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Energy & Fuels 2006, 20, 812-817
Continuous Production of Biodiesel via Transesterification from Vegetable Oils in Supercritical Methanol Kunchana Bunyakiat, Sukunya Makmee, Ruengwit Sawangkeaw, and Somkiat Ngamprasertsith* Fuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn UniVersity, Bangkok 10330, Thailand
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ReceiVed October 6, 2005. ReVised Manuscript ReceiVed December 8, 2005
The continuous production of biodiesel (fatty acid methyl esters) by the transesterification reaction of coconut oil and palm kernel oil was studied in supercritical methanol without using any catalyst. Experiments were carried out in a tubular flow reactor, and reactions were studied at 270, 300, and 350 °C at a pressure of 10 and 19 MPa with various molar ratios of methanol-to-oils from 6 to 42. It was found that the best condition to produce methyl esters from coconut oil and palm kernel oil was at a reaction temperature of 350 °C, molar ratio of methanol-to-vegetable oil of 42, and space time 400 s. The % methyl ester conversions were 95 and 96 wt % for coconut oil and palm kernel oil, respectively. The regression models by the least-squares method were adequate to predict % methyl ester conversion with temperature, molar ratio of methanol-to-oil, and space time as the main effects. The produced methyl ester fuel properties met the specification of the ASTM biodiesel standards.
1. Introduction Biodiesel (fatty acid alkyl esters) is an alternative fuel for diesel engines. It is an alcohol ester product from the transesterification of triglycerides in vegetable oils or animal fats. This can be accomplished by reacting lower alcohols such as methanol or ethanol with triglycerides. The reaction proceeds well in the presence of some homogeneous catalysts such as sodium hydroxide and sulfuric acid, or heterogeneous catalysts such as metal oxides or carbonates or enzymes. Sodium hydroxide is very well accepted and widely used because of its low cost and high product yield, but the solubility of potassium hydroxide in methanol is higher than that of sodium hydroxide. Although the reaction system is simple, one drawback that prevents wider use of biodiesel is its high energy consumption and production cost, partly resulting from the complicated separation and purification of the product. Therefore, to perform the reaction without the presence of a catalyst is one effective way to reduce the biodiesel cost. Various biodiesel production processes employing homogeneous, heterogeneous catalytic, and noncatalytic supercritical methods as reported in the literature are summarized in Table 1.1-5 Recently, there have been some reports on the noncatalytic transesterification reaction employing supercritical methanol conditions.3,6,7 Saka and Kusdiana3,6 have proposed that the * Corresponding author. Fax: +66-2255-5831. E-mail: somkiat@ sc.chula.ac.th. (1) Ma, F.; Hanna, A. M. Bioresour. Technol. 1999, 70, 1-15. (2) Branwal, B. K.; Sharma, M. P. Renewable Sustainable Energy ReV. 2005, 9, 363-378. (3) Saka, S.; Kusdiana, D. Fuel 2001, 80, 225-231. (4) Du, W.; Xu, Y.; Liu, D.; Zeng, J. J. Mol. Catal. B: Enzym. 2004, 30, 125-129. (5) Noureddini, H.; Gao, X.; Philkana, R. S. Bioresour. Technol. 2005, 96, 769-777. (6) Kusdiana, D.; Saka, S. Fuel 2001, 80, 693-698. (7) Demirbas, A. Energy ConVers. Manage. 2002, 43, 2349-2356.
reactions of rapeseed oil were complete within 240 s at 350 °C, 19 MPa, and molar ratio of methanol-to-oil at 42. Demirbas7 studied the transesterification reaction under supercritical methanol employing six potential vegetable oils (cottonseed, hazelnut kernels, poppy seed, rapeseed, safflower seed and sunflower seed) at varying molar ratios of alcohol-to-vegetable oil and reaction temperatures. It was found that, when the molar ratio of methanol-to-oil was 24, at 250 °C, and at 300-s reaction time, the best methyl ester yield from hazelnut kernels and cotton seed oil was 95%. The properties of biodiesel were also tested and found to be similar to those of No. 2 diesel fuel but were slightly more viscous. This study was carried out to investigate the effects of temperature and molar ratio of methanol-to-oil on the biodiesel production from palm kernel oil and coconut oil with supercritical methanol in a continuous system. The critical properties of the mixtures at various molar ratios of methanol-to-oils were calculated in the following manner. First, the critical properties of a vegetable oil that is a mixture of various triglycerides is represented by a single pseudo-triacylglyceride with the following molecular structure:8
[(CH2COO)2CHCOO](CHdCH)m(CH2)n(CH3)3
(1)
The term in brackets represents the triglyceride functional group. The values of m and n reproduce the molecular weight and degree of unsaturation of the vegetable oil and are calculated from the fatty acid composition of the oil. The critical temperature and pressure were then calculated using Lydersen’s method of group contributions.9 Finally, the critical temperature and pressure of the mixtures of oil and methanol were calculated (8) Espinosa, S.; Fornari, T.; Bottini, S. B.; Brignole, E. A. J. Supercrit. Fluids 2002, 23, 91-102. (9) Chopey, N. P. Handbook of Chemical Engineering Calculation; McGraw-Hill: New York, 1994; pp 1-8.
10.1021/ef050329b CCC: $33.50 © 2006 American Chemical Society Published on Web 02/04/2006
Production of Biodiesel from Vegetable Oils
Energy & Fuels, Vol. 20, No. 2, 2006 813
Table 1. Various Biodiesel Production Processes1-5 homogeneous catalytic method
heterogeneous catalytic method
enzymatic method
SC MeOH method
0.5-3 h 0.1-5.0 MPa, 30-200 °C metal oxide or carbonate methyl esters normal methanol
1-8 h 0.1 MPa, 35-40 °C immobilized lipase methyl esters low to high methanol or methyl acetate
120-240 s >8.09 MPa, >239.4 °C none methyl esters high methanol
waste glycerin purity
0.5-4 h 0.1 MPa, 30-65 °C acid or alkali saponified products normal to high methanol, catalyst, and saponified product wastewater low
none low to normal
none high
process
complicated
complicated
none normal or triacetylglycerol as byproduct complicated
reaction time reaction conditions catalyst free fatty acids yield removal for purification
simple
by Lorentz-Berthelot-type mixing rules10 as the following equations:
TcmVcm )
∑i ∑j xixjTcijVcij ) xi2TciVci + 2xixjTcijVcij + xj2TcjVcj (2)
Vcm )
∑i ∑j xixjVcij ) xi2Vci + 2xixjVcij + xj2Vcj
(3)
∑i ∑j xixjzcij ) xi2zci + 2xixjzcij + xj2zcj
(4)
zcmRTcm Vcm
(5)
zcm )
Pcm )
The terms Vcij, Tcij, and zcij were calculated by the combining rules as the following equations:
Tcij ) xTciTcj Pcij )
1 P P V V Vcijx ci cj ci cj
(6) (7)
zcij ) 0.5(zci + zcj)
(8)
1 Vcij1/3 ) (Vci1/3 + Vcj1/3) 2
(9)
where i and j are subscripts for vegetable oil and methanol, respectively, x is the mole fraction of vegetable oil or methanol, Tc is the critical temperature of vegetable oil or methanol, Vc is the molar volume of vegetable oil or methanol, zc is the compressibility factor of vegetable oil or methanol, Tcm is the critical temperature of vegetable oil and methanol mixture, Vcm is the molar volume of vegetable oil and methanol mixture, zcm is the compressibility factor of vegetable oil and methanol mixture, and Pcm is the critical pressure of vegetable oil and methanol mixture. 2. Experimental Methods Two potential vegetable oils were studied: coconut oil (CCO) and palm kernel oil (PKO). The CCO was supplied by Tab Sakae Co. Ltd., and the PKO was supplied by Cheeva Mongkol Co. Ltd. Both samples were warmed and filtered prior to use. Analytical grade methanol (Fisher) was used with no further purification. The experiments were performed using a tubular flow reactor shown in Figure 1. The oil and methanol were pumped in two different lines by high-pressure high-performance liquid chromatographic pumps (Jasco, model PU-1580) up to 19 MPa (total flow rate of 1.5-9.0 (10) Walas, S. M. Phase Equilibria in Chemical Engineering; Butterworth: Boston, 1985; pp 29-33.
Figure 1. Schematic diagram of the continuous transesterification reactor system. 1. High-pressure pumps, 2. methanol reservoir, 3. vegetable oil reservoir, 4. nitrogen cylinder, 5. preheaters, 6. reactor, 7. salt bath, 8. temperature monitoring system, 9. cooling bath, 10. inline filter, 11. pressure monitoring system, 12. back pressure regulator, and 13. sample collector.
mL/min depending on space time and molar ratio of methanol-tooil), preheated while flowing in the preheat lines (SUS316 tubing of 1/8-in. o.d., 0.035-in. thickness, and 2-m length). After being preheated, the two lines were mixed at the reactor inlet using a SUS316 mixing tee, and the temperature of the fluid was monitored directly using a thermocouple located within this mixing tee. The reactor was constructed from a 5.5-m length of 3/8-in. o.d., 0.035in. thickness SUS316 tubing. The preheat lines and the reactor were immersed in an electrically heated salt bath. The fluid product exiting from the reactor was promptly cooled by an external watercooling bath and depressurized using a back-pressure regulator. After pressure and temperature were constant, approximately 10 mL of liquid product was collected, and then methanol was evaporated by a rotary evaporator. The liquid product was checked for % methyl esters by gas chromatography to ensure that the system reached steady state, which was indicated by a constant value, after more than 90 min. The product was then collected until the total volume was sufficient for further analysis. The final liquid product was collected and left to settle for several hours, preferably overnight, to ensure complete separation. Two liquid phases were obtained: ester and crude glycerin. The top ester layer was separated by a separatory funnel and put in a rotary evaporator to remove any excess methanol. The % methyl ester in liquid product was then analyzed by gas chromatography (Shimadzu model GC14BSPL) with a flame ionization detector. A 30-m-long, 0.25-mm-diameter capillary column coated with poly(ethylene glycol) was used with helium as a carrier gas. The ester product was diluted with n-heptane (analytical grade) before injection and standardized by standard methyl esters. Chemical analyses of the oil samples were performed according to AOCS standards,11 while the other standard test methods for fuel properties were performed according to ASTM standards,12 as shown in Table 2. (11) Methods Cd 3d-63, Cd 3b-73, and Ce 2-66. American Oil Chemical Society: Champaign, IL, 1997. (12) Methods D1298, D976, D93, D240, and D445. American Society for Testing and Materials Annual Book of ASTM Standards, Part 26; ASTM: Philadelphia, PA, 2004.
814 Energy & Fuels, Vol. 20, No. 2, 2006
Bunyakiat et al.
Table 2. Standard Test Methods for Oil and Fuel Properties items
AOCS test no.
acid value (mg of KOH/g of oil) saponification value (mg of KOH/g of oil) fatty acid composition higher heating value (MJ/kg) kinematic viscosity (mm2/s) flash point (°C) sp.gr. 15.6 °C cetane No.
Cd 3D-63 Cd 3B-76
ASTM test no.
Ce 2-66 D240 D445 D93 D1298 D976
Table 3. Properties of Coconut Oil, Palm Kernel Oil, and Low Speed Diesel Fuel properties
coconut oil
palm kernel oil
low speed diesel fuel
0.919 38.43 27.0 231 16 204
0.925 38.59 31.1 264 31 244
0.836 46.0 3.4 77 -
sp.gr. 15.6 °C higher heating value (MJ/kg) kinematic viscosity (mm2/s) flash point (°C) acid value (mg of KOH/g of oil) saponification value (mg of KOH/g of oil)
Table 4. Fatty Acid Composition in Coconut and Palm Kernel Oil Samples (Mass % of Fatty Acids) as Analyzed by AOCS Ce - 2 66 Standard
fatty acid
formula
C in fatty acids: C in double bonds
caproic acid caprylic acid capric acid lauric acid myristic acid palmitic acid stearic acid oleic acid linoleic acid
C5H11COOH C7H15COOH C9H19COOH C11H23COOH C13H27COOH C15H31COOH C17H35COOH C17H33COOH C17H31COOH
C6:0 C8:0 C10:0 C12:0 C14:0 C16:0 C18:0 C18:1 C18:2
MW
coconut oil % mass
palm kernel oil % mass
116 144 172 200 228 256 284 282 280
0.39 5.75 5.09 44.56 19.08 10.04 3.57 8.8 2.72
3.77 3.67 48.38 17.37 9.13 2.72 12.56 2.4
total
100
100
3. Results and Discussion 3.1. Vegetable Oil Properties. The vegetable oil properties, together with low speed diesel fuel, are given in Table 3, and fatty acid compositions of the vegetable oil samples are given in Table 4. From Table 3, it is clear that both CCO and PKO had lower heating values and much higher viscosities than those of low speed diesel fuel. This would indicate troublesome atomization and poor engine performance if these oils are used as neat fuels. Table 4 shows that both the CCO and PKO samples contained lauric acid as the major fatty acid. The CCO sample contained 44.56% lauric, 19.08% myristic, and 10.04% palmitic acids, while the PKO sample contained 48.38% lauric, 17.37% myristic, and 12.56% oleic acids, respectively. 3.2. Critical Property Estimation. The critical temperatures and pressures of CCO and PKO were calculated as 629 °C, 6.1
atm and 677 °C, 5.8 atm, respectively. The critical temperature and pressure of the mixtures of oil and methanol are calculated using eqs 2-9, and the values are shown in Table 5. At a molar ratio of methanol-to-oils of 6, the critical temperatures of mixtures of CCO and PKO with methanol were 396.0 and 415.6 °C, respectively. As the methanol content in mixture increases, the critical temperature decreases, while the critical pressure increases. These properties are used to better explain the effect of operating conditions in the following sections. 3.3. Effect of Temperature. (1) % Methyl ester conversion was calculated from GC analyses and standardization by the following equation:
% methyl ester conversion )
WME × 100 WFA
(10)
where WME is the weight of methyl ester in liquid product that is obtained from gas chromatography; and WFA is the weight of fatty acid in each vegetable oil (Table 4). (2) Space time was calculated from the oil and methanol flow rates to obtain the real gas volumes employing the compressibility factor, Z, obtained by the Pitzer method.13 Coconut Oil. Figure 2a-d shows the effect of temperature on % methyl ester conversion during various space times at increasing molar ratios of methanol-to-oils of 42, 24, 12, and 6, respectively. By increasing the temperature from 270 to 300 and 350 °C, the % methyl ester conversion increased. At the space time of 400 s and molar ratio of methanol-to-oil at 42, % methyl ester conversion increased from 50 to 85 and 95% when the reaction temperature increased from 270 to 300 and 350 °C. This indicates that, at higher temperatures of 300 and 350 °C, the conversion rate is higher than that at 270 °C. One can use the calculated critical temperature (Table 5) to better explain this effect. At the molar ratio of methanol-to-oil of 42, the critical temperature of the mixture is 282 °C. This means that the system at 270 °C was a little below the critical temperature. At this temperature, the conversion rate would be low, presumably due to the subcritical state or the instability of the supercritical state of mixture.14 Palm Kernel Oil. Figure 3a-d shows the effect of temperature on % methyl ester conversion during various space times at palm kernel molar ratios of methanol-to-oil of 42, 24, 12, and 6, respectively. By increasing the temperature from 270 to 300 and 350 °C, % methyl ester conversions increased. At the space time of 400 s and molar ratios of methanol-to-oil at 42, % methyl ester conversions increased from 38 to 94 and 96% when the reaction temperature increased from 270 to 300 and 350 °C, respectively. Again, this indicates that, at higher temperatures of 300 and 350 °C, the conversion rate is higher than that at 270 °C. 3.4. Effect of Molar Ratio of Methanol-to-Oil. At 350 °C, % methyl ester conversions at the molar ratio of methanol-to-
Table 5. Calculated Critical Properties of Oil and Methanol Mixture at Various Compositionsa molar ratio of methanol-to-oil 6
12
24
42
properties
PKO
CCO
PKO
CCO
PKO
CCO
PKO
CCO
Tc, K Tc, °C Pc, atm Vc, L/mol zc
688.60 415.45 37.22 0.33 0.22
669.02 395.87 37.26 0.32 0.22
632.72 359.57 50.36 0.23 0.22
619.08 345.93 50.31 0.22 0.22
587.75 314.60 61.80 0.17 0.22
579.00 305.85 61.68 0.17 0.22
561.10 287.95 68.52 0.15 0.22
555.35 282.20 68.39 0.15 0.22
a T , P , V , and z of methanol: 512.6 K, 79.9 atm, 118.0 L/mol, and 0.224, respectively. Calculated T , P , V , and z of coconut oil: 879.93 K, 6.21 c c c c c c c c atm, 2.366 L/mol, and 0.20; palm kernel oil: 926.12 K, 5.936 atm, 2.476 L/mol, and 0.19, respectively.
Production of Biodiesel from Vegetable Oils
Energy & Fuels, Vol. 20, No. 2, 2006 815
Figure 2. Effect of temperature on the % methyl ester conversion at various molar ratios of methanol-to-coconut oil, P ) 19 MPa. (a) 42, (b) 24, (c) 12, (d) 6.
Figure 3. Effect of temperature on the % methyl ester conversion at various molar ratios of methanol-to-palm kernel oil, P ) 19 MPa. (a) 42, (b) 24, (c) 12, (d) 6.
coconut oil of 42, 24, 12, and 6 are plotted against space time, as shown in Figure 4. When the methanol content in the supercritical fluids increased, % methyl ester conversion also increased. The higher methanol content is favorable not only because more molecules of methanol surround the oil molecules but also because it contributes to the lower critical temperature of the mixture. (13) C¸ engel, Y. A.; Boles, M. A. Thermodynamics: An Engineering Approach; McGraw-Hill: Boston, 1998; p 81. (14) Cao, W.; Han H.; Zhang J. Fuel 2005, 84, 347-351.
At a space time of 450 s, % methyl ester conversions increased from 50, 52, 93, to 95% when molar ratio of methanolto-oil increased in the mixture from 6, 12, 24, to 42, respectively. The same conclusion holds true for palm kernel oil, as shown in Figure 5. When the methanol content in the supercritical fluid increased, % methyl ester conversion also increased. At a space time of 450 s, % methyl ester conversions increased from 48, 50, and 85 to 96% when molar ratio of methanol-to-oil increased in the mixture from 6, 12, and 24 to 42, respectively. The maximum methyl ester conversion is 96% at a molar ratio of
816 Energy & Fuels, Vol. 20, No. 2, 2006
Bunyakiat et al. Table 7. Analysis of Variance of Palm Kernel Oil Regression Model
Figure 4. Effect of the molar ratio of methanol-to-coconut oil on the % methyl ester conversion at 350 °C, 19.0 MPa.
source
sum of squares
DF
mean square
F value
P value
model A B C A2 B2 C2 AB AC BC residual
23160 4206 1805 1964 2781 1.13 91.69 216.0 202.6 322.3 3388
9 1 1 1 1 1 1 1 1 1 31
2573 4206 1805 1964 2781 1.13 91.69 216.0 202.6 322.3 109.3
23.54 38.48 16.52 17.97 25.45 0.01 0.84 1.98 1.85 2.95
