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Kinetics of the Esterification of Low-Concentration Naphthenic Acids and Methanol in Oils with or without SnO as a Catalyst Yan-zhen Wang,*,† Yan-ping Liu,‡ and Chen-guang Liu† State Key Laboratory of HeaVy Oil Processing, Key Laboratory of Catalysis, China National Petroleum Corporation (CNPC), China UniVersity of Petroleum, Dongying, China 257061, and China Petroleum Eastchina Design Institute, China National Petroleum Corporation (CNPC) Number A-113, Yan’an 3 Road, Qingdao, Shandong, China 266071 ReceiVed February 23, 2008. ReVised Manuscript ReceiVed April 9, 2008
Removal of naphthenic acids is very important for oil refineries to lower its corrosion of equipments. Esterification can be an interesting method to lower the acid concentration of high acid number oils. The kinetics of esterification of low-concentration naphthenic acids and methanol in oils with or without SnO as a catalyst have been investigated at 180, 200, 220, 250, and 280 °C in an autoclave with a methanol and naphthenic acid molecular ratio 10:1. The experiment data follows a second-order reaction equation with or without SnO as a catalyst. The activation energy, “Ea”, the frequency factor, “A”, and reaction rate constant, “k”, were also determined. The use of SnO can obviously lower the activation energy.
1. Introduction Naphthenic acids are the main organic acids in petroleum.1–3 Naphthenic acids may cause serious corrosion to the equipment of oil processing and the corrosion changes with temperature and acid concentration.4,5 Naphthenic acids can also make the corrosion more serious if the petroleum contains sulfur.6 Naphthenic acids also have bad effects on the oil performance, for example, lubricating and diesel oils.7 Esterification can lower the acid number of petroleum distillate fractions. SnO can be the catalyst for the esterification.8,9 The research on the esterification reaction kinetics of naphthenic acids and methanol with or without SnO as a catalyst plays an important role for reactor design and process optimization. The kinetics of naphthenic acids and methanol has not been reported up to now. However, the kinetics of the esterification of some organic acids and fatty alcohols had been investigated before. E1-Kinawy et al. investigated the kinetics of esterifi* To whom correspondence should be addressed. Telephone: 86-5468392147. Fax: 86-546-8391971. E-mail:
[email protected]. † China University of Petroleum. ‡ China Petroleum Eastchina Design Institute. (1) Fan, T.-P. Characterization of naphthenic acids in petroleum by fast atom bombardment mass spectrometry. Energy Fuels 1991, 5, 371–375. (2) 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. (3) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Naphthenic acids in crude oils characterized by mass spectrometry. Energy Fuels 2000, 14, 217–223. (4) Elizabeth, B. K.; Lee, C. H.; Gary, R. L.; et al. Naphthenic acid corrosion in refinery settings. Mater. Perform. 1993, 32 (4), 50–55. (5) Slavcheva, E.; Shone, B.; Turnbull, A. Review of naphthenic acid corrosion in oil refining. Corros. J. 1999, 34 (2), 125–131. (6) Yepez, O. Influence of different sulfur compounds on corrosion due to naphthenic acid. Fuel 2005, 84 (1), 97–104. (7) Danzik, M. U.S. Patent 4,634,519, Jun 11, 1985. (8) Savage, S. G.; Dalrymple, D. W.; et al. U.S. Patent 6,251,305, Oct 6, 1998. (9) Wang, Y. Z.; Sun, X. Y.; Liu, Y. P.; Liu, C. G. Removal of naphthenic acids from a diesel fuel by esterification. Energy Fuels 2007, 21 (2), 941–943.
cation of various saturated fatty acids with isobutanol.10 In this study, sulfuric acid was used as a catalyst at a concentration equivalent to 0.02 M [H+]; the activation energy Ea and frequency factor “a” were estimated. The activation energies are 9.648, 12.817, 5.357, 16.58, and 2.51 kcal/mol for propionic, butyric, lauric, myristic, and stearic acids, respectively. Kulawska et al. studied the esterification of maleic anhydride over an acid ion-exchange resin catalyst in an isothermal semi-batch reactor, and first-order kinetics with respect to acid has been observed.11 Tesser et al. investigated the kinetics of oleic acid esterification with methanol in the presence of triglycerides. The data interpreted a second-order, pseudo-homogeneous kinetic model.12 Shintre et al. studied the kinetics of esterification of lauric acid with C1-C18 fatty alcohols by lipase using isooctane as a solvent. When lauric acid and fatty alcohols were taken in the mole ratio of 1:1, first-order kinetics were observed for all of the alcohols.13 Basheer et al. investigated the esterification of long-chain fatty acids and fatty alcohols with a surfactantcoated lipase in n-hexane. They found that the esterification reaction follows Michaelis-Menten kinetics and the kinetics of the lipase-catalyzed esterification reaction model follow a ping-pong bi-bi mechanism with no substrate or product inhibition.14 2. Experimental Procedure 2.1. Determination of Acid Number and Acid Removal. The acid number was measured according to American Society for Testing and Materials (ASTM) D664. (10) El-Kinawy, O. S.; Megahed, O. A.; Zaher, F. A. Kinetics of esterification of various saturated fatty acids with isobutanol. Modell., Meas Control, C 2002, 63 (1-2), 27–38. (11) Kulawska, M.; Sadlowski, J. Z.; Skrzypek, J. Kinetics of the esterification of maleic anhydride with octyl, decyl or dodecyl alcohol over dowex catalyst. React. Kinet. Catal. Lett. 2005, 85 (1), 51–56. (12) Tesser, R.; Di Serio, M.; Guida, M.; Nastasi, M.; Santacesaria, E. Kinetics of oleic acid esterification with methanol in the presence of triglycerides. Ind. Eng. Chem. Res. 2005, 44 (21), 7978–7982. (13) Shintre, M. S.; Ghadge, R. S.; Sawant, S. B. kinetics of esterification of lauric acid with fatty alcohols by lipase: Effect of fatty alcohol. J. Chem. Technol. Biotechnol. 2002, 77 (10), 1114–1121.
10.1021/ef800138h CCC: $40.75 2008 American Chemical Society Published on Web 06/11/2008
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Table 1. Properties of the Base Stock properties
results
0.935 density 20/4 °C (g/cm3) total acid number (mg of KOH/g) 3.69 dynamic viscosity at 100 °C (mm2/s) 5.4 dynamic viscosity at 40 °C (mm2/s) 54.3 total acid number of the pure naphthenic acid (mg of KOH/g) 154 average molecular weight (g/mol) 260
The acid removal was determined according to the formula acid number of esterificated oil × 100% acid number of the base stock 2.2. Base Stock. The base stock was the second vacuum fraction processed by a corporation of China. The properties of the base stock were listed in Table 1. It can be observed that the base stock has high density, viscosity and total acid number. The naphthenic acid concentration in the fraction is about 0.059 mol/L. The other reagents used in the experiments, for example, ethyl alcohol, methanol, and SnO, were analytical-grade. 2.3. Equipments and Experiments. The reaction apparatus was an autoclave with a volume of 1000 mL. The autoclave was equipped with a constant electric stirrer and a temperature controller. A glass tube that was used to contain methanol was linked with an inlet of the autoclave, and the other inlet of the glass tube was linked with a nitrogen bottle by a gas regulator. A total of 700 mL of base stock and catalyst was placed into the autoclave. Then, the autoclave was closed and swept with nitrogen to displace the air, and at last, the autoclave was vacuumed with a vacuum pump. Afterward, the autoclave was heated to the reaction temperature with constant stirring, and then the stirring was stopped and the methanol was pressured into the autoclave by nitrogen. After that, the stirring was continued and the reaction time began. Samples were taken at intervals, and the acid numbers of the samples were measured. 2.4. Experimental Theory. The naphthenic acids and the methanol react according to the following formula:
(
)
acid removal ) 1 -
Figure 1. Esterification kinetics plots without catalysts.
Figure 2. Arrhenius plot without catalysts.
In the equation, A is naphthenic acids, B is methanol, C is methyl naphthenate, and D is water. In this experiment, because the concentration of naphthenic acids is low (about 0.059 mol/L) and the volume of the methanol used is in far excess of the theory amount and its concentration was constant; therefore, the influence of the methanol can be ignored, and the reaction velocity equation can be described as below in eq 2 -
dCA ) kCAn dt
(2)
where CA represents the concentration of naphthenic acid in oils, t represents the reaction time, k represents the reaction rate constant, and n represents the reaction order. Thus, the equation is simple, and n and k are easy to calculate by experimental data.
3. Results and Discussion 3.1. Reaction Kinetics without Catalysts. 3.1.1. Reaction Order. First, the esterification was finished without catalysts. Methanol and the naphthenic acid molecular ratio was 10, and the reaction temperature was 180, 200, 220, 250, and 280 °C, (14) Basheer, S.; Cogan, U.; Nakajima, M. Esterification kinetics of longchain fatty acids and fatty alcohols with a surfactant-coated lipase in n-hexane. J. Am. Oil Chem. Soc. 1998, 75 (12), 1785–1790.
respectively. As may be seen in Figure 1, a reasonable linear dependence of the reaction time and 1/CA was observed. Data in Figure 1 reveal that the esterification of naphthenic acid and methanol fulfill a second-order reaction equation, whose k and 1/CA are in line relationship as follows in eqs 3 and 4: kt )
1 1 CA CA0
(3)
dCA ) kCA2 (4) dt where CA0 is the original concentration of naphthenic acids in oils in eq 3. 3.1.2. Constant of the Reaction Rate. Five fitting equations and their relation coefficient (R2) under different reaction temperatures were obtained from Figure 1 as follows in Table 2. R2 represents the relation coefficient of the equation. The fact that R2 values are more than 0.99 realizes that the equation and the experimental data have a high relationship in the above equations. The reaction rate constant (k) under different temperatures without catalysts are listed in Table 3. It can be seen that the reaction rate constant becomes large with the increase of the reaction temperature. -
Esterification of Naphthenic Acids and Methanol
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Table 2. Fitting Equations and R2 without Catalysts reaction temperature (°C)
fitting equation
relation coefficient, R2
180 200 220 250 280
1/CA ) 0.0020t + 17.02 1/CA ) 0.0070t + 17.17 1/CA ) 0.0223t + 17.17 1/CA ) 0.245t + 17.35 1/CA ) 3.26t + 17.53
0.997 0.994 0.994 0.994 0.993
from the Arrhenius equation, then,
R2 ) 0.994 k ) A exp(-Ea/RT)
ln k ) ln A - Ea/RT
453
473
493
523
553
0.0020
0.0070
0.0223
0.245
3.26
Table 4. Fitting Equations and R2 Using 1 wt % SnO as Catalyst reaction temperature (°C)
fitting equation
relation coefficient, R2
180 200 220 250 280
1/CA ) 0.137t + 17.3 1/CA ) 0.559t + 16.8 1/CA ) 1.82t + 15.5 1/CA ) 6.82t + 16.6 1/CA ) 21.2t + 17.4
0.992 0.994 0.997 0.999 0.993
(5) (6) (7)
where A is the frenquency factor and Ea is the activation energy. It can be obtained from eqs 6 and 7 that ln A ) 34.2 and Ea/R ) 1.85 × 104. Then, A ) 7.15 × 1014 L mol-1 min-1 and Ea ) 153.7 kJ/ mol. The esterification of naphthenic acids and methanol is a thermopositive reaction. The esterification reaction kinetic model of naphthenic acids and methanol without catalysts is listed in eq 8. -
reaction temperature, T (K) reaction rate constant, k (L mol -1 min-1)
3.1.3. ActiVation Energy. The activation energy of esterification of naphthenic acids and methanol without catalysts can be calculated from the Arrhenius plot15 as seen in Figure 2. The fitting plot equation in Figure 2 is listed below lnk ) 34.2 - 1.85 × 104/T
Table 3. Reaction Rate Constant under Different Temperatures without Catalysts
Table 5. Reaction Rate Constant under Different Temperatures Using SnO as a Catalyst reaction temperature, T (K) reaction rate coefficient, k (L mol-1 min-1)
453
473
493
523
553
0.137
0.559
1.82
6.82
21.2
dCA ) kCA2 dt ) A exp(-Ea/RT )CA2 ) 7.15 × 1014 exp(-1.537 × 105/RT )CA2
(8)
3.2. Reaction Kinetics Using SnO as a Catalyst. The esterification experiments were finished with 1 wt % SnO in oils as catalyst. Methanol and the naphthenic acid molecular ratio was 10, and the reaction temperature was 180, 200, 220, 250, and 280 °C, respectively. 3.2.1. Reaction Order. Figure 3 is the relationship and their fitting curves of the naphthenic acid concentration and the
Figure 4. Arrhenius plot using 1 wt % SnO as a catalyst.
reaction time under different reaction temperatures using 1 wt % SnO as a catalyst. It can be seen from Figure 3 that 1/CA and the reaction time are in linear relationship and the reaction order also fulfills the second-order reaction equation, whose k and 1/CA also follow eqs 3 and 4. 3.2.2. Constant of the Reaction Rate. Five fitting equations and their relation coefficient (R2) under different reaction temperatures were obtained from Figure 3 as follows in Table 4. The fact that R2 values are more than 0.99 realizes that the equation and the experimental data have a high relationship in the above equations. The reaction rate constants under different temperatures using 1 wt % SnO as a catalyst are listed in Table 5. It can be seen that the reaction rate constant also becomes big with the increase of the reaction temperatures, and the reaction rate constant with SnO as a catalyst is much more than that without catalysts under the same temperature in Table 3. Figure 3. Relationship of the naphthenic acid concentration and the reaction time with SnO.
(15) Physical Chemistry Institute of Tianjin University. Physical Chemistry II, 2nd ed.; High Education Press: Peking, China, 1985; pp 234245.
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3.2.3. ActiVation Energy. The Arrhenius fitting plot using 1 wt % SnO as a catalyst is a straight line, as seen in Figure 4. The fitting equation in Figure 4 is calculated as follows in eq 9: ln k ) 25.9 - 1.25 × 104/T from the Arrhenius equation,
R2 ) 0.990
ln k ) ln A - Ea/RT
-
dCA ) kCA2 dt ) A exp(-Ea/RT )CA2 ) 1.70 × 1011 exp(-1.04 × 105/RT )CA2
(10)
(9) 4. Conclusions
(7)
then, ln A ) 25.9 and Ea/R ) 1.25 × 104. In addition, A ) 1.70 × 1011 L min-1 and Ea ) 104.2 kJ/ mol. The activation energy is much smaller using 1 wt % SnO as a catalyst than that without a catalyst. This is the main theory of the catalysis for the esterification of naphthenic acid and methanol. However, the frequency factor using SnO as a catalyst is also smaller than that without a catalyst. The esterification reaction kinetic model for naphthenic acids and methanol with 1 wt % SnO as a catalyst is seen in eq 10.
From the above analysis, we can draw the conclusions: The esterification of naphthenic acids and methanol is in accordance with a second-order reaction equation with or without SnO as a catalyst. The reaction rate constant becomes large with the increasing reaction temperatures. The activation energy of the esterification is 153.7 kJ/mol without catalysts and 104.2 kJ/ mol using 1 wt % SnO as a catalyst. The use of SnO can obviously lower the reaction activation energy and speed the reaction. EF800138H
