In the product separation units of ethylene cracking plants the ethylene stream contains acetylene. This acetylene has to be removed, because it hampers the ethylene polymerization. Usually this is done in a catalytic adiabatic fixed bed reactor, where the acetylene is selectively hydrogenated with an excess of hydrogen. Ideally it is converted into ethylene only, but small amounts of ethylene are also cohydrogenated. The two reactions are:
The hydrogenation reactors are known to exhibit thermal runaway rather frequently; suddenly the ethylene cohydrogenation starts full scale, thereby completely consuming the excess of hydrogene and increasing the temperature difference over the bed manifold. A project was started in our laboratories to investigate the causes of the runaway and the dynamic behaviour of the reactor. During experiments on the reaction kinetics some unexpected phenomena were observed: it took long periods of time to reach stable conditions in a test reactor. This observation led us to a study of the behaviour of a single catalyst pellet. In this study a temperature overshoot was observed with a time constant in the order of magnitude of minutes; it is this phenomenon that shall be reported here in more detail.
2 Literature survey
Plant conditions for the acetylene removal by hydrogenation is described a. o. in [I -51; Lam and Lloyd [5] provide many operating details. The C2H2 content of the ethylene stream is usually reduced from 2000 to 20 000 ppm to around 1 to 5 ppm with a Pd catalyst on a alumina carrier. The reactor inlet temperature is 50 to 80 'C and the exit temperature depends on the amount of hydrogen converted, the adiabatic temperature rise being 30 to 4 0 T per mole percent of H2 converted. The reactor pressure varies from 1.0 to 3.5 MPa. Carbon monoxide is often added to the feed stream to moderate the catalytic activity and increase the selectivity. The literature is not conclusive as t o the reaction rate equations, but all agree upon a Lungmuir-Hinshelwood or an Eley-Rideul type. Most experiments were done at atmospheric pressure. On a Pd catalyst the selectivity towards the acetylene hydrogenation is high, which is explained by the strong adsorption of CzH2. Probably C2H2 and C2H4 are adsorbed on different sites and the C2H2 is replaced by Hz, as it is consumed. The selectivity deteriorates, when the C2H2 is replaced by H2 . C O adsorbs on the same sites as Hz , an addition of CO leads to a reduction of the catalyst activity, but restores its selectivity. Beeck [6] reported on a reversible deactivation of the catalyst without mentioning an influence on the catalyst pellet temperature. Catalyst pellets can exhibit multiplicity and instabilities, e. g. oscillations. These phenomena are amply discussed in literature [6-101. Concentration overshoot phenomena on catalyst pellets after a step