In the English and Scotch systems, the receivers are changed for collecting what is known as " second light oils," and there is no doubt as to the advantage of this, especially where large quantities of tar are treated ; by the French method, no fraction is obtained corresponding to this.
The distillation is carried on until a few drops of the distillate will sink in water ; but it is better to stop the collection of second light oils at an earlier stage, for the distillation of some tars, as those yielding paraffin, can be carried on at a very high temperature without yielding a product heavier than water. It is better to use a thermometer, and to change the receiver as soon as the temperature is sufficiently high. With ordinary English tar, the first and second light oils are collected until the distillate sinks in water, when the receivers are changed for the " heavy oils," " creosote oils," or " creosote." As the product now contains much naphthalene, which may choke the condenser, the latter must be kept warm ; diminishing the flow of cold water will secure this under ordinary circumstances, but it may be necessary to heat the condensing worm, if large quatitities of naphthalene are present.
After a time, the distillate changes again, and has a green colour ; this is known as " anthracene," or "green oils," and is collected in a separate receiver. Towards the end, the distillate becomes black, and the receivers are finally changed for what are termed " last runnings." The residue contained in the retort constitutes pitch, which, while still liquid, is drawn off (or blown out by superheated steam) into a closed tank, to cool down to about 204° (400° F.), before being placed in its ultimate receptacles. The reason for this is that its temperature is so high that it would readily ignite, if immediately exposed to the air.
Neglecting tbe ammonia, which has been already treated of at p. 232, the products of the fractional distillation of coal-tar are:— 1. " First light oils,'' with water and ammonia, passing over at .. 100° (212° F.).
2. " Second light oils," or " once run" (or crude) naphtha, passing over at 149° (300° F.).
3. " Heavy oils," or " creosote oils," or " creosote" 171° (340° F.).
9, 71 4. " Green oils," or " anthracene" 204° (400° F.).
5. " Black oils " 425° (800° F.).
6. Pitch, left as a residue.
The temperatures given are approximately those at which the receivers are changed. On the French system, the fractions are collected at slightly different temperatures. The next consideration will be the products derived from each of the above-nientioned groups, omitting all theoretical or purely scientific elements, as well as compounds of no immediate importance.
Once-run (or Crude) Naphtha.—Thie liquid is almost colourless when freshly drawn over, but rapidly acquires a brownish tinge, by oxidation, on exposure to the air and light. It is rather curious that those constituents of coal-tar which are so readily affected by light appear to have escaped the attention of the scientific photographer. A test tbr the purity of rectified naphtha is its non-changeability in colour on exposure to light ; and the treatment to which it is subjected is for the purpose of removing those principles which render it liable so to change. If the receiver has been changed after the water has passed over, the crude naphtha will be con siderably less rich in those principles which boil at or under 100° (212° F.), and the watery vapour will have carried over much of those having a hoiliug-point above that of water. This is more
perceptible with those tars which are lighter than water, for the vapour, in passing through the tar, mechanically carries over A little of all the principles contained in it, and which of themselves would not go over except at a higher temperature. If the receiver has not been changed, the crude naphtha will contain the first and second light oils. Iu the case of tars containing much creosote, it is more economical and convenient to split the crude uaphtha iuto first and second light oils at the first operation. Many manufacturers of benzol, solvent naphtha, and aniline products, purchase crude naphtha from the tar distillers; and as this product varies in composition and value, the manufacturer should know what he is actually purchasing. Its value chiefly depends upon how much it contains of benzol, toluol, xylol, and cymol, which come over at different temperatures, and upon their sp. gr. It is submitted to a rectifieation or second distillation, with or without the oily portion which separates out of the ammoniacal liquor. The latter is best treated separately, as it consists mostly of benzol and toluol, and is consequently a convenient source of high-class benzols. The object of the second distillation is to separate the more volatile benzol and toluol, and to clear the solvent naphtha of a great portion of the heavy oils. It is pumped into a still, heated by steam or hot water, and covered with felt and cement, to prevent the heat escaping and raising the temperature of the room. The process is governed by the temperature indicated by a thermometer immersed in the vapour. This operation is carried on with extra care ; the vapours are led away to an efficient condenser at some distance from the building, and the liquor is received in covered tanks sunk in the ground. As a rule, the crude naphtha is only fractionized once; but it may be split up into different products, according to the requirements of the manu facturer. To produce high-class benzol, the receivers may he changed as soon as the temperature rises to 100°-104° (212°-220° F.). The usual plan is to allow the distillate to flow into a closed vessel supplied with two or more cocks, and containing either a hydrometer or some excisemen's beads, so as to guide the distiller in changing the receivers. Scarcely two distillers fractionize naphtha in exactly the same way ; everything depends upon what It is wished to extract, without rendering the residue unsaleable. Some collect the distillate until the temp. rises to 115°-121° (240°-250° F.); others go so far as 132°-138° (270°-280° F.); and others, again, make only one change of the receivers, collecting, at 121°-132° (250°-270° F.), all that comes over until the temp. rises to 149°-160° (300°-320° F.). The capacity of the stills rnust depend on the number and extent of the fractionizings. A disadvantage is experienced from splitting the distillates iuto too many products ; but this is removed by collecting the products of the first distillation, in such a way that two fractionizings will be sufficient for the second distillation of each of the first products. In the rectification of the crude naphthii, whatever comes over at the lowest temperature should he added to the second product obtained in the first distillation, and thus the products should be treated throughout.