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Energy & Fuels 2009, 23, 888–893
Compositional and Structural Analysis of Lubricating Oil Feedstock Obtained from a Light Crude Oil Juan J. Espada, Baudilio Coto,* and Jose´ L. Pen˜a† Department of Chemical and EnVironmental Technology, ESCET., UniVersidad Rey Juan Carlos, c/ Tulipa´n s/n, 28933, Mo´stoles (Madrid), Spain ReceiVed October 24, 2008. ReVised Manuscript ReceiVed NoVember 19, 2008
Vacuum distillate cuts are used as feedstock in the lubricating oil manufacturing process. In this work, compositional and structural characterization of three vacuum distillate cuts was carried out. Composition of distillate cuts was determined in term of saturates, aromatics, and polars using the ASTM D2007 standard test method. The aromatic fraction was analyzed by HPLC to obtain the distribution of aromatic compounds in mono-, di-, and polyaromatic hydrocarbons. Saturate and aromatic fractions were analyzed for elemental carbon, hydrogen, and sulfur and for molecular weight. Proton and carbon nuclear magnetic resonance spectroscopy were applied to determine the average molecular structure of the hydrocarbons present in both fractions. The average molecular structure of the saturate hydrocarbons contained one naphthenic ring with an n-alkyl chain length of 18-22 carbon atoms and different branches. The average molecular structure of the aromatic fractions was made up of an aliphatic and an aromatic part. The aliphatic part is formed by one (light distillates cuts) or two cycloparaffinic rings (heavy distillate cut) and the n-alkyl chain length increasing with the boiling point of the distillate cut. The aromatic part was similar for all distillate cuts. Mono- and diaromatics showed, respectively, one or two aromatic rings fused to cycloparaffinic structures. Polyaromatics showed similar structure to that for the diaromatics, but including a thiophenic ring bridged to two aromatic rings.
1. Introduction Petroleum heavy distillate cuts and residues are raw materials for the production of valuable products such as lubricating oils. The residue from the crude oil atmospheric distillation (long residue) is transferred to a vacuum distillation column and separated into different lube oil cuts and characterized by their boiling range temperature and viscosity. Five raw lubricating cuts are commonly obtained. They are called SPD (spindle distillate), LND (light neutral distillate), MND (medium neutral distillate), HND (heavy neutral distillate), and BSD (bright stock distillate).1,2 Further refining of these fractions (such as removal of aromatic and long n-paraffin compounds) are required to obtain lubricating oils with suitable properties. The knowledge of the physicochemical properties, especially the composition, of petroleum fractions is crucial to describe processes in which these mixtures are involved. Likewise, it determines the quality of the final products. The composition of light petroleum fractions (FBP 220 °C) can be accurately determined in terms of individual components by well established methodologies, such as gas chromatography (GC) and GC-mass spectrometry.3 However, the complexity of heavier fractions makes the individual determination impossible * To whom correspondence should be addressed: Phone: 34 91 4887089. Fax: 34 91 4887068. E-mail:
[email protected]. † Alfonso Cortina Technology Centre, REPSOL-YPF, 28933, Mo ´ stoles (Madrid), Spain. (1) Sequeira A., Jr. Lubricant Base Oil and Wax Processing; Dekker: New York, 1994. (2) Speight J. G. The Chemistry and Technology of Petroleum; Dekker: New York, 1991. (3) Altgelt, K. H.; Boduszynski, M. Composition and Analysis of HeaVy Petroleum Fractions; Dekker: New York, 1994; pp 175-199.
and, therefore, the composition is frequently expressed in terms of the hydrocarbon-type, with regard to their chemical nature.4,5 The ASTM D2007 standard test method provides the composition of heavy petroleum fractions in terms of saturates, aromatics, and polar compounds.6 SARA analysis determines the content of saturates, aromatics, resins, and asphalthenes.7 These standard test methods are time- and money-consuming and, in some cases, not very reliable. For this reason, new methods based on experimental techniques have been investigated 8-11 and some chromatographic procedures have been developed and validated to carry out these analyses.12,13 Complete characterization of heavy petroleum fractions requires not only separation in different solvent fractions, but also structural analysis. Despite the complexity of heavy petroleum fractions, different experimental techniques have been successfully used to develop reliable methodologies applied to (4) Van Grieken, R.; Coto, B.; Romero, E.; Espada, J. J. Ind. Eng. Chem. Res. 2005, 44, 8106–8112. (5) Coto, B.; van Grieken, R.; Pen˜a, J. L.; Espada, J. J. Chem. Eng. Sci. 2006, 61, 4381–4392. (6) Standard Test Method for Characteristic Groups in Extender and Processing Oils and Other Petroleum-DeriVed Oils by the Clay-Gel Absorption Chromatographic Method, ASTM Standard D-2007-98; ASTM 2003 Annual Book of Standards, Vol. 5; American Society for Testing Materials: West Conshohocken, PA. (7) Paraffin Wax Content of Petroleum Oils and Asphalts, UOP Method 46-85; UOP Methods, UOP Inc., 1985. (8) Jorikova, L.; Ruman, J.; Davidova, M. Pet. Coal 2000, 42, 185– 187. (9) Ali, M. A.; Hassan, A. Pet. Sci. Technol. 2002, 20, 751–762. (10) Ali, M. A. Pet. Sci. Technol. 2003, 21, 963–970. (11) Kaminski, M.; Kartanowicz, R.; Przyjazny, A. J. Chromatogr., A 2004, 1029, 77–85. (12) Islas-Flores, C. A.; Buenrostro-Gonzalez, E.; Lira-Galeana, C. Energy Fuels 2005, 19, 2080–2088. (13) Kaminski, M.; Kartanowicz, R.; Gilgenast, E.; Namiesnik, J. Crit. ReV. Anal. Chem 2005, 35, 193–216.
10.1021/ef800930w CCC: $40.75 2009 American Chemical Society Published on Web 01/20/2009
Lubricating Oil Feedstock from Light Crude Oil
the structural analysis of these mixtures. Proton and carbon nuclear magnetic resonance (1H NMR and 13C NMR) spectroscopy has been reported as a useful tool to determine hydrocarbontype composition of both light and heavy petroleum fractions, and it is widely applied to the structural analysis of such mixtures.3,14-18 The combination of NMR information with that provided by other techniques yields average molecular parameters of hydrocarbons present in heavy petroleum fractions.19 Al-Zaid et al. 20 separated different heavy distillate cuts into saturates and aromatics by using an anion resin. Aromatics were split into mono-, di-, and polyaromatic hydrocarbons using opencolumn chromatography. Proton and carbon nuclear magnetic resonance were combined to obtain average molecular structures for the aromatic fractions of heavy distillate cuts. Ali et al. 21 fractionated heavy distillate cuts into saturates, monoaromatic, diaromatic, polyaromatic, and sulfur-enriched hydrocarbons using gel permeation chromatography (GPC). Average structure of the isolated fractions was determined by combining 1H and 13C NMR, GPC, and elemental analysis. The compositional and structural characterization of petroleum fractions is of great interest to model the processes in which these mixtures are involved. This kind of study allows average molecular structures to be obtained, and therefore thermodynamic models based on model-molecule approach can be applied. Although the application of this approach requires the deep work of characterization, the results obtained for the liquid-liquid equilibrium (LLE) process are similar or even more accurate than those provided by pseudocomponent-based methods. Thus, for instance, UNIFAC has been checked to describe the removal of aromatic compounds in the lubricating oil manufacturing process, obtaining good results.22,23 In this work, three different vacuum distillate cuts were separated into saturates, aromatics, and polars following the ASTM D2007 standard test method.6 Due to the very low content of polars, these compounds were included in the aromatic fraction. Both saturate and aromatic fractions were analyzed by elemental analysis, GPC, 1H, and 13C NMR experimental techniques. In addition, the aromatic fraction of each distillate was analyzed by a high performance liquid chromatography (HPLC) experimental technique to obtain the distribution in monoaromatic, diaromatic, and polyaromatic hydrocarbons. The combination of the obtained results was used to construct average molecular structures for each fraction. Obtained results were compared to those reported in the literature for similar distillate cuts showing reasonable agreement. 2. Experimental Section Three vacuum distillate cuts (VD_1, VD_2, and VD_3) were provided by REPSOL-YPF and analyzed as follows. (14) Kapur, G. S.; Chopra, A.; Sarpal, A. S. Energy Fuels 2005, 19, 1065–1071. (15) Netzel, D. A.; McKay, D. R.; Heppner, A.; Guffey, F. D.; Cooke, S. D.; Varie, D. L.; Linn, D. E. Fuel 1981, 60, 307–320. (16) O’Donnell, D. J.; Sigle, S. O.; Berlin, K. D.; Sturm, G. P.; Vogh, J. W. Fuel 1980, 59, 166–174. (17) Gillet, S.; Rubini, P.; Delpuech, J. J.; Escalier, J. C.; Valentin, P. Fuel 1981, 60, 221–225. (18) Gillet, S.; Rubini, P.; Delpuech, J. J.; Escalier, J. C.; Valentin, P. Fuel 1981, 60, 226–30. (19) Petrakis, L.; Allen, D. NMR for Liquid Fossil Fuels; Elsevier: Amsterdam, 1987; pp 91-112. (20) Al-Zaid, K.; Khan, Z. H.; Hauser, A.; Al-Rabiah, H. Fuel 1998, 77, 453–458. (21) Ali, F.; Khan, Z. H.; Ghaloum, N. Energy Fuels 2004, 18, 1798– 1805. (22) Espada, J. J.; Coto, B.; Pen˜a, J. L. Fluid Phase Equilib. 2007, 259, 201–209. (23) Vakili-Nezhaad, G. R.; Modarres, H.; Mansoori, G. A. Chem. Eng. Technol. 1999, 22, 847–853.
Energy & Fuels, Vol. 23, 2009 889 2.1. Composition Analysis. The composition was determined in terms of saturate, aromatic, and polar hydrocarbons following the ASTM D2007 standard test method.6 Both the vacuum distillate cuts and their aromatic fraction obtained by means of the ASTM D2007 were analyzed by HPLC to determine the distribution in monoaromatic, diaromatic, and polyaromatic hydrocarbons. An Agilent 1100 Series Chromatograph was used. A 0.1-g portion of sample was dissolved in 50 mL of heptane, and 8 µL of the mixture was injected into a column with a 5-µm thick amino stationary phase following a standard test method.24 2.2. Elemental Analysis. An Elementar Vario EL III CHNS analyzer was used to determine the content of the carbon and hydrogen of the saturate and aromatic fractions (of the studied distillate cuts). An oxygen flow of 65 mL/min was used in the combustion of the sample. Combustion gases were selectively separated by flowing them through different columns and were detected by thermal conductivity. Sulphanilic acid was used as standard for the calibration and analyzed after each experiment to check the quality of measurements. The precision for each determination was ( 0.3 wt%. 2.3. GPC Analysis. The average molecular weight of the different fractions was determined by gel permeation chromatography (GPC). An Alliance GPCV 2000, equipped with refractive index and viscosimeter detectors and three different columns (two PLgel 10 µm MIXTED-B, 300 × 7, 5 mm and a PLgel 10 µm 10E6 Å, 300 × 7, 5 mm) was used. The mobile phase was 1,2,4trichlorobenzene, the flow rate was set at 1 mL/min, and the temperature at 145 °C. Samples were dissolved in 1,2,4-trichlorobenzene, and the concentration was around 1.2-1.5 mg/mL. Universal calibration obtained from pure n-paraffins was used in this work. 2.4. NMR Analysis. A Bruker DRX 500 NMR spectrometer (C/H dual 5 mm probe, frequency 500 MHz) was used to quantify in different functional groups of hydrogen and carbon atoms. Samples for 1H NMR measurements were prepared by desolving 15-20 mg of the sample in 0.45 mL of CDCl3, used as the solvent, in 5 mm samples tubes. The number of scans was 64, with a 30° pulse and a 1 s delay time between scans. For 13C NMR inverse gated decoupling was applied. The conditions of the analysis were spectral widths of 20 kHz, pulse width of 6 µs (45°) and pulse delays of 20 s as reported elsewhere.21 A total of 70-80 mg of the sample was desolved in 0.80 mL of CDCl3 solvent with 15-20 mg of relaxation agent Cr (acac)3. Such agent was added to decrease the delay time between the pulse cycles.
3. Results and Discussion 3.1. Characterization of Vacuum Distillate Cuts. The distillation curve and composition in terms of hydrocarbon-type were determined for the studied distillate cuts. 3.1.1. Distillation CurVe. The distillation curves were determined for the studied distillate cuts following the ASTM D2887 standard test method25 and shown in Figure 1. As can be observed, the three cuts were in the temperature range of lubricating oil feedstock (SPD-HND).4 3.1.2. Composition Analysis. The fractionation of the distillate cuts was determined following the ASTM D2007 standard test method. Experiments were replicated to validate the results. Table 1 shows the compositions in weight percent obtained by means of the ASTM D2007 standard test method in saturates (XS), aromatics (XA) and polars (XP) and by HPLC analysis in monoaromatics (XMA), diaromatics (XDA), and polyaromatics (XPA). The absolute deviation between two experiments was also (24) European Patent prEN 12916. 1997. (25) Standard Test Method for Boiling range Distribution of Petroleum Fractions by Gas Chromatography, ASTM Standard D-2887-99; ASTM 2003 Annual Book of Standards, Vol. 5; American Society for Testing Materials: West Conshohocken, PA.
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Figure 1. ASTM D2887 distillation curve of the vacuum distillates ( · - · - · VD_1; · · · · · · · VD_2; s VD_3).
included and in all cases they were within the reproducibility limits of both techniques. Both the aromatic and polar contents increased as the boiling temperature of the distillate cut became higher, as expected. All distillate cuts studied in this work showed polar contents less than 1% by weight, thus revealing the low content of heteroatoms. As reported elsewhere,22 polars were included in the aromatic fraction as sulfur-containing aromatic compounds for this kind of distillate fraction. Therefore, the composition of the mixtures can be expressed in terms of only two chemical classes: saturate and aromatic hydrocarbons. The vacuum distillate cuts were analyzed by HPLC to determine the contents of aromatic compounds in mono-, diand polyaromatics. The obtained results shown in Table 1 revealed that the aromatic compounds present in all distillate cuts were mainly composed of monoaromatic hydrocarbons. Diand polyaromatic hydrocarbons are much less, but they increased for the heavier cuts, as expected. This indicated a low content of complex aromatic structures, which is of interest for lubricating oil feedstock since those compounds make the viscosity of the mixture strongly temperature dependent.1 Figure 2 shows the comparison between the total aromatic content determined by both the ASTM D2007 standard test method and by HPLC analysis. Good agreement was found in all cases, thus validating the experimental results. In this work, polyaromatic hydrocarbons were considered as sulfur-containing aromatic hydrocarbons as reported elsewhere for this kind of mixtures.22 Good agreement (deviations
