Extractive distillation is to add a third component (called extractant or solvent) to the mixed solution to change the volatility of the original component and make it separate. It is required that the boiling point of the extractant is much higher than that of the components, and it does not form azeotropic liquid with the components. Extractive distillation is often used to separate solutions with small differences in boiling points (volatility) of each component. For extractive distillation, a variety of extractants can often be selected. Generally speaking, the main basis for selecting extractant is that (1) extractant has high selectivity. The relative volatility of separated components in extractant is called the selectivity of extractant. The relative volatility of the separated components in the extractant is much higher, and the separation is easier, that is, the selected extractant is more selective. Selectivity is the main basis for selecting extractant. Because selectivity also determines the separation difficulty of light and heavy key components in the separation components. Therefore, the number of trays and reflux ratio (affecting the tower diameter) are also closely related to it. (2) The extractant has high solubility for the separated components, so that the liquid on the tower plate is homogeneous without stratification. (3) The boiling point of the extractant should be much higher than the boiling point of the separated component, otherwise the extractant will easily volatilize from the top of the tower. (4) Good thermal stability and chemical stability, non-toxic and non-corrosive to equipment. (5) Easy to recycle, cheap and easy to obtain.