chapter one the estimation of physical properties

PURE COMPONENT CONSTANTS
2.22 CHAPTER TWO
A discussion comparing the QSPR techniques with other methods for the properties
of this chapter is presented below and in Sec. 2-5.
Discussion and Recommendations for Critical Properties. The methods of Joback
(1984; 1987), Constantinou and Gani (1994), Wilson and Jasperson (1996)
and Marrero and Pardillo (1999) were evaluated. Summaries of comparisons with
data from Appendix A are given in Tables 2-1, 2-2, 2-4, and 2-6. A few results
from QSPR methods are given in Table 2-7. Overall, the methods are all comparable
in accuracy.
A useful method for determining consistency among T c , P c , and V c is to use Eq.
(2-3.2) relating the critical compressibility factor, Z c P c V c /RT c , to the acentric
factor (Sec. 2-3). The theoretical basis of the acentric factor suggests that except
for substances with T c 100 K, Z c must be less than 0.291. When Eq. (2-3.2) was
tested on the 142 substances of Appendix A for which reliable values of Z c and
are available and for which the dipole moment was less than 1.0 debye, the average
absolute percent error in Z c was 2% with only 9 substances having errors greater
than 5%. When applied to 301 compounds of all types in Appendix A, the average
percent error was 5% with 32 errors being larger than 10%. Some of these errors
may be from data instead of correlation inadequacy. In general, data rather than
estimation methods should be used for substances with one or two carbon atoms.
Critical Temperature, T c . The methods all are broadly applicable, describing
nearly all the substances of Appendix A; the average percent of error is around 1%
with few, if any substances being off by more than 10%. If an experimental T b is
available, the method of Marrero and Pardillo has higher accuracy than does that
of Wilson and Jasperson. On the other hand, for simplicity and breadth of substances,
the Wilson/Jasperson method is best since it has the fewest groups to
tabulate, is based mostly on atom contributions, and treats inorganic substances as
well as organics. Finally, Joback’s method covers the broadest range of compounds
even though it is somewhat less accurate and more complex.
However, if there is no measured T b available and estimated values must be
used, the errors in these methods increase considerably. Then, if the substance has
fewer than 3 carbons, either the Wilson/Jasperson or Marrero/Pardillo method is
most reliable; if the substance is larger, the Constantinou/Gani approach generally
gives better results with Second Order calculations being marginally better than
First Order. The Joback method is somewhat less accurate than these.
The molecular descriptor method of Jurs is as accurate as the group/bond/atom
methods, at least for the substances compared here, though the earlier method of
Grigoras is not. While the method is not as accessible, current applications show
that once a user has established the capability of computing descriptors, they can
be used for many properties.
Critical Pressure, P c . The methods all are broadly applicable, describing nearly
all the substances of Appendix A. All methods give average errors of about 5%
with about the same fraction of substances (20%) having errors greater than 10%.
The Wilson/Jasperson method has the lowest errors when an experimental value
of T c is used; when T c is estimated the errors in P c are larger than the other methods.
All show better results for substances with 3 or more carbons, except for a few
species. The Constantinou/Gani Second Order contributions do not significantly
improve agreement though the Second Order contributions to the Wilson/Jasperson
method are quite important. Thus, there is little to choose among the methods. The
decision can be based less on accuracy and reliability than on breadth of applicability
and ease of use.
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