Adsorption of Native Resins on Asphaltene Particles: A Correlation

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Adsorption of Native Resins on Asphaltene Particles: A Correlation between Adsorption and Activity Olga Leoń,*,† Eliasara Contreras,† Estrella Rogel,† Gilberto Dambakli,‡ Soćrates Acevedo,‡ Lante Carbognani,† and Joussef Espidel† PDVSA-Intevep, Apdo. 76343, Caracas-1070A, Venezuela, and Escuela de Quı´mica, Universidad Central de Venezuela, 47102, Caracas-1041, Venezuela Received September 4, 2001. In Final Form: March 7, 2002 Adsorption isotherms of two native resins on two different asphaltene surfaces were obtained using the UV spectrophotometric technique. The shape of the curves obtained can be attributed both to multilayer adsorption and to penetration of resins in the microporous structure of the asphaltenes. The relationship between adsorption behavior and activity of the native resins as asphaltene stabilizers indicates that their effectiveness is related to their adsorbed amount on asphaltene particles and also to their capacity to dissolve asphaltenes. The results obtained support a model for asphaltene stabilization where the resins are incorporated into the bulk asphaltenes helping them to diffuse in the solvent. Significant differences in the behavior of native resins and alkylbenzene-derived amphiphiles as asphaltene stabilizers were observed. At the same equilibrium concentration, the adsorbed amount of native resins is lower than the adsorbed amount of amphiphiles. However, the native resins exhibit a higher asphaltene dissolution power than amphiphiles and a comparable effectiveness as asphaltene stabilizers.

Introduction The success of crude oil production operations depends on the colloidal stability of crude oils. During production of crude oils, changes in pressure, temperature, and composition can significantly disrupt the colloidal stability inducing asphaltene precipitation.1 The consequences of this phenomenon are economically devastating. Reservoir damage, reduction of well productivity, and clogging of tubing and production facilities are some of them. Annually, the oil industry spends significant amounts of money in well cleaning operations and treatments to avoid asphaltene precipitation.1-3 Despite this, the main causes of asphaltene deposition are not completely understood at the present. It is currently accepted that crude oils are colloidal systems where asphaltenes and resins compose the disperse phase and maltenes are the continuous phase.4 The precipitation of asphaltenes depends on the colloidal stability of this complex system.5 Among the causes that originate this phenomenon, it has been found that composition plays a major role.6-10 In particular, the characteristics of the disperse phase6-8 and the peptizing power of the resins10,11 are considered fundamental factors * To whom correspondence should be addressed. E-mail: [email protected]. Phone: 58-212-908-7804. Fax: 58-212-908-7524. † PDVSA-Intevep. ‡ Universidad Central de Venezuela. (1) Leontariris, K. J. Oil Gas J. 1998, Sept 01, 122. (2) Taylor, S. E. Fuel 1992, 71, 1338. (3) Galoppini, M.; Tambini, M. SPE European Production Operations Conference and Exhibition, Aberdeen, U.K., 1994; SPE 27622. (4) Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142, 497. (5) Laux, H.; Rahimian, I.; Butz, T. Fuel Process. Technol. 1997, 53, 69. (6) von Hatke, A.; Rahimian, I.; Neumann, H. J. Erdo¨ el Erdgas Kohle 1993, 109, 73. (7) Leoń, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14, 6. (8) Rogel, E.; Leoń, O.; Espidel, J.; Gonza´lez, J. SPE Prod. Facil. 2001, May, SPE72050. (9) Loeber, L.; Muller, G.; Morel, J.; Sutton, O. Fuel 1998, 77, 1443. (10) Taylor, S. E. Fuel 1998, 77, 821. (11) Koots, J. A.; Speight, J. G. Fuel 1975, 54, 179.

for the stabilization of asphaltenes in crude oil. In fact, the separation of the resins from crude oil originates the precipitation of asphaltenes.11 The resins seem to provide a transition between the most polar (asphaltenes) and the relatively nonpolar (maltenes) fractions in petroleum, making it possible to maintain asphaltenes in solution.4 It has been proposed that resins attach to asphaltene aggregates to form a steric stabilization layer around them.12 The strong interactions between asphaltenes and resins have been proved qualitatively using spectroscopic techniques. However, severe difficulties in using these techniques for quantification of asphaltene-resin interactions have limited their use.13 For this reason, the interactions between asphaltenes and resins have been studied using alkylbenzene-derived amphiphiles as model systems of native resins. From these earlier studies, it has been shown that the strength of the asphalteneamphiphile interactions and the capacity of the amphiphiles to form a steric stabilization layer are the key factors in the effectiveness of amphiphiles as asphaltene stabilizers.13 Even more, it has been shown that the activity of the amphiphiles is related to the maximum amount of amphiphile adsorbed on the asphaltene surface.14,15 There seems to be no doubt regarding the important role of resins in stabilizing asphaltene colloids in crude oil and organic solvents. However, the mechanism for such stabilization is not yet completely clear. A steric mechanism and desorption of resins after dilution with the precipitating solvent has been suggested.12 However, adsorption of resin solutions of n-heptane on an asphaltene surface leading to multilayer formation has been reported.16 This finding is not clearly consistent with a steric (12) Leontaritis, K. J.; Mansoori, G. A. SPE International Symposium on Oilfield Chemistry, San Antonio, TX, 1987; SPE 16258. (13) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10, 1749. (14) Leoń, O.; Rogel, E.; Urbina, A.; Andu´jar, A.; Lucas, A. Langmuir 1999, 15, 7653. (15) Leoń, O.; Contreras, E.; Rogel, E. Colloids Surf., A 2001, 189, 123. (16) Acevedo, S.; Escobar, G.; Ranaudo, M. A.; Gutierrez, L. B. Fuel 1994, 73, 1807.

10.1021/la011394q CCC: $22.00 © 2002 American Chemical Society Published on Web 05/21/2002

Adsorption of Native Resins on Asphaltene

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Table 1. Characteristics of the Crude Oils crude oils OL1 OL2 OL3 OL4 OL5 OL6 a

operational stability of crudes unstable stable unstable unstable unstable unstable

API gravity

saturatesa (wt %)

aromaticsa (wt %)

resina (wt %)

asphaltenea (wt %)

22.4 18.3 22.8 25.6 25.0 26.3

36.9 32.3 43.6 44.3 45.6 51.9

37.9 42.2 35.5 38.9 34.2 38.9

19.4 19.8 14.3 11.6 17.0 8.1

5.8 5.8 6.6 5.2 3.2 1.1

SARA by TLC/FID (ref 20). (Results reported on a C13+ (>220 °C) basis.)

stabilization mechanism generally based on repulsion of adsorbed moieties on colloidal particles.17 On the other hand, it has been shown that about 70% of asphaltenes are actually insoluble in toluene. However, the whole asphaltene fraction is completely soluble in toluene forming a colloid. This indicates that the 70% of asphaltenes is maintained in solution by the other 30% of asphaltenes which plays the role usually attributed to resins.18 On the basis of this finding and consideration of the high polydispersity of asphaltenes, steric hindrance as the unique mechanism of colloidal stabilization is not plausible. Instead, a model for asphaltene colloids in toluene would be constituted by a core of the insoluble asphaltene fraction surrounded by a periphery permeable to solvent molecules and constituted by the soluble asphaltene fraction. In this simple model, both the desorption of the soluble fraction and draining of solvent from the colloid periphery would be required for flocculation to occur.18 Such a model implicitly postulates that the solubility of the asphaltene fraction in the colloidal particle would increase from the center to the particle surface. By extension of this argument to the adsorption of resins on asphaltenes, a multilayer adsorption is expected, where resin components with the higher affinity for asphaltenes would adsorb first and those components with the higher affinity for solvent would adsorb last. As is the case for asphaltene colloids, this sequential adsorption would result in a lyophilic colloid easily dispersed by the solvent media. On the basis of these previous results, this work is directly focused on the study of asphaltene-native resin interactions employing the same techniques that have been successfully used to evaluate asphaltene-amphiphile interactions.14,15 The main objective of this work is to explore the mechanism of asphaltene stabilization by native resins. A better understanding of this mechanism can help to unveil the relationship between activity and structure, providing useful information for the design of more efficient asphaltene stabilizers to be used in oilfield operations. In this work, adsorption isotherms of two native resins on two different asphaltene surfaces were obtained using the UV spectrophotometric technique.14,15 For these systems, the relationship between adsorption behavior and activity of the native resins as asphaltene stabilizers was evaluated. Also, a comparison with previous results obtained for amphiphile-asphaltene systems is made. Materials and Methods Experimental Methods. Hydrocarbon Group Type Preparative Isolation. Asphaltenes (AOL1-AOL6) were extracted from Venezuelan crude oils (OL1-OL6) with a 30:1 volume ratio of (17) Evans, F.; Wennerstro¨m, H. The colloidal domain where physics, chemistry, biology and technology meet; VCH: New York, 1994; Chapter 7, p 318. (18) Gutie´rrez, L. B.; Ranaudo, M. A.; Meńdez, B.; Acevedo, S. Energy Fuels 2001, 15, 624.

Table 2. Characterization of Resins and Asphaltenes Resins molecular weighta (g/mol) ROL1 ROL2

683 1024

%C

%H

84.60 10.37 85.57 8.59

%(N + S + O)

fab

5.03 5.84

0.35 0.40

Ci/C1c Ard 0.52 1.07

4 9

Asphaltenes

AOL1 AOL2

molecular weighta (g/mol)

surface area (m2/g)

%C

%H

%(N + S + O)

2100 3098

42 172

84.40 84.90

6.75 8.60

8.85 6.50

a Determined by VPO in CH Cl at 25 °C. b Aromaticity (ratio 2 2 of aromatic carbons to total carbons). c Aromatic condensation index (ratio of internal aromatic to nonbridging aromatic carbons). d Number of aromatic rings.

n-heptane to crude oil according to the isolation method formerly described.15 From the remaining maltene-soluble portions of oils OL1 and OL2, native resins (ROL1 and ROL2) were obtained using a modification of the method described in ASTM D-2007.19 These maltenes were separated into saturates, aromatics, and resins using a chromatographic column packed with attapulgus clay. These fractions were sequentially eluted and collected using different solvents: saturates plus aromatics were eluted using toluene/n-heptane mixtures. Resins were trapped on the clay and were recovered using a mixture of methanol, acetone, and chloroform (15:15:70). Table 1 shows a summary of the main characteristics of studied crude oils. Characterization of Asphaltenes and Resins. Elemental composition of asphaltenes and resins was determined with a LECO CHNS 244 elemental analyzer. Average number molecular weights were determined with a Knauer vapor pressure osmometer, using CH2Cl2 as the solvent and a temperature of 25 °C. Table 2 shows resin and asphaltene characterization.15 Resin NMR spectra were obtained on a Bruker ACP-400 spectrometer, at a resonance frequency of 400 MHz for protons. A flip angle of 45° was used, with a repetition rate of 3 s and a spectral width of 12 ppm, and the chemical shift was referenced relative to tetramethylsilane (TMS). Samples of 25 mg were dissolved in 1 cm3 of dichloromethane, and 5% by weight hexamethyl cyclosiloxane was added as an internal standard. Table 2 also shows the average molecular parameters (AMP) calculated according to a method previously reported.21 Adsorption Isotherms. The asphaltenes used as adsorbents were extracted from crude oils OL1 and OL2. Four adsorption isotherms were obtained: resins ROL1 and ROL2 on asphaltenes AOL1 and AOL2. Solutions of a known concentration of each native resin were prepared in n-heptane. A 10 cm3 aliquot was added to 10 mg of asphaltenes in a flask. The flask was covered, and the suspension was stirred in a thermostatic bath at 25 °C during 16 h. After reaching equilibrium, the liquid was separated by centrifugation and its absorbance was measured on a PerkinElmer UV-vis spectrophotometer, using 1 cm path length cells (19) Standard Test Method for Characteristic Groups in Rubber Extender and Processing Oils and Other Petroleum-Derived Oils by the Clay-Gel Absorption Chromatographic Method; ASTM D-2007; American Society for Testing and Materials: West Conshohocken, PA, 2001. (20) Sol, B.; Romero, E.; Carbognani, L.; Sańchez, V.; Sucre, L. Revista Te´ cnica de Intevep 1984, 4, 127. (21) Leon, V. Fuel 1987, 66, 145.

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Figure 1. UV-vis spectra of n-heptane solutions (200 ppm) of ROL1 (a) and ROL2 (b) before and after the equilibrium with asphaltenes. Table 3. Swelling Data for Asphaltenes asphaltene volumetric swelling

Q′va

asphaltene

ROL1

ROL2

AOL1 AOL2

0 17

16 20

a Q′ ) h /h (h , swelled in n-heptane; h , swelled in resin + 2 1 1 2 v n-heptane).

the stabilizers was determined by titration of 30 mg of asphaltene in 10 cm3 of toluene. The native resin-asphaltene weight ratio ranged from 0.1 to 3. Temperature, titration rate, stirring speed, and time of sample preparation were the same during all titrations. Solutions without native resins were used as references. The activity of the resins was calculated as the difference in the flocculation onset for asphaltene solutions with resins minus the flocculation onset for pure asphaltene solutions.

Results and Discussion at the wavelength of 400 nm. Calibration curves of absorbance as a function of concentration allowed the determination of the total amount of native resin adsorbed on the asphaltene surface. Solvent Swelling of Asphaltenes. Nearly 300 mg (weighed to the nearest 0.1 mg) of ground asphaltenes was poured into a centrifuge tube and tightly packed by repeated compression with a metal piston, until constant height (h0). The n-heptane (2 cm3) was poured on the compacted asphaltene, and the mixture was stirred, until releasing of air bubbles ceased. The tubes were capped, and solvent swelling was carried out for 22 h. The swelled solid was compacted by centrifugation and forced bumping until constant height (h1) of the material was observed. Volumetric asphaltene solvent swelling (Qv) was determined as the ratio of swelled/dried solid height (Qv ) h1/h0).22 Duplicate determinations were run, and average values were reported in Table 3. The influence of the native resins on the volumetric expansion of the asphaltene structure was tested during swelling experiments. A small amount of native resin (24 mg) was dissolved in the 2 cm3 of n-heptane. The difference in the volumetric swelling using the neat solvent and the native resin solutions is a measure of the influence of resins on the volumetric expansion of asphaltenes Q′v ) h2/h1 (h1, swelled in n-heptane; h2, swelled in resin + n-heptane). Flocculation Onset. The activity of native resins (ROL1 and ROL2) as asphaltene stabilizers was evaluated for the six asphaltene samples (AOL1-AOL6) by means of flocculation onset measurements. In this work, the flocculation points were determined by a titration method: n-heptane is added at a constant rate (1 cm3/min) to a solution of asphaltene and native resin in toluene under intensive stirring. The titration is monitored by means of a guided wave NIR spectrophotometer at the wavelength of 768 nm. The flocculation point is defined as the amount of n-heptane needed to obtain the maximum of the light intensity, which corresponds to the beginning of aggregation and coagulation of the asphaltenes. The activity of (22) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935.

Adsorption Isotherms. UV-vis determination of adsorption isotherms is a very simple technique. However, careful selection of the appropriate wavelength is required for meaningful results. As can be seen in Figure 1, low wavelengths, that is, less than 350 nm, are not appropriate for quantitative analysis since there are large absorbance variations for narrow wavelength changes. Long wavelengths, that is, more than 450 nm, are also inadequate due to low sensitivity that hampers analysis precision. The best trade-off is achieved by selecting a wavelength of 400 nm. However, some crude oils, particularly Venezuelan medium-heavy crude oils, display an absorption band centered at 408 nm. This signal corresponds to the so-called Soret band of petro-porphyrins. This Soret band can be clearly observed for resins ROL2 in Figure 1. One analytical issue would arise if these compounds were preferentially adsorbed on asphaltenes. However, for the studied cases it was observed that petro-porphyrin Soret bands, where noticeable, follow the same decrease observed for the other signals in the whole spectral range. This is illustrated in Figure 1 for the ROL2 resin solution equilibrated with asphaltenes. From the above, the selection of 400 nm to carry out the isotherm studies seems to be the best choice. The adsorption isotherms of native resins ROL1 and ROL2 on asphaltene particles are shown in Figures 2 and 3, respectively. The isotherms of the native resins ROL1 exhibit a continuous increase in the amount of adsorbed resins on both asphaltenes. In contrast, the isotherms of resins ROL2 show a plateau at low concentration and a sharp increase in the adsorption at higher concentrations. The adsorption isotherms obtained are complex making their analysis difficult. The curves observed could be


Langmuir, Vol. 18, No. 13, 2002 5109

attributed to multilayer adsorption and also to penetration of resins in the microporous structure of the asphaltenes.23,24 Additionally, the heterogeneity of the resin fraction, which is a polydisperse distribution of molecules, introduces more difficulties in interpreting the adsorption isotherms. The adsorption isotherms of ROL1 on asphaltenes can be classified as C-type. Basically, this type of isotherm is

characterized by a linear slope of the isotherm at any point. In the case of S- and L-isotherms, the slope changes as the concentration increases. For L-isotherms, the slope falls with a rise in concentration because vacant sites become scarce with the covering of the surface. For S-isotherms, the slope at first increases with the concentration because of cooperative adsorption and then falls until no vacant sites remain. C-Isotherms are consistent with conditions in which the number of sites remains constant. This means that the surface available for adsorption expands proportionally with the amount of solute adsorbed. Theoretical analysis and experimental results attributed the shape of this kind of isotherm to the penetration of the microporous structure by solute molecules.23 This behavior is observed for solutes having a higher affinity for the adsorbent than for the solvent. It is also supposed that the penetration of the solute in the microporous structure of the substrate can lead to the partial breakdown of the structure, making available new internal surface for additional adsorption of the solute. The adsorption mechanism suggested by this model is consistent with the observation that many of the solutes giving C-isotherms cause the adsorbent to disintegrate completely and even to dissolve.23 In fact, the dissolution of asphaltenes during the adsorption experiments was observed and precluded the collection of adsorption data at higher concentrations. X-ray diffraction of adsorbents in systems that show C-curves revealed that the substrate lattice is progressively expanded with the amount of solute adsorbed in accordance with the theoretical model.23 Swelling measurements of asphaltenes AOL2 immersed in n-heptane solutions of ROL1 indicate a volumetric expansion of the asphaltene microstructure of 17% just before the beginning of the asphaltene dissolution. In the case of asphaltenes AOL1 immersed in n-heptane solutions of ROL1, no swelling was observed. The adsorption isotherms of resins ROL2 on asphaltenes shown in Figure 3 reveal the presence of a plateau at low concentration, and then, as the concentration exceeds a certain threshold, a drastic increase of the adsorption in a narrow concentration region is observed. Assuming that the observed plateaus correspond to the formation of a close-packed monolayer, the molecular area of resins ROL2 was calculated based on the active surface area of asphaltenes AOL1 and AOL2 previously determined.15 The calculations yield molecular areas in close agreement: 697 Å2 for resins on asphaltenes AOL1 and 714 Å2 for resins on asphaltenes AOL2. The calculation of the area occupied by the average polyaromatic ring for the resins ROL2, using weighted holistic invariant molecular descriptors,25 yields a value of 313 Å2. This is less than half the estimated area from the adsorption experiments. This can indicate that naphthenic and aliphatic moieties of resin molecules should also be in direct contact with the asphaltene surface. The steeply rising part of the isotherms for resins ROL2 indicates a higher affinity for the substrate than for the n-heptane. A similar sharp increase in the adsorption isotherm is observed for different systems in proximity to phase separation. This phenomenon is more related to the phase separation itself than to the adsorbent material. Examples of this behavior are the isotherms of ethoxylated amphiphiles from water on different adsorbents at high temperatures near the critical temperature of phase separation.27,28 The solubility of resins ROL2 in n-heptane

(23) Giles, C. H.; Smith, D.; Huitson, A. J. Colloid Interface Sci. 1974, 47, 755. (24) Acevedo, S.; Ranaudo, M. A.; Escobar, G.; Gutie´rrez, L.; Ortega, P. Fuel 1995, 74, 595.

(25) Todeschini, R.; Gramatica, P.; Provenzani, R.; Marengo, E. Chemom. Intell. Lab. Syst. 1995, 27, 221. (26) Findenegg, G. H.; Pasucha, B.; Strunk, H. Colloids Surf. 1989, 37, 223.

Figure 2. Adsorption isotherms for native resins ROL1 on asphaltenes AOL1 and AOL2.

Figure 3. Adsorption isotherms for native resins ROL2 on asphaltenes AOL1 and AOL2.

5110


was estimated to be 0.000 78 mol/L. In contrast, the solubility of resins ROL1 in n-heptane was found to be 0.001 32 mol/L. At high equilibrium concentrations, the comparison of the adsorption isotherms for both resins indicates that the curves agree with the expected tendency from their solubilities. Considering the trends for resins ROL2 at high equilibrium concentration, the shape of the isotherms might also suggest the adsorption in micropores. In fact, expansion of the asphaltene microstructure was observed also for asphaltenes AOL1 and AOL2 immersed in n-heptane solutions of ROL2. The volumetric expansion was 16% and 20% for AOL1 and AOL2, respectively. The penetration of resins in the microporous structure of the asphaltenes followed by the dissolution of asphaltenes is also in agreement with the reported behavior as solid solutions of the resin-asphaltene mixtures.29 However, despite these results, multilayer adsorption cannot be ruled out, since resin aggregation in different solvents has been reported.24,30 During all of the adsorption experiments, dissolution of asphaltenes was observed and prevented the collection of adsorption data at higher concentrations. It has been shown that polyaromatic compounds have a special capacity to dissolve asphaltene deposits.31 According to previous studies, the asphaltenes are more efficiently dissolved by condensed aromatic hydrocarbons than by alkylbenzenes. In fact, the effectiveness of petroleum fractions and derived materials to dissolve asphaltene deposits increases as the hydrogen deficiency increases.32 Table 2 shows that both resins contain polyaromatic compounds with an average of 4-9 aromatic rings per molecule. Then, both resins contain a high proportion of condensed aromatic hydrocarbons, and therefore a high asphaltene dissolution capacity is expected from these fractions. Effectiveness of the Resins as Asphaltene Stabilizers. The effectiveness of the studied resins was determined by flocculation measurements. Figure 4 shows the effectiveness of the resins to stabilize asphaltenes AOL1 and AOL2 at different concentrations. At each concentration, the effectiveness was calculated as the difference in the flocculation onset between asphaltenesresins and the reference solutions (pure asphaltenes). Higher volumes of n-heptane indicate higher effectiveness of the resins as asphaltene stabilizers. In previous studies, it was found that the activity of different amphiphiles as asphaltene stabilizers is related to the maximum amount of amphiphile adsorbed on the asphaltene surface.14,15 This means that the larger the concentration of amphiphile on the asphaltene surface, the larger the volume of n-heptane needed to begin the flocculation of asphaltenes. According to Figure 4, resins ROL2 are better stabilizers than resins ROL1. This is the expected result since resins ROL2 show a sharp increase (27) Kochkodan, O. D.; Klimenko, N. A.; Karmazina, T. V. Colloid J. 1996, 58, 330. (28) Cox, A. R.; Mogford, R.; Vincent, B.; Harley, S. Colloids Surf., A 2001, 181, 205. (29) Acevedo, S.; Ranaudo, M. A.; Escobar, G.; Gutierrez, L. B.; Gutierrez, X. A Unified View of the Colloidal Nature of Asphaltenes. Asphaltenes: Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995; Chapter 4. (30) Bardon, Ch.; Barre´, L.; Espinat, D.; Guille, V.; Li, M. H.; Lambard, J.; Ravey, J. C.; Rosenberg, E.; Zemb, T. Fuel Sci. Technol. Int. 1996, 14, 203. (31) Carbognani, L.; Contreras, E.; Guimerans, R.; Leoń, O.; Flores, E.; Moya, S. SPE International Symposium on Oilfield Chemistry, Houston, TX, 2001; SPE 64993. (32) Del Bianco, A.; Stroppa, F. SPE International Symposium on Oilfield Chemistry, San Antonio, TX, 1995; SPE 28992.

Leo´ n et al.

Figure 4. Effectiveness of native resins to stabilize asphaltenes AOL1 and AOL2 as a function of their concentration. Effectiveness ) (flocculation onset of resin + asphaltene solution) (flocculation onset of asphaltene solution).

Figure 5. Effectiveness of native resins to stabilize different asphaltenes. Solutions of resin-asphaltene were prepared in a ratio of 3:1 by weight. Effectiveness ) [(flocculation onset of resin + asphaltene solution) - (flocculation onset of asphaltene solution)]/moles of native resins.

in the amount adsorbed, whereas resins ROL1 show a continuous slight increase in adsorption at the studied concentrations (Figures 2 and 3). The effectiveness of both resins to stabilize asphaltenes from other sources was compared. A native resinasphaltene weight ratio of 3:1 was used in these experiments. Figure 5 shows the results expressed as the effectiveness per mole of resins. It has been reported that resins from certain crude oils are unable to disperse asphaltenes from other crude oils.33 In this work, both resins have shown a good asphaltene-stabilizing activity for all of the asphaltene samples studied. In general, resins ROL2 are better stabilizers than resins ROL1 as can be seen in Figure 5. This can be related to the different adsorption behavior shown by both resins on asphaltenes. In the same concentration range, resins ROL2 are more prone to adsorb in a higher amount than resins ROL1. The chemical reasons for this difference in behavior could be linked to structural characteristics: resins ROL2 are more aromatic, and therefore their average interaction with asphaltenes is expected to be more favorable. Also, (33) Andersen, S. I.; Speight, J. G. Pet. Sci. Technol. 2001, 19, 1.


Figure 6. Comparison of the adsorption isotherms of NP (nonyl phenol) and native resins ROL1 and ROL2 on asphaltenes AOL2.

Langmuir, Vol. 18, No. 13, 2002 5111

Figure 7. Comparison of the effectiveness of NP (nonyl phenol), NPR (nonyl phenolic resin), and native resins to stabilize the unstable asphaltene AOL1.

resins ROL2 came from a crude oil without asphaltene production problems. In contrast, resins ROL1 were extracted from a crude oil that has frequently shown production problems due to asphaltene deposition, mainly clogging of the production tubing. Recently, it has been shown that resins from stable crude oils show higher activity for stabilization of asphaltenes compared to resins from unstable crude oils.34 Comparison with Amphiphile Behavior. Previously, the adsorption of different alkylbenzene-derived amphiphiles from n-heptane on asphaltene particles AOL1 and AOL2 has been studied.14,15 In these earlier works, the isotherms obtained could be classified as a two-plateau type (LS): at low concentrations, the surface excess is very small, and as the concentration exceeds a certain threshold, a drastic increase of the surface excess in a narrow concentration region is observed. This kind of isotherm can be related to a two-step adsorption mechanism where the amphiphile molecules are first adsorbed through interactions with the solid surface and then, in the second step, through the interaction with the amphiphiles adsorbed.35 The comparison of the adsorption isotherms of an alkylbenzene-derived amphiphile (nonyl phenol) and native resins on asphaltenes AOL2 is shown in Figure 6. The adsorption experiments revealed two important differences between the adsorption behavior of native resins and amphiphiles: (1) At the same equilibrium concentration, native resins adsorb in a lower amount than amphiphiles. (2) Asphaltene dissolution was observed at lower concentrations for native resins than for the alkylbenzene-derived amphiphiles. In fact, for the amphiphiles studied, asphaltene dissolution begins after reaching the second plateau, above the critical aggregate concentration (cac) of the amphiphile in n-heptane. The differences in dissolution behavior can be explained by the fact that asphaltenes are more efficiently dissolved by condensed aromatic hydrocarbons (that mainly composed native resins) than by alkylbenzenes.32 On the other hand, the formation of resin-asphaltene solid solutions indicates that the resins can be easily incorporated into the bulk asphaltenes helping them to be dissolved. Also, the mixture of asphaltene precipitation inhibitors and aromatic solvents does not dissolve asphaltene deposits more efficiently than the solvent alone. In other

words, the addition of the inhibitor does not improve the dissolution of the deposits.36 The effectiveness of native resins and two amphiphiles to stabilize AOL1 and AOL2 asphaltenes is compared in Figures 7 and 8, respectively. According to the adsorption isotherms, it can be expected that the effectiveness of the native resins would be much lower than the effectiveness of the studied amphiphiles because of their lower adsorption. However, as can be seen in Figures 7 and 8, the effectiveness of native resins and amphiphiles is comparable for the stable asphaltene AOL2 (Figure 8) and not widely diverse for the unstable asphaltene AOL1 (Figure 7). The reason for this unexpected behavior is not clearly understood, but it can be related to the high capacity of native resins to dissolve asphaltenes at low concentrations. Vapor pressure osmometry (VPO)37 and small-angle X-ray scattering30 measurements have shown that the presence of resins can significantly decrease the size of asphaltene aggregates. These results have been related to the antiflocculant action of resins. In contrast, small-angle X-ray scattering measurements indicate that for the pair nonyl phenol/asphaltene, colloids are slightly larger than

(34) Rogel, E.; Leoń, O.; Contreras, E.; Carbognani, L.; Torres, G.; Espidel, J.; Zambrano, A. Energy Fuels, submitted. (35) Zhu, B. Y.; Gu, T. Adv. Colloid Interface Sci. 1991, 37, 1.

(36) Carbognani, L. Energy Fuels 2001, 15, 1013. (37) Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Ind. Eng. Chem. Res. 2000, 39, 2916.

Figure 8. Comparison of the effectiveness of NP (nonyl phenol), NPR (nonyl phenolic resin), and native resins to stabilize the stable asphaltene AOL2.

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Figure 9. Schematic representation of the adsorption of resins followed by the dissolution of asphaltenes: (I) formation of a resin layer, (II) penetration of resins into asphaltene micropores, (III) partial breakdown of the asphaltene structure, and (IV) diffusion of a mixed particle in the solvent.

asphaltene colloids.38 This suggests that the addition of nonyl phenol to asphaltene solutions does not decrease the size of the asphaltene aggregates. These previous results support the differences found in this study for the adsorption and effectiveness of alkylbenzene-derived amphiphiles and native resins. Amphiphiles are currently considered as good models to simulate the behavior of native resins in different experiments. The results obtained in the present work indicate that there are significant differences in their behavior that preclude the use of amphiphiles as models for native resins. The results obtained in the present work together with experimental evidence reported earlier could be used to compose a rough model of the adsorption process of resins on asphaltenes. The proposed model is based on the following experimental evidence: (1) Formation of asphaltene/resin solid solutions. (2) The size of asphaltene aggregates decreases by the addition of resins. (3) There is a substantial increase in the volumetric swelling of asphaltenes due to the presence of resins in the n-heptane solution in comparison to the pure n-heptane. (4) The shape of the adsorption isotherm that indicates an expansion of the surface available to adsorption. Figure 9 shows a schematic representation of the model. In the proposed model, we suggest that the resins adsorb in the asphaltene surface and penetrate together with the n-heptane the internal structure of the solid. This process induces the swelling of the asphaltene. The “swollen asphaltene” regime has been recently identified by X-ray scattering measurements.39 According to the classical theory of swelling, the driving force for swelling is the decrease in Gibbs energy during the mixing of solvent with the solid particle; the elastic forces caused by the extension of the particle act against the swelling.40 The proposed model also suggests that the resins must disrupt the asphaltene-asphaltene interac(38) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10, 1758. (39) Sirota, E. B. Pet. Sci. Technol. 1998, 16, 415.

Leo´ n et al.

tions to induce the swelling of the material and therefore the expansion of the surface available to adsorption. This recurrent process eventually leads to the diffusion of asphaltene/resin aggregates in the solution. In a recent study of the swelling behavior of asphaltenes in different solvents, it was found that the lowest swelling ratios correspond to polar solvents such as methanol, acetone, and so forth.41 This finding suggests that these solvents interact with specific sites in the asphaltene, mainly by H-bonding, without disrupting the asphalteneasphaltene interactions. Then, as resins are able to break asphaltene-asphaltene interactions, one might expect that resins interact with asphaltenes in a different way as these polar solvents do. A plausible explanation for this diverse behavior can be found in the capacity of the resins to break the stacking of the aromatic sheets in the asphaltene solid particles, while specifically interacting solvents cannot do that. This physical picture of the adsorption of resins on asphaltenes will be developed and tested more formally in a forthcoming publication. Conclusions The shape of the adsorption isotherms of native resins on asphaltene particles can be attributed both to multilayer adsorption and penetration of resins in the microporous structure of the asphaltenes. Volumetric expansion of the asphaltene samples immersed in n-heptane solutions of native resins was observed and supports the mechanism of penetration of resins in the asphaltene microstructure. Significant differences in the behavior of native resins and alkylbenzene-derived amphiphiles as asphaltene stabilizers were observed. At the same equilibrium concentration, native resins adsorb in a lower amount than amphiphiles. However, they exhibit a higher asphaltene dissolution power and a comparable effectiveness as asphaltene stabilizers. On the basis of these results, the use of amphiphiles as models for native resins must be revised. The results obtained support a model for asphaltene stabilization where the resins are incorporated into the bulk asphaltenes helping them to diffuse in the solvent. The proposed model is composed of the following steps: (I) formation of a resin layer on the asphaltene surface, (II) penetration of the asphaltene microporous structure by resins, (III) breaking of the asphaltene microporous structure by resins, and (IV) diffusion of a mixed asphaltene-resin particle in the solvent. Acknowledgment. The authors thank PDVSA-Intevep for support and permission to publish this work. LA011394Q (40) Munk, P. Introduction to Macromolecular Science; John Wiley & Sons: New York, 1989. (41) Carbognani, L.; Rogel, E. Energy Fuels, submitted.

Adsorption of Native Resins on Asphaltene Particles: A Correlation

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