Analysis of the reduction kinetics of a Fischer-Tropsch Co/TiO2 catalyst using temperature programmed reduction: the implications and applications to industry
Date
2010-04-09T10:31:44Z
Authors
Chansongo, Mukuka S. Bowa
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Abstract
Reduction is a critical step in catalyst preparation in chemical processes.
Stability, activity and selectivity of catalysts are affected when certain
parameters, such as temperature and water partial pressures, are not
controlled during reduction.
The main objective of this work is to use information from Temperature
Programmed Reduction (TPR) that is both quantitative and qualitative, based
on the existing literature, experimental data, and the researcher’s
assumptions, to understand the implications of the reduction conditions on
catalyst in an industrial fixed-bed reactor. This investigation has been carried
out using simple mass balance calculations; evaluating reduction parameters;
developing and applying methods of kinetic analysis and modeling, for a
better comparative insight into real operating conditions.
A Co/TiO2 catalyst used in Fischer-Tropsch (FT) synthesis of hydrocarbons
from syngas generated by reforming natural gas and/or coal has been used
to illustrate this analysis. The catalyst was prepared by incipient wetness.
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To obtain accurate results for kinetic analysis, a custom-built TPR was
modified and optimized. The parameters used in the analysis to determine
the position of the maximum rate and shape of H2 consumption peaks were
also optimized.
Simple mass balance calculations were made on TPR system to determine
the rate of reduction and P during the process. This information (and
relevant literature on the subject) was used to evaluate the implications
P on catalyst reducibility in a 12m long tube.
To evaluate the effect of different parameters on catalyst reducibility, flow
rate, ramping rate, catalyst grain size and drying time prior to reduction were
studied. It was found that heating rate and drying time prior to reduction had a
significant impact on catalyst reducibility.
It has been established that a lower ramping rate maximizes the extent of
reduction to active Co metal at low temperatures, while at the same time
ensuring an equilibrium particle size which is stable against sintering.
v
The study of the effect of water content in the catalyst prior to reduction led to
the conclusion that it has a significant effect on catalyst reducibility. However,
to arrive at a more conclusive explanation of the effect it was recommended
that further FT experiments should be performed on the catalyst with varying
amounts of water content to investigate the effect, not only on the reducibility
but also the activity and selectivity of the catalyst.
It has been shown that kinetic analysis using TPR can be used to determine
the optimum reduction temperature among those that occur at different
stages (in the case of multi-step reduction). These can then be used to
predict the amount of H2 that will be consumed with increasing temperatures.
Furthermore, this study has established that the mechanism of reduction
obtained from kinetic analysis can help understand the degree of reduction
observed at various temperatures. It can also contribute to an explanation of
the stages of reduction and underlying gas-solid reactions, which in turn
make it useful as a guide to monitor and control P throughout the reduction
process in a 12m long tube.