Energy Fuels 2010, 24, 1043–1050 Published on Web 12/01/2009
: DOI:10.1021/ef901119y
Extraction of Basic Components from Petroleum Crude Oil Sebastien Simon,* Andreas L. Nenningsland, Emily Herschbach, and Johan Sj€ oblom Ugelstad Laboratory, Department of Chemical Engineering, the Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway Received October 2, 2009. Revised Manuscript Received November 12, 2009
An easy-to-implement solid/liquid extraction method has been developed to extract the basic molecules from petroleum crude oil in significant amounts (700-800 mg). The basic molecules are first extracted from crude oil using a cation-exchange sorbent, then recovered using methylamine. After checking the quantitativity of the method with model systems composed of commercially available basic and nonbasic molecules, four different crude oils spanning different properties (from light oil to extra-heavy crude oils) were tested. The efficiency of the method has been checked by determining the extraction yield from elemental analysis and titration measurements (total base number determination). Results show that 80-90% of bases are extracted from crude oils having densities comprised between 0.875 and 0.939 g.cm-3 at 15 °C. The extraction yield is significantly lower for an extra-heavy crude oil (around 70%).
Different methods have been developed to extract basic components from crude oil. They can be ranked in liquid/ liquid (L/L), solid/liquid (S/L), and precipitation as hydrochloride salt methods. The L/L methods consist in putting crude oil into contact with an acidic water solution. The bases are protonated and as a result their solubility in water increases. Like that, the aqueous phase containing protonated bases initially present in crude oil is recovered.9,10 It is worth noticing that some special devices must be applied to avoid formation of emulsions detrimental to the method. The S/L methods are based on the use of a cation-exchange column. The packing material can be either cation-exchanger macroreticulated resins (generally Amberlyst-15)11-13 or acid-modified silica.10,14,15 Finally, more anecdotal, the precipitation method consists to pass gaseous hydrogen chloride to the sample to precipitate the base hydrochlorides that are filtered off and washed with benzene. To our knowledge this method has only been applied to asphaltenes.16 From the results published in the literature, it seems that the S/L extraction method provides the highest basic component extraction yield among the three methods.10,16 The lower extraction yield for the L/L method is due to poor solubility of high molecular weight compounds in water. The goal of this article is to develop an easy-to-implement method allowing to extract basic components from crude oil
1. Introduction Nitrogen is present in petroleum crude oils at concentrations varying from 0.05 to 0.9% wt/wt.1 Despite this low concentration, its presence is associated to problems in some processes: for instance, they are known to be responsible for the poisoning of cracking catalysts2 and also contribute to gum formation in fuel oil.3 They are generally classified into basic and nonbasic, that is, respectively, titrable and nontitrable by a mineral acid.2 The basic components that are the subject of this article are mainly represented by derivatives of pyridines and its benzologs.4 The classical method to assess the basic molecule content in crude oil is performed by titrating the bases by an acid. This method provides the total base number (TBN), which is defined as the amount of KOH in milligrams that would require the same amount of acid titrant as 1 g of oil. Different methods exist differing by the nature of the titrant and the crude oil solvent.5 Practically, the determination is all but straightforward principally because the titration equivalence point is difficult to detect.6 This is explained by the fact the bases form a complex mixture and their pKa values are expected to be spread.6 A variation of this method, using acetic anhydride as solvent, allows one to titrate very weak basic nitrogen compounds (such as amides) present in crude oils and crude oil fractions.7 Similar methods exist to titrate bases in asphaltenes.8
(9) Barth, T.; Hoiland, S.; Fotland, P.; Askvik, K. M.; Myklebust, R.; Erstad, K. Energy Fuels 2005, 19 (4), 1624–1630. (10) Merdrignac, I.; Behar, F.; Albrecht, P.; Briot, P.; Vandenbroucke, M. Energy Fuels 1998, 12 (6), 1342–1355. (11) McKay, J. F.; Weber, J. H.; Latham, D. R. Anal. Chem. 1976, 48 (6), 891–898. (12) Rosset, R.; Caude, M.; Escalier, J. C.; Bollet, C. J. Chromatogr. 1978, 167, 125–131. (13) Conceic-~ao Oliveira, E.; Vaz de Campos, M. C.; Sant’Ana Lopes, A.; Rodrigues Vale, M. G.; Bastos Caram~ao, E. J. Chromatogr. A 2004, 1027 (1-2), 171–177. (14) Schmitter, J. M.; Ignatiadis, I.; Arpino, P.; Guiochon, G. Anal. Chem. 1983, 55 (11), 1685–1688. (15) Laredo, G. C.; Leyva, S.; Alvarez, R.; Mares, M. T.; Castillo, J.; Cano, J. L. Fuel 2002, 81 (10), 1341–1350. (16) Wallace, S.; Crook, M. J.; Bartle, K. D.; Pappin, A. J. Fuel 1986, 65 (1), 138–139.
*To whom correspondence should be addressed. Telephone: (þ47) 73 59 16 57. Fax: (þ47) 73 59 40 80. E-mail: sebastien.simon@chemeng. ntnu.no. (1) Thompson, K. F. M. Org. Geochem. 1994, 21 (8-9), 877–890. (2) Speight, J. G., In The Chemistry and Technology of Petroleum, 4th ed; CRC Press: Boca Raton, FL, 2007; p 190. (3) Dahlin, K. E.; Daniel, S. R.; Worstell, J. H. Fuel 1981, 60 (6), 477–480. (4) Arkenov, V. S.; Titov, V. I.; Kam’yanov, V. F. Chem. Heterocycl. Compd. 1979, 15 (2), 119–135. (5) Zheng, J.; Powers, S. E. J. Contam. Hydrol. 1999, 39 (1-2), 161– 181. (6) Dubey, S. T.; Doe, P. H. SPE Reservoir Eng. 1993, 8 (3), 195–200. (7) Buell, B. E. Anal. Chem. 1967, 39 (7), 756–761. (8) Dutta, P. K.; Holland, R. J. Fuel 1984, 63 (2), 197–201. r 2009 American Chemical Society
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: DOI:10.1021/ef901119y
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Table 1. Characteristics of Crude Oil Samples Used in This Work SARA analysisa
crude oil 1 2 3 4 5w 5a
density @ 15 °C
TAN (mg 3 g-1)
0.875 0.908 0.939 0.946 0.995 n.d.
0.0 2.23 2.15 c
9.67 9.81
water content (wt %)b 0.012 0.91 0.11 12 3.3 0.036
saturates (wt %)
aromatics (wt %)
resins (wt %)
asphaltenes, hexane insoluble (wt %)
51 52 37 31 27 n.d.
37 37 44 43 46 n.d.
9.8 6.3 16 21 22 n.d.
1.2 0.5 2.5 5.0 3.1 n.d.
a The SARA composition determination method by HPLC is described by Hannisdal et al.24 b Determined by Karl-Fisher titration. c Ttitration curve inconsistent. n.d.: not determined.
in significant amount (700-800 mg). The recovery of such an amount of basic components would allow their subsequent analysis by a set of experimental techniques. As mentioned by Merdrignac et al.,10 the extraction yield (ratio between the mass of extracted basic molecules to their total mass in crude oil) of methods available in the literature were rarely reported, which is why mass balances and extraction yields were carefully determined hereafter to test the efficiency of the developed method.
3-5 mL) was solubilized in 40 mL of methylisobutyl ketone (MIBK) and titrated by means of a 0.025 M perchloric acid solution in acetic acid. A total of 10 mL of titrant was added by increment of 0.1 mL every 3 min. The titration was controlled by a Titrando unit (Metrohm) fitted with a 6.0229.100 LL solvotrode with 2 M LiCl in ethanol as electrolyte (Metrohm). The TBN was then calculated from the following equation: M KOH CV eq TBN ¼ ð2Þ moil where MKOH is the molar weight of potassium hydroxide, C is the molar concentration of the perchloric acid solution, Veq is the equivalence volume (taken at the maximum of the derivative curve, see Section 3.1.1) and moil is the mass of crude oil. 2.3. Base Extraction Procedure. A glass column was filled with 7.5 g of sorbent (Biotage Isolute SCX-2, propylsulfonic acidmodified silica, 0.51 meq 3 g-1) and conditioned by 300 mL of CH2Cl2. The amount of crude oil corresponding to 760 μmol of basic functions (calculated from the crude oil TBN) diluted with 145 mL of CH2Cl2 was then loaded and after the sample had adsorbed onto the column, nonbasic components were removed by eluting with 350 mL of CH2Cl2 to obtain a colorless filtrate. The basic fraction was eluted with 11.55 g of a 2 M methylamine in THF solution diluted with 67 mL of CH2Cl2 and then 100 mL of pure CH2Cl2. The fraction containing the nonbasic molecules (NB fraction) was obtained by evaporating the solvent of the first collected fraction, first with a rotary evaporator then at 38 °C under a stream of nitrogen in a water bath up to a precalculated weight (95% of the mass of crude oil loaded into the column). Indeed, tests performed with crude oil 2 showed that no constant weight was reached during the drying of the NB fraction. This means, after evaporating dichloromethane, that the lightest molecules present in crude oil were also being evaporated. The fraction containing basic molecules (B) was obtained in the following way: After evaporation of the solvent of the second collected fraction to a volume of 40 mL with a rotary evaporator, the solution was washed with 40 mL of 50% v/v water pH = 13/50% ethanol. The biphasic mixture was centrifuged and the aqueous phase discarded. The B fraction was then dried using a block heater (GRANT UBR) set at 38 °C then 60 °C at the end under a stream of nitrogen. The evaporation was considered terminated when the variation of mass was lower than 1 mg in 1/2 h. 2.4. Asphaltene Extraction. A 160 mL portion of n-hexane was added to 4 g of crude oil, and the mixture was stirred overnight with a magnetic stirrer. After mixing, the asphaltenes were filtrated from the maltenes using a 0.45 μm membrane filter and then further washed with hexane to remove other crude oil components completely. Finally, the asphaltenes were dried in a desiccator filled with nitrogen gas, overnight. The maltenes were obtained by evaporating the hexane with a rotary evaporator, until the mass of the maltenes was constant. 2.5. Elemental Analysis. The elemental composition (C, H, N, O, S) was determined by the Laboratory SGS Multilab (Evry,
2. Experimental Section 2.1. Chemicals. The sample set comprised five samples named 1-5. Their features (density; total acid number (TAN); water content; and saturate, aromatic, resin, and asphaltene (SARA) compositions) are summarized in Table 1. Crude oil 5 underwent a special treatment to remove the water in the oil. The samples are named 5w (with water) and 5a (anhydrous) if they, respectively, have undergone the treatment or not. The procedure is the following: 50 g of the crude oil 5w was added to 35 g of tetrahydrofuran. After shaking the sample for 1-2 h, the sample was then centrifuged at 11 000 rpm for 30 min. The bottom water phase was removed with a syringe. The tetrahydrofuran from the crude oil was removed using a rotary evaporator until no change in weight occurred over a 30 min period. It was then assumed that all the tetrahydrofuran had been evaporated from the crude oil. All the other chemicals and solvents were used with no further purification and were of analytical grade. 2.2. Total Acid Number (TAN) and Total Base Number (TBN) Determination. The TAN values were determined according to the D664-95 ASTM method. The crude oil to be analyzed (mass adjusted to obtain an equivalence volume close to 5 mL) was mixed with 50 mL of toluene, 50 mL of isopropanol, and 0.5 mL of water and titrated by means of a 0.1 M tetrabutylammonium hydroxide solution in a mixture of isopropanol and methanol 10:1 (v/v). The titration was controlled by a Titrando unit (Metrohm) fitted with a 6.0229.100 LL solvotrode with a 0.4 M tetraethylammonium bromide in ethylene glycol solution as electrolyte (Metrohm). The TAN is given by the following equation: M KOH CV eq TAN ¼ ð1Þ moil where MKOH is the molar weight of potassium hydroxide, C is the molar concentration of the titrant solution, Veq is the equivalence volume and moil is the mass of crude oil. The TBN values were determined according to a method similar to the one described by Dubey and Doe6 and subsequently used by Zheng and Powers5 and Barth et al.9,17 The sample to be analyzed (mass adjusted to obtain an equivalence volume close to (17) Barth, T.; Hoeiland, S.; Fotland, P.; Askvik, K. M.; Pedersen, B. S.; Borgund, A. E. Org. Geochem. 2004, 35 (11-12), 1513–1525.
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Figure 1. Titration curves and their derivatives of crude oil samples 2 and 3 with a 0.025 M perchloric acid solution (mass of crude oil titrated = 7 and 2.5 g respectively).
3.1.2. 1 or 2 Equivalence Points. While titration curves of crude oils 1, 2, and 3 by a perchloric acid solution unambiguously show only one equivalence point allowing to calculate the TBN, the situation is different for crude oil 5w. Indeed its titration curve shows the presence of two equivalence points (Figure 2), the first one at ≈2 mL (calculated TBN = 5.7 mg 3 g-1) and the second at ≈ 6 mL (calculated TBN = 16 mg 3 g-1). As this sample contains 3.2% of water, that is, significantly more than in crude oils 1, 2, and 3, we wondered if the presence of water could be responsible of the second equivalence point. To check this hypothesis we have dehydrated this crude oil using the procedure described in Section 2.1. This procedure is successful since the water content is reduced to 360 ppm (Table 1) and the oil composition seems not to have changed: the TAN and the elemental composition have not varied (Tables 1 and 5). Moreover, the water separated during this study was turbid and crystals were visible with a microscope, which proves the presence of salt in the water. The titration curve of the crude 5a by perchloric acid presents a single equivalence point at 2 mL (TBN = 6.1 mg 3 g-1) corresponding to the first equivalence point of crude 5w. We can therefore conclude that the second equivalence point was an artifact induced by the presence of water. Moreover, we can notice that the equivalence point is better defined in the absence of water. Consequently, the oil samples should be dehydrated before determination of their TBN values. 3.1.3. Correlation TBN/Density. Table 2 presents results of the determination of the TBN for all the crude oil studied. We can notice the values vary from about 1 to 6 mg 3 g-1, that is, the same order of magnitude as the TAN values (compared with Table 1). As it is known that the heavy crude oils have a higher acid content (measured by their TAN) than lighter crude oils18 due to biodegration process,19 we have checked if such a
France) by thermal conductivity measurements for C, H, and N and by infrared measurements for O and S. 2.6. Gas Chromatography. Gas chromatography of test mixtures was performed on a Hewlett-Packard HP 6890 GC system equipped with a flame ionization detector. The column was an HP-5MS ((5%-Phenyl)-methylpolysiloxane). The temperature was programmed at 5 °C/min from 100 to 280 °C. Hexadecane was added as an internal standard.
3. Results and Discussion 3.1. TBN Measurements. 3.1.1. Applicability of the TBN Method. To be able to check the efficiency of the separation method for crude oil, we must use a method allowing us to determine the concentration of bases in the different samples before and after extraction to calculate the extraction yield: the TBN method. Consequently, we have checked if this method is applicable to the studied crude oils. Figure 1 presents the titration curves of crude oil samples 2 and 3. These titration curves are representative of all the other crude oils. We can notice that the equivalence point is not well marked, even if each point was measured after a long equilibration time (3 min, total time of the titration = 5 h) to improve the accuracy of the titration. The derivative curves (Figure 1) present a maximum that allows us to determine the equivalence points, in this case comprised between 5.3 and 6.1 mL for crude oil 2 and between 4.7 and 5.7 for crude oil 3. These maxima are not well-defined and can only be determined with uncertainties close to 10% (or so). Other solvents were tested (mixture of toluene and acetonitrile) without visible improvement. As mentioned in the Introduction section, several authors have also noticed the problem to accurately determine the equivalence point.6 Finally, in order to validate our titration, we have varied the mass of crude oil to be titrated by perchloric acid (results not shown) and found no variation of TBN with the mass of crude oil, which is an indication the results are consistent and meaningful. To conclude, even though the TBN method must be carefully implemented, it can be used to determine the amount of base in the investigated samples.
(18) Hurtevent, C.; Rousseau, G.; Bourrel, M.; Brocart, B., Production issues of acidic petroleum crude oils. In Surfactant Science Series, 132 (Emulsions and Emulsion Stability, 2nd ed; CRC Press LLC: Boca Raton, FL, 2006; pp 477-516. (19) Meredith, W.; Kelland, S. J.; Jones, D. M. Org. Geochem. 2000, 31 (11), 1059–1073.
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Figure 2. Titration curves and their derivatives of crude oil sample 5w (water content = 3.3%) and 5a (water content = 360 ppm) with a 0.025 M perchloric acid solution (mass of crude oil titrated = 0.5 g). Table 2. Values of TBN Determined for the Crude Oil Set
Table 5. Elemental Composition of Crude Oils and Their Fractions and the Recovery Yield for Every Element
-1
TBN (mgKOH 3 g )
crude oil
Sample
1.26 ( 0.17 1.11 ( 0.07 2.81 ( 0.24 4.93 ( 0.63 5.68 ( 0.37 6.06 ( 0.15
1 2 3 4 5w 5a
1
2
Table 3. Separation of a Test Mixturea
component
3
injected amount (10-3 mol) in 160 mL of CH2Cl2
% recovery in NB fraction
% recovery in B fraction
2.5 2.5 2.5 0.25 0.25
96 95 97 0 0
0 0 0 95 103
0.25
0
99
anthracene carbazole tricosane quinoline 2,6-dimethylquinoline acridine
5w
5a
a Note: the cleaning step of the B fraction with a 50% v/v water pH = 13 and 50% ethanol solution was not implemented. Concentrations were determined by gas chromatography.
1
2.58
TBNcrude (mg 3 g-1) 1.26
2 3
1.85 6.18
1.11 2.81
5w 5a
7.31 9.04
5.68 6.06
TBNNB (mg 3 g-1) ambiguous determination, see text 0 ambiguous determination, see text 0 1.64
TBNB (mg 3 g-1) 39.6
RTBN (%)
54.8 37.5
91 80
56.3 47.1
72 69
%C
%H
%N
%S
C/H atomic ratio
87.0 86.1 83.9 95 85.7 85.7 84.4 96 86.5 85.9 84.4 102 83.1 85.1 84.1 105 85.9 85.3 83.8 102
12.41 12.74 10.29 98 12.19 12.05 9.63 95 11.72 11.66 9.94 101 11.05 10.9 9.29 100 10.97 10.9 9.58 101
0.17 0.14 1.4 98 0.14 0.12 1.66 102 0.29 0.21 1.8 107 0.38 0.24 1.86 95 0.37 0.21 2.19 106
0.82 0.69 2.31 85 0.64 0.63 1.74 97 0.84 0.78 1.62 102 0.94 0.91 1.33 104 0.86 0.86 1.39 109
0.584 0.563 0.680 0.586 0.592 0.730 0.615 0.614 0.708 0.626 0.651 0.755 0.653 0.652 0.730
exists between the TBN and the asphaltene content in crude oils: the values show that the TBN increases with the asphaltene content for the set of crude oils investigated (results not shown). These results are consistent with the work by Barth et al.9,17 3.2. Development of the Extraction Procedure. 3.2.1. Presentation of the Extraction Procedure. The extraction method we have developed is based on the work by Merdrignac et al.10 It consists of a solid/liquid method of extraction of bases by a sulfonic acid-modified silica column. However, this method has been significantly improved. The recovery of the bases uses a solution of methylamine in dichloromethane instead of a mixture of a methanol, dichloromethane, and ammonia used by Merdrignac et al. in order to prevent the precipitation of the higher molecular weight component (asphaltene-like molecules). The method also presents an extra washing step with a 50% v/v water pH = 13 and 50% ethanol solution to eliminate methylammonium (see hereafter). Finally, the method is sized to obtain 700-800 mg of basic components.
Table 4. Weight Fraction of Bases Recovered (wB) and Recovery Yields of Bases Determined from TBN Determinations Crude wB (%) oil
crude NB fraction B fraction recovery % crude NB fraction B fraction recovery % crude NB fraction B fraction recovery % crude NB fraction B fraction recovery % crude NB fraction B fraction recovery %
81
relationship also exist for basic components. Figure 3 presents a plot of TBN values as a function of the density of crude oil samples. We can notice that the TBN roughly increases with the density of crude oils. That means heavy crude oils tend to have a higher content of basic molecules than light crude oils. We have also checked if a relationship 1046
Energy Fuels 2010, 24, 1043–1050
: DOI:10.1021/ef901119y
Simon et al. Table 6. TAN Values of Crude Oils and NB Fractionsa Sample 2 3 5w 5a a
TANcrude (mg 3 g-1)
TANNB (mg 3 g-1)
RTAN (%)
2.23 2.15 9.67 9.81
2.51 2.07 11.2 10.1
106 93 106 97
RTAN is the recovery yield based on TAN values.
for each crude oil samples to test the reproducibility of the method. The weight fraction (Table 4), TBN values (Table 4), elemental compositions (C, H, N, O, S) (Table 5), and TAN values (Table 6) of the fractions were determined for all the extractions. The 1H NMR spectra were also acquired for crude 2 to determine the residual solvent content in the fractions. Table 4 presents the weight fraction of the B fractions (wB). They vary from 1.85 to 9.04%, and the parallels show that the reproducibility is close to 5% in relative value. We can notice that water has an influence on the extraction, as shown by the comparison between the samples 5w and 5a. We have measured the TBN values of all the B and NB fractions in order to determine the extraction yields. The determination of the equivalence points for all the B fractions was straightforward, as shown in the Figure 4. This allows an accurate and reliable determination of their TBN values, especially compared with pure crude oils. Values of the TBN for B fraction calculated from these titration curves are given in Table 4. These values are comprised between 38 and 56 mg 3 g-1 depending on the sample, that is, between 8 and 49 times the values of their corresponding crude oils. That shows that we have a concentration of bases in the B fractions. From the TBN values, we have calculated the TBN recovery yield RTBN: wB TBNB ð3Þ RTBN ¼ TBNcrude
Figure 3. Relationship between the TBN and density of crude oil samples.
3.2.1. Test Mixture. Before applying the method on real crude oil samples, we have tested it on a test mixture to evaluate the selectivity and quantitative aspect of extraction of basic components. A mixture of six standards was selected (Table 2): three nonbasic molecules (one alkane, one aromatic, and one aromatic molecule with a nonbasic nitrogen) and three basics (pyridine derivatives as they are known to be present in crude oil4). As shown in Table 3, a complete separation is obtained between basic and nonbasic molecules. A 96% recovery of nonbasic molecules is obtained in the NB fraction whereas all the basic molecules (95-103%) are recovered in the B fractions. From these data, we can conclude that the method is selective and quantitative. Although the B fraction appears clear just after separation, we can notice the appearance of a precipitate in the capped bottle the next day. To determine where this precipitate comes from, we have recovered it by filtration and characterized by 1H and 13C. It appears this precipitate is composed of CH3NH3þ and nothing else organic, which means the anion is inorganic. We have performed different experiments to pinpoint how this salt can be created: The wash of the column with water and THF during the conditioning step has no effect. Moreover, if we filter the B fraction and discard the precipitate, there is appearance of a new precipitate the next day. As far as we are concerned, from this set of observation, the most plausible explanation is there is reaction between CH3NH2 present in the B fraction and CO2 from air to form methylammonium carbonate. In order to remove this precipitate, after evaporation of the solvent to a volume of 40 mL with a rotary evaporator, the B fraction was washed with 40 mL of a 50% v/v water pH = 13 and 50% ethanol solution to neutralize and extract methylamine in aqueous phase. The biphasic mixture was centrifuged and the aqueous phase discarded. The oil phase was then dried under a stream of nitrogen and the B fraction recovered. After solubilizing in chloroform the B fraction obtained this way, we have not noticed the presence of any precipitate, which is an indication of the effectiveness of the washing procedure. 3.2.3. Crude Oil. We have then implemented the extraction method to separate the crude oils 1,2,3, 5w, and 5a into two fractions: NB and B. At least two parallels were carried out
where wB is the weigh percent of the B fraction, and TBNB and TBNcrude are the TBN values of, respectively, the B fraction and the crude oil. The values reported in Table 4 show that the yield is high for samples 1, 2, and 3 (80-90% of bases initially present in the crude oils are extracted in the B fractions), but is significantly lower for crude oils 5w and 5a (around 70%), that is, the extraction yield decreases for extra-heavy crude oil. This decrease is either due to more complex interactions between the bases and the crude oil matrix or to difference in the bases population for crude oils 5a and 5w compared with crude oils 1, 2, and 3. Moreover, we cannot rule out the possibility there is a selective extraction of base components for crudes 5. However, more characterization is required to confirm if such a selective extraction effectively happens. We have also titrated the NB fractions but the results are ambiguous. Indeed, as indicated in Table 4, titration of NB fractions from crude oils 2 and 5w present no visible equivalence point, which means that this fraction contains no (below the detection limit) titrable base. Crude oil 5a presents a very distinct equivalence point and, finally, for crude oils 1 and 3, a weak equivalence point can perhaps be distinguished but we are not sure. To conclude, the determination of the TBN in the NB fractions are far from being conclusive. This lack of conclusion could result from the low base content left in the NB fraction after extraction. We have determined the 1H NMR spectra of NB and B fractions for crude oil 2 in deuterated chloroform to assess 1047
Energy Fuels 2010, 24, 1043–1050
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Figure 4. Titration curve of B fractions extracted from crude oil samples 1, 2, and 5a with a 0.025 M perchloric acid solution (mass of B fraction titrated = 0.1 g).
the presence of residual solvent in these fractions. The 1H NMR spectrum of the NB fraction (not shown) evidence the presence of remaining CH2Cl2 (singulet at 5.30 ppm). We can estimate the residual CH2Cl2 content with the following formula:
with areaH_CH2Cl2 and areaH_total being the areas of, respectively, the peak at 5.30 ppm and all the peaks of the 1H NMR spectrum, and %HNB and %HCH2Cl2 being the weight percent of hydrogen in the NB fraction (table 5) and CH2Cl2 (2.35%), respectively. We found that the NB sample contains 6 wt % of CH2Cl2. We applied the same methodology for the B fraction. Its 1 H NMR spectrum presents peaks corresponding to residual THF (multiplets at 1.85 and 3.8 ppm) and ethanol (3.7 and 1.2 ppm), but no CH2Cl2. By applying an equation similar to eq 3, we found fraction B contains a total of 4 wt % of ethanol and THF. To conclude, every fraction contains 4-6% of residual solvent. Table 5 presents the elemental composition (C, H, N, S) of crude oils 1, 2, 3, 5w, and 5a and their NB and B fractions. We have not included data for oxygen due to relatively high water content in some crude oils which could mislead us on the significance of the variations of the oxygen content in the different fractions. Form the data presented in Table 5 we have determined the percent recovery of every element with the following formula: %XNB wNB þ %XB wB ð5Þ RX ¼ %Xcrude
wN and wBN are the weight percent of the B and NB fractions, respectively. The values obtained (Table 5) show a recovery close to 100% for all the elements and all the crude oils. That means no part of the samples were lost during the extraction process. If we take a closer look at Table 5, we can notice that the composition of the crude oils and their NB fractions are similar, whereas the B fractions present different features: first its composition in nitrogen is significantly higher (1.4-2.2% compared with 0.1-0.2% for the NB fractions and the crude oils), which proves that we have enriched the B fraction in nitrogen compounds and most likely bases as previously proved by the TBN measurements. Second, B fractions are more aromatic (higher C/H atomic ratio) than the initial crude oils, which is in good agreement with the known structure of the basic molecules present in crude oil: derivatives of pyridines and its benzologs.4 Finally, the B fractions are richer in sulfur than their respective crude oils. This result is in good agreement with data obtained by Qian et al.20 These authors, using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS), have determined the elemental compositions of the nitrogen-containing aromatic compounds present in a heavy crude oil sample and have shown the presence of basic molecules containing both nitrogen and sulfur. By assuming an average molar mass of basic molecule of 1000 g 3 mol-1 (average value of MTBN and MN, see below), we can calculate that about one out of every two basic molecules contains a sulfur atom. The TAN values of the crude oil and NB fraction were also determined in order to check that the naphthenic acids are not modified (ionized, for instance) during the extraction method. These values are indicated in Table 6 in addition to
where RX is the percent recovery of the element X; %XNB, % XB, and %Xcrude are the weight percent of the element X in the NB and B fractions and in the crude oil, respectively; and
(20) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Energy Fuels 2001, 15 (2), 492–498.
CH2 Cl2 content ¼
100 areaH CH2 Cl2 %HNB areaH total %HCH2 Cl2
ð4Þ
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: DOI:10.1021/ef901119y
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from elemental composition and the ones calculated from TBN measurements. The latter are significantly higher: more than 1000 g 3 mol-1 compared with 700-800 for MN. MN values also seem too high compared with molar masses reported in the literature. The asphaltenes, for instance, which are considered as the higher molar mass molecules present in crude oils, have a molar mass ranging from 500 to 1000 g 3 mol-1 with an average of 750 g 3 mol-1.21-23 Qian et al.20 have analyzed by FT-ICR MS the nitrogen-containing compound of an heavy crude oil and found that their molar masses vary from 250 to 1250 g 3 mol-1 with an average at 617. Barth et al. have found values comprised between 400 and 600 g 3 mol-1 by titrating the basic fraction extracted by a L/L extraction method, but the extraction yield of their method was only 25-37%, which means they have not characterized all the basic molecules and probably not the heaviest ones. Several hypothesis can be put forward to explain the anomalous higher values found for MTBN: (1) There could be more than one basic function per molecule, but the calculated molar mass would be even higher. (2) Nonbasic molecules could be present in the B fraction and perhaps even associated with basic molecules, but in this case the calculated values of MN would be affected as well. However, if this hypothesis is true, further cleaning of the isolated base components by repetitive dissolution-precipitation would help removal of entrained nonbase components. This further cleaning step would nevertheless reduce the simplicity of the method. (3) Finally, some basic molecules present in crude oils could not be titrated by perchloric acid. This would explain the discrepancy between MN and MTBN. However, we do not know any method that could validate or invalidate this hypothesis. 3.3.3. Percentage of Basic Nitrogen. The ratio of basic to total nitrogen %Nbasic can be calculated with the following equation: %NB ð9Þ %Nbasic ¼ wB %Ncrude
Table 7. Results from the Titration of Maltenes from Crude Oil 3 fraction from crude oil 3 maltenes
TANmaltenes (mg 3 g-1)
Acids (%)
TBNmaltenes (mg 3 g-1)
Bases (%)
2.06
93
2.44
85
Table 8. Elemental Composition of Asphaltenes and the Basic Fractions Recovered from Crude Oil 3 fraction from crude oil 3
%C
%H
%N
%S
C/H atomic ratio
B fraction asphaltenes
84.4 86.1
9.94 8.28
1.80 1.29
1.62 2.10
0.708 0.867
Table 9. Percentage of Basic Nitrogen Atoms (%Nbasic) and Average Molar Masses Calculated from TBN Measurement (MTBN) and Elemental Analysis (MN) crude oil 1 2 3 5w 5a
MTBN (g 3 mol-1)
MN (g 3 mol-1)
%Nbasic
1417 1034 1497 1000 1104
1000 843 778 754 640
21 22 37 36 54
acid recovery yield RTAN calculated with the following formula: wNB TANNB ð6Þ RTAN ¼ TANcrude where wNB is the weight percent of the NB fraction; and TANNB and TANcrude are the TAN values of the B fraction and the crude oil, respectively. The obtained values show that the naphthenic acids present in crude oil are quantitatively recovered under their protonated form in the NB fraction at the end of the extraction method. 3.3. Properties of Bases. 3.3.1. Bases: Asphaltenes or Resins?. In this section we determine if the bases are a subfraction of asphaltenes or maltenes. To do that, sample 3 was separated into asphaltenes and maltenes, and the latter was titrated to find its TAN and TBN values (results presented in Table 7). By comparing with values from the pure crude oil, we can calculate that 85% of all the bases and 93% of all the acids in sample C are found in the maltenes. Consequently, the bases are a subfraction of maltenes (most likely resins). This result is corroborated by the comparison between the elemental composition of aspahltenes and the basic fraction (Table 8). This table shows the bases are less aromatic than asphaltenes precipitated with hexane, which is consistent with their maltene nature. This result is also consistent with the study by Barth et al.9 3.3.2. Molar Masses of Bases?. The average molar mass of the basic compounds can be obtained from two different set of data: (1) From the TBN value of the B fraction (TBNB), assuming there is only one basic function per molecule: M KOH ð7Þ M TBN ¼ TBNB
Its application (Table 7) shows there is an evolution of the ratio %Nbasic from light to heavy and extra-heavy crude oils. Indeed, results show that 21-22% of nitrogen atoms present in crude oil 1 and 2 have basic properties and this number increases to higher values 37-54% for crude oils 3, 5e, and 5a. 4. Conclusion In this work we have developed a method to extract basic components from crude oil. By characterizing extracted fractions by acid and base titrations and determining their elemental compositions, we have shown that this method is quantitative (extraction of 80-90% of bases) and repeatable (repeatability of 5% on the extracted mass). However, the extraction yield is significantly lower (around 70%) for the heaviest crude oil sample tested. This method allows us to obtain bases in sufficient amount (700 mg) to study
(2) From the nitrogen atomic composition of the B fraction (%NB). Here also we assume there is one nitrogen atom per molecule: 1400 ð8Þ MN ¼ %NB
(21) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103 (50), 11237–11245. (22) Groenzin, H.; Mullins, O. C.; Eser, S.; Mathews, J.; Yang, M. G.; Jones, D. Energy Fuels 2003, 17 (2), 498–503. (23) Hortal, A. R.; Martı´ nez-Haya, B.; Lobato, M. D.; Pedrosa, J. M.; Lago, S. J, Mass Spectrom. 2006, 41 (7), 960–968. (24) Hannisdal, A.; Hemmingsen, P. V.; Sjoblom, J. Ind. Eng. Chem. Res. 2005, 44 (5), 1349–1357.
The obtained values are presented in Table 9. We can notice there is a discrepancy between the values obtained 1049
Energy Fuels 2010, 24, 1043–1050
: DOI:10.1021/ef901119y
Simon et al.
their properties. This will be presented in a forthcoming article. Another important point of this article is the fact that the titration of a crude oil by perchloric acid allows to determine TBN values, but this method must be carefully implemented owing to the small “jump” of potential at the equivalence point.
Acknowledgment. The authors are grateful for the financial contributions from the members of the Joint Industrial Project on Waxy Crude Oils and Emulsions: Start-up and Separation, including Aibel AS, Akzo Nobel, Baker Petrolite, BP, Champion Technologies, Maersk, Shell, StatoilHydro, Total, and Vetco. Erland L. Nordgard is acknowledged for the NMR experiments.
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