Efficient removal of alkali and alkaline earth metals from biodiesel using Ion-exchange resin: Performance and mechanism

Abstract

Raw materials, production process, and other factors cause several metal ions to exist in biodiesel. Furthermore, the existence of alkali and alkaline earth metals seriously affects the quality and performance of biodiesel. To improve fuel quality, a 732 ion-exchange resin was used to reduce the contents of alkali and alkaline earth metal ions in Jatropha biodiesel. The adsorption efficiency of the ion-exchange resin for alkali and alkaline earth metal ions in biodiesel was evaluated using ion chromatography, while the adsorption mechanism of ion change resin on alkali and alkaline earth metal ions in biodiesel was elucidated via FTIR and XPS. In addition, the effects of ion-exchange resin on several key physiochemical properties of biodiesel, including oxidation stability, corrosivity, heat value and kinematic viscosity, were also explored. The results showed that the total content of alkali and alkaline earth metal ions in biodiesel was reduced by 88.19%, and the K+, Na+, Mg2+, and Ca2+ contents were reduced to 0.11 mg/kg, 0.89 mg/kg, 0.53 mg/kg, and 0.78 mg/kg, respectively, after removing the ion-exchange resin. In the removal process, the hydroxyl group of sulfonic acid group on the ion-exchange resin is broken, and the substitution adsorption reaction between H+ and the alkali and alkaline earth metal ions in biodiesel is carried out. Moreover, the oxidation induction period of biodiesel was prolonged by 40.8% owing to the reduction in the content of alkali and alkaline earth metal ions, which significantly blocked the decomposition pathways of unstable oxidation intermediates in biodiesel. The degree of copper corrosion in biodiesel did not change significantly before and after ion-exchange resin adsorption during short-term corrosion (3 h); however, the copper corrosion grade decreased from 2c to 1b after ion-exchange resin adsorption during long term corrosion (240 h). Nevertheless, the heat value and kinematic viscosity of biodiesel were observed to be independent of the ionic metal content.