Thermodynamic equilibrium distribution of light olefins in catalytic pyrolysis

• Olefin systems with different carbon numbers showed a similar thermodynamic equilibrium distribution.
• Catalyst could greatly increase the yields of ethene and propene compared with thermal pyrolysis, but the yields were still lower than the equilibrium data.
• Catalytic pyrolysis was carried out in the interaction zone where both catalytic conversion and thermal conversion were important.
• The shape-selective catalysts could prevent olefins with large carbon numbers from isomerization and could improve the equilibrium yields of ethene and propene.

Catalytic pyrolysis is a promising technology to produce light olefins. Gibbs free-energy minimization method was used to study the thermodynamic equilibrium distribution of olefins in catalytic pyrolysis with Aspen Plus software. The result showed that olefin systems with different carbon numbers demonstrated a similar thermodynamic equilibrium distribution. The ethene equilibrium composition increased with increasing reaction temperature and decreased with increasing total hydrocarbon pressure. By contrast, the propene equilibrium composition reached a maximum of 40 wt% at 850–950 K under 0.1 MPa. Ethene yield and propene yield of thermodynamic equilibrium, catalytic pyrolysis and thermal pyrolysis were compared. The use of catalyst greatly increased the yields of ethene and propene, but the yields were still lower than the equilibrium data. Catalytic pyrolysis was carried out in the interaction zone where both catalytic conversion and thermal conversion were important. Propene yield was close to ethene yield at about 950 K from the thermodynamic view. Given the shape-selective effect of the catalyst on branched olefins with large carbon number, the equilibrium carbon number distribution of olefins possibly shifted from large carbon numbers to low carbon numbers, resulting in enhanced ethene and propene yields.

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