Effect of Additives in the Reaction Medium on Noncatalytic Ester Production from Used Frying Oil with Supercritical Ethanol Ana Carolina de Araú jo Abdala, † Talita Amabile da Silva Colonelli, † Caroline Portilho Trentini, ‡ J. Vladimir Oliveira, § Lú cio Cardozo-Filho, ∥ Edson Antonio da Silva, † and Camila da Silva* ,†,‡ † Program of Post-Graduation in Bioenergy, State University of West Parana ́ (UNIOESTE), Faculdade Street 645, Jardim La Salle, Toledo, Parana ́ 85903-000, Brazil ‡ Department of Technology, Maringa ́ State University, (UEM), Umuarama, Parana ́ 87506-370, Brazil § Department of Chemical and Food Engineering, Federal University of Santa Catarina (UFSC), C.P. 476, Florianó polis, Santa Catarina 88040-900, Brazil ∥ Department of Chemical Engineering, Maringa State University (UEM), Av. Colombo 5790, Maringa, Parana ́ 87020-900, Brazil ABSTRACT: In this study, the noncatalytic production of ethyl esters from used frying oil (UFO) in a continuous process was evaluated under supercritical conditions. Experiments were performed with the objective of evaluating the effect of the addition of water, a co-solvent (n-hexane), and ethyl esters to the reaction medium, applying different temperatures and keeping the oil:ethanol mass ratio (1:1), pressure (20 MPa), and residence time (40 min) fixed. The results demonstrated that the formation of fatty acid ethyl esters (FAEE) is favored at higher temperatures. The addition of 5 wt % water increased the yield, while no significant effect (p > 0.05) was noted with the addition of 10 wt % water. The addition of cosolvent and ethyl esters in the range investigated plays a vital role in maximizing the FAEE yields for most conditions studied. The presence of water and cosolvent reduced the degree of fatty acids decomposition, while the addition of ethyl esters increased this parameter. The effect of the residence time was investigated applying the best conditions observed and good reaction yields (>85 wt %) were achieved under different conditions. 1. INTRODUCTION In the Brazilian context, soybean oil is the main feedstock used for biodiesel production; 1 however, recently, other feedstocks have been considered, because of the fact that the cost of the raw materials represents the highest percentage of the total production cost. 2 In this context, the use of waste oil as a raw material for biodiesel production is an interesting alternative, considering its low cost and high availability; it also involves the reuse of a material with high pollution potential. 3,4 In Brazil, cooking oil consumption by the population generates ∼3 billion liters of waste oil per year. 5 The quality standards for the use of frying oils in Brazil are established by Anvisa, 6 which determined that the free fatty acids (FFA) content should not exceed 0.9% and the content of polar compounds should not be greater than 25%. It has been reported that waste oil as a raw material can easily be adapted to biofuel production, since it is appropriate for reuse. 7 Ruiz-Me ́ ndez et al. 8 published useful information regarding the compounds present in used frying oils (UFOs), along with data on the characterization of the biodiesels obtained from them. The same authors reported that, during frying, polymers, dimers, oxidized triacylglycerides, diacylglycerides, and FFA are formed and some of these compounds cannot be converted to alkyl esters. The disadvantages associated with the use of biodiesel include higher emissions of nitrogen oxides, compared with mineral diesel fuel, 9 and the use of UFOs can be associated with the emission of nitrogen compounds derived from proteinaceous compounds present in these oils. However, this increase in the nitrogen oxide emissions can be addressed through the proper treatment of the combustion exhausts. This feedstock has high levels of FFA and water, which can compromise the performance achieved in the conventional homogeneous alkali-catalyzed process. To avoid this adverse effect, the used of an alcohol under supercritical conditions can be applied. In this method, the presence of FFA can lead to simultaneous transesterification, hydrolysis, and also esterifica- tion 10 (and, hence, higher reaction yields 11,12 ). In relation to alcohol, Brazil is one of the largest producers of ethanol in the world, the production technology is well-established and the industrial plants installed have a large capacity. The use of supercritical conditions in the transesterification of vegetable oils provides better solubility between the phases and decreased limitations to mass transfer. Also, the reaction rate increases significantly in the supercritical state and thus the reactions are completed in shorter periods and simpler separation and purification steps are involved. 13,14 In addition, it has been shown that the energy consumption associated with the supercritical method is very similar to that of processes involving homogeneous alkaline catalysis. 15,16 The transesterification of vegetable oils under supercritical conditions requires operation at elevated temperatures and pressures as well as the use of a high amount of alcohol in order to obtain satisfactory yields. This results in the disadvantage of Received: November 15, 2013 Revised: April 22, 2014 Published: April 22, 2014 Article pubs.acs.org/EF © 2014 American Chemical Society 3122 dx.doi.org/10.1021/ef402253e | Energy Fuels 2014, 28, 3122−3128