Page 114 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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94 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS

FIG. 3.9—A GPC from waters model 150-C plus (courtesy of Chemical Engineering
Department at KU).
in the fraction. The SEC method is mainly used to determine of about 0.001 C. The relation to obtain molecular weight of
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molecular weights of polymers in the range of 2000 to 2 × 10 . a sample is [6]
6
This method is also called gel permeation chromatography
1000 × K f × m 1
(GPC) and is described in the ASTM D 5296 test method. In (3.2) M =
the GPC method, by comparing the elution time of a sample T × m 2
with that of a reference solution the molecular weight of the where K f is molal freezing point depression constant of the
sample can be determined. A GPC instrument is shown in solvent and is about 5.12 C/mol. T is the freezing point de-
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Fig. 3.9. The SEC experiment is usually performed for heavy pression and the reading from the cryoscope. m 1 is the mass
residues and asphaltenes in crude oils and gives the wt% of of solute and m 2 is the mass of solvent both in grams. It gener-
various constituents versus molecular weight as will be dis- ally consists of refrigerator, thermometer and the apparatus
cussed in Chapter 4. to hold the sample. A cryoscope is shown in Fig. 3.10.
The vapor pressure method is based on the measurement of
the difference between vapor pressure of sample and that of 3.1.4 Refractive Index
a known reference solvent with a vapor pressure greater than
that of the sample. A solution of about 1 g of sample in 25 mL Refractive index or refractivity is defined in Section 2.1.4 and
of the reference solvent is prepared. This solution, which has its values at 20 C for pure hydrocarbons are given in Table
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vapor pressure less than that of the solvent, tends to condense 2.1. Refractive indexes of hydrocarbons vary from 1.35 to 1.6;
the vapors of solvent on the sample thus creating a temper- however, aromatics have refractive index values greater than
ature difference which is measured by two thermistors. The naphthenes, which in turn have refractive indexes greater
molarity of the solution is calculated using calibrated curves. than paraffins. Paraffinic oils have lower refractive index val-
This method is described by the ASTM D 2503 test method ues. It was shown in Chapter 2 that refractive index is a useful
and is applicable to oils with an initial boiling point greater parameter to characterize hydrocarbon systems and, as will
than 220 C [5]. A typical experimental error and uncertainty be seen later in this chapter, it is needed to estimate the com-
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in measuring the molecular weight is about 5%. position of undefined petroleum fractions. Refractive index is
The third and most widely used method of determining the ratio of the speed of light in a vacuum to that of a medium.
the molecular weight of an unknown petroleum mixture is In a medium, the speed of light depends on the wavelength
by the cryoscopy method, which is based on freezing point and temperature. For this reason refractive index is usually
depression. The freezing point of a solution is a measure of measured and reported at 20 C with the D line sodium light.
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the solution’s concentration. As the concentration of the so- For mixtures, refractive index is a bulk property that can
lute increases, the freezing point of the solution will be lower. be easily and accurately measured by an instrument called
The relation between freezing point depression and concen- a refractometer. Refractive index can be measured by digital
tration is linear. For organic hydrocarbons, benzene is usually refractometers with a precision of ±0.0001 and temperature
used as the solvent. Special care should be taken when work- precision of ±0.1 C. The amount of sample required to mea-
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ing with benzene [6]. Calibration curves can be prepared by sure refractive index is very small and ASTM D 1218 provides
measuring the freezing points of different solute concentra- a test method for clear hydrocarbons with values of refractive
tions with a known solute and a known solvent. A cryoscope indexes in the range of 1.33–1.5 and the temperature range
can measure the freezing point depression with an accuracy of 20–30 C. In the ASTM D 1218 test method the Bausch and
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