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 Hydrodealkylation of toluene
  • Hydrodealkylation of toluene
  • Today, the most important industrial processes arc those for the manufacture of benzene from toluene:

    The percentage of toluene production used for the hydrodcalkylation to benzene varies between countries。
     
    The hydrodealkylation is conducted either purely thermally at 550-800°C and 30-100 bar or catalytically at somewhat lower temperatures of 500-650 °C and 30-50 bar Over Cr2O3, Mo2O3, or CoO on supports (e.g., Al2O3) or, as in a recent development, at 400-480°C over Rh/Al2O3.

    In the commercial catalytic process, toluene is passed over the catalyst together with a recycle gas consisting of a mixture of H2 and CH4. The high working temperatures result in relatively strong coking, and two parallel reactors are generally used so the reaction can proceed in one while the catalyst is being regenerated in the other.
     
    Because of the large amount of CH4 formed - about 250 scm/tonne toluene - special measures are necessary to remove it from the recycle gas. The CH4/H2 mixture can, for this purpose, be totally or partially subjected to low temperature separation. By cracking with H2O2 the CH4 can be converted into H2 to cover the process requirements for the hydrodealkylation.

    After dealkylation, the benzene is isolated by distillation from the unreacted toluene, which is recycled. Higher boiling byproducts such as biphenyl and condensed aromatics (e. g., fluorene) can be separated off or recycled to the process, since they are also subject to cleavage.

    In thermal processes, e. g., THD (Gulf), HDA (Arco-Hydrocarbon Research), and MHD (Mitsubishi Petrochemical), benzene selectivities of about 95% are attained at a 60-90% toluene conversion.

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