Chemical Engineering and Processing, 32 ( 1993) l-7 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Review Paper Mechanism and kinetics of the selective hydrogenation of ethyne and ethene zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA A. N. R. Bos and K. R. Westerterp” Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, University of Twente, P.O. Box 217, 7500 AE Enschede (Netherlands) Abstract A review of the kinetics and mechanism for the selective hydrogenation of ethyne and ethene on palladium catalysts is presented. The progress made in the last fifteen years is mainly discussed. It has become clear that the classical view, where the selectivity of the reaction was believed to be due to the thermodynamic factor is an over-simplifica- tion. Currently, it is generally assumed that at least two different sites are active during the selective hydrogenation, one of these might possibly involve the support. Ethene hydrogenation also occurs in the presence of high ethyne concentrations, which cannot be explained by the classical theory. Besides the two main hydrogenation reactions and the oligomerisation, there exists a direct route from ethyne to ethane, which, however, is only of minor importance. Possibly due to the rather complex nature of the system, there have been relatively few kinetic studies presenting practical rate expressions. Introduction In the manufacturing of polymer grade ethene, the removal of ethyne from the hydrocarbon mixtures ob- tained in cracking plants, is an important step. Typi- cally, ethyne is present as approximately 1% in either complex gas mixtures containing lo-20% of hydrogen (front-end mixtures) or in essentially ethene and ethane only (tail-end mixtures). An elegant and widely used method is the catalytic hydrogenation of ethyene. With regard to the ethyne hydrogenation, the process must be highly selective since the ethyne content has to be reduced to less than 5 ppm, while higher ethene losses are economically in- tolerable. Palladium based catalysts have proven to be capable of meeting these demands. The main reactions involved are: CzHz + Hz - CzHz, AHzg8 K = - 172 MJ/kmol (1) C2H, + H, - C,H, AH,,, K = - 137 MJ/kmol (2) Recent studies [l] have revealed that the direct hydro- genation of ethyne to ethane can also take place: C,H, + 2H2 - C,H, AHzg8 k = - 309 MJ/kmol (3) *Author to whom correspondence should be addressed. Besides these main reactions oligomerisation also oc- curs, yielding a complex mixture of C,C compounds, commmonly named ‘green oil’. In order to obtain a good selectivity and to reduce or prevent a net loss of ethene, small amounts of carbon monoxide are added to the feed gas. Industrially this hydrogenation is usually carried out in adiabatic packed bed reactors. This reaction is ac- companied by several chemical reaction engineering problems, in particular the phenomenon of thermal runaway, which is known to occur rather often in industrial practice. Here we present a brief literature review on the kinetics and catalytic aspects of the selective hydrogenation of ethyne/ethene on palladium based catalysts, that is Pd/Al,O,. First, we shall discuss the properties of palladium catalysts with respect to adsorption of hydrogen, ethyne and ethene on the metal surface. After a brief discussion on the oligomeri- sation, the literature on the mechanisms and kinetics of the hydrogenation reactions will be reviewed. Properties of PI/Al, 0, catalysts For the selective hydrogenation of ethynejethene, low surface area alumina supports are mostly used, while the palladium content typically amounts to only 0.01-O. 1% by weight. Industrially, for the selective hydrogenation 0255-2701/93/$24.00 G 1993~ Elsevier Sequoia. All rights reserved