Polymerisation kinetics of the Fischer-Tropsch CO hydrogenation using ironand cobalt based catalysts

Many equations describing the intrinsic rate of Fischer-Tropsch CO hydrogen ation have been proposed in literature. Most of them are empirical, but a n umber of them have been based on a mechanism with a postulated rate determi ning step. For the derivation of these rate equations it is usually assumed that the rate of formation of the monomer is rate limiting, i.e. the slowe st step in the product formation in the Fischer-Tropsch synthesis. This app roach is fundamentally flawed since then methane would be the most abundant product. Furthermore, it does not take into account the polymerisation cha racter of the Fischer-Tropsch synthesis and it cannot account for the obser ved product distributions. It will be shown that a different approach, whic h accounts for the polymerisation character of the Fischer-Tropsch synthesi s and the irreversibility of this reaction, leads to a rate expression whic h is similar to the ones derived previously in many aspects. The theoretica l implications of the rate determining step, however, no longer apply. The new rate equation proposed is based on the assumption that the rate of the reaction in the Fischer-Tropsch synthesis is governed by the rate of hydrog enation of surface carbon. The proposed rate equation will be used successf ully to describe the kinetics of the consumption of carbon monoxide for the formation of organic compounds in the Fischer-Tropsch synthesis for both i ron and cobalt based catalysts. (C) 1999 Elsevier Science B.V. All rights r eserved.