Molecular Distillation - Pharmaceutical Engineering B. Pharm Third Semester PDF Notes

Molecular Distillation


• It is a distillation process in which each molecule in the vapour phase travels mean free path and gets condensed individually without inter-molecular collisions on application of vacuum

• Molecular distillation is based on the principle of the simple distillation with some modifications

• This is also called evaporation distillation or short path distillation


• The substances to be distilled have very low vapour pressures.  Examples are viscous liquids, oils, greases, waxy materials and high molecular weight substances. These boil at very high temperature.

• In order to decrease the boiling point of the liquids, high vacuum must be applied

• The vapour pressure above the liquid is much lower.

• At very low pressure, the distance between the evaporating surface and the condenser is approximately equal to the mean free path of the vapour molecules

• Molecules leaving the surface of the liquid are more likely hit the condenser surface nearby. Each molecule is condensed individually

• The distillate is subsequently collected


The mean free path of a molecule is defined as the average distance through which a molecule can move without coming into collision with another

The mean path (λ.) can be expressed mathematically as:

λ = η v 3 


where. p = vapour pressure, kPa

p = density, kg/m3

η = viscosity, Pa's

λ = mean path length, m

For example, mean path (heavy molecules) of butyl phthalate is about 30 mm and of olive oil is 20 mm when measured at a pressure of 0.1 pascal

The mean free path can be increased by decreasing the viscosity which can be obtained at high temperature and low pressure

Thus, nonvolatile substances may become volatile and distillation is possible

Requirements for design the equipment

• The evaporating surface must be close to the condensing surface.

This ensures the molecules to come in contact with the condenser as soon as they leave the evaporating surface

For this reason, this process is also known as short path distillation

• The molecular collisions should be minimized because they change the direction of the path of molecules

• In other words, intermolecular distances should be fairly high.

• It can be achieved under very high vacuum, usually of the order of 0.1 to 1.0 pascals

• The liquid surface area must be as large as possible as so that the vapour is evolved from the surface only, but not by boiling.

• Thus this process is also called evaporation distillation


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