Molecular distillation applied to the purification of biodiesel from ethanol and soybean oil

Abstract

In the biodiesel production and purification process, the washing step, where impurities such as free glycerol, mono, di and triglycerides are eliminated, is maybe the most questioned step for two reasons: i) for the generation of about 0.5 to 10 L of residual water each biodiesel liter, ii) for the complexity and the cost of productive process. The washing water is eliminated from biodiesel using separation methods that imply high operative and the maintenance costs (ultracentrifuges). Furthermore, this washing water is highly emulsified with high organic load and must be treated before being thrown away. For these reasons the reduction in the water consumption in the biodiesel purification is a priority. This paper presents the use of molecular distillation in the biodiesel purification process as an alternative to the washing conventional process. The employ of molecular distillation would avoid the water consume, a resource of highly value. This operation was employed as a purification technique to decrease the free glycerol and mono, di and triglycerides contained in biodiesel which is obtained by alkaline transesterification of soybean oil with ethanol, with sodium hydroxide as catalyzer. The effect of evaporation temperature and stirring rate on the mass and concentrations in the residue and distillated streams was studied. With two molecular distillation steps it was possible to obtain biodiesel with concentrations of free glycerol and glycerides that respect the limit set by the ASTM D 6751-08 and DIN EN 14,214 international norms.