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.
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condensed in a condenser and withdrawn as a product.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
