Kinetics of Asphaltene Aggregation in Crude Oil Studied by Confocal

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Kinetics of Asphaltene Aggregation in Crude Oil Studied by Confocal Laser-Scanning Microscopy Christine M. Seifried, John Crawshaw, and Edo S. Boek* Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom ABSTRACT: This paper presents results of our study of the kinetics of asphaltene aggregation as a function of different precipitants, solvents, and the Hildebrand solubility parameter δ. The aggregation was studied in situ in the crude oil using confocal laser-scanning microscopy. We observe that the kinetics of asphaltene aggregation strongly depends upon the Hildebrand solubility parameter of the mixture. The aggregation process, which occurs when adding precipitant close to the precipitation onset point, can be described by a diffusion-limited aggregation (DLA) process. A slight change of the Hildebrand solubility parameter away from the precipitation onset point into the precipitation regime leads to a different kinetic mechanism. In this regime, we observe that the aggregate size increases almost linearly over time, which is an indication of the crossover between DLA and reaction-limited aggregation (RLA). The maximum mean aggregate diameter increases from 18 μm near the onset point to 90 μm away from the onset point. Precipitating the asphaltenes with n-heptane leads to a faster aggregation rate with bigger flocs at a short time when compared to n-hexane.

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INTRODUCTION Asphaltenes are qualitatively defined as the most complex and heaviest components of crude oils or the bottom of a vacuum residue. They can also be defined as a solubility class: the portion of crude oil that is insoluble in light n-alkanes and soluble in aromatic solvents.1−5 Asphaltenes can flocculate, then precipitate, and deposit on surfaces. Because of their flocculation, asphaltenes can cause severe problems during processing, transportation, and storage of crude oil. They have a significant effect on the rheological properties of crude oils; hence, they affect efficiency and cost of production. They cause fouling during transport and, consequently, reduce flow as well as plugging of downstream separators, exchangers, and distribution towers and blocking of reservoir rock pores.6−8 Therefore, it is important to study the precipitation and aggregation behavior of asphaltenes because the kinetics may occur on time scales comparable to the deposition. To overcome these challenges, the petroleum industry has put a lot of effort into studying the asphaltene colloidal properties for more than 50 years.5,9 Below the so-called “precipitation onset point”, the asphaltenes are believed to be stable and do not precipitate.10 Maqbool et al.11 state in their work that the crude oil/ precipitant ratio determines the time required for the precipitation onset and can vary from minutes to month. Within a narrow range, the amount of precipitant needed for the precipitation onset point depends upon the time of observation, and therefore, short-term experiments may overpredict the solubility. Two main mechanisms are associated with irreversible asphaltene aggregation in crude oil: diffusion-limited aggregation (DLA) and reaction-limited aggregation (RLA). The characteristic feature of DLA is that any contact between the particles results in their sticking together. Therefore, the particle size increases with flocculation time. For RLA, on the other hand, not every contact between two particles results in their aggregation and it is a function of the number of particles © 2013 American Chemical Society

in an aggregate. In this process, a larger number of collisions is required before two particles aggregate. This decreases the aggregation rate when compared to the DLA process.12 The particle growth for DLA is described in eq 113 R = R 0(1 + t /τD)1/ d f

(1)

while the RLA kinetics can be described with eq 213 R = R 0 exp(t /τR df )

(2)

where τD is the diffusion time, τR is the reaction time, t is the flocculation time, R0 is the initial particle size, R is the mean radius of the asphaltene aggregates, and df is the fractal dimensionality. If τD > τR, the aggregation process is controlled by diffusion, while for τD < τR the aggregation is controlled by reaction. The characteristic times depend upon the volume of precipitant added. The particle-size growth can be measured as a function of time, and then the data can be fitted with eqs 1 and 2. τD and τR depend upon the amount of precipitant added and the initial asphaltene concentration. The adjustable parameter df relates the mean aggregate size to the number of particles in an aggregate.14,15 Yudin et al.12 observed a crossover behavior between RLA and DLA above the critical micelle concentration (cmc) (Figure 1). At the initial stage of the aggregation process, the RLA process dominates, and then the aggregation mechanism goes to DLA with time and size increasing. The aggregated asphaltenes can precipitate from the crude oil and change its solvent properties. A parameter to estimate these changes is the Hildebrand solubility parameter δ, which Special Issue: 13th International Conference on Petroleum Phase Behavior and Fouling Received: September 28, 2012 Revised: February 12, 2013 Published: February 12, 2013 1865

dx.doi.org/10.1021/ef301594j | Energy Fuels 2013, 27, 1865−1872

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asphaltene content of 18.8 wt %. The density (at 15 °C) was measured to be 0.983 g/cm3, and the kinematic viscosity (at 40 °C) was 20 020 mm2/s. Toluene (RI = 1.4969) and 1-methylnaphthalene (1-MNA) (RI = 1.6167) were used as solvents. To initiate the precipitation of the asphaltenes, n-heptane (RI = 1.3888) and n-hexane (RI = 1.3759) were used as precipitants. All chemicals used correspond to the degree of purity for spectroscopy. Methods. The crude oil sample contained water droplets in the size range of several micrometers, which disturb the observation of the asphaltenes with the CLSM. To disperse the water droplets, 1 mL of crude oil was diluted with 1 mL of solvent and stirred for 12 h using a magnetic stirrer. After that period of time, no more water droplets were detected with the microscope. The dilution with solvent also reduced the viscosity and optical density of the sample. After the precipitant was added, the crude oil/solvent/precipitant mixture was stirred for 20 s. For the microscopy, two droplets of the sample were placed between a glass slide and a cover sheet. Then, a drop of immersion oil was placed onto the cover sheet. Light passing through the glass slide and air causes a refraction of light. Immersion oil was used to match the RI of oil and glass, which can avoid refraction of light and, thus, prevent the loss of resolution. The experiments were carried out under ambient conditions. Time zero (t = 0 min) was defined when visual observation of the sample with the microscope has started. This results in an actual time lag of 30−35 s (stirring, t = 20 s; transfer to microscope, t = 10−15 s). To study the asphaltene aggregation in situ from crude oil, a stateof-the-art confocal laser-scanning microscope LSM 700 was used (Carl Zeiss, Germany). The sample was irradiated with a laser wavelength λ = 405 nm. To use the confocal principle, the oil-immersion 63× objective lens was chosen (theoretical resolution in lateral direction = 0.176 μm). The CLSM was mostly operated in the transmission mode. To obtain statistically reliable results, about 30 images were taken per sample and the aggregate size was measured and analyzed with the Zen 2010 software and the Zen 2009 Light Edition software. Determination of the Precipitation Onset Point. Prior to the study of asphaltene aggregation, the onset point of precipitation was determined. For our work, the onset point was defined at that oil/ solvent/precipitant ratio, where the first aggregates were detected with the confocal microscope. The onset point was determined in two ways: measuring the RI of the crude oil/solvent/precipitant system and simultaneous visual observation with the CLSM. The RI of the mixture can be used to characterize the precipitation onset point because precipitated asphaltenes lower the values for RI in a crude oil/solvent/precipitant mixture.17 As in the experiments for the kinetic studies, 1 mL of crude oil was diluted with 1 mL of solvent to reduce the viscosity and optical density and to disperse the water droplets in the crude oil to a smaller size. The mixture was stirred for 12 h using a magnetic stirrer. The precipitant was then gradually added, and for each volume of precipitant added, the mixture was stirred for 20 s and the RI was measured as a function of the crude oil volume fraction in steps of 0.02 and, after the detection of the onset point, in steps of 0.01. The RI measurements were carried out at T = 20 °C using an automatic refractometer (Index Instruments GPR 12-70E) with a sodium-D-line (λ = 589 nm). It uses the critical angle technique, which allows for determination of the RI for the opaque crude oil. The FRI values were calculated with eq 5; the Hildebrand solubility parameter for the crude oil, solvents, and the precipitant was calculated with eq 4; and the Hildebrand solubility parameter for the mixture was calculated with eq 6. Two droplets of each mixture were placed on the CLSM and observed for the appearance of precipitated asphaltenes. This results in a time lag of 40−45 s between the addition of precipitant and the observation with the microscope (stirring the mixture, t = 20 s; placing the droplet in the refractometer, t = 10 s; and transfer to the microscope, t = 10−15 s).

Figure 1. Crossover behavior for the asphaltene aggregation. This figure was reprinted from ref 12 with permission from Elsevier.

describes the degree of solubility for nonpolar or slightly polar substances without hydrogen bonding and is given in eq 3 as16 δ=

ΔH − RT v

(3)

where ΔH is the heat of vaporization, R is the ideal gas constant, T is the temperature, and v is the molar volume. Measuring the refractive index (RI) is a straightforward method to estimate the Hildebrand solubility parameter for a mixture.17 For nonpolar substances, an empirical linear relationship was found by Wang and Buckley,18 i.e. δi (MPa 0.5) = 52.042FRI + 2.904

(4)

where FRI is defined as FRI =

n2 − 1 n2 + 2

(5)

where n is the RI. δ for the mixture can then be calculated from the volume fractions of the components i18,19 δmixture =

∑ φδi i

(6)

where φ is the volume fraction of the component i. A step forward in studying in situ asphaltene precipitation from crude oil is the use of confocal laser-scanning microscopy (CLSM).13 This optical microscope is known for its nondestructive and non-contact features. One of the most important components in the CLSM is the pinhole aperture. It acts as a spatial filter at the confocal plane and suppresses the out-of-focus and stray light, which results in images with a higher resolution in comparison to those from conventional microscopy. The information can be gathered from different focal planes. With the variation of the diameter of the pinhole, the degree of confocality can be tuned. A fully open aperture corresponds to a non-confocal image. Using high-resolution imaging, the asphaltene aggregation and precipitation can be studied.20−22 Here, we make use of state-of-the-art confocal imaging to observe the aggregation process in greater detail than hitherto possible.

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RESULTS Determination of the Precipitation Onset Point. The graph in Figure 2 shows the results for the determination of the

EXPERIMENTAL SECTION

Materials. The crude oil used in these experiments was obtained from Shell, The Netherlands (δ = 19.96 MPa0.5), with an n-heptane 1866


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insufficiently precise because they only give an indication that we are working close to the onset point (δ ≈ δonset). Kinetics of the Asphaltene Aggregation. To study the kinetics of the asphaltene aggregation, five experiments were carried out, where the asphaltenes in the crude oil were precipitated with different precipitants and solvents: (i) adding an amount of n-heptane to initiate the precipitation, where δ ≈ δonset, (ii) adding an amount of n-hexane at the precipitation onset point, where δ ≈ δonset, (iii) adding a higher volume of nheptane than required for the onset point, where δ < δonset, (iv) adding a higher volume of n-hexane required to initiate precipitation, where δ < δonset, and (v) diluting the crude oil with 1-MNA and precipitating the asphaltenes with the volume of n-heptane required to initiate the precipitation, where δ ≈ δonset. Experiment i. The crude oil was diluted with toluene, and the asphaltenes were first precipitated with 4.3 mL of n-heptane at a crude oil volume fraction = 0.16. This corresponds to a Hildebrand solubility parameter of 16.43 MPa0.5 for the mixture. It was found that these were the conditions close to the precipitation onset point for this crude oil/solvent/ precipitant mixture. The graph in Figure 3 represents the

Figure 2. Determination of the precipitation onset point for the crude oil/toluene/n-heptane system, with FRI as a function of the crude oil volume fraction.

precipitation onset point for the crude oil/toluene/n-heptane system. The calculated values for FRI are plotted as a function of the crude oil volume fraction. The graph is separated by a dashed line into two parts, which indicates the precipitation onset point. The solid square symbols indicate that no asphaltenes were precipitated from the mixture, and the open triangles represent the RI values where asphaltenes precipitated from the crude oil. The red line shows the linear fit for the square points only. The mixture FRI for a stable crude oil can be linearly related to the volume fraction of the crude oil. Precipitated asphaltenes contribute to the RI and, thus, result in a deviation from the line, when the RI of the precipitation onset point is reached. Table 1 summarizes the results obtained for the measurement of the RI for different solvents and precipitants used. Table 1. Results of the Determination of the Precipitation Onset Point from the RI Measurements solvent precipitant RI (onset) FRI (onset) δ ≈ δonset (MPa0.5) precipitant/oil ratioonset solvent/oil ratioonset

mixture 1

mixture 2

mixture 3

toluene n-heptane 1.4331 0.26 16.43 4.3 1

toluene n-hexane 1.4288 0.26 16.24 4.0 1

1-MNA n-heptane 1.4444 0.27 16.71 5.1 1

Figure 3. Mean aggregate diameter as a function of the flocculation time. The solid curve represents the fit for DLA (experiment i).

measured mean aggregate diameter as a function of the flocculation time. Each triangle in the graph represents a mean aggregate diameter of the sample taken at specific times. The solid triangles represent the aggregate diameter during the aggregation process, i.e., increasing mean aggregate diameter, whereas the open triangles indicate that the sedimentation of flocs has started. The vertical bars denote the calculated standard deviation of the aggregate diameter. Figure 4 represents the optical images of the increasing asphaltene aggregate diameter as a function of time, and Figures 5−7 show the size evolution for experiment i at flocculation time t = 0 min and after 5 and 45 min. The initial mean aggregate diameter was found to be 1.7 ± 1.0 μm at t = 0 min, with a maximum of around 1 μm (Figure 5). The mean aggregate diameter increased over time, with a faster increase spanning a short time. The confocal image at t = 5 min illustrates a fractal structure of the aggregates with a size range between 5.0 and 34 ± 4.7 μm. The size distributions in Figures 5−7 show an increase in the diameter and a decrease in the number of smaller diameters. After 15 min, the size of the flocs increased because of aggregation of the existing flocs. This can be seen in Figure 4b, with the increasing floc diameter and

Table 2 summarizes the results obtained for the observation with the CLSM for different solvents and precipitants used. For both detection methods, the precipitation onset point was detected at the same volume of precipitant, which indicates a good agreement between both detection methods and, hence, a good estimate of the Hildebrand solubility parameter of the precipitation onset point. However, those methods are still Table 2. Results of the Determination of the Precipitation Onset Point with the CLSM

solvent precipitant precipitant/oil ratioonset solvent/oil ratioonset

mixture 1

mixture 2

mixture 3

toluene n-heptane 4.3 1

toluene n-hexane 4.0 1

1-MNA n-heptane 5.1 1 1867


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Figure 6. Aggregate diameter distribution for the crude oil with 4.3 mL of n-heptane at flocculation time t = 5 min for experiment i.

Figure 7. Aggregate diameter distribution curve for the crude oil with 4.3 mL of n-heptane at flocculation time t = 45 min for experiment i. Figure 4. Confocal images of the asphaltene aggregates in the crude oil precipitated with 4.3 mL of n-heptane at different flocculation times for experiment i.

by the increasing degree of polydispersity. After this time, sedimentation of the aggregates started. The data points were fitted to eq 1 for DLA and indicated DLA of the asphaltenes. The solid curve in Figure 3 represents the fit function. The adjustable parameters were found to be τD = 0.0068 min and df = 3.8. Experiment ii. In the next experiment, the crude oil/ toluene/n-hexane system close to the precipitation onset point was studied (δ = 16.24 MPa0.5) with a precipitant volume of 4 mL. The graph in Figure 8 represents the measured mean aggregate diameter as a function of the flocculation time, and Figures 9 and 10 show the size distributions obtained in experiment ii for 7 and 50 min of flocculation time. As the time increased, the mean aggregate diameter increased as well, with a faster increase at early time. At t = 20 min, the mean diameter increased up to 12 ± 8.6 μm. At early times, the aggregates formed a narrow size distribution. The polydispersity increased because of the presence of a higher number of fractal aggregates. The data points in Figure 8 were fitted to the equation for DLA, where τD and df were treated as adjustable parameters. These were found to be τD = 1.5 min and df = 1.5. Experiment iii. In this experiment, the asphaltenes were precipitated from crude oil with 4.7 mL of n-heptane, which corresponds to a Hildebrand solubility parameter of 16.35 MPa0.5 for the mixture. The higher volume of precipitant resulted in a lower Hildebrand solubility parameter for the mixture further away from the onset point. The graph in Figure

Figure 5. Aggregate diameter distribution for the crude oil with 4.3 mL of n-heptane at flocculation time t = 0 min for experiment i.

decreasing number of flocs, when compared to Figure 4a. Panels c−e of Figure 4 show the increasing size because of aggregation. However, fractal aggregates were still present. The maximum mean diameter of 17 ± 11 μm was detected at 45 min of the flocculation time. The flocs formed increased to 10 times more than the diameters detected at t = 0 min (Figure 7). The aggregates both increased in number and size, confirmed 1868


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Figure 11. Mean aggregate diameter as a function of the flocculation time (experiment iii).

Figure 8. Mean aggregate diameter as a function of the flocculation time. The solid curve represents the fit for DLA (experiment ii).

Figure 9. Aggregate diameter distribution for the crude oil with 4 mL of n-hexane at flocculation time t = 7 min for experiment ii.

Figure 12. Confocal images of the asphaltene aggregates in the crude oil precipitated with 4.7 mL of n-heptane at different flocculation times for experiment iii.

μm at t = 15 min. The curve could not be fitted to eq 1 for DLA. The aggregation character of the crude oil at this volume of precipitant is probably governed by another mechanism and does not only depend upon the growth and diffusion of the aggregates. The results obtained in the experiment raised the question whether the kinetic mechanism changes with changing Hildebrand solubility parameter. For the next experiment, the Hildebrand solubility parameter for the crude oil/toluene/nhexane system was lowered further away from the onset point. Experiment iv. The next experiment of the kinetic studies was carried out by precipitating the asphaltenes with 6.0 mL of n-hexane (δ = 15.89 MPa0.5). The graph in Figure 13 represents the measured mean aggregate diameter as a function of the flocculation time. Figure 14 shows two examples of the confocal images taken in experiment iv.

Figure 10. Aggregate diameter distribution for the crude oil with 4 mL of n-hexane at flocculation time t = 50 min for experiment ii.

11 represents the measured mean aggregate diameter as a function of the flocculation time. Figure 12 shows the confocal images of the increasing asphaltene aggregate diameter as a function of time. The higher volume of precipitant added resulted in an increase of the initial aggregate size from 1.7 ± 1.0 μm to approximately 11 ± 4.7 μm. The curve shows a steeper increase because the mean aggregate diameter increased almost linearly over time. As time increased, more aggregates joined and stuck to the flocs formed. The maximum mean aggregate diameter observed was 26 ± 10 1869


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mixture where the Hildebrand solubility parameter is close to the Hildebrand solubility parameter for the onset point (δ = 16.71 MPa0.5). The graph in Figure 15 represents the measured mean aggregate diameter as a function of the flocculation time.

Figure 13. Mean aggregate diameter as a function of the flocculation time (experiment iv).

Figure 15. Mean aggregate diameter as a function of the flocculation time. The solid curve represents the fit for DLA (experiment v).

Figures 16−18 show the size evolution for the experiment v after 0, 5, and 50 min of flocculation time. The initial aggregate

Figure 14. Confocal images of the asphaltene aggregates in the crude oil precipitated with 6.0 mL of n-hexane at flocculation times t = 5 min (left) and t = 15 min (right) for experiment iv.

The particles observed at t = 0 min had a mean size of approximately 1.8 ± 0.70 μm. However, the mean aggregate size did not increase gradually as it did in experiment i. The mean diameter rather increased faster with a shorter time and reaches a size of 20 ± 10 μm at 2 min. The mean size continued to increase over the flocculation time. The flocs undergo further aggregation with an increasing time before sedimentation was observed. The maximum mean size found was approximately 88 ± 71 μm at t = 15 min with a high degree of polydispersity. The smallest flocs found in this sample were in the range of 7− 22 μm, whereas the biggest floc size was 260 μm. The network morphology of the asphaltene aggregates disappeared and changed to a denser isolated network, but the fractal aggregates were still present. The appearance of smaller and bigger flocs indicates that not all of the aggregates undergo further aggregation to big flocs at the same velocity. The diffusion through the solution depends upon the particle size, and maybe not all of the collisions between the asphaltenes resulted in their sticking (RLA). This would explain the appearance of a high number of fractal aggregates. We observe that the curve for the experiment iv could not be fitted to eq 1 for DLA. This indicates, for this system, different kinetics and a different aggregation process when compared to the classical colloidal system. It could imply that we are at a crossover behavior between DLA and RLA, where other workers have found a linear relationship.23 Experiment v. The next experiment covered the question whether the different solvents influence the kinetic mechanism. The crude oil was diluted in 1-MNA, and the asphaltenes were precipitated with 5.1 mL of n-heptane, which corresponds to a

Figure 16. Aggregate diameter distribution for the crude oil with 5.1 mL of n-heptane in 1-MNA at flocculation time t = 0 min for experiment v.


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Figure 19 compares the mean aggregate diameter as a function of the flocculation time for the three experiments


Figure 19. Mean aggregate diameter as a function of the flocculation time in different solvent−precipitant mixtures in double logarithm scaling (experiments i, ii, and v) and the work by Hung et al.13

diameter at t = 0 min was found to be 3.8 ± 2.1 μm. As the time increased, the mean aggregate diameter increased as well, with a faster increase during a short time. At t = 15 min, fractal aggregates appeared in the size range from 2.0 to 17 ± 4.0 μm. The size of the asphaltene flocs increased because of the aggregation of smaller flocs. The flocs observed at t = 30 and 50 min had a mean size of 16 ± 6.4 and 18 ± 8.4 μm, respectively. The aggregation occurs over a wide size range up to 40 μm (Figure 18). The size distribution shows a more multimodal distribution when compared to the previous experiments. This indicates an increase of the mean diameter in size rather than in number. After the flocs were aggregated to this diameter, sedimentation started. The data points were fitted to eq 1 for DLA, and the parameters were found to be τD = 1.1 min and df = 2.4.

listed in Table 3 and the work by Hung et al. (Boscan crude oil, 66% n-heptane)13 for DLA behavior. The change in the precipitant and solvent did not qualitatively affect the kinetic mechanism because all aggregation patterns show DLA behavior. The graph in Figure 19 is drawn on a log−log scale, and therefore, the slope is 1/df as in eq 1. The detailed parameters, however, depend upon the solvent. The method that we have used to determine the precipitation onset point requires further work to make it more precise, such as filtration experiments or titration. The systems studied for this work were therefore close but not exactly at the onset point. This may result in the different aggregation rate for the three experiments. Yudin et al.24 and Ashoori et al.14 studied the kinetics of precipitated, extracted, and redissolved asphaltenes in toluene. They found an average value of df = 1.7 ± 0.2 for DLA, whereas τD varied for different n-heptane concentrations. Ashoori et al.14 found similar results for df, which agree with the values obtained by Yudin et al.24 In both of these sets of experiments with extracted asphaltenes, the rate of aggregation was very much smaller than seen in our experiments with the whole crude oil. The large value for df found in our experiment i is unexpected; however, Hung et al.13 also found values of df > 3 for aggregation in crude oil. Further experiments are planned to verify this. When 1-MNA was used as a solvent, the aggregate size increased more gradually when compared to experiment i. The overall mean aggregate size was smaller at the same flocculation times. However, for both experiments, the maximum mean size was 17 μm. With increasing time, sedimentation dominated the process for both experiments. In the experiments iii and iv, the kinetic mechanism was studied as a function of the higher precipitant volume (δ < δonset). The curves with the data points show a faster increase, approaching crossover behavior. The aggregation between the asphaltenes is not only governed by diffusion anymore.

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DISCUSSION The kinetic mechanism depends upon the Hildebrand solubility parameter δ of the given crude oil/solvent/precipitant mixture. Close to the onset point, the kinetic process followed a mechanism where the measured mean aggregate size over time could be fitted to the equation of DLA. However, with a lower Hildebrand solubility parameter, the aggregates grew faster at a short time, with an almost linear increase over time, and a higher maximum mean aggregate size was observed. Table 3 summarizes the results obtained for the experiments i, ii, and v, where the kinetics were studied starting close to the predicted onset point of precipitation. Table 3. Kinetics of the Asphaltene Aggregation in Different Solvent−Precipitant Mixtures experiment i solvent precipitant (volume) δ (MPa0.5) R0 (μm) maximum mean diameter (μm) sedimentation start (min) τD (min) df

experiment ii

experiment v

toluene n-heptane (4.3 mL) 16.43 1.8 17

toluene n-hexane (4.0 mL) 16.24 1.8 17

1-MNA n-heptane (5.1 mL) 16.71 3.8 18

45

50

50

0.0068 3.8

1.5 1.5

1.1 2.4

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CONCLUSION We have observed, for the first time, a change in the kinetic mechanism of asphaltene precipitation from crude oil as a function of the Hildebrand solubility parameter. Close to the precipitation onset point (δ ≈ δonset), the kinetic mechanism follows DLA behavior and the data points could be fitted to a 1871


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(13) Hung, J.; Castillo, J.; Reyes, A. Kinetics of asphaltene aggregation in toluene−heptane mixtures studied by confocal microscopy. Energy Fuels 2005, 19, 898−904. (14) Ashoori, S.; Abedini, A.; Saboorian, H.; Nasheghi, K. Q.; Abedini, R. Mechanisms of asphaltene aggregation in toluene and heptane mixtures. J. Jpn. Pet. Inst. 2009, 52 (5), 283−287. (15) Mullins, O. C.; Sheu, E.; Hammami, A.; Marshall, A. G. Asphaltenes, Heavy Oils, and Petroleomics; Springer: New York, 2007. (16) Hildebrand, J. H.; Scott, R. L. The Solubility of Nonelectrolytes, 3rd ed.; Reinhold Publishing: New York, 1950. (17) Buckley, J. S. Predicting the onset of asphaltene precipitation from refractive index measurements. Energy Fuels 1999, 13, 328−332. (18) Wang, J. X.; Buckley, J. S. Asphaltene stability in crude oil and aromatic solventsThe influence of oil composition. Energy Fuels 2003, 17, 1445−1451. (19) Wang, J. X.; Buckley, J. S. A two-component solubility model of the onset of asphaltene flocculation in crude oils. Energy Fuels 2001, 15, 1004−1012. (20) Castillo, J.; Hung, J.; Goncalves, S.; Reyes, A. Study of asphaltenes aggregation process in crude oils using confocal microscopy. Energy Fuels 2004, 18, 698−703. (21) Claxton, N. S.; Fellers, T. L.; Davidson, M. W. Microscopy, Confocal Encyclopedia of Medical Devices and Instrumentation; John Wiley and Sons: Hoboken, NJ, 2006. (22) Pawley, J. B. Handbook of Biological Confocal Microscopy, 3rd ed.; Springer: Berlin, Germany, 2006. (23) Weitz, D. A.; Huang, J. S.; Lin, M. Y.; Sung, J. Limits of the fractal dimension for irreversible kinetic aggregation of gold colloids. Phys. Rev. Lett. 1985, 54 (13), 1416−1419. (24) Yudin, I. K.; Nikolaenko, G. L.; Gorodetskii, E. E.; Markhashov, E. L.; Agayan, V. A.; Anisimov, M. A.; Sengers, J. V. Crossover kinetics of asphaltene aggregation in hydrocarbon solutions. Phys. A 1998, 251, 235−244.

power law equation. With a Hildebrand solubility parameter further away from the onset point (δ < δonset), the aggregation process seems to approach crossover behavior between DLA and RLA. Therefore, the equations for DLA might not be completely capable of describing the kinetics of asphaltene aggregation in situ for a given crude oil. The study of different precipitants and solvents at the respective asphaltene onset Hildebrand solubility parameter resulted in a change in the aggregation rate, but it did not qualitatively change the kinetic mechanism. The differences in the dynamics of the aggregation can be related to the stability of the crude oil. The stability is therefore not a function of the asphaltene concentration present in the crude oil; it rather depends upon the rate of the aggregation and the sedimentation rate.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Frans van den Berg and Shell for providing the crude oil. REFERENCES

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Kinetics of Asphaltene Aggregation in Crude Oil Studied by Confocal

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