Page 135 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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3. CHARACTERIZATION OF PETROLEUM FRACTIONS 115
amount in the fraction is known. However, based on our expe-
rience the PNA three-pseudocomponent model is sufficiently deviation of 9%. From the pseudocomponent approach, M P ,
M N , and M A are calculated from Eqs. (3.41)–(3.43) as 79.8,
accurate for olefin-free petroleum fractions. For coal liquids 76.9, and 68.9, respectively. The mixture molecular weight is
with a high percentage of aromatic content, splitting aromat- calculated through Eq. (3.40) as M = 0.82 × 79.8 + 0.155 ×
ics into two subgroups may greatly increase the accuracy of 76.9 + 0.025 × 68.9 = 79, with relative deviation of 1.3%. If
model predictions. In using this method the minimum data values of M P , M N , and M A are substituted in Eq. (2.42) for
needed are at least one characterizing parameter (T b or M) the specific gravity, we get SG P = 0.651, SG N = 0.749, and
and the PNA composition. SG A = 0.895. From Eq. (3.40) the mixture specific gravity is
Properties of pseudocomponents may be obtained from in- SG = 0.673, with AD of 2.3%. It should be noted that when
terpolation of values in Tables 2.1 and 2.2 to match boiling Eq. (3.40) is applied to molecular weight, it would be more
point to that of the mixture. As shown in Section 2.3.3, prop- appropriate to use composition in terms of mole fraction
erties of homologous groups can be well correlated to only rather than volume fraction. The composition can be con-
one characterization parameter such as boiling point, molec- verted to weight fraction through specific gravity of the three
ular weight, or carbon number, depending on the availability components and then to mole fraction through molecular
of the parameter for the mixture. Since various properties of weight of the components by equations given in Section
pure homologous hydrocarbon groups are given in terms of 1.7.15. The mole fractions are x mP = 0.785, x mN = 0.177, and
molecular weight by Eq. (2.42) with constants in Table 2.6, x mA =0.038 and Eq. (3.39) yields M = 78.8 for the mixture
if molecular weight of a fraction is known it can be used di- with deviation of 1%. The difference between the use of
rectly as the characterizing parameter. But if the boiling point volume fraction and mole fraction in Eq. (3.40) is minor and
is used as the characterizing parameter, molecular weights of within the range of experimental uncertainty. Therefore, use
the three model components may be estimated through rear- of any form of composition in terms of volume, weight, or
rangement of Eq. (2.42) in terms of boiling point as following: mole fraction in the pseudocomponent method is reasonable
without significant effect in the results. For this reason, in
1
3/2
(3.41) M p = [6.98291 − ln(1070 − T b )] most cases the PNA composition of petroleum fractions are
0.02013 simply expressed as fraction or percentage and they may
3/2 considered as weight, mole, or volume.
1

(3.42) M N = [6.95649 − ln(1028 − T b )]
0.02239
In the above example the method of pseudocomponent pre-
3/2
1

(3.43) M A = [6.91062 − ln(1015 − T b )] dicts molecular weight of the fraction with much better ac-
0.02247 curacy than the use of Eq. (2.50) with bulk properties (%AD
of 1.3% versus 9%). This is the case for fractions that are
where M P , M N , and M A are molecular weights of paraffinic,
naphthenic, and aromatic groups, respectively. T b is the char- highly rich in one of the hydrocarbon types. For this frac-
acteristic boiling point of the fraction. Predicted values of M P , tion paraffinic content is nearly 80%, but for petroleum frac-
M N , and M A versus T b were presented in Fig. 2.15 in Chapter 2. tions with normal distribution of paraffins, naphthenes, and
As shown in this figure the difference between these molecu- aromatics both methods give nearly similar results and the
lar weights increase as boiling point increases. Therefore, the advantage of use of three pseudocomponents from different
pseudocomponent approach is more effective for heavy frac- groups over the use of single pseudocomponent with mixture
tions. If ASTM D 86 distillation curve is known the tempera- bulk properties is minimal. For example, for a petroleum frac-
ture at 50% point should be used for T b , but if TBP distillation tion the available experimental data are [36] M = 170, T b =
data are available an average TBP would be more suitable to 487 K, SG = 0.802, x p = 0.42, x N = 0.41, and x A = 0.17. Equa-
be used for T b . Once M P , M N , and M A are determined, they tion (2.50) gives M = 166, while Eq. (3.40) gives M = 163 and
should be used in Eq. (2.42) to determine properties from cor- SG = 0.792. Equation (3.40) is particularly useful when only
responding group to calculate other properties. The method one bulk property (i.e., T b ) with the composition of a frac-
is demonstrated in the following example. tion is available. For highly aromatic (coal liquids) or highly
paraffinic mixtures the method of pseudocomponent is rec-
ommended over the use of bulk properties.
Example 3.10—A petroleum fraction has ASTM D 86 50%
temperature of 327.6 K, specific gravity of 0.658, molecular
weight of 78, and PNA composition of 82, 15.5, and 2.5 in 3.3.5 Estimation of Molecular Weight, Critical
vol% [36]. Estimate molecular weight of this fraction using Properties, and Acentric Factor
bulk properties of T b and SG and compare with the value esti-
mated from the pseudocomponent method. Also estimate the Most physical properties of petroleum fluids are calculated
mixture specific gravity of the mixture through the pseudo- through corresponding state correlations that require pseu-
component technique and compare the result with the exper- docritical properties (T pc , P pc , and V pc ) and acentric factor
imental value. as a third parameter. In addition molecular weight (M)is
needed to convert calculated mole-based property to mass-
Solution—For this fraction the characterizing parameters based property. As mentioned in Section 1.3, the accuracy
are T b = 327.6 K and SG = 0.672. To estimate M from these of these properties significantly affects the accuracy of es-
bulk properties, Eq. (2.50) can be applied since the boiling timated properties. Generally for petroleum fractions these
point of the fraction is within the range of 40–360 C(∼C 5 – basic characterization parameters are calculated through ei-
◦
C 22 ). The results of calculation is M = 85.0, with relative ther the use of bulk properties and correlations of Chapter 2

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