Energy & Fuels 2002, 16, 1571-1575
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Rapid and Accurate SARA Analysis of Medium Gravity Crude Oils Tianguang Fan and Jill S. Buckley* New Mexico Petroleum Recovery Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico Received May 31, 2002
Crude oils can be described compositionally by a number of methods. SARA analysis is widely used to divide crude oil components according to their polarizability and polarity using a family of related analytical techniques. Problems arise because the analytical techniques do not necessarily produce identical results. Users of the data, however, rarely distinguish between the different techniques, assuming that SARA fraction values generated by any of the commonly used methods are essentially interchangeable. We examine this assumption for medium gravity crude oils and three SARA analysis methods: gravity-driven chromatographic separation, thinlayer chromatography (TLC), and high-pressure liquid chromatography (HPLC). Results for a suite of six crude oil samples show that a significant volume of volatile material that contains both saturates and aromatics is lost in the TLC analysis. An improved HPLC method is introduced that gives analyses comparable to the ASTM-recommended chromatographic method in less time than that required for TLC analysis. An internal consistency test is recommended for evaluating SARA fraction data.
Introduction Analysis of the composition of crude oils can be endlessly complex; the amount of detail collected should be dictated by the application for which the data is needed. One simple analysis scheme is to divide an oil into its saturate, aromatic, resin, and asphaltene (SARA) fractions. The saturate fraction consists of nonpolar material including linear, branched, and cyclic saturated hydrocarbons. Aromatics, which contain one or more aromatic rings, are more polarizable. The remaining two fractions, resins and asphaltenes, have polar substituents. The distinction between the two is that asphaltenes are insoluble in an excess of heptane (or pentane), whereas resins are miscible with heptane (or pentane). This classification system is useful because it identifies the fractions of the oil that pertain to asphaltene stability; it thus should be useful in identifying oils with the potential for asphaltene problems. SARA analysis began with the work of Jewell et al.1 Three main approaches have been used to separate crude oils and other hydrocarbon materials into SARA fractions. A clay-gel adsorption chromatography method is the basis of ASTM D2007-93.2 This method requires a fairly large oil sample, is time-consuming and difficult to automate, and requires large quantities of solvents. Improved methods fall into two groups. In the first group are high-pressure liquid chromatographic (HPLC) * Corresponding author. Fax: 1-505-835-6031. E-mail: jill@ prrc.nmt.edu. (1) Jewell, D. M.; Weber, J. H.; Bunger, J. W.; Plancher, H.; Latham, D. R. Anal. Chem. 1972, 44, 1391-1395. (2) ASTM D2007-93: “Standard Test Method for Characteristic Groups in Rubber Extender and Processing Oils by the Clay-Gel Adsorption Chromatographic Method,” ASTM, 1993.
methods, first introduced by Suatoni and Swab.3 Early HPLC techniques used silica or alumina columns to separate lighter petroleum fractions. The developments in preparation of the bonded phase of HPLC columnss especially NH2-bonded materialssmade it practical to separate heavier fractions of petroleum samples.4-8 HPLC techniques are faster, more reproducible, and more readily automated than the ASTM column technique. In both cases, however, it is necessary to remove the asphaltene fraction before proceeding with the chromatography. Asphaltenes are either irreversibly adsorbed or precipitated during the saturate elution step, and quantitative recovery cannot be achieved.9 The fastest separation method uses thin-layer chromatography (TLC) on quartz rods that are coated with sintered silica particles. Unlike the column and HPLC techniques, asphaltenes need not be separated from other crude oil components before chromatographic analysis. A popular technology known as the Iatroscan that combines TLC with flame ionization detection (TLC-FID) was first applied by Suzuki10 to automate quantitative SARA separations, a method which has (3) Suatoni, J. C.; Swab, R. E. J. Chromatogr. Sci. 1975, 13, 361366. (4) Miller, R. Anal. Chem. 1982, 54, 1742-1746. (5) Radke, M.; Willsch, H.; Welte, D. H. Anal. Chem. 1984, 56, 25382546. (6) Grizzle, P. L.; Sablotny, D. M. Anal. Chem. 1986, 58, 2389-2396. (7) Fe´lix, G.; Thoumazeau, E.; Colin, J. M.; Vion, G. J. Liq. Chromatogr. 1987, 10, 2115-2132. (8) Chaffin, J. M.; Lin, M. S.; Liu, M.; Davison, R. R.; Glover, C. J.; Bullin, J. A. J. Liq. Chromatogr., Relat. Technol. 1996, 19, 1669-1682. (9) McLean, J. D.; Kilpatrick, P. K. Energy Fuels 1997, 11, 570585. (10) Suzuki, Y. 21st Annual Meeting of the Japan Society for Analytical Chemistry, 1972; 47 (in Japanese).
10.1021/ef0201228 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/29/2002
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Energy & Fuels, Vol. 16, No. 6, 2002
Fan and Buckley
Table 1. Crude Oil Sample Properties oil
°API
density at 20 °C (g/cm3)
A-95 C-LH-99 C-R-00 S-Ven-39 SQ-95 Tensleep-99
25.2 22.6 31.3 28.8 37.2 31.1
0.8956 0.9161 0.8673 0.8795 0.8409 0.8685
since been used extensively.11,12 Barman13 compared SARA analyses of heavy hydrocarbon distillates by the clay-gel and TLC-FID methods. TLC-FID uses very small amounts of sample. SARA fractions in a crude oil sample are often well-resolved using established development procedures. Quantitative results are obtained from peak areas, assuming that each SARA fraction has an identical FID response factor. Comparisons of SARA fraction measurements by different techniques, usually from different laboratories, can show large differences. To some extent, these differences might be realscaused by the use of different samples. In this work, identical samples were examined by several different techniques to illuminate the strengths and weaknesses of each. An improved HPLC technique and a test for internal consistency in SARA data are presented. Experimental Materials and Methods Crude Oils. Six medium-gravity, dead crude oils, varying in API gravity from 22.6 to 37.2°, were used in this study. Selected physical and chemical properties are shown in Table 1 (reproducible to at least (5 in the least significant digit shown for each measurement). Densities were measured using a Mettler/Paar DMA 40 with a circulating water bath for temperature control. API gravities were calculated from measured densities, corrected to 60 °F. Average molecular weight (MW) was determined by freezing point depression (Precision Systems Cryoscope 5009). Kinematic viscosities were measured in Cannon-Fenske viscometers and converted to absolute viscosity units. The amounts of asphaltene precipitated by n-heptane (1 g oil:40 mL heptane) ranged from 1.3 to 8.7%. Refractive index (RI) was measured with an Index Instruments GPR11-37 automatic refractometer. PRI is the RI of a mixture of oil and the least amount of heptane in which asphaltene aggregates can be observed microscopically (at a magnification of about 320×). The difference between RI of the oil sample and PRI is a measure of asphaltene stability.14 ASTM Column Separation. Each of the crude oils was tested using the full ASTM-recommended procedure (ASTM D2007-932) using n-hexane to separate the asphaltenes. The ASTM procedure is a chromatographic separation of the nonasphaltic oil components through two columns: an Attapulgite clay-packed column adsorbs the resins and a second column packed with activated silica gel separates aromatics from the saturate fraction. A 50:50 mixture of toluene and acetone is used to recover the resin fraction from the clay packing. The aromatics can be recovered by Soxhlet extraction of the silica gel in hot toluene. Volatile components lost during the process are calculated by weight difference. TLC-FID. An MK-5 Iatroscan (Iatron Labs Inc., Tokyo), equipped with a flame ionization detector (FID), interfaced (11) Karlsen, D. A.; Larter, S. R. Org. Geochem. 1991, 17, 603-617. (12) Vela, J.; Cebolla, V. L.; Membrado, L.; Andres, J. M. J. Chromatogr. Sci. 1995, 33, 417-424. (13) Barman, B. N. J. Chromatogr. Sci. 1996, 34, 219-225. (14) Buckley, J. S.; Hirasaki, G. J.; Liu, Y.; Von Drasek, S.; Wang, J. X.; Gill, B. S. Pet. Sci. Technol. 1998, 16, 251-285.
MW (g/mol)
RI at 20 °C
PRI
n-C7 asph (%)
236 268 235 240 213 270
1.5128 1.5137 1.4851 1.4976 1.4769 1.4906
1.4513 1.4231 1.4444 1.4465 1.4223 ∼1.44
8.7 2.8 1.9 5.8 1.3 4.1
Figure 1. Separation of paraffins from single- and doublering aromatics. with a model 203 PeakSimple data system (SRI Instrument), was used to scan silica-coated quartz rods (Chromarod-SIII, Iatron Laboratories). The rods were 15.2 cm long and 1.0 mm in diameter, with a uniform coating of 5.0 µm silica particles (pore diameter 60 Å). The FID detector was operated with a pure grade of hydrogen at a flow rate of 160 mL/min; air at a flow rate of 2.0 L/min was supplied by a pump; scan speed was 60s/scan. Crude oil samples were dissolved in HPLC grade dichloromethane (DCM) at a concentration of 20 mg/mL. A 1 µL repeating syringe (Alltech Associate, Inc.) was used to spot 10 to 20 µg of sample on freshly activated rods. Development steps included exposure to HPLC grade hexane for 30 min, HPLC grade toluene for 10 min, and a 95:5 mixture of DCM and methanol for 4 min. The rods were dried in air for 3 min between solvent exposures. This treatment produced four wellresolved peaks representing saturates, aromatics, resins, and asphaltenes. Evaporative losses during the development steps were evaluated by tests of similarly treated preparative thinlayer chromatography plates for which changes in weight could be measured and by comparison of peak sizes for undeveloped rods. HPLC. The HPLC chromatographic separation system consisted of a Model 110A pump (Beckman), an R401 differential refractometer (Waters), a U6K universal injector (Waters), a Model 7040 high-pressure switching valve (Rheodyne), and a Waters 486 UV detector (Millipore). Analogue signals from the RI detector were interfaced to the model 203 PeakSimple data acquisition system (SRI Instrument). Two 3.9 × 300 mm µBondapak NH2 columns with 10 µm packing (Waters) were used in series for chromatographic separation of crude oil. The UV detector was operated at a wavelength of 254 nm to monitor elution of each fraction. Selectivity of the columns was tested using mixtures of known compounds. Mixtures of n-decane (C10), n-tridecane (C13), n-pentadecane (C15), and n-octadecane (C18) in hexane eluted as a single peak, as did hexane solutions of toluene and 1,3-diisopropylbenzene (DIPB). Hexane solutions of a singlering aromatic (DIPB) with a two-ring compound (1-methylnaphthalene or 1-MN) eluted as two well-resolved peaks. Mixtures of the hexane solutions of DIPB, 1-MN, and C15 eluted as three distinct peaks (Figure 1). Crude oil (1-mL) was weighed and dissolved in 40 mL of hexane in an open-top screw cap vial with a Teflon/silicon septum. After 48 h, a sample of maltenes dissolved in hexane was withdrawn through a 0.2 µm PTFE syringe filter. The withdrawn maltene solution was sealed in a 5-mL crimp-top
SARA Analysis of Medium Gravity Crude Oils
Energy & Fuels, Vol. 16, No. 6, 2002 1573 Table 3. Pairwise t-tests for Significance of Differences between SARA Methodsa p-valuesb
Figure 2. HPLC separation of C-R-00 maltenes. Table 2. RI Detector Calibration class
hydrocarbon
signal (area/mg)
saturates 1-ring aromatics 2-ring aromatics 3-ring aromatics
n-heptadecane DIPB 1-MN phenanthrene
4647 7088 12517 16022
glass vial with an aluminum seal and PTFE/silicon septum. The precipitated asphaltenes were recovered by filtration through a 0.22 µm filter, dried, and weighed. Several 0.5 mL aliquots of the maltene/hexane mixture were injected onto the HPLC columns, using a gastight HPLC syringe (Hamilton). Saturates and aromatics were eluted with hexane at a flow rate of 1.5 mL/min. The amounts of saturates and aromatics were calculated from peak areas using calibration factors measured for known compounds (Table 2). Standard deviations for peak areas of known compounds were less than 1% of the measured values. A typical chromatogram is shown in Figure 2. Resins were strongly adsorbed and did not elute with hexane. A 30% (v/v) dichloromethane/hexane backflush was used to elute the resins accumulated from at least three replicate injections to ensure that sufficient resins could be collected for accurate gravimetric determination of the amount of resin remaining after solvent evaporation. The principal improvements in the HPLC method described here over previously proposed HPLC techniques include the following: (i) analysis of the saturate and aromatic fractions without solvent evaporation, thus avoiding the uncertainties due to evaporation of some of the more volatile material along with the solvent; and (ii) improved gravimetric quantification of the amount of resins eluted by backflushing the columns after repeated injections of maltenes.
Results and Discussion The SARA fractions measured by all three techniques are summarized in Figure 3. The results of the ASTM method provide a baseline against which other SARA separation methods can be compared. Table 3 shows the results of t-tests applied to TLC-FID and HPLC results paired with those obtained by the ASTM method for the same oil to determine statistical significance of the differences in the data. TLC-FID vs ASTM. Comparisons between ASTM and TLC-FID results show consistent, statistically significant differences for all fractions except the asphaltenes (Table 3). The amount of volatile material lost before the detector response is recorded is not routinely recognized or measured. That amount is as high as 60% in this study, all of which comes from the saturate and aromatic fractions. If fractions were apportioned on the basis of peak areas summed to 100%, ignoring the volatile material, the amounts of resins and asphaltenes
fraction
ASTM vs TLC-FID
ASTM vs HPLC
saturates saturates + volatiles aromatics resins asphaltenes
