The influence of syngas composition on the initial behaviour of a Co/Al2O3 catalyst in Fischer–Tropsch reaction has been studied in a continuous perfectly mixed slurry reactor for an inlet H2/CO ratio between 1.6 and 3.35 keeping other conditions constant (T = 220 °C, P = 2 MPa). Significantly different behaviors of initial deactivation for CO conversion have been observed with different H2/CO ratios. It was observed that the deactivation increases with increase in H2/CO ratio and in carbon monoxide conversion. The computed liquid concentrations of CO, H2 and H2O have shown that water is the most abundant species in the liquid phase of the reactor during our experiments. The concentration of the water produced by the FT reaction seems to be the key parameter responsible of the initial behavior and then of the initial deactivation. For moderate levels of water (Click to view the MathML source corresponding to PH2O/PH2<0.4), a simple kinetic model assuming a reversible oxidation of cobalt active sites by water in competition with their reduction by hydrogen seems to represent satisfactorily the initial behaviour of the catalyst. For higher water concentrations, the irreversible deactivation should be probably taken into consideration.
