291
Structural Group Analysis
and
R TS = 1 if R T > 1
Thus, a relation between specific refraction, molecular weight, and percentage carbon in naphthene
rings for saturated hydrocarbons or atomic- and olefin-free oil fractions can be defined (Brooks
et al., 1954, p. 451). Similarly, a graphic relation between aniline point, specific refraction, and
molecular weight of saturated oil fractions has also been defined (Brooks et al., 1954, p. 453).
The most important sources of error of the Waterman ring analysis are the specific refraction, the
molecular weight, and the aniline point, and although the accuracy of the Waterman ring analysis
is questionable, the method has found wide acceptance.
11.2.1.3 Density Method
This method is based on the use of the density, d, specific refraction, r LL , and the molecular weight, M
(Chapter 10). This method is based on correlation of oil composition, determined by the direct method,
with the physical properties of a large number of petroleum fractions obtained by various means. The
procedure used for this method is analogous to that employed for the Waterman ring analysis, but the
main difference is that the density is used instead of the aniline point.
Thus, the density corresponding to the observed molecular weight M and specific refraction r LL
is determined (Brooks et al., 1954, p. 453); the difference ∆d between this value and the observed
value is noted. Thus,
%C A = 420 ∆d/(1 + 3.2)d
C R is obtained graphically; thence,
%C P = 100 − %C R
%C N = %C R − %C A
Compared with the Waterman ring analysis, the experimental procedure is simplified, the applicability is more general, and there is better agreement with data obtained by the direct method.
Disadvantages, which are also inherent in the Waterman ring analysis, are that the molecular weight
range is limited to 200–500 and extrapolation of the data to the higher range is uncertain; indeed, a
high aromatic content causes the specific refraction to fall outside the correlation. The accuracy of
the density method is in general fairly good, although according to the basic data used the application should be limited to fractions with %C A <1.5% C N or with R A <0.5 R T .
11.2.1.4 n–d–M Method
Linear relationships between the composition of petroleum fractions (as determined by the direct
method) and the refractive index, density, and molecular weight (Chapter 10) led to the development
of a method based on these three physical characteristics of petroleum fractions.
Thus, as the name (n–d–M method) implies, the refractive index, n, density, d, and molecular
weight, M, of the sample are determined; if the sulfur content of the sample is expected to be greater
than 0.206, this value should also be determined. Substitution of the data into the formulae allows
an estimation of the carbon distribution and ring content.
The n–d–M method is especially intended for petroleum fractions boiling above the gasoline
range and for similar products after extraction, hydrogenation, or other treatment. The validity of
the method is good for petroleum fractions with up to %C R = 75% (aromatic + naphthenic), provided
that %C A (as found by the n–d–M method) is not higher than 1.5 times %C N . The method is also
valid for samples containing up to four rings per molecule if not more than half of these are aromatic.
Structural Group Analysis
and
R TS = 1 if R T > 1
Thus, a relation between specific refraction, molecular weight, and percentage carbon in naphthene
rings for saturated hydrocarbons or atomic- and olefin-free oil fractions can be defined (Brooks
et al., 1954, p. 451). Similarly, a graphic relation between aniline point, specific refraction, and
molecular weight of saturated oil fractions has also been defined (Brooks et al., 1954, p. 453).
The most important sources of error of the Waterman ring analysis are the specific refraction, the
molecular weight, and the aniline point, and although the accuracy of the Waterman ring analysis
is questionable, the method has found wide acceptance.
11.2.1.3 Density Method
This method is based on the use of the density, d, specific refraction, r LL , and the molecular weight, M
(Chapter 10). This method is based on correlation of oil composition, determined by the direct method,
with the physical properties of a large number of petroleum fractions obtained by various means. The
procedure used for this method is analogous to that employed for the Waterman ring analysis, but the
main difference is that the density is used instead of the aniline point.
Thus, the density corresponding to the observed molecular weight M and specific refraction r LL
is determined (Brooks et al., 1954, p. 453); the difference ∆d between this value and the observed
value is noted. Thus,
%C A = 420 ∆d/(1 + 3.2)d
C R is obtained graphically; thence,
%C P = 100 − %C R
%C N = %C R − %C A
Compared with the Waterman ring analysis, the experimental procedure is simplified, the applicability is more general, and there is better agreement with data obtained by the direct method.
Disadvantages, which are also inherent in the Waterman ring analysis, are that the molecular weight
range is limited to 200–500 and extrapolation of the data to the higher range is uncertain; indeed, a
high aromatic content causes the specific refraction to fall outside the correlation. The accuracy of
the density method is in general fairly good, although according to the basic data used the application should be limited to fractions with %C A <1.5% C N or with R A <0.5 R T .
11.2.1.4 n–d–M Method
Linear relationships between the composition of petroleum fractions (as determined by the direct
method) and the refractive index, density, and molecular weight (Chapter 10) led to the development
of a method based on these three physical characteristics of petroleum fractions.
Thus, as the name (n–d–M method) implies, the refractive index, n, density, d, and molecular
weight, M, of the sample are determined; if the sulfur content of the sample is expected to be greater
than 0.206, this value should also be determined. Substitution of the data into the formulae allows
an estimation of the carbon distribution and ring content.
The n–d–M method is especially intended for petroleum fractions boiling above the gasoline
range and for similar products after extraction, hydrogenation, or other treatment. The validity of
the method is good for petroleum fractions with up to %C R = 75% (aromatic + naphthenic), provided
that %C A (as found by the n–d–M method) is not higher than 1.5 times %C N . The method is also
valid for samples containing up to four rings per molecule if not more than half of these are aromatic.