VACCUM DISTILATION
Vacuum distillation is the process of lowering the pressure in the column above the
solvent to less than the vapor pressure of the mixture, creating a vacuum, and
causing the elements with lower vapor pressures to evaporate off.
Industrial distillation can be achieved in a variety of ways. If the elements of your
solution have distinct vapor pressures and boiling points, a basic distillation system
might be all you need. However, vacuum distillation is the perfect solution when
you have elements that decompose when heated at atmospheric pressure – or – that
have high boiling points and would have high system heating requirements.
Vacuum distillation is the process of lowering the pressure in the column above the
solvent to less than the vapor pressure of the mixture, creating a vacuum, and
causing the elements with lower vapor pressures to evaporate off. Lowering the
pressure below atmospheric results in lower temperature requirements to achieve
evaporation for elements of the distillate, lowering overall heating requirements for
the system.
Of course, there are trade-offs – while you can reduce the system temperature
requirements, you do have to add equipment (usually a pump) to create the vacuum
inside the distillation column. This is where an experienced distillation system
designer is a big help; determining which trade-offs in method and equipment will
result in the biggest savings overall.
PURPOSE OF VACCUM DISTILATION
Vacuum distillation is the technique used to separate higher boiling fractions of
crude oil. The underlying theory and the process are analogous to those used to
separate the lighter fractions in the atmospheric distillation process. The difference
between the two physical separation methods is that atmospheric distillation occurs
under atmospheric pressure, whereas vacuum distillation occurs at a significantly
reduced pressure, thus reducing the boiling point of a substance. Hence, high
boiling components can be boiled at lower temperatures, without the risk of
cracking. Vacuum towers are much shorter than atmospheric towers in order to
minimize the pressure differential from top to bottom. Generally, the pressure used
in vacuum distillation is in the range of 50 to 100 mmHg, although some
lubricating oil stocks may require even lower pressure operating conditions.
STEPS FOR VACCUM DISTILATION
Safety note: Inspect every piece of glassware to be used with the vacuum
distillation, checking for stars, cracks, or other weaknesses in the glass, as
these may allow for implosion when the pressure is reduced.
A stir bar needs to be used for bump prevention. Boiling stones cannot be
used with vacuum distillation as air trapped in the stone's pores is rapidly
removed under vacuum, causing the stones to fail to produce bubbles.
Although greasing is some what of a personal choice with simple and
fractional distillations, all joints must be greased in vacuum distillations or
the system will leak and fail to achieve a low pressure.
Begin assembly of the apparatus near the vacuum source. If using a water
aspirator, test to be sure that the aspirator works well as some are more
functional than others. To test an aspirator, apply thick vacuum hosing to the
nub on the aspirator, turn on the water and feel for suction at the end of the
hose with your finger.
A Claisen adapter should be included in the apparatus as solutions under
vacuum tend to bump violently.
Attach thick-walled tubing to the vacuum adapter on the distillation
apparatus and connect to a vacuum trap. A trap suitable for a water aspirator
is shown in , but a more substantial trap cooled with dry ice and acetone
should be used with a portable vacuum to prevent solvent vapors from
degrading the oil pump.
Connect the trap to the vacuum source (aspirator or vacuum pump). It is best
to not bend or strain the tubing as much as is practical, as this may create a
leak in the system.
Stir plate with wood block to allow for lowering of the apparatus, b) Active
distillation, c) Insulating the Claisen and three-way adapters with foil.
Insert a wood block or lab jack beneath the stirring plate to allow for
lowering of the heat source when the distillation is complete.
Begin the Distillation
Before heating, turn on the vacuum source to begin reducing pressure inside
the apparatus. There should not be a hissing sound or else there is a leak in
the system.
The purpose of reducing the pressure before heating is for removal of very
low-boiling liquids (e.g. residual solvent). If the system were heated at the
same time, the low-boiling liquids might boil violently in the flask.
If a manometer is available, take note of the pressure inside the apparatus.
This may be used to predict the boiling point of the sample.
When confident that the apparatus is adequately evacuated and any low-
boiling compounds have been removed, begin heating the sample.
If it is difficult to achieve more than a reflux, the Claisen and three-way
adapter can be insulated by wrapping them tightly with glass wool then
aluminum foil. Insulation allows the column to maintain heat and the sample
to remain in the gas phase longer. A small gap should be left in the
insulation near the distilling flask to "peek in" and make sure the stirring
mechanism continues to work properly.
Record the temperature over which material is collected, making sure the
value corresponds to a temperature when the thermometer bulb is fully
immersed in vapors. If a manometer is used, also record the pressure. If no
manometer is used, record the vacuum source (e.g. aspirator).
Pure liquids do not always distill at a constant temperature when under
vacuum, as variations in pressure so easily occur and affect the boiling
temperature. A range of 5oC is not uncommon for pure liquids. This is
especially true when the vacuum source is a water aspirator, where
variations in water flow alter the pressure.
If more than one fraction of distillate is desired, the distillation must be
stopped before changing the receiving flask (see the next section for how). If
available, a "cow" receiving flask can be used to collect different fractions
without ceasing the vacuum.
Slowly reinstate the atmospheric pressure into the flask by opening the pinch
clamp at the vacuum trap , or by removing the rubber tubing at the vacuum adapter
or aspirator . You will know the system is open to the atmosphere when there is an
increase in water flow at the aspirator, or if a hissing sound is heard. Then turn off
the vacuum source.
It is important to first cool the system before allowing air back in as the
superheated residue in the flask may react unexpectedly with oxygen in the air.
It is also important to first allow air back into the system before turning off the
vacuum source. If the vacuum is turned off first, sometimes changes in pressure
inside the apparatus (as it cools) cause back-suction. If a water aspirator is used,
this may cause water from the sink to be pulled into the vacuum line. The vacuum
trap prevents this back suction from ruining the distillate.
Disassemble and clean up the distillation apparatus as quickly as is practical, as the
joints can sometimes freeze if left connected for prolonged periods.
