Page 80 - Characterization and Properties of Petroleum Fractions

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60 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
Riazi-Daubert or Riazi methods. These equations are recom-
Riazi and Daubert may be used [67]:
mended only for hydrocarbons in the molecular weight range
−0.1616
0.1157
(2.61) SG = 0.7717 ν 38(100) × ν 99(210) of 70–300 and have been widely used in industry [2, 47, 49, 51,

in which ν 38(100) and ν 99(210) are kinematic viscosities in cSt at 54, 70]. However, these correlations were replaced with more
100 and 210 F (37.8 and 98.9 C), respectively. Equation (2.61) accurate correlations presented by Eq. (2.40) and Table 2.5 in
◦
◦
is shown in Fig. 2.7 and has also been adopted by the API and terms of T b and SG as given below:
is included in 1987 version of API-TDB [2]. This equation −4
gives an AAD of about 1.5% for 158 fractions in the molecular T c = 9.5233[exp(−9.314 × 10 T b − 0.544442SG
0.53691
0.81067
−4
weight range of 200–500 (∼SG range of 0.8–1.1). (2.65) + 6.4791 × 10 T b SG)]T b SG
For coal liquids and heavy residues that are highly aro-
5
−3
matic, Tsonopoulos et al. [58] suggest the following relation P c = 3.1958 × 10 [exp(−8.505 × 10 T b − 4.8014SG
in terms of normal boiling point (T b ) for the estimation of (2.66) + 5.749 × 10 T b SG)]T −0.4844 SG 4.0846
−3
specific gravity. b
These correlations were also adopted by the API and have
2
SG = 0.553461 + 1.15156T ◦ − 0.708142T + 0.196237T 3
◦ ◦ been used in many industrial computer softwares under the
(2.62) API method. The same limitations and units as those for Eqs.
(2.63) and (2.64) apply to these equations. For heavy hydro-
where T ◦ = (1.8T b − 459.67) in which T b is in kelvin. This
equation is not recommended for pure hydrocarbons or carbons (>C 20 ) the following equations are obtained from
petroleum fractions and has an average relative deviation of Eq. (2.46a) and constants in Table 2.9:
−4
about 2.5% for coal liquid fractions [58]. For pure homolo- T c = 35.9413[exp(−6.9 × 10 T b − 1.4442SG
gous hydrocarbon groups, Eq. (2.42) with constants given in (2.67) + 4.91 × 10 T b SG)]T 0.7293 SG 1.2771
−4
Table 2.6 for SG can be used. Another approach to estimate b
specific gravity is to use the Rackett equation and a known −2
density data point at any temperature as discussed in Chap- P c = 6.9575[exp(−1.35 × 10 T b − 0.3129SG
−3
ter 5 (Section 5.8). A very simple and practical method of (2.68) + 9.174 × 10 T b SG)]T b 0.6791 SG −0.6807
estimating SG from density at 20 C, d, is given by Eq. (2.110), If necessary these equations can also be used for hydrocar-
◦
which will be discussed in Section 2.6.1. Once SG is esti- bons in the range of C 5 –C 20 with good accuracy. Equation
mated the API gravity can be calculated from its definition, (2.67) predicts values of T c from C 5 to C 50 with %AAD of 0.4%,
i.e., Eq. (2.4).
but Eq. (2.68) predicts P c with AAD of 5.8%. The reason for
this high average error is low values of P c (i.e, a few bars) at
higher carbon numbers which even a small absolute deviation
2.5 PREDICTION OF CRITICAL shows a large value in terms of relative deviation.
PROPERTIES AND ACENTRIC FACTOR
2.5.1.2 API Methods
Critical properties, especially the critical temperature and The API-TDB [2] adopted methods developed by Riazi and
pressure, and the acentric factor are important input param- Daubert for the estimation of pseudocritical properties of
eters for EOS and generalized correlations to estimate phys- petroleum fractions. In the 1982 edition of API-TDB, Eqs.
ical and thermodynamic properties of fluids. As shown in (2.63) and (2.64) were recommended for critical temperature
Chapter 1 even small errors in prediction of these proper- and pressure of petroleum fractions, respectively, but in its
ties greatly affect calculated physical properties. Some of the editions from 1987 to 1997, Eqs. (2.65) and (2.66) are included
methods widely used in the petroleum industry are given in after evaluations by the API-TDB Committee. For pure hy-
this section. These procedures, as mentioned in the previous drocarbons, the methods recommended by API are based on
sections, are mainly developed based on critical properties of group contribution methods such as Ambrose, which requires
pure hydrocarbons in which validated experimental data are the structure of the compound to be known. These methods
available only up to C 18 . The following correlations are given are of minor practical use in this book since properties of
in terms of boiling point and specific gravity. For other in- pure compounds of interest are given in Section 2.3 and for
put parameters, appropriate correlations given in Section 2.3 petroleum fractions the bulk properties are used rather than
should be used.
the chemical structure of individual compounds.
2.5.1 Prediction of Critical Temperature 2.5.1.3 Lee–Kesler Method
and Pressure Kesler and Lee [12] proposed correlations for estimation of
T c and P c similar to their correlation for molecular weight.
2.5.1.1 Riazi–Daubert Methods
Simplified equations to calculate T c and P c of hydrocarbons T c = 189.8 + 450.6SG + (0.4244 + 0.1174 SG)T b
in the range of C 5 –C 20 are given by Eq. (2.38) as follows [28]. (2.69) + (0.1441 − 1.0069 SG)10 /T b
5
(2.63) T c = 19.06232T b 0.58848 SG 0.3596
ln P c = 5.689 − 0.0566/SG
−3
2
7
(2.64) P c = 5.53027 × 10 T −2.3125 SG 2.3201 − (0.43639 + 4.1216/SG + 0.21343/SG ) × 10 T b
b 2
where T c and T b are in kelvin and P c is in bar. In the litera- (2.70) + (0.47579 + 1.182/SG + 0.15302/SG ) × 10 −6 × T b 2
2
ture, Eqs. (2.50), (2.63), and (2.64) are usually referred to as − (2.4505 + 9.9099/SG ) × 10 −10 × T 3
b
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