Methanol synthesis over industrial Cu/Zn/Al₂O₃ catalyst: kinetics selection and analysis of intraparticle diffusion limitations

Experimental data on methanol synthesis were obtained in a laboratory flow reactor over an industrial Cu/Zn/Al₂O₃ catalyst (Katalko 51-8) using fractions of 0.5-1 mm, 1-3 mm, and whole pellets (5×5 mm cylinders) at pressures of 31, 41, and 49 bar and temperatures of 200-270 °C. Based on the literature kinetics of Graaf et al. (1988, 1990) and a comparison of the modeling results for an isothermal laboratory reactor with experimental data in the kinetic regime, a two-stage thermodynamically consistent reaction scheme was selected, which includes methanol formation from CO and H₂ and the reverse water-gas shift reaction, along with the corresponding kinetic constants. A numerical analysis of the effect of intraparticle diffusion limitations on the apparent reaction rates was performed for the porous catalyst pellet using a parallel pore model and the proposed kinetics. The spherically symmetric boundary value problem was based on the Stefan-Maxwell equations for multicomponent diffusion, accounting for the contribution of Knudsen diffusion within the isothermal pellet. It was shown that at a pressure of 49 bar and with increasing temperature from 190 °C to 250 °C, the calculated effectiveness factor for the 5 mm industrial catalyst pellets decreases from 93% to 36% for the methanol synthesis and from 93% to 52% for the reverse water-gas shift reaction.

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