Fractionating Column :
A fractionating column or fractionator consists of (i) a cylindrical shell divided into sections by a series of perforated trays, (ii) a reboiler and (iii) a condenser. The liquid mixture to be separated is introduced in the cylindrical column more or less centrally. The column itself is divided into two sections - rectifying and stripping section. The section above the feed plate or tray is called the rectifying section, wherein the vapour is washed to remove the less volatile component with the liquid returned to the column from top (known as reflux). The portion below the feed plate including the feed plate is called the stripping section wherein the liquid is stripped off more volatile component by rising vapour. Perforated trays are nothing but gas-liquid contacting devices on which gas/vapour and liquid are brought into intimate contact for mass transfer to occur.
Fig. Fractional Distillation Column
Vapours are generated in a reboiler (generally steam heated) and are fed to the bottom of the column. The liquid removed from the fractionator rich in the less volatile component is called the bottoms or bottom product. The vapour issuing from the top of the column is fed to a condenser where the latent heat is removed with the help of a circulated coolant through the condenser. A part of the condensed liquid is returned to the column (reflux) and the remaining part is withdrawn as the top product or distillate which is rich in the more volatile component. As we move up the column, the vapour becomes richer and richer in the more volatile component and as we move down the column, the liquid becomes richer and richer in the less volatile component. As the liquid is at its bubble point and the vapour is at its dew point, temperature is maximum at the bottom and minimum at the top. The part of the condensed liquid returning to the top of the column is called as reflux.