Page 86 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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66 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
f. Lee–Kesler method with T c , P c obtained from Part a in
Example 2.6 QC: —/— T1: IML August 16, 2007 16:6 density generally decreases with temperature. Variation of
density with temperature is discussed in Chapter 6. However,
g. Lee-Kesler method with T c , P c obtained from Part b in in this section methods of estimation of density at 20 C, d 20 ,
◦
Example 2.6 are presented to be used for the characterization methods dis-
h. Lee–Kesler method with T c , P c obtained from Part c in cussed in Chapter 3. The most convenient way to estimate d 20
Example 2.6 is through specific gravity. As a rule of thumb d 20 = 0.995 SG.
i. Lee–Kesler method with T c , P c obtained from Part d in However, a better approximation is provided through calcu-
Example 2.6 lation of change of density with temperature ( d/ T), which
j. Edmister method with T c , P c obtained from Part h in Ex- is negative and for hydrocarbon systems is given as [7]
ample 2.6 −3
k. Lee–Kesler method with T c , P c obtained from Part e in (2.110) d/ T =−10 × (2.34 − 1.9d T )
Example 2.6 where d T is density at temperature T in g/cm . This equation
3
l. Lee–Kesler method with T c , P c obtained from Part g in may be used to obtain density at any temperature once a value
Example 2.6 of density at one temperature is known. This equation is quite
m. Tabulate %D for estimated value of acentric factor in each accurate within a narrow temperature range limit. One can
method. use the above equation to obtain a value of density, d 20 ,at
3
20 C (g/cm ) from the specific gravity at 15.5 Cas
◦
◦
Lee–Kesler method refers to Eq. (2.105) and Kesler–Lee to
−3
Eq. (2.107). (2.111) d 20 = SG − 4.5 × 10 (2.34 − 1.9SG)
Equation (2.111) may also be used to obtain SG from density
Solution—All three methods of Lee–Kesler, Edmister, and at 20 or 25 C.
◦
Korsten require T b , T c , and P c as input parameters. The
method of Kesler–Lee requires K W in addition to T br . From (2.112) SG = 0.9915d 20 + 0.01044
definition of Watson K, we get K W = 13.64. Substituting these SG = 0.9823d 25 + 0.02184
values from various methods one calculates the acentric fac-
tor. A summary of the results is given in Table 2.12. The least Similarly density at any other temperature may be calculated
accurate method is the Kesler–Lee correlations while the most through Eq. (2.110). Finally, Eq. (2.38) may also be used to
accurate method is Korsten combined with Eqs. (2.67) and estimate d 20 from T b and SG in the following form:
(2.68) for the critical constants. (2.113) d 20 = 0.983719T 0.002016 SG 1.0055
b
This equation was developed for hydrocarbons from C 5 to C 20 ;
2.6 PREDICTION OF DENSITY, however, it can be safely used up to C 40 with AAD of less than
REFRACTIVE INDEX, CH WEIGHT 0.1%. A comparison is made between the above three meth-
RATIO, AND FREEZING POINT ods of estimating d for some n-paraffins with actual data taken
from the API-TDB [2]. Results of evaluations are given in
Table 2.13. This summary evaluation shows that Eqs. (2.111)
Estimation of density at different conditions of temperature, and (2.113) are almost equivalent, while as expected the rule
pressure, and composition (ρ) is discussed in detail in Chap- of thumb is less accurate. Equation (2.111) is recommended
ter 5. However, liquid density at 20 C and 1 atm designated by for practical calculations.
◦
3
d in the unit of g/cm is a useful characterization parameter
which will be used in Chapter 3 for the compositional analy-
sis of petroleum fractions especially in conjunction with the 2.6.2 Prediction of Refractive Index
definition of refractivity intercept by Eq. (2.14). The sodium
◦
D line refractive index of liquid petroleum fractions at 20 C The refractive index of liquid hydrocarbons at 20 C is corre-
◦
and 1 atm, n, is another useful characterization parameter. lated through parameter I defined by Eq. (2.14). If parameter
Refractive index is needed in calculation of refractivity inter- I is known, by rearranging Eq. (2.14), the refractive index, n,
cept and is used in Eq. (2.40) for the estimation of various can be calculated as follows:
1/2
properties through parameter I defined by Eq. (2.36). More- 1 + 2I

over refractive index is useful in the calculation of density and (2.114) n = 1 − I
transport properties as discussed in Chapters 5 and 8. Carbon-
to-hydrogen weight ratio is needed in Chapter 3 for the esti- For pure and four different homologous hydrocarbon com-
mation of the composition of petroleum fractions. Freezing pounds, parameter I is predicted from Eq. (2.42) using molec-
point, T F , is useful for analyzing solidification of heavy com- ular weight, M, with constants in Table 2.6. If boiling point is
ponents in petroleum oils and to determine the cloud point available, M is first calculated by Eq. (2.48) and then I is cal-
temperature of crude oils and reservoir fluids as discussed in culated. Prediction of I through Eq. (2.42) for various hydro-
Chapter 9 (Section 9.3.3). carbon groups is shown in Fig. 2.9. Actual values of refractive
index from API-TDB [2] are also shown in this figure.
For all types of hydrocarbons and narrow-boiling range
2.6.1 Prediction of Density at 20 C petroleum fractions the simplest method to estimate param-
◦ ◦
eter I is given by Riazi and Daubert [28] in the form of
Numerical values of d 20 for a given compound is very close Eq. (2.38) for the molecular weight range of 70–300 as follows:
to the value of SG, which represents density at 15.5 C in the
--`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
◦
3
unit of g/cm as can be seen from Tables 2.1 and 2.3. Liquid (2.115) I = 0.3773T −0.02269 SG 0.9182
b

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