New size-dependent kinetic equations for hydrocarbon production rates from Fischer–Tropsch synthesis on an iron-based catalyst

Abstract

The effect of catalyst nanoparticle size on hydrocarbon production rates in the Fischer–Tropsch synthesis (FTS) has been investigated on an iron-based catalyst. A series of iron oxide particles was prepared via precipitation by a microemulsion method. New size-dependent kinetic models for hydrocarbon production rates were developed using a thermodynamic analysis method. Size-dependent parameters of the models were evaluated using the experimental results with a non-linear optimisation routine by minimising the mean absolute relative residual. Because of the role of iron carbide in the FTS reaction by iron catalysts, the iron carbide particle sizes was considered as an important factor in this paper. Experimental results show that the hydrocarbon product distribution shows a slight shift to lower molecular weight hydrocarbons by decreasing catalyst particle sizes and increasing the reaction temperature. The value of the surface tension energy (σ) for paraffin and olefin production on the iron catalyst is calculated in the range 1.2–0.9 J m^–2 and 0.62–0.49 J m^–2 respectively. These values are lower than for metals and are related to the presence of iron carbide in the catalytic reaction. Also, σ for paraffin production is higher than that for olefin production. The size- and carbon number-independent activation energies for paraffin and olefin formation are 9.1 and 14.9 kJ mol⁻¹, respectively.