What is Molecular distillation?

Molecular distillation is a type of short-path vacuum distillation, characterized by an extremely low vacuum pressure, 0.01 torr or below, which is performed using a molecular still. It is a process of separation, purification and concentration of natural products, complex and thermally sensitive molecules for example vitamins and polyunsaturated fatty acids. This process is characterized by short term exposure of the distillate liquid to high temperatures in high vacuum (around 10−4 mmHg) in the distillation column and a small distance between the evaporator and the condenser around 2 cm.In molecular distillation, fluids are in the free molecular flow regime, i.e. the mean free path of molecules is comparable to the size of the equipment.The gaseous phase no longer exerts significant pressure on the substance to be evaporated, and consequently, rate of evaporation no longer depends on pressure. The motion of molecules is in the line of sight, because they do not form a continuous gas anymore. Thus, a short path between the hot surface and the cold surface is necessary, typically by suspending a hot plate covered with a film of feed next to a cold plate with a line of sight in between. 

Principles of Molecular Distillation

According to the principle of thermodynamics shows that, under certain temperature and pressure of the molecular mean free path L=0.707K*T/(πd2P), in D for effective molecular diameter; P for molecular space in which the pressure; T for the molecular environment temperature; K is the Boltzmann constant (Fu and Chen, 1979).

By the molecular mean free path of the movement of the formula, we can see that different kinds of molecules, because of the difference of the effective diameter of the molecule, the mean free path is different, that is, different kinds of escaping from the molecular level not and other molecular collision of flight distance is not the same. Light molecular mean free path, small molecular mean free path, if in away from the liquid surface is smaller than the mean free path of molecular light rather than heavy molecular mean free path set a condensing surface, making molecular light falling on the condensing surface chant condensation, and heavy molecules reach the condensing surface and return to the original level, so as to realize the mixture separation.

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