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A Novel Approach for the Quantitation of the Hydrocarbon Groups in Heavy Petroleum Fractions by HPLC-RI Analysis N. Pasadakis* and N. Varotsis PVT and Core Analysis Laboratory, Department of Mineral Resources Engineering, Technical University of Crete, GR 73100, Chania, Greece Received April 10, 2000. Revised Manuscript Received September 11, 2000
A novel approach is presented for the accurate quantitative determination of the main hydrocarbon groups (saturates and aromatics) in heavy petroleum fractions using high-pressure liquid chromatography and refractive index detection (HPLC-RI). The concentration of each component group is determined from the RI peak areas, which are corrected using an experimentally derived function of the response factors with respect to the elution time. The calibration function is determined by analyzing a series of successively eluting subfractions, obtained by open column chromatography. The method was applied for the determination of the saturates and of the aromatic content of 16 heavy petroleum fractions, exhibiting significant differences in their compositions, and the results obtained were found to agree within 1.5 wt % with the values measured by the ASTM D2549-91 method.
Introduction Accurate compositional data describing the distribution of the main hydrocarbon groups in heavy petroleum fractions are required in several applications related to the production of lubricant oils, including optimization of the operating conditions, prediction of the product quality, product performance evaluation, structureproperty correlation, etc. A significant number of experimental methods including standard ASTM ones have been developed for the accurate determination of the component groups that are present in oil samples.1-3 HPLC is nowadays the most commonly used analytical method for hydrocarbon group type analysis, as it requires relatively simple experimental setup and procedures. The major drawbacks of the method are the lack of a universal mass sensitive detector and the incomplete separation achieved between the different component groups. The RI detector, which is the most commonly utilized detector for this type of analysis, cannot provide directly quantitative results as each component group exhibits a different response factor depending on the refractive index values of its constituents. The calibration procedure that uses pure hydrocarbons as representative compounds of each component group leads to inaccurate results as the responses of the * Corresponding author. Tel: +30-821-37469. Fax: +30-821-37468. E-mail:
[email protected]. (1) Separation of Representative Aromatics and Nonaromatics Fractions of High-Boiling Oils by Elution Chromatography. ASTM D 254991; 1995; ASTM Vol. 05.01 (2) Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption. ASTM D-1319-89; American Society for Testing and Materials: West Conshohocken, Pennsylvania, 1993; ASTM Vol. 0501. (3) Altgelt, K. H.; Gouw, T. H. Separation Schemes in Chromatography in Petroleum Analysis. In Chromatography in Petroleum Analysis; Marcell Dekker: New York, 1979; pp 185-214. (4) Sink, C. W. Fuel Sci. Technol. Int. 1994, 12, 1081-1103.
actual components contained within each group deviate to a lesser or greater extent from the RI response of the standards. Several experimental procedures have been developed to overcome this problem. C. W. Sink4 proposed a method to calculate the response factor of each component group based on a double analysis of the sample, using a different mobile phase for each run. The method is troublesome for routine analysis especially in industrial applications, due to time constraints as every time the mobile phase composition changes, the RI signal takes a long time to stabilize. W. A. Dark5 proposed to quantify the component groups based on the RI peak areas using response factors obtained from equations, which were derived experimentally as functions of the specific gravity (API) of the crude oil. For the separation and quantitation of the aromatics, the IP 391 method6 enables the determination of the mono-, di-, tri-, and tetracyclic aromatic content in petroleum distillates with boiling points in the range of 90-430 °C. The calibration is based on response factors for each aromatic group derived from the analysis of pure hydrocarbons (o-xylene, 1-methylnaphthalene, and phenanthrene, respectively). An alternative approach was proposed by Sukhhal et al.7 according to which the total saturates and total aromatics were determined using the RI signal calibrated using subfractions obtained from samples of the same origin by conventional column chromatography. This technique can be applied with accuracy for the analysis of samples exhibiting similar composition. (5) Dark, W. A. J. Liq. Chromatogr. 1982, 5, 1645-1652. (6) IP Standard Methods for Analysis and Testing of Petroleum and Related Products; John Wiley & Sons: London, 1995; p 391.1-3. (7) Sarowha, S. L.; Sharma, B. K.; Sharma, C. D.; Bhagat, S. D. Fuel 1996, 75, 1323-1326
10.1021/ef000072b CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000
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Table 1. Physicochemical Characterization of the Petroleum Fractions and Separation Results Obtained by the ASTM D2549-91 and the Proposed HPLC-RI Methods (wt %) HPLC-RI
sample F-52 F-54 F-106 F-107 F-201 F-303 F-405 E-52 R-52 W-52 D-52 B-52 B-308 B-001 B-34 B-725
viscosity cSt
S content wt %
3.16 2.91 6.00 5.30 13.24 13.84 24.69 4.12 3.05
2.46 2.25 2.51 2.42 2.72 2.74 3.26 4.55 0.76 0.21 1.05 0.52 0.84 0.60 1.05 0.69
3.18 3.12 10.42 10.77 10.19 11.37
specific gravity 0.9026 0.8972 0.9168 0.9130 0.9364 0.9362 0.9503 0.9885 0.8567 0.8560 0.8560 0.8625 0.8752 0.8630 0.8790
boiling point (°C) initial
final
saturates
aromatics
recovery
saturates
aromatics
total calculated vs injected sample mass (%)
295 290 320 315 350 360 370 300 290 310 290 290 355 360 285 360
510 512 517 510 520 530 545 512 510 510 500 500 525 530 500 525
54.1 53.0 49.0 50.6 43.4 43.8 40.6 13.5 74.3 91.2 70.6 73.6 66.0 68.0 78.7 70.0
45.9 47.0 51.0 49.4 56.6 56.2 59.4 86.5 25.7 8.8 29.4 26.4 34.0 32.0 21.3 30.0
99.6 99.5 99.9 99.9 99.8 100.1 99.1 99.1 99.1 99.6 99.2 100.7 100.2 100.4 99.6 99.5
55.2 53.3 49.8 51.8 44.4 43.0 42.0 14.9 73.3 89.7 68.7 71.9 67.3 69.8 78.5 71.4
44.8 46.7 50.2 48.2 55.6 57.0 58.0 85.1 26.7 10.3 31.3 28.1 32.7 30.2 21.5 28.6
100.2 100.1 99.5 99.2 99.6 98.5 98.4 98.5 100.8 101.0 101.1 100.5 101.6 100.9 100.3 98.9
ASTM D2549-91
Table 2. Solvents Used for B-34 Sample Separationa solvent
volume (mL)
n-pentane n-pentane, dichloromethane (97:3 vol) n-pentane, dichloromethane (80:20 vol) diethyl ether, dichloromethane (80:20 vol) methanol, diethyl ether (90:10 vol)
70 60 60 70 110
a
Sample mass 2 g.
The objective of this work was to improve the calibration procedure for the quantitative determination of the main hydrocarbon groups present in heavy gas oil fractions by HPLC-RI analysis. It was found that the response factors of individual subfractions could be expressed satisfactorily as a function of their elution time. The quantitative results obtained by the proposed method from the analysis of 16 heavy petroleum fractions were found to be within 1.5 wt % with the results derived using the standard ASTM D2549-91 method. Experimental Section Samples. Sixteen heavy petroleum fractions obtained from the lube oil production unit of a Greek refinery were used for the development of the method. The sample set includes feedstock, intermediate, and final products of the production unit. They are referred as F, E, R, D, W, and B representing feed, extract, raffinate, dewaxed oil, wax, and base oil, respectively. All the samples were preliminary separated into nonaromatic and aromatic fractions, according to the ASTM D254991 method. The separation results are shown in Table 1. The sample B-34 (2 g) was subsequently separated into a series of progressively eluted subfractions using an open column chromatography method, based on the analytical scheme proposed by Lappas et al.9 The separation of the sample was performed using the quantities of solvents (analytical grade from Labscan) presented in Table 2. Eluate fractions of 10 mL each were successively collected and their sample contents were determined gravimetrically after solvent evaporation. The sample content in each eluate fraction, referred as Si and Ai for the nonaromatics and the aromatics respectively, are presented in Table 3. (8) Pasadakis, N.; Gaganis, V.; Varotsis, N. International Conference, Instrumental Methods of Analysis. Modern Trends and Applications; Chalkidiki, Greece, 1999; pp 472-478. (9) Lappas, A. A.; Patiaka, D. T.; Dimitriadis, B. D.; Vasalos, I. A. Appl. Catal. A 1997, 152, 7-26.
Table 3. Results of the Open Column Separation of the B-34 Sample subfraction
eluate volume (mL)
sample content wt %
S1 S2 S3 S4 S5 total saturates
10 10 10 10 10
18.3 35.9 18.1 5.0 1.4 78.7
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 residual total aromatics
10 10 10 10 10 10 10 10 10 10 10
2.0 2.5 2.5 2.3 1.9 6.2 1.4 0.6 1.0 0.5 0.2 0.3 21.3
HPLC Analysis. A Waters modular HPLC system consisting of a pump (model 600) and a refractive index (RI) detector (model 410) was used, with 95% n-hexane and 5% isopropyl ether as mobile phase at a flow rate of 1 mL/min. The analytical part consisted of two columns Versapack NH2 4.1 × 300 mm from Alltech, connected in series and kept thermostatically controlled at 35 °C. The experimental procedure is presented in details in an earlier work.8 All the samples and the subfractions obtained from the sample B-34 were analyzed. A typical HPLC-RI chromatogram of the samples F-52 and B-34 together with the chromatogram of a mixture consisting of tetradecane, benzene, 1-methylnaphthalene, and 9-methylanthracene to indicate the position of the elution windows of the hydrocarbon groups are presented in Figure 1.
Results and Discussion The first five subfractions obtained by column chromatography of the B-34 sample were attributed to the saturated fraction as indicated by the ASTM D-2549 method. This was also confirmed by analyzing these subfractions using HPLC-UV-DAD where no traceable signal was detected. From the separation results (Table 3) it can be noticed that more than 99% wt of the sample components were eluted with the first 170 mL of solvents. This is due to the nature of the B-34 sample,
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Figure 1. HPLC-RI chromatogram of the samples F-52, B-34, and of the standard mixture.
Figure 2. HPLC-RI elution profiles of the subfractions separated from the B-34 sample.
which is a light, low aromatic hydrofinished base oil. The total amound of the solvents (Table 2) ensure the complete elution of all the components from heavy oil fractions with high aromatic content. The RI signals of the first thirteen subfractions, normalized for equal peak height, are presented in
Figure 2. As expected, strong overlapping was observed between successively eluting subfractions. The RI signal of each subfraction after baseline subtraction was expressed as a two-column matrix (elution time-intensity). Each intensity value represents the area of a 0.01 min time segment in the signal. The
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response factor of the detector for each time segment RFi can be expressed as
RFi )
Ei Ci
(1)
where Ei is the area of the ith segment in the RI signal and Ci the concentration (mass per unit of injected volume) of the sample eluting within this segment. The RFi was modeled as a function of the elution time using a second-degree homographic equation:
RFi )
a*RTi2 + b*RTi + c d*RTi + e
(2)
where RTi is the retention time of the corresponding time segment. An iterative calculation procedure was performed to determine the values of the coefficients of the above equation in order that the calculated overall concentration of each subfraction to become equal to the injected one. The minimization of the error between the injected and the calculated concentrations was accomplished with a least-squares regression algorithm. The equation coefficients determined from the subfractions were found to exhibit similar values. The calculated RFi’s of all the subfractions, for each time segment, were found to deviate less than 3% from their average value. Therefore, the following average coefficients were adapted:
a b c d e
) ) ) ) )
142 703 262 24 584 976 4 237 640 31 838 350 5 457 154
The RI signals of the time segments of the 16 samples involved in this study were multiplied by the corresponding RFi’s obtained from eq 2 to produce the mass concentration elution profile of each sample. The inte-
gration of these profiles (duplicate analysis) gave total concentrations with maximum absolute error less than 2% of the injected in the HPLC system ones, as shown in Table 1. The good agreement between the experimental and the calculated concentrations indicates that the calibration equation describes satisfactorily the RI response factors with elution time. The obtained mass fractions of the nonaromatic and aromatic groups are shown in Table 1 in comparison to the mass fractions determined by the ASTM D2549-91 method. The observed differences in the results of the two methods are shown to be within the range of the repeatability of the standard method. The corrected, according to this method, RI signal can be further used to calculate the concentration of the aromatic subgroups (mono-, di-, and tri-), based on the shape of the chromatogram and the elution times of representative pure hydrocarbons. The results of this study indicate that accurate and reliable quantitation of the major hydrocarbon classes in heavy petroleum fractions can be obtained using elution time dependent response factors for the HPLCRI signal. According to the presented method, each time segment is processed with its own response factor. Therefore, the quantitation of the hydrocarbon groups takes into account the specific shape of their peaks. As the peak shape is indicative for the type of the compounds that belong to each hydrocarbon group, the quantitative results are more accurate than those derived using a single response factor for the entire hydrocarbon group. The proposed method can successfully replace timeconsuming open column chromatography methods providing additionally quantitative information on the aromatic subgroups. The method is especially suitable for industrial laboratories, where large sets of samples of similar origin have to be analyzed on a routine basis. Acknowledgment. We would like to thank the Greek General Secretariat of Research and Technology for partially financing this work. EF000072B
