Research Article Heterogeneously Catalyzed Esterification of FFAs in Vegetable Oils The production of methyl esters (biodiesel) from free fatty acids (FFAs) contained in vegetable oils was studied using a heterogeneous acid catalyst. The feedstock was a by-product of a vegetable oil refinery. The experiments were performed in a batch reactor, in a temperature range of 363.15–393.15 K, with an initial molar ratio of methanol to FFAs of 6.6/1, while the catalyst mass was fixed at 2 wt % of the total vegetable oil mass. A technical kinetic model has been developed which accounts for the reversible esterification reaction. Kinetic parameters were deter- mined by fitting experimental data to the model. Keywords: Biofuels, Catalysts, Esterification, Oils Received: March 27, 2006; revised: July 20, 2006; accepted: August 18, 2006 DOI: 10.1002/ceat.200600109 1 Introduction Humanity shows an unsatisfied thirst for energy. As a conse- quence, an amount of energy almost equivalent to 10 000 mil- lion tons of oil is spent every day. Although society has based its future on energy, most of the energy used derives from fos- sil fuels such as coal, natural gas, and especially petroleum. The use of these sources contributes to the increase of green- house gas emissions and global warming. Therefore, fulfill- ment of the requirements set by Kyoto targets has become a crucial factor. To this end, the European Community has decided to replace by the year 2010, at least 5.75 % of the yearly consumed fossil fuels by biofuels, the use of which does not contribute to the growth of greenhouse gases (2003/30/EC directive). Among these biofuels, biodiesel – consisting of fatty acid methyl esters (FAME) – has been widely recognized as a promising biofuel and its market is growing fast. Biodiesel can be produced from a variety of feedstock such as vegetable oils, animal fats, and waste grease, through the transesterification reaction of triglycerides (TGs) [1, 2] or the esterification of free fatty acids (FFAs) present in the feed, with low molecular weight alcohols. Biodiesel has similar physical properties to diesel and therefore it can be used as a substitute for diesel fuel, either in neat form or in blends with it. Com- pared to petroleum-based diesel, biodiesel is more rapidly bio- degradable and less toxic, with a higher flash point than diesel and an ultra-low sulfur content [3]. In addition, biodiesel in- creases the lubricity of diesel fuel in their mixtures. The deep desulfurization process for the production of low sulfur-con- tent diesel has a negative effect on fuel lubricity [4], while an addition of 1 % biodiesel, or even less, restores the required fuel lubricity [5]. Furthermore, biodiesel is an oxygenated fuel (oxygen content about 10 %) with low particulate emissions when it burns. Particulate mater emissions can be reduced up to 45–50 % when biodiesel is used instead of diesel in conven- tional engines, while the reduction can reach 75–83 % for the engines with the latest technology [6]. Nowadays, refined vegetable oils such as Soybean, Rapeseed, Sunflower, and Palm oils are the major feedstocks for biodiesel production, worldwide. Refined oils can be easily converted to biodiesel with the current industrial technology of basic homogeneous catalysis. The main disadvantage of using re- fined oils as raw material for biodiesel production is their high cost, which has a major effect on the final product price. How- ever, waste grease such as yellow grease from used cooking oils and animal fats, are attractive alternatives for biodiesel pro- duction due to their low cost. Production of biodiesel from used oils and fats also contributes to waste recycling. However, the conversion of feedstocks with high FFA content to biodie- sel requires a more complicated process. When a basic homogeneous catalyst is used for the transes- terification of feeds with FFAs, soaps are produced as by-prod- ucts through the unwanted saponification reaction [7] of car- boxylic acids, with the base (e.g., KOH, NaOH, and CH 3 ONa) as shown below (see reaction (a)): ROOH + NaOH (or MeONa) → ROO – Na + + H 2 O (or MeOH) (a) ROOMe + H 2 O > ROOH + MeOH (b) © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com S. Pasias 1 N. Barakos 1 C. Alexopoulos 1 N. Papayannakos 1 1 School of Chemical Engineering, National Technical University of Athens, Greece. – Correspondence: Prof. N. Papayannakos (npap@central.ntua.gr), School of Chemical Engineering, National Technical University of Athens, Heroon Polytechniou 9, 157 80 Zografos, Athens, Greece. Chem. Eng. Technol. 2006, 29, No. 11, 1365–1371 1365