Page 394 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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374 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
petroleum fractions as pellets of 34–40 microns and are main-
tained in suspension by resins [16, 18, 19]. Petroleum fluids compound such as n-C 7 is added to a petroleum mixture, the
asphaltene components (heavy aromatics) begin to precipi-
with low-resin contents or under specific conditions of tem- tate. If propane is added to the same oil more asphaltenes
perature, pressure, and LMP concentration may demonstrate precipitate as the difference in solubilities of C 3 -asphaltene
asphaltene deposition in oil-producing wells. Asphaltene de- is greater than that of nC 7 -asphaltene. Addition of an aro-
position may also be attributed to the reduction of pressure matic hydrocarbon such as benzene will not cause precip-
in the reservoirs or due to addition of solvents as in the case itation of asphaltic compounds as both are aromatics and
of CO 2 injection in enhanced oil recovery (EOR) processes. similar in structure; therefore they are more soluble in each
Resins play a critical role in the solubility of the asphaltenes other in comparison with LMP hydrocarbons. When three
and must be present for the asphaltenes to remain in the so- parameters for a petroleum fluid change, heavy deposition
lution. Although the exact mechanism is unknown, current may occur. These parameters are temperature, pressure, and
theory states that resins act as mutual solvent or form sta- fluid composition that determine location of state of a sys-
bility peptide bonds with asphaltenes [16]. Both oils and as- tem on the PT phase diagram of the fluid mixture. Precipita-
phaltenes are soluble in resins. Structure of resins is not well tion of a solid from liquid phase is a matter of solid–liquid
known, but it contains molecules with aromatic as well as equilibrium (SLE) with fundamental relations introduced in
naphthenic rings. Resins can be separated from oil by ASTM Sections 6.6.6 and 6.8.3.
D 2006 method. Resins are soluble in n-pentane or n-heptane Estimation of the amount of asphaltene and resins in crude
(while asphaltenes are not) and can be adsorbed on surface- oils and derived fractions is very important in design and
active material such as alumina. Resins when separated are operation of petroleum-related industries. As experimental
red to brown semisolids and can be desorbed by a solvent determination of asphaltene or resin content of various oils
such as pyridine or a benzene/methanol mixed solvent [15]. is time-consuming and costly, reliable methods to estimate
The amount of sulfur in asphaltenes is more than that of asphaltene and resin contents from easily measurable or
resins and sulfur content of resins is more than that of oils available parameters are useful. Waxes are insoluble in 1:2
[15]. Oils with higher sulfur contents have higher asphaltene mixture of acetone and methylene chloride. Resins are insol-
content. Approximate values of molecular weight, H/C weight uble in 80:20 mixture of isobutyl alcohol–cyclohexane and as-
ratio, molar volume, and molecular diameter of asphaltenes, phaltenes are insoluble in hexane [15]. ASTM D 4124 method
resins and oils are given in Table 9.6. In the absence of actual uses n-heptane to separate asphaltenes from oils. Other ASTM
f
data typical values of M, d 25 , H , and T M are also given for test methods for separation of asphaltenes include D 893 for
i
monomeric asphaltene separated by n-heptane as suggested separation of insolubles in lubricating oils [21]. The most
by Pan and Firoozabadi [20]. In general M asph. > M res. > M wax widely used test method for determination of asphaltene con-
and (H/C) wax > (H/C) resi > (H/C) asph . Waxes have H/C atomic tent of crude oils is IP 143 [22]. Asphaltene proportions in a
ratio of 2–2.1 greater than those of resins and asphaltenes typical petroleum residua is shown in Fig. 9.6. Since these
because they are mainly paraffinic. are basically polar compounds with very large molecules,
In general, crude oil asphaltene content increases with de- most of correlations developed for typical petroleum fractions
crease in the API gravity (or increase in its density) and for the and hydrocarbons fail when applied to such materials. Meth-
residues the asphlatene content increases with increase in car- ods developed for polymeric solutions are more applicable to
bon residue. Approximately, when Conradson carbon residue asphaltic oils as shown in Section 7.6.5.4.
increases from 3 to 20%, asphaltene content increases from 5 Complexity and significance of asphaltenes and resins in
to 20% by weight [15]. For crude oils when the carbon residue petroleum residua is clearly shown in Fig. 9.6. Speight [15]
increases from 0 to 40 wt%, asphaltene, sulfur, and nitrogen as well as Goual and Firoozabadi [23] considered a petroleum
contents increase from 0 to 40, 10, and 1.0, respectively [15]. fluid as a mixture of primarily three species: asphaltenes,
Oils with asphaltene contents of about 20 and 40 wt% exhibit resins, and oils. They assumed that while the oil component
6
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viscosities of about 5 × 10 and 10 × 10 poises, respectively. is nonpolar, resins and asphaltene components are polar. The
As discussed in Section 6.8.2.2, generally two substances degrees of polarities of asphaltenes and resins for several
with different structures are not very soluble in each other. oils were determined by measuring dipole moment. They re-
For this reason, when a low-molecular-weight n-paraffin ported that while dipole moment of oil component of various
crudes is usually less than 0.7 debye (D) and for many oils
zero, the dipole moment of resins is within 2–3 D and for
asphaltenes (separated by n-C 7 ) is within the range of 4–8 D.
TABLE 9.6—Properties of typical asphaltenes, resins and oils.
Hydrocarbons M H% H/C V d, ˚ A D Dipole moment of waxy oils is zero, while for asphaltic crudes
Asphaltene 1000–5000 9.2–10.5 1.0–1.4 900 14.2 4–8 is about 0.7 D. Therefore, one may determine degree of as-
Resin 800–1000 10.5–12.5 1.4–1.7 700 13 2–3 phaltene content of oil through measuring dipole moment.
Oil 200–600 12.5–13.1 1.7–1.8 200–500 8–12 0–0.7 Values of dipole moments of some pure compounds are given
M is molecular weight in g/mol. H% is the hydrogen content in wt%. H/C is in Table 9.7. n-Paraffins have dipole moment of zero, while
the hydrogen-to-carbon atomic ratio. V is the liquid molar volume at 25 C.
◦
d is molecular diameter calculated from average molar volume in which for hydrocarbons with double bonds or branched hydrocarbons
methane molecules is about 4 ˚ A(1 ˚ A = 10 −10 m). D is the dipole moment in have higher degree of polarity. Presence of heteroatoms such
Debye. These values are approximate and represent properties of typical as- as N or O significantly increases degrees of polarity.
phaltenes and oils. For practical calculations for resins one can assume M =
800 g/mol and for a typical monomeric asphaltene separated by n-heptane ap- The problems associated with asphaltene deposition are
proximate values of some properties are as follows: M = 1000 g/mol. Density even more severe than those associated with wax deposition.
3
of liquid ≈ density of solid ≈ 1.1 g/cm . Enthalpy of fusion at the melting Asphaltene also affects the wettability of reservoir fluid on
point: H M = 7300 cal/mol, melting point: T M = 583 K. Data source: Pan and
Firoozabadi [20]. solid surface of reservoir. Asphaltene may cause wettability

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