Steady-state modelling and concentration-forcing operation applied to the methanol synthesis reaction over two industrial copper oxide, zinc oxide, aluminum oxide catalysts

The kinetics of the industrially important methanol synthesis reaction is not well understood. In particular, there is controversy over the relative contributions of carbon monoxide and carbon dioxide during methanol synthesis as well as over the identity of the active site(s) on the copper oxide, zinc oxide, aluminum oxide catalyst. In this study a steady-state rate expression was developed based on mechanisms reported in the literature as well as on kinetics data obtained in our laboratory. This rate expression was the first reported in the literature which included a mechanism-based term accounting for the contribution of carbon dioxide and was able to predict, better than other rate expressions previously reported in the literature, the kinetics behavior relative to data for two industrial methanol synthesis catalysts with different performance characteristics. In particular, the proposals that carbon monoxide hydrogenation and carbon dioxide hydrogenation mechanistically occur on separate sites and that carbon dioxide inhibits carbon monoxide hydrogenation but not vice versa appear to be justified.; Concentration-forcing operation was applied to methanol synthesis in a differentially operated plug-flow reactor in order to determine if production rates greater than that of the optimal steady-state production rate could be obtained. Improvement relative to the optimal steady-state methanol production rate were obtained over the BASF S 3-85 catalyst at {dollar}tau{dollar} = 12 seconds and a {dollar}gammasb{lcub}rm CO{rcub}{dollar} = 0.20, 0.27, and 0.40 for pure component cycling between hydrogen and carbon monoxide with a constant molar concentration of 2% carbon dioxide present in both parts of the cycle. The maximum improvement of 1.25 times the optimal steady-state rate was obtained at {dollar}tau{dollar} = 12 seconds and {dollar}gammasb{lcub}rm CO{rcub}{dollar} = 0.20. Improvement was also obtained over the ICI 51-2 methanol synthesis catalyst at {dollar}tau{dollar} = 12 and 24 seconds and {dollar}gammasb{lcub}rm CO{rcub}{dollar} = 0.15 and 0.20 for pure component cycling with a constant molar concentration of 3% carbon dioxide present in both parts of the cycle. The maximum improvement of 1.15 times the optimal steady-state rate was obtained at {dollar}tau{dollar} = 24 seconds and {dollar}gammasb{lcub}rm CO{rcub}{dollar} = 0.15. No improvement over the optimal steady-state methanol production rate was found in pure-component cycling between carbon monoxide and carbon dioxide with a constant molar concentration of 61% hydrogen or between hydrogen and a 19.6/78.4 mole percent mixture of CO/H{dollar}sb2{dollar} at the conditions studied.