I.

Introduction

Distillation, process of heating a liquid until its more volatile constituents pass into the vapor phase, and then cooling the vapor to recover such constituents in liquid form by condensation. The main purpose of distillation is to separate a mixture of several components by taking advantage of their different volatilities, or the separation of volatile materials from nonvolatile materials. In evaporation and in drying, the purpose usually is to obtain the less volatile constituent; the more volatile constituent, in most cases water, is discarded. In distillation, on the other hand, the principal object of the operation is to obtain the more volatile constituent in pure form. The removal of water from glycerin by vaporizing the water, for example, is called evaporation, but the removal of water from alcohol by vaporizing the alcohol is called distillation, although similar apparatus is used in both cases.

If the difference in volatility (and hence in boiling point) between the two constituents is great, complete separation may be easily accomplished by a single distillation (see Boiling Point). Seawater, for example, which contains about 4 percent dissolved solids (principally common salt), may be readily purified by vaporizing the water, condensing the steam thus formed, and collecting the product, distilled water. This product is, for most purposes, equivalent to pure water, although actually it contains some impurities in the form of dissolved gases, the most important of which is carbon dioxide.

If the boiling points of the constituents of a mixture differ only slightly, complete separation cannot be achieved in a single distillation. An important example is the separation of water, which boils at 100° C (212° F), and alcohol, which boils at 78.5° C (173° F). If a mixture of these two liquids is boiled, the vapor that rises is richer in alcohol and poorer in water than the liquid from which it came, but it is not pure alcohol. In order to concentrate a 10 percent solution of alcohol (such as might be obtained by fermentation) to obtain a 50 percent solution (common for whiskey), the distillate must be redistilled once or twice, and if industrial (95 percent) alcohol is desired, many redistillations are needed.