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Name: Prem Shanker R.
Status: other
Age: 40s
Location: N/A
Country: N/A
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 !!!

Alcir Grohmann

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

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

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.
Assistant Director
PG Research Foundation, Darien, Illinois

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