chapter one the estimation of physical properties

PURE COMPONENT CONSTANTS
2.34 CHAPTER TWO
tions where most of the methods were of the group/bond/atom type and were
limited in the classes of properties or types of substances treated, recent work in
these techniques has both improved their accuracy and broadened their range. Thus,
there are now two methods (Joback and Constantinou/Gani) that provide all pure
component constants and heat capacities and properties of formation of ideal gases
(see Chap. 3) with a single group formulation.
All of the group/bond/atom methods examined here were set up on spreadsheets
since their application was the same regardless of the property and component.
Some methods required larger data bases than others, but implementation and execution
for new substances and properties would be straightforward. It is also possible
to obtain a complete suite of estimation methods in the program Cranium
(1998). The Constantinou/Gani method for the broadest set of properties is available
directly from CAPEC (1999). It is expected that methods of currently limited
application, such as the Marrero/Pardillo approach, will be expanded to include
other properties. There are still enough errors and limitations in the methods that
new research will continue with this approach. It is likely that an individual user
with a typical individual computer will be able to use both current and future
versions of these methods.
This edition has also introduced the molecular descriptor and QSPR relations
which add another dimension to the methodology since they can be applied not
only to pure component constants but to a variety of solution systems (Mitchell
and Jurs, 1998; Katritzsky, et al., 1998). This presents users with opportunities to
obtain more reliable values, but also may require greater expertise and investment
in the selection of computer software for estimations. As mentioned above, there
is no tabulation of contributions for these methods since the molecular structure
and descriptors of each new substance are computed from molecular and quantum
mechanical programs. While complex, the estimation methods are established by a
generally agreed upon process of fitting limited data (Mitchell and Jurs, 1996; St.
Cholakov, et al., 1999) to establish the weights of the significant descriptors from
a large set of possibilities. As described above, the results can be very good and it
is likely that further improvements in computational techniques will add even
greater reliability and applicability. However, the computational power required is
extensive and care must be exercised to use the same computational programs as
the developer in order to insure that the values for the descriptors will be consistent
with those fitted. This is likely to require expertise and computers of a large organization
and beyond that of an individual. At this time, these methods have not
been implemented in process simulation software, but that would certainly be possible
in the future.
It will be important that users follow the developments in this area so that the
most prudent decisions about investment and commitment can be made.
2-6 DIPOLE MOMENTS
Dipole moments of molecules are often required in property correlations for polar
materials such as for virial coefficients (Chap. 4) and viscosities (Chap. 9). The
best sources of this constant are the compilations by McClellan (1963] and Nelson,
et al. [1967). There also is a large number of values in the compilation of Lide
(1996). For the rare occasions when an estimated value is needed, vector group
contribution methods such as summarized by Minkin, et al. (1970) can be used. In
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