Chemical Engineering zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Science, Vol. 47, No. 9-I 1, pp. 2647-2652, 1992. ooo5-2509f92 $s.oo+o.oo Printed in Great Britain. Q 1992 Pergamon Press Ltd zyxwv KINETIC MODEL FOR METHANE OXIDATIVE COUPLING REACTORS ENIRICERCHE SSonato Milsnese. Milan Italy M.DENTE, E.RANZI POLITECNLCO Chemical Engineering Department Milan Italy ABSTRACT A heterogeneous kinetic scheme, describing the catalyst surface reactions involved in the oxidative coupling of methane. is connected to a detailed homogeneous kinetic scheme with the aim of simulating the reacting behaviour of mixtures of oxygen and light hydrocarbons. The homogeneous put of the scheme involves more than 1000 elementary reaction steps and 60 reacting species; it has been demonstrated as able to describe the experimental behaviour of the light hydrocarbon pyrolysis (methane included) up to the partial oxidation of those hydrocarbons and combustion of them and thair intermediate products (as CHgO. CH30H etc.). Due to its generality, the homogeneous portion of the kinetic scheme can be related to every catalytic heterogeneous scheme. depending on the particular catalyst used. The performance of the overall scheme in oxidative coupling conditions (IWO-900°C and large excess of methane) is verified in comparison with experimental data obtained in a lab scale tubular reactor. The kinetic parameters of the heterogeneous reactions have been tuned on the basis of the experimental results. This overall scheme allows studies on possible process and reactor alteruatives. l-Introduction zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA For a long time there has been an industrial interest in transforming natural gas into more valuable products (chemicals or fuels); this interest has increased in the last ten years and a lot of research activities have been performed in connection with that purpose. One of the recent and potentially attractive prospects seems to be the oxidative coupling of methane. In this process methane reacts with 02 in the presence of a catalyst and C2 hydrocarbons (mainly ethanc and ethylene) are formed, together with oxygenated products like H20, CO and CO2 and other minor components such as CHs0I-I and CH20. If the proper catalyst and operating conditions are adopted, practically all the oxygen is converted and satisfactory selectivities on the transformed methane are obtained. The oxydative properties of the catalyst are active also on most of the primary products, and of course, this fact can influence the selectivities, giving by itself problems of optimal conditions. Depending on the catalyst, the process temperature ranges from 700 up to 900°C, the pressure being close to atmospheric. As a consequence, the temperature levels adopted for the methane oxidative coupling are such as to make it unavoidable that a significant amount of homogeneous radicalic reactions spontaneously take place in the gaseous phase, in all the empty volume (including the catalyst pores) of the reactor within and outside the catalyst bed. These oxidative and pyrolysis side reaction paths involve methane, oxygen and products as well, creating an interaction and a competition with the catalytic reactions. Moreover, following the most diffused and realistic interpretations about the prevailing role of the catalyst, the active centers of it behave as a source of radicals that diffuse towards the gas phase; therefore, the catalyst becomes the effective initiator of the homogeneous reaction chain (Ito et al.. 1985; Ekstrom et al., 1989; Driscoll et al., 1985). Every empirical interpretative trial of the experimental results (Otsuka et al., 1986; Wada et al., 1989; Carreiro et aL.1988). that would neglect the importance of the homogeneous reactions, considering only the catalytic aspects. would fail in being extrapolated, both on the basis of the detailed mechanistic description and on the basis of a representation of the data coherent with the behaviour of the system in absence of the catalyst. 2647