Enhanced Heat Transfer Catalyst Structures for Fischer-Tropsch Synthesis

Abstract

Highly exothermic catalytic reactions are problematic from a thermal management perspective and often dictate the type of reactor, heat exchanger, level of conversion/recycle and contacting schemes employed. Those reactions, such as Fischer-Tropsch synthesis (FTS), require catalyst beds with enhanced heat transfer characteristics. Novel catalyst structures, microfibrous entrapped catalyst (MFEC) structures, made of highly conductive metals were compared with traditional packed beds (PB) based on the experimental determinations of thermal parameters and performances in FTS process. Conductive metal MFECs had higher effective thermal conductivities and higher inside wall heat transfer coefficients than PBs including those made of pure copper particles. The radial effective thermal conductivity of copper MFEC was 56-fold higher than that of alumina PB in a stagnant gas, while the inside wall heat transfer coefficient was 10 times higher. A modified resistance network model, the junction factor model, is developed to predict the effective thermal conductivity of sintered microfibrous materials (MFM) made of conductive metals. It contains two characteristic variables: metal volume fraction (y) and junction factor (∅). The junction factor representing the fibers’ connection quality can be easily determined by the measurement of electrical resistance, so this model provides a practical and convenient method to estimate the effective thermal conductivity of sintered MFM. Moreover, various methods to improve the junction factor and the effective thermal conductivity of copper MFM are investigated. High sintering temperatures and long sintering times increase both the junction factor and effective thermal conductivity of MFM. Electroplating and impregnation methods were also employed to enhance the junction conductivity. Electroplating provides a significant improvement in the junction factor and the effective thermal conductivity of the media. Computational Fluid Dynamics (CFD) was used to compare the micro scale heat transfer inside a packed bed and a MFEC structure. Simulations conducted in stagnant gas determined the thermal resistance of the gas in the micro gaps between the particle-to-particle contact points in the resistance network model of a packed bed. It was shown that thermal resistance at the contact points accounted for 90% of the thermal resistance of the solid path. In the MFEC, the thermal resistance of the continuous metal fibers was relatively smaller than that of contact points. As a result, 97.2% of the total heat flux was transported by continuous fiber cylinders, which was the fundamental function of fibers on improving the heat transfer of MFEC structures. Enhanced heat transfer characteristics of MFEC were further demonstrated by simulations performed in flowing gas, where both heat conduction and heat convection were significant. To investigate the performance of the enhanced heat transfer characteristics of MFEC, 15wt% Co/Al2O3 catalyst particles (149-177µm dia.) were examined in both a packed bed configuration and after being entrapped within a 7.4 vol.% network of sintered Cu fibers (12µm dia.). FTS at 225-255°C, 20bar, H2/CO of 2.0, was utilized as the probe reaction due to its exothermicity and temperature dependent selectivity. Both the hot spot and runaway state were prevented by utilizing metal MFEC compared to the packed bed. In a 41mm ID reactor, the maximum temperature deviation from the centerline to the reactor wall was only 6.4°C for the copper MFEC. In contrast, the packed bed diluted to the same catalyst density and operated at an equivalent condition had a centerline temperature deviation of 460°C indicating ignition. The more isothermal temperature profile through the catalyst bed of the copper MFEC led to a higher selectivity of heavy products than that of the packed bed. Also, it enabled a larger reactor diameter to be used with more precise and robust temperature control.