Hydrocarbon-Type Analysis of Middle Distillates by Mass

Aug 7, 2004 - In the present work, mass spectroscopy (MS) and 1H nuclear magnetic resonance (NMR) spectroscopy methods have been used to determine the...
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Energy & Fuels 2004, 18, 1505-1511

1505

Hydrocarbon-Type Analysis of Middle Distillates by Mass Spectrometry and NMR Spectroscopy TechniquessA Comparison V. Bansal,* S. Vatsala, G. S. Kapur, B. Basu, and A. S. Sarpal Indian Oil Corporation, Research and Development Centre, Sector-13, Faridabad, Haryana, India Received February 25, 2003

In the present work, mass spectroscopy (MS) and 1H nuclear magnetic resonance (NMR) spectroscopy methods have been used to determine the total aromatics content in diesel-range products. The data indicate that both the methods exhibit good correlation among each other, although the MS method has a tendency to provide higher aromatic results for the case of samples that contain a greater amount of sulfur. The possible factors responsible for variations in the results from both techniques have been discussed. The reproducibility of the aromatic data is dependent on the intrinsic nature of the technique and the analytical method applied. This systematic comparative study has proved that NMR spectroscopy is a fast and reliable alternative method to MS for estimation of the total aromatics content in diesel-range products.

Introduction The need for reliable and rapid analytical methods in energy research is essential, where fuel quality is correlated to their physicochemical properties and performance. Among the different classes of hydrocarbons in fuels, the aromatics content is identified as one of the predominant characteristics that influences combustion performance. Given the stringent environmental regulations, the optimization of product quality and product performance becomes critical. There are various standard methods and literature reports available for the determination of hydrocarbon types in liquid petroleum fractions.1-7 These include the FIA method (based on elution chromatography (ASTM D-1319)), liquid chromatography (IP-391), mass spectrometry (ASTM D- 2789, D-2425, D-3239), and supercritical fluid chromatography (ASTM D-5186). Methods based on the aforementioned techniques are wellestablished and are now routinely used in various laboratories. Although these methods are considered to be reliable, they are also laborious and time-consuming and they demand the utmost care on the part of the * Author to whom correspondence should be addressed. E-mail address: [email protected]. (1) Drews, A. W. ASTM Manual on Hydrocarbon Analysis, 4th ed.; ASTM: Philadelphia, PA, 1989. (2) Bundt, J.; Herbel, W.; Steinhart, H.; Francke, W. J. High Resolut. Chromatogr. 1991, 14 (2), 91-98. (3) Fuhr, B. J.; Klein, L. L.; Reichert, C.; Lee, S. W. LC-GC 1990, 8 (10), 800, 802-804. (4) Chen, E. N., Jr.; Cusatis, P. D.; Popid, E. J. J. Chromatogr. 1993, 637 (2), 181-186. (5) Trisciani, A.; Munari, F. J. High Resolut. Chromatogr. 1994, 17 (6), 452-456. (6) Li, W.; Malik, A.; Lee, M. L.; Jones, B. A.; Porter, N. L.; Richter, B. E. Anal. Chem. 1995, 67 (3), 647-654. (7) Malhotra, R.; Coggiola, M. A.; Young, S. E.; Spondt, C. A. Presented at the Division of Petroleum Chemistry, 212th National Meeting, American Chemical Society, Orlando, FL, August 25-29, 1996.

analyst (particularly, in regard to methods based on elution chromatography). In addition, these methods require various standards for calibration and other input data, such as boiling range, response factors, etc., for reliable results. Considerable efforts and research have been applied to optimize the conditions in each technique to improve upon the methods. Nuclear magnetic resonance (NMR) spectroscopy has emerged as an alternative technique for the determination of hydrocarbon composition, because of its rapidness, directness, and ease of analysis. Studies on the estimation of the aromatics content and average structural parameters used for structure-property correlation have been performed by various workers who are engaged actively in hydrocarbon-type analyses.8-12 Oneand two-dimensional NMR methods have been used to elucidate the structural characteristics of a set of monoaromatic fractions that were separated from petroleum fuels.10 Extensive work has been performed for hydrocarbontype analyses during the past decade, and there is a scope for further improvement. Lee et al.12 coordinated a study that statistically compared results obtained via ASTM D-1319 (FIA), NMR, mass spectroscopy (MS), and supercritical fluid chromatography (SFC) for hydrocarbon-type analysis. The average differences calculated from the average of individual variations showed that the NMR method has a tendency to overestimate the aromatics content, whereas FIA and MS have a (8) Von Deutsh, K. J. Prakt. Chem. 1977, 319, 439-443. (9) Muhl, J.; Srica, V.; Mimica, B.; Tomaskovie, M. Anal. Chem. 1982, 54, 1871-1874. (10) Cookson, D. J.; Smith, B. E. Energy Fuels 1987, 1, 111-120. (11) Glavincevski, B.; Gulder, O. L.; Gardner, L. Presented at the American Chemical Society Symposium, Miami, FL, September 1015, 1989. (12) Lee, S. W.; Coulombe, S.; Glavinceski, B. Energy Fuels 1990, 4, 20-23.

10.1021/ef030046o CCC: $27.50 © 2004 American Chemical Society Published on Web 08/07/2004

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Figure 1.

Bansal et al.

1

H NMR (300 MHz) spectrum of a representative diesel (HSD) sample.

tendency to underestimate it, compared to that obtained using SFC. The NMR method developed by Lee and coworkers for the estimation of the total aromatics in distillate fuels uses a formula that involves hydrogenand carbon-type distribution data to derive equations based on a substructure relationship, similar to those developed by Glavincevski et al.11 and Muhl et al.9 The aromatics content is expressed as a function of molecular structure parameters, such as R-CHn hydrogen and the ratio of R-hydrogen to carbon, which are normally obtained from the spectral features of 1H and 13C NMR spectra, and also uses boiling-point information. As described previously, the spectroscopic methods that are based on MS, NMR, and other standard methods have shown variations in the results from each technique. However, no plausible explanation for the observed discrepancies has been given. In the present work, the aromatics content of middle distillates has been determined using the NMR method that was developed in our laboratory13 and the MS14 method. Factors responsible for the variations in the results for both the techniques have been identified, and solutions have been suggested to further narrow down the differences in the results. Experimental Section Materials. Samples of high-speed diesel (HSD) and superior kerosine oil (SKO) in the boiling range of 140-350 °C were obtained from different Indian refineries. These refineries were processing crude from different sources. NMR Measurements. 1H NMR spectra have been obtained on a 300 MHz NMR spectrometer under the following conditions: spectral width, 5000 Hz (0.0-12.0 ppm); spectral size, 16K; digital resolution, 0.49 Hz/point; 90° pulse, 18 µs; relaxation delay, 10 s; number of scans, 64; and solution concentration, 5% in CDCl3 and tetramethylsilane (TMS) as an internal reference. Integration has been conducted after baseline correction, and a mean of three integration values has been taken for each calculation. Mass Spectrometric Measurements. Mass spectra of the samples were generated under the following conditions: sample introduction through an all-glass heated inlet system (AGHIS), (13) Bansal, V.; Kapur, G. S.; Sarpal, A. S.; Kagdiyal, V.; Jain, S. K.; Srivastava, S. P.; Bhatnagar, A. K. Energy Fuels 1998, 12, 12231227. (14) Teeter, R. H. Mass Spectrom. Rev. 1985, 4, 123-43.

dynamic resolution of 5000 at a scanning speed of 100 s/decade, and an interscan delay of 1 s. Seven scans were acquired and averaged, and the data were processed using hydrocarbon-type analysis software (HC-22) that was obtained from PCMASPEC, USA. The analysis required input of the distillation data, which were generated via gas chromatography (GC), per standard method ASTM D-2887. The analysis provided eight classes of saturates, 10 classes of aromatics, and four classes of sulfur aromatics. Open Column Chromatography. Column chromatography work has been performed according to the standard method ASTM D-2549-91 (which was reapproved in 1995).

Results and Discussions Nuclear Magnetic Resonance Methodology. Figure 1 shows the 1H NMR spectrum of a high-speed diesel (HSD), along with the expanded 2.0-9.0 ppm region. Various resonance signals assigned to different groups have been marked in the spectrum. The spectrum has been divided into various regions: aromatic ring protons (region A), 6.5-9.0 ppm; R-alkyl (CH2, CH3) groups to aromatic ring (regions B and C), 2.0-4.0 ppm; β-CH and CH2 groups to aromatic rings and -CH and CH2 groups of cycloalkanes and normal and isoparaffins (region D), 1.0-2.0 ppm; and -CH3 of branched and normal paraffins (region E), 0.5-1.0 ppm. The integral areas of these regions have been denoted by letters A, B, C, D, and E, respectively. The detail of obtaining the total aromatics content from 1H NMR has been described in our earlier publication,13 and only a summary of the method is described here. From an NMR point of view, any molecule containing even one aromatic ring will be considered to be an aromatic compound. After complete assignment of the 1H NMR spectrum and the relative contribution of the bridgehead aromatic (Arb) and substituted aromatic (Arq) carbons have been realized, the total aromatics content of the samples was calculated. The main steps involved in estimation of the total aromatics content are (i) estimation of the total relative number of carbons (Tc) and (ii) the total group molecular weight (Tw) of the sample. The Tc value for the sample is calculated by dividing the individual regions in the 1H NMR spectrum by the number of protons causing the signal. Tw is then obtained by multiplying these

Analysis of Middle Distillates by MS and NMR

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numbers by the respective molecular weight of the groups (12 for C, 13 for CH, 14 for CH2, and 15 for the CH3 group). Therefore, Tc and Tw are given as

Table 1. Comparative Studies of Aromatic Data by Nuclear Magnetic Resonance (NMR) and Mass Spectroscopy (MS) Techniques (Dataset I) Total Aromatics (%, w/w)

A B C D E Tc ) + + + + + Arb + Arq 1 2 3 2 3 Tw )

(A1 )13 + (B2 )14 + (C3 )15 + (D2 )14 + (E3 )15 +

(Arb + Arq)12

Substituting the contributions of Arb and Arq as described in our earlier work,13 the aforementioned equation simplifies to

Tw ) 13(A + B) + 9C + 7D + 5E + 8.2Id To estimate the total aromatics content, the group molecular weight of the aromatics (Aw) is estimated, which is given as

Aw )

(A1 )13 + n(B2 )14 + (C3 )15 + (Ar

b

+ Arq)12

Substituting the value of the average alkyl chain length of the aromatic substitutents (n) ) 3 and those values for Arb and Arq as described in our previous work,13 the equation simplifies to

Aw ) 13A + 27B + 9C + 8.2Id The total aromatics content of the sample (given as a weight percentage) can then be estimated as

total aromatics content (TA) )

( )

Aw × 100 Tw

(1)

The total aromatics content for various samples obtained from the 1H NMR-based equation is given in Tables 1 and 2 for datasets I and II, respectively. To cover a broad range, samples that contained a varying amount of aromatics and a total sulfur content of 0.0%-2.0% have been analyzed. Mass Spectroscopy Methodology. Hydrocarbontype analysis was performed, per the method described by Teeter.14 The sample was introduced into the mass spectrometer through an AGHIS system, and data acquisition was conducted as described in the Experimental Section. Seven scans were acquired for each run and averaged for processing. Each sample was repeated three times to ensure repeatability/reproducibility of the data. The mass intensity list of the averaged spectrum was processed using HC-22 software. The basis of an HC-22 type of analysis is the “Z” number, which is described as the number of H atoms relative to the number of C atoms, as expressed by the letter Z in the general empirical formula CnH2n+Z. All members of the homologous series have the same value for Z. For example, for paraffins, Z ) +2. The other saturate classes (namely, naphthenes) have Z values of 0, -2, -4, -6, -8, -10, and -12, which corresponds to naphthenes with one, two, three, four, five, six, or seven rings, respectively. For compounds that have the same nominal masses, the fragmentation pathways determine the fragment to be considered. Hydrocarbon fragmentation is dependent

Difference, NMR - MS

sample

via NMR

via MS

difference

S-1 S-2 S-3 S-4 S-6 S-7 S-8 S-9 S-10 S-15 S-16 S-17 S-18 S-19 S-21 S-22 S-24 S-25 S-26 S-33 S-34 S-37 S-38 S-39 S-40 S-41 S-42 S-43 S-48 S-49 S-50 S-51

30.2 37.2 48.3 31.8 17.2 19.7 6.6 6.4 19.1 17.2 18.6 18.0 28.4 27.4 20.6 31.7 27.8 29.3 23.8 20.0 29.7 27.9 24.1 28.2 2.3 1.8 4.1 31.7 1.2 1.5 3.2 1.4

29.4 38.2 50.3 30.8 19.3 20.4 7.9 6.8 20.8 19.2 19.0 19.3 28.2 23.5 18.0 29.5 29.5 26.4 23.0 21.6 33.1 31.8 24.1 30.8 4.6 2.8 6.7 35.4 1.5 2.1 4.2 3.6

0.8 -1.0 -2.0 1.0 -2.1 -0.7 -1.3 -0.4 -1.7 -2.0 -0.4 -1.3 0.2 3.9 2.6 2.2 -1.7 2.9 0.8 -1.6 -3.4 -3.9 0.0 -2.6 -2.3 -1.0 -2.6 -3.7 -0.3 -0.6 -1.0 -2.2

absolute sulfur aromatics value via MS (% w/w) 0.8 1.0 2.0 1.0 2.1 0.7 1.3 0.4 1.7 2.0 0.4 1.3 0.2 3.9 2.6 2.2 1.7 2.9 0.8 1.6 3.4 3.9 0.0 2.6 2.3 1.0 2.6 3.7 0.3 0.6 1.0 2.2

0.1 0.9 4.1 3.7 0.7 0.8 0.1 0.1 0.0 0.8 0.0 0.0 4.6 0.6 0.6 1.5 3.2 1.9 3.5 2.5 2.4 2.3 0.2 1.9 0.0 0.0 0.1 1.8 0.0 0.1 0.0 0.1

on structure; therefore, the abundance of the characteristic fragment ion is dependent on the specific compound present in the sample. Hence, it is necessary to consider the abundance of several ions of each group to be summed. In addition, data from distillation via standard ASTM D-2887 is also used as input data, which allows the selection of the matrix to be used, for calculation purposes, by the HC-22 software used. The calculation yields 8 classes of saturates, 10 classes of aromatics, and 4 classes of sulfur aromatics. The percentage of aromatics and sulfur aromatics were added together to obtain the total aromatics content of the sample. Comparison of Data by Nuclear Magnetic Resonance and Mass Spectroscopy Methods. A comparison of aromatics content determined by the 1H NMR and MS techniques is presented in Tables 1 and 2. This comparison of NMR and MS data reveals the following facts: (1) There is a good correlation between the NMR and MS results in dataset I, where the absolute difference of aromatics between the two techniques is 0%-4.0% (see Figure 2a, correlation of R ) 0.988). (2) In the remaining samples (dataset II), the results obtained using the MS method had a tendency to be higher, by 25%-35%, compared to the NMR method, where the absolute difference of aromatics between the two techniques is higher and lies in the range of 4%16.0% (see Figure 2b, correlation of R ) 0.785). The data obtained by the two methods under investigations (i.e., NMR and MS) was further evaluated

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Bansal et al.

Table 2. Comparative Studies of Aromatic Data by NMR and MS Techniques (Dataset II) Total Aromatics (%, w/w)

Difference, NMR - MS

Aromatics Content (%, w/w)

sample

NMR

MS

difference

absolute value

sulfur, via MS

mono-ring

di-ring

tri-ring

tetra-ring

S-5 S-11 S-12 S-13 S-14 S-20 S-23 S-27 S-30 S-31 S-32 S-35 S-28 S-29 S-36 S-48 S-44 S-45 S-46 S-47

32.6 25.7 26.6 31.4 27.9 31.7 25.5 27.8 27.2 29.3 26.1 34.2 26.4 24.6 25.3 29.1 29.0 29.3 22.5 32.9

39.8 32.2 32.4 39.8 32.3 40.2 33.3 40.6 34.1 42.9 38 40.6 34.1 30.1 33.5 42.3 36.7 35.3 30.4 48.6

-7.2 -6.5 -5.8 -8.4 -4.4 -8.5 -7.8 -12.8 -6.9 -13.6 -11.9 -6.4 -7.7 -5.5 -8.2 -13.2 -7.7 -6.0 -7.9 -15.7

7.2 6.5 5.8 8.4 4.4 8.5 7.8 12.8 6.9 13.6 11.9 6.4 7.7 5.5 8.2 13.2 7.7 6.0 7.9 15.7

6.5 7.4 7.6 6.5 10.3 10.1 6.3 7.1 5.8 7.6 5.5 7.6 1.3 2.9 2.2 16.4 3.0 1.5 8.4 18.3

9.9 11.1 10.4 10.5 11.8 12 14.2 18.3 15.7 20.8 19.3 19.1 20.9 18.1 21.8 10.3 23.3 25.3 12.6 12.5

18.8 10.9 11.7 16 8.2 12.7 12 12 11.7 11.7 11.4 11.2 11.1 8.4 8.7 10.5 8.5 6.9 8.5 11.3

4.4 2.6 2.6 3.2 1.9 1.4 1.4 2.9 1.5 2.6 1.8 2.5 0.8 0.7 0.8 5.1 1.8 1.5 0.9 5.5

0.2 0.2 0.1 0.2 0.1 0 0 0.3 0 0.2 0 0.2 0 0 0 0 0.1 0.1 0 1

Figure 2. Plot of NMR versus MS data, using (a) dataset I and (b) dataset II. Table 3. Statistical Comparison of Data Regarding the Aromatics Content Obtained via Nuclear Magnetic Resonance (NMR) and Mass Spectroscopy (MS) parameter correlation, R average difference standard deviation of the difference average absolute deviation, AAD standard deviation of the absolute difference

dataset I (NMR vs MS)

dataset II (NMR vs MS)

0.988 -0.794 1.883 1.694 1.111

0.785 -8.605 3.112 8.605 3.112

separately, in a statistical manner, for datasets I and II. The results are given in Table 3. The average difference calculated from the averages of individual variations show that the MS method has a tendency to overestimate the total aromatics content, compared to the NMR method, for both datasets. Alternatively, it can be construed that the NMR method is underestimating the aromatics content. However, as stated previously, the difference is much greater for dataset II, where the absolute difference of aromatics

data between the MS and NMR methods is in the range of 4.0%-16.0%. The values of the absolute deviations also show a similar trend. Similarly, the standard deviations associated with these averages confirm that values are more scattered for dataset II, compared to dataset I, which shows better correlation between the two methods. To investigate the possible causes for differences in the results of aromatic data between the two techniques as observed in dataset II, efforts were made to correlate the absolute difference of the aromatics content between that obtained using MS and NMR versus various groups of aromatics, i.e., mono-ring, di-ring, tri-ring, tetra-ring, and sulfur aromatics of the samples as obtained by the MS technique (see Table 2 and the Supporting Information). A very poor correlation is observed among the absolute differences of the aromatics content versus mono-ring aromatics and di-ring aromatics (R ) 0.002 and 0.170, respectively); however, a moderate correlation exists among the absolute differences of the aromatics content versus tri-ring, tetra-ring, and sulfur aromatics (R ) 0.576, 0.534, and 0.557, respectively). This indicates that the presence of tri-ring, tetra-ring, and sulfur aromatics have a bearing on the values of total aromatics determined by NMR and MS techniques. The statistical treatment of the results has brought the following facts to light: (i) the presence of higherring aromatics, including sulfur aromatics, might be interfering in the methodologies adopted by the NMR and MS techniques; and (ii) because determination via the MS technique has shown higher variations in the samples that have a higher sulfur content, sulfur aromatics might be affecting the results, to some extent. This would require further detailed investigation. The results obtained using the NMR technique will also be affected marginally, because the C-S resonances will not appear in the 1H NMR spectra. Critical assessment of the data for various samples by NMR and MS has revealed the fact that, generally, samples that contained a higher amount of sulfur aromatics (5.0%-16.0%; see Table 2) showed higher aromatics contents, according to MS. The comparison is generally very good in samples that contained a low percentage of sulfur aromatics (0.0%-5.0%; see Table

Analysis of Middle Distillates by MS and NMR

Figure 3. Plot of NMR data (total aromatics) versus MS data (total aromatics - sulfur aromatics). Table 4. Estimation of Sulfur Content sample

sulfur content by XRF (%)

sulfur aromatics content, by MS (%)

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

0.2 0.61 0.2 0.17 0.19 0.17 0.16 0.2 0.69 0.25 1.35 0.01

2.9 7.6 2.5 2.3 2.3 2.2 1.9 2.4 7.6 3.4 16.4 0.1

1). When the contribution of sulfur-containing aromatics was removed from the total aromatics, the results obtained by the MS technique for the entire range of samples are found to be in excellent agreement with the NMR results (see Figure 3, R ) 0.998). This observation indicates that the main reason for the difference in results is the sulfur-containing species, which are either being underestimated by the NMR method or overestimated by the MS method. Given the aforementioned findings, an examination was made to determine if any direct correlation existed between the absolute difference of values by the NMR and MS techniques and the sulfur content of the sample. However, sulfur data obtained by X-ray fluorescence (XRF) techniques were available for only a few samples, whereas the sulfur aromatics content that was determined via MS was available for all the samples. Therefore, sulfur content data via the XRF technique was correlated with sulfur aromatics data obtained by MS (see Table 4), and, as expected, a high level of correlation was obtained (R ) 0.998; see Supporting Information). Furthermore, a secondary correlation was drawn between the absolute difference of values obtained by the NMR and MS techniques and the sulfur aromatics data obtained via the MS method, which should also reflect the correlation between the absolute difference of values and the sulfur content of the sample. A correlation between the absolute difference of values obtained via the NMR and MS techniques and the sulfur aromatics obtained via the MS method gave a moderately high correlation value (R ) 0.785), indicating that the sulfur content of the sample is one of the main factors that influences the results obtained using the NMR and MS techniques.

Energy & Fuels, Vol. 18, No. 5, 2004 1509

To further confirm the effect of sulfur-containing compounds on the estimation of aromatics by both techniques, pure aromatics and saturate fractions (see Figure 4a and 4b, respectively) were obtained from samples that contained a greater amount of sulfur aromatics (>5.0%), via column chromatographic separation, using an alumina:silica (1:3) mixture as an adsorbent and eluting with solvents of increasing polarity. The blends of these saturates and aromatics thus separated were prepared in different proportions and analyzed by both NMR and MS methods (Table 5). The MS method again shows higher values of aromatics than the blended one, whereas the NMR results were found to be match those of the blended values. Contribution due to Sulfur Aromatics. Sulfurcontaining aromatics, which are normally found in middle distillates, include compounds such as benzothiophenes, dibenzothiophenes, naphthanothiophene, and their substituted derivatives.15-18 The possibility of the NMR method underestimating the aromatics content because of the presence of sulfur compounds has been ruled out, per the following discussion. In the 1H NMR spectrum, proton signals due to the aforementioned types of sulfur-containing aromatics will also appear in the aromatic region (6.0-9.0 ppm), along with the signals that are due to other non-sulfurcontaining aromatics.19 However, the contribution of S atoms will not be realized by 1H NMR spectroscopy, because these behave similar to nonprotonated C atoms. The atomic weight of sulfur is 2.7 times that of carbon; therefore, a slightly lower value of the total group molecular weight of the total aromatics is estimated. This is likely to introduce a small error in the estimation and subsequently will estimate a slightly lower value of aromatics. However, it has been estimated that the decrease in the value will not be more than 5.0% of the total aromatics for a sample that contains ∼10.0% sulfur aromatic compounds, which is the maximum expected percentage. This has been demonstrated in Appendix A, using a theoretical blend of components. The exercise in Appendix B shows that the total aromatics content is underestimated by only ∼1.5%, because of the nonvisibility of sulfur in the 1H NMR spectrum. This is valid for a sample that contains ∼10.0% sulfur aromatics (or 40.0% total aromatics). This has proved that the contribution that is due to sulfur-containing compounds is mostly observed in the 1H NMR spectral analysis and the introduction of error is marginal. On the other hand, the HC-22 class, which uses the MS method, has limitations for samples that contain a greater amount of sulfur. This is due to the fact that, under the presently used MS resolution, the amount of benzothiophene may be overestimated, because there is an overlap between a characteristic fragment of C15H9 (which has a mass of 189.0704) with that of C12H13S (15) Bhatia, V. K.; Singh, H.; Kaul, S.; Parasada Rao, T. S. R. Hydrocarbon Technol. 1999, 56-66. (16) Chawla Birbal; Di Sanzo F., J. Chromatogr. 1992, 589, 27179. (17) Jewell, D. M.; Ruberto, R. G.; Swansiger, J. T. Presented at the Division of Petroleum Chemistry, Philadelphia Meeting, April 6-11, 1975, Paper No. p-19. (18) Ali, M. F.; Perzanowski, H.; Koreish, S. A. Fuel Sci. Technol. Int. 1991, 9 (4), 397-424. (19) Pouchert, C. J.; Behnke, J. The Aldrich Library of 13C and 1HFT NMR Spectra, Edition-1; Aldrich Chemical Co., Inc.: Milwaukee, WI.

1510 Energy & Fuels, Vol. 18, No. 5, 2004

Figure 4.

Bansal et al.

1

H NMR spectra of (a) pure saturates and (b) aromatic fractions separated via column chromatography.

Table 5. Nuclear Magnetic Resonance (NMR) and Mass Spectroscopy (MS) Data of Blends sample 1 2 3 4

Total Aromatics (%, w/w) actual blends NMR MS 27.4 48.9 23.9 47.4

28.4 50.2 24.5 49.5

35.7 58.5 30.2 57.6

sulfur aromatics, via MS (%, w/w) 9.8 15.0 8.5 14.5

(which has a mass of 189.0737); the resolution of these two masses would require an instrument resolving power of 57 000. The overestimation by the MS method might also be due to the different response factors of sulfur aromatics, compared to aromatic compounds. However, to further substantiate the findings, moredetailed investigations are required to study the effect of sulfur aromatic and polyaromatic compounds on the determination of aromatics via the MS technique. Assessment of Nuclear Magnetic Resonance and Mass Spectroscopy Methodologies. The determination of the aromatics using the two techniques has been subjected to ANOVA (analysis of variance) analysis, which is a statistical tool that separates and estimates the possible causes of variation and, hence, systematic and random errors. ANOVA analysis has been performed to examine the variation in the NMR and MS data separately, including variations that are due to the operator, the recording time during the day, instrumental operating parameters, etc. The parameters that were noted while the 1H NMR spectra were recorded are sample preparation/ sample concentration (5% v/v), sensitivity/resolution of the spectrometer, shimming of instrument for homogeneity of field, and baseline correction. The parameters that were noted during analysis using the MS method are instrument resolution, sample quantity, AGHIS heater temperature, scanning speed, and total ion current (TIC) of each scan. The dataset shown in Table 6 indicated that there was no significant variation in the NMR data, with respect

Table 6. ANOVA Analysis of Nuclear Magnetic Resonance (NMR) Data sample replicate 1.0 replicate 2.0 replicate 3.0 replicate 4.0 A A A A B B B B

28.7 27.3 29.3 27.9 27.4 28.2 27.8 27.7

26.9 26.6 29.2 27.6 27.6 27.2 27.2 27.5

27.4 26.5 27.8 26.8 27.4 28.5 29.3 26.9

26.5 27.7 29.2 28.5 27.0 28.4 26.9 27.3

Table 7. ANOVA Analysis of Mass Spectroscopy (MS) Data sample replicate 1.0 replicate 2.0 replicate 3.0 replicate 4.0 1 2 3 4 5 6

25.4 29.1 37.4 41.5 26.2 30.9

26.7 29.1 33.2 43.4 26.2 30.7

28.1 31.1 34.5 40.8 27.5 32.7

27.3 32.5 33.9 41.6 27.5 32.9

to operator and time of analysis. Day-to-day variations that affected the precision of the method were also found to be insignificant (Fcalculated ) 1.519, Fcritical ) 3.634). Standard deviation and repeatability were determined to be 0.70 and 2.55, respectively. The aromatic data set obtained using the MS method (shown in Table 7) also indicated that there was no significant variation in the MS data, with respect to the instrument (Fcalculated ) 0.5188, Fcritical ) 2.4377), the operator (Fcalculated ) 0.0764, Fcritical ) 2.534), and the time of analysis. Day-to-day variations were also found to be insignificant, which affects the precision of the method, and proper setting of the instrumental parameters is the main prerequisite for achieving better precision of the data. Standard deviation and repeatability were determined to be 1.33 and 4.83, respectively. The aforementioned exercise proved that the difference bewteen the results obtained using the NMR and

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Energy & Fuels, Vol. 18, No. 5, 2004 1511

Table B-1. Composition of a Theoretical Blend of Saturates and Aromatic Hydrocarbons

Table B-2. Relative Integral Intensities region

A

B

C

D

E

Id

absolute number of protons in a region, N relative integral intensity, C C × GMWa

18 24.98 324.74

2 4.048 28.336

3 6.072 30.36

20 111.46 780.22

9 39.51 197.55

4 4.806 57.672

a

GMW denotes group molecular weight. Table B-3. Total Aromatics Content, Determining by Ignoring and Including the Contribution from Sulfur

a

parameter

ignoring the contribution of sulfur

including the contribution of sulfur in region Id

total group molecular weight of sample, Twa group molecular weight of aromatics, Awb total aromatics (%)

1467.454 532.188 36.3

1467.454 + 32 ) 1499.454 532.188 + 32 ) 564.188 37.6

Tw ) 13(A + B) + 9C + 7D + 5E + 8.2Id. b Aw ) 13A + 27B + 9C + 8.2Id.

MS methods could not be ascribed to the experimental conditions or the repeatability/reproducibility of the methods. Conclusions This systematic comparative study has proved that nuclear magnetic resonance (NMR) is an fast and reliable alternative method for estimating the total aromatics in middle distillate fuels. The method is applicable to a variety of samples available from different refineries that are processing crude of different quality. The NMR method provides results that are comparable to those obtained using the HC-22 method; however, the former methodology has been proven to be superior when samples contain higher sulfur compounds. The error introduced because of the high sulfur percentage is much higher in the MS method, compared to the NMR method. Moreover, compositional analysis in terms of total aromatics and saturates can be obtained within a short span of time (i.e., ∼10 min), and the NMR method is independent of any standards. Appendix A Calculation of the total aromatics content can be made using blends of high-speed diesel (HSD) with different contents of sulfur aromatics. For example, in the 1H NMR spectrum of sample S-50 with no sulfur compounds, the following intensities of the proton NMR integrals are obtained: A ) 0.25, B ) 0.5, C ) 0.5, D ) 66, E ) 26, Id ) 0. The total aromatics content (TA) is 3.2% (w/w), using the NMR method (see eq 1). Case 1. When 4.48% (w/w) of sulfur aromatics are blended in sample S-50, the integral intensities become A ) 2.0, B ) 0.5, C ) 0.5, D ) 67, E ) 26, and Id ) 0.7. The total aromatics content is TA ) 7.3% (w/w), using the NMR method (eq 1), and the actual total aromatics content in blend 1 is 7.5% (w/w).

Case 2. When 3.2% of sulfur aromatics are blended in sample S-50, the integral intensities become A ) 1.8, B ) 0.5, C ) 0.5, D ) 75, E ) 30, and Id ) 0.6. The total aromatics content is TA ) 6.1% (w/w), using the NMR method (eq 1), and the actual total aromatics content in blend II is 6.3% (w/w). The difference in the total aromatics contents (between actual values and those estimated using the NMR method) in case 1 and case 2 is within the repeatability of the present NMR method. Appendix B Calculation of the total aromatics content in a theoretical blend of saturate and aromatic hydrocarbons can be made by (a) ignoring and (b) including the contribution of sulfur, using the present NMR method. For example, consider a theoretical blend that contains benzothiophene (I), 1,3-methyl ethyl benzene (II), naphthalene (III), and dodecane (IV) in the respective proportions of 10%, 20%, 10%, and 60% (w/w). Theoretically, the total aromatics content should be 40% (w/w) and the total saturates content should be 60% (w/w). Component content, in terms of mole percent, are given in Table B-1. Per the present NMR method, the relative integral intensities, which are given in Table B-2, will be obtained in the 1H NMR spectrum of the blend with the aforementioned composition. The total aromatics content, obtained by ignoring (and including) the contribution of sulfur (as a percentage), is given in Table B-3. Supporting Information Available: Figures showing plots of the absolute difference of aromatics between data obtained using NMR and MS methods, and a figure showing a plot of sulfur content obtained via XRF versus the sulfur aromatics content obtained via MS. This material is available free of charge via the Internet at http://pubs.acs.org. EF030046O