DISTILLATION is a special application of the process of evaporation which has for its object the separation of the component parts of a complex or compound substance. The vapori zation is done generally by heat with, in some cases, the assistance of a vacuum, and the vapors are collected and condensed generally into a liquid form. When the resulting distillate is a solid and no liquid form has intervened the process is called "sublimation.* The apparatus required in a simple distilling plant is: (1) the still or retort in which the vapors are produced; (2) the condenser in which the vapors are liquefied, and (3) the re ceiver in which the product or distillate is col lected.
The process of distillation is applied to both liquid and solid substances. In the latter case it is called dry distillation when the substance dis tilled does not liquefy in the operation.
The theory of distillation depends on the fact that a simple substance boils or vaporizes at a certain constant temperature —or, in other words, when its vapor pressure exceeds by the smallest fraction the atmospheric pressure upon it. As a rule the various components of a com plex substance will vaporize each at its indi vidual boiling point. By collecting separately the different vapors which come over from the retort at different temperatures the several com ponents are secured. There are some substances which decompose at the normal atmospheric pressure when raised .to a temperature high enough to vaporize them. In order to distil such substances it is necessary to reduce the pressure in the retort by producing therein a partial vacuum,. which has the effect also of lowering the boiling point. Vaporization of a substance below its boiling point may also be accomplished by passing a current of steam through it.
Another economic use of distillation is the concentration of volatile substances throw repeated distillations, as in the case of e rectification of alcohol and the production of glycerine.
Among the articles in common use made by the distillation process are gasoline, kerosene, lubricating oils, paraffin wax, coal-tar color intermediates, aniline oil, alcohol, spirituous liquors, perfumes, essential oils, glycerine, cam phor, ether, chloroform, turpentine, sulphuric acid and many others.
When distillation is properly conducted many liquids as for instance, water, can be purified.
Distillation of Water is never found absolutely pure in nature; it generally contains salts and other mineral substances, organic matter and decomposition products, some volatile and some non-volatile. The water is readily separated from the non-volatile com ponents by distillation. Inasmuch as the volatile constituents are highly evaporative they are received in the very first distillate of the water, so that it is customary to reject this first por tion. Experience has taught that pure water is obtained by distilling three-fifths of the entire amount and rejecting the first one-fifth. In
some special cases where a water has many volatile impurities which can be detected by its appearance, odor and taste, the distillation might be rendered more difficult as it becomes neces sary to add certain chemicals to the water in order to combine with the volatile components and transform them into non-volatile bodies. The purifying of water by means of distillation, or, in other words, the preparation of distilled water, is of much practical value. In the phar macy, in the chemical laboratory and in many chemical industries distilled water is an indis pensable article. The transformation of the salt water of the ocean into a potable water by means of distillation is also of inestimable value. Pure distilled water, however, has often an empyreumatic odor and a repulsive flat taste. According to the most recent medical investiga tions pure distilled water when constantly used is, on account of its very purity, deleterious to health. It is claimed that this water possesses high solvent properties and also absterges the mucous membrane of the stomach too much. For this reason the odor of the water is improved by thorough aeration or carbonization, and it is made palatable by the addition of pure salt or sugar or any other desired substance. The large ocean steamers have an equipment for producing potable water from sea water. The fundamental elements in this apparatus are a steam generator or evaporator in which the sea water is vapor ized by means of superheated steam, which is obtained either from a special boiler or directly from the engine boiler, a condenser combined with an aerator and a refrigerator with which in many cases a filter is combined. In recent times many ships supply themselves with an especially good spring water for drinking, while for cooking, washing and boiler feeding they distil the sea water as it is required. For these latter purposes carbonating and aerating are of course unnecessary. In most cases the evapo rator is connected directly with the ship's en fine. The sea water which is to be evaporated is taken from the water cooling the condenser, while the steam is obtained from the steam jacket or an intermediate compartment of the engine. In this way the cylinder is constantly drained and supplied with fresh steam. One form of still utilizes the escaping gases for heating the water so that the entire apparatus is situated in the chimney. In general the distilla tion of water is very simple, because it is a liquid which has a constant boiling point. More heat in the distillation process simply occasions more rapid evaporation. The escaping steam, unless it is heated in a special apparatus as is done in the case of superheated steam, retains the boiling point temperature until it is cooled.