Page 78 - Characterization and Properties of Petroleum Fractions

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58 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
fraction distillation data is not usually available. When d 20 is
of extremely large molecules (M →∞) approaches a finite
not available it may be estimated from SG using the method This relation is based on the assumption that the boiling point
given in Section 2.6.1. value of 1071.28 K. Soreide [52] compared four methods for
the prediction of the boiling point of petroleum fractions:
2.4.1.6 Other Methods (1) Eq. (2.56), (2) Eq. (2.58), (3) Eq. (2.50), and (4) Twu
Twu [30] proposed a set of correlations for the calculation of method given by Eqs. (2.89)–(2.92). For his data bank on
M, T c , P c , and V c of hydrocarbons. Because these correlations boiling point of petroleum fractions in the molecular weight
are interrelated, they are all given in Section 2.5.1. The com- range of 70–450, he found that Eq. (2.50) and the Twu cor-
puterized Winn method is given by Eq. (2.93) in Section 2.5.1 relations overestimate the boiling point while Eqs. (2.56)
and in the form of chart in Fig. 2.12. and (2.58) are almost identical with AAD of about 1%. Since
Eq. (2.56) was originally based on hydrocarbons with a molec-
Example 2.5—For n-Butylbenzene estimate the molecular ular weight range of 70–300, its application to heavier com-
weight from Eqs. (2.50), (2.51), (2.54), and (2.55) using the pounds should be taken with care. In addition, the database
input data from Table 2.1 for evaluations by Soreide was the same as the data used to
derive constants in his correlation, Eq. (2.58). For heavier hy-
drocarbons (M > 300) Eq. (2.57) may be used.
Solution—From Table 2.1, n-butylbenzene has T b = 183.3 C,
◦
SG = 0.8660, d = 0.8610, and M = 134.2. Applying various For pure hydrocarbons from different homologous families
equations we obtain the following: from Eq. (2.50), M = 133.2 Eq. (2.42) should be used with constants given in Table 2.6 for
with AD = 0.8%, Eq. (2.51) gives M = 139.2 with AD = 3.7%, T b to estimate boiling point from molecular weight. A graph-
Eq. (2.54) gives M = 143.4 with AD = 6.9%, and Eq. (2.55) ical comparison of Eqs. (2.42), (2.56), (2.57), and (2.58) for
gives M = 128.7 with AD = 4.1%. For this pure and light hy- n-alkanes from C 5 to C 36 with data from API-TDB [2] is shown
drocarbon, Eq. (2.50) gives the lowest error because it was in Fig. 2.6.
mainly developed from the molecular weight of pure hydro-
carbons while the other equations cover wider range of molec- 2.4.3 Prediction of Specific Gravity/API Gravity
ular weight because data from petroleum fractions were also
used in their development. Specific gravity of hydrocarbons and petroleum fractions is
normally available because it is easily measurable. Specific
gravity and the API gravity are related to each other through
2.4.2 Prediction of Normal Boiling Point Eq. (2.4). Therefore, when one of these parameters is known
2.4.2.1 Riazi–Daubert Correlations the other one can be calculated from the definition of the API
gravity. Several correlations are presented in this section for
These correlations are developed in Section 2.3. The best in- the estimation of specific gravity using boiling point, molec-
put pair of parameters to predict boiling point are (M, SG) or ular weight, or kinematic viscosity as the input parameters.
(M, I). For light hydrocarbons and petroleum fractions with
molecular weight in the range of 70–300, Eq. (2.40) may be 2.4.3.1 Riazi–Daubert Methods
used for boiling point (T b in K):
These correlations for the estimation of specific gravity re-
T b = 3.76587[exp(3.7741 × 10 −3 M + 2.98404SG quire T b and I or viscosity and CH weight ratio as the input
(2.56) − 4.25288 × 10 −3 MSG)]M 0.40167 SG −1.58262 parameters (Eq. 2.40). For light hydrocarbons, Eq. (2.40) and
Table 2.5 can be used to estimate SG from different input
For hydrocarbons or petroleum fractions with molecular parameters such as T b and I.
weight in the range of 300–700, Eq. (2.46b) is recommended:
T b = 9.3369[exp(1.6514 × 10 −4 M + 1.4103SG SG = 2.4381 × 10 exp(−4.194 × 10 T b − 23.5535I
−4
7
(2.57) − 7.5152 × 10 −4 MSG)]M 0.5369 SG −0.7276 (2.59) + 3.9874 × 10 T b I )T −0.3418 I 6.9195
−3
b
Equation (2.57) is also applicable to hydrocarbons having where T b is in kelvin. For heavy hydrocarbons with molecular
molecular weight range of 70–300, with less accuracy. Esti- weight in the range 300–700, the following equation in terms
mation of the boiling point from the molecular weight and of M and I can be used [65]:
refractive index parameter (I ) is given by Eq. (2.40) with con-
4
stants in Table 2.5. The boiling point may also be calculated SG = 3.3131 × 10 exp(−8.77 × 10 −4 M − 15.0496I
through K W and API gravity by using definitions of these pa- (2.60) + 3.247 × 10 MI )M −0.01153 I 4.9557
−3
rameters given in Eqs. (2.13) and (2.4).
Usually for heavy fractions, T b is not available and for this rea-
2.4.2.2 Soreide Correlation son, M and I are used as the input parameters. This equation
Based on extension of Eq. (2.56) and data on the boiling point also may be used for hydrocarbons below molecular weight
of some C 7+ fractions, Soreide [51, 52] developed the follow- of 300, if necessary. The accuracy of this equation is about 0.4
ing correlation for the normal boiling point of fractions in the %AAD for 130 hydrocarbons in the carbon number range of
range of 90–560 C(T b in K). C 7 –C 50 (M ∼ 70–700).
◦
For heavier fractions (molecular weight from 200 to 800)
4
T b = 1071.28 − 9.417 × 10 exp(−4.922 × 10 −3 M and especially when the boiling point is not available the fol-
(2.58) − 4.7685SG + 3.462 × 10 −3 MSG) M −0.03522 SG 3.266 lowing relation in terms of kinematic viscosities developed by

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