Aromatic compounds from coal
Coal is a complex mixture of hydrocarbons. It also contains some organic compounds containing nitrogen and sulphur in small amounts. It can be approximated to the formula (C3H4)n.
Coal is a hydrogen-deficient substance. It is assumed that the basic structure of coal is probably built up of a large number of interlocked benzene rings, upto thirty, in bituminous/anthracite coal. Hydrogen is present in the aliphatic side chains. Bituminous coal has to be thermally decomposed, for obtaining organic compounds from coal. This process is called destructive distillation or 'pyrolysis' of coal.Destructive distillation of coal
Destructive distillation of coal occurs when coal is heated in the absence of air, at high temperatures. To achieve this, coal is heated in the absence of air in iron retorts, wherein the volatile material evolved is collected as distillate/condensate. The process can be carried out under different temperature conditions, depending upon the nature of the products required.
Low temperature pyrolysis
Coal when heated in the absence of air at about 723 - 973 K gives a soft solid, smokeless coke called coalite. The yield of coal tar and coal gas is doubled here. Coal tar so obtained contains more of aliphatic compounds.
High temperature pyrolysis
In this process, coal is heated in the absence of air to about 1273 - 1473 K. The major products of high temperature pyrolysis are,
Coal
Coke Coal gas Coal tar Ammonium sulphate Light oil 680 kg 30 m3 15 - 22 kg 1 kg 5 - 6 kgThe coal tar obtained is a heavy viscous almost black liquid due to carbon content with an unpleasant odor. It is a valuable by-product in the destructive distillation of coal. Its composition depends upon the nature of coal used during distillation. However, coal tar generally contains the following compounds: Acidic compounds (Phenol and cresols etc.), basic compounds (Pyridine, etc.) and Neutral compounds (Benzene, toluene, naphthalene and anthracene etc.)
Fractional distillation of coal tar
The outgoing hot vapors from the main iron still preheat coal tar obtained from the destructive distillation of coal, in a pre-heater. In this process, coal tar loses most of the water it contains, along with some low boiling hydrocarbons. It is then sent to the main iron still (a side retort) heated directly in a furnace. The vapors are condensed to obtain various fractions at different temperatures. Various fractions obtained during fractional distillation of coal-tar are given below.
