Molecular Distillation

Molecular distillation may be considered as a special version of evaporative distillation in which the liquid is evaporated without boiling, but in such circumstances that the evaporating molecules reach the condenser surface without obstruction. Three conditions for molecular distillation are:

1. Pressure must be lower than 0.001 mmHg. This low pressure is required to ensure that the molecules do not collide with each other.

2. The distance between evaporation and condensation surfaces is of the same order of magnitude as the mean free path of the molecules and the free motion of the molecule is not mechanically hindered.

3. The temperature of the condenser surface should be between 50 and 100°C lower than that of the evaporation surface to prevent re-evaporation of molecules.

The mean free path X is given by:

j2%o2N

2NbQ0-5

n2-5d2w0-5 tan Nb nd.

Hede HeNb deHs den tan.

where a is the diameter of the molecule in centimetres and N is the number of molecules in 1 cm3. By reducing the pressure to very low values, N decreases, giving values for X of 1-3 cm, in the range of the distance between evaporator and condenser surface.

In molecular distillation the maximum or theoretical rate of evaporation was proposed by Langmuir in 1916:

where M is the molecular mass and T is the absolute temperature.

The relative volatility is given by:

p2 72 y Mj

From these two equations it may be seen that molecular weight of the compounds involved is an important consideration.

Equipment for molecular distillation and medium vacuum is expensive, but it is economically justified for the separation of high value products such as vitamins, fats, essential oils and hormone concentration.

Typical unit operations and applications for medium and high vacuum using WFE, short path

Table 5 Specific applications of low pressure distillation

Unit operations where wiped-film evaporators (WFE) are used

Evaporation Drying

Concentration Degassing

Distillation Reactions

Stripping Deodorizing

Heating

Distillation

Concentration

Application of WFE for distillation and concentration at pressure of 1-1000 mbar

Acrylonitrile Extract solutions

Essential oils Insulin

Amines caprolactam Peroxides

Quinoline derivatives Phospholipids

Dioctyl phthalate Pyrethrum

WFE for distillation and concentration of viscous products Fatty alcohols Resins

Waxes Honey

Glue

Polymers Lubricant oils Silicone fluids

WFE for distillation and concentration of products liable to encrust the heated surface

Used oils Contaminated effluents

Solvents containing impurities Residue products from rectification and evaporation plants equipment or molecular distillation are shown in Table 5.

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