Purification of the Ester Phase by Water Extraction in Biodiesel Production

Water extraction is an effective method for removal of impurities from the post-transesterification reaction product in biodiesel production. It is commonly used in downstream processing alongside evaporation/distillation and filtration. The effects of process parameters on biodiesel purity, biodiesel loss, and capacity of water extraction were investigated. Soaps were found to be key impurities, which significantly influenced the process and are the only emulsifier species that have an impact on the process. The acid value of biodiesel after water extraction was dependent on the initial soap content. Difficulties with soaps can be solved by adding an adequate amount of citric acid (0.01-0.10%) in wash water. The effects of temperature, pH of water extract, and mixing were investigated. Higher temperature (>25 degrees C) and higher pH (>7.0) increased biodiesel loss during the extraction. Higher pH and lower temperature increased the ester content. With an optimal amount of citric acid in the wash water, high intensity mixing (>1.0kWm(-3)) is desired for the most effective purification. Correlation for calculation of an optimal amount of citric acid based on the desired pH of extract, the ratio of wash water to ester, and the initial soap content in ester was found. Excess hydrolysis of biodiesel during extraction was not observed. For the first time, engineering aspects of process-settling time, and mixing parameters and their influence on removal of impurities and subsequent separation were determined. Optimal mixing power per unit of volume was 2.25kWm(-3), mixing time 2.5min, and settling time 30min. From the results and data obtained, an optimal system for water extraction of crude biodiesel was also proposed.