Page 119 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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3. CHARACTERIZATION OF PETROLEUM FRACTIONS 99

FIG. 3.17—A chrompack model 940 PIONA analyzer (courtesy of Chemical Engineering
Department at KU).

that can be obtained is on the C/H ratio and sulfur content tion. Metallic constituents are associated with heavy com-
of a petroleum mixture from which one can determine the pounds and mainly appear in residues. There is no general
quality of oil. As boiling points of fractions increase or their method to determine the composition of all metals at once
API gravity decrease the amount of C/H ratio, sulfur content, but ASTM [4] provides test methods for determination of
nitrogen content, and the metallic constituents increase, sig- various metallic constituents (i.e., ASTM D 1026, D 1262,
nifying a reduction in the quality of an oil. Sulfur content of D 1318, D 1368, D 1548). Another method is to burn the oil
very heavy fractions can reach 6–8% and the nitrogen content sample in which metallic compounds appear in inorganic
can reach 2.0–2.5 wt%. There are specific methods to measure ashes. The ash should be digested by an acid and the so-
these elements individually. However, instruments do exist lution is examined for metal species by atomic absorption
that measure these elements all together; these are called ele- spectroscopy [7].
mental analyzers. One of these apparatuses is CHN analyzers
in which there is a simultaneous combustion in pure oxygen 3.1.6 Viscosity
at 1000 C. Carbon is reduced to CO 2 , H is reduced to H 2 O,
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and N is converted to nitrogen oxides. Nitrogen oxides are Absolute and kinematic viscosities are defined in Section
then reduced over copper at 650 C to nitrogen by eliminating 2.1.8 and experimental data for the kinematic viscosity of
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oxygen. A mixture of CO 2 ,H 2 O, and N 2 is separated by gas some pure hydrocarbons are given in Table 2.2. Viscosity of
chromatography with TCD. In a similar fashion, sulfur is ox- petroleum fractions increase with a decrease in the API grav-
idized to SO 2 and is detected by TCD after detection of CO 2 , ity and for residues and heavy oils with the API gravity of
N 2 , and H 2 O. Oxygen is determined by passing it over carbon less than 10 (specific gravity of above 1), viscosity varies from
at high temperature and converted to CO, which is measured several thousands to several million poises. Viscosity is a bulk
by a GC [5]. ASTM test methods for elemental analysis of property that can be measured for all types of petroleum frac-
petroleum products and crude oils include hydrogen content tions in liquid form. Kinematic viscosity is a useful character-
(ASTM D 1018, D 3178, D 3343), nitrogen content (ASTM ization parameter for heavy fractions in which boiling point
D 3179, D 3228, D 3431), and sulfur content (ASTM D 129/IP data are not available due to thermal decomposition during
61, D 1266/IP 107, D 1552, D 4045). An elemental analyzer distillation. Not only is viscosity an important physical prop-
Model CHNS-932 (Leco Corp., St. Joseph, MI, USA) is shown erty, but it is a parameter that can be used to estimate other
in Fig. 3.18. In this analyzer, the CO 2 ,H 2 O, and SO 2 are physical properties as well as the composition and quality of
detected by infrared detector (IRD) while N 2 is determined undefined petroleum fractions as shown later in this chap-
by the TCD method. ter. Since viscosity varies with temperature, values of viscos-
Another group of heteroatoms found in petroleum mix- ity must be reported with specified temperature. Generally,
tures are metallic constituents. The amount of these met- kinematic viscosity of petroleum fractions are measured at
als are in the range of few hundreds to thousand ppm and standard temperatures of 37.8 C (100 F) and 98.9 C (210 F).
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their amounts increase with increase in boiling points or de- However, for very heavy fractions viscosity is reported at tem-
crease in the API gravity of oil. Even a small amount of these peratures above 38 C, i.e., 50 C (122 F) or 60 C (140 F). When
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metals, particularly nickel, vanadium, iron, and copper, in viscosity at two temperatures are reported from the method
the feedstocks for catalytic cracking have negative effects on of Section 2.7 one can obtain the viscosity at other tempera-
the activity of catalysts and result in increased coke forma- tures. Measurement of viscosity is easy but the method and

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