Page 115 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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3. CHARACTERIZATION OF PETROLEUM FRACTIONS 95

FIG. 3.10—Model 5009 wide range cryoscope to measure molecular weight (courtesy of
Chemical Engineering Department at KU).

Lomb refractometer is used. Refractive index of viscous oils
with values up to 1.6 can be measured by the ASTM D 1747
test method. Samples must have clear color to measure their
refractive index; however, for darker and more viscous sam-
ples in which the actual refractive index value is outside the
range of application of refractometer, samples can be diluted
by a light solvent and refractive index of the solution should be
measured. From the composition of the solution and refrac-
tive index of pure solvent and that of the solution, refractive
index of viscous samples can be determined. A Model Abbe re-
fractometer (Leica) is shown in Fig. 3.11. This refractometer
measures refractive index of liquids within the temperature
range of −20 to 100 C with temperature accuracy of ±0.01 C.
◦
◦
Because of simplicity and importance of refractive index it
would be extremely useful if laboratories measure and report
its value at 20 C for a petroleum product, especially if the
◦
composition of the mixture is not reported.
3.1.5 Compositional Analysis

Petroleum fractions are mixtures of many different types of
hydrocarbon compounds. A petroleum mixture is well defined
if the composition and structure of all compounds present in
the mixture are known. Because of the diversity and number
of constituents of a petroleum mixture, the determination of
such exact composition is nearly impossible. Generally, hy-
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drocarbons can be identified by their carbon number or by
their molecular type. Carbon number distribution may be
determined from fractionation by distillation or by molec-
ular weight distribution as discussed earlier in this section.
However, for narrow boiling range petroleum products and
petroleum cuts in which the carbon number range is quite
limited, knowledge of molecular type of compounds is very
important. As will be seen later, properties of petroleum frac-
tions with detailed compositional analysis can be estimated
with a higher degree of accuracy than for undefined fractions. FIG. 3.11—Leica made Abbe refractometer (courtesy of
After distillation data, molecular type composition is the Chemical Engineering Department at KU).
most important characteristic of petroleum fractions. In this

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