Process Safety and Environmental Protection 9 2 ( 2 0 1 4 ) 179–185 Contents lists available at ScienceDirect Process Safety and Environmental Protection journal h om ep age: www.elsevier.com/locate/ps ep Optimization of biodiesel production by alkali-catalyzed transesterification of used frying oil Mehdi Atapour, Hamid-Reza Kariminia ∗ , Parivash Moslehi Moslehabadi Department of Chemical and Petroleum Engineering, Sharif University of Technology, P.O. Box 11155-9465, Azadi Ave., Tehran, Iran a b s t r a c t Biodiesel as an alternative fuel for fossil diesel has many benefits such as reducing regulated air pollutants emissions, reducing greenhouse gases emissions, being renewable, biodegradable and non-toxic. In this study, used frying oil was applied as a low cost feedstock for biodiesel production by alkali-catalyzed transesterification. The design of experiments was performed using a double 5-level-4-factor central composite design coupled with response surface methodology in order to study the effect of factors on the yield of biodiesel and optimizing the reaction conditions. The factors studied were: reaction temperature, molar ratio of methanol to oil, catalyst concentration, reaction time and catalyst type (NaOH and KOH). A quadratic model was suggested for the prediction of the ester yield. The p- value for the model fell below 0.01 (F-value of 27.55). Also, the R 2 value of the model was 0.8831 which indicates the acceptable accuracy of the model. The optimum conditions were obtained as follows: reaction temperature of 65 ◦ C, methanol to oil molar ratio of 9, NaOH concentration of 0.72% w/w, reaction time of 45 min and NaOH as the more effective catalyst. In these conditions the predicted and observed ester yields were 93.56% and 92.05%, respectively, which experimentally verified the accuracy of the model. The fuel properties of the biodiesel produced under optimum conditions, including density, kinetic viscosity, flash point, cloud and pour points were measured according to ASTM standard methods and found to be within specifications of EN 14214 and ASTM 6751 biodiesel standards. © 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Biodiesel; Used frying oil; Design of experiments; Optimization; Response surface methodology 1. Introduction Since fossil fuel resources are finite and nonrenewable, using renewable sources of energy such as biodiesel is increasing in different countries. Chemically, biodiesel is a mixture of mono-alkyl esters of long chain fatty acids (Meher et al., 2006). It is an ecological fuel because of its qualitative com- position (Tomasevic and Siler-Marinkovic, 2003). Biodiesel has many environmental benefits such as reducing regulated air pollutants emissions (Dorado et al., 2003), reducing green- house gases emissions (Bouaid et al., 2007), together with being biodegradable and non-toxic in nature (Murugesan et al., 2009) over petroleum diesel. Biodiesel can be utilized in diesel engines because of having fuel properties close to petroleum diesel (Dorado, 2008). However, the high cost of produced biodiesel mainly because of the cost of raw material i.e. veg- etable oil, is the main obstacle for its commercialization. ∗ Corresponding author. Tel.: +98 21 66166426; fax: +98 21 66166426. E-mail address: kariminia@sharif.ir (H.-R. Kariminia). Received 18 November 2011; Received in revised form 17 August 2012; Accepted 22 December 2012 Therefore, using a low cost feedstock such as non-edible oils, low-cost edible oils, by products of the refining vegetable oils, soap stocks and animal fats, restaurant greases and used fry- ing oil (UFO) is necessary (Canakci and Sanli, 2008). Enormous amount of UFO is generated in the world which may lead to environmental and health problems if not properly managed. On the other hand, using UFO as raw material for biodiesel pro- duction can significantly reduce the total biodiesel production cost. Therefore, production of biodiesel from UFO and other greasy wastes has the advantage of recycling waste materi- als without competing with the food market. However, UFO might include food remaining, particles and other impurities that requires pretreatment before being utilized as raw mate- rial for biodiesel production. Also, some properties of UFO such as viscosity, acid value, sulfur content and heating value are different with those of virgin vegetable oils. (Dias et al., 2008; Lam et al., 2010; Marchetti, 2012). 0957-5820/$ – see front matter © 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.psep.2012.12.005