The Methods of Distillation a Binary Systems | Distillation Definition

Definition of distillation: distillation unit operation in which the constituents of a liquid mixture (solution) are separated using thermal energy. Basically, the difference in vapour pressure (volatilities) of different constituents at the same temperature is responsible for such a separation. This unit operation is also termed fractional distillation or fractionation.

With this technique, it is possible to separate the liquid mixture into its components in almost pure form and this fact has made distillation perhaps the most important of all mass transfer operations.

In distillation the phases involved are

Liquid and vapour or gas (the vapour phase is created by supplying heat to the liquid) and mass is transferred from both the phase to one another, by vaporization from the liquid phase and by condensation from the vapour phase. The net effect is an increase in the composition of the more volatile component in the vapour (phase) and that of the less volatile component in the liquid. The basic requirement for the separation of components by distillation is that the composition of the vapour be different from the composition of the liquid with which it is in the equilibrium-the vapour is always richer in the more volatile component than the liquid from which it is formed. If the vapour composition is the same as the liquid composition, the distillation technique will not affect separation.

The methods of distillation

Distillation is commonly encountered in chemical and petroleum industries as a means of separating the liquid mixture into its component parts. Separation of ethanol and water mixture, production of absolute alcohol and from 95% ethanol using benzene, and separation of petroleum crude into gasoline, kerosene, fuel oils etc. are typical examples of distillation.

Evaporation is concerned with the separation of a solution containing a non-volatile solute and volatile solvent, whereas distillation is concerned with the separation of a solution where all the components are appreciably volatile. Thus, the separation of solution where all the components are appreciably volatile. Thus, the separation of brine into salt and water is evaporation whereas the separation of a mixture of alcohol and water into its components is distillation.  

The methods of distillation -a binary systems

Basically, distillation is carried out in two ways:

The liquid mixture to be separated is heated to create a vapour. The vapour formed is condensed in a condenser and withdrawn as a product. As there is no reflex, products of relatively low purities are obtained.

The liquid mixture to be separated is heated to create a vapour, and the vapour formed is condensed in a condenser. A part of the condensed liquid is returned to the distillation still ( as a reflex) and the remaining part is withdrawn as a product. In this method, the liquid and vapour are brought into intimate contact a number of times and an almost pure product can be achieved. The part of the vapour returned as a liquid to the distillation unit is called reflux and the operation is called rectification or fractionation. The term rectification urinated in the alcohol industry whereas the term fractionation is popular in the petroleum industry.

Standard methods used in distillation practice are

Out of these three methods, distillation with rectification or simply called rectification is the most important. The first two methods are carried out without reflux and the third one is carried out with reflex (which is nothing but returning a part of the condensed liquid back to the distillation system).

(i)                  Differential or simple distillation

(ii)                Flash or equilibrium distillation

(iii)               Rectification or fractionation.

(iv)        Steam Distillation process

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

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