Distillation is a common laboratory practice used to isolate and purify liquids on the basis of their volatilities characterized by boiling point differences. Distillation, however, results only in partial isolation of liquids present in mixtures.
It is important to have clarity on boiling point concept as well as the boiling points of constituents of mixture of liquids. Boiling point of a liquid is the temperature at which the vapour pressure of a liquid equals the atmospheric pressure. On reaching the boiling point the partial pressures of all the components contribute to the overall vapour pressure. In other words the liquid mixture will have a single boiling point and not several boiling points. The proportion of each component present in the vapour will depend on its volatility. Thus the vapour will comprise of all the components present in the mixture but the proportion of the more volatile component will be highest.
The common laboratory distillation processes are:
- Simple Distillation
- Fractional Distillation
- Steam distillation
- Azeotropic Distillation
The different types of distillations are briefly discussed in the article.
Simple distillations are useful in resolving components of liquid mixtures having boiling points differing by \(25-30^0C\).The vapour generated is led through a condenser to a collection vessel where it is liquefied. The liquefied component will not be 100% pure but will have a higher proportion of the more volatile component. The distillation should be stopped when the temperature of the vapour begins to rise above the boiling point of the required component. For liquids having closer boiling points the collected liquid can be subjected to one or more successive stages of distillation.
Fractional distillation is essentially same as simple distillation and is useful when the boiling points of constituent liquids lie within a narrower range. The distillation process is repetitive but takes place inside the condenser tube .It affords saving of time over the repetitive simple distillations and is more efficient than multiple steps in simple distillations. The same re-distillation is affected repetitively inside the fractionating column.
The rising vapour recondenses on the rings, plates or packings inside the condenser tube. It gets revaporized by the rising hot vapour. Each vaporization- condensation cycle enriches the vapour with the volatile component.
Steam distillation should be preferred over fractional distillation for separation of heat sensitive compounds which can degrade on the uncontrolled heating of the solid supports inside the fractionating column. Steam on the other hand promotes the same purpose in a more controlled manner. However, the collected distillate is in two layers, namely, an oily layer and water layer which will require further steps to isolate the pure compound.
Compounds having very high boiling point require higher temperatures for distillation to be effective. The stability of the compound can be retained if pressure inside condenser tube is lowered instead of raising the temperature. Lowering the temperature lowers the boiling point of the higher boiling component. Vacuum distillation is generally carried out by making use of a rotary evaporator.
Interactions at molecular level in liquid mixtures can result in alteration of the boiling point of the mixture. The mixture boils at a temperature which is lower or higher than the boiling points of the constituent pure compounds. On reaching the boiling point a constant composition vapours results which distil over without separation of the original components. A common example is ethanol and water which forms an azeotrope of 96.5% at \(78.1^0 C\) .The azeotropic composition can be altered if further purification is required. A drying agent, such as potassium carbonate, can be added to remove the residual water and distil the ethanol to higher purity levels.
The same laboratory scale preparations form the basis of large scale industrial distillation processes which provide separations of tons of liquid fractions on daily basis. Such industrial separations are common in petroleum, petrochemicals and organic chemical industries.