Page 55 - Characterization and Properties of Petroleum Fractions

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2. CHARACTERIZATION AND PROPERTIES OF PURE HYDROCARBONS 35
paraffins and aromatics have very high octane numbers. The
purpose of definition of this factor was to classify the type
octane number of a fuel can be improved by adding tetra- T b is the mean average boiling point (also see Chapter 3). The
ethyl-lead (TEL) or methyl-tertiary-butyl-ether (MTBE). Use of hydrocarbons in petroleum mixtures. The naphthenic hy-
of lead (Pb) to improve octane number of fuels is limited in drocarbons have K W values between paraffinic and aromatic
many industrial countries. In these countries MTBE is used compounds. In general, aromatics have low K W values while
for octane number improvement. However, there are prob- paraffins have high values. However, as will be discussed in
lems of groundwater contamination with MTBE. MTBE has Chapter 3 there is an overlap between values of K W from dif-
MON and RON of 99 and 115, respectively [8]. Lead gener- ferent hydrocarbon groups. The Watson K was developed in
ally improves octane number of fuels better than MTBE. The 1930s by using data for the crude and products available in
addition of 0.15 g Pb/L to a fuel of RON around 92 can im- that time. Now the base petroleum stocks in general vary sig-
prove its octane number by 2–3 points. With 0.6 g Pb/L one nificantly from those of 1930s [10, 11]. However, because it
may improve the octane number by 10 points [8]. However, combines two characterization parameters of boiling point
as mentioned above, because of environmental hazards use and specific gravity it has been used extensively in the devel-
of lead is restricted in many North American and West Euro- opment of many physical properties for hydrocarbons and
pean countries. Values of the octane number measured with- petroleum fractions [2, 11, 12].
out any additives are called clear octane number. For pure
hydrocarbons values of clear MON and RON are given in Sec- 2.1.16 Refractivity Intercept
tion 2.2. Estimation of the octane number of fuels is discussed
in Chapter 3. Kurtz and Ward [13] showed that a plot of refractive index
against density for any homologous hydrocarbon group is
2.1.14 Aniline Point linear. For example, plot of refractive index of n-paraffins ver-
sus density (d 20 ) in the carbon number range of C 5 –C 45 is a
The aniline point for a hydrocarbon or a petroleum fraction is straight line represented by equation n = 1.0335 + 0.516d 20 ,
defined as the minimum temperature at which equal volumes with R value of 0.9998 (R = 1, for an exact linear relation).
2
2
of liquid hydrocarbon and aniline are miscible. Aniline is an Other hydrocarbon groups show similar performance with
aromatic compound with a structure of a benzene molecule an exact linear relation between n and d. However, the inter-
where one atom of hydrogen is replaced by the NH 2 group cept for various groups varies and based on this observation
(C 6 H 5 NH 2 ). The aniline point is important in characteriza- they defined a characterization parameter called refractivity
tion of petroleum fractions and analysis of molecular type. intercept, R i , in the following form:
As discussed in Chapter 3, the aniline point is also used as a
characterization parameter for the ignition quality of diesel (2.14) d
fuels. It is measured by the ASTM D 611 test method. Within R i = n − 2
a hydrocarbon group, aniline point increases with molecu- where n and d are refractive index and density of liquid hy-
lar weight or carbon number, but for the same carbon num- drocarbon at the reference state of 20 C and 1 atm in which
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ber it increases from aromatics to paraffinic hydrocarbons. density must be in g/cm . R i is high for aromatics and low for
3
Aromatics have very low aniline points in comparison with
naphthenic compounds, while paraffins have intermediate R i
paraffins, since aniline itself is an aromatic compound and it values.
has better miscibility with aromatic hydrocarbons. Generally,
oils with higher aniline points have lower aromatic content.
Values of the aniline point for pure hydrocarbons are given 2.1.17 Viscosity Gravity Constant
in Table 2.2, and its prediction for petroleum fractions is dis- Another parameter defined in the early years of petroleum
cussed in Chapter 3. characterization is the viscosity gravity constant (VGC). This
parameter is defined based on an empirical relation developed
2.1.15 Watson K between Saybolt viscosity (SUS) and specific gravity through
Since the early years of the petroleum industry it was desired a constant. VGC is defined at two reference temperatures of
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to define a characterization parameter based on measurable 38 C (100 F) and 99 C (210 F) as [14]
parameters to classify petroleum and identify hydrocarbon 10SG − 1.0752 log (V 38 − 38)
10
molecular types. The Watson characterization factor denoted (2.15) VGC = 10 − log (V 38 − 38)
by K W is one of the oldest characterization factors originally 10
defined by Watson et al. of the Universal Oil Products (UOP) SG − 0.24 − 0.022 log (V 99 − 35.5)
in mid 1930s [9]. For this reason the parameter is sometimes (2.16) VGC = 0.755 10
called UOP characterization factor and is defined as --`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
where
(1.8T b ) 1/3
(2.13) K W = V 38 = viscosity at 38 C (100 F) in SUS (Saybolt Universal
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SG Seconds)
where V 99 = Saylbolt viscosity (SUS) at 99 C (210 F)
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T b = normal boiling point K Conversion factors between cSt and SUS are given in Sec-
SG = specific gravity at 15.5 C tion 1.7.18. Equations (2.15) and (2.16) do not give identical
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In the original definition of K W , boiling point is in degrees values for a given compound but calculated values are close to
Rankine and for this reason the conversion factor of 1.8 each other, except for very low viscosity oils. Equation (2.16)
is used to have T b in the SI unit. For petroleum fractions is recommended only when viscosity at 38 C (100 F) is not
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