Isolation of Benzene, Toluene and Xylene

•Benzene, Toluene and Xylene (BTX) are the compounds have boiling points 80.1oC, 110.6oC, 144oC (at atm pressure) respectively.

•The mixture of these components recovered from coke gas in benzol section of a coal chemical process plant.

•Extractive distillation technology provides effective separation of these compounds in pure liquid form.

•Initially xylene is separated from the mixture by distillation than the left out benzene and toluene mixture is subjected to extractive distillation.

•Solvent called N-formylmoropholine (NFM) act as the azeotrope breaker to separate benzene and toluene.

•The process requires the following distillation and separation unit operation.

ØPressure distillation

ØExtractive distillation

ØSolvent recovery column

ØAromatic stripper

ØBT separation column

ØXylene solvent column

     Azeotropes are a mixture of at least two different liquids. Their mixture can either have a higher boiling point than either of the components or they can have a lower boiling point.

Pressure distillation:

•The BTX solvents mixtures is pumped to the feed surge drum and from it, the mixture continuously pumped to pressure distillation column through four heat exchangers.

• General distillation column used in most industries designed with 50 bubble cap trays is sufficient to perform the distillation operation.

•At its 25th trays feed in injected and utilized as feed tray, its operating pressure maintained at 15 kg/cm2. These conditions may vary based on the regular optimization experiments.

•Hot oil is used as heating medium to the column re-boiler and passes through its shell side. The benzene-toluene (BT) vapors from the top of the column are collected in reflux drum before which they are condensed. Benzene-Toluene fraction continuously accumulated in the reflux drum. Some of it is used as reflux to the column and remaining fraction is passed to pre-heater.

BUBBLE CAP TRAYS

    A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is a space between riser and cap to allow the passage of vapour. Vapour rises through the chimney and is directed downward by the cap, finally discharging through slots in the cap, and finally bubbling through the liquid on the tray.

Extractive distillation column:

•The BT excess is pumped from vessel via heat exchangers  and fed to extractive distillation column. The engineering model separation column has 60 trays and feed is introduced at the middle of the column. For extraction, the solvent N-formylmoropholine (NFM) is introduced on to the top tray of the extractive distillation column with flow ratio of 56 kg NFM per kg of feed at 92оC. The solvent NFM temperature directly effects the separation of aromatics from non-aromatics.

•Extractive distillation column makes it possible to separate non-aromatics contained in the feed, which is not possible under normal distillation conditions. The solvent changes the boiling points of non-aromatics from higher to lower and makes it easy to withdrawn at the top of the extractive distillation column. Aromatics dissolve in the NFM solvent and removed from the bottom of the extractive distillation column.

Uses of Tar Products

•      Benzene is used as a starting material in the manufacture of synthetic phenol, chlorobenzene, etc

•      Toluene is used as a raw material in the manufacture of saaharin, benzaldehyde, benzoic acid, chloromine T and TNT

•      Para xylene is used for making anthraquinone an important dyestuff intermediate

•      Pyridine and other tar products are used manufacture of dyestuff intermediate, accelerators in vulcanization of rubber and manufacture of pharmaceuticals

•      Phenols, cresols and xyenols are used to prepare salicylic acid, aspirin, phenacetin etc. Phenol is also used in manufacture of adipic acid as a raw material of Nylon

•      Carbazole and phenanthrene are used in the synthesis of many organic chemicals.

Fractional distillation of coal tar

•      Tar obtained by destructive distillation of coal (heating coal around 1000^oC 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.

      •Distillation of dehydrated tar is carried out in a cast iron cylindrical horizontal plate tank.
•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.

Fractional distillation of coal and its products

What is coal Tar?

•       Coal tar is a black, thick, oily liquid obtained by destructive distillation of coal

•       Tar obtained by high temperature carbonization are more dark coloured (Blackish) than tar obtained from low temperature carbonization.

•       The actual composition of tar varies with the nature of coal and temperature of destructive distillation.

•       Coal tar is important raw material for the manufacture of various products like explosives, synthetic dyes, perfumes, drugs, photo goods etc