(32b) 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.