Molecular distillation is a distillation method operated under high vacuum, where the average free path of the vapor molecules is greater than the distance between the evaporating surface and the condensing surface.Thus, the liquid mixture can be separated by the difference of evaporation rate of each component in the feed liquid. At a given temperature, the lower the pressure, the greater the average free path of the gas molecules.When the pressure in the evaporation space is very low (10-2 ~ 10-4 mmHg) and the condensation surface is close to the evaporation surface, and the vertical distance between them is less than the average free path of the gas molecules, the vapor molecules vaporized from the evaporation surface can directly reach the condensation surface without colliding with other molecules and condense.