Energy & Fuels 2008, 22, 4177–4181
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Removal of Naphthenic Acids from Beijiang Crude Oil by Forming Ionic Liquids Lijuan J. Shi, Benxian X. Shen,* and Gongqun Q. Wang State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, China ReceiVed June 23, 2008. ReVised Manuscript ReceiVed August 6, 2008
More and more acidic crude oil has been exploited in the world. The presence of naphthenic acids in crude oil has a great influence on petroleum refiners. A new method was introduced to separate naphthenic acids from Beijiang highly acidic crude oil in this paper. The 2-methylimidazole solution in ethanol was used as the acid-removal reagent by mixing with the crude oil and then allowing the two phases to separate, with the naphthenic acids being extracted from the crude oil. Data indicated that the optimal content of 2-methylimidazole in ethanol was 20% (w/w) and the optimal extraction time was 10 min, with the reagent/oil ratio being 0.4:1 (w/w). The suitable reaction temperature could be room temperature. The total acid number of the crude oil was lowered from 4.18 to 1.38 mg of KOH/g, and the acid-removal rate could reach up to 67.0%.
1. Introduction A considerable amount of crude oils that contain a high concentration of naphthenic acids (NAs) have been produced. Crude oils are considered acidic if their total acid number (TAN) exceeds 0.5 mg of KOH/g by titration. Naphthenic acids are classified as carboxylic monoacids of the general formula RCOOH, where R represents any cycloaliphatic structure.1 Generally, the term “naphthenic acids” is used to account for all carboxylic acids present in crude oil, including acyclic and aromatic acids, which are complicated mixtures. It was shown them to be C10-C50 compounds with 0-6 fused saturated rings and with the carboxylic acid group apparently attached to a ring with a short side chain. Aromatic rings or fused aromatics are usually present in high-molecular-weight acids.2-5 In refinery processes, naphthenic acids, which are found to a greater or lesser extent in all crude oils, tend to cause operational problems, such as foaming in the desalter or other units and carrying cations through the desalting process, which can cause deactivation of catalysts. On the other hand, pure naphthenic aids are important raw material in the chemical industry, which can be applied as a timber antiseptic, a paint drying reagent, an additive in petroleum, and so on.6 Therefore, separation and purification of naphthenic acids in crude oil are expected. Many efforts have been attempted to solve the problem of naphthenic acids in crudes and crude fractions.7-12 In these methods, aqueous base washing can remove NAs effectively, but serious * To whom correspondence should be addressed. Telephone: 86-2164253346. Fax: 86-21-64252160. E-mail:
[email protected]. (1) Havre, T. E. Colloid Polym. Sci. 2004, 282, 270–279. (2) Fan, T. P. Energy Fuels 1991, 5, 371–375. (3) Schmitter, J. M.; Arpino, P.; Guiochon, G. J. Chromatogr., A 1978, 167, 149–158. (4) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Energy Fuels 2000, 14, 217–223. (5) Rudzinski, W. E.; Oehlers, L.; Zhang, Y. Energy Fuels 2002, 16, 1178–1185. (6) Jones, D. M.; Watson, J. S.; Jones, D. M.; Meredith, W.; Chen, M.; Bennett, B. Anal. Chem. 2001, 73, 703–707. (7) Petersen, P. R.; Robbins, F. P.; Winston, W. G. U.S. Patent 5,182,013, Jan 26, 1993. (8) Varadaraj, R.; Savage, D. W. U.S. Patent 6,030,523, Feb 29, 2000.
emulsion is formed in the process. Treatments of heating, hydrogenation, and esterification all have destroyed the valuable resource of naphthenic acids; therefore, a further study should be taken about the removal of naphthenic acids from crude oil. Ionic liquids are typically nonvolatile, nonflammable, and thermally stable. Ionic liquids have been studied for applications related to green chemical processes, such as liquid/liquid extractions, gas separations, electrochemistry, and catalysis.13-21 Adams et al.22 achieved the cracking of polyethylene to light alkanes in ionic liquid systems, such as 1-ethyl-3-methylimidazolium chloride-aluminum(III) chloride. Bo¨smann et al.23 disclosed a method for the deep desulfurization of diesel fuels by extraction with ionic liquids, especially with regard to those sulfur compounds that are very difficult to remove by common hydrodesulfurization techniques. Zhang et al.24 revealed that EMIMBF4, BMIMPF6, and BMIMBF4 and other heavier AMIMPF6 ones showed remarkable selectivity for the absorption of aromatic S- and N-containing molecules from transportation fuels. Meindelsma et al.25 observed that ionic liquids were (9) Sartori, G.; Savage, D. W.; Ballinger, B. H. U.S. Patent 6,121,411, Sept 19, 2000. (10) Blum, S. C.; Olmstead, W. N.; Bearden, R. U.S. Patent 5,820,750, Oct 13, 1998. (11) Halbert, T. R.; Riley, K. L.; Trachte, K. L. U.S. Patent 5,910,242, June 8, 1999. (12) Sartori, G.; Savage, D. W.; Blum, S. C.; Dalrymple, D. C.; Wales, W. E. U.S. Patent 5,948,238, Sept 7, 1999. (13) Howard, K. A.; Mitchell, H. L.; Waghore, R. H. U.S. Patent 4,359,596, Nov 16, 1982. (14) Boate, D. R.; Zaworotko, M. J. U.S. Patent 5,220,106, June 15, 1993. (15) Sherif, F. G.; Shyyu, L.; Greco, C. C. U.S. Patent 5,824,832, Oct 20, 1998. (16) Koch, V. R.; Nanjundiah, C.; Carlin, R. T. U.S. Patent 5,827,602, Oct 27, 1998. (17) Silvu, S. M.; Suarcz, P. A. Z.; de Souza, R. F.; Doupont, J. Polym. Bull. 1998, 40, 401–405. (18) Carmichael, A. J.; Haddletton, D. M.; Bon, S. A. F.; Seddon, K. R. Chem. Commun. 2000, 1237–1238. (19) Carlin, R. T.; Wilkes, J. S. J. Mol. Catal. 1990, 63, 125–129. (20) Goledzinowski, M.; Birss, V. I.; Galuszka, J. Ind. Eng. Chem. Res. 1993, 32, 1795–1797. (21) Boon, J. A.; Levisky, J. A.; Pflug, J. L.; Wilkes, J. S. J. Org. Chem. 1986, 51, 480–483. (22) Adams, C. J.; Earle, M. J.; Seddon, K. R. Green Chem. 2000, 2 (1), 21–23.
10.1021/ef800497p CCC: $40.75 2008 American Chemical Society Published on Web 10/02/2008
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Shi et al. °C. It revealed that Beijiang crude oil had a high total acid number and more macromolecular components. 2.2. Determination of the TAN of Crude Oil and Naphthenic Acids, Acid-Removal Rate, Oil Yield Rate, Reagent Yield Rate, and Material Accounting. The determination of TAN in crude oil is routinely carried out by a standard method in the oil industry using American Society for Testing and Materials (ASTM) D664. The TAN of naphthenic acids was measured according to SH/T 0092-91. The acid-removal rate was determined according to eq 1
Table 1. Properties of the Beijiang Crude Oil density (g/cm3, 20 °C) TAN (mg of KOH/g) viscosity (mPa s, 80 °C) gel point (°C) ASTM distillation range (°C) initial boiling point (IBP) 10% 20% 36%
0.8903 4.18 18 2 94 220 260 320
Table 2. Effect of Different Imidazole Derivatives on the Acid-Removal Rate reagent
acid-removal rate (%)
ethanol In 10% ethanol solution imidazole 1-methylimidazole 1-ethylimidazole 2-methylimidazole 2-ethylimidazole 2-propylimidazole 4-methylimidazole 1,2-dimethylimidazole 2-ethyl-4-methylimidazole benzimidazole
17.6 48.3 44.4 40.5 59.1 57.1 53.3 54.2 45.6 52.1 44.1
(
acid-removal rate ) 1 -
)
TANd × 100% TAN
(1)
where TANd was the TAN of deacidified crude oil and TAN was the TAN of the base stock. The oil yield rate was determined according to eq 2
oil yield rate )
or × 100% oa
(2)
where or was the mass of crud oil recovered and oa was the mass of the initial crude oil. The reagent yield rate was determined according to eq 3
reagent yield rate )
Table 3. Effect of Different Polar Solvent on the Acid-Removal Rate acid-removal rate (%)
solvents
pure solvent
20% of 2-methylimidazole solution
difference value
dimethyl sulfoxide N,N-dimethylform amide ethanol n-propyl alcohol n-butanol isopropanol ethylene alcohol triglycol
26.1 26.3 36.6 39.2 36.7 33.2 25.7 31.2
43.1 48.6 71 77.2 73.7 70.1 45.8 41.2
17 22.3 34.4 38 37 36.9 20.1 10
Table 4. Effect of Different Polar Solvent on Separation solvents
oil yield rate (%)
reagent yield rate (%)
material accounting (%)
dimethyl sulfoxide N,N-dimethylform amide ethanol n-propyl alcohol n-butanol isopropanol ethylene alcohol triglycol
106.7 103.0 95.3 91.3 78.4 93.0 102.5 104.2
91.2 89.3 96.9 114.1 136.7 108.2 90.5 89.4
100.9 97.8 95.9 99.8 100.2 98.7 99.1 100.0
suitable for the separation of aromatic hydrocarbons from mixtures of aromatic and aliphatic hydrocarbons. In this paper, naphthenic acids were separated from Beijiang highly acidic crude oil by forming ionic liquid with 2-methylimidazole. 2. Experimental Procedure 2.1. Base Stock. All of the chemicals were used as supplied. The crude oil was from the Beijiang oil field of China. The properties of the crude oil were listed in Table 1. A total of 10% of the components in crude oil could be distillated from the initial boiling point (IBP) to 220 °C and 36% distillated from IBP to 320 (23) Bo¨smann, A.; Datsevich, L.; Jess, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. Chem. Commun. 2001, 66 (23), 2494–2495. (24) Zhang, S. G.; Zhang, Q. L.; Zhang, Z. C. Ind. Eng. Chem. Res. 2004, 43 (2), 614–622. (25) Meindelsma, G. W.; Podt, J. G.; Haan, A. B. Fuel Process. Technol. 2005, 87 (1), 59–79.
rr × 100% ra
(3)
where rr was the mass of reagent recovered and ra was the mass of the initial reagent added in the crude oil. The material accounting was determined according to eq 4
material accounting )
rr + or × 100% ra + oa
(4)
2.3. Acid-Removal Process. Beijiang crude oil and the reagent were put into a three-neck flask equipped with a magnetic stirrer, a temperature controller, and water-cooling tube. The mixture was stirred at a constant temperature during the process. After the extraction, the mixture was put into a separating funnel for 30 min at room temperature to gravity separate the reagent with the acid compounds extracted from the crude oil. In the top of the funnel was mainly the reagent with ionic liquid, and in the bottom was mainly the deacidified crude oil. 2.4. Reagent Recovery and Naphthenic Acids Refining. The top phase of the funnel was heated to 100 °C to distill ethanol. Afterward, the residue was cooled to ambient temperature and then extracted by petroleum ether (60-90 °C) to remove neutral oils that were coextracted. Then, the precipitation of 2-methylimidazole could be observed, which could be separated with ionic liquids by filtration. HCl solution (aqueous) was added to the ionic liquids to make naphthenic acids generating finally. 2.5. Mechanisms of Ionic Liquids Generating. Imidazole and its derivatives are five-membered heterocyclic compounds containing two nitrogen atoms, in which one is pyrrole-N and the other is pyridine-N. Pyrrole-N participates in the conjugated system, while pyridine-N, which does not participate in the conjugated system, has strong alkalinity and can easily combine with H+ to form an imidazole-cation, which is a common cation of ionic liquid. The reaction process of imidazole derivatives and naphthenic acids is as follows:
RemoVal of NAs from Beijiang Crude Oil
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Figure 1. Effect of the 2-methylimidazole content and temperature.
Figure 3. Effect of the reagent/oil ratio on separation.
3. Results and Discussion
Figure 2. Effect of the reagent/oil ratio on the acid-removal rate.
The ionic liquids that have larger polar differences with the crude oil can be quickly separated from crude oil. There is no water in this process; therefore, the emulsification problem associated with the common aqueous solution will be overcome. Refined naphthenic acids are used to react with equimolar 2-methylimidazole solution in ethanol to form imidazolium salts, and the purer imidazolium salts can be obtained after removing ethanol by distillation. The melting point of the salts is 5 °C; therefore, they can be taken as ionic liquids.
3.1. Effect of Different Imidazole Derivatives. A total of 15% (w/w) of imidazole derivative solutions in ethanol were mixed with Beijiang crude oil. The extraction was fixed at 30 °C for 20 min, with the reagent/oil ratio being 0.4 (w/w). Table 2 showed the effect of different imidazole derivatives. It could be seen that ethanol could extract naphthenic acid from crude oil, the deacidification rate of which could reach up to 17.6%. According to the similar dissolve mutually theory, the polar naphthenic acids could be partly dissolved in alcohol; especially, the smaller molecular weight ones with larger polarity were more easily soluble in ethanol. The deacidification rate had been significantly improved when imidazole derivatives were added in ethanol. This was because the addition of imidazole derivatives changed the extraction process from physical extraction to reaction extraction. The formed ionic liquid with a stronger polarity was more easily soluble in ethanol; therefore, the deacidification rate had a qualitative change. Apart from the influence of the extraction process, the chemical reaction itself was an important factor affecting the deacidification rate. It was obvious that the ionic liquid was the product of acid-alkali neutralization. Naphthenic acid was a weak acid; therefore, the stronger the alkalescence of imidazole derivatives, the easier the reaction of them. Table 2 showed that the deacidification rate of 2-methylimidazole was the highest. It was considered that alkalescence of 2-methylimidazole was the strongest. The alkalescence order of the imidazole
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Figure 4. Effect of the reaction time on the acid-removal rate.
Figure 5. Effect of the reaction temperature on the acid-removal rate and viscosity of the crude oil.
derivatives with the same substituent group at different substitutional positions was 2-methylimidazole > 4-methylimidazole > 1-methylimidazole > midazolam; the order of the derivatives with different substituent groups at the same substitutional positions was methylimidazolium > ethylimidazole > propylmidazolam. 3.2. Effect of Different Solvents. The deacidification results of the selected polar solvents were shown in Table 3. It was seen that the acid-removal rates of ethanol, n-propyl alcohol, n-butanol, and isopropanol were better and the imidazole derivative solutions gave a larger increase to the acid-removal rates compared to the pure solvents. The yield rate of crude oil and reagent should also be considered in the deacidification process. The results were shown in Table 4. The results showed that all of the selected solvents could be isolated from Beijiang crude oil. The densities of dimethyl sulfoxide, N,N-dimethylform amide, ethylene alcohol, and triglycol were more than crude oil; therefore, the separated reagents, which were brown-red transparent solutions, were at the bottom of the separatory funnel. The deacidification rate was less than 50%; the oil yield rate was generally more than 100%; and the reagent yield rate was quite low; otherwise, the material accounting was close to 100%. It indicated that a small portion of reagent mixed in crude oil and had not been separated. The reason for the lower deacidification rate might be because of the selective dissolution of the ionic liquids. The densities of short-chain alkyl alcohols were less than crude oil; therefore, the separated reagents were in the top of
Shi et al.
the separatory funnel, which was a dark brown solution with a worse transparency than aforementioned. Furthermore, with the increase of the alkyl alcohol carbon number, the reagent would become more feculent. It was because alkyl alcohol with fewer carbon numbers had a stronger polarity and weak solubility in crude oil. The polarity of alcohol weakened with the carbon number increasing; therefore, more hydrocarbons in crude oil would dissolve into it, which would result in the decline of the oil yield rate. Taking all of these factors into account, ethanol was selected as the solvent for the naphthenic acids removal. 3.3. Effect of the 2-Methylimidazole Concentration. To study the influence of the 2-methylimidazole concentration on the acids removal efficiency, a series of ethanol solutions with different 2-methylimidazole concentrations were mixed with crude oil at 30 and 60 °C. The effect of 2-methylimidazole concentrations was shown in Figure 1. It was observed that the acid-removal rate increased, with the 2-methylimidazole concentration increasing from 10 (w/w) to 20% (w/w) at both 30 and 60 °C, but showed very small improvement when the 2-methylimidazole was more than 20% (w/w). The temperature had a very small effect on the acid-removal rate, but the material accounting at 60 °C was lower than at 30 °C; therefore, the optimal 2-methylimidazole concentration should be 20%, and the reaction temperature should be 30 °C. 3.4. Effect of the Reagent/Oil Ratio on the Acid-Removal Efficiency. The reagent/oil ratio was an important factor effecting the acid-removal rate. Figure 2 revealed the effect of the reagent/ oil ratio. Some interesting conclusions could be obtained from Figure 2. It could be observed that the acid-removal rate was gradually increased at 30 °C with the increase of the reagent/ oil ratio. There was a very interesting region between 0.3 and 0.4. Although the increase of the reagent/oil ratio was only 0.1, the acid-removal rate had a qualitative leap; after that, the change of the deacidification rate inclined to slow down. This phenomenon was particularly obvious in high concentrations. This interesting region was between 0.2 and 0.3 at 60 °C. The reagent/oil ratio had a homologous effect on the acidremoval rate at different contents of 2-methylimidazole. It was worth noting that, before the singularity, the content of 2-methylimidazole had a negative impact on the acid-removal rate and, afterward, a positive impact. This was probably because the reaction was not sufficient or the reacted reagent could not effectively separate from the crude oil when the reagent/oil ratio was smaller. Above all, a useful conclusion could be obtained that the reaction temperature could be improved to reduce the load of the reagent/oil ratio at the actual operation. The acid-removal efficiency had been comprehensively analyzed, including the acid-removal rate, oil yield rate, and reagent yield rate. The results were shown in Figure 3. The results further indicated that the main reason for the low deacidification rate was due to the ineffective separation of crude oil and the reagent at a low reagent/oil ratio. It was seen from Figure 3 that the acid-removal rate and reagent yield rate were very low, and the oil yield rate was more than 100% when the reagent/oil ratio was low. This indicated that most of the reagent mixed with the crude oil had not separated. In conclusion, it could be seen that 0.4-0.6 was a suitable reagent/oil ratio. If the reagent/oil ratio is too low, the reagent could not be separated from the oil; if the reagent/oil ratio is too large, the crude oil dissolved in the reagent increased and the loss of the oil became more. To make the acid-removal rate maximal at a minimal reagent/oil ratio, 0.4 was considered as the optimal reagent/oil ratio.
RemoVal of NAs from Beijiang Crude Oil
3.5. Effect of the Reaction Time. To study the influence of the reaction time on the acid-removal rate, 20% (w/w) of 2-methylimidazole solution in ethanol was employed and the reaction temperature was fixed at 30 °C, with the reagent/oil ratio being 0.4 (w/w). The influence of the reaction time on the acid-removal rate was shown in Figure 4. It was observed that the acid-removal rate increased with the reaction time, but when the reaction time was more than 10 min, the acid-removal rate increased very slowly. Therefore, the optimal reaction time should be 10 min. 3.6. Effect of the Reaction Temperature. Figure 5 showed the effect of the reaction temperature on the acid-removal rate and the viscosity of the crude oil. It could be observed that the acid-removal rate increased with the reaction temperature increasing. Every curve had a knee-point temperature, after which the acid-removal rate increased slowly. The knee-point temperature was 50 °C when the reagent/oil ratio was 0.3 (w/ w) and 30 °C when the reagent/oil ratio was 0.4 (w/w). It revealed that the knee-point temperature decreased with the reagent/oil ratio increasing. This may be related to the viscosity-temperature nature of crude oil. It could be seen that, along with the increase of temperature, viscosity decreased gradually. When the temperature was more than 50 °C, the effect of the temperature on the viscosity became smaller. Above all, the viscosity of the crude oil had no effect on the mixture and
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separation at a larger reagent/oil ratio (0.4), however, it distinctly affected the mixture and separation at a lower reagent/oil ratio (0.3). 3.7. Solvent Recovery. It could be obtained that 95% of 2-methylimidazole and 98% of ethanol could be recycled for reuse in the process. There were 14.6% of neutral oils coextracted in the solvent phase. A total of 0.95 g of naphthenic acids, with the TAN being 222 mg of KOH/g, could be obtained when 100 g of crude oil was treated in this method. 4. Conclusions It was suitable for the removal of naphthenic acids from highly acidic crude oil by forming ionic liquids through the reaction of naphthenic acids and imidazole derivatives. The optimal imidazole derivative was 2-methylimidazole, and ethanol was the optimal solvent. The acid-removal rate was influenced by the 2-methylimidazole content, reagent/oil ratio, reaction time, and reaction temperature, all of which had a positive effect on the acid-removal rate. The reagent/oil ratio had a negative effect on the oil yield rate. High-purity naphthenic acids could be obtained in this process. Acknowledgment. We thank Liang Xiao-dong and Yin Xiao-li for their help in the experiments. EF800497P
