Catalyst Deactivation by Coke Formation in Acetylene Hydrogenation

Doctoral thesis, 1996

Several aspects of catalyst deactivation have been studied with reference toselective acetylene hydrogenation, a reaction used industrially to purify ethylene from unwanted acetylene. A mathematical model has been developed for an industrial packed-bed reactor, employing a supported Pd catalyst. Based on the reactor model, anextended Kalman filter was developed to estimate catalyst activity from process data. This framework was found to be an excellent tool for combining information from measurements and partly uncertain process models. All systems using process data are sensitive to occasional gross errors. An improved scheme is presented for detection of such errors, based on balance equations and specified variable bounds. Within the scope of modelling, a new numerical algorithm has been developed for preparative liquid chromatography problems. The proposed algorithm is based on a novel view of axial dispersion and is superior to conventional algorithms for high-efficiency columns. Coke formation and its effects on reaction rates have been studied in an internal recycle reactor. The catalysts used were Pd/.alpha.-Al2O3 and Pd/.gamma.- Al2O3. Acetylene hydrogenation rate was only slightly affected by catalyst aging, while the product distribution changed more. Accumulation of coke in the porous catalysts was found to increase the intraparticle diffusion resistance leading to an increased ethane formation. This effect could be readily explained by a model for the intraparticle mass transfer. Aside from mass transfer effects, no direct relation was found between observed reaction rates and the amount of coke deposited on the catalyst. In contrast, ethane formation increased markedly when a catalyst was aged at low H2/C2H2 ratios or in the presence of a liquid. TPO analyses showed no differences between coke formed at different conditions, suggesting that the increased ethane formation is caused by only a small fraction of the deposited species. The incorporation of deuterium in the deposited coke showed that hydrogen or deuterium from the gas is needed for coke formation. A half- hydrogenated species is suggested as the coke precursor.