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Quantitative Extraction Procedure of Naphthenic Acids Contained in Crude Oils. Characterization with Different Spectroscopic Methods J. Saab,*,† I. Mokbel,† A. C. Razzouk,† N. Ainous,† N. Zydowicz,‡ and J. Jose† Universite´ Claude Bernard Lyon 1, UMR 5180 Sciences Analytiques, Equipe Analyze des Syste` mes Polyphasiques, Baˆ t J.Raulin, 2e` me e´ tage, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France, and Universite´ Claude Bernard Lyon 1, UMR 5627sLaboratoire des Mate´ riaux Polyme` res et Biomate´ riaux, Baˆ timent ISTIL, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received March 18, 2004. Revised Manuscript Received December 3, 2004
The organic acids present in petroleum, commonly called “naphthenic acids” (NA), are identified as carboxylic acids of the general formula “RCOOH”, where R represents a hydrocarbon chain that does not necessarily show cycloaliphatic structure. The presence of stable oil-in-water emulsions in the crude oils hinders the dehydration process, which is required during the production step. Some compounds, such as the organic acids (NA) present in the crude oils, are involved in the stabilization of the emulsions, because of their amphiphilic structure. The emulsion-breaking process is improved if the organic acids are determined qualitatively and quantitatively. We proposed the study of a quantitative extraction procedure of NA contained in crude oils. First of all, we performed the liquid-liquid extraction of the organic acids, using alcoholic solutions. Because this method did not allow the quantitative recovery of the acids, we developed a separation process based on an ion-exchange resin (QAE Sephadex A25). The isolated acid fraction was then derivatized as methyl esters and quantified by gas chromatography experiments, using the internal standard method allowing the determination of the NA composition. The extraction yield was checked via the total acid number (TAN) measurement, using the standard ASTM D664-95 (IP 177/96), which is the standard method commonly used in the oil industry. The extraction of NA on ion-exchange resins required the preliminary study of a model molecule mixture of carboxylic acids, to ensure the complete control of the procedure. Four crude oil samples that were provided by Total (France) (Y1, Y2, Y3, Y4) were then analyzed. The results confirmed the presence of such acids in the crude oils.
1. Introduction Crude oils are known to contain organic acids that are often called naphthenic acids (NA).1 Their presence in crude oils can result from two different sources:2 acids resulting from the original rock and acids synthesized from the crude oil biodegradation by the microorganisms. They are considered as biomarkers, related to the maturity and the biodegradation level of oil reservoirs.3,4 During the oil production, the degasification (in the decanter) of CO2 due to the pressure gradient leads to the dispersion of water in the organic phase (oil).5 The formed emulsion is thermodynamically unstable. How* Author to whom correspondence should be addressed. E-mail address:
[email protected]. † UMR 5180 Sciences Analytiques, Equipe Analyze des Syste ` mes Polyphasiques. ‡ UMR 5627sLaboratoire des Mate ´ riaux Polyme`res et Biomate´riaux. (1) Meredith, W.; Kelland, S. J.; Jones, D. M. Org. Geochem. 2000, 30, 1059-1073. (2) Watson, J. S.; Jones, D. M.; Swannell, R. P. J.; Duin Van, A. C. T. Org. Geochem. 2002, 33, 1153-1169. (3) Meredith, W.; Kelland, S.-J.; Jones, D. M. Org. Geochem. 2000, 30, 1059-1073. (4) Dzidic, I.; Somerville, A. C.; Raia, J. C.; Hart, H. V. Anal. Chem. 1988, 60, 1318-1323.
ever, the natural emulsion stabilizers present in the oil (such as asphaltenes, carboxylic acids, and their soaps) lead to emulsion stability, because of their role in the oil/water (O/W) interface.6-10 The stability of the O/W emulsions could be due to the formation of strong interfacial films of asphaltenes. Furthermore, the presence of the carboxylic acids would induce the decrease of the interfacial tension required for the formation of a stable emulsion. The NA chemical structure and their amount have an important role in regard to the interfacial tension (γ) values. (5) Sjoblom, J.; Johnsen, E. E.; Westvik, A.; Ese, M. H.; Djuve, J.; Auflem, I. H.; Kallevik, H. Demulsifiers in the Oil Industry. In Encyclopaedic Handbook of Emulsion Technology; Marcel Dekker: New York, 2000; pp 595-619. (6) Goldszal, A.; Hurtevent, C.; Rousseau, G. Scale and Naphthenate Inhibition in Deep-Offshore Fields. Presented at the Society of Petroleum Engineers Oilfield Scale Symposium, 2002, Aberdeen, U.K., Paper No. SPE 74661. (7) Pathak, A. K.; Kumar, T. Study of Indigenous Crude Oil Emulsions and Their Stability. In Proceedings of PETROTECH-95, Technology Trends in Oil Industry, New Dehli, India, 1995; pp 217224. (8) Strassner, J. E. J. Pet. Technol. 1968, 20 (3), 303-312. (9) Pickett, J. M.; Ellway, K. A. J. Pharm. Pharmacol. 1976, 28 (8), 625-628. (10) Mendez, Z.; Anton, R. E.; Salager, J. L. J. Dispersion Sci. Technol. 1999, 20 (30), 883-892.
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Nevertheless, the NA are responsible for some problems observed in the refining of oil, such as the deactivation of heterogeneous catalysts used in the refineries and their contribution to the salt deposits in the pipelines [(RCOO)2Ca].11 Knowledge of the acid origin, their extraction, the quantitative and structural study, the phase equilibria of the water-oil-carboxylic acid systems, and the interfacial activity is required a the better understanding of the organic acid chemistry. The organic acids are known as products of oil biodegradation. Because the crude oil is formed in anaerobic mediums, its biodegradation by the anaerobic bacteria was studied.12,13 However, it is difficult to perform the degradation of hydrocarbons under anaerobic conditions. This resistance to the oxidation is due to the great stability of the C-H and C-C bonds in hydrocarbons.14 Some authors15-17 showed that some radicalar enzymatic reactions are implied in the initial activation of hydrocarbons under anoxic conditions. To study the effect of the acids formed by oil degradation (emulsion, formation of salt deposit, corrosion), it is necessary to realize a quantitative extraction of these acids. Five methods of acid extraction are mainly cited in the literature: (1) The extraction method by supercritical fluid.18 (2) Liquid-liquid extraction by a potassium alcoholic solution. This technique often causes the formation of an emulsion, which makes the quantitative recovery of the acids difficult.19,20 (3) Extraction on activated silica gel by a potassium alcoholic solution. This method requires a long time (∼72 h) and a high quantity of extraction solvent.21,22 (4) Extraction on an ion-exchanging resin in an aqueous medium,23 using cyclohexane as the solvent of the sample. The cyclohexane makes the asphaltene precipitate on the functional groups of the resin, causing a loss of acids with the interferent compounds. (5) Extraction on an ion-exchanging resin in a nonaqueous medium,1,24 which is a method for routine and rapid analysis of carboxylic acids in crude oils. However, the conditioning of the resin by the hexane does not allow one to eliminate the Cl- (original counterion), (11) Nordli, K. G.; Sjoblom, J.; Kizling, J.; Stenius, P. Colloids Surf. 1991, 57, 83-98. (12) Rueter, P.; Rabus, R.; Wilkes, H.; Aeckersberg, F.; Rainey, F. A.; Jannasch, H. W.; Widdel, F. Nature 1994, 372, 455-457. (13) Zengler, K.; Richnow, H. H.; Rossello-Mora, R.; Michaelis, W.; Widdel, F. Nature 1999, 401, 226-269. (14) Boll, M.; Fuchs, G.; Heider, J. Curr. Opin. Chem. Biol. 2002, 6 (5), 604-611. (15) Atlas, R. M, Ed. Petroleum Microbiology; Collier Macmillan Publishers: London, 1984. (16) Wilkes, H. Molecular Insights into the Anaerobic Biodegradation of Hydrocarbons. Presented at the 2nd Meeting: How Do We Study The Deep Biosphere in Europe?, AberWrac’h, 2-3 October 2-3, 2000. (17) Rabu, R.; Fukui, M.; Wilkes, H.; Widdel, F. Appl. Environ. Microbiol. 1996, 62, 3605-3613. (18) McDaniel, L. H.; Taylor, L. T. J. Chromatogr. A 1999, 858 (2), 201-207. (19) Rogers, V. V.; Liber, K.; Mackinnon, M. D. Chemosphere 2002, 48 (5), 519-527. (20) Holwenko, F. M.; MacKinnon, M. D.; Fedorak, P. M. Water Res. 2002, 36 (11), 2843-2855. (21) McCarthy, R. D.; Duthie, A. R. J. Lipid Res. 1962, 3, 117-119. (22) Ramijak, Z.; Solc, A.; Arpino, P.; Schmitter, J. M.; Guiochon, G. Anal. Chem. 1977, 49, 1222-1236. (23) Ovalles, C.; Garcia, M. C.; Lujano, E.; Aular, W.; Bermudez, R.; Cotte, E. Fuel 1998, 77 (3), 121-126. (24) Jones, D. M.; Watson, J. S.; Meredith, W.; Chen, M.; Bennett, B. Anal. Chem. 2001, 73, 703-707.
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which makes the exchange between the carboxylic acids (contained in crude oil) and the resin functional groups very difficult. In this method, the elimination of interferences and the sample dissolution are performed by hexane, which precipates the heavy paraffins and the asphaltenes. The aim of this work is to develop a new extraction method for the carboxylic acids contained in crude oils on an ion-exchanging resin and their quantification by gas chromatography (GC) and the total acid number (TAN) measurement. In this paper, we study the influence of the carboxylic acid fraction on oil acidity by comparing the TAN values of four crude oils (Y1, Y2, Y3, and Y4) and studying the relationship between the oil TAN and the amount of carboxylic acid. The recovered acids from crude oil Y4 have been characterized by various spectroscopic methods. (Note: In our work, we represent exclusively the characterization of the acidic fraction “Y4” by various methods such as elemental analysis, infrared spectroscopy, 1H and 13C nuclear magnetic resonance (NMR).) 2. Experimental Section 2.1. Extraction Procedure and Gas Chromatography Analysis of Synthetic Acids. 2.1.1. Extraction Procedure. 2.1.1.1. Resin Conditioning Step. Five grams of QAE Sephadex A25 quaternary amine (diethyl [2-hydroxypropyl] aminoethyl), which is a strong anion-exchanging resin, are used. The resin, with a capacity of 3.4 meq/g, is placed in a column and conditioned with 150 mL of an aqueous buffer solution of NaHCO3/Na2CO3 (1 M) to permute the original counterion (Cl-) by HCO3-. Effectively, HCO3- has less affinity to the resin and, thus, will facilitate the exchange with the carboxylic acid ion. The resin is then washed with methanol and dried under a stream of nitrogen. 2.1.1.2. Anion Exchange by Batch. The conditioned resin is transferred into a flask containing 25 mL of toluene (the solvent of dissolution) and 73.732 mg of synthetic acids from C3 to C24 (Table 1). The solution is stirred for 24 h, using an oscillating stirrer, to ensure an optimal exchange between the functional groups of the resin and the carboxylic acids. The toluene is then recovered and the resin is washed (three times) with 10 mL of dichloromethane (DCM) to eliminate the interferences. The DCM wash is recovered for a control analysis via GC. 2.1.1.3. Carboxylic Acids Elution Step. The resin is impregnated with 50 mL of the elution solvent, which is composed of CH2Cl2/HCOOH (9:1, v/v) and subjected to the action of an oscillating stirrer for 4 h. The acid fraction is recovered by a simple decantation, followed by filtration of the resin. The resin is then rinsed with 20 mL of the extraction solvent. The extracted acids are thus contained in 70 mL of CH2Cl2/HCOOH (9:1, v/v) solution. This solution is called the “extract”. 2.1.2. Analysis by Gas Chromatography. Prior to GC analysis, the extracted synthetic acids are derivatized in methyl esters, according to the method of Bannon et al.25 Seven milliliters of the acid extract are spiked with 200 µL of lauric acid (7.34 mg of C12H24O2/mL of DCM) as an internal standard. The volatile solvent is carefully removed under a stream of nitrogen, and the acid fraction is dissolved in 1 mL of 14% BF3/methanol (v/v) (as a methylating agent). The glass bulb containing the solution is sealed and plunged into an oil bath that is maintained at 100 °C for 20 min. After cooling, the derivatized sample is transferred to a flask containing 2 mL of hexane and 1 mL of water for liquid-liquid extraction. The (25) Bannon, C. D.; Craske, J. D.; Hai, N. T.; Harper, N. L.; O’Rourke, K. L. J. Chromatogr. A 1982, 247, 63-69.
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Table 1. Weight of the Recovered Carboxylic Acids, as Determined by Gas Chromatography (GC), and the Response Factors of Various Acids carboxylic acid
mass of acid deposited on the resin (mg)
response factor of GC-FID
mass of recovered acid, mCn (mg)
C3H6O2 C4H8O2 C6H12O2 C8H16O2 C10H20O2 C12H24O2 C14H28O2 C16H32O2 C18H36O2 C20H40O2 C22H44O2 C24H48O2
7.364 7.294 7.919 7.650 7.317
0.700 0.740 0.848 0.888 0.959 1.000 1.022 1.056 1.058 1.089 1.091 1.112
eluted with the solvent eluted with the solvent 6.140 6.959 6.684 internal standard 6.639 9.400 7.126 3.769 3.368 3.407
7.097 9.983 7.428 4.094 3.693 3.893
total
73.732
53.492
organic phase (hexane) that contains the methyl esters of synthetic acids is quantified (yield of extraction) using gas chromatography-flame ionization detection (GC-FID) peak areas via the internal standard (lauric acid, “C12”). The flame ionization detection (FID) equipment does not have an identical response for the different compounds; therefore, the relative response factor (Fr) for each esterified acid is first determined and reported in Table 1. The chromatograph used is from Hewlett-Packard (model HP 6890A Plus) and is equipped with a cool on-column injector and a flame ionization detector. The signal is integrated using the HP-GC Chemstation software. The capillary column used (model HT-5, 12 m × 0.32 mm × 0.1 µm) allows the analysis of methyl esters from C1 to C40. The oven temperature is held at 50 °C for 6 min and then programmed at a heating rate of 10 °C/min to 400 °C, where it is held for 30 min. Nitrogen is used as the carrier gas. 2.2. Study Conducted on Four Crude Oils (Y1, Y2, Y3, and Y4). 2.2.1. Extraction of Carboxylic Acids Contained in Crude Oils: GC Analysis. We studied four crude oils provided by Total (Y1, Y2, Y3, and Y4), by applying the same procedure of extraction and analysis. Five grams of crude oil dissolved in 20 mL of toluene are doped with 8 mg of octadecanoı¨c acid as an extraction standard. The toluene over the resin is recovered to determine the nonfixed acids by the TAN method. The elution of the acids by the “DCM/formic acid”solvent mixture constitutes the last step of the extraction process. The recovered acidic fraction is derivatized in methyl esters, according to the method described by Bannon et al.25 NA methyl esters were quantified by measurement of the total GC-FID chromatogram areas above the baseline of a blank (hexane) analysis through the internal standard peak (C12) (see Figure 2, presented later in this work), assuming a response factor of 1.24 The weight of recovered acids is calculated using eq 1:
m(Cn) )
A(Cn) × m(C12) A(C12) × Fr(Cn)
(1)
where A, m, and Fr are the respective area, weight, and response factor of the carboxylic acids contained in crude oils. The extraction yields (f) are determined by TAN measurement. 2.2.2. Determination of the Extraction Yield by Total Number of Acids (TAN) Measurement. This method (acid titration by KOH) provides only information on the total acidity. The total number of acids (TAN) is the quantity of KOH, expressed as shown in eq 2, in terms of milligrams per gram of sample (mg/ g):
TAN(mgKOH/g sample) )
(VA - VB) × M × 56.1 W
(2)
where VA is the volume (in milliliters) of KOH solution obtained at the equivalent point, VB the volume (in milliliters) corresponding to the blank (the blank:solvent mixture is 2-propanol/toluene/water), M the concentration of the KOH solution (in units of mol/L), and W the weight of the sample (in grams). The molecular weight of KOH is 56.1 g/mol. The titration is performed according to standard ASTM D664-95 (IP 177/96), which is the standard method in the oil industry. The extraction yield (f) is the ratio of the TANextracted acids and the TANcrude oil (eq 3), which is calculated by the TAN measurements of the crude oil sample and the extracted acids (eq 2).
f)
TAN(extracted acids) TAN(crude oil)
(3)
Because of the presence of formic acid (elution solvent) in the extract, the molar amount of the extracted acids could not be directly determined by titration. Thus, the formic acid is removed by liquid-liquid extraction with water (extract/water; 1/10 v/v); this action is repeated three times, to totally eliminate the formic acid. The aqueous phase of the third washing is recovered and analyzed by ionic chromatography with a gradient elution, to control the formic acid elimination. The resulting acidic fraction is then titrated by the TAN method. 2.3. Characterization of Naphthenic Acids Contained in Crude Oil Y4. Before each stage of characterization, formic acid is eliminated by liquid-liquid extraction with water and by evaporation under a flow of nitrogen, to dry the sample. 2.3.1. Determination of the Number-Average Molecular Weight of the Recovered Acids. The number-average molecular weight (Mn) is the ratio of the extracted acid weight and the number of acid moles contained in the crude oil. The acid weight is calculated from the GC results, by taking into account the extraction yield. The acid number of moles is calculated by the crude oil TAN measurement. 2.3.2. Characterization of the Naphthenic Acids.26-29 Elemental analysis, infrared spectroscopy (carbonyl resonances, OH broad), 1H NMR spectroscopy, and 13C NMR spectroscopy were conducted in conventional equipment, using conventional techniques. These techniques confirm the presence of NA in crude oil Y4. 1H NMR spectroscopy allows the percentage of aromatic, aliphatic, or alicyclic acids to be determined. The 13 C NMR spectrum proves the presence of carboxylic, methylene/methyl, and aromatic groups in the recovered acids. The (26) Hsu, C. S.; Decher, G. J.; Robbins, W. K.; Fukuda, E. K. Energy Fuels 2000, 14, 217-223. (27) Midttun, Ø.; Kvalheim, O.-M. Fuel 2001, 80, 717-730. (28) Leenheer, J. A.; Wershaw, R. L.; Brown, G. K.; Reddy, M. M. Appl. Geochem. 2003, 18, 471-482. (29) John, W. P. S.; Rughani, J.; Green, S. A.; McGinnis, G. D. J. Chromatogr. A 1998, 807, 241-251.
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Figure 1. Gas chromatography (GC) analysis of synthetic acids after extraction and derivatization.
Figure 2. Chromatogram of the acids extracted from crude oil Y3. Mn value and the elemental analysis permit us to determine the NA average formula. 2.3.3. Double Bound Equivalent (DBE).30 The double bound equivalent (DBE) can be obtained from eq 4:
DBE )
2C + 2 - H + N 2
(4)
where C, H, and N are the number of carbon, hydrogen, and nitrogen atoms in the formula. Indeed, the DBE allows the determination of the average unsaturation function number that is present in the chemical structure of NA.
3. Results and Discussion 3.1. Synthetic Acids: Gas Chromatography Analysis. Figure 1 represents the chromatogram of the esterified acids (GC analysis). The peak at 13.99 min corresponds to the internal standard, which is the lauric acid. The compounds with four or fewer carbons are drowned in the solvent peak. The mass of each recovered (30) Acevedo, S.; Escobar, G.; Ranaudo, M. A.; Khazen, J.; Borges, B.; Pereira, J. C.; Mendez, B. Energy Fuels 1999, 13, 333-335.
acid is calculated by eq 1, with Fr representing the response factor of the synthetic esterified acid (see Table 1). The total weight of extracted acids (mr) from C6 to C24 (identified on the chromatogram) is 53.492 mg, whereas the corresponding weight of deposited acids on the resin (md) is 59.074 mg (see Table 1). The extraction yield is 90%, which is satisfactory. 3.2. Percentage of Naphthenic Acids Contained in the Crude Oils. Extraction Yields. For example, in regard to the GC analysis of the esterified acids, the chromatogram of the acids extracted from crude oil Y3 has been represented in Figure 2. The peaks observed at ∼17 and ∼23 min correspond to the lauric and octadecanoı¨c acids (the internal standard and extraction standards), respectively. To verify the reproducibility of the extraction method, three runs were performed on each crude oil. The response factors of the analyte compounds were assumed to be unity.24 The average percentages of carboxylic acids in crude oils (taking into account the extraction yield determined by eq 3) and the relative
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Figure 3. Correlation between the acidity (total acid number, TAN) and carboxylic acid content of crude oils. Table 2. Percentage of Acids, with the Relative Standard Deviation (RSD), and the Extraction Yield (of Crude Oils)
a
crude oil
average percentage of acids (GC analysis)
TANcrude oil (mg/g)
TANextracted acids (mg/g)
yield of extraction (%)a
Y1 Y2 Y3 Y4
1.20 (RSD ) 4.3%) 1.00 (RSD ) 2.7%) 3.00 (RSD ) 7.2%) 4.10 (RSD ) 5.8%)
1.41 (RSD ) 2.8%) 0.63 (RSD ) 4.8%) 2.40 (RSD ) 1.7%) 3.40 (RSD ) 3.2%)
1.11 (RSD ) 2.7%) 0.32 (RSD ) 6.5%) 1.98 (RSD ) 1.6%) 2.80 (RSD ) 2.3%)
79.0 51.0 82.5 82.0
TAN measurement.
corresponding standard deviation are reported in Table 2. The extraction method is reproducible, with a relative standard deviation (RSD) of 3000 cm-1). The cyclization decreases the frequency of the CH2 scissoring vibration. This shift makes it possible to observe the distinct bands (1370-1410 cm-1) for methylene and methyl absorption in this region. In this case, the C-H stretching frequency of methylene is
