Increasing oil prices, strict environmental regulations and lack of sufficient growth in renewable energy sector have led to renewed interest in Fischer- Tropsch technology. Improved understanding of reaction mechanisms and development of detailed kinetic models for Fischer-Tropsch synthesis (FTS) would facilitate better design and optimization of all FTS reactor configurations.

In this work, detailed kinetic models have been developed utilizing mechanistic approach. Experiments were conducted over 25% Co/0.27%Ru/AlO (in parts by weight) catalyst in a 1L stirred tank slurry reactor over a wide range of conditions. Langmuir-Hinshelwood-Hougen-Watson type rate expressions were derived for the entire product spectrum. Models are based on the assumption that 1-olefins re-adsorb on active sites. Effective pressure of olefin (PCnHn*) at the catalyst surface was assumed to vary exponentially with carbon number (according to Henry's law).

The genetic algorithm followed by Levenberg-Marquardt method was used to estimate kinetic parameters for 13 models using a single set of process conditions (i.e. T = 220°C, P = 2.4 MPa, H/CO feed ratio of 2.1, and gas space velocity of 6 NL/g-cat/h). Two models FT-6 and FT-8 showed carbon number dependent chain growth probability and olefin to paraffin ratios. The model predictions were in good agreement with experimental data. Model FT-6 is based on dissociative adsorption of CO, followed by Eley-Rideal reaction with molecular H, while FT-8 follows dissociative adsorption of CO and H, to form building block monomer CH. For both of the models, chain growth takes place by alkyl mechanism and oE-olefins are formed by aB-hydride elimination reaction. Formation of paraffin occurs via single-site reaction with molecular hydrogen (FT-6) or via dual site reaction with adsorbed hydrogen (FT-8).

Parameter estimation resulted in at least one negative parameter in both models. However, to assess the physical meaningfulness of results and the true values of kinetic parameters, one has to use data at multiple sets of process conditions. This work is in progress. Nevertheless, from a qualitative point of view initial results provide valuable insight into selection of reaction mechanisms and rate determining steps for future developments and refinements.


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