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Energy & Fuels 2004, 18, 1324-1328
Asphaltene Flocculation, Precipitation, and Liesegang Ring Fabricio Arteaga-Larios,† Eric Y. Sheu,*,‡ and Elias Pe´rez†,§ Instituto de Fı´sica, Universidad Auto´ noma de San Luis Potosı´, AÄ lvaro Obrego´ n #64, 78000 SLP, Me´ xico, Vanton Research Laboratory, Inc., 7 Olde Creek Place, Lafayette, California 94549, and Programa de Ingenierı´a Molecular, Instituto Mexicano del Petro´ leo, 152 Eje Central L. Ca´ rdenas 07720, Me´ xico Received October 6, 2003
Precipitations of Ratawi and Kuparuk asphaltene in toluene/heptane mixed solvents were investigated using a newly developed dripping test method. Precipitation of asphaltene solution at a critical heptane volume ratio can be readily identified. In addition, for the first time, a Liesegang-like phenomenon [Liesegang, R. E. Naturwiss. Wochenschr. 1896, 11, 353] was observed for the Ratawi asphaltene system. Similar but less-profound Liesegang rings were observed for asphaltene derived from Kuparuk crude oils. Clear crossover and a diffusive hyperfine structure were observed in the Ratawi asphaltene precipitation pattern, whereas asphaltene derived from Kuparuk crude oil showed only marginal crossover phenomenon. The patterns of the rings from Ratawi asphaltene seem to fit the space law of the Liesegang pattern. Both the post-nucleation Ostwald-Prager process and the pre-nucleation spinonal decomposition phase separation process are possible precipitation mechanisms.
I. Introduction Asphaltene has been known to precipitate and hinder oil production whenever the production pressure and temperature parameters drive the oil liquid into the precipitation envelope. To avoid asphaltene precipitation, dispersants or solvents have been developed and/ or are under development to combat this problem. A commonly used method to evaluate the effectiveness of these dispersants is to test the precipitation behavior of asphaltene-containing petroleum liquids in various mixed solvents (e.g., toluene/heptane mixtures) or use live oil under the pressure and temperature regulation directly. In a laboratory experiment, one can measure the flocculation or precipitation as a function of heptane content with or without the dispersants. Another option is to measure the precipitation behavior of the extracted asphaltene, instead of crude oils, in similar mixedsolvent series. In the study of asphaltene precipitation, it is wellknown that it differs from the pure chemical substances because asphaltene contains many compounds of similar yet different molecular structures. Because of this uniqueness, asphaltene precipitation may take a very different thermodynamic path from that of a pure substance. Many groups had reported asphaltene aggregation (or micellization) in organic solvents.1-6 Apparently, the aggregation mechanism is more complex * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Universidad Auto ´ noma de San Luis Potosı´. ‡ Vanton Research Laboratory. § Instituto Mexicano del Petroleo. (1) Sheu, E. Y.; Storm, D. A.; De Tar, M. M. J. Non-Cryst. Solids 1991, 131-133, 347. Sheu, E. Y.; De Tar, M. M.; Storm, D. A.; DeCanio, S. J. Fuel 1992, 71, 299.
than the commonly known hydrophobic energy driven micellization process. A full understanding of the aggregation process is still lacking. Nevertheless, previous experiments had revealed very slow aggregation kinetics,1,7 which is consistent with the kinetics of a nonhomogeneous system, based on a simple packing argument. Moreover, these micelle-like aggregates, or, more precisely, the reverse micelles do not grow in size,1,4 which is, again, consistent with the packing argument of a nonhomogeneous substance. Using Yen’s terminology,8 these nongrowing “micelles” are the “elementary particles”, consisting of 5-10 asphaltene molecules of different molecular structures.9,10 These molecules interlock themselves, thus preventing further growth in size. However, these elemental particles do interact with each other to form a fractal structure when the concentration is high enough.4,11-13 The extension of the correlation length (2) Anderson, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142, 497. (3) Taylor, S. E. Fuel 1992, 71, 1338. (4) Roux, J.-N.; Broseta, D.; Deme´, B. Langmuir 2001, 17, 5085. (5) Anisimov, M. A.; Yudin, I. K.; Nikitin, V.; Nikolaenko, G.; Chernoutsan, A.; Toulhoat, H.; Frot, D.; Briolant, Y. J. Phys. Chem. 1995, 99, 9576-9580. (6) Hayasaka, K.; Takanohashi, T.; Iino, M. Energy Fuels 1996, 10, 262. (7) Sheu, E. Y.; Liang, K. S.; Sinha, S. K.; Overfield, R. E. J. Colloid Interface Sci. 1992, 153, 399-410. (8) Dickie, J. P.; Yen, T. F. Anal. Chem. 1967, 39, 1847. Pollack, S. S.; Yen, T. F. Anal. Chem. 1970, 42, 623. Yen, T. F. In The Future of Heavy Crude Oils and Tar Sands; Meyer, R. F., Steele, C. T., Eds.; McGraw-Hill: New York, 1980; p 174. Yen, T. F. Adv. Chem. Ser. 1980, 195, 39-51. (9) Sheu, E. Y., Mullins, O. C., Eds. Asphaltenes: Fundamentals and Applications; Plenum Press: New York, 1995. (10) Brandt, H. C. A.; Hendriks, E. M.; Michels, M. A. J.; Visser, F. J. Phys. Chem. 1995, 99, 10430-10432.
10.1021/ef030168g CCC: $27.50 © 2004 American Chemical Society Published on Web 06/29/2004
Asphaltene Flocculation and Precipitation
in the fractal structure is likely the first step toward precipitation. Although interaction between elementary asphaltene particles has been shown and early-stage flocculation being observed, the precipitation mechanism and kinetics are not well understood. This may be attributed to its non-well-defined molecular structures, which draw less attention from academic researchers for studying its underlying physics. On the other hand, the engineers, who deal with production, require clear precipitation data to adjust the production strategies, instead of understanding the underlying physics that may or may not relate to the parameters that engineers need for field operations. Under such circumstances, detailed physics and its relevance to the production have been overlooked in petroleum research. The objective of this study is to find a link between the physical properties of asphaltene and its precipitation mechanism, thereby allowing scientists and engineers to develop better dispersants or processing strategy for preventing asphaltene precipitation. One requirement for field application is that the parameter to be used in the field should be measurable, obvious, and simple to use. In this work, a very simple dripping test method was developed to investigate asphaltene precipitation behavior. Other than providing a laboratory method for evaluating precipitation and its kinetics, this method surprisingly shows its potential to reveal the entire precipitation mechanism in a simple phenomenological manner. A Liesegang-like ring14 can be produced when the nonsolvent content reaches a critical concentration. This seems to be relevant to asphaltene flocculation. Further analysis of the rings suggests that the flocculation-diffusion-precipitation process resembles a typical Liesegang phenomenon, in terms of the position of the precipitated bands. II. Experimental Section Samples. Ratawi and Kuparuk asphaltenes were respectively extracted from vacuum residue (>1000 °F) and crude oil, using heptane as the precipitating solvent (weight-tovolume ratio of 1:40). The extracted asphaltenes were redissolved in toluene, votexed, and followed by slow rotation mixing for 24 h. The resulting asphaltene solutions were allowed to settle for another 24 h, to allow the systems to reach thermodynamic equilibrium. Asphaltenes were completely dissolved in toluene for the two concentrations that were studied (2 and 5 wt %). Before the dripping test, a known amount of heptane was added and then votexed for 15 s. The first dripping test was taken right after vortex mixing, and followed by certain time interval, which was dependent on each sample’s flocculation kinetics. Dripping Test. Figure 1 illustrates the dripping test setup. A micropipet is fixed in a lab stand 0.4 in. above a leveled microscope slide. The micropipet was set at 10 µL intake volumes. To do the test, 10 µL of sample was drawn from the (11) Liu, Y. C.; Sheu, E. Y.; Chen, S. H.; Storm, D. A. Fuel 1995, 74, 1352. (12) Espinat, D.; Rosenberg, E.; Scarsella, M.; Barre, L.; Fenistein, D.; Broseta, D.; Chapter, V. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998. (13) Janardhan, A. S.; Mansoori, G. A. J. Pet. Sci. Eng. 1993, 9, 17-27. (14) Liesegang, R. E. Naturwiss. Wochenschr. 1896, 11, 353.
Energy & Fuels, Vol. 18, No. 5, 2004 1325
Figure 1. Schematic depiction of the ring creator. The pattern is formed by dripping a 10 µL drop of asphaltene solution onto the glass slide. A 100 µL full-scale pipet was used to create an evenly distributed ring or a Liesegang-like pattern. sample vial (without moving the micropipet) and the 10 µL sample was gently dripped onto the microscope slide. Following the dripping, the slide was kept in place until the solvent was completely dried and no liquid moving was occurring. It is a rather simple experiment. However, one needs to maintain consistency, to ensure the formation of a symmetric pattern and an analyzable Liesegang pattern, if it happens.
III. Liesegang Ring Liesegang phenomena has been found in a variety of formations in nature, from agate rocks and gold veins to the growth of bacteria in agar and gallstones.15 Using two reacting reagents, noted here as A and B, a typical Liesegang pattern can be formed. Reagent A is the outer electrolyte and the mobile phase, whereas reagent B is the inner electrolyte predissolved in a gel and situated in a test tube. At time t ) 0, reagent A, which has a much higher concentration than reagent B (usually ∼200 times greater than that of reagent B) is poured into the test tube. As reagent A diffuses down the test tube, its concentration is above a threshold needed to react with reagent B and produce compound C. C is insoluble in its environment, thereby leading to precipitation. The reaction front continues to move down the tube via gravitational forces, leaving behind a stationary zone for precipitant C, which forms the Liesegang band. As the Liesegang band is formed, the zone between the Liesegang band and the moving reaction front becomes heavily depleted, in regard to reagent A. In this zone, the concentration of reagent A is below the threshold and no reaction occurs; this becomes the clear band. At the beginning, this Liesegang clear zone alternates frequently, because of the high concentration of reagent A traveling through it. As it goes down farther, amount of reagent A become less and less. The reaction takes longer and longer, resulting in the spacing between Liesegang bands becoming larger and larger. The aforementioned process description suggests that the Liesegang rings result from a discontinuously precipitation process and the precipitated bands are parallel to the surface of the diffusion fronts. Liesegang (15) Barkema, G. T. Phys. Rev. E 1996, 53, 2017.
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patterns are known to follow certain scaling laws, although it is never quantitatively rigorous. There are three well-known scaling laws, in regard to a Liesegang pattern. First, the band position, which is an indication of the formation kinetics, follows a simple scaling law:
xn ≈ tR
(1)
where xn is the position of the band (measured from the origin to the center of the band) and t is the time required for the diffusion front to travel to the xn position. This law seems to be true for all measurable experiments. The second scaling law is known as the spacing law. This is the interval between two rings increases according to a geometric law
xn ) Q(1 + p)n
(2)
Figure 2. Photograph of the dripping test for 5% Ratawi asphaltene in a 60/40 heptane/toluene mixed solvent. No distinctive pattern was observed. The lightly tilted straight line is the edge of the slide.
where p is the space coefficient, which is often found to be in the range of 0.05-0.4. In some cases, p was found to be negative, which is known as the reverse spacing. The last law is the width law, which describes the scaling of the bandwidth with its position:
wn ≈ xβn
(3)
where wn is the width of the nth band. This law is less certain, often because of bandwidth fluctuation in the measurement. Although eqs 1-3 are still under modification and discussion, they nevertheless explain many electrolyteelectrolyte reaction-diffusion processes.15-19 In this work, the dripping test follows a different process. Instead of reaction-initiated precipitation, it is more similar to a process of evaporation or surface wetting and/or a viscosity-induced process. Nevertheless, the patterns so obtained were describable by the same equations used to describe the Liesegang patterns. Recent work from Warner et al.20-22 clearly showed that patterns could be formed via the evaporation of solvent, especially when nucleation holes are present. These holes can be nanoparticles or the two-dimensional (2D) hetreogeneities of the material. This might be suitable for describing the patterns formed in this work by asphaltene solutions. IV. Results Figure 2 shows a typical dripping test of a 5% Ratawi in a toluene/heptane (40/60) mixed solvent. No Liesegang-like patterns were observed. This pattern was sustained for over 4000 min, suggesting that the system is, at most, in the flocculation state but is not precipitat(16) Ga´lfi, L.; Ra´acz, Z. Phys. Rev. A 1988, 38, 3151. (17) Larralde, H.; Araujo, M.; Havlin, S.; Stanley, H. E. Phys. Rev. A 1992, 46, 855. (18) Ben-Naim, E.; Redner, S. J. Phys. A: Math. Gen. 1992, 25, L575. (19) Araujo, M.; Havlin, S.; Rosenbaum, R.; Stanley, H. E. Phys. Rev. Lett. 1993, 70, 1461. (20) Warner, M. R. E.; Craster, R. V.; Matar, O. K. J. Colloid Interface Sci. 2003, 267, 92-110. (21) Warner, M. R. E.; Craster, R. V.; Matar, O. K. J. Colloid Interface Sci. 2003, 268, 448-463. (22) Warner, M. R. E.; Craster, R. V.; Matar, O. K. Phys. Fluids 2002, 14, 40404054.
Figure 3. Photograph of the dripping test for 5% Ratawi asphaltene in a 64/36 heptane/toluene mixed solvent at 5 min after mixing. A clear Liesegang-like pattern was profoundly exhibited.
ing. As the heptane content increases to 64%, the precipitation was clearly shown (see Figure 3) and a Liesegang-like pattern appeared ∼5 min after mixing. Other than its fast precipitation kinetics, one notes that the there are trace amounts of asphaltene residing between bands. This pattern quickly evolved into a thick fluid after ∼1 h (see Figure 4), where some ring patterns were observed. Figures 5 and 6 show the same tests for Kuparuk asphaltene. For a 60/40 toluene/heptane mixed solvent (Figure 5), no pattern was observed; however, the edge seems to be much better defined and thicker than Ratawi asphaltene. For 64/36 toluene/heptane mixtures, rings were observed at the outer fronts, similar to that for Ratawi at 1 h. This pattern was sustained for as long as 4000 h, suggesting that the system is not forming a network, although its viscosity seems to increase, to make the inner area much darker (Figure 6). Interestingly, a relatively wide darker band formed between the inner lighter range and the outer range where rings were exhibited. V. Analysis and Discussion Figure 3 was analyzed according to eqs 1-3. Because the kinetics was too fast in this simple experiment to
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Figure 7. Analysis of Figure 3 using the position law of a Liesegang pattern. The positions that are measured closely follow a Liesegang pattern with p ) 0.155.
Figure 4. Photograph of the dripping test for 5% Ratawi asphaltene in a 64/36 heptane/toluene mixed solvent at 1 h after mixing. Some rings can be seen at the outer fronts. They may be Liesegang-like.
Table 1. Extracted Q and P Values Using Eq 2 at Four Directions angle of the line used for analysis
amplitude, Qa
P valuea
3.17 3.22 3.1 3.27
0.156 0.157 0.155 0.156
0 (positive x-axis) 180 (negative x-axis) 90 (positive y-axis) 270 (negative y-axis) a
See eq 2.
explain the relative positions very well. The p values can be extracted by using any diameter of the ring. Table 1 shows the p values extracted from the different diameters that have been drawn. All p values are ∼0.155. VI. Discussion
Figure 5. Photograph of the dripping test for 5% Kuparuk asphaltene in a 60/40 heptane/toluene mixed solvent. No distinctive pattern was observed. The lightly tilted straight line is the edge of the slide. Apparently, the edge is more concentrated in asphaltene.
Figure 6. Photograph of the dripping test for 5% Kuparuk asphaltene in a 64/36 heptane/toluene mixed solvent at 5 min after mixing. Some rings were formed at the outer region.
quantify the time for each band to form, only the space law was used to examine Figure 3. Figure 7 shows the analysis using the position law. With the five identifiable bands, the space law can
Asphaltene Precipitation. From a practical application point of view, the method proposed here is a simple method, yet allows one to quickly distinguish a system that remains stable (Figures 2 and 5), undergoes flocculation only (Figure 6), or undergoes flocculation and then precipitation (Figures 3 and 4). This is a method that is based on rather complicated physical processes that may combine wetting, evaporation, flocculation, and precipitation processes. However, the effect of their integration tells a good story for engineers or petroleum researchers to develop chemicals or processes for treating asphaltene-related field issues. Liesegang Ring. Although there are many models proposed to explain the Liesegang pattern, there is not a single model that can fully explain all of the Liesegang patterns observed, such as reverse spacing, crossover, the limited amount of precipitated particles between bands, and hyperfine ring or double banding structures. The supersaturation theory of Ostwald is the simplest theory and has been considered plausible by many researchers.23-27 In Ostwald’s supersaturation theory, the formation of bands is considered to be a spatially discontinuous nucleation process, with the sharp rings being preceded by the onset of a moving front with characteristic turbidity. It originates from the reaction of the moving reagents A and B. As the insoluble C particles are produced, the particles agglomerate to form flocs and the subsequent turbid larger structure, and these particles gradually precipitate to form the sharp (23) Galfi, L.; Raacz, Z. Phys. Rev. A 1988, 38, 3151. (24) Dee, G. T. Phys. Rev. Lett. 1986, 57, 275. (25) Smith, D. A. J. Chem. Phys. 1984, 81, 3102. (26) Pillari, K. M.; Vaidyan, V. K.; Ittyachan, M. A. Colloid Polym. Sci. 1980, 258, 831. (27) Talanquer, V. J. Chem. Educ. 1994, 71, 58.
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rings. This implies that it is a post-nucleation process. Because asphaltene has been known to form a fractal structure at high concentration, the growing process is similar to the C particles in an A + B f C process. In this regard, the asphaltene flocculation and precipitation may be modeled as another Liesegang process. However, one notes that the solvent evaporation is a crucial ingredient that induced the flocculation and precipitation. Thus, the recent theory proposed by Warner et al.20-22 should be used, although one may still be able to phenomenologically argue from a simple precipitation point of view. This simple argument can be stronger if all three scaling laws are met. Unfortunately, there are not enough rings to perform a detailed analysis. However, there are several noticeable points worth mentioning. First, the smeared precipitated material is clearly observed in Figure 3, indicating that an aggregation process exists after the formation of C. Second, crossover patterns were observed. This supports a post-nucleation scenario, as discussed in a recent report.28 However, Figure 6 provides a very different view. There are large amount of smeared precipitated material entrapped in the center region inside the first well-defined ring. This seems to suggest a pre-nucleation process. Moreover, the pattern in Figure 3 fits a space law, which can only be predicted from a pre-nucleation argument.24,29,30 If one treats the asphaltene precipitation as a normal Liesegang pattern, then post-nucleation may not be the case. On the other hand, if one uses the pre-nucleation view, the space law is describable, and the Figure 6 phenomenon also may be explainable. This is to say that the formation of C should follow a “right” dynamics, to form the immovable precipitate D. Racz31 proposed the dynamics to be “hydrodynamical” while imposing a conservation of C materials and used the Chan-Hilliard equation with the Landau-Ginsburg free energy. Furthermore, Racz ignored the thermal noise and any (28) Cabarcos, L. E.; Kuo, C. S.; Scala A.; Bensil, R. Phys. Rev. Lett. 1996, 77, 2834. (29) Chopard, B.; Luthi, P.; Droz, M. Phys. Rev. Lett. 1994, 72, 1384. (30) Antal, T.; Droz, M.; Magnin, J.; Racz, Z.; Zrinyi, M. J. Chem. Phys. 1998, 109, 9479. (31) Antal, T.; Droz, M.; Magnin, J.; Racz, Z. Phys. Rev. Lett. 1999, 83, 2880.
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source terms from the reaction that may perturb the spinodal decomposition. Racz conducted his simulation according to this assumption and arrived at a satisfactory space law and the Matalon-Packter law. However, one must be aware of the fact that a band can be formed by nucleation; the spinodal decomposition scenario should not be taken as a universal law. This is to say that one should collect enough evidence before confirming what mechanism is really followed to form the asphaltene rings. Warner’s approach20-22 seems to be a reasonable one. It is to be investigated in a future publication. VII. Conclusion A Liesegang-like pattern was observed for the first time when asphaltenes were precipitated using toluene/ heptane mixed solvent and a dripping test method. There was a subtle difference between the Ratawi asphaltene that was derived from vacuum residue and the Kuparuk asphaltenes that were derived from crude oil. Ratawi asphaltene forms a much more pronounced Liesegang-like ring pattern. As noted by Cabarcos et al.,28 crossover and hyperfine patterns were observed. Appropriate theoretical development for describing this phenomenon is needed to explain the precipitation process properly. Both nucleation and spinodal decomposition arguments seem to take some credit from our observation. In addition to the Ratawi case (Figure 3), the Kuparuk precipitation forms a somewhat different pattern from a typical Liesegang pattern, with apparent interference from dynamics after the formation of the stationary precipitant. This unusual pattern is observed for the first time and should be a fruitful subject for Liesegang theorists. Acknowledgment. E.Y.S. would like to thank the Chevron Research and Technology Company and Mr. Steve Chang for funding this research (under Contract No. SAP4517159). The supplying of the Kuparak asphaltene sample, provided by Dr. Carlson, is greatly appreciated. F.A.L. and E.P. thank IMP (Grant No. FIES-98-101-I) for the financial support. EF030168G
