Enhanced removal of Free Fatty Acid from waste oil for Biodiesel production

Introduction The possibility of using vegetable oils as fuel has been documented in the beginning of diesel engines. Biodiesel produced from waste oil by trans-esterification process can be used as a substitute for diesel in internal combustion engines [1]. Biodiesel can be produced by the transesterifcation of vegetable oils [2] such as soyabean oil, rapeseed oil, sunflower oil etc. [3]. Transesterification is a well known process since 1864. Rochleder described glycerol preparation through the ethanolysis of castor oil [4]. Solvent used for the production of biodiesel affects the process in terms of conversion efficiency [5] and the conversion differs according to oil quality. Transesterification is the general term used to describe the organic reactions where triglyceride is transformed into methyl ester through interchange of the alkoxy moiety. The general equation for a transesterification reaction is shown in Figure 1 [6]. Transesterification of oils can be carried out using either an acid or base catalyst [7]. The use of homogenous acid catalysts for the esterification of oil has several disadvantages such as the catalyst could not be recovered and limited use for continuous process [8]. Acid catalyzed transesterification reaction rate is much slower than alkali. The ester conversion efficiency is strongly affected by molar ratio of alcohol to oil. In acid catalyzed transesterification a high molar ratio is required than alkali catalyzed. If acid catalyzed reaction takes place at room temperature, the reaction is very slow and poor ester conversion is obtained [9]. Alkaline catalyzed transesterification process is faster than acid catalyzed reaction and also requires less time when compared to acidcatalyzed transesterification. Hence most commercial processes employ homogenous base catalysts for biodiesel production [10]. Figure 1. General equation for a transesterification reaction Many edible and non-edible oil sources are available for transesterification. However these oils cannot be used as a viable source for biodiesel production due to their high FFA content. Hence a suitable solvent system for the removal of high FFA oil is essential to make these sources reliable for base catalyzed transesterification. The yield of base-catalyzed transesterification decreases when the FFA value of oil is greater than 2%. It has been found that transesterification would not occur if FFA content in the oil is high [11]. The difficulty with FFA content transesterification is that it rapidly reacts with the alkaline catalyst forming soap [12], as shown in reaction equation (1). This soap formation inhibits the separation of ester and glycerol. Increase in acid value of oil reduces the conversion efficiency [13, 14].