Ask A Scientist Chemistry Archive

name         Prem Shanker R.
status       other
age          40s
Question -   What is vacuum distillation?
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  Hi, Prem !!!
  Well, as you know, water boils at 100 degree Celsius
  under the atmospheric pressure. At this temperature,
  the molecules of water acquire sufficient energy level
  and can leave the bounding forces acting in water as
  a liquid, and reach the vapour phase, where dominates a
  higher energy level.
  On the other side, if you heat water at a condition where
  the pressure is lower than the atmospheric pressure, then
  the boiling temperature will be lower. In other words,
  to leave the liquid phase and go into the vapour phase,
  the molecules need NOT so energy as before.
  A pressure under the atmospheric pressure is called vacuum.
  If you heat a liquid solution of two or more substances
  the temperature will reach a level where they boil, and
  go into the vapour phase. It is likely that each of them boils
  at certain temperature and then the first one reaches the
  vapour. Later on, the second reaches the vapour phase at
  a higher temperature... and so on !! If the vapours is cooled,
  it becomes liquid again. Such process is called distillation.
  If this process is conducted under vacuum, then you have
  vacuum distillation !!
  What are the advantages of this process ?? Well, letīs
  suppose for a moment that you want to separate two
  liquid substances using distillation process. But, letīs
  suppose that they can be destroyed by higher level of
  temperature !!!  Then, to slow down the temperature,
  you use vacuum, and so it is possible to separate them
  without destroying them !!!
  At the oil refinery industry, for instance, the residue of
  the atmospheric distillation column follows to the vacuum
  column, to recover more diesel and gasoil ( raw material for
  lubricants ). The residue of the vacuum column follows
  to the FCC process ( Fluidised Catalitic Column ) to produce
  more LGP, gasoline and diesel. The residue of this column
  becomes asphalt for the streets or oil for combustion in
  heat vessels. But...this is another history !!!
  Regards
  Alcir Grohmann
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Vacuum distillation is used for compounds that have high boiling points
(usually above 200 °C).  These compounds tend to decompose at the T(bp)
required at atmospheric pressure.  Since pulling a vacuum on the flask will
reduce the temperature required to have boiling you avoid decomposition.

-Darin Wagner
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 > Distillation occurs when the vapor pressure of the liquid equals the
applied pressure. Water boils at 100 C, or there about, because of the
applied pressure from the atmosphere (ignoring solubility of atmospheric
gases). When the applied pressure is reduced (using a vacuum pump), the
applied pressure decreases, and so the liquid boils at a lower temperature.
In the case of water for example if the vacuum pump is capable of reducing
the applied pressure to 25 mm of Hg, the water will boil at about 25 C. The
25/25 pressure/temperature is just a coincidence.

Vince Calder
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Distillation under vacuum.  I.E., the boiling flask, the condenser, and
the collection flask are all at sub-atmospheric pressure.  This allows
the distillation to occur at lower temperatures so there is less chance
that the molecules will be decomposed by heat.
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Distilling a liquid at a pressure less than one atmosphere.  It requires
applying a vacuum to the apparatus to lower the pressure.  It is used
because substances boil at lower temperatures when under vacuum, so the
distillation can be carried out at a lower temperature.  This saves heating
costs and also keeps sensitive substances from decomposing under the high
temperatures that would be required at atmospheric pressure.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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