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Kinetics of Palm Oil Transesterification in a Batch Reactor
D. Darnoko1 and Munir Cheryan 2
University of Illinois, Department of Food Science and Human Nutrition, Agricultural Bioprocess Laboratory, Urbdna, Illinois 61801

ABSTRACT: Methyl esters were produced by transesterification of palm oil with methanol in the presence of a catalyst (KOH). The rate of transesterification in a batchreactor increased with temperature up to 60° C. (Higher temperatures did not reduce the time to reach maximal conversion. The conversion of triglycerides (-TG), diglycerides (-DG) and monoglycerides (-MG) appeared to be second order up to 30 mins of reaction time. Reaction rate constants for TG, DG and MG hydrolysis reactions were 0.018 -0.191 (wt % @min)-1, and were higher at higher temperatures andhigher for the MG reaction than for the TG hydrolysis. Acti vation energies were : 14.7, 14.2 and 6.4 kcal/mol for the TG, DG and MG hydrolysis reactions respectively. The optimal catalyst concentration was 1% KOH. Paper no. 19574 in JAOCS 77, 1263-1267 (December 2000). KEY WORDS: Biodiesel, kinetics, methyl esters, palm oil, transesterification. Methyl esters derived from vegetable oil(biodiesel) have good potential as an alternative diesel fuel. The cetane number, energy content., viscosity, and phase changes of biodiesel are similar to those of petroleum-based diesel fuel (1). Biodiesel is produced by transesterification of large, branched triglycerides (TG) into smaller, straight-chain molecules of methyl esters, using an alkali or acid as catalyst (2). There are three stepwisereactions with intermediate formation of diglycerides (DG) and monoglycerides (MG) resulting in the production of 3 mol of methyl esters (ME) and 1 mol of glycerol (GL) as follows (3,4). The overall reaction is:
Catalyst

k2

DG + ROH

W MG + R'CO R
2

[3]

k5 k3

MG + ROH

W GL + R'CO R
2

[4]

k6.

Transesterification reactions have been studied for many vegetable oils such as.soybean, rapeseed, sunflower, and safflower seed (5,6). Common catalysts for transesterification are NaOH, KOH, or sodium methoxide. However, sodium methoxide causes formation of several by-products, mainly sodium salts, which have to be treated as waste (7). In addition, a high-quality oil is required with this catalyst. Potassiurn hydroxide has an advantage in that, at the end of the reaction,the reaction mixture can be neutralized with phosphoric acid resulting in potassium phosphate, which can be used as fertilizer. Isigigur et al. (6) reported that potassium hydroxide was superior to sodium hydroxide as a catalyst for the transesterification of safflower seed oil. A study of the kinetics of transesterification will provide parameters that can be used to predict the extent of thereaction at any time under particular conditions. Among several kinetic studies published on transesterifieation of simple esters, only a few were concerned with the transesterifieation of vegetable oil fatty esters, such as those by Freedman et al. (3) and Noureddini and Zhu (4). Although methyl esters from palm oil have been produced on a pilot scale in Malaysia, there are no published reports onits kinetics. EXPERIMENTAL PROCEDURES Refined, bleached, and deodorized palm oil was obtained from Archer, Daniels Midland (Decatur, IL). Reference standards such as tripalmitin, triolein, diolein, monoolein, methyl oleate, methyl palmitate, methyl heptanoate, and glycerol of >99% purity were purchased from Nu-Chek-Prep. Inc. (Elysian, MN) and Sigma Chemical Co. (St. Louis, MO).. Methanol andpotassium hydroxide were of analytical grade obtained from Fisher Scientific Co. (Pittsburgh, PA). Batch reaction. Transesterifieation reactions were performed in a 1-L three-necked flask equipped with a reflux condenser, a thermometer, and a sampling port. The reactor was immersed in a constant-temperature water bath equipped

TG + 3ROH

)))))))))))> 3R'CO2-R + GL

[1]

The stepwise reactions...
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