(143b) Thermodynamic Prediction of the Olefin Product Distribution in the Fischer-Tropsch Synthesis

A substantial number of papers postulate various mechanisms for chain growth in Fischer-Tropsch synthesis. Most agree that the synthesis mechanism may be divided into a number of steps: (a) Adsorption of reactants, (b) chain initiation, (c) chain growth, (d) chain termination, (e) desorption of products, and (f) readsorption and further reaction. It was postulated in 1926 that the synthesis involved oxygenated intermediates. This proposal was later extended in 1951 to a mechanism that explained many characteristics of synthesis products, such as carbon number and isomer distributions. Here the intermediate was postulated to be similar to an alcohol. The incorporation of radioactive alcohols into the synthesis largely confirmed these predictions.

In 1953 some concluded that primary alcohols, aldehydes, and 1-olefins were the primary synthesis products. Apparently this conclusion demanded no major revision of the concept of oxygenated intermediates in the synthesis. Further developments showed that á-olefins are primary products and that paraffins are formed from them.

As a primary assumption we suggest that the product distribution may be determined by Equilibrium considerations. Thus this work uses thermodynamics to predict the equilibrium distribution assuming that á-olefins are the primary products. The results obtained show that á-olefins follow a single alpha product distribution at Fischer-Tropsch reaction conditions which is consistent with many experimental results. However, assuming that the alcohols and parraffins are the primary products is not consistent with experimental results. This further supports that á-olefins are the primary products of the FT synthesis and paraffins are formed from them since the whole product distribution is determined by olefins.

Keywords: Fischer-Tropsch Reaction, á-olefins Primary products, FT Thermodynamic Equilibrium, Product Distribution Model, Chain propagation probability (á), ASF.