Oxidation of ethylene to ethylene oxide: catalyst deactivation in an industrial run

Abstract

The deactivation of a silver catalyst for the selective oxidation of ethylene to ethylene oxide was investigated after six years of industrial use. Some tubes of the reactor were unloaded and the catalyst was divided into five parts: sample U1 at the gas inlet (top of the reactor) to sample U5 at the gas outlet were compared with fresh catalyst (F). This permitted the determination of a deactivation profile along the reactor. The performances of the fresh and discharged catalysts were examined in an internal recycle reactor in the range 503 – 543 K. The rate of ethylene oxidation (at a constant feed composition and reaction temperature) and the selectivity to ethylene oxide (at a constant ethylene conversion) varied according to the sequence F ≫ U5 ≅ U4 ≅ U3 > U2 > U1. At the same temperature, U1 and U2 were not only the least active, but also the least selective catalysts. The specific rate of ethylene oxidation (based on O 2 chemisorption data) followed a reverse sequence: U1 ≅ U2 > U4 > F. The apparent activation energies for the formation of ethylene oxide and of carbon dioxide were lower for U1 than for the other samples. Catalyst U1 was peculiar also for the temperature oscillations occurring in the reactor. The physico-chemical characterization of the samples has shown that (i) contamination by sulphur decreases from U1 to U5, (ii) oxygen uptake varies according to the sequence F > U4 > U2 ≅ U1, (iii) silver sintering occurred in the discharged catalysts and seems to be more pronounced for the top levels of the reactor tube and (1v) for some silver particles, changes of morphology accompany the metal sintering. Based on the previous literature, these findings were examined as possible explanations of the modified performances of the discharged catalyst.