Energy & Fuels 2005, 19, 1065-1071
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Estimation of Total Aromatic Content of Vacuum Gas Oil (VGO) Fractions (370-560 °C) by 1H NMR Spectroscopy G. S. Kapur, Anju Chopra, and A. S. Sarpal* R&D Centre, Indian Oil Corporation, Ltd., Sector-13, Faridabad, Haryana, India Received July 31, 2004. Revised Manuscript Received December 28, 2004
In this paper, a quick and direct method has been developed for the quantitative estimation of total aromatics and saturates in a vacuum gas oil (VGO) fractions boiling range (370-560 °C), using 1H NMR spectroscopic techniques. The method is based on the estimation of the average alkyl chain length of substituents on aromatics and the relative group molecular weight of aromatics and saturates. The total aromatics are estimated from the derived equation, which takes into account the relative distribution of CHn (n ) 0, 1, 2, 3) groups of aromatics substituents and saturates. The exact assignment of aromatics substituents of R-CHn (n ) 1, 2, 3) has been performed by the application of 1H/13C multipulse NMR techniques, such as DEPT-135 and 2D HSQC (edited) (heteronuclear single quantum coherence). The results obtained for several VGO samples from different sources have been in close agreement with those from the open chromatography method (ASTM D-2549-91) and the thin-layer chromatography-flame ionization detection (TLC-FID) method (IP-469).
Introduction The physicochemical characteristics of vacuum gas oils (VGOs, 370-560 °C), which are used as feedstocks for various refining processes such as catalytic cracking, hydrotreatment, etc., are governed by their composition at the molecular level, which can be described as the hydrocarbon group-type composition. The refiners for process optimization and catalyst selection and for assessing the end product quality require the hydrocarbon group-type composition of VGOs. Unlike lighter fractions (such as light naphtha, heavy naphtha (130-220 °C), and middle distillates (220-345 °C)), where suitable methodologies have been established to obtain information about individual components or compound classes, the analysis of heavier petroleum fractions (such as VGOs, vacuum residue (>540 °C), and nondistillate residue (>700 °C)) has shown that they are, by far, the most difficult to handle.1 The composition fraction that has a boiling range of IBP-130 °C can be readily determined, in terms of individual components by gas chromatography (GC)2-4 or as compound classes by nuclear magnetic resonance (NMR).5-8 The composition of heavy naphtha (130-220 °C) is more difficult to measure, and analysis using a * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Altgelt, K. H., Boduszynski, M. H., Eds. Composition and Analysis of Heavy Petroleum Fractions; Chemical Industries Series, Vol. 54; Marcel Dekker: New York, 1993. (2) Standard Test Method for Detailed Analysis of Petroleum Naphthas through n-Nonane by Capillary Gas Chromatography; ASTM Standard D-5134-98, ASTM 2003 Annual Book of Standards, Vol. 05.02; American Society for Testing and Materials: West Conshohocken, PA. (3) Standard Test Method for Oxygenates and Paraffin, Olefin, Naphthene, Aromatic (O-PONA) Hydrocarbon Types in Low-Olefin Spark Ignition Engine Fuels by Gas Chromatography, ASTM Standard D-6293-98; ASTM 2003 Annual Book of Standards, Vol. 05.03; American Society for Testing and Materials: West Conshohocken, PA.
GC-PIONA analyzer reflects these difficulties by providing five compound classes rather than resolving individual components. However, the detailed compositional analysis of heavy and heavy naphtha fractions is conveniently performed using gas chromatography/ mass spectrometry (GC/MS)8,9 and a standard GC method (ASTM Methods D-6729, D-6730, and D-6733). The challenge further increases with the analysis of middle distillates (∼220-345 °C cuts), where only group-type analysis is obtained by mass spectroscopy (MS) techniques.10,11 Liquid chromatography (LC) and supercritical fluid chromatography (SFC) techniques are used to analyze such middle distillates, in term of the compound groups of saturates, aromatics, and polars.12-14 (4) Standard Test Method for Paraffin, Naphthene, and Aromatic Hydrocarbon Type Analysis in Petroleum Distillates through 200 °C by Multi-Dimensional Gas Chromatography, ASTM Standard D-544393; ASTM 1998 Annual Book of Standards; American Society for Testing and Materials: West Conshohocken, PA. (5) Sarpal, A. S.; Kapur, G. S.; Mukherjee, S.; Tiwari, A. K. Fuel 2001, 80 (4), 521-528. (6) Kapur, G. S.; Singh, A. P.; Sarpal, A. S. Fuel 2000, 79 (9), 10231029. (7) Meusinger, R. Fuel 1996, 75 (10), 1235. (8) Singh, A. P.; Mukherjee, S.; Tiwari, A. K.; Kalsi, W. R.; Sarpal, A. S. Fuel 2003, 82, 23-33. (9) Zadro, S.; Haken J. K.; Pinczewski, W. V. J. Chromatogr. 1985, 323, 305-322. (10) Teeter, R. M. Mass Spectrom. Rev. 1985, 4, 123-143. (11) Standard Test Method for Hydrocarbon Types in Middle Distillates by Mass Spectrometry, ASTM Standard D-2425-99; ASTM 1999 Annual Book of Standards; American Society for Testing and Materials: West Conshohocken, PA. (12) Test Method for Determination of Aromatic Hydrocarbon Types in Middle DistillatessHigh Performance Liquid Chromatography Method with Refractive Index Detection; Standard IP-391, Institute of Petroleum: London. (13) Standard Test Method for Determination of Aromatic Hydrocarbon Types in Aviation Fuels and Petroleum DistillatessHigh Performance Liquid Chromatography Method with Refractive Index Detection, ASTM Standard D-6379-99; ASTM 1999 Annual Book of Standards; American Society for Testing and Materials: West Conshohocken, PA. (Also see Institute of Petroleum Test Method IP-436.)
10.1021/ef040069i CCC: $30.25 © 2005 American Chemical Society Published on Web 04/06/2005
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Structural complexities further increase, in the case of VGO fractions (∼345-560 °C) and further heavier ends, such as vacuum residues (greater than ∼540 °C) and nondistillables (greater than ∼700 °C). In such heavier ends, including asphaltenes, limited analysis is possible, in terms of group-type composition and carbon-type analysis, and average structural parameters can be obtained using MS and NMR spectroscopic techniques, each of which has its own limitations.15,16 Recently, a method based on thin-layer chromatography-flame ionization detection (TLC-FID) has gained recognition for the analysis of heavier petroleum fractions, where saturates, aromatics, polars, and asphaltenes (SARA) can be determined quantitatively.17 Liquid chromatography (open column chromatography) has been used to characterize the hydrocarbon group composition of crude oils and other petroleum products since the beginning of this century. The fluorescent indicator absorption (FIA) method (ASTM Method D-1319) has been used to estimate the content of saturates, olefins, and aromatics in fuel range products. The method that is based on open column chromatography is ASTM Method D-2549, which is generally used for the separation of saturates and total aromatics for samples boiling in the range of 205-540 °C. As a part of long drawn program in our laboratory, in regard to the development of analytical methods based on NMR spectroscopic technique for various petroleum fractions,5-7,18-23 a new 1H NMR based method has been proposed for the hydrocarbon-type analysis of VGO fractions (370-560 °C). The refiners for process optimization and catalyst selection, and for assessing the quality of the end product, require the hydrocarbon group-type composition of VGOs. Open column chromatography has generally been used for the separation of saturates and total aromatics for samples boiling in the range of 205-540 °C (ASTM D-2549). However, the method is time-consuming, laborious, and sometimes does not ensure complete elution of the sample. TLC-FID analysis offers the separation of saturates, aromatics, and polars (total aromatics ) the sum of aromatics and polars) in heavier ends (bp > 300 °C), but the repeatability offered by the technique is in the range of (5%-8% for the different classes. Moreover, the time required for the analysis is 2 h. (14) Standard Test Method for Determination of Aromatic Content and Polynuclear Aromatic Content of Diesel Fuels and Aviation Turbine Fuels by Supercritical Fluid Chromatography, ASTM D 518699; ASTM 1999 Annual Book of Standards; American Society for Testing and Materials: West Conshohocken, PA. (15) Bouquet, M.; Brument, J. Fuel Sci. Technol. Int. 1990, 8 (9), 961-986. (16) Netzel, D. A.; Guffey, F. D. Energy Fuels 1989, 3, 455-460. (17) Test Method for Determination of Saturated, Aromatic and Polar Compounds in Petroleum Products by Thin-Layer Chromatography and Flame Ionization Detection, Standard IP-469; Institute of Petroleum: London. (18) Christopher, J.; Sarpal, A. S.; Kapur, G. S.; Krishna, A.; Tyagi, B. R.; Jain, M. C.; Bhatnagar, A. K. Fuel 1996, 75 (8), 999-1008. (19) Sarpal, A. S.; Kapur, G. S.; Bansal, V.; Jain, S. K.; Srivastava, S. P.; Bhatnagar, A. K. Pet. Sci. Technol. Int. 1998, 16 (7&8), 851868. (20) Bansal, V.; Kapur, G. S.; Sarpal, A. S.; Kagdiyal, V.; Jain, S. K.; Srivastava, S. P. Energy Fuels 1998, 12, 1223-1227. (21) Sarpal, A. S.; Kapur, G. S.; Chopra, A.; Jain, S. K.; Srivastava, S. P.; Bhatnagar, A. K. Fuel 1996, 75, 483-490. (22) Sarpal, A. S.; Kapur, G. S.; Chopra, A.; Mukhejee, S.; Jain, S. K. Fuel 1997, 76, 931-937. (23) Sarpal, A. S.; Mukherjee, S.; Bansal, V.; Kapur, G. S. Fuel Int. 2000, 1-1, 3-15.
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In the present work, a simple and rapid method has been developed, based on 1H NMR techniques, for estimation of the total aromatics in VGO samples boiling in the range of 370-560 °C. The data so generated have been validated by comparison with ASTM Method D-2549 and TLC-FID methods. Experimental Details Materials. Various VGO samples were obtained from different Indian refineries that are involved in processing crudes from indigenous and imported sources and used as such. The samples were thoroughly homogenized by uniform heating before any analytical measurements. NMR Analysis. 1H NMR Spectra. An ∼10% solution of the samples in CDCl3 were prepared. 1H NMR spectra were recorded on a Bruker ACP-300 NMR spectrometer, using the following conditions: spectral width, 5000 Hz; pulse angle, 90°; delay, 10 s; and number of transients, 32. Data were processed with 0.5 Hz of line broadening. The 10 s delay was sufficient to relax all the protons. This was checked by giving a different delay value, in the range of 5-10 s. Under these sample preparation and experimental conditions, a very good signalto-noise (S/N) ratio was achieved. 13 C Spectra. The quantitative 13C NMR and DEPT-135 spectra were obtained, as per the conditions described in our previous work.21 Quantitative 13C NMR spectra were recorded by adding a relaxation agent (chromium triacetylacetonate, Cr(acac)3, 0.2 M) in the inverse gated condition. The amount of relaxation agent was sufficient to relax all the quaternary carbons in the aromatic region, because there was no change in the intensity with further increases in the amount of relaxation agent. The relaxation delay was 5 s, and 10 00015 000 scans were given to improve the S/N ratio sufficiently. 2D HSQC (Edited) (Heteronuclear Single Quantum Coherence) Experiments and DEPT-135 Spectra Recording. The DEPT spectral recordings were conducted, in accordance with the procedure given in the work of Sarpal and co-workers.21-23 The 2D HSQC (edited) spectra were obtained by the pulse sequence and program reported by Davis.24 Open Column Chromatography. All the samples were subjected to open column chromatography, as per ASTM Method D-2549. The saturate and aromatic fractions were collected, and the weight of the fractions was determined gravimetrically. TLC-FID Analysis. TLC-FID analysis has been performed on a Iatroscan TLC-FID instrument (from Iatran Laboratories, Japan) that was equipped with an FID detector. The rods used for the analysis were silica-coated quartz rods of the type S-II, manufactured by Iatran Laboratories. Approximately 1 µL of the 2% sample solution was spotted on the chromrods and developed sequentially in hexane, toluene, and tetrahydrofuran (THF). The first development was performed up to 10 cm in hexane, the second development up to 5 cm in toluene, and the third development up to 2 cm in THF. The rods were dried in an oven at 70 °C for ∼2 min after each development. The peaks were detected by scanning the rods in an oxygen-hydrogen flame at a scan rate of 35 s per scan. The hydrogen and air flow rates were 160 and 2000 mL/min, respectively. An Iatro recorder TC-21 was used as the integrator.
Results and Discussion Basis of the 1H NMR Method. To estimate the total aromatics content in VGO fractions, a 1H NMR method that was based on the group molecular weight has been proposed. The method is simple, straightforward, and (24) Davis, D. G. J. Magn. Reson. 1991, 91, 665-672.
Estimating the Total Aromatics in VGO Fractions
does not require the use of any internal standards. The method relates the proton NMR spectral integral intensity to the relative carbon content, which, in turn, rely on the exact assignments of 1H NMR spectrum to different group types (such as -CH, -CH2, -CH3), rather than to the detailed structural groups. The advantage of this approach is that the overlap of signals in various chemical shift regions does not affect the results significantly. In VGO-range fractions, aromatics molecules consist of monoaromatic, diaromatic, triaromatic, and even tetra-aromatic ring structures. In addition, there could be structures that consist of condensate aromatic and naphthenic rings and heteroaromatics as well. In the present methodology, aromatics are perceived as any molecule that contains at least one benzene ring. Per this observation, all benzo-naphthenic and polar aromatics are determined as part of the total aromatics content of a VGO fraction. The aromatic ring structures in petroleum fractions are also associated with some degree of substitution (σ) by groups such as CH3 or longer alkyl groups. Therefore, the total aromatics content will be the contribution of ring structures as well as the various substituents attached to them. Hence, knowledge about the average alkyl chain length attached to the aromatics (other than R-CH3 groups), denoted as “n”, is essential for the estimation of the total aromatics content, using the present methodology. It is very convenient to identify and quantify R-CH3 groups from a 1H NMR spectrum, because the signals due to this group appear in the 2-2.35 ppm range. The average length of alkyl substituents greater than CH3 groups cannot be estimated directly, because signals due to β-, γ-, and δ-groups to the aromatic rings are merged with the signals of paraffins (0.5-2.0 ppm) in the 1H NMR spectra. Similarly, the contribution of substituted aromatic ring carbons (denoted as Arq) and bridgehead aromatic carbons (denoted as Cb) must also be estimated. However, these carbons cannot be estimated directly, because they are nonprotonated and, hence, are invisible to the 1H NMR spectroscopy. In the present method, an indirect approach has been adopted to obtain information about the parameters Arq, Cb, and n: (a) The relative Arq carbons have been realized from the signals in the region of 2.05-4.0 ppm, which has been assigned to the R-CH3 , R-CH2, and R-CH groups. Because these signals are overlapped in this region, these have been resolved by multipulse NMR techniques. (b) Cb carbons have been realized by the developed equations. (c) An average value of n has been estimated from the 1H NMR spectral analysis of the pure aromatic fractions of few of the VGO samples from different sources, obtained using open column spectroscopy. The 300 MHz 1H NMR spectra of a representative VGO sample is shown in Figure 1. The spectrum exhibit the structural specificity of the hydrogen-type distribution that is associated with the chemical-shift regions that include the following: (1) hydrogens of aromatic rings (region A; 6.5-9.0 ppm)
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Figure 1. sample.
1
H NMR spectrum of the VGO-G (370-560 °C)
Figure 2. 1H NMR spectra of aromatic fractions of VGO (370-560 °C) from three different sources (denoted as R, G, and C).
(2) hydrogens on R-carbon, relative to aromatic rings (regions B and C, due to R-CH, R-CH2, and R-CH3; 2.054.0 ppm) (3) hydrogens on β-CH/CH2 groups, relative to aromatic rings, and -CH/CH2 groups of alkanes and cycloalkanes (region D; 1.5-2.05 ppm) (4) methylene groups of longer alkyl chains (region E; 1.05-1.5 ppm), and (5) γ- and δ-methyl hydrogens, relative to aromatic rings, and methyl hydrogens of alkanes and cycloalkanes (region F; 0.5-1.05 ppm) The region of 7.4-9.0 ppm also provides information about the presence of di-plus aromatic ring compounds in VGO samples. The similar 1H NMR spectral behavior is shown by other VGO samples from different sources (Figure 2). The complete assignment of the 1H NMR spectrum, in terms of various hydrocarbon groups, is included in Table 1. The assignment of various regionssparticularly, from 2.05 to 4.0 ppmshas been facilitated with the help of the 2D HSQC (edited) experiment, as described in the succeeding section. 2D NMR Analysis. To estimate the average alkyl chain length of the substitutents on the aromatic ring, it is essential to assign the resonances in the overlapped
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Table 1. Assignment and Division of the 1H NMR Spectrum of Vacuum Gas Oil Sample (370-560 °C) for Measuring the Integral Intensity, as per Figure 1 region (ppm) 6.5-9.0 7.4-9.0 2.55-4.0 2.05-2.5 2.05-1.5 1.05-1.5 0.5-1.05
assignment CH groups of all types of aromatics rings CH groups of di-plus aromatic rings R-CH2 to aromatic rings R-CH3 to aromatic rings (average value of chemical shift region) CH, CH2 of naphthenes, β CH2/CH of aromatics and iso-paraffins (average CH(1.5) groups) CH2 CH3
integral intensity A Id B C D E F
region of 2-4.0 ppm. This region contains overlapped resonance, because of the R-CH, R-CH2, and R-CH3 substituents on aromatic rings. Because aromatics are composed of monoaromatics, diaromatics, and polyaromatic hydrocarbons (PAHs), it may not be possible to demark the region or signals that are due to R-CHn (n ) 1, 2, 3) accurately, because the signal of these multiplicities appear in the full region from 2 ppm to 4.0 ppm. The overlapped signals in the region between 2 and 4.0 ppm have been resolved satisfactorily with the help of the 2D HSQC (edited) technique.24 The 2D NMR HSQC multipulse experiment correlates, edits, and separates the signals due to the R-CHn multiplicity in the 1H NMR spectral region of 2-4.0 ppm with the corresponding signals in the 13C NMR spectral region of 18-50 ppm. The 2D NMR spectra of the aromatic fraction of a representative VGO sample are given in Figure 3. The edited 2D spectra due to R-CH2 and the combination of R-CH3 and CH are given in Figure 4. The edited spectra indicate that it is possible to unambiguously assign and mark the region due to R-CH2 and R-CH3. The signals due to R-CH are also observed in the region of 3-4.0 ppm; however, their intensities are much smaller than those of R-CH3 and R-CH2 protons. The chemical-shift region of 2-2.8 ppm is assigned to R-CH3 susbtituents; a small amount of interference of R-CH2 is observed in this region (see Figure 4). The region from 2.8 ppm onward is due to the R-CH2 signals of longer alkyl chains or a naphthenic ring sandwiched between aromatic rings, such as 9,10-dihydroanthracene or a similar type of naphthoaromatic compound. Very little interference from R-CH3 is observed up to 3.0 ppm. These types of protons have shown connectivity with the corresponding carbon signals in the region of 25-50 ppm in the 13C NMR spectra. The 2D NMR HSQC experiment has also separated the signals due to -CH3 groups of isoparaffins, which otherwise appeared to be overlapped in the 13C NMR spectra region of 18-24. The correlation of isomethyl protons of the region of 0.5-1.0 ppm in the 1H NMR spectra with corresponding carbons in the 13C NMR spectra have been clearly established. The following conclusions could be drawn from the analysis of the 2D HSQC (edited) spectra: (a) The chemical-shift region of 2-2.8 ppm is primarily due to R-CH3 substituents on aromatics with a marginal extension up to 3.0 ppm. The region can be further distinguished between R-CH3 signals of monoaromatics (2-2.40 ppm) and polyaromatics (2.4-2.8 ppm). This information has been derived by comparing the
Figure 3. 2D HSQC (edited) contour plot of VGO-R; the dark and light shaded contours denote (R-CH3, -CH3, -CH) and (R-CH2, -CH2) groups, respectively.
spectra with corresponding standard spectra of a monoaromatic reference compound given in the Aldrich spectral library of 13C and 1H FT-NMR spectra.25 Because the R-CH2 signal of longer alkyl chain substituents and naphthoaromatics start from 2.5 ppm and have only minor interference from R-CH3, the region of R-CH2 has been assumed to be 2.5-4 ppm. (b) The chemical-shift region of 2.5-4 ppm is assigned to R-CH2 signals of longer alkyl substituents on aromatics or naphthoaromatics. This region has a small amount of interference from R-CH3, which extends to 2.8 ppm. (c) The R-CH substituent may appear between 3 ppm and 4.0 ppm. However, no conclusive evidence could be obtained; R-CH signals of weak intensity were observed to spread in the 2D HSQC (edited) and DEPT 135 spectra. (d) The appearance of intense signals in the chemicalshift regions of 0.5-1.0 ppm (CH3) and 1.0-2.0 ppm (CH2 + CH) in the 1H spectra and sharp signals at 14.1, 22.7, 31.9, and 29-30 ppm in the 13C NMR spectra are indicative of the presence of longer alkyl chain substituents on the aromatic ring system. This could be due to either the presence of a naphthenic ring of aromatics (such as hydropyrene or hydronaphthalene) or to the monoaromatic or polyaromatics ring. As explained previously, the R-CH2 and CH signals in the region between 2.5-2.8 ppm and 4.0 ppm have been assigned to naphthoaromatics and monoaromatic signals. This has also been confirmed by the analysis of monoaromatic and PAH aromatic fractions from the column separately. The monoaromatics content has been determined to be 10%-20% of the total aromatics, as confirmed by the high-performance liquid chromatography (HPLC) and column chromatographic separation analysis of the aromatic fraction and sample as such. Therefore, signals due to longer alkyl chains primarily originate from the chain attached to the naphthenic ring of the aromatic. (e) The NMR analysis of VGO samples from different sources indicates a similar type of nature of aromatics (25) Pouchert, C. J., Behnke, J., Eds. The Aldrich Library of 13C and 1H FT NMR Spectra, Edition 1, Volume 2; Aldrich Chemical Company: Milwaukee, WI, 1993.
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Figure 4. 2D HSQC (edited) contour plot of VGO samples (G, R, and C); the -CH2 and -CH3 groups are indicated separately.
and substituents pattern (degree of substitution ≈ 40%45%), except for the distribution of monoaromatics and PAH, as confirmed by the 2D HSQC experiment of aromatic fractions from different sources (see Figure 3). Total Group Molecular Weight of the Sample. After the assignment of the 1H NMR spectrum has been made, the total aromatics content can be estimated using the developed equations, which relate the spectral integral intensities to the composition. The first step involves the calculation of the total group molecular weight (Tw) of the sample. The Tw for the sample is calculated by first dividing the individual regions in the spectrum by the number of protons causing the signals (see Table 1) and then multiplying by the molecular weight of the groups (12 for the C group, 13 for the CH group, 14 for the CH2 group, and 15 for the CH3 group). Tw is given as
D × (A1 × 13) + (B2 × 14) + (C3 × 15) + (1.5 E F 13.5) + 6B + 4C + ( × 14) + ( × 15) + (X × 12) 2 3
Tw )
(1) The quantity X in eq 1 is the contribution of substituted aromatic carbons (Arq) and bridgehead carbons (Cb); it is given as
X ) Arq + Cb
(2)
where the relative value of the quantity Arq has been estimated from the integrals of R-groups, relative to the aromatic rings, i.e., of spectral regions B and C.
Arq )
(B2 + C3)
(3)
The relative Cb content can be obtained from the spectral region of 7.4-9.0 ppm (Id). The signals in this region shall appear only due to the presence of di-plus aromatic ring protons, which also contain Cb carbons. It can also be appreciated that Cb is directly related to the concentration of the di-plus ring aromatic compounds. For example, in naphthalene-type molecules, out of a total of 10 carbons, there are 2 Cb carbons. The contribution of 8 of the carbons (unsubstituted and substituted) can be realized from the 1H NMR spectrum, whereas the 2 Cb carbons can approximately be realized from the integral intensity obtained in the region of 7.49.0 ppm. The derivation of the quantity Cb from Id has been explained in detail in our earlier publication,20 where Cb has been related to Id by the following equation:
Cb ) 0.7 × Id
(4)
It has also been shown in the previous publication that, because the quantity Cb appears both in the numerator and the denominator in the final equation, which calculates the total aromatics content, an error in the
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aforementioned equation does not produce any significant change in the final results. Therefore, the quantity X is given as
X)
(B2 + C3) + (0.7 × Id)
(5)
Average Alkyl Chain Length of Substituents on Aromatic Rings. The average alkyl chain length of the substituents on the aromatic rings (nsub) is generally obtained by applying the following equation on the 1H NMR spectrum of the pure aromatic fractions:26
nsub )
B+C+D+E B+C
(6)
However, the aforementioned equation gives a higher value of nsub, which could easily be appreciated by applying the equation to a model compound such as linear alkyl benzene (LAB), where n ) 8. The LAB samples from different souces have been found to contain both R-CH3 and R-CH2 substituents, as indicated by the 1HNMR spectral analysis. In the model compound, the theoretical value of nsub is 8, whereas eq 6 gives nsub to be in the range of 9-10. Moreover, nsub is an average of R-CH3 and higher groups. Because R-CH3 groups of 2.05-2.5 ppm can be separately identified and quantified, determination of the average alkyl chain length of long alkyl substituents (including naphthene rings of naphthoaromatic substituents other than methyl groups, denoted as n) will give a better understanding of the structure of aromatic fractions. The determination of n is also a prerequisite for estimating the total aromatics content of the VGO samples using the proposed 1H NMR method. The estimation of a higher value of nsub by the aforementioned equation by Knight,26 in the case of a model compound, has been attributed to the nonavailability of a multiplicity factor of R-protons (R-CHn, n ) 1, 2, 3) in the region of 2-4.0 ppm or 0.5-2.0 ppm. In the present case, the identity of the signals in the region of 2-4.0 ppm (R-CHn) and 0.5-2.0 ppm (-CH, -CH2, -CH3) have been revealed by the DEPT and 2D HSQC spectral analysis. The multiplicities are known and can be used in the aforementioned equations to estimate the average alkyl chain length correctly. A more realistic value of nsub and n can be obtained if relative moles of carbons or a multiplicity factor obtained from the 1H NMR spectrum are used. The value of n can be estimated using the following equation:
n)
(F/3) + (E/2) + (D/2) + (B/2) B/2
(7)
A value of n has been determined using the aforementioned equation on six 1H NMR spectra of aromatic fractions generated from six different VGO samples. The average value of n has been determined to be 5.0. It is reasonable to assume that the value of n will not vary much in other VGO samples that have a similar boiling range (370-560 °C). Aromatic Content of VGO Samples. To estimate the total aromatics content, the group molecular weight (26) Knight, S. K. Chem. Ind. (London) 1967, 1920.
Table 2. Total Aromatics Content of Several VGO Samples (370-560 °C) Obtained Using the Developed 1H NMR Method, Along with the Data Obtained Using Open Column Chromatography (ASTM Method D-2549) and TLC-FID Methods Total Aromatics Content sample
ASTM D-2549
NMR
TLC-FID
VGO-M VGO-P-1 VGO-H-1 VGO-H-2 VGO-BLEND VGO-U-1 VGO-B VGO-U-2 VGO-P-2 VGO-P-3 VGO-P-4 VGO-P-5 VGO-F VGO-S VGO-G VGO-R VGO-C
52 31 50 38 40 1.5 4 3 40 28 32 22 32 50 33.4 63.3 38.7
47 28.5 52 34 39 1 3 3.2 38 25.7 37.8 18 32 50 35 64.4 40.2
50 29.5 48.0 36.5 40.5
41.5 27.0 33.5 20 31 52 36.9 68.5 36.2
of the aromatics (Aw) is calculated, which is given as
Aw ) 13A + (n × 7B) + 5C + 12X
(8)
Substituting the value of X in the previous equation yields
[B2 + C3 + (0.7 × Id)] (9)
Aw ) 13A + (n × 7B) + 5C + 12
where n is the average chain length of the alkyl substituents on the aromatic ring, other than methyl groups. As explained previously, an average value of 5.0 has been obtained for the VGO-range fractions. Substituting n in the previous discussion, the total aromatics content of the sample can then be estimated as
total aromatics )
Aw × 100 Tw
(10)
The total aromatics content for various samples obtained from 1H NMR method is given in Table 2. For the purpose of further convenience, the calculations are performed using a small program written in BASICS. The integral intensities of various regions in the 1H NMR spectrum (Table 1) are provided as inputs, and the total aromatics content is obtained. To get reproducible results, the spectrum should be integrated three times and average values for the respective chemicalshift regions, as per Table 1, are used for calculation. Repeatability of the NMR Method. The repeatability of the developed NMR method has been estimated on three samples that have different average values of the total aromatics content. A single operator recorded each of the samples at least five times under similar experimental conditions on the same NMR equipment. Each time, a fresh sample solution was prepared in CDCl3 in a new NMR tube. The total aromatics content was estimated from the recorded NMR spectra using the previously developed equations. The value of the standard deviation (at a 95% confidence level) for the samples was 1.5.
Estimating the Total Aromatics in VGO Fractions
Energy & Fuels, Vol. 19, No. 3, 2005 1071
open column chromatography methods, which shows a high degree of correlation (R2 ) 0.99). This shows that the developed methodology has taken into consideration the accurate demarcation between the regions at -CH, -CH2, and -CH3 substituents in the 1H NMR spectra. This has resulted in the accurate estimation of aromatics in VGO-range samples. Conclusions A simple, direct, and rapid method that is based on NMR spectroscopy has been developed for estimating the total aromatics content (weight percentage) of vacuum gas oil (VGO, 370-560 °C) fractions. Substitution of 1H NMR spectral integral intensity data in the derived equations can yield the total aromatics content of VGOs, which agrees well with the results obtained using standard ASTM D-2549 and thin-layer chromatography-flame ionization detection (TLC-FID) methods. The developed NMR method can be applied with ease, because no instrumental intricacies are involved, which are the case with other methods. The proposed NMR-based method would be very useful in routine quality and process control applications, where a large number of such samples is required to be analyzed for their total aromatics content. 1H
Figure 5. Statistical correlation of aromatics results by the NMR-developed method and column data (ASTM D-2549).
Validation of the Proposed NMR Method. The proposed 1H NMR-based method has been validated by comparing the results with those obtained using other standard methods such as open column chromatography (ASTM D-2549) and TLC-FID (IP-469) methods. The percentage of total aromatics for several samples, as determined by these methods, is included in Table 2. The data obtained by the proposed NMR method are in very good agreement with those obtained by the standard methods. Figure 5 shows a correlation plot between the total aromatics content obtained by the NMR and
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