Fischer-Tropsch synthesis fixed bed reactor intensification

Abstract

A 2D pseudo-homogeneous fixed bed reactor model was developed using ANSYS Fluent, based on an actual bench-scale Fischer-Tropsch reactor. The single-tube reactor had a diameter of 0.05m, which is representative of the diameters used in industrial applications. Using a specially-designed temperature measurement, the detailed temperature distribution in the bench-scale reactor was reported for the first time. A position-dependent heat transfer coefficient, which is considered more accurate for temperature predictions, was applied. The model was validated against the experimental data, including both the reaction results and the measured temperatures, which obtained with a Co-based catalyst under conditions of 2MPa and 458K at various syngas partial pressures and space velocities. The model was qualified to carry out temperature-prediction related simulations, for example the following studies of Fischer-Tropsch reactor intensification approaches. The changes in the maximum temperature in the bed and hot spot region are discussed in terms of different N2 flowrates and gas hourly space velocity rates. The inferred properties within the reactor were analysed to obtain insight into how to increase the production capacity of the reactor. One Fischer-Tropsch reactor intensification approach proposed was tubular reactor internals with a simple structure and easy installation, in order to suppress hot spot formation in the catalyst bed. The reactor internals were designed to adjust the effective inner diameter in the front region of the catalyst bed where the hot spot was most likely to form. A bench-scale CFD reactor model was employed to verify the performance of the reactor internals under Fischer-Tropsch reaction conditions and to optimize its key parameters: neck diameter and frustum cavity height. The simulation results for the optimised reactor internals showed that: he maximum temperature rise decreased by as much as 22.6%; the change in the rate of CO conversion was less than 2.13%; the C3+ product selectivity increased slightly. The design of reactors for small scale Fischer-Tropsch Synthesis (FTS) plants needs a different approach to that used for mega plants, because of the drivers from environmental, economic and social aspects. A novel reactor design approach for small-scale FTS reactors was proposed, in which the reactor diameter was optimised for the maximum productivity of heavy hydrocarbon products at a given temperature rise limit. The catalyst activity and the space velocity were varied so as to keep the maximum temperature in the catalyst bed at a specified value. Using a multi-tube fixed bed reactor of given dimensions as an example, CFD simulation was carried out to verify the proposed design approach. According to the simulation results for a single tube, the highest productivity was achieved with the reactor tube with the largest diameter. However, when looking at the multi-tube fixed bed reactor itself, because fewer tubes could be fitted into the reactor as the reactor tube diameter increased, the results for maximum productivity became more complicated. According to the simulation results, a maximum heavy hydrocarbon productivity of 15.3kg/h can be achieved in a multi-tube reactor fitted into a commercial container when using: an optimal tube diameter of 3/4’; a SV of 300h-1; a required catalyst activity of 695% of that of the base case. The multi-plate reactor, of which the spaces between the plates were used for thermal fluid and catalyst alternately, integrates the fixed bed reactor with the plate heat exchanger. The proposed reactor design approach was applied in the optimization of the plate pitch of a multi-plate reactor with the same aforementioned given dimension for maximum productivity at the same temperature rise limit. Similarly, the catalyst activity and the space velocity were set as parameters to adjust the temperature peak in the catalyst bed. The simulation results showed that a maximum productivity of 21.3kg/h can be obtained in the multi-plate reactor that meets the specification requirements of a containerized FT application with a 0.5 inch plate distance and 300 h-1, while the required catalyst activity is 582% of that of the base case.