Mesoscale Organization in a Physically Separated Vacuum Residue

Article pubs.acs.org/EF

Mesoscale Organization in a Physically Separated Vacuum Residue: Comparison to Asphaltenes in a Simple Solvent Joel̈ le Eyssautier,†,‡ Didier Espinat,† Jérémie Gummel,§ Pierre Levitz,‡ Mildred Becerra,∥ John Shaw,∥ and Loïc Barré*,† †

IFP Energies nouvelles, 1-4 avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex, France Physique de la Matière Condensée, Centre National de la Recherche Scientifique (CNRS)−École Polytechnique, UMR 7643 CNRS, 91128 Palaiseau Cedex, France § European Synchrotron Radiation Facility (ESRF), BP 220, 38043 Grenoble Cedex 9, France ∥ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada ‡

S Supporting Information *

ABSTRACT: Physical separation of heavy oils and bitumen is of particular interest because it improves the description of the chemical and structural organization in these industrial and challenging fluids (Zhao, B.; Shaw, J. M. Composition and size distribution of coherent nanostructures in Athabasca bitumen and Maya crude oil. Energy Fuels 2007, 21, 2795−2804). In this study, permeates and retentates, differing in aggregate concentrations and sizes, were obtained from nanofiltration of a vacuum residue at 200 °C with membranes of varying pore size. Elemental composition and density extrapolations show that aggregates are best represented as n-pentane asphaltenes, while the dispersing phase corresponds to n-pentane maltenes. Small-angle X-ray scattering (SAXS) measurements are processed, on this basis, to calculate the size and mass of the aggregates. Aggregates in the vacuum residue are similar in size and mass to asphaltenes in toluene, and temperature elevation decreases the size of the aggregates. Wide-angle X-ray scattering (WAXS) highlights a coherent domain observed for fluids containing aggregates, corresponding to aromatic stacking described for dry asphaltenes. The scattered signal in this region, not observed in maltenes, grows as aggregate content increases, and the signal persists up to 300 °C. A generic behavior of aggregation in the vacuum residue is depicted, from nanoaggregates to large fractal clusters with high aggregation numbers, that is similar to the organization in toluene. free nanofiltration at 200 °C was performed with various pore size ceramic membranes. Saturates, aromatics, resins, and asphaltenes (SARA) and chemical composition (C, H, O, N, S, metals, and minerals) analyses were performed on permeates and retentates. Asphaltene size distributions in fractionated heavy oil and vacuum residue (VR) were approximated on the basis of membrane pore size, which was used as a surrogate for the leading dimensions of the structures. A significant separation was obtained, highlighting the aggregate polydispersity in shape, size, and composition, while the nominal size of the asphaltene structures was overestimated, a common artifact associated with filtration studies. In the present work, these preliminary size distributions are re-evaluated using small-angle X-ray scattering (SAXS) measurements. Samples were solvent-free permeates obtained from a nanofiltered VR under the same experimental conditions, 200 °C and near atmospheric pressure, as this prior work. Comparisons between aggregate and dispersing medium characterization (concentration and composition) visa-vis standard definitions of asphaltenes and maltenes are the

1. INTRODUCTION Heavy oils, vacuum residua, and bitumen all possess high asphaltene contents and, consequently, exhibit complex properties, such as aggregation, high viscosity, and low diffusivity, which affect both up- and downstream processing. Asphaltenes, the family of heavy components at issue, are defined operationally by a solubility class. They represent the insoluble part of oil in alkane (usually n-pentane or n-heptane). This convenient definition facilitates molecular and structural analysis and characterization of asphaltenes in model solvents. A large number of properties of interest have been obtained for asphaltene + solvent mixtures: molecular weights and molecular structure,1,2 colloidal structure,3−6 aggregation states,4,7−9 temperature and pressure effects,10−12 and rheological properties.11,13 Because of their polydispersity in shape, size, and composition, property variation results have also been obtained from asphaltene fractionation, using centrifugation,14 nanofiltration,15 membrane diffusion,16 and selective precipitation.17,18 However, from an industrial perspective, properties of asphaltenes in heavy oils and bitumen under reservoir and processing conditions, which do not include possible artifacts introduced by solvent addition, are of pressing importance and largely lacking in the literature. In a recent paper, Zhao et al.19 presented a characterization study of coherent nanostructures in bitumen and heavy oil based on physical separation. Solvent© 2011 American Chemical Society

Special Issue: 12th International Conference on PetroleumPhase Behaviorand Fouling Received: September 16, 2011 Revised: November 24, 2011 Published: November 28, 2011 2680

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2.1.4. Elemental Analysis. The carbon, hydrogen, and nitrogen contents of nanofiltered samples were determined using American Society for Testing and Materials (ASTM) D5291. Oxygen contents were determined by SGS using MO238 LA2008 [pyrolysis at 1100 °C and CO infrared (IR) detection]. Sulfur contents were determined using MO240 LA2008 (combustion at 1150 °C and SO2 IR detection). The elemental composition of a binary mixture is defined by the elemental composition of each constituent or phase weighted by their respective mass fractions.

focus of this contribution. The structure and properties of aggregates are shown to be similar to those of n-pentane asphaltenes, and the non-aggregated phase has properties similar to n-pentane maltenes. The SAXS characterization permits size and mass determinations using the same methodology as used previously for asphaltenes in model solvents.3 In this work, wide-angle scattering (WAXS) measurements provide data on the organization of structure at the molecular level, as performed previously for dry asphaltenes.13,20−23 A few studies are also reported for vacuum residua.24,25 Finally, ultra-small-angle X-ray scattering (USAXS) was performed to study the macroscopic length scale. Thanks to the high boiling point of the VR, higher temperatures (300 °C) were easily investigated to assess the persistence of structures near refining temperatures.

ni1 + 2 = ϕmni1 + (1 − ϕm)ni2

2.2. X-ray Scattering. 2.2.1. Equipment. 2.2.1.1. Combined SAXS/WAXS. SAXS and WAXS measurements were performed simultaneously on the ID02 instrument at the European Synchrotron Radiation Facility in Grenoble, France. The SAXS detector is a Frelon 4 M charge-coupled device (CCD) camera with a 2048 × 2048 pixel chip of a total 10 × 10 cm2 surface placed inside an evacuated flight tube. Two sample−detector distances were used: 1 and 10 m along with a 1 Å wavelength X-ray beam, giving a q range lying from 8.8 × 10−4 to 4.8 × 10−1 Å−1. The WAXS detector is an Aviex PCCD-4284 camera. This detector is mounted in an upright configuration, in air, close to the sample. The WAXS q range lies from 3.4 × 10−1 to 4 Å−1. During data acquisition, standard beamline-specific corrections are made to the recorded CCD images that account for transmission and monitor, detector efficiency, and distortion. Pure water is used to calibrate the instrument. For WAXS measurements, geometrical and distortion corrections are made. Poly-n-benzyl-L-aspartate (PBBA) was used to calibrate WAXS angles. SAXS two-dimensional (2D) images were then azimuthally regrouped after applying a mask to remove the impact of the beam stop and faulty regions of the images. The impact of the mica windows was then subtracted, and the intensities were normalized by sample thickness to obtain I = f(q) spectra in absolute intensity (cm−1).4 2.2.1.2. USAXS. USAXS was performed on an ID02 line in the Bonse−Hart configuration using a multiple-bounce crossed analyzer over a q range from 3 × 10−4 to 2 × 10−2 Å−1.26 The measured intensity profiles were normalized to an absolute scale using the peak intensity, and the known acceptance angle was defined by the analyzer crystals. Mica windows were subtracted from the signal, and the signal was normalized by sample thickness to obtain I = f(q) spectra in absolute intensity (cm−1). For all scattering setups (USAXS, SAXS, and WAXS), samples were placed in an in-house brass cell with a 1.86 mm optical path and mica windows. The sample cell was placed vertically on a temperaturecontrolled hot stage (HFS350 V-MU, Linkam) with an upper operating temperature of 335 °C. Samples were kept under a nitrogen blanket to avoid oxidation during experiments. The temperature was raised at 10 °C/min and stabilized for 10 min in the 25−300 °C temperature range. Data at 200 °C (the filtration temperature) were the focus of this study. 2.2.2. Data Processing. From a SAXS experiment, the scattered intensity I(q) probes spatial correlations between particle-rich regions on a scale of the order of q−1. The general equation for a two-phase mixture of particles, at volume fraction ϕ, in a solvent, is

2. EXPERIMENTAL SECTION 2.1. Sample Preparation and Characterization. Safaniya VR from a Saudi Arabian field, containing 23 and 13.4 wt % n-pentane and n-heptane asphaltenes, respectively, was the base sample. Chemical and physical properties of the VR and its fractions are listed in Table 1.

Table 1. Chemical and Physical Properties of Safaniya VR and Its Fractions C (wt %) H (wt %) O (wt %) N (wt %) S (wt %) Ni (ppm) V (ppm) H/C density (g/cm3)

VR

n-pentane asphaltenes

n-pentane maltenes

83.9 10.5 0.43 0.39 5.29 175 580 1.48 1.027

81.8 7.8 1.2 0.7 8.0

83.7 11.0 0.5 0.2 4.7

1.13 1.173

1.56 0.985

2.1.1. Nanofiltration. Safaniya VR was separated by solvent-free nanofiltration using 5, 10, 20, 50, and 100 nm ceramic membranes at 200 °C. A detailed description of the experimental procedure is presented elsewhere.19 Abbreviations “P” and “R” refer to permeate and retentate, respectively. All of the Safaniya VR passed through the 100 nm membrane, while none passed through the 5 nm membrane. 2.1.2. Asphaltene Content. n-Pentane and n-heptane asphaltene contents (AC5 and AC7, respectively) for permeate and retentate samples were determined by precipitation following the addition of pentane or heptane at a 40:1 volume ratio. These mixtures were stirred at a reflux temperature (31 and 98 °C for nC5 and nC7, respectively) for 20 min and vacuum-filtered at 80 °C with a 0.45 μm Millipore membrane. The filtration membranes and the flask were washed with small volumes of solvent to eliminate residual oil until the filtrate became colorless. The membranes with precipitated material (AC5 or AC7) were dried overnight at 80 °C in an oven and weighted. The solvent was removed from the maltene samples by evaporation under controlled vacuum in a rotary evaporator. The maltene samples were dried overnight at 80 °C in an oven and weighted, so that the mass balance could be completed. 2.1.3. Mass Density. Density measurements of nanofiltered samples (permeates and retentates) were performed with an Anton Paar DMA5000 densitometer at 20 °C. The specific volume of solutions versus the sample mass fraction shows a linear trend in the range of 0.01−25 wt %. This permits extrapolation to null dilution because the mass density of a binary mixture is defined by the density of each constituent or phase weighted by their respective volume fraction.

d1 + 2 = ϕ V d1 + (1 − ϕ V )d2

(2)

I(q) = ϕ(1 − ϕ)Δρ2F(q)S(q)

(3)

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]. Δρ is the electronic density difference between particles and a solvent, determined from density and elemental composition (n) of the solvent and particles

ρ=

le ∑1N niZi V

(4)

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

(1) 2681

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Table 2. Mass Density, Asphaltene Content (n-Pentane and n-Heptane), Elemental Analysis, and Electronic Density of Nanofiltered Samples, Safaniya VR, Asphaltenes, and Maltenes (n-Pentane and n-Heptane) MC5 MC7 P10 P20 P50 VR R50 R20 R10 AC5 AC7

d (g/cm3)

[AC5] (wt %)

0.985 1.017 0.993 0.997 0.998 1.027 1.073 1.086 1.092 1.173 1.195

0 4.8 6.2 7.8 22.9 47.9 53.3 55.5 100

[AC7] (wt %) 0 0.7 1.4 2.8 13.4 29.6 35.5 36.7 100

C (wt %)

H (wt %)

O (wt %)

N (wt %)

S (wt %)

H/C

ρ (e−/Å3)

83.7 83.4 83.7 84.0 84.2 83.9 83.3 83.3 82.8 81.8 82.5

11.0 10.5 11.0 10.9 10.9 10.5 9.6 9.3 9.1 7.8 7.6

0.5 0.6 0.4 0.4 0.4 0.4 0.7 0.7 0.7 1.2 1.3

0.2 0.3 0.3 0.3 0.3 0.4 0.6 0.6 0.6 0.7 1.0

4.7 5.2 4.7 4.7 4.7 5.3 6.1 6.4 6.6 8.0 7.6

1.56 1.50 1.56 1.55 1.54 1.48 1.37 1.33 1.30 1.13 1.09

0.327 0.332 0.330 0.332 0.332 0.341 0.353 0.357 0.356 0.377 0.386

atom i. On this length scale, a coarse graining approach is used and the electronic density of a binary mixture is defined by the electronic density of each constituent or phase weighted by their respective volume fractions.

ρ1 + 2 = ϕ V ρ1 + (1 − ϕ V )ρ2

(5)

For dilute mixtures [S(q) = 1], in the Guinier region (i.e., at scales larger than the characteristic size of particles or qRg < 1), the Zimm approximation27 can be used to determine the scattering cross-section at q = 0 and the radius of gyration Rg of the particles.

⎛ q 2R g 2 ⎞ 1 1 ⎜ ⎟ 1+ = 3 ⎟⎠ I(q) I(0) ⎜⎝

(6)

From eq 3, I(0) takes a simple form for dilute solutions from which the particle volume v can be extracted.

v=

Figure 1. (Open symbols) Mass density of the nanofiltered samples and Safaniya VR as a function of (bottom x scale) n-pentane asphaltene content (circles) and (top x scale) n-heptane asphaltene content (squares). Dotted lines are linear regressions on open symbols. (Full symbols) Mass density of n-pentane maltenes and npentane asphaltenes (circles) and mass density of n-heptane maltenes and n-heptane asphaltenes (squares).

I(0) ϕ(1 − ϕ)Δρ2

(7)

The apparent molar mass MW can then be derived

MW = dNav

(8)

where d is the density of the particles and Na is Avogadro’s number. Equations 6−8 are model-independent.

respectively). As anticipated from eq 1, these data are linear. Data shown at ϕ = 0 and 1 comprise the mass density of chemically separated maltenes and asphaltenes, separated by npentane and n-heptane, respectively. Extrapolated density values for nanofiltered samples are also presented for ϕ = 0 and 1 to facilitate comparison. As noted previously,19,30 the npentane chemical definition of asphaltenes corresponds closely to the dispersed phase in Safaniya VR, from the perspective of phase density. Table 3 shows the deviations between extrapolated values and chemical definitions at ϕ = 0 and 1. A similar observation can be made with respect to the elemental composition and hydrogen/carbon ratio data, shown in Figure 2 and Table 3. Linear trends are obtained for both chemical definitions of asphaltenes and maltenes. Extrapolated values at ϕ = 0 and 1 are closer to the n-pentane chemical definition of asphaltenes, although oxygen and nitrogen contents, the least repeatable of the analysis measurements,19 show large relative but small absolute deviations. The small magnitudes of the O and N contents, measurement error, and uncontrolled variations in sample handling readily account for the poor resolution of these data. SAXS data processing, eqs 3 and 4, makes use of the particle and the solvent electronic density to calculate the size and mass of the scattering aggregates. With data from Figures 1 and 2, the electronic density was calculated and values at ϕ = 0 and 1 were

3. RESULTS AND DISCUSSION One main hypothesis in this study concerns the impact of the nanofiltration separation on the size distribution of the particles in the VR. Removing part of the particles is assumed to have no effect on the size distribution of the remaining particles. This assumption is under investigation currently for asphaltenes in model solvents and will be reported separately. 3.1. Aggregate Concentration and Characterization. The characteristics of the scattering particles and the dispersing solvent are key concerns for small-angle scattering measurements. As shown in eqs 6−8, calculation of the mass and radius of gyration requires knowledge of the concentration, the elemental composition, and the mass density of the particles and solvent. In natural heavy oils and bitumen, these data are hard to obtain. Scattering aggregates appear to be related to asphaltenes.28,29 However, for a detailed study, the scattering phase must be carefully defined. In the present work, VR samples were fractionated using solvent-free nanofiltration. The n-pentane and n-heptane asphaltene contents, densities, and elemental analyses of the resulting permeates and retentates varied over a broad range, as shown in Table 2. Figure 1 shows the mass density of the nanofiltered samples versus their npentane and n-heptane asphaltene contents (AC5 and AC7, 2682

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Table 3. Extrapolated Scatterer and Solvent Density, Elemental Analysis, and Electronic Density Values Based on n-Pentane and n-Heptane Asphaltenes and Maltenes and Corresponding Deviation from Each Extrapolated Value extrapolations

3

d (g/cm ) C (wt %) H (wt %) O (wt %) N (wt %) S (wt %) H/C ρ (e−/Å3)

ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ

= = = = = = = = = = = = = = = =

0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

asphaltenes/ maltenes

deviation (%)

C5

C7

C5

C7

C5

C7

0.985 1.191 84.1 82.2 11.2 7.6 0.3 1.0 0.2 0.9 4.4 8.1 1.58 1.11 0.329 0.384

0.991 1.293 84.0 81.4 11.1 6.0 0.4 1.3 0.3 1.2 4.6 9.8 1.57 0.92 0.330 0.411

0.985 1.173 83.7 81.8 11.0 7.8 0.5 1.2 0.2 0.7 4.7 8.0 1.56 1.13 0.327 0.377

1.017 1.195 83.4 82.5 10.5 7.6 0.6 1.3 0.3 1.0 5.2 7.6 1.50 1.09 0.332 0.386

0.01 1.5 0.54 0.60 1.9 1.9 26.4 17.7 1.4 34.0 4.6 1.5 1.3 2.3 0.57 1.9

2.5 8.2 0.76 1.3 5.5 20.4 39.8 2.0 21.9 18.9 12.6 29.6 4.7 16.0 0.59 6.5

Figure 3. (Open symbols) Electronic density of the nanofiltered samples and the initial VR as a function of (bottom x scale) n-pentane asphaltene content (circles) and (top x scale) n-heptane asphaltene content (squares). Dotted lines are linear regressions on open symbols. (Full symbols) Electronic density of n-pentane maltenes and n-pentane asphaltenes (circles) and electronic density of n-heptane maltenes and n-heptane asphaltenes (squares).

3.2. Size and Mass of Aggregates in Permeates. Figure 4 shows SAXS spectra of Safaniya VR and the permeates, together with n-pentane and n-heptane maltenes (MC5 and MC7), at 200 °C. Spectra in the 80−200 °C range can be found in the Supporting Information. The general behaviors of the VR and MC7 are the same as observed by Espinat et al.28 The n-pentane maltenes present a weak qdependence signal, close to a solvent flat background. Zimm approximation (eq 6) gives a radius of gyration of Rg = 6.9 Å, which is consistent with small aggregates or large molecules. The scattering spectrum of n-heptane maltenes shows a greater q dependence, far from a solvent behavior. Zimm approx-

obtained by extrapolation using eq 5. The results are shown in Figure 3 and Table 3. Again, n-pentane asphaltenes correspond closely to the scattering particles. Thus, from the perspectives of X-ray scattering, elemental analysis and particle density, n-pentane asphaltenes represent the scattering aggregates better than n-heptane asphaltenes. While n-pentane asphaltenes are not expected to be an exact match, only this chemical definition of asphaltenes is used for further SAXS data processing.

Figure 2. (Open symbols) Elemental analysis (C, H, O, N, and S) (wt %) and H/C ratio (atomic ratio) of the nanofiltered samples and the initial VR as a function of (bottom x scale) n-pentane asphaltene content (circles) and (top x scale) n-heptane asphaltene content (squares). Dotted lines are linear regressions on open symbols. (Full symbols) Elemental analysis (C, H, O, N, and S) and H/C ratio of n-pentane maltenes and n-pentane asphaltenes (circles) and elemental analysis (C, H, O, N, and S) and H/C ratio of n-heptane maltenes and n-heptane asphaltenes (squares). 2683

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Figure 4. SAXS spectra at 200 °C of (1) (lines) nanofiltered VR samples (VR, initial vacuum residue; P50, P20, and P10, permeates), (2) (dots) n-pentane maltenes with Zimm approximation (eq 6) (line), yielding I0 = 0.23 cm−1 and Rg = 6.9 Å, and (3) (squares) nheptane maltenes with Zimm approximation (eq 6) (line), yielding I0 = 0.53 cm−1 and Rg = 11.2 Å.

Figure 5. SAXS spectra from Figure 4 subtracted from the n-pentane maltenes and normalized by the contrast term and the volume fraction (eqs 6−8). Lines are the Zimm approximation of each spectrum (see Table 3). (Dotted line) SAXS spectrum of a nanoaggregate4 normalized by the contrast term (eqs 6−8).

imation yields a radius of gyration Rg = 11.2 Å. Considering MC5 as the dispersing phase of the null aggregate concentration is therefore more appropriate than considering MC7, containing larger aggregates. Moreover, we see that P10 shows a lower scattered intensity than MC7. As expected from eq 3, the scattering intensity of the sample is greater as the aggregate concentration increases. Thanks to accurate characterization of the dispersed and dispersing phase (AC5 and MC5, respectively), we were able to calculate the scattering contribution of the aggregates (i.e., AC5) by subtraction of the solvent contribution to the spectra of Figure 4 and normalization by the contrast term and the volume fraction, as required from eqs 6−8. This procedure enables calculation of the molar mass of the aggregates. The result is shown in Figure 5. First of all, the spectrum corresponding to the AC5 scattering contribution in the VR shows a similar behavior as asphaltenes in toluene.3 The calculated radius of gyration and molar mass are listed in Table 4 for each permeate sample at every processed temperature.31 Experimental SAXS data at each temperature can be found in section A of the Supporting Information. As expected, the radius of gyration and mass decrease as the membrane pore size becomes smaller. At 200 °C, Rg decreases from 32.6 Å for Safaniya VR to 22.7 Å for P10 and MW decreases from 4.66 × 104 to 1.42 × 104 g/mol. With an increasing temperature, the size and mass decrease, a behavior similar to what was previously observed for asphaltenes in toluene.10,11 The asphaltene aggregation scheme in toluene was carefully described by SAXS and small-angle neutron scattering (SANS).4 As a comparison, the SAXS spectrum of the socalled nanoaggregate in Figure 5 gives a radius of gyration Rg = 20 Å and a molecular weight MW = 9.6 × 103 g/mol. We see in Table 3 that, as the pore size of the filtration membrane is reduced, the mass and radius of the aggregates are closer to those of the nanoaggregate.

3.3. Qualitative Coherent Scattering Domain on the Molecular Scale. 3.3.1. Concentration Effect. WAXS measurements were performed for retentates, Safaniya VR, and n-pentane asphaltenes and maltenes. Experimental data are shown in Figure 6. Interestingly, the two bands thoroughly described by Yen et al.23 on dry asphaltenes are observed in liquid phase also. The large band centered at 2θ ≈ 17° is the γ band assimilated to the saturated structure, while the shoulder centered at 2θ ≈ 25° is the (002) band. The latter represents the aromatic structure, corresponding to the characteristic spacing distance between two aromatic sheets (3.354 Å).32 All VR samples show the γ and (002) bands, except for n-pentane maltenes (0% AC5) not exhibiting the (002) band. With an increasing asphaltene concentration, the scattering signal of the (002) band grows stronger. A coherent scattering domain corresponding to the crystallite size of the aromatic stacking appears in the VR and is definitively attributable to asphaltenes. The quantitative interpretation of these spectra with determination of crystallite parameters, as performed elsewhere25,20 is not performed here because baseline subtraction and spectra decomposition are not adequately robust.20 3.3.2. Temperature Effect. The effect of the temperature on the (002) band was assessed. Results are shown in Figure 7 for the R50 sample (44 vol %) in the 25−300 °C temperature range. The coherent scattering domain is observed up to 300 °C, and similar results were obtained for all of the samples listed in Figure 6. 3.4. Large-Scale Fluctuations. Espinat et al.28 and Headen et al.29 observed significant intensity increases in the large-scale domain for VR. This phenomenon was attributed to large asphaltene aggregates comprising part of the VR.29 The USAXS measurements performed as part of the present contribution can be found in section B of the Supporting Information. These results confirm the presence of large-scale fluctuations in both VR and maltenes. No signal is obtained from the P50 sample. The 50 nm membrane removed large 2684

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Table 4. Radius of Gyration Rg and Molar Mass MW of Nanofiltered Samples in the 80−200 °C Temperature Range, Resulting from the Zimm Approximationa P10 T (°C)

Rg (Å)

80 100 130 200

25.3 24.8 23.7 22.7

a

P20

MW (g/mol)

Rg (Å)

× × × ×

31.2 30.4 29.0 27.6

2.00 1.82 1.63 1.42

104 104 104 104

P20−P10

MW (g/mol)

Rg (Å)

× × × ×

37.4 36.0 34.2 31.6

3.24 3.05 2.76 2.42

104 104 104 104

P50

MW (g/mol)

N

Rg (Å)

× × × ×

7.7 7.3 6.6 5.7

41.2 38.8 34.1 33.1

7.39 6.95 6.36 5.50

104 104 104 104

P50−P20

MW (g/mol)

Rg (Å)

× × × ×

57.8 50.6 43.2 40.2

5.29 4.74 3.99 3.70

104 104 104 104

VR

MW (g/mol)

N

Rg (Å)

× × × ×

15.4 12.4 10.0 9.2

40.4 39.1 37.0 32.6

14.8 11.9 9.57 8.80

104 104 104 104

MW (g/mol) 6.39 5.83 5.57 4.66

× × × ×

104 104 104 104

Data for P20−P10 and P50−P20 result from a separation of aggregates by class. N is the aggregation number (see the Discussion).

n-heptane insolubles best describe the aggregated phase in the VR, from an X-ray point of view. This finding augments elemental analysis results linking aggregates in crudes with n-pentane asphaltenes.19 Detailed aggregate size and mass characterizations from SAXS analyses can be compared to the approximate dimensional boundaries identified using nanofiltration. 4.2. Order at the Molecular Level and Persistence at High Temperatures. The study at the molecular level showed that a coherent scattering domain is unequivocal in the VR, which implies a certain degree of aggregation. These structures are persistent at 300 °C, a result of great interest for refinery operations, where more knowledge on dynamic properties at temperatures higher than 300 °C is required. This short-range organization drives the properties of the VR at larger length scales. 4.3. Aggregate Size and Mass. Mean size and mass determinations of aggregates in permeates provide measures of average dimensions in the VR and how they are affected by separation and temperature. Although the samples do not have the same polydispersity, P50 is more polydispersed than P10. This affects the results on average calculations of Rg and MW. We separated the aggregates by class by subtraction of the scattering contribution of the smaller portion of aggregates in the samples (P50−P20 and P20−P10). As shown in Table 4, masses and radii for P20−P10 and P50−P20 are greater than for P20 and P50, where average dimensions were dragged toward lower values because of small aggregates. The aggregate dimensions in the VR are lower than the range of sizes and masses for asphaltenes in toluene at 25 °C.3 Aggregation is also not as extended in maltenes as it is in toluene. A correlation between the membrane pore size and radius of gyration can be attempted: P50, P20, and P10 have equivalent pore radii from membrane pore sizes of 250, 100, and 50 Å, respectively. Radii of gyration are about 2−5 times smaller than the nominal pore size. This difference is expected because, as a filter cake forms on the membranes, the nominal pore size of the membrane is reduced. Figure 8 shows the mass-to-radius of gyration dependence for each aggregate class. Two distinct generic behaviors are observed. The first generic behavior, attributed to P10, shows that mass increases rapidly with the radius. It corresponds to a compact growth of particles. The second generic behavior is drawn for the larger aggregate classes. The power law relation between mass and radius of gyration indicates lose aggregates, typically mass fractals. The exponents found for both behaviors and shown in Figure 8 are tentative, because the parameter range is small, but the two distinct behaviors are evident. The data obtained for nanoaggregates in toluene4 fall in line with the mass-to-radius dependence for small aggregates in P10 and reinforce the similarity of the asphaltene aggregation behavior in toluene and maltenes.

Figure 6. WAXS spectra at 130 °C of n-pentane maltenes (MC5), Safaniya VR, retentates (R50 and R10), and dry n-pentane asphaltenes (AC5).

Figure 7. WAXS spectra of R50 retentate in the 25−300 °C temperature range. Peaks on the 25 °C spectrum at 2θ = 21.5° and 2θ = 23.9° are assigned to crystals of paraffin, melting at T > 50 °C.

particles. The intensity increase in the large-scale domain (low q values) decreases slightly between 25 and 80 °C and, together with the disappearance of sharp diffraction peaks in the very high q range (observed in Figure 7), is attributed to the melting of paraffins (section B of the Supporting Information). Part of the high intensity at low q is ascribed to crystals of paraffins. However, the slope remains unchanged between 80 and 300 °C, contrary to the intermediate length scale, where an aggregate size decrease is observed (Table 4). These macroscopic particles are believed to be mineral particles, with no or little effect on the length scale of interest.19

4. DISCUSSION 4.1. Characterization of the Aggregates. Solvent-free nanofiltration was useful to obtain samples of various aggregate contents without a solvent precipitation step. The characterization of the aggregates as a comparison to standard chemical asphaltene definitions shows that n-pentane rather than 2685

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5. CONCLUSION The study of nanofiltered VR samples using a multi-scale approach enables a better description of the aggregated phase in the VR and a better description of the dispersing phase. Size and mass of aggregates in the VR are determined in the 80−200 °C temperature range on this basis. With reference to common chemical definitions of asphaltenes, n-pentane asphaltenes best represent the aggregates, while the balance of the VR is best represented as n-pentane maltenes. The methodology commonly applied for studying the structure of asphaltenes in toluene is shown to be directly transposable to petroleum feedstocks. A hierarchical aggregation, from molecules to nanoaggregates that then assemble into fractal clusters in a second aggregation step, is identified in the VR. This multi-scale organization, from free monomers to high aggregation number fractal clusters, persists from room temperature to at least 300 °C and is similar to the multiscale organization arising in asphaltene + toluene mixtures. An additional coherent scattering domain, attributed by others to the crystallite size of aromatic stacking, also persists at 300 °C.

Figure 8. Molar mass MW versus radius of gyration Rg for nanofiltered samples in the 80−200 °C temperature range, for each aggregate class. (Gray square) Nanoaggregate in toluene from Eyssautier et al.4 The right y scale represents the aggregation number, in terms of the number of nanoaggregates (see the text).

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For the foregoing analyses, aggregates are considered to have the same average composition and dry density, irrespective of their size. Small variations may arise depending upon their state of aggregation. For example, small aggregates may be less aromatic and, thus, less dense. Molecular weights would be affected slightly if this were the case, but radii of gyration are independent of the composition and density. 4.4. Generic Behavior. From the previous considerations, the following aggregation scheme can be drawn for aggregates in the VR. This scheme, depicted in Figure 9, shows two

ASSOCIATED CONTENT S Supporting Information * SAXS spectra of the temperature effect and USAXS data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

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ACKNOWLEDGMENTS The authors thank J. M’Hamdi [IFP Energies nouvelles (IFPEN)] for helping with the SAXS measurements, J. Verstraete (IFPEN) for providing a large quantity of sample, and Dymtro Statiychuk-Dear (University of Alberta) for his help with nanofiltration experiments. Funding from the sponsors of the Natural Sciences and Engineering Research Council of Canada (NSERC) Industrial Research Chair (IRC) in Petroleum Thermodynamics supported the nanofiltration work. The authors thank the European Synchrotron Radiation Facility (ESRF) for allocating beamtime (proposal SC-3108) and for technical support during the experiments. The constructive comments of the reviewers are acknowledged.

Figure 9. Schematic representation of the generic behavior of aggregation in the VR. (a) MW ∝ Rg3, nanoaggregate polydispersity. (b) MW ∝ Rg1.7, fractal aggregate polydispersity.

characteristic aggregation levels. Size and mass polydispersity of nanoaggregates is illustrated in Figure 9a for the small dimensions seen in Figure 8. A mass-to-radius of gyration dependence with a lower exponent observed for larger dimensions is explained by a further fractal aggregation of these nanoaggregates. The variations in size and mass are interpreted in terms of aggregation numbers, illustrated on the right y scale in Figure 8. The aggregation number is simply defined as N = Mcluster/Mnanoaggregate. N varies between 5 and 16 for large clusters found in P20 and P50. The retentates were not studied on this length scale, because of their high asphaltene and mineral contents. A correct model for the interaction factor or dilution of the samples would give some insight into the maximum dimensions and aggregation numbers.

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REFERENCES

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