Kinetic models of the molecular mass distribution of the products of the Fischer-Tropsch synthesis at cobalt catalysts

Abstract

Kinetic models were used to investigate the Fischer-Tropsch synthesis at increased pressures. The general mechanism proposed for the process at low and increased pressures was based on quantitative analysis of the products obtained with cobalt catalysts at pressures of 2.5-10 MPa using various kinetic models, the previously developed reference point method, and published data.The molecular mass distributions (MMD) of the hydrocarbons formed in the Fischer-Tropsch (FT) synthesis often differ from the Anderson-Schulz-Flory (ASF) distribution [1]. Various schemes have been proposed for the mechanism of the process to explain the deviations from the ASF distribution [2][3][4][5][6][7][8][9]. On the basis of such schemes mathematical models establishing a relationship between the concentrations of the individual products and the carbon number were derived. As kinetic coefficients they contain the probabilities or functions of the probabilities that the reactions occur with the participation of intermediates. They are models of the selectivity, but they make it possible to calculate the absolute rates of all stages of the kinetic scheme, starting from the rate of transformation of CO or H 2 [2]. A whole series of reasons for the deviations from the ASF distribution have been discussed in the literature.The reduced or increased methane content of the products is explained by the fact that the C 1 and C 2+ hydrocarbons are formed at different centers on the surface of the catalyst [10] or as a result of reactions leading to termination of the chain [1]. The deviation from the ASF distribution is sometimes attributed to increase in the probability of growth of the chain of C-C bonds (a) according to Herington scheme [11] with increase in the carbon number [1].The molecular mass distributions of the hydrocarbons often take the form of concave curves with a clearly defined kink [1,12,13]. Anderson approximated such distributions by two straight lines with different slopes in relation to the abscissa axis [13]. The existence of two probabilities of chain growth in the Fischer-Tropsch synthesis (usually a 1 < a 2 ) was thus mentioned first in this paper. Later Huff and Sutterfield [12] proposed an equation describing such distributions. This equation was obtained on the basis of the idea that the experimental molecular mass distributions represent the superimposition of the distributions of the a 1 -and a 2 -hydrocarbons.Initially the existence of two values for the probability of chain growth was explained by the fact that the growth of the C-C bonds takes place at different centers of the catalyst [12, 14]. However, subsequent data indicated that the two values of a arise as a result of simultaneous growth of the chain by two different mechanisms through two types of intermediates [9,[15][16][17][18]. The distributions of the hydrocarbons formed at Fe catalysts usually obey the Huff-Sutterfield equation [1,[11][12][13].(Here, they can often be approximated with sufficient accuracy by two straight lines.) The distributions o...