Name: Prem Shanker R.
Date: 2001 - 2002
What is vacuum distillation?
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, let´s
suppose for a moment that you want to separate two
liquid substances using distillation process. But, let´s
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 !!!
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.
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.
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.
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.
PG Research Foundation, Darien, Illinois
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Update: June 2012