1. Prediction of Asphaltene Self-Precipitation in Dead Crude Oil
M. Bayat, M. Sattarin,* and M. Teymouri
Petroleum Refining DiVision, Research Institute of Petroleum Industry (RIPI), Tehran, Iran
ReceiVed September 8, 2007. ReVised Manuscript ReceiVed NoVember 21, 2007
Asphaltene precipitation in dead crude oil can occur not only by adding saturated solvents such as heptane
but also occurs at elevated temperatures. In this work, the relation between asphaltene self-precipitation onset
and refractive index (RI) at elevated temperatures is investigated. Experimental measurements of RI for three
crude oils are reported at different temperatures. Determination of RI at the onset of precipitation showed that
that precipitation occurred at a characteristic RI of 1.42 for each crude oil. The asphaltene content of these
samples were in the range 1–11.6 wt %. The sizes of the asphaltene particles formed at elevated temperature
were smaller than those formed upon solvents addition.
Introduction
Asphaltene is known to flocculate in crude oil and is thought
to be responsible for its instability. Crude oil stability depends
on composition, pressure, and temperature. The effect of
composition and pressure on asphaltene precipitation is generally
believed to be stronger than the effect of temperature.1 However,
it appears that composition and origin of the oil affect instability
of the oil more than the actual amount of asphaltenes.2
Instability and chemical composition incompatibly of crude
oils are defined as the colloidal instability index (CII), which is
the ratio of the sum of asphaltenes and saturated hydrocarbons
to the sum of the resins and aromatics.3
CII ) (asphaltenes + saturates) ⁄ (aromatics + resins)
CII of larger than one means unstable crude oil, and precipitation
of asphaltene is likely to occur.4
CII can also be used for determination of crude oil stability
at room temperature. However, since asphaltene precipitation
does not occur just at CII larger than 1, therefore it cannot be
used as a quantitative measure to determine asphaltene precipi-
tation onset.
The equilibrium of a well-peptized asphaltene system can
easily be disturbed by addition of a paraffinic solvent, application
of heat, oxidation, or ultraviolet irradiation. In each case, the
chemical composition is altered and the aromaticity is decreased,
thereby causing a disruption of the equilibrium of colloidal
system.3
Addition of n-alkane, as a solvent, to stock-tank oils has often
been used for understanding the asphaltene precipitation phe-
nomena. Hirschberg et al.5 and Chang et al.6 carried out their
experiments by adding a series of n-alkanes from n-pentane to
n-decane to the oil samples and reported precipitation onset
composition, amount of material precipitated, and solubility
properties of asphaltene.
Even though there is no existing standard method to determine
oil stability, especially at elevated temperatures, different
methods such as spot test,7 P value,8 and turbiscan9 are being
used to compare the stability of different oils. However, outcome
of a spot test is visual, and hence, the result may be somewhat
inaccurate. Even though, determination of P value can show
the total amount of n-heptane that can be added to the oil before
it becomes unstable, it is time-consuming and is only used for
comparing the stability of different oils. Ostlund et al.9
investigated oil stability by utilizing an instrument consisting
of an optical scanning device (turbiscan). This method was found
to be quick and sensitive, so that the detection of a very small
difference in stability is possible.
Buckley et al.10 used an improved method for predicting
asphaltene precipitation onset according to refractive index (RI).
Measurements of RI at the onset of precipitation have shown
that for each oil-precipitant combination the onset occurs at a
characteristic RI of 1.42-1.44 and is independent of asphaltene
content.
On the basis of Buckley’s works, asphaltene precipitation
onset at characteristic RI between 1.42 and 1.44 and its
independence of crude oil type, asphaltene content, and nearly
solvent type may be attributed to the transmission of asphaltene
from liquid bulk to solid phase.
Asphaltene precipitation occurs in crude oil not only by
adding n-alkane as a solvent but also happens by increasing
temperature, which may be the cause of fouling of crude oil
* Corresponding author: e-mail sattarinm@ripi.ir.
(1) Leontaritis, K. J.; Mansoori, G. A. Asphaltene Flocculation During
oil Prduction and Processing: A Thermodynamic Colloidal Model, SPE
Internartional Symposium on Oil Field Chemistry, San Antonio, TX, Feb
1987, SPE 16258.
(2) Andersen, S. I. Dissolution of Solid Boscan Asphaltenes in Mixed
Solvents. Fuel Sci. Technol. Int. 1994, 12, 51.
(3) Mushrush, G. W.; Speight, J. G. Petroleum Products: Instability and
Incompatibility; Taylor and Francis: London, 1995; p 298.
(4) Barker, K. Understanding Paraffin and Asphaltene Problems in Oil
and Gas Wells, PTTC Workshop, July 2003.
(5) Hirschberg, A. L.; de Jong, N. J.; Schipper, B. A.; Meyers, J. G.
Influence of Temperature and Pressure on Asphaltene Flocculation. Soc.
Pet. Eng. J. 1984, 283.
(6) Chang, F.; Sarthi, P.; Jones, R. Modeling of Asphaltene and Wax
Precipitation; U.S. Department of Energy, Jan 1991; Topical Report NIPER
498.
(7) ASTM D-4740, Standard Test Method for Cleanliness and Compat-
ibility of Residual Fuel by Spot Test, Annual Book of ASTM Standards,
2002; Vol. 5.2.
(8) Heithaus, J. J. Measurement and Significance of Asphaltene Pepti-
zation. J. Inst. Pet. 1962, 48, 45.
(9) Östlund, J. A.; Russel, T.; Walker, S.; Hakansson, R.; Greek, L.;
Richards, G. Evalution of a Novel Method to Study Oil Stability (Internet).
(10) Buckley, J. S.; Hirasaki, G. J.; Liu, Y.; Von Drasek, S.; Wang,
J. X.; Gill, B. S. Asphaltene Precipitation and Solvent Properties of Crude
Oils. Pet. Sci. Technol. 1998, 16, 251.
Energy & Fuels 2008, 22, 583–586 583
10.1021/ef700536z CCC: $40.75  2008 American Chemical Society
Published on Web 12/19/2007

2. heat exchangers. In this work, the onset temperature of self-
precipitation of asphaltene in relation with characteristic RI has
been investigated. Increasing temperature caused self-precipita-
tion of asphaltene to occur at RI of 1.42, which confirmed
Buckley’s work.
Experiment
Materials and Equipment. Analytical grade heptane (42 wt %
normal heptane and 58 wt % isoheptane, benzene, and toluene free)
was used for preparing crude oil-heptane mixtures.
A high-pressure reactor, equipped with stirrer (Parr model 4560
mini bench-top reactor), was used for heating of crude oil.
A DUR refractometer equipped with a sodium lamp was used
for measurement of RI according to the ASTM D-1218 test
method.11 A visual analyzer, Quantimet model 570, equipped with
an optical microscope was used for taking photographs of the
samples. Asphaltene, resin, saturate, and aromatic contents were
determined by the SARA method,12 and density was measured
according to the ASTM D-5002 test method.13
Procedure. To determine crude oil RI, several mixtures of crude
oil and heptane were prepared with ratio of heptane to crude of
10, 20, 35, or 40 by weight. RI of these mixtures was measured at
temperatures of 20, 30, 40, and 50 °C.
An optical microscope was used for detecting asphaltene
precipitation. One drop of each mixture was spotted on a lamella
and placed under a microscope for observation.
To determine the onset temperature of self-precipitation, 150 mL
of each crude oil was placed into the pressure reactor and gradually
heated to elevated temperatures while the stirrer speed was set at
300 rpm. Sampling was done through a dipped pipe, after each
5-10 °C temperature increase. Immediately after sampling, a drop
of heated crude oil was placed on a lamella for detecting asphaltene
precipitates by an optical microscope. In addition by microscopic
observation, the rheology of crude oil layer between two glass plates
was investigated. Formation of separate smudge was another
evidence, for showing asphaltene self-precipitation (Figure 6).
Results and Discussion
Table 1 shows some specifications of three different crude
oils. Asphaltene content of these crude oils are varied from low
(1.0 wt %) to high (11.6 wt %). Also, CII of crude oils A, B,
and C are 0.77, 0.84, and 1.23, respectively. This means that
crude oil C is in its unstable form. According to Buckley’s
results, RI of different ratios of crude oilA/heptane mixtures
was measured at 20 °C, and a plot of RI vs wt % of crude oil
was drawn. The result is shown in Figure 1. According to Figure
1, by increasing the crude oil content, refractive index of the
mixture is increased and reaches 1.42 at 33 wt % of crude oil,
and hence, according to Buckley,10 precipitation of asphaltene
is expected to occur.
Figure 2 shows optical microscopic photographs of the crude
oil A/heptane mixtures with different percentage of the crude
oil. These pictures were identical for mixtures containing 10,
20, 30, and 31.5 wt % of crude oil (Figure 2a). These mixtures
contain some particle with average size of 2 µm. For the mixture
containing 33 wt % crude oil (Figure 2b), large dark aggregates
with average size of 50 µm start to appear and the number of
aggregates increases for the mixture containing 40 wt % crude
oil. According to the above results, the onset of asphaltene
precipitation occurs at refractive index of 1.42, which exactly
matches Buckley’s results.
To detect self-precipitation of asphaltene in crude oil and its
relation to RI, measurement of RI at different temperature is
required. On the other hand, because of the dark color of the
crude oil, measuring its refractive index is almost impossible.
To overcome this problem, varying concentrations of crude oil
in heptane were prepared and their refractive index was
measured. A function of RI (FRI), according to eq 1, was plotted
against concentration of the oil and RI of crude oil was obtained
by extrapolation.
FRI ) (RI2
- 1) ⁄ (2 + RI2
) (1)
Figure 3a-c shows plots of FRI vs concentration (wt %) of
crude oils A, B, and C at temperatures of 20, 30, and 50 °C,
and the crude oil RI that were obtained by extrapolation is shown
in Table 2.
Guseva et al.14 found a linear relationship between RI of crude
oil and temperature. This relation was found to hold for crude
oils A, B, and C at low temperatures and seems to be true at
high temperatures, too (Figure 4).
(11) ASTM D-1218, Standard Test Method for Refractive Index and
Refractive Dispersion of Hydrocarbon Liquids, Annual Book of ASTM
Standards, 2002; Vol. 5.1.
(12) Wauquier, J. P. Crude Oil Petroleum Products, Process Flowsheets;
Institut Francais du Petrole: Paris, France; 1995; p 45.
(13) ASTM D-5002, Standard Test Methode for Density and Relative
Density of Crude Oils by Digital Density Analayzer, Annual Book of ASTM
Standards, 2002; Vol. 5.3.
(14) Guseva, A. N.; Leifman, I. E. Study of Contraction of Paraffin
Waxes Based on Refractometric Data. Chem. Technol. Fuels Oils 1966, 2,
728.
Table 1. Specification of Crude Oil Samples
specification
crude oil
A
crude oil
B
crude oil
C
API 18.9 25.8 33.5
kinematics viscosity, cSt, at 40 °C 219.7 16.28 6.37
asphaltene content, wt % 11.6 5.5 1.0
resin content, wt % 14.4 17.9 5.0
saturated content, wt % 32 33.7 54
aromatic content, wt % 42 37.9 39.5
CII 0.77 0.84 1.23
Figure 1. Refractive index of crude oil A with n-heptane at 20 °C.
Figure 2. Optical microscopic photographs of crude oil A-heptane
mixture, magnitude 5000: (a) containing 10, 20, 30, and 31.5 wt %
crude oil; (b) crude oil A-heptane mixture containing 33 wt % crude
oil.
584 Energy & Fuels, Vol. 22, No. 1, 2008 Bayat et al.

3. Using the curve-fitting method, eqs 2a, 2b, and 2c were
derived respectively for refractive index of crude oils A, B, and
C as a function of temperature.
RI(crude oil A) ) 1.5331 - 0.0006T (°C) (2a)
RI(crude oil B) ) 1.4961 - 0.0006T (°C) (2b)
RI(crude oil C) ) 1.4944 - 0.0006T (°C) (2c)
These equations show that a refractive index of 1.42 is obtained
for crude oils A, B, and C at temperatures of 188.5, 126.8, and
124 °C, respectively. Therefore, it can be expected that self-
precipitation of asphaltene on these crude oils starts at these
temperatures. Microscopic inspection of crude oil samples shows
that, when temperature reaches 200, 130, and 115 °C for crude
oils A, B, and C, respectively, a small dark spot is observed in
the layer between two glass plates (Figure 5). These tempera-
tures are somewhat higher than those predicted by the above
equations for crude oils A and B. This difference could be the
result of either temperature drop of crude oil flashing through
Figure 3. (a) FRI of mixtures of crude oil A with n-heptane at 20, 30, and 50 °C. (b) FRI of mixtures of crude oil B with n-heptane at 20, 30, and
50 °C. (c) FRI of mixtures of crude oil C with n-heptane at 20, 30, and 50 °C.
Table 2. Refractive Index of Crude Oils A, B, and C at
Temperatures 20, 30, and 50 °C
RI
temp (°C) crude oil A crude oil B crude oil C
20 1.5216 1.4834 1.4834
30 1.5148 1.4766 1.4767
50 1.5035 1.4641 1.4641
Asphaltene Self-Precipitation in Dead Crude Oil Energy & Fuels, Vol. 22, No. 1, 2008 585

4. sampling or low sensitivity of visual observation to detect onset
of self-precipitation or both phenomena. The predicted temper-
ature for crude oil C is a little lower than real one, which may
relate to instability of crude oil C. However, the size of these
spots is reduced as the temperature drops and disappears
completely at temperatures below the ones predicted by the
equations. These dark points are asphaltene particles with
average size of less than 10 µm, which are much smaller than
the particles formed in the presence of heptane, at room
temperature. In addition, adsorptions of asphaltene particles on
the wall of sampling glass bottle can be observed only at
precipitation temperatures (Figure 6). The number of observed
asphaltene particles increases when temperatures goes higher
than onset temperature for crude oils A and B, which shows
continuation of asphaltene precipitation. Crude oil C contains
only a little asphaltene content, and nearly all of its asphaltene
precipitates at the beginning of the precipitation; therefore,
continuation of asphaltene precipitation was not found for this
crude oil.
Therefore, the above results show that self-precipitation of
asphaltene occurs nearly at temperatures at which according to
Buckley RI reaches 1.42.
These observations suggest that onset temperature of self-
precipitation of asphaltene may be predict by measuring of crude
oil refractive index at least at three different temperatures below
50 °C.
Since adsorption of asphaltene on the tube surface of the crude
oil preheaters and its subsequent oxidation to cock is the origin
of fouling, and it occurs only at temperature above self-
precipitation temperature, therefore determination of this tem-
perature can be helpful in prediction of crude oil heat exchangers
fouling.
Conclusions
Asphaltene precipitation in dead crude oils can occur not only
by adding saturated solvents such as heptane but also occurs at
elevated temperatures. Self-precipitation of asphaltene in crude oils
with different compositions occurs at different temperatures. This
phenomenon is only dependent on the composition compatibility
of the oil, and it does not depend on the asphaltene content.
Because of the composition compatibility, heavy crude oils
with high asphaltene content may be more stable than light ones
with low asphaltene. The refractive index of crude oil is a
suitable parameter for determination of asphaltene precipitation
onset. Self-precipitation of asphaltene occurs at characteristic
RI of 1.42. Therefore, the onset temperature of self-precipitation
can be determined using a linear curve of RI vs temperature.
This curve can be obtained by measuring RI at three different
temperatures below 50 °C. This is a fast and easy method for
determination of onset temperature of self-precipitation of
asphaltene.
EF700536Z
Figure 4. RI of crude oils A, B, and C versus temperature.
Figure 5. Image of the crude oil layer between two glass plates at
onset temperature of asphaltene precipitation.
Figure 6. Image of the crude oil layer on the wall of sampling bottle:
(a) before asphaltene precipitation; (b) after asphaltene precipitation.
586 Energy & Fuels, Vol. 22, No. 1, 2008 Bayat et al.