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Electrokinetic Characterization of Asphaltenes and the Asphaltenes-Resins Interaction Gaspar Gonza´lez,*,† Guilherme B. M. Neves,† Sandra M. Saraiva,†,‡ Elizabete F. Lucas,§ and Marcia dos Anjos de Sousa§ Petrobras Research Center, Cidade Universita´ ria, Q.7, Rio de Janeiro, RJ 21949-900, Brazil, and Instituto de Macromole´ culas/UFRJ, Centro de Tecnologia, bloco J., P.O. Box 68525, 21945-970, Rio de Janeiro, Brazil Received October 27, 2002
The electrophoretic mobility of asphaltene particles formed by precipitation induced by the addition of n-heptane to asphaltene solutions in toluene was measured in water and in nonpolar media. Dispersed in water, the particles presented a negative electrophoretic mobility, whereas in toluene their mobility was positive and around 2.0 × 10-9 m2/V s. When resins were present in the precipitating medium, a coprecipitation of these fractions occurred, indicating an adsorption or binding process of the resins on the nascent asphaltene particles. This interaction, however, did not significantly change the electrophoretic mobility of the asphaltene particles. In contrast with commercial dispersants, resins fail to stabilize asphaltene dispersions. This result indicate that the descriptions representing asphaltenes as a solid phase intrinsically insoluble in hydrocarbon media dispersed by adsorbed resins molecules does not accurately represent the structure of these fractions
Introduction Asphaltenes are generally defined as the portion of the crude oil insoluble in n-alkanes, such as n-heptane or n-pentane, yet soluble in benzene or toluene.1 More accurate definition describes the asphaltenes as hard solids with melting points well above 150 °C and specific gravity up to 1.2. They are 0-60% soluble in petroleum naphtha but 50-60% soluble in carbon disulfide and are readily precipitated from oils by n-heptane, an occurrence with forms the basis of method of quantitative assessment of asphaltene content of oils. Resins and asphaltenes are usually distinguished on the basis of their separation procedure. Precipitation by propane separates resins and asphaltenes from the remainder of the crude, while the addition of n-heptane separates the soluble resins from the insoluble asphaltenes. The resins included minor amounts of free acids and esters but the bulk of the resins is composed of molecules similar to, but less aromatic than, the asphaltenes.2 More recently, the asphaltenes and resins fractions have been considered an almost continuous spectrum of different polyaromatic species, each of them having its own solubility parameter.3 Although electric charge and potential may represent important factors in the colloidal stability of crude oil * To whom correspondence should be addressed. Fax: 55 21 3865 6796. E-mail:
[email protected]. † Petrobras. ‡ Present address: COPPE, UFRJ, Rio de Janeiro, Caixa Postal 68505, Rio de Janeiro, RJ, Brazil. § UFRJ. (1) Gawrys, K. L.; Spiecker, P. M.; Kilpatrick, P. K. Prepr.-Pet. Chem. Div. 2002, 47, 332-335. (2) Kinghorn, R. R. F. An Introduction to the Physics and Chemistry of Petroleum; John Wiley & Sons: New York, 1983. (3) Porte, G.; Zhou, H.; Lazzeri, V. Langmuir 2003, 19, 40-47.
and asphaltene dispersions, the sign, magnitude and/ or the mechanism of formation of the charge on the surface of the asphaltene particles is still not well established. Furthermore, neither the contribution of surface charge to asphaltene dispersions stability nor the effect of resins has been experimentally substantiated so far. Since the early work of Preckshot4 and Katz,5 many authors6,7 have studied the electrodeposition of crude oil or asphaltenes dissolved in different solvents as a method to infer the charge on the asphaltene particles. An obvious drawback of the electrodeposition studies is the high potentials usually applied to induce deposition, since at least in one work8 it has been reported that the electrolyzed oil contained a considerable quantity of solid asphaltic material that was not present in the original oil. Electrophoretic mobility measurements permit direct access to this information. Write and Minesinger,9 using a conventional electrophoresis apparatus, measured the electrophoretic mobility of asphaltenes dispersed in nitromethane at a concentration of 0.1%. The authors studied 24 samples of asphaltenes separated from asphalts from different sources and observed positively charged particles with electrophoretic mobilities in the range of 3.2-5.4 µm s-1/V cm-1. Values within this range for asphaltenes extracted from (4) Preckshot, G. W.; DeLisle, N. G.; Cottrell, C.; E. Katz, D. L. Trans. AIME 1943, 151, 188-205. (5) Katz, D. L.; Beu, K. E. Ind. Eng. Chem. 1945, 37, 195-200. (6) Lichaa, P. M.; Herrera, L. 1975, Soc. Pet. Eng. AIME, Paper No. 5304. (7) Taylor, E. S. Fuel 1998, 77, 821-827. (8) Moore, E. W.; Crowe, C. W.; Hendrickson, A. R. J. Pet. Technol. 1965, 1023-1028. (9) Wright, J. R.; Minesinger, R. R. J. Colloid Sci. 1963, 18, 223236.
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Preparations of Asphaltenes and Resins Samples. The polar fractions, asphaltenes, and resins were separated from samples of crude oil from Marlim and Santos basin, from a solid deposit collected from a producing well from Santos field, and from the residue of the deasphalting unit from a local
refinery. The composition of the original sources of asphaltenes and resins are presented in Table 1. The asphaltene fractions were extracted from the samples following the IP 143 standard procedure using an aliphatic solvent (n-pentane or n-heptane) as precipitating agent. In this standard procedure the alkaneinsoluble residue obtained from crude oil is dissolved in a small volume of toluene, filtered through Whatman 42 filter paper to remove solid particles, and dried under vacuum in a rotating evaporator. This solid material, alkane insoluble but toluene soluble, was subsequently ground in a mortar to obtain a powder that we called “consolidated asphaltene particles”. For some samples, the n-heptane-soluble material was eliminated by extracting with n-heptane until the brownish color typical of the asphaltic fractions was no longer evident in the solvent. In another preparation, the asphaltenes were dissolved in toluene and precipitated by the addition of n-heptane, leaving out the drying step. In this case, dispersion in the toluene/ heptane mixture is formed. We called this sample “nonconsolidated asphaltene particles”. The resins were separated by adsorption onto an attapulgus clay adsorbing resin, followed by elution with a chloroformmethanol solution according to the ASTM D2007-93 method. Solvents and Additives. n-Pentane and n-heptane were purchased from Grupo Quı´mica (Rio de Janeiro, Brazil). The additives used were oil-soluble polymeric dispersants developed by companies that supply chemicals for the petroleum industry. Triton X-100 was obtained from Rohm and Haas. Solvents and additives were used as received. Electrophoretic Mobility. The electrophoretic mobility was measured using a Zetasizer 2c (Malvern Instruments). The asphaltene dispersions used to determine the electrophoretic mobility in aqueous media were prepared according to the following procedure. To ensure the wetting of the solid particles by the liquid phase,13 a 100 mg sample of asphaltenes was initially dispersed in 10 mL of an aqueous mixture containing 90% of ethyl alcohol. Subsequently, the dispersion was diluted with a solution containing potassium or calcium nitrate and the nonionic surfactant Triton X-100 as dispersant. The final concentrations were 1 mM in electrolyte and 0.1 mM Triton X-100. At this low concentration, the nonionic surfactant does not change to a great extent the surface charge of the solid particles.14 The samples of asphaltenes containing adsorbed resins were prepared following the procedure described below for the adsorption on consolidated asphaltene particles. In the case of nonaqueous dispersions containing nonconsolidated particles, the cell for measurements in organic media supplied with the Malvern apparatus was used. The particles were prepared as described in the section on the adsorption of resins on nonconsolidated asphaltene particles. Adsorption of Resins on Asphaltene Particles. Two alternative procedures were used to determine the electrophoretic mobility of asphaltene particles: (1) Adsorption on Consolidated Particles. Asphaltenes separated from Marlim crude oil by precipitation with n-pentane were powdered using a laboratory mortar and their n-heptanesoluble material was eliminated by extraction with n-heptane. A certain amount of this powdered material was added to solutions of resins in n-heptane with different concentrations. The final asphaltene contents in the solutions were 44.5 and 189 ppm and the resin concentration ranged from 0 to 1000 ppm. The tubes containing the resin were sealed and maintained under agitation in a shaker with thermal control at 30 °C for 4 h. Studies on the adsorption of asphaltic fraction on minerals have shown that this period is enough to attain adsorption equilibrium.15 The samples were centrifuged, and the adsorption of resins was determined from its reduction in
(10) Kokal, S.; Thompson, T.; Schram, L.; Sayegh, S. Colloids Surf., A 1995, 94, 253-265. (11) Henry, J. D.; Jacques, M., T. AIChE J. 1977, 239, 607-609. (12) Leontaritis, K. J.; Mansoori, G. A. J. Pet. Sci. Eng. 1988, 1, 229-239.
(13) Zisman, W. A. In Contact Angle, Wetting and Adhesion; Fowkes, M., Ed.; Advances in Chemistry Series, 43; American Chemical Society: Washington, DC, 1964; pp 1-51. (14) Kratohvil, S.; Matijevic, E. Colloids Surf. 1982, 4, 179-183. (15) Gonza´lez, G.; Middea, A. Colloids Surf. 1988, 33, 217-229.
Table 1. Composition and Physical Properties of the Samples Used To Separate Asphaltenes and Resins Marlim crude oil asphaltenes (wt %) resins (wt %) API density (g/mL) viscosity (cSt) paraffin (Shell) saturated (%) aromatic (%) a
Santos crude oil
Santos deposit
2.0-2.3
0.18
22.0
25.1 19.2 0.9352 418 (20 °C) 1.25 26 21.2
29.3 4.0 40.2 0.8191 7.6 (20 °C) 5.3 52.0 62.0 13.5 12.0
refinery residuea 16 17.23 1.049 6670 (60 °C) 3.3 43.0
Typical composition of the local refinery residue.
a sample of crude oil from Alberta have also been obtained more recently.10 Henry et al.,11 through direct microscopic observation of the motion of the particles contained in coal derived liquids in an electric field, concluded that the particles presented a net positive surface charge ascribed to positively charged asphaltenes. With the exception of some heavy crude oils,7 most of the results indicate that asphaltenes present a positive charge. Partially on the basis of these results, a picture has been widely used in the literature in which the positively charged asphaltene particles would be maintained dispersed or peptizised in the organic media by adsorbed, negatively charged, resins. In this representation, the resins would act as dispersants for the intrinsically unstable asphaltene particles. The stability of the asphaltene fraction as a colloidal dispersion is very important during the production operations. The precipitation and deposition of solid asphaltenes in pipe lines and production facilities may cause serious and expensive problems.12 During catalytic hydrocracking, large amounts of sludge and sediment can form, ostensibly due to the flocculation of asphaltenes during processing. Asphaltenes deposition within reservoir rocks has been blamed for pronounced reductions in well productivity. Asphaltenes have also been found to facilitate the formation of extremely stable water-in-crude oil emulsions. Understanding asphaltene chemistry and the fundamental mechanism of colloid formation has been the driving force behind much petroleum research for the last half-century.1 In this article we study the electrophoretic mobility of asphaltenes dispersed in aqueous electrolyte solutions and in hydrocarbon solvents. In addition, the effect of resins on the electrophoretic mobility and stability of asphaltene dispersions is assessed. Using this experimental information, the mechanisms of surface charge generation, models for the colloidal stability of asphaltenes, and the asphaltenes-resins interactions in nonaqueous media are briefly discussed. Experimental
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Figure 1. Electrophoretic mobility against pH for asphaltenes dispersed in 1 mM KNO3 solution. (a) Asphaltenes extracted from Santos basin crude oil. (b) O, Santos crude oil deposit; b, refinery residue. concentration measured using a spectrophotometer, UV-240 Shimadzu at 400 nm. A small amount of asphaltenes was dissolved during the agitation process: this was measured in a blank, without resins, and discounted to calculate the adsorption. (2) Adsorption on Nonconsolidated Asphaltene Particles. In an alternative procedure to determine the adsorption of resins by asphaltenes, a concentrated solution of asphaltenes in toluene (1.2 mL of a solution containing 20 mg of asphaltenes) was mixed with 20 mL of a solution of resins in n-heptane. The final content of asphaltenes in the solutions was 94 mg/L and the resin concentration ranged from 0 to 900 mg/L. The adsorption of resins was determined using the previously described procedure. Stabilization of Asphaltene Dispersions. A 1.2 mL sample of a solution containing 40 mg of asphaltenes in toluene were mixed with 20 mL of a solution containing 0-300 mg of resins in n-heptane. The final concentrations in the flocculation media were 188 mg/L for asphaltenes and 0, 94.3, 188.7, and 283.0 mg/L for resins. The mixtures were shaken during 10 min, and samples were removed to measure the changes of absorbance at different times, using a double-beam UV3101PC Shimadzu spectrophotometer with 10 nm optical path cells. This procedure was repeated after 4 h, when the adsorption process was accomplished. This test permits to evaluate qualitatively the stability of the asphaltene dispersions. A similar procedure was used to assess the effect of polymeric deposition inhibitors on the stability of asphaltene dispersions. Two commercial samples of asphaltene deposition inhibitors (here called inhibitor A and inhibitor B) were also used.
Results Electrophoretic Mobility of Asphaltenes Dispersed in Aqueous Media. Figures 1 and 2 present the electrophoretic mobility as a function of pH for asphaltenes dispersed in aqueous KNO3 solution. The samples were extracted from four different sources by precipitation with n-pentane. One sample was extracted from Santos basin crude oil, the other from a deposit collected in a producing well of Santos field, the third
Figure 2. Electrophoretic mobility against pH for asphaltenes extracted from Marlim crude oil dispersed in electrolyte solutions. (2) Ca(NO3)2 1 mM. (0) KNO3 1 mM.
was separated from the solid residue produced in the deasphalting plant of a local refinery, and the fourth was extracted from crude oil from the Marlin field. The effect of calcium ions is included in Figure 2. All the samples presented a negative surface charge in the whole pH range, and their electrophoretic mobilities increased as the pHs of the solutions increased. The isoelectric points may be extrapolated to a value of around pH 2. When calcium nitrate was used instead of sodium nitrate as supporting electrolyte, a reduction in electrophoretic mobility and a shift of the isoelectric point toward higher pH values were observed. As shown in Figure 3, when the asphaltenes separated with n-pentane were extracted with n-heptane, the negative electrophoretic mobilities of the particles
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Gonza´ lez et al. Table 3. Adsorption of Resin on Consolidated Asphaltene Particles concentration of resins (mg/L) initial concn final concn 330.6 454.4 570.1 621.6 741.3 1039.6
299.4 421.4 471.5 541.2 615.5 1007.0
adsorption (mg of resins/g of asphaltenes) 87.5 34.5 162.8 88.0 199.2 116.0
Table 4. Electrophoretic Mobilities for Recently Formed Asphaltene Particles and for Asphaltenes-Resins Aggregates
Figure 3. Electrophoretic mobility against pH for asphaltenes extracted from Marlim crude oil dispersed in KNO3 1mM. (b) Asphaltenes separated with n-pentane and extracted with n-heptane. (O) Same sample, containing 34.5 mg of resins per gram of asphaltenes. (9) Same sample, containing 116 mg of resins per gram of asphaltenes. Table 2. Electrophoretic Mobilities at Various pH Values for Asphaltenes and Asphaltenes Containing Adsorbed Resins Molecules pH 3
4
10
adsorption (mg of resin/g of asphaltenes)
mobility (108 m2/V s)
0; original 0; extracted 34.52 88.0 116.0 0; original 0; extracted 34.52 88.0 116.0 0; original 0; extracted 34.52 88.0 116.0
-0.5 -0.6 0.5 -0.2 0.60 -0.8 -0.75 -0.25 0.10 -0.06 -1.70 -1.56 -1.45 -1.20 -1.00
increased. The adsorption of resins on these particles caused a reduction in the electrophoretic mobility, and for high surface coverage, a charge reversal was observed at low pH. Table 2 summarizes part of the results for various pH values. Electrophoretic Mobility of Nonconsolidated Asphaltene Particles in Nonaqueous Media. To assess the electrokinetic potential of the asphaltene particles in nonaqueous media, the electrophoretic mobility for recently formed asphaltene dispersions was measured. The particles were flocculated from a concentrated solution of asphaltenes in toluene by the addition of n-heptane, and the measurements were carried out directly in the precipitating media. The particles were positively charged presenting a low electrophoretic mobility, around 1.9 × 10-9 m2 V-1 s-1 (Table 4). Similar values were obtained for other asphaltene concentrations indicating that the value was independent of concentration. The addition of resins to the solution of asphaltenes, prior to precipitation with
resins concn (mg/L)
mobility (108 m2/V s)
0 100 200 300 1000
0.194 ( 0.103 0.168 ( 0.014 0.180 ( 0.027 0.184 ( 0.022 0.190 ( 0.02
n-heptane, did not change the electrophoretic mobility of the flocculated particles. Adsorption of Resins by Asphaltene Particles. The data for the adsorption of resins by consolidated asphaltene particles are presented in Table 3. Although it is clear that resins are removed from the solution by the asphaltene particles, the process did not follow a well-defined adsorption pattern. The amount adsorbed was dependent on the initial concentration of asphaltene particles as well as on the initial concentration of the dissolved resins. The amounts of resins adsorbed, up to 162 mg/g, also appears rather high for consolidated solid particles. The values presented in Table 3 must be compared with the adsorptions of asphaltic fractions on minerals, where surface saturations values lower than 25 mg/g have always been obtained.16,17 Furthermore, Langmuir or Freundlich type isotherms are, in general, followed.18 The adsorption on nonconsolidated particles is shown in Figure 5. In this case a saturation concentration of 600 mg of resins per gram of asphaltenes was attained, at an equilibrium resins concentration of about 600 mg/L. Stability of Asphaltene Dispersions and the Effect of Resins and Other Additives. Typical sedimentation curves obtained for asphaltene suspensions are shown in Figure 6; in all the cases it was observed that sedimentation is fast, showing no evidence of enhanced stabilization by the addition of resins. The results indicate that resins did not show any effect on the flocculation of asphaltene dispersions. Even for a concentration of 300 ppm corresponding to ratio resins/asphaltenes of 1.5, this fraction was ineffective in the prevention of the flocculation and sedimentation of the asphaltene dispersion, which was observed within 5-10 min. Similar results were obtained in studies intending to assess the effect of resins on the precipitation onset of asphaltenes. In this case, concentrations of resins 3-5 times higher than those of asphaltenes (16) Gonza´lez, G.; Middea, A. J. Dispersion Sci. Technol. 1987, 8, 525-548. (17) Acevedo, S.; Ranaudo, M. A.; Garcı´a, C.; Castill, J.; Ferna´ndez, A.; Caetano, M.; Gonc¸ alves, S. Colloids Surf., A 2000, 166, 145-152. (18) Gonza´lez, G.; Moreira., M. B. C. In Asphaltenes and Asphalt; Yen, T. F., Chiligarian, G. V., Eds.; Elsevier Science: New York, 1994; pp 207-231.
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Figure 4. Schematic representation of the particle charge generation process by acidic amphiphiles. (A) For dispersions in aqueous media the adsorption of dissociated acidic amphiphiles produce negatively charged particles. (B) For dispersions in nonpolar media, the adsorption of nondissociated amphiphiles followed by ion exchange at the particle surface produce positively charged particles.
Figure 5. Adsorption of resins by nonconsolidated asphaltene particles.
were necessary to increase the precipitation onset from a ratio n-heptane/toluene of 1.5 to 2.0 or 2.5.19 Figures 7 and 8 present the performance of commercial asphaltene deposition inhibitors. In contrast to resins, these additives provide a long-term stability to the asphaltene dispersions at very low concentrations. Additive A seems to be more effective since a concentration of 0.125 ppm was enough to prevent the deposition of the asphaltene particles. For additive B, a concentration of 1 ppm was necessary to attain stability, which also represents an excellent performance. (19) Sousa, M. A. M. Sc. Thesis, Instituto de Macromole´culas, Universidade Federal de Rio de Janeiro, Rio de Janeiro, Brazil, March, 2000.
Figure 6. Effect of resins on the flocculation of asphaltene particles. Resins Concentrations (mg/L): (4) 0, (2) 100, (0) 200, (9) 300.
Discussion The origin of the asphaltene samples assayed was very different. Santos crude oil is unstable and the asphaltenes from this oil tend to form deposits. Therefore, the asphaltenes separated from this oil should represent the fraction remaining in the oil that did not separate during the production process. On the contrary, the asphaltenes separated from the production deposit should preferentially contain those components with a more evident tendency to flocculate. Marlin oil contains a larger amount of asphaltenes than that of Santos, but it is stable, and depositional problems would not be
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Figure 7. Effect of the asphaltene deposition inhibitor A on the flocculation of asphaltene particles. Additive concentration (mg/L): (O) 0, (b) 0.006, (0) 0.020, (9) 0.025, (4) 1.25.
Figure 8. Effect of the asphaltene deposition inhibitor B on the flocculation of the asphaltene particles. For additive concentrations, see Figure 7.
expected for this oil. Finally, the refinery residue corresponds to the whole asphaltic material contained in a blend of crude oils used as the inlet in the refining process. The asphaltenes separated from this residue correspond to a mixture of asphaltenes from different origins. It is interesting to note that, independently of their origin or composition of the original source, the four asphaltene samples assayed presented similar electrophoretic mobility-pH diagrams. This behavior reflect the fact that asphaltenes are separated by a solvent fractionation process and, therefore, corresponds to an oil fraction containing, in all the cases, components with similar solution behavior that should present similar overall macroscopic properties. The results of Figures 1 and 2 are typical of particles presenting functional or amphoteric groups undergoing
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acidic dissociation on the solid surface20 and indicate that negatively charged surface groups predominate over the positive groups. Hydroxyl groups associated to carboxylic acids or alcohols and phenols containing alkyl chains and SH groups associated to mercaptans or organic sulfides may be considered as the probable chemical species responsible for this negative charge. Another source of surface charge may be the χ-potential,21 associated with the orientation of water molecules on the solid surface. The high electron density associated with the polyaromatic sheets characteristic of the asphaltenes’ structures may orient the water dipoles with their positive ends pointed toward the solid phase, resulting in an additional negative contribution to the particle surface potential. The increase in the electrophoretic mobility and the shift of the isoelectric point toward higher pH values caused by calcium nitrate indicates a specific adsorption of calcium ions on the electrical double layer of the asphaltene particles. This effect may be caused by a chemical interaction of the Ca2+ ions with the surface groups of the particles.22 The results reported in Figure 3 and Table 2 indicate that the asphaltene particles extracted with n-heptane present a larger negative surface charge than that of the original, nonextracted, sample. Adsorbed resins reduced the electrophoretic mobility and for higher surface coverage caused a charge reversal. Even being rather small, these differences indicate that positively charged species are predominantly present in the pentane-insoluble, heptane-soluble fraction and in the resins fractions. Considering that the resins were separated by adsorption on a media containing a large number of SiOH groups, it seems reasonable to find cationic amphiphiles in this fraction. The nonconsolidated asphaltene particles in heptane/ toluene present a positive surface charge. Their electrophoretic mobility is relatively low but corresponds, under the low dielectric constant conditions, to a ζ-potential of around 70 mV, calculated using Hu¨ckel equation. This value is comparable to the potentials usually observed for colloidal particles dispersed in aqueous media. The mechanisms of charge generation of polar or partially polar molecules in nonaqueous media are different than those operating in water. In nonaqueous media, basic molecules or aggregates produce negatively charged particles and acidic molecules or aggregates produce positively charged particles.23 This trend is opposite to the pattern observed in aqueous dispersions. In nonpolar solvents the uncharged, nondissociated amphiphiles adsorb on the particles and exchange ions with the surface groups. As a result, two oppositely charged species are formed. Subsequently, the charged amphiphiles desorb from the particle to form the fluid part of the double layer.24 This mechanism of charge generation is illustrated schematically in Figure 4. Our results are in good agreement with this picture. Asphaltene particles dispersed in water presented negative (20) Parks, G. A.; de Bruyn, P. L. J. Phys. Chem. 1962, 66, 967973. (21) Hunter, R. J. Zeta Potential in Colloid Sciences; Academic Press: New York, 1981. (22) Modi, H. J.; Fuerstenau, D. W. Trans. AIME 1960, 217, 381387. (23) Morrison, I. D. Colloids Surf., A1993, 71, 1-37. (24) Fowkes, F. M. Discuss. Faraday Soc. 1966, 46, 246-247.
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surface charge, whereas, when formed by aggregation of single molecules (or smaller aggregates) in hydrocarbon media, their charge was positive. This indicates, in both cases, the involvement of acidic groups in the surface charge generation process. One of the factors that must be considered to explain the adsorption results or resins uptake by asphaltene consolidated particles reported in Table 3 is that at relatively high concentrations resins may also associate to form different species that may display different adsorption behaviors.25 For low resin concentrations, interactions with the solid asphaltenes may be favored because single molecules can reach and interact with the various different surface groups of the solid. As the concentration increases, the resin-resin interactions may form soluble aggregates that may maintain the molecules in solution, resulting in a lower apparent adsorption. However, if these aggregates adsorb on the particles, adsorption levels higher than expected would be obtained. Thus, for a fixed resins/asphaltenes ratio, different adsorption values would be obtained at different resins concentration ranges. Another point to consider is the partial solubility of the solid particles in the medium. In fact, even after extensive extraction of the asphaltenes with n-heptane prior to the adsorption tests, some dissolution of the solid during the agitation period to attain adsorption equilibrium was evident by a slightly brownish color detected in the blank, containing no resins. The adsorption on nonconsolidated asphaltene particles followed a better defined pattern. The apparent surface saturation in this case (600 mg/g at an equilibrium concentration of about 600 mg/L) may also be considered too high. However, as the particles are being formed from dissolved asphaltene molecules during the adsorption process, a very large surface area may be expected. An alternative way to interpret the results presented in Figure 5 is to assume that mixed particles, containing asphaltenes and resins, are formed as a result of binding during the precipitation process. In this case the interaction may be analyzed using a binding isotherm, for instance, the Hughes-Klotz doublereciprocal plot:26
Kd 1 1 ) + r n n[R]
(1)
where r represents the ratio between the concentrations of resins bound to the asphaltenes and the total concentration of asphaltenes, n the number of binding sites for the resins in the asphaltene molecule, Kd the average dissociation constant for each surface site, and [R] the concentration of free resins at equilibrium. In a previous work, we reported the values of 1200 and 750 dalton for the number average molecular weight of asphaltenes and resins, extracted from Marlim crude oil and dissolved in toluene.19 These data are in good agreement with results reported previously by Travalloni and (25) Espinat, D.; Ravey, J. C. SPE Paper No. 25187. In Proceedings of the SPE International Symposium on Oilfield Chemistry, SPE International Symposium on Oilfield Chemistry, March 2-5, 1993, New Orleans; Society of Petroleum Engineers: Richardson, TX, 1993; pp 365-373. (26) Price, N. C.; Dwek, R. A Principles and Problems in Physical Chemistry for Biochemists; Oxford Science Publications: London, 1979.
Figure 9. Hughes-Klotz double-reciprocal plot for the binding of resins by asphaltenes.
Freire27 for these samples. Using these values, the plot presented in Figure 9 was obtained, and it may be concluded that the asphaltene molecules contain around three equivalent and independent sites for the binding of resins with an average dissociation constant of 6.3 × 10-4 mol/L. This interpretation of the results points to an interaction between resins and asphaltenes at a molecular level rather than the adsorption of dispersing agents on solid particles. The results presented in Figure 6 indicate that resins fail to provide stability to asphaltene dispersions. In hydrocarbon media, aggregates are formed because the interactions between the hydrophilic groups of the particles or amphiphilic molecules are stronger than the interactions between these molecules and the solvent. The acid-base or electron acceptor/donor characteristic of the interacting particles or molecules may provide a mechanism for this aggregation process and for the surface charge generation. However, for the case of resins, this mechanism does not provide colloidal stability to the asphaltene dispersion. This result contrasts with the efficiency of commercial asphaltene deposition inhibitors that ensure long-term stability to the dispersions at very low concentrations (Figures 7 and 8). This type of behavior represents what it would be expected from a dispersing agent, i.e., good dispersion stability at low additive concentrations. In this context we speculate that the descriptions representing asphaltenes as a solid phase intrinsically insoluble in hydrocarbon media dispersed by adsorbed resins molecules, extensively used to model asphaltene precipitation and depositional problems,28 do not accurately represent the structure of these fractions. A picture in which asphaltenes and resins represent a family of compounds (27) Travalloni, A. M.; Freire, N. In Proceedings of the 1st International Symposium on Colloid Chemistry in Oil Production; Gonza´lez, G., Ed.; Cenpes/Petrobras, Rio de Janeiro; 1995; pp 252-261. (28) Leontaritis, K. J.; Mansoori, G. A. SPE Paper No. 16258. In Proceedings of the International Symposium on Oilfield Chemistry, SPE International Symposium on Oilfield Chemistry, Jan 1987, San Diego, Texas; Society of Petroleum Engineers: Richardson, TX, 1987; pp 149157.
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combined to form a lyophilic colloid,29 susceptible to phase separation under unfavorable composition, temperature, or pressure conditions, in the terms described by Hirshberg et al.,30 seems to better represent these rather complex oil fractions. Conclusions From the experimental evidence gathered in this work, it may concluded that asphaltenes effectively present a surface charge ascribed, according to our (29) Kruyt, H. R. Colloid Science; Elsevier: Amsterdam, 1949; Vol. 1. (30) Hirschberg, A.; de Jong, L. N. J.; Schiper, B. A.; Meijers, J. G. SPE J. 1984, 283-293.
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results, to acidic groups existing on the particles surface. Asphaltenes interact with resins in a process that corresponds to binding or coprecipitation rather than to adsorption. This resins-asphaltenes interaction does not provide stability to asphaltene dispersions, at least at the concentrations in which dispersing agents are expected to be effective. Acknowledgment. The authors thank Petrobras for permitting the publication of this article. E. F. Lucas and M. A. Sousa acknowledge the partial financial support received from CAPES and CNPq. EF020249X
