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.