Benzene production from coal-tar


Benzene is considered as simplest form of arene and cyclic hydrocarbon. In 1825 benzene was first isolated by Faraday. Benzene is a natural component of crude oil, and is one of the most elementary petrochemicals. In 1845, Hoffmann isolated benzene from coal-tar.

Benzene molecular formula is C6H6 and benzene structure is having alternative double bonds with hexagon shape. Hydrogens present in the benzene can be replaced by some of the other functional groups. As a result number benzene derivatives will be generated. Derivatives of benzene are ethylbenzene, cumene.

Benzene structure

Benzene ring structure was deduced by Friedrich August Kekule. The carbons present in benzene ring are arranged in a hexagon, and he suggested alternating double and single bonds between them. Each carbon atom has two hydrogens instead of four hydrogens. To satisfy the tetravalency of carbon, the benzene ring consisted of alternate single and double bonds.

August Kekule in dream saw a snake coil up, and grabs its own tail then he proposed benzene might be a ring structure.

Benzene preparation methods

  • Sodium benzoate is heated with soda-lime (NaOH) and when it gets decarboxylated benzene is obtained.
  • Phenol vapors are passed over heated zinc dust, benzene is formed.
  • When ethyne is passed through a red hot copper tube, it polymerizes to benzene
  • Benzene is formed on the reduction of benzene diazonium chloride with sodium stannite or hypophosphorus acid.
  • Benzene sulphonic acid on hydrolysis with superheated steam gives benzene.

C6H5.SO3H + H2O — > C6H6 + H2SO4

Following are the other major processes for production of benzene

  • Catalytic reforming
  • Toluene hydrodealkylation
  • Toluene disproportionation
  • Pyrolysis gasoline
  • Production from coal tar

Pyrolysis gasoline

Pyrolysis gasoline is the by-product of steam cracking of petroleum by products like paraffin gases, naphthas, gas oils. Pyrolysis gasoline contains 5 per cent diolefins. In addition it also contains 60 per cent aromatic compounds, 50 per cent of benzene. Different techniques are applied on diolefins to produce benzene, these are

  • Distillation of diolefins to olefins
  • Saturation of olefins to remove sulfur content
  • Execution of solvent extraction and distillation process to obtain benzene

Production from coal tar

This process improved methods of recovery and purification that coke-oven benzene has been able to withstand the competition of petroleum-derived benzene as well as it has. Production of benzene from coal tar involves recovering benzene from coal tar.

  • Extraction of lowest boiling point fractions
  • Applying of caustic soda for the removal of tar acids
  • Crude oil distillation
  • Crude oil purification through hydrodealkylation

Toluene Hydrodealkylation

  • Toluene hydrodealkylation reaction takes place is as follows
  • Mixing of toluene with aromatics or paraffins
  • At specific pressures these mixtures are heated in the presence of hydrogen gas
  • The steam which is formed in previous step then moved to reactor containing dealkylation catalyst
  • In this reactor toluene reacts with hydrogen as a result, benzene and methane products will form
  • At high pressures benzene is separated from methane
  • Then methane also removed from reactor
  • Here in this step benzene can be recovered from fractionalization column and then be stored.

Catalytic Reforming

Catalytic reforming involves the dehydrogenation of naphthenes to aromatics, or the isomerizatoin of alkylnaphthenes and it follows dehydrogenation process. The feed for this process is naptha.

  • First the naptha is hydrotreated to remove sulfur contaminant.
  • Recycled hydrogen is then added, mixed and heated.
  • Conversion of paraffins to aromatic compounds in catalytic reactors and in this reactors platinum or rhenium chloride is acts as catalyst.
  • In further step a stream is formed which is rich in aromatic compounds.
  • Then stream is sent to separation section to separate hydrogen and this hydrogen recycled as basic feedstock.
  • Liquid portion of stream fed to a stabilizer which separates hydrocarbons from liquids.
  • The liquid is then sent to a debutanizer
  • Benzene, toluene and xylenes are then extracted using glycol and sulfonate solvents

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