Page 113 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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atmospheric boiling points are given in the ASTM Manual [4]
3.1.3 Molecular Weight
and will be discussed later in this chapter. 14:23 3. CHARACTERIZATION OF PETROLEUM FRACTIONS 93
Molecular weight is another bulk property that is indicative of
molecular size and structure. This is an important property
3.1.2 Density, Specific Gravity, and API Gravity that usually laboratories do not measure and fail to report
when reporting various properties of petroleum fractions.
Specific gravity (SG) or relative density and the API gravity are This is perhaps due to the low accuracy in the measurement
defined in Section 2.1.3 and for pure hydrocarbons are given of the molecular weight of petroleum fractions, especially for
in Tables 2.1 and 2.2. Aromatic oils are denser than paraffinic heavy fractions. However, it should be realized that experi-
oils. Once specific gravity is known, the API gravity can be de- mental uncertainty in reported values of molecular weight
termined by Eq. (2.4), which corresponds to the ASTM D 287 is less than the errors associated with predictive methods for
method. The standard temperature to measure the specific this very useful parameter. Since petroleum fractions are mix-
gravity is 15.56 C (60 F); however, absolute density is usually tures of hydrocarbon compounds, mixture molecular weight
◦
◦
reported at 20 C. Specific gravity or density for a petroleum is defined as an average value called number average molec-
◦
mixture is a bulk property that can be directly measured for ular weight or simply molecular weight of the mixture and it
the mixture. Specific gravity is a property that indicates the is calculated as follows:
quality of a petroleum product and, as was shown in Chapter
2, is a useful property to estimate various physical proper- (3.1) M =
ties of petroleum fluids. A standard test method for density x i M i
i
and specific gravity of liquid petroleum products and distil-
lates in the range of 15–35 C through use of a digital density where x i and M i are the mole fraction and molecular weight of
◦
meter is described in ASTM D 4052 method [4]. The appa- component i, respectively. Molecular weight of the mixture,
ratus must be calibrated at each temperature and this test M, represents the ratio of total mass of the mixture to the total
method is equivalent to ISO 12185 and IP 365 methods. An- moles in the mixture. Exact knowledge of molecular weight
other method using a hydrometer is described under ASTM D of a mixture requires exact composition of all compounds in
1298 test method. Hydrometer is a glass float with lead ballast the mixture. For petroleum fractions such exact knowledge is
that is floated in the liquid. The level at which hydrometer is not available due to the large number of components present
floating in the liquid is proportional to the specific gravity in the mixture. Therefore, experimental measurement of mix-
of the liquid. Through graduation of the hydrometer the spe- ture molecular weight is needed in lieu of exact composition
cific gravity can be read directly from the stalk of hydrometer. of all compounds in the mixture.
This method is simpler than the ASTM 4052 method but is There are three methods that are widely used to measure
less accurate. The French standard procedure for measuring the molecular weight of various petroleum fractions. These
density by hydrometer is described under NFT 60-101 test are cryoscopy, the vapor pressure method, and the size ex-
method. With some hydrometers densities with accuracy of clusion chromatography (SEC) method. For heavy petroleum
0.0005 g/mL can be measured. A digital density meter model fractions and asphaltenic compounds the SEC method is
DMA 48 from PARA (Austria) is shown in Fig. 3.8. commonly used to measure distribution of molecular weight

FIG. 3.8—PARA model DMA 48 digital density meter (courtesy of Chemical Engineering
Department at KU).

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