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An electron spin resonance probe method for the understanding of petroleum asphaltene macrostructure
Journal of Petroleum Science and Engineering 28 Ž2000. 55–64 www.elsevier.nlrlocaterjpetscieng
An electron spin resonance probe method for the understanding of petroleum asphaltene macrostructure Gary K. Wong, Teh Fu Yen ) Department of CiÕil and EnÕironmental Engineering, UniÕersity of Southern California, 3620 South Vermont AÕenue 224A, Los Angeles, CA 90089-2531, USA Received 12 January 2000; accepted 14 July 2000
Abstract Molecularly, petroleum asphaltenes are induced dipoles, which agglomerate into nanometer-sized colloids of different aggregation states. The electron spin resonance ŽESR. vanadyl probe method is used to investigate the asphaltene macrostructures under different temperatures and microwave powers. Oxovanadium complexes native to an asphaltene isolated from Boscan crude oil, Venezuela, function as tracers to examine the behavior of micelle agglomerates when subjected to a microwave field. Both mobile and bounded oxovanadium compounds in colloidal asphaltene solution are in a state of equilibrium. It is noted that a greater amount of mobile vanadyl complexes can be stabilized in a dispersing medium Žsingle-aromatic ring solvent series. with a higher-valued Hansen hydrogen bonding solubility parameter. We found that conversion of ESR vanadyl hyperfine lines occurs from anisotropic to isotropic as the temperature of a 4% Boscan asphaltene solution in o-xylene increased from 258C to 1008C. Free tumbling of total vanadyl complexes in organic solvent signifies dissociation of micelles at packing imperfections prior to their release from aromatic hosts. Coupling of petroleum asphaltenes with microwave power can overcome charge transfer and charge balance interactions within micelle agglomerates. The relative content of mobile to bounded vanadyl complexes in 4% Boscan asphaltene solution of o-xylene was found to increase with microwave power at 458C. Microwave energy will enable effective dispersion of colloidal asphaltene in heavy oil refining and upgrading. q 2000 Elsevier Science B.V. All rights reserved. Keywords: asphaltenes; electron spin resonance; microwave power; vanadyl complexes
1. Introduction Formation of petroleum coke and oil sludge during heavy oil refining can lead to serious problems of equipment maintenance and residue management. Flocculation of asphaltene often causes fouling of ) Corresponding author. Tel.: q1-213-740-0586; fax: q1-213744-1426. E-mail address: [email protected] ŽT.F. Yen..
heat exchangers as well as carbonaceous deposition in reactors, transportation pipes, and storage tanks. The end result of reduction of reactor throughput and liquid product yield will greatly affect the overall refining economics. Conversion of asphaltics to liquid fuel oils, therefore, is essential and necessary. However, it is the unique colloidal nature and stable free radical behavior, which cause petroleum asphaltenes to be the key obstacles in the upgrading of heavy oil.
0920-4105r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 4 1 0 5 Ž 0 0 . 0 0 0 6 7 - X
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Stability of asphaltene colloids in heavy oil is attained by both steric peptization with resin and miscibility in the dispersing medium. Petroleum resins with lower average molecular weights are structural analogues of asphaltenes ŽWiehe, 1992.. Hydrogen bonding between polar functional groups is the main driving force for the association of resin molecules with colloidal asphaltene. Stability enhancement of asphaltene suspension against critical solvent conditions as well as the incompatibility of asphaltene and gas oil fractions have suggested that petroleum resin functions as peptization agents in heavy oil ŽLian et al., 1994; Koots and Speight, 1975.. Asphaltic colloids coexist with aromatics and saturates in the state of equilibrium, which is governed by mutual solubilities of saturates–aromatics– resins–asphaltenes ŽSARA. fractions. Clustering of particles by a diffusion-limited aggregation process takes place in heavy oils containing high concentrations of asphaltenes ŽLin et al., 1991.. Asphaltene clusters are considered as precursors for flocs and mesophase ŽYen, 1994a,b.. Overall, the criteria for achieving maximum stability and compatibility of heavy oils is a function of, but is not limited to: Ž1. low asphaltene content, Ž2. high resin to asphaltene ratio, and Ž3. dispersing oil of high aromaticity. Disturbance in the compatibility state of heavy oil is unavoidable under thermal cracking conditions. Redistribution and changes in the chemical characters of SARA fractions occur throughout the course of hydroconversion reactions. Homolytic cleavage of chemical bonds to produce smaller free radical fragments is the principal reaction pathway to decomposition of the macromolecular structures in petroleum asphaltics. Native asphaltenes are thermally reactive with higher first-order rate constants than the nheptane soluble fractions ŽWiehe, 1993.. When compared to oil molecules, however, asphaltenes exhibit the least amount of conversion due to their colloidal nature. Thermal cracking of native asphaltenes proceeds with dealkylation and aromatization of naphthenic portions leading to the newly polycondensed aromatic cores as product asphaltenes ŽCalemma et al., 1994.. Simultaneous saturation of dispersing aromatic oil and hydrocracking of resin further interrupts the solubility equilibrium of the system until phase separation takes place. Asphaltene flocculation is believed to be responsible for the sludge formation
during visbreaking or catalytic hydroconversion of heavy oil. The amount of sludge formed has been empirically determined to be dependent on the feedstock characteristics, such as resin to asphaltene ratio, ring condensation index, aromaticity, and degree of alkyl substitution of aromatic ring systems in asphaltenes ŽStorm et al., 1994.. As a result, rapid and extensive cracking of colloidal asphaltene in conjunction with a relatively moderate conversion of maltene fraction would be advantageous for maintaining compatibility and stability of the processed oil. However, the refractory character of polycondensed aromatic systems has rendered asphaltenes as coke precursors. Reactivity of asphaltenes is mainly governed by the forms and behavior of free radicals evolved in the process environment. The thermally induced radicals are stabilized as delocalized spins through p–p resonance within the aromatic host structures of polycondensates. Retrogressive reactions are predominant within micelles and clusters due to the combined effect of the stable free radical nature and insufficient hydrogen delivery. Limited conversion of polycondensed aromatic cores eventually leads to phase separation and coke formation. Fine grain mosaics are formed during carbonization of petroleum residue with low hydrogen donating capacity, which is observed as inversely proportional to the spin concentration ŽYokono et al., 1986.. Mesophase pitch samples above 4008C show a common pattern of spin increase along with linewidth narrowing, which implicates electron spin exchange interactions of aromatic structures ŽKaneko et al., 1990.. All the facts indicate that high conversion of colloidal asphaltene requires both efficient hydrogen termination reactions to counter-play the intrinsic stable free radical nature, and high dispersion of nanocolloids to reduce irreversible particle growth ŽStorm et al., 1998.. Many heavy oil-upgrading technologies adopt novel catalysts to specifically promote conversion of heavy petroleum components. Catalyst designs place emphasis on the caging of colloidal asphaltene in close vicinity of active sites for cracking andror hydrogenation reactions. Both surface association onto micron-sized metal sulfide particles and diffusion into the specialized pore structures of Asphaltenic Bottom Cracking catalyst do serve this funda-
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mental purpose ŽBearden and Aldridge, 1981; Takeuchi et al., 1983.. A catalytic environment rich in chemisorbed hydrogen can greatly retard free radical polymerization. Deagglomeration of asphaltene clusters and micelles is beneficial for the acceleration of hydrogen quenching reactions, as well as lowering the diffusion limitation of catalyst pores. Particle aggregation, however, tends to occur under high hydrogen over-pressure, which is typical in residual hydroprocessors ŽSyunyaev and Abid, 1994.. A pertinent technological issue is the effective application of specific energy to overcome cohesive forces of colloidal asphaltene in order to improve conversion kinetics and minimize fouling of catalytic sites. The objective of the current study was to conduct an examination of the ability of microwave power to dissociate petroleum asphaltene macrostructures. Electron spin resonance ŽESR. vanadyl probe method utilizing 9 GHz microwave energy is adopted for this investigation. Oxovanadium complexes function as a tracer to elucidate the dynamic behavior of asphaltene micelles when subjected to a microwave field.
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o-xylene, which was prepared in an amber-colored bottle. Interpretations of ESR vanadyl spectra and determination of Arrhenius energy both follow the methods of Tynan and Yen Ž1969..
3. Results and discussion 3.1. Release of the bonded form of oxoÕanadium The ESR spectra of 4% Boscan asphaltene solution in o-xylene at 258C, 608C and 1008C are shown in Figs. 1–3, respectively. An increase of sample temperature results in a transition from anisotropic to isotropic form. The room temperature measurement ŽFig. 1. shows anisotropic features resembling the spectrum of powder asphaltene ŽFig. 4.. Actually, a Boscan asphaltene revealed the 16 features of the vanadium anisotropic spectra with eight parallel ŽNo. 1 I aniso , No. 2 I aniso , etc.. and eight perpendicular ŽNo. 1 H aniso , No. 2 H aniso , etc.. features ŽYen et al.,
2. Experimental 2.1. Materials and methods The asphalt sample designated as AAK-1 was obtained from the material library of the Strategic Highway Research Program ŽSHRP.. Asphalt AAK-1 was acquired from the distillation of Boscan crude oil from Venezuela. The asphaltene sample was prepared using the solvent fractionation method. An asphalt–toluene solution Ž1:1 vrv ratio. was diluted with an excess amount of n-pentane Ž1:50 vrv ratio., and the mixture was stirred for 4 h. The asphaltene precipitate was filtered and purified with hot n-pentane for 48 h by Soxhlet extraction. All spectra were recorded with the Bruker X-band ESR spectrometer ŽModel ESP-300.. A standard TE Žtransverse electric. resonant cavity was used with 100 kHz magnetic field modulation. A modulation amplitude of 6.30 G and receiver gain of 10,000 were employed for all of the measurements. Sample temperature was calibrated with a copper-constantan thermocouple. All ESR measurements were conducted with the 4% solution of Boscan asphaltene in
Fig. 1. ESR spectrum of 4% Boscan asphaltene solution in o-xylene at 258C. The incident microwave power is 3.2 mW. ŽThis spectrum and other spectrum in this paper contain the free radical peak of asphaltene, which is the predominant one. Since we are only concerned with the weaker feature, we allow the spectrum to fall outside the scale of the figure..
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Fig. 2. ESR spectrum of 4% Boscan asphaltene solution in o-xylene at 608C. The incident microwave power is 3.2 mW.
1969.. This represents the solid frozen state of vanadyl complexes associated with colloidal asphal-
Fig. 4. ESR spectrum of Boscan asphaltene in the powder form at 258C. Notice that there are no isotropic lines in this spectrum.
tene in o-xylene. The high-temperature ESR spectrum ŽFig. 3. reveals an isotropic system leading to eight resonance lines ŽNo. 1 iso , No. 2 iso , etc.., which is observed for oxovanadium acetylacetonate dissolved in toluene ŽWilson and Kivelson, 1966.. Vanadyl complexes associated with asphaltene in o-xylene are transformed to a free tumbling state at 1008C. Superposition of isotropic and anisotropic resonance components in the ESR vanadyl spectrum ŽFig. 2. indicates that both mobile and bounded oxovanadium compounds are present in asphaltene solution at 608C. Under the considerations of line overlappings and signal intensities, the relative content of mobile to bounded vanadyl complexes in asphaltene solution can only be estimated by the ESR peak-to-peak height ratio of No.5iso to No.6 H aniso ŽNo.6 perpendicular resonance feature. line structures ŽTynan and Yen, 1969.. 3.2. Packing imperfection in A micelleB or in A assemblageB
Fig. 3. ESR spectrum of 4% Boscan asphaltene at 1008C. The incident microwave power is 3.2 mW.
Petroleum asphaltics is a complex mixture, which requires a combination of techniques for the chemical and structural elucidation. Asphaltenes in the
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natural state have been repeatedly verified as colloidal particles dispersed in organic solvents, petroleum crudes, and distillation residues by small angle neutron scattering, small angle X-ray scattering, and electron microscopy ŽSheu et al., 1991; Storm and Sheu, 1994; Pollack and Yen, 1970; Reerink, 1973.. The majority of these colloids possess a characteristic distance of approximately 10–15 nm, and they are commonly referred to as asphaltene AmicellesB or AassemblageB. Colloidal asphaltene belongs to the material class of multipolymers, which are distinguished as polycondensation of random polymers having a great variety of building blocks, Fig. 5 ŽYen et al., 1961; Dickie and Yen, 1967; Yen, 1981.. Detection of the Ž002.-band from X-ray diffraction verifies the existence of polycondensed
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aromatic regions in the Boscan asphaltene sample fractionated from AAK-1 asphalt, Fig. 6. The Ž002.band is attributed to the interplanar spacing between layers of polynuclear aromatic fused-ring systems, such as found in graphites and carbon blacks. Comparable aromaticity values of petroleum asphaltenes obtained from powder X-ray diffraction and nuclear magnetic resonance ŽNMR. carried out in solution have confirmed aromatic polycondensates as structural entities within micelle agglomerates ŽYen et al., 1984.. Electron micrographs of mica sprayed with diluted Boscan asphaltene solution have revealed polyhedral shaped particles possessing an average diameter of 2–3 nm ŽDickie et al., 1969.. Packing imperfections among aromatic crystallites Žreference to Fig. 5., therefore, must exist within asphaltene
Fig. 5. Macrostructure of petroleum asphaltics. ŽDickie and Yen, 1967..
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polymeric network of asphaltene macromolecules. Coordination of vanadyl ion with nitrogen, sulfur, andror oxygen donor ligands at the defect center of polynuclear aromatic fused-ring system can also be identified as non-porphyrin metal complex ŽYen, 1982a,b; Boucher et al., 1969; Malhotra and Buckmaster, 1985.. Integration of non-porphyrin structures within the macromolecular framework of asphaltene is one of the main reasons for their high AapparentB molecular weights. Fig. 6. Detection of Ž002.-band by X-ray powder diffraction of Boscan asphaltene fractionated from AAK-1 asphalt.
micelles. In this description, the crystallites are not restricted to a solid state, since one-, two- and threedimensional order can be in a liquid or gaseous state. 3.3. Porphyrin Õanadium and non-porphyrin Õanadium complexes Petroporphyrins are mainly identified as homologous series of etioporphyrin III ŽEtio. and deoxophylloerythroetioporphyrin ŽDPEP. ŽYen, 1982a,b.. Together with the more generally defined nonporphyrins, they are the two major classes of vanadium complexes present in petroleum asphaltenes. The elution profile of isooctane-separated Boscan asphaltene by size exclusion chromatography–inductively coupled plasma–atomic emission spectroscopy ŽSEC–ICP–AES. has illustrated a bimodal distribution of vanadyl compounds having maxima at approximate molecular weights of 800 and 9000 Žpolystyrene equivalent. ŽReynolds and Biggs, 1986.. The maximum at 800 has been identified as metalloporphyrins, which are released into the mobile solvent during chromatographic separation. The nonporphyrins Ži.e. 9000 MW peak. remain in colloidal asphaltene of molecular weights comparable with aromatic polycondensates ŽYen, 1994a,b.. It is evident that both petroporphyrins and nonporphyrins are bounded to asphaltene micelles in a different manner. Vanadium porphyrins with diaza-annulene base structures are believed to associate through p–p electron-clouds overlapping with the aromatic systems of asphaltenes. On the other hand, nonporphyrins can exist as structural units within the
3.4. Effect of Õanadyl on other p-system in the A micelleB Some evidences have pointed to the relation between vanadium complexes and the association sites of colloidal particles. A comparison of SEC–ICP– AES vanadium profiles has shown the formation of higher molecular weight nonporphyrins in isooctane-precipitated asphaltene, which was not detected in the original Boscan q10008F residuum ŽReynolds and Biggs, 1986.. Electron micrograph reveals that vanadium enrichment of native Boscan asphaltene can lead to particle agglomeration in benzene ŽDickie et al., 1969.. The sixteen anisotropic features of ESR spectra indicate single axial symmetry of vanadyl coordination in petroleum asphaltenes ŽTynan and Yen, 1969; Yen et al., 1969.. The observed nitrogen superhyperfine structures of native Boscan and vanadium-doped asphaltenes are attributed to the reduced distance from VO 2q ion to the basal ligand plane Ž d V ™ p .. This fact will result in enhanced interaction between vanadium d xy orbital and the nitrogen bonding orbitals ŽYen et al., 1969; Yen, 1971.. It has been suggested that in-plane movement of VO 2q ion toward the coordination center is affected by donor–acceptor interactions with the highly aromatic host as well as the extended p-system localized in packing microenvironment ŽYen, 1982a,b; Yen, 1994a,b.. These experimental results altogether explain that vanadyl complexes are bounded and situated within the aromatic systems of asphaltenes, and some of their association sites can be the basis of disclinations in micelle agglomerates ŽYen, 1994a,b.. Thus, free tumbling of vanadium compounds in oxylene at 1008C requires initial dissociation of as-
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phaltene micelles at the packing imperfections prior to their release from aromatic hosts. 3.5. Effects of polar solÕents to micelle Mutual miscibility of asphaltene and the dispersing solvent is governed by Gibbs free energy of mixing: DGm s D Hm y TD Sm
Ž 1.
2
Ž 2.
D Hm s Vm Ž d 1 y d 2 . f 1 f 2
where Vm is the total volume of mixture, f 1 and f 2 are the volume fractions, and d 1 and d 2 are the solubility parameters of solvent and solute, respectively. The change in enthalpy of mixing is almost independent of temperature and it must be small or equal to zero ŽBarton, 1975., and entropy change is always positive in order to have a negative Gibbs energy. There is a greater tendency of mutual miscibility in an asphaltene-solvent system at a higher temperature, which is due to an enhancement in motion of both dispersed and dispersing phases. Yet, small angle neutron scattering measurements of 1% and 5% asphaltene solutions in toluene have indicated that the mean radius of the asphaltene micelles remains constant as temperature increases from 258C to 1008C ŽSheu et al., 1992.. Since Hildebrand solubility parameters of o-xylene and toluene are comparable, thermal energy alone under ESR experimental conditions at 1008C is clearly not sufficient to cause deagglomeration of micelles in o-xylene. This is in agreement with results from electron-nuclear double resonance study of 0.5% asphaltene in a crudertoluene mixture, which concludes that there is no destruction of micelles after heat treatment at 908C ŽGaltsev et al., 1995.. On the other hand, ESR detection of free tumbling vanadyl complexes has been observed with a 0.4% asphaltene solution in tetrahydrofuran ŽTHF. at 208C ŽTynan and Yen, 1969.. The characteristic distance of asphaltene colloids in 1% solution of Boscan cruderTHF, 0.5% solution of maltenerTHF, and 0.4% solution of THF all fall within the distance range of micelles at room temperature ŽLin, 1992; Ravey et al., 1988.. The dispersion energy of a polar organic solvent surely is not capable of overcoming the cohesiveness of micelles within this concentration range. However, a
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solvent of high donor ability can stabilize mobile metalloporphyrins and non-porphyrin macrocycles through donor–acceptor interactions at the available ligand sites. The estimated ratio of mobile to bounded vanadyl complexes in an asphaltene solution progressively increases in a dispersing medium Žsinglearomatic ring organic solvent series. with a higher value of Hansen hydrogen bonding solubility parameter Ž d h . at room temperature, Fig. 7 ŽTynan and Yen, 1969; Barton, 1991.. 3.6. Dipole–dipole interaction The induced dipole nature of asphaltene micelles has been identified in asphaltenertoluene gel systems ŽSheu et al., 1994.. These electric dipoles are attributed to stable free radicals at defect centers in polycondensed aromatics. A charge transfer through propagation of Wannier excitons is the responsible intermolecular force within and among most of these polynuclear p–p systems ŽYen and Young, 1973., and its associative strength can be affected by the vanadium content ŽYen et al., 1962.. Asphaltene micellization is a thermodynamically reversible pro-
Fig. 7. Ratio of mobile to bounded vanadyl complexes in asphaltene solution shows a progressive increase in a dispersing medium Žsingle-aromatic ring organic solvent series. with higher value of Hansen hydrogen bonding solubility parameter at room temperature. The data point of o-xylene is for Boscan asphaltene from the present study, and the remaining ESR data is for Bachaquero asphaltene from Venezuela ŽTynan and Yen, 1969.. Literature values of Hansen hydrogen bonding solubility parameter are taken from the CRC Handbook of Solubility Parameters and other Cohesion Parameters ŽBarton, 1991..
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cess ŽLin, 1992; Sheu, 1996., which proceeds as charge transfer, charge balance, and hydrogen bondings all driving aromatic polycondensates into compact assemblage ŽYen, 1992; Andersen and Birdi, 1991.. Further clustering of micelles is mainly through induced dipole–dipole and dipole–dipole interactions ŽEnsley, 1975.. The fractal aggregation of asphaltene at the mica–toluene interface due to charge neutralization and destabilization is clearly illustrated by atomic force micrographs ŽToulhoat et al., 1994.. The electric charge character of asphaltenes in an organic medium is well established ŽSiffert et al., 1990; Taylor, 1998; Storm and Sheu, 1993.. The interaction of asphaltene micelles with high frequency alternating electric field leads to a drastic increase in conductivity response, which is temperature independent ŽSheu et al., 1991.. Clusters of asphaltene induced dipoles can undergo orientation polarization when subjected to an applied microwave power. Observing the mobile state of total oxovanadium complexes in 4% asphaltene solution ŽFig. 3. implies that electromagnetic wave of microwave frequency has the ability of overcoming charge transfer and charge balance interactions within micelle agglomerates. A significant decrease in spin content of 0.4% asphaltene solution has been determined in a dispersing solvent of greater dipole moment at 258C ŽKhulbe et al., 1992.. A higher ratio of No.5isorNo.6 H aniso is observed with an increasing microwave power incident to asphaltene solution, Fig. 8. Finally, the association energy of the Boscan
Fig. 9. Temperature dependence study of 4% solution of Boscan asphaltene in o-xylene is conducted under incident microwave power of 3.2 mW. The ESR peak-to-peak height ratio of No. 5iso r No. 6 H aniso is correlated with reciprocal absolute temperature following Arrhenius’ law.
asphaltene fractionated from AAK-1 asphalt is found to be 17.4 kcalrmol ŽFig. 9., which is within the range of 14–20 kcalrmol as determined for a number of petroleum asphaltenes ŽTynan and Yen, 1969; Shibata et al., 1978.. In conclusion, microwave power is an efficient energy source capable of causing dissociation of asphaltene micelles when the field strength is above the threshold level. Symbols and acronyms used DGm D Hm D Sm d1 0d 2 dh
Fig. 8. ESR peak-to-peak height ratio of No. 5iso r No. 6 H aniso increases at higher microwave power incident to 4% Boscan asphaltene solution in o-xylene at 458C.
change in Gibbs free energy of mixing change in enthalpy of mixing change in entropy of mixing solubility parameter of solvent solubility parameter of solute Hansen hydrogen bonding solubility parameter DPEP deoxophylloerythroetioporphyrin ESR electron spin resonance spectroscopy f1 volume fraction of solvent in a mixture f2 volume fraction of solute in a mixture H magnetic field NMR nuclear magnetic resonance SARA saturates–aromatics–resins–asphaltenes SEC–ICP–AES size exclusion chromatography–inductively coupled plasma–atomic emission spectroscopy
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T TE THF Vm
absolute temperature transverse electric tetrahydrofuran total volume of a mixture
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Takeuchi, C., Fukul, Y., Nakamura, M., Shiroto, Y., 1983. Asphaltene cracking in catalytic hydrotreating of heavy oils: 1. Processing of heavy oils by catalytic hydroprocessing and solvent deasphalting. Ind. Eng. Chem. Process Des. Dev. 22, 236–242. Taylor, S.E., 1998. The electrodeposition of asphaltenes and implications for asphaltene structure and stability in crude and residual oils. Fuel 77 Ž8., 821–828. Toulhoat, H., Prayer, C., Rouquet, G., 1994. Characterization by atomic force microscopy of adsorbed asphaltenes. Colloids Surf., A: Physicochem. Eng. Aspects 91, 267–283. Tynan, E.C., Yen, T.F., 1969. Association of vanadium chelates in petroleum asphaltenes as studied by ESR. Fuel 43, 191–208. Wiehe, I.A., 1992. A solvent-resid phase diagram for tracking resid conversion. Ind. Eng. Chem. Res. 31 Ž2., 530–536. Wiehe, I.A., 1993. A phase-separation kinetic model for coke formation. Ind. Eng. Chem. Res. 32, 2447–2454. Wilson, B.R., Kivelson, D., 1966. ESR linewidths in solution: I. Experiments on anisotropic and spin-rotational effects. J. Chem. Phys. 44, 154–168. Yen, T.F., 1971. Nitrogen superhyperfine splittings of vanadyl porphyrins in native asphaltenes. Naturwissenschaften 58, 267–268. Yen, T.F., 1981. Structural difference between asphaltenes isolated from petroleum and from coal liquids. In: Bunger, J.W., Li, N.C. ŽEds.., Chemistry of Asphaltenes. American Chemical Society, Washington, DC, pp. 39–51. Yen, T.F., 1982a. Chemical aspects of metals in native petroleum. In: Yen, T.F. ŽEd.., The Role of Trace Metals in Petroleum. Ann Arbor Sci. Publ., Ann Arbor, MI, pp. 1–30. Yen, T.F., 1982b. Vanadium and its bonding in petroleum. In: Yen, T.F. ŽEd.., The Role of Trace Metals in Petroleum. Ann Arbor Sci. Publ., Ann Arbor, MI, pp. 167–182.
Yen, T.F., 1992. The colloidal aspect of a macrostructure of petroleum asphalt. Fuel Sci. Technol. Int. 10 Ž4–6., 723–733. Yen, T.F., 1994a. Multiple structural orders of asphaltenes. In: Yen, T.F., Chilingarian, G.V. ŽEds.., Asphaltenes and Asphalts vol. 1 Elsevier, Amsterdam, pp. 111–123. Yen, T.F., 1994b. Meso-scaled structure and membrane mimetic chemistry. In: Yen, T.F., Gilbert, R.D., Fendler, J.H. ŽEds.., Membrane Mimetic Chemistry and Its Applications. Plenum, New York, pp. 255–279. Yen, T.F., Erdman, J.G., Pollack, S.S., 1961. Investigation of the structure of petroleum asphaltenes by X-ray diffraction. Anal. Chem. 33 Ž11., 1587–1594. Yen, T.F., Erdman, J.G., Saraceno, A.J., 1962. Investigation of the nature of free radicals in petroleum asphaltenes and related substances by electron spin resonance. Anal. Chem. 34 Ž6., 694–700. Yen, T.F., Tynan, E.C., Vaughan, G.B., Boucher, L.J., 1969. Electron spin resonance studies of petroleum asphaltics. In: Friedel, R.A. ŽEd.., Advances in Spectroscopy of Fuels. Plenum, New York, pp. 187–201. Yen, T.F., Young, D.K., 1973. Spin excitations of bitumens. Carbon 11 Ž1., 33–41. Yen, T.F., Wu, W.H., Chilingarian, G.V., 1984. A study of the structure of petroleum asphaltenes and related substances by proton nuclear magnetic resonance. Energy Sources 7 Ž3., 275–304. Yokono, T., Obara, T., Sanada, Y., 1986. Characterization of carbonization reaction of petroleum residues by means of high-temperature ESR and transferable hydrogen. Carbon 24 Ž1., 29–32.
An electron spin resonance probe method for the understanding of petroleum asphaltene macrostructure Gary K. Wong, Teh Fu Yen ) Department of CiÕil and EnÕironmental Engineering, UniÕersity of Southern California, 3620 South Vermont AÕenue 224A, Los Angeles, CA 90089-2531, USA Received 12 January 2000; accepted 14 July 2000
Abstract Molecularly, petroleum asphaltenes are induced dipoles, which agglomerate into nanometer-sized colloids of different aggregation states. The electron spin resonance ŽESR. vanadyl probe method is used to investigate the asphaltene macrostructures under different temperatures and microwave powers. Oxovanadium complexes native to an asphaltene isolated from Boscan crude oil, Venezuela, function as tracers to examine the behavior of micelle agglomerates when subjected to a microwave field. Both mobile and bounded oxovanadium compounds in colloidal asphaltene solution are in a state of equilibrium. It is noted that a greater amount of mobile vanadyl complexes can be stabilized in a dispersing medium Žsingle-aromatic ring solvent series. with a higher-valued Hansen hydrogen bonding solubility parameter. We found that conversion of ESR vanadyl hyperfine lines occurs from anisotropic to isotropic as the temperature of a 4% Boscan asphaltene solution in o-xylene increased from 258C to 1008C. Free tumbling of total vanadyl complexes in organic solvent signifies dissociation of micelles at packing imperfections prior to their release from aromatic hosts. Coupling of petroleum asphaltenes with microwave power can overcome charge transfer and charge balance interactions within micelle agglomerates. The relative content of mobile to bounded vanadyl complexes in 4% Boscan asphaltene solution of o-xylene was found to increase with microwave power at 458C. Microwave energy will enable effective dispersion of colloidal asphaltene in heavy oil refining and upgrading. q 2000 Elsevier Science B.V. All rights reserved. Keywords: asphaltenes; electron spin resonance; microwave power; vanadyl complexes
1. Introduction Formation of petroleum coke and oil sludge during heavy oil refining can lead to serious problems of equipment maintenance and residue management. Flocculation of asphaltene often causes fouling of ) Corresponding author. Tel.: q1-213-740-0586; fax: q1-213744-1426. E-mail address: [email protected] ŽT.F. Yen..
heat exchangers as well as carbonaceous deposition in reactors, transportation pipes, and storage tanks. The end result of reduction of reactor throughput and liquid product yield will greatly affect the overall refining economics. Conversion of asphaltics to liquid fuel oils, therefore, is essential and necessary. However, it is the unique colloidal nature and stable free radical behavior, which cause petroleum asphaltenes to be the key obstacles in the upgrading of heavy oil.
0920-4105r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 4 1 0 5 Ž 0 0 . 0 0 0 6 7 - X
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Stability of asphaltene colloids in heavy oil is attained by both steric peptization with resin and miscibility in the dispersing medium. Petroleum resins with lower average molecular weights are structural analogues of asphaltenes ŽWiehe, 1992.. Hydrogen bonding between polar functional groups is the main driving force for the association of resin molecules with colloidal asphaltene. Stability enhancement of asphaltene suspension against critical solvent conditions as well as the incompatibility of asphaltene and gas oil fractions have suggested that petroleum resin functions as peptization agents in heavy oil ŽLian et al., 1994; Koots and Speight, 1975.. Asphaltic colloids coexist with aromatics and saturates in the state of equilibrium, which is governed by mutual solubilities of saturates–aromatics– resins–asphaltenes ŽSARA. fractions. Clustering of particles by a diffusion-limited aggregation process takes place in heavy oils containing high concentrations of asphaltenes ŽLin et al., 1991.. Asphaltene clusters are considered as precursors for flocs and mesophase ŽYen, 1994a,b.. Overall, the criteria for achieving maximum stability and compatibility of heavy oils is a function of, but is not limited to: Ž1. low asphaltene content, Ž2. high resin to asphaltene ratio, and Ž3. dispersing oil of high aromaticity. Disturbance in the compatibility state of heavy oil is unavoidable under thermal cracking conditions. Redistribution and changes in the chemical characters of SARA fractions occur throughout the course of hydroconversion reactions. Homolytic cleavage of chemical bonds to produce smaller free radical fragments is the principal reaction pathway to decomposition of the macromolecular structures in petroleum asphaltics. Native asphaltenes are thermally reactive with higher first-order rate constants than the nheptane soluble fractions ŽWiehe, 1993.. When compared to oil molecules, however, asphaltenes exhibit the least amount of conversion due to their colloidal nature. Thermal cracking of native asphaltenes proceeds with dealkylation and aromatization of naphthenic portions leading to the newly polycondensed aromatic cores as product asphaltenes ŽCalemma et al., 1994.. Simultaneous saturation of dispersing aromatic oil and hydrocracking of resin further interrupts the solubility equilibrium of the system until phase separation takes place. Asphaltene flocculation is believed to be responsible for the sludge formation
during visbreaking or catalytic hydroconversion of heavy oil. The amount of sludge formed has been empirically determined to be dependent on the feedstock characteristics, such as resin to asphaltene ratio, ring condensation index, aromaticity, and degree of alkyl substitution of aromatic ring systems in asphaltenes ŽStorm et al., 1994.. As a result, rapid and extensive cracking of colloidal asphaltene in conjunction with a relatively moderate conversion of maltene fraction would be advantageous for maintaining compatibility and stability of the processed oil. However, the refractory character of polycondensed aromatic systems has rendered asphaltenes as coke precursors. Reactivity of asphaltenes is mainly governed by the forms and behavior of free radicals evolved in the process environment. The thermally induced radicals are stabilized as delocalized spins through p–p resonance within the aromatic host structures of polycondensates. Retrogressive reactions are predominant within micelles and clusters due to the combined effect of the stable free radical nature and insufficient hydrogen delivery. Limited conversion of polycondensed aromatic cores eventually leads to phase separation and coke formation. Fine grain mosaics are formed during carbonization of petroleum residue with low hydrogen donating capacity, which is observed as inversely proportional to the spin concentration ŽYokono et al., 1986.. Mesophase pitch samples above 4008C show a common pattern of spin increase along with linewidth narrowing, which implicates electron spin exchange interactions of aromatic structures ŽKaneko et al., 1990.. All the facts indicate that high conversion of colloidal asphaltene requires both efficient hydrogen termination reactions to counter-play the intrinsic stable free radical nature, and high dispersion of nanocolloids to reduce irreversible particle growth ŽStorm et al., 1998.. Many heavy oil-upgrading technologies adopt novel catalysts to specifically promote conversion of heavy petroleum components. Catalyst designs place emphasis on the caging of colloidal asphaltene in close vicinity of active sites for cracking andror hydrogenation reactions. Both surface association onto micron-sized metal sulfide particles and diffusion into the specialized pore structures of Asphaltenic Bottom Cracking catalyst do serve this funda-
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mental purpose ŽBearden and Aldridge, 1981; Takeuchi et al., 1983.. A catalytic environment rich in chemisorbed hydrogen can greatly retard free radical polymerization. Deagglomeration of asphaltene clusters and micelles is beneficial for the acceleration of hydrogen quenching reactions, as well as lowering the diffusion limitation of catalyst pores. Particle aggregation, however, tends to occur under high hydrogen over-pressure, which is typical in residual hydroprocessors ŽSyunyaev and Abid, 1994.. A pertinent technological issue is the effective application of specific energy to overcome cohesive forces of colloidal asphaltene in order to improve conversion kinetics and minimize fouling of catalytic sites. The objective of the current study was to conduct an examination of the ability of microwave power to dissociate petroleum asphaltene macrostructures. Electron spin resonance ŽESR. vanadyl probe method utilizing 9 GHz microwave energy is adopted for this investigation. Oxovanadium complexes function as a tracer to elucidate the dynamic behavior of asphaltene micelles when subjected to a microwave field.
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o-xylene, which was prepared in an amber-colored bottle. Interpretations of ESR vanadyl spectra and determination of Arrhenius energy both follow the methods of Tynan and Yen Ž1969..
3. Results and discussion 3.1. Release of the bonded form of oxoÕanadium The ESR spectra of 4% Boscan asphaltene solution in o-xylene at 258C, 608C and 1008C are shown in Figs. 1–3, respectively. An increase of sample temperature results in a transition from anisotropic to isotropic form. The room temperature measurement ŽFig. 1. shows anisotropic features resembling the spectrum of powder asphaltene ŽFig. 4.. Actually, a Boscan asphaltene revealed the 16 features of the vanadium anisotropic spectra with eight parallel ŽNo. 1 I aniso , No. 2 I aniso , etc.. and eight perpendicular ŽNo. 1 H aniso , No. 2 H aniso , etc.. features ŽYen et al.,
2. Experimental 2.1. Materials and methods The asphalt sample designated as AAK-1 was obtained from the material library of the Strategic Highway Research Program ŽSHRP.. Asphalt AAK-1 was acquired from the distillation of Boscan crude oil from Venezuela. The asphaltene sample was prepared using the solvent fractionation method. An asphalt–toluene solution Ž1:1 vrv ratio. was diluted with an excess amount of n-pentane Ž1:50 vrv ratio., and the mixture was stirred for 4 h. The asphaltene precipitate was filtered and purified with hot n-pentane for 48 h by Soxhlet extraction. All spectra were recorded with the Bruker X-band ESR spectrometer ŽModel ESP-300.. A standard TE Žtransverse electric. resonant cavity was used with 100 kHz magnetic field modulation. A modulation amplitude of 6.30 G and receiver gain of 10,000 were employed for all of the measurements. Sample temperature was calibrated with a copper-constantan thermocouple. All ESR measurements were conducted with the 4% solution of Boscan asphaltene in
Fig. 1. ESR spectrum of 4% Boscan asphaltene solution in o-xylene at 258C. The incident microwave power is 3.2 mW. ŽThis spectrum and other spectrum in this paper contain the free radical peak of asphaltene, which is the predominant one. Since we are only concerned with the weaker feature, we allow the spectrum to fall outside the scale of the figure..
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Fig. 2. ESR spectrum of 4% Boscan asphaltene solution in o-xylene at 608C. The incident microwave power is 3.2 mW.
1969.. This represents the solid frozen state of vanadyl complexes associated with colloidal asphal-
Fig. 4. ESR spectrum of Boscan asphaltene in the powder form at 258C. Notice that there are no isotropic lines in this spectrum.
tene in o-xylene. The high-temperature ESR spectrum ŽFig. 3. reveals an isotropic system leading to eight resonance lines ŽNo. 1 iso , No. 2 iso , etc.., which is observed for oxovanadium acetylacetonate dissolved in toluene ŽWilson and Kivelson, 1966.. Vanadyl complexes associated with asphaltene in o-xylene are transformed to a free tumbling state at 1008C. Superposition of isotropic and anisotropic resonance components in the ESR vanadyl spectrum ŽFig. 2. indicates that both mobile and bounded oxovanadium compounds are present in asphaltene solution at 608C. Under the considerations of line overlappings and signal intensities, the relative content of mobile to bounded vanadyl complexes in asphaltene solution can only be estimated by the ESR peak-to-peak height ratio of No.5iso to No.6 H aniso ŽNo.6 perpendicular resonance feature. line structures ŽTynan and Yen, 1969.. 3.2. Packing imperfection in A micelleB or in A assemblageB
Fig. 3. ESR spectrum of 4% Boscan asphaltene at 1008C. The incident microwave power is 3.2 mW.
Petroleum asphaltics is a complex mixture, which requires a combination of techniques for the chemical and structural elucidation. Asphaltenes in the
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natural state have been repeatedly verified as colloidal particles dispersed in organic solvents, petroleum crudes, and distillation residues by small angle neutron scattering, small angle X-ray scattering, and electron microscopy ŽSheu et al., 1991; Storm and Sheu, 1994; Pollack and Yen, 1970; Reerink, 1973.. The majority of these colloids possess a characteristic distance of approximately 10–15 nm, and they are commonly referred to as asphaltene AmicellesB or AassemblageB. Colloidal asphaltene belongs to the material class of multipolymers, which are distinguished as polycondensation of random polymers having a great variety of building blocks, Fig. 5 ŽYen et al., 1961; Dickie and Yen, 1967; Yen, 1981.. Detection of the Ž002.-band from X-ray diffraction verifies the existence of polycondensed
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aromatic regions in the Boscan asphaltene sample fractionated from AAK-1 asphalt, Fig. 6. The Ž002.band is attributed to the interplanar spacing between layers of polynuclear aromatic fused-ring systems, such as found in graphites and carbon blacks. Comparable aromaticity values of petroleum asphaltenes obtained from powder X-ray diffraction and nuclear magnetic resonance ŽNMR. carried out in solution have confirmed aromatic polycondensates as structural entities within micelle agglomerates ŽYen et al., 1984.. Electron micrographs of mica sprayed with diluted Boscan asphaltene solution have revealed polyhedral shaped particles possessing an average diameter of 2–3 nm ŽDickie et al., 1969.. Packing imperfections among aromatic crystallites Žreference to Fig. 5., therefore, must exist within asphaltene
Fig. 5. Macrostructure of petroleum asphaltics. ŽDickie and Yen, 1967..
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polymeric network of asphaltene macromolecules. Coordination of vanadyl ion with nitrogen, sulfur, andror oxygen donor ligands at the defect center of polynuclear aromatic fused-ring system can also be identified as non-porphyrin metal complex ŽYen, 1982a,b; Boucher et al., 1969; Malhotra and Buckmaster, 1985.. Integration of non-porphyrin structures within the macromolecular framework of asphaltene is one of the main reasons for their high AapparentB molecular weights. Fig. 6. Detection of Ž002.-band by X-ray powder diffraction of Boscan asphaltene fractionated from AAK-1 asphalt.
micelles. In this description, the crystallites are not restricted to a solid state, since one-, two- and threedimensional order can be in a liquid or gaseous state. 3.3. Porphyrin Õanadium and non-porphyrin Õanadium complexes Petroporphyrins are mainly identified as homologous series of etioporphyrin III ŽEtio. and deoxophylloerythroetioporphyrin ŽDPEP. ŽYen, 1982a,b.. Together with the more generally defined nonporphyrins, they are the two major classes of vanadium complexes present in petroleum asphaltenes. The elution profile of isooctane-separated Boscan asphaltene by size exclusion chromatography–inductively coupled plasma–atomic emission spectroscopy ŽSEC–ICP–AES. has illustrated a bimodal distribution of vanadyl compounds having maxima at approximate molecular weights of 800 and 9000 Žpolystyrene equivalent. ŽReynolds and Biggs, 1986.. The maximum at 800 has been identified as metalloporphyrins, which are released into the mobile solvent during chromatographic separation. The nonporphyrins Ži.e. 9000 MW peak. remain in colloidal asphaltene of molecular weights comparable with aromatic polycondensates ŽYen, 1994a,b.. It is evident that both petroporphyrins and nonporphyrins are bounded to asphaltene micelles in a different manner. Vanadium porphyrins with diaza-annulene base structures are believed to associate through p–p electron-clouds overlapping with the aromatic systems of asphaltenes. On the other hand, nonporphyrins can exist as structural units within the
3.4. Effect of Õanadyl on other p-system in the A micelleB Some evidences have pointed to the relation between vanadium complexes and the association sites of colloidal particles. A comparison of SEC–ICP– AES vanadium profiles has shown the formation of higher molecular weight nonporphyrins in isooctane-precipitated asphaltene, which was not detected in the original Boscan q10008F residuum ŽReynolds and Biggs, 1986.. Electron micrograph reveals that vanadium enrichment of native Boscan asphaltene can lead to particle agglomeration in benzene ŽDickie et al., 1969.. The sixteen anisotropic features of ESR spectra indicate single axial symmetry of vanadyl coordination in petroleum asphaltenes ŽTynan and Yen, 1969; Yen et al., 1969.. The observed nitrogen superhyperfine structures of native Boscan and vanadium-doped asphaltenes are attributed to the reduced distance from VO 2q ion to the basal ligand plane Ž d V ™ p .. This fact will result in enhanced interaction between vanadium d xy orbital and the nitrogen bonding orbitals ŽYen et al., 1969; Yen, 1971.. It has been suggested that in-plane movement of VO 2q ion toward the coordination center is affected by donor–acceptor interactions with the highly aromatic host as well as the extended p-system localized in packing microenvironment ŽYen, 1982a,b; Yen, 1994a,b.. These experimental results altogether explain that vanadyl complexes are bounded and situated within the aromatic systems of asphaltenes, and some of their association sites can be the basis of disclinations in micelle agglomerates ŽYen, 1994a,b.. Thus, free tumbling of vanadium compounds in oxylene at 1008C requires initial dissociation of as-
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phaltene micelles at the packing imperfections prior to their release from aromatic hosts. 3.5. Effects of polar solÕents to micelle Mutual miscibility of asphaltene and the dispersing solvent is governed by Gibbs free energy of mixing: DGm s D Hm y TD Sm
Ž 1.
2
Ž 2.
D Hm s Vm Ž d 1 y d 2 . f 1 f 2
where Vm is the total volume of mixture, f 1 and f 2 are the volume fractions, and d 1 and d 2 are the solubility parameters of solvent and solute, respectively. The change in enthalpy of mixing is almost independent of temperature and it must be small or equal to zero ŽBarton, 1975., and entropy change is always positive in order to have a negative Gibbs energy. There is a greater tendency of mutual miscibility in an asphaltene-solvent system at a higher temperature, which is due to an enhancement in motion of both dispersed and dispersing phases. Yet, small angle neutron scattering measurements of 1% and 5% asphaltene solutions in toluene have indicated that the mean radius of the asphaltene micelles remains constant as temperature increases from 258C to 1008C ŽSheu et al., 1992.. Since Hildebrand solubility parameters of o-xylene and toluene are comparable, thermal energy alone under ESR experimental conditions at 1008C is clearly not sufficient to cause deagglomeration of micelles in o-xylene. This is in agreement with results from electron-nuclear double resonance study of 0.5% asphaltene in a crudertoluene mixture, which concludes that there is no destruction of micelles after heat treatment at 908C ŽGaltsev et al., 1995.. On the other hand, ESR detection of free tumbling vanadyl complexes has been observed with a 0.4% asphaltene solution in tetrahydrofuran ŽTHF. at 208C ŽTynan and Yen, 1969.. The characteristic distance of asphaltene colloids in 1% solution of Boscan cruderTHF, 0.5% solution of maltenerTHF, and 0.4% solution of THF all fall within the distance range of micelles at room temperature ŽLin, 1992; Ravey et al., 1988.. The dispersion energy of a polar organic solvent surely is not capable of overcoming the cohesiveness of micelles within this concentration range. However, a
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solvent of high donor ability can stabilize mobile metalloporphyrins and non-porphyrin macrocycles through donor–acceptor interactions at the available ligand sites. The estimated ratio of mobile to bounded vanadyl complexes in an asphaltene solution progressively increases in a dispersing medium Žsinglearomatic ring organic solvent series. with a higher value of Hansen hydrogen bonding solubility parameter Ž d h . at room temperature, Fig. 7 ŽTynan and Yen, 1969; Barton, 1991.. 3.6. Dipole–dipole interaction The induced dipole nature of asphaltene micelles has been identified in asphaltenertoluene gel systems ŽSheu et al., 1994.. These electric dipoles are attributed to stable free radicals at defect centers in polycondensed aromatics. A charge transfer through propagation of Wannier excitons is the responsible intermolecular force within and among most of these polynuclear p–p systems ŽYen and Young, 1973., and its associative strength can be affected by the vanadium content ŽYen et al., 1962.. Asphaltene micellization is a thermodynamically reversible pro-
Fig. 7. Ratio of mobile to bounded vanadyl complexes in asphaltene solution shows a progressive increase in a dispersing medium Žsingle-aromatic ring organic solvent series. with higher value of Hansen hydrogen bonding solubility parameter at room temperature. The data point of o-xylene is for Boscan asphaltene from the present study, and the remaining ESR data is for Bachaquero asphaltene from Venezuela ŽTynan and Yen, 1969.. Literature values of Hansen hydrogen bonding solubility parameter are taken from the CRC Handbook of Solubility Parameters and other Cohesion Parameters ŽBarton, 1991..
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cess ŽLin, 1992; Sheu, 1996., which proceeds as charge transfer, charge balance, and hydrogen bondings all driving aromatic polycondensates into compact assemblage ŽYen, 1992; Andersen and Birdi, 1991.. Further clustering of micelles is mainly through induced dipole–dipole and dipole–dipole interactions ŽEnsley, 1975.. The fractal aggregation of asphaltene at the mica–toluene interface due to charge neutralization and destabilization is clearly illustrated by atomic force micrographs ŽToulhoat et al., 1994.. The electric charge character of asphaltenes in an organic medium is well established ŽSiffert et al., 1990; Taylor, 1998; Storm and Sheu, 1993.. The interaction of asphaltene micelles with high frequency alternating electric field leads to a drastic increase in conductivity response, which is temperature independent ŽSheu et al., 1991.. Clusters of asphaltene induced dipoles can undergo orientation polarization when subjected to an applied microwave power. Observing the mobile state of total oxovanadium complexes in 4% asphaltene solution ŽFig. 3. implies that electromagnetic wave of microwave frequency has the ability of overcoming charge transfer and charge balance interactions within micelle agglomerates. A significant decrease in spin content of 0.4% asphaltene solution has been determined in a dispersing solvent of greater dipole moment at 258C ŽKhulbe et al., 1992.. A higher ratio of No.5isorNo.6 H aniso is observed with an increasing microwave power incident to asphaltene solution, Fig. 8. Finally, the association energy of the Boscan
Fig. 9. Temperature dependence study of 4% solution of Boscan asphaltene in o-xylene is conducted under incident microwave power of 3.2 mW. The ESR peak-to-peak height ratio of No. 5iso r No. 6 H aniso is correlated with reciprocal absolute temperature following Arrhenius’ law.
asphaltene fractionated from AAK-1 asphalt is found to be 17.4 kcalrmol ŽFig. 9., which is within the range of 14–20 kcalrmol as determined for a number of petroleum asphaltenes ŽTynan and Yen, 1969; Shibata et al., 1978.. In conclusion, microwave power is an efficient energy source capable of causing dissociation of asphaltene micelles when the field strength is above the threshold level. Symbols and acronyms used DGm D Hm D Sm d1 0d 2 dh
Fig. 8. ESR peak-to-peak height ratio of No. 5iso r No. 6 H aniso increases at higher microwave power incident to 4% Boscan asphaltene solution in o-xylene at 458C.
change in Gibbs free energy of mixing change in enthalpy of mixing change in entropy of mixing solubility parameter of solvent solubility parameter of solute Hansen hydrogen bonding solubility parameter DPEP deoxophylloerythroetioporphyrin ESR electron spin resonance spectroscopy f1 volume fraction of solvent in a mixture f2 volume fraction of solute in a mixture H magnetic field NMR nuclear magnetic resonance SARA saturates–aromatics–resins–asphaltenes SEC–ICP–AES size exclusion chromatography–inductively coupled plasma–atomic emission spectroscopy
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T TE THF Vm
absolute temperature transverse electric tetrahydrofuran total volume of a mixture
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