Page 446 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

408 Production

be made and the approach described in the flash at fixed temperature-pressure
conditions can be used quite effectively. The logic diagram for this type of cal-
culation can be deduced from earlier diagrams.
Predlcting the Properties of Hexane Plus (CJ Fractions

Physical properties light hydrocarbons are given in Table 61. In naturally
occurring gas and oil C6+ is unknown and makes a problem. Since the C, is a
combination of paraffins (P), naphthenes (N) and aromatics (A) of varying
molecular mass (M), these fractions must be defined or characterized in some
way. Changing the characterization of C, fractions present in even small
amounts (at 1.0% mole level) can have a significant effect on the predicted phase
behavior of a hydrocarbon system. The dew point of the gas is heavily dependent
upon the heaviest components in the mixture.
The SRK (Equation 9-11) and PR (Equation 9-13) require the smallest number
of parameters of any of the equations of state. They require the critical
temperature, the critical pressure and the acentric factor. There are many
different approaches that can be utilized to predict these parameters for C,
fractions or other mixtures of undefined components.
Some minimum of information must be available on the C, fraction, usually
it is specific gravity (S) average boiling point (T,,) and molecular mass (M) of
the fraction.
The following equation is used [14] to estimate the molecular mass (M) of
petroleum fractions

M = 2.0438 X lo2 exp(0.00218T)exp(-3.07S)To~11sS1~" (6-27)
where T = mean average boiling point of petroleum fraction, OR (from ASTM
D86 test, see Figure 6-29)
S = specific gravity, 60°F/600F
The following equation is to be used to calculate the initial temperature (TJ
of pure hydrocarbons; it is applicable for all families of hydrocarbons:
log T, = A + B log S + C log Tb (6-28)

T, (OR); T, and S given; A, B and C as below:
Type Compound A B C
Paraffln 1.47115 0.43684 0.56224
Napthene 0.70612 4.071 65 0.81196
Olefin 1.18325 0.27749 0.65563
Acetylene 0.79782 0.30381 0.79987
Diolefin 0.14890 4.39618 0.99481
Aromatic 1.14144 0.22732 0.66929

For petroleum fractions, physical properties can be predicted more accurately
if the fraction of paraffins (P), naphthenes (N) and aromatics (A) are known.
If 8 is a physical property to be predicted and the molecular type fractions are
known, a pseudocompound, i.e., a compound having the same boiling point and
specific gravity as the fraction, for each molecular type can be defined. These
properties can be combined by