The mode of heating varies with the substance to be distilled. For highly volatile liquids, e.g. ether, ligroin, &c., immersion of the flask in warm water suffices; for less volatile liquids a directly heated water or sand bath is used; for other liquids the flask is heated through wire gauze or asbestos board, or directly by a Bunsen. The condensing apparatus must also be conditioned by the volatility. With difficulty volatile substances, e.g. nitrobenzene, air cooling of the retort neck or of a straight tube connected with the distilling flask will suffice; or wet blotting-paper placed on the tube and the receiver immersed in water may be used. For less volatile liquids the Liebig condenser is most frequently used. In its original form, this consists of a long tube surrounded by an outer tube so arranged that cold water circulates in the annular space between the two. The vapours pass through the inner tube, and the cold water enters at the end farthest from the distilling flask. For more efficient condensation—and also for shortening the apparatus—the central tube may be flattened, bent into a succession of V’s, or twisted into a spiral form, the object in each case being to increase the condensing surface. Of other common types of condenser, we may notice the “spiral” or “worm” type, which consists of a glass, copper or tin worm enclosed in a vessel in which water circulates; and the ball condenser, which consists of two concentric spheres, the vapour passing through the inner sphere and water circulating in the space between this and the outer (in another form the vapour circulates in a shell, on the outside and inside of which water circulates). A very effective type is shown in fig. 2. The condensing water enters at the top and is conducted to the bottom of the inner tube, which it fills and then flows over the outside of the outer tube; it collects in the bottom funnel and is then led off. The vapours pass between the inner and outer tubes.
Practically any vessel may serve as a receiver—test tube, flask, beaker, &c. If noxious vapours come over, it is necessary to have an air-tight connexion between the condenser and receiver, and to provide the latter with an outlet tube leading to an absorption column or other contrivance in which the vapours are taken up. If the substances operated upon decompose when heated in air, as, for example, the zinc alkyls which inflame, the air within the apparatus is replaced by some inert gas, e.g. nitrogen, carbon dioxide, &c., which is led in at the distilling flask before the process is started, and a slow current maintained during the operation.
2. Distillation under Reduced Pressure.—This method is adopted for substances which decompose at their boiling-points under ordinary pressure, and, generally, when it is desirable to work at a lower temperature. The apparatus differs very slightly from that employed in ordinary distillation. The “receiver” must be connected on the one side to the condenser, and on the other to the exhaust pump. A safety vessel and a manometer are generally interposed between the pump and receiver. For the purpose of collecting the distillates in fractions, many forms of receivers have been devised. Brühl’s is one of the simplest. It consists of a number of tubes mounted vertically on a horizontal circular disk which rotates about a vertical axis in a cylindrical vessel. This vessel has two tubulures: through one the end of the condenser projects so as to be over one of the receiving tubes; the other leads to the pump. By rotating the disk the tubes may be successively brought under the end of the condenser. Boiling under reduced pressure has one very serious drawback, viz. the liquid boils irregularly or “bumps.” W. Dittmar showed that this may be avoided by leading a fine, steady stream of dry gas-air, carbon dioxide, hydrogen, &c., according to the substance operated upon—through the liquid by means of a fine capillary tube, the lower end of which reaches to nearly the bottom of the flask. “Bumping” is common in open boiling when the liquid is free from air bubbles and the interior of the vessel is very smooth. It may be diminished by introducing clippings of platinum foil, pieces of porcelain, glass beads or garnets into the liquid. “Frothing” is another objectionable feature with many liquids. When cold, froth can be immediately dissipated by adding a few drops of ether. In boiling liquids its formation may be prevented by adding paraffin wax; the wax melts and forms a ring on the surface of the liquid, which boils tranquilly in the centre.
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| Wurtz. | Linnemann. | Le Bel-Henninger. | Glynsky. | Young. | Kreusler. |
| Fig. 3. | |||||
3. Fractional Distillation.—By fractional distillation is meant the separation of a mixture having components which boil at neighbouring temperatures. The distilling flask has an elongated neck so that the less volatile vapours are condensed and return to the flask, while the more volatile component passes over. The success of the operation depends upon two factors: (1) that the heating be careful, slow and steady, and (2) that the column attached to the flask be efficient to sort out, as it were, the most volatile vapour. Three types of columns are employed: (1) the elongation is simply a straight or bulb tube; (2) the column, properly termed a “dephlegmator,” is so constructed that the vapours have to traverse a column of previously condensed vapour; (3) the column is encircled by a jacket through which a liquid circulates at the same temperature as the boiling-point of the most volatile component. To the first type belongs the simple straight tube, and the Wurtz tube (see fig. 3), which is simply a series of bulbs blown on a tube. These forms are not of much value. Several forms of the second type are in use. In the Linnemann column the condensed vapours temporarily collect on platinum gauzes (a) placed at the constrictions of a bulbed tube. In the Le Bel-Henninger form a series of bulbs are connected consecutively by means of syphon tubes (b) and having platinum gauzes (a) at the constrictions, so that when a certain amount of liquid collects in any one bulb it syphons over into the next lower bulb. The Glynsky form is simpler, having only one syphon tube; at the constrictions it is usual to have a glass bead. The “rod-and-disk” form of Sidney Young is a series of disks mounted on a central spindle and surrounded by a slightly wider tube. The “pear-shaped” form of the same author consists of a series of pear-shaped bulbs, the narrow end of one adjoining the wider end of the next lower one. In this class may also be placed the Hempel tube, which is simply a straight tube filled with glass beads. Of the third type is the Warren column consisting of a spiral kept at a constant temperature by a liquid bath. Improved forms were devised by F. D. Brown. Kreusler’s form is easily made and manipulated. A tube closed at the bottom is traversed by an open narrower tube, and the arrangement is fitted in the neck of the distilling flask. Water is led in by the inner tube, and leaves by a side tube fused on the wider tube. Many comparisons of the effectiveness of dephlegmating columns have been made (see Sidney Young, Fractional Distillation, 1903). The pear-shaped form is the most effective, second in order is the Le Bel-Henninger, which, in turn, is better than the Glynsky. The main objection to the Hempel is the retention of liquid in the beads, and the consequent inapplicability to the distillation of small quantities.
4. Distillation with Steam.—In this process a current of steam, which is generated in a separate boiler and superheated, if necessary, by circulation through a heated copper worm, is led into the distilling vessel, and the mixed vapours condensed as in the ordinary processes. This method is particularly successful in the case of substances which cannot be distilled at their ordinary boiling-points (it will be seen in the following section that distilling with steam implies a lowering of boiling-point), and which can be readily separated from water. Instances of its application are found in the separation of ortho- and para-nitrophenol, the o-compound distilling and the p- remaining behind; in the separation of aniline from the mixture obtained by reducing nitrobenzene; of the naphthols from the melts produced by fusing the naphthalene monosulphonic acids with potash; and of quinoline from the reaction between aniline, nitrobenzene, glycerin, and sulphuric acid (the product being first steam distilled to remove any aniline, nitrobenzene, or glycerin, then treated with alkali, and again steam distilled when quinoline comes over). With substances prone to discolorization, as, for example, certain amino compounds, the operation may be conducted in an atmosphere of carbon dioxide, or the water may be saturated with sulphuretted hydrogen. Liquids other than water may be used: thus alcohol separates α-pipecoline and ether nitropropylene.
5. Theory of Distillation.—The general observation that under a constant pressure a pure substance boils at a constant temperature leads to the conclusion that the distillate which comes over while the thermometer records only a small variation is of practically constant composition. On this fact depends “rectification or purification by distillation.” A liquid boils when its vapour pressure equals the superincumbent pressure (see [Vaporization]); consequently any process which diminishes the external pressure must also lower the boiling-point. In this we have the theory of “distillation under reduced pressure.” The theory of fractional distillation, or the behaviour of liquid mixtures when heated to their boiling-points, is more complex. For simplicity we confine ourselves to mixtures of two components, in which experience shows that three cases are to be recognized according as the components are (1) completely immiscible, (2) partially miscible, (3) miscible in all proportions.
When the components are completely immiscible, the vapour pressure of the one is not influenced by the presence of the other. The mixture consequently distils at the temperature at which the sum of the partial pressures equals that of the atmosphere. Both components come over in a constant proportion until one disappears; it is then necessary to raise the temperature in order to distil the residue. The composition of the distillate is determinate (by Avogadro’s law) if the molecular weights and vapour pressure of the components at the temperature of distillation be known. If M1, M2, and P1, P2 be the molecular weights and vapour pressures of the components A and B, then the ratio of A to B in the distillate is M1P1/M2P2. Although, as is generally the case, one liquid (say A) is more volatile than the other (say B), i.e. P1 greater than P2, if the molecular weight of A be much less than that of B, then it is obvious that the ratio M1P1/M2P2 need not be very great, and hence the less volatile liquid B would come over in fair amount. These conditions pertain in cases where distillation with steam is successfully practised, the relatively high volatility of water being counterbalanced by the relatively high molecular weight of the other component; for example, in the case of nitrobenzene and water the ratio is 1 to 5. In general, when the substance to be distilled has a vapour pressure of only 10 mm. at 100° C., distillation with steam can be adopted, if the product can be subsequently separated from the water.
When distilling a mixture of partially miscible components a distillate of constant composition is obtained so long as two layers are present, i.e. A dissolved in B and B dissolved in A, since both of these solutions emit vapours of the same composition (this follows since the same vapour must be in equilibrium with both solutions, for if it were not so a cyclic system contradicting the second law of thermodynamics would be realizable). The composition of the vapour, however, would not be the same as that of either layer. As the distillation proceeded one layer would diminish more rapidly than the other until only the latter would remain; this would then distil as a completely miscible mixture.
The distillation of completely miscible mixtures is the most common practically and the most complex theoretically. A coordination of the results obtained on the distillation of mixtures of this nature with the introduction of certain theoretical considerations led to the formation of three groups distinguished by the relative solubilities of the vapours in the liquid components.