Energy & Fuels 2007, 21, 941-943
941
Removal of Naphthenic Acids from a Diesel Fuel by Esterification Yanzhen Z. Wang,* Xueying Y. Sun, Yanping P. Liu, and Chenguang G. Liu State Key Laboratory of HeaVy Oil Processing, Key Laboratory of Catalysis, CNPC, China UniVersity of Petroleum, Dongying, China 257061 ReceiVed October 7, 2006. ReVised Manuscript ReceiVed NoVember 14, 2006
The presence of relatively high levels of naphthenic acids in diesel fuels has great influence on petroleum refiners. In this paper, a new method of catalytic esterification was introduced to remove the naphthenic acids from a diesel fuel. The paper used SnO/Al2O3 as the catalyst system, and a fixed bed catalyst was made. The experiments were finished in a fixed bed reactor. The total acid number of the diesel fuel was lowered from 1.7 mg KOH/g to less than 0.1 mg of KOH/g at a methanol/oil ratio of 0.010. This method can attain the effectiveness of aqueous base washing and has no loss of oil. Data indicates that the acid removal is influenced by the reaction temperature, methanol/oil ratio, and space velocity. A high reaction temperature and methanol/ oil ratio or low space velocity are favorable for the esterification. The color of the de-acided oils can apparently be improved during the esterification. The optimum reaction temperature is 280 °C.
1. Introduction The presence of relatively high levels of naphthenic acids in diesel fuels has great influence on petroleum refiners. Essentially, these acids are corrosive, may cause the fuel to deteriorate, tend to cause equipment failures, lead to high maintenance costs, and may pose environmental disposal problems.1-3 Naphthenic acids occurring in petroleum oil are predominantly monocarboxylic acids.4 Generally, the carboxyl group is attached to a cyclopentane or a cyclohexane ring through a -CH2- group or a chain containing five or more -CH2groups. Aromatic rings or fused aromatic rings are usually present in high molecular weight acids.5-8 Efforts to minimize organic acid corrosion have included a number of approaches based on neutralization and removal of the acids from the oil. For example, Yanzhen Wang used ethylene glycol and ammonium to remove the naphthenic acids * To whom correspondence should be addressed. Telephone: 86-5468392147. Fax: 86-546-8391971. E-mail:
[email protected]. (1) Jing, H. M.; Zhong, Y. G.; Yao, Z. M.; Ke, W. Corrosion of Naphthenic acids and control. Corros. Prot. Petrochem. Ind. (Chinese) 1999, 16 (1), 1-7. (2) Gao Y. M.; Chen J. J.; Lei L. C.; Yang H. Y. The status-quo of reseach on naphthenic acids corrosion and protection. Corros. Prot. Petrochem. Ind. (Chinese) 2000, 17 (2), 6-11. (3) Hu Y.; Xue G. T. Corrosion of equipment in processing high acid value crude and development of corrosion prevention technologies. Corros. Prot. Petrochem. Ind. (Chinese) 2004, 21 (4), 5-8. (4) Dzidic, I.; Sommervile, A. C.; Rais, J. C.; Hart, H. V. Determination of naphthenic acids in California crudes and refinery wastewaters by fluoride ion chemical ionization mass spectrometry. Anal. Chem. 1988, 60, 13181329. (5) Fan, T.-P. Characterization of naphthenic acids in petroleum by fast atom bombardment mass spectrometry. Energy Fuels 1991, 5, 371-375. (6) Schmitter, J. M.; Arpino, P.; Guiochon, G. Investigation of highmolecular-weight carboxylic acids in petroleum by different combinations of chromatography (gas and liquid) and mass spectrometry (electron impact and chemical ionization). J. Chromatogr. 1978, 167, 149-158. (7) Chang, S.; Hsu, Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Naphthenic acids in crude oils characterized by mass spectrometry. Energy Fuels 2000, 14, 217-223. (8) Rudzinski, W. E.; Oehlers, L.; Zhang, Y. Tandem mass spectrometric characterization of commercial naphthenic acids and a Maya crude oil. Energy Fuels 2002, 16, 1178-1185.
from a second vacuum fraction.9 Ohsol disclosed a method to remove naphthenic acids and sulfur by treating the oils with an alkaline earth metal oxide. The naphthenic acids were converted into acidic compounds and alkaline earth carbonates. The alkaline earth carbonate was then separated from the oil stream.10 Wu Jinhui used an isopropyl alcohol-ammonia-water solvent to extract naphthenic acids: the naphthenic acids were converted into ammonium naphthnate and dissolved into an alcohol-water solution.11 Franklin used various base treatments of oils and crude fractions to neutralize the naphthenic acids.12 Ferguson invented a method to produce a neutralization number of less than 1.0 mg KOH/g by neutralizing the organic acidity in petroleum and petroleum fractions.13 Verachtert and Tomas used a dilute aqueous NaOH or KOH solution to dispose a liquid hydrocarbon. This method may result in the treated oils containing higher concentrations of base.14 Fuqua used amines and an organic liquid contact agent to remove organic acids from crude oils.15 Sartori et al. invented an esterification method to lower the total acid number (TAN) of petroleum; because they did not use catalyst, the methanol/oil ratio was very large, and the reaction temperature was very high (above 300 °C).16 Although the aqueous base washing is a simple method for petroleum refiners to remove the naphthenic acids from highacid diesel fuel, the method may also cause serious emulsion and a great loss of diesel fuel. So there remains a need to eliminate or reduce the acid concentration of diesel fuels that is both low cost and refinery friendly, particularly for diesel fuels where theTAN is above 1 mg KOH/g. This paper describes (9) Wang, Y. Z.; Chu, Z. S.; Qiu, B.; Liu, C. G. Removal of naphthenic acids from a vaccum fraction oil with an Ammonia solution of ethylene glycol. Fuel 2006, 85, 2489-2493. (10) Ohsol, E. O.; Gillespie, T. E.; Pinkerton, J. W. U.S. Patent 5,985,137, Feb 26, 1998. (11) Wu, J. H. A study on deacidifying processes of the vacuum cut 3 distillate oil with a isopropyl alcohol-ammonia-water mixed solvent. Lubricant (Chinese) 1999, 14 (3), 8-12. (12) Waterkins, F. M. U.S. Patent 2,302,281, Nov 17, 1942. (13) Ferguson, S.; Reese, D. D. U.S. Patent 4,752,381, May 18, 1987. (14) Verachtert, T. A. U.S. Patent 4,199,440, Aug 24, 1978. (15) Fuqua, M. C.; Lovell, J. B. U.S. Patent 2,424,158, July 17, 1947. (16) Sartori, G.; Savage, D. W.; Dalrymple, D. C.; Ballinger, B. H.; Blum, S. C.; Wales, W. E. U.S. Patent 6,251,305, Oct 6 1998.
10.1021/ef060501r CCC: $37.00 © 2007 American Chemical Society Published on Web 02/08/2007
942 Energy & Fuels, Vol. 21, No. 2, 2007
Wang et al.
Table 1. Properties of the Base Stock density 20/4 °C (g/cm3) flashing point (°C) TAN (mg KOH/g) ASTM distillation range (°C) IBP 10% 50% 90%
0.8812 85 1.7 195 223 285 359
Scheme 1 Esterification Reactor and Flow Chart
Figure 1. Effect of reaction temperature and space velocity (methanol/ oil ratio (w/w) ) 0.0050).
a new method for the catalytic esterification of naphthenic acids with methanol to turn the naphthenic acids into methyl naphthenate. The catalyst was based on SnO and Al2O3. SnO was used as a catalyst for the synthesis of isoamylacetate and isooctyl naphthenate.17,18 Al2O3 is a good catalyst carrier. Therefore, the two substances were used in the synthesis of the catalyst. 2. Experimental Procedure 2.1. Determination of Acid Number and Acid Removal. The acid number was measured according to ASTM D664. The acid removal was determined according to the formula
(
acid removal ) 1 -
acid number of esterificated oil × 100% acid number of the base stock
)
2.2. Base Stock. The base stock was the diesel fuel fraction processed by a corp. of China. The properties of the base stock are listed in Table 1. It can be observed that the base stock has a high density and a high total acid number. The naphthenic acid concentration in the diesel fuel is about 0.027 mol/L. 2.3. Preparation of Catalyst. The esterification was finished in the presence of a catalyst. The catalyst was prepared by blending SnO, Al2O3, and water, and then the mixture was extruded and baked to form a solid catalyst with a diameter of about 1 mm and a length of about 2 mm. 2.4. Esterification. The esterification was finished in a fixed bed reactor as shown in Scheme 1. The base stock and methanol were mixed uniformly according to a specified ratio and poured into the base stock container. The mixture was then pumped into the reactor. The reactor is separated into two areas: in the top is the preheating area, and the bottom is reaction area. The reactor was fixed in two furnaces, and two temperature controllers were used to control the temperatures. The catalysts were fixed in the (17) Yang, S. J.; Luo, Y. Synthesis of iso-amylacetate using SnO as catalyst. J. Baoji Coll. Arts Sci., Nat. Sci. (Chinese) 2002, 22 (1), 46-48. (18) Qian, J. H. Synthesis of isooctyl naphthenate over SnO catalyst. J. Fushun Pet. Inst. (Chinese) 1995, 15 (4), 15-17.
Figure 2. Effect of reaction temperature and space velocity (methanol/ oil ratio (w/w) ) 0.010).
reaction area, and quartz particles were fixed in the preheating area. In the preheating area, the mixture was heated to the reaction temperature; then it was moved into the reaction area. The reaction area was maintained at a constant temperature, and the naphthenic acids reacted with methanol to produce methyl naphthenate. The product was sampled at intervals during the reaction and was finally pumped into the product container or allowed to flow out of the equipment. The reaction was finished under the atmospheric pressure.
3. Results and Discussion 3.1. Effect of Reaction Temperature and Space Velocity. Figures 1 and 2 show the effect of reaction temperature and space velocity on the acid removal with methanol/oil ratios (w/ w) of 0.0050 and 0.010, respectively. It can be observed that the acid removal increases with increasing reaction temperature. Every curve has a knee-point temperature, after which the acid removal increases slowly with the temperature. When the methanol/oil ratio is determined, the knee-point temperature increases with increasing space velocity. 3.2. Effect of the Methanol/Oil Ratio. The dosage of methanol has an important influence on acid removal. Table 2 reveals the effect of the methanol/oil ratio. It can be observed from Table 2 that the total acid number decreases with an increasing methanol/oil ratio. When the methanol/oil ratio (w/ w) is 0.010 and the reaction temperature is more than 250 °C, the total acid number of the de-acided oil can be less than 0.1 mg KOH/g. This is similar to the effect of aqueous base
RemoVal of Naphthenic Acids by Esterification
Energy & Fuels, Vol. 21, No. 2, 2007 943
Table 2. Effect of Methanol/Oil Ratio
Table 3. Effect of Esterification on the Color of De-acided Oils
TAN of de-acided oil (mg KOH/g)
methanol/ space oil velocity ratio (w/w) (h-1) 150 °C 180 °C 200 °C 250 °C 280 °C 300 °C 0.0050 0.0075 0.010
1.2 1.2 1.2
1.1 1.1
0.69 0.78
0.55 0.51 0.19
0.20 0.18 0.083
0.18 0.16 0.085
0.18 0.16 0.089
washing. Because the process is finished in a fixed bed reactor, the ester generated is not separated from the oil (it was a component of the oil), the oil has no loss at all, and because the molecular weight of the ester is more than that of the naphthenic acid, the total yield is slightly more than 100%. Methanol is much cheaper than diesel fuel, so this method is cheap and convenient. From the above table and figures, we can find that more than 90% of the acid can be removed. Although the naphthenic acids decompose at high temperatures,19 they do not decompose at 150-300 °C. We have performed many experiments in an autoclave to test the catalysts, and the data indicate that the total acid number does not change at 150-330 °C in 5 h if methanol is not added to the mixture. Therefore, it can be confirmed that the decrease of the total acid number is the result of esterification in this paper. Although the amount of methanol is small, the methanol/naphthenic acids molecular ratio is more than 5. The esterification is fast when the water generated in the reaction is removed. Because the concentration of naphthenic acids is very small, the amount of water generated is also very small in the oil; thus the water has a very small influence on the reaction, and the esterification is fast and complete. 3.3. Effect of Esterification on the Color of De-acided Oils. During the experiment, we observed that the de-acided oils had better color than the base stock. Therefore, we selected a diesel fuel that had been sampled for about 10 days; the color of the sample became dark with a color number of 25, and the total acid number did not change. This sample was used to test the color improvement caused by esterification. The space velocity was 1.6 h-1, and the methanol/oil ratio (w/w) was 0.0050. The
space velocity (h-1)
reaction temp (°C)
TAN of de-acded oils (mg of KOH/g)
acid removal (%)
color number of de-acided oils
1.6 1.6 1.6 1.6 1.6 1.6
150 180 200 250 280 300
1.15 1.0 0.51 0.31 0.26 0.25
37.7 45.9 72.4 83.2 85.9 86.5
24 20 20 16 15 16
esterification was finished under different temperatures. The color number and the total acid number of the de-acided oils are listed in Table 3. The color numbers of the oils were measured according to the Sinopec standard SH/T0168-92.20 It can be seen from Table 3 that the total acid number of the de-acided oils decreased with increasing reaction temperature and the color number of the de-acided oils became smaller with increasing reaction temperature, up to 280 °C. When the reaction temperature was greater than 280 °C, the color number increased slowly with the temperature. So the optimum reaction temperature for color improvement was 280 °C. This result reveals that the esterification of diesel fuel can improve the oil color. Methanol reacts with the matter which had darkened the oil color deep to make it lighter. 4. Conclusions Esterification is suitable for the removal of napthenic acids from high-acid concentration diesel fuels. SnO/Al2O3 is a good catalyst for the esterification. The acid removal is influenced by the reaction temperature, methanol/oil ratio, and space velocity. A high reaction temperature and methanol/oil ratio or low space velocity are favorable for the esterification. The color of the de-acided oils can apparently be improved during the esterification. Acknowledgment. The authors thank Guangyou Liu and Xinlong Yan for their help in the experiments. EF060501R
(19) Shen, H. P.; Wang, Y. Z.; Li R. Removal of petroleum acids by thermal treatment. Pet. Process. Petrochem. (Chinese) 2004, 35 (2), 3235.
(20) Standard Test Method of Petroleum and Petroleum Products; China Standard Press: Peking, 1994; pp 441-446.
