Petroleum Science and Technology, 29:2432–2440, 2011

Copyright © Taylor & Francis Group, LLC

ISSN: 1091-6466 print/1532-2459 online

DOI: 10.1080/10916461003735061

The Effect of Solvent Nature and Dispersant

Performance on Asphaltene Precipitation from

Diluted Solutions of Instable Crude Oil

J. C. PEREIRA,1;2 J. DELGADO-LINARES,2 A. BRIONES,2

M. GUEVARA,2 C. SCORZZA,2 AND J.-L. SALAGER2

1Lab. Petróleo, Hidrocarburos y Derivados (PHD), Universidad de Carabobo,

Facultad Experimental de Ciencias y Tecnología, Departamento de Química,

Valencia, Venezuela2Laboratorio de Formulación, Interfases, Reología y Procesos, Facultad de

Ingeniería, Escuela de Ingeniería Química, Mérida, Venezuela

Abstract The asphaltene precipitation experiments were studied on El Furrial crudeoil from western Venezuela, which is known to exhibit serious instability problems.

A Turbiscan backscattering apparatus was used to evaluate the precipitation of as-phaltenes with different solvents. The transmittance variation with time was studied

as the crude was diluted with heptane, pentane, and cyclohexane. Linear alkanes-containing systems exhibit a two-stage behavior, whereas only one is found when

diluting with cyclohexane. Dispersing agents were tested by using the precipitateheight as a criterion of effectiveness. Results are reported for ethoxylated nonylphenols

and a commercial dispersant.

Keywords asphaltene, crude oil, dispersant, precipitation

Introduction

During crude oil production and refining, unwanted asphaltene precipitation can take

place in pipelines, separation equipment, and conversion units. Asphaltenes are generally

defined as the petroleum fraction soluble in aromatic solvents (i.e., benzene and toluene)

but insoluble in n-alkanes (i.e., n-pentane and n-heptane; Acevedo et al., 1985; Speight

2004; Labrador et al., 2007).

The precipitation of asphaltenes has been shown to be intimately related to their

behavior in solution, which has been modeled according to liquid–liquid or solid–liquid

equilibria (Qin et al., 2000; Angle et al., 2005).

Most publications on asphaltenes considered some kind of aggregation and aggregate

peptization by resins (Pfeiffer and Saal, 1940; Priyanto et al., 2001; Pereira et al., 2007).

This aggregation often boosts the asphaltene films adsorbed at the interface that stabilize

water-in-crude emulsions (McLean and Kilpatrick, 1997).

Ekulu and coworkers (2005) have studied the microstructure of asphaltenes in crude

oil with differential scanning calorimetry. Their results suggest that the thermal effects

produced when the crude is mixed with n-heptane or toluene are characteristics of the

Address correspondence to Juan Carlos Pereira, Lab. PHD, FACYT, Universidad de Carabobo,Avenue Salvador Allende, 2001 Carabobo, Venezuela. E-mail: jcpereir@uc.edu.ve

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Effect of Solvent on Asphaltene Precipitation 2433

solvent. This conforms with the well-known influence of the medium on the aggregation

of asphaltenes (Yudin et al., 1998; Castillo et al., 2001; Ortega-Rodríguez et al., 2003).

Synthetic dispersing agents have been used as stabilizers of asphaltenes in crude oil,

which is essentially the role played by some species of resins to stop the aggregation at

some aggregate size and avoid precipitation. Carnahan et al. (1999) found that Boscan

crude resins are better stabilizers than those from Hamaca crude. An increase in stabi-

lization results from the addition of Boscan resins to Hamaca crude. A correlation was

suggested between the resin polarity and its stabilizing capacity.

It has been shown that the effectiveness of asphaltenes dispersants depend on their

molecular structures (Wiehe and Jermansen, 2003). Deasphalted oil (DO) and resins have

been used by Al-Sahhaf et al. (2002) as inhibitors of the asphaltene precipitation.

Various surfactant molecules were tested for the same application, and a correlation

was found between their effectiveness as asphaltene dispersants and the acidity of their

polar group (Rogel and León, 2001; Rocha et al., 2006).

Recent studies on the aggregation and precipitation of asphaltenes (Marugán et al.,

2009) were based on the variation of light reflectance and transmittance versus time.

Kraiwattanawong et al. (2009) related the effectiveness of several asphaltene dispersants

to the average transmittance of the blend (crude oil, dispersants, n-heptane), which is

essentially a direct measurement of the sample turbidity.

In the present article, the variation of transmittance versus time will be used to

study the effect of solvent type. Additionally, the influence of synthetic dispersants on

asphaltene precipitation was monitored for the height of the precipitate.

Materials and Methods

Crude Oil

Asphaltenes and maltenes were extracted from El Furrial petroleum, a crude oil from

western Venezuela, which is known for its strong tendency to precipitate asphaltenes.

Some properties of this crude oil are presented in Table 1 (Pereira, 2000).

Asphaltenes Extraction

A 10 g sample of crude oil was diluted with 400 mL of n-heptane. The sample was

stirred for 1 hr and then left to rest for 24 hr at ambient temperature.

The precipitate was filtered and washed with hot n-heptane in a Soxhlet extractor until

the solvent became colorless. The remaining n-heptane was eliminated by evaporation

Table 1

Properties of El Furrial crude oil and its asphaltene (Pereira, 2000)

Elemental analysis, wt%a

ıAPI

Asphaltenes

in the crude,

wt% C H Ob N S H/C

21 7.6 85.5 6.9 2.5 1.73 3.4 0.97

aIn asphaltene.bIs calculated by difference with the other elements (C, H, N, and S).

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under vacuum (18 mmHg) at 60ıC until a constant mass was reached. Asphaltenes were

stored in dark glass flasks to prevent oxidation.

Synthetic Dispersants

Lauric alcohol was donated by Lipesa SA (El Tigre, Anzoategui, Venezuela), and

polyethoxylated nonylphenol nonionic surfactants with different degrees of ethoxylation

(labeled nonyl-phenol [NPEON] where EON is the average number of ethylene oxide

group per molecule) were obtained from Witco Chemicals (Houston, TX). A commercial

asphaltene dispersant of the ethoxylated nonylphenol-formol resin was supplied by

Resinas Multiples SA (Cagua Aragua, Venezuela). The dispersants were first dissolved

in toluene before being added to the crude according to the procedures discussed next.

Sample Preparation

Solutions containing toluene and crude oil (50:50 by volume) were prepared to achieve

a low-viscosity fluid. Thirty milliliters of n-heptane was mixed with 0.16 mL of the

toluene/crude oil solution and 0.16 mL of dispersant solution to be tested. The dispersant

was added so that its concentration in the final mixture (containing crude C toluene C

heptane C dispersant) was 1 and 25%. The latter value was only for dodecanol, which is

effective only in very high proportions. A part of the mixture (10 mL) was then poured

into a glass tube (Turbiscan cell) and stirred for 5 min in an ultrasonic bath. Precipitation

experiments were carried out as indicated next.

Experiments of Asphaltene Precipitation

Asphaltene precipitation was determined by measuring the variation of transmittance

versus the sample height; see Figure 1. These measurements were carried out as a function

of time, with a Turbiscan MA2000 apparatus from Formulaction (L’Union, France).

The curves of percentage transmittance (%T) versus height of test tube transmittance of

asphaltene solution. Each curve corresponds to a particular time (see column on right-

hand side of figure). The values of transmittance increased as a function of time due

to a reduction in the amount of asphaltene in solution. This value of %T was constant

throughout the tube test. Furthermore, it is shown in the circle the started curve %T for

each system, which is indicative of an increase in the height in the asphaltene precipitate

for each system.

Initially, the transmittance was essentially zero, as a consequence of the high opacity

of the sample. When asphaltenes begin to precipitate, the liquid becomes clearer and the

transmittance increases in the tube, whereas the precipitate settles at the bottom. This is

the principle of the technique proposed in recent publications to evaluate the performance

of asphaltene dispersants (Kraiwattanawong et al., 2009). The transmittance of the sample

is measured at some selected time intervals by scanning a nephelometric device from the

top to the bottom of the tube. The transmittance of the solution is essentially the same

everywhere in the upper part of the tube. The asphaltene precipitate that settles at the

bottom of the tube blocks the light beam and keeps the transmittance at zero; this allows

the determination of the precipitate height, which is used here to estimate the effectiveness

of the tested dispersing agent. The experiments were carried out at ambient temperature

(22 ˙ 2ıC).

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Effect of Solvent on Asphaltene Precipitation 2435

Figure 1. Curves of percentage transmittance (%T) versus height (mm) of the test tube. Each curve

corresponds to a particular time (see column on right-hand side). The %T value at solution bulk is

constant throughout the tube test. (color figure available online)

Results and Discussion

Effect of the Medium

Figure 2 shows the variation of transmittance (%T) of the solution as a function of time

(hr) for systems containing El Furrial crude oil and different diluents: heptane, pentane,

and cyclohexane. Although toluene was added to the crude at first (50:50 volume) in order

to reduce its original viscosity without precipitating asphaltenes, the amount of toluene in

the final system was negligible; that is, less than 0.3%. There was a considerable excess

of diluent (375:1 vol) with respect to the crude (and toluene) to warrant the precipitation

of asphaltenes and to result in a low optical density of the liquid. Such an amount of

diluent was found to be appropriate to monitor the asphaltene precipitation process. The

zero time in Figure 2 actually corresponds to the first scan, which was carried out a few

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Figure 2. Asphaltene precipitation from El Furrial crude oil by different solvents through the

measurement of the solution transmittance versus time. Each point of %T corresponds to the value

of transmittance in solution.

minutes after the diluent was added, stirred in an ultrasonic bath for 5 min, and poured

into the tube. The non-zero (but close to zero) transmittance at zero time indicates that

the dilution provided a suitable initial turbidity for the purpose of the technique.

Each one of the curves plotted in Figure 2 indicates an increase in transmittance of

the upper solution as time elapses. The heptane and pentane curves were found to rise

slowly over the first 300 hr and to exhibit some irregularities that are not very significant

but are definitely larger than the experimental error. This could indicate the presence of

different stages in the precipitation process.

Cyclohexane exhibited a much quicker and smoother variation with time. A plateau

value was attained after 20–25 hr and corresponded to about 60% transmittance; that is,

much less than the value attained with heptane after 300 hr.

The initial increase in transmittance is shown in Figure 3, which is a blowup of

Figure 2 data during the first hours. The initial slope may be considered as an indirect

measurement of the initial precipitations, which is seen to be much faster for cyclohexane

than for both n-alkanes; that is, 10 times or more in the 1- to 4-hr intervals. Because

the agglomeration-precipitation process is likely to start with an association into small

size agglomerates sometimes referred to as aggregation, it may be conjectured that

cyclohexane favors such a first step, which results in the reduction of the scattering centers

from molecules to core aggregates and thus a reduction of the turbidity. Cyclohexane is

a better asphaltenes solvent than n-alkanes; hence, it might keep a part of the asphaltenes

in a molecular state while favoring the association of only some other part, probably the

higher molecular weight species. The lower molecular weight asphaltenes would hence

remain as single molecules or in a very low aggregation (dimer or trimer) state and

be responsible for the relatively high turbidity that remains after 20–25 hr, when some

equilibrium appears to be reached.

With n-alkanes, the initial precipitation is much slower, thus indicating that the for-

mation of the first aggregates is delayed. This may be explained by the fact that the resins

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Effect of Solvent on Asphaltene Precipitation 2437

Figure 3. Initial kinetics for three solvents used. Data correspond to initial values shown in

Figure 2.

are less soluble in n-alkanes than in cyclohexane and hence are more likely to interact

with asphaltenes to keep them as smaller aggregates. The fact that the precipitation is

even slower for pentane than for heptane is consistent with this interpretation, because

resins are less soluble in pentane.

On the other hand, Figure 2 indicates that the transmittance continued to increase

over 200 hr; that is, that the formation of aggregates, probably of larger and larger size,

proceeded with the systems containing n-alkanes. This is consistent with the fact that

less asphaltene will be soluble at the end in these solvents, particularly n-heptane, which

is the most aliphatic (McLean and Kilpatrick, 1997; Spiecker et al., 2003).

Effect of Synthetic Dispersants

The dispersant activity of some surfactants (NPEON, 1-dodecanol, and commercial dis-

persant) was evaluated with El Furrial crude oil. The dilution and asphaltene precipitation

were carried out with n-heptane, which is the most effective solvent for precipitation of

asphaltenes.

The excess heptane added to the crude–toluene mixture led to asphaltene precipitation

and allowed evaluation of the effectiveness of the dispersants. The Turbiscan was used

to measure the precipitate height in the test tube as time elapsed, and the precipitation

efficiency criterion was taken as this height after 200 hr, which, according to Figure 2,

is probably close to the plateau equilibrium value.

The % dispersion index is defined by the following equation:

%Dispersion index D

�

.HnoD � HD/

HnoD

�

� 100 (1)

where HnoD is the precipitate height without addition of any dispersant and HD is

the corresponding height with dispersant D at the indicated concentration. This criterion

provides an estimate of the asphaltenes that would have been precipitated by the n-heptane

alone but were kept suspended in the solution by the dispersant.

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Table 2

Height of asphaltene precipitate for several dispersants

after 200 hr

Dispersant wt%

Precipitate

height,

mm

%

Dispersion

index

No dispersant 2.57 —

NP4 1 2.87 —

NP5 1 2.85 —

NP7 1 2.52 2.0

NP11 1 2.15 16.3

Dodecanol 1 2.86 —

Dodecanol 25 1.96 23.7

Commercial 1 0.68 73.5

Figure 4. Height of asphaltene precipitate after 200 hr versus the type of ethoxylated nonylphenol

dispersant. The solvent employed as precipitating agent was heptane.

The data in Table 2 suggest that this method is appropriate to evaluate the perfor-

mance of a dispersing agent. In the present case, the commercial dispersant is the best

agent, with a 73% dispersion index. For the ethoxylated nonylphenols (see Figure 4),

it is clear that the performance increased with the degree of ethoxylation and that it is

significant only for more polar species (EON > 7).

In the case of dodecanol, it is worth noting that for obtained a similar dispersion

index is attained only at extremely high concentrations. This result is consistent with the

fact that it is much less polar than the NP4 species.

Conclusions

The Turbiscan is a useful and simple tool for the study of asphaltene precipitation when

El Furrial crude oil is diluted with n-alkanes. The variation of transmittance versus

time brings a clear information on the asphaltene precipitation in excess of several

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Effect of Solvent on Asphaltene Precipitation 2439

solvents. With n-alkanes, the initial precipitation is much slower, thus indicating that

the formation of the first aggregates is delayed. This may be explained by the fact that

the resins are less soluble in n-alkanes than in cyclohexane and hence are more likely

to interact with asphaltenes to keep them as smaller aggregates. Measurement of the

precipitate height provides an estimate of the performance of an asphaltene dispersing

agent. The commercial dispersant was the best agent, with a 73% dispersion index. For

the ethoxylated nonylphenols, the performance increased with the degree of ethoxylation,

which is significant only for more polar species (EON > 7).

Acknowledgment

The authors are grateful to the Council of Scientific and Humanistic Development of the

University of Los Andes for financial support under Project I-1039-07-08-F. P.J.C. thanks

the University of Carabobo for a doctoral fellowship.

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