Influence of forced cycling on the fischer‐tropsch synthesis. Part II. Response to feed concentration square‐waves

The influence of forced feed cycling was investigated on the product distribution from C 1 to C 9 of the Fischer‐Tropsch synthesis. A promoted iron catalyst in a small‐scale differential reactor at 246°C and 384 kPa total pressure was employed with square‐wave composition cycles generated at a constant flow velocity. Except for methane, the product distribution during feed composition cycling followed the steady‐state Schulz‐Flory model. For some combinations of period, cycle‐split and mean composition, the time‐average methane rate was higher than the corresponding steady‐state rate. Decreasing the period increased the time‐average rate both for methane and other hydrocarbon products. High methane rates, found only during a partial cycle with pure hydrogen, indicated that methane was probably being produced by a parallel reaction path, the hydrogenation of a surface carbide. The methane reaction rate via this mechanism should be strongly inhibited by CO. Product selectivity may be altered by forced composition cycling.