## 1 Introduction  
Biodiesel is a promising alternative to diesel, which is gained from fossil resources. Biodiesel from vegetable oils provides lower carbon dioxide and greenhouse gas emission compared to fossil fuels and, therefore, has a smaller environmental impact [1]. Nowadays, biodiesel is industrially produced through transesterification of high fatty acids (triacylglycerols), which are mostly gained from soybean, rapeseed and palm oil [2], and methanol or ethanol as aliphatic alcohol in a base-catalyzed transesterification reaction. As catalyst NaOH or KOH can be used. Since methanol is obtained from mineral oil, ethanol can be produced in the fermentation process from biomass. The food industry is deprived if using soybean oil or other cooking oils for biodiesel production. Applying waste cooking oil or non-edible oils like algae oil for the ethanolysis provides environmentally friendlier biodiesel [3]. The suitability of the oil for the reaction depends on the number of triacylglycerlycerols [2]. The reaction mechanism includes three stages. In the first stage (1), triacylglycerols (T) is converted into diacylglycerol (D) while one equivalent of fatty acid ester (E) with ethanol is formed. In the second stage (2) another equivalent of alcohol (R-OH) reacts with D and forms monoacylglycerols (M) and glycerol (G) in the last step (3). With each of the reversible steps, one equivalent of R-OH is needed and one equivalent of fatty acid ethyl (FAEE) ester is formed.  
\[\text{T}+\text{ROH}\rightleftharpoons\text{D}+\text{E} \tag{1}\]  
\[\text{D}+\text{ROH}\rightleftharpoons\text{M}+\text{E} \tag{2}\]\[\mathrm{M}+\mathrm{ROH}\rightleftharpoons\mathrm{G}+\mathrm{E} \tag{3}\]  
Previous studies have shown that it is possible to conduct the transesterification reaction in continuously operated microreactors [4]. Microreactors provide advantages like higher controllability, safer dealing with toxic chemicals and easy scale-up by numbering-up. Moreover, microreactors make available micromixing of the reactants, which can lead to higher yields [5]. Oil and alcohol phase (with the catalyst) are involved in the ethanolysis reaction leading to mass transfer limitations [2]. Due to micromixing, a higher phase boundary interface can be obtained because of a higher surface-to-volume-ratio of the reactants and short diffusion distances [6]. Methanolysis or ethanolysis reaction was investigated in several microreactors [5; 7; 8; 9]. [PERSON] et al. [7] used zig-zag microreactors and observed a reaction yield of 97.3% at 56 \({}^{\circ}\)C, methanol-oil-ratio of 6:1, 1.6 wt.-% KOH and 18 s residence time. [PERSON] et al. [8] worked with a T-mixer and different microreactors, and obtained a maximum yield of 93.8% in a split-and-recombine micromixer at 50 \({}^{\circ}\)C, molar ethanol/oil ratio of 6:1, 1 wt.-% KOH and 11.3 min residence time. [PERSON] et al. [9] showed that at 1 wt.-% KOH catalyst, molar methanol/oil ratio of 6:1 and 60 \({}^{\circ}\)C reaction temperature the alcoholysis of rapeseed and cottonseed oil in a capillary microreactor with an inner diameter of 0.25 mm led to maximum yields of 99.4% FAEE after a reaction time of 5.89 min. The mixing of the reactants was performed in a separate container before the mixture was pumped through the capillary reactor. The authors found out that further increasing the catalyst concentration, the molar ethanol/oil ratio, the temperature or the reaction time was counterproductive for the investigated reactor system.  
The aim of this paper is to determine optimal reaction conditions for base-catalyzed transesterification of different oil types and in different reactor systems using design of experiments.