δi δj–( )2
RT
---------------------------= Ri j,
Ai j,
Aj i,
--------= Vi j, Ri j,= Si j, Ri j,=
(A.11)vi
L
vω i, 5.7 3Tr i,+( )=
Model Ai,j Ri,j Si,j Vi,j
van Laar
Margules
Scatchard Hamer
γi j,
∞
ln Ai j,
Aj i,
-------- Ri j, Ri j,
2 γi j,
∞
ln
1
γi j,
∞
ln
γj i,
∞
ln
----------------
 
 
 
+
-------------------------------
Ai j,
Aj i,
-------- 1 1
2 γi j,
∞
ln
1
γi j,
∞
ln
γj i,
∞
ln
----------------
 
 
 
+
------------------------------- Ai j,
Aj i,
--------
vi
∞
vj
∞-----
vi
∞
vj
∞-----
(A.12)γi j,
∞
ln ai j,
bi j,
T
------- cijT+ +=
If you have regressed 
parameters using HYPROP for 
any of the Activity Models 
supported under the Chien 
Null, they will not be read in.
A-22 Property Methods
A-22
The NRTL form for the Chien Null uses:
The expression for the τ term under the Chien Null incorporates the R 
term of HYSYS’ NRTL into the values for aij and bij. As such, the values 
initialized for NRTL under Chien Null will not be the same as for the 
regular NRTL. When you select NRTL for a binary pair, aij will be empty 
(essentially equivalent to the regular NRTL bij term), bij will be 
initialized and cij will be the α term for the original NRTL, and will be 
assumed to be symmetric. 
The General Chien Null equation is:
In all cases:
With the exception of the Regular Solution option, all models can 
utilize 6 constants, ai,j, aj,i, bi,j, bj,i, ci,j and cj,i for each component pair. 
For all models, if the constants are unknown they can be estimated 
internally from the UNIFAC VLE or LLE methods, the Insoluble option, 
or using Henry’s Law coefficients for appropriate components. For the 
general Chien Null model, the cij’s are assumed to be 1.
Extended and General NRTL
The Extended and General NRTL models are variations of the NRTL 
model. More binary interaction parameters are used in defining the 
component activity coefficients. You may apply either model to 
systems:
• with a wide boiling point range between components.
• where you require simultaneous solution of VLE and LLE, and 
there exists a wide boiling point range or concentration range 
between components.
(A.13)Ai j, 2τi j, Vi j,= Ri j, 1= Vi j, ci j,– τi j,( )exp= Si j, 1= τi j, ai j,
bi j,
T K( )
-----------+=
(A.14)
(A.15)
Ai j, ai j,
bi j,
T K( )
-----------+= Ri j,
Ai j,
Aj i,
--------= Vi j, Ci j,= Si j, Ci j,=
Ai i, 0= Ri i, Si i, Vi i, 1= = =
Property Methods and Calculations A-23
A-23
With the General NRTL model, you can specify the format for the 
Equations of  τij and aij to be any of the following:
Depending on which form of the equations that you have chosen, you 
will be able to specify values for the different component energy 
parameters. The General NRTL model provides radio buttons on the 
Binary Coeffs tab which access the matrices for the Aij, Bij, Cij, Fij, Gij, 
Alp1ij and Alp2ij energy parameters.
τij and αij Options
The equations options can be 
viewed in the Display Form 
drop down list on the Binary 
Coeffs tab of the Fluid 
Package property view.
τij Aij
Bij
T
------
Cij
T
2
------ FijT Gij T( )ln+ + + +=
αij Alp1ij Alp2ijT+=
τij
Aij
Bij
T
------+
RT
-------------------=
αij Alp1ij=
τij Aij
Bij
T
------ FijT Gij T( )ln+ + +=
αij Alp1ij Alp2ijT+=
τij Aij Bijt
Cij
T
------+ +=
αij Alp1ij Alp2ijT+=
where: T is in K and t is °C
τij Aij
Bij
T
------+=
αij Alp1ij=
A-24 Property Methods
A-24
The Extended NRTL model allows you to input values for the Aij, Bij, Cij, 
Alp1ij and Alp2ij energy parameters by selecting the appropriate radio 
button. You do not have a choice of equation format for τij and αij. The 
following is used:
where: T is in K
 t is in °C
Margules
The Margules equation was the first Gibbs excess energy representation 
developed. The equation does not have any theoretical basis, but is 
useful for quick estimates and data interpolation. HYSYS has an 
extended multicomponent Margules equation with up to four 
adjustable parameters per binary.
The four adjustable parameters for the Margules equation in HYSYS are 
the aij and aji (temperature independent) and the bij and bji terms 
(temperature dependent). The equation will use parameter values 
stored in HYSYS or any user supplied value for further fitting the 
equation to a given set of data. 
The Margules activity coefficient model is represented by the following 
equation: 
where: γi = activity coefficient of component i
xi = mole fraction of component i
(A.16)
τij Aij Bijt
Cij
T
------+ + 
 =
αij Alp1ij Alp2ij+=
The equation should not be 
used for extrapolation beyond 
the range over which the 
energy parameters have been 
fitted.
(A.17)γiln 1.0 xi–[ ]2
Ai 2xi Bi Ai–( )+[ ]=
Ai xj
aij bijT+( )
1.0 xi–( )
--------------------------
j 1=
n
∑=
Bi xj
aji bjiT+( )
1.0 xi–( )
--------------------------
j 1=
n
∑=
Property Methods and Calculations A-25
A-25
T = temperature (K)
n = total number of components
aij = non-temperature dependent energy parameter between 
components i and j
bij = temperature dependent energy parameter between 
components i and j [1/K]
aji = non-temperature dependent energy parameter between 
components j and i
bji = temperature dependent energy parameter between 
components j and i [1/K]
NRTL
The NRTL (Non-Random-Two-Liquid) equation, proposed by Renon 
and Prausnitz in 1968, is an extension of the original Wilson equation. It 
uses statistical mechanics and the liquid cell theory to represent the 
liquid structure. These concepts, combined with Wilson’s local 
composition model, produce an equation capable of representing VLE, 
LLE and VLLE phase behaviour. Like the Wilson equation, the NRTL is 
thermodynamically consistent and can be applied to ternary and 
higher order systems using parameters regressed from binary 
equilibrium data. It has an accuracy comparable to the Wilson 
equation for VLE systems.
The NRTL equation in HYSYS contains five adjustable parameters 
(temperature dependent and independent) for fitting per binary pair. 
The NRTL combines the advantages of the Wilson and van Laar 
equations, and, like the van Laar equation, it is not extremely CPU 
intensive and can represent LLE quite well. It is important to note that 
because of the mathematical structure of the NRTL equation, it can 
produce erroneous multiple miscibility gaps. 
Unlike the van Laar equation, 
NRTL can be used for dilute 
systems and hydrocarbon-
alcohol mixtures, although it 
may not be as good for 
alcohol-hydrocarbon systems 
as the Wilson equation.
A-26 Property Methods
A-26
The NRTL equation in HYSYS has the following form:
where: γi = activity coefficient of component i
xi = mole fraction of component i
T = temperature (K)
n = total number of components
aij = non-temperature dependent energy parameter between 
components i and j (cal/gmol)
bij = temperature dependent energy parameter between 
components i and j (cal/gmol-K)
αij = NRTL non-randomness constant for binary interaction 
note that αij  = αji  for all binaries
The five adjustable parameters for the NRTL equation in HYSYS are the 
aij, aji, bij, bji, and αij terms. The equation will use parameter values 
stored in HYSYS or any user supplied value for further fitting the 
equation to a given set of data.
UNIQUAC
The UNIQUAC (UNIversal QUAsi Chemical) equation proposed by 
Abrams and Prausnitz in 1975 uses statistical mechanics and the quasi-
chemical theory of Guggenheim to represent the liquid structure. The 
equation is capable of representing LLE, VLE and VLLE with accuracy 
comparable to the NRTL equation, but without the need for a non-
randomness factor. The UNIQUAC equation is significantly more 
(A.18)γiln
τjixjGji
j 1=
n
∑
xkGki
k 1=
n
∑
---------------------------
xjGij
xkGkj
k 1=
n
∑
----------------------- τij
τmjxmGmj
m 1=
n
∑
xkGkj
k 1=
n
∑
-----------------------------------–
 
 
 
 
 
 
 
 
j 1=
n
∑+=
Gij τijαij–[ ]exp=
τij
aij bijT+
RT
---------------------=
Property Methods and Calculations A-27
A-27
detailed and sophisticated than any of the other activity models. Its 
main advantage is that a good representation of both VLE and LLE can 
be obtained for a large range of non-electrolyte mixtures using only two 
adjustable parameters per binary. The fitted parameters usually exhibit 
a smaller temperature dependence which makes them more valid for 
extrapolation purposes. 
The UNIQUAC equation utilizes the concept of local composition as 
proposed by Wilson. Since the primary concentration variable is a 
surface fraction as opposed to a mole fraction, it is applicable to 
systems containing molecules of very different sizes and shape, such as 
polymer solutions. The UNIQUAC equation can be applied to a wide 
range of mixtures containing H2O, alcohols, nitriles, amines, esters, 
ketones, aldehydes, halogenated hydrocarbons and hydrocarbons. 
HYSYS contains the following four-parameter extended form of the 
UNIQUAC equation. The four adjustable parameters for the UNIQUAC 
equation in HYSYS are the aij and aji terms (temperature independent), 
and the bij and bji terms (temperature dependent). The equation will 
use parameter values stored in HYSYS or any user supplied value for 
further fitting the equation to a given set of data.
where: γi = activity coefficient of component i
xi = mole fraction of component i
T = temperature (K)
n = total number of components
(A.19)γiln
Φi
xi
----- 
 ln 0.5Zqi
θi
Φi
----- 
  Li
θi
Φi
----- 
  Ljxj qi 1.0 θjτji
j 1=
n
∑ln–
 
 
 
 
qi
θjτij
θkτkj
k 1=
n
∑
----------------------
 
 
 
 
 
 
 
 
j 1=
n
∑–+
j 1=
n
∑–+ln+=
A-28 Property Methods
A-28
Lj = 0.5Z(rj-qj)-rj+1
Z = 10.0 co-ordination number
aij = non-temperature dependent energy parameter between 
components i and j (cal/gmol)
bij = temperature dependent energy parameter between 
components i and j (cal/gmol-K)
qi = van der Waals area parameter - Awi /(2.5e9)
Aw = van der Waals area
ri = van der Waals volume parameter - Vwi /(15.17)
Vw = van der Waals volume
van Laar 
The van Laar equation was the first Gibbs excess energy representation 
with physical significance. The van Laar equation in HYSYS is a 
modified form of that described in "Phase Equilibrium in Process 
Design" by H.R. Null. This equation fits many systems quite well, 
particularly for LLE component distributions. It can be used for 
systems that exhibit positive or negative deviations from Raoult’s Law, 
however, it cannot predict maxima or minima in the activity coefficient. 
Therefore, it generally performs poorly for systems with halogenated 
hydrocarbons and alcohols. Due to the empirical nature of the 
equation, caution should be exercised in analyzing multi-component 
systems. It also has a tendency to predict two liquid phases when they 
do not exist. 
The van Laar equation has some advantages over the other activity 
models in that it requires less CPU time and can represent limited 
miscibility as well as three phase equilibrium. HYSYS uses the following 
extended, multi-component form of the van Laar equation.
θi
qixi
qjxj∑---------------=
τij
aij bijT+
RT
---------------------–exp=
The van Laar equation also 
performs poorly for dilute 
systems and CANNOT 
represent many common 
systems, such as alcohol-
hydrocarbon mixtures, with 
acceptable accuracy.
(A.20)γiln Ai 1.0 zi–[ ]2
1.0 Eizi+( )=
Property Methods and Calculations A-29
A-29
where: γi = activity coefficient of component i
xi = mole fraction of component i 
T = temperature (K)
n = total number of components
aij = non-temperature dependent energy parameter between 
components i and j
bij = temperature dependent energy parameter between 
components i and j [1/K]
aji = non-temperature dependent energy parameter between 
components j and i
bji = temperature dependent energy parameter between 
components j and i [1/K]
The four adjustable parameters for the van Laar equation in HYSYS are 
the aij, aji, bij, and bji terms. The equation will use parameter values 
stored in HYSYS or any user supplied value for further fitting the 
equation to a given set of data. 
Wilson
The Wilson equation, proposed by Grant M. Wilson in 1964, was the 
first activity coefficient equation that used the local composition model 
to derive the Gibbs Excess energy expression. It offers a 
thermodynamically consistent approach to predicting multi-
component behaviour from regressed binary equilibrium data. Our 
experience also shows that the Wilson equation can be extrapolated 
with reasonable confidence to other operating regions with the same 
set of regressed energy parameters. 
Ai xj
aij bijT+( )
1.0 xi–( )
--------------------------
j 1=
n
∑=
Bi xj
aji bjiT+( )
1.0 xi–( )
--------------------------
j 1=
n
∑=
Ei 4.0 if Ai Bi < 0.0, otherwise 0.0–=
zi
Aixi
Aixi Bi 1.0 xi–( )+[ ]
------------------------------------------------=
The Wilson equation CANNOT 
be used for problems involving 
liquid-liquid equilibrium.
A-30 Property Methods
A-30
Although the Wilson equation is more complex and requires more CPU 
time than either the van Laar or Margules equations, it can represent 
almost all non-ideal liquid solutions satisfactorily except electrolytes 
and solutions exhibiting limited miscibility (LLE or VLLE). It performs 
an excellent job of predicting ternary equilibrium using parameters 
regressed from binary data only. The Wilson equation will give similar 
results as the Margules and van Laar equations for weak non-ideal 
systems, but consistently outperforms them for increasingly non-ideal 
systems.
The Wilson equation in HYSYS requires two to four adjustable 
parameters per binary. The four adjustable parameters for the Wilson 
equation in HYSYS are the aij and aji (temperature independent) terms, 
and the bij and bji terms (temperature dependent). Depending upon 
the available information, the temperature dependent parameters may 
be set to zero. 
Although the Wilson equation contains terms for temperature 
dependency, caution should be exercised when extrapolating. The 
Wilson activity model in HYSYS has the following form:
where: γi = activity coefficient of component i
xi = mole fraction of component i
T = temperature (K)
n = total number of components
aij = non-temperature dependent energy parameter between 
components i and j (cal/gmol)
bij = temperature dependent energy parameter between 
components i and j (cal/gmol-K)
Note that setting all four 
parameters to zero does not 
reduce the binary to an ideal 
solution, but maintains a 
small effect due to molecular 
size differences represented by 
the ratio of molar volumes.
(A.21)γiln 1.0 xjAij
xkAki
xjAkj
j 1=
n
∑
---------------------
k 1=
n
∑–
j 1=
n
∑ln–=
Aij
Vj
Vi
----
aij bijT+( )
RT
--------------------------–exp=
Property Methods and Calculations A-31
A-31
Vi = molar volume of pure liquid component i in m3/kgmol 
(litres/gmol)
The equation will use parameter values stored in HYSYS or any user 
supplied value for further fitting the equation to a given set of data. 
Henry’s Law
Henry’s Law cannot be selected explicitly as a property method in 
HYSYS. However, HYSYS will use Henry’s Law when an activity model is 
selected and "non-condensable" components are included within the 
component list.
HYSYS considers the following components "non-condensable":
The extended Henry’s Law equation in HYSYS is used to model dilute 
solute/solvent interactions.  "Non-condensable" components are 
defined as those components that have critical temperatures below the 
temperature of the system you are modelling. The equation has the 
following form:
where: i = solute or "non-condensable" component
j = solvent or condensable component
Component Simulation Name
CH4 Methane
C2H6 Ethane
C2H4 Ethylene
C2H2 Acetylene
H2 Hydrogen
He Helium
Ar Argon
N2 Nitrogen
O2 Oxygen
NO NO 
H2S H2S 
CO2 CO2 
CO CO
(A.22)Hijln A
B
T
--- C T( ) DT+ln+ +=
A-32 Property Methods
A-32
Hij = Henry’s coefficient between i and j in kPa
A = A coefficient entered as aij in the parameter matrix 
B = B coefficient entered as aji in the parameter matrix 
C = C coefficient entered as bij in the parameter matrix
D = D coefficient entered as bji in the parameter matrix
T = temperature in degrees K
An example of the use of Henry’s Law coefficients is illustrated below. 
The NRTL activity model is selected as the property method. There are 
three components in the Fluid Package, one of which, ethane, is a "non-
condensable" component. On the Binary Coeffs tab of the Fluid 
Package property view, you can view the Henry’s Law coefficients for 
the interaction of ethane and the other components. By selecting the Aij 
radio button, you can view/edit the A and B coefficients. Choose the Bij 
radio button to enter or view the C and D coefficients in the Henry’s 
Law equation.
If HYSYS does not contain pre-fitted Henry’s Law coefficients and 
Henry’s Law data is not available, HYSYS will estimate the missing 
coefficients. To estimate a coefficient (A or B in this case), select the Aij 
radio button, highlight a binary pair and press the Individual Pair 
button. The coefficients are regressed to fugacities calculated using the 
Chao-Seader/Prausnitz-Shair correlations for standard state fugacity 
and Regular Solution. To supply your own coefficients you must enter 
them directly into the Aij and Bij matrices, as shown previously. 
 Figure A.3
HYSYS does not contain 
a pre-fitted Henry’s Law 
A coefficient for the 
ethane/ethanol pair. You 
can estimate it or provide 
your own value.
Henry’s Law A 
coefficient for the 
interaction 
between C2 and 
H2O.
Normal binary 
interaction 
coefficient for the 
H2O/Ethanol pair.
C2 is a "non-condensable" 
component.  Henry’s Law  will be 
used for the interaction between 
C2 and the other components in 
the Fluid Package. 
Henry’s Law B 
coefficient for the 
interaction 
between C2 and 
H2O.
Henry’s Law D 
coefficient for the 
interaction 
between C2 and 
H2O.
Henry’s Law C 
coefficient for the 
interaction 
between C2 and 
H2O.
Property Methods and Calculations A-33
A-33
No interaction between "non-condensable" component pairs is taken 
into account in the VLE calculations.
A.2.3 Activity Model Vapour 
Phase Options
There are several models available for calculating the Vapour Phase in 
conjunction with the selected liquid activity model. The choice will 
depend on specific considerations of your system. However, in cases 
when you are operating at moderate pressures (less than 5 atm), 
choosing Ideal Gas should be satisfactory. The choices are:
Ideal
The ideal gas law will be used to model the vapour phase. This model is 
appropriate for low pressures and for a vapour phase with little 
intermolecular interaction.
Peng Robinson, SRK or RK
To model non-idealities in the vapour phase, the PR, SRK or RK options 
can be used in conjunction with an activity model. The PR and SRK 
vapour phase models handle the same types of situations as the PR and 
SRK equations of state (refer to Section A.2.1 - Equations of State).
When selecting one of these options (PR, SRK or RK) as the vapour 
phase model, you must ensure that the binary interaction parameters 
used for the activity model remain applicable with the chosen vapour 
model. You must keep in mind that all the binary parameters in the 
HYSYS Library have been regressed using the ideal gas vapour model.
For applications where you have compressors or turbines being 
modelled within your Flowsheet, PR or SRK will be superior to either 
the RK or ideal vapour model. You will obtain more accurate 
horsepower values by using PR or SRK, as long as the light components 
within your Flowsheet can be handled by the selected vapour phase 
model (i.e. C2H4 or C3H6 are fine, but alcohols will not be modelled 
correctly).
A-34 Property Methods
A-34
Virial
The Virial option enables you to better model vapour phase fugacities 
of systems displaying strong vapour phase interactions. Typically this 
occurs in systems containing carboxylic acids, or compounds that have 
the tendency to form stable H2 bonds in the vapour phase. In these 
cases, the fugacity coefficient shows large deviations from ideality, even 
at low or moderate pressures. 
HYSYS contains temperature dependent coefficients for carboxylic 
acids. You can overwrite these by changing the Association (ii) or 
Solvation (ij) coefficients from the default values.22
If the virial coefficients need to be calculated, HYSYS contains 
correlations utilizing the following pure component properties:
• critical temperature
• critical pressure
• dipole moment
• mean radius of gyration
• association parameter
• association parameter for each binary pair
This option is restricted to systems where the density is moderate, 
typically less than one-half the critical density. The Virial equation 
used is valid for the following range:
A.2.4 Semi-Empirical Methods
The Chao-Seader10 and Grayson-Streed11 methods are older, semi-
empirical methods. The GS correlation is an extension of the CS 
method with special emphasis on H2. Only the equilibrium results 
produced by these correlations is used by HYSYS. The Lee-Kesler 
method is used for liquid and vapour enthalpies and entropies as its 
results have been shown to be superior to those generated from the CS/
GS correlations. This method has also been adopted by and is 
recommended for use in the API Technical Data Book. 
(A.23)P
T
2
--
yiPci
i 1=
m
∑
yiTci
i 1=
m
∑
--------------------≤
Property Methods and Calculations A-35
A-35
The following table gives an approximate range of applicability for 
these two methods, and under what conditions they are applicable. 
The GS correlation is recommended for simulating heavy hydrocarbon 
systems with a high H2 content, such as hydrotreating units. The GS 
correlation can also be used for simulating topping units and heavy 
ends vacuum applications. The vapour phase fugacity coefficients are 
calculated with the Redlich Kwong equation of state. The pure liquid 
fugacity coefficients are calculated via the principle of corresponding 
states. Modified acentric factors are included in HYSYS’ GS library for 
most components. Special functions have been incorporated for the 
calculation of liquid phase fugacities for N2, CO2 and H2S. These 
functions are restricted to hydrocarbon mixtures with less than five 
percent of each of the above components. As with the Vapour Pressure 
models, H2O is treated using a combination of the steam tables and the 
kerosene solubility charts from the API data book. This method of 
handling H2O is not very accurate for gas systems. Although three 
phase calculations are performed for all systems, it is important to note 
that the aqueous phase is always treated as pure H2O with these 
correlations. 
A.2.5 Vapour Pressure Property 
Packages
Vapour pressure K value models may be used for ideal mixtures at low 
pressures. This includes hydrocarbon systems such as mixtures of 
ketones or alcohols where the liquid phase behaves approximately 
ideal. The models may also be used for first approximations for non-
ideal systems.
Method Temp. (°C) Temp.  (°C) Press.  (psia) Press. (kPa)
CS 0 to 500 18 to 260 < 1500  < 10000
GS 0 to 800 18 to 425 < 3000 < 20000
Conditions of Applicability
For all hydrocarbons (except 
CH4):
0.5 < Tri < 1.3 and Prmixture < 0.8
If CH4 or H2 is present: • molal average Tr < 0.93 
• CH4 mole fraction < 0.3
• mole fraction dissolved gases < 0.2
When predicting K values for:
Paraffinic or Olefinic Mixtures
Aromatic Mixtures
liquid phase aromatic mole fraction < 0.5
liquid phase aromatic mole fraction > 0.5
A-36 Property Methods
A-36
 
Vapour pressures used in the calculation of the standard state fugacity 
are based on HYSYS’ library coefficients and a modified form of the 
Antoine equation. Vapour pressure coefficients for pseudo components 
may be entered or calculated from either the Lee-Kesler correlation for 
hydrocarbons, the Gomez-Thodos correlation for chemical 
compounds or the Reidel equation. 
The Vapour Pressure options include the Modified Antoine, BraunK10, 
and EssoK packages.
Approximate ranges of application for each vapour pressure model are 
given below:
Modified Antoine Vapour Pressure Model 
The modified Antoine equation assumes the form as set out in the 
DIPPR data bank. 
where A, B, C, D, E and F are fitted coefficients and the units of Pvap and 
T are kPa and K. These coefficients are available for all HYSYS library 
components. Vapour pressure coefficients for pseudo components may 
be entered or calculated from either the Lee-Kesler correlation for 
hydrocarbons, the Gomez-Thodos correlation for chemical 
compounds, or the Reidel equation. 
The Lee-Kesler model is used for enthalpy and entropy 
calculations for all vapour pressure models and all components 
with the exception of H2O, which is treated separately with the 
steam property correlation.
All three phase calculations are performed assuming the 
aqueous phase is pure H2O and that H2O solubility in the 
hydrocarbon phase can be described using the kerosene 
solubility equation from the API data book (Figure 9A1.4).
Because all of the Vapour 
Pressure options assume an 
ideal vapour phase, they are 
classified as Vapour Pressure 
Models.
Model Temperature Press. (psia) Press.  (kPa)
Mod. Antoine < 1.6 Tci < 100 < 700
BraunK10 0°F (-17.78°C) < 1.6 Tci < 100 < 700
EssoK < 1.6 Tci < 100 < 700
(A.24)Pvapln A
B
T C+
------------- D T ET
F
+ln+ +=
Note that all enthalpy and 
entropy calculations are 
performed using the Lee-
Kesler model.
Property Methods and Calculations A-37
A-37
This model is applicable for low pressure systems that behave ideally. 
For hydrocarbon components that you have not provided vapour 
pressure coefficients for, the model converts the Lee-Kesler vapour 
pressure model directly. As such, crude and vacuum towers can be 
modelled with this equation. When using this method for super-critical 
components, it is recommended that the vapour pressure coefficients 
be replaced with Henry’s Law coefficients. Changing Vapour Pressure 
coefficients can only be accomplished if your component is being 
installed as a Hypothetical.
Braun K10 Model 
The Braun K10 model is strictly applicable to heavy hydrocarbon 
systems at low pressures. The model employs the Braun convergence 
pressure method, where, given the normal boiling point of a 
component, the K value is calculated at system temperature and 10 
psia. The K10 value is then corrected for pressure using pressure 
correction charts. The K values for any components that are not 
covered by the charts are calculated at 10 psia using the modified 
Antoine equation and corrected to system conditions using the 
pressure correction charts. 
Accuracy suffers with this model if there are large amounts of acid gases 
or light hydrocarbons. All three phase calculations assume that the 
aqueous phase is pure H2O and that H2O solubility in the hydrocarbon 
phase can be described using the kerosene solubility equation from the 
API data book (Figure 9A1.4). 
Esso K Model
The Esso Tabular model is strictly applicable to hydrocarbon systems at 
low pressures. The model employs a modification of the Maxwell-
Bonnel vapour pressure model in the following format:
The Lee-Kesler model is used 
for enthalpy and entropy 
calculations for all 
components with the 
exception of H2O which is 
treated with the steam tables.
(A.25)Pvaplog Aix
i∑=
A-38 Property Methods
A-38
where: Ai = fitted constants
Tb
i = normal boiling point corrected to K = 12
T = absolute temperature
K = Watson characterisation factor
For heavy hydrocarbon systems, the results will be comparable to the 
modified Antoine equation since no pressure correction is applied. For 
non-hydrocarbon components, the K value is calculated using the 
Antoine equation. Accuracy suffers if there is a large amount of acid 
gases or light hydrocarbons. All three phase calculations are performed 
assuming the aqueous phase is pure H2O and that H2O solubility in the 
hydrocarbon phase can be described using the kerosene solubility 
equation from the API data book (Figure 9A1.4). 
A.2.6 Miscellaneous - Special 
Application Methods
Amines Property Package
The amines package contains the thermodynamic models developed 
by D.B. Robinson & Associates for their proprietary amine plant 
simulator, called AMSIM. Their amine property package is available as 
an option with HYSYS giving you access to a proven third party 
property package for reliable amine plant simulation, while 
maintaining the ability to use HYSYS’ powerful flowsheeting 
capabilities.
The chemical and physical property data base is restricted to amines 
and the following components:
x
i
Tb
i
T
----- 0.0002867Tb
i
–
748.1 0.2145Tb
i
–
-------------------------------------------=
Note that the Lee-Kesler model 
is used for enthalpy and 
entropy calculations for all 
components with the 
exception of H2O which is 
treated with the steam tables.
For the Amine property 
method, the vapour phase is 
modelled via the PR model.
Component Class Specific Components
Acid Gases CO2, H2S, COS, CS2 
Hydrocarbons CH4   C7H16 
Olefins C2=, C3=
Mercaptans M-Mercaptan, E-Mercaptan
Non Hydrocarbons H2, N2, O2, CO, H2O
Property Methods and Calculations A-39
A-39
The equilibrium acid gas solubility and kinetic parameters for the 
aqueous alkanolamine solutions in contact with H2S and CO2 have 
been incorporated into their property package. The amines property 
package has been fitted to extensive experimental data gathered from a 
combination of D.B. Robinson’s in-house data, several unpublished 
sources, and numerous technical references.
The following table gives the equilibrium solubility limitations that 
should be observed when using this property package. 
It is important to note that data have not been correlated for H2S and 
CO2 loadings greater than 1.0 mole acid gas/mole alkanolamine. 
The absorption of H2S and CO2 by aqueous alkanolamine solutions 
involves exothermic reactions. The heat effects are an important factor 
in amine treating processes and are properly taken into account in the 
amines property package. Correlations for the heats of solution are set 
up as a function of composition and amine type. The correlations were 
generated from existing published values or derived from solubility 
data using the Gibbs-Helmholtz equation. 
The amines package incorporates a specialized stage efficiency model 
to permit simulation of columns on a real tray basis. The stage 
efficiency model calculates H2S and CO2 component stage efficiencies 
based on the tray dimensions given and the calculated internal tower 
conditions for both absorbers and strippers. The individual component 
stage efficiencies are a function of pressure, temperature, phase 
compositions, flow rates, physical properties, mechanical tray design 
and dimensions as well as kinetic and mass transfer parameters. Since 
kinetic and mass transfer effects are primarily responsible for the H2S 
selectivity demonstrated by amine solutions, this must be accounted 
for by non unity stage efficiencies. See Chapter 7 - Column of the 
Steady State Modeling manual for details on how to specify or have 
HYSYS calculate the stage efficiencies. 
Note: this method does not 
allow any hypotheticals.
Alkanolamine
Alkanolamine 
Concentration (wt%)
Acid Gas Partial 
Pressure (psia)
Temperature 
(°F)
Monoethanolamine, MEA 0 - 30 0.00001 - 300 77 - 260 
Diethanolamine, DEA 0 - 50 0.00001 - 300 77 - 260 
Triethanolamine, TEA 0 - 50 0.00001 - 300 77 - 260 
Methyldiethanolamine, MDEA* 0 - 50 0.00001 - 300 77 - 260 
Diglycolamine, DGA 50 - 70 0.00001 - 300 77 - 260 
DIsoPropanolAmine, DIsoA 0 - 40 0.00001 - 300 77 - 260 
* The amine mixtures, DEA/
MDEA and MEA/MDEA are 
assumed to be primarily 
MDEA, so use the MDEA value 
for these mixtures.
A-40 Property Methods
A-40
Steam Package
HYSYS includes two steam packages:
• ASME Steam
• NBS Steam
Both of these property packages are restricted to a single component, 
namely H2O.
ASME Steam accesses the ASME 1967 steam tables. The limitations of 
this steam package are the same as those of the original ASME steam 
tables, i.e., pressures less than 15000 psia and temperatures greater 
than 32°F (0°C) and less than 1500°F. The basic reference is the book 
"Thermodynamic and Transport Properties of Steam" - The American 
Society of Mechanical Engineers - Prepared by C.A. Meyer, R.B. 
McClintock, G.J. Silvestri and R.C. Spencer Jr.20
Selecting NBS_Steam utilizes the NBS 1984 Steam Tables, which 
reportedly has better calculations near the Critical Point. 
MBWR
In HYSYS, a 32-term modified BWR equation of state is used. The 
modified BWR may be written in the following form:
where:
 F = exp (-0.0056 r2) 
(A.26)P RTρ NiXi
i 1=
32
∑+=
X1   = ρ2T X8  = ρ3/T X15  = ρ6/T2 X22 =  ρ5F/T2 X29 =  ρ11F/T3
X2   = ρ2T1/2 X9  = ρ3/T2 X16  = ρ7/T X23 = ρ5F/T4 X30 = ρ13F/T2
X3   = ρ2 X10 = ρ4T X17 = ρ8/T X24 = ρ7F/T2 X31 = ρ13F/T3
X4   = ρ2/T X11 = ρ4 X18 = ρ8/T2 X25 = ρ7F/T3 X32 = ρ13F/T4 
X5   = ρ2/T2 X12 = ρ4/T X19 = ρ9/T2 X26 = ρ9F/T2
X6   = ρ3T X13 = ρ5 X20 = ρ3F/T2 X27 = ρ9F/T4
X7   = ρ3 X14 = ρ6/T X21 = ρ3F/T3 X28 = ρ11F/T2
Property Methods and Calculations A-41
A-41
The modified BWR is applicable only for the following pure 
components:
A.3 Enthalpy and Entropy 
Departure 
Calculations
The Enthalpy and Entropy calculations are performed rigorously by 
HYSYS using the following exact thermodynamic relations:
Component Temp. (K) Temp. (R)
Max. Press. 
(MPa)
Max. Press. 
(psia)
Ar 84 - 400 151.2 - 720 100 14504
CH4 91 - 600 163.8 - 1080 200 29008
C2H4 104 - 400 187.2 - 720 40 5802
C2H6 90 - 600 162. - 1080 70 10153
C3H8 85 - 600 153. - 1080 100 14504
i-C4 114 - 600 205.2 - 1080 35 5076
n-C4 135 - 500 243. - 900 70 10153
CO 68 - 1000 122.4 - 1800 30 4351
CO2 217 - 1000 390.6 - 1800 100 14504
D2 29 - 423 52.2 - 761.4 320 46412
H2 14 - 400 25.2 - 720 120 17405
o-H2 14 - 400 25.2 - 720 120 17405
p-H2 14 - 400 25.2 - 720 120 17405
He 0.8 - 1500 1.4 - 2700 200 29008
N2 63 - 1900 113.4 - 3420 1000 145038
O2 54 - 400 97.2 - 720 120 17405
Xe 161 - 1300 289.8 - 2340 100 14504
Note that mixtures of different 
forms of H2 are also 
acceptable.  The range of use 
for these components is shown 
in this table.
Note that with semi-empirical 
and vapour pressure models, a 
pure liquid water phase will 
be generated and the solubility 
of H2O in the hydrocarbon 
phase will be determined from 
the kerosene solubility model.
(A.27)
(A.28)
H H
ID
–
RT
------------------- Z 1–
1
RT
------ T
T∂
∂P
 
 
V
P– Vd
∞
V
∫+=
The Ideal Gas Enthalpy basis 
(HID) used by HYSYS  is equal 
to the ideal gas Enthalpy of 
Formation at 25°C and 1 atm. S S°
ID
–
RT
------------------ Zln
P
P°
-----ln–
1
R
--
T∂
∂P
 
 
V
1
V
---– Vd
∞
V
∫+=
A-42 Enthalpy and Entropy Departure 
A-42
A.3.1 Equations of State
For the Peng-Robinson Equation of State:
where: 
For the SRK Equation of State:
A and B term definitions are provided below:
(A.29)
(A.30)
(A.31)
The Ideal Gas Enthalpy basis 
(HID) used by HYSYS  changes 
with temperature according to 
the coefficients on the TDep 
tab for each individual 
component.
H H
ID
–
RT
------------------- Z 1–
1
2
1.5
bRT
------------------- a T
td
da
–
V 2
0.5
1+( )b+
V 2
0.5
1–( )b+
------------------------------------
 
 
 
ln–=
S S°
ID
–
R
------------------ Z B–( )ln
P
P°
-----ln–
A
2
1.5
bRT
-------------------
T
a
--
td
da V 2
0.5
1+( )b+
V 2
0.5
1–( )b+
------------------------------------
 
 
 
ln–=
a xixj aiaj( )0.5
1 kij–( )
j 1=
N
∑
i 1=
N
∑=
(A.32)
(A.33)
H H
ID
–
RT
------------------- Z 1–
1
bRT
---------- a T
da
dt
-----– 1
b
V
--+ 
 ln–=
S S°
ID
–
RT
------------------ Z b–( )ln
P
P°
-----ln–
A
B
---
T
a
--
da
dt
----- 1
B
Z
---+ 
 ln+=
Peng - Robinson Soave -Redlich - Kwong
bi
ai
aci
0.077796
RTci
Pci
---------- 0.08664
RTci
Pci
----------
aciαi aciαi
0.457235
RTci( )2
Pci
------------------ 0.42748
RTci( )2
Pci
------------------
Property Methods and Calculations A-43
A-43
where:
R = Ideal Gas constant
H = Enthalpy
S = Entropy
subscripts:
ID = Ideal Gas
o = reference state
PRSV
The PRSV equation of state is an extension of the Peng-Robinson 
equation utilizing an extension of the κ expression as shown below: 
This results in the replacement of the αi term in the definitions of the A 
and B terms shown previously by the αi term shown above.
mi
Peng - Robinson Soave -Redlich - Kwong
αi
1 mi 1 Tri
0.5
–( )+ 1 mi 1 Tri
0.5
–( )+
0.37646 1.54226ωi 0.26992ωi
2
–+ 0.48 1.574ωi 0.176ωi
2
–+
a xixj aiaj( )0.5
1 kij–( )
j 1=
N
∑
i 1=
N
∑=
(A.34)
αi 1 κi 1 Tr
0.5
–( )+[ ]
2
=
κi κ0i 1 Tri
0.5
+( ) 0.7 Tri–( )=
κ0i 0.378893 1.4897153ωi 0.17131848ωi
2
– 0.0196554ωi
3
+ +=
A-44 Enthalpy and Entropy Departure 
A-44
A.3.2 Activity Models
The Liquid enthalpy and entropy for Activity Models is based on the 
Cavett Correlation:
for Tri < 1:
for Tri  1:
where:
where a1, a2, and a3 are functions of the Cavett parameter, fitted to 
match one known heat of vapourization. 
The Gas enthalpies and entropies are dependent on the model chosen 
to represent the vapour phase behaviour:
Ideal Gas: 
(A.35)
H
L
H
ID
–
Tci
---------------------- max
Hi°∆ L sb( )
Tci
-------------------------
Hi°∆ L sb( )
Tci
-------------------------,
 
 
 
=
≥
(A.36)
H
L
H
ID
–
Tci
---------------------- max
Hi°∆ L sb( )
Tci
-------------------------
Hi°∆ L sp( )
Tci
-------------------------,
 
 
 
=
(A.37)
(A.38)
Hi°∆ L sb( )
Tci
------------------------- a1 a2 1 Tri
–( )
1 a3 Tri
0.1–( )–
+=
Hi°∆ L sp( )
Tci
------------------------- max 0 b1 b2Tri
2
b3Tri
3
b4Tri
4
b5Tri
2
+ + + +,( )=
(A.39)
(A.40)
H H
ID
=
S S°
ID Cv Td
T
------------
T1
T2
∫ R
V2
V1
-----ln+= =
Property Methods and Calculations A-45
A-45
Redlich-Kwong:
Virial Equation:
where: B = second virial coefficient of the mixture
A.3.3 Lee-Kesler Option
The Lee and Kesler method is an effort to extend the method originally 
proposed by Pitzer to temperatures lower than 0.8 Tr. Lee and Kesler 
expanded Pitzer’s method expressing the compressibility factor as:
where: Z o = the compressibility factor of a simple fluid
Z r = the compressibility factor of a reference fluid
They chose the reduced form of the BWR equation of state to represent 
both Z o and Z r:
(A.41)
(A.42)
H H
ID
–
RT
------------------- Z 1–
1.5
bRT
---------- 1
b
V
---+ 
 ln–=
S S°
ID
–
RT
------------------ Z b–( )ln
P
P°
-----ln–
A
2B
------ 1
B
Z
---+ 
 ln+=
(A.43)
(A.44)
H H
ID
–
RT
-------------------
T
V B–
------------
dB
dt
------– Z 1–( )+=
S S°
ID
–
R
------------------
RT
V B–
------------
dB
dT
------– R
V
V B–
------------ln– R
V
V°
-----ln+=
The SRK and PR are given in 
Section A.2.1 - Equations of 
State.
(A.45)Z Z° ω
ωr
----- Zr Z°–( )+=
(A.46)Z 1
B
Vr
-----
C
Vr
2
-----
D
Vr
5
-----
D
Tr
3
Vr
3
----------- β γ
Vr
2
-----–
 
 
 
e
γ
Vr
2
-----
 
 –
+ + + +=
A-46 Enthalpy and Entropy Departure 
A-46
where:
The constants in these equations were determined using experimental 
compressibility and enthalpy data. Two sets of constants, one for the 
simple fluid (ωo = 0) and one for the reference fluid (ωr=0.3978, n-C8) 
were determined. 
The Enthalpy and Entropy departures are computed as follows:
Vr
VPc
RTc
---------=
B b1
b2
Tr
----–
b3
Tr
2
-----–
b4
Tr
4
-----–=
C c1
c2
Tr
----–
c3
Tr
3
-----+=
D d1
d2
Tr
----+=
(A.47)
(A.48)
(A.49)
H H
ID
–
RTc
------------------- Tr Z 1–
b2 2
b3
Tr
---- 3
b4
Tr
2
-----+ +
TrVr
------------------------------------–
c2 3
c3
Tr
2
-----–
2TrVr
2
--------------------–
d2
5TrVr
5
--------------– 3E+
 
 
 
 
 
 
 
=
S S°
ID
–
R
------------------ Zln
P
P°
----- 
 ln–
b1
b3
Tr
2
----- 2
b4
Tr
3
-----+ +
Vr
---------------------------------–
c1 3
c3
Tr
2
-----–
2Vr
2
--------------------–
d1
5Vr
2
--------– 2E+=
E
c4
2Tr
3γ
----------- β 1 β 1
γ
Vr
2
-----+ +
 
 
 
e
γ
Vr
---- 
 –
–+
 
 
 
 
 
=
Property Methods and Calculations A-47
A-47
for mixtures, the Critical Properties are defined as follows:
Fugacity Coefficient
Soave-Redlich-Kwong
Peng Robinson
ω xiωi
i 1=
N
∑=
zci
0.2905 0.0851ωi–=
Vci
Zci
RTci
Pci
-----------------=
Vc
1
8
-- xixj Vci
1
3
--
Vcj
1
3
--
+
 
 
 
3
j 1=
N
∑
i 1=
N
∑=
Tc
1
8Vc
-------- xixj Vci
1
3
--
Vcj
1
3
--
+
 
 
 
3
Tci
Tcj
( )0.5
j 1=
N
∑
i 1=
N
∑=
Pc 0.2905 0.085ω–( )
RTc
Vc
--------=
(A.50)φiln Z
Pb
RT
------– 
 ln– Z 1–( )
bi
b
---
a
bRT
----------
1
a
-- 2ai
0.5
xjaj
0.5
1 kij–( )
j 1=
N
∑
 
 
 
  bi
b
---– 1
b
V
--+ 
 ln–+=
(A.51)φiln Z
Pb
RT
------– 
 ln– Z 1–( )
bi
b
---
a
2
1.5
bRT
-------------------
1
a
-- 2ai
0.5
xjaj
0.5
1 kij–( )
j 1=
N
∑
 
 
 
  bi
b
---–
V 2
0.5
1+( )b+
V 2
0.5
1–( )b–
------------------------------------ln–+=
A-48 Physical and Transport Properties
A-48
A.4 Physical and Transport 
Properties
The physical and transport properties that HYSYS calculates for a given 
phase are viscosity, density, thermal conductivity and surface tension. 
The models used for the transport property calculations have all been 
pre-selected to yield the best fit for the system under consideration. For 
example, the corresponding states model proposed by Ely and Hanley 
is used for viscosity predictions of light hydrocarbons (NBP<155), the 
Twu methodology for heavier hydrocarbons, and a modification of the 
Letsou-Stiel method for predicting the liquid viscosities of non-ideal 
chemical systems. A complete description of the models used for the 
prediction of the transport properties can be found in the references 
listed in each sub-section. All these models have been modified by 
Hyprotech to improve the accuracy of the correlations. 
In the case of multiphase streams, the transport properties for the 
mixed phase are meaningless and are reported as , although 
the single phase properties are known. There is an exception with the 
pipe and heat exchanger operations. For three-phase fluids, HYSYS 
uses empirical mixing rules to determine the apparent properties for 
the combined liquid phases.
A.4.1 Liquid Density
Saturated liquid volumes are obtained using a corresponding states 
equation developed by R. W. Hankinson and G. H. Thompson13 which 
explicitly relates the liquid volume of a pure component to its reduced 
temperature and a second parameter termed the characteristic volume. 
This method has been adopted as an API standard. The pure 
compound parameters needed in the corresponding states liquid 
density (COSTALD) calculations are taken from the original tables 
published by Hankinson and Thompson, and the API data book for 
components contained in HYSYS’ library. The parameters for 
hypothetical components are based on the API gravity and the 
generalized Lu equation. Although the COSTALD method was 
developed for saturated liquid densities, it can be applied to sub-cooled 
liquid densities, i.e., at pressures greater than the vapour pressure, 
using the Chueh and Prausnitz correction factor for compressed fluids. 
It is used to predict the density for all systems whose pseudo-reduced 
temperature is below 1.0. Above this temperature, the equation of state 
compressibility factor is used to calculate the liquid density. 
Property Methods and Calculations A-49
A-49
Pseudo components generated in the Oil Characterization 
Environment will have their densities either calculated from internal 
correlations or generated from input curves. Given a bulk density, the 
densities of the pseudo components are adjusted such that:
The characteristic volume for each pseudo component is calculated 
using the adjusted densities and the physical properties. The calculated 
characteristic volumes are then adjusted such that the bulk density 
calculated from the COSTALD equation matches the density calculated 
via above equation. This ensures that a given volume of fluid will 
contain the same mass whether it is calculated with the sum of the 
component densities or the COSTALD equation.
A.4.2 Vapour Density
The density for all vapour systems at a given temperature and pressure 
is calculated using the compressibility factor given by the equation of 
state or by the appropriate vapour phase model for Activity Models. 
A.4.3 Viscosity
HYSYS will automatically select the model best suited for predicting the 
phase viscosities of the system under study. The model selected will be 
from one of the three available in HYSYS: a modification of the NBS 
method (Ely and Hanley), Twu’s model, or a modification of the Letsou-
Stiel correlation. HYSYS will select the appropriate model using the 
following criteria:
All of the models are based on corresponding states principles and have 
been modified for more reliable application. Internal validation 
showed that these models yielded the most reliable results for the 
chemical systems shown. Viscosity predictions for light hydrocarbon 
liquid phases and vapour phases were found to be handled more 
(A.52)
ρbulk
1.0
xi
ρi°
------∑
-------------=
Chemical System Vapour Phase Liquid Phase
Lt Hydrocarbons (NBP<155°F) Mod Ely & Hanley Mod Ely & Hanley
Hvy Hydrocarbons (NBP>155°F) Mod Ely & Hanley Twu
Non-Ideal Chemicals Mod Ely & Hanley Mod Letsou-Stiel
A-50 Physical and Transport Properties
A-50
reliably by an in-house modification of the original Ely and Hanley 
model, heavier hydrocarbon liquids were more effectively handled by 
Twu’s model, and chemical systems were more accurately handled by 
an in-house modification of the original Letsou-Stiel model. 
A complete description of the original corresponding states (NBS) 
model used for viscosity predictions is presented by Ely and Hanley in 
their NBS publication. The original model has been modified to 
eliminate the iterative procedure for calculating the system shape 
factors. The generalized Leech-Leland shape factor models have been 
replaced by component specific models. HYSYS constructs a PVT map 
for each component using the COSTALD for the liquid region. The 
shape factors are adjusted such that the PVT map can be reproduced 
using the reference fluid.
The shape factors for all the library components have already been 
regressed and are included in the Pure Component Library. Pseudo 
component shape factors are regressed using estimated viscosities. 
These viscosity estimations are functions of the pseudo component 
Base Properties and Critical Properties.
Pseudo components generated in the Oil Characterization 
Environment have the additional ability of having their shape factors 
regressed to match kinematic or dynamic viscosity assays. 
The general model employs CH4 as a reference fluid and is applicable to 
the entire range of non-polar fluid mixtures in the hydrocarbon 
industry. Accuracy for highly aromatic or naphthenic crudes will be 
increased by supplying viscosity curves when available, since the pure 
component property generators were developed for average crude oils. 
The model also handles H2O and acid gases as well as quantum gases. 
Although the modified NBS model handles these systems very well, the 
Twu method was found to do a better job of predicting the viscosities of 
heavier hydrocarbon liquids. The Twu model9 is also based on 
corresponding states principles, but has implemented a viscosity 
correlation for n-alkanes as its reference fluid instead of CH4. A 
complete description of this model is given in the paper entitled 
"Internally Consistent Correlation for Predicting Liquid Viscosities of 
Petroleum Fractions"15. 
For chemical systems the modified NBS model of Ely and Hanley is 
used for predicting vapour phase viscosities, whereas a modified form 
of the Letsou-Stiel model is used for predicting the liquid viscosities. 
This method is also based on corresponding states principles and was 
found to perform satisfactorily for the components tested.
Property Methods and Calculations A-51
A-51
The shape factors contained in the HYSYS Pure Component Library 
have been fit to match experimental viscosity data over a broad 
operating range. Although this will yield good viscosity predictions as 
an average over the entire range, improved accuracy over a narrow 
operating range can be achieved by using the Tabular features (see 
Chapter 1 - Fluid Package for more information).
A.4.4 Liquid Phase Mixing Rules 
for Viscosity
The estimates of the apparent liquid phase viscosity of immiscible 
Hydrocarbon Liquid - Aqueous mixtures are calculated using the 
following "mixing rules":
• If the volume fraction of the hydrocarbon phase is greater than 
or equal to 0.5, the following equation is used17:
where: µeff  = apparent viscosity
µoil  = viscosity of Hydrocarbon phase
νoil  = volume fraction Hydrocarbon phase
• If the volume fraction of the hydrocarbon phase is less than 
0.33, the following equation is used18:
where: µeff = apparent viscosity
µoil = viscosity of Hydrocarbon phase
µH2O= viscosity of Aqueous phase
νoil = volume fraction Hydrocarbon phase
• If the volume of the hydrocarbon phase is between 0.33 and 
0.5, the effective viscosity for combined liquid phase is 
calculated using a weighted average between Equation 
(A.53) and Equation (A.54). 
(A.53)µeff µoile
3.6 1 νoil–( )
=
(A.54)µeff 1 2.5νoil
µoil 0.4µH2O+
µoil µH2O+
-----------------------------------
 
 
 
+ µH2O=
A-52 Physical and Transport Properties
A-52
The remaining properties of the pseudo phase are calculated as follows:
A.4.5 Thermal Conductivity
As in viscosity predictions, a number of different models and 
component specific correlations are implemented for prediction of 
liquid and vapour phase thermal conductivities. The text by Reid, 
Prausnitz and Poling16 was used as a general guideline in determining 
which model was best suited for each class of components. For 
hydrocarbon systems the corresponding states method proposed by 
Ely and Hanley14 is generally used. The method requires molecular 
weight, acentric factor and ideal heat capacity for each component. 
These parameters are tabulated for all library components and may 
either be input or calculated for hypothetical components. It is 
recommended that all of these parameters be supplied for non-
hydrocarbon hypotheticals to ensure reliable thermal conductivity 
coefficients and enthalpy departures. 
The modifications to the method are identical to those for the viscosity 
calculations. Shape factors calculated in the viscosity routines are used 
directly in the thermal conductivity equations. The accuracy of the 
method will depend on the consistency of the original PVT map.
The Sato-Reidel method16 is used for liquid phase thermal conductivity 
predictions of glycols and acids, the Latini et al. method16 is used for 
esters, alcohols and light hydrocarbons in the range of C3 - C7, and the 
Missenard and Reidel method16 is used for the remaining components.
For vapour phase thermal conductivity predictions, the Misic and 
Thodos, and Chung et al.16 methods are used. The effect of higher 
pressure on thermal conductivities is taken into account by the Chung 
et al. method.
As with viscosity, the thermal conductivity for two liquid phases is 
approximated by using empirical mixing rules for generating a single 
pseudo liquid phase property. The thermal conductivity for an 
(A.55)
MWeff xiMWi∑=
ρeff
1
xi
ρ
i
---- 
 ∑
-----------------=
Cpeff
xiCpi∑=
(molecular weight)
(mixture density)
(mixture specific heat)
Property Methods and Calculations A-53
A-53
immiscible binary of liquid phases is calculated by the following 
equation21:
where:  λLmix = mixture liquid thermal conductivity at temperature 
T(K)
κij = liquid thermal conductivity of pure component i or j at 
temperature T
λL1 = liquid thermal conductivity of liquid phase 1
λL2 = liquid thermal conductivity of liquid phase 2
φ1 = 
φ2 = 
xi = mole fraction of component i
Vi = molar volume of component i
(A.56)λLmix
φ1
2λL1
2φ1φ2λ12 φ2
2λL2
+ +=
λLmix φiφjkij
j
∑
i
∑=
kij
2
1 ki⁄( ) 1 kj⁄( )+
-------------------------------------=
x1V1
xiVi
i 1=
2
∑
-------------------
x2V2
xiVi
i 1=
2
∑
-------------------
A-54 Physical and Transport Properties
A-54
A.4.6 Surface Tension
Surface tensions for hydrocarbon systems are calculated using a 
modified form of the Brock and Bird equation8. The equation expresses 
the surface tension, σ, as a function of the reduced and critical 
properties of the component. The basic form of the equation was used 
to regress parameters for each family of components.
where: σ = surface tension (dynes/cm2)
Q = 0.1207[1.0 + TBR ln Pc /(1.0 - TBR)] - 0.281
a = parameter fitted for each chemical class
b = c0 + c1 ω + c2 ω2 + c3 ω3 (parameter fitted for each chemical 
class, expanded as a polynomial in acentricity)
For aqueous systems, HYSYS employs a polynomial to predict the 
surface tension. It is important to note that HYSYS predicts only liquid-
vapour surface tensions. 
A.4.7 Heat Capacity
Heat Capacity is calculated using a rigorous Cv value whenever HYSYS 
can. The method used is given by the following equations:
However, when ever this equation fails to provide an answer HYSYS will 
fall back to the semi-ideal Cp/Cv method by computing Cp/Cv as Cp/
(Cp-R), which is only approximate and valid for ideal gases. Examples of 
when HYSYS will use the ideal method are:
• Equation (A.58) fails to return an answer
• The stream has a solid phase
• abs(dV/dP) < 1e-12
• Cp/Cv < 0.1or Cp/Cv > 20 - this is outside the range of 
applicability of the equation used so HYSYS falls back to the 
ideal method
(A.57)σ Pc
2 3⁄
Tc
1 3⁄
Q 1 TR–( )a
b=
(A.58)Cp Cv– T– dV dT⁄( )2
dV dT⁄( )⁄⋅=
Property Methods and Calculations A-55
A-55
A.5 Volumetric Flow Rate 
Calculations
HYSYS has the ability to interpret and produce a wide assortment of 
flow rate data. It can accept several types of flow rate information for 
stream specifications as well as report back many different flow rates 
for streams, their phases and their components. One drawback of the 
large variety available is that it often leads to some confusion as to what 
exactly is being specified or reported, especially when volumetric flow 
rates are involved. 
In the following sections, the available flow rates are listed, each 
corresponding density basis is explained, and the actual formulation of 
the flow rate calculations is presented. For volumetric flow rate data 
that is not directly accepted as a stream specification, a final section is 
provided that outlines techniques to convert your input to mass flow 
rates.
A.5.1 Available Flow Rates
Many types of flow rates appear in HYSYS output. However, only a 
subset of these are available for stream specifFications.
Flow Rates Reported In The Output
The flow rate types available via our numerous reporting methods - 
property views, workbook, PFD, specsheets etc. are:
• Molar Flow
• Mass Flow 
• Std Ideal Liq Vol Flow
• Liq Vol Flow @Std Cond
• Actual Volume Flow 
• Std Gas Flow
• Actual Gas Flow
A-56 Volumetric Flow Rate Calculations
A-56
Flow Rates Available For Specification
You will find that the following flow rate types are available for stream 
specifications:
• Molar Flows
• Mass Flows
• LiqVol Flows
A.5.2 Liquid and Vapour Density 
Basis
All calculations for volumetric stream flows are based on density. 
HYSYS utilizes the following density basis:
Calculation of Standard and Actual Liquid 
Densities
The Standard and Actual liquid densities are calculated rigorously at 
the appropriate T and P using the internal methods of the chosen 
property package. Flow rates based upon these densities automatically 
take into account any mixing effects exhibited by non-ideal systems. 
Thus, these volumetric flow rates may be considered as "real world".
Calculation of Standard Ideal Liquid Mass 
Density
Contrary to the rigorous densities, the Standard Ideal Liquid Mass 
density of a stream does not take into account any mixing effects due to 
its simplistic assumptions. Thus, flow rates that are based upon it will 
not account for mixing effects and are more empirical in nature. The 
calculation is as follows:
The volumetric flow rate 
reference state is defined as 
60°F and 1 atm when using 
Field units or 15°C and 1 atm 
when using SI units.
Actual Densities are 
calculated at the stream 
Temperature and Pressure.
Density Basis Description
Std Ideal Liq Mass 
Density 
This is calculated based on ideal mixing of pure 
component ideal densities at 60°F.
Liq Mass Density 
@Std Cond 
This is calculated rigorously at the standard 
reference state for volumetric flow rates.
Actual Liquid 
Density 
This is calculated rigorously at the flowing conditions 
of the stream (i.e. at stream T and P).
Standard Vapour 
Density 
This is determined directly from the Ideal Gas law.
Actual Vapour 
Density 
This is calculated rigorously at the flowing conditions 
of the stream (i.e. at stream T and P).
Property Methods and Calculations A-57
A-57
where: xi = molar fraction of component i
ρi
Ideal = pure component Ideal Liquid density
HYSYS contains Ideal Liquid densities for all components in the Pure 
Component Library. These values have been determined in one of three 
ways, based on the characteristics of the component, as described 
below:
Case 1 - For any component that is a liquid at 60°F and 1 atm, the data 
base contains the density of the component at 60°F and 1 atm.
Case 2 - For any component that can be liquefied at 60°F and pressures 
greater than 1 atm, the data base contains the density of the 
component at 60°F and Saturation Pressure.
Case 3 - For any component that is non-condensable at 60°F under any 
pressure, i.e. 60°F is greater than the critical temperature of the 
component, the data base contains GPA tabular values of the 
equivalent liquid density. These densities were experimentally 
determined by measuring the displacement of hydrocarbon liquids by 
dissolved non-condensable components.
For all hypothetical components, the Standard Liquid density (Liquid 
Mass Density @Std Conditions)  in the Base Properties will be used in 
the Ideal Liquid density (Std Ideal Liq Mass Density) calculation. If a 
density is not supplied, the HYSYS estimated liquid mass density (at 
standard conditions) will be used. Special treatment is given by the Oil 
Characterization feature to its pseudo components such that the ideal 
density calculated for its streams match the assay, bulk property, and 
flow rate data supplied in the Oil Characterization Environment.
(A.59)
Ideal DensityStream
1
xi
ρi
Ideal
-------------∑
---------------------=
A-58 Volumetric Flow Rate Calculations
A-58
A.5.3 Formulation of Flow Rate 
Calculations
The various procedures used to calculate each of the available flow 
rates are detailed below, based on a known molar flow:
Molar Flow Rate
Mass Flow
Std Ideal Liq Vol Flow
This volumetric flow rate is calculated using the ideal density of the 
stream and thus is somewhat empirical in nature.
Liq Vol Flow @Std Cond
This volumetric flow rate is calculated using a rigorous density 
calculated at standard conditions, and will reflect non-ideal mixing 
effects.
(A.60)Total Molar Flow Molar FlowStream=
(A.61)Mass Flow Total Molar Flow MWStream×=
Note that even if a stream is all 
vapour, it will still have a 
LiqVolume flow, based upon 
the stream’s Standard Ideal 
Liquid Mass density, whose 
calculation is detailed in the 
previous section.
(A.62)LiqVolFlow
Total Molar Flow MWStream×
Ideal DensityStream
--------------------------------------------------------------------------=
(A.63)Std Liquid Volume Flow
Molar Flow MW×
 Std Liq Density
--------------------------------------------=
Property Methods and Calculations A-59
A-59
Actual Volume Flow
This volumetric flow rate is calculated using a rigorous liquid density 
calculation at the actual stream T and P conditions, and will reflect 
non-ideal mixing effects.
Standard Gas Flow
Standard gas flow is based on the molar volume of an ideal gas at 
standard conditions. It is a direct conversion from the stream’s molar 
flow rate, based on the following:
• Ideal Gas at 60°F and 1 atm occupies 379.46 ft3/lbmole
• Ideal Gas at 15°C and 1 atm occupies 23.644 m3/kgmole
Actual Gas Flow
This volumetric flow rate is calculated using a rigorous vapour density 
calculation at the actual stream T and P conditions, and will reflect 
non-ideal mixing and compressibility effects.
A.5.4 Volumetric Flow Rates as 
Specifications
If you require that the flow rate of your stream be specified based on 
actual density or standard density as opposed to Standard Ideal Mass 
Liquid density, you must utilize one of the following procedures:
Liq Vol Flow @Std Cond 
1. Specify the composition of your stream. 
2. Use the standard ideal liquid mass density reported for the stream 
and calculate the corresponding mass flow rate either manually, or 
in the SpreadSheet.
3. Use this calculated mass flow as the specification for the stream.
(A.64)Actual Volume Flow
Molar Flow MW×
Density
--------------------------------------------=
(A.65)Actual Gas Flow
Molar Flow MW×
 Density
--------------------------------------------=
A-60 Flash Calculations
A-60
Actual Liquid Volume Flow 
1. Specify the composition and the flowing conditions (T and P) of 
your stream.
2. Use the density reported for the stream and calculate the 
corresponding mass flow rate either manually, or in our 
spreadsheet.
3. Use this calculated mass flow as the specification for the stream. 
A.6 Flash Calculations
Rigorous three phase calculations are performed for all equations of 
state and activity models with the exception of Wilson’s equation, 
which only performs two phase vapour-liquid calculations. As with the 
Wilson Equation, the Amines and Steam property packages only 
support two phase equilibrium calculations.
HYSYS uses built-in intelligence to determine when it can perform a 
flash calculation on a stream, and then what type of flash calculation 
needs to be performed on the stream. This is based completely on the 
degrees of freedom concept. Once the composition of a stream and two 
property variables are known, (vapour fraction, temperature, pressure, 
enthalpy or entropy) one of which must be either temperature or 
pressure, the thermodynamic state of the stream is defined. When 
HYSYS recognizes that a stream is thermodynamically defined, it will 
perform the correct flash automatically in the background. You never 
have to instruct HYSYS to perform a flash calculation.
Property variables can either be specified by you or back-calculated 
from another unit operation. A specified variable is treated as an 
independent variable. All other stream properties are treated as 
dependent variables and are calculated by HYSYS.
In this manner, HYSYS also recognizes when a stream has been 
overspecified. For example, if you specify three stream properties plus 
composition, HYSYS will print out a warning message that an 
inconsistency exists for that stream. This also applies to streams where 
an inconsistency has been created through HYSYS calculations. For 
example, if a stream Temperature and Pressure are specified in a 
Flowsheet, but HYSYS back-calculates a different temperature for that 
stream as a result of an enthalpy balance across a unit operation, 
HYSYS will generate an Inconsistency message.
Specified variables can only be 
re-specified by you or via 
RECYCLE ADJUST, or 
SpreadSheet operations. They 
will not change through any 
heat or material balance 
calculations.
If a flash calculation has been 
performed on a stream, HYSYS 
will know all the property 
values of that stream, i.e., 
thermodynamic, physical and 
transport properties.
Property Methods and Calculations A-61
A-61
A.6.1 T-P Flash Calculation
The independent variables for this type of flash calculation are the 
temperature and pressure of the system, while the dependent variables 
are the vapour fraction, enthalpy and entropy.
With the equations of state and activity models, rigorous calculations 
are performed to determine the coexistence of immiscible liquid 
phases and the resulting component distributions by minimization of 
the Gibbs free energy term. For Vapour Pressure models or the Semi 
Empirical methods, the component distribution is based on the 
Kerosene solubility data (Figure 9A1.4 of the API Data Book). 
If the mixture is single-phase at the specified conditions, the property 
package calculates the isothermal compressibility (dv/dp) to determine 
if the fluid behaves as a liquid or vapour. Fluids in the dense-phase 
region are assigned the properties of the phase that best represents 
their current state. 
Note that material solids will appear in the liquid phase of two-phase 
mixtures, and in the heavy (aqueous/slurry) phase of three-phase 
systems. Therefore, when a separator is solved using a T-P flash, the 
vapour phase will be identical regardless of whether or not solids are 
present in the feed to the flash drum. 
A.6.2 Vapour Fraction Flash
Vapour fraction and either temperature or pressure are the 
independent variables for this type of calculation. This class of 
calculation embodies all fixed quality points including bubble points 
(vapour pressure) and dew points. To perform bubble point calculation 
on a stream of known composition, simply specify the Vapour Fraction 
of the stream as 0.0 and define the temperature or pressure at which the 
calculation is desired. For a dew point calculation, simply specify the 
HYSYS will automatically perform the appropriate flash 
calculation when it recognizes that sufficient stream 
information is known.  This information may have been either 
specified by the user or calculated by an operation.
Depending on the known stream information, HYSYS will perform 
one of the following flashes: T-P, T-VF, T-H, T-S, P-VF, P-H, or P-S.
See Section 1.4.5 - Stability 
Test Tab for options on how to 
instruct HYSYS to perform 
phase stability tests.
Use caution in specifying 
solids with systems that are 
otherwise all vapour.  Small 
amounts of non-solids may 
appear in the "liquid" phase.
A-62 Flash Calculations
A-62
Vapour Fraction of the stream as 1.0 and define the temperature or 
pressure at which the dew point calculation is desired. Like the other 
types of flash calculations, no initial estimates are required. 
Note that the vapour fraction is always shown in terms of the total 
number of moles. For instance, the vapour fraction (VF) represents the 
fraction of vapour in the stream, while the fraction, (1.0 - VF), 
represents all other phases in the stream (i.e. a single liquid, 2 liquids, a 
liquid and a solid).
Dew Points 
Given a vapour fraction specification of 1.0 and either temperature or 
pressure, the property package will calculate the other dependent 
variable (P or T). If temperature is the second independent variable, 
HYSYS will calculate the dew point pressure. Likewise, if pressure is the 
independent variable, then the dew point temperature will be 
calculated. Retrograde dew points may be calculated by specifying a 
vapour fraction of -1.0. It is important to note that a dew point that is 
retrograde with respect to temperature can be normal with respect to 
pressure and vice versa.
Bubble Points/Vapour Pressure
A vapour fraction specification of 0.0 defines a bubble point 
calculation. Given this specification and either temperature or 
pressure, the property package will calculate the unknown T or P 
variable. As with the dew point calculation, if the temperature is known, 
HYSYS will calculate the bubble point pressure and conversely, given 
the pressure, HYSYS will calculate the bubble point temperature. For 
example, by fixing the temperature at 100°F, the resulting bubble point 
pressure is the true vapour pressure at 100°F. 
Quality Points
Bubble and dew points are special cases of quality point calculations. 
Temperatures or pressures can be calculated for any vapour quality 
between 0.0 and 1.0 by specifying the desired vapour fraction and the 
corresponding independent variable. If HYSYS displays an error when 
calculating vapour fraction, then this means that the specified vapour 
fraction doesn't exist under the given conditions, i.e., the specified 
pressure is above the cricondenbar, or the given temperature lies to the 
right of the cricondentherm on a standard P-T envelope.
All of the solids will appear in 
the liquid phase.
Vapour pressure and bubble 
point pressure are 
synonymous.
HYSYS will calculate the 
retrograde condition for the 
specified vapour quality if the 
vapour fraction is input as a 
negative number.
Property Methods and Calculations A-63
A-63
A.6.3 Enthalpy Flash
Given the enthalpy and either the temperature or pressure of a stream, 
the property package will calculate the unknown dependent variables. 
Although the enthalpy of a stream cannot be specified directly, it will 
often occur as the second property variable as a result of energy 
balances around unit operations such as valves, heat exchangers and 
mixers. 
If HYSYS responds with an error message, and cannot find the specified 
property (temperature or pressure), this probably means that an 
internally set temperature or pressure bound has been encountered. 
Since these bounds are set at quite large values, there is generally some 
erroneous input that is directly or indirectly causing the problem, such 
as an impossible heat exchange.
A.6.4 Entropy Flash
Given the entropy and either the temperature or pressure of a stream, 
the property package will calculate the unknown dependent variables.
A.6.5 Handling of Water
Water is handled differently depending on the correlation being used. 
The PR and PRSV equations have been enhanced to handle H2O 
rigorously whereas the semi-empirical and vapour pressure models 
treat H2O as a separate phase using steam table correlations. 
In these correlations, H2O is assumed to form an ideal, partially-
miscible mixture with the hydrocarbons and its K value is computed 
from the relationship:
where: p° = vapour pressure of H2O from Steam Tables
 P = system pressure
xs = solubility of H2O in hydrocarbon liquid at saturation 
conditions.
If a specified amount of energy 
is to be added to a stream, this 
may be accomplished by 
specifying the energy stream 
into either a COOLER/
HEATER or BALANCE 
operation.
(A.66)Kω
p°
xsP( )
------------=
A-64 Flash Calculations
A-64
The value for xs is estimated by using the solubility data for kerosene as 
shown in Figure 9A1.4 of the API Data Book19. This approach is 
generally adequate when working with heavy hydrocarbon systems. 
However, it is not recommended for gas systems.
For three phase systems, only the PR and PRSV property package and 
Activity Models will allow components other than H2O in the second 
liquid phase. Special considerations are given when dealing with the 
solubilities of glycols and CH3OH. For acid gas systems, a temperature 
dependent interaction parameter was used to match the solubility of 
the acid component in the water phase.
The PR equation considers the solubility of hydrocarbons in H2O, but 
this value may be somewhat low. The reason for this is that a 
significantly different interaction parameter must be supplied for cubic 
equations of state to match the composition of hydrocarbons in the 
water phase as opposed to the H2O composition in the hydrocarbon 
phase. For the PR equation of state, the latter case was assumed more 
critical. The second binary interaction parameter in the PRSV equation 
will allow for an improved solubility prediction in the alternate phase.
With the activity coefficient models, the limited mutual solubility of 
H2O and hydrocarbons in each phase can be taken into account by 
implementing the insolubility option (please refer to Section A.2.2 - 
Activity Models). HYSYS will generate, upon request, interaction 
parameters for each activity model (with the exception of the Wilson 
equation) that have been fitted to match the solubility of H2O in the 
liquid hydrocarbon phase and hydrocarbons in the aqueous phase 
based on the solubility data referred to in that section. 
Note that the Peng-Robinson and SRK property packages will always 
force the water rich phase into the heavy liquid phase of a three phase 
stream. As such, the aqueous phase is always forced out of the bottom 
of a three phase separator, even if a light liquid phase (hydrocarbon 
rich) does not exist. Solids will always be carried in the second liquid 
phase.
Property Methods and Calculations A-65
A-65
A.6.6 Solids
HYSYS does not check for solid phase formation of pure components 
within the flash calculations, however, incipient solid formation 
conditions for CO2 and hydrates can be predicted with the Utility 
Package (for more information refer toChapter 8 - Utilities of the 
User’s Guide).
Solid materials such as catalyst or coke can be handled as user-defined, 
solid type components. The HYSYS property package takes this type of 
component into account in the calculation of the following stream 
variables: stream total flow rate and composition (molar, mass and 
volume), vapour fraction, entropy, enthalpy, specific heat, density, 
molecular weight, compressibility factor, and the various critical 
properties. Transport properties are computed on a solids-free basis. 
Note that solids will always be carried in the second liquid phase, i.e., 
the water rich phase.
Solids do not participate in vapour-liquid equilibrium (VLE) 
calculations. Their vapour pressure is taken as zero. However, since 
solids do have an enthalpy contribution, they will have an effect on 
heat balance calculations. Thus, while the results of an Temperature 
flash will be the same whether or not such components are present, an 
Enthalpy flash will be affected by the presence of solids.
A solid material component is entered as a hypothetical component in 
HYSYS. See Chapter 2 - Hypotheticals for more information on 
Hypotheticals.
A-66 Flash Calculations
A-66
A.6.7 Stream Information
When a flash calculation occurs for a stream, the information that is 
returned is dependent on the phases present within the stream. The 
following table shows the stream properties that will be calculated for 
each phase.
Steam Property Applicable PhasesA
Vapour Phase Mole Fraction F V L S
Vapour Phase Mass Fraction F V L S
Vapour Phase Volume Fraction F V L S
Temperature F V L S
Pressure F V L S
Flow F V L S
Mass Flow F V L S
Liquid Volume Flow (Std, Ideal) F V L S
Volume Flow F V L S
Std. Gas Flow F V L S
Std. Volume Flow F L S
Energy F V L S
Molar Enthalpy F V L S
Mass Enthalpy F V L S
Molar Entropy F V L S
Mass Entropy F V L S
Molar Volume F V L S
Molar Density F V L S
Mass Density F V L S
Std. Liquid Mass Density FD L S
Molar Heat Capacity F V L S
Mass Heat Capacity F V L S
CP/CV F V L S
Thermal Conductivity FB,D V L
Viscosity FB,D V L
Kinematic Viscosity FB,D V L
Surface Tension FB L
Molecular Weight F V L S
Z Factor FB V L S
Air SG FB V
Watson (UOP) K Value F V L S
Component Mole Fraction F V L S
Component Mass Fraction F V L S
Property Methods and Calculations A-67
A-67
AStream phases:
BPhysical property queries are allowed on the feed phase of single 
phase streams.
CPhysical property queries are allowed on the feed phase only for 
streams containing vapour and/or liquid phases.
DPhysical property queries are allowed on the feed phase of liquid 
streams with more than one liquid phase.
A.7 References
 1 Peng, D. Y. and Robinson, D. B., "A Two Constant Equation of State", 
I.E.C. Fundamentals, 15, pp. 59-64 (1976).
 2 Soave, G., Chem Engr. Sci., 27, No. 6, p. 1197 (1972).
 3 Knapp, H., et al., "Vapor-Liquid Equilibria for Mixtures of Low Boiling 
Substances", Chemistry Data Series Vol. VI, DECHEMA, 1989.
 4 Kabadi, V.N., and Danner, R.P. A Modified Soave-Redlich-Kwong 
Equation of State for Water-Hydrocarbon Phase Equilibria, Ind. 
Eng. Chem. Process Des. Dev. 1985, Volume 24, No. 3, pp 537-541.
Component Volume Fraction F V L S
Component Molar Flow F V L S
Component Mass Flow F V L S
Component Volume Flow F V L S
K Value (y/x)
Lower Heating Value
Mass Lower Heating Value
Molar Liquid Fraction F V L S
Molar Light Liquid Fraction F V L S
Molar Heavy Liquid Fraction F V L S
Molar Heat of Vapourization FC V L
Mass Heat of Vapourization FC V L
Partial Pressure of CO2 F V L S
Steam Property Applicable PhasesA
F Feed
V Vapour
L Liquid
S Solid
A-68 References
A-68
 5 Stryjek, R., Vera, J.H., J. Can. Chem. Eng., 64, p. 334, April 1986.
 6 API Publication 955, A New Correlation of NH3, CO2 and H2S 
Volatility Data From Aqueous Sour Water Systems, March 1978.
 7 Zudkevitch, D., Joffee, J. "Correlation and Prediction of Vapor-Liquid 
Equilibria with the Redlich-Kwong Equation of State", AIChE 
Journal, Volume 16, No. 1, January pp. 112-119.
 8 Reid, C.R., Prausnitz, J.M. and Sherwood, T.K., "The Properties of 
Gases and Liquids", McGraw-Hill Book Company, 1977.
 9 Prausnitz, J.M., Lichtenthaler, R.N., Azevedo, E.G., "Molecular 
Thermodynamics of Fluid Phase Equilibria", 2nd. Ed., McGraw-
Hill, Inc. 1986.
 10Chao, K. D. and Seader, J. D., A.I.Ch.E. Journal, pp. 598-605, 
December 1961.
 11Grayson, H. G. and Streed, G. W., "Vapour-Liquid Equilibria for High 
Temperature, High Pressure Systems", 6th World Petroleum 
Congress, West Germany, June 1963.
 12Jacobsen, R.T and Stewart, R.B., 1973. "Thermodynamic Properties of 
Nitrogen Including Liquid and Vapour Phases from 63 K to 2000K 
with Pressure to 10 000 Bar." J. Phys. Chem. Reference Data, 2: 757-
790.
 13Hankinson, R.W. and Thompson, G.H., A.I.Ch.E. Journal, 25, No. 4, p. 
653 (1979).
 14Ely, J.F. and Hanley, H.J.M., "A Computer Program for the Prediction 
of Viscosity and Thermal Conductivity in Hydrocarbon Mixtures", 
NBS Technical Note 1039.
 15Twu, C.H., I.E.C. Proc Des & Dev, 24, p. 1287 (1985).
 16Reid, R.C., Prausnitz, J.M., Poling, B.E., "The Properties of Gases & 
Liquids", McGraw-Hill, Inc., 1987.
 17Woelflin, W., "Viscosity of Crude-Oil Emulsions", presented at the 
spring meeting, Pacific Coast District, Division of Production, Los 
Angeles, Calif., Mar. 10, 1942.
 18Gambill, W.R., Chem. Eng., March 9, 1959.
 19API Technical Data Book, Petroleum Refining, Fig. 9A1.4, p. 9-15, 5th 
Edition (1978).
Property Methods and Calculations A-69
A-69
 20Keenan, J. H. and Keyes, F. G., Thermodynamic Properties of Steam, 
Wiley and Sons (1959).
 21Perry, R. H.; Green, D. W.; “Perry’s Chemical Engineers’ Handbook 
Sixth Edition”, McGraw-Hill Inc., (1984).
 22Passut, C. A.; Danner, R. P., “Development of a Four-Parameter 
Corresponding States Method: Vapour Pressure Prediction”, 
Thermodynamics - Data and Correlations, AIChE Symposium 
Series; p. 30-36, No. 140, Vol. 70.
A-70 References
A-70
Petroleum Methods/Correlations B-1
B-1
B Petroleum Methods/
Correlations
B.1  Characterization Method............................................................................. 3
B.1.1  Generate a Full Set of Working Curves ................................................... 3
B.1.2  Light Ends Analysis ................................................................................. 4
B.1.3  Auto Calculate Light Ends ....................................................................... 7
B.1.4  Determine TBP Cutpoint Temperatures................................................... 7
B.1.5  Graphically Determine Component Properties ........................................ 8
B.1.6  Calculate Component Critical Properties................................................. 9
B.1.7  Correlations ............................................................................................. 9