1028
Energy & Fuels 2000, 14, 1028-1031
Estimation of Bromine Number of Petroleum Distillates by NMR Spectroscopy V. Bansal, A. S. Sarpal,* G. S. Kapur, and V. K. Sharma Indian Oil Corporation Limited, R & D Centre, Sector-13, Faridabad, India Received February 21, 2000. Revised Manuscript Received June 5, 2000
In the present work, a direct and fast method based on NMR spectroscopic techniques (1H/13C) has been developed for the estimation of bromine number of gasoline and coker kerosine (CK) fuel samples of boiling range 50-250 °C. Two approaches have been adopted. In the first, the percentage unsaturation (HIO%) in the chemical shift range of 4.6-6.6 ppm (1H NMR) has been correlated with the predetermined bromine number of the samples. The HIO% versus bromine number gives a straight line, with proportionality constant K ) 16.2 (FCC) and 9.3 (Coker) products. In the second approach, the relative number of double bonds (D) in the sample has been determined by 1H/13C NMR method. The bromine number has been estimated using developed equations. A very good correlation (correlation coefficient, R2 ) 0.98) has been observed between the NMR methods and the standard method (IP: 129/81, reapproved 1988). The 13C NMR method is independent of the nature of olefins, source of feed, and catalyst/cracking conditions compared to the 1H NMR spectroscopy method.
Introduction Bromine number is useful as a measure of aliphatic unsaturation in petroleum distillates. It is included in the quality control specifications of these distillates and has great significance in defining the storage stability. Standard test methods for the estimation of bromine number of petroleum distillates and commercial aliphatic olefins include the following: electrometric titration (ASTM D-1159/84, IP: 130/85),1 and color indicator method (IP:129/81, reapproved 1988), (IS 1448 [P:43]: 1991).2,3 Although routinely used, these methods are quite laborious, time-consuming, require various standards for calibration, demand utmost care on part of the analyst, and have certain limitations. An electrometric method is not satisfactory for normal R-olefins, which are abundant in coker kerosine (CK) streams of petroleum distillates. Also, the accuracy of the method depends on the content of sulfur, nitrogen, or oxygen compounds and certain polycyclic aromatic hydrocarbons, which have been reported to react with bromine. Ruzicka and Vadum4 modified the ASTM method for determining the bromine number of petroleum distillates for application to petroleum fractions and residues, and the modified method was tested on selected model compounds. Therefore, a strong need has been felt for a quick and reliable analytical method for the estimation of bromine number. * Corresponding author. (1) Annual Book of ASTM Standards, ASTM D-1159-84 (IP: 130/ 85), Standard Test Method for Bromine Numbers of Petroleum Distillates and Commercial Aliphatic Olefins by Electrometric Titration. (2) IP129/81 (Reapproved 1988), Bromine Number by ColourIndicator Method. (3) IS 1448 [P: 43]: 1991, Indian Standard Methods of Test for Petroleum and its Products; Bromine Number by Colour-Indicator Method. (4) Ruzicka, D. J.; Vadum K. Oil Gas J. 1987, 85 (31), 48-50.
In the 1H NMR spectra of cracked (FCC/coker) petroleum products, the proton of unsaturated compounds exhibits resonances in the chemical shift region of 4.6-6.6 ppm. This region is free from the interference of resonances due to aromatics and saturates. Therefore, this region can be exploited for the estimation of characteristic properties of fuels such as unsaturation value, bromine number, iodine number, etc. The aim of the present investigation is to correlate the unsaturation proton content of fuel range samples (50-250 °C) determined by 1H NMR with the bromine number estimated by the standard test procedure (IP: 129/81, reapproved 1988). The bromine number of the samples has also been estimated directly from the relative number of the double bonds determined by 13C NMR spectral analysis. Experimental Section Samples. The fuel samples, viz., fluid catalytic cracked (FCC) gasoline, coker gasoline, straight run (SR) (no olefin) gasoline, and coker kerosine (CK), were obtained from different Indian refineries. These refineries are processing feed for FCC and coker processes at optimum conditions. Standards olefinic compounds, e.g., 1-octene, 1-decene, used were procured from Aldrich. Blends of different olefinic standards and SR gasolines were prepared in w/w ratio. Various blends of different FCC, SR gasolines, and CK were prepared in different w/w ratio. Quantitative NMR Spectra. 1H NMR.1H NMR spectra of around 10% solution of a sample in deuterated chloroform (CDCl3) were recorded on a 300 MHz NMR spectrometer operating at 300 MHz frequency. The spectral parameters used were as follows: number of scans NS ) 64, relaxation delay ) 10 s, and spectral size of 16 K with time domain size 8 K. All the chemical shift values are reported with respect to tetramethylsilane (TMS) equals 0.0 ppm as internal standard. 13C NMR. 75.4 MHz 13C NMR spectra of the samples were recorded at around 30-40% solution in CDCl3. The quantita-
10.1021/ef000028w CCC: $19.00 © 2000 American Chemical Society Published on Web 08/02/2000
Bromine Number of Petroleum Distillates
Energy & Fuels, Vol. 14, No. 5, 2000 1029
Figure 2.
13C
NMR spectra of representative gasoline sample.
Figure 1. 1H NMR spectra of representative gasoline samples. (a) Straight run naphtha, (b) FCC naphtha, and (c) coker gasoline. tive spectra were obtained in the inverse gated mode and using 0.1 M Cr(acac)3 as relaxation agent. Under these conditions, reproducible quantitative spectra were obtained. The spectral parameters were as follows: number of scans ) 3000, relaxation delay 5 s, and spectral size 16 K with time domain size 8K. The 1H and 13C NMR spectra were integrated thrice between 0.5 and 10.0 ppm and between 5 and 190 ppm, respectively, and average of the three values was used for further calculations. Bromine Number. The bromine number was determined as per standard method (IP: 129/81, reapproved 1988).
Results and Discussion General Features of 1H and 13C NMR Spectra. The 1H NMR spectrum of a SR (Figure 1a), FCC (Figure 1b), and coker kero (Figure 1c) exhibits signals due to unsaturated protons in the region of 4.6-6.3 ppm due to both R-substituted and internal olefinic components. Different types of olefinic structures present in these petroleum distillates have been differentiated through 1H NMR spectrum at 300 MHz.5 The percentage of R-olefinic protons in CK is much higher than that of gasoline. The other part of the spectrum represents signals due to aromatic protons (6.6-7.6 ppm), substituents of aromatics (2.2-3.0 ppm), and paraffinic and naphthenic protons (0.5-2.0 ppm). The 13C NMR spec(5) Sarpal, A. S.; Kapur, G. S.; Mukherjee, S.; Tiwari A. K. Fuel, submitted.
Figure 3. 1H NMR spectra of the olefinic region of (a) FCC, and (b) coker gasoline.
trum of the FCC gasoline (Figure 2) shows wide spread of the different types of carbon signals belonging to olefins, aromatics, and naphthenes. The olefinic and aromatic carbons overlapped and appeared between 100 and 160 ppm. The other signals in the region 5-60 ppm correspond to R-substituents, naphthenic and paraffinic carbons. Bromine Number Estimation. 1H NMR Method. The intensity of the olefinic protons in the region 4.66.6 ppm has been considered for correlation with the bromine number determined by standard test procedure. The bromine number of a variety of samples of FCC gasolines and coker kero (CK) having varying degree of olefinic content (5-70%) estimated by the standard procedure, has been correlated with 1H NMR spectral intensity of the olefinic regions by the following equation:
bromine number ) K × HIO% H
IO% ) HIΟ/HIT × 100
(1) (2)
where HIT and HIO are the total integral intensities of the region 0.5-8.0 ppm and 4.6-6.6 ppm, respectively. The value of K (Figure 4) has been found to be dependent upon the nature of olefins (16.2 for FCC and 9.3 for
1030
Energy & Fuels, Vol. 14, No. 5, 2000
Bansal et al. Table 1. Comparison of Bromine Number Data by Standard and Developed Methodsa sample no.
Figure 4. Plot of bromine number versus % I unsaturation.
CK) and independent of alkyl chain length. The theoretical value of K for R-olefins (1-octene, 1-decene, etc.) has been found to be 7.6. This has been determined by dividing the bromine number by the actual percentage of olefinic protons in the R-olefinic standard compounds. The low value of K for CK has been attributed to higher R-olefinic content (Figure 3). Equation 1 has been validated for a number of samples and standard blends having large variations in the total olefinic content, and excellent correlation has been obtained (R2 ) 0.98) (Table 1). Equations 1 and 2 will give accurate determination of bromine number if there is not much variation in the types and composition of different olefinic compounds appearing in the region 4.6-6.6 ppm. To visualize these variations, the percentage of integral areas of various regions corresponding to different types of olefinic hydrogens was measured for a large number of samples collected from different refineries which are engaged in processing the different varieties of feed for FCC processes. The average values are given in Table 2. It is quite obvious that there are no significant variations observed among the various samples containing different types of olefinic components. The comparison in Table 1 has also shown that these variations are well taken care of, as the agreement is excellent between the two methods. The 1H NMR method may not give accurate values of bromine number, if the nature of olefins in the samples is different from those present in FCC or coker gasoline. There can be a possibility of samples containing blends of both FCC and coker gasolines. In that case, the slope of the curve will change and eqs 1 and 2 will not be applicable. To confirm these observations, blends of FCC and coker gasoline were analyzed by 1H NMR method and the bromine number was estimated using K ) 16.3. The bromine number as determined by 1H NMR method was not found to match the blended values. Therefore, for samples of unknown identity, the 13C NMR method will be most preferable for the accurate determination of the bromine number, which has been discussed in the following section. 13C NMR Method. In comparison to the 1H NMR spectrum, the olefinic carbon signals in the 13C NMR spectrum cannot be estimated directly for correlation with bromine number as these are overlapped with the aromatic carbon signals. Therefore the contribution of olefinic carbons from the aromatic region has been estimated indirectly. The 13C NMR spectral data have
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
sample G-1 G-2 G-3 G-4 G-5 G-6 G-7 G-8 G-9 G-10 G-11 G-12 G-13 G-14 G-15 G-16 F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 SR CK-1 CK-2 CK-3 CK-4 CK-5 CK-6 CK-7 CG-1 CG-2 CG-3 CG-4 B-1 B-2
1H
NMR
52.3 54.0 51.2 40.2 23.9 39.5 21.2 14.7 nil 130.0 30.0 64.8 39.5 10.5 15.4 30.9 105.3 82.6 94.3 71.3 60.6 38.1 28.8 126.4 nil 25.0 12.3 18.1 4.7 2.4 23.3 nil 11.5 9.4 10.0 114.0 71.1 56.2
13C
NMR
IP-129/81
53.5 56.3 56.8 38.2 21.9 48.6 32.4 nil 136.0 28.0 65.8 36.0 24.4 35.3 81.2 81.2 84.4 72.3 60.8 41.1 30.0 102.6 nil 24.0 10.5 11.0 5.2 3.8 23.8 nil
72.0 57.3
51.0 52.0 59.5 36.9 22.0 49.6 23.8 22.2 nil 139.0 29.0 68.0 38.0 15.0 20.0 31.0 109.0 85 95.0 72.0 57.0 43.0 30.0 121.0 nil 25.0 12.2 17.6 5.8 3.8 22.0 0.8 9.0 13.0 13.0 110.0 73.0 55.2
a G ) FCC gasoline, F) blends of FCC and SR gasoline, CK) coker kero, CG ) coker gasoline, B ) blends of SR and olefinic standard compounds.
Table 2. Average Percentage Hydrogena of Various Olefinic Region in the 1H-NMR Spectra of Different Categories of Gasoline chemical shift region (ppm) type of gasoline 4.5-4.8 4.8-5.1 5.1-5.3 5.3-5.6 5.6-6.6 FCC gasoline coker gasoline
10-16 8-13
11-17 40-43
18-24 7-10
34-40 12-14
12-18 24-28
a The % hydrogen of different olefinic regions has been calculated from samples from different sources for the period 19961999.
been used to estimate relative number of double bonds (D) present in the sample. The bromine number is thus estimated by the following equation:
bromine number ) (D × 159.8 × 100)/(F × CIT)
(3)
where CIT is the total integral intensity of the sample in the 13C NMR spectral region 5-160 ppm excluding signals due to solvent CDCl3; 159.8 is the molecular weight of the bromine and factor F is the relative average group molecular weight of the C, CH, CH2, and CH3 groups (13.8). The procedure for the estimation of factors F and D has been discussed below. Estimation of Factor (F) and Relative Number of Double Bonds (D). The relative average group molecular weights of the sample are determined from
Bromine Number of Petroleum Distillates
Energy & Fuels, Vol. 14, No. 5, 2000 1031
their respective relative areas of CHn groups by the following equation:
