80
Energy & Fuels 1989, 3, 80-84
Development of a Supercritical Fluid Chromatographic Method for Determination of Aromatics in Heating Oils and Diesel Fuels S. Win Lee* Energy Research Laboratories, CANMET, Energy, Mines & Resources Canada, Ottawa, Ontario, Canada K1A OGl
Bryan J . Fuhr, Larry R. Holloway, and Charles Reichert Alberta Research Council, Box 8330, Station F,Edmonton, Alberta, Canada T6H 5x2 Received August 1 , 1988. Revised Manuscript Received October 10, 1988
A new analytical method using supercritical fluid chromatography with flame ionization detection was developed for determining the total aromatic content of middle distillates. Separation between saturates and aromatics is accomplished on a packed silica column with supercritical C02 as the carrier fluid. The method is simple, fast, and applicable to heavy distillates with final boiling points up to 450 OC and is not restricted by sample color. The similar response of the FID to saturates and aromatics collected from separation of actual samples indicates that calibration is not required. Aromatic concentrations in a number of middle distillates were also determined by proton nuclear magnetic resonance and the ASTM standard fluorescent indicator adsorption methods. Data indicate good correlations among the methods. Because of its simplicity and speed, the SFC method would be suitable for applications in petroleum refinery control laboratories.
Introduction Energy conservation technologies and energy efficiency programs have prompted the oil industry’s effort to maximize the distillate product yield from the crude barrel in recent years. It is anticipated that the future fuel trend in Canada is toward low-quality component blends from conventional crudes and distillates from synthetic crudes derived from oil sand bitumens and heavy oils. Studies have shown that combustion of those oils generates higher particulate concentrations than normal fuels in both diesel engines and heating appliances, due to their high content of aromatic compounds. It is essential to have a simple and reasonably fast method for aromatics determination, applicable to moderately high-boiling oils such as low-grade fuels. Quantitative studies on the effects of aromatics on combustion performance of equipment could be made effectively only by utilizing a reliable analytical technique. The method normally used in refineries for determining aromatics in oils is ASTM D1319, the fluorescent indicator adsorption (FIA) methodel FIA is best suited to colorless samples, since the quantitation is carried out by measuring different colored zones of a chromatographic column. Poor color separation is usually observed for oils with final boiling points higher than 315 “C, resulting in less accurate results. The main advantage of the FIA method is its simplicity and ease of operation since it employs rather unsophisticated instrumentation. Nuclear magnetic resonance (NMR) and mass spectroscopic (MS) methods are powerful analytical tools that provide detailed structural information on fuel components, in addition to total aromatic concentrations. These techniques, however, have not received as wide an application as FIA by the oil industry due to their higher capital costs, long analysis times, and requirement of specially trained operators. (1) Manual on Hydrocarbon Analysis, 3rd ed.; ASTM D1319; American Society for Testing and Materials: Philadelphia, PA, 1977.
0887-0624/89/2503-0080$01.50/0
Several methods based on high-performance liquid chromatography (HPLC) have been employed for saturates and aromatics determination.24 These methods are generally characterized by good separation, short analysis time, and adaptability to high-boiling samples. Ultraviolet (UV) and refractive index (RI) detectors are commonly used for detection of the separated species, but extensive calibration of these detectors is required due to their specificityto compound types. Certain improvementshave been made by using a dielectric constant (DC) detector, which has a more uniform response.6 The flame ionization detector (FID), which is also a relatively uniform response detector over a wide range of hydrocarbon types, presents a problem when used with HPLC due to the interferences from commonly used solvents. Supercritical fluid chromatography (SFC) can be easily used with FID for hydrocarbon type analyses. This has been demonstrated by Norris and Rawdon6 and Schwartz and Brownlee7 for separation of saturates, olefins, and aromatics in gasolines. Both workers employed packed columns. Recently, Fuhr et al.*J’ reported on the application of an SFC/FID technique employing a silica column and carbon dioxide mobile phase for determining aromatics in heating oils and diesel fuels. Campbell et al.1° have subsequently described a more involved SFC/FID method for determining saturates, olefins, and aromatics in gaso(2) Suatoni, J. C.; Swab, R. E. J. Chromatogr. Sci. 1975,13,361-366. (3) Alfredson, T. V. J . Chromatogr. 1981,218, 715-728. (4) Cookson, D. J.;Rix, C. J.; Shaw,I. M.; Smith, B. E. J. Chromutogr. 1984,312, 237-246. (5) Hayes, P. C.; Anderson, S. D. Anal. Chem. 1985,57, 2094-2098. (6) Norris, T. A,; Rawdon, M. G. Anal. Chem. 1984, 56, 1767-1769. (7) Schwartz, P. E.; Brownlee, R. G. J. Chromatogr. 1986,353,77-93. (8) Fuhr, B. J.; Holloway,L. R.; Richert, C. Development of an Analytical Method to Determine Aromatica in Heating Oila and Diesel Fuels. A contract report to CANMET, Energy, Mines & Resources Canada, 1987. (9) Fuhr, B. J.; Holloway, L. R.; Richert, C.; Lee, S.W.; Hayden, A. C. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1987, 32(4), 30-38. (10) Campbell, R. M.; Djordjevic, N. M.; Markides, K. E.; Lee, M. L. Anal. Chem. 1988, 60,356-362.
Published 1989 by the American Chemical Society
Supercritical Fluid Chromatographic Method
6
Energy & Fuels, Vol. 3, No. 1, 1989 81
Pressure gauge
L I .
FID
1
Circulalor bath
chromalograpt Gas
column
r
0
m
''1
17-
1
Compuling integrator
U
I
0,
Figure 1. Schematic of SFC/FID instrument.
lines and middle-distillates fuels. With a mixed mobile phase of sulfur hexafluoride and carbon dioxide, a silica column was used to isolate aromatics while a silver ion loaded strong cation-exchange silica column was used to separate the saturates from the olefins and aromatics. Olefins were determined by difference or by using a column switching technique. The present work details the methodology of the previously reported method of Fuhr et al.9 with a special emphasis on its simplicity and suitability for petroleum refinery applications. Experimental Section Equipment and Materials. A Varian Model 8500 syringe pump and a Shimadzu Model GC-8A gas chromatograph equipped with a flame ionization detector were used. The syringe pump was utilized to pressurize the carbon dioxide in the chromatographic column to achieve inlet pressures between 3250 and 4200 psi (22400 and 28900 kPa). A Lauda Model K4R circulator bath was used in the initial experiments to keep the syringe pump cooled while it was being filled with liquid carbon dioxide. In the latter part of the work, commercial carbon dioxide cylinders pressurized to 1200 psi (8300 kPa) with helium were used and eliminated the need for a cooling bath. Distillate samples were introduced to the column by means of a Rheodyne Model 7520 valve equipped with a 0.2-pL internal sample loop mounted on the exterior of the gas chromatograph. A Spectra Physics Model 4290 computing integrator was used for collection and reduction of data. A schematic of the SFC/FID equipment is illustrated in Figure 1. A packed column of 250 mm X 2.1 mm i.d. with 5 pm silica adsorbosphere stationary phase was employed in this method development. The restrictor was a 130 mm long piece of 20 pm i.d. flexible fused silica capillary tubing attached to the column with a 0.4 mm 40% graphite ferrule with a low dead volume fitting. It was inserted approximately 90 mm into the FID and was found to eliminate "spiking", a common problem in SFC.I1 ACS grade carbon disulfide (CS2)was used for dilution of the samples prior to injection. (11) Schwartz, H. E.; Higgins, J. W.; Brownlee, R. G. LC-GC 1986,4,
639-642.
2 3 4
5 E 7 8 Q I O I I 12 13 1 4 1 5 1 8 17 18182021 SAMPLE CODE
Figure 2. Boiling point ranges of middle-distillate samples as determined by GC simulated distillation. Calibration of the instrument was achieved by injecting different concentrations of saturate and aromatic fractions separated from actual middle-distillate oils by using a modified silica-alumina column chromatographic method reported by Sawatzky et a1.12 In this method, the saturates were eluted from the silicaalumina column with n-pentane and the aromatics were eluted with toluene followed by careful removal of solvents by air drying. Pure samples of hexane, hexadecane, toluene, and naphthalene were also injected in different concentrations to determine their response factors. The middle d i s t i l l a h analyzed included heating oils and diesel fuels whoee boiling point ranges are shown in Figure 2. Please note that only 21 actual samples are shown in the f i e . One sample was given twice with different labels to the operator, adding to a total of 22 samples to be analyzed. Procedure. If the refrigerated circulator bath was used, the bath and gas chromatograph were turned on and allowed to equilibrate for at least 30 min prior to the operation. When the syrine pump reservoir cooled sufficiently,the pump was filled with liquid C02 and adjusted to achieve operating pressure. If the pressurized COz was used, the pump was filled a t ambient temperature. When all instrumental conditions stabilized, a sample previously diluted 10 times with CSz was injected. The chromatogram WBS recorded until the signal returned to the base line. Optimum conditions selected in this study for aromatics in diesel and heating fuels are as follows: stationary phase, 5 pm silica packed column; column temperature, 35 "C; carrier fluid pressure, 3600 psi (24800 kPa); detector temperature, 400 O C ; syringe pump flow rate, 8 mL COz/h; FID air flow rate, 300 mL/min; FID hydrogen flow rate, 40 mL/min. The helium shown in Figure 1was used to operate the valve in the syringe pump. For these experimental conditions, a chromatogram of a middle-distillate sample is normally obtained within 15 min, which is essentially the sample analysis time. Other Comparison Techniques. Distillate fuel samples were also analyzed by using the standard ASTM D1319 procedure. The percentage of aromatic carbons (aromaticity %) was determined by the proton NMR technique according to the procedure reported by Muhl et al.13 The spectrometer used was a Varian EM-390 instrument, operated a t 90 MHz. Samples were introduced as a 50:50 volume mixture of sample with chloroform-d and with a drop of tetramethylsilane as reference.
Results and Discussion Optimum Conditions for Separation of Saturates and Aromatics. Effects of temperature on the separation
efficiency are discussed in terms of capacity factors as reported in Table I. The experimental conditions for the (12)Sawatzky, H.;George, A. E.; Smiley, G. T.; Montgomery, D. S. Fuel 1976,55, 16-20. (13) Muhl, J.; Srica, V.; Mimica, B.; Tomaskovic, M. Anal. Chem. 1982, 54, 1871-1874.
82 Energy & Fuels, Vol. 3, No. I , 1989
Lee et al.
-
-VI
P VI
8
N 0
00
1
5
0
0 co
I
j
10
0
MIN
5
I
I
10
0
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I
5
10
MIN
Figure 4. SFC chromatogram of a middle-distillate sample at 35 O C under different pressures using a silica column.
I
I
0
4
8
Time (min)
Figure 3. SFC chromatogram of a middle-distillate sample at temperatures of 90 and 35 O C and a pressure of 3600 psi using a silica column. data represent the silica stationary phase, mobile phase pressure of 3600 psi, and column temperatures of 90 and 35 "C. The capacity factor, k , is defined by k = [t,- t O ] / t o where t, is the retention time of the given model compound and to is the retention time of a compound not retained by the column. In this case, to is the retention time of the CS2 solvent. At both temperatures, the saturates span a much smaller range of k values than do the aromatics. However, it can be seen that at 35 "C the saturates as a group are better separated from the monoaromatics. For example, at 90 "C, the CzOpeak overlaps with that for benzene, but at 35 "C, these peaks are well separated. The significant improvement in saturates-aromatics separation for an actual sample upon decreasing the column temperature is shown in Figure 3. The sharp initial eluting peak in the chromatogram is due to the saturates. At 35 "C there is almost base-line separation between the saturates and the monoaromatic group. Operating at a lower temperature also allows shorter analysis time as seen from Figure 3, which is attributed to the increased density of the supercritical C02at the lower temperature, which increases its solvating ability.14 The effect of mobile-phase pressure is less dramatic than the effect of column temperature on the saturates-aromatics separation. At 35 "C there was virtually no change in the separation over the pressure range of 3000 and 4200 psi, as shown in Figure 4. Stationary phases other than silica, such as amino~ilane'~ and silver impregrenated silica,16have been employed for saturates-aromatics separations. Limited data obtained from the present study (but not shown) indicate that although some of the other stationary phases offer better (14) Peaden, P. A.; Lee, M. L. J.Liq. Chromatogr. 1982,5(Suppl. 2), 179-221. (15) Grizzle, P. L.; Seblotny, D. M. Anal. Chem. 1986,58,2389-2396. (16) Lundanes, E.; Greibrokk,T. J.Chromatogr. 1985,349,439-446.
i Aromatlca
L
0
1
5
10
1 1s
Time (min)
Figure 5. SFC chromatograms obtained at 90 "C and 3250 psi of a whole middle-distillate sample and its saturates and aromatics separated by column chromatography. separation of aromatics ring types, the silica at 35 "C produces the best separation between saturates and monoaromatics. Calibration Curves. Figure 5 shows a SFC chromatogram of a high-aromatic diesel sample that illustrates a clear distinction between saturates and aromatics and two other chromatograms that represent the separated saturates and aromatics fractions. The separation into fractions was achieved by silica-alumina column chro-
Energy & Fuels, Vol. 3, No. 1, 1989 83
Supercritical Fluid Chromatographic Method
Table I. Comparison of Capacity Factors of Model ComDounds at Two Different TemDeratures" ~~
~
silica comad saturates n-hexane 3-methylheptane n-decane cyclohexane dimethylcyclohexane hexadecane decalin eicosane (C,) triacontane ('2%) monoaromatics benzene toluene 3-ethyltoluene o-xylene indan 1-phenyldecane tetralin diaromatics naphthalene polyaromatics phenanthrene pyrene chrysene
90 O
35 O
C ~
~
C ~~~
0.04 0.04
b b 0.0 b
0.05
0.04
0.11 0.13 0.18 b
b 0.05 0.04 0.09
0.16
0.39 0.41
0.17 0.17 0.16 0.20 0.25 0.24 b
0.53
0.37
1.27 1.92 b
0.85
0.00 0.02
0.19
0.24 0.26 0.33
0
b
1.89
"All runs were carried out at a mobile phase pressure of 3600 psi. Not determined.
matography.I2 High purity of the separated fractions, as indicated by the chromatograms, permitted their use for calibration. The calibration curves were prepared by using saturates and aromatics fractions recovered from a number of actual middle-distillate oils and several pure compounds. Figure 6 illustrates the FID response from different concentrations of saturates and aromatics prepared as carbon disulfide solutions. Saturates show saturation of FID response after certain concentrations whereas aromatics seem to exhibit linearity to high concentrations. Therefore, linear portions of both curves up to concentrations of 140 cLg/pL were used in quantitation. Relative response factors as given by the slopes of the lines representing the saturates and aromatics are 1.05 and 1.00 respectively. Differences of this magnitude have been reported for GC/FID work on model compounds." Relative response factors for the following model compounds are given in parentheses: toluene (1x4, naphthalene (1.08), hexane (1.08), and hexadecane (0.97). These differ slightly more than those for the saturates and aromatics fractions. However, since the fractions were isolated from actual samples, they were considered to represent a better average than individual compounds. Because of the similar response factors, it was decided to simply use the integrated area percentages of corresponding chromatographic peaks as the weight percentages of saturates and aromatics. The limitation is that components must be within the linear range of detector response, which could be accomplished with a 10-fold dilution of the sample in carbon disulfide. This solvent was chosen because of its low FID response although correction is sometimes necessary when the CS2peak interferes with the peak area calculations. Precision and Accuracy. The pooled standard deviationlaas determined by multiple analyses of 22 distillate (17) Deitz, W. A. J. Gas Chromatogr. 1967, February, 68-72. (18) Snedecor, G. W.; Cochran, W. G. Statistical Methods; 7th ed.; Iowa State University Press: Ames, IA, 1980; p 10.
+ 400 Concentration 0
0
500
SO0
(mg/mL)
Saturates Aromatics
Figure 6. SFC calibration curves using saturates and aromatics fractions separated from middle distillates by column chromatography.
samples is 0.4 wt % . For this value the repeatability by ASTM defiiitionlg is 1.1wt % of aromatics. The accuracy of the method, however, could not be determined by using actual samples since the absolute aromatic contents are not known. Comparison of SFC/FID with Other Methods. A total of 22 middle distillate fuels including 3 secondary standards, with different concentrations of aromatics, were selected. They were analyzed by SFC/FID, FIA (ASTM D1319), and NMR techniques. FIA was selected as a basis for comparison since it is the most widely used standard in the oil industry for aromatic determination despite its disadvantages. The NMR method provides additional structural information. Figure 7 shows a good correlation between SFC/FID aromatics (wt 5%) and FIA aromatics (vol %) data. SFC does overestimate the aromatics compared to FIA, which is expected since the former is based on weight and the latter is based on volume. Three points appear to be outliers. The chromatograms of these three samples all exhibit poor separation between their saturate and aromatic components, the region in which olefins are expected to elute, and have higher olefin contents (by FIA) than the rest. These facts, as well as the possibility of inaccuracies in the FIA data, may explain these outliers. The correlation between the weight percent aromatics by SFC/FID and NMR aromaticity is illustrated in Figure 8. Linear regression gave a slope of 0.69. The aromatics by SFC/FID are a measure of the amount of molecules (19) Preparing Precision Statements for Test Methods for Construction Materials. Annu. Book ASTM Stand. 1984, C 670-81.
