Page 407 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

T1: IML
QC: IML/FFX
P2: IML/FFX
P1: IML/FFX
June 22, 2007
AT029-09
14:25
AT029-Manual-v7.cls
AT029-Manual
9. APPLICATIONS: PHASE EQUILIBRIUM CALCULATIONS 387
3
where he assumed ρ A = 1.28 cm /g and M A = 1000 g/mol. δ A
and δ L are the solubilities of asphaltene and liquid solvent 1.1
3 1/2
(i.e., oil), respectively. If δ A and δ L are in (cal/cm ) and T is 1.0
in kelvin then R = 1.987 cal/mol · K. This equation provides
only a very approximate value of asphaltene solubility in oils. 0.9
In fact one may obtain Eq. (9.38) from Eq. (9.35) by assuming 0.8
molar volumes of both oil and asphaltene in liquid phase are
L
L
equal: V = V . As this assumption can hardly be justified, Fraction of Asphaltene Precipitated_ 0.7
A
m
one may realize the approximate nature of Eq. (9.38).
A mixture of asphaltenes and oil may be considered ho- 0.6
mogenous or heterogeneous. Kawanaka et al. [30] have de-
veloped a thermodynamic model for asphaltene precipitation 0.5 n-C5
n-C6
based on the assumption that the oil is a heterogeneous solu- 0.4 points: experimental data n-C7
tion of a polymer (asphaltenes) and oil. The asphaltenes and lines: calcualted n-C8
the C 7+ part of the oil are presented by a continuous model (as 0.3 n-C10
discussed in Chapter 4) and for each asphaltenes component 0 5 10 15 20 25 30 35
the equilibrium relation has been applied as Solvent Dilution Ratio (mL solvent added/g oil)
FIG. 9.22—Calculated versus experimental
(9.39) S S L L
ˆ μ (T, P, x ) = ˆμ (T, P, x ) i = 1, ... , N A
Ai Ai Ai Ai amount of asphaltene precipitated by various
n-alkanes solvents added to Suffield crude oil.
where ˆμ S and ˆμ L are chemical potentials of ith component
Ai Ai Taken with permission from Ref. [39].
of asphaltene in the solid and liquid phase, respectively. Sim-
ilarly x S and x L are the composition of asphaltene compo-
Ai Ai specific gravity of 0.952 and average molecular weight of 360
S
nents in the solid and liquid phases. The sum x is unity but with resin and asphaltene contents of 8 and 13 wt%, respec-
Ai
L
L
the sum x is equal to x the mole fraction of asphaltenes
Ai A tively. Effects of temperature and pressure on asphaltene and
in the liquid phase after precipitation. N A is the number of solid precipitation were discussed in Section 9.3.1.
asphaltenes components determined from distribution model To avoid complex calculations for quick and simple esti-
as it was discussed in Chapter 4. In this model, the solid phase mation of asphaltene and resin contents of crude oils, at-
is a mixture of N A pseudocomponents for asphaltenes. tempts were made to develop empirical correlations in terms
In this thermodynamic model, several parameters for as- of readily available parameters similar to those presented in
phaltenes are needed that include molecular weight (M A ), Section 3.5.1.2 for composition of petroleum fractions. Be-
mass density (ρ A ), binary interaction coefficient between as- cause of the complex nature of asphaltenes and wide range
phaltene and asphaltene-free crude (k AB ), and the asphaltene
L
of compounds available in a crude, such attempts were not
solubility parameter in liquid phase δ A . As discussed in Sec- as successful as those developed for narrow-boiling range
tion 9.3.1, in lieu of experimental data on M A and ρ A they can petroleum fractions. However, Ghuraiba [53] developed the
3
be assumed as 1000 and 1.1 g/cm , respectively. Kawanaka following simple correlation based on limited data collected
et al. [30] recommends the following relations for calculation from the literature for prediction of asphaltene and resin con-
L
of δ and k AB as a function of temperature:
A tents of crude oils:
−4
L
(9.40) δ = 12.66(1 − 8.28 × 10 T)
A wt% of asphaltene or resin in crude oil = a + bR i + cSG
(9.41) k AB =−7.8109 × 10 −3 + 3.8852 × 10 −5 M B (9.42)
3 0.5
L
where δ is the asphaltene solubility parameter in (cal/cm ) where R i is the refractivity intercept defined in Eq. (2.14)
A
and T is temperature in kelvin. M B is the molecular weight of as R i = n 20 − d 20 /2. Amounts of asphaltenes and resin in a
S
S
asphaltene-free crude oil. To calculate ˆμ , values of δ and crude mainly depend on the composition of the crude. In Sec-
Ai
A
S
S
S
ρ are needed. δ can be calculated from Eq. (6.154) and ρ is tion 3.5.1.2, parameters R i and SG were used to predict the
A A A
L
assumed the same as ρ . It should be noted that in these rela- composition of petroleum fractions. Calculation of n 20 and d 20
A
tions asphaltene-free crude refers to the liquid phase in equi- for a crude is not as accurate as for a fraction since the crude
librium with precipitated solid phase, which include added has a very wide boiling point range. For this reason, the above
solvent (i.e., C 3 , n-C 5 ,or n-C 7 ) and the original crude. The equation gives only an approximate value of asphaltene and
phase diagram shown in Fig. 9.11 was developed based on resin contents. Coefficients a, b, and c in Eq. (9.42) are given in
this compositional model [30]. Equation (9.40) gives value of Table 9.11. These coefficients have been determined based on
9.5 (cal/cm ) at 25 C, which is consistent with the value re- the calculation of n 20 and d 20 from Eq. (4.7) and Table 4.5. Only
3 0.5
◦
ported by other investigators. Equation (9.40) is named after M is required for calculation of these two properties. If M is
Hirschberg who originally proposed the relation [52]. not available it may be estimated from other properties such
Most of the thermodynamic models discussed in this sec- as viscosity and SG (i.e., Eq. (2.52) or reversed form of Eq.
tion predict data with good accuracy when the adjustable (4.7) and Table 4.5). The above correlation generally predicts
parameters in the model are determined from experimen- amount of asphaltene and resin contents with absolute devi-
tal data on asphaltene precipitation. Results of a thermody- ation of 1.5–2 wt%. Experimental data points for resin con-
namic model based on the colloidal model and SAFT theory tents were very limited and for this reason predicted values
for Suffield crude oil are shown in Fig. 9.22. The crude has must be taken with caution. Data to develop these correlations
--`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
Copyright ASTM International
Provided by IHS Markit under license with ASTM Licensee=International Dealers Demo/2222333001, User=Anggiansah, Erick
No reproduction or networking permitted without license from IHS Not for Resale, 08/26/2021 21:56:35 MDT