Several modes of heating are adopted. In some cases, especially in dry distillations, the furnace flames play directly on the retorts, in others, such as in the case of nitric acid, the whole still comes under the action of the furnace gases to prevent condensation on the upper part of the still, while in others the furnace gases do not play directly on the base or upper portion of the still but are conducted around it by a system of flues (see [Coal-Tar]). Steam heating, dry or live, is employed alone and also as an auxiliary to direct firing.
The condensing plant varies with the volatility of the distillate. Air cooling is adopted whenever possible. For example, in the less modern methods for manufacturing nitric acid the vapours were conducted directly into double-necked bottles (bombonnes) immersed in water. A more efficient arrangement consists of a stack of vertical pipes standing up from a main or collecting trough and connected at the top in consecutive pairs by a cross tube. By an arrangement of diaphragms in the lower trough the vapours are circulated through the system. As an auxiliary to air cooling the stack may be cooled by a slow stream of water trickling down the outside of the pipes, or, in certain cases, cold water may be injected into the condenser in the form of a spray, where it meets the ascending vapours. Horizontal air-cooling arrangements are also employed. A common type of condenser consists of a copper worm placed in a water bath; but more generally straight tubes of copper or cast iron which cross and recross a rectangular tank are employed, since this form is more readily repaired and cleansed. Wood-spirit, petroleum and coal-tar distillates are condensed in plant of the latter type. In cases where the condenser is likely to become plugged there is a pipe by means of which live steam can be injected into the condenser. The supply of water to the condenser is regulated according to the volatility of the condensate. When the vapours readily condense to a solid form the condensing plant may take the form of large chambers; such conditions prevail in the manufacture of arsenic, sulphur and lampblack: in the latter case (which, however, is not properly one of distillation) the chamber is hung with sheets on which the pigment collects. Large chambers are also used in the condensation of mercury.
Dephlegmation of the vapours arising from such mixtures as coal-tar fractions, petroleum and the “wash” of the spirit industry, is very important, and many types of apparatus are employed in order to effect a separation of the vapours. The earliest form, invented by C. B. Mansfield to facilitate the fractionation of paraffin and coal-tar distillates, consisted in having a pipe leading from the inclined delivery tube of the still to the still again, so that any vapour which condensed in the delivery tube was returned to the still. Of really effective columns Coupier’s was one of the earliest. The vapours rising from the still traverse a tall vertical column, and are then conveyed through a series of bulbs placed in a bath kept at the boiling-point of the most volatile constituent. The more volatile vapours pass over to the condensing plant, while the less volatile ones condense in the bulbs and are returned to the column at varying heights by means of connecting tubes. The French column is similar in action. The Coffey still is one of the most effective and is employed in the spirit, ammonia, coal-tar and other industries. It consists of a vertical column divided into a number of sections by horizontal plates, which are perforated so that the ascending vapours have to traverse a layer of liquid. Above this “separator” is a reflux condenser, termed the “cooler,” maintained at the correct temperature so that only the more volatile component passes to the receiver. The success of the operation chiefly depends upon the proper management of the cooler.
8. Commercial Distillation of Water.—Distilled water, i.e. water free from salts and to some extent of the dissolved gases which are always present in natural waters, is of indispensable value in many operations both of scientific and industrial chemistry. The apparatus and process for distilling ordinary water are very simple. The body of the still is made of copper, with a head and worm, or condensing apparatus, either of copper or tin. The still is usually fed continuously by the heated water from the condenser. The first portion of the distillate brings over the gases dissolved in the water, ammonia and other volatile impurities, and is consequently rejected; scarcely two-fifths of the entire quantity of water can be safely used as pure distilled water.
Apparatus for the economic production of a potable water from sea-water is of vital importance in the equipment of ships. The simple distillation of sea-water, and the production thereby of a certain proportion of chemically fresh water, is a very simple problem; but it is found that water which is merely evaporated and recondensed has a very disagreeable flat taste, and it is only after long exposure to pure atmospheric air, with continued agitation, or repeated pouring from one vessel to another, that it becomes sufficiently aerated to lose its unpleasant taste and smell and become drinkable. The water, moreover, till it is saturated with gases, readily absorbs noxious vapours to which it may be exposed. For the successful preparation of potable water from sea-water, the following conditions are essential:—1st, aeration of the distilled product so that it may be immediately available for drinking purposes; 2nd, economy of coal to obtain the maximum of water with the minimum expenditure of fuel; and 3rd, simplicity of working parts, to secure the apparatus from breaking down, and enable unskilled attendants to work it with safety. The problem is a comparatively old one, for we find that R. Fitzgerald patented a process in 1683 having for its purpose the “sweetening of sea-water.” A history of early attempts is given in S. Hales’s Philosophical Experiments, published in 1739. Among the earlier of the modern forms of apparatus which came into practical adoption are the inventions of Dr Normandy and of Chaplin of Glasgow, the apparatus of Rocher of Nantes, and that patented by Gallé and Mazeline of Havre. Normandy’s apparatus, although economical and producing water of good quality, is very complex in its structure, consisting of very numerous working parts, with elaborate arrangements of pipes, cocks and other fittings. It is consequently expensive and requires careful attention for its working. It was extensively adopted in the British navy, the Cunard line and many other important emigrant and mercantile lines. Chaplin’s apparatus, which was invented and patented later, has also since 1865 been sanctioned for use on emigrant, troop and passenger vessels. The apparatus possesses the great merit of simplicity and compactness, in consequence of which it is comparatively cheap and not liable to derangement. It was adopted by many important British and continental shipping companies, among others by the Peninsular & Oriental, the Inman, the North German Lloyd and the Hamburg American companies.
The modern distilling plant consists of two main parts termed the evaporator and condenser; in addition there must be a boiler (sometimes steam is run off the main boilers, but this practice has several disadvantages), pumps for circulating cold water in the condenser and for supplying salt water to the evaporator, and a filter through which the aerated water passes. The evaporator consists of a cylindrical vessel having in its lower half a horizontal copper coil connected to the steam supply. The cylindrical vessel is filled to a certain level with salt water and the steam turned on. The water vaporizes and is led from the dome of the evaporator to the head of the condenser. The water level is maintained in the evaporator until it contains a certain amount of salt. It is then run off, and replaced by fresh sea-water. The condenser consists of a vertical cylinder having manifolds at the head and foot and through which a number of tubes pass. In some types, e.g. the Weir, the condensing water circulates upwards through the tubes; in others, e.g. the Quiggins, the water circulates around the tubes. Various forms of the tubes have been adopted. In the Pape-Henneberg condenser, which has been adopted in the German navy, they are oval in section and tend to become circular under the pressure of the steam; this alteration in shape makes the tubes self-scaling. In the Quiggins condenser, which has been widely adopted, e.g. in the “Lusitania,” the steam traverses vertical copper coils tinned inside and outside; the coils are crescent-shaped, a form which gives a greater condensing surface and makes the coils self-scaling. The aeration of the water is effected by blowing air into the steam before it is condensed; as an auxiliary, the storage tanks have a false bottom perforated by fine holes so that if air be injected below it, the water is efficiently aerated by the air which traverses it in fine streams. After condensation the water is filtered through charcoal. The filter is either a separate piece of plant, or, as in the Quiggins form, it may be placed below the coils in the same outer vessel. In this plant the aeration is conducted by blowing in air at the base of the condenser. After filtration the water is pumped to the storage tanks. Many types of distilling plant are in use in addition to those mentioned above, for example the Rayner, Kirkaldy, Merlees, Normand; the United States navy has adopted a form designed by the Bureau of Engineering.
Bibliography.—The general practice of laboratory distillation is discussed in all treatises on practical organic chemistry; reference may be made to Lassar-Cohn, Manual of Organic Chemistry (1896), and Arbeitsmethoden für organisch-chemische Laboratorien (1901); Hans Meyer, Analyse und Konstitutionermittlung organischer Verbindungen (1909). The theory of distillation finds a place in all treatises on physical chemistry. Of especial importance is Sidney Young, Fractional Distillation (1903). The history of distillation is to be studied in E. Gildemeister and F. Hoffmann, Die ätherischen Öle (Berlin, 1899; Eng. tr. by E. Kremers, Milwaukee Press, 1900). The technology of distillation is best studied in relation to the several industries in which it is employed; reference should be made to the articles [Coal-Tar], [Gas], [Petroleum], [Spirits], [Nitric Acid], &c.
(C. E.*)
DISTRACTION (from Lat. distrahere, to pull asunder), a drawing away or apart; a word now used generally of a state of mind, to mean a diversion of attention, or a violent emotion amounting almost to madness.