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Unlike traditional kinetic models describing the dependence of the concentrations of all the products on the number of carbon atoms and widely used for analysis of the mechanism of the Fischer-Tropsch synthesis, fragmentary models establish a relationship between the concentrations of a limited number of products. They can be constructed if it is possible to obtain a dimensionless function of the concentrations that does not change with change in the concentrations themselves. Such models are based on fragments of the kinetic schemes of the process and are designed both for identification and discrimination among the mechanisms with data obtained in both differential and integral reactors. The general principles for the construction of fragmentary models are described. Examples of identification and discrimination among the various mechanisms from the results of fragmentary modelling using previously obtained experimental data for a cobalt catalyst are presented.Kinetic modelling has recently been used widely to investigate the mechanism of the Fischer-Tropsch (FT) synthesis [1][2][3][4]. In this case, mathematical models describing the dependence of the concentrations of the products on the carbon number have been derived on the basis of hypothetical schemes for the mechanism of the synthesis. As kinetic coefficients they contain the probabilities of the occurrence of reactions with the participation of various intermediates. The initial kinetic schemes usually include reactions leading to the formation of products with numbers of carbon atoms between 1 and ¥. As a rule the mathematical models establish a relationship between the concentrations of all the reaction products.Earlier [5,6] to study the mechanism of the FT synthesis it was proposed to use models constructed from fragments of the kinetic schemes of the mechanism. In the present work the general approach to the construction of such models and examples of their application to identification and discrimination among the various versions of the mechanism are examined. The isolated fragments of the kinetic schemes and the mathematical models based on them will subsequently be called fragmentary models (FM). PRINCIPLES OF THE CONSTRUCTION OF FRAGMENTARY MODELSLet us assume that it is possible from a kinetic scheme containing an arbitrary number of reactions to single out a fragment consisting of several reactions that take place in an adsorbed layer. Let such a fragment consist of m nonequilibrium reactions, the rate of which obeys the Langmuir equation 0040-5760/06/4202-0071

Increasing the working time of the catalyst led to a change in the surface concentration of various intermediates. The activity of the catalyst decreased principally as a result of the decrease in the rate of methane formation. The probability of chain growth increases via alkyl intermediates and the contribution of secondary reactions in the process decreases. In this way the selectivity with respect to C 5+ hydrocarbons increases, but the content of these products in the C 2+ fraction decreases.