Kinetics of Slurry Phase Fischer-Tropsch Synthesis

Abstract

This report covers the fourth year of a research project conducted under the University Coal Research Program. The overall objective of this project is to develop a comprehensive kinetic model for slurry-phase Fischer-Tropsch synthesis (FTS) employing iron-based catalysts. This model will be validated with experimental data obtained in a stirred-tank slurry reactor (STSR) over a wide range of process conditions. The model will be able to predict molar flow rates and concentrations of all reactants and major product species (water, carbon dioxide, linear 1-and 2-olefins, and linear paraffins) as a function of reaction conditions in the STSR.During the fourth year of the project, an analysis of experimental data collected during the second year of this project was performed. Kinetic parameters were estimated utilizing product distributions from 27 mass balances. During the reporting period two kinetic models were employed: a comprehensive kinetic model of Dr. Li and co-workers and a hydrocarbon selectivity model of Van der Laan andBeenackers (1998, 1999) The kinetic model of Yang et al. (2003) has 24 parameters (20 parameters for hydrocarbon formation, and 4 parameters for the water-gas-shift (WGS) reaction). Kinetic parameters for the WGS reaction and FTS synthesis were estimated first separately, and then simultaneously. The estimation of these kinetic parameters employed the Levenberg-Marquardt (LM) method and the trust-region reflective Newton large-scale (LS) method. A genetic algorithm (GA) was incorporated into estimation of parameters for FTS reaction to provide initial estimates of model parameters.All reaction rate constants and activation energies were found to be positive, but at the 95% confidence level the intervals were large. Agreement between predicted and experimental reaction rates has been fair to good. Light hydrocarbons are predicted fairly accurately, whereas the model underpredicts values of higher molecular weight hydrocarbons.Van der Laan and Beenackers hydrocarbon selectivity model provides a very good fit of the experimental data for hydrocarbons up to about C 20. However, the experimental data shows higher paraffin formation rates in C 12 -C 25 region which is likely due to hydrocracking or other secondary reactions. The model accurately captures the observed experimental trends of decreasing olefin to paraffin ratio and increasing α (chain growth length) with increase in chain length.