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Energy & Fuels 2004, 18, 30-36
1H
NMR Spectroscopic-Based Method for the Estimation of Total Aromatic Content of Aviation Turbine Fuels (ATF): Comparison with Liquid Chromatographic Methods S. Mukherjee, G. S. Kapur, Anju Chopra, and A. S. Sarpal* Indian Oil Corporation Limited, Research & Development Centre, Sector -13, Faridabad-121007, India Received November 19, 2002. Revised Manuscript Received April 17, 2003
This work describes the development of a direct and quick method based on 1H NMR spectroscopy for the determination of total aromatic content of jet and aviation turbine fuels (ATF). The method is based on the calculation of the average group molecular weight of aromatics from the integral intensities of characteristic regions of aromatics 6.5-8.0 ppm and R-subsituents (2.1-3.0 ppm). The average chain length of the alkyl subsituents on aromatic rings (other than methyl groups) has been determined from the 1H NMR spectrum of pure aromatic fractions of a few aviation fuels. The NMR results have been compared, and have been found to be well correlated with the other standard methods based on liquid chromatography (LC) (ASTM-D1319 (FIA) and IP-436 (HPLC)). The results obtained from the three investigated methodss NMR, FIA, and HPLCshave been statistically evaluated and found to give equivalent results. The developed 1H NMR method is quite fast and reliable, and offers an alternative to long and tedious LC-based methods in routine quality control.
Introduction It has been well recognized that the total aromatic hydrocarbons in the petroleum fractions significantly affect the combustion process in different engines. The control specifications of aviation turbine fuels (ATF) limit the total aromatics content to below 20-25 vol % and olefins to below 5% vol. The aviation industry is more concerned about specification of the amount of aromatics due to their poorest combustion performance. The incomplete combustion of aromatics results in the black exhaust increasing the visual identification of enemy aircraft. The fuel system components, which are made from certain elastomers, are also affected when the aromatics content is more than 30% w/w. Similarly, the limit on olefins is necessary in order to prevent the fuel flow problem during flight on account of the gumforming tendency of unsaturated compounds. An accurate determination of total aromatic content in a fuel sample (gasoline, kerosene, aviation turbine fuel, diesel, etc.) is always very critical, and a number of analytical methods have been developed for this purpose. In general, there are two approaches for the analysis of complex petroleum fuels. In one approach, separation into single components or groups of components is carried out, followed by subsequent identification and quantitation. This is performed by chromatographic methods, e.g., gas chromatography (GC),1,2 * Corresponding author. E-mail:
[email protected]. (1) Protic-Lovasic, G.; Jambrec, N.; Deur-Siftar, D.; Prostenik, M. V. Fuel 1990, 69, 525. (2) Haas, A.; McElhiney, G.; Ginzel, W.; Buchsbaum, A. Erdol Kohle, Erdgas, Petrochem. 1990, 43, 21.
supercritical fluid chromatography (SFC),3 high-performance liquid chromatography (HPLC),4 or fluorescent indicator adsorption (FIA).5 The aromatic content of ATF is usually determined by the standard methods based on fluorescence indicator adsorption5 and high performance liquid chromatograph, IP- 436.4 Both methods, particularly ASTM-1319, are least precise and time-consuming, and results are affected by conditioning of columns, substrate purity, and overlapping of bands in the case of higher amounts of olefins.6-8 Some of the other documented limitations of the FIA method are the following: long analysis time, poor resolution between the hydrocarbon groups, poor precision due to operator’s inability to distinguish the borders of the colored bands, limited applicability to colored samples (i.e., higher molecular weight samples), inability to analyze samples with light hydrocarbons (below hexane) or with end points over 315 °C by FIA, and error due to variation in the dye-composition. Similarly, the other widely used method, i.e., IP-436, has also been reported to have a possibility of error in determining total aromatic content. This is mainly associated with the detector response, e.g., the UV detector gives varying response for different aromatics; (3) Lee, S. W.; Fuhr, B. J.; Holloway, L. R.; Reichert, C. Energy Fuels 1989, 3, 80. (4) IP-436/98 Method, Determination of aromatic hydrocarbon types in aviation fuels and petroleum distillates - High Performance Liquid Chromatography Method with refractive index detector. (5) American Society for Testing and Material, ASTM Test Method D1319, In Manual on Hydrocarbon Analysis; Philadelphia, 1977. (6) Lysaght, M. J.; Kelly, J. J.; Calles, J. B. Fuel 1993, 72 (5), 623. (7) Campbell, R. M.; Djordjevic, N. M.; Markides, K. E.; Lee, M. L. Anal. Chem. 1988, 66, 356. (8) Norris, A. T.; Ramdon, M. G. Anal. Chem. 1984, 56, 1767.
10.1021/ef020275v CCC: $27.50 © 2004 American Chemical Society Published on Web 11/05/2003
Estimation of Total Aromatic Content of ATF
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the RI detector requires calibration for “typical” saturates, olefin, and aromatics; and the FID detector requires mobile phase evaporation and is not suitable for volatile samples. Since toluene and xylenes are used as standards for calibration in IP-436, results might be different due to the different response factors of various other types of aromatic components present in a sample. The reproducibility limits are also high, and critical examination of data is necessary in borderline cases. In our laboratory, also, we have observed a large variation in the results obtained from the ASTM-1319 and IP-436 methods.
Experimental Section
The other approach for hydrocarbon-type analysis is based on molecular spectroscopic techniques such as middle infrared (IR)9 and nuclear magnetic resonance (NMR)10-18 spectroscopy. Such methods rely upon the identification of various structural groups due to different hydrocarbon classes in a sample and provide average compositional information. These non standard methods based on NMR, IR, and MS are also applied routinely to carry out hydrocarbon type analyses. Though these approaches are rapid and accurate, these are not cost-effective due to high cost of the instruments. Although, a large number of methods exist for determining the aromatic content of a fuel sample, many of the techniques used are associated with problems of incomplete chromatographic separation or sample cleanup, nonuniform detector response, and some times long and tedious analytical procedures. Therefore, care must be taken to evaluate the results obtained by two different methods. Lee et al.19 had carried out a systematic study investigating various methods such as FIA, NMR, SFC, and mass spectrometric (MS) for determining aromatics in middle distillate fuels. As a part of a long drawn-out program in our laboratory, we are involved in developing direct, fast, and reliable methods for determining aromatic content in fuel samples using proton (1H) NMR spectroscopy.11-13 Such a method has been found to be very useful in situations where a large number of samples are required to be analyzed in a short time period. In the present work, a direct method based on 1H NMR spectroscopy has been developed for the estimation of total aromatic content of aviation turbine fuels. The validity and reliability of the proposed NMR method has been checked by comparing the NMR results with those from FIA and HPLC methods. (9) Iob, A.; Ali, M. A.; Tawabini, B. S.; Anabtawi, J. A.; Ali, S. A.; Al-Farayedhi, A. Fuel 1995, 74, 227. (10) Muhl, J.; Srica, V.; Mimica, B.; Tomaskovic, M. Anal. Chem. 1982, 54, 1871. (11) Kapur, G. S.; Singh, A. P.; Sarpal, A. S. Fuel 2000, 79, 1023. (12) Sarpal, A. S.; Kapur, G. S.; Mukherjee, S.; Tiwari, A. K. Fuel 2001, 80, 521. (13) Bansal, V.; Kapur, G. S.; Sarpal, A. S.; Kagdiyal, V.; Jain, S. K.; Srivastava, S. P. Energy Fuels 1998, 12, 1223. (14) Lee, S. W.; Glavincevski, B. Fuel Process. Technol. 1999, 60, 81. (15) Singh, A. P.; Mukherjee, S.; Tiwari, A. K.; Kalsi, W. R.; Sarpal, A. S. Fuel 2003, 8, 233-333. (16) Joo, H. S.; Guin, J. A. Fuel Process. Technol. 1998, 57, 25-40. (17) Meusinger, R. Fuel 1996, 75 (10), 1235-1243. (18) Myers, M. E., Jr.; Stollsteimer, J.; Wins, A. M. Anal. Chem. 1975, 47, 2010. (19) Lee, S. W.; Coulombe, S.; Glavincevski, B. Energy Fuels 1990, 4, 20.
Sample. The samples of aviation turbine fuels were obtained from different Indian refineries processing crudes of different origins. Around 70 samples have been used in this study. The average hydrogen content (wt %) of these samples calculated on the basis of NMR parameters lies in the range 13.4-14.4. The density of a number of ATF samples determined as per ASTM-D 4052-96 lies in the range 0.780-0.820 g/mL. The typical density of aromatics present in ATF is taken as 0.89 g/mL (calculated from the average values of densities of aromatics normally present in the boiling range of ATF samples). These density values were taken from the Aldrich Book of Reference Compounds. NMR Spectrometry. All the 1H NMR spectra were recorded on a Bruker ACP 300 MHz NMR spectrometer. The concentration of the sample as ≈ 5-10% w/w in CDCl3 (deuterated chloroform) containing TMS (tetramethyl silane) as internal reference was used. The spectra were recorded between 0 and 10 ppm. The experimental instrumental parameters were optimized using different relaxation delays (RD). It has been observed that with RD ) 10, completely relaxed spectra are obtained. The automated phase and baseline corrections were applied in order to get the reproducible integral values. The spectra were integrated thrice, and average values were taken for the purpose of calculations. High-Performance Liquid Chromatography. The HPLC analysis was carried out on a Waters 515 chromatographic instrument equipped with a 20-µL loop, rheodyne injector, and a M-2410 refractive index detector. The separation was achieved on two chemically bonded amino propyl columns (4.6 mm × 250 mm) connected in series which produced a back pressure of ≈ 900 psi with n-hexane as the mobile phase. A flow rate of 1.2 mL/min was maintained throughout the analysis. The mobile phase was dried on 5 Å molecular sieves, filtered through a 0.45 µm membrane filter, and degassed prior to analysis. The back-flushing was performed by an electronic switching valve. The chromatographic data was processed on Waters Millenium-32 chromatographic software. For estimation of total aromatic content of the samples, HPLC analysis has been carried out as per the IP-436 procedure. The method is applicable to aviation kerosene and petroleum distillates boiling in the range 50 to 300 °C. It determines mono-aromatic (MAHs) and di-aromatic (DAHs) hydrocarbons. The sum of MAHs and DAHs is reported as the total aromatic content of the sample. Saturates are calculated by difference. The column chosen for the analysis is a polar amino column, which has little affinity for nonaromatic hydrocarbons, but exhibits pronounced selectivity for aromatic hydrocarbons. Consequently, aromatic hydrocarbons are separated into nonaromatic hydrocarbons with distinct bands according to their ring structure, i.e., MAHs and DAHs. The percentage of MAHs and DAHs has been calculated against calibration standards 1,2-dimethylbenzene and 1-methylnaphthalene, respectively (Figure 1). FIA Method. The FIA method has been performed as per the ASTM-D 1319/95 procedure. Open Column Chromatography. The ATF samples were fractionated into saturated and aromatic fractions as per the ASTM-2549/85 method.
Results and Discussions Basis of NMR Method. Estimation of the total aromatic content in an ATF sample using 1H NMR spectrum is based on the group molecular weight (GMW) method and the determination of the relative number of carbon atoms from the 1H NMR spectrum. The philosophy of the group molecular weight method is to first assign the 1H NMR spectrum in terms of CHn
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Figure 2. 300 MHz 1H NMR spectrum of a representative ATF sample, along with division of various regions.
Figure 1. HPLC chromatogram of representative ATF sample and Calibration Standards A, B & C.
groups (n ) 1, 2, and 3) in order to calculate the relative number of carbon atoms. Another requirement would be realization of complete contributions made by the hydrocarbon class under estimation (say aromatics) in the spectrum. The later part requires estimation of the quaternary carbons and average chain length of the substituents attached to aromatic rings. The kerosene and ATF samples are rich in saturates. The aromatics present in such samples belong predominately to the class of mono-aromatics, whereas diaromatics constitute only a minor class ( 15%). To check the applicability of the developed NMR method, few hydrocracked samples and few blends of commercial samples with dodecane were also analyzed. These samples contained total aromatics in the range of 2 to 15%. A comparison of the data in Table 2 indicates that the results by the developed NMR method are in good agreement with those obtained using the other two methods. A correlation plot between the results obtained by the NMR and HPLC methods (70 samples) is shown in Figure 4a. Similarly, the results obtained by the NMR and FIA methods (Figure 4b; 51 samples) and those obtained by FIA and HPLC methods (Figure 4c; 43 samples) have been found to be highly correlated as well. The data obtained by the three methods under investigations i.e., FIA, HPLC, and NMR, was further evaluated statistically and the results are given below:
method
correlation coefficient (R)
average difference
standard deviation
FIA vs NMR FIA vs HPLC NMR vs HPLC
0.921 0.937 0.976
-0.33 -0.39 -0.22
1.98 1.58 1.31
The statistical data given above indicate that all the three methods give comparable results. The average difference calculated from the averages of individual
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variations show that both HPLC and NMR tend to overestimate total aromatic content compared to the FIA method. Similarly, the HPLC results are on the higher side compared to those obtained by the NMR method. Similarly, the standard deviation of the differences is somewhat comparable; however, it is maximum for the FIA vs NMR comparison. Tests of Significance. Though the comparison of results on total aromatic content in ATF samples determined by FIA, HPLC, and the NMR methods has shown a very high degree of correlation, in view of the importance of this determination, it was thought appropriate to subject the total data to various tests of significance in order to evaluate the reliability of measurements by the proposed NMR method. For such purposes, a visual inspection of the results and simple correlations is usually inadequate. Therefore, the ANalysis Of VAriance (ANOVA) has been carried out on the set of data obtained by different techniques comparing two techniques at a time.20 The test starts with the null hypothesis H0: “there is no difference between the two techniques”. ANOVA is a technique by which variations associated with defined sources may be isolated and estimated. These are the conditions, which an analyst may not be able to control during a determination by a particular technique. The “two-way ANOVA without (20) Applied Multivariate Data Analysis, Vol.1, Regression and Experimental Design; Jobson, J. D., Ed.; Springer-Veriag: New York, 1991.
Mukherjee et al.
replicate” has been carried out for pairs of FIA-NMR, FIA-HPLC, and NMR-HPLC results. For all the three comparisons, it was found that the calculated F-ratio was less than the critical F-ratio at the 95% confidence level. It was concluded that there is no significant difference between the results (total aromatic content in the samples) obtained by NMR and HPLC methods, between NMR and FIA methods, and from FIA and HPLC methods. The proposed NMR method provides the total aromatic content, which is statistically equivalent to that obtained by the established FIA and HPLC (IP-436) methods. Conclusions The above analyses have shown that using 1H NMR spectral integral intensity data in the derived equations, the total aromatic content (weight percentage) of aviation turbine fuel samples can be conveniently estimated. The total aromatic content estimated by the NMR method agrees well with that estimated using the LCbased methods (FIA and HPLC). The proposed NMRbased method is fast, reliable, and less time-consuming compared to the commonly used LC-based methods. Besides, the NMR method is simple and can be applied with ease as no instrumental intricacies are involved, which is the case with other methods. The proposed NMR-based method would be very useful in routine quality and process control applications. EF020275V
