Physical State and Aging of Flocculated Asphaltenes - Energy & Fuels

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Energy & Fuels 2007, 21, 1256-1262

Physical State and Aging of Flocculated Asphaltenes† Wojciech Marczak,‡ Driss Dafri,§ Ali Modaressi,§ Honggang Zhou,| and Marek Rogalski*,§ Institute of Chemistry, UniVersity of Silesia, Szkolna 9, 40-006 Katowice, Poland, Department of Chemistry, UniVersity of Metz, 1, bd Arago, 57070 Metz, France, and Total, Centre Scientifique et Technique Jean Feger (CSTJF), AVenue Larribau, 64018 Pau, France ReceiVed August 16, 2006. ReVised Manuscript ReceiVed December 5, 2006

The phase transition leading to the petroleum asphaltene flocculation is considered as either a liquid-solid or liquid-liquid transition. Despite the fact that asphaltenes obtained by the usual procedure are amorphous solids, numerous arguments support the second hypothesis. In this work, we present studies of asphaltene flocculation kinetics and compare the results with the kinetics of coalescence of flocculated asphaltenes that have been redispersed mechanically in the solvent. Redispersion of flocculated asphaltenes was realized using an ultrasound bath. Kinetic measurements were performed using a dynamic light scattering technique. The Malvern 4800 apparatus used in this study makes it possible for us to observe the growth of aggregates in the range of 2-5000 nm. It was found that kinetic curves for redispersed asphaltenes are very similar to those of the initial flocculation induced by an antisolvent. This finding confirms the liquid-like behavior of flocculated asphaltenes. Moreover, the energy of cohesion of asphaltene aggregates is very low, and consequently, the amount of energy needed for redispersion is also low. The aging of the flocculated sample accelerates the aggregation kinetics, which points to very slow reorganization of the asphaltene structure, leading to more compact, solid-like flocs.

1. Introduction Flocculation of petroleum asphaltenes is usually discussed in terms of the flocculation onset. The latter corresponds to the formation of asphaltene aggregates that may be observed by either optical or spectroscopic methods and is governed by phase-transition thermodynamics. Nevertheless, the growth and precipitation of aggregates depend not only upon thermodynamic parameters, such as the concentration and temperature, but also kinetic factors. Results obtained with optical1 microscopy revealed the growth pattern of aggregates: first as spots which grew into strings and then as strings clustered into small flocs, which grew to large masses of fractal-like flocs. The first stage of this process corresponds probably to the spinodal liquidliquid-phase decomposition.2 The array of spots regularly dispatched in the solvent matrix confirms this hypothesis. This stage is thermodynamically driven. The further stages correspond to the coalescence process and are mainly driven by dynamic factors. The structure of the asphaltene precipitate during formation and growth depends upon the aging period as well as the chemical environment of the precipitation reaction.1 The present study is focused on measurements and the interpretation of aggregation kinetics determined by dynamic light scattering (DLS) spectroscopy. At first, we studied the aggregation and flocculation phenomena occurring in either asphaltene or crude oil solutions. Next, the effect of aging on † Presented at the 7th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed. E-mail: rogalski@ sciences.univ-metz.fr. ‡ University of Silesia. § University of Metz. | Total. (1) Angle, C. A.; Yicheng Long, Y. L.; Hamza, L.; Lue, L. Fuel 2006, 85, 492-506. (2) Kawasaki, K.; Koga, T. Physica A 1996, 224, 1-8.

the physical properties of the precipitate was investigated. Last, the effect of the chemical environment on the aggregation mechanism was studied. 2. Experimental Section 2.1. Materials. The crude oil used in this study was furnished by the Petroleum Company Total and contained 9% (w/w) of asphaltenes. Asphaltenes were obtained from this oil according to the protocol described previously.3 All chemicals were purchased from Acros Organics. 2.2. DLS. The mean particle size was determined using the autocorrelation function of the scattered light with a 7032 Malvern, 8 bit, high-speed, 72 channel correlator. A Malvern 4800 photon correlation spectrometer (Malvern Instruments, Malvern, U.K.) was used for this purpose. The laser beam was from a 35 mW, 633 nm He-Ne laser from Spectraphysics. Measurements were carried out at a scattering angle of 90° and a constant temperature of 25 °C. The scattering vector q4 was calculated from the equation q ) (4πn/ λ)sin(θ/2), where n is the refractive index of the medium, λ is the wavelength, and θ is the scattering angle (90°). The particle size was measured from the autocorrelation function of the intensity fluctuation of the scattered light. These measurements yielded the diffusion coefficient, D. The latter was used to calculate the particle hydrodynamic radius RH using the Stokes-Einstein equation D ) kT/6πηRH, where k is the Boltzmann constant, T is the absolute temperature, and η is the viscosity of the continuous phase, calculated from viscosities of chemicals forming that phase. The photon correlation spectroscopy (PCS) data were analyzed using a computer program provided by Malvern with the apparatus, operating in standard mode or implementing the CONTIN method, (3) Mutelet, F.; Ekulu, G.; Solimando, R.; Rogalski, M. Energy Fuels 2004, 18, 667-673. (4) Cotton, J. P. Neutron, X-Ray and Light Scattering: Introduction to an InVestigatiVe Tool for Colloidal and Polymeric Systems; NorthHolland: Amsterdam, The Netherlands, 1991.

10.1021/ef0603840 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/02/2007

Physical State and Aging of Flocculated Asphaltenes

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which led to average values of the particle size Zav.5,6 All samples were sonicated for 15 min immediately before the measurement.

3. Results and Discussion DLS measurements may be performed with transparent solutions only. Therefore, investigated samples of crude oils and asphaltenes were highly diluted with a suitable mixture of solvents. It was demonstrated3 that the flocculation threshold occurs at the constant ratio of n-heptane/toluene or, more generally, at the floculant/solvent ratio. Because the flocculation threshold depends upon the quality of the solvent only, the flocculation process is thermodynamically driven. As was recently shown,1 precipitation kinetics near the onset point depends upon the initial concentration of the heavy oil (asphaltenes) in toluene. Increasing the oil concentration increases the rate of the aggregate growth and the asphaltene flocculation. However, both the mechanism of the aggregate growth and the intermediate stages of the flocculation process are very similar in diluted and concentrated oil solutions. Consequently, measurements preformed with diluted oil solutions afford valuable information, valid with more concentrated solutions also. Measurements were performed with solutions below, at, and above the flocculation onset. 3.1. Aggregation of Asphaltenes below the Flocculation Onset. The flocculating power of n-alkanes decreases with an increasing chain length.7 The maximum flocculating power corresponds to n-alkanes with the chain length close to the average lateral chain of asphaltenes that was explained by n-alkane-bonded chain interactions. We have studied asphaltene aggregation in solutions containing 3 volume parts of n-dodecane with 1 volume part of 2-methyl naphthalene. Aggregation and flocculation processes are expected to be much slower in 2-methyl naphthalene/n-dodecane as compared to the usual toluene/n-heptane mixed solvent. Thus, we hoped that all stages of the aggregation process could be observed. In this case of this solvent, no precipitation was observed up to the 2-methyl naphthalene/n-dodecane ratio of higher than 4. Samples containing 1.2 g of asphaltenes/1 L of solvent were studied during several days. Figures 1 and 2 present the average size of aggregates, Zav, with a function of time, and corresponding functions of the intensity of scattered light, with the latter expressed as raw counts of the photomultiplier. The number of counts is related to the number and size of the scattering particles. Consecutive measurements were performed immediately after sample preparation, Figure 1a; 3 days later, Figure 1b; 6 days later, Figure 2a; and 9 days later, Figure 2b. Results obtained after 3 days are very similar to those measured directly after the sample preparation. The low value of counts indicates the low number of aggregates present in the solution. Aggregates were small, with the average size of about 100 nm. However, after 6 days, the situation changes dramatically. The size and number of aggregates increase. The size is about 1500 nm and remains constant within the experience time. A total of 3 days later (9 days after sample preparation), the aggregate size increases to several micrometers, which corresponds to the size of precipitating flocs in n-heptane/ toluene solutions. Contrary to the results obtained immediately after the sample preparation and after 3 days, measurements performed after 6 and 9 days indicate that aggregates are (5) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213-227. (6) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 229-242. (7) Stachowiak, C.; Viguie´, J. P.; Grolier, J.-P. E.; Rogalski, M. Langmuir 2005, 21, 4824-4829.

Figure 1. Average size of aggregates, Zav (nm), and intensity of scattered light expressed as a number of the photomultiplier counts, n, with a function of time. Aggregation kinetics of the system: asphaltenes (0.50 g), 2-methyl naphtalene (100 mL), and n-dodecane (300 mL). (O) Measurements immediately after preparation of the mixture. ([) Measurements 3 days after preparation of the mixture.

Figure 2. Average size of aggregates, Zav (nm), and intensity of scattered light expressed as a number of the photomultiplier counts, n, with a function of time. Aggregation kinetics of the system: asphaltenes (0.50 g), 2-methyl naphtalene (100 mL), and n-dodecane (300 mL). (O) Measurements 6 days after preparation of the mixture. ([) Measurements 9 days after preparation of the mixture.

growing in the first stage of the experiment up to a stable size. This finding may be explained in the following way. As was mentioned in the Experimental Section, samples were sonicated before measurements. The sonication energy was sufficient to overcome the cohesion forces in big aggregates and cause their partial disintegration. Consequently, the starting size of an aggregate is of the order of 1 µm in the two last experiments. However, kinetics of the aggregate growth corresponding to “reaggregation” differs substantially in the two cases. While aggregates rapidly attain a stable size in solutions aged during 6 days, the aggregation growth observed with the “oldest” sample is more rapid and very similar (see below) to the pattern

1258 Energy & Fuels, Vol. 21, No. 3, 2007

Figure 3. Average size of aggregates, Zav (nm), and intensity of scattered light expressed as a number of the photomultiplier counts, n, with a function of time. Aggregation kinetics of the system: crude oil (0.49 g), toluene (100 mL), and n-heptane (133 mL). ([) Sample prepared 24 h before measurement, with sonication for 15 min. (O) Sample of the previous experiment sonicated for 15 min.

observed with flocculated solutions. It must be noted that no precipitation was observed in the latter sample. The abovediscussed results indicate that the chemical factors influence the aggregation rate and aggregate stability. The aggregation process in the 2-methyl naphthalene/n-dodecane solvent is very slow. Moreover, the stability of aggregates in solution is much higher than that observed in the usual toluene/n-heptane solvent. In this case, the aggregation process cannot be analyzed in terms of the flocculation onset and asphaltene precipitation. The asphaltic phase forms an emulsion-like dispersion in the solvent phase. Consequently, the stability of this system with respect to the asphaltene precipitation is not governed by phase-splitting thermodynamics as compared to the model toluene/n-heptane solvent. Certain crude oils contain big aggregates that may be easily observed with an optical microscope. We think that aggregation and flocculation processes occurring in these oils are similar to those observed in the oil/2-methyl naphthalene/ n-dodecane system. 3.2. Aggregation of Asphaltenes above the Flocculation Onset. Results presented in Figure 3 indicate that the cohesion energy of asphaltene aggregates is weak. Consequently, these aggregates may be considered as a dispersed liquid phase. To confirm this hypothesis, we have determined the aggregation kinetics of mixtures [crude oil (0.49 g), toluene (100 mL), and n-heptane (133 mL)]. If the flocculated asphaltenes display a liquid-like character, the cohesion energy of flocs should be weak and flocs could be easily dispersed. We determined the aggregation kinetics of flocculated asphaltenes that were dispersed in a ultrasound bath. Measurements were repeated several times after consecutive dispersions of the sample by ultrasounds. Figure 3 presents the average size of aggregates, Zav, with a function of time, and corresponding values of the photomultiplier counts (n). The ratio of heptane/toluene in the solvent just slightly exceeded the value necessary to give rise to the flocculation. Prepared samples were equilibrated for 24 h and sonicated during 15 min just before measurement. Thus, flocculated and precipitated asphaltenes were redispersed before the experiment. The data set “a” corresponds to these results.

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The data set “b” was assessed just after the first experiment followed by 15 min of sonication. An important finding is that the sonication allows us to disperse completely flocculated asphaltenes. Indeed, particle size observed at the starting point of the experiment is very low. The aggregate growth continues during 60 min and is followed by a massive precipitation of asphaltenes as indicated by the curve of photomultiplier counts. The next stage of the aggregation process leads to the formation of large, slowly precipitating domains. The kinetic curves of series “a” and “b” are very similar. These results do not encourage a straightforward explanation of the flocculation process. Indeed, the liquid-like character of the aggregated asphaltene phase was confirmed, but the coalescence of the dispersed phase is more complex than that of a simple emulsion. A consecutive growth of two families of aggregates suggests the multiphase (at least three-phase) character of the system. Figure 4 shows that, despite the precipitation of the major part of asphaltenes, the distribution of aggregate size remains bimodal with a function of time. These results may be explained on the basis of the idea8,9 that asphaltenes are composed of two main fractions with principally different solubilities. The slightly soluble asphaltenes (A1) form stacked aggregates that are stabilized via interactions with more soluble molecules with higher solubility (A2). The later molecules (A2) adsorb at the A1 stacked aggregates in the edge-surface manner and stretch their aliphatic groups outward to form a steric-stabilization shell (corona) around the A1 core.8 According to this theory, the asphaltene colloidal particles have core-shell (core-corona) structures. The stabilizing role played by lateral alkyl chains could be explained in terms of the entropic balance of the system.7,10 In that approach, the alkyl moieties act as an entropy reservoir, tuning the equilibrium between two energetically close phases.10 This theory lets us interpret the split into two phases as a process thermodynamically driven by the entropy factor. The binary model based on the Flory-Huggins polymer theory lets us interpret the flocculation onset as a separation into the asphaltene-poor and asphaltene-rich phases.11 However, this model fails to predict either a complete phase diagram accounting for the precipitation of asphaltene or dynamic properties of systems containing asphaltene. These problems can be resolved by considering asphaltenes8 as a mixture of two components. Thermodynamic and dynamic properties of asphaltene solutions are well-represented with this model. The next series of measurements was performed with the system [asphaltenes (0.50 g), toluene (100 mL), and n-heptane (a, 150 mL; b, 200 mL; c, 300 mL)]. Results are presented in Figure 5. The essential difference between this experiment and the previous one is the use of asphaltenes instead of crude oils. Kinetic curves were obtained using solvents composed of n-heptane with toluene in the ratio varying from 1.5:1, which just ensures the flocculation onset, up to 3:1. In each case, aggregation kinetics were assessed immediately after the sample preparation and 4 days later. Before each experiment, precipitated asphaltenes were redispersed by sonication during 15 min. Results obtained in this case indicate that aggregates form a highly polydispersed system. The presence of aggregates with a diameter of several micrometers renders a trustful estimation of the size distribution impossible. Low values of photomultiplier counts indicate that only a small part of the flocculated (8) Gutierrez, L. B.; Ranaudo, M. A.; Mendez, B.; Acevedo, S. Energy Fuels 2001, 15, 624-628. (9) Evdokimov, I. N.; Eliseev, N. Yu.; Akhmetov, B. R. Fuel 2006, 85, 1465-1472. (10) Sorai, M.; Saito, K. Chem. Rec. 2003, 3, 29-39. (11) Wang, J. X.; Buckley, J. S. Energy Fuels 2001, 15, 1004-1012.



Figure 4. Size distribution curves illustrating the progress of aggregation. Measurement: (a) after 10 min, (b) after 40 min, (c) after 200 min, and (d) after 240 min. Data for the first experiment reported in Figure 3. Smaller values of the aggregates size in Figure 3 as compared with this figure result from the calculation procedure. Calculations in weight (Figure 3) favor greater values than calculations in the number of particles (this figure).

asphaltenes can be redispersed. We notice that the photomultiplier counts are stable in time in all experiments. Moreover, the number of counts increases with an increasing ratio of n-heptane/toluene and with the sample aging. The number of counts increases 6 times after the 6-day-long aging in the mixture of the n-heptane/toluene ratio close to 1.5:1 and doubles in that of the ratio 3:1. This can be explained by the presence of a stable particle population growing with the sample aging. Very slow reorganization of the structure of the asphaltene precipitate probably causes an increase of the fraction of redispersed aggregates. A comparison of results obtained for the crude oil and asphaltene solutions leads to the conclusion that oil components give a liquid-like character to the asphaltene phase and contribute to the stabilization of the bimodal distribution of asphaltene domains. 3.3. Impact of the Chemical Environment on the Flocculation. The stability of crude oil with respect to flocculation is mainly governed by the content and polarity of asphaltenes and the composition and properties of dispersing media.1,12-15 In the last part of the present study, we describe the influence of the chemical environment on the mechanism of flocculation. (12) Carbognani, L. Energy Fuels 2001, 15, 1013-1020. (13) Savvidis, T.; Fenistein, D.; Barre, D.; Behar, E. AIChE J. 2001, 206-211. (14) Hammami, A.; Phelps, C. H.; Monger-McClure, T. Energy Fuels 2000, 14, 14-18. (15) Mansouri, G. A. J. Pet. Sci. Eng. 1997, 17, 101-111.

Recent small-angle neutron scattering (SANS) studies16 suggested that the solvent entrainment within the aggregates varied from 30 to 50% (v/v). 3.3.1. Effect of AdditiVes. A chemical compound, the amount of which is of the order of the amount of the crude oil present in the system, was considered as an additive. We tested the influence of two additives: tetrahydrofuran (THF), a polar compound and an excellent solvent of heavy petroleum cuts, and n-octadecane, a long chain n-alkane with weak flocculating properties. Results presented in Figure 6 were obtained with the solution [crude oil (0.50 g), toluene (100 mL), n-heptane (133 mL), and THF (1.2 g)]. The average size of aggregates, Zav, and the photomultiplier counts, n, change with time following the same pattern as previously observed for the system [crude oil, toluene, and n-heptane] (Figure 3). Nevertheless, THF significantly modifies the aggregation mechanism. The maximal population of aggregates observed after 60 min corresponds to smaller aggregates with a smaller propensity toward precipitation, which is indicated by slowly decreasing the value of the photomultiplier counts. Figure 7 illustrates results obtained using n-octadecane as the additive [crude oil (0.41 g), toluene (100 mL), n-heptane (133 mL), and n-octadecane (0.45 g)]. The most interesting findings are multiple maxima on the Zav and n curves that probably correspond to the precipitation of different families (16) Gawrys, K, L.; Blankenship, G. A.; Kilkpatrick, P. K. Langmuir 2006, 22, 4487-4497.


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Figure 6. Average size of aggregates, Zav (nm), and intensity of scattered light expressed as a number of the photomultiplier counts, n, with a function of time. Aggregation kinetics of the system: crude oil (0.50 g), toluene (100 mL), n-heptane (133 mL), and THF (1.2 g).

of aggregates. It could be noticed that n-octadecane causes a decrease in the size of aggregates observed in the experiment. Two examples discussed above provide evidence for the sensitivity of the flocculating systems to different chemicals. A more systematic study of these effects would be necessary for understanding the flocculating behavior of asphaltenes in a multicomponent complex environment.

Figure 5. Average size of aggregates, Zav (nm), and intensity of scattered light expressed as a number of the photomultiplier counts, n, with a function of time. Aggregation kinetics of the system sonicated for 15 min immediately before the measurement: asphaltenes (0.50 g), toluene (100 mL), and n-heptane (a, 150 mL; b, 200 mL; c, 300 mL). (O) Measurements immediately after sample preparation. ([) Measurements 4 days after sample preparation. The flocculation onset corresponds to the ratio of n-heptane/toluene ) 1.3.

It may be noted that kinetic curves obtained with asphaltene and crude oil solutions differ. This can be easily explained in terms of the above discussion on the role played by additives. Resins and other component of the oil may stabilize asphaltene dispersions. 3.3.2. Effect of SolVents. It is well-known that aggregation and flocculation of asphaltenes are influenced by chemical properties of the solvent used. We have determined aggregation kinetics of asphaltenes in chloroform and in 2,3-dimethylnaphthalene using the following solutions: [crude oil (0.97 g), 2,3dimethylnaphtalene (100 mL), and n-heptane (235 mL)] and


Figure 7. Average size of aggregates, Zav (nm), and intensity of scattered light expressed as a number of the photomultiplier counts, n, with a function of time. Aggregation kinetics of the system: crude oil (0.41 g), toluene (100 mL), n-heptane (133 mL), and n-octadecane (0.45 g).


Figure 9. Average size of aggregates, Zav (nm), and intensity of scattered light expressed as a number of the photomultiplier counts, n, with a function of time. Aggregation kinetics of the system: crude oil (0.41 g), toluene (100 mL), n-heptane (133 mL), and chloroform (66.5 g). (O) Solution prepared immediately before the experiment. ([) Same sample as used in experiment 1, redispersed by sonication.

(coalescence of redispersed asphaltenes) in the initial 30 min. While the monotonous growth occurred in the first run, the characteristic maxima corresponding to the precipitation of different families of aggregates were noted in the further experiments. After the half hour, all three results became nearly identical. Similar results were obtained with chloroform solutions as illustrated in Figure 9. Thus, the initial aggregation of asphaltenes does not influence the aggregation and flocculation mechanism induced by n-alkanes. 4. Conclusions

Figure 8. Average size of aggregates, Zav (nm), and intensity of scattered light expressed as a number of the photomultiplier counts, n, with a function of time. Aggregation kinetics of the system: crude oil (0.97 g), 2-methyl naphtalene (100 mL), and n-heptane (235 mL). (O) Solution prepared immediately before the experiment. ([) Solution prepared 24 h before the experiment and sonicated for 15 min. (×) Same sample as used in experiment 2, redispersed by sonication.

[crude oil (0.41 g), toluene (100 mL), n-heptane (133 mL), and chloroform (66.5 g)]. The results are presented on Figures 8 and 9. The first data set presented in Figure 8 corresponds to the solutions prepared immediately before the experiment. Two further series were obtained with the same sample in two consecutive runs: the second run started 24 h after the sample preparation, and the third run started immediately after the second run was over. In both cases, the sample was sonicated just before the experiment. Significant differences were observed between the first run (flocculation in situ) and the next two runs

Results discussed above show that the phase-splitting transition leading to the formation of the asphaltene phase is of the liquid-liquid type. This transition is probably a spinodal decomposition, resulting in metastable phases. Such an assumption makes it possible for us to explain the aging of asphaltenes. According to results shown in Figure 5, the asphaltic phase transforms with time, changing the composition and structure. Because the phase splitting is probably entropy-driven, the asphaltene phase is an ordered mesophase, intermediate between the amorphous and liquid-crystalline phases. The aging entrains structure modification that eventually leads the configuration to an equilibrium. The kinetics of this process is very slow, and steady state is achieved in several days. We have shown, in section 3.3, that physical properties of the asphaltic phase are very sensitive to chemical parameters of this process. A comparison of the asphaltic phase precipitated from dissolved asphaltenes with that precipitated from dilute oil provides the most striking example. In the first case, the precipitate is solidlike, while it is liquid-like in the second. Our results clearly show that flocculating systems are multiphasic rather than biphasic. This approach makes it possible to integrate the concept of resins that can be considered as one of the aspaltene fractions. All results were obtained with the classical DLS setup. This technique imposes the work with very diluted solutions of petroleum materials. We have shown that the asphaltene


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aggregation and flocculation mechanism may be studied at these conditions. Yudin et al.17-19 proposed recently a new DLS method based on the analysis of the reflected light. This technique, particularly suited for petroleum fluids, was used to determine the flocculation kinetics in the crude oils. Kinetic

curves obtained in concentrated and highly diluted fluids display a similar behavior as found in the present work.

(17) Yudin, I. K.; Nikolaenko, G. L.; Gorodetskii, E. E.; Melikyan, V. R.; Markhashov, E. L.; Agayan, V. A.; Anisimov, M. A.; Sengers, J. V. Physica A 1998, 251, 235-244. (18) Yudin, I. K.; Nikolaenko, G. L.; Gorodetskii, E. E.; Markhashov, E. L.; Frot, D.; Briolant, Y.; Agayan, V. A.; Anisimov M. A. J. Pet. Sci. Technol. 1998, 395-414.

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Acknowledgment. We thank TOTAL - France for financing this research.

(19) 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.

Physical State and Aging of Flocculated Asphaltenes - Energy & Fuels

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