Characterization of the Nonstable Fraction of Hassiâ Messaoud

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Energy & Fuels 2008, 22, 3134–3142

Characterization of the Nonstable Fraction of Hassi-Messaoud Asphaltenes Mortada Daaou,†,‡ Ali Modarressi,† Dalila Bendedouch,‡ Youcef Bouhadda,‡ Gabriel Krier,§ and Marek Rogalski*,† LCME, and LCMS, UniVersity of Metz, 1, bd Arago, 57070 Metz, France, and LCPM, UniVersity of Oran, Post Office Box 1524, 31000 El-M’naouer, Algeria ReceiVed February 4, 2008. ReVised Manuscript ReceiVed April 13, 2008

The objective of this work was to separate and study a least stable fraction of Hassi-Messaoud asphaltenes that flocculate during oil transport and storage. The unstable fraction was studied by consecutive fractioning with solvents differing in polarity. A correlation between structural parameters of asphaltenes and the flocculation onset was established. It was demonstrated that the spontaneously flocculating part of asphaltenes is composed of relatively small molecules that are less aromatic and contain more heteroatoms than more stable asphaltenes. The length of the side aliphatic chain was shown to be an important parameter influencing the flocculation onset.

1. Introduction During the last 2 decades, petroleum asphaltenes have been extensively studied because of technical problems that they cause during petroleum production, transportation, and storage.1–6 The solid deposit lowering the crude oil production contains asphaltenes, mineral particles, and waxes. The co-precipitation of asphaltenes and waxes was recently studied, and the mechanism of this process was discussed.7–11 Divergent opinions were formulated concerning the synergic interactions between paraffins and asphaltenes promoting the deposit formation. Several authors7,8 consider that the asphaltene aggregate surface may induce the crystallization of paraffins that may be explained by interactions between the aliphatic chains of paraffins and asphaltenes.9 However, this opinion was not confirmed by recent results,10 suggesting an independent precipitation of two families of compounds. Probably, the formation of the solid deposit is a complex process, the mechanism of which is dependent upon not only the oil chemical properties but also thermodynamic and hydrodynamic conditions during the oil transportation. Despite the fact that the crude oil from the Hassi-Messaoud field contains a small amount of asphaltenes only, it is very * To whom correspondence should be addressed. E-mail: rogalski@ univ-metz.fr. † LCME, University of Metz. ‡ University of Oran. § LCMS, University of Metz. (1) Haskett, C. E.; Tartera, M. J. Pet. Technol. 1965, April, 387–391. (2) Speight, J. G., Moschopedis, S. E. On the molecular structure of petroleum asphaltenes, In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; American Chemical Society: Washington, D.C., 1981; Advances in Chemistry Series, Vol. 195, pp 1-15. (3) Speight, J. G. Fuel 1971, 50, 102. (4) Tuttle, R. N. J. Pet. Technol. 1983, June, 1192–1196. (5) Leontaritis, K. J.; Mansoori, G. A. J. Pet. Sci. Eng. 1988, 1, 229. (6) McLean, J. D.; Kilpatrick, P. K. J. Colloid Interface Sci. 1997, 189, 242. (7) Mahmoud, R.; Gierycz, P.; Solimando, R.; Rogalski, M. Energy Fuels 2005, 19, 2474–2479. (8) Garcia, M. C. Energy Fuels 2000, 14, 1043–1048. (9) Kriz, P.; Andersen, S. I. Energy Fuels 2005, 19, 948–953. (10) Yang, X.; Kilpatrick, P. Energy Fuels 2005, 19, 1360–1375. (11) Boukadi, A.; Philp, R. P.; Thanh, N. X. Appl. Geochem. 2005, 20, 1974–1983.

unstable with respect to the flocculation process. Physical reasons of this strong propensity to precipitate asphaltenes are not well-established. Indeed, the value of the precipitation onset indicates that this oil is stable. However, both the fact that asphaltenes precipitated from this oil are highly unstable and the high propensity of asphaltenes to co-precipitate with n-paraffins indicate that the concept of dispersion-force-driven aggregation12,13 is not sufficient to assess the flocculation processes. Indeed, a probable flocculation mechanism should take into account the structural features of asphaltene molecules leading to asphaltene self-assembly14 and the entropic interactions between aliphatic chains.7,15 The purpose of this work was to characterize the part of the Hassi-Messaoud asphaltenes that co-precipitate with n-paraffin in the storage tank. The distance between the Hassi-Messaoud field and the storage tank was about 850 km. The crude oil was transported by pipeline after the usual operations, aiming to reduce the oil viscosity. The precipitate forms probably during transportation in the pipeline. The precipitation may be induced by fluctuations of temperature and pressure during transportation. Another possible reason may be the shear strain created in the pumps during transportation. While two former factors have thermodynamic character and their influence on the ordering and aggregation of heavy molecules of the oil is in principle reversible, the later one may induce irreversible aggregation in a purely mechanical way. We believe that the later mechanism plays a major role in precipitation of fine particles of the organic solid mainly transported by the oil flow. Indeed, this mechanism conforms to the above mentioned aggregation model based on the entropic interactions between aliphatic chains.7,15 Fine particles precipitate in the storage tanks, forming deposits investigated in the present study. This deposit contained about 6% (w/w) of asphaltenes. The paraffin content represented about (12) Buckley, J. S.; Hirasaki, G. J.; Liu, Y.; Von Drasek, S.; Wang, J. X.; Gill, B. S. Pet. Sci. Technol. 1998, 16, 251–285. (13) Buckley, J. S. Energy Fuels 1999, 13, 328–332. (14) Porte, G.; Zhou, H.; Lazzeri, V. Langmuir 2003, 19, 40–47. (15) Stachowiak, C.; Viguie´, J. P.; Grolier, J.-P. E.; Rogalski, M. Langmuir 2005, 21, 4824–4829.

10.1021/ef800078u CCC: $40.75  2008 American Chemical Society Published on Web 07/12/2008

Nonstable Fraction of Hassi-Messaoud Asphaltenes

Energy & Fuels, Vol. 22, No. 5, 2008 3135

Figure 1. Asphaltene fractioning.

70%. The remaining part of the deposit was composed of aromatics, naphthenics, and resins. We have studied the flocculation propensity and the chemical composition of asphaltenes co-precipitated with paraffins. These asphaltenes may be considered as the most unstable part of the asphaltenic fraction. Properties of co-precipitated asphaltenes were compared to properties of asphaltenes obtained from the crude oil extracted directly from the well. Next, the deposit asphaltenes were fractioned by extraction with solvents of different polarity, such as toluene, dichloromethane, and tetrahydrofuran, and the corresponding fractions were characterized. A great deal of research oriented toward unravelling asphaltene chemistry was based on the concept of asphaltene fractioning.16–20 Avid et al.16 fractioned Brazilian asphaltenes by stepwise extraction using n-heptane/toluene mixtures, and (16) Avid, B.; Sato, S.; Takanohashi, T.; Saito, I. Energy Fuels 2004, 18, 1792–1797.

they separated asphaltenes in two fractions. The first fraction was extracted with a higher n-heptane/toluene ratio and presented a high aromaticity with a smaller coke content. Buenrostro-Gonzalez et al.17 flocculated asphaltenes from toluene/ acetone and toluene/n-heptane mixtures. Their results showed significant structural differences between acetone- and nheptane-precipitated asphaltenes. Groenzin et al.18 studied asphaltenes fractioned by precipitation with n-pentane/toluene mixtures at different ratios of the two components. Their results indicate that the solubility of asphaltenes diminish with an (17) Gonzalez, E. B.; Andersen, S. I.; Garcia-Martinez, J. A.; LiraGaleana, C. Energy Fuels 2002, 16, 732–741. (18) Groenzin, H.; Mullins, O. C.; Eser, S.; Mathews, J. M.; Yang, G.; Jones, D. Energy Fuels 2003, 17, 498–503. (19) Grijalva-Monteverde, H.; Arellano-Tanori, O. V.; Valdez, M. A. Energy Fuels 2005, 19, 2416–2422. (20) Fossen, M.; Kallevik, H.; Knudsen, K. D.; Sjoblom, J. Energy Fuels 2007, 21, 1030–1037.

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Daaou et al.

Table 1. Extraction of Asphaltenes and the Procedure of Asphaltene Fractioning (Figure 1, with the Symbols Corresponding to

Different Fractions) step

fractions

nomenclatures

oil storage tank deposit (1 g) + n-heptane (40 mL) (according to norm IP143/82) asphaltene obtained directly from the oil storage tank deposit (AS-D)tot solution (AS-D)tot + toluene [46% (v/v)] + n-heptane [(57% (v/v)] (AS-D)tol asphaltene obtained by precipitation with n-heptane (AS-E)tol fraction obtained by evaporation of the solvent after precipitation solution (AS-D)tol + dichloromethane [46% (v/v)] + n-heptane [(57% (v/v)] (AS-D)DCM asphaltene obtained by precipitation with n-heptane (AS-E)DCM fraction obtained by evaporation of the solvent after precipitation solution (AS-D)DCM + tetrahydrofurane [46% (v/v)] + n-heptane [(57% (v/v)] (AS-D)THF asphaltene obtained by precipitation with n-heptane (AS-E)THF fraction obtained by evaporation of the solvent after precipitation

1 2 3 4

Table 2. Expressions Used To Calculate the Average Molecular Parameters of Asphaltene Molecules17,a CT ) %C × MW/1200 HT ) %H × MW/100 Cs ) HT(HR/2 + Hβ/2 + Hγ/3) CA ) CT - Cs fa ) CA/CT

Cp ) HT(Har + HR/2) C I ) CA - C p Ra ) (CI + 2)/2 Φ ) Cp/CA n ) (HR + Hβ + Hγ)/HR

a C and H are the total carbon and hydrogen, respectively. H , H , T T R β and Hγ are the percentage of the aliphatic protons in R, β, and γ position obtained as the corresponding ratio of the 1H NMR spectrum integration area to the total integration area of the 1H NMR spectrum. Har is the ratio of the 1H NMR spectrum integration area of the aromatic proton to the total integration area of the 1H NMR spectrum. Cs is the total saturated carbon. CA is the total aromatic carbon. Cp is the peripheral carbon in condensed aromatic sheets. CI is the internal carbon atoms in condensed aromatic sheets. %C and %H are the weight percent of carbon and hydrogen obtained with the elemental analysis of asphaltene samples. MW is the molecular weight determined using LDI-TOF mass spectroscopy.

Table 3. Results of the Elemental Analysis of Asphaltene Fractions fraction elemental analysis (wt %) (AS-D)field (AS-D)tot (AS-E)tol (AS-E)DCM (AS-E)THF C H N S H/C

88.7 6.3 0.4 0.8 0.85

74.0 7.9 0.5 3.1 1.28

77.3 8.2 0.4 2.6 1.27

74.3 7.5 0.6 3.0 1.21

73.0 7.4 0.6 2.9 1.22

increasing size of asphaltene molecules. Grijalva-Monteverde et al.19 fractioned Mexican asphaltenes using mixtures of methylene chloride and n-pentane at different ratios of the two solvents. Results obtained showed that the polar asphaltene fraction is more hydrophilic, with a smaller molecular area, while the weakly polar fraction is more hydrophobic, with a higher molecular area. Fossen et al.20 fractioned asphaltenes originated from West Africa and the North Sea. They precipitated asphaltenes directly from the crude oil using different crude oil/n-pentane ratios. Their results showed that asphaltene fractions differ in solubility and interfacial properties. Results of these studies proved that the fractioning technique may be successfully used to investigate the mechanism of asphaltene flocculation. With this view, we have used the fractioning technique to characterize structural properties and the flocculation behavior of asphaltenes co-precipitated with paraffins from Hassi-Messaoud crude oil. The flocculation onset of all asphaltene samples was determined with UV-vis spectroscopy. Chemical properties of fractions were established with Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR), fluorescence spectroscopy, and laser desorption ionization-time of flight (LDI-TOF) mass spectroscopy. Results obtained allowed us to propose relationships between

the propensity to flocculate and the average chemical and physical properties of asphaltene and asphaltene fractions. 2. Materials and Methods The crude oil and deposit from the storage tank were from the Hassi-Messaoud petroleum field and were supplied by the Arzew Petroleum Refinery Complex, Algeria. The toluene, tetrahydrofurane, and dichloromethane were from Fisher Chemicals with 99% purity, while the n-heptane was from Prolabo Chemicals. Asphaltene Extraction. Asphaltenes were obtained either from the crude oil sampled directly from the well or from the storage tank deposit. The former was named (AS-D)field, and the latter was named (As-D)tot. Asphaltenes were precipitated by the addition of an excess of n-heptane with a 40:1 (volume/mass) ratio. The n-heptane/oil (or deposit) mixture was gently stirred during 24 h. The precipitated asphaltenes were filtered through a 0.45 µm pore size membrane and then washed with n-heptane to remove co-precipitated maltenes until the washing solvent was colorless. Fractioning of the (As-D)tot Asphaltenes. The (As-D)tot asphaltenes were fractionated according to the procedure summarized in Figure 1. The sample of (As-D)tot, 625 mg, was dissolved in 114.6 mL of toluene [0.625% (w/w)]. The solution was gently stirred until the total dissolution. Then, n-heptane was added to the concentration of 57% (v/v). The mixture was maintained at room temperature during 24 h. After vacuum filtration through a 0.22 µm pore size membrane, the supernatant was evaporated. The dry residue obtained after evaporation was named (As-E)tol and represented 70% (w/w) of (As-D)tot. The precipitate, (As-D)tol, was dissolved in dichloromethane to obtain a 0.625% (w/w) solution. The same procedure as this described above yielded the second asphaltene fraction (AS-E)DCM that represented 10% (w/w) and the precipitate (AS-D)DCM. The third fractioning was performed dissolving (AS-D)DCM in tetrahydrofuran and then using the same separation protocol. This operation yielded the (AS-E)THF fraction [8.5% (w/w) of (AS-D)tot] and (AS-D)THF [11.5% (w/w) of (ASD)tot]. It should be pointed out that the (AS-D)THF deposit was insoluble in nonpolar and mildly polar organic solvents. However, this fraction was soluble in highly polar solvents, such as N-methyl2-pyrrolidone. This result indicates that the (AS-D)THF fraction is free of mineral matter and contains highly polar compounds. The nomenclature of these fractions is summarized in Table 1. All fractioning was performed to obtain the concentration of asphaltenes in organic solvent of 0.625% (w/w) at the n-heptane/ organic solvent volume ratio of 4:3 [57% (v/v)]. In the present work, we studied chemical and physical properties of raw asphaltenes obtained from the crude oil and the storage tank deposit as well as of three fractions, namely, (AS-E)tol, (AS-E)DCM, and (AS-E)THF. Characterization of Asphaltene Fraction. IR. A FTIR spectrometer spectrum one of Perkin-Elmer with a spectral resolution of 4 cm-1 in the 700-4000 cm-1 spectral range was used. The asphaltene samples were introduced directly in the attenuated total reflectance (ATR) crystal. NMR. 1H NMR spectra were obtained using a 250 MHz spectrometer (Bruker 250, tube diameter of 5 mm and spectral width of 18 ppm with a pulse width of 3.5 µs, 30° flip angle) operated



Figure 2. FTIR spectra of asphaltenes and asphaltene fractions.

with recycle delay of 2 s with 400 scans. Chemical shifts (δ) are reported relative to tetramethylsilane (TMS) used as an internal standard. All spectra were recorded in deutered chloroform (CDCl3). Fluorescence Spectroscopy. A fluorescence spectroscopy setup Fluoromax-3 by Jobin Yvon Horiba was used in emission mode in the range from 350 to 700 nm with an excitation wavelength of 256 nm. The samples of asphaltenes in toluene at 5 mg L-1 were analyzed. Mass Spectroscopy. Molecular weights were determined by mass spectroscopy (Reflex VI Bruker Daltonics) using a LDI-TOF technique in a reflector mode and optimized nitrogen laser (λ ) 337 nm) energy at 80% of the full power. All fractions were dissolved in toluene at a concentration of 0.001 g L-1. Elemental Analysis. Carbon, hydrogen, nitrogen, and sulfur content were determined using a Thermo Finningan EA 1112 analyzer. Experimental accuracy of these measurements was of 0.2%. Results obtained with 1H NMR, LDI-TOF mass spectroscopy, and elemental analysis made it possible to calculate the aromaticity factor (fa), the average number of carbons per alkyl side chain (n), the aromatic ring number (Ra), and shape factor of the aromatic sheet (Φ). Calculations were performed with the formulas reported in the literature17,21,22 and given in Table 2. Determination of the Asphaltene Flocculation Onset. The flocculation onset was determined with the UV-vis spectroscopy as described in the literature.23–25 The spectrophotometer (Analytik Jena Specord 205) was operated at the wavelength of 750 nm. The quartz cells with 1 and 10 mm optical path were used as a function of the opacity of investigated solutions. Solutions of asphaltenes in toluene (0.1-10 g L-1) were titrated with n-heptane. The flocculation onset was assigned to the minimum optical density observed during titration.

3. Results and Discussion 3.1. Asphaltene Characterization. 3.1.1. Elemental Analysis. Results of C, H, N, and S analysis of asphaltenes and asphaltene fractions are given in Table 3. We note a big difference in the H/C ratio between asphaltenes extracted from the crude oil and from the storage tank deposit, indicating that (21) Seki, H.; Kumata, F. Energy Fuels 2000, 14, 980–985. (22) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. Energy Fuels 1995, 9, 225–230. (23) Bartholdy, J.; Andersen, S. I. Energy Fuels 2000, 14, 52–55. (24) Leon, O.; Contreras, E.; Rogel, E.; Dambakli, G.; Espidel, J.; Acevedo, S. Energy Fuels 2001, 15, 1028–1032. (25) Gonzalez, G.; Sousa, M. A.; Lucas, E. F. Energy Fuels 2006, 20, 2544–2551.

the less stable asphaltenes are less aromatic. The N and S content is substantially higher in the deposit asphaltenes. Therefore, the heteroatoms confer to asphaltenes the higher propensity to flocculate. It can be seen that the (AS-D)tot and the toluenesoluble fraction (AS-E)tol have the same value of the H/C ratio (1.28), while the dichloromethane, (AS-E)DCM, and tetrahydrofurane-soluble (AS-E)THF fractions display the H/C ratio of 1.22. Because the H/C ratio correlates well with the aromaticity (i.e., a lower H/C ratio implies a greater aromaticity26), these results suggest that the aromaticity of (AS-E)DCM and (AS-E)THF is higher than the aromaticity of (AS-D)tot and (AS-E)tol. (ASE)THF and (AS-E)DCM contain more nitrogen and sulfur atoms than the toluene-soluble fraction, (AS-E)tol. The highest content of sulfur corresponds to the fraction (AS-D)tot and may be explained by transfer of the sulfur compounds toward the insoluble fraction, (As-D)THF. 3.1.2. FTIR Analysis. FTIR spectra of asphaltenes and asphaltene fractions are shown in Figure 2. All spectra show a low intensity band in the range between 3500 and 3100 cm-1, indicating the presence of a small amount of N-H and O-H groups. The bands at 2935/1640 and 2845/1367 cm-1 correspond to CH3 and CH2 aliphatic groups. In the range between 1800 and 1640 cm-1, all spectra show a small peak around 1772 cm-1 correspondingtoalkane(R-COO-R′)oraromatic(Ar-COO-Ar′) ester groups. A distinct peak is observed at 1708 cm-1 (CdO signal of ketones, aldehydes, or carboxylic acids) for (AS-E)tol, (AS-E)DCM, and (AS-E)THF. In the case of the (AS-D)tot fraction, only a low-intensity peak is observed at this wavelength. The aromatic CdC band is observed at around 1600 cm-1, with the intensity depending upon the fraction aromaticity. Peaks differing in intensity are observed with all asphaltene fractions at 1263 cm-1 and may be attributed to the vibrational stretching of the C-O bond of the ester group, the carbonyl part of which was identified at 1772 cm-1. Nevertheless, this peak may correspond to ether or an amine group also. Finally, a sulfoxide group peak appears for all fractions at 1030 cm-1, with different intensities in agreement with elemental analysis results. 3.1.3. Molecular-Weight Distribution by LDI-TOF Mass Spectroscopy. Mass spectroscopy experiments were performed with highly diluted asphaltene solutions (∼0.001 g L-1 of toluene) to reduce aggregation of asphaltenes. Figure 3 shows (26) Bouhadda, Y.; Bendedouch, D.; Sheu, E.; Krallafa, A. Energy Fuels 2000, 14, 845–853.


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Figure 3. LDI-TOF spectrum of asphaltene fractions: (a) (AS-D)field, (b) (AS-D)tot, (c) (AS-E)tol, (d) (AS-E)DCM, and (e) (AS-E)THF.

the LDI-TOF spectra corresponding to asphaltenes and asphaltene fractions. Mass experiments indicate the monomodal distribution with the maximum of ion abundance found at around m/z 500. This result was found with all samples obtained from the storage tank deposit. Therefore, the fractioning does not change the average molar mass of asphaltenes. On the other hand, it must be pointed out that the spectrum corresponding to the asphaltenes extracted from the crude oil, (AS-D)field, is quite different. In this case, two maxima were found: one corresponding to the mass of 1000 and the second corresponding to the mass of 2000. The general pattern of obtained spectra is very similar to results published in the literature.27,28 3.1.4. Fluorescence Spectroscopy Analysis. Fluorescence

emission spectra of asphaltenes and asphaltene fractions presented in Figure 4 are similar to those found in the literature.17,29–31 The peak location (Table 4) exhibits a shift that can be explained (27) Rizzi, A.; Cosmina, P.; Flego, C.; Montanari, L.; Seraglia, R.; Traldi, P. J. Mass Spectrom. 2006, 41, 1232–1241. (28) Acevedo, S.; Gutierrez, L. B.; Negrin, G.; Pereira, J. C.; Mendez, B.; Delolme, F.; Dessalces, G.; Broseta, D. Energy Fuels 2005, 19, 1548– 1560. (29) Ralston, C. Y.; Kirtley, S. M.; Mullins, O. C. Energy Fuels 1996, 10, 623–630. (30) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103, 11237– 11245. (31) Badre, S.; Goncalves, C. C.; Norinaga, K.; Gustavson, G.; Mullins, O. C. Fuel 2006, 85, 1–11.



Figure 4. Emission fluorescence spectra of asphaltenes and asphaltene fractions: (s) (AS-D)tot, (-) (AS-e)tol, (- · -) (AS-E)DCM, (- - -) (ASE)THF, and ( · · · ) (AS-D)fields. Table 4. Fluorescence Analysis of Asphaltene Fractionsa fractions

peak position (nm)

(AS-D)field (AS-D)tot (AS-E)tol (AS-E)DCM (AS-E)THF

515 461 457 473 404

linear aromatic compounds benzene naphthalene anthracene tetracene pentacene

shortest emission wavelength (nm)31 280 320 390 485 590

a The location of the emission fluorescence peak compared to the shortest wavelength emission for a series of polyaromatics.

in terms of the rate of polyaromatic structure condensation. Accordingly, (AS-E)THF is the least condensed aromatic system among the asphaltene fractions studied. The analysis of the present results in terms of literature data of the shortest emission wavelength of a series of polyaromatic compounds31 (Table 4) suggests that asphaltene samples contain 3-4 fused aromatic rings. According to this analysis, (AS-D)field asphaltenes contain more fused rings than deposit asphaltenes and corresponding fractions. 3.1.5. 1H NMR Analysis. 1H NMR spectra are presented in Figure 5a. Four signals are observed at 0-1.0, 1.0-2.0, 2.0-4.0, and 6.5-9.0 ppm, corresponding respectively to aliphatic protons in R (HR), β (Hβ), and γ (Hγ) positions to aromatic ring and aromatic protons (Har). The atomic abundances of the HR, Hβ, Hγ, and Har obtained from the corresponding integrated band areas are reported in Table 5. These results indicate that the aromatic hydrogen percentage in (AS-D)tot and (AS-E)tol is lower than in (AS-E)DCM and (AS-E)THF. Therefore, the use of polar or polarizable solvents enhances solubilization of aromatic moieties. Asphaltenes (AS-D)field display significantly higher content of the aromatic hydrogen than asphaltenes (AS-D)tot. 3.1.6. Structural Analysis. The structural parameters calculated using equations given in Table 2 are reported in Table 6. These results suggest that (AS-D)tot, (AS-E)tol, and (AS-E)DCM have approximately four fused aromatic rings, whereas (ASE)THF has three fused aromatic rings per molecule. The similar conclusion was obtained with fluorescence spectroscopy measurements. The aromaticity factor is of 0.54 for (AS-E)DCM and (AS-E)THF and is higher than observed with (AS-E)tol and (ASD)tot. These results are in good agreement with the H/C ratio value determined by elemental analysis. It is also observed that the highest average number of carbons per alkyl side chain is observed with (As-D)tot, which indicates that the long-chain molecules were concentrated in the (AS-D)THF insoluble fraction.

Therefore, the increasing lateral chain length enhances the asphaltene propensity to flocculate. Indeed, when the mean number of carbons per alkyl side chain is 4.37 with (AS-D)tot, the values found with fractions are within the interval 2.50-3.79. It is interesting to observe that the n value of (AS-D)field, 3.78, is significantly lower than the corresponding value of (AS-D)tot. The fact that the storage tank deposit is enriched in long-chain moieties confirms the idea that the side-chain length is related to the flocculation propensity of asphaltenes. Interesting results were obtained analyzing the shape factor of the aromatic sheet, Φ, that indicates the rate of condensation of polyaromatic structures. The value of Φ corresponding to the model compounds, such as naphthalene, anthracene, pyrene, and ovalene, is respectively 0.80, 0.71, 0.63, and 0.4417 and decreases with an increasing condensation. Accordingly, data from Table 5 indicate that polyaromatic structures are less condensed in (ASD)tot than in (AS-D)field asphaltenes. Therefore, asphaltenes with a low condensation rate are less stable and deposit spontaneously in the storage tank. On the other hand, the Φ value of fractions obtained from the deposit asphaltenes increases in the order of increasing extraction stages. This finding suggests that the consecutive asphaltene extractions concentrate the condensed polyaromatics in the deposit. Results obtained from this analysis are in agreement with the florescence spectroscopy results. 3.2. Asphaltenes Flocculation Onset. Figure 6a shows changes of absorbance at 750 nm wavelengths during titration with n-heptane of samples dissolved in toluene at the concentration of 10 g L-1. The initial decrease of absorbance is due to the dilution effect, whereas the absorption enhancement reflects the increasing contribution of light scattering from growing particles.32,33 The volume of n-heptane corresponding to the minimum of absorbance is usually identified as the flocculation onset. The results are reported in Table 7. These experiments were repeated using asphaltene solution in toluene at different dilutions. Results are shown in Figure 7 displaying the plots of (n-heptane volume)/(asphaltene mass) ratio at flocculation onset versus the (toluene volume)/(asphaltene mass) ratio with all asphaltene samples studied. As observed previously,33,34 the flocculation onsets are linear within this figure. The solubility parameters of asphaltene samples (δas) were calculated from flocculation onset data using the method proposed by Donnagio et al.35–37 Resulting values are listed in Table 7. We note a significant difference between the flocculation onset of (AS-D)tot and (AS-D)field. In the case of (AS-D)tot, the lower stability may be related to the lower aromaticity and the higher mean chain length of the side aliphatic chains. The flocculation onset of fractions precipitated from different solvents does not depend upon the solvent polarity only. While (AS-E)tol and (AS-E)DCM have nearly the same flocculation onset, the flocculation onset of (AS-E)THF is significantly lower. On the other hand, dipole moments of DCM and THF are similar and higher than the dipole moment of toluene. We have studied the correlation between the flocculation onset and structural parameters. We observed that the aromaticity of asphaltenes increases their stability. The most striking example comes from the comparison of Ra of asphaltenes obtained from field oil and the storage tank deposit. The asphaltenes obtained from the crude oil contain 4.4 times more aromatic rings than (32) Hotier, G.; Robin, M. ReV. Inst. Fr. Pet. 1983, 38 (1), 101. (33) Oh, K.; Ring, T. A.; Deo, M. D. J. Colloid Interface Sci. 2004, 271, 212. (34) Andersen, S. I. Energy Fuels 1999, 13, 315–322. (35) Donaggio, F.; Correra, S.; Lockhart, T. P. 3rd International Conference on Petroleum Phase Behavior and Fouling, New Orleans, LA, March 10-14, 2002.


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Figure 5. 1H NMR spectra of asphaltenes: (a) (AS-D)field, (b) (AS-D)tot, (c) (AS-E)tol, (d) (AS-E)DCM, and (e) (AS-E)THF. Table 5. Atomic Percentage of the Aliphatic and Aromatic Hydrogen in Asphaltenes and Asphaltene Fractions Obtained with 1H-RMN Spectraa fraction

HR (%)

Hβ (%)

Hγ (%)

Har (%)

Hal (%)


20.70 19.59 22.89 22.78 31.43

48.28 51.55 48.19 44.31 35.71

9.20 14.43 15.66 12.66 11.43

21.82 14.43 13.26 20.25 21.43

78.18 85.57 86.74 79.75 78.57

a H , H , and H corresponds respectively to aliphatic protons in R, R β γ β, and γ positions.

the deposit asphaltenes. This means that the asphaltene fractions unstable with respect to the flocculation contain less aromatic moieties. The same tendency is observed with fractioned asphaltenes also. The general tendency is shown in Figure 8. On the other hand, we observed that the asphaltene stability depends upon n, the number of carbon atoms in the side aliphatic chains. Indeed, the n value corresponding to the crude oil

Table 6. Average Molecular Parameters of the Asphaltenes and Asphaltene Fractions fraction

Cs

Car

fa


23.66 15.79 17.43 14.61 14.31

50.26 14.80 16.10 17.42 17.06

0.68 0.48 0.48 0.54 0.54

Cp

Ci

Ra

φ

n

n/Ra

20.27 29.99 15.99 0.40 3.78 0.24 9.47 5.33 3.67 0.64 4.37 1.19 10.57 5.53 3.77 0.66 3.79 1.01 12.24 5.18 3.59 0.70 3.50 0.97 14.21 2.84 2.42 0.83 2.50 1.03

asphaltenes is of about 16% lower than in the case of the storage tank asphaltenes. Therefore, the less stable fraction of asphaltenes contains the longest side chains. Figure 9 shows changes of the flocculation onset with an increasing value of n. We conclude that both short and long side aliphatic chains favor (36) Cimino, R.; Correra, S.; Del Bianco, A.; Lockhart, T. P. In Asphaltenes Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995.



Figure 6. Flocculation onset determination using UV-vis. Absorbance variation of asphaltenes and asphaltene fractions in toluene (10 g L-1) versus n-heptane added volume at 750 nm: (9) (AS-D)field, (b) (ASD)tot, (2) (AS-E)tol, (1) (AS-E)DCM, and ([) (AS-E)THF.

Figure 8. Variation of asphaltene flocculation onset versus the number of aromatic rings (Ra).

Table 7. Flocculation Onset of Asphaltenes and Asphaltene Fractionsa fraction

onset (% vol.)

R2

slope

δas (MPa)1/2


50.0 32.0 41.5 42.4 37.0

0.999 56 0.999 75 0.999 12 0.999 21 0.998 73

1.6040 0.5264 0.7300 0.7949 0.6367

19.6 20.5 20.2 20.1 20.3

a Results of the linear regression, slope, and regression coefficient of flocculation data reported in Figure 7 as well as solubility parameters of asphaltene fractions.

Figure 9. Variation of asphaltene flocculation onset versus the number of carbon in the side aliphatic chain (n).

idea that Ra and n play a crucial role in the mechanism of asphaltene aggregation. 4. Conclusion

Figure 7. Results of asphaltene flocculation experiments. [n-heptane (mL)/asphaltenes (g)] versus [toluene (mL)/asphaltenes (g)]: (9) (ASD)field, (b) (AS-D)tot, (2) (AS-E)tol, (1) (AS-E)DCM, and ([) (AS-E)THF.

aggregation. The problem of aliphatic chain interaction and their influence on the flocculation mechanism was recently discussed in the literature.7,15 We combined these two structural parameters in the corresponding ratio (n/Ra). Figure 10 shows that there exists a roughly linear relationship between the flocculation onset and this parameter. Results presented in Figure 10 confirm the

(37) Mutelet, F.; Ekulu, G.; Solimando, R.; Rogalski, M. Energy Fuels 2004, 18, 667–673.

We observed important differences between flocculation onset and chemical properties of asphaltenes obtained from the crude oil, (AS-D)field, and from the deposit, (AS-D)tot. The molar mass of the unstable fraction of asphaltenes, (AS-D)tot, was of about 500 as compared to higher values, 1000-2000, observed in the case of (AS-D)field. As expected, (AS-D)field displays a much higher flocculation onset. They present more aromatic character with the lower length of the side aliphatic chains, n. The value of n influences the stability of asphaltenes. As shown in Figure 9, both very short and very long side chains lower asphaltene stability. The aromatic character of asphaltenes as expressed with the number of aromatic cycles in the molecule, Ra, favors their stability, which is evident with results presented in Figure 8. Finally, we observed a roughly linear correlation between the flocculation onset and the ratio, n/Ra, as shown in Figure 10. The flocculation onset was found similar to all asphaltene fractions, except a significantly lower value observed with (ASE)THF. Therefore, the aggregation of asphaltene fractions obtained by consecutive extractions with solvents differing in


Daaou et al.

such as N-methyl pyrrolidone, and nonsoluble in mildly polar or nonpolar solvents. We hypothesize that (AS-E)THF contains components enabling solubilization of (AS-D)THF via colloidal sterical protection. In our opinion, this issue is important to the comprehension of the mechanism of asphaltene flocculation. Our results indicate that it is impossible to understand the flocculation mechanism considering asphaltenes as one pseudo-component. Indeed, the flocculation of about 88% (w/w) of asphaltenes contained in the deposit studied is related to the solvent solubility parameter (effect of dispersion forces). The flocculation of “nonsoluble”, highly polar fraction, 12% (w/w) of asphaltenes, depends upon the presence of molecules conferring the sterical protection (colloidal stability).

Figure 10. Variation of asphaltene flocculation onset versus the structural parameter (Ra/n).

polarity follows a similar mechanism. It must be pointed out that the lowest value of the flocculation onset was observed with asphaltenes (AS-D)tot. Indeed, these asphaltenes contain the fraction (AS-D)THF that is soluble in very polar solvents,

Acknowledgment. M.D. acknowledges the financial support of the “Ministe`re de l’Enseignement Supe´rieure et de la Recherche Scientifique” and the hospitality of the University Paul Verlaine Metz. The authors thank Complexe de Raffineries de Sonatrach for supplying samples of the crude oil and the storage tank deposit. We are grateful to Dr. N. Oget, Universite´ de Metz, for technical and scientific assistance in 1H NMR analysis of asphaltene fractions. EF800078U

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