SCIENCE AND INDUSTRY. 
2. DESTRUCTIVE DISTILLATION OF COAL. 
When a solid or a liquid is converted into vapour by the application of 
heat and the vapour is then condensed by cooling, the process is called one 
of “‘ distillation.” If the body which is distilled decomposes at the tempera- 
ture necessary to convert it into a gas, so that the products cannot be again 
condensed into the original substance, it is termed “ destructive distillation.’’ 
The effect of heat upon coal is to decompose it into the solids which remain 
behind in the retort, the liquids which are condensed, and the gaseous 
products. - 
When an ordinary gas coal is subjected to destructive distillation, the 
volume of the gas, its heating and illuminating value, and also the quality 
and quantity of the tar are subject to great variations, according to the 
temperature at which the distillation is carried out. The following table — 
shows the average results that may be obtained with a good sample of gas 
coal. In addition to the tar, the distillates include an upper aqueous portion 
containing ammonia in solution from which the ammonium salts of commerce 
are obtained. 
AVERAGE YIELDS oF GAS AND TAR PER TON or Coan CarBonizen.* 
Temperature of Distillation. Volume of Gas, Tar. hela EN 
SG. edith Cub. Ft. Gals. 
S00 mee al 652 cae rr a 11,000 9 1.200 
SOOM eal 4 725 Xe Ne 10,000 12 1.170 
TN ys GE pA _ ar 9,000 15 1.140 
Gite ge. re x 7,750 18 1.115 
Ut San BYE sp! ore he 6,400 21 1.087 
400 .. 1b tees ie at 5,000 23 1.060 
* See ‘‘ The Carbonization of Coal,” by Prof. V. B. Lewes, London, Benn Bros., 1918, p. 122. 
Not only is there a great variation in the quantities of gas and tar yielded 
_ at different temperatures, but their composition also varies greatly. Thus 
the percentage of hydrogen in the gases obtained in the above experiments 
ranged from 21.2 at the lowest temperature to 54.5 at the highest. Similarly 
the nature of the tar varies greatly, as is seen from the difference in the 
specific gravities. Broadly speaking, high-temperature carbonization gives 
high yields of gas and aromatic oils (benzene, &c.), while lower 
temperatures result in a smaller amount of gas and a larger proportion of tar 
yielding light oils (paraffins, &c.). This statement is, however, subject to 
qualifications respecting recent developments in low-temperature carboniza- 
tion processes referred to hereinafter. When coal tar is distilled, different 
distillates pass over within certain definite temperature limits, as is shown 
in the following statement :— 
Coat TAR DISTILLATES. 
Temperature. Distillates. 
20:0 Gaen .. Light oils. 
LO 250 gree '.. Middle oils. 
2307-270 Cae a .. Creosote oils. 
Above 270° C. .. Anthracene oils. 
~ Residue—pitch (soft, medium, or hard, according to the extent to whicH 
the anthracene oils have been distilled). 
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