Page 147 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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3. CHARACTERIZATION OF PETROLEUM FRACTIONS 127
where
v = 2.51(n − 1.475) - (d − 0.851) June 22, 2007 14:23 hydrogen content or lower carbon content have higher heat-
ing value and contain more saturated hydrocarbons. Predic-

430 if v > 0 tive methods for such elements are rare and limited so there
a = is no possibility of comparison of various methods but the
670 if v < 0
presented procedures are evaluated directly against experi-

0.055 if v > 0
b = mental data.
0.080 if v < 0
w = (d − 0.851) − 1.11(n − 1.475) 3.5.2.1 Prediction of Carbon and Hydrogen Contents

820 w − 3%S + 10000/M if w > 0 The amount of hydrogen content of a petroleum mixture is
%C R =
1440 w − 3%S + 10600/M if w < 0 directly related to its carbon-to-hydrogen weight ratio, CH.

1.33 + 0.146M (w − 0.005 × %S) if w > 0 Higher carbon-to-hydrogen weight ratio is equivalent to lower
R T =
1.33 + 0.180M (w − 0.005 × %S) if w < 0 hydrogen content. In addition aromatics have lower hydrogen
content than paraffinic compounds and in some references
Once carbon distribution is calculated from Eq. (3.86), the hydrogen content of a fraction is related to the aromatic con-
PNA composition can be determined as follows: tent [57] although such relations are approximate and have
x P = %C P /100 low degrees of accuracy. The reason for such low accuracy
is that the hydrogen content of various types of aromatics
(3.87) x N = %C N /100
varies with molecular type. Within the aromatic family, dif-
x A = %C A /100 ferent compounds may have different numbers of rings, car-
As mentioned above the n-d-M method cannot be applied bon atoms, and hydrogen content. In general more accurate
to light fractions with molecular weights of less than 200. prediction can be obtained from the CH weight ratio method.
However, when it was evaluated against PNA composition Several methods of estimation of hydrogen and carbon con-
of 70 fractions for the molecular weight range of 230–570, tents are presented here.
AAD of 0.064, 0.086, and 0.059 were obtained for x P , x N , and
x A , respectively. Accuracy of the n-d-M method for prediction 3.5.2.1.1 Riazi Method—This method is based on calcula-
of composition of fractions with M > 200 is similar to the tion of CH ratio from the method of Riazi and Daubert given
accuracy of Eqs. (3.79)–(3.82). But accuracy of Eqs. (3.73) in Section 2.6.3 and estimation of %S from Riazi method in
and (3.74) in terms of viscosity (API method) is more than Section 3.5.2.2. The main elements in a petroleum fraction
the n-d-M method [30, 36]. are carbon, hydrogen, and sulfur. Other elements such as ni-
In addition to the above methods there are some other trogen, oxygen, or metals are in such small quantities that
procedures reported in the literature for estimation of the on a wt% basis their presence may be neglected without se-
PNA composition of petroleum fractions. Among these rious error on the composition of C, H, and S. This is not to
methods the Bergman’s method is included in some refer- say that the knowledge of the amounts of these elements is
ences [48]. This method calculates the PNA composition in not important but their weight percentages are negligible in
weight fraction using the boiling point and specific gravity comparison with weight percentages of C, H, and S. Based
of the fraction as input data. The weight fraction of aromatic on this assumption and from the material balance on these
content is linearly related to K W . The x P and x N are calculated three main elements we have
through simultaneous solution of Eqs. (3.72) and (3.46) (3.88) %C + %H + %S = 100
when they are applied to specific gravity. Specific gravity
of paraffinic, naphthenic, and aromatic pseudocomponents (3.89) %C = CH
(SG P ,SG N , and SG A ) are calculated from boiling point of %H
the fraction. Equation (2.42) may be used to calculate SG for From simultaneous solution of these two equations, assum-
different groups from T b of the fraction. Except in reference ing %S is known, the following relations can be obtained for
[48] this method is not reported elsewhere. There are some %H and %C, all in weight%:
other specific methods reported in various sources for
each hydrocarbon group. For example, ASTM D 2759 gives a (3.90) %H = 100 − %S
graphical method to estimate naphthene content of saturated 1 + CH
hydrocarbons (paraffins and naphthenes only) from refrac- CH
tivity intercept and density at 20 C. In some sources aromatic (3.91) %C = 1 + CH × (100 − %S)
◦
content of fractions are related to aniline point, hydrogen
content, or to hydrogen-to-carbon (HC) atomic ratio [57]. An where %S is the wt% of sulfur in the mixture, which should
example of these methods is shown in the next section. be determined from the method presented in Section 3.5.2.2
if the experimental value is not available. Value of CH may
be determined from the methods presented in Section 2.6.3.
3.5.2 Prediction of Elemental Composition
In the following methods in which calculation of only %H is
As discussed earlier, knowledge of elemental composition presented, %C can be calculated from Eq. (3.88) if the sulfur
especially of carbon (%C), hydrogen (%H), and sulfur con- content is available.
tent (%S) directly gives information on the quality of a fuel.
Knowledge of hydrogen content of a petroleum fraction helps 3.5.2.1.2 Goossens’ Method—Most recently a simple re-
to determine the amount of hydrogen needed if it has to go lation was proposed by Goossens to estimate the hydrogen
through a reforming process. Petroleum mixtures with higher content of a petroleum fraction based on the assumption of

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