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Differences in Average Chemical Structures of Asphaltene Fractions Separated from Feed and Product Oils of a Mild Thermal Processing Reaction George Michael,*,† Mohammad Al-Siri,‡ Zahida Hameed Khan,† and Fatima A. Ali§ Petroleum Research and Studies Center, Kuwait Institute for Scientific Research (KISR), P.O. Box 24885, Safat 13109, Kuwait, Joint Technologies Company (JT), P.O. Box 4918, 22050 Salmiya, Kuwait, and Central Analytical Laboratory, Kuwait Institute for Scientific Research (KISR), P.O. Box 24885, Safat 13109, Kuwait Received June 22, 2004. Revised Manuscript Received May 10, 2005
The differences in the chemical structures of the asphaltene fractions present in a vacuum residue feed and in product oil after mild thermal processing, in the presence of N2 and H2 gases (activation gases), was studied using measurements of molecular weight, elemental analysis, nuclear magnetic resonance (NMR) spectroscopy, and X-ray diffraction (XRD). The asphaltene fractions were separated from the feed and product oil according to standard procedures. The percentage intensities of H and C nuclei in different chemical environments were obtained from 1H NMR and 13C NMR spectra of these fractions. The numbers of H atoms and C atoms in various building blocks were thus calculated. The average structural data for the feed and product asphaltene molecules were derived. The macrostructure and aromaticity parameters were determined using XRD and compared with the average structural parameters calculated from NMR. The data show that the aromatic ring structures were similar, although it was the aliphatic structures that made the difference between these fractions.
Introduction The quality of the petroleum crude processed in the refineries has diminished continuously over the past two decades. The refineries must install secondary processing units to produce liquid fuel products from crude that has lower API gravity and higher sulfur content. As the supply of lighter crude oil has continued to decline, there has been an increasing need to develop processing methods to upgrade the abundant supply of heavy-oil reserves into more value-added products. Such development work requires a clear understanding of the chemical transformations occurring in the different types of hydrocarbon molecules during the process. The important classes of hydrocarbon compounds present in heavy oil such as vacuum residues are saturates, aromatics, resins, and asphaltenes. Asphaltenes are the toluene-soluble and heptane-insoluble fraction of petroleum. Asphaltenes are hydrocarbon molecules that have a hydrogen-to-carbon (H/C) ratio in the range of 1.0-1.3, and these elements account for 85%-95% of the mass of the molecule, with molar masses of ∼1000-2000 daltons.1 The molecules derive much of their polarity from the presence of sulfur (3-5 * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Petroleum Research and Studies Center, Kuwait Institute for Scientific Research. ‡ Joint Technologies Company. § Central Analytical Laboratory, Kuwait Institute for Scientific Research. (1) McLean, J. D.; Kilpatrick, P. K. J. Colloid Interface Sci. 1997, 196 (1), 23-24.
wt %) and nitrogen (1-2 wt %) heteroatoms. Asphaltenes have a tendency to be large flat molecules with considerable fused aromatic sheets and polar functional groups. Asphaltene particles are believed to exist in petroleum partially in dissolved forms and partially in steric-colloidal and/or micellar forms, depending on the polarity of their oil medium and the presence of other compounds in oil.2 These molecules are believed to be responsible for the formation of coke precursors and for the deactivation of catalytic reactions. It is of tremendous interest and potential benefit to understand the chemical differences of the asphaltene fractions present in feed and product oil of a mild thermal processing reaction. The important thermal processing methods used in refineries worldwide are visbreaking and delayed coking. The constituent molecules of the petroleum feedstocks undergo thermal decomposition when the oil is heated to temperatures of >350 °C. The severity of the process condition is a combination of the residence time of the oil in the reactor and the temperature required to achieve a given conversion.3 Experiments for the present study were conducted at relatively lower temperature (430 °C) than delayed coking or visbreaking (480-500 °C), to achieve a better quality of products while maintaining good conversion of the residue to lighter products. A mixture of H2 and N2 gases was preheated to the reaction temperature and (2) Branco, V. A. M.; Mansoori, G. A.; Xavier, L. C. D.; Park, S. J.; Manafi, H. J. Pet. Sci. Eng. 2001, 32, 217-230. (3) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999.
10.1021/ef049854l CCC: $30.25 © 2005 American Chemical Society Published on Web 06/15/2005
Average Chemical Structures of Asphaltenes
purged through the feed to distribute the heat energy uniformly. Asphaltene fractions were separated from feed and product oil to study the differences in chemical structure of these fractions as a result of the process. Nuclear magnetic resonance (NMR) methods, especially 1H NMR and 13C NMR, have gained a prominent place in the structural analysis of petroleum fractions. NMR measures the aromatic and aliphatic carbon content directly, as well as the carbon and hydrogen distributions. It can also determine C and H atoms in various structural groupings in a molecule.4-6 The spectra of hydrocarbon mixtures consist of broad bands, in contrast to those of individual hydrocarbons.7 In the case of single hydrocarbons, a visual comparison of the NMR spectra of feed and product may indicate structural changes that occurred to a molecule during a process, whereas in the case of a mixture of hydrocarbon molecules (e.g., petroleum fractions), a visual comparison may not show these changes. However, an average chemical structure of the hydrocarbon molecules can be derived from these spectra via the application of different correlations. These correlations require the percentage of H and C nuclei in different chemical environments obtained from 1H NMR and 13C NMR spectra, the values of elemental analysis (carbon, hydrogen, sulfur, and nitrogen), and the average molecular weights of the petroleum fraction for the calculation of average structural parameters. The structural parameters thus derived represent a statistical average of the molecules in the fraction and largely determine the chemical behavior of the fraction. This makes it possible to visualize the chemical structure of a feed fraction and to compare it with that of a reaction product. The limitation of the method is that, in a mixture, NMR spectroscopy cannot distinguish between resonances of paraffins and paraffinic chains connected to aromatic rings. Therefore, the basic separation of the petroleum fraction into aromatics and nonaromatics is required before applying NMR correlations to obtain average structural data on those separated classes of compounds. Asphaltene fractions are condensed aromatic structures with aliphatic or naphthenic substitutions, and NMR correlations can be applied to these molecules to study the differences in average chemical structures of these fractions from different sources. The measurement of molecular weight by various techniques is a subject of controversy in asphaltene science. The results from field ionization mass spectroscopy, laser desorption mass spectroscopy, atmospheric pressure chemical ionization mass spectroscopy, and electrospray ionization mass spectrometry, along with the recently developed fluorescence depolarization techniques, suggest a mean asphaltene molecular weight of ∼750 amu, with a factor of 2 in the width of the mass distribution.8 These results are coupled with 13C NMR spectroscopy, infrared spectroscopy, and high-resolution (4) Brown, J. K.; Ladner, W. R. Fuel 1960, 39, 87-96. (5) Snape, C. E.; Ladner, W. R.; Bartle, K. D. Anal. Chem. 1979, 51, 2184-2198. (6) Petrakis, L.; Allen, D. NMR for liquid fossil fuels. In Analytical Spectroscopy Library, Vol. 1; Elsevier: New York, 1987; Chapter V, pp 91-111. (7) Gray, M. Upgrading Petroleum Residues and Heavy Oils; Marcel Dekker: New York, 1994. (8) Buch, L.; Groenzin, H.; Gonzalez, E. B.; Andersen, S. I.; Galeana, C. L.; Mullins, O. C. Fuel 2003, 82, 1075-1084.
Energy & Fuels, Vol. 19, No. 4, 2005 1599
Figure 1. Schematic depiction of a cross-sectional view of an asphaltene cluster.
transmission electron microscopy. The techniques that yield much-higher molecular weights include gel permeation chromatography and colligative methods such as vapor pressure osmometry (VPO), which was used to determine the molecular weights in the present study. X-ray diffraction (XRD) is a very popular technique currently to study the crystalline structure of asphaltene.9 Yen and co-workers10-12 used this technique to study the aromaticity and crystallite parameters of asphaltenes. Shirokoff et al.13 studied asphaltene macrostructure and crystallite parameters by combining XRD and NMR. Asphaltenes are believed to form a layer or sheet structure, where the condensed aromatic sheets have a tendency to stack, bearing naphthenic and alkyl systems on their periphery (Figure 1). XRD can be used to calculate the average distance between the aromatic sheets (dm), the average distance between the aliphatic chains (dγ), the average diameter of aromatic sheets (La), the average diameter of the cluster (Lc), the average number of aromatic rings per sheet (Rn), the number of C atoms per aromatic structural unit (Cau), the number of aromatic structural unit per molecule (N), and the average number of aromatic sheets per stack (M). Experimental Section Materials. The vacuum residue feed to an industrial delayed coker unit was used as the feedstock for the reaction. The sulfur content in the feedstock was low, because it was produced from desulfurized atmospheric residue. Pilot-Plant Experiment. The thermal processing was conducted in a pilot-plant unit that consisted of a main reactor unit and three product receiver tanks. The vacuum residue was fed to the reactor of the unit at the rate of 60 g/min, using a metering pump. The feed temperature was maintained at 430 °C inside the reactor, and H2 (4 L/min) and N2 (16 L/min) gases were purged through the feed. The distilled product oil from the top of the reactor was collected in two tanks. The heavy product was collected in the first tank, and the volatile vapors were cooled by a condenser and collected in a second tank. The bottom residue was collected in a third tank at the bottom of the reactor. The three fractions were combined together to get the total product oil from the reaction. Asphaltene Separation. Asphaltene fractions were separated from the vacuum residue and product oil, following the procedures mentioned in the standards published by the (9) Robert, E. C.; Merdrignac, I.; Rebours, B.; Harle, V.; Kressmann, S.; Colyar. J. Pet. Sci. Technol. 2003, 21 (3 & 4), 615-627. (10) Yen, T. F.; Erdman, J. G.; Pollack, S. Anal. Chem. 1961, 33, 1587-1590. (11) Wen, C. S.; Chilingarian, G. V.; Yen, T. F. Properties and Structure of Bitumens. In Bitumens, Asphalts and Tar Sands; Elsevier Science Publishers: Amsterdam, 1978; Chapter 7, p 155. (12) Schwager, I.; Farmanlan, P. A.; Kwan, J. T.; Weinberg, V. A.; Yen, T. F. Anal. Chem. 1983, 55, 42-45. (13) Shirokoff, J. W.; Siddiqui, M. N.; Ali, M. F. Energy Fuels 1997, 11, 561-565.
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Institute of Petroleum (IP).14 Maltene fractions were concentrated to remove traces of n-heptane solvent and were used for the hydrocarbon type analysis. Hydrocarbon Type Analysis. The procedure developed by Khan and Marron15 was followed for the separation and estimation of saturates, aromatics, and resins in the feed and product oils. High-Temperature Simdis Analyzer. The Simdis analyzer, which is capable of simulating the boiling-point range of petroleum fractions up to 750 °C, was used. A basic gas chromatograph (Agilent model 6890) modified by Analytical Controls (software and hardware modification for the application) was used. The program used n-paraffin standards (with carbon numbers in the range of 5-120) to calibrate and simulate the higher boiling range up to 750 °C. A nonpolar capillary column (5 m in length, internal diameter of 530 µm, film thickness of 0.09 µm) was used to separate the hydrocarbons according to boiling point. A reference oil with a known boiling range was used as an external standard for system validation and response factor calculation. The response factor was used for the calculation of recovery. The gas chromatography (GC) analysis was controlled according to the following parameters: injector temperature, 100 °C for 0 min, increasing at a rate of 15 °C/min to 430 °C and being held at 430 °C for 22 min; flame ionization detector temperature, 430 °C; oven temperature, 40 °C for 0 min, increasing at a rate of 10 °C/ min to 430 °C and being held at 430 °C for 5 min; carrier gas, helium at an average linear velocity of 139 cm/s. fuel gas, hydrogen at a flow rate of 35 mL/min; oxidant, air at a flow rate of 350 mL/min; and makeup gas, helium at a flow rate of 45 mL/min. CHNS Analysis. An elemental analyzer (Perkin-Elmer, model 2400) was used to analyze the samples for total carbon and hydrogen content. Calibration factors (the K-factor) were developed for individual elements by testing standard materials that have similar elemental composition of the sample to be analyzed. A quantitative standard prepared from known components such as n-hexadecane, dibenzothiophene, pyrrole, and naphthalene gave reliable factors and repeatable results. Vapor Pressure Osmometry. VPO equipment (Knaur, model K-7000) was used to estimate the molecular weight, according to American Society for Testing and Materials (ASTM) Standard 2503, using benzil as the standard and toluene at 50 °C as the solvent.16 Molecular weights were also determined, using o-dichlorobenzene as the solvent at 120 °C.17 Poly(methyl methacrylate) (PMMA, from Polymer Laboratories, Ltd., U.K.), with a molecular weight (Mn) of 3006 (as determined by VPO), was used as the standard at concentrations of 0.005, 0.01, and 0.015 mol/kg. The calibration constant (Kmn) was determined at zero dilution by extrapolation. Sulfur and Nitrogen Analysis. An Antek analyzer (model 7000) was used to determine the total sulfur and nitrogen in the samples. Sulfur and nitrogen standards in the same concentration range as the samples were used to calibrate the equipment. Nuclear Magnetic Resonance Analysis. All NMR spectra were recorded in the pulse Fourier transform mode, using a Bruker Avance-300 spectrometer that was operating at 75.47 MHz for the 13C NMR spectra. 1H NMR measurements were performed with a spectral sweep width of 3.59 kHz, a pulse angle of 20 µs (90°), and a delay time of 5 s. Parameters for 13 C measurements were a spectral width of 18.8 kHz, a pulse (14) Standard Methods: for Analysis and Testing of Petroleum and Related Products; Institute of Petroleum: London, 1995; pp 143.1143.4. (Also published by Wiley (New York).) (15) Khan, Z. H.; Marron, K. J. Liq. Chromatogr. 1988, 11 (8), 16051613. (16) 16. 1996 Annual Book of ASTM Standards; American Society for Testing and Materials: Philadelphia, PA, 1996; Vols. 05.01, 05.02, 05.03. (17) Wiehe, I. A. Ind. Eng. Chem. Res. 1992, 31 (2), 530-536.
Michael et al. width of 13.2 µs (45°), and a pulse delay of 60 s. Samples for 1 H NMR measurements were prepared by adding 0.5 mL of CDCl3 solvent to 5-10 mg of the petroleum fraction in a 5-mm tube. Tetramethylsilane was used as an internal reference. For 13 C measurement, 1.5 mL of CDCl3 was added to 150-250 mg of petroleum fraction. XRD Measurements. XRD patterns of fine asphaltene powder were recorded using a system (Philips, X’Pert-model) that contains two vertical goniometers with a diffractometer control unit (Philips, model PW3710). Copper tubing was used with a high-voltage generator (model PW 1830) operating at 35 kV and 45 mA. The following instrument parameters were used for the measurements: a starting angle of 6° 2θ, an ending angle of 80° 2θ, a step size of 0.02° 2θ, and time/steps of 1.2 s. After the XRD measurements, the profile fitting was performed using the proFit program software and the full width at half-maximum (FWHM) of the gamma (γ), 002, and 01 band were calculated, and, in turn, the structural parameters were deduced.
Results and Discussion The vacuum residue feed and the product oil from the thermal processing reaction were characterized according to the procedures published by ASTM16 and IP.14 The data are given in Table 1. The density of the product oil was lower than that of the feed oil, indicating the formation of lighter hydrocarbons during the reaction. Simulated distillation results indicated the formation of 25-30 wt % of lighter hydrocarbons, mostly in the middle distillate range (kerosene and diesel). The residues, at certain distillation temperatures (350, 450, and 550 °C), in the feed and products were estimated using high-temperature simulated distillation results. The percentage reduction in the quantity of residue in the product oil compared to that in the feed oil was reported as the conversion for that residue. The conversions for residues at these temperatures were 21.11, 37.07, and 50.56 wt %, respectively. There was partial desulfurization of the feed, as proven by the presence of hydrogen sulfide (H2S) in the product gas stream. However, there was no evidence for any denitrogenation during the reaction. The hydrocarbon analyses of the feed and product oil indicated that the total saturate content of the product oil increased by 0.82 wt % and the total aromatic content increased by 1.99 wt %, whereas the total resin content decreased by 3.22 wt % The total asphaltene contents in feed and product oil were similar (5.63 and 6.04 wt %, respectively), and the difference of 0.41% was within the repeatability criteria for the method of analysis. The differences in the average chemical structures of the asphaltene molecules in the feed and in the product oil of the process were studied by separating the asphaltene fractions from these oils. The asphaltene fractions were characterized for carbon, hydrogen, sulfur, and nitrogen, as well as molecular weight (see Table 1). The 1H NMR and 13C NMR were recorded and are shown in Figures 2 and 3, respectively. The correlations shown in Table 2 were used to calculate the average structural data. The percentage of hydrogen and carbon in different chemical environments were obtained for feed and product asphaltenes from the respective 1H NMR and 13C NMR spectra and are given in Table 3. Molecular weights were determined using toluene at 50 °C and o-dichlorobenzene at 120 °C as the solvents.
Average Chemical Structures of Asphaltenes
Energy & Fuels, Vol. 19, No. 4, 2005 1601
Table 1. Characteristic Properties of Vacuum Residue, Composite Product Oil, and Asphaltenes Separated from These Oils Value test density at 15 °C API gravitya sulfur content nitrogen content molecular weight carbon content hydrogen content hydrocarbon analysis saturates aromatics resins asphaltenes high-temperature simulated distillation initial boiling point, IBP 5 wt % 10 wt % 20 wt % 30 wt % 40 wt % 50 wt % 60 wt % 70 wt % final boiling point, FBP total recovery recovery at 350 °C recovery at 450 °C recovery at 550 °C conversion for 350 °C plus residue conversion for 450 °C plus residue conversion for 550 °C plus residue asphaltene composition carbon hydrogen sulfur nitrogen molecular weight of asphaltenes in toluene in o-dichlorobenzene a
vacuum residue
composite product oil
1.0031 g/mL 9.44 1.26 wt % 0.41 wt % 837 86.44 wt % 11.90 wt %
0.9340 g/mL 19.87 1.19 wt % 0.29 wt % 380 86.87 wt % 11.35 wt %
25.20 wt % 59.89 wt % 9.28 wt % 5.63 wt %
26.02 wt % 61.88 wt % 6.06 wt % 6.04 wt %
418.0 °C 500.5 °C 524.0 °C 551.5 °C 575.5 °C 602.5 °C 633.0 °C 669.0 °C 713.0 °C 750.0 °C 73.1 wt % 0.0 wt % 1.1 wt % 19.4 wt %
94.0 °C 201.5 °C 266.5 °C 355.0 °C 418.5 °C 469.0 °C 511.5 °C 554.5 °C 629.5 °C 750.0 °C 76.7 wt % 19.3 wt % 36.1 wt % 59.1 wt % 21.11 wt % 37.07 wt % 50.56 wt %
85.75 wt % 9.13 wt % 4.17 wt % 0.95 wt %
89.03 wt % 5.84 wt % 3.83 wt % 1.30 wt %
1790 1763
980 959
API ) American Petroleum Institute.
Figure 2. 1H nuclear magnetic resonance (NMR) spectra of asphaltenes from feed (bottom spectrum) and product oil (top spectrum).
The feed was a vacuum residue produced from desulfurized atmospheric residue and was lower in sulfur content and nitrogen content, compared to a vacuum residue produced from straight-run atmospheric residue. The asphaltenes separated from the feed and product oils were lower in molecular weight, compared to the asphaltenes separated from crude oil or straightrun products. Because of the desulfurization reaction undergone by the feed, the aromatic core of the asphalt-
ene molecules was less polar. During the thermal processing reaction of the feed oil, the aromatic core of the asphaltene molecules did not undergo many changes and remained less polar, because of the desulfurization reaction that occurred in the previous stage. The asphaltene molecules with less polar aromatic core structures are more easily soluble and have lesser tendency for associating in solutions.8 This may be the reason the asphaltenes separated from the feed and
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Figure 3.
Michael et al.
13
C NMR spectra of asphaltenes from feed (bottom spectrum) and product oil (top spectrum). Table 2. Correlations Used To Calculate the Average Structural Parameters formulaa
property total number of H atoms per molecule, #H total number of C atoms per molecule, #C total number of S atoms per molecule, #S total number of N atoms per molecule, #N total number of aliphatic hydrogens per molecule, #Hal total number of aromatic hydrogens per molecule, #Har total number of aliphatic hydrogens in R position, #HR total number of aliphatic hydrogens in β position, #Hβ total number of aliphatic hydrogens in γ position, #Hγ total number of aliphatic carbons per molecule, #Cal total number of aliphatic carbons in CH3 groups per molecule, #Cal; CH3 total n-alkyl carbons, #Cal;n-alkyl average number of C atoms on chain, n total number of aromatic carbons per molecule, #Car total number of tertiary aromatic carbons per molecule, #Car,t total number of quaternary aromatic carbons per molecule, #Car,q H/C atomic ratio for aliphatic component total number of substituted aromatic carbons per molecule, #Car,sub total number of heteroatom-substituted aromatic carbons per molecule, #Car,X total number of bridged aromatic carbons per molecule, #Car,b total number of nonbridged aromatic carbons per molecule, #Car,nb aromaticity, fa degree of substitution of aromatic carbons, σ degree of condensation of aromatic carbons, γ average C/H weight ratio of alkyl groups, fc total number of aromatic rings per molecule, Rar branchiness index, BI r total number of naphthenic rings per molecule, Rna total number of naphthenic carbons per molecule, #Cna a
(MW × % H)/100 (MW × % C)/1200 (MW × % S)/3200 (MW × % N)/1400 (TI for aliphatic hydrogen × #H)/100 (TI for aromatic hydrogen × #H)/100 (TI for aliphatic hydrogen in R-position × #H)/100 (TI for aliphatic hydrogen in β-position × #H)/100 (TI for aliphatic hydrogen in γ-position × #H)/100 (TI for aliphatic carbon × #C)/100 (TI for methyl carbon × #C)/100 [(TI for CH2 in aliphatic chain) + (5 × TI for terminal CH3) × #C]/100 (TI of aliphatic hydrogen)/(TI of R-H) (TI for aromatic carbon × #C)/100 (TI for aromatic hydrogen × #H)/100 #Car - #Car,t #Hal/#Cal (TI for aliphatic hydrogen in R-position × #H)/(100 × (H/C) atomic ratio for aliphatic part) (TI for heteroatom substituted aromatic carbon × #C)/100 #Car - #Car,t - #Car,sub - #Car,X #Car - #Car,b (TI of aromatic C)/100 #Car,sub/(#Car,sub + #Car,t) #Car,b/#Car [(TI for aliphatic carbon) × % C]/[(TI of aliphatic hydrogen) × % H] 1 + (#Car - #Car,nb)/2 #Hγ/#Hβ (n - 1) × {[0.250(BI + 4.12) - 1]/2} #Car,sub × r 3.5 × Rna
MW ) molecular weight; C ) carbon; H ) hydrogen; S ) sulfur; N ) nitrogen; TI ) total percent integration.
product oil, showing monomolecular concentrations in both solvents, irrespective of the difference in polarity of the two solvents. The molecular weights determined by VPO for each sample using two solvents (toluene at 50 °C and o-dichlorobenzene at 120 °C) were within the repeatability criteria, according to ASTM Standard D-2503 (see Table 1). The results in Table 4 give the average structural data for asphaltene molecules in feed and product oils. The asphaltene fraction separated from the product oil had a lower molecular weight (980) than that of the asphaltene from feed (1790). The reduction in the numbers of C atoms, H atoms, S atoms, and N atoms per molecule were down from 128 to 73, 163 to 57, 2.33 to 1.17, and 1.21 to 0.91, respectively. The reductions in aliphatic carbon and aliphatic hydrogen (down from 69 to 18 and 150 to 42, respectively) were much more prominent, compared to those of aromatic carbon and
aromatic hydrogen. In fact, there was an increase in the number of aromatic H atoms per molecule of the thermally processed asphaltene (up from 13 to 15) and a small reduction in the numbers of aromatic carbons (down from 59 to 55). There was a considerable reduction in the number of H atoms at the γ-position of the thermally processed asphaltene molecule, relative to that of the asphaltene molecule of the feed (down from 38 to 9). This can be attributed to the reductions in the number and length of the aliphatic side chains in the product asphaltene. The number of alkyl-substituted aromatic carbons decreased from 7 to 5, the number of alkyl carbon atoms per molecule decreased from 43 to 10, and the average number of C atoms on chains decreased from 10 to 3. Irrespective of the reduction in molecular weight, the total number of aromatic rings per molecule of the feed and product asphaltene remained almost the same (18 and 17, respectively). The
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Energy & Fuels, Vol. 19, No. 4, 2005 1603
Table 3. Percentage Intensity of Carbon and Hydrogen in Different Chemical Environment for Asphaltene Molecules from Feed and Product Oil 1H
NMR chemical shift (ppm)
H-type total aliphatic hydrogen total hydrogens on the R-position of an aromatic ring total hydrogens on the β-position of an aromatic ring total hydrogens on the γ-position of an aromatic ring total aromatic hydrogen 13C
C-type total aliphatic carbon total methyl carbon total terminal methyl carbon total branched methyl carbon total tertiary (CH) carbon total secondary (CH2) carbon total aromatic carbon total alkyl-substituted aromatic carbon total heteroatom-substituted aromatic carbon
Integral (%) feed product oil
0.5-4.5 1.9-4.5
92.28 9.13
73.34 22.41
1.0-1.9
59.83
35.31
0.5-1.0
23.32
15.62
6.5-9.9
7.72
26.66
NMR chemical shift (ppm)
feed
10-70 10-22.7 14.3 19.7 28.2 29.7 100-178 138-150
53.72 8.59 3.76 3.22 1.61 15.04 46.3 11.8
24.00 6.70 1.62 1.84 1.19 5.19 76.00 14.06
150-178
4.01
3.80
product oil
Table 4. Average Structural Data for Asphaltenes Separated from Feed and Composite Product Oil by Nuclear Magnetic Resonance (NMR) Spectra Average Structural Data (atoms per average molecule) property
feed
product oil
total hydrogen aromatic hydrogen aliphatic hydrogen aliphatic hydrogens in the R-position aliphatic hydrogens in the β-position aliphatic hydrogens in the γ-position total carbon aromatic carbon tertiary aromatic carbon quaternary aromatic carbon substituted aromatic carbon bridged aromatic carbon nonbridged aromatic carbon aliphatic carbon naphthenic carbon n-alkyl carbon aliphatic carbon in CH3 group average number of C atoms on chains, n total number of aromatic rings per molecule total number of naphthenic rings per molecule
163 13 150 15
57 15 42 13
98
20
38
9
128 59 13 47 7 35 25 69 14 43 11 10
73 55 15 40 5 32 23 18 3 10 5 3
18
17
4
1
branchiness index, BI aromaticity, fa degree of substitution of aromatic carbon, σ degree of condensation of aromatic carbon, γ average C/H weight ratio of alkyl groups, fc total sulfur total nitrogen empirical formula
0.39 0.46 0.35
0.48 0.76 0.26
0.59
0.58
5.47
5.03
2.33 1.17 1.21 0.91 C128H163S2.33N1.21 C73H57S1.17N0.91
total numbers of naphthenic rings per molecule of feed asphaltene and product asphaltene were 4 and 1, respectively. These results indicate that the main structural differences between these fractions involve the aliphatic side chains and naphthenic rings. The asphaltene molecules in the product oil were formed by the cleavage of aliphatic-carbon-to-carbon bonds at the
R-, β-, or γ-positions from the aromatic rings, resulting in shorter aliphatic side chains and increased aromaticity for the product asphaltene molecule (up from 0.46 to 0.76). The carbon skeleton of the aromatic rings was highly condensed, and the reaction did not lead to a significant fragmentation of these rings. The reduction in the degree of substitution of these molecules (down from 0.35 to 0.26) during the thermal processing was very small, indicating that the reaction did not lead to the complete cracking of aliphatic side chains from the aromatic rings. The degree of condensation remained almost the same (0.62 and 0.58) for both types of structures, indicating the similarity of the condensed structure of the aromatic units. Because the aromatic ring structure remained intact during the reaction, the S or N atoms on these rings remained in the same aromatic structure. The S atoms in the aliphatic side chains or naphthenic rings or thiophenic structures that broke away from the condensed aromatic structures contributed to the sulfur content of the lighter distillates. Some of these atoms also underwent a desulfurization reaction to form H2S, which was detected in the product gas stream. From the difference in sulfur contents of the feed and product asphaltene fractions, it is assumed that the S atoms were almost equally distributed in the chemical structure of feed asphaltene molecules, with one S atom in the aromatic rings and another S atom on the aliphatic chain or naphthenic ring. Also, for every third feed asphaltene molecule, there was an additional S atom in the aromatic rings. The product asphaltene molecule had one S atom per molecule in the aromatic structure, and, for every fifth molecule, there was an additional S atom in the aromatic ring. With respect to nitrogen, each feed asphaltene molecule had one N atom in the aromatic ring structure, and every fifth molecule had another N atom, probably in a monoaromatic ring on the aliphatic chain. The product asphaltene had a single N atom in the aromatic structure, and every tenth molecule had no N atoms in the chemical structure. Because there was a reduction in molecular weight from the feed asphaltene to the product asphaltene and the total asphaltene content remained almost same in the feed and product oil, it is clear that the original asphaltene structure in the feed contributed only to a portion (∼50%) of the asphaltene fraction in the product oil. The cleavage of side chains and removal of polar heteroatoms, along with aggregation of the condensed aromatic ring structure of some of the bigger resin molecules in the feed composition, might have produced the remaining portion of the asphaltene. The total resin contents in the feed and product oil were 9.28 and 6.06 wt %, respectively. The asphaltene XRD patterns (Figures 4, 5, and 6) had primarily three characteristic bands: the gamma (γ) band, the 002 band, and the 10 band. A fourth band (the 11 band) was generally very weak and is not shown in the figures. The structural data of the asphaltene clusters, the average distance between two aliphatic chains or saturated rings (dγ), the average distance between the aromatic sheets (dm), the cluster diameter (Lc), and the average diameter of the aromatic sheets (La), respectively, were obtained from the γ-band position, the 002 band position, and the FWHM of the 10
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Michael et al. Table 5. Correlations Used To Calculate the X-ray Diffraction (XRD) Parameters property
Figure 4. X-ray diffraction (XRD) patterns of asphaltenes from feed and product oil.
formula
average distance between aromatic sheets, dm (Å) average distance between aliphatic chains, dγ (Å) average diameter of aromatic sheets, La (Å) cluster diameter, Lc (Å) diameter of the actual aromatic sheets excluding naphthenic carbons, L/a (Å) average number of aromatic rings per sheet, Ra average number of aromatic sheets per stack, M number of carbons per aromatic structural unit, Cau number of aromatic structural unit per molecule, N
dm ) λ/(2 sin Θ) dγ) 5λ/(8 sin Θ) La) 0.92/(FWHM (10 band)) Lc) 0.45/(FWHM (002 band)) L/a) La[Car/Car + Cna] Ra) La/2.667 M) (Lc/dm) + 1 Cau) (L/a + 1.23)/0.615 N) Car/Cau
Table 6. Structural Parameters for Asphaltenes Separated from Feed and Composite Product Oil by XRD Value propertya
feed
product oil
molecular weight dγ dm La Lc Carb Cnab L/a Cau Ra M N
1790 4.4 Å 3.5 Å 8.9 Å 14.2 Å 59 14 7.2 Å 13.7 3 5 4
983 4.4 Å 3.5 Å 9.8 Å 14.2 Å 55 3 9.3 Å 17.1 4 5 3
a For definitions of the variables listed, see Table 5. b As determined from NMR analysis.
Figure 5. Profile fit for the feed asphaltene.
Figure 6. Profile fit for the product asphaltene.
and 002 bands. The parameter La measured the total diameter of the aromatic sheets, which include the naphthenic groups attached to it. Combining the XRD and NMR data and assuming a catacondensed aromatic structure, the diameter of the actual aromatic sheets excluding naphthenic carbons (L/a), the number of car-
bons per aromatic structural unit (Cau), and the number of aromatic structural units per molecule (N) were calculated, according to the correlations in Table 5. XRD results are listed in Table 6. For the feed asphaltenes, the size of the actual aromatic sheet L/a (7.2 Å) clearly was smaller than La (8.9 Å), which indicates that the original feed asphaltene had several naphthenic carbons attached to the aromatic rings. In contrast, for the product asphaltenes, L/a (9.2 Å) was almost equal to La (9.8 Å), which indicates the absence of any naphthenic carbons. The feed and product asphaltenes had five aromatic sheets per cluster. However, the feed asphaltenes had four aromatic structural units per molecule, each having 3 aromatic rings (Ra) that were composed of 14 C atoms, and the product asphaltene had three aromatic structural units per molecule, each having 4 aromatic rings (Ra) that were composed of 17 C atoms. These results agree with the NMR data where the aromaticity increases in the product, because it has fewer alkyl and naphtenic carbons. Combining NMR and XRD results, the possible proposed chemical structures for the feed and product asphaltenes may be represented as shown in Figure 7. Conclusions Nuclear magnetic resonance (1H NMR and 13C NMR), along with information on molecular weight, elemental analysis, and X-ray diffraction (XRD) were used to study the differences in average chemical structures of as-
Average Chemical Structures of Asphaltenes
Energy & Fuels, Vol. 19, No. 4, 2005 1605
plant thermal processing reaction. The total asphaltene contents in the feed and the composite product oil were almost the same (5.63 and 6.04 wt %, respectively). However, the NMR studies indicated that the chemical structures of the asphaltene molecules were totally different in the two oils. The asphaltene molecule in the product oil was lighter than that in the feed, as a result of the selective cracking reaction that is undergone by the aliphatic or naphthenic side chain of the molecules. The highly condensed aromatic ring structure was almost refractive toward the reaction and resulted in asphaltene molecules with lower molecular weight, shorter aliphatic side chains, and fewer naphthenic rings and higher aromaticity. Results were also confirmed by XRD studies.
Figure 7. Average chemical structures for the feed and product asphaltenes.
phaltene fractions separated from the feed (desulfurized vacuum residue) and the product oils of a mild pilot-
Acknowledgment. The authors acknowledge the management of the Kuwait Institute for Scientific Research (KISR) for their support of the studies described in this article. The authors also thank the staff of the Petroleum Evaluation Facilities of KISR’s Petroleum Research and Studies Center for the sample analyses and Dr. Hauser of KISR’s Central Analytical Laboratories for valuable discussions. EF049854L
