For the production of larger quantities of tar it is preferable to use the rotating drum, illustrated in Fig. 6.
It admits of performing the distillation without decomposition and in a current of steam.
The rotating drum consists of a cylindrical vessel made of sheet metal. It turns on rollers, is closed at one end by a screw cover, and takes a charge of from 10 to 20 kg. of coal. At the other end the cylinder is provided with a dust separator and a stuffing-box through which the gas and tar vapours pass to a condenser. The drum is heated by gas, and the temperature is measured with the aid of a thermo-couple. With a charge of 15 kg. of gas flame coal, the drum yields 15 kg. of tar in two hours. We have been working with a furnace of this type since 1916, and have examined bv its aid nearly all German coals and many coals from other countries. We have also applied this apparatus to the study of brown coal, peat, and oil shales.
The distillation of coal at the lowest possible temperatures, with a maximum of 450 to 500 degrees celsius, leaves a coke which contains practicallv the total nitrogen of the coal and about 10 per cent. volatile matter, i.e., it still contains hydrogen compounds decomposable at higher temperatures with evolution of hydrogen.
The tar and gas differ considerably from the corresponding high-temperature products. Tar is formed in much greater quantity and cloes not contain any aromatic compounds such as benzene, toluene, napllthalene or anthracene. The hydrocarbons of primary tar from all fuels resemble certain kinds of crude petroleum and are more or less rich in paraffin wax. Beside hydrocarbons, many tars contain large quantities of phenols, which, with some kinds of coal, amount to more than 50 per cent. While the gas liquor in high-temperature carbonisation reacts alkaline owing to its ammonia content, the low-temperature liquor has an acid reaction,
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