Deep eutectic solvents for deacidification of waste biodiesel feedstocks: an experimental study

The key challenge in reaching the goals set by Renewable Energy Directive is developing a sustainable, profitable, and environmentally acceptable biodiesel production process. In order to achieve balance between the above criteria, any use of high-quality edible oils as feedstocks needs to be avoided and utilization of waste feedstocks, such as used coffee grounds and waste animal fats, should be encouraged. The main drawback of these waste feedstocks is their high impurity content which usually requires an additional purification step. It is the purpose of this research to investigate the deacidification of three different waste biodiesel feedstocks by means of liquid-liquid extraction with deep eutectic solvents, in order to identify possible connections between solvent properties and extraction efficiency. Eight deep eutectic solvents were chosen to cover a wide range of different properties and the three used feedstocks varied in free fatty acid content. The relationship between solvent properties and extraction efficiency was determined by Spearman's rank-order correlation. Strong, statistically significant positive correlation was found for solvent pH values, while a strong negative correlation was observed for polarities and molar volumes. The most effective solvent was potassium carbonate/ethylene glycol (1:10, mol.). Depending on the initial total acid number, solvent to feedstock mass ratios 0.1:1 and 0.25:1 were enough to reduce the acidity of waste animal fat below 2 mg of potassium hydroxide/g fat and the solvent was successfully reused up to four times.