Steady-State Attainment Period for Fischer–Tropsch Products

Abstract

In studying the mechanism of the Fischer–Tropsch (FT) reaction, deuterium tracer techniques have been widely used and several important conclusions have been reported. A novel combination of experiment and modelling to quantify the residence times of long chain hydrocarbons using deuterium tracing has been devised. In this study, the effect of variation of residence time with carbon number on the olefin to paraffin ratio is investigated and also the time required for each hydrocarbon to reach steady state is determined. The results show that the olefin to paraffin ratio decreases with increasing carbon number which is consistent with olefin readsorption but not necessarily diffusion enhanced olefin readsorption. Therefore, chain length-dependent solubility in the liquid phase should be the predominant cause for chain length-dependencies of secondary olefin reactions in FT synthesis. Furthermore, the results show that it takes around 100 h for the overall/total mole fractions to reach steady state. Therefore, actual compositions (mole fractions) equilibrate faster even though actual flows (moles/h) might still be changing. Hence, for practical purposes the total mole fraction can be used as a guide to establish steady state.