Molecular distillation is
a type of short-path vacuum distillation, characterized by extremely low vacuum
pressure (≈ 0.01 torr). It is a process of separation, purification, and
concentration of natural products, complex and thermally sensitive molecules.
This process is characterized by short-term exposure of the distillate liquid
to high temperatures in a high vacuum 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. 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, the 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.
Principle of Molecular Distillation
Molecular distillation is
considered the safest mode of separation and to purify the thermally unstable
molecules and related compounds with low volatility and elevated boiling
points. The process distinguishes the short residence time in the zone of the
molecular evaporator exposed to heat and low operating temperature due to
vacuum in the space of distillation. The separation principle of molecular
distillation is based on the difference of molecular mean free path. The
passage of a free path for molecules should be collision-free.
Langmuir and Knudsen
derived an equation that describes the yield of distillate for molecular
distillation.
`Q=k\times P\times F\left(\frac MT\right)^\frac1 2 ...(1)`
Where,
- Q is the evaporated quantity kg/h,
- P = pressure (mbar),
- F = evaporator surface (m2 ), k = 1577,
- M = molecular weight g/mole and
- T is absolute temperature (K).
Theoretically, molecular
distillation could be used for separating mixtures that have a close boiling
point or forming azeotrope.
According to the
principle of thermodynamics under certain temperature and pressure conditions,
the molecular mean free path (L) is expressed as;
`L=\frac{0.707K\times T}{\pi d^2P} ...(2)`
Where,
- D is the effective molecular diameter;
- P is molecular space in which the pressure;
- T is the molecular environment temperature;
- K is the Boltzmann constant.
Construction of Molecular Distillation
A simple molecular
distillation has a unit that is placed on a hot surface. The distillate moves a
very short distance before it gets condensed, Fig.1. If the substance is not
too viscous, it will drip from the point on the glass condensing surface and
run down to the receiving point. The sophisticated apparatus with a different
design will have the liquid distilled down on a heated surface close to the
condenser. The movement of a film prevents a build-up of non-volatile materials
on the surface of the material to be distilled as this might cause the
distillation to stop.
Fig.1: Typical Molecular Distillation Unit
Centrifugal Molecular Distillation
This is a technique of
purification applied under molecular distillation utilized worldwide for food
processing, pharmaceutical applications, petroleum industries, and chemical
industries. The main principle behind the unit is low pressure and very short
residence time. Degassing of feed material leads to the next stage where the
material is flown into a spinning disc which is pre-heated. The entire process
of distillation gets over in less than a fraction of a second because the
material that is fed into expands on the spinning pre-heated disc. The
distillate finally condenses on the outer extremes of the shell and then slowly
flows into the collecting vessel due to gravity. The residual matter is
collected in the gutter around the spinning disc and finally into the
collecting vessel below Fig.2. The molecular distillation process occurs at a
very low temperature and hence can avoid thermal decomposition. The high vacuum
applied helps in eliminating the oxidation due to exposure to atmospheric air.
The free path distillation is carried out at a very low pressure of 10−2
Torr while in molecular distillation the pressure is kept at 10−3
Torr.
Fig.2: Centrifugal Molecular Distillation Unit
Working of Molecular Distillation
The molecular
distillation process is carried out at very low pressure so that the distance
between the hot and condensing surface is less than the mean free path of the
molecules. Each of the units is a single-stage but has several units in series.
Molecular distillation is applied to thermally sensitive high molecular weight
materials. The contact times in commercial units may be low as 0.001 seconds.
The film thickness is of the order of 0.05 – 0.1 mm. In vacuum operations, air
ingress is very much possible, whereas in pressure operations vapor emissions
are likely to occur. The distillation process is inherently hazardous with
flammables and the presence of a huge volume of flammables in reboilers, in
column internals and adjacent piping pose huge explosion hazards in these
distillation units. The concentration gradient between the top and bottom of
the column has a bearing on safety. The concentration of impurities in the
column can lead to hazards.
Advantages of Molecular Distillation
- Toxicity: Avoids the problem of toxicity of solvents used as the separating agent.
- Thermal stability: Minimizes losses due to thermal decomposition.
- Continuous process: It can be used in a continuous feed process to harvest distillate without having to break the vacuum.
- Stability: The vacuum allows oils to be processed at minimal temperatures, reducing the risk of oxidative damage.
- Purity: Separating the oil’s components by weight allows contaminants to be reduced far below industry standards.
- Concentration: Weight grouping allows the processor to concentrate fatty acids.
- Short residence time: This process has a short residence time of the feed liquid.
- It works at a significantly lowered temperature due to high vacuum capability
- It has optimal efficiency in mass and heat transfer.
- It is suitable for processing high-value products.
Disadvantages of Molecular Distillation
- Cost: The cost for this complicated technology is relatively high.
- Natural form: The starting natural triglyceride form is lost in the distillation process.
Applications of Molecular Distillation
- It is used for the separation of vitamins and polyunsaturated fatty acids.
- Molecular distillation is used industrially for the purification of oils.
- It is also used to enrich borage oil in γ-linolenic acid (GLA) and recover tocopherols from deodorizer distillate of soybean oil.
- It can be used for the production of synthetic and natural vitamin E,
- Capsicum red pigment containing 1% to 2% of the solvent can be separated after two-stage molecular distillation.
- It is used for the separation of strong spices like volatile substances.
- It is used for highly heat-sensitive materials.
- It is a common process in deodorization, decolonization, and purification.
- It is used for deoxidation, level off odor or purification, and bleaching or purification.
- It is used in fractionation of dimers of fatty acids, separation of radioactive nuclides from melts of irradiated media, lanolin purification, preparation of highly concentrated monoglycerides, recovery of carotenoids from palm oil,
- It is used in the synthesis of pure diglicyde ether of bisphenol-A.
Make sure you also check our other amazing Article on : Steam Distillation