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 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 trans- esterification 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 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 metha- nol 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 super- critical 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 fol- lowing molecular structure:8 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 * 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. (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. [(CH2COO)2CHCOO](CHdCH)m(CH2)n(CH3)3 (1) 812 Energy & Fuels 2006, 20, 812-817 10.1021/ef050329b CCC: $33.50 �� 2006 American Chemical Society Published on Web 02/04/2006

by Lorentz-Berthelot-type mixing rules10 as the following equations: The terms Vcij, Tcij, and zcij were calculated by the combining rules as the following equations: 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 mL/min depending on space time and molar ratio of methanol-to- oil), 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., 0.035- in. 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 water- cooling 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. (10) Walas, S. M. Phase Equilibria in Chemical Engineering Butter- worth: Boston, 1985 pp 29-33. (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. Table 1. Various Biodiesel Production Processes1-5 homogeneous catalytic method heterogeneous catalytic method enzymatic method SC MeOH method reaction time 0.5-4 h 0.5-3 h 1-8 h 120-240 s reaction conditions 0.1 MPa, 30-65 ��C 0.1-5.0 MPa, 30-200 ��C 0.1 MPa, 35-40 ��C 8.09 MPa, 239.4 ��C catalyst acid or alkali metal oxide or carbonate immobilized lipase none free fatty acids saponified products methyl esters methyl esters methyl esters yield normal to high normal low to high high removal for purification methanol, catalyst, and saponified product methanol methanol or methyl acetate methanol waste wastewater none none none glycerin purity low low to normal normal or triacetylglycerol as byproduct high process complicated complicated complicated simple 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. TcmVcm ) ������xixjTcijVcij i j ) xi2TciVci + 2xixjTcijVcij + xj2TcjVcj (2) Vcm ) ������xixjVcij i j ) xi2Vci + 2xixjVcij + xj2Vcj (3) zcm ) ������xixjzcij i j ) xi2zci + 2xixjzcij + xj2zcj (4) Pcm ) zcmRTcm Vcm (5) Tcij ) xTciTcj (6) Pcij ) 1 Vcij xPciPcjVciVcj (7) zcij ) 0.5(zci + zcj) (8) Vcij1/3 ) 1 2 (Vci1/3 + Vcj1/3) (9) Production of Biodiesel from Vegetable Oils Energy & Fuels, Vol. 20, No. 2, 2006 813