and its specific gravity. If carried too far, not merely is coke formed, but the pitch is porous and almost useless, and the anthracene oil is contaminated with high-boiling hydrocarbons which may render it almost worthless as well. Hard pitch proper should soften at 100° C., or little above.
Where the distillation is to stop at soft pitch it is, of course, not carried up to the same point, but wherever the pitch can be disposed of during the colder season or without a long carriage, even the hard pitch is preferably softened within the still by pumping back a sufficient quantity of heavy oil, previously deprived of anthracene. This makes it much easier to discharge the still. When the contents consist of soft pitch they are run off without much trouble, but hard pitch not merely emits extremely pungent vapours, but is mostly at so high a temperature that it takes fire in the air. Hard pitch must, therefore, always be run into an iron or brick cooler where it cools down out of contact with air, until it can be drawn out into the open pots where its solidification is completed.
Most of the pitch is used for the manufacture of "briquettes" ("patent fuel"), for which purpose it should soften between 55° and 80° C. according to the requirements of the buyer. In Germany upwards of 50,000 tons are used annually in that industry; much of it is imported from the United Kingdom, whence also France and Belgium are provided. Apart from the softening point the pitch is all the more valued the more constituents it contains which are soluble in xylene. The portion insoluble in this is denoted as "fixed carbon." If the briquette manufacturer has bought the pitch in the hard state he must himself bring it down to the proper softening point by re-melting it with heavy coal-tar oils.
We now come to the treatment of the various fractions obtained from the tar-stills. These operations are frequently not carried out at the smaller tar-works, which sell their oils in the crude state to the larger tar-distillers.
Working up of the Light-Oil Fraction.—The greatest portion of the light-oil fraction consists of aromatic hydrocarbons, about one-fifth being naphthalene, four-fifths benzene and its homologues, in the proportion of about 100 benzene, 30 toluene, 15 xylenes, 10 trimethylbenzenes, 1 tetramethylbenzene. Besides these the light-oil contains 5–15% phenols, 1–3% bases, 0.1 sulphuretted compounds, 0.2–0.3% nitriles, &c. It is usually first submitted to a preliminary distillation in directly fired stills, similar to the tar-stills, but with a dephlegmating head. Here we obtain (1) first runnings (up to 0.89 spec. grav.), (2) heavy benzols (up to 0.95), (3) carbolic oil (up to 1.00). The residue remaining in the still (chiefly naphthalene) goes to the middle-oil fraction.
The "first runnings" are now "washed" in various ways, of which we shall describe one of the best. The oil is mixed with dilute caustic soda solution, and the solution of phenols thus obtained is worked up with that obtained from the next fractions. After this follows a treatment with dilute sulphuric acid (spec. grav. 1.3), to extract the pyridine bases, and lastly with concentrated sulphuric acid (1.84), which removes some of the aliphatic hydrocarbons and "unsaturated" compounds. After this the crude benzol is thoroughly washed with water and dilute caustic soda solution, until its reaction is neutral. The mixing of the basic, acid and aqueous washing-liquids with the oils is performed by compressed air, or more suitably by mechanical stirrers, arranged on a perpendicular, or better, a horizontal shaft. Precisely the same treatment takes place with the next fraction, the "heavy benzols," and the oils left behind after the washing operations now go to the steam-stills. The heaviest hydrocarbons are sometimes twice subjected to the operation of washing.
The washed crude benzols are now further fractionated by distillation with steam. The steam-stills are in nearly all details on the principle of the "column apparatus" employed in the distillation of alcoholic liquids, as represented in fig. 4. They are usually made of cast iron. The still itself is either an upright or a horizontal cylinder, heated by a steam-coil, of a capacity of from 1000 to 2000 gallons. The superposed columns contain from 20 to 50 compartments of a width of 2½ or 3 ft. The vapours pass into a cooler, and from this the distillate runs through an apparatus, where the liquor can be seen and tested, into the receivers. The latter are so arranged that the water passing over at the same time is automatically removed. This is especially necessary, because the last fraction is distilled by means of pure steam.
The fractions made in the steam distillation vary at different works. In some places the pure hydrocarbons are net extracted and here only the articles called: "90 per cent. benzol," "50 per cent. benzol," "solvent naphtha," "burning naphtha" are made, or any other commercial articles as they are ordered. The expression "per cent." in this case does not signify the percentage of real benzene, but that portion which distills over up to the temperature of 100° C., when a certain quantity of the article is heated in glass retorts of a definite shape, with the thermometer inserted in the liquid itself. By the application of well-constructed rectifying-columns and with proper care it is, however, possible to obtain in this operation nearly pure benzene, toluene, xylene, and cumene (in the two last cases a mixture of the various isomeric hydrocarbons). These hydrocarbons contain only a slight proportion of thiophene and its isomers, which can be removed only by a treatment with fuming sulphuric acid, but this is only exceptionally done.
|Fig. 4.—Benzol Still (elevation).|
Sometimes the pyridine bases are recovered from the tarry acid which is obtained in the treatment of the light oil with sulphuric acid, and which contains from 10 to 30% of bases, chiefly pyridine and its homologues with a little aniline, together with resinous substances. The latter are best removed by a partial precipitation with ammonia, either in the shape of gas or of concentrated ammoniacal liquor. This reagent is added until the acid reaction has just disappeared and a faint smell of pyridine is perceived. The mixture is allowed to settle, and it then separates into two layers. The upper layer, containing the impurities, is run off; the lower layer, containing the sulphates of ammonia and of the pyridine bases, is treated with ammonia in excess, where it separates into a lower aqueous layer of ammonium sulphate solution and an oil, consisting of crude pyridine. This is purified by fractionation in iron stills and distillation over caustic soda. Most of it is used for denaturing spirit of wine in Germany, for which purpose it is required to contain 90% of bases boiling up to 140° C. (see Alcohol).
Working up of the Middle-Oil Fraction (Carbolic Oil Fraction).—Owing to its great percentage of naphthalene (about 40%) this fraction is solid or semi-solid at ordinary temperatures. Its specific gravity is about 1.2; its colour may vary from light yellow to dark brown or black. In the latter case it must be re-distilled before further treatment. On cooling down, about four-fifths of the naphthalene crystallizes out on standing from three to ten days. The crystals are freed from the mother oils by draining and cold or hot pressing; they are then washed at 100° C. with concentrated sulphuric acid, afterwards with water and re-distilled or sublimed. About 10,000 tons of naphthalene are used annually in Germany, mostly for the manufacture of many azo-colours and of synthetic indigo.
The oils drained from the crude naphthalene are re-distilled and worked for carbolic acid and its isomers. For this purpose the oil is washed with a solution of caustic soda, of specific gravity 1.1; the solution thus obtained is treated with sulphuric acid or with carbon dioxide, and the crude phenols now separated are fractionated in a similar manner as is done in the case of crude benzol. The pure phenol crystallizes out and is again distilled in iron stills with a silver head and cooling worm; the remaining oils, consisting mainly of cresols, are sold as "liquid carbolic acid" or under other names.