Study of Fischer-Tropsch Synthesis (FTS) In a Batch Reactor with TiO2 Supported Co Catalyst

INTRODUCTION: Laboratory scale research for low temperature FTS is normally carried out in a fixed bed or a slurry bed reactor. A batch reactor in the gas-solid reaction regime is seldom used by researchers in either evaluating a catalyst or studying the mechanism of the FTS It is however of value in investigating the FTS as it creates an even reactant distribution environment for all the catalyst pellets in the reactor, and eliminates the effect of the solvent, which is used in a slurry bed. The continuous mode reactors (PFR and CSTR) are operated at steady state in most cases, while the batch reactor is operated at unsteady state and the pressure of the reactor and partial pressure of the reactants and products change with the extent of the reaction. This may offer extra information for us to understand the behavior of FT reaction.

METHODS: The Fischer-Tropsch reaction was conducted in a batch reactor on a TiO₂ supported cobalt catalyst (10%Co/90%TiO₂, BET area 28.6 m²/g, average pore diameter 35.8 nm) in a gas-solid regime to present another way of looking at the Fischer-Tropsch Synthesis. The batch operation was initiated after the reactor was first operated at steady state in CSTR mode. The reaction conditions applied were that of typical low temperature FTS for cobalt catalyst with a reaction temperature 210 ^oC, starting pressure 20 bar(g), H₂/CO = 2 in the feed. The reaction duration was varied from 20 minutes to 22.5 hours

RESULTS: The conversion at various reaction durations was tracked and the reaction rate was compared to that in the CSTR mode. The concentrations of the reactants were correlated to reaction time and the reaction rate could be expressed as first order with H₂ concentration. CH₄ selectivity was investigated and the olefin to paraffin ratios for the light hydrocarbons was compared at different reaction durations and to that for the steady state in the CSTR mode. The product distribution for C₁-C₉ is given and the results were obscure as an ascending trend was observed with the increase of carbon number. The pressure of the reactor was monitored.

Fig. 1: The pressure in the reactor at different reaction durations with the corresponding CO conversions (starting at 20bar)

Fig. 2: The reaction rates in the CSTR and batch operation modes

The reactor system pressure at different reaction durations in the batch reactor are given in Figure 1. The corresponding CO conversions are given as well. The reaction rates for the CSTR and batch operational modes are presented in Figure 2.

DISCUSSION & CONCLUSIONS: For the results presented in Figure 1, if one assumes that all the hydrocarbons condense, the calculated pressure in the batch would be significantly higher than the measured. Thus one can only presume that at least a fraction of the water condenses. We have attempted to simulate this behaviour. For the reaction rates in CSTR mode and Batch mode presented in Figure 2, an apparent increase is observed when the reactor was switched from CSTR to batch mode. As there would no expectation of a change of the catalyst properties, this is believed to be caused by the accumulation of the products in the reactor.