In this study the effect of catalyst nanoparticle size on hydrocarbon production rates in the Fischer- Tropsch synthesis is investigated on iron catalyst. A series of iron oxide particles were prepared via precipitation by microemulsion method. New size-dependent kinetic models for hydrocarbon production rates were developed using a thermodynamic analysis method. Size-dependent parameters of models were evaluated using the experimental results with a non-linear optimization routine by minimizing the mean absolute relative residual. Because of the role of iron carbide in FTS reaction by iron catalysts, the iron carbide particle sizes was considered as an active phase in this paper. Experimental results show that hydrocarbon product distribution shows a slight shift to the lower molecular weight hydrocarbons by decreasing catalyst particle sizes and increasing reaction temperature. The value of surface tension energy (σ) for paraffin and olefin productions on iron catalyst is calculated in range 1.2- 0.9 J/m2 and 0.62 – 0.49 J/m2, respectively. This value is lower than metals and related to presence of iron carbide in catalytic reaction. Also, the surface tension energy (σ) for paraffin productions is higher than olefin. The size and carbon number independent activation energies for paraffin and olefin formations are 9.1 and 14.9 kJ mol-1, respectively.

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