The conditions of molecular distillation.
1. The partial pressure of the residual gas must be very low so that the mean free path length of the residual gas is the difference in the distance between the distiller and the condenser surface.
2. At saturation pressure, the average free path length of the vapor molecules must be the same as the distance between the evaporator and the condenser surface.
Under this ideal condition, evaporation occurs from residual gas molecules without hindrance. The vapor molecules reach the condenser surface without encountering other molecules and returning to the liquid. The evaporation rate reaches a possible value at the temperature at which it is located. The evaporation rate is proportional to the pressure, and thus the amount of distillate in the molecular distillation is relatively small. In large and medium-sized distillation, the distance between the condenser and the heating surface is about 20 to 50 mm, and when the residual gas pressure is 10-3 mbar, the average free path length of the residual gas molecules is about 2 times longer.