Acentric factor (equation of state)

[reservoir_engineer]

dear friends
one of the parameters that used for substances characterization is the acentric factor.
it has many uses, the most important in equation of sate
it = (-log(Pv/Pc)@(t=0.7TC))-1
where Pv is the vapor pressure at temperature equal to 70% of critical temperature.
Pc critical pressure.
can any one helpe me:
what is the physical meaning of the acentric factor?
what is the conditions that make necessity for this factor?
thanks

[Shakespear]

This factor is in the equation defining alpha which is a correction term to the constant "a" in the cubic equation. Alpha in turn has a parameter "m" which has this acentric factor in its equation. This was developed by Soave and is found in the SRK EOS model.

This is well explained in that Whitson PVT class set of video files. In this one

PERA WHITSON Advanced PVT EOS Tuning Course Wednesday-1.wmv

The reason for using it was that the RK EOS model had problems vapor pressure prediction which this parameter nicely fixed. Too large of an error at vs_L and an error at the vs_V end but smaller in magnitude.

[reservoir_engineer]

Alsalam 3alikom
Dear Shekspear
thanks for your help, it is very greatful help from you.
i searched in many places about the acentric factor, i found may helpful informations in the manual of PVTP.
you will found it in the attached doc file.
actually i was reading about EOs and i started to learn how to perform PVT modeling using EOS using PVTP or winprop or PVTSIm.
if you have courses or papers about EOS PVT modeling, it will be apprecciated to share it with us.
also if you have any tutorials, courses about any of previous mentioned softwares.
last thing,
i think we need to start new thread discussing the PVT modeling steps,
do you agree
Regards,
___________________

[Shakespear]

I agree, PVT properties are important and should have a sticky for its discussion.

I am not a guru in this area. In most places it was something that was given or field data was not available and correlations were used. The latter ruled in my case. It all depends who and where you work.

The Whitson lectures start this discussion (EOS tuning) on the Wed. video. I will go through this very carefully and will scan my notes (if anyone can read them) and share them.

In terms of material there is only books which have been mentioned on this site. That is all I have.

PS: Whitson mentioned that the acentric factor was related to the vapor pressure curve slope near Critical Point, for all that's worth. I would recommend again to listen to his lectures as seen the equations or reading about the tuning is not the same. I get more out of listening to him. :-)

[Sequestrator]

accentric factor describes asymmetry of molecules EOS tuning purposes. General assumption for cubic EOS is that molecules cannot be infinitely compressed, and that they interact on sub-molecular scale. Tacitly, molecules were approximated as spheres. However, more complex molecules can cause errors in EOS calculation with these assumptions, which are most apparent at phase equilibria predictions. However, accf for pseudo components is tuned as statistical parameter, i.e. almost by random, neglecting physical meaning.

[Lotfi_RE]

dear all,
I've some videos and nice notes in PDF, very useful for understinding EOS generation.
Email me: isbn2005@yahoo.fr

[Lotfi_RE]

dear all,
I've some videos and nice notes in PDF, very useful for understinding EOS generation.
Email me: isbn2005@yahoo.fr

[zanlog]

from riyazi book:
Acentric Factor
Acentric factor is a parameter that was originally defined by
Pitzer to improve accuracy of corresponding state correla-
tions for heavier and more complex compounds [5, 6]. Acen-
tric factor is a defined parameter and not a measurable quan-
tity. It is a dimensionless parameter represented by w and is
defined as
w = - log10 (PI vap) -- 1.0 (2.10)
where
pvap
pvap
= reduced vapor pressure, pvap/Pc, dimensionless
= vapor pressure at T = 0.7 Tc (reduced temperature
of 0.7), bar
Pc -- critical pressure, bar
T = absolute temperature, K
Tc = critical temperature, K
Acentric factor is defined in a way that for simple fluids such
as argon and xenon it is zero and its value increases as the
size and shape of molecule changes. For methane w -- 0.001
and for decane it is 0.489. Values reported for acentric fac-
tor of pure compounds are calculated based on Eq. (2.10),
which depends on the values of vapor pressure. For this rea-
son values reported for the acentric factor of a compound may
slightly vary from one source to another depending on the re-
lation used to estimate the vapor pressure. In addition, since
calculation of the acentric factor requires values of critical
temperature and pressure, reported values for w also depend
on the values of Tc and Pc used.

[zanlog]

from riazi book:
Acentric Factor
Acentric factor is a parameter that was originally defined by
Pitzer to improve accuracy of corresponding state correla-
tions for heavier and more complex compounds [5, 6]. Acen-
tric factor is a defined parameter and not a measurable quan-
tity. It is a dimensionless parameter represented by w and is
defined as
w = - log10 (PI vap) -- 1.0 (2.10)
where
pvap
pvap
= reduced vapor pressure, pvap/Pc, dimensionless
= vapor pressure at T = 0.7 Tc (reduced temperature
of 0.7), bar
Pc -- critical pressure, bar
T = absolute temperature, K
Tc = critical temperature, K
Acentric factor is defined in a way that for simple fluids such
as argon and xenon it is zero and its value increases as the
size and shape of molecule changes. For methane w -- 0.001
and for decane it is 0.489. Values reported for acentric fac-
tor of pure compounds are calculated based on Eq. (2.10),
which depends on the values of vapor pressure. For this rea-
son values reported for the acentric factor of a compound may
slightly vary from one source to another depending on the re-
lation used to estimate the vapor pressure. In addition, since
calculation of the acentric factor requires values of critical
temperature and pressure, reported values for w also depend
on the values of Tc and Pc used.