4 replies to this topic

[siretb]

Well, it has do to with the very basics of distillation and chemical engineering. Please refer to a book.
You may view reflux as a plain requisite for separation. If , for the sake of explanation, we assume there is almost no reflux, even if the liquid is in equilibrium with the vapors, because the amount of liquid is almost zero, the vapor composition will not change from tray to tray. Reflux is , in a way, a stream that will absorb the composition change in the vapor. This explains, why, for a given separation there is a minimum reflux.
Of course, in addition to that, the technology we use calls for a minimum wetting rate, so in the upper part of a column (above the last feed) , reflux is needed.

[gvdlans]

Zili,

You posted many questions lately on distillation columns, some of which are indeed quite basic. I agree with Siretb that it may be a good idea for you to buy or borrow some good books on the subject. I can recommend H.Z. Kister "Distillation Design" New York [etc.] : McGraw-Hill, 1989. - XIII, 710 p. ISBN: 0-07-034909-6.

[niallmacdowell]

Zili,
one book that I feel explains the necessity for a reflux in a distillation column is McCabe, Smith & Harriott's Unit Operations of Chemical Engineering.
Basically, the reason you use reflux/rectification in distillation is to get a purer distillate. Without reflux, your top/rectification section of your column is just a piece of pipe. What happens is that the LIQUID that is refluxed is rich in the desired top product. It is contacted with the VAPOUR that is rising, which contains some of (for binary distillation) both of the top and bottoms products. As the top product will have a lower bp that the bottoms, upon contacting the vpr stream, some of the reflux will vapourise, and at the same time some of the bottoms product will condense, and fall back down the column. See that reflux is vital to the act of separation, mechanical operating considerations aside.
Niall