Simulation of the transient and steady state behaviour of a bubble column slurry reactor for Fischer–Tropsch synthesis

This paper develops a model to simulate the dynamic and steady-state behaviour of a commercial scale (diameter 7.5 m and dispersion height 30 m) Fischer–Tropsch bubble column slurry reactor operating in the churn-turbulent regime. A distinction is made between ‘large’ and ‘small’ bubble classes and the axial dispersion model is used to simulate their mixing behaviour and also of the liquid and catalyst particle phases. The results of the dynamic simulations indicate that no thermal runaways are to be expected and that steady-state is achieved within about 7 min from start-up. Analysis of the steady-state behaviour shows that the hydrogen conversion in the reactor is mainly dictated by the ‘large’ bubbles, which account for a major fraction of the gas throughput. Comparison of the commercial scale reactor performance with that of a smaller scale demonstration unit (of diameter 1 m and dispersion height 30 m) shows that due to significantly improved staging in the liquid phase, higher conversions are achieved in the demonstration unit. Furthermore, steep temperature gradients are to be expected in the demonstration unit, while these are absent in a reactor of commercial scale. The study underlines the need for accurate information on the liquid phase backmixing for scale up purposes.