21.3 Benzene production using hydrodealkylation route

21.3.1 Reactions

• • Toluene does not have much market value.

• Therefore, Toluene is reacted with H₂ to produce Benzene and Methane

• Main reaction: Toluene + H₂ → Benzene + Methane

• Reactor operating conditions: 600 – 650°C and 35 – 40 atms

• Catalyst: Chromia on porous carrier.

• Other reactions: Alkyl aromatics + H₂ → Benzene + Alkanes

21.3.2 Side reactions

• • Toluene → Diphenyl + H₂

• Toluene + Benzene → Methyl diphenyl

• Toluene + H₂ → Alkanes (Cracking reaction)

21.3.3 Process Technology (Figure 21.2)

Figure 21.2 Flow sheet of Benzene production using hydrodealkylation route

• • The process corresponds to a simple reactor-separator-recycle system facilitated with heat integration.

• Alkyl-aromatics (Toluenes and other higher order alkyl aromatics) along with makeup H₂ and recycle stream consisting of unreacted toluene is allowed to enter a feed pre-heater.

• The feed pre-heater increases the temperature of the reactants at the desired pressure.

• After the reaction, the reactor products are cooled using heat integration concept by exchanging heat with the cold feed stream. Further cooling of the stream is carried out using a water based cooler.

• The stream enters a phase separation unit where the fuel gas components such as H₂ and CH₄ are removed as vapour stream. The liquid stream consists of H₂, CH₄, Light ends, Benzene, Toluene, Diphenyl and other higher order aromatics.

• The vapour stream is partially purged and to a large extent recycled as the stream has good amount of H₂ . The gas phase purge stream is for controlling the concentration of methane in the reactor.

• The liquid stream enters a gas stripper which removes the light ends as a gaseous product stream from the top tray. The bottom product is then sent to a fractionator.

• The fractionator separates benzene from all heavy ends. The heavy end product consisting of unreacted toluene, diphenyl etc. is largely sent back to the reactor by allowing mixing with the fresh feed.

• A purge stream is facilitated to purge components such as diphenyl in order to not allow their build up in the reactor.