Kinetics of Asphaltene Aggregation in Toluene−Heptane Mixtures

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Energy & Fuels 2005, 19, 898-904

Kinetics of Asphaltene Aggregation in Toluene-Heptane Mixtures Studied By Confocal Microscopy J. Hung,*,† J. Castillo,‡ and A. Reyes‡ Centro de Fı´sica, Instituto Venezolano de Investigaciones Cientı´ficas, Caracas 1020A Apartado 21827, Venezuela, and Escuela de Quı´mica, Facultad de Ciencias, Universidad Central de Venezuela, Caracas, Venezuela, 1020A, Apartado 47102, Venezuela Received November 2, 2004

The kinetics of asphaltenes in crude oils of different stability was studied by confocal microscopy. The physical nature of flocculated asphaltenes of Boscan and Furrial crude oils is shown through of high-resolution micrographics image, and their colloidal structural evolutions are described by an analysis of size distribution of flocculated asphaltenes particles. Two different behaviors associated with the crude oil stability were found. The size aggregates increase rapidly at the first time of the flocculation for a stable crude oil such as the Boscan and gradually for the Furrial crude oil, which is known as a crude oil of low stability. The results obtained demonstrate that the aggregation process of Boscan crude oil is initially determined by the diffusion of the aggregates by the attractive interactions between flocks and aggregate asphaltenes. On the other hand, the aggregation process of Furrial crude oil occurs in the initial stage due to the increment of the number of particles and not by the particle growth until reaching a limited one where the aggregation process becomes governed by the diffusion of colloid particles.

1. Introduction It is well known that the importance of asphaltene in the petroleum industry is due to its negative impact on various petroleum operations, such as exploration, production, transportation, and refinement.1-3 In exploration, asphaltene can alter the flow phase of the reservoir; in production, it can plug the wellbore; in transportation, it can precipitate and eventually clot up the pipeline; and in refinement, it can hinder the refining yield. These are well-known phenomena experienced during many years of operations and/or processes.2 Many of the problems described above are related to a fundamental characteristic of the asphaltene colloidal state and how the asphaltene molecules have a strong self-association to give rise to more- or less-extended aggregates. The concept that asphaltene molecules are present as a colloidal system is credited to Nellensteyn.4 He proposed that flocks and/or aggregates of asphaltene protected by adsorbed resin and hydrocarbon medium forms asphaltic compounds. Several studies about the colloidal nature of asphaltenes and its self-association phenomenon were reported a long time ago.5-9 In general, small-sized asphaltene particles can be dissolved in a petroleum fluid, whereas relatively big * Author to whom correspondence should be addressed. E-mail: [email protected]. Phone: +58212-5041580. Fax: +58212-5041148. † Instituto Venezolano de Investigaciones Cientı´ficas. ‡ Universidad Central de Venezuela. (1) Levent, A.; Yan, S.; Yoshihisa, H.; Masahiro, H. Energy Fuels 1999, 13, 287. (2) Sheu, E. Y.; Storm, D. A. Colloidal Properties of Asphaltenes in Organic Solvents. In Asphaltenes-Fundamental and Applications; Sheu E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995; Chapter I, p 1. (3) Buckley, J. S. Energy Fuels 1999, 13, 328. (4) Nellensteyn, F. J. J. Inst. Pet. Technol. 1924, 10, 311-323.

asphaltene particles can flocculate and decant in the solution due to the high paraffin content of the oil, forming random aggregates. Flocculation of asphaltene in paraffin is known to be almost irreversible, and in some cases, it has hysterises when the conditions are returned to the preflocculation point.10 This colloidal structural evolution of the flocculated asphaltene has had various postulates for its mechanism; recently, Kyebongseok et al.11 postulated the following mechanism for asphaltene flocculation in organic solvent: (1) molecular self-association in solution, (2) nucleation of asphaltene particles, (3) growth of asphaltene particles, and (4) aggregation of asphaltene particles resulting in phase change. Two of the more employed theories to explain the particle aggregation are the known as theory of diffusion-limited aggregation (DLA) and reaction-limited aggregation (RLA),12 which predicts the possible mechanism of asphaltene aggregation similar to that ob(5) Espinat, D.; Rosenberg, E.; Scarsella, M.; Barre, L.; Fenistein, D.; Broseta, D. Colloidal structural evolution from stable to flocculated state of asphaltene solutions and heavy crudes. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; Chapter V, p 145. (6) Sheu, E. Y. Self- Association of Asphaltenes. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; Chapter IV, p 115. (7) Acevedo, S.; Ranaudo, M. A.; Escobar, G.; Gutierrez, L. B; Gutierrez, X. A unified view of the colloidal nature of asphaltenes. In Asphaltenes -Fundamental and Applications; Sheu, E. Y., Mullins, O. C., Eds; Plenum Press: New York, 1995; Chapter IV, p 131. (8) Acevedo, S.; Escobar, O.; Echevarria, L.; Gutierrez, L.; Mendez, B. Energy Fuels 2004, 2, 305. (9) Oh, K.; Ring, T. A.; Deo, M. D. J. Colloid Interface Sci. 2004, 271, 212-219. (10) Branco, V. A. M.; Mansoori, G. A.; De Almeida Xavier, L. C.; Park, S. J.; Manafi, H. J. Petr. Sci. Eng. 2001, 32, 217-230. (11) Oh, K.; Ring, T. A.; Deo, M. J. D. J. Colloid Interface Sci. 2004, 271, 212-219.

10.1021/ef0497208 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/04/2005

Aggregation Kinetics in Toluene-Heptane Mixtures

Energy & Fuels, Vol. 19, No. 3, 2005 899

served in classical colloidal system.12 Generally, the aggregation process involves two characteristic times: a diffusion time, τD, and a reaction time, τR; when the diffusion time is bigger than the reaction time, the aggregation kinetics is controlled by diffusion (DLA), and if it is contrary, the reaction is controlled by reaction-limited aggregation (RLA). DLA and RLA can be described by

DLA: R ) R0 (1 + t/τD)1/df

(1)

RLA: R ) R0 exp(t/τR df)

(2)

where R0 is the initial particle size and df the fractal dimensionality. These mechanisms are studied by the measurement of the rate of particle-size growth and the average size of the particles as a function of time. Then, the data obtained are fitted with the eqs 1 and 2 using τD, τR, and df as adjustable parameters. These equations permit us to determinate the kind of process that dominates in the aggregation. A number of investigations in this way have been researched through techniques such as light scattering5,13-14 and microscopy.5,15-16 Scattering methods have been widely used to study aggregation kinetics. The advantage of scattering is that it does not require sample manipulation, although it has to be sufficiently diluted and it only can measure properties that correspond to a certain micelle size distribution. On the other hand, techniques such as scanning electron microscopy (SEM) and Cryo-SEM are microscopic techniques widely used to visualize the aggregation and flocculation process. Unfortunately, this technique is extremely cumbersome and additionally has the disadvantage that the sample may be altered during the preparation of the sample. Recently, a technique based on confocal microscopy has been introduced to study the colloidal nature of asphaltenes. This technique is known by its noncontact and nondestructive features and has been used extensively in the biomedical field.17,18 The use of a confocal microscope is starting to increased in the areas of colloid physics.18-20 At present, the mechanism of aggregation of asphaltene is still not fully understood. Its complexity is due mainly to the fact that the asphaltenes are not molecules; they are a fraction of crude oil composed by different kinds of insoluble molecules in paraffin. Additionally, the colloidal nature of asphaltenes depends (12) Yudin, I. K.; Nikolaenko, G. L.; Gorodetskii, E. E.; Kosov, V. I.; Melikyan, V. R.; Markhashov, E. L.; Frot, D.; Briolant, Y. J. Pet. Sci. Eng. 1998, 20, 297-301. (13) Liu, Y. C.; Sheu, E. Y.; Chen, S. H.; Storm, D. A. Fuel 1995, 74, 9, 1352-1356. (14) Cametti, C.; Codastefano, P; Tartaglia, P. J. Colloid Interface Sci. 1989, 131, 2, 409-422. (15) Sharma, A.; Groenzin, H.; Tomita, A.; Mullins, O. Energy Fuels 2002, 16, 490-496. (16) Ferworn, K. A.; Svrcek, W. Y. Characterization and Phase Behavior of Asphaltenic Crude Oils. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998, pp 227-266. (17) Sheppard, C. J. R., Shotton, D. M. Confocal Laser Scanning Microscopy; Bios Scientific Publishers: New York, 1997. (18) Shotton, D. M. J. Cell Sci. 1989, 94, 175. (19) Mikula, R. J.; Munoz, V. A. Colloids Surf. 2000, 174, 26-36. (20) Hung, J.; Castillo, J.; Goncalves, S; Reyes, A. Energy Fuels 2004, 18, 698-703.

Figure 1. Transmitted intensity vs time for Boscan (A) and Furrial (B) crude oil at different % n-heptane.

on the source of the crude oil and the procedure used for precipitation of the asphaltenes. In a previous paper,20 using confocal microscopy, we reported high-resolution micrographic images of asphaltene flocculation induced by n-heptane addition, demonstrating the asphaltenes flocculation process. This process depends on the characteristics of the flocks formed that depend on the crude oil nature and its stability. This technique permitted us to visualize the morphology of flocculated asphaltene. In this paper, an analysis of the flocculation kinetics of asphaltene colloidal particles was carried out using confocal microscopy. The dynamic flocculation of asphaltenes from different crude oils was shown through high-resolution image micrographics and its colloidal structural evolution by an analysis of its size-distribution curve. The size distribution of flocculated asphaltenes of the different crude oils studied as a function of time was determined, and the data were fitted to determine the behavior of the aggregation process. Two different behaviors associated with the crude oil stability were found. 2. Experimental Method 2.1. Sample Preparation. Two types of crude oils from Venezuela were used in this study. Boscan (11° API, and 17% asphaltene) and Furrial (21° API, and 4.3% asphaltene) crude oil that are from Monagas State in eastern Venezuela. The crude oils were diluted with a minimum of toluene to reduce the viscosity and diminish the optical density of the sample. The onset of the precipitation threshold was obtained through automatic titration of diluted toluene solutions of crude oil described in ref 20. All solvents used were HPLC grade. 2.2. Flocculation and Microscopy of Flocculated Asphaltene. The studies of flocculation kinetics were carried out using the apparatus described previously.21 To induce the aggregation, 15 mL of n-heptane are added to 10 mL of 1% crude oil in toluene; this ratio of crude oil/n-heptane ensures (21) Hung, J.; Castillo, J.; Jimenez, G.; Hasehawa, M.; Rodriguez, M. Spectrochim. Acta, Part A 2003, 59/13, 3177-3183.

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Figure 2. Confocal images of Boscan crude oil flocculated with 66% n-heptane at different times: (a) 5, (b) 20, (c) 30, (d) 60, (e) 90, and (f) 120 min. the flock formation.20 With this apparatus, is possible to measure the transmittance as a function of time, and from this plot, the aggregation time is obtained. Simultaneity at selected times, 3 drops of the sample were taken to be observed in the confocal microscope. Figure 1 shows a plot of the transmitted intensity as a function of time for Boscan and Furrial crude oil at different initial volumes of n-heptane added. The diminishing of the transmitted intensity as a function of time is indicative of the flock formation dynamic. This system is sensitive to the aggregation process, but it does not discriminate in the nature of the flock formed Microscopic images of flocculated asphaltenes of the different crude oils were obtained with a confocal microscope.20 The light source consisted of a Tungsten lamp. The samples were observed with a 40× objective (10 000× magnification). The microscope is connected to a Sony CCD camera. The images of the flocculated asphaltenes obtained from the microscope were stored in raw format of 640 × 480 pixels. To obtain a statically reliable particles size, more than 20 images were measured per samples. More than 200 images were stored to analyze the size as a function of time. The determination of the flock particle size of the different crude oil was processed and analyzed with the EPA software (designed in our laboratory) that works under Windows XP.

3. Results and Discussion Figure 1 shows a plot of the transmitted intensity as a function of time for Boscan and Furrial crude oil at different initial volumes of n-heptane added. The curve shows two important characteristics: for Boscan crude oil (A), solutions with more than 75% n-heptane added show relatively fast aggregation dynamics, presumably due to the formation of big flocks and an incremental increase in size with time. For Furrial crude oil (B), the curve reveals that the aggregation growth dynamic is almost 1.5 times slower than that of Boscan crude oil. We interpret this behavior as indicative of the effect of the growing process more predominant in the number of small aggregates in the solution than in the increment in size of the flocks formed. Figure 2 shows confocal images for Boscan crude oil at 66% n-heptane added as a function of the time of flocculation. Figure 2a shows the image of the flocks for 5 min of flocculation; this image depicts a fractal structure whose size is near 2 µm. After 30 min, the size of the flocks of asphaltenes increased by aggregation



Figure 3. Confocal images of Furrial crude oil flocculated with 66% n-heptane at different time: (a) 5, (b) 30, (c) 60, (d) 60, (e) 90, and (f) 120 min.

of small flocks (Figure 2b-c). It is noted that the Boscan crude oil contains flocks whose size is between 1 and 4 µm. After 30 min, Figure 2c-e shows clusters (flocks/ aggregates) formed because of the aggregation of flocks of asphaltene particles resulting in structural change. The network morphology has disappeared to give rise to denser, isolated aggregates. Fractal aggregates after 120 min are still present. Figure 2f shows dark and large flocks inside of aggregates that are formed by the basic cluster (flocks) of aggregate fractal between 2 and 5 µm. The image exhibits heterogeneous aggregates of a complex network formed by the self-association of small flocks. Figure 3 shows confocal images for Furrial crude oil at 66% n-heptane added as a function of the time of flocculation. We must point out that this crude oil did not exhibit particles detected by the naked eye before n-heptane addition in crude oil. After n-heptane addition, Figure 3a shows small flocks of dark asphaltene particles formed. The fractal structure is formed by flocculation of these small particles whose size is

between 800 and 1000 nm. After 20 min, the Figure 3c-e shows a larger number of flocks whose size is between 2 and 4 µm. The clusters formed within of the 60-180 min interval have sizes between 5 and 8 µm. In this crude oil, the growing process of the aggregates is faster in number than in size. Figure 3f is just like a macrostructure. An excess of time results in a collapse of the aggregates into a single huge aggregate, arranging in a compact structure of large size. Figure 4 shows the size distribution curve obtained for the Boscan crude oil after 5 (a) and 60 min (b) periods of flocculation. The plot (a) in Figure 4 shows a unimodal distribution with a maximum around of 1 µm. The lack of particles of small dimensions of 1 µm is due to the lack of sensibility of the micrograph. In plot (b), it is possible to observe a multimodal distribution indicating the increase in the size of the flocks and a clear diminishing in the number of particles of small size and also an increment in the relative number of big particles. At lower times, the small flocks of asphaltene particles formed had a narrower size distribution, implying an

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Figure 4. Particles size distribution curve for Boscan crude oil flocculated with 66% n-heptane at 5 (top) and 60 min (bottom) of period flocculation, respectively.

instantaneous aggregation of the asphaltene molecules. After 60 min, the cluster (flocks/aggregates) formed increased in size to three or four times more than the initial value due to the flocculation of the asphaltene molecules. The growth of aggregates occurs over a wide size range from 12 to 34 µm. Figure 5a shows the size distribution of the asphaltene Furrial crude oil for 5 (inserted in a), 30, and 120 min periods of flocculation. It is interesting to note that, at short times, the size distribution is monodisperse, the formation of large aggregates is gradual, and the process that controls the flock size distribution is not the growth of aggregate but the increase of flocks more in number than in size. This observation is consistent with the transmittance curves (Figure 1). Figure 5b shows the size distributions curve after a 120 min period of flocculation; this curve reflects that the clusters formed increased in size to between 5 and 8 µm. In this crude oil, the growing process of the aggregates is initially slower, but after 30 min, the growing in number of flocks is faster. This difference in the dynamics of the aggregation process can be related with the crude oil stability and their solubility. This aggregation process involves an increment of flocks at the first time of flocculation followed by a growing process causing high sedimentation rates which are reflected in Figure 2f. Figure 6 shows the overall trend in the mean particle size as a function of the time of flocculation for Boscan and Furrial crude oil at 66% and 88% n-heptane,

Hung et al.

Figure 5. Particles size distribution curve for Furrial crude oil flocculated with 66% n-heptane at 5 (insert), 30 (top curve), and 120 min of period flocculation, respectively.

respectively. For Boscan crude oil at 66% n-heptane, the mean particle size increases exponentially with a faster increase at short time. The solid curve reported in this figure is the result of the fit of eq 1 to the experimental data. The τD and df in eq 1 were treated as adjustable parameters, and these were found to be tD ) 0.84 and 1.02 and df ) 4.5 and 2.8 for 66% and 80% of n-heptane, respectively. As can be observed, the mean particle size growth increases exponentially with the time, and it can be fitted to eq 1 for DLA but with high values of df. The difference in the values obtained and those reported in the literature22 is indicative of the different mechanisms of aggregation that can be present in the crude oil. The attractive interactions among asphaltene particles can be first controlled by diffusion-limited aggregation. However, the interaction’s range can increase to levels, and models of short-range interactions, such as DLA, are not capable of describing completely all the aggregation process in the crude oils. The growth of the flocks at low % n-heptane is faster initially and then increase until they reach a maximum of size. At high % nheptane, the growth of flocks is faster than at low % n-heptane. On the other hand, asphaltene particle size for Furrial crude oil increases gradually until it reaches an approximately constant mean size. As the time is increased, more aggregates join the flocks formed, and (22) Yudin, I. K.; Nikolaenko, G. L.; Gorodetskii, E. E.; Markhashov, E. L.; Agayan, V. A.; Anisimov, M. A.; Sengers, J. V. Physica A 1998, 251, 235-244.


Figure 6. Mean particle size of flocks for Boscan and Furrial crude oil as a function of time of flocculation. The solid curve represents fit of the eq 1.

thereby the mean particle size of the distribution increases. This curve cannot be fitted to eq 1 for DLA. The aggregation character of this crude oil does not depend solely on the particle growth, but it is governed by other mechanism. An important difference in the behavior is the relative number of particles formed for each crude oil. In the Boscan case, this crude oil is considered of high stability and the relative number of flocks formed is low (∼60) in comparison with the Furrial crude oil (120-300). This difference in the relative number of flocks formed and the difference in the aggregation kinetics mechanism are the way to understand the stability of the crude oil. Although the percentage of asphaltenes in Boscan crude oil is higher than in Furrial crude oil, the second is more unstable due to the tendency to form a greater number of aggregates followed by high sedimentation rates. Figure 7 shows the evolution of the number of particles of asphaltene as a function of the time of flocculation for Boscan and Furrial crude oil, respectively. As is illustrated in this curve, from 5 to 60 min for Boscan crude oil, the number of small aggregates diminished gradually due to the formation of clusters (flocks/aggregates) of asphaltene particles. The solid curve represents a fit to the exponential decay. As the time of flocculation increases, the aggregates become larger and the character of the aggregation is predominantly determined by the diffusion of the aggregates. Every contact of the flock particles results in their coupling. For Furrial crude oil, the number of asphaltene particle increments from 5 to 80 min, and then it diminished gradually due to the aggregation of asphaltene particles by the large number of asphaltenes flocks formed. In this case, the initial stage of the aggregation


Figure 7. Particle number of flocks as a function of time of flocculation for Boscan and Furrial crude oil. The solid curve represents a fit to the exponential decay.

is determined by the increment of the average number of particles of flocks, until it reaches a limited one where the aggregation process becomes governed mainly by the diffusion of colloid particles. The colloidal structural evolution and aggregation process of the asphaltenes Boscan and Furrial crude oil should be described depending on the nature of the crude oil and how it is affected by the percentage of asphaltene present in the crude oil. In general, we suggest that the self-association of small flocks forms different complex aggregates of asphaltene. In different papers,20,23 we reported that the evidence of the molecular asphaltenes association takes place at low concentrations and at very low volumes of n-heptane. The aggregates increase rapidly at the first time of flocculation for the Boscan crude oil and gradually for the Furrial crude oil (Figure 6). Groezin and Mullins et al.24 suggested that the aggregation process of asphaltene is governed by the formation of large asphaltene molecular aggregates from the smaller asphaltene molecular aggregates. The results presented in this paper show clearly and directly the formation of these flocks starting from the aggregates. The aggregates formed from “asphaltene flocks” in some crude oils evidence also the studies reported by Yudin et al.22 They postulated that, at low aspahaltene concentrations, the aggregates contain “asphaltene molecules” and diffusion-limited particle aggregation kinetics is obeyed. (23) Hung, J.; Goncalves, S.; Castillo, J.; Fernandez, A. Fuel 2004, 83, 1823-1828. (24) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677.

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4. Conclusions The flocculation dynamics of asphaltenes from different crude oils was demonstrated through high-resolution micrographic images and size distribution of the asphaltenes flocks. For Boscan crude oil, the flocks of asphaltenes particles size increase rapidly at the first time of flocculation and gradually for the Furrial crude oil. The aggregation process for Boscan crude oil is determined initially by the diffusion of the aggregates that depend on the growth of asphaltenes aggregates with the time and of the attractive interactions among asphaltene aggregates. In the case of Furrial crude oil, the aggregation process occurs due to the increment of the number of particles and not by particle growth. The

Hung et al.

colloidal structural evolution and aggregation process of the asphaltenes Boscan and Furrial crude oils depends on the nature of the crude oil, and it is affected by the percentage of asphaltene present in the crude oil. Acknowledgment. We would like to thank FONACIT, Caracas, Venezuela, for financial support. This work was supported by the FONACIT-Venezuela through Grant Nos. G97000593, G97000722, S1-2001000877, and AP 97004022 Consejo de Desarrollo Cientı´fico y Humanı´stico de la Universidad Central de Venezuela Grant No. 03124338. EF0497208

Kinetics of Asphaltene Aggregation in Toluene−Heptane Mixtures

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