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Chemistry · Glossary

What is Fractional distillation?

Also known as: theoretical plate · reflux ratio

Definition 7.13 University Chemistry — Year 2 · Chapter 7 — Binary Liquid–Vapour Diagrams and Distillation

Fractional distillation separates the components of a liquid mixture in a column where a rising vapour and a descending liquid are brought into repeated contact. A theoretical plate is a stage of the column whose leaving vapour and liquid are in equilibrium with each other. The reflux ratio is the ratio of the amount of condensate returned to the top of the column to the amount drawn off as distillate.

Left: a plate column. Liquid flows across each plate and down the downcomer to the plate below; vapour rises through the plates and bubbles through the liquid. The condensed vapour is partly returned as reflux; the reboiler at the bottom provides the vapour. Right: a distillation column of a refinery (Cologne, 2014), which holds dozens of plates. (Photograph: CEphoto, Uwe Aranas, CC BY-SA 3.0, Wikimedia Commons.)
Left: a plate column. Liquid flows across each plate and down the downcomer to the plate below; vapour rises through the plates and bubbles through the liquid. The condensed vapour is partly returned as reflux; the reboiler at the bottom provides the vapour. Right: a distillation column of a refinery (Cologne, 2014), which holds dozens of plates. (Photograph: CEphoto, Uwe Aranas, CC BY-SA 3.0, Wikimedia Commons.)
Left: a plate column. Liquid flows across each plate and down the downcomer to the plate below; vapour rises through the plates and bubbles through the liquid. The condensed vapour is partly returned as reflux; the reboiler at the bottom provides the vapour. Right: a distillation column of a refinery (Cologne, 2014), which holds dozens of plates. (Photograph: CEphoto, Uwe Aranas, CC BY-SA 3.0, Wikimedia Commons.)
Plates at total reflux for benzene–toluene, from a distillate of 0.95 down: each horizontal step goes from a vapour to the liquid in equilibrium with it (one plate), each vertical step from that liquid to the vapour of the plate below. Seven steps take the liquid below x_B = 0.05; Fenske’s formula gives 6.5.
Plates at total reflux for benzene–toluene, from a distillate of 0.95 down: each horizontal step goes from a vapour to the liquid in equilibrium with it (one plate), each vertical step from that liquid to the vapour of the plate below. Seven steps take the liquid below xB=0.05x_B = 0.05; Fenske’s formula gives 6.5.
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