Structural Studies of Vacuum Gas Oil Distillate Fractions of Kuwaiti

Oct 15, 2004 - The SAs had average alkyl chain lengths of n = 13−15 and two naphthenic rings, and they were free of aromatics. The average molecule ...
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Energy & Fuels 2004, 18, 1798-1805

Structural Studies of Vacuum Gas Oil Distillate Fractions of Kuwaiti Crude Oil by Nuclear Magnetic Resonance Fatima Ali,† Zahida Hameed Khan,*,‡ and Nargis Ghaloum‡ Central Analytical Labrotories and Petroleum Refining Department, Kuwait Institute for Scientific Research, PO Box 24885, Safat 13109, Kuwait Received January 6, 2004. Revised Manuscript Received July 19, 2004

Nitrogen-free vacuum gas oil (VGO) distillate was separated, using preparative column chromatography, into five hydrocarbon groups: saturates (SAs), monoaromatics (MAs), diaromatics (DAs), polycyclic aromatic hydrocarbons (PCAHs), and polar hydrocarbons (POLHs). The hydrocarbon groups MA, DA, PCAH, and POLH were separated into low- and high-sulfur compounds (LSCs and HSCs, respectively) on silica gel impregnated with PdCl2 (aqueous). SAs and LSCs obtained from MAs, DAs, PACHs, and POLHs were further fractionated into subfractions using gel permeation chromatography (GPC), based on their molecular size in solution. All the GPC subfractions were analyzed for elemental carbon, hydrogen, nitrogen, and sulfur, and for molecular weight. Nuclear magnetic resonance (NMR) spectroscopic techniques also were applied to elucidate the average molecular structure of selected GPC subfractions, POLHs, and HSCs. The SAs had average alkyl chain lengths of n ) 13-15 and two naphthenic rings, and they were free of aromatics. The average molecule in the three groups (MAs, DAs, and PCAHs) contained (i) one aromatic ring, two cyclo-paraffinic rings, two alkyl groups on aromatic rings, and no bridged aromatic carbon; (ii) two aromatic rings, two naphthenic rings, five alkyl-substituted groups, and two bridged aromatic C atoms; or (iii) three aromatic rings, one naphthenic ring, four bridged aromatic C atoms, and four alkyl-substituents attached to the aromatic or naphthenic ring(s), with an average chain length of seven C atoms. The average molecule of the POLH group contained one S atom, four aromatic rings, four naphthenic rings, five bridged aromatic C atoms and four alkyl substituents attached to the aromatic or naphthenic ring with an average chain length of seven C atoms.

Introduction The economic importance of vacuum gas oil (VGO) distillates stems from the global demand for transportation fuels, which are difficult to obtain from resources other than fossil fuels. The demand for light distillates is growing rapidly, and the market for residual fuel is decreasing progressively.1 Refiners must utilize highboiling feed for the production of low-boiling distillates. Information on the feed composition will help the refiner in choosing suitable processing conditions. Knowledge of the compositional analysis of heavy petroleum fractions is essential for the conversion of heavy materials into useful, profitable, and clean low-boiling distillate fuels. VGO distillates are important feedstocks for the production of gasoline, diesel, and jet fuels. It is known that 1H and 13C nuclear magnetic resonance (NMR) spectroscopy are appropriate methods for the characterization, both qualitative and quantitative, of crude petroleum oils and their fractions.2 The ap* Author to whom correspondence should be addressed. Telephone: (965) 3987675. Fax: (965) 3987653. E-mail address: zhameed@ prsc.kisr.edu.kw, [email protected]. † Central Analytical Laboratories. ‡ Petroleum Refining Department. (1) Absi-Halabi, M.; Stanislaus, A.; Qabazard, H. Hydrocarbon Process. 1997, 2, 45-55.

plication of spectroscopy, especially 1H and 13C NMR, to elucidate the chemical structure of heavy oils from petroleum, vacuum residues, asphaltenes, and liquid coal has been used successfully.3-9 Kurashova et al.10 applied gas liquid chromatography (GLC), mass spectrometry, and 13C NMR spectroscopy, which enabled them to identify many of the branched-structure alkanes and cyclanes of the hydrocarbons of the 320-500 °C boiling-point fractions for Khar’yag crude oil. The information derived merely from 1H NMR is very limited, because of the close similarity of the proton chemical shifts, which results in broad, unresolved signals. Also, difficulties are encountered in converting the hydrogen structure to the basic carbon skeleton. The (2) Petrakis, L.; Allen, D. NMR for Liquid Fossil Fuels; Elsevier: Amsterdam, The Netherlands, 1987. (3) McKay, J. F.; Amend, P. J.; Harnsberger, P. M.; Cogswell, T. E.; Latham, R. Fuel 1981a, 60, 14-16. (4) McKay, J. F.; Harnsberger, P. M.; Erickson, R. B.; Cogswell, T. E.; Latham, R. Fuel 1981b, 60, 17-26. (5) Mckay, J. F.; Lathman, R.; Haines, W. Fuel 1981c, 60, 27-32. (6) Hasan, M. A.; Ali, M. F.; Bukhari, A. Fuel 1983, 62, 518-523. (7) Hasan, M. A.; Bukhari, A.; Ali, M. A. Fuel 1985, 64, 839-841. (8) Siddique, M. N.; Ali, M. A. Fuel 1999, 78, 1407-1416. (9) Sharma, B. K.; Tyaga, O. S.; Aloopwan, M. K. S.; Bhagat, S. D. Pet. Sci. Technol. 2000, 18 (3&4), 249-272. (10) Kurashova, E. K.; Musayev, I. A.; Smirnov, M. B.; Simanyuk, R. N.; Mikaya, A. I.; Ivanov, A. V.; Sanin, P. I. Pet. Chem. 1989, 29 (3), 206-220.

10.1021/ef040004f CCC: $27.50 © 2004 American Chemical Society Published on Web 10/15/2004

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Table 1. Characteristics of Crude Oil and Vacuum Gas Oil (VGO) Distillates Value property boiling range (°C) density at 15 °C (g/mL) gravity (API) total sulfur (wt %) total nitrogen (wt %) hydrocarbons (wt %) SAs MAs LSCs HSCs DAs LSCs HSCs PCAHs LSCs HSCs POLHs LSCs HSCs a

crude 0.8744 30.18 2.61 0.15

VGO

Scheme 1. Flowchart of VGO Separation (GPC Subfractions were Subjected to Elemental Analysis, Molecular Weight Analysis, and 1H NMR and 13C NMR Analyses)

350-550 0.9211 22.04 3.00 0.09 49.7 19.7 14.4 (73.1)a 5.3 (26.9)a 15.9 6.8 (42.9)a 9.1 (57.1)a 7.2 0.8 (10.8)a 6.4 (89.2)a 7.5 0.1 (1.1)a 7.4 (98.9)a

Numbers enclosed in parentheses are based on the fraction.

combination of both 1H and 13C NMR has proven to be useful in the elucidation of the average molecular structure of petroleum fractions. The 13C NMR technique, which is known as 13C J-modulated NMR, produces further valuable information about hydrogen and carbon connectivity, and allows tertiary and quaternary carbon as well as secondary and primary aliphatic carbon to be distinguished. The NMR technique has the potential to provide accurate saturateto-aromatic ratios more rapidly than conventional methods. Primary, secondary, tertiary, and quaternary C atoms in the saturate and aromatic moieties within the same molecule are distinguished by 13C NMR spectra. In the present paper, the structural parameters of VGO subfractions, as determined using 1H and 13C NMR spectroscopy, are discussed. Experimental Section Preparation and Characterization of VGO Distillate. One crude oil of medium API gravity was secured from the Kuwait Oil Company (KOC) and characterized according to standard methods of the Institute of Petroleum11 and the American Society for Testing and Materials12 (Table 1). It was distilled on Autodest 80-l model 800 and 20-l high-vacuum distillation units to produce a suitable quantity of high-boilingrange distillate (i.e., 350-550 °C) and was characterized (see Table 1). Material Acquisition. The VGO distillate was separated into saturates (SAs), monoaromatics (MAs), diaromatics (DAs), polycyclic aromatic hydrocarbons (PCAHs), and polar hydrocarbons (POLH) in preparative chromatography columns. Details of the procedure were reported elsewhere.13 These subgroup hydrocarbons (except SAs) were separated into lowand high-sulfur compounds (LSCs and HSCs) via ligand exchange chromatography.14 The SAs and LSCs obtained from MAs, DAs, and PCAHs were further fractionated, according (11) Institute of Petroleum. Methods for Analysis and Testing of Petroleum and Related Products; Wiley: London, 1996; Vols. 1 and 2. (12) 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. (13) Khan, Z. H.; Ghaloum, N. Fuel, in press. (14) Ghaloum, N.; Michael, G.; Khan, Z. J. Liq. Chrom. Relat. Technol. 2002, 25 (9), 1409-1420.

to their molecular size, on a Waters Associates liquid chromatograph.15 In this paper, the MAs, DAs, and PCAHs are used for the respective LSCs (see Scheme 1). Elemental Analysis. Elemental analysis was performed using an elemental analyzer (CE Instruments, model EA 1110 CHNS). The total sulfur, in low concentrations, was determined by an elemental analyzer (Antek model 7000). Molecular Weight. The molecular-weight determinations of the subfractions were obtained on analytical gel permeation chromatography (GPC) columns.15 NMR Analysis. All NMR spectra were recorded using a Bruker AMX-300 spectrometer operating at 75.47 MHz for 13C and 300 MHz for 1H. 1H measurements were performed with a spectral sweep width of 4.5 kHz, a pulse angle of 18 µs (90°), and a delay time of 3 s. Parameters for 13C inverse gated decoupling and J-modulated measurements were as follows: spectral widths of 20 kHz, pulse widths of 6 µs (45°) and 13 µs (90°), respectively; and pulse delays of 20 and 3 s, respectively.2,16 Samples for 1H 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 (TMS) was used as an internal reference. For 13C measurement, 1.5 mL of CDCl3 was added to 150-250 mg of the petroleum 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 allow a shortened delay time between the pulse cycles. (15) Khan, Z. H.; Hussain, K. Fuel 1989, 68, 1198-1202. (16) Al-Zaid, K.; Khan, Z. H.; Hauser, A.; Al-Rabiah, H. Fuel 1998, 77 (9), 453-458.

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Figure 1.

1

H NMR and

Ali et al.

13

C NMR spectra for vacuum gas oil (VGO) groups.

Results and Discussion Preparation of Hydrocarbon Groups. VGO distillate was prepared by distilling the crude oil using the 80-l and 20-l distillation units and characterized for important parameters (see Table 1). The acidic, basic, and neutral nitrogen compounds were separated from the VGO distillate with treatments of anionic and cationic Amberlite resins and ferric chloride impregnated on cellulose. VGO distillate was separated into SAs, MAs, DAs, PCAHs, and POLHs (see Scheme 1), according to the methodology established in our laboratories.13 All hydrocarbons except the SAs were separated into LSCs and HSCs using ligand exchange chromatography. Details of the experiments were reported elsewhere.14 The preparative GPC was used to separate the SAs and LSCs into several subfractions, according to their molecular size in solution. Structural Analysis by NMR. In the present work, three NMR techniques (i.e., 1H NMR, 13C NMR, and J-coupling) were applied for the systematic determination of the average molecular parameters of Kuwaiti VGO distillate. Selected subfractions obtained by GPC from LSCs of various hydrocarbon groups (i.e., SAs and MAs, DAs, PCAHs, POLHs, and HSCs obtained from ligand exchange chromatography) were also studied by 1H and13C NMR spectroscopy. Typical spectra for each group (i.e., SAs, MAs, DAs, PCAHs, and POLHs) are documented in Figure 1. The integrated areas of different chemical-shift regions for both 1H NMR and 13C NMR spectra were used to quantify the distinct structural units in the subfractions.

Table 2.

1H

NMR and 13C NMR Chemical-Shift Assignment

moiety 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 carbon aromatic carbon attached to heteroatoms alkyl-substituted aromatic carbon without CH3 tertiary aromatic carbon, aromatic carbon attached to CH3 total aliphatic carbon carbon in n-alkyl chains (n > 6) carbon in branching position of a terminal isopropyl group carbon in CH3 branches carbon in terminal position of n-alkyl chains (n > 6) total carbon in CH3 groups

chemical-shift region (ppm) 6.3-9.3 0.5-4.5 1.9-4.5 1.0-2.2 0.5-1.0 118-170 150-170 138-150 118-128.5 0-70 29.1-31.5 27.6-28.6 17.6-20.4 13.7-15.5 0-20.5

For instance, these integral values yield information on the relative percentages of paraffinic, naphthenic, and aromatic structures in VGO, which are important parameters for refinery product quality assurance, in geological studies, and in crude oil assessment. The different chemical-shift assignments for 1H NMR and 13C NMR occurring in hydrocarbons are shown in Table 2. The naphthenic carbons (Cna) were estimated from the integral values over the 20-45-ppm range after

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Table 3. Percentage of Carbon in Structural Units Obtained by 13C NMR Spectroscopy for Gel Permeation Chromatography (GPC) Subfractions of Saturates (SAs) average structural data yield (wt %) carbon (wt %) hydrogen (wt %) molecular weight average molecular formula number of carbons number of hydrogens number of carbons in branches total CH3 number of n-alkyl carbons (n) naphthenic carbon

SA 3

SA 4

Value SA 5

SA 6

SA 7

24.38 85.50 14.60 500 C36H72

26.96 85.60 14.40 460 C33H66

17.49 85.60 14.40 460 C33H66

12.48 85.80 14.20 450 C32H63

7.93 85.70 14.30 450 C31H64

36 73 2

33 66 2

33 66 2

32 65 2

32 65 2

7 16

6 14

6 15

6 14

6 14

12

12

12

12

11

subtracting the contributions of the peaks of n- and isoparaffins (SAs) or straight and branched alkyl substituents of the aromatics (MAs, DAs, PCAHs, and POLHs) in this region. The aromaticity (fa) is obtained as a ratio of the aromatic carbons to the total carbon content estimated from 13C spectra. The calculations of the average structural parameters were performed by applying method number 4 in Petrakis and Allen2 and AlZaid et al.16 Average molecular structures were constructed from NMR parameters, molecular weights, and elemental analysis data. SA GPC Subfractions. The selected GPC subfractions of SAs obtained from VGO distillate were analyzed by 1H NMR and 13C NMR spectroscopy. The visual observation of both the 1H NMR and 13C NMR spectra for all GPC subfractions revealed the complete absence of a broad envelope in the aromatic region. This demonstrates that they were free from aromatic hydrogen and carbon signals. Proton NMR spectroscopy does not provide much information on the SA fraction; however, 13C NMR spectroscopy provided information on straight, branched, terminal, and cyclic hydrocarbons. Average structural parameters of various fractions were calculated using the carbon-hydrogen distributions obtained from 1H NMR and 13C NMR data, and the data obtained from molecular weight and elemental analysis. Data thus obtained are presented in Table 3. The empirical formula for SAs may take the form of CnH2n+2 for linear or branched SAs. It is obvious that the identification of each compound present in such a mixture is not possible, nor it is necessary. All subfractions from VGO vary in their contents of various types of saturated carbon. The length of the carbon alkyl chain (n) is smaller with the low-molecularweight fraction (n ) 14) and longer (n ) 14-16) with the high-molecular-weight fraction. The concentration of naphthenic carbon marginally increases in the subfractions (carbon numbers of 11-12). It indicates that between two and three 5- and 6-membered naphthenic rings are present. These SAs are composed of a mixture of n-paraffinic chains and naphthenic rings with alkyl substitution. Their average chemical structures are shown in Figure 2. MA GPC Subfractions. The average molecular structural parameters derived from 1H NMR and 13C NMR spectra, molecular weight data, and elemental analysis data are presented in Table 4.

The numbers of aromatic carbons for all MA subfractions were 6 or 7, which is consistent with the numbers expected for MA hydrocarbons. The resonance in the aliphatic region is due to linear and branched alkane and cycloalkane substituents attached to aromatic ring(s). The NMR spectra of the GPC subfractions eluted early (due to high molecular weights) show a predominant, fairly long, alkyl chain (n ) 11), which is reflected by the ratio of the two signals at 29.7 and 14.1 ppm. The data for later eluting fractions indicate shorter side chains (n ) 9) on the ring. These aliphatic substituents can be either on the aromatic or the naphthenic rings. The number of naphthenic carbons (Cna) varied in a range of 6-8, which is consistent with the number of naphthenic rings (1-2) for the entire MA subfraction. The number of alkyl substituents (2) on the rings was constant across the GPC subfractions, as indicated in Table 4. The total number of carbons in the n-alkyl chain decreased from 22 in subfraction 2 to 12 in subfraction 7. The aromaticity factor (fa), i.e., the ratio of aromatic carbon to the total number of both aliphatic and aromatic carbons, was in the range of 0.15-0.21. All the subfractions were free of bridged aromatic carbon. This demonstrates that there was no detectable contamination of DA hydrocarbons, and excellent separation of hydrocarbons on the column was achieved. Some sulfur was detected; it was distributed in all of the GPC subfractions. The data thus obtained indicate that alkylbenzenes and their naphthenologues, and benzothiophenes and their naphthenologues, are present. The average molecular structure of the MA hydrocarbon contains one aromatic ring, two cycloparaffin rings, two alkyl substituents with an average of nine carbons on aromatic rings, no bridged aromatic carbon, and sulfur in every other molecule. Based on the structural parameters, possible average molecular structures for MA are presented in Figure 2. DA GPC Subfractions. As shown in Figure 1, the aromatic region for a typical 13C spectrum of DA fractions has a broad envelope with few humps in it, whereas the aliphatic region has well-resolved lines. The information obtained from 1H NMR spectra was combined with the 13C NMR data, and several structural parameters related to functional groups in the aromatic and aliphatic moieties were calculated. As the number of aromatic carbons increased from the MA subfractions to that for the DA subfractions, the chain length of the aliphatic substituent (n-alkyl carbon) decreased. This is clearly observed, because this number was as high as 22 (see Table 4) for MA subfraction 2 and decreased to a maximum of 11 for DA subfraction 2 (Table 5). Also, the value of the average chain length (n) decreased from 11 down to 8 for DA subfraction 2. The average number of naphthenic carbons (Cna) was 4 or 6 for the DA subfractions, which is consistent with either 5- or 6-membered naphthenic rings. The number of bridged aromatic carbons was 2, which corroborates very well with the two aromatic rings. The aromaticity (fa) of all the subfractions was 0.37-0.43. As expected, as the aromaticity increases, the total number of H atoms decreases. The number of the

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Figure 2. Average molecular structures of VGO hydrocarbon groups.

aromatic C atoms was consistent with the number for two aromatic rings in all subfractions. The number of bridged aromatic C atoms was 2 or 3. This result indicates that the two aromatic rings are adjacent and not separated. The average molecular structure of DA hydrocarbons, derived from the aforementioned data, contains two aromatic rings, one naphthenic ring, and three alkyl substituents, with chain lengths of 7 or 8 (see Figure 2). PCAH GPC Subfractions. The structural parameters were calculated by following the same methodologies and equations as those used for SA and other aromatic subfractions (Table 6). The aromatic carbon number increased in subfractions 5 and 7. The aromatic carbon attached to hydrogen was almost constant in all of the subfractions (10%-11%). Also, 11%-19% of the aromatic carbon was present as bridged carbon and was highest (19%) in the middle two subfractions. The total number of carbon n-alkyl chains also decreased as the subfraction number increased. The ratio of

aliphatic C atoms to aliphatic hydrogen decreased. The values for aromaticity (fa) for most of the subfractions were higher (0.37-0.63) than those for the MA and DA subfractions (0.15-0.43). The elemental analysis of these subfractions showed a high content of sulfur. The average molecular structure drawn from these results may contain one S atom, one naphthenic ring, 3-5 aromatic rings, 3-4 bridged aromatic carbons, and an average chain length of seven C atoms. The possible average molecular structures are shown in Figure 2. Sulfur-Enriched Aromatic GPC Subfractions (HSCs). The MAs, DAs, PCAHs, and POLH hydrocarbon groups were separated into LSCs and HSCs by ligand exchange chromatography.14 The POLHs had high sulfur contents (7.70 wt %), and the yield for LSCs was very low; therefore, the latter was not fractionated by GPC. The total POLHs group was considered as HSCs. The sulfur-enriched, aromatic hydrocarbons (MA-HSCs, DA-HSCs, PCAH-HSCs, and POLHs) were

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Table 4. Average Structural Data for GPC Subfractions from the Monoaromatics (MAs) Fraction from VGO Distillate Value average structural data

MA 2

MA 3

MA 4

MA 5

MA 7

yield of the fraction carbon (wt %) hydrogen (wt %) sulfur (wt %) molecular weight average molecular formula total number of hydrogens aromatic hydrogen aliphatic hydrogen aliphatic hydrogen in the R-position aliphatic hydrogen in the β-position aliphatic hydrogen in the γ-position total carbons aromatic carbon aliphatic carbon quaternary aromatic carbon tertiary aromatic carbon alkyl-substituted aromatic carbon naphthenic carbon bridged aromatic carbon aromatic rings naphthenic rings number of carbons in n-alkyl chain average length of alkyl chain (n) number of carbons per alkyl substitute, n* aromaticity, fa

8.97 84.58 12.04 3.38 600 C42H72S0.6 72 6 66 11 43 12 42 6 36 3 3 2 6 0 1 2 22 11 6 0.15

27.12 85.27 11.73 3.00 500 C35H58S0.5 58 3 55 11 34 10 36 7 29 4 3 2 7 0 1 2 16 9 5 0.20

21.58 85.52 11.54 2.94 460 C33H53S0.4 53 4 49 10 26 13 33 7 26 4 3 2 8 0 1 2 13 9 4 0.20

13.34 85.39 11.68 2.93 460 C33H54S0.4 54 4 49 7 30 12 33 7 26 4 3 2 8 0 1 2 13 9 7 0.21

10.89 85.45 11.65 2.90 450 C31H52S0.4 52 3 49 9 30 10 31 6 24 3 3 2 8 0 1 2 12 9 5 0.21

Table 5. Average Structural Data for GPC Subfractions from the Diaromatics (DAs) Fraction from VGO Distillate Value average structural data

DA 2

DA 3

DA 4

DA6

yield of the fraction (wt %) carbon (wt %) hydrogen (wt %) sulfur (wt %) molecular weight average molecular formula total number of hydrogens aromatic hydrogen aliphatic hydrogen aliphatic hydrogen in the R-position aliphatic hydrogen in the β-position aliphatic hydrogen in the γ-position total carbon aromatic carbon aliphatic carbon quaternary aromatic carbon tertiary aromatic carbon alkyl-substituted aromatic carbon naphthenic carbon bridged aromatic carbon aromatic rings naphthenic rings number of carbons in n-alkyl chain average length of alkyl chain (n) number of carbons per alkyl substitute, n* aromaticity, fa

2.09 85.34 10.47 4.19 460 C33H48S0.6 48 6 42 9 23 10 33 12 21 6 5 3 5 2 2 1 11 8 5 0.37

18.30 85.30 9.83 4.87 440 C31H43S0.7 43 5 38 9 19 10 31 12 19 7 5 3 5 3 2 1 9 7 4 0.40

28.71 85.08 9.58 5.34 430 C30H41S0.7 41 6 35 9 17 9 30 12 18 7 5 3 6 3 2 1 8 7 4 0.42

10.91 85.21 9.63 5.16 410 C29H39S0.7 39 5 34 8 16 10 29 12 17 7 5 3 4 2 2 1 9 7 6 0.43

analyzed by 1H and 13C NMR spectroscopy (Table 7). All the fractions contained >5 wt % sulfur, and their molecular weights were in the range of 400-530. This means that the average molecular structure must contain at least one S atom. The aromatic signals were broadened, because of the presence of the high sulfur content, especially in the PCAHs and POLHs (see Figure 1). The average structural parameters of the HSCs show that the number of aromatic rings increased in the following order: MA-HSCs, DA-HSCs, PCAHHSCs, and POLHs (1 > 2 > 3 > 5 rings, respectively; see Table 7). One naphthenic ring was present in all cases except that in PCAH-HSCs. It is observed also that the length of the alkyl chain decreased from

MA-HSCs to POLHs, as expected. Aromaticity also increased from fa ) 0.26 to fa ) 0.61. The POLHs had a higher aromatic portion, a short n-alkyl chain, the highest number of bridged carbons, and the highest aromaticity (fa ) 0.61). Possible average structures are presented in Figure 2. Comparison of GPC Subfraction. Subfraction 4 of the SAs, MAs, DAs, PCAHs, and POLHs hydrocarbon groups was selected arbitrarily for comparing the chemical composition. Visual inspection of the spectra (see Figure 2) reveals that there are major differences between the five hydrocarbon groups. The SAs are the largest class of compounds obtained from the preparative column chromatography of the VGO distillate. The

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Table 6. Average Structural Data for GPC Subfractions from the Polycyclic Aromatic Hydrocarbons (PCAHs) Fraction Value average structural data

PCAH 3

PCAH 4

PCAH 5

PCAH 7

PCAH 8

yield of the fraction (wt %) carbon (wt %) hydrogen (wt %) sulfur (wt %) molecular weight average molecular formula total number of hydrogens aromatic hydrogen percentage of aromatic hydrogen PCAH hydrogen MA hydrogen aliphatic hydrogen percentage of aliphatic hydrogen aliphatic hydrogen in the R- position aliphatic hydrogen in the β-position aliphatic hydrogen in the γ-position total carbon aromatic carbon % of aromatic carbon aliphatic carbon % of aliphatic carbon quaternary aromatic carbon % of quaternary aromatic carbon tertiary aromatic carbon alkyl-substituted aromatic carbon naphthenic carbon bridged aromatic carbon number of aromatic rings number of naphthenic rings number of carbons in n-alkyl chain average length of alkyl chain (n) carbons per alkyl substitute (n*) aromaticity, fa

19.19 82.70 10.20 7.10 390 C27H40S0.9 40 4 10 3 1 36 90 7 21 8 27 10 37 17 63 6 22 4 4 4 3 2 1 9 8 5 0.37

29.09 83.10 8.60 8.20 390 C27H34S1 34 6 18 4 2 28 82 10 12 6 27 15 56 12 44 8 30 7 4 4 4 3 1 6 7 3 0.56

18.41 83.90 8.30 7.80 390 C27H32S1 32 7 22 6 1 25 78 10 10 5 27 17 63 10 37 8 30 9 5 5 5 4 1 4 7 3 0.63

6.47 83.40 8.60 8.00 380 C26H33S1 33 6 18 5 1 27 82 10 11 6 26 16 62 10 38 7 27 9 5 5 5 3 1 4 7 3 0.62

5.03 84.30 8.80 6.90 370 C26H33S0.8 33 6 18 4 2 27 82 9 12 6 26 15 58 11 42 9 35 6 4 6 4 3 2 4 7 3 0.58

Table 7. Average Structural Data for Sulfur-Enriched Compounds from VGO Distillate Value

a

average structural data

MA-HSC

DA-HSC

PCAH-HSC

POLHa

yield of the fraction (wt %) carbon (wt %) hydrogen (wt %) sulfur (wt %) nitrogen (wt %) molecular weight average molecular formula total hydrogen aromatic hydrogen PCAH hydrogen MA hydrogen aliphatic hydrogen aliphatic hydrogen in the R-position aliphatic hydrogen in the β-position aliphatic hydrogen in the γ-position total carbon aromatic carbon aliphatic carbon quaternary aromatic carbon tertiary aromatic carbon alkyl-substituted aromatic carbon naphthenic carbon bridged aromatic carbon aromatic rings naphthenic rings number of carbon in n-alkyl chain average length of alkyl chain (n) carbon per alkyl, substitute (n*) aromaticity, fa

5.30 80.96 11.58 7.46 0.00 400 C27H46S0.9 46 2 1 1 44 7 27 10 27 7 20 5 2 4 5 1 1 1 11 8 6 0.26

9.10 81.67 10.59 7.74 0.00 440 C30H46S1.1 47 5 3 2 42 6 24 13 30 10 20 5 4 3 7 2 2 1 9 8 7 0.33

6.40 85.84 9.03 5.13 0.01 400 C29H36S0.6N0.03 36 8 6 2 28 9 13 6 29 16 13 8 8 5 3 4 3 2 7 7 3 0.55

7.50 84.08 8.02 7.70 0.20 530 C37H38S1.2N0.08 39 9 8 1 30 13 11 6 38 23 15 14 9 6 3 8 5 1 7 7 2 0.61

Total polar aromatic hydrocarbon.

quantity of SAs was almost the sum of all of the aromatic fractions. The data are compared and summarized in Table 8. Subfraction 4 of the SAs contained 33 carbons and the highest number of hydrogen atoms

(i.e., 66). Naphthenic carbon was also highest in this subfraction (i.e., 12 naphthenic C atoms). This indicates the presence of several long-chain and branched alkanes.

VGO Distillate Fractions of Kuwaiti Crude Oil Table 8.

1H

NMR and 13C NMR Spectral Data for GPC Fraction 4 from VGO Distillate

average structural data yield on VGO distillate (wt %) yield of subfraction (wt %) total hydrogen aromatic hydrogen PCAH hydrogen MA hydrogen aliphatic hydrogen aliphatic hydrogen in the R-position aliphatic hydrogen in the β-position aliphatic hydrogen in the γ-position total carbon aromatic carbon aliphatic carbon quaternary aromatic carbon tertiary aromatic carbon alkyl-substituted aromatic carbon naphthenic carbon bridged aromatic carbon aromatic ring naphthenic ring n-alkyl carbon average length of alkyl chain (n) carbon per alkyl substitute (n*) aromaticity, fa a

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Value SA 4 MA 4 DA 4 PCAH 4 POLHa 49.7

19.7

15.9

7.2

27.0

21.6

28.7

29.1

66 0.0 0.0 0.0 66

54 4

41 6

50 10

35 9

34 6 4 2 28 10

39 9 8 1 30 13

26

17

12

11

13

9

6

6

33 7 26 4

30 12 18 7

27 15 12 8

38 23 15 14

3 2

5 3

7 4

9 6

8 0 1 2 13 9

6 3 2 1 8 7

4 4 3 1 6 7

3 8 5 1 7 7

4

4

3

2

0.20

0.42

0.56

0.61

33 0.0

12

6 14

7.2

Total polar aromatic hydrocarbon.

The NMR data for subfraction 4 from MAs and DAs confirm that these two hydrocarbon groups were well-separated from each other. These subfractions showed a gradual decrease in the number and sharpness of the peaks in the aliphatic region, whereas the aromatic region grew into a broader envelope. The calculated number of aromatics was consistent with the expected numbers, i.e., 1 and 2 rings for MAs subfraction 4 and DA subfraction 4, respectively. The 13C NMR spectra of the PCAHs subfraction 4 and POLHs have very broad envelopes in both the aliphatic and aromatic regions. This suggests that abundant carbons were substituted in various ways. These fractions were also deficient in hydrogen. The percentage of aliphatic hydrogen in the R-position was almost constant in MAs subfraction 4, DAs subfraction 4, and PCAHs subfraction 4, but slightly increased in the POLHs. There was a sharp decrease of hydrogen in the β-position from MAs subfraction 4 to DAs subfraction 4 and PCAHs subfraction 4, which indicates the presence of short paraffinic chains in the PCAH subfraction 4. The same observations were also noticed in regard to the average chain length. Both the aromatic carbon and aromaticity increased from the MAs subfraction 4 (0.20) to the POLHs (0.61). This implies that the POLHs were more condensed than the other aromatic hydrocarbons. The content of bridged carbon increased gradually and steadily from MAs subfraction 4 to DAs subfraction 4 to PCAHs subfraction 4 to POLHs (0 > 3 > 4 > 8).

Conclusions 1H and 13C NMR spectroscopy was applied successfully to various fractions of VGO distillates for obtaining chemical structures. The hydrocarbon groups (SAs, MAs, DAs, PACHs, and POLHs) separated from the VGO distillates gave reliable average structural parameters for their GPC subfractions. The same holds for the sulfur-rich aromatic hydrocarbons obtained from these groups. The percentage of carbon in key hydrocarbon structures such as aliphatics, aromatics, n-alkanes, naphthenes, and branched alkanes were determined. Further methyl or methylene carbon, aromatic, quaternary or tertiary carbon, aromatic-bridged carbons, and aromatic-substituted carbon were determined. The average chemical structures of the fractions were calculated by combining data from NMR and elemental analysis and the average molecular weights. The developed methodology of applying separation procedures together with elemental and metal analysis and 1H and 13C NMR spectroscopy may be used to characterize any VGO distillates of Kuwait crude oils, regardless of the source of the crude oil. The information thus obtained is very important, because it provides compositional data that are useful for predicting, developing, and optimizing appropriate refining processes for VGO distillates. The information is also important for the petrochemical industry, to assist it in deciding which of the available feedstocks will meet its specifications.

Acknowledgment. The authors acknowledge the financial support of the Kuwait Foundation for the Advancement of Sciences (KFAS). The authors thank the Ministry of Oil for permitting them to utilize Kuwaiti crude oils. The authors also acknowledge the Central Analytical Laboratory staff (Dr. Andre Hauser for useful discussions, Ms. Hanadi Abdullah for her assistance in measuring some of the samples) and Dr. George Michael (from the Petroleum Refining Department). List of Abbreviations al ) aliphatic hydrogen art ) aromatic tertiary carbon br ) branching DA ) diaromatic hydrocarbon fa ) aromaticity factor GLC ) gas liquid chromatography GPC ) gel permeation chromatography HSC ) high-sulfur compound i-pr ) isopropyl group KFAS ) Kuwait Foundation for the Advancements of Sciences LSC ) low-sulfur compound MA ) monoaromatic hydrocarbon KOC ) Kuwait Oil Company Nb ) number of nonbridged aromatic carbons NMR ) nuclear magnetic resonance PCAH ) polycyclic aromatic hydrocarbon PFT ) pulsed Fourier transform POLH ) polyaromatic hydrocarbon SA ) saturate hydrocarbon TMS ) tetramethylsilane VGO ) vacuum gas oil EF040004F