Page 141 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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3. CHARACTERIZATION OF PETROLEUM FRACTIONS 121
such a parameter is the Watson characterization factor, which
eroatoms also increase as shown in Fig. 3.22.
along with other parameters is introduced and discussed in aromatic content, amounts of sulfur, nitrogen, and other het-
this section. However, the first known method to predict the
PNA composition is the n-d-M method proposed by Van Nes 3.5.1.1 Characterization Parameters for Molecular
and Van Westen [30] in the 1950s. The n-d-M method is Type Analysis
also included in the ASTM manual under ASTM D 3238 test A characterization parameter that is useful for molecular type
method. The main limitation of this method is that it can- prediction purposes should vary significantly from one hy-
not be applied to light fractions. Later in the 1980s Riazi and drocarbon type to another. In addition, its range of varia-
Daubert [36, 47] proposed a series of correlations based on tion within a single hydrocarbon family should be minimal.
careful analysis of various characterization parameters. The With such specifications an ideal parameter for character-
unique feature of these correlations is that they are appli- izing molecular type should have a constant value within a
cable to both light and heavy fractions and identify various single family but different values in different families. Some
types of aromatics in the mixture. In addition various meth- of these characterization parameters (i.e., SG, I, VGC, CH,
ods are proposed based on different bulk properties of the and K W ), which are useful for molecular type analysis, have
mixture that might be available. The Riazi–Daubert methods been introduced and defined in Section 2.1. As shown in Table
have been adopted by the API Committee on characteriza- 2.4, specific gravity is a parameter that varies with chemical
tion of petroleum fractions and are included in the fourth structure particularly from one hydrocarbon family to an-
and subsequent editions of the API-TDB [2] since the early other. Since it also varies within a single family, it is not a
1980s. The other method that is reported in some literature perfect characterizing parameter for molecular type analysis
sources is the Bergamn’s method developed in the 1970s [48]. but it is more suitable than boiling point that varies within a
This method is based on the Watson K and specific gravity single family but its variation from one family to another is
of the fraction as two main characterization parameters. One not significant. One of the earliest parameters to characterize
common deficiency for all of these methods is that they do hydrocarbon molecular type was defined by Hill and Coats in
not identify n-paraffins and isoparaffins from each other. In 1928 [49], who derived an empirical relation between viscos-
fact compositional types of PIONA, PONA, and PINA can- ity and specific gravity in terms of viscosity gravity constant
not be determined from any of the methods available in the (VGC), which is defined by Eq. (2.15) in Section 2.1.17. Def-
literature. These methods provide minimum information on inition of VGC by Eqs. (2.15) or (2.16) limits its application
the composition that is predictive of the PNA content. This is to viscous oils or fractions with kinematic viscosity at 38 C
◦
mainly due to the complexity of petroleum mixtures and dif- (100 F) above 38 SUS (∼3.8 cSt.). For quick hand estima-
◦
ficulty of predicting the composition from measurable bulk tion of VGC from viscosity and specific gravity, ASTM [4] has
properties. The method of Riazi–Daubert, however, is capable provided a nomograph, shown in Fig. 3.23, that gives VGC
of predicting the monoaromatic (MA) and polyaromatic (PA) values close to those calculated from Eq. (2.15). Paraffinic
content of petroleum fractions. oils have low VGC, while napthenic oils have high VGC val-
In general low boiling point fractions have higher paraffinic ues. Watson K defined by Eq. (2.13) in terms of MeABP and
and lower aromatic contents while as boiling point of the frac- SG was originally introduced to identify hydrocarbon type
tion increases the amount of aromatic content also increases. [9, 50, 51], but as is shown later, this is not a very suitable
In the direction of increase in boiling point, in addition to parameter to indicate composition of petroleum fractions.

FIG. 3.22—Variation of composition of petroleum fractions with boiling point. Reprinted
from Ref. [7], p. 469, by courtesy of Marcel Dekker, Inc.

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