Fractional distillation
of coal tar
•
Tar
obtained by destructive distillation of coal (heating coal around 1000oC
coke and coal tar with ammonia is produced) still contain some ammonical
liquor. It is necessary to remove it before distillation. Otherwise it may
create froth. It can be removed by dehydrating the tar by heating in a tank
fitted with steam coil.
•
Ammonical
liquor being lighter and come on upper layer and thus removed.
•It is provided with main hole ‘M’ at the top for charging and withdrawal of pitch (Solid material) by pumping.
•It is also provided with curved vapour pipe V, a thermostate tube T, a safety valve S and an inlet pipe P for coal tar.
•The vapour pipe V is joined to the water cooled condenser.
•The cylinder is set in a brick built furnace. The brick work arc A protects the bottom coming in direct contact with the flame.
•The fuel may be coal, oil or gas.
•The still is adjusted in a way in the furnace so that more than half the area of the still is exposed to the hot gases produced by burning fuel in combustion chamber.
•Tar is agitated by blowing the superheated steam through the steam coil S1 to reduce the temperature of distillation and hence to facilitate the distillation of heavy oil.
•The specific gravity of the various fraction, may also be noted occasionally in the pot G, before passing them to their receivers respectively.
•The tar is allowed to pass into still through the inlet pipe P, while it is already hot from the previous operation when the still is half full.
•It is then heated in the still carefully, till the cracking noise in the still ceases. It indicates that water is not present in the tar.
•Completely dehydrated tar is then heated at a higher temperature so that the distillates at different temperature range are collected separately in different receivers R1, R2,R3 and R4 at a rate about 4-6 gallons per min.
The soft pitch is then collected in coolers and then in storage.
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