Biodiesel is considered to be a cleaner-burning diesel fuel made from natural, renewable sources such as vegetable oils or animal fats. Various processes have been developed to produce biodiesel. Transesterification of vegetable oils and animal fats using methanol to produce biodiesel is a two-phase reaction in which methanolysis occurs only in the methanol phase. However, oil has a limited solubility in methanol so that vigorous mechanically mixing and/or heating are necessary to improve the mass transfer between the two phases. To resolve this problem, we examined the application of microreactor process to biodiesel production because of the properties of short diffusion distance and large specific surface area.

Transesterification of sunflower oil with methanol by using KOH catalyst to fatty acid methyl esters (FAMEs) was performed in microtube reactors. Influenced of microtube diameter, methanol/oil molar ratio and reaction temperature on the oil conversions were investigated. Flow behaviors in the transparent microtube (inner diameter = 0.8 mm, length = 1000 mm) under different operating conditions were observed and characterized by optical measurements. Methanol was colored to get clear images of the flow patterns in the microtube by inert red phloxine B. The relationship between the flow pattern and the oil conversion was discussed.

The segmented two-phase flow of the red methanol and the nearly-colorless oil phases was observed in all regions of the microtube in the absence of the reaction. The segmented flow was also observed at the entrance region with low conversion of sunflower oil, and then fine red droplets were dispersed and circulated in the oil segments with the progress of the reaction. In the case of a methanol/oil molar ratio of 23.9 at 60oC, quasi-homogeneous phase was formed at the place about 300 mm apart from the inlet because of high miscibility of the produced FAME and glycerol in methanol solution. As a result of the measurement of the oil conversion at each point of the microtube, sunflower oil was found to be completely converted to FAMEs in the quasi-homogeneous phase.