(249j) Fischer-Tropsch Synthesis Under Periodic Operation in a Microstructured Reactor

The terms “periodic”, “pulsed” or “cyclic operation” describe a similar process control strategy consisting in the modification of a process controlling parameter according to a time repeated protocol. As far as catalytic reactors are concerned periodic operations were defined as engineering tools which can allow to tune or control selectivity and/or conversion. It can namely be applied to any control parameter whatever it is composition, temperature or pressure. Most widespread applications mainly concerned composition cycling so far.

The applications of the Fischer-Tropsch synthesis (FT) under periodic operation were reviewed by Adesina et al. Their purpose was essentially aiming at modifying the product distribution and the productivity of a given hydrocarbon product range. Other kind of pulsed applications, in order to gather data to validate a reaction pathway or to do mechanistic studies, were reported too. The application of a forced feed policy showed in all cases some effects upon FT catalytic activity and selectivity. The pulse frequency was the main influencing parameter studied. For Co based catalysts the periodic hydrogen injection was mainly studied either under single step change strategies or as a cyclic modification of P_H2. A basic assumption was that the chain propagation of FT product distribution could be restricted over narrowed lengths by periodic H₂ injection. In all cases the periodic operation evidenced the existence of an optimum adjustment of the feed parameters to maximize the activity and the product distribution. These feed parameters were the applied frequencies (or periods (T)) and the duty cycles (dc, i.e. the fractional time of exposure to a given reactant in respect of the entire period length)).

The scope of this study is to investigate the influence of high frequency pulses with various duty cycles in a microchamber reactor in order to determine if this managment policy can be eventually scalable to production. The range of frequencies applied is up to one or two order of magnitude higher than in the previous studies published. The effects on the FT performances are expressed as the CO conversion and the associated C₁₀₊ selectivity on a 20 wt% Co/SiO₂ catalyst.

Some discrepancies from the behaviour at steady-state were observed:

i) the C₁₀₊ selectivity is influenced by the duty cycle

ii) there is an optimum period length to get an increased CO conversion level for each duty cycle. For the investigated experimental space, up to 3.5 fold C₁₀₊ yield increase was obtained in respect of a continuously fed reactor at steady state.

These differences cannot be explained on the basis of the kinetic rate form available for the catalyst used.