Structure Representation of Asphaltene GPC Fractions Derived from

Nov 24, 2005 - Center, Kuwait Institute for Scientific Research, Post Office Box 24885, Safat 13109, Kuwait. ReceiVed May 4, 2005. ReVised Manuscript ...
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Energy & Fuels 2006, 20, 231-238

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Structure Representation of Asphaltene GPC Fractions Derived from Kuwaiti Residual Oils Fatima A. Ali,*,† Narjes Ghaloum,‡ and Andre Hauser† Central Analytical Laboratory, and Petroleum Refining Department, Petroleum Research and Studies Center, Kuwait Institute for Scientific Research, Post Office Box 24885, Safat 13109, Kuwait ReceiVed May 4, 2005. ReVised Manuscript ReceiVed October 26, 2005

Two asphaltenes obtained from a residual oil before and after hydrotreatment were subjected to preparative GPC. Selected GPC fractions were analyzed by NMR and XRD to derive structural parameters to investigate whether there is a relationship between the molecular size of the asphaltene fraction and its structural features. Under the GPC experimental conditions (∼5 wt % THF solution), some eluting species are aggregates rather than monomeric molecules. Nevertheless, a correlation between the molecular size of the GPC fraction and its structure was found. The higher the molecular weight (MW), the more aromatic and condensed is the fraction. Moreover, the asphaltene fractions with the highest MW of both residues, before and after hydrotreating, show very similar structural features. This fact suggests that during hydrotreatment the low MW asphaltenes are converted to distillates, whereas the higher MW fractions (>6000 Da) stay unaltered and can cause severe coking in the downstream processes because of the highly refractory asphaltenes, which are still present. Structure presentations derived from NMR and XRD data demonstrate that the higher MW fractions (≈5250 Da) of both residues consist of clusters of 4-5 aromatic rings, which are interlinked by aliphatic chains of 9-11 carbon atoms.

1. Introduction Asphaltenes as the heaviest and least reactive components in crude oil and oil residues from distillation units have received great attention by petroleum chemists over the past decade1-9 because of the fact that asphaltens are held accountable for a number of difficulties faced during residue upgrading. It is a matter of fact that heteroatoms such as S, N, V, and Ni are * To whom correspondence should be addressed. Telephone: ++9654836100. Fax: ++965-4815197. E-mail: [email protected]. † Central Analytical Laboratory. ‡ Petroleum Refining Department, Petroleum Research and Studies Center. (1) George, M.; Al-Siri, M.; Khan, Z. H.; Ali, F. A. Differences in average chemical structure of asphaltene fractions separated from feed and product oils of a mild thermal processing reaction. Energy Fuels 2005, 19, 15981605. (2) Stanislaus, A.; Hauser, A.; Marafi, M. Investigation of mechanism of sediment formation in residual oil hydrocracking process though characterization of sediment deposits. Catal. Today, in press. (3) Hauser, A.; Marafi, A.; Stanislaus, A. Relation between feed quality and coke formation in a three-stage atmospheric residue desulfurization (ARDS) process. Energy Fuels 2005, 19, 544-553. (4) Gray, M. R. Consistency of asphaltene chemical structures with pyrolysis and coking behavior. Energy Fuels 2003, 17, 1566-1569. (5) Ancheyta, J.; Centeno, G.; Trejo, F.; Marroquin, G. Changes in asphaltene properties during hydrotreating of heavy crudes. Energy Fuels 2003, 17, 1233-1338. (6) Higashi, H.; Takashi, T.; Kai, T. The effect of start-up conditions on deactivation of hydrotreating catalyst for heavy residue with high asphaltene content. Catal. SurV. Fap. 2002, 5, 111-119. (7) Seki, H.; Yoshimoto, M. Deactivation of hydrodesulfurization catalysts in two-stage reside desulfurization process (part 3). Influence of asphaltene content of feedstock and hydrodesulfurization temperature at second stage. Sekiyu Gakkaishi 2001, 44, 102-108. (8) Bartholdy, J.; Andersen, S. I. Changes in asphaltene stability during hydrotreating. Energy Fuels 2000, 14, 52-55. (9) Absi-Halabi, M.; Stanislaus, A.; Owaysi, F.; Khan, Z. H.; Diab, S. Hydroprocessing of heavy residues: Relation between operating temperature, asphaltene conversion and coke formation. Stud. Surf. Sci. Catal. 1998, 53, 201-212.

concentrated in asphaltenes10 and that asphaltenes are prone to precipitate during heavy oil upgrading.11-14 These properties make them responsible for catalyst deactivation in the refining process and equipment fouling in the downstream units.2,3,9,15-18 Asphaltenes are dark amorphous solids, which by definition are insoluble in n-heptane but soluble in toluene.19 In solid form, asphaltenes build a semicrystalline structure (stacks) that shows an X-ray scattering.20-23 In oil, asphaltenes occur in a micellar structure where the asphaltenic core is surrounded by resin and (10) Dembirbas, A. Physical and chemical characterizations of asphaltenes from different sources. Pet. Sci. Technol. 2002, 20, 485-495. (11) Wiehe, I. A.; Kennedy, R. J.; Dickakain, G. Fouling of nearly incompatible oils. Energy Fuels 2001, 15, 1057-1058. (12) Wiehe, I. A.; Kennedy, R. J. The oil compatibility model and crude oil incompatibility. Energy Fuels 2000, 14, 56-59. (13) Wiehe, I. A.; Kennedy, R. J. Application of the oil compatibility model to refinery streams. Energy Fuels 2000, 14, 60-63. (14) Storm, D. A.; Decanio, S. J.; Sheu, E. Y. in Asphaltene Particles in Fossil Fuel Exploration, RecoVery and Refining Processes, Sludge Formation During HeaVy Oil ConVersion; Sharma, M. K., and Yen, T. F., Eds.; Plenum Press: New York, 1994; p 81. (15) Seki, H.; Yoshimoto, M. Deactivation of HDS catalyst in two-stage RDS process. II Effect of crude oil and deactivation mechanism. Fuel Process. Technol. 2001, 69, 229-238. (16) Hauser, A.; Marafi, A.; Stanislaus, A.; Al-Adwani, A. Initial coke deposition on hydrotreating catalysts: Part II. Structure elucidation of initial coke on hydrodemetallation catalysts. Fuel 2005, 84, 259-269. (17) Marafi, A.; Stanislaus, A.; Absi-Halabi, M.; Hauser, A.; Matsushita, K. An investigation of the deactivation behavior of industrial Mo/Al2O3 and Ni-Mo/Al2O3 catalysts in hydrotreating Kuwait atmospheric residue. Pet. Sci. Technol. 2005, 2, 385-408. (18) Matsushita, K.; Marafi, A.; Hauser, A.; Stanislaus, A. Relation between relative solubility of asphaltenes in product oil and coke deposition in residue hydroprocessing. Fuel 2004, 83, 1669-1674. (19) Institute of Petroleum. Methods for Analysis and Testing of Petroleum and Related Products; Wiley and Sons: London, U.K., 1996; Vols. 1 and 2; Petroleum products, lubricants, and fossil fuels. 1996 Annual Book of ASTM Standards, Vols. 5.01 and 5.02; American Society for Testing and Materials: Philadelphia, PA, 1996.

10.1021/ef050130z CCC: $33.50 © 2006 American Chemical Society Published on Web 11/24/2005

232 Energy & Fuels, Vol. 20, No. 1, 2006

Ali et al. Table 1. Characteristics of AR and VR

Figure 1. GPC fractions obtained from asphaltens of two residual oils: (above) AR/AS and (below) VR/AS.

aromatic molecules12,24,25 that prevent the asphaltene molecules from separation from the oil matrix. Strausz et al.26 could show that monomeric molecules of asphaltenes in >5% solutions possess a high tendency to form aggregates. Because of this fact, it is most likely that the asphaltenic core does not contain a monomeric molecule but an aggregate of monomers. Therefore, studies on the molecular size of asphaltenes have shown that the molecular weight (MW) of asphaltenes spans over a region as wide as 30 000 Da.27,28 Typical MW distributions for asphaltenes separated from atmospheric and vacuum residues of Kuwaiti crude is shown in Figure 1. On the basis of average structural parameters derived from elemental analysis, MW determination, and NMR measurements, many researchers have proposed an average structure of an asphaltene molecule.3,16,29-33 Some caution, however, (20) Tanaka, R.; Sato, E.; Hunt, J. E.; Winans, R. E.; Sato, S.; Takanohashi, T. Characterization of asphaltene aggregates using X-ray diffraction and small-angle X-ray scattering. Energy Fuels 2004, 18, 11181125. (21) Bansal, V.; Patel, M. B.; Sarpal, A. S. Structural aspects of crude oil derived asphaltenes by NMR and XRD and spectroscopic techniques. Pet. Sci. Technol. 2004, 22, 1401-1426. (22) Christopher, J.; Sarpal, A. S.; Kapur, G. S.; Krishna, A.; Tyagi, B. T.; Jain, M. C.; Jain, S. K.; Bhatnagar, A. K. Chemical structure of bitumenderived asphaltenes by nuclear magnetic resonance spectroscopy and X-ray diffraction. Fuel 1996, 75, 999-1008. (23) Schwager, I.; Farmanian, P. A.; Kwan, J. T.; Weinberg, V. A.; Yen, T. F. Characterization of microstructure and macrostructure of coal-derived asphaltenes by nuclear magnetic resonance and X-ray diffraction. Anal. Chem. 1983, 55, 42-45. (24) Andersen, S. I.; Christensen, S. D. The critical micelle concentration of asphaltenes as measured by calometry. Energy Fuels 2000, 14, 38-42. (25) Evdokimov, I. N.; Eliseev, N. Y.; Akhmetov, B. R. Initial stages of asphaltene aggregation in dilute crude oil solution: Studies of viscosity and NMR relaxation. Fuel 2002, 82, 817-823. (26) Strausz, O. P.; Peng, P.; Murgich, J. About the colloidal nature of asphaltenes and the MW of covalent monomeric units. Energy Fuels 2002, 16, 809-822. (27) Batholdy, J.; Lauridsen, R.; Mejlholm, M.; Andersen, S. I. Effect of hydrotreatment on product sludge stability. Energy Fuels 2001, 15, 10591062. (28) Acevedo, S. A.; Escobar, G.; Ranaudo, M. A.; Rizzo, A. Molecular weight properties of asphaltenes calculated from GPC data for octylated asphaltenes. Fuel 1998, 77, 853-858. (29) Ibrahim, Y. A.; Abdulhameed, M. A.; Al-Sahhaf, T. A.; Fahim, M. A. Structural characterization of different asphaltenes of Kuwaiti origin. Pet. Sci. Technol. 2003, 21, 825-837.

test

unit

AR

VR

density at 15 °C density at 65 °C gravity total sulfur total nitrogen conradson carbon residue asphaltenes maltenes (oil) molecular weight carbon hydrogen vanadium nickel high-temperature-simulated distillation IBP 5 wt % recovered at 10 wt % recovered at 50 wt % recovered at 80 wt % recovered at FBP total recovery at 750 °C

g/mL g/mL API wt % wt % wt % wt % wt % wt % wt % wt % µg/g µg/g

1.0414 1.0091 8.6 4.28 0.60 17.00 14.49 85.51 560 84.43 11.00 69 21

1.0031 0.9703 9.44 1.26 0.41 12.07 9.48 90.52 840 86.44 11.90 ND ND

°C °C °C °C °C °C wt %

270.5 345.0 379.5 542.5 691.0 750.0 88.0

372.7 491.2 514.5 608.9 709.6 750.0 83.6

should be applied while describing a structural diverse and in molecular size wide-spanning material like asphaltenes by only a single structure. Therefore, we think it is worthwhile to verify the structural homogeneity of the asphaltenes obtained from residual oils. In view of the above and the importance of asphaltenes in catalytic residue upgrading, two asphaltens isolated from typical Kuwaiti residual oils were studied by NMR and XRD. Before subjecting the asphaltenes to the detailed analysis, they were separated into narrow MW fractions by preparative GPC. In the course of this work, special emphasis was placed on finding a relationship between the molecular size and average structural parameters. 2. Experimental Section 2.1. Asphaltenes. Asphaltenes were isolated from two residues, one obtained from the atmospheric distillation of Kuwaiti crude (AR/AS) and another one obtained from the vacuum distillation of a hydrodesulfurized feed that is used as a decoker feedstock (VR/ AS). The asphaltenes were separated from the residual oils following the IP standard IP-143/90 and a method laid down in the American Society for Testing Materials.19 The recovery of asphaltenes and maltenes was better than 99.0%. Main characteristics of both residual oils are compiled in Table 1. 2.2. Gel-Permeation Chromatography (GPC). 2.2.1. Preparative GPC. The preparative separation of the asphaltenes into several fractions according to their MW was carried out on a Waters Associate’s liquid chromatograph equipment34,35 operated by Millenium 2010 software. The chromatograph consisted of a Waters Associate’s solvent delivery system 510, Waters Associate’s (30) Acevedo, S. A.; Escobar, O.; Echevarria, L.; Gutierres, L. B.; Mendes, B. Structural analysis of soluble and insoluble fractions of asphaltenes isolated using the PNP method. Relation between asphaltene structure and solubility. Energy Fuels 2004, 18, 305-311. (31) Strausz, O. P.; Mojelsky, T. W.; Lown, E. The molecular structure of asphaltnes: An unfolding story. Fuel 1992, 17, 1355-1363. (32) Storm, D. A.; Edwards, J. C.; DeCanio, S. J.; Sheu, E. Y. Molecular representation of Ratawi and Alaska north slope asphaltenes based on liquidand solid-state NMR. Energy Fuels 1994, 8, 561-566. (33) Sheremata, J. M.; Gray, M. R.; Deiiman, H. D.; McCaffrey, W. C. Quantitative molecular representation and sequential optimization of Athabasca asphaltens. Energy Fuels 2004, 18, 1377-1384. (34) Ali, F.; Khan, Z. H.; Ghaloum, N. Structural studies of vacuum gas oil distillate fractions of Kuwaiti crude oil by nuclear magnetic resonance. Energy Fuels 2004, 18, 1798-1805. (35) Ghaloum, N.; Michael, G.; Khan, Z. H. Separation of sulfur compounds from vacuum gas oil distillates by liquid exchange chromatography. J. Liq. Chromatogr. Relat. Technol. 2002, 25, 1409-1420.

Asphaltene GPC Fractions DeriVed from Kuwaiti Residual Oils differential refractromenter 410, and Waters Associate’s prepacked ultra-stryragel columns (diameter, 19 mm; length, 300 mm; pore sizes, 1000 and 500 Å; particle size, 7 µm). Analar-grade chloroform (Merck) as a mobile phase was used. The Waters Organic Solvent Clarification Kit (WAT 200543) was used to remove microparticulate matter above 0.45 µm. The separation efficiency of the columns was sufficient enough to separate the materials having a MW up to 10 000 Da. The instrument was operated at ambient temperature, while the refractrometer was maintained at 35 ( 1 °C. The flow rate of the solvent was maintained at 0.5 mL/min. 2.2.2. Analytical GPC. The MW determinations of the subfractions obtained from the preparative GPC were carried out on the same equipment.35,36 Waters Associate’s prepacked ultra-styragel columns (diameter, 7.2 mm; length, 300 mm; pore sizes, 1000 and 500 Å; particle size, 7 µm) and HPLC-grade tetrahydrofuran (Merck) as a mobile phase were used. The Waters Organic Solvent Clarification Kit (Wat 200543) was used to remove microparticulate matter above 0.45 µm. The instrument was operated at ambient temperature, while the refractrometer was maintained at 35 ( 1 °C. The flow rate of the solvent was maintained at 1.0 mL/min. All samples were dissolved in THF and filtered through 0.45 µm fluoropore filter paper (Millipore Corp.) prior to injection (5 wt % solution). Injection volumes were between 100 and 150 µL. Millenium 2010 Chromatography Manager software was used to control the GPC modular system, and collect, save, and retrieve the collected data at a later stage. 2.2.3. Calibration of Analytical GPC Column. A calibration curve was constructed from the narrow MW fractions of petroleum products in the range of 280-2300 Da36 and polystyrene in the range of 68 000 and 110 000 Da. Solutions of 5 wt % were made and filtered, and 150-200 µL injection volumes were used for all standards. On the analytical GPC columns, all of the fractions were well-resolved. 2.3. Elemental Analysis. Elemental analysis was carried out on a CE Instruments model EA 1110. Total nitrogen and sulfur in low concentrations were determined by an elemental analyzer from ANTEK model ANTEK 7000. Appropriate standard reference samples were used to prepare the calibration curves. 2.4. NMR Analysis. All NMR spectra were recorded using a Bruker AVANCE300 spectrometer operating at 75.47 MHz for 13C and 300 MHz for 1H. 1H measurements were carried out with a spectral width of 4.5 kHz, a pulse angle of 18 µs (90°), and a delay time of 3s. Parameters for 13C inverse-gated decoupling measurement were spectral widths, 20 kHz; pulse widths, 6 µs (90°); and pulse delay, 20 s. Samples for 1H NMR measurements were prepared by adding 0.5 mL of CDCl3 solvent to 5-10 mg of the asphaltene fraction in a 5-mm tube. Tetramethylsilane (TMS) was used as an internal reference. For 13C measurement, 1.5 mL of CDCl3 was added to 150-250 mg of the asphaltene fraction with 15-20 mg of relaxation reagent Cr(acac)3 in 10-mm tubes. The relaxation reagent was added to cause an appreciable reduction of the spin-lattice relaxation time, T1, and consequently, to the delay time between the pulse cycles. 2.5. X-ray Diffraction (XRD). XRD data was collected on a Philips X-pert PW 1830 diffractometer in Bragg Brentano reflection geometry, applying the following conditions: starting angle, 2Θ ) 6°; end angle, 2Θ ) 80°; step size, 0.02°; and time/step, 1.2 s. A copper (Cu) tube was used where the wavelength of the incident radiation was 1.540 56 Å. Curve deconvolution was performed using the Profit software program applying the asymmetrical function.

3. Results 3.1. Preparative GPC Separation of Asphaltenes. The asphaltenes isolated from two residues, AR/AS and VR/AS, (36) Khan, Z. H.; Hussain, K. Non-destructive analysis of crude oil by GPC. Fuel 1989, 68, 1198-1202.

Energy & Fuels, Vol. 20, No. 1, 2006 233 Table 2. GPC Fraction Yields Obtained from AS of AR and VR fraction number

AR/AS (wt %)

VR/AS (wt %)

1 2 3 4 5 6 7 8 9 10 11 12 13

0.29 4.11 12.39 16.71 18.83 18.30 11.43 8.04 4.04 1.81 0.79 0.38 0.36

1.01 4.34 10.50 13.28 16.22 16.88 13.74 10.22 6.21 3.33 0.90 0.30 0.38

recovery

97.48

97.31

were further fractionated into 11 subfractions according to their molecular size using preparative GPC columns. The fractions were collected into 5 mL of solvent (CHCl3), each after a void volume of 95 mL of pure solvent was eluted. Repetitions (68) were carried out to collect enough material for further analysis. The solvent was removed from the eluted fractions by a rotary evaporator; the residual was transferred to a preweighed beaker, dried, and weighed to a constant weight to the nearest 0.1 mg. The fraction yields and the total recovery of the preparative GPC separation of asphaltenes from both residues, AR/AS and VR/AS, are presented in Table 2. The recovery of the material (>97%) was excellent in both cases, indicating that negligible amounts of asphaltenes are retained on the GPC column. The largest quantities (AR/AS, 19 wt %; VR/AS, 17 wt %) were eluted in subfraction numbers 5 and 6, respectively. All of the subfractions were analyzed for average MWs and elemental analysis. NMR and XRD data were also obtained for selected subfractions to derive their average structural parameters. 3.2. Average Molecular Presentation of Asphaltene GPC Fractions. 3.2.1. Average Molecular Formula. On the basis of the elemental analysis data and average MWs of the original asphaltenes, AR/AS and VR/AS, and their GPC fractions, the average molecular formulas have been calculated applying the methodology described by Khan and Hussain.36 The average MW data (Table 3) may indicate that, under the GPC experimental conditions applied, some eluting species are rather aggregates than monomeric asphaltene molecules. The latter may especially hold for the lower MW region of the GPC chromatogram. Strausz at al.26 showed for Athabasca asphaltenes that about 80% of the low MW fractions dissociated to monomeric covalent molecules of less than 1000 Da. Table 2 demonstrates that the GPC fractions with a MW of 6) C in branching position of a terminal i-propyl group C in CH3 branches C in terminal position of n-alkyl chains (n > 6) total C in CH3 groups

NMR spectra (Figure 2) of the two asphaltenes, AR/AS and VR/AS, and their asphaltene GPC fractions 2, 4, 6, and 8. The integral area of 1H, 13C signals in defined chemical-shift regions (38) Petrakis, L.; Allen, D. NMR for Liquid Fossil Fuels; Elsevier: Amsterdam, The Netherlands, 1987.

(Tables 4 and 5) was used to quantify hydrogen and carbon in distinct structural units of the studied asphaltenes and their fractions. The integral values normalized to 100% obtained from the predefined chemical-shift regions are shown in Table 4 (AR/ AS) and Table 5 (VR/AS). On the basis of these NMR data combined with the MW and elemental analysis data of the GPC

Asphaltene GPC Fractions DeriVed from Kuwaiti Residual Oils Table 5. 1H and

13C

Energy & Fuels, Vol. 20, No. 1, 2006 235

NMR Data of GPC Fractions Obtained from AS of VR chemical shift (ppm)

spectral parameter

GPC fraction number

total AR/AS

2

4

6

8

6.3-9.3 0.5-4.5 1.9-4.5 1.0-1.9 0.5-1.0

5.85 94.15 8.14 61.58 24.43

8.04 91.96 8.73 61.65 21.58

6.43 93.57 25.24 50.92 17.41

4.60 95.40 20.11 52.82 22.45

5.00 95.00 15.83 60.26 18.91

100-170 150-170 138-150 128.5-170 100-128.5 0-70 29.1-31.5 27.6-28.6 17.6-20.4 13.7-15.5 0-20.5

46.20 2.05 12.32 25.67 20.53 53.80 14.21 1.02 3.05 2.54 8.12

57.00 4.65 13.96 33.73 23.27 43.00 12.59 1.57 3.15 2.10 8.39

51.9 1.73 11.25 30.26 21.64 48.1 13.49 1.26 3.37 2.53 8.43

43.00 2.21 8.82 24.26 18.74 57.00 14.78 1.06 3.69 2.11 8.44

43.00 2.05 8.19 23.55 19.45 57.00 14.25 0.71 3.56 2.49 9.62

1H

NMR total aromatic H total aliphatic H CH/CH2/CH3 in R position CH2/CH in β position CH3 in γ position 13C NMR total aromatic C aromatic C attached to heteroatoms alkyl-substituted aromatic C without CH3 quaternary aromatic C without triple-bridged aromatic C tertiary aromatic C, triple-bridged aromatic C total aliphatic C C in n-alkyl chains (n > 6) C in branching position of a terminal i-propyl group C in CH3 branches C in terminal position of n-alkyl chains (n > 6) total C in CH3 groups

Table 6. Structural Parameters of GPC Fractions Obtained from AS of AR

Table 7. Structural Parameters of GPC Fractions Obtained from AS of VR

GPC fraction number

GPC fraction number

parameter

total AR/AS

predicted parametera

2

4

6

8

parameter

total VR/AS

predicted parametera

2

4

6

8

MW C Cal Cal,n-alkyl n Cal,CH3 Car Car,t Car,q fa {H/C}al Car,sub Car,X Car,nb Car,b Car,b3 Car,b2 σ γ Rar n* BI Rna Cna

3230 224 47% 26% 10 15% 53% 11% 42% 0.53 2.20 14% 3% 28% 25% 12% 13% 0.55 0.47 29 3 0.46 5 8%

4590 321 44% 24% 10 9% 56% 7% 49% 0.56 2.43 10% 3% 20% 35% 17% 19% 0.60 0.63 59 4 0.37 7% 7%

5990 423 44% 27% 10 6% 56% 8% 48% 0.56 2.36 9% 3% 21% 35% 13% 22% 0.52 0.63 75 5 0.56 13 10%

5260 367 43% 25% 9 10% 57% 6% 51% 0.57 2.50 8% 3% 17% 40% 16% 24% 0.57 0.70 74 5 0.41 8 8%

4750 336 44% 21% 12 8% 56% 7% 49% 0.56 2.46 12% 3% 21% 35% 20% 15% 0.64 0.62 59 4 0.30 6 6%

2120 149 48% 30% 9 10% 52% 8% 44% 0.52 2.26 13% 3% 24% 27% 12% 16% 0.61 0.53 21 4 0.35 3 7%

MW C Cal Cal,n-alkyl n Cal,CH3 Car Car,t Car,q fa {H/C}al Car,sub Car,X Car,nb Car,b Car,b3 Car,b2 σ γ Rar n* BI Rna Cna

2670 191 54% 27% 11 8% 46% 8% 39% 0.46 2.24 5% 2% 14% 32% 13% 19% 0.38 0.69 32 12 0.40 6 11%

3899 280 53% 26% 11 8% 47% 6% 41% 0.47 2.08 11% 2% 20% 27% 14% 14% 0.62 0.58 41 6 0.36 7 9%

6254 442 43% 23% 11 8% 57% 9% 48% 0.57 2.45 4% 5% 18% 39% 14% 25% 0.31 0.69 87 11 0.35 10 8%

5360 385 48% 26% 10 8% 52% 7% 45% 0.52 2.23 13% 2% 22% 30% 14% 16% 0.64 0.57 58 4 0.34 8 7%

3220 233 57% 25% 12 8% 43% 5% 38% 0.43 1.94 12% 2% 20% 23% 13% 10% 0.69 0.55 28 5 0.42 7 11%

2120 153 57% 27% 11 10% 43% 6% 37% 0.43 1.94 10% 2% 17% 26% 14% 12% 0.62 0.60 21 6 0.31 4 9%

a Calculated as the sum of the percent values of the respective data of the fraction numbers 2, 4, 6, and 8.

a Calculated as the sum of the percent values of the respective data of the fraction numbers 2, 4, 6, and 8.

fractions, a number of structural parameters were calculated applying the method number 4 in Petrakis and Allen38 and equations proposed by Ali et al.34 and Al-Zaid et al.39 The formulas used are compiled in Table A1 of the Appendix. 3.2.3. Structural Representation of GPC Fractions. The structural parameters in terms of the percentage of carbon in certain functional groups of selected, asphaltenic GPC fractions from both asphaltenes, AR/AS and VR/AS, are shown in Tables 6 and 7. These parameters have been used to construct structural presentations of the narrow MW fraction number 4 of both asphaltenes [Figure 3 (AR/AS) and Figure 4 (VR/AS)]. 3.3. Macrostructure of Asphaltene GPC Fractions. Because of the semicrystalline structure in the solid state, asphaltenes typically exhibit in XRD a pattern of three broad bands at 2Θ ∼20 (d ∼ 4.5 Å), ∼25 (d ∼ 3.5 Å), and ∼45 (d ∼ 2.0

Å)20-23 as shown in Figure 5 for the GPC fraction number 4 of VR asphaltenes. The position and full width at half-maximum (fwhm) obtain information about the macrostructure of asphaltenes, such as the distance between two aliphatic (dγ) or two aromatic sheets (dm), the size of an aromatic sheet (La), the diameter of a stack (Lc), the number of aromatic rings per sheet (Ra), and the number of aromatic sheets in a stack (M). The procedure to derive these macrostructural parameters from XRD patterns is described elsewhere.23 When NMR and XRD data are combined, the real average layer diameter of an aromatic sheet (La′ ) La[Car/(Car + Can])23 excluding the naphtenic carbons is obtained, which can in turn be related to the number of the actual aromatic carbons per aromatic structural unit (Cau ) [(La′ + 1.23)/0.615]) and the number of aromatic structural units per molecule (N ) Car/Cau).23 Table 8 compiles the XRD data and the derived macrostructural parameters of the stack for both asphaltenes, AR/AS and VR/AS, and for each of the GPC fraction number 4.

(39) Al-Zaid, K.; Khan, Z. H.; Hauser, A.; Al-Rabiah, H. Composition of high boiling petroleum distillates of Kuwait crude oils. Fuel 1998, 77, 453-458.

236 Energy & Fuels, Vol. 20, No. 1, 2006

Ali et al.

Figure 3. Structure presentation of GPC fraction number 4 obtained from AR asphaltenes.

4. Discussion As previously mentioned, the scope of this work was to investigate the relationship between the molecular size and the average molecular structure of asphaltenes. Tables 6 and 7 show selected structural parameters derived from NMR versus MW for GPC fractions obtained from AR and VR asphaltenes. The structural parameters (Tables 6 and 7) demonstrate an obvious relationship between the molecular size of the GPC fraction and the percentage of carbon present in different functional groups. This indicates that the number of carbons in distinct structural groups does not increase proportionally with the MW of the fraction. For instance, the percentage of aliphatic carbon (% Cal) in each fraction decreases with an increasing molecular size of the fraction. As for the relationship between the molecular size and aromaticity (fa ) % Car/% C), there are contradicting results in the literature. For instance, Strausz et al.26 reported for Athabasca asphaltenes a slight increase of aromatic carbon with a declining MW. Dettman et al.,40 however, found for Athabasca bitumen asphaltenes that the early eluting GPC fractions (bp > 750 °C) have the greatest aromatic contents. Beside changes in the aromaticity of the fractions, the aromatic features of each fraction changes as well. The fractions with the highest MW show a higher percentage of quaternary carbon (% Car,q) present in substituted, bridged, or heteroatomsubstituted aromatic ring positions, while fractions with low MW are less aromatic and the aromatic rings are less condensed (less aromatic carbon in the bridge-head position). This means that the different structural units in the asphaltenes do not only (40) Dettman, H.; Inman, A.; Salmon, S. Chemical characterization of GPC-fractions of Athabasca bitumen asphaltenes isolated before and after thermal treatment. Energy Fuels 2005, 19, 1399-1405.

enlarge proportionally in size from one to another GPC fraction but also alter in their structure. The general tendency, for both asphaltenes, is that asphaltenes with the largest MW (size) show the highest aromaticity and contain the highest percentage of aromatic carbon in the bridge-head position (% Car,b) and the highest amount of heteroatoms. This can have serious consequences for the refining of asphaltene-rich feedstocks. While the low MW asphaltenes are converted to distillates, the high MW asphaltenes are concentrated in the product oil. Although the total amount of asphaltenes in the product oil has been lowered, the aromaticity and the degree of condensation (γ ) % Car,b/% Car) of the remaining asphaltenes increased, while their number of aliphatic side chains decreased. Consequently, the asphaltene stability in the oil becomes disturbed.12,13,27 Further processing of such oils can cause severe coking,3 because on one hand these high MW asphaltenes in a more saturated oil matrix have a high tendency to flocculate2 and on the other hand the polynuclear aromatic cores of the asphaltenes undergo further condensation reactions, especially under severe operating conditions, leaving the asphaltenes with even larger aromatic cores.15,41 Pilot plant hydrotreating experiments conducted with the atmospheric residue from Kuwait export crude and with the partly deasphaltened atmospheric residue of the same crude showed a higher coke generation for the deasphaltened residue than for the original feed.3 Tables 6 and 7 also demonstrate that some structural parameters of the original, unseparated asphaltenes (first column in Tables 6 and 7) differ clearly from those of the GPC fraction numbers 2, 4, 6, and 8. Therefore, conclusions drawn from the structural parameters of the entire asphaltene can easily result (41) Seki, H.; Kumata, F. Structural changes in petroleum asphaltenes and resins by hydrodemetallization. Energy Fuels 2000, 14, 980-985.

Asphaltene GPC Fractions DeriVed from Kuwaiti Residual Oils

Energy & Fuels, Vol. 20, No. 1, 2006 237

Figure 4. Structure presentation of GPC fraction number 4 obtained from VR asphaltenes.

The structure presentation of the fraction number 4 of both asphaltenes, exhibited in Figure 3 (AR/AS) and Figure 4 (VR/ AS), demonstrates that the average asphaltene consists of several cores of aromatic rings, which bear alkyl changes of about 11 carbon atoms. The archipelago structure results from the XRD measurements that show that the average number of aromatic rings per sheet is low (≈4), which correlates with an aromatic sheet diameter of about 7-9 Å.

Figure 5. Deconvoluted XRD pattern of GPC fraction number 4 obtained from VR asphaltenes.

in misleading predictions of the chemical behavior of the asphaltenes in the refining process.

When the structural features of the two sets of GPC fractions are compared (NMR, Tables 6 and 7; XRD, Table 8), it is seen that hydrotreating the residual oil results in (i) an appreciable reduction of the aromatic carbon in the lower MW fractions, whereas the high MW fractions of both asphaltenes (refractory fraction) show the same aromaticity (fa ≈ 0.57); (ii) along with the lower aromaticity of the hydrotreated asphaltenes (VR/AS), the number of aromatic carbons in bridge-head positions also reduce except for the heaviest GPC fraction (number 2), which shows the same value as the heaviest GPC fraction of AR asphaltenes (Car,b ≈ 37%); and (iii) XRD of the fraction number 4 of both asphaltenes, AR/AS and VR/AS, reveal that hydrotreatment decreases the real layer diameter of an average aromatic sheet by about 2 Å.

238 Energy & Fuels, Vol. 20, No. 1, 2006

Ali et al.

Table 8. Macrostructural Parameters of Asphaltenes of AR and VR and of Their GPC Fraction Number 4 sample

MW

dg (Å)

dm (Å)

La (Å)

Lc (Å)

Ra

M

Car (NMR)

Cna (NMR)

La′ (Å)

Cau

N

AR asphaltenes GPC fraction number 4 VR asphaltenes GPC fraction number 4

3230 5260 2670 5360

4.4 4.5 4.6 4.5

3.5 3.6 3.5 3.6

8.9 9.9 8.4 8.0

16.6 13.6 14.6 11.4

3 4 3 3

6 5 5 4

118 209 88 200

19 29 21 27

7.7 8.7 6.8 7.0

15 16 13 13

8 13 7 14

Table A1. Description of Acronyms descriptiona

acronym #H #C #S #N #Hal #Har #HR #Hβ #Hγ #Cal #Cparaffins

TN of H atoms/molecule TN of C atoms/molecule TN of S atoms/molecule TN of N atoms/molecule TN of aliphatic H atoms/molecule TN of aromatic H atoms/molecule TN of aliphatic H atoms in R position TN of aliphatic H atoms in β position TN of aliphatic H atoms in γ position TN of aliphatic C atoms/molecule TN of C atoms on chains with n > 6

n n* #CCH3 #Car #Car,t #Car,q (H/C)al #Car,sub #Car,X #Car,b #Car,b3 #Car,b2 #Car,nb fa σ γ Rar BI r Rna #Cna

average number of C atoms in n-alkyl chains with n > 6 average number of C atoms in alkyl substituents TN of C atoms in CH3 groups/molecule TN of aromatic C atoms/molecule TN of tertiary aromatic C atoms/molecule TN of quaternary aromatic C atoms/molecule H/C atomic ratio for aliphatic part TN of substituted aromatic C atoms/molecule TN of heteroatom-substituted aromatic C atoms/molecule TN of bridged aromatic C atoms/molecule TN of triple-bridged aromatic C atoms/molecule TN of double-bridged aromatic C atoms/molecule TN of nonbridged aromatic C atoms/molecule aromaticity degree of substitution of aromatic C degree of condensation of aromatic C TN of aromatic rings per molecule branching index number of naphthenic rings per substituent TN of naphthenic rings per molecule TN of naphthenic C/molecule

formulab MW × percent H/100 MW × percent C/1200 MW × percent S/3200 MW × percent N/1400 TI for aliphatic H × #H/100 TI for aromatic H × #H/100 TI for aliphatic H in R position × #H/100 TI for aliphatic H in β position × #H/100 TI for aliphatic H in γ position × #H/100 TI for aliphatic C × #C/100 (TI for C in CH2 of aliphatic chain + 5 × TI for C in terminal CH3 of chain) × #C/100 (TI for CH2 in aliphatic chain/TI for terminal CH3) + 5 TI of aliphatic H/TI of R-H TI for methyl C × #C/100 TI for aromatic C × #C/100 TI for aromatic H × #H/100 #Car - #Car,t #Hal/#Cal TI for aliphatic H in R position × #H/[100 × (H/C)al] TI for heteroatom-substituted aromatic C × #C/100 #Car - #Car,t - #Car,sub - #Car,X (TI for aromatic C from 100 to 128.5 ppm × #C/100) - #Car,t #Car,b - #Car,b3 #Car - #Car,b TI of aromatic C/100 #Car,sub/(#Car,sub + #Car,t) #Car,b/#Car 1 + (#Car - #Car,nb)/2 #Hγ/#Hβ (n* - 1)((0.250(BI + 4.12) - 1)/2) #Car,sub × r 3.5Rna

a TN ) total number. b MW ) molecular weight, C ) carbon, H ) hydrogen, S ) sulfur, N ) nitrogen, TI ) total percent integration; formulas from George et al.1 and Petrakis, L. and Allen, D.38

5. Summary Two asphaltenes, one obtained from the atmospheric distillation of Kuwaiti crude (AR/AS) and another one from the vacuum distillation of a hydrodesulfurized feed (VR/AS), were subjected to preparative GPC. The lack of appreciable amounts of asphaltenes with a MW of