Page 75 - Characterization and Properties of Petroleum Fractions

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2. CHARACTERIZATION AND PROPERTIES OF PURE HYDROCARBONS 55
therefore at temperature 288 or 293 K it cannot be a liquid
and values reported for density at these temperatures are fic- specific gravity. Equation (2.38) for molecular weight is [28]
titious. In any case the values given here for density of N 2 , (2.50) M = 1.6607 × 10 T b 2.1962 SG −1.0164
−4
CO 2 ,C 1 ,C 2 , and H 2 S should not be taken as real values and
they are only recommended for use in Eq. (2.47). It should This equation fails to properly predict properties for hydro-
be noted that d 20 is the same as the specific gravity at 20 C carbons above C 25 . This equation was extensively evaluated
◦
20
in the SI system (d ). This equation was developed based on for various coal liquid samples along with other correlations
4
the fact that nonhydrocarbons are mainly polar compounds by Tsonopoulos et al. [34]. They recommended this equation
and a two-parameter potential energy relation cannot rep- for the estimation of the molecular weight of coal liquid frac-
resent the intermolecular forces between molecules, there- tions. Constants in Eq. (2.40) for molecular weight, as given
fore a third parameter is needed to characterize the system. in Table 2.5, were modified to include heavy hydrocarbons up
This method would be particularly useful to estimate the bulk to molecular weight of 700. The equation in terms of T b and
properties of petroleum fluids containing light hydrocarbons SG becomes
as well as nonhydrocarbon gases. Evaluation of this method
is presented in Section 2.9. M = 42.965[exp(2.097 × 10 T b − 7.78712SG
−4
−3
(2.51) + 2.08476 × 10 T b SG)]T 1.26007 SG 4.98308
b
2.4 PREDICTION OF MOLECULAR WEIGHT,
BOILING POINT, AND SPECIFIC GRAVITY This equation can be applied to hydrocarbons with molecular
weight ranging from 70 to 700, which is nearly equivalent to
boiling point range of 300–850 K (90–1050 F), and the API
◦
Molecular weight, M, boiling point, T b , and specific gravity, gravity range of 14.4–93. These equations can be easily con-
SG, are perhaps the most important characterization param- verted in terms of Watson K factor (K W ) and API degrees
eters for petroleum fractions and many physical properties using their definitions through Eqs. (2.13) and (2.4). A graph-
may be calculated from these parameters. Various methods ical presentation of Eq. (2.51) is shown in Fig. 2.4. (Equation
commonly used to calculate these properties are presented (2.51) has been recommended by the API as it will be dis-
here. As mentioned before, the main application of these cor- cussed later.) Equation (2.51) is more accurate for light frac-
relations is for petroleum fractions when experimental data tions (M < 300) with an %AAD of about 3.5, but for heavier
are not available. For pure hydrocarbons either experimental fractions the %AAD is about 4.7. This equation is included in
data are available or group contribution methods are used to the API-TDB [2] and is recognized as the standard method
estimate these parameters [4]. However, methods suggested of estimating molecular weight of petroleum fractions in the
in Chapter 3 to estimate properties of petroleum fractions are
based on the method developed from the properties of pure industry.
hydrocarbons in this chapter. For heavy petroleum fractions boiling point may not be
available. For this reason Riazi and Daubert [67] developed
a three-parameter correlation in terms of kinematic viscosity
2.4.1 Prediction of Molecular Weight based on the molecular weight of heavy fractions in the range
of 200–800:
For pure hydrocarbons from homologous groups, Eq. (2.42)
can be reversed to obtain the molecular weight from other
(−1.2435+1.1228SG) (3.4758−3.038SG) SG −0.6665
properties. For example, if T b is available, M can be estimated M = 223.56 ν 38(100) ν 99(210)
from the following equation: (2.52)
1/c
1 The three input parameters are kinematic viscosities (in cSt)

(2.48) M = [a − ln(T b∞ − T b )]
b at 38 and 98.9 C (100 and 210 F) shown by ν 38(100) and ν 99(210) ,
◦
◦
respectively, and the specific gravity, SG, at 15.5 C. It should
◦
where values of a, b, c, and T b∞ are the same constants as be noted that viscosities at two different temperatures repre-
those given in Table 2.6 for the boiling point. For example, sent two independent parameters: one the value of viscosity
for n-alkanes, M can be estimated as follows:
and the other the effect of temperature on viscosity, which is
3/2
1
another characteristic of a compound as discussed in Chap-
(2.49) M p = [6.98291 − ln(1070 − T b )] ter 3. The use of a third parameter is needed to character-
0.02013
ize complexity of heavy hydrocarbons that follow a three-
in which M p is molecular weight of n-alkane (n-paraffins) parameter potential energy relation. Equation (2.52) is only
whose normal boiling point is T b . Values obtained from recommended when the boiling point is not available. In
Eq. (2.49) are very close to molecular weight of n-alkanes. a case where specific gravity is not available, a method is
Similar equations can be obtained for other hydrocarbon proposed in Section 2.4.3 to estimate it from viscosity data.
groups by use of values given in Table 2.6. Once M is deter- Graphical presentation of Eq. (2.52) is shown in Fig. 2.5 in
--`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
mined from T b , then it can be used with Eq. (2.42) to obtain terms of API gravity. To use this figure, based on the value of
other properties such as specific gravity and critical constants. ν 38(100) a point is determined on the vertical line, then from
values of ν 99(210) and SG, another point on the chart is speci-
2.4.1.1 Riazi–Daubert Methods fied. A line that connects these two points intersects with the
The methods developed in the previous section are commonly line of molecular weight where it may be read as the estimated
used to calculate molecular weight from boiling point and value.

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