chapter one the estimation of physical properties

PURE COMPONENT CONSTANTS
2.4 CHAPTER TWO
Group k N k N k (tck) N k (pck) N k (vck)
—CH 3 1 0.0141 0.0012 65
—CH 2 — 1 0.0189 0 56
CH(ds)
4 0.0328 0.0044 164
C(ds)
2 0.0286 0.0016 64
—ACOH (phenol) 1 0.0240 0.0184 25
5
k1
0.1184 0.0232 324
N k F k
The value of N atoms 19, while T b 477.67 K. The Joback estimation method (Sec.
2-4) gives T b 489.74 K.
T T [0.584 0.965(0.1184) (0.1184) ]
c
b
2 1
698.1 K (with exp. T b), 715.7 K (with est. T b)
P [0.113 0.0032(19) 0.0232]
c
2
44.09 bar
3 1
Vc
17.5 324 341.5 cm mol
Appendix A values for the critical temperature and pressure are 703 K and 43.00
bar. An experimental V c is not available. Thus the differences are
Tc
Difference (Exp. T b) 703 698.1 4.9 K or 0.7%
Tc
Difference (Est. T b) 703 715.7 12.7 K or 1.8%
P Difference 43.00 44.09 1.09 bar or 2.5%.
c
A summary of the comparisons between estimations from the Joback method
and experimental Appendix A values for T c , P c , and V c is shown in Table 2-1. The
results indicate that the Joback method for critical properties is quite reliable for
T c of all substances regardless of size if the experimental T b is used. When estimated
values of T b are used, there is a significant increase in error, though it is less for
compounds with 3 or more carbons (2.4% average increase for entries indicated by
b
in the table, compared to 3.8% for the whole database indicated by a ).
For P c , the reliability is less, especially for smaller substances (note the difference
between the a and b entries). The largest errors are for the largest molecules,
especially fluorinated species, some ring compounds, and organic acids. Estimates
can be either too high or too low; there is no obvious pattern to the errors. For V c ,
the average error is several percent; for larger substances the estimated values are
usually too small while estimated values for halogenated substances are often too
large. There are no obvious simple improvements to the method. Abildskov (1994)
did a limited examination of Joback predictions (less than 100 substances) and
found similar absolute percent errors to those of Table 2-1.
A discussion comparing the Joback technique with other methods for critical
properties is presented below and a more general discussion of group contribution
methods is in Sec. 2-5.
Method of Constantinou and Gani (CG). Constantinou and Gani (1994) developed
an advanced group contribution method based on the UNIFAC groups (see
Chap. 8) but they allow for more sophisticated functions of the desired properties
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