Simple Distillation Process | Differential Distillation process

Simple distillation in this technique, a known quantity of a liquid mixture is charged in to a jacketed kettle or still. The jacket is provided for heating the liquid mass in the still with the help of a heating media such as steam. The charge is boiled slowly, vapours formed are withdrawn and fed to a condenser where they are liquified and collected in a receiver a distillate. 

In the early stage of distillation, vapours leaving the still are richest in the more volatile component and as the distillation proceeds the still becomes lean with respect to the more volatile component. 

The composition of the less volatile component thereby increased and hence the boiling point increases. The product from such units can be collected in several receivers, called cuts, to give products of various purities over the length of distillation period. 

The distillation is continued till the boiling point of liquid reaches a predetermined value and the content of the still is finally removed as residual liquid containing majority of the less volatile component.

Material balance-a binary mixture

As the composition of the vapour issuing from the distillation still and that of liquid remaining in it changes during the course of operation, the mathematical approach should be differential. 

Let F be the kmol of a liquid mixture (A+B) containing xF mole fraction of A which is charged to a distillation still. Let D be kmol of distillate and W be kmol of residual liquid in the still which are obtained at the end of operation. Let yDavg and xW be the mole fraction of A in distillate and bottom residual liquid. 

Let L be kmol of liquid in the still at any time during the course of distillation and let x be the mole fraction of A in the liquid. Let a very small amount dD kmol of the distillation of composition y in equilibrium with the liquid is vaporised. Then, the composition and the quality of liquid decreases from x to x-dx and L - dL respectively. 

Then, 

The overall materials balance at any times is

L = L - dL + dD

dL = dD

If yD, avg is the composition distillate composition then. 

Material balance of the component A is:

xF. F = yD, avg .D + xw. W

Thought the simple or differential distillation as a method of separation is not effective many such units are used, especially where

(i) the component to be separated have widely different boiling points. 

(ii) methods giving sharp separations are not necessary. 

>Molecular diffusion in solids, liquid and gases 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane