Continuous Fractionating Still

Fractionating Still
Fig. 3-23

The "recycling" principle at the top of a Compound Still applies all the way down inside a Continuous Fractionating Still. The Coffey still mentioned previously was an early move in this direction. Changing the rectifier section to a packed column with a reflux condenser on top creates the modern Continuous Fractionating Still used in many industries like oil refining and industrial alcohol production, better known as a "Fractionating Column".

In a long column at equilibrium, each fraction (component of a mix)will concentrate at a level corresponding to its boiling point. The temperature above this point is lower than this fraction can exist as a vapor, but the higher temperature below makes it difficult for that fraction to remain liquid. Picture it as a hot "fountain" of material that rises as far as it can as a mixture of liquid and vapor. The top of this "fountain" is behaving in the same way as the "reservoir" we made at the top of a compound still by putting a reflux condenser on top, and the constantly re-circulating material at the top of the "fountain" becomes very rich in the component that has reached the limit of how far up it can go. The column of a continuous still has many such "fountains" inside it, each representing a particular fraction. Of course, as these are not individual "fountains", but are mixed up together, none of these fractions are completely pure, because more volatile compounds are passing through on their way to the top. However, by extracting fractions at the top of their individual "fountains", we can ensure that we have the richest concentration possible.

Feeding the raw material into the middle of the column improves the quality of the separation. The upper half of the column deals only with the lightest and most volatile materials, while the lower half deals only with the heaviest and least volatile. The lighter materials do not have to pass through zones of all the heavier ones, and purer fractions are collected.

These regions are usually named for the principle component extracted there, eg. propanol, naphtha, etc. Industrial production is usually more interested in quantity than absolute purity, so the slight contamination of the fractions is not a problem. When completely pure compounds are needed, smaller quantities may be purified in other equipment.

If you are thinking that this might be a good way of running a small still, please remember the word we underlined at the beginning: "LONG". There just isn't the room inside a small, 1 meter long column to set up the dynamic equilibrium described above. A commercial column can be as high as a 20 floor building! The best we can do is use a small compound still and draw off the fractions one at a time from the top. Mind you, this is not really a disadvantage, for it's the fraction that reaches the top that is really pure!

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Responses

  • anacleto
    What the vapor temperature in fractionating still?
    8 years ago

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