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.