Separation by distillation can be described with a boiling point diagram.
The important process of distillation can now be investigated. From the boiling point diagram one can see that if a small amount of vapour were removed from a liquid of composition a, the vapour would have a composition higher in the more voltaic component than the original solution a. such a single step is, of course, inadequate for any appreciable separation of two components unless they have extremely different boiling points. In practice, a process of fractional distillation is used, in which the separation step is just described is, in effect, repeated by condensing some of the vapour, boiling off some vapour from this new liquid, collecting and vaporizing this product, and so forth. This procedure has the effect of stepping across the boiling point diagram.
The efficiency of a distillation column is determined by the number of theoretical plates to which the separation it performs corresponds. For example, a column supplied with a charge of composition a, is operated at total reflux until equilibrium is established. A small sample of distillate is then drawn off and analyzed and has, say, composition b. the separation that has resulted corresponds to four evaporations and condensations, and the column is said to have four theoretical plates.
For a solution showing a maximum vapour pressure and a maximum boiling point, the distillation process is indicated by the dashed lines. Regardless of the initial solution, distillation in a fractional distillation unit results ultimately in a distillate of the composition of the maximum boiling point mixture. One or the other of the pure components could be prepared only by working with the residue. The most important commercial solution that shows this behavior is the water ethanol system. Fermentation processes result in an ethanol concentration of about 10 percent. The object of distillation is to increase this concentration and possibly to yield pure ethanol. The boiling point diagram shows that distillation at atmospheric pressure can yield, at best, a distillate of 95 percent ethanol.
A different situation arises with the solutions that shows a maximum in their boiling points curves, like the system of such a solution which is merely boiled away, the residue will approach the composition corresponding to the maximum of the boiling point curve and the boiling point at this temperature and will not been reached, the remaining solution will boil at this temperature and will not change its composition.
Although in the case of an azeotrope we are dealing with a constant temperature constant composition boiling mixture, this mixture is not to be regarded as a compound that is formed between the two components. A change in the total pressure is usually sufficient to show that the azeotropic composition can be changed.