By
M. Fort, M.Sc. (Leeds) Late Lecturer in Dyeing in the Bradford Technical College and L. L. Lloyd, Ph.D. (Bern) Lecturer in Organic and Technical Chemistry in the Bradford Technical College
Cambridge: at the University Press 1919
(First edition 1917, reprinted 1919)Production of Tar
I. The most plentiful supply of tar is obtained during the decomposition of bituminous coals for the production of coal gas. The coal is heated in retorts at about 900°C. to 1000°C., at slightly reduced pressure, to remove the gas from the hot retorts as quickly as possible. The gas is cooled artificially in order to condense the substances of low boiling point After cooling, it is necessary to remove tar that is suspended in a very finely divided condition in the gas. This is done by passing the gas under about 20 inches of water pressure through a "Tar Separator." The whole of the condensed portion is then allowed to mix. This separates into an aqueous upper layer and a lower layer of tar. The aqueous layer contains ammonia and small quantities of tarry substances in solution.
II. A large, increasing supply of tar is obtained from coke ovens. The coal is heated to a higher temperature than in the production of coal gas.
The tar is much thicker and contains a larger quantity of free carbon than coal gas tar.
III. A fairly large amount of tar is obtained by cooling the waste gases from blast furnaces, and this tar contains phenols of high molecular weight and of high germicidal value, with a large amount of free carbon.
IV. During the preparation of charcoal by the distillation of wood, there is obtained a distillate which separates into two layers; the upper aqueous layer is used for the manufacture of acetone, methyl alcohol, and acetic acid; the lower layer furnishes wood tar from which creosote (crude guaiacol) is obtained.
Different classes of tar are also obtained from the following processes: the distillation of bituminous shale; the manufacture of oil gas; the partial decomposition of oils by "cracking," the oils being delivered in thin streams into very hot retorts , the distillation of crude mineral oils, etc.
A different variety of coal tar is obtained by the distillation of coal at comparatively low temperature, viz., at about 400°C. to 450°C.
Composition of Tar.
The composition of tar varies with the source, and in the case of coal tar with the coal and temperature of distillation.
Fatty and aromatic hydrocarbons have been extracted from coal by means of solvents, but the amounts of such substances are usually very small. Paraffin hydrocarbons and hexahydrofluorene have been obtained.
The yield, as well as the relative amounts of the constituents of the tar, varies with the class of coal. The brown or young coals (lignite) give more fatty compounds and also more tar than the short flaming or older coals. The older or the less bituminous the coals, the smaller will be the amount of low boiling point constituents in the tar, and the higher will be the amount of anthracene and like compounds.
By the distillation of coal, for the manufacture of patent fuel, at low temperature, viz. 400°C. to 450°C., there is obtained a tar, which is fairly rich in low boiling hydrocarbons, a large percentage consisting of paraffin hydrocarbons. The so-called benzol from this tar, on account of the presence of other than aromatic hydrocarbons, is of no use for the manufacture of nitrobenzene.
In the distillation of caking coals, for the manufacture of coal gas, it is general to calculate upon a yield of 10 gallons of tar per ton of coal. This figure is only a rough guide, the quantity varying very much with the kind of coal.
It has been shown recently by Pictet and others that coal from Montrambert yields a mixture of hydrocarbons by extraction with boiling benzol. Among others hexahydrofluorene has been separated. By the distillation of coal in vacuum a tar is obtained, free from phenolic substances, containing a large proportion of basic compounds, but practically free from the aromatic bodies naphthalene, anthracene. This vacuum tar is decomposed by ordinary distillation with production of benzol, naphthalene and anthracene. From this it appears that the aromatic compounds in tar are pyrogenetic decomposition products.
At about 900°C. to 1000°C. there is a maximum yield of benzene compounds with a good illuminating gas; higher temperatures give more gas, but of less illuminating power, and the tar is richer in the more complex aromatic compounds.
Coal tar is a black viscous liquid, Sp. Gr. 1'1 to 1'2, and contains many aromatic compounds, the chief of which are now given.
Hydrocarbons: benzene, toluene, xylenes and other homologues, naphthalene and its homologues, acenaphthene, fluorene, anthracene, phenanthrene, pyrene, chrysene, etc.
Neutral compounds: carbon disulphide, thiophene and its homologues, carbazol, etc.
Bases: pyridine and its homologues, quinoline, isoquinoline, aniline, acridine, etc.
Phenolic substances: carbolic acid, cresylic acids, naphthols, etc.
Practically all of these compounds are employed in the colour industry, although many are not separated as such from tar, but may be made from a more easily obtainable constituent, e.g., aniline from benzene, naphthols from naphthalene.
Gas tar contains roughly about 2 per cent, benzol, 0,5 per cent, toluol, 0,6 per cent, phenol, 5 to 6 per cent, naphthalene and 0,6 per cent, anthracene.
On distillation the tar is divided, as described later, into four or five parts.
The light oil contains roughly 5 to 15 per cent, phenols, 1 to 3 per cent, basic substances, 0,1 per cent. thiophene and homologues, 0,3 per cent, nitriles, 1,5 per cent, neutral bodies containing oxygen, the remainder being benzene and its homologues.
The middle oil contains roughly 40 per cent, naphthalene, 25 to 35 per cent, phenol and homologues, the remainder consisting chiefly of pyridine and quinoline bases.
The heavy oil contains mainly cresols, quinoline bases, naphthalene and its homologues.
The anthracene oil contains 2,5 to 3,5 per cent, anthracene, 2,5 to 3,5 per cent, phenanthrene, 1,5 to 2,5 per cent, carbazol; along with fluorene, pyrene, chrysene, and phenolic substances.
Distillation of Tar.
The tar as obtained from the gas works contains some aqueous ammoniacal liquor; this is partly separated by allowing to stand in tanks. If the tar is run directly into the stills (Fig. I, Appendix) in this condition it must be distilled very slowly at first in order to avoid sudden boiling over; this may happen with careless working, sometimes attaining the violence of an explosion. Consequently in most works the tar is heated up to about 150°C. before running into the stills, the latter being usually hot from the last charge. Several devices are employed to secure this preliminary heating. The tar may be heated (dehydrated) by utilising the waste gases from the stills before passing up the chimney stack. This is sometimes done in large boilers or by means of a shallow tubular still; the products that pass off are condensed and collected. Water, carbon disulphide, pyridine, benzene, etc., are thus obtained. The water layer is then separated and the oil treated along with the light oils. The dehydrated tar is then run into the stills and directly distilled. The distillate is collected in fractions according to the specific gravity, or in some cases the fractions may be divided according to the temperature at which they distil.
The following table shows the approximate specific gravity and boiling points of the different fractions:
Fractions | Specific Gravity | Boiling Point | Average Yield I. First Runnings | or Light oil 0,9 to 0,95 | up to 170 C. | 2 to 6 % II. Second Runnings| or Middle oil... 0,95 to 1-01 | up to 230 C. | 10 to 12 % III. Third Runnings| or Creosote oil or Heavy oil 1,01 to 1,05 | up to 280 C. | 8 to 10 % IV. Anthracene oil | 1,06 to 1,1+ | to close of distillation | 16 to 18 % V. Pitch | - | still content | about 50 %
During the distillation of the light oil it is necessary to cool the distillation products in a water-cooled worm, but the higher boiling fractions are condensed without water-cooling on account of the danger of sealing up the worm by deposition of solid naphthalene. As soon as the creosote oil fraction has been distilled from the tar the distillation is completed by means of superheated steam. When distillation is complete the tar is run into a cooler, which consists in most cases of a large tank or boiler, in order to cool it below its firing point.
The tar is then run out and exposed to air in large shallow pits to solidify.
Continuous distillation of Tar. Several types of continuous distillation plant are in use. One of the best forms of these installations is briefly as follows. The tar is gradually warmed up by using it in the first place to run around the condensing worm of the first still, from which it flows to the other tanks through which the condensing worms pass. In this manner the tar is partly distilled, the volatile portions being collected before it enters the first still. The tar, as a rule, passes through three stills, the first and second being directly heated, and the third distillation conducted by means of steam. The tar flows continuously through the three stills, in shallow layers, in a circuitous course, and finally to a cooling chamber or in some cases directly to the solidifying pits. In this manner four fractions are collected which are similar to those stated above.
Treatment of Tar distillates.
The separate fractions as obtained by the distillation are now further treated in order to produce purer compounds as used in the chemical trades. The light oil is well mixed with caustic soda of 15°Tw. The acidic substances (phenols) are dissolved, the upper layer is run off, washed with water to remove alkali and then treated with sulphuric acid (1 part H2 SO4and 2 parts water). The sulphuric acid forms salts with bases, these remain in solution. The oil that collects above is well washed and then fractionated (Fig. II, Appendix), a Savalle column, a dephlegmator, or in some cases a combination of these, being used in order to obtain, by one distillation, practically pure benzene, toluene and commercial xylene.
Intermediate fractions are also obtained, these are added to fresh quantities of treated light oils, or are again redistilled.
The "dephlegmator" is the class of still-head in which the condensed liquid is caused, by means of suitable obstructions, to collect in. shallow pools, through which the ascending vapour has to force its way; very good contact is thus obtained between vapour and liquid at definite intervals. The excess of liquid is carried back from pool to pool, and finally to the still by suitable trapped reflux tubes or intermittent syphons.
The Savalle column contains a dephlegmator at the lower portion, and then a surface, or multittibular, condenser, provided with a water supply, so regulated that its temperature is about that of the boiling point of the liquid required; the liquid condensed in the regulated temperature still-head returns to the dephlegmator, and the purified vapour passes on to the cold condenser.
The benzene and toluene so obtained still contain sulphur compounds, viz. thiophene and thiotolenes; these may be removed by extracting three or four times with small quantities of concentrated sulphuric acid. Thiophene and its homologues are sulphonated in the cold, whereas benzene is only slightly affected. For most purposes the thiophene is seldom removed from the benzene. The portions boiling above the boiling point of the xylenes furnish solvent naphtha or cumol (commercial), which is used in the rubber industry.
The middle oil on standing deposits dark brown crystals of crude naphthalene; these are separated by filter pressing or by a centrifugal machine. The oil so obtained is treated with caustic soda of 15°Tw. at about 40°C. and the lower caustic layer run off. Stronger caustic soda solution may be used for the extraction of the phenols; such a solution is, however, a better solvent for the neutral oils, consequently the phenols obtained will be less valuable. The upper layer on cooling deposits a further quantity of crude naphthalene. The alkali extracted oil is, as a rule, added to the creosote oil, or in some cases is treated with sulphuric acid exactly the same as with the light oils. The neutral oil is then added to the creosote oil.
The caustic soda extracts of the first and second distillates are mixed together and treated for the separation of carbolic acid and cresols.
In some cases the alkali extract is just acidified with sulphuric acid, the phenols collect as an upper oily layer; the latter is separated, washed free from acid and distilled, a fractionating column (Savalle) being used to aid in the separation of the carbolic acid from the cresols. By using sulphuric acid for "cracking out" the phenols, there is a loss of about two to three per cent, of phenol which remains dissolved in the aqueous liquor. On account of this loss it is more general to separate the phenolic substances from the alkaline extracts by means of carbon dioxide. The furnace gases are employed for this purpose. They are first washed to remove sulphur dioxide, and then passed through the alkaline extract until the phenols are precipitated. The upper oil layer is washed and then distilled, and the lower aqueous solution of sodium carbonate containing a little phenol, etc. is recausticised with milk of lime and again used for extraction purposes.
The distillates from the phenols are collected and classified according to the melting points; 60 phenol means that the phenol has a mean melting point of 60°F. By repeatedly recrystallising it is possible to obtain a carbolic acid of melting point 38°C., but further purification, to obtain pure phenol, is impossible by simple crystallisation, owing to the presence of orthocresol, and, since separation cannot be effected by distillation, a method of purification, based on the formation of a crystallisable hydrate of phenol, is employed. The mixture is churned with warm water and allowed to cool, a phenol hydrate, having the formula C6H3OH.H2O, crystallises out, whereas orthocresol forms a liquid hydrate. The crystals are separated and the phenol obtained by distillation. The phenol so obtained turns red on exposure to air and light, but is sufficiently pure for manufacturing purposes. The red colour is probably due to some sulphur compound which may be present in the original tar or produced by the action of too strong sulphuric acid in precipitation, etc. Since phenol "cracked out" by furnace gases also turns red it is most probably due to some compound originally present in the tar.
The cresols are mainly used for the manufacture of disinfectants, such as Lysol, Jeyes' Fluid, Cresolin.
In this connection it has been observed that the higher the boiling point of the phenol homologues the higher is the antiseptic value. Consequently the phenols from blast furnace gases are particularly important for the manufacture of these compounds.
The crude naphthalene as obtained above is melted and extracted with caustic soda to remove phenols, then with diluted sulphuric acid as above to remove basic compounds. The partly treated naphthalene is. then hot pressed, similarly to the separation of stearin for candle manufacture. The homologues of naphthalene, etc., melt at a lower temperature and are expressed from the mixture. The now practically pure naphthalene is redistilled or sublimed in a hooded iron pot. It is used very extensively in the colour industry, also as a deodorant.
The basic compounds are separated from the sulphuric acid extracts by means of caustic soda or more generally by ammonia. The bases, pyridine and its homologues, are separated and distilled. The pyridine is used for "denaturing" alcohol and as a solvent for the purification of anthracene.
Pyridine and Quinoline bases. Pyridine and its homologues are at present only used as solvents for the purification of organic compounds. Dyestuffs are obtainable from these compounds, but they have not yet found commercial application.
Crude quinoline bases are obtained from coal tar and bone oil, and the portion boiling between 235°C. and 245°C. is employed to a small extent in the production of Quinoline Red and Quinoline Blue.
The synthetic compound quinaldine [structure image, not available] (B.P. 243°C.) is obtained by boiling aniline, hydrochloric acid, and para-acetaldehyde under a reflux condenser. The reaction mixture is made alkaline and the base distilled.
It is employed in the production of Quinoline Yellow.
The creosote oil contains higher phenols, naphthols, naphthalene homologues, etc.; it is used to a large extent for creosoting timber. For this purpose the wood is packed in tanks, the air pumped out, and when a good vacuum is obtained the oil is run in. In this manner the wood is well impregnated with the oil. The oil also finds application as a fuel. It is used for the manufacture of lamp-black, and in the waterproofing of felt. It is also added to pitch to soften it.
The anthracene oil contains anthracene, methylanthracene, naphthalene, phenanthrene, acenaphthene, fluorene, chrysene, pyrene, acridine, some naphthols and some quinoline bases. On allowing to stand for some time it separates into a crystalline and a liquid portion. This is filtered by filter-presses or by hydroextractors (centrifugated). The liquid portion so obtained, which no longer crystallises on standing, is termed "green oil"; it is used for the preparation of cheap lubricants, and is also added to pitch for softening purposes.
The solid portion is expressed warm, which leaves a cake containing about 25 to 35 per cent, anthracene. The warm expressed oily portion is treated again after it has crystallised.
The crude 25 to 35 per cent, anthracene is purified by means of solvents until about 80 per cent, anthracene is obtained. For this purpose the following solvents may be used: solvent naphtha, crude pyridine, liquid ammonia, liquid sulphur dioxide, etc. (Fig. IX, Appendix.) When solvent naphtha is used, the extract is separated hot, leaving the anthracene of about 70 to 80 per cent, purity.
Crude pyridine gives a better separation than naphtha, the anthracene being only sparingly soluble is thus obtained as a greenish solid of about 80 per cent, purity.
Tar Distillation
The anthracene in this form is distilled in steam, and the distillate quickly chilled in order to obtain the substance in a finely divided form, in which state it is oxidised by sulphuric acid and sodium bichromate into anthraquinone.
Phenanthrene is not so readily oxidised as anthracene and may be extracted by solvents from the crude anthraquinone. The anthraquinone may be further purified by heating with concentrated sulphuric acid to sulphonate impurities, which may be then removed with alkali washing, after which 95 to 98 per cent, anthraquinone is obtained.
Phenanthrene, acenaphthene, fluorene, and carbazole are obtained from the naphtha and pyridine extracts. The commercial importance of these bodies has greatly increased of late years.
Carbazole is obtained by heating the crude or treated anthracene distillate with strong caustic potash; the resulting melt is allowed to cool and the upper layer stripped from the lower one of potassium carbazole, which on treating with water gives carbazole and caustic potash solution. Carbazole may be nitrated, sulphonated, halogenated, etc., the products being employed for the production of dyestuffs, e.g., Carbazol Yellow from diazotised diamino-carbazole and salicylic acid, Hydron Blue by polysulphide melt of carbazole indophenol.
The pitch, which remains in the stills, is run into cooling vessels, then into shallow pits, and is then broken up. The pitch is used for road material, asphalt, for the manufacture of briquettes, etc.
For some purposes the pitch is required free from suspended carbon; this is obtained by dissolving in twice its volume of hot high-boiling point naphtha, allowing the carbon to deposit, running off the clear hot liquid portion and then distilling off the solvent. Soft pitches, e.g., for modern road making requirements, are made by working up crude pitch with creosote and waste anthracene distillates.
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Further information may be obtained from Lunge's Coal Tar and Ammonia (2 vols), and "Tar Distillation" by H. P. Hird, Jour. Soc. Dyers, April, 1916.
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