Molecular distillation is a distillation process that occurs under high vacuum conditions. This allows molecules to travel through the vapor phase without intermolecular collisions before condensing individually on a nearby surface. There are two main types of molecular stills - falling film stills and centrifugal stills. Falling film stills use a heated surface and wipers to create a thin liquid film for evaporation, while centrifugal stills rotate a bucket at high speeds to create an evaporating liquid film on the inner wall. Both utilize a short path between evaporation and condensation surfaces to minimize molecular collisions in the vapor phase. Molecular distillation is useful for separating compounds with similar boiling points.

1. MOLECULAR DISTILLATION

2. MOLECULAR DISTILLATION
Evaporative distillation
 short path distillation

3. MOLECULAR DISTILLATION
• Molecular distillation is defined as a
distillation process in which each molecule in
the vapor phase travels mean free path and get
condensed individually without intermolecular
collisions on application of vacuum

4. PRINCIPLE
• Mean free path of a molecule is defined as
the average distance through which a
molecule can move without coming in to
collision with another
• Mean free path

5. PRINCIPLE
P vapor pressure ,Kpa
density
viscosity
mean path length

6. MOLECULAR DISTILLATION
It is necessary to design the equipment based on
the requirement of the molecular distillation
• The evaporating surface must be close to the
condensing surface
This ensure the molecule come in contact with the
condenser as soon as they leave the evaporating
surface – so it is known as short path distillation

7. MOLECULAR DISTILLATION
• High vacuum is applied (0.1-1.0 Pascal) so
intermolecular distance increases and thus
molecular collisions minimized
• The liquid surface area must be as large as
possible .so that vapor evolved from the
surface without boiling
• So it is called evaporative distillation

8. MOLECULAR DISTILLATION
• Based on the method of formation of liquid
film, apparatus is divided in to two types
 falling film molecular still or wiped film
molecular still
Centrifugal molecular still

9. FALLING FILM MOLECULAR STILL

10. FALLING FILM MOLECULAR STILL
PRINCIPLE
• WIPED FILM MOLECULAR STILL
• Vaporization occurs from a film of liquid
flowing down a heated surface under high
vacuum
• The vapor travels a short distance and strikes
the condenser nearby

11. FALLING FILM MOLECULAR STILL
PRINCIPLE
• Each molecule is condensed individually
• The distillate is collected

12. CONSTRUCTION
• It consists of a jacketed vessel – 1m diameter
• Wipers are provided adjacent to the vessel
wall (evaporating surface)

13. CONSTRUCTION
• Vacuum pump is connected to a large pipe at
the centre of the vessel
• Provisions for collecting distillate and un-
distilled liquid residue at the bottom

14. WORKING
• The vessel is heated by suitable means
• Vacuum is applied at the centre of the vessel
• Wipers are allowed to rotate

15. WORKING
• Feed is entered through the inlet of the vessel
• As liquid flows down the walls, it spread to
form a film by PTFE wipers(moving a rate of
3m/s)

16. WORKING
• The velocity of the film is 1.5m/s
• Since the surface is heated liquid film
evaporates directly
• The vapor travels its mean free path and strike
the condenser

17. WORKING
• The condensate is collected in to a vessel
• The residue is collected from the bottom of
the vessel and recirculated through the feed
port for further distillation

18. CENTRIFUGAL MOLECULAR STILL

19. CENTRIFUGAL MOLECULAR STILL
PRINCIPLE
• Liquid feed introduced in to a vessel which is
rotated at very high speed
• On heating vaporization occurs from a film on
the side of the vessel wall

20. PRINCIPLE
• The vapor travels a short distance and get
condensed on the adjacent condenser
• Each molecule condensed individually
• Distillate is subsequently collected

21. CONSTRUCTION
• A bucket shaped vessel – 1 to 1.5 m diameter
• Rotated at high speed using a motor
• Radiant heaters are provided externally to
heat the fluid in the bucket
• Condenser arranged very close to the
evaporating surface

22. CONSTRUCTION
• vacuum pump is connected to the entire vessel at
the top
• Provisions for
– introducing the feed in to the centre of the
bucket
– discharging the residue
– collecting the distillate

23. CONSTRUCTION
• Radiant heaters provide heat so the liquid
directly evaporate from the film
• The vapor travels its mean free path and
strikes the condenser
• The condensate is collected in to another
vessel

24. CONSTRUCTION
• The residue collected from the bottom of the
vessel and is recirculated through the feed
port for further distillation

25. DISADVANTAGE
• Construction and operation are more
complicated compared to falling film still

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