Separation process of Components Inside Distillation column / What Happens Inside Distillation Column

Phenomenon of partial vaporization and partial condensation brings about enrichment in the column.
If a mixture of benzene and toluene is heated in a vessel, closed in such a way that the pressure remains atmospheric and no material can escape and the mole fraction of  the more volatile component in the liquid, that is benzene, is plotted as abscissa, and the temperature at which the mixture boils as ordinate, then the boiling curve is obtained as shown by ABCJ in the following figure. The corresponding dew point curve ADEJ shows the temperature at which a vapor of composition y starts to condense.

     If a mixture of composition x2 is at a temperature T3 below its boiling point, T2, as shown by point G on the diagram, then on heating at constant pressure the following changes will occur:

a) When the temperature reaches T2, the liquid will boil, as shown by point B, and some vapor of composition y2, shown by point E, is formed.

b) On further heating the composition of the liquid will change because of the loss of the more volatile component to the vapor and the boiling point will therefore rise to some temperature T'. At this temperature the liquid will have a composition represented by point L, and the vapor a composition represented by point N. Since no material is lost from the system, there will be a change in the proportion of liquid to vapor, where the ratio is

Liquid / Vapor = MN / ML

c) On further heating to a temperature T1, all of the liquid is vaporized to give vapor D of the same composition y1 as the original liquid.

     It may be seen that partial vaporization of the liquid gives a vapor richer in the more volatile component than the liquid. If the vapor initially formed, as for instance at point E, is at once removed by condensation, then a liquid of composition x3 is obtained represented by point C. The step BEC may be regarded as representing an ideal stage, since the liquid passes from composition x2 to a liquid of composition x3, which represents a greater enrichment in the more volatile component than can be obtained by any other single stage of vaporization.

     Starting with superheated vapor represented by point H, on cooling to D condensation commences, and the first drop of liquid has a composition K. Further cooling to T' gives liquid L and vapor N. Thus, partial condensation brings about enrichment of the vapor in the more volatile component in the same manner as partial vaporization. The industrial distillation column is, in essence, a series of units in which these two processes of partial vaporization and partial condensation are effected simultaneously.