ARTICLE pubs.acs.org/EF
Organization of Asphaltenes in a Vacuum Residue: A Small-Angle X-ray Scattering (SAXS)Viscosity Approach at High Temperatures Jo€elle Eyssautier,†,‡ Isabelle Henaut,† Pierre Levitz,‡ Didier Espinat,† and Loïc Barre*,† † ‡
IFP Energies nouvelles, 1-4 avenue de Bois-Preau, 92852 Rueil-Malmaison Cedex, France cole Polytechnique, UMR 7643 CNRS, Physique de la Matiere Condensee, Centre National de la Recherche Scientifique (CNRS)E 91128 Palaiseau Cedex, France
bS Supporting Information ABSTRACT: Temperature-dependent rheological behavior of heavy oils and bitumen is usually modeled with a colloidal approach, taking into account a temperature-dependent solvation effect (Storm, D. A.; Barresi, R. J.; Sheu, E. Y. Rheological study of Ratawi vacuum residue in the 298673 K temperature range. Energy Fuels 1995, 9, 168176). In addition to viscosity measurements for vacuum residue at various asphaltene contents, in the present study, we make use of small-angle X-ray scattering (SAXS) data on the 80240 °C temperature range to propose an interpretation on asphaltene aggregation, consistent with both approaches. The radius of gyration Rg and molecular weight MW of asphaltenes in a vacuum residue are measured and are of the same magnitude as asphaltenes in toluene. Dimensions and masses decrease with the temperature, while the small length scale remains unchanged, reinforcing the hierarchical aggregation scheme previously described in toluene. These findings enrich the viscosity data interpretation. A solvation factor has to be accounted for, as noticed in previous works. Its signification is made clear by SAXS data: lose asphaltene clusters in maltenes dissociate with temperature, decreasing their solvation.
1. INTRODUCTION Vacuum residua are obtained from heavy crude oils after all volatile hydrocarbons, easier to recover, have been removed by atmospheric and vacuum distillations. This fraction is, by definition, made of the heaviest petroleum molecules. Process intensification tries to extract the most out of these residua to produce valuable cuts, transformed into a light energy resource by refinery operations. Their main operating conditions are high temperatures (up to 440 °C), hydrogen pressure, and the use of specific nanoporous catalysts. The process design for vacuum residue refinery operations requires a refined knowledge on the properties of the residue, particularly in processing conditions.1,2 Properties of interest include viscosity behavior, structural organization, and temperature effect on dynamics of molecules. These properties have been shown to be greatly dependent upon the asphaltene content.3 This group of petroleum molecules is defined as the part of oil that is insoluble in n-alkane (usually npentane or n-heptane). In consequence to their considerable effect on residue hydrodynamic and structural properties, asphaltenes have been extensively studied since then. Characterization of this ill-defined petroleum fraction was performed in model solvent, usually toluene. During the past decade, noticeable advances were made on molecular-weight measurements, colloidal and structural characteristics,47 and dynamics.4,8 High-resolution mass spectrometry9 shows that asphaltenes have an average molecular weight of 750 ( 250 g/mol, and fluorescence spectroscopy describes the molecule as made of an aromatic core surrounded by alkyl chains.10 Nanoaggregates, formed from the self-association of molecular aromatic sheets, are commonly considered, especially from hydrodynamic properties.1115 Recently, the description of the nanoaggregate in solution was refined, and size and shape were specified.7 From r 2011 American Chemical Society
small-angle neutron and X-ray scattering (SANS and SAXS) measurements, nanoaggregates are 32 Å discs of low height, made of a dense core and a highly aliphatic shell. This structural organization is analogous to aromatic stacking of about three sheets and is confirmed by molecular simulations.16 On a larger length scale, small-angle scattering sees a further aggregation, with mass fractal organization.5,7 Fractal clusters are made of about 12 nanoaggregates, but the interaction forces still remain to be understood. The temperature is shown to decrease the size of the clusters1720 in the 20400 °C temperature range. Viscous properties of asphaltenes in solvents were studied on a colloidal approach as solute particles, spherical or anisotropic and possibly solvated. The viscosities of asphaltene solutions present a nonlinear exponential-like increase as a function of the asphaltene concentration.5,2124 A combination of viscosity modeling and SAXS measurements on various asphaltene fractions enabled the determination of the fractal dimension of the aggregates.5 For a structural study of the vacuum residue, the literature is not as dense as for asphaltenes in toluene. The molecular complexity of maltenes compared to a model solvent is limiting for most techniques. A few scattering studies have shown that the intense intensity scattered by a vacuum residue actually comes from the asphaltene fraction.16,25,26 On the other hand, viscosity of heavy oils and bitumen is largely explored and modeled in the literature.27 Since the pioneer Special Issue: 12th International Conference on Petroleum Phase Behavior and Fouling Received: September 16, 2011 Revised: November 14, 2011 Published: November 16, 2011 2696
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Table 1. Chemical and Physical Properties of Safaniya Vacuum Residue and Its Fractions vacuum residue
AC5
MC5
C (wt %)
83.9
81.8
83.7
H (wt %) O (wt %)
10.5 0.43
7.8 1.2
11.0 0.46
N (wt %)
0.39
0.67
0.23
S (wt %)
5.29
8.01
4.65
Ni (ppm)
175
V (ppm)
580
H/C
1.48
1.13
1.56
density
1.027
1.173
0.985
3
work of Mack, asphaltene content implications on vacuum residue viscosity and temperature effect were assessed.27,28 Reerink29 suggested that asphaltenes were solvated associated aggregates made of primary particles and that high temperatures break the aggregates to reach the primary particles at temperatures higher than 120 °C. Later on, Storm et al.30 showed that one could interpret this solvation by a solvation layer made of resins and surrounding the spherical asphaltene particles. The solvation decrease is said to come from the thickness decrease of the layer during temperature elevation. Luo et al.31 used the same approach on heavy oils and added a shape factor to avoid the sphere hypothesis. More recent work was performed on reconstituted heavy oil samples compared to asphaltenes in solvent and on nanofiltered samples,32,33 highlighting a multiphase behavior on a large temperature range and artifacts introduced during measurements.34 In the present study, we propose a combined approach made of structural measurements from SAXS and viscosity measurements in the 80240 °C temperature range. Common samples for these two techniques are made of the recombined vacuum residue at various asphaltene contents. Coupling these two complementary techniques, well-adapted for dark and viscous systems, increases the power of data interpretation.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation and Characterization. Safaniya vacuum residue was used for sample preparation. It is concentrated in asphaltenes, 23 and 13.4 wt % for n-pentane and n-heptane insolubles, respectively. Density measurements of the vacuum residue and its fractions (maltenes and asphaltenes) were performed with an Anton Paar DMA5000 densitometer at 20 °C. The specific volume of solutions versus the asphaltene mass fraction shows a linear trend in the range of 0.0125%, which allows us to extrapolate to null dilution. In the following study, the asphaltene density will be considered constant in the 20240 °C temperature range for simplification. Previous results showed that, between 8 and 73 °C, asphaltene density decreases from 1.21 to 1.19 g/cm3.35 Chemical and physical properties of the vacuum residue and its fractions can be found in Table 1. Seven samples of recombined vacuum residue at varying amount of n-pentane asphaltenes (AC5), from 1.8 to 15.6 vol % (i.e., 2.117.9 wt %), were made by diluting the initial vacuum residue by its n-pentane maltenes (MC5) and mixing at 473 K under a nitrogen blanket to avoid oxidation. MC5 were previously obtained by precipitating the AC5 from the vacuum residue. n-Pentane was used as a precipitating solvent for preparing maltenes because it was determined that aggregates in vacuum residue are best described by n-pentane precipitation.36
Figure 1. Typical rheogram obtained. Viscosity versus shear rate for the vacuum residue in the 80240 °C temperature range.
2.2. Viscosity Measurements and Modeling. 2.2.1. Viscosity Measurements. Viscosity measurements were carried out on a controlled stress rheometer AR2000 (TA Instrument) in the steady-state flow mode, equipped with an environment testing chamber (ETC) for temperature control up to 600 °C. The heated chamber was filled with nitrogen to avoid sample oxidation. Parallel plate geometry was used (25 and 40 mm diameter for the upper and lower plates, respectively) with a 1 mm gap. Viscosity (via shear stress) was measured at shear rates going from 5 to 316 s1, corresponding to the Newtonian domain (see Figure 1). The temperature was raised at 10 °C/min and stabilized for 10 min prior to measurement in the 80240 °C temperature range. The lowest temperature was set to 80 °C, so that Newtonian behavior was still found. In addition, this temperature remains higher than the melting point of paraffins, measured by polarized light optical microscopy. The highest temperature (240 °C) was defined by the limit of sensitivity of the equipment toward low shear stress. 2.2.2. Viscosity Modeling. On the basis of principles of hydrodynamics, the relative viscosity, defined as the ratio of the viscosity η of the colloidal dispersion to the viscosity η0 of the dispersing liquid (i.e., the continuous phase), can be expressed as37,38 ηr ¼ 1 þ ½ηϕ þ k1 ϕ2 þ :::
ð1Þ
where [η] is the intrinsic viscosity, ϕ is the volume fraction of the dispersed phase, and k1 is a constant. The intrinsic viscosity [η] can be probed from the slope of eq 1 at low concentrations. For non-interacting spherical particles in the dilute regime, [η] is predicted to be 2.5 and eq 1 is derived into the well-known Einstein equation. ηr ¼ 1 þ 2:5ϕ
ð2Þ
However, when hydrodynamic interactions occur, eq 2 becomes inapplicable. In the literature, different viscosity models for colloidal dispersion have been proposed to take into account these factors. Roscoe showed that, for a suspension of spheres of different sizes39 ηr ¼ ð1 ϕÞ2:5
ð3Þ
40
Pal and Rhodes proposed that, in most cases, particles are solvated by the solvent and, therefore, have a larger volume than in the dry state. This increase in hydrodynamic volume can be corrected by scaling the dry volume of the particles with a solvation constant K ϕeff ¼ Kϕ
ð4Þ
where ϕeff is the effective volume fraction to take into account. Equation 3 can be rewritten and is known as the PalRhodes semi-empirical model.40 ηr ¼ ð1 KϕÞ2:5 2697
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A plot of ηr1/2.5 versus ϕ is predicted to yield a straight line with the intercept at 1 and the slope being equal to K. One can assess the shape of the particles by replacing 2.5 by a shape factor ν and determining the ν value that provides the best linear fit of the data. Because of the solvation effect, the intrinsic viscosity will be greater than without solvation because particles are larger. If spherical particles are considered, we can calculate the corrected intrinsic viscosity as28 ½η ¼ 2:5K
ð6Þ
According to eq 5, the relative viscosity diverges as the effective volume fraction approaches 1. The Krieger equation is another well-known semi-empirical representation, this time accounting for the jamming effect on relative viscosity divergence through a maximum packing fraction ϕmax.41 !2:5 ϕ ηr ¼ 1 ð7Þ ϕmax Equations 5 and 7 show that both solvation and jamming effects are present and cannot be easily separated from viscosity data alone. 2.3. SAXS. 2.3.1. SAXS Measurements. SAXS measurements were performed on an in-house experimental setup, made of a copper rotating anode (MicroMax-007, Rigaku) operating at 0.8 kW. The beam is reflected on a parabolic multilayer mirror (Fox-2D, Xenocs). The reflected beam (λ = 1.54 Å) is collimated by two pairs of scatterless crossed slits (Xenocs), and the beam scattered by the sample is captured by a two-dimensional (2D) proportional multiwire detector (Rigaku). The range of scattering angles 2θ enables a range of the wave scattering vector q, defined as q = 4π sin θ/λ, from 1 102 to 0.3 Å1 to be covered. The vacuum residue sample is introduced in an in-house brass cell with a 1.86 mm optical path and mica windows. The sample cell is placed on a hot stage (Linkam) controlled to heat the sample up to 335 °C. The temperature was raised at 10 °C/min and stabilized for 10 min prior to measurement in the 80240 °C temperature range. The lowest temperature (80 °C) was set after the melting point temperature of paraffins. The highest temperature (240 °C) was set before the critical point is observed, where a high-intensity increase is observed because of high-density fluctuations. After normalization with respect to thickness, transmission, and measuring time, the raw intensities were converted to the scattering cross-section I(q) in absolute scale (cm1). The MC5, considered as the dispersing phase, was subtracted from the signal, weighed by its volume fraction. 2.3.2. Data Processing. 2.3.2.1. Dilute Systems. From a SAXS experiment, the scattered intensity I(q) probes the correlations between particle-rich regions on a scale of the order of q1. The general equation describing the scattered intensity for a two-phase system of particles at volume fraction ϕ, in a solvent, is IðqÞ ¼ ϕð1 ϕÞΔF2 FðqÞSðqÞ
ð8Þ
where S(q) is the structure factor, which accounts for interactions between particles, and F(q) is the form factor normalized by the scattering volume v [F(0) = v]. At low concentrations, interactions between particles can be neglected [S(q) = 1]. ΔF is the scattering length density difference between the particle and solvent, determined from the density and elemental composition (n) of the solvent and particles
approximation42 can be used to determine the scattering cross-section at q = 0 and the radius of gyration Rg of the particles. ! q2 R g 2 1 1 ¼ 1 þ ð10Þ IðqÞ Ið0Þ 3 The validity of eq 10 can be extended for swollen particles.5 For polymers, Burchard43 considers the Zimm approximation valid up to qRg = 2. From eq 8, I(0) takes a simple form for dilute solutions, from which the particle volume v can be extracted. v¼
Ið0Þ ϕð1 ϕÞΔF2
The “molar mass” MW can be derived using the usual expression, with the density d of the particles and the Avogadro number Na. MW ¼ dNa v
le
∑1 ni Zi V
ð12Þ
Combining eqs 1012 and considering concentration c in grams per unit volume, one obtains ΔF2 c c 1 ¼B ¼ Ið0Þ MW d2 Na Ið0Þ
ð13Þ
It is worth mentioning that eqs 1013 are model-independent. 2.3.2.2. Interactions. For moderate concentrations c of particles, interparticle interactions have to be taken into account, and the previous equations are modified through the virial expansion to give 2 3 ! q2 Rg 2 Bc 1 4 þ 2A2 MW c þ :::5 for A2 MW c < 0:25 1 þ ¼ IðqÞ MW 3
ð14Þ This equation can be simplified in the Guinier region to yield Bc 1 ¼ ð1 þ 2A2 MW cÞ for A2 MW c < 0:25 and qRg < 1 Ið0Þ MW ð15Þ where A2 is the second virial coefficient. A positive value of A2 means a repulsive interaction between particles, leading to a lowering of the I(q)/ϕ(1 ϕ) ratio at small q values as the concentration increases. The reverse of the right end part of eq 15 can be defined as the “apparent mass”, which tends toward the real molar mass at low concentrations. The three parameters, Rg, MW, and A2, can be extracted through to the “Zimm plot”, i.e., the representation of eq 14 for various concentrations c. This application reduces the data dispersion, and instead of apparent mass and radius of gyration (eq 15), it gives proper mass and radius. As illustrated in Figure 2, the plot of Bc/I(q) = f(q2 + αc), where α is an arbitrary constant, gives two extrapolated lines, where y intercepts and slopes enable determination of Rg, MW, and A2. for q ¼ 0
Bc 1 ¼ þ 2A2 c IðqÞ MW
ð16Þ
for c ¼ 0
Rg 2 2 Bc 1 ¼ þ q IðqÞ MW 3MW
ð17Þ
N
F¼
ð11Þ
ð9Þ
where V is the volume considered in the chemical composition, le is the scattering length of one electron, and Zi is the atomic number of atom i. For dilute solutions [S(q) = 1], in the Guinier region (i.e., at scales larger than the characteristic size of particles or qRg < 1), the Zimm
3. RESULTS 3.1. Viscosity Measurements on a Temperature Range. Viscosity measurements were conducted at six different constant temperatures in the range of 80240 °C, on the nine samples of 2698
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Figure 2. Typical Zimm plot obtained. Representation of eq 14 for vacuum residue samples at various concentrations at 100 °C. The dotted line is the extrapolation at q = 0, and the black solid line is the extrapolation at c = 0.
ARTICLE
Figure 5. PalRhodes representation (eq 5) for vacuum residue samples as a function of the AC5 volume fraction, for temperatures between 80 and 240 °C. (0) Data from Barre et al.,5 n-heptane asphaltenes in toluene at 25 °C.
Table 2. Solvation Coefficient K, Maximum Volume Fraction ϕmax, and Intrinsic Viscosity [η] at Various Temperatures, Considering Spherically Shaped Particles
a
Figure 3. Viscosity of vacuum residue samples as a function of the AC5 volume fraction, for temperatures between 80 and 240 °C.
Figure 4. Relative viscosity of vacuum residue samples as a function of the AC5 volume fraction, for temperatures between 80 and 240 °C. (0) Data from Barre et al.,5 n-heptane asphaltenes in toluene at 25 °C.
vacuum residue at AC5 concentrations going from 0 to 20.0 vol %. For the shear rate range applied, all samples show a Newtonian behavior, from which we can determine the zeroshear viscosity. Figure 3 shows the viscosity as a function of the volume fraction on the temperature range observed. As expected, the viscosity increases with the asphaltene volume fraction and decreases when the temperature increases. Relative viscosity was calculated, and MC5 was considered as the solvent phase.
T (°C)
K
ϕmax
[η]
25a
3.6
0.28
9.1
80
3.3
0.30
8.4
100
3.1
0.33
7.7
130
2.7
0.37
6.8
170
2.3
0.43
5.8
200 240
2.1 1.8
0.49 0.56
5.2 4.5
Calculated from data from Barre et al.5
Figure 4 shows the relative viscosity versus the volume fraction for the temperature range explored. We observe that the viscosity divergence appears (or will appear) at a higher volume fraction when the temperature is increased. We have plotted on the same graph the relative viscosity of n-heptane asphaltenes in toluene at 25 °C, obtained from previous measurements.5 Interestingly, it shows the same trend as asphaltenes (AC5) in the vacuum residue, with a viscosity divergence at an even lower volume fraction than AC5 in the vacuum residue at 80 °C. AC5 in maltenes are considered as spherically shaped. Deviation from a spherical shape is appraised and detailed in Section A of the Supporting Information. Viscosity data were processed through the PalRhodes model to probe any solvation effect, as observed by Storm et al.30 and Luo et al.31 If any, the effective volume fraction has to be taken into account on the graph of Figure 4, instead of the dry volume fraction. Figure 5 shows a representation of eq 5 that enables calculation of the solvation constant K, with a spherical model for aggregate shape. As expected again,30,31 linear trends with an intercept at 1 are found on the temperature range explored, as well as for asphaltenes in toluene.44 Data obtained at 240 °C are more scattered, because of a lack of sensitivity. K decreases with the temperature, from 3.3 at 80 °C to 1.8 at 240 °C, which means that the effective volume of the particle is more than 3 times the dry volume at 80 °C and about 2 times the dry volume at 240 °C. Results for K values can be found in Table 2. With the solvation constant being known for each temperature, Figure 4 can be scaled with the effective volume fraction, as shown in Figure 6. All data, including data in toluene,5 scale 2699
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Figure 6. Relative viscosity of vacuum residue samples as a function of the AC5 effective volume fraction, for temperatures between 80 and 240 °C. (0) Data from Barre et al.,5 n-heptane asphaltenes in toluene at 25 °C. (Line) PalRhodes representation (eq 5). The inset is a zoom of the main graph. Figure 9. SAXS spectra from Figure 8 subtracted from MC5 and normalized by the AC5 volume fraction.
Figure 7. Krieger representation (eq 7) of relative viscosity data for each temperature.
Figure 8. SAXS spectra at 100 °C of (1) dots, vacuum residues at various AC5 concentrations, from 1.8 to 20.0% (initial vacuum residue); (2) crosses, MC5 with Zimm approximation (eq 11) (line), yielding I0 = 0.23 cm1 and Rg = 6.7 Å; and (3) squares, n-heptane maltenes with Zimm approximation (eq 10) (line), yielding I0 = 0.53 cm1 and Rg = 11.2 Å.
remarkably on the same trend. The intrinsic viscosity is calculated as the intrinsic viscosity of non-interacting solvated spheres
(eq 6). Values in Table 2 show that [η] is equal to 8.4 at 80 °C and drops to 4.5 at 240 °C. One can plot the same data correspondingly to the Krieger representation (eq 7). Figure 7 shows that ϕmax increases with an increasing temperature. ϕmax values are reported in Table 2, from 0.30 at 80 °C to 0.56 at 240 °C. 3.2. SAXS Measurements on a Temperature Range. 3.2.1. Preliminary Observations. SAXS measurements were conducted at six different constant temperatures in the range of 80240 °C, on the nine samples of vacuum residue at AC5 concentrations going from 0 to 20.0 vol %. Figure 8 shows the SAXS spectra obtained at 100 °C for the range of concentrations studied. Scattered intensity increases with the asphaltene concentration, as mentioned in eq 8. It is worth mentioning that a concentration as low as 1.8% of asphaltenes in maltenes still produces a significant scattering signal. MC5 presents a weak q-dependent signal, close to a solvent flat background. Zimm approximation (eq 10) gives a radius of gyration of 6.7 Å, which is the result of small aggregates or large molecules. The scattering spectrum of n-heptane maltenes (MC7) is also shown on this graph, for comparison. It is clear that MC7 shows a greater q dependence and a higher intensity, far from a solvent behavior. Zimm approximation yields Rg = 11.2 Å. It is indeed more appropriate to consider MC5 as the dispersing phase of the null aggregate concentration rather than MC7, containing larger aggregates. We show in Section B of the Supporting Information the behavior of MC5 with temperature elevation. It is a characteristic behavior for solvents. Figure 9 shows the asphaltene scattering contribution at 100 °C. Spectra are subtracted from the MC5 scattering contribution and normalized by their asphaltene concentration. These spectra show the same characteristic behavior as asphaltenes in toluene:5 a smooth decrease with q, ending by a power law of the order of q2. Interestingly, all concentrations superimpose on the whole spawned q range. With reference to eq 15 and its explanation, the superimposition at low q values means that the interactions between aggregates are low or that repulsive and attractive forces are balanced. Concerning large q values, there is no structural change on the small length scale with dilution. 3.2.2. Asphaltene Behavior in Temperature. Figure 10 shows the effect of the temperature increase for one sample containing 2700
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Figure 13. Second virial coefficient A2 of AC5 versus the temperature in the vacuum residue, resulting from the Zimm formalism (eqs 16 and 17).
Figure 10. SAXS spectra of the vacuum residue at 8.9 vol % AC5 concentration, subtracted from MC5, on the 80240 °C temperature range.
Table 3. Radius of Gyration Rg, Molar Mass MW, and Second Virial Coefficient A2 on the 80240 °C Temperature Range, Resulting from the Zimm Formalism T (°C)
Rg (Å)
MW (g/mol)
A2 (mol cm3 g2)
80 100
41.1 39.0
6.86 104 6.19 104
8.9 106 8.0 106
130
37.6
5.96 104
9.4 106
34.6
5.35 10
4
1.0 105
33.3
4.99 10
4
1.2 105
32.7
4.71 10
4
1.3 105
170 200 240
Figure 11. Radius of gyration Rg and molar mass MW of AC5 versus the temperature in the vacuum residue, resulting from the Zimm formalism (eqs 16 and 17).
Figure 12. Molar mass MW versus the radius of gyration Rg of AC5 in the vacuum residue in the 80240 °C temperature range. Linear trend yields MW µ Rg1.5.
8.9 vol % asphaltenes. We observe an intensity decrease for the low-q part of the graph as the temperature increases, in the same way that Espinat et al.17 observed n-heptane asphaltenes in toluene but to a lower extent. It reveals a decrease in aggregate
size that will be confirmed below by the Zimm formalism. On the other hand, the spectra perfectly superimpose on the high-q part. Considering a constant density in this temperature range is reasonable for the following data treatment. It is also unequivocal to say that there is no structural change at small length scale in the 80240 °C temperature range. 3.2.3. Zimm Formalism. Data such as Figures 8 and 9 were carried out for six temperatures between 80 and 240 °C. Each set of data was processed through the Zimm formalism (eq 14) to extract Rg, MW, and A2. Plots such as Figure 2 were obtained for the six temperatures. The results are summarized through the temperature effect on the radius of gyration, the molar mass, and the second virial coefficient, as shown in Figures 11, 12, and 13, respectively, and Table 3. The size of the asphaltene aggregates, expressed by their radius of gyration, decreases from 41.1 to 32.7 Å from 80 to 240 °C. The same trend is observed for the molar mass, decreasing from 6.9 104 to 4.7 104 g/mol. Similar to previous observations in toluene,45 the radius of gyration and molar mass follow a power law (Figure 12). As shown in Figure 13, the second virial coefficient, resulting from interparticle interactions, is positive. It means that interaction forces are rather repulsive. However, in line with our comment on the superimposition of all spectra in Figure 9, this coefficient is low, around 105 mol cm3 g2. A temperature dependence is observed: A2 increases with the temperature, meaning that the aggregates are more repulsive at high temperatures. This should yield a more stable system.
4. DISCUSSION 4.1. Viscosity Behavior. The first part of the present work concerning viscosity measurements of the vacuum residue at 2701
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Figure 14. Schematic view of Storm et al.30 interpretation of solvation constant reduction with a temperature increase.
various asphaltene concentrations and various temperatures confirmed the work performed by Storm et al.30 and Luo et al.31 on similar systems. The PalRhodes model with a spherical shape assumption succeeded in quantifying the effective volume fraction of the aggregates. The determination of a solvation constant K for each temperature enables all data to be scaled on the same model. The Krieger representation highlights a maximum volume fraction, accounting for the jamming effect, increasing with the temperature. Storm et al. found that K tended to 1 at T > 250 °C. The variation of K with the temperature shows that the volume of the particles is 3 times greater at 80 °C than the volume of the same amount of particles at 250 °C and beyond. Storm et al. explained this phenomenon by the presence of an adsorbed layer surrounding the asphaltene aggregates, and the thickness of the solvation layer reduces as the temperature increases. Such a suggestion is depicted in Figure 14. The present result on viscosity behavior also accounts for this schematic view. 4.2. Structural Behavior. The second part of the work concerns SAXS measurements for vacuum residue at various asphaltene concentrations and various temperatures. This type of experiment was conducted for the first time. Special attention has to be made on experimental conditions because the scattering contrast between aggregates and maltenes is low. Similar measurements were conducted for asphaltenes in toluene5 and at various temperatures,17 where the scattering contrast is enhanced by the model solvent. The general behavior of asphaltenes in maltenes is similar to asphaltenes in toluene. Figure 11 shows that the mass and radius decrease when the temperature increases, which was previously observed in toluene by Roux et al.35 Espinat et al.17 also showed with light scattering that the aggregate radius rapidly increased at T < 20 °C. In the present study, Figure 13 shows that, when the temperature decreases, the second virial coefficient decreases, meaning that interactions between aggregates become more attractive. It goes in line with the formation of large flocs at temperatures lower than 20 °C. This phenomenon can also cause non-Newtonian behaviors observed for the vacuum residue at low temperatures. The shear thinning properties of a heavy crude oil at low temperatures were previously observed by Pierre et al.46 They were attributed to the slowdown of the asphaltene dynamics in cold conditions. The power law formed by the molecular mass and the radius of gyration of the aggregates (Figure 12) is characteristic of a fractal organization, as explained for asphaltenes in toluene.5 The size and mass range investigated is narrow; therefore, no precision can be expected on the fractal dimension df. However, the general behavior is similar to the organization of asphaltenes in toluene. 4.3. Structure/Viscosity Relationship: Combined Interpretation. The reduction of aggregate size with a temperature increase is coherent with the solvation coefficient variation and Storm et al. interpretation.30 Indeed, the size of the overall aggregates goes down when the thickness layer goes down, as
Figure 15. SAXS spectra simulations of Storm et al. interpretation. Sphere of 32 Å radius (30% polydispersity) and shell thickness set between 0 and 11 Å, simulating the desorption of resins as the temperature increases.
Figure 16. Schematic view of the solvation constant and size reductions in the vacuum residue with a temperature increase in the 80240 °C temperature range.
shown in Figure 14. However, accounting for a solvation layer made of adsorbed “resins” or other components requires consideration of a shell of different density (thus, different scattering contrast) surrounding the asphaltenic core. The X-ray scattering behavior of such a structure will be modified on both small and high q values, which is not what is observed in Figure 10, where only small q values are modified. Figure 15 illustrates this behavior thanks to SAXS spectra simulations, with a shell thickness set between 0 and 11 Å. Although we observe an intensity decrease at small q values (with Rg decreasing from 58.9 to 55.7 Å when the shell disappears), modifications at high q values are obvious. The addition of SAXS measurements to previous viscosity measurements brings another interpretation for asphaltene behavior through a temperature increase. Asphaltene aggregates have been described in model solvent as mass fractal clusters made of small entities called nanoaggregates.7 Throughout the present study, we observed that asphaltenes in maltenes behave very similarly to asphaltenes in toluene: conclusions on the contribution to system viscosity and colloidal organization are the same as the conclusions drawn in the model solvent. Therefore, we believe that the structural organization of asphaltenes, well-defined in toluene,7 is relevant in maltenes as well. If asphaltene clusters are a superaggregation of nanoaggregates, that we will call fractals, the solvation coefficient will refer to the 2702
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’ ACKNOWLEDGMENT The authors thank J. M’Hamdi for technical support in preparing the SAXS measurements and D. Isra€el from TA Instruments for fruitful discussions on viscosity measurements. The constructive comments of the reviewers are also acknowledged. ’ REFERENCES
Figure 17. Cubic radius of gyration Rg3 versus the solvation constant K for AC5 in the vacuum residue in the 80240 °C temperature range. (Line) Linear trend. (Square) Rg3 calculated for the nanoaggregate, according to Eyssautier et al.7
entrapped solvent between the nanoaggregates.27 Considering that no modification at small length scale occurs through temperature elevation but the solvation coefficient and cluster size decrease, we suggest that clusters dissociate when the temperature increases, as illustrated in Figure 16. The temperature reaching K = 1 will correspond to a system made of entirely dissociated clusters, i.e., a system made of nanoaggregates. According to Storm et al.,30 this happens at T g 300 °C. To reinforce our statement, we show in Figure 17 the aggregate “gyration volume”-to-solvation constant dependence. An acceptable linear trend is observed, and extrapolation to K = 1 gives Rg = 24.3 Å, reached at 307 °C from the linear extension of K data in Table 2, while the radius of gyration of the nanoaggregate is 20 Å.7
5. CONCLUSION This combined structure/viscosity approach helped to make evidence of the colloidal organization in the vacuum residue, highlighting the temperature effect on aggregation mechanisms. We conclude that asphaltenes behave similarly in maltenes as in toluene, forming clusters made of nanoaggregates. Size and mass of clusters decrease with temperature elevation. On the other hand, the small length scale remains unchanged, reinforcing the hierarchical aggregation scheme. Viscosity behavior through a temperature increase is fully predicted with a classical approach accounting for temperature-dependent solvation. Taking into account both structure and viscosity data, we suggest that clusters in the vacuum residue most likely dissociate into nanoaggregates with the temperature, yielding a decrease of solvation and cluster size. Moreover, repulsive interactions between clusters decrease when the temperature is lowered, which can explain the flocculation phenomenon and non-Newtonian behavior frequently described at low temperatures. ’ ASSOCIATED CONTENT
bS
Supporting Information. Results for non-spherical aggregates hypothesis, behavior of maltenes in temperature, and SAXS data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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