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EXTRACTION TECHNOLOGIES FOR MEDICINAL AND AROMATIC PLANTS
8.5 Molecular Distillation or Short Path
Distillation
Molecular distillation (MD), also known as short path distillation,
is a fairly well established technique. In view of this, the discussion of
MD is restricted to its principle, advantages and applications in the processing
of MAPs.
8.5.1 Principle of MD
The term MD refers to a non-equilibrium process. The still used
has an evaporating surface very close to a condensing surface. Under very
low pressures, this results in a situation where the distance traveled by
the evaporating molecules is comparable to the mean free path of the molecules.
The nomenclature MD is derived from this particular condition under
which the so-called distillation is carried out.
8.5.2 Advantages of MD
1. Operating pressures as low as 0.001 mbar can yield relatively
low processing temperatures, thereby reducing thermal
degradation.
2. Agitated fi lm MD units can process high viscosity feeds with
very good turndown.
3. Combination of low pressures and high temperatures (up to
300° C) allows processing of extremely high-boiling materials
without degradation.
4. Short exposure to high temperature (low residence time)
prevents degradation.
5. Very low liquid hold-up allowing use in applications involving
low volume, high value materials.
6. Available in low (laboratory scale) to high heat transfer areas
to suit the requirements.
8.5.3 Separation Efficiency of MD
For high viscosity liquid fi lms falling under gravity, agitated fi lm
MD units perform far better than those without agitation of the fi lm. This is
due to the fact that, particularly for high viscosity liquids, the agitation of the
fi lm renews the surface more frequently than when there is no agitation. The
surface renewal model is useful for predicting the effi ciency η of MD stills
without mechanical control.
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