Molecular Distillation

Molecular Distillation

Molecular distillation is carried out without boiling at very low pressures of the order 0.001 mmHg. At these pressures, collision of molecules in the evolving vapor and reflection back to the liquid surface are greatly decreased and the mean free path of the molecules is of the same order as the distance between the evaporating surface and a condenser placed a short distance away. It then becomes possible to distill liquids of very high boiling point, although the degree of separation cannot exceed one theoretical plate. The process is therefore used primarily to concentrate nonvolatile components in a high–boiling point medium. The vitamins in cod liver oil can be concentrated in this way. For the separation of liquids of comparable volatility, several separate distillation stages will be necessary.

Since agitation due to boiling is absent, an alternative method of main-taining the more volatile component at the evaporating surface must be adopted. In the industrial molecular still shown in Figure 10.8, the feed is introduced at the bottom of a heated conical rotor and flows upward as a thin liquid layer under the action of centrifugal force. The residue is caught in a gutter at the top. The vapor is condensed on a concentric, water-cooled con-denser a short distance away and discharged.