Page 54 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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34 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
As discussed in Section 1.7.18, kinematic viscosity is ex-
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pressed in cSt, SUS, and SFS units. Values of kinematic vis- flash point greater than 80 C (ASTM D 92 or ISO 2592 test
methods). Flash point should not be mistaken with fire point,
cosity for pure liquid hydrocarbons are usually measured and which is defined as the minimum temperature at which the
reported at two reference temperatures of 38 C (100 F) and hydrocarbon will continue to burn for at least 5 s after being
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99 C (210 F) in cSt. However, other reference temperatures ignited by a flame.
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of 40 C (104 F), 50 C (122 F), and 60 C (140 F) are also used
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to report kinematic viscosities of petroleum fractions. Liq- 2.1.11 Autoignition Temperature
uid viscosity decreases with an increase in temperature (see
Section 2.7). Kinematic viscosity, as it is shown in Chapter 3, This is the minimum temperature at which hydrocarbon va-
is a useful characterization parameter, especially for heavy por when mixed with air can spontaneously ignite without
fractions in which the boiling point may not be available. the presence of any external source. Values of autoignition
temperature are generally higher than flash point, as given
in Table 2.2 for some pure hydrocarbons. Values of autoigni-
2.1.9 Freezing and Melting Points
tion temperature for oils obtained from mineral sources are
Petroleum and most petroleum products are in the form of a in the range of 150–320 C (300–500 F), for gasoline it is about
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liquid or gas at ambient temperatures. However, for oils con- 350 C (660 F), and for alcohol is about 500 C (930 F) [7].
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taining heavy compounds such as waxes or asphaltinic oils, With an increase in pressure the autoignition temperature
problems may arise from solidification, which cause the oil decreases. This is particularly important from a safety point
to lose its fluidity characteristics. For this reason knowledge of view when hydrocarbons are compressed.
of the freezing point is important and it is one of the ma-
jor specifications of jet fuels and kerosenes. For a pure com- 2.1.12 Flammability Range
pound the freezing point is the temperature at which liquid
solidifies at 1 atm pressure. Similarly the melting point, T M , To have a combustion, three elements are required: fuel (hy-
is the temperature that a solid substance liquefies at 1 atm. drocarbon vapor), oxygen (i.e., air), and a spark to initiate the
A pure substance has the same freezing and melting points; combustion. One important parameter to have a good com-
however, for petroleum mixtures, there are ranges of melting bustion is the ratio of air to hydrocarbon fuel. The combustion
and freezing points versus percent of the mixture melted or does not occur if there is too much air (little fuel) or too lit-
frozen. For a mixture, the initial melting point is close to the tle air (too much fuel). This suggests that combustion occurs
melting point of the lightest compound in the mixture, while when hydrocarbon concentration in the air is within a certain
the initial freezing point is close to the freezing point (or melt- range. This range is called flammability range and is usually
ing point) of the heaviest compound in the mixture. Since the expressed in terms of lower and upper volume percent in the
melting point increases with molecular weight, for petroleum mixture of hydrocarbon vapor and air. The actual volume per-
mixtures the initial freezing point is greater than the initial cent of hydrocarbon vapor in the air may be calculated from
melting point. For petroleum mixtures an equivalent term of Eq. (2.11) using vapor pressure of the hydrocarbon. If the cal-
pour point instead of initial melting point is defined, which culated vol% of hydrocarbon in the air is within the flamma-
will be discussed in Chapter 3. Melting point is an important bility range then the mixture is flammable by a spark or flame.
characteristic parameter for petroleum and paraffinic waxes.
2.1.13 Octane Number
2.1.10 Flash Point
Octane number is a parameter defined to characterize an-
Flash point, T F , for a hydrocarbon or a fuel is the minimum tiknock characteristic of a fuel (gasoline) for spark ignition
temperature at which vapor pressure of the hydrocarbon is engines. Octane number is a measure of fuel’s ability to re-
sufficient to produce the vapor needed for spontaneous igni- sist auto-ignition during compression and prior to ignition.
tion of the hydrocarbon with the air with the presence of an Higher octane number fuels have better engine performance.
external source, i.e., spark or flame. From this definition, it is The octane number of a fuel is measured based on two ref-
clear that hydrocarbons with higher vapor pressures (lighter erence hydrocarbons of n-heptane with an assigned octane
compounds) have lower flash points. Generally flash point number of zero and isooctane (2,2,4-trimethylpentane) with
increases with an increase in boiling point. Flash point is assigned octane number of 100. A mixture of 70 vol% isooc-
an important parameter for safety considerations, especially tane and 30 vol% n-heptane has an octane number of 70.
during storage and transportation of volatile petroleum prod- There are two methods of measuring octane number of a fuel
ucts (i.e., LPG, light naphtha, gasoline) in a high-temperature in the laboratory. The methods are known as motor octane
environment. The surrounding temperature around a storage number (MON) and research octane number (RON). The MON
tank should always be less than the flash point of the fuel is indicative of high-speed performance (900 rpm) and is mea-
to avoid possibility of ignition. Flash point is used as an in- sured under heavy road conditions (ASTM D 357). The RON
dication of the fire and explosion potential of a petroleum is indicative of normal road performance under low engine
product. Estimation of the flash point of petroleum fractions speed (600 rpm) city driving conditions (ASTM D 908). The
is discussed in Chapter 3, and data for flash points of some third type of octane number is defined as posted octane num-
pure hydrocarbons are given in Table 2.2. These data were ob- ber (PON), which is the arithmetic average of the MON and
tained using the closed cup apparatus as described in ASTM RON [PON = (MON + RON)/2]. Generally isoparaffins have
D 93 (ISO 2719) test method. There is another method of higher octane number than do normal paraffins. Naphthenes
measuring flash point known as open cup for those oils with have relatively higher octane number than do corresponding
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