## 1 Introduction  
Due to an increase in crude oil price, environmental concern for vehicle exhaust emission, and the demand for the reduction of greenhouse gas emission, the development of renewable energies has grown dramatically over the past years. Among the applicable renewable energies, biodiesel has recently attracted extensive attention because it is biodegradable, non-toxic, and environmentally friendly [1; 2; 3]. Biodiesel is defined as fatty acid methyl ester (FAME) or fatty acid ethyl ester (FAEE) from vegetable oils or animal fats. The significant advantages of biodiesel over conventional fossil-based diesel fuels include cleaner engine emissions, its status as a renewable energy source and superior lubricating property, making it an excellent alternative fuel [4]. In general, transesterification of triglyceride (TG) to diglyceride (DG), monoglyceride (MG), methyl ester (ME) and glyceride (GL) can be catalyzed by bases, acids, and enzymes [5; 6]. In many production processes for biodiesel, alkaline catalysts are typically used. The commonly used homogenous base catalysts are potassium hydroxide (KOH), sodium hydroxide (NaOH), and the corresponding potassium or sodium methoxide [7; 8]. However, the use of homogeneous alkaline catalysts in the production of biodiesel requires acid neutralization and a washing step to remove the catalyst and salt from the ester phase, which generates a large amount of wastewater. In addition, it is difficult to separate the homogeneous catalyst from the glycerol phase [9]. In order to overcome these problems, the use of heterogeneous catalysts as a potential solution is preferred [10]. The heterogeneous catalyst process has recently become attractive as an effective biodiesel production process [11]. Due to their facile separation and recycled use, solid catalysts, such as zeolites, alkali earth oxides, alkali metals or alkali earth salts loaded on metal oxide, metallic complexes, and silica-alumina, have been explored as promising alternatives [12]. More recently, there has been an increasing development of heterogeneous catalysts, such as NaOH, KOH and a series of potassium catalysts supported on alumina, alkali-doped metal oxide, zeolite and activated carbon. The activity of these solid bases generally increases with base strength. Besides being more active than metal compounds, the use of solid bases requires catalytic reactions to occur under milder conditions than acids [13]. However, the small pore size and low surface area of these catalysts have limited their use in biodiesel synthesis [14]. Due to its unique properties, it is now well established that activated carbon can be used as a highly effective catalyst support in liquid and vapor phase reactions such as transesterification reactions. Not only can activated carbon be produced from a variety of raw materials [15], but also it has the large surface area, which allows that catalyst to be highly dispersed in an effective manner. With regard to concerns about recycling and utilization of agricultural biomass resources [16], the effective uses of agricultural waste biomass for the production of activated carbons by thermo-chemical conversion has gained much attention over the last decade [17]. _Jatropha curcas_ fruit shell (JS) is a promising biomass waste that can be utilized as a precursor for activated carbon. Their seeds also contain high oil levels and have been widely used for biodiesel production in several Asian countries. However, the solid residues that remain after the seed-pressing process are typically regarded as mere waste. Accumulation of these wastes for a long period of time often creates environmental problems due to its unpleasant odor. Since the fixed carbon content in JS is quite high, the utilization of this waste as activated carbon can minimize the environmental problem [18]. Themain objective of this work was to optimize the continuous process for biodiesel production from waste frying oil (WFO) using potassium hydroxide supported on _Jatropha curcas_ fruit shell activated carbon (KOH/JS).