Molecular distillation is a type of
short-path vacuum distillation
Vacuum distillation or distillation under reduced pressure is a type of distillation performed under reduced pressure, which allows the purification of compounds not readily distilled at ambient pressures or simply to save time or energy. This te ...
, characterized by an extremely low vacuum pressure, 0.01
torr
The torr (symbol: Torr) is a Pressure#Units, unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (unit), atmosphere (101325 Pa). Thus one torr is exactly (≈ ).
Historically, one torr was intended to be ...
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 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
Free molecular flow describes the fluid dynamics of gas where the mean free path of the molecules is larger than the size of the chamber or of the object under test. For tubes/objects of the size of several cm, this means pressures well below 10− ...
regime, i.e. the
mean free path
In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
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.
This process has the advantages of avoiding the problem of toxicity that occurs in techniques that use solvents as the separating agent, and also of minimizing losses due to thermal decomposition. and can be used in a continuous feed process to harvest distillate without having to break vacuum.
Molecular distillation is used industrially for purification of oils. It is also used to enrich
borage oil in
γ-linolenic acid (GLA) and also to recover
tocopherol
Tocopherols (; TCP) are a class of organic compounds comprising various methylated phenols, many of which have vitamin E activity. Because the vitamin activity was first identified in 1936 from a dietary fertility factor in rats, it was named ''t ...
s from
deodorizer
Air fresheners are products designed to reduce unwanted odors in indoor spaces, to introduce pleasant fragrances, or both. They typically emit fragrance to mask odors but may use other methods of action such as absorbing, bonding to, or chemically ...
distillate of
soybean oil
Soybean oil (British English: soyabean oil) is a vegetable oil extracted from soybean (''Glycine max'') legumes. It is one of the most widely consumed cooking oils and the second most consumed vegetable oil. As a drying oil, processed soybean oil ...
(DDSO).
Molecular stills were historically used by
Wallace Carothers
Wallace Hume Carothers (; April 27, 1896 – April 29, 1937) was an American chemist, inventor, and the leader of organic chemistry at DuPont, who was credited with the invention of nylon.
Carothers was a group leader at the DuPont Experimen ...
in the synthesis of larger
polymers
A polymer () is a substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeating subunits derived from one or more species of monomers. Due to their broad spectrum of properties, b ...
, as a reaction product, water, interfered with polymerization by undoing the reaction via hydrolysis, but the water could be removed by the molecular still.
[pg113, ''Enough for One Lifetime: Wallace Carothers, Inventor of Nylon'', Hermes 1996]
"Du Pont Strikes Pay Dirt at Purity Hall: Groundbreaking studies by Wallace Carothers paved the way for the development of nylon and neoprene."
''Today's Chemist''
See also
*
References
{{Authority control
Distillation
Separation processes
Laboratory techniques