Page 132 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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112 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
TABLE 3.15—Presentation of a petroleum fraction (diesel fuel) by a matrix of 30 pseudocomponents.
Carbon number n-Paraffins Isoparaffins Olefins Naphthenes Aromatics
C 11 1 2 3 4 5
C 12 6 7 8 9 10
C 13 11 12 13 14 15
C 14 16 17 18 19 20
C 15 21 22 23 24 25
C 16 26 27 28 29 30
groups, which can be identified by a PIONA analyzer. As an are rich in one hydrocarbon type such as coal liquids that
example in Table 1.3 (Chapter 1), carbon number ranges for may have up to 90% aromatics, it would be appropriate to di-
different petroleum products are specified. For a diesel fuel vide the aromatics into further subgroups of monoaromatics
sample, carbon number varies from C 11 to C 16 with a boiling (MA) and polyaromatics (PA). Therefore, creation of a matrix
range of 400–550 C. If each single carbon number hydrocar- of pseudocomponents, such as Table 3.15, largely depends on
◦
bon cut is further separated into five pseudocomponents from the nature and characteristics of the petroleum mixture as
different groups, the whole mixture may be represented by a well as availability of experimental data.
group of 30 pseudocomponents as shown in Table 3.15. Al-
though each pseudocomponent is not a pure hydrocarbon but 3.3.2 Narrow Versus Wide Boiling Range Fractions
their properties are very close to pure compounds from the
same family with the same carbon number. If the amounts of In general, regardless of molecular type, petroleum fractions
all these 30 components are known then properties of the mix- may be divided into two major categories: narrow and wide
ture may be estimated quite accurately. This requires exten- boiling range fractions. A narrow boiling range fraction was
sive analysis of the mixture and a large computation time for defined in Section 3.2.1 as a fraction whose ASTM 10–90%
estimation of various properties. The number of pseudocom- distillation curve slope (SL) is less than 0.8 C/%, although
◦
ponents may even increase further if the fraction has wider this definition is arbitrary and may vary from one source to
boiling point range such as heptane plus fractions as will be another. Fractions with higher 10–90% slopes may be consid-
discussed in Chapter 4. However, many petroleum fractions ered as wide boiling range. However, for simplicity the meth-
are olefin free and groups of n-paraffins and isoparaffins may ods presented in this section for narrow fractions may also
be combined into a single group of paraffins. Therefore, the be applied to wider fractions. For narrow fractions, only one
number of different families reduces to three (paraffins, naph- carbon number is considered and the whole fraction may be
thenes, and aromatics). In this case the number of compo- characterized by a single value of boiling point or molecu-
nents in Table 3.15 reduces to 6 × 3 or 18. If a fraction is nar- lar weight. For such fractions, if molecular type is known
row in boiling range then the number of rows in Table 3.15 (PNA composition), then the number of pseudocomponents
decreases indicating lower carbon number range. In Chapter in Table 3.15 reduces to three and if the composition is not
4, boiling points of various single carbon number groups are known the whole mixture may be considered as a single pseu-
given and through a TBP curve it would be possible to deter- docomponent. For this single pseudocomponent, properties
mine the range of carbon number in a petroleum fraction. In of a pure component whose characteristics, such as boiling
Table 3.15, if every two carbon number groups and all paraf- point and specific gravity, are the same as that of the fraction
fins are combined together, then the whole mixture may be can be considered as the mixture properties. For mixtures the
represented by 3 × 3 or 9 components for an olefin-free frac- best characterizing boiling point is the mean average boiling
tion. Similarly if all carbon numbers are grouped into a single point (MeABP); however, as mentioned in Section 3.2.1, for
carbon number group, the mixture can be represented by only narrow fractions the boiling point at 50 vol% distilled may
three pseudocomponents from paraffins (P), naphthenes (N), be considered as the characteristic boiling point instead of
and aromatics (A) groups all having the same carbon number. MeABP. The specific gravity of a fraction is considered as the
This approach is called pseudocomponent technique. second characteristic parameter for a fraction represented by
Finally the ultimate simplicity is to ignore the difference in a single pseudocomponent. Therefore, the whole mixture may
properties of various hydrocarbon types and to present the be characterized by its boiling point (T b ) and specific gravity
whole mixture by just a single pseudocomponent, which is (SG). In lieu of these properties other characterization pa-
the mixture itself. The simplicity in this case is that there is rameters discussed in Chapter 2 may be used.
no need for the composition of the mixture. Obviously the Treatment of wide boiling range fractions is more compli-
accuracy of estimated properties decreases as the number of cated than narrow fractions as a single value for the boiling
pseudocomponents decreases. However, for narrow boiling point, or molecular weight, or carbon number cannot rep-
range fractions such as a light naphtha approximating the resent the whole mixture. For these fractions the number of
mixture with a single pseudocomponent is more realistic and constituents in the vertical columns of Table 3.15 cannot be
more accurate than a wide boiling range fraction such as an reduced to one, although it is still possible to combine var-
atmospheric residuum or the C 7+ fraction in a crude oil sam- ious molecular types for each carbon number. This means
ple. As discussed in Chapter 2, the differences between prop- that the minimum number of constituents in Table 3.14 for
erties of various hydrocarbon families increase with boiling a wide fraction is six rather than one that was considered
point (or carbon number). Therefore, assumption of a single for narrow fractions. The best example of a wide boiling
pseudocomponent for a heavy fraction (M > 300) is less ac- range fraction is C 7+ fraction in a crude oil or a reservoir
curate than for the case of light fractions. For fractions that fluid. Characterization of such fractions through the use of a

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