Page 453 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Properties of Hydrocarbon Mixtures 413

4. A partial TBP analysis. A true boiling point (TBP) distillation has been
performed on the C,, fraction. The TBP distillation is a batch distillation
similar to an ASTM distillation but the distillation apparatus contains
several trays (usually 10 or more or the equivalent amount of packing) and
a high reflux ratio is used. The TBP gives a sharper separation between
the subfractions than an ASTM distillation. Normally, at least five tem-
peratures are reported as a function of liquid volume percent distilled over.
Frequently, more than 20 temperatures will be reported. The specific
gravity and molecular mass of the total fraction are usually reported.
5. A comphte TBP analysis. A true boiling point distillation has been performed
on the total C, fraction. The specific gravity and molecular mass have been
measured for each of the reported distillate subfractions. Between five and
fifty temperatures and subfraction properties will be reported.
Table 6-15 shows typical information as it may be reported for each of the
five categories of C, characterization. The complete TBP analysis is believed
to be the best form of C, analysis to be used with today’s thermodynamic
property prediction procedures. Consequently, it is recommended that all
noncomplete TBP analyses be converted to this form. This section deals with
these conversion techniques. These techniques are based on empirical correla-
tions and, in some cases, experience and judgment. There is also one basic
constraint that must be used in these conversion techniques-that is, maintenance
of volume-mass-molar relationships in the C, fraction along with consistency
in the composition of the total stream. One cannot capriciously change the
molecular mass or specific gravity of the total C, fraction without simultaneously
adjusting the reported composition. All of the procedures reported here strive
to maintain consistency of the specific gravity, molecular mass and, when
possible, the boiling point(s) of the total C, fraction.
The various procedures for converting noncomplete TBP analyses to complete
TBP analyses are illustrated in the following section. A common sample problem
is used to illustrate the basic conversion procedure. In addition, the results of
several equilibrium calculations are reported for each type of characterization.
The gas composition, true boiling point date, gravity and molecular weight
measurements for the C,+ fraction are shown in Table 6-16. Though the parti-
cular system chosen shows C, as a basis for the heavy and characterization C,
will be used. There are several isomers of hexane, as well as other materials,
that can appear in the C, subfraction. The molecular mass tabulated for the
fractions in Table 616 makes them appear to be normal paraffins. This, however,
is not true and a complete TBP analysis was made on the C, fraction.
Calculations made based on the different C, characterizations are compared
with experimental values, Table 6-17 and Figure 6-30. The complete TBP
characterization provides the best predictions of the phase behavior and the
liquid formation, though there is only a little difference between the full TBP and
the partial TBP results. The lumped specific gravity-molecular mass charactenza-
tion and the lumped n-paraffin characterization give the poorest predictions.
All of the characterizations in Table 6-18 are in better agreement with experi-
mental values than one would normally expect.

Vapor-Liquid Equilibrium by Equation of State
Prediction of a vapor-liquid mixture is more complicated than prediction of
pure component VLE.
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