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Distillation

water vapour chemical condenser steam mixture liquid vapours heat distillate

DISTILLATION, a generic term for a class of chemical operations which all agree in this, that the substance operated upon is heated in a close vessel (" retort," " still ") and thereby wholly or,partially converted into vapour, which vapour is then condensed, by the application of cold, in another apparatus (the " condenser '') connected with the vessel, and allowed to collect in a third portion of the apparatus, called a " receiver." In most cases the substance isla liquid, or assumes the liquid form previous to emitting vapours, and the product obtained (the " distillate ") is also in greater proportion liquid. The comparatively few and special cases of distillation, wherein solids are converted into vapours which condense directly from the gaseous into the solid form, are designated. " sublimations." Thus we speak of the "distillation " of water or of spirits, while we speak of the "sublimation " of sal-ammoniac. Distillations may be divided into two classes - viz., 1st, those which are not, and 2d, those which are, accompanied by chemical changes. The word " distillation," in a narrower sense, is generally understood to apply to the first class only. The gecond might be called " destructive distillations," if it were not customary to reserve this, term for the particular case in which the substance operated on consists of vegetable or animal matter which is being decomposed by the application of heat alone, i.e., without the aid of re-agents, The general object of simple distillation is the separation of substances of different degrees of volatility. The apparatus used varies very much according to the nature of the substance operated on and of the product extracted, and according to the scale on which the operation is carried out. Of the various contrivances used in chemical laboratories, the simplest is a glass retort, the descending neck of which is inserted into, and goes to near the bottom of, a slanting globular flask. The retort serves for the reception of the substance to be distilled, and is heated by means of char-coal or gas fire ; the vapours pass into the flask, which is kept cool by a continuous current of cold water running over it, or, in the case of more volatile substances, by being immersed in ice or some freezing mixture. This somewhat primitive arrangement works satisfactorily only when the vapours are easily condensible, and when the product is meant to be collected as a whole. In the majority of cases, however, the distillate bas to be " fractionated," i.e., collected in a number of separate, consecutive portions; and it is then desirable that the apparatus should be so constructed as to enable one at any moment to examine the distillate as it is coming over. For this purpose -it is necessary to condense the vapours on their way to, and not within, the receiver, so that the latter can, at any time, be removed and, replaced by another. 'The condenser most generally used in chemical laboratories is that known as Lielny's condenser. It consists of a straight glass or metal tube, 1 to 3 feet long and to 1 inch wide, fitted co-axially, by means of corlss or india-rubber tubes, into a wider tube (made of glass or iron) which communicates at the lower end with a water tap, and at the upper with a sink, so that a stream of cold water can be made to run aganst the current of the vapour. The condenser tube is fixed in a slanting position, and the vapours made to enter at the upper end. The dimensions of the condenser and rate of water-flow depend on the speed at which the vapour is driven over, and on the temperature of that vapour, and, last not least, on the latent heat of the tTapour and specific heat of the distillate. To show the importance of the last-named point, let us compare the quantities of heat to be withdrawn from 1 lb of steam. and 1 lb of bromine vapour respectively, to reduce them to liquids at 0° C. We have in the case of water and bromine - Water. Bromine.

For the temperature of the vapour... 100° 63° For the latent heat 536° 45°.6 For the specific heat of the liquids 1° 0°.106 For the total heats of the vapours 636° 52°.3 /100 drawal of x 52.3 = ) 83 units from the steam, as an easy calculation shows, only 0.10 lb of liquid water, of even 100', could be produced - hence vzore than 0.84 lb of steam remains uncondensed (at a temperature of about 96° C., assuming the steam to remain saturated, and to have the temperature of the condensed water). But obviously a condenser under all circumstances is the more efficacious the greater its surface and the thinner its body. It is also obvious, cceteris paribus, that the most suitable material for a condenser tube is that which conducts heat best. Hence a metal tube will generally condense more rapidly than one of glass, and for metal tubes copper is better than tin, and silver better than either. In chemical laboratories glass is the 0„ly material which is quite generally applicable. In chemical works, on the other hand, glass, on account of its fragility, is rarely -used ; condensers there, wherever possible, are made of metal, usually fashioned into spirals (" worms ") and set in tub-shaped refrigerators Where acids have to be condensed, stoneware worms are generally employed. In the distillation of acetic acid pla-tinum worms, notwithstanding their high price, have been found to work best, and in the long run to be cheapest.

The theory and successful execution of the process assume their greatest simplicity when the substances to be separated differ so greatly in their volatility that, without appreciable error, one Call be assumed to be non-volatile at the boiling point of the other. A good illustration of this special case is afforded by the customary- process used for the purification of water. A natural sweet water may in general be assumed to consist of three parts - lst, water proper, which always forms something like 98 per cent. or more of the whole ; 2d, non-volatile salts ; 3d, gases. To obtain pure water from such material, we need only boil it in a distillation apparatus, so as to raise from it dry steam, which steam when condensed yields water con-taminated only with the gases. To expel these all that is necessary is to again boil it for a short time ; the gases go off with the first portions of steam, so that the residue, when allowed to cool in absence of air, constitutes pure water. To pass to a less simple case, let us assume that the substance to be distilled is a solution of ether in water and the object is the separation of these two bodies. Ettier boils at 35° C., water at 100° C. The elastic force of saturated steam at 35° is 42 = 744_ = of an atmosphere. Assuming now the mixture to be distilled from a flask, what will go on? Neglecting for the sake of simplicity the small tension of the steam at 35°, we should expect that at first the ether would simply boil away, so to speak, from a, bath of warm water at .35° C. ; that the vapour would be pure ether, and maintain that composition until all the ether had boiled off ; then there would be a break - the tempera-ture of the liquid would gradually rise to 100°, and the water then distil over in its turn. And so it is approxi-mately, but not exactly. Our theory obviously neglects some important points. Water at 35° has a tension of th atmosphere, ether of one atmosphere ; hence the two saturated vapours together should press with a force of of its volume of ether vapour, and 0.06 of its volume of steam, supposing both substances to have the same chances of forming saturated vapour, which, of course, holds only so long as they both are present in appreciable quantities. We easily see that, as the distillation progresses, the ether vapour must get more and more largely charged with vapour of water, until at last what goes off is steam, con-taminated with less and . less of ether vapour. A thermometer placed near the entrance end of the condenser will, of course, record lower than one plunged into the boiling liquid, because the vapour in rising undergoes partial condensation, and the thermometer being bedewed with the condensed vapour will approximately indicate tho boiling point of that dew, i.e., of that which is just going over. The composition of the vapour as above given inust not be confounded with the composition by weight of the distillate. To obtain the latter we must multiply each of the two volumes by the density of the respective vapour, or,. what COIlleA to the same thing, by its molecular weight as expressed by the chemical formula. In our case the vanour volume ratio This consideration strips of its apparently anomalous character what we observe when vegetable substances con-tainin, essential oils are distilled with water, when we find that tlpiese oils, although boiling far above 100° C., go over with the first fractions of the water. Take the case of lemon oil, which boils at about 174° C. The molecular weight of the oil is 136 its vapour tension at = ; 100° is 70 inm. Hence what goes over at first when lemon peel is distilled with water should contain oil and water in the proportion - DISTILLATION The oil, although the less volatile substance of the two, being present in small quantity, but finely- diffused, is soon completely driven over. No doubt the latent heats of vaporization of the two constituents lia-ve some-thing to do with the composition of the vapour formed, as the chance of every particle of the mixture to be va.porized is obviously the greater the less its latent heat of vaporization.

After what has been said it will be clear that in the dis-tillation of a mixture of two substances of approximately equal molecular weight and latent heats of vaporization, supposing neither to predominate overwliehningly over the other, the one with the lower boiling point will predominate in.the early, and the other will gradually a.ccrimulate in the later, fractions of the distillate. And similarly with mixtures of three or more bodies. The further the respec-tive boiling points are removed. from one another the more complete a separation can be effected ; but in no case is the separation perfect. It is, however, easily- seen that the analytic effect of a distillation can be increased by causing the vapour, before it reaches the condenser, to undergo partial condensation, when naturally the less volatile parts chiefly will run back. This artifice is largely employed by chemists, technical as well as scientific. The simplest mode is to let the vapour ascend through a lour?, vertical tube before it reaches the condenser, and to distirso slowly that a sufficiently large fraction of the vapour originally formed fails to survive the ascent through the cooling influence of the atmosphere. A more effective method is to let the condensed vapour accumulate in a series of small receptacles insertedbetween flask and condenser, constructed_ so that the vapour cannot pass through the receptacles without bubbling through their liquid contents, and so that the liquid in the receptacles cannot rise above a certain level, the excess flowing back into the next lower receptacle or into the still. But the most effective method is to let the vapour aicend through a slanting condenser kept by means of a bath at a certain temperature, which is controlled so that while the liquid in the flask- boils rapidly, the dis-tillation only just progresses and no more.

The general principles flaw stated regarding fractional distillation are liable to not a few exceptions, of which the following may be cited as examples. A solution of one part of hydrochloric acid gas in four parts of water boils (constant) at 110° C. - i.e., 10° above the boiling point of water, although the acid constituent is an almost permanent gas. This, however, is easily explained ; there can be no doubt that such an acid is a mixture of real hydrates, i.e., does not contain either free water or free hydrochloric acid. A. similar explanation applies to the case of aqueous oil of vitriol, which boils the further above 100° the stronger it is, although the vapour may be, and in the case of acids COD tain-mg less than 84 per cent. of real acid really is, pure steam. The following cases, however, can scarcely be disposed of by the assumption of the interference of chemical action. Propyl alcohol boils at 97° C., water at 100° ; and yet a mixture of the two, as Pierre and Puchot found, when distilled always commences to boil at 88°.5 with formation of a distillate of the approximate composition C31%0 + 2.78H20 ; and this particular aqueous alcohol boils without apparent decomposition at 88°•3. Some time later Dittmar and Steuart made a precisely analogous observation with regard to aqueous allyl alcohol. A strong temptation exists to explain these anomalies by the assumption of definite hydrates in the aqueous alcohols, and this hypothesis would serve in the meantime were it not for the curious fact, discovered by the two French chemists named, that amyl alcohol and water (two liquids which do not mix), when distilled simultaneously out of the same retort, go over at a constant temperature less than 100', and with formation of a distillate which, although it is not even a mixture, has a constant composition. The most natural explanation of these phenomena is to assnme them to be owing, not to chemical action, but rather to an exceptional absence of chemical affinity between the two components of the mixture, which for once gives the physical forces fair play.

DRY (DESTRUCTIVE) DISTILLATION. - Of the great number of chemical operations falling under this head, we can notice only those which are carried out industrially for the manufacture of useful products. Of such the most important are those in which wood, coal, shale, and bones form the materials operated upon. But as these processes form so many important industries, which have all special articles devoted to them, we must confine ourselves here to summing up shortly the features common to all.

In all cases the " retorts " consist of iron or fire-clay semi-cylinders placed horizontally in a furnace and con-nected by iron pipes with refrigerators, and through these with gas-holders. Within these retorts the materials are brought up, more or less gradually, to a red heat, which is maintained until the formation of vapours practically ceases. Each of the materials pained is a complex mixture of different chemical species. Wood consists mainly of cellulose and other carbo-hydrates, i.e., bodies composed of carbon aud the elements of water ; coal and shale the combustible part consists of compounds of carbon and hydrogen, or carbon, hydrogen, and oxygen, richer in carbon than the components of wood ; bones consist of about half of incombustible and infusible phosphate of lime (bone earth) and half of organic matter, of which the greater part is gelatine (conipounels of carbon, nitrogen, hydrogen, and oxygen), and the lesser is fat (compounds of carbon, hydrogen, and oxygen). The chemical decomposi-tion in each case is highly- complex. An infinite variety of products is invariably fonned, which, however, always readily divide into three :-1st, a non-volatile residue, con-sisting of mineral matter and elementary carbon (" wood charcoal," " coke," &c.) which, in the case of animal matter, contains chemically combined nitrogen ; 2d, a part condensible at ordinary temperatures which always readily separates into two distinct layers, viz. : - (a) an aqueous portion (" tar-water "), and (b) a semifluid, viscid, oily, or resinous portion (" tar ") ; and 3d, a gaseous por-tion.

The " tar-water " is the one, of all the four products, of which the qualitative composition most directly depends on the nature of the material distilled. In the case of wood it has an acid_ reaction, from the presence in it of acetic acid, which is associated (amongst many other things) with a-cetone and methyl alcohol. In the case of coal it is alkaline, from ammonia, present as carbonate, sulphide, sulphocyanide, and in other forms. Alcohols and oxygenated acids are absent.

The " tar" is a complex mixture of carbon com-pounds, all combustible, but, although all directly derived from a vapour, not by any means all of them vo/ati/e. (Regarding the components, see TAR.) The quantity and quality of the tar naturally depend on the kind of material used, but perhaps yet more on the mode in which the dis-tillation is conducted. Thus, for instance, a coal tar pro-duced at low temperature contains a considerable per-centage of paraffins. If, on the other hand, the dis-tillation is conducted at a high temperature, the paraffins ere almost absent, while the proportion of benzols con-siderably increases. A similar remark applies to the gaseous portion, as will readily be understood when we say that all volatile tar constituents, when passed through red hot tubes, are decomposed with formation of hydrogen and gaseous hydrocarbons, which latter again, when submitted to the same operation, are all liable to undergo dissociation into simpler compounds and associa-tion into more complex.

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