(22e) Experimental Investigation and Simulation On the Phase of the Product Water in Low Temperature Fischer-Tropsch Synthesis

Water
is the main by-product in the process of converting the synthesis gas (mixture
of CO and H₂) to a liquid fuel by applying Fischer-Tropsch Synthesis (FTS). The phase of this
by-product water inside the reactor is critical in both the catalysis and
reaction engineering aspects as it is an important factor affecting
the deactivation of the catalyst, the mass transfer in the catalyst and the
heat transfer in the reactor. Fischer-Tropsch Synthesis is presumed to
take place within catalyst pores that are believed to be filled with waxy
liquid hydrocarbon products [1], while the phase of the water is either
regarded as a gas under the reaction conditions or often avoided by
the researchers. In this work, an FT experiment was designed and performed and
the simulation taking into consideration of the non-ideality
behaviour of the mixture of the hydrocarbons and water was conducted. Both the
results suggest that a considerable proportion of the water product is in
liquid phase in a low temperature FTS reactor. Experimental

The investigation of the phase of the water in the low temperature (190-230^oC) Fischer-Tropsch Synthesis
was carried out both from the experimental work and simulation. The experimental investigation was designed using an indirect method by means of monitoring the pressure
of the reactor system when the FT experiment was performed in a batch reactor. By
comparing the pressure reading of the reactor and the system pressure calculated
from reactants' conversion, the phase of the
water then could be suggested.

Flushing experiments
were further designed and performed. At the end of batch operation, the gaseous
material (including products and un-reacted reactants) in the reactor tank
(with catalyst inside) was replaced rapidly with inert gas argon. The reactor
then was flushed with argon at a low flow rate and the flushed stream was
sampled and analysed continuously. As the gaseous material has been replaced,
the products in the flushed out stream were from the liquid phase product
remained in the reactor. Results and discussion

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
Figure illustrates three total pressures in the reactor based on different
assumptions. The CO conversions for the range of reaction durations are also
given as a reference. Curve A is the experimental pressure reading of the
reactor. To be conservative we assumed all the hydrocarbons except the methane
were in the liquid phase. If we assumed all the water was in the gas-phase P_system = P_CO+P_H2+P_N2+P_CH4+P_H2O
(Curve B) we obtained a pressure-time curve that was significantly higher than
the measured curve. If we assumed all the water was in the liquid-phase, P_system = P_CO+P_H2+P_N2+P_CH4,we obtained Curve C which did give a graph much lower than the measured pressure
curve. We therefore conclude at least some of the water must have been in the
liquid-phase. We notice there is a crossing in curves A and B at around 3 hours.
This could be corrected if we took the phase of C₂ and above
hydrocarbons into consideration. The information these results provide strongly
suggest that a considerable proportion of the water formed by the reaction is
in the liquid phase.

Figure 1 The pressure in the reactor at different
reaction time (CO conversion is plotted as a reference.)

Figure 2 The vapour
content in the flushed out streams

when the FT reaction had been conducted at different temperatures

Figure 2 presents the vapour content in the flushed
out streams when the FT reaction had been conducted at different temperatures.
As it has been explained above, the material flushed out were from the liquid
phase product remained in the reactor, so that the concentration of the vapour
in the flushed out stream could tell the concentration of the water in the
liquid. By integrating the flushed out water with flushing period, the amount
of water in the liquid at different reaction conditions could be suggested.

References

1.    Madon, R. J.; Reyes, S. C.; Iglesia, E. The
Journal of Physical Chemistry. 1991.
95, 7795-7804.