(149d) Effect of Space Velocity and Long Residence Time to Fischer Tropsch Products and Catalysts Reduced with Different Gases

Authors: 
Gorimbo, J., University of South Africa
Muleja, A., University of South Africa
Hildebrandt, D., University of South Africa
In this paper, we carried out Fischer-Tropsch synthesis (FTS) over an industrial iron-based catalyst (FeCuKSiO2) under unusual laboratory operating conditions, involving an extremely low reaction pressure (1.85 bar absolute) and prolonged reaction times on stream (TOS) that are representative of industrial practice (5500 h). Three different reducing gases (syngas H2/CO = 2, H2 and CO) were used to activate the catalyst. FTS was conducted under similar conditions (gas hourly space velocity {GHSV} at 2592 h-1, 1.85 bar (abs) and temperature 533 K) under each reducing gas. The evolution of the morphology and microstructures of the spent catalysts that had been differently reduced were characterized by HRTEM and XRD. Running FTS for long TOS results in a change in the product distribution due to catalyst speciation as deactivation occurs. Magnetite is the dominant phase in a deactivated catalyst, and graphitic carbon is found on the catalyst surface. The deposited carbon tends to be a hindrance. Diffusion concert can be used to explain the trend observed in this work. The catalyst speciation that could be observed as the catalyst deactivated with TOS resulted in the formation of magnetite, which is a water gas shift (WGS)-active catalyst. FTS reactions are accompanied by the formation of carbonaceous by-products (like coke), which deactivate the catalyst. GHSV is preferential for an increase in light FTS products, while the opposite trend is to be seen in heavy products. From the observed results, it can be concluded that at very long TOS a trend inversion in terms of product selectivity occurs if the GHSV is changed. An analysis of the link between GHSV and product distribution has also been given using diffusion concepts.
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