Heterogeneous free fatty acids esterification in waste cooking oil using ion-exchange resins Suyin Gan a, ⁎, Hoon Kiat Ng b , Park Hinn Chan b , Fook Lim Leong b a Department of Chemical and Environmental Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia b Department of Mechanical, Materials and Manufacturing Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia abstract article info Article history: Received 8 August 2011 Received in revised form 6 April 2012 Accepted 10 April 2012 Available online 15 May 2012 Keywords: Esterification Free fatty acids Waste cooking oil Ion-exchange resins Biodiesel A Taguchi orthogonal array was used to investigate the effects of catalyst type, catalyst concentration, tem- perature and methanol to oil molar ratio on the heterogeneous esterification of free fatty acids (FFA) in waste cooking oil (WCO) using ion-exchange resins. Analysis of Variance (ANOVA) was also applied to study the final FFA conversion in response to the investigated factors. The F-test indicated that catalyst type and methanol: oil molar ratio significantly influenced the FFA conversion. The optimal parameters of FFA esterification in WCO using ion-exchange resins obtained within the experimental ranges studied are as follows: Catalyst Amberlyst-15 at 4 wt.%, temperature of 65 °C and a methanol to oil molar ratio of 15:1. Under these conditions, a maximum FFA conversion of 60.2% could be obtained. The kinetics of the optimised FFA esterification catalysed by 4 wt.% Amberlyst-15 fitted well to a Langmuir–Hinshelwood (L–H) based model. © 2012 Elsevier B.V. All rights reserved. 1. Introduction In recent years, research into sustainable alternative fuels has been highly prioritised in many countries around the world. One of the major fossil fuel substitutes is biodiesel, a renewable fuel comprising alkyl monoesters of fatty acids originating from vegetable oils or animal fats. Biodiesel is a more attractive replacement of fossil diesel because of its relatively lower cost, compatibility with existing fossil diesel infra- structure and availability of production technology [1]. Furthermore, combustion studies of biodiesel fuels in engines and burners have dem- onstrated that these fuels have the potential to reduce pollutant emis- sions [2,3]. Biodiesel can be produced via transesterification which is a catalysed reaction between an alcohol such as methanol and vegetable oils or other fats. Refined vegetable oils including palm oil, rapeseed oil and soybean oil are commonly used as feedstocks for biodiesel production. However, for biodiesels produced from refined edible oils, the feedstock cost contributes more than 70% of the overall production cost [4,5]. This represents a major challenge in the commercialisation and widespread use of biodiesel [6]. Additionally, the biodiesel industry faces pressure in light of recent ‘food versus fuel’ debate. Thus, waste cooking oil (WCO) has been proposed as a cheaper, environmentally friendly alternative feedstock for biodiesel production [7–9]. However, the disadvantage of using WCO is its typically high FFA con- tent due to hydrolysis of triglycerides during frying. The high FFA content causes saponification during base-catalysed transesterification which consumes the catalyst and lowers the yield of biodiesel [10]. To address this, several researchers have proposed a two-step process in which the WCO first undergoes an acid catalysed esterification to lower the FFA con- tent followed by the conventional base-catalysed transesterification [4,11]. Methyl esters and water are produced in the first esterification step. Further refining occurs in the second step, i.e. transesterification, where the water content might lead to saponification. Sulphuric acid is often used in the first step because of its high conversion and low cost but its usage is associated with effluent disposal problems, loss of catalyst and high equipment cost due to the corrosive nature of acids [12,13]. These drawbacks have led to research into the potential use of solid acid catalysts for the esterification process such as zeolites, heteropolyacids immobilised on silica and ion-exchange resins [14–16]. From an organic chemistry viewpoint, the ability of acidic ion- exchange resins to catalyse esterification processes with different acids/ alcohols has been studied in the late 1990s [17,18]. Nonetheless, the role of ion-exchange resins in biodiesel production has only been recently investigated [16]. To date, a statistical approach to optimise the FFA ester- ification in WCO catalysed by ion-exchange resins without the expense of high experimental costs and time has yet to be attempted. The present work is focused firstly on the combined use of a Taguchi orthogonal array and an Analysis of Variance (ANOVA) to study and optimise four factors governing the FFA conversion in the first esterification step using ion-exchange resins. The Taguchi methodology allows a partial fac- torial design to be conducted such that the number of laboratory-scale experiments can be minimised while simultaneously retaining sufficient statistical accuracy in the results. The second part of this work is focused on conducting single factor experiments under optimised conditions in Fuel Processing Technology 102 (2012) 67–72 ⁎ Corresponding author. Tel.: + 60 3 8924 8162; fax: + 60 3 8924 8017. E-mail address: suyin.gan@nottingham.edu.my (S. Gan). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2012.04.038 Contents lists available at SciVerse ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc