Analysis of the reduction kinetics of a Fischer-Tropsch Co/TiO2 catalyst using temperature programmed reduction: the implications and applications to industry

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. iv 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.