Lactic acid-based deep eutectic solvents and activated carbon for soap removal from crude biodiesel

The physical properties of deep eutectic solvents (DESs) such as density, viscosity, conductivity, melting point and thermogravimetric analysis (TGA) prepared from tetramethylammonium chloride (TMAC) and lactic acid (LA) were investigated at temperatures from 303.15 K to 373.15 K. Three adjustable parameter correlations such as the Vogel-Fulcher-Tamman (VFT), Arrhenius and Litovitz models were used in the viscosity experiments. The DESs and activated carbon (AC) were employed for the removal of soap from biodiesel (BD) as an alternative to traditional water-based washing operations. The DESs molar ratio, DESs to biodiesel volume ratio, stirring speed and extraction time were optimized. The results showed that the VFT model gave the closest match in the viscosity experiments. From the extraction results, the optimal conditions for the decrease of the soap concentration in accordance with the ASTM D6751 standards were achieved using 1:1 volume ratio of DES to biodiesel, 1:4 molar ratio of DES, mixing speed of 150 rpm and extraction time of 6 minutes. An addition of 0.03 g of AC into the DES improved the oil BD purification. In contrast, the incorporation of AC into the DES enhanced the extraction yields while shortening the extraction time dramatically compared to the pure DESs. The enhanced interaction between the functional groups of the soap molecules with the AC surface is due to the large surface area with rough edges and high porosity of the AC. The utilization of AC and DES can be considered as a new route for the downstream processing of crude BD and could replace water as the washing agent.