Energy & Fuels 1990,4, 20-23
20
Investigation of Methods for Determining Aromatics in Middle-Distillate Fuels S. Win Lee* and S. Coulombe Energy Research Laboratories, CANMET, Energy, Mines & Resources Canada,' Ottawa, Canada, K 1 A OG1
B . Glavincevski Fuels and Lubricants Laboratory, National Research Council of Canada, Ottawa, Canada, K 1 A OR6 Received July 6, 1989. Revised Manuscript Received August 24, 1989
The need for reliable analytical techniques is critical in energy research where combustion characteristics of the fuels are interpreted in terms of their chemical and physical properties. The fuel aromatic content is identified as one of the predominant characteristics that influence the combustion performance. Four analytical methods applicable for determining aromatics in middle distillates were studied. Selected middle-distillate fuels were analyzed for aromatics by using the fluorescent indicator adsorption method, nuclear magnetic resonance spectrometry, mass spectrometry, and supercritical fluid chromatography. Interlaboratory comparisons were made by using nuclear magnetic resonance and supercritical fluid chromatography results. Data indicated that all methods exhibit a good correlation among each other while mass spectrometric method tends to provide lower aromatic results. The nuclear magnetic resonance results appears to exhibit the highest values. The reproducibility of the aromatics data depends on the intrinsic nature of the technique, the analytical method applied, and the performance of the laboratory.
Introduction Increased demand for petroleum products and depleting conventional resources reflect the industry's effort to maximize the product yield from the crude barrel and to explore suitable fossil fuel supply sources. However, with degrading feedstock qualities and stringent environmental regulations, the optimization of product quality and product performance becomes critical. In Canada, where energy demand per capita is one of the highest in the world, it is expected that, by 1995, more than half of the total domestic production will be derived from the oil sands bitumens, heavy crudes, and heavy-oil deposits from western Canada.' Compositional analysis of synthetic distillates showed larger proportions of aromatics than paraffins compared to conventional distillates.2 The problems associated with the use of highly aromatic fuels are widely documented and have prompted performance evaluations on various combustion equipment. A t the Canadian Combustion and Carbonization Research Laboratory, a research program is being carried out to study the influence of fuel quality on burner performance in residential heating appliances. In the course of the study, it has become apparent that a reliable analytical technique for fuel aromatics determination is critical for accurate interpretation of combustion performance. The search for such a method has led to the development of a new technique utilizing supercritical fluid chromatography (SFC):p4 and the review process of several techniques has provided relevant information associated with each technique. Initial survey (1)Canadian National Energy Board, "Canadian Energy Supply and Demand 1983-2005",September 1984. (2)Steere, D.E.; Nunn, T. J. SAE Technical Paper Series; Society of Automotive Engineers: Warrendale, PA, 1979; Paper No. 790922. (3) Fuhr, B. J.; Holloway, L. R.; Reichert, C.; Lee, S. W.; Hayden, A. C. S. Prepr. Pap. Am. Chem. SOC.,Diu. Fuel Chem. 1987,32(4),30-38. (4)Lee, S.W.; Fuhr,B. J.; Holloway, L. R.; Reichert, C. Energy Fuels 1989,3,8C-84.
0887-0624/90/2504-0020$02.50/0
of analytical data from two laboratories indicated considerable variance between nuclear magnetic resonance (NMR) and fluorescent indicator adsorption (FIA) data and caused concern over the accurate interpretation of results provided by different techniques. Without accurate fuel property data, accurate prediction of combustion performance cannot be achieved. This paper reports data from the analyses of middle-distillate oils by four independent laboratories. RRsults provided by the techniques of ASTM D1319 (FIA), NMR, mass spectrometry (MS), and SFC are described. Equal emphasis is given to the data comparison between independent laboratories.
Experimental Section Fuel Variety. A total of 52 distillate fuels were used in this investigation. One group of fuels (23 samples) was analyzed by FIA, NMR, MS, and SFC. The other groups of fuels were analyzed for aromatics by two independent laboratories using NMR and SFC techniques. Fluorescent Indicator Adsorption. The standard equipment and procedure described in the ASTM D1319 method5 were used. Nuclear Magnetic Resonance Spectrometry. 'H NMR spectra were recorded on a Varian Model EM-390 and VXR-200 NMR spectrometers operated a t 90 and 200 MHz, respectively. Solutions containing 50% (v/v) of the oil in CCl, were used for hydrogen-type determination a t 90 MHz. Less concentrated solutions (less than 10 wt %) in CDC13 were used for the proton runs a t 200 MHz. Sixteen transients were accumulated with a pulse width of 90" and a 20-s relaxation delay period between consecutive pulses. Carbon-13 NMR (13C NMR) was used for confirmation of carbon aromaticity derived from 'H NMR. Conventional inverse-gated 13C NMR experiments were performed by using a pulse width of 90" and a relaxation delay period of 12 s. Solution concentrations were 35-40% in CDC13, containing a relaxation reagent, tris(acetylacetonato)chromium(III). All chemical shifts were measured relative to tetramethylsilane. Total aromatics in (5) Manual on Hydrocarbon Analysis, 3rd ed.; ASTM D1319; American Society for Testing and Materials: Philadelphia, PA, 1977.
Published 1990 by t h e American Chemical Society
Energy & Fuels, Vol. 4, No. 1, 1990 21
Aromatics in Middle-Distillate Fuels the distillate fuels were determined from hydrogen- and carbon-type distribution data and formula-substructure relationships.%' The relation expresses the aromatic content as a function of some molecular-structureparameters that are normally obtained from the spectral features of proton and carbon-13NMR spectra. The following analytical data are required for the determination of aromatic content: average molecular mass, mass percentage of hydrogen,and hydrogen- and carbon-type concentrations from proton and 13C NMR data. Mass Spectrometry. All mass spectra were obtained on a Kratos MS25Q, double-focusing,medium-resolutionmass spectrometer. The samples were introduced into the ion source via an all-glass batch inlet system. The inlet system and ion source temperature were maintained at 300 "C. All data were collected at electron ionizing voltage of 70 eV. The laboratory ensures the authors that distillate samples are completely volatilized under the selected operating conditions. Hydrocarbon-typeinformation was calculated by using a modified version of the "Robinson method".8 The method is not published in the literature since it is protected under the properietary ownership of a company. Supercritical Fluid Chromatography. Details of the SFC procedure were reported recently by Lee et. al.4 Two independent laboratories took part in this study utilizing the same procedure but different instrumentation. One laboratory used a Lee Scientific Model 600 SFC system. The column was a 500 mm X 1.0 mm, 10 pm, Alltech silica microbore. The other laboratory used a Varian Model 8500 syringe pump to maintain the mobile-phase pressure and a Shimadzu Model GC-8A gas chromatograph for chromatographic separation. The column was a 250 mm X 2.1 mm, 5 pm, silica adsorbosphere supplied by Alltech.
Table I. Variety of Middle-Distillate Fuels
fuel type A conventional crude furnace oil B" cat-cracked gas oil blend C" cat-cracked gas oil blend D western Canadian conventional crude distillate E high aromatic diesel F" light cycle oil blend G western Canadian diesel fuel H western Canadian diesel fuel I" conventional type B diesel fuel J" cat-cracked components blended to simulate future diesel fuels K" cat-cracked components blended to simulate future diesel fuels L" diesel fuel M synthetic crude furnace oil N conventional crude furnace oil 0 conventional crude furnace oil P" hydrotreated cat-crackedgas oil and cycle oil &" cat-crackedgas oil and light cycle oil R" heavy gas oil and furnace oil blend
final bp, "C 405 367 401 402 366 396 343 368 351 399 405 369 350 351 329 379 395 398 344 378 310 344 380 341
S" synthetic and conventional distillate blend T" cat-crackedgas oil and diesel blend U light synthetic crude distillate V furnace oil and light cycle oil blend W high-viscosityNorth Sea crude furnace oil X synthetic and conventional distillate blend " Fuels prepared in the laboratory using refinery stocks to attain
specific properties.
Results and Discussion Middle distillates are characterized normally by using a combination of spectroscopic methods such as mass spectrometry, NMR, IR, and UV and a number of physical and chemical properties. Such characterization provides detailed information on the composition of the hydrocarbon mixture and enables comparison of the results. The use of a single analytical method, however informative, is often inadequate. Nonetheless, the results obtained by various methods cannot be directly compared as they are derived differently. The mass spectrometric analysis gives the group composition, which describes relative concentrations of different compound groups. Other methods of structural and group analysis that are based on physical and chemical characteristics provide the number of aromatic and naphthenic rings and the distribution of carbon types in aliphatic, naphthenic, and aromatic structures. For conventional petroleum distillate products of moderate boiling point range, the FIA method offers the best features in terms of cost, simplicity, and its outstanding long track record of application by the industry. However, current requirements by the industry for aromatic analysis of products from nonconventional sources and high boiling distillates are not satisfied sufficiently by FIA. Mass spectrometry has been for many years a major tool for characterizing petroleum-derived and coal-derived fractions. Conventional NMR spectroscopy has been applied to complex mixtures for over 25 years, partly because it provides quantitative, chemically comprehensible information for the whole sample. NMR procedures and data reduction processes are relatively simpler than those of MS. However, in areas where information on the total aromatic content is a primary requirement, the SFC method offers the highest degree of practicability, in terms of its moderate capital cost, its speed, and its good re(6)Glavincevski, B.; Gulder, 0. L.; Gardner, L. Prepr. Am. Chem. SOC., Diu. Pet. Chem. 1989, 34(4), 897-899. (7) Muhl, J.; Srica, V.; Mimica, B.; Tomaskovic, M. Anal. Chem. 1982, 54. - - , 1871-1874.
(8) Robinson, C. J. Anal. Chem. 1971, 43, 1425.
peatability. The attractive features of the SFC method in petroleum refinery applications have been reported re~ently.~ In combustion research, the requirement for accurate fuel aromatics data is of high priority. The availability of a reliable analytical laboratory is critical, and gaining access to such a laboratory is not easy. The Energy Research Laboratories (ERL) is anxious to respond to the industry's requirement of a fast and accurate analysis method. The SFC method, recently developed by ERL, is new and the evaluation of its standing with respect to other techniques appears worthwhile. These concerns were the major driving force behind this study. Petroleum distillates included in the present study were selected from fuels used in the combustion evaluation program at the Combustion and Carbonization Research Laboratory. The original distillates were contributed by several Canadian major oil companies, and fuel blends were prepared to obtain specific fuel properties. The National Research Council of Canada contributed some diesel fuel blends that were specifically prepared to simulate Canadian future transportation fuels. In this paper, three sets of data from different groups of fuels are presented. One group of fuels was analyzed by four laboratories using NMR, MS, SFC, and FIA. Table I describes the fuel-type information, and Table I1 presents the total aromatics results. The second group of fuels was analyzed by two laboratories using proton NMR (Table 111). The last set of data (Table IV) represents a comparison of SFC data provided by two different laboratories. The FIA results are given in volume percent and must be interpreted with some caution. Typical SFC chromatograms of fuels B and K are shown in Figures 1 and 2. The first single peak in the SFC chromatogram is for total saturates, and it is followed by aromatics peaks. The notation mark "C" denotes the cut point between the saturates and aromatics that was used to separate the peak areas. The first peak after the cut point represents the monoaromatics, the second is for the diaromatics, and the following peaks represent various
22 Energy & Fuels, Vol. 4 , No. 1, 1990 Table 11. Aromatic Content of Middle-Distillate Fuels As Determined bv Various Methods fuel MS, wt % NMR, wt % SFC, wt % FIA, V O ~% A 25.5 33.4 26.1 30.1 B 12.3 25.6 28.0 26.9 C 73.4 76.5 75.0 77.0 D 57.0 67.7 55.2 61.8 E 73.0 74.2 77.7 83.7 F 30.9 30.6 31.8 33.5 G 26.9 29.1 28.9 28.8 H 51.0 57.5 63.9 52.7 I 27.1 21.2 24.4 31.6 J 46.3 48.9 38.4 43.6 K 45.7 49.0 38.5 44.2 L 29.0 37.4 26.2 30.4 M 30.5 38.4 30.7 36.3 N 34.5 39.0 naa 40.3 36.1 46.3 35.2 0 36.8 P 34.7 31.3 35.5 35.7 57.7 61.9 64.0 60.5 Q R 30.8 36.7 33.8 34.3 62.2 S 58.3 67.1 56.5 T 41.7 43.3 42.8 na" 29.4 U 29.6 29.3 28.3 v 73.9 48.9 65.7 63.9 W 26.3 33.3 28.5 29.8 X 67.1 73.3 58.7 na" ~
"na, not available.
Table 111. NMR Carbon Aromaticity Percent As Determined by Two Different Laboratories" aromaticitv '70 fuel lab 1 lab 2 1 28.8 20.0 19.1 13.0 20.9 14.0 24.1 21.0 29.8 24.0 27.2 21.0 21.4 14.0 33.5 24.0 35.1 28.0 18.0 27.0 59.9 50.0 22.4 12.0 15.2 12.0 34.6 27.0 33.1 25.0 33.7 29.0 47.2 42.0 51.9 51.0 14.7 16.0 14.6 16.0 19.3 16.0 11.0 19.6
Lee et al. Table IV. Total Aromatics by SFC and FIA As Determined by Three Independent Laboratories SFC 1, SFC 2, FIA, vol fuel" type wt% wt% % SFC 1 fuel blend from 38.3 40.3 38.2 synthetic and conventional crudes 14.2 SFC 2 hydrotreated 15.8 16.5 distillate from 100% synthetic crude SFC 3 conventional 41.3 40.2 38.2 distillate with cat-cracked components SFC 4 high-boiling 33.9 31.6 29.6 conventional cat-cracked distillate low-boiling SFC 5 28.1 27.3 26.1 commercial diesel fuel SFC 6 jet fuel type A1 20.5 22.8 18.9 SFC 7 21.2 22.3 low pour diesel 19.7 distillate from SFC 8 42.4 46.2 41.6 100% synthetic distillate SFC 9 Arctic diesel 17.0 18.1 13.5 high-density , 32.1 SFC 10 36.1 38.2 high-power diesel SFC 11 25.2 jet fuel type B 26.8 22.8 SFC 12 (M) synthetic crude 34.9 36.3 30.7 distillate furnace oil a Fuel given in parentheses designates the same samples described in Table I.
"Fuels given in parentheses designate the same samples described in Table I. The rest are middle-distillate fuels having a wide range of properties.
triaromatic components. Similar response factors of pure hydrocarbon type compounds and the corresponding fuel fractions allow the use of the corresponding chromatographic peak areas in calculating weight percentages of saturates and aromatic^.^ While it is not appropriate to compare the absolute values of fuel aromatic contents, some observations may be made from the data in Table 11. MS weight percent data show the lowest values while NMR results appear the highest. MS data reduction applies the calibration derived from mathematical factors assigned to various component types, and it is possible that some compound types might have been omitted.* The somewhat lower results by the SFC technique may be due to the lower separation efficiency between saturates and aromatics and a possible
0
5
10
15
20
25
35
40min
Retention time
Figure 1. SFC chromatogram of fuel B, a blend of cracked gas oil components.
difference in their respmse factors. The overestimation of fuel aromatics by NMR calculation method is also a possibility. The following statistical data (where R2is the correlation coefficient) are derived from regression analyses of data sets in Table 11. methods R2 slope av diff std dev NMR vs MS, wt % SFC vs NMR, wt % SFC, wt 90,vs FIA, V O ~70 SFC vs MS, wt 70
0.968 0.962 0.893 0.978
1.07 0.93 0.91 1.02
5.65 -3.42 3.06 2.23
3.32 3.37 5.66 2.47
Energy & Fuels, Vol. 4, No. 1, 1990 23
Aromatics in Middle-Distillate Fuels
A
0
I
I
5
10
I
I
I
20 25 Retention time 15
I
I
30
35
I
40min
Figure 2. SFC chromatogram of fuel K, a cat cracked component blend prepared to simulate future diesel fuels.
The statistical data indicate a good linear relationship among MS, NMR, and SFC, and the best correlation between SFC and MS. A slightly poorer correlation between SFC and FIA may be due to the fact that two different laboratories provided part of the FIA data set. The SFC results also represent combined data from two laboratories, but results agree better than FIA data as evidenced by a good correlation between SFC and MS. The average differences calculated from the average of individual variations show that NMR tends to overestimate the aromatic content while FIA and MS tend to underestimate it compared to SFC. The standard deviations associated with these averages confirm that data are more scattered when FIA and SFC are compared whereas MS and SFC show the best correlation. Fuel aromaticity values for a set of distillate fuels determined by two independent laboratories are listed in Table 111. It is important to note that the aromatic weight percent of the fuels discussed above (Table 11)is different from the fuel aromaticity. The aromaticity is a measure of the percentage of aromatic ring carbons present in the fuel and is occasionally used to describe fuel aromatics. Data show a significant difference between the two laboratories, one laboratory being consistently lower than the other. Laboratory 1 derived the results from hydrogenand carbon-type distribution and formula-substructure relationships: and laboratory 2 used the Brown-Ladner m e t h ~ d . ~This minor difference may not be the only
reason for the large difference between the two data sets. Other variables such as operator manipulation and data reduction techniques may also be responsible for such a discrepancy.l0 The SFC results of a selected group of fuels determined by two independent laboratories are given in Table IV. The results show excellent agreement, despite the fact that each laboratory used a different instrument. One used a commercial instrument, and the other utilized a laboratory-built unit. That indicates that purchase of a higher cost commercial SFC instrument is not necessary to carry out the analysis. One can use existing laboratory equipment to build a similar unit. An improved correlation between SFC and FIA data can be noted as well. The linear regression analysis of the data gave a correlation coefficient (R2) of 0.951 with a slope of 1.06. This is better than the correlation coefficient (0.893) between the SFCFIA data in Table 11, probably because FIA data was provided by one laboratory alone. In addition, data in Table I indicate that most of the samples analyzed had final boiling points higher than 315 OC.
Conclusions This study does not cover all technological aspects of mass spectrometry or NMR techniques. There are numerous developed methods and modified versions applied daily in different analytical laboratories. The methods discussed in this paper are the ones used in the laboratories the authors have access to. From this study, the following conclusions can be made. (1)The selection of an aromatic analysis method mainly depends on the user's requirement that in turn depends on the nature of sample and the technique available. (2) The MS technique is recommended for detailed structural information within a number of samples. NMR provides not only the aromatic contents but also the distribution of carbon and hydrogen structural group. SFC stands as the most promising method for petroleum refinery applications. (3) It is important to have the analysis done in a laboratory where good quality control and quality assurance are proven. Acknowledgment. We thank A. C. S. Hayden from the Combustion and Carbonization Research Laboratory, Dr. B. J. Fuhr from the Alberta Research Council of Canada, and D. V. Rasmussen of R & D Centre, Petro-Canada Products, for their assistance in this work. (9) Brown, J. K.; Ladner, R. W. Fuel (London)1960, 39, 87-96. (10)Lee, S. W. Prepr. Am. Chem. Soc., Diu. Fuel Chem. 1989,33(4),
883-890.
