Page 56 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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36 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
available. VGC varies for paraffinic hydrocarbons from 0.74 to
clohexanes (naphthenes) have formula of C n H 2n , and alkyl-
0.75, for naphthenic from 0.89 to 0.94, and for aromatics from general formula of C n H 2n+2 , alkylcyclopentanes or alkylcy-
0.95 to 1.13 [15]. In Chapter 3, VGC along with other param- benzenes (aromatics) have formula of C n H 2n−6 (n ≥ 6). This
eters has been used to estimate the composition of petroleum shows that at the same carbon number, the atomic ratio of
fractions. Values of VGC for some hydrocarbons are given in number of carbon (C) atoms to number of hydrogen (H)
Table 2.3. The main limitation in use of VGC is that it cannot atoms increases from paraffins to naphthenes and aromat-
be defined for compounds or fractions with viscosities less ics. For example, n-hexane (C 6 H 14 ), cyclohexane (C 6 H 12 ), and
than 38 SUS (∼3.6 cSt) at 38 C. A graphical method to esti- benzene (C 6 H 6 ) from three different hydrocarbon groups all
◦
mate VGC of petroleum fractions is presented in Chapter 3. have six carbon atoms, but have different CH atomic ratios
ASTM D 2501 suggests calculation of VGC using specific of 6/14, 6/12, and 6/6, respectively. If CH atomic ratio is mul-
2
gravity and viscosity in mm /s (cSt) at 40 C, ν 40 in the follow- tiplied by the ratio of atomic weights of carbon (12.011) to
◦
ing form: hydrogen (1.008), then CH weight ratio is obtained. For ex-
ample, for n-hexane, the CH weight ratio is calculated as
SG − 0.0664 − 0.1154 log (ν 40 − 5.5)
(2.17) VGC = 10 (6/14)×(12.011/1.008) = 5.107. This number for benzene is
0.94 − 0.109 log (ν 40 − 5.5) 11.92. Therefore, CH weight ratio is a parameter that is ca-
10
Values of VGC calculated from Eq. (2.17) are usually very pable of characterizing the hydrocarbon type. In addition,
close to values obtained from Eq. (2.15). If viscosity at 40 C within the same hydrocarbon group, the CH value changes
◦
is available, use of Eq. (2.17) is recommended for calculation from low to high carbon number. For example, methane has
of VGC. Another relation to calculate VGC in metric units was CH value of 2.98, while pentane has CH value of 4.96. For
proposed by Kurtz et al. [16] in terms of kinematic viscosity extremely large molecules (M →∞), the CH value of all hy-
and density at 20 C, which is also reported in other sources drocarbons regardless of their molecular type approaches the
◦
[17]. limiting value of 5.96. This parameter is used in Section 2.3
to estimate hydrocarbon properties, and in Chapter 3 it is
d − 0.1384 log (ν 20 − 20) used to estimate the composition of petroleum fractions. In
(2.18) VGC = 10 + 0.0579
0.1526[7.14 − log (ν 20 − 20)] some references HC atomic ratio is used as the character-
10
izing parameter. According to the definition, the CH weight
in which d is density at 20 C and 1 atm in g/cm and ν 20 is the ratio and HC atomic ratio are inversely proportional. The
3
◦
kinematic viscosity at 20 C in cSt. In this method viscosity limiting value of HC atomic ratio for all hydrocarbon types
◦
of oil at 20 C must be greater than 20 cSt. However, when is 2.
◦
there is a choice Eq. (2.15) should be used for the procedures Another use of CH weight ratio is to determine the quality of
described in Chapter 3.
a fossil-type fuel. Quality and the value of a fuel is determined
from its heat of combustion and heating value. Heating value
Example 2.1—API RP-42 [18] reports viscosity of some heavy of a fuel is the amount of heat generated by complete com-
hydrocarbons. 1,1-Di-(alphadecalyl)hendecane (C 31 H 56 )isa bustion of 1 unit mass of the fuel. For example, n-hexane has
naphthenic compound with molecular weight of 428.8 and the heating value of 44734 kJ/kg (19232 Btu/lb) and benzene --`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
specific gravity of 0.9451. The kinematic viscosity at 38 C has the heating value of 40142 kJ/kg (17258 Btu/lb). Calcula-
◦
(100 F) is 20.25 cSt. Calculate the viscosity gravity constant tion of heating values are discussed in Chapter 7. From this
◦
for this compound. analysis it is clear that as CH value increases the heating value
decreases. Hydrogen (H 2 ), which has a CH value of zero, has a
Solution—Using Eq. (1.17), the viscosity is converted from heating value more than that of methane (CH 4 ) and methane
cSt to SUS: V 38 = 99.5 SUS. Substituting values of V 38 and has a heating value more than that of any other hydrocarbon.
SG = 0.9451 into Eq. (2.15) gives VGC = 0.917. The VGC may Heavy aromatic hydrocarbons that have high CH values have
be calculated from Eq. (2.17) with direct substitution of vis- lower heating values. In general, by moving toward lower CH
cosity in the cSt unit. Assuming there is a slight change in value fuel, not only do we have better heating value but also
viscosity from 38 to 40 C, the same value of viscosity at 38 C better and cleaner combustion of the fuel. It is for this reason
◦
◦
2
is used for ν 40 . Thus ν 40 = 20.25 cSt (mm /s) and Eq. (2.17) that the use of natural gas is preferable to any other type of
∼
gives VGC = 0.915. The small difference between calculated fuel, and hydrogen is an example of a perfect fuel with zero
values of VGC because in Eq. (2.17) viscosity at 40 C must CH weight ratio (CH = 0), while black carbon is an example
◦
be used, which is less than the viscosity at 38 C. Calculated of the worst fuel with a CH value of infinity. Values of CH for
◦
VGC is within the range of 0.89–0.94 and thus the hydrocar- pure hydrocarbons are given in Section 2.2 and its estimation
bon must be a naphthenic compound (also see Fig. 3.22 in methods are given in Section 2.6.3.
Chapter 3).
2.1.18 Carbon-to-Hydrogen Weight Ratio 2.2 DATA ON BASIC PROPERTIES
OF SELECTED PURE HYDROCARBONS
Carbon-to-hydrogen weight ratio, CH weight ratio, is defined
as the ratio of total weight of carbon atoms to the total weight 2.2.1 Sources of Data
of hydrogen in a compound or a mixture and is used to
characterize a hydrocarbon compound. As was discussed in There are several sources that provide data for physical prop-
Section 1.1.1, hydrocarbons from different groups have dif- erties of pure compounds. Some of these sources are listed
ferent formulas. For example, alkanes (paraffins) have the below.

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