chapter one the estimation of physical properties

PURE COMPONENT CONSTANTS
2.26 CHAPTER TWO
As shown by Liu and Chen (1996), the sensitivity of to errors of input information
is very great. The recommended procedure for obtaining an unknown value
of is to use a very accurate correlation for P vp such as Eqs. (7-3.2), (7-3.3) or
(7-3.7) directly in Eq. (2-3.1). The next most reliable approach is to use accurate
experimental values of T c , P c , T b in Eq. (2-3.3). Finally, the method of Constantinou
and Gani with Eq. (2-3.4) can be used with some confidence.
Estimated property values will not yield accurate acentric factors. For example,
approximate correlations for P vp such as the Clausius-Clapeyron Equation (7-2.3)
as used by Edmister (1958) or the Antoine Equation (7-3.1) as used by Chen, et
al. (1993) are about as good as (2-3.4). Further, we tried using this chapter’s best
estimated values of T c , P c , T b , or estimated T c and P c with experimental T b , or other
combinations of estimated and experimental data for nearly 300 substances in Appendix
A. The results generally gave large errors, even for ‘‘normal’’ substances.
Earlier methods for described in the 4th Edition are not accurate or have limited
applications.
Along these lines, Chappelear (1982) has observed that ‘‘accepted’’ values of
the acentric factor can change due to the appearance of new vapor pressure or
critical constant data, changing predicted properties. In addition, using revised acentric
factors in a correlation developed from earlier values can lead to unnecessary
errors. Chappelear’s example is carbon dioxide. In Appendix A, we show
0.225; others have quoted a value of 0.267 (Nat. Gas Proc. Assoc., 1981). The
differences result from the extrapolation technique used to extend the liquid region
past the freezing point to T r 0.7. Also, Eq. (2-3-2) yields 0.213. Yet, in the
attractive parameter in the Peng-Robinson equation of state (1976) (see Chapter 4),
the value should be 0.225, since that was what was used to develop the equation
of state relations. One should always choose the value used for the original correlation
of the desired property.
2-4 BOILING AND FREEZING POINTS
Boiling and freezing points are commonly assumed to be the phase transition when
the pressure is 1 atm. A more exact terminology for these temperatures might be
the ‘‘normal’’ boiling and ‘‘normal’’ freezing points. In Appendix A, values for T fp
and T b are given for many substances. Note that estimation methods of Sec. 2-2
may use T b as input information for T c . The comparisons done there include testing
for errors introduced by using T b from methods of this section; they can be large.
A number of methods to estimate the normal boiling point have been proposed.
Some were reviewed in the previous editions. Several of group/bond/atom methods
described in Sec. 2-2 have been applied to T fp and T b , as have some of the molecular
descriptor techniques of Sec. 2-2. We describe the application of these in a similar
manner to that used above for critical properties.
Method of Joback for T ƒp and T b . Joback (1984; 1987) reevaluated Lydersen’s
group contribution scheme, added several new functional groups, and determined
new contribution values. His relations for T fp and T b are
T 122 N (tƒpk) (2-4.1)
ƒp
k
k
T 198 N (tbk) (2-4.2)
b
where the contributions are indicated as tƒpk and tbk. The group identities and
k
k
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