Alternative energy sources, such as renewable sources and biofuels, are increasingly being used as a substitute for fossil fuels. The use of waste cooking oil as an energy source could lead to a reduction in dependence on fossil fuels and a reduction in environmental pollution. One of the alternative methods of oil pretreatment for biodiesel synthesis is extractive deacidification using environmentally friendly deep eutectic solvents. In this paper, the influence of hydrodynamic conditions on the mixing of heterogeneous two-phase systems in three geometrically similar Rushton type mixing vessels was investigated. It was investigated how the initial free fatty acid content, hydrodynamic conditions and solvent to oil ratio affect the batch efficiency, ie how the pulsation pulsation frequencies and solvent-oil ratio affect the efficiency of continuous extractive deacidification of waste cooking oil using deep eutectic solvent potassium carbonate-ethylene-glycol (1:10). Solvent capacity and phase equilibrium were determined experimentally and it was found that the investigated heterogeneous systems are not prone to coalescence, which facilitates the mathematical description of mixing and interfacial mass transfer. Volumetric mass transfer coefficient is larger for the continuous phase (oil), which means that the rate of mass transfer is controlled by the mass transfer on the side of the dispersed phase. The conditions for batch and continuous process, which achieve high efficiency in the removal of free fatty acids from waste cooking oil, were determined experimentally.