This article discusses the origins of tars and pitches from the pyrolysis of organic matter in general; those derived from coal are dealt with in detail. The physical characteristics of tars and pitch residues are summarized, as are the chemical composition of the fractions produced and the distillation processes by which they are obtained from coal tar.
Change has taken place in the twentieth century in the role of coal tar as the principal source of aromatic organic chemicals, and for road construction and surfacing, which has become an important source of components for carbon products, binders, and coating materials. Attention is drawn to the remaining chemical products of commercial importance, principally naphthalene, creosote oil, anthracene, and modest amounts of phenols and aromatic bases. There have also been a few newer uses for naphthalene and a novel purification process, as well as different requirements for pitches used as carbon binders and for the composition of creosote. Concern about the health and safety aspects of coal‐derived substances has been considered by the authorities, particularly in the European Union and directives, and regulations have been introduced to reduce risks in both the environment and workplace.
BETTS et a1.-DEALKYLATIONOF AROMATIC HYDROCARBONS 497 oxidation to take place. Bradford 'A' grease, having its origin in sewage, has been exposed t o the action of many types of micro-organism. It is clear, however, that none of these conditions is suitable for producing cholestenone from the potential precursors in wool wax.
The studies of thermal dealkylation1, 2 have been extended to the polycyclic hydrocarbons. In the temperature interval of 560 to 660°, at a pressure of 100 atm. and with a large excess of hydrogen, α‐ and β‐methylnaphthalene, a methylnaphthalene oil and a number of coal‐tar fractions were studied. On the assumption of the same mechanism as for toluene dealkylation1 the results were fitted to a 1.5‐order reaction mechanism. The energies of activation for α‐ and β‐methylnaphthalenes agree well with the proposed mechanism.
Naphthalene oils of coke‐oven origin should be capable of giving quantitative yields of naphthalene. Oils of vertical‐retort origin are too rich in paraffin content. Little dealkylation was obtained with a coke‐oven anthracene oil.
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