Petroleum Heavy Ends Stability: Evolution of Residues Macrostructure

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Energy & Fuels 1999, 13, 739-747

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Petroleum Heavy Ends Stability: Evolution of Residues Macrostructure by Aging M. Scarsella* and D. Mastrofini Universita` degli Studi di Roma “La Sapienza”, Dipartimento di Ingegneria Chimica, dei Materiali, delle Materie Prime e Metallurgia, Via Eudossiana, 18, 00184 Roma, Italy

L. Barre´, D. Espinat, and D. Fenistein Institut Franc¸ ais du Pe´ trole, 1&4 Ave du Bois-Pre´ au, 92852 Rueil Malmaison, France Received November 3, 1998. Revised Manuscript Received January 18, 1999

Rheological and small-angle scattering techniques were used to investigate the evolution of the colloidal structure of the Safaniya vacuum residue during the age hardening process. Rheological measurements revealed an increase in the molecular weight of the vacuum residue and maltene fraction after the RTFO aging test. Even the molecular-weight distribution became slightly wider, the amount of this variation being greater for aged maltenes than for the whole aged vacuum residue. Small-angle scattering results confirmed the importance of maltene aging on the whole sample viscoelastic properties; the macrostructure of asphaltenes both in solution and in a vacuum residue did not appear to be representative of the age hardening process, in contrast to previous findings.

Introduction The increased interest in the use and refining of heavy crudes and residues rich in asphaltenes has clearly shown the need for a deeper knowledge of the composition and structure of these materials, particularly of their troublesome asphaltene fraction.1-5 The composition, molecular structure, and colloidal state of asphaltenes in crude oils and especially in residues have been the subject of an increased number of studies in petroleum research,1,6-7 owing to their influence on the performances of the related materials.8 The stability of crude oils and residues is strictly connected to the nature and composition of their polar “alkylnaphthenoaromatic” heterocompounds, mostly asphaltenes and resins, which are considered the main groups responsible of aging,9 i.e., of the evolution of the rheological and physicochemical properties with time, during stockage and use. (1) Speight, J. G. The Chemistry and Technology of Petroleum, 2nd ed.; Marcel Dekker Inc.: New York, 1991. (2) Billon, A.; Morel, F.; Morrison, M. E.; Peries, J. P. Rev. Inst. Fr. Pe´ t. 1994, 49 (5), 495. (3) Zou, R.; Liu, L. In Asphaltenes and Asphalts, 1; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science: Amsterdam, The Netherlands, 1994; pp 339-363. (4) Speight, J. G. Annu. Rev. Energy 1986, 11, 253. (5) Speight, J. G. The desulfuration of Heavy Oils and Residua; Marcel Dekker Inc.: New York, 1981. (6) Asphaltenes and Asphalts, 1; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science: Amsterdam, The Netherlands, 1994. (7) Asphaltenes Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995. (8) Speight, J. G. Symposium on the Role of Asphaltenes in Petroleum Exploration, Production and Refining, Division of Petroleum Chemistry; 207th National Meeting of the American Chemical Society, San Diego, CA, March 13-18, 1994; American chemical Society: Washington, DC, 1994; pp 200-203. (9) Petersen, J. C. Trans. Res. Rec. 1984, 999, 13.

The main rheological alteration is related to a variation of the flow properties (increase of viscosity) due to stiffening or hardening of the material. The factors responsible for such rheological change are the loss of volatile oily components,10 the change of chemical composition due to asphaltene oxidation,11-14 and the molecular structuring process involving molecules and clusters.15-17 Previous studies18-20 showed that volatility generally does not give a large contribution to age hardening; when detectable, the loss of the more volatile oily components gives rise to a change of rheological properties, which is easily understood, the oily components acting as solvent for the more polar fractions. (10) Zupanick, M.; Baselice, V. Transportation Research Board; 76th Annual Meeting, Washington, DC, January 12-16, 1997; Paper No. 971223. (11) Choquet, F. Le vieillissement du bitume; Preprint of International Conference: Strategic Highway (SHRP) and Traffic Safety on Two Continents, The Hague, The Netherlands, September 22-24, 1993. (12) Van Oort, P. W. P. Ind. Eng. Chem. 1956, 48, 1169. (13) Doumenq, P.; Guillano, M.; Mille, G.; Kister, K. Anal. Chim. Acta 1991, 242, 137. (14) Gray, M. R.; Choi, J. H. K.; Egiebor, N. O.; Lirchen, R. P.; Sanford, E. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1988, 33 (2), 277. (15) Traxler, R. N. In Bituminous Materials; Hoiberg, A. J., Ed.; Interscience Publishers: New York, 1964; Vol. 1, pp 143-211. (16) Brown, A. B.; Sparks, J. W.; Smith, F. M. Proc. Assoc. Asphalt Paving Technol. 1957, 26, 486. (17) Vanderhart, D. L.; Manders, W. F.; Campell, G. C. Investigation of Structural Inhomogeneity and Physical Aging in Asphalts by Solid NMR, American Chemical Society, Division Fuel Chemistry, Washington, DC, August 26-31, 1990. (18) Corbett, L. W.; Mertz, R. F. Trans. Res. Rec. 1975, 544, 27. (19) Smith,. R. J. Laboratory Measurement of the Durability of Paving Asphalts, ASTM STP 532, American Society for Testing and Materials, 1973; pp 79-99. (20) Petersen, J. C. Proc., Assoc. Asphalt Paving Technol. 1989, 58, 220.

10.1021/ef980238x CCC: $18.00 © 1999 American Chemical Society Published on Web 03/02/1999

740 Energy & Fuels, Vol. 13, No. 3, 1999

The oxidative aging represents a much more important phenomenon,9,21-22 mostly related to the reaction of the asphaltenes with the atmospheric oxygen. Such reaction produces ketones and sulfoxides as major functional groups, with small amounts of anhydrides and carboxylic acids.23 The intermolecular associative interactions among polar oxygen-containing functional groups, which increase during oxidative aging, cause an increase of the viscosity and a shift to a gel structure for the asphaltenic product considered. Finally, age hardening could originate from a structuring reversible phenomenon concerning molecules and clusters:24-27 in this case, the changes in the flow properties are associated with a new reorganization of the molecules and not with a change in the chemical composition of the asphaltene molecules. This type of aging process, which is very slow at room temperature and not yet well understood, affects all the industrial applications, where long-term resistance to physical changes is an important parameter. Today aging is one of the main parameters connected with the stability of petroleum heavy ends and the evaluation of the bituminous binders. For heavy ends, the standard methods (ASTM D-2274 and D-1661) reported for evaluation of the oxidative and thermal stability are based on the determination of the total amount of insoluble matter formed. Several standard methods have been proposed for the evaluation of the time-related stability of bituminous binders, such as the TFOT (thin film oven test, ASTM D-1754) and the RTFOT (rolling thin film oven test, ASTM D-2872). In both ASTM tests, the aging hardening is evaluated by measuring the change of the mass and viscosity, although the penetration and softening point are also considered. Even if the colloidal structure of crude oils, vacuum residues, and asphaltene solutions was widely studied,28-29 data are not available on the evolution and behavior of their colloidal structures during the aging process. A better understanding of such evolution is, therefore, needed in order to improve the stability of the above-mentioned products to aging. The purpose of this work is the characterization of the colloidal structure of a vacuum residue, before and after a standard aging test, by considering its rheological behavior and investigating the macrostructure of the whole system and of asphaltene and maltene fractions. In addition, it will be shown how the coupling of techniques such as dynamic rheology and small-angle (21) Petersen, J. C. Trans. Res. Rec. 1986, 1096, 1. (22) Edler, A. C.; Hattingh, M. M.; Servas, V. P.; Marais, C. P. Proc., Assoc. Asphalt Paving Technol. 1985, 54, 118. (23) Lin, M. S.; Lunsford, K. M.; Glover, C. J.; Davison, R. R.; Bullin, J. A. In Asphaltenes Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995; pp 155-176. (24) Hoiberg, A. J. Ind. Eng. Chem. 1951, 43, 1419. (25) Traxler, R. N.; Schweyer, H. W. Proc., Am. Soc. Testing Mater. 1936, 36 (II), 544. (26) Traxler, R. N.; Coombs., C. E. Proc., Am. Soc. Testing Mater. 1937, 37 (II), 549. (27) Ensley, E. K. J. Colloid Interface Sci. 1975, 53, 452. (28) Storm, D. A.; Sheu, E. Y. In Asphaltenes and Asphalts, 1; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science: Amsterdam, The Netherlands, 1994; pp 125-157. (29) Speight, J. G.; Wervick, D. L.; Gould, K. A.; Overfield, R. E.; Rao, B. M. L.; Savage, D. W. Rev. Inst. Fr. Pe´ t. 1985, 40 (1), 52.

Scarsella et al.

scattering can be helpful for the study of aging of petroleum heavy ends and how their results can be correlated. Experimental Section Samples. The vacuum residue (VR) used in this work was obtained by vacuum distillation of Safaniya crude oil originated from Middle East. Original Safaniya vacuum residue was aged using a RTFOT (Rolling Thin Film Oven Test) according to ASTM D-2872. This test was preferred to those used specifically for heavy ends stability, since they are related to the presence of flocculated insoluble matter that represents an extreme evolution of the colloidal state. The SARA analysis for the original and aged Safaniya vacuum residues was performed according to ASTM D-412486. Asphaltenes and maltenes were obtained according to the IP 143/81 method. Asphaltene solutions were prepared by dissolving them in the selected solvent and allowing the solutions to stand overnight. The concentration of asphaltene solutions in toluene was about 2.5 wt %. Original and aged vacuum residues were characterized by the standard ASTM procedures. The penetration value and softening point were determined according to ASTM D5 and D36, respectively. Aging was expressed as

aging )

[

]

η0(after RTFOT) - η0(original) η0(original)

100

where η0 is the zero shear viscosity at 60 °C, measured as the complex dynamic shear viscosity η* for very low frequencies. Considering the colloidal nature of the analyzed products, all the reported experimental data extracted both from rheological and small-angle scattering analyses (such as complex viscosity, average molecular weight, molecular weight distribution, radius of gyration, and intensity scattered at zero angle) will be referred to the colloidal aggregates rather than to single molecules. Techniques. Dynamic Rheological Tests. Viscoelastic materials, such as residues and bitumens, exhibit both viscous and elastic behavior and display a time-dependent relationship between the applied stress and the resultant strain. Within the linear viscoelastic region, the interrelation of stress and strain is influenced only by time and not by the magnitude of the stress.30-31 In dynamic mechanical analysis, the peak stress, the peak strain, and the phase relationship between the stress and strain are measured. All the rheological parameters are determined from such data:

G* ) peak stress/peak strain The ratio of the peak stress to the peak strain is the absolute value of the modulus, referred to as the complex shear modulus G*. The elastic in-phase component of G* is called the shear storage modulus G′

G′ ) G* cos δ where δ is the phase angle between the applied maximum strain and the maximum stress. The viscous out-of-phase component of G* is called the shear loss modulus G′′: (30) Ferry, J. D. Viscoelastic properties of polymers; John Wiley: New York, 1980. (31) Macosko, C. W. Rheology principles, measurements, and applications; VCS Pubblishers, Inc.: New York, 1994.

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Energy & Fuels, Vol. 13, No. 3, 1999 741

G′′ ) G* sin δ

I(Q) ) nV2(FA - FS)2 F(Q)S(Q) ) φV(FA - FS)2F(Q)S(Q)

The complex dynamic shear viscosity η* is computed from the complex shear modulus and the strain frequency ω (radians/ s):

where n is the number per unit volume of particles with volume V (V is proportional to the molecular weight M of the particles), φ ) nV is the particle volume fraction, FA and FS are the scattering length densities of aggregates and of solvent or surrounding medium, their difference being the contrast term. F(Q) is the particle form factor, and S(Q) is the structure factor that accounts for interparticle correlations. In the case of asphaltene solutions or asphaltenic products, the contrast term between the scattering length densities of asphaltenes FA and of solvent medium FS must be high enough that detectable intensities will be measured. The relationship used to calculate the scattering length densities for X-ray scattering are

η* ) G*/ω The loss tangent (tan δ) is the ratio of the energy lost to the energy stored in a cyclic deformation:

tan δ ) G′′/G′ If dynamic testing is done using small strain (within the linear viscoelastic region), the data obtained at higher and lower temperatures can be simply equated with the lower and higher frequency values, respectively, according to the principle of time-temperature superposition. This principle represents a powerful tool for evaluating dynamic loading data. Data sets obtained over a wide range of temperatures and a narrow range of frequencies can be shifted along the frequency axis to form a smooth curve, named the “master curve”. When a viscoelastic material is analyzed, one of the main features of the master curve is the position of the crossover point defined as the frequency ωc at which G′ and G′′ assume the same value Gc. Further information on the rheological properties of a sample material can be obtained from stress relaxation measurements. Recording the profile of the stress relaxation modulus G(λ) against relaxation time (λ), a relaxation spectrum, i.e., a weighting factors H(λ) versus λ plot, can be calculated. The continuous relaxation spectrum is defined as

G(t) ) Ge +

∫

+∞

-∞

H(ln λ)e-t/λ d ln λ

Ge being the equilibrium shear modulus. The magnitude of the weighting factor H(λ) at a given λ can be thought of as describing the effect that the corresponding weight average molecular weight Mw has on the mechanical behavior of the material. In this work, dynamic rheological tests were performed with a controlled shear rheometer (Rheometrics RDA II) using a parallel plate geometry, in temperature sweep at constant frequency (50 rad/s) and in frequency sweep at constant temperature and strain; in both cases, tests were performed within the linear viscoelastic range of the materials. Small-Angle Scattering Measurements. Measurement of the scattering of radiation at small angles is a powerful tool for probing the colloidal structure from nanometric to submicronic scales.32-34 In a small-angle neutron scattering (SANS) or a small-angle X-ray scattering (SAXS) experiment, the intensity I(Q) is measured over a range of scattering vectors Q. The scattering vector Q module is defined as

Q)

4π sin θ λ

where λ is the wavelength of the incident radiation and 2θ is the angle of observation. The scattering intensity I(Q) is usually interpreted within the two-component (i.e., the colloidal aggregates and the solvent or surrounding medium) approximation: (32) Guinier, A.; Fournet, G. Small Angle Scattering of X-rays; Wiley: New York, 1955. (33) Espinat, D. Rev. Inst. Fr. Pe´ t. 1990, 45 (6), 775. (34) Neutron, X.-Ray and Light Scattering: Introduction to an Investigative Tool for Colloidal and Polymeric Systems; Linden, P., Zemb, T., Eds.; Elsevier Science: Amsterdam, The Netherlands, 1991.

F)

∑nN

Le-Navd

i i

∑MN i

i

i

i

and for neutron scattering

F)

∑bN

Navd

i i

∑MN i

i

i

i

where Nav is Avogadro’s number, d is the mass density, and Le- is the scattered amplitude of a single electron, the sums run over every element of the chemical composition labeled by index i; ni, Ni, bi, and Mi are number of electrons, number fraction, scattering length, and molar mass of the ith element of asphaltenes or solvent, respectively. For neutron scattering of asphaltene solutions, the scattering length density depending on the nature of the nuclei, the perdeuterated solvent provides a strong contrast with asphaltenes owing to the very different values of bi for deuterium and hydrogen. For neutron scattering of real products, both the mass densities and the chemical compositions of asphaltenes and the surrounding medium do not differ enough to give an important contrast term. In practice, the use of SANS is limited to asphaltenes and resins in deuterated solvents. With X-rays, the scattering length density is proportional to the electronic density. Asphaltenes, being the heaviest, the most polar and aromatic among the constituents of crude oils and residues, exhibit a “natural” contrast with the other organic compounds of the product, thus allowing SAXS measurements to be carried out both on asphaltene solutions and on real oils and residues. The structure factor S(Q) is considered constant and equal to 1, in the whole Q range, for dilute asphaltene solutions, whereas it varies according to Q when the concentration increases. Therefore, in the dilute regime, useful information on the molecular weight, radius of gyration, and internal structure of the asphaltene aggregates may be obtained from SANS spectra. In this case, at low scattering angles (Guinier region), the scattering intensity is exponentially related to the radius of gyration Rgz2, as follows:

I(Q) ) φ(FA - FS)2Vw(1 - Q2Rgz2/3) ) I(Q)Qf0(1 - Q2Rgz2/3)

(QRgz e 1)

where the intensity scattered at zero angle I(Q)Qf0 and the radius of gyration are determined by the intercept and the slope of a semilogarithmic plot of I(Q)/φ versus Q2. The weight average molecular weight Mw can be deduced from I(Q)Qf0:

Mw ) VwdNAv )

dNAv

I(Q)Qf0 φ (FA - FS) 2

742 Energy & Fuels, Vol. 13, No. 3, 1999

Scarsella et al.

Table 1. Elemental Analysis of Original and Aged Safaniya Vacuum Residue Asphaltenes elemental composition (wt %)

Table 3. Main Properties of Original and Aged Safaniya Vacuum Residues

atomic ratios

sample

C

H

N

O

S

H/C

O/C

original asphaltenes aged asphaltenes

82.6 83.4

7.6 7.2

1.0 1.0

1.3 1.4

6.8 7.0

1.104 1.022

0.012 0.013

sample original vacuum residue aged vacuum residue

penetration softening viscosity aging (1/10 mm) point (°C) η0a (Pa s) (%) 68 37

50 57

312 806

158

Zero shear viscosities η0 were measured at the reference temperature T ) 60 °C. a

Table 2. SARA Analysis of Original and Aged Safaniya Vacuum Residues (wt %) sample original vacuum residue aged vacuum residue

asphaltenes resins aromatics + ratio ratio (A) (B) saturates (C) B/A C/A 15.1 16.8

27.6 29.7

57.3 53.5

1.83 3.79 1.76 3.18

At intermediates angles, for scattering vectors Q higher than 1/Rgz, the scattering intensity decays as a power law

I(Q) ≈ Q-D where D is an apparent fractal dimension.35 The exponent D may give information on the internal structure of the aggregates: because asphaltene solutions are polydisperse systems, D may be representative of both the internal structure and size distribution of asphaltene aggregates. In the more concentrated regime, when residues are studied, S(Q) is not constant and, as a consequence, the approximation applied in the case of dilute solutions is not valid anymore. X-ray scattering data on asphaltene solutions and residues give evidence of swelling of asphaltenes.36 Following this phenomenon, both the measured radius and intensity are smaller; coupled with this lower intensity, a less negative slope D is observed.36 The same trend is observed in polymers when the critical concentration c*, where the polymers begin to overlap, is approached.37 In this case, the values extrapolated from the spectra, D, I(Q)Qf0 and Rgz2, are only apparent values, strictly dependent on the concentration. Anyway, a comparison between them is still possible when the examined samples are in the same asphaltene concentration range. In this work, SANS measurements were performed at the Leon Brillouin Laboratory (L.L.B.-CEN CNRS, Saclay, France) on the PACE instrument. The Q-domain accessible with this neutron spectrometer was ranged from 3 × 10-3 to 0.2 Å-1. For SAXS measurements, a Huxley-Holmes-type camera was used. The total accessible Q-range with this instrument was from 10-2 to 0.2 Å-1.

Results and Discussion The elemental composition of asphaltenes from Safaniya vacuum residue before and after RTFO test is given in Table 1. Variations in the elemental composition indicate the occurrence of an oxidation process, due to the addition of oxygen to asphaltene molecules and to a dehydrogenation reaction.9 The degree of this oxidation aging process is small, as indicated by the differences in the atomic O/C and H/C ratios. The loss of volatile fractions does not contribute significantly to aging, since the differences of weight between the original and aged samples were found to be negligible. The results of SARA analysis are reported in Table 2; they show differences of composition between the original and aged samples. Similar variations of com(35) Teixeira, J. J. Appl. Crystallogr. 1988, 21, 781. (36) Sirota, E. B. Pet. Sci. Technol. 1988, 16 (3/4), 415. (37) DeGennes, P. G. Scaling Concepts in Polymer Physics; Cornell: Ithaca, 1979.

Table 4. Rheological Properties of Original and Aged Safaniya Vacuum Residues and of Their Maltenes sample original vacuum residue aged vacuum residue maltenes from original vacuum residue maltenes from aged vacuum residue

ωc (rad/s) 212 22 2 × 107

Gc (Pa)

PI (Pa-1)

η0 (Pa s)

8× 0.01 1.7 × 106 4 × 106 0.02 7.1 × 106 4.7 × 108 2 × 10-4 570 106

1.4 × 106 1.1 × 108 9 × 10-4 1310

position were found in blown (industrially oxidized) bitumens and in residues treated with phosphorus compounds, where the phenomenon is much more enhanced.38 As expected, a conversion of aromatics and saturates to resins and of resins to asphaltenes is also evident. Such conversion gives rise to a light increase of peptized material (asphaltenes) and a decrease of the ratio of resins (peptizing material) against asphaltenes and of the ratio of aromatics and saturates (solvent) against asphaltenes. Some notable properties of the original and aged vacuum residues are listed in Table 3. A higher softening point and a lower penetration value are found as far as the RTFOT sample is concerned, showing that an aging process occurred. The relatively small value of the aging index (158%), calculated by viscosity data at 60 °C, confirms the good aging resistance properties of the Safaniya vacuum residue. This behavior is confirmed by the small variations in the SARA composition of the vacuum residue and in the asphaltene elemental analysis. The small changes in composition and properties occurring during age hardening have a large influence on the rheological behavior of the original and aged samples, reflecting the importance of even moderate variations in all structuring factors (concentration, composition, and polarity of asphaltenes, resins, and aromatics and saturates) to the flow properties. In Table 4 the main rheological properties of the original and aged Safaniya vacuum residues and of their maltenic fractions are reported. Measurements of the loss tangent (tan δ) as a function of temperature (T) from temperature scans from about 0 to 100 °C at a fixed angular frequency (50 rad/s) were made. Figure 1a gives semilogarithmic plots of tan δ versus T for both the original and the aged Safaniya vacuum residue samples. As expected, an increase in tan δ is observed in the whole temperature range, suggesting a variation from a more gellike to a more sollike behavior occurs when temperature increases. In addition, it can be seen that at the fixed frequency of 50 rad/s for temperatures lower than 40 °C, both the original and the aged samples show the same G′′/G′ ratio, suggesting a similar gellike behavior, while for (38) De Filippis, P.; Giavarini, C.; Scarsella, M. Fuel 1995, 74, 836.

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Energy & Fuels, Vol. 13, No. 3, 1999 743

Figure 2. Master curve obtained for the original Safaniya vacuum residue: semilogarithmic plot of complex viscosity η* (]), loss modulus G′ (O), and storage modulus G′′ (3) versus angular frequency.

Figure 1. (a) Semilogarithmic plots of the loss tangent versus temperature for the original and aged Safaniya vacuum residues. (b) log-log plots of the loss tangent versus angular frequency for the original (×) and aged (2) Safaniya vacuum residues.

higher temperatures, the aged sample shows a more gellike behavior. The higher asphaltenes/resins ratio for the aged sample confirms its more gellike nature. For a deeper analysis of the rheological response, especially for temperatures lower than 40 °C, the master curve can be analyzed. The temperature ranged from -20 to 60 °C with a temperature of 20 °C selected as the reference temperature. The frequency dependence of the loss modulus tan δ for both the original and aged Safaniya vacuum residues is reported in Figure 1b. It provides more reliable information about the viscous/elastic balance, such measurements being far more sensitive than isochronal tan δ vs temperature. In addition, since it was obtained by applying the time-temperature equivalence principle, this graphical representation allows an overall view on the viscoelastic behavior of both residues examined in a wide range of frequencies (from 10-6 to 108 rad/s). Despite the fact that Figure 1 (parts a and b) provides almost the same information on the rheological behavior of such residues, the latter allows one to calculate the crossover frequencies values for both the samples. In Figure 2 the master curve obtained for the Safaniya vacuum residue is reported: the trend of G′, G′′, and η* versus the angular frequency ω is typical for the asphaltic materials.39-40 log-log plots of η* versus ω for (39) Jongepier, R.; Kuiliman, B. Rheol. Acta 1970, 9, 102.

Figure 3. log-log plots of the complex viscosity versus angular frequency for the original (×) and aged (2) Safaniya vacuum residues.

the original and aged Safaniya vacuum residues are compared in Figure 3. Several quantitative parameters may be extracted from Figures 1b and 3: some of them are reported in Table 4 and discussed successively in the text. The comparison between the complex viscosity vs frequency plots (Figure 3) shows that in the highfrequency region (power-law field) the curves do coincide. On the contrary, in the low-frequency region, the complex viscosity values are always higher for the aged sample. The gap is at its maximum when ω f 0, i.e., when the viscosity is frequency independent (zero shear viscosity). These results confirm that oxidative aging causes no variations of the viscoelastic properties of the Safaniya vacuum residue in the low-temperature range; as temperature increases, the aged sample shows higher stiffness. The behavior of the complex viscosity vs ω curves (see Figure 3) are similar to those found when polymers with different average molecular weights (Mw), but with same molecular weights distribution (MWD), were studied.41-43 In the latter case, it was found that whereas the viscosity at higher frequencies hardly changed when (40) Jongepier, R.; Kuiliman, B. Proc., Assoc. Asphalt Paving Technol. 1969, 38, 98. (41) Franck, A. J. P. Colloid Polym. Sci. 1980, 258 (1), 88. (42) Franck, A.; Meissner, J. J. Rheol. Acta 1984, 23 (2), 117. (43) Franck, A. J. Rheology 1991, 1 (2), 84.

744 Energy & Fuels, Vol. 13, No. 3, 1999

Figure 4. Relaxation spectra for the original (×) and aged (2) Safaniya vacuum residues.

increasing the molecular weight, the zero shear viscosity η0 increased. With increasing molecular weights, even the onset of frequency-independent viscosity is shifted to lower frequency, as well. The position of the crossover point (ωc, Gc) of G′ and G′′ provides information on the molecular weight and molecular weight distribution of polymers, but it is not sensitive to changes in polymer structure.41-43 The crossover frequency ωc is seen to be lower with increasing Mw and almost independent of the molecular weight distribution; on the contrary, the crossover point modulus (Gc) does not depend on the molecular weight, while it increases with decreasing broadness of the MWD. On the basis of these observations, the results listed in Table 4 can be interpreted as follows. An increase of molecular weight occurs from the original Safaniya vacuum residue to the aged one, as shown by the lowering of ωc, which confirms again the indication of η0 values. Even the MWD of the vacuum residue becomes slightly wider after RTFOT is performed. It is worth noticing that the amount of this variation is small. In fact, the relaxation spectra for the original and aged Safaniya vacuum residue, reported in Figure 4, confirm the small amount of MWD variation, since no difference is observed in the transition from Newtonian flow to power-law behavior.44-46 To better understand the vacuum residue viscoelastic behavior, rheological measurements of the maltene fractions from both the original and the aged Safaniya vacuum residue were carried out. The evaluation of the maltene flow properties can elucidate the degree of association in maltenes and their role in a vacuum residue colloidal nature. The effect on of the temperature maltene viscoelastic properties was determined, and the semilogarithmic tan δ versus T plot is given in Figure 5. The explored temperatures ranged from 30 to 100 °C, at 50 rad/s angular frequency. A linear dependence of tan δ against T was found for both the original and aged maltenes when the temperature was lower than 40 °C. The trend of tan δ vs T suddenly changed at higher temperatures: tan δ ran into a peak, for T ) Tmax, then decreased until a region, where it was almost temperature independent. (44) Bersted, B. H.; Slee, J. D. J. Appl. Polym. Sci. 1977, 21, 2631. (45) Penwell, R. C.; Graessley, W. W. J. Polym. Sci. 1957, 12, 1771. (46) Menefee, E. J. Appl. Polym. Sci. 1972, 16, 2215.

Scarsella et al.

Figure 5. Semilogarithmic plots of the loss tangent versus temperature for the original (×) and aged (2) maltenes from Safaniya vacuum residue.

A similar tan δ versus T trend was observed when mixtures of a block copolymer and an endblock-associating resin were studied.47-48 In this case, the shift in Tmax indicated that a transition from a more to a less associated state occurred. On the basis of the previous observation, the presence of a peak of tan δ could suggest two different association degrees, for both the original and aged maltenes, at temperatures lower and higher than the respective Tmax temperature. In the linearity range of tan δ against T (up to 40 °C for original sample and up to 50 °C for the aged one), the curves show the same slope; this could suggest that the same association mechanism occurs for the two samples. For temperatures higher than 40 °C, corresponding to Tmax for the original maltenes, the aged maltenes have a smaller gellike character than the original ones, as shown by the tan δ value, always greater for the aged sample. In the whole temperature range a tan δ value much greater than 1 is found, both for the original and the aged maltenic fractions, which indicates the main contribution of the loss modulus to the viscoelastic behavior. The viscoelastic behavior can be more clearly analyzed in Figure 6, where it is reported that, in the semilogarithmic G* versus T plot, the complex modulus G* is greater for the original maltenes than for the aged ones, until a temperature of 60 °C is reached, while at higher temperature it becomes smaller. The aging process affects the maltene viscoelastic behavior both in the viscous and elastic contribution, decreasing both of them at low temperatures whereas increasing both of them at high temperatures. In Table 4 the main rheological properties of maltene samples, calculated from their respective master curves, are listed. The aged maltenes have a greater zero shear viscosity than the original ones, which would indicate a larger average molecular weight. This is also confirmed by the shift at lower values of ωc frequency for the maltenes after RTFO test. The smaller Gc value obtained for the aged maltenes supports the conclusion that oxidative aging leads to a broader molecular weight distribution. The characterization by small-angle scat(47) Kim, J.; Han, C. D.; Chu, S. G. J. Polym. Sci. Polym. Phys. Ed. 1988, 26, 667. (48) Han, C. D.; Kim, J.; Baek, D. M.; Chu, S. G. J. Polym. Sci. Polym. Phys. Ed. 1990, 28, 315.

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Energy & Fuels, Vol. 13, No. 3, 1999 745 Table 5. Main Diffusion Values Extrapolated from SAXS and SANS Spectra Guinier region

high Q region

I(Q)Qf0/c (cm-1) RGZ (Å)

sample Safaniya vacuum residue Safaniya aged vacuum residue Safaniya asphaltenes Safaniya aged asphaltenes

0.24 ( 0.04 0.21 ( 0.04 2.4 ( 0.2 1.9 ( 0.2

D

61 ( 5 1.30 ( 0.05 56 ( 3 1.30 ( 0.05 62 ( 5 2.1 ( 0.1 57 ( 3 2.1 ( 0.1

Figure 6. Semilogarithmic plots of the complex modulus versus temperature for the original and aged maltenes from Safaniya vacuum residue.

tering techniques was carried out both on the original and aged Safaniya vacuum residues and on their asphaltenes in toluene. The behavior of asphaltenes, both in their natural medium and in solution is governed by association phenomena:29,49-57 colloidal asphaltenic particles have been observed by small-angle X-ray scattering in crude oil58-61 and in atmospheric or vacuum residues;62-65 in the same way, SANS and SAXS spectra showed that asphaltenes self-associate forming colloidal particles in various organic solvents,59-62,64-71 such as toluene, tetrahydrofurane, chloroform, pyridine, nitrobenzene. Experimental scattering data for asphaltene solutions are very useful to investigate the self-association ability and to compare different asphaltenes under simplified conditions consisting of an identical and good solvent like toluene and in a concentration regime where no (49) Dickie, J. P.; Yen, T. F. Anal. Chem. 1967, 39, 1847. (50) Pfeiffer, J. P.; Saal, R. N. J. J. Phys. Colloid Chem. 1940, 44, 139. (51) Ray, B. R.; Whiterspoon, P. A.; Grim, R. J. J. Phys. Chem. 1957, 61, 1296. (52) Eldib, I. A.; Dunning, H. N.; Bolen, R. G. J. Chem. Eng. Data 1960, 5 (4), 550. (53) Wales, M.; Wan der Waarden, M. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1964, 9 (2), B21-B24. (54) Reerink, H. Ind. Eng. Chem. Prod. Res. Dev. 1973, 12, 82. (55) Moschopedis, S. E.; Fryer, J. F.; Speight, J. G. Fuel 1976, 55, 227. (56) Maruska, H. P.; Rao, B. M. L. Fuel Sci. Technol. Int. 1987, 5 (2), 119. (57) Yen, T. F. In International Symposium Chemistry of Bitumens, Proceedings; Giavarini, C., Speight, J. G., Eds.; Rome, Italy, June 5-8, 1991; The University of Wyoming Research Corporation: Laramie, WY, 1991; Vol. I, pp 382-407. (58) Dwiggings, C. W., Jr. J. Phys. Chem. 1965, 69, 3500. (59) Dwiggings, C. W., Jr. J. Appl. Crystallogr. 1978, 11, 615. (60) Carnahan, N. F.; Quintero, L.; Pfund, D. M.; Fulton, J. L.; Smith, R. D.; Capel, M.; Leontaritis, K. Langmuir 1993, 9, 2035. (61) Barre´, L.; Espinat, D.; Rosemberg, E.; Scarsella, M. Rev. Inst. Fr. Pet. 1997, 52 (2), 161. (62) Pollack, S. S.; Yen, T. F. Anal. Chem. 1970, 42, 623. (63) Klm, H.; Long, R. B. Ind. Eng. Chem. Fundam. 1977, 18, 60. (64) Herzog, P.; Tchoubar, D.; Espinat, D. Fuel 1988, 67, 245. (65) Storm, D. A.; Sheu, E. Y.; DeTar, M. M. Fuel 1993, 72, 977. (66) Ravey, J. C.; Ducouret, G.; Espinat, D. Fuel 1988, 67, 1560. (67) Overfield, R. E.; Sheu, E. Y.; Simha, S. K.; Liangs, K. S. Fuel Sci. Technol. Int. 1989, 7, 611. (68) Sheu, E. Y.; Storm, D. A.; DeTar, M. M. J. Non-Cryst. Solids 1991, 131-133, 341. (69) Sheu, E. Y.; DeTar, M. M.; Storm, D. A. Macromol. Rep. 1991, A28, 159. (70) Xu, Y.; Koga, Y; Straustz, O. P. Fuel 1995, 74, 960. (71) Thiyagarajan, P.; Hunt, J. E.; Winans, R. E.; Anderson, K. B.; Miller, J. T. Energy Fuels 1995, 9, 829.

Figure 7. SANS spectra of the original (2) and aged (b) asphaltenes Safaniya in perdeuterated toluene solution.

interactions between aggregates are present. The small angle scattering spectra of asphaltenes display generic features. The scattering intensity is characterized by a quasiplateau IQf0 at low scattering angles, followed by a strong decrease at higher scattering angles, with a scaling dependence of the type

I(Q) ≈ Q-D The main parameters extracted from the neutron and X-ray spectra are presented in Table 5. The SANS spectra of original and aged asphaltenes Safaniya in perdeuterated toluene solution are reported in Figure 7. The I(Q) values experimentally found are divided by asphaltene concentration c. A similar degree of selfassociation is evident, according to the I(Q)Qf0/c (which are proportional to the asphaltene average volume or molecular weight) and the Rgz values, almost identical for original and aged asphaltenes. Small-angle X-ray scattering was also applied to the real products. Figure 8 shows SAXS spectra of the original and aged Safaniya vacuum residues. The apparent Rgz and I(Q)Qf0/c values of the scattering entities for the original and aged vacuum residues are almost equal. Despite the fact that they are not dilute systems, since these values were obtained for systems in the same concentration range, it is reasonable to make a comparison between them. The scattering profile of the original and aged vacuum residues in the low Q range, representative of the larger particle sizes, is mainly originated by asphaltenes as supported by the following observations: (a) the radius of gyration of asphaltenes in toluene by SANS is almost as great as the one found for the real products; it may be due to the same scattering objects, the asphaltene aggregates, both for the vacuum residue and for dilute asphaltene solution. (b) The I(Q)Qf0/c values are 10

746 Energy & Fuels, Vol. 13, No. 3, 1999

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sured by small angle scattering techniques), the comparison must be made between scattering data measured at 20 °C and the master curves at the same temperature. From the crossover point modulus (Gc) it is possible to calculate the polydispersion index PI, defined as:

PI )

Figure 8. SAXS spectra of the original (2) and aged (b) Safaniya vacuum residues

times more important for neutron scattering of toluene solutions than for X-ray scattering of vacuum residue. This is in good agreement with the related average values of contrast terms, in a 1:10 ratio (1.8 × 10-3 Å-4 for SANS of asphaltenes in toluene solutions and 1.3 × 10-4 Å-4 for SAXS of asphaltenes in maltenes). As a consequence, once the average volumes and molecular weights were calculated, almost the same value will be obtained for asphaltenes both in toluene solution and in real products. Therefore, the mass of asphaltene aggregates does not change with aging, both for toluene solutions and for the real product. On the contrary, there are some differences in the high Q region, representative of the internal structure of the aggregates. The differences in the D values between real products and related asphaltenes in toluene solution are probably due both to the asphaltene concentration and to the differences in the peptizing power of the medium. While in toluene solutions the asphaltene concentration is sufficiently low (about 2.5 wt %) to ensure no interactions occur among aggregates, in real products the high asphaltene concentration could cause extensive interactions between aggregates, lowering the D value.34 In addition, in real products asphaltene self-association is less important: asphaltenes seem to constitute the polar core of a micellar entity stabilized by resins (the peptizing agent) and then by the other constituents of the medium; according to Pfeiffer and Saal,50 a nearly continuous transition exists from the most polar entities (asphaltenes) at the core of micellar structure to the less polar ones (saturates) forming the surrounding medium. When the comparison is made between aged and original asphaltenes in the same medium (i.e., maltenes or toluene), no significant differences appear. This suggests that the RTFO test does not lead to meaningful modifications in the asphaltene structure both in model solution and in real product. Therefore, no changes in asphaltene size or internal structure is detected by scattering measurements, despite elemental analysis showing some differences. Although there is no evidence that rheological and small-angle scattering techniques are sensitive to the same range of colloidal particle sizes, it is interesting comparing the respective results. Since the rheological behavior of viscoelastic materials is strongly temperature dependent (like colloidal particles’ properties mea-

105 Gc

It is a measure of the ratio between the weight average molecular weight and the number average molecular weight. The PI values show a negligible difference from the original and the aged Safaniya vacuum residues; on the contrary, the PI of maltenes from aged Safaniya vacuum residue is greater in comparison with the PI value found for maltenes from the original Safaniya vacuum residue. The variation of the maltene structure and of the related flow properties is evident from rheological measurements, while the same structural effects cannot be seen by X-ray measurements. Rheological measurements and X-ray scattering data give complementary results, the former being sensitive to a global effect of all the components of the sample, the latter only reflecting the behavior of the asphaltenic part according to its contrast term with maltenes. The increase of molecular weight and polydispersity due to aging of the Safaniya vacuum residue reflects the micelle growth as a consequence of the SARA composition variation: both asphaltenes-to-resins ratio and resins:(aromatics plus saturates) ratio become greater. A conversion of the less polar entities to the most polar ones occurs, i.e., aromatics and saturates are converted to resins and resins to asphaltenes. This transition leads to a decrease in the resins content, whose shortage gives rise to attraction forces between the polar cores of the micelles: more extended association phenomena are observed but are not sufficient to create a gel structure. Since the resins are mainly affected by this trend, the aging has the most important effect on the viscoelastic properties of maltenes. It is important to underline that the effect of aging on the rheological behavior of Safaniya vacuum residue must be related to the structural change of the nonasphaltenic part of the real product. The change of the maltene structure seems to be able to strongly affect the rheological properties of the whole sample. In the low-temperature range, aggregation of asphaltenes is so important to conceal the rheological differences noticed for maltenes before and after aging: the similar concentration and aggregation power found for original and aged asphaltenes give rise to the same gellike behavior for the residue before and after aging. In the high-temperature range, the viscoelastic behavior of the residue mainly depends on its maltenic fraction: the higher η* value found for the aged sample is due to an increase both in the elastic and viscous contribution of asphaltene and maltene fractions, respectively. The effects of aging on maltene structure are not detectable by SAXS measurements on the real product because of the small contrast term of maltenes, screened by the much more important signal given by the asphaltenes.

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Conclusions The effect of the changes in the composition on the Safaniya vacuum residue macrostructure is summarized here; the age hardening does not seem to be only connected with the composition evolution of the asphaltenes. Both the rheological and scattering measurements performed have experimentally supported the above hypothesis. On one side, rheological measurements have proved the important role of the maltenes in the changes occurring in the vacuum residue colloidal structure due to aging. The effects of age hardening are more detectable by rheology in maltene fractions than in real products. The presence of asphaltenes in the real products seems to be responsible for a reorganization in the colloidal structure, making such rheological differences minor, especially at low temperature. On the other hand, the only self-association of asphaltenes does not appear to be representative of the time evolution of their inner structure, as shown by SANS experiments on asphaltene solutions, although the contribution they have on the technological properties and the macrostructure of the real products is well-known.

Energy & Fuels, Vol. 13, No. 3, 1999 747

In short, it is the effect of aged maltene on the viscosity that is predominant, as well as the crossed interactions between asphaltenes and resins, because of the high asphaltene concentration involved; the aged asphaltene effect on viscosity does not seem so important. The evolution of the rheological properties of Safaniya vacuum residue following aging is mainly ruled by maltene behavior; no main changes in the asphaltene macrostructure are involved in such a phenomenon. Acknowledgment. The Authors thank Prof. C. Giavarini for the helpful discussions. They also thank Dr. J. P. Cotton (L.L.B.-CEN CNRS, Saclay, France) and Dr. J. Lambard (CEN, Saclay, France) for the help they provided during SANS experiments and the related discussions. The authors are grateful to Dr. F. Farcas (Laboratoire Central des Ponts et Chausse´es, Paris) for the aging of the samples. EF980238X